The Wolfson Department of Chemical Engineering
Technion – Israel Institute of Technology

Teem LabPublications

Publications

Chain entanglement and free-volume effects in branched poly (arylene piperidinium) anion-exchange membranes: random-branched versus end-branched

Authors:
Jiandang Xue, John C Douglin, Tong Huang, Haifei Jiang, Junfeng Zhang, Yan Yin, Dario R Dekel, Michael D Guiver
Journal:Journal of Membrane Science

Poly(arylene piperidinium) (PAP) anion-exchange membranes (AEMs) are attractive platform materials currently used for anion-exchange membrane fuel cells (AEMFCs) and chain branching modifications on PAP polymer structures have recently drawn much attention for effectively regulating the membrane properties. To better understand the influence and interplay of the polymer chain entanglement and free volume effect from various branched architectures, herein, lightly end-branched EB-PTP/TPB AEMs, using poly(p-terphenyl dimethylpiperidinium) (PTP) as the polymer backbone and bulky rigid 1,3,5-triphenylbenzene (TPB) as the branching agent, are designed and compared to previously reported random-branched RB-PTP/TPB and linear-chain PTP membranes. Intrinsic viscosity, bulk density and positron annihilation lifetime spectroscopy suggest a lesser extent of chain entanglement, but more evident free-volume elements for end-branched EB-PTP/TPB, particularly for higher branched EB-PTP/TPB-1% containing 1 mol% of TPB branching agent, compared with the random-branched RB-PTP/TPB that exhibits a more predominant chain entanglement effect. EB-PTP/TPB AEMs with increased free volume exhibit faster water sorption kinetics and slightly higher ion conductivity, but the concurrent high water sorption levels are disadvantageous to high-temperature ion transport, alkaline stability and water balance management. Even so, EB-PTP/TPB AEMs still show sufficient mechanical robustness, restricted swelling and higher alkaline stability than linear-chain PTP membranes, aided by the considerable extent of chain entanglement. This work reveals branched architecture–AEM property relationships and provides valuable insights towards branched architecture design.

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Wet-coating Amino-functionalized PDMS Interlayer Boosting Thin Film Composite Membranes for Efficient CO2 Separation

Authors:
Zhicong Liang, Jiali Tang, Dario R Dekel, Xuezhong He
Journal:Journal of Membrane Science

Membrane separation is in the spotlight as one of the most cost-effective technologies without chemicals for carbon capture. This work aims to fabricate thin film composite (TFC) membranes for boosting CO₂ separation. Commercial polysulfone (PSf) flat sheet membranes pre-treated with ethanol solutions were used as support materials by coating with amino-functionalized cross-linked polydimethylsiloxane (PDMS) interlayer. It is worth noting that the developed amino-functionalized PDMS interlayer based on a novel wet-coating method to avoid the penetration of coating solution, which provides better compatibility with the coated polyamide selective layer and also the facilitated transport for CO₂ permeation. A thin CO₂-selective layer was obtained by interfacial polymerization (IP) between trimesoyl chloride (TMC) in the organic phase and diethylene glycol bis(3-aminopropyl) ether (DGBAmE or EO3) in the aqueous phase to enhance the CO₂/N₂ selectivity. Both the IP process and the membrane preparation parameters such as heat-treatment temperature, and monomer concentration were systematically optimized. It was found that the best membrane prepared with 9.9 mmol/L TMC monomer solution presents a CO₂ permeance of 81 GPU and a CO₂/N₂ selectivity of 65, which was significantly enhanced from the non-selective supports. This work provides a facile approach for tuning the TFC membrane performance for CO₂ separation and can be extended to make high-performance membranes for industrial CO₂ capture by selecting more permeable supports.

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Anisotropic membrane with high proton conductivity sustaining upon dehydration

Authors:
Jian Li, Jay Prakash Singh, Vadim Neklyudov, Mikhail Stolov, Ziyi Yuan, Yaelle Schilt, Uri Raviv, Dario R Dekel, Viatcheslav Freger
Journal:Science Advances

In fuel cells and electrolyzers, suboptimal proton conductivity and its dramatic drop at low humidity remain major drawbacks in proton exchange membranes (PEMs), including current benchmark Nafion. Sustained through-plane (TP) alignment of nanochannels was proposed as a remedy but proved challenging. We report an anisotropic composite PEM, mimicking the water-conductive composite structure of bamboo that meets this challenge. Micro- and nanoscale alignment of conductive pathways is achieved by in-plane thermal compression of a mat composed of co-electrospun Nafion and poly(vinylidene fluoride) (PVDF) nanofibers stabilizing the alignment. This translates to pronounced TP-enhanced proton conductivity, twice that of pure Nafion at high humidity, 13 times larger at low humidity, and 10 times larger water diffusivity. This remarkable improvement is elucidated by molecular dynamics simulations, which indicate that stronger nanochannels alignment upon dehydration compensates for reduced water content. The presented approach paves the way to overcoming the major drawbacks of ionomers and advancing the development of next-generation membranes for energy applications.

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Anion-Exchange Membrane Oxygen Separator

Authors:
Maisa Faour, Karam Yassin, Dario R Dekel
Journal:ACS Organic & Inorganic Au

Anion-exchange membranes (AEMs), known for enabling the high conductivity of hydroxide anions through dense polymeric structures, are pivotal components in fuel cells, electrolyzers, and other important electrochemical systems. This paper unveils an unprecedented utilization of AEMs in an electrochemical oxygen separation process, a new technology able to generate enriched oxygen from an O₂/N₂ mixture using a small voltage input. We demonstrate a first-of-its-kind AEM-based electrochemical device that operates under mild conditions, is free of liquid electrolytes or sweep gases, and produces oxygen of over 96% purity. Additionally, we develop and apply a one-dimensional time-dependent and isothermal model, which accurately captures the unique operational dynamics of our device, demonstrates good agreement with the experimental data, and allows us to explore the device’s potential capabilities. This novel technology has far-reaching applications in many industrial processes, medical oxygen therapy, and other diverse fields while reducing operational complexity and environmental impact, thereby paving the way for sustainable on-site oxygen generation.

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Metal-free Advanced Energy Materials for the Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells

Authors:
Ramesh K Singh, John C Douglin, Vipin Kumar, Polina Tereshchuk, Pietro G Santori, Eduardo B Ferreira, Gregory Jerkiewicz, Paulo J Ferreira, Amir Natan, Frédéric Jaouen, Dario R Dekel
Journal:Applied Catalysis B: Environment and Energy

The success of the next generation of anion-exchange membrane fuel cells (AEMFCs) depends on the development of active, reliable, and economical oxygen reduction reaction (ORR) catalysts. Here, we synthesize a series of ultra-low-cost metal-free ORR catalysts by doping a common pristine graphite precursor with chemically singular-type heteroatoms, namely I, S, N, or B, using single-step planetary ball milling technique. All doped-graphites show substantially enhanced ORR performance relative to the pristine (undoped) graphite. Among all the tested catalysts, N-graphite exhibited the highest ORR onset potential of 0.87 V vs. reversible hydrogen electrode. These results are supported by density functional theory calculations. The ORR catalysts also exhibit remarkable stability as evaluated through electrochemical tests. Most importantly, the AEMFCs prepared using these ultra-low-cost doped graphites deliver notable peak power densities with impressive voltage efficiencies, which further supports their efficacy in ORR catalysis and the broad implementation of this technology.

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Carrier-driving CO2 Separation by Amine-rich Membranes with Intercalated Graphene Oxide

Authors:
Yang Li, Dario R Dekel, Xuezhong He
Journal:Journal of Membrane Science

Amine-rich facilitated transport membranes (FTMs) attract great interest in intensifying the membrane-based CO₂ separation processes. The high-molecular-weight polyvinylamine (PVAm) polymers containing fixed-site carriers of the amino groups were used to prepare highly CO₂-permeable membranes. The sterically hindered PVAm polymers of poly(N-methyl-N-vinylamine) and poly(N-isopropyl-N-vinylamine) were obtained by functionalization of PVAm to provide superior CO₂ solubility. By loading the mobile carriers of amino acid salt (AAS) and CO₂-philic graphene oxide (GO), the prepared FTMs render enhanced CO₂ permeance and CO₂/N₂ selectivity. The d-spacing of 8.8 Å and the ultramicropores of 3.5 Å from GO nanosheets provide the combination of both selective surface flow and molecular sieving mechanisms to achieve improved CO₂ permeance and CO₂/N₂ selectivity. In addition, the intercalation of GO hinders N₂ transport through the membrane due to a longer pathway, while the mobile carriers of AAS introduced into the PVAm matrix facilitate CO₂ transport through the selective layer. Therefore, the CO₂/N₂ selectivity of the prepared FTMs was significantly enhanced to 171 based on the intensified carrier-driving transport mechanism. It can be concluded that amine-rich membranes based on both fixed and mobile carriers of the amino groups together with intercalated GO can synergistically improve the CO₂/N₂ separation performance, and be potentially applied for CO₂ capture from flue gas.

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Lignin-Derived Precious Metal-Free Electrocatalysts for Anion-Exchange Membrane Fuel Cell Application

Authors:
Umber Sajjad, Ave Sarapuu, John C Douglin, Arvo Kikas, Alexey Treshchalov, Maike Käärik, Jekaterina Kozlova, Jaan Aruväli, Jaan Leis, Vambola Kisand, Kaupo Kukli, Dario R Dekel, Kaido Tammeveski
Journal:ACS Catalysis

A facile method for the preparation of precious metal-free catalysts for the oxygen reduction reaction (ORR) from lignin, dicyandiamide, and transition metal salts is presented. Magnesium acetate was employed as a precursor for a sacrificial template, enhancing the porous structure of the catalysts. Iron content in the catalyst materials was optimized and a bimetallic catalyst containing Fe and Co was also prepared. The physicochemical analysis revealed uniform dispersion of various nitrogen moieties and transition metal centers on sheet-like carbon structures, along with some carbon-encapsulated metal-rich nanoparticles. Rotating disc electrode tests in an alkaline solution demonstrated the dependence of the ORR performance of the catalysts on their iron content and confirmed the high stability of both iron and bimetallic catalysts over 10,000 potential cycles. Anion-exchange membrane fuel cell (AEMFC) studies revealed that the bimetallic catalyst outperforms the Fe-containing material, achieving a very promising peak power density of 675 mW cm^–2 at 60 °C and 833 mW cm^–2 at 80 °C.

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Unraveling the Mechanism of Ammonia Electrooxidation by Coupled Differential Electrochemical Mass Spectrometry and Surface-Enhanced Infrared Absorption Spectroscopic Studies

Authors:
Hongsen Wang, Dario R Dekel, Héctor D Abruña
Journal:Journal of the American Chemical Society

Ammonia electrooxidation has received considerable attention in recent times due to its potential application in direct ammonia fuel cells, ammonia sensors, and denitrification of wastewater. In this work, we used differential electrochemical mass spectrometry (DEMS) coupled with attenuated total reflection–surface-enhanced infrared absorption (ATR–SEIRA) spectroscopy to study adsorbed species and solution products during the electrochemical ammonia oxidation reaction (AOR) on Pt in alkaline media, and to correlate the product distribution with the surface ad-species. Hydrazine electrooxidation, hydroxylamine electrooxidation/reduction, and nitrite electroreduction on Pt have also been studied to enhance the understanding of the AOR mechanism. NH₃, NH₂, NH, NO, and NO₂ ad-species were identified on the Pt surface with ATR–SEIRA spectroscopy, while N₂, N₂O, and NO were detected with DEMS as products of the AOR. N₂ is formed through the coupling of two NH ad-species and then subsequent further dehydrogenation, while the dimerization of HNO_ad leads to the formation of N₂O. The NH–NH coupling is the rate-determining step (rds) at high potentials, while the first dehydrogenation step is the rds at low potentials. These new spectroscopic results about the AOR and insights could advance the search and design of more effective AOR catalysts.

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Polymer Backbone Chemistry Shapes the Alkaline Stability of Metallopolymer Anion‐Exchange Membranes

Authors:
Kanika Aggarwal, Songlin Li, Sally Nijem, Dario R Dekel, Charles E Diesendruck
Journal:Chemistry–A European Journal

Anion-exchange membrane fuel cells and water electrolyzers have garnered significant attention in past years due to their potential role in sustainable and affordable energy conversion and storage. However, the chemical stability of the polymeric anion-exchange membranes (AEMs), the key component in these devices, currently limits their lifespan. Recently, metallopolymers have been proposed as chemically stable alternatives to organic cations, using metal centers as ion transporters. In metallopolymer AEMs, various properties such as alkaline stability, water uptake, flexibility, and performance, are determined by both the metal complex and polymer backbone. Herein we present a systematic study investigating the influence of the polymer backbone chemistry on some of these properties, focusing on the alkaline stability of low-oxophilicity gold metallopolymers. Despite the use of a common N-heterocyclic carbene ligand, upon gold metalation using the same reaction conditions, different polymer backbones end up forming different gold complexes. These findings suggest that polymer chemistry affects the metalation reaction in addition to the other properties relevant to AEM performance.

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High-temperature anion-exchange membrane fuel cells with balanced water management and enhanced stability

Authors:
Jiandang Xue, John C Douglin, Karam Yassin, Tong Huang, Haifei Jiang, Junfeng Zhang, Yan Yin, Dario R Dekel, Michael D Guiver
Journal:Joule

Over the last decade, anion-exchange membrane fuel cells (AEMFCs) have continued to show steady power output and durability improvements at low temperatures of 60oC–80oC. However, AEMFC durability still lags, largely due to the critical issue of water management. High-temperature operation (R100oC) enables simplified water management, but additional material stability challenges remain, particularly concerning the chemical stability of the anionexchange membranes (AEMs). Herein, we report the synthesis of lightly branched poly(arylene piperidinium) AEMs, leading to balanced water management and sufficient stability. The optimized membranes demonstrate high-temperature H2/O2 AEMFC operation at 100oC, with a peak power density of 2 W cm-2 and durability over a 195-h period under a constant current density of 600 mA cm-2 with only 4% voltage decay. This work illustrates an effective AEM design strategy through high temperature operation to resolve water management issues, thereby improving AEMFC performance and durability.

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How can we design anion-exchange membranes to achieve longer fuel cell lifetime?

Authors:
Karam Yassin, Igal G Rasin, Simon Brandon, Dario R Dekel
Journal:Journal of Membrane Science

The substantial advancements and availability of new cost-effective materials have drawn significant attention to the anion-exchange membrane fuel cell (AEMFC) technology. The anion exchange membrane (AEM) is a core component in AEMFCs, essential for conducting hydroxide ions and controlling water transport between the fuel cell electrodes. In this study, we apply a numerical model to investigate the relationship and sensitivity of AEMFC performance and its stability to key AEM properties. Our findings show that membrane maximum hydration level (λ_max) has the most significant impact on AEMFC lifetime, followed by membrane ion-exchange capacity, water diffusivity, and membrane thickness. We also demonstrate the significance of improving the stability of the functional group to AEMFC lifetime, while AEM hydroxide conductivity shows a negligible effect on AEMFC lifetime. Finally, we provide a simple algebraic functional relationship between key dimensionless parameters as well as a machine learning-based analysis of the relationship between AEM parameters and AEMFC lifetime. Through these analyses, we calculate the lifetime of selected membranes from the literature and compare them to the measured operation time. This study summarizes and highlights important AEM property targets suggested to improve AEM design to boost the performance stability of AEMFCs.

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Electrostatic Potential of Functional Cations as a Predictor of Hydroxide Diffusion Pathways in Nanoconfined Environments of Anion Exchange Membranes

Authors:
Tamar Zelovich, Dario R Dekel, Mark E Tuckerman
Journal:The Journal of Physical Chemistry Letters

Nanoconfined anion exchange membranes (AEMs) play a vital role in emerging electrochemical technologies. The ability to control dominant hydroxide diffusion pathways is an important goal in the design of nanoconfined AEMs. Such control can shorten hydroxide transport pathways between electrodes, reduce transport resistance, and enhance device performance. In this work, we propose an electrostatic potential (ESP) approach to explore the effect of the polymer electrolyte cation spacing on hydroxide diffusion pathways from a molecular perspective. By exploring cation ESP energy surfaces and validating outcomes through prior ab initio molecular dynamics simulations of nanoconfined AEMs, we find that we can achieve control over preferred hydroxide diffusion pathways by adjusting the cation spacing. The results presented in this work provide a unique and straightforward approach to predict preferential hydroxide diffusion pathways, enabling efficient design of highly conductive nanoconfined AEM materials for electrochemical technologies.

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Advanced nickel-based catalysts for the hydrogen oxidation reaction in alkaline media synthesized by reactive spray deposition technology: Study of the effect of particle size

Authors:
Zhiqiao Zeng, Jiale Xing, Leonard Bonville, Dario R Dekel, Radenka Maric, Stoyan Bliznakov
Journal:International Journal of Hydrogen Energy

Anion exchange membrane fuel cells (AEMFCs) have been regarded as a promising low-cost alternative to proton exchange membrane fuel cells (PEMFCs) due to their potential to utilize platinum group metal (PGM) free catalysts and their recently demonstrated improvement in power density. However, the development of highly active and stable PGM-free electrocatalysts for the hydrogen oxidation reaction (HOR) in alkaline solutions remains a significant challenge. In this study, reactive spray deposition technology (RSDT) is used to fabricate a set of Ni/CeO₂/C catalysts, and their activity toward the HOR is investigated as a function of the nanoparticle size. The structural and morphological characterization of as synthesized Ni/CeO₂/C catalysts confirms that the RSDT is capable of precise control of the nanoparticle size by adjusting the deposition parameters. The electrochemical rotating disk electrode study shows that the Ni/CeO₂/C catalyst with an average particle size distribution of 3 nm has the highest mass activity of 14.8 A g_Ni⁻¹, which is among the highest values reported in the literature for Ni-based catalysts. Moreover, the post-test characterization, performed after the accelerated stress test (AST) measurements, reveals that both the catalyst’s corrosion and particle size increase are the main contributors to the mass activity loss. The measured electrocatalytic performance confirms the feasibility of the RSDT fabricated Ni/CeO₂/C nanoparticles as an efficient PGM-free HOR catalyst for application in AMEFCs, with additional development required to improve their durability.

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High-performance ionomerless cathode anion-exchange membrane fuel cells with ultra-low-loading Ag–Pd alloy electrocatalysts

Authors:
John C Douglin, José A Zamora Zeledón, Melissa E Kreider, Ramesh K Singh, Michaela Burke Stevens, Thomas F Jaramillo, Dario R Dekel
Journal:Nature Energy

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Hydrogenated TiO2 Carbon Support for PtRu Anode Catalyst in High-Performance Anion-Exchange Membrane Fuel Cells

Authors:
John C Douglin, Archana Sekar, Ramesh K Singh, Zihua Chen, Jun Li, Dario R Dekel
Journal:Small

The availability of durable, high-performance electrocatalysts for the hydrogen oxidation reaction (HOR) is currently a constraint for anion-exchange membrane fuel cells (AEMFCs). Herein, a rapid microwave-assisted synthesis method is used to develop a core–shell catalyst support based on a hydrogenated TiO₂/carbon for PtRu nanoparticles (NPs). The hydrogenated TiO₂ provides a strong metal-support interaction with the PtRu NPs, which improves the catalyst’s oxophilicity and HOR activity compared to commercial PtRu/C and enables greater size control of the catalyst NPs. The as-synthesized PtRu/TiO₂/C-400 electrocatalyst exhibits respectable performance in an AEMFC operated at 80 °C, yielding the highest current density (up to 3× higher) within the catalytic region (compared at 0.80–0.90 V) and voltage efficiency (68%@ 0.5 A cm⁻²) values in the compared literature. In addition, the cell demonstrates promising short-term voltage stability with a minor voltage decay of 1.5 mV h⁻¹. This “first-of-its-kind in alkaline” work may open further research avenues to develop rapid synthesis methods to prepare advanced core–shell metal-oxide/carbon supports for electrocatalysts for use in the next-generation of AEMFCs with potential applicability to the broader electrochemical systems research community.

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Boosting CO2/N2 Separation from Cellulose Nanocrystal-Reinforced and Amine-Rich Facilitated Transport Membranes Comprising Deprotonated Amino Acid Salt and Ultrahigh-Molecular-Weight Polyvinylamine

Authors:
Yang Li, Mingyuan Li, Zhihong Lin, Moris S Eisen, Dario R Dekel, Xuezhong He
Journal:Industrial & Engineering Chemistry Research

An ultrathin amine-rich selective layer comprising the fixed-site carrier from polyvinylamine (PVAm) and the mobile carrier with an amino acid salt was successfully coated on top of the polysulfone (PSf) substrate for enhanced CO₂-facilitated transport. PVAm with an ultrahigh molecular weight was synthesized via the inverse emulsion polymerization method, which allows the excellent dissipation of the reaction heat and reduces gel formation drastically. Several batches of PVAm with different hydrolysis degrees and molecular weights were successfully synthesized. The mobile carriers of 2-(1-piperazinyl)ethylamine salts of sarcosine (PZEA-Sar) were synthesized and introduced to strengthen the facilitated transport contribution, while cellulose nanocrystals (CNCs) were incorporated into the polymer matrix to enhance the mechanical strength and also increase the free volume of the polymer matrix. The amine-rich composite membranes made from 0.5 wt % PVAm/PZEA-Sar-0.5 wt % CNC with a high-molecular-weight PVAm of 4.4 MDa at a degree of hydrolysis (DOH) of 78% demonstrate a significantly enhanced CO₂ permeance of 160 GPU with a good CO₂/N₂ selectivity of 48 at 35 °C and a feed pressure of 2 bar. The developed amine-rich facilitated transport membrane shows excellent potential for industrial carbon capture.

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Migration and Precipitation of Platinum in Anion‐Exchange Membrane Fuel Cells

Authors:
Aniket Raut, Haoyan Fang, Yu‐Chung Lin, Shi Fu, David Sprouster, Ryuichi Shimogawa, Anatoly I Frenkel, Chulsung Bae, John C Douglin, Jaana Lillojad, Kaido Tammeveski, Zhiqiao Zeng, Stoyan Bliznakov, Miriam Rafailovich, Dario R Dekel
Journal:Angewandte Chemie International Edition

Despite the recent progress in increasing the power generation of Anion-exchange membrane fuel cells (AEMFCs), their durability is still far lower than that of Proton exchange membrane fuel cells (PEMFCs). Using the complementary techniques of X-ray micro-computed tomography (CT), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) spectroscopy, we have identified Pt ion migration as an important factor to explain the decay in performance of AEMFCs. In alkaline media Pt⁺² ions are easily formed which then either undergo dissolution into the carbon support or migrate to the membrane. In contrast to PEMFCs, where hydrogen cross over reduces the ions forming a vertical “Pt line” within the membrane, the ions in the AEM are trapped by charged groups within the membrane, leading to disintegration of the membrane and failure. Diffusion of the metal components is still observed when the Pt/C of the cathode is substituted with a FeCo−N−C catalyst, but in this case the Fe and Co ions are not trapped within the membrane, but rather migrate into the anode, thereby increasing the stability of the membrane.

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Size and Electronic Effects on the Performance of (Corrolato) cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis

Authors:
Arik Raslin, John C Douglin, Amit Kumar, Marcelo Fernandez-Dela-Mora, Dario R Dekel, Zeev Gross
Journal:Inorganic Chemistry

Considering the worldwide efforts for designing catalysts that are not based on platinum group metals while still reserving the many advantages thereof, this study focused on the many variables that dictate the performance of cathodes used for fuel cells, regarding the efficient and selective reduction of oxygen to water. This was done by investigating two kinds of porous carbon electrodes, modified by molecular cobalt(III) complexes chelated by corroles that differ very much in size and electron-withdrawing capability. Examination of the electronic effect uncovered shifts in the Co^II/Co^III redox potentials and also large differences in the affinity of the cobalt center to external ligands. Spontaneous absorption of the catalysts was found to depend on the size of the corrole’s substituents (C₆F₅ ≫ CF₃ ≫ H) and the metal’s axial ligands (PPh₃ versus pyridine), as well as on the porosity of the carbon electrodes (BP2000 > Vulcan). The better-performing cobalt-based catalysts were almost as active and selective as 20% platinum on Vulcan in terms of the onset potential and the only 2–10% undesirable formation of hydrogen peroxide. Durability was also addressed by using the best-performing modified cathode in a proper anion-exchange membrane fuel cell setup, revealing very little voltage change during 12 h of operation.

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Quantifying the resistive losses of the catalytic layers in anion‐exchange membrane fuel cells

Authors:
John C Douglin, Kalimuthu Vijaya Sankar, Ana Laura G Biancolli, Elisabete I Santiago, Yoed Tsur, Dario R Dekel
Journal:ChemSusChem

The existing gap in the ability to quantify the impacts of resistive losses on the performance of anion-exchange membrane fuel cells (AEMFCs) during the lifetime of their operation is a serious concern for the technology. In this paper, we analyzed the ohmic region of an operating AEMFC fed with pure oxygen followed by CO₂-free air at various operating currents, using a combination of electrochemical impedance spectroscopy (EIS) and a novel technique called impedance spectroscopy genetic programming (ISGP). Presented here for the first time in this work, we isolated and quantified the individual effective resistance (R_eff) values occurring in the AEMFC and their influence on performance as operating conditions change. We believe that this first work is vital to help distinguish the influence of the individual catalytic and mass-transfer processes in this technology thereby providing valuable data to the AEMFC community, with potentially wider applicability to other electrochemical devices where individual physical processes occur simultaneously and need to be sequestered for deeper understanding.

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Functional groups in anion exchange membranes: Insights from Ab initio molecular dynamics

Authors:
T Zelovich, D R Dekel, M E Tuckerman
Journal:Journal of Membrane Science

Fuel cell deployable anion exchange membranes (AEMs) constitute some of the cleanest and most affordable electrochemical devices. Elucidation of key design principles underlying these electrolytes requires a fundamental understanding of the effect of different cationic functional groups (FGs) on the performance of an AEM. In this study, we use fully atomistic ab initio molecular dynamics simulations to study the effect of the trimethyl alkyl ammonium (TMA) and imidazolium (IMI) FGs on the hydroxide ions and water diffusivity in AEMs under low hydration conditions using nano-confined structures. The IMI FG was found to be a better chaotropic ion, resulting in a higher water diffusivity. Exploration of the hydroxide diffusion revealed that at high temperatures, both systems achieved high hydroxide diffusivity. However, only AEM-based TMA showed high hydroxide diffusivity at room temperature. We find that differences in the hydroxide diffusivity are a result of the FG structure. We anticipate that a molecular-level understanding of the effect of FGs on water and hydroxide diffusivity will ultimately guide the synthesis and experimental characterization of AEMs toward new, stable polymer electrolyte materials with high hydroxide ion conductivity and water diffusivity. This will be beneficial for the advancement and implementation of emerging AEM-based technologies.

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Evaluation of Semi-Automatic Compositional and Microstructural Analysis of Energy Dispersive Spectroscopy (EDS) Maps via a Python-Based Image and Data Processing Framework for Fuel Cell Applications

Authors:
M Batool, A O Godoy, M Birnbach, D R Dekel, J Jankovic
Journal:Journal of The Electrochemical Society

Computer-aided data acquisition, analysis, and interpretation are rapidly gaining traction in numerous facets of research. One of the subsets of this field, image processing, is most often implemented for post-processing material microstructural characterization data to understand better and predict materials’ features, properties, and behaviors at multiple scales. However, to tackle the ambiguity of multi-component materials analysis, spectral data can be used in combination with image processing. The current study introduces a novel Python-based image and data processing method for in-depth analysis of energy dispersive spectroscopy (EDS) elemental maps to analyze multi-component agglomerate size distribution, the average area of each component, and their overlap. The framework developed in this study is applied to examine the interaction of Cerium Oxide (CeO_x) and Palladium (Pd) particles in the membrane electrode assembly (MEA) of an Anion-Exchange Membrane Fuel Cell (AEMFC) and to investigate if this approach can be correlated to cell performance. The study also performs a sensitivity analysis of several parameters and their effect on the computed results. The developed framework is a promising method for semi-automatic data processing and can be further advanced towards a fully automatic analysis of similar data types in the field of clean energy materials and broader.

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Single Solution‐Phase Synthesis of Charged Covalent Organic Framework Nanosheets with High Volume Yield

Authors:
T Huang, H Jiang, J C Douglin, Y Chen, S Yin, J Zhang, X Deng, H W, Yan Yin, D R Dekel, M D Guiver, Z Jiang
Journal:Angewandte Chemie International Edition

Covalent organic framework nanosheets (COF-NSs) are emerging building blocks for functional materials, and
their scalable fabrication is highly desirable. Current synthetic methods suffer from low volume yields resulting from
confined on-surface/at-interface growth space and complex multiple-phase synthesis systems. Herein, we report the
synthesis of charged COF-NSs in open space using a single-phase organic solution system, achieving magnitudes higher
volume yields of up to 18.7 mgmL 1. Charge-induced electrostatic repulsion forces enable in-plane anisotropic secondary
growth from initial discrete and disordered polymers into large and crystalline COF-NSs. The charged COF-NS colloidal
suspensions are cast into thin and compact proton exchange membranes (PEMs) with lamellar morphology and oriented
crystallinity, displaying outstanding proton conductivity, negligible dimensional swelling, and good H2/O2 fuel cell
performance.

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Advanced nickel-based catalysts for the hydrogen oxidation reaction in alkaline media synthesized by reactive spray deposition technology: Study of the effect of particle size

Authors:
Z Zeng, J Xing, L Bonville, D R Dekel, R Maric, S Bliznakov
Journal:International Journal of Hydrogen Energy

Anion exchange membrane fuel cells (AEMFCs) have been regarded as a promising low-cost alternative to proton exchange membrane fuel cells (PEMFCs) due to their potential to utilize platinum group metal (PGM) free catalysts and their recently demonstrated improvement in power density. However, the development of highly active and stable PGM-free electrocatalysts for the hydrogen oxidation reaction (HOR) in alkaline solutions remains a significant challenge. In this study, reactive spray deposition technology (RSDT) is used to fabricate a set of Ni/CeO₂/C catalysts, and their activity toward the HOR is investigated as a function of the nanoparticle size. The structural and morphological characterization of as synthesized Ni/CeO₂/C catalysts confirms that the RSDT is capable of precise control of the nanoparticle size by adjusting the deposition parameters. The electrochemical rotating disk electrode study shows that the Ni/CeO₂/C catalyst with an average particle size distribution of 3 nm has the highest mass activity of 14.8 A g_Ni⁻¹, which is among the highest values reported in the literature for Ni-based catalysts. Moreover, the post-test characterization, performed after the accelerated stress test (AST) measurements, reveals that both the catalyst’s corrosion and particle size increase are the main contributors to the mass activity loss. The measured electrocatalytic performance confirms the feasibility of the RSDT fabricated Ni/CeO₂/C nanoparticles as an efficient PGM-free HOR catalyst for application in AMEFCs, with additional development required to improve their durability.

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Operando EPR study of radical formation in anion-exchange membrane fuel cells

Authors:
S Wierzbicki, J C Douglin, R K Singh, D R Dekel, K Kruczała
Journal:ACS Catalysis

The alkaline operating environment of anion-exchange membrane (AEM) fuel cells (AEMFCs) makes it possible to employ a wide variety of catalysts, including platinum group metals (PGMs) as well as PGM-free materials. However, little is understood about radical formation during AEMFC operation with different catalysts and their implications. In this investigation, we utilized spin-trapping and electron-paramagnetic resonance measurements to measure and identify the radicals produced on selected PGM and PGM-free oxygen-reduction reaction catalysts. To the best of our knowledge, this is an original study exploring radical formation on different classes of catalysts in operando AEMFCs. This work highlights an unexplored radical degradation mechanism in AEMFCs and calls for innovative strategies in designing radical attack-resistant AEMs.

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Model-based insights into the decarbonation dynamics of anion-exchange membranes

Authors:
F Kubannek, A Zhegur-Khais, S Li, D R Dekel, U Krewer
Journal:Chemical Engineering Journal

We present a model-based analysis of the transport of anions in anion exchange membranes (AEMs) with the aim of understanding the decarbonation process and its dynamics. The dynamic simulation model covers the diffusive and migrative transport of water, carbonate anions, and hydroxide anions through an AEM. To the best of our knowledge, this is the first model studying decarbonation process in AEMs. The model is validated using decarbonation data from anion conductivity measurements of ten different AEM materials. Driven by migrative transport, strong concentration gradients develop inside the membranes. A parameter study reveals that the ion-exchange capacity and the applied current have the largest effect on the decarbonation dynamics. Further, high hydroxide diffusivities increase the decarbonation time constant, whereas high carbonate diffusivities decrease it. The results further indicate that high diffusivities of both carbonates and hydroxide result in slower membrane decarbonation. This work provides new insights into properties determining the carbonation and decarbonation of AEMs, which is critical for AEM-based electrochemical devices such as fuel cells and electrolysers.

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Role of Fe in the hydrogen oxidation reaction in a NiFe-based catalyst: An in situ Mössbauer spectroscopic investigation

Authors:
L Ni, E S Davydova, R K Singh, L K-Shmuel, D R Dekel, U I Kramm
Journal:Energy

Nickel-based catalysts reach a high activity for the hydrogen oxidation reaction (HOR) in anion exchange membrane fuel cells. While incorporation of iron significantly decreases the HOR overpotential on NiFe-based catalysts, the reason for the enhanced activity remains only partially understood. For the first time, in situ 57Fe Mössbauer spectroscopy is used to gain insights into the iron-related composition at different potentials. The aim is to evaluate which changes occur on iron at potentials relevant for the HOR on the active Ni sites. It is found that different pre-conditionings at low potentials stabilize the iron at a low oxidation state as compared to the as-prepared catalyst powder. It is likely that the lower average oxidation state enables a higher exchange current density and a more efficient OH adsorption, which make the Volmer step much faster in the HOR. Insights from in situ Mössbauer spectroscopy enlighten the role of iron in the nickel-iron catalyst, paving the way for developing improved Ni-based catalysts for HOR catalysis.

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CoO x-Fe 3 O 4/N-rGO Oxygen Reduction Catalyst for Anion-Exchange Membrane Fuel Cells

Authors:
R K Singh, J C Douglin, L Jiang, K Yassin, S Brandon, D R Dekel
Journal:Energies

Platinum group metal (PGM)-free oxygen reduction reaction (ORR) catalysts are of utmost importance for the rapid development of anion-exchange membrane fuel cell (AEMFC) technology. In this work, we demonstrate the improved ORR performance and stability of Co and Fe oxide-decorated/N-doped reduced graphene oxide (CoO_x-Fe₃O₄/N-rGO) prepared via a hydrothermal method at the low temperature of 150 °C. The catalysts were characterized thoroughly using transmission electron microscopy, high-angle annular dark field-scanning electron microscopy, X-ray diffraction, N₂ physisorption, Raman spectroscopy, and X-ray photoelectron spectroscopy to obtain information about morphology, elemental distribution, phases, porosity, defects, and surface elemental compositions. Significant ORR activity improvement (130 mV@-1.5 mA cm⁻²) was achieved with this catalyst compared to the pristine graphene oxide, and the ORR limiting current was even 12%@0.5 V higher than the commercial Pt/C. The enhanced ORR activity of CoO_x-Fe₃O₄/N-rGO was attributed to the uniform dispersion of Co, Fe, and N on reduced graphene oxide (rGO) sheets. Furthermore, ORR accelerated stress tests revealed excellent durability, suggesting that this material could be a promising and durable catalyst. With a cathode layer of the CoO_x-Fe₃O₄/N-rGO catalyst, we achieved a peak power density of 676 mW cm⁻² in an operando H₂-O₂ AEMFC. To the best of our knowledge, this is the highest reported power density per cathode catalyst mass in a reported PGM-free cathode catalyst. Finally, we quantified the various cell polarization losses as a function of cathode catalyst loadings to obtain insights for future work with AEMFCs based on this catalyst. The improvement in the AEMFC performance using CoO_x-Fe₃O₄/N-rGO as a cathode catalyst can be attributed to the synergistic effects of (i) the high turnover frequency of the transition metals (Co and Fe) for ORR and (ii) the enhancement provided by N doping to the metal distribution and stability.

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Modeling direct ammonia anion-exchange membrane fuel cells

Authors:
D R. Dekel, K Yassin, I G. Rasin, S Brandon
Journal:Journal of Power Sources

Ammonia has recently been proposed as a promising candidate fuel for anion-exchange membrane fuel cell (AEMFC) technology. Direct ammonia AEMFCs (DA-AEMFCs) are a carbon-free technology that combines the ammonia’s high energy density with the fuel cells’ high efficiency. However, two major challenges face this technology: ammonia crossover (due to ammonia’s high solubility in water) and sluggish ammonia oxidation reaction (AOR). We have developed, applied, and presented a one-dimensional and transient model of a DA-AEMFC system to address these challenges. Excellent agreement is obtained between the experimentally-measured and computationally-simulated performance of DA-AEMFCs operating at 100 and 120 °C with KOH-free anode feed. As the current density increases, the initial cell performance analysis reveals a reduction in the parasitic AOR rate through the cathode. More intriguingly, the results demonstrate a positive impact of ammonia crossover on cell longevity. Crossover drives an AOR within the cathode, which has a detrimental effect on performance but comes with an associated benefit of water generation in this region. The resultant improvement in the cathode hydration reduces the degradation rate of ionomeric material, ultimately increasing cell lifetime. Further studies are required to determine the desired rate of ammonia crossover and its influence on the system’s cost-effectiveness.

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Alkaline Stability of Anion-Exchange Membranes

Authors:
S Willdorf-Cohen, A Zhegur-Khais, J Ponce-González, S Bsoul-Haj, J R. Varcoe, C E. Diesendruck, D R. Dekel
Journal:ACS Applied Energy Materials

Recently, the development of durable anion-exchange membrane fuel cells (AEMFCs) has increased in intensity due to their potential to use low-cost, sustainable components. However, the decomposition of the quaternary ammonium (QA) cationic groups in the anion-exchange membranes (AEMs) during cell operation is still a major challenge. Many different QA types and functionalized polymers have been proposed that achieve high AEM stabilities in strongly alkaline aqueous solutions. We previously developed an ex situ technique to measure AEM alkaline stabilities in an environment that simulates the low-hydration conditions in an operating AEMFC. However, this method required the AEMs to be soluble in DMSO solvent, so decomposition could be monitored using ¹H nuclear magnetic resonance (NMR). We now report the extension of this ex situ protocol to spectroscopically measure the alkaline stability of insoluble AEMs. The stability ofradiation-grafted (RG) poly(ethylene-co-tetrafluoroethylene)-(ETFE)-based poly(vinylbenzyltrimethylammonium) (ETFE-TMA) and poly(vinylbenzyltriethylammonium) (ETFE-TEA) AEMs were studied using Raman spectroscopy alongside changes in their true OH^– conductivities and ion-exchange capacities (IEC). A crosslinked polymer made from poly(styrene-co-vinylbenzyl chloride) random copolymer and N,N,N′,N′-tetraethyl-1,3-propanediamine (TEPDA) was also studied. The results are consistent with our previous studies based on QA-type model small molecules and soluble poly(2,6-dimethylphenylene oxide) (PPO) polymers. Our work presents a reliable ex situ technique to measure the true alkaline stability of AEMs for fuel cells and water electrolyzers.

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Chemically Folded Polyelectrolytes with Superior Alkaline Stability

Authors:
I Agami, N Gjineci, S Li, S Srebnik, D R. Dekel, C E. Diesendruck
Journal:CS Appl. Polym. Mater. 4, 9250−9256, 2022

In recent years, the development of anion-exchange membranes (AEMs) for anion-exchange membrane fuel cell (AEMFC) applications has been rapidly growing due to their numerous advantages over mainstream proton-exchange membrane fuel cells. However, a major challenge in the development of practical AEMs is the low chemical stability of the AEM quaternary ammonium (QA) functional groups in the strongly alkaline and the relatively dry environment produced during operation of the AEMFC. Herein, we investigate the effect of polymer chain folding on the chemical stability of the QA groups. While these polymers have virtually the same chemical composition, their molecular architectures are quite different, significantly affecting the kinetics of nucleophilic attacks on the QAs embedded inside the folded chains. The stability tests reveal a remarkable improvement in the stability of the folded chains compared to the linear (unfolded) control, resulting in polyelectrolytes that are two orders of magnitude more stable. We provide here a simple method for the preparation of chemically stable AEMs with different QA groups and polymer backbones. These folded architectures present a very promising family of polyelectrolyte membranes for AEMFCs and other electrochemical applications.

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Templated Nitrogen-, Iron-, and Cobalt-Doped Mesoporous Nanocarbon Derived from an Alkylresorcinol Mixture for Anion-Exchange Membrane Fuel Cell Application

Authors:
K Kisand, A Sarapuu, J C Douglin, A Kikas, A Treshchalov, M Käärik, H-M Piirsoo, P Paiste, J Aruväli, J Leis, V Kisand, A Tamm, D R Dekel, K Tammeveski
Journal:ACS Catalysis, 12, 14050-14061, 2022

In this work, doped nanocarbon electrocatalysts for electrochemical oxygen reduction reaction (ORR) are prepared by high-temperature pyrolysis from Honeyol, cobalt and iron salts, and dicyandiamide. MgO-based inorganic templates are further used to increase the mesoporosity of the prepared catalyst materials. The templated bimetallic electrocatalyst containing iron and cobalt (FeCoNC-MgOAc) showed excellent stability and remarkable ORR performance in rotating disk electrode testing in alkaline conditions. The catalyst was further tested in anion-exchange membrane fuel cells (AEMFCs), where FeCoNC-MgOAc performed significantly better than the nontemplated material (FeCoNC), yielding a peak power density (P_max) of 0.92 W cm^–2 surpassing that of the commercial Pt/C (20 and 40 wt %) catalysts (P_max = 0.85 and 0.69 W cm^–2, respectively). The high AEMFC performance was attributed to the mesoporous morphology and high density of active sites in the nanocarbon-based cathode catalyst.

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Hydroxide Chemoselectivity Changes with Water Microsolvation

Authors:
S Willdorf-Cohen, A Kaushansky, D R Dekel, C E Diesendruck
Journal:J. Phys. Chem. Lett. 13, 10216-10221, 2022

Authors: Sapir Willdorf-Cohen, Alexander Kaushansky, Dario R. Dekel, and Charles E. Diesendruck

Solvent molecules are known to affect chemical reactions, especially if they interact with one or more of the reactants or catalysts. In ion microsolvation, i.e., solvent molecules in the first solvation sphere, strong electronic interactions are created, leading to significant changes in charge distribution and consequently on their nucleophilicity/electrophilicity and acidity/basicity. Despite a long history of research in the field, fundamental issues regarding the effects of ion microsolvation are still open, especially in the condensed phase. Using reactions between hydroxide and relatively stable quaternary ammonium salts as an example, we show that water microsolvation can change hydroxide’s chemoselectivity by differently affecting its basicity and nucleophilicity. In this example, the hydroxide reactivity as a nucleophile is less affected by water microsolvation than its reactivity as a base. These disparities are discussed by calculating and comparing oxidation potentials and polarizabilities of the different water–hydroxide clusters.

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The effect of membrane thickness on AEMFC Performance: An integrated theoretical and experimental study

Authors:
K Yassin, J C Douglin, I G Rasin, P G Santori, B Eriksson, N Bibent, F Jaouen, S Brandon, D R Dekel
Journal:Energy Conversion & Management 270, 116203, 2022

Authors: Karam Yassin, John C. Douglin, Igal G. Rasin, Pietro G. Santori, Bjorn Eriksson, Nicolas Bibent, Fr´ed´eric Jaouen, Simon Brandon, Dario R. Dekel

We present a comprehensive theoretical and experimental study of the effect of membrane thickness on the anion-exchange membrane (AEM) fuel cell (AEMFC) performance. AEMFC tests are carried out with several AEMs with thickness in the range of 5 – 50 µm and assembled with a PtRu anode, and two different cathode catalysts (Pt/C or Fe single bond NC). Dramatic improvements in cell performance are observed as the membrane thickness decreases, which is mainly attributed to reduced ohmic losses and enhanced water transport between the electrodes. The simulated cell performance obtained using our previously developed AEMFC model qualitatively and quantitatively explains the experimental results in the entire range of current densities (0–4 Acm⁻²). Simulated results show that thinner membranes enhance water transport between the electrodes, mitigating the anode flooding, and resulting in increased local hydration in the membrane and cathode catalytic layer. These high hydration values enhance the anionic conductivity of the ionomeric materials, thereby improving cell performance. Furthermore, the enhanced water transport towards the cathode electrode provides sufficient water to participate in the oxygen reduction reaction, thus reducing the activation losses. Simulation modeling allows for a thorough understanding of cell behavior and aids in the development of the next generation of advanced AEMFCs.

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Improving Alkaline Hydrogen Oxidation Activity of Palladium through Interactions with Transition-Metal Oxides

Authors:
M V Pagliaro, C Wen, B Sa, B Liu, M Bellini, F Bartoli, S Sahoo, R K Singh, S P Alpay, H A Miller, D R Dekel
Journal:ACS Catal. 12, 10894-10904, 2022

Authors: Maria V. Pagliaro, Cuilian Wen, Baisheng Sa, Baoyu Liu, Marco Bellini, Francesco Bartoli, Sanjubala Sahoo, Ramesh K. Singh, S. Pamir Alpay, Hamish A. Miller, Dario R. Dekel

Anion-exchange membrane fuel cells (AEMFCs) are a promising electrochemical power generation technology that will most likely find applications in stationary power supply and mobile applications such as electric vehicles in the future. One of the main technological challenges with AEMFCs is developing catalysts with improved activities to reduce the current overpotential losses in the cell. A historically underappreciated challenge for AEMFC catalyst development is the sluggish hydrogen oxidation reaction (HOR) kinetics at the anode that still requires high loadings of platinum group metals. Here, we demonstrate that the alkaline HOR activity of palladium nanoparticles is enhanced through strong interactions with transition-metal oxides. A series of transition-metal oxides in the IV oxidation state (ZrO₂, CeO₂, SnO₂, and RuO₂) were deposited onto Vulcan XC-72 carbon. Pd nanoparticles (20 wt %) were then grown on each support. This series of catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The alkaline HOR activity was investigated using cyclic voltammetry and linear sweep voltammetry. Of the several systems that were considered, Pd–RuO₂/C displayed the highest HOR surface-specific activity (49 μA cm_Pd^–2). It had an onset for CO electro-oxidation at around 200 mV, which is lower than the other materials analyzed herein. The results were explained using first-principles calculations by investigating Tafel and Volmer reactions. These results suggested that increased HOR activity is favored by the population of the Pd surface with OH^– groups at lower overpotentials. These insights are valuable for the future design of next-generation catalysts with high activity toward alkaline HOR required by future high-performance AEMFCs.

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Performance optimization of PGM and PGM‑free catalysts in anion‑ exchange membrane fuel cells

Authors:
J C Douglin, R K Singh, E R Hamo, M B Hassine, P J Ferreira, B A Rosen, H A Miller, G Rothenberg, D R Dekel
Journal:Journal of Solid State Electrochemistry (2022) 26:2049–2057

Authors: John C. Douglin, Ramesh K. Singh, Eliran R. Hamo, Mohamad B. Hassine, Paulo J. Ferreira, Brian A. Rosen, Hamish A. Miller, Gadi Rothenberg, Dario R. Dekel

This study focuses on H₂-O₂ operando AEMFC performance based on several platinum group metal (PGM) and PGM-free catalysts. Specifically, we evaluate the AEMFC performance of commercial PtRu/C, as-synthesized PtRu/C, and Pd-CeO₂/C catalysts as anodes and commercial Pt/C and as-synthesized N-doped carbon catalysts as cathodes. The evolution of cell performance with varying cathode catalyst layer compositions, back pressure, and dew points is underscored. The as-synthesized PtRu/C catalyst is characterized by advanced transmission electron microscopy and showed significantly high performance reaching a peak power density of 1900 mW cm⁻² at 80 °C with remarkably promising initial stability. We further showed the cell performances using Pd-CeO₂/C anodes paired with both standard Pt/C and metal-free N-doped carbon as a cathodes, which resulted in peak power densities of 890 and 537 mW cm^‒2, respectively. Data reported in this study sheds light on the development of AEMFCs using PGM and PGM-free catalysts.

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N Heterocyclic Carbene Ligands’ Electronic Effects on Metallopolymer Anion Exchange Membranes

Authors:
Kanika Aggarwal, Songlin Li, Elisa Ivry, Dario R. Dekel, and Charles E. Diesendruck
Journal:Organometallics 2022, 41 (11), 1419-1425

Authors: Kanika Aggarwal, Songlin Li, Elisa Ivry, Dario R. Dekel, and Charles E. Diesendruck

Metallopolymers are intriguing prospective materials for use as anion exchange membranes (AEMs) in fuel cells and water electrolysis applications. Metallopolymers potentially offer high ion conductivity due to the multivalent nature of metal cations; however, similar to organic cations, the durability of the organic component can be compromised by the alkaline environment. To develop AEMs with long operational lifetimes, the fundamental relationship between the metal–ligand pair and alkaline stability needs to be understood. Here, we synthesize metallopolymers with different N-heterocyclic carbene ligand side chains that are connected to gold cations to study the ligand electronic effects on the AEM relevant properties. Interestingly, when tested under the same conditions, the different metallopolymers present markedly different alkaline stabilities, ion conductivities, and water uptake values. Explanation for such discrepancies is provided based on data from organometallic complexes. Designing optimized complexes with the correct electronic parameters can advance the development of practical AEMs for alkaline fuel cells and water electrolyzers.

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Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Authors:
M Chatenet, B G Pollet, D R Dekel, F Dionigi, J Deseure, P Millet, R D Braatz, M Z Bazant, M Eikerling, I Staffell, P Balcombe, Y Shao-Horn, H Schafer
Journal:Chem. Soc. Rev., 2022, 51, 4583–4762

Authors: Marian Chatenet, Bruno G. Pollet, Dario R. Dekel, Fabio Dionigi, Jonathan Deseure, Pierre Millet, Richard D. Braatz, Martin Z. Bazant, Michael Eikerling, Iain Staffell, Paul Balcombe, Yang Shao-Horn and Helmut Schafer

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO₂ footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the ‘junctions’ between the field’s physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

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Communication—Electropolymerization of Anion-Conducting Polymer Films

Authors:
Z-Y Xiao, C E Diesendruck, V Freger, D R Dekel
Journal:Journal of The Electrochemical Society, 2022, 169 064506

Authors: Zi-Ye Xiao, Charles E. Diesendruck, Viatcheslav Freger, and Dario R. Dekel

We successfully electropolymerize homopolymer and copolymer from vinylbenzyltrimethylammonium chloride (VBTMA) and divinylbenzene (DVB) by cyclic voltammetry to form ultra-thin anion-conducting polymer films with significant anion conductance. The morphologies of electropolymerized polymers with different monomer compositions are analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). In-situ electrochemical impedance spectroscopy (in situ EIS) is performed to track the electropolymerization (EP) process and film properties. High anion conductance of up to 2 mS is found in the electropolymerized thin films, suggesting that this technique can be suitable for making anion-conducting electrodes for advanced electrochemical devices.

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The impact of carbonation on hydroxide diffusion in nano-confined anion exchange membranes

Authors:
T Zelovich, C Simari, I Nicotera, D R Dekel, M E Tuckerman
Journal:J. Mater. Chem. A 10, 11137-11149, 2022

Authors: Tamar Zelovich, Cataldo Simari, Isabella Nicotera, Dario R. Dekel and Mark E. Tuckerman

Exposing anion exchange membrane (AEM) fuel cells to ambient air is known to decrease fuel cell efficiency significantly due to the presence of CO₂. In this combined theoretical and experimental study, we examine the hydration conditions that promote reactions between CO₂ and hydroxide ions in nano-confined AEMs, and we explore the effect of the carbonation process on the solvation structure and diffusion of hydroxide ions. Using fully atomistic ab initio molecular dynamics (AIMD) simulations, we find that increasing hydration can delay the carbonation reaction between OH⁻ and CO₂. Once reacted, HCO₃⁻ ions exist along the simulation, which significantly reduce the diffusion of the hydroxide ions. We confirm these results using ¹H- and ¹³C-pulsed field gradient nuclear magnetic resonance. AIMD simulations further reveal that the HCO₃⁻ actively “blocks” the diffusion path of hydroxide ions along the simulation cell. We expect that elucidating the key design principles underlying the atomistic effects of carbonate ions on hydroxide ions diffusion mechanisms in model AEMs will provide useful guidelines for the synthesis and experimental characterization of novel highly conductive AEM fuel cell technologies in the presence of CO₂-containing air.

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Impact of the Relative Humidity on the Performance Stability of Anion Exchange Membrane Fuel Cells Studied by Ion Chromatography

Authors:
J Lorenz, H Janßen, K Yassin, J Leppin, Y-W Choi, J-E Cha, M Wark, S Brandon, D R Dekel, C Harms, A Dyck
Journal:ACS Appl. Polym. Mater., 4, 3962−3970, 2022

Authors: Julian Lorenz, Holger Janßen, Karam Yassin, Janine Leppin, Young-Woo Choi, Jung-Eun Cha, Michael Wark, Simon Brandon, Dario R. Dekel, Corinna Harms, and Alexander Dyck

Although substantial improvement of the performance of anion exchange membrane fuel cells (AEMFCs) was achieved, longevity is still the main challenge for the AEMFC technology, which is attributed to the degradation of the functional groups of applied membranes and ionomers. Contrary to ex situ material stability studies, we demonstrate here the application of ion chromatography to quantify the amounts of degradation products in the exhaust water during different fuel cell operation conditions on the example of trimethylbenzyl ammonium as a functional group. Higher amounts of degradation products were detected directly after equilibration and completion of polarization curves compared to performance stability measurements under constant load. Moreover, the performance stability dependent on the relative humidity of the anode and cathode feed gases was evaluated. Elevated losses of ionic groups were observed in the anode exhaust water at high humidity fuel cell operation, although higher degradation rates were determined for the cathode side by modeling the performance stability. In contrast, higher amounts of degradation products were detected in the cathode exhaust water under low humidity conditions. However, the mobility of water and degradation products under different fuel cell operation conditions impedes a detailed allocation of the observed degradation to one electrode. The demonstrated combination of in situ electrochemical measurements, corresponding ex situ degradation measurements, and modeling data gives comprehensive insights into the evaluation of the performance stability of anion exchange membrane materials under fuel cell operation, which could exceed ex situ durability experiments based on the membrane materials itself.

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Isoindolinium Groups as Stable Anion Conductors for Anion-Exchange Membrane Fuel Cells and Electrolyzers

Authors:
K Aggarwal, N Gjineci, A Kaushansky, S Bsoul, J C Douglin, S Li, I Salam, S Aharonovich, J R Varcoe, D R Dekel, C E Diesendruck
Journal:ACS Mater. Au, 2, 367−373, 2022

Authors: Kanika Aggarwal, Nansi Gjineci, Alexander Kaushansky, Saja Bsoul, John C. Douglin, Songlin Li, Ihtasham Salam, Sinai Aharonovich, John R. Varcoe, Dario R. Dekel, and Charles E. Diesendruck

Anion-exchange membrane (AEM) fuel cells (AEMFCs) and water electrolyzers (AEMWEs) have gained strong attention of the scientific community as an alternative to expensive mainstream fuel cell and electrolysis technologies. However, in the high pH environment of the AEMFCs and AEMWEs, especially at low hydration levels, the molecular structure of most anion-conducting polymers breaks down because of the strong reactivity of the hydroxide anions with the quaternary ammonium (QA) cation functional groups that are commonly used in the AEMs and ionomers. Therefore, new highly stable QAs are needed to withstand the strong alkaline environment of these electrochemical devices. In this study, a series of isoindolinium salts with different substituents is prepared and investigated for their stability under dry alkaline conditions. We show that by modifying isoindolinium salts, steric effects could be added to change the degradation kinetics and impart significant improvement in the alkaline stability, reaching an order of magnitude improvement when all the aromatic positions are substituted. Density functional theory (DFT) calculations are provided in support of the high kinetic stability found in these substituted isoindolinium salts. This is the first time that this class of QAs has been investigated. We believe that these novel isoindolinium groups can be a good alternative in the chemical design of AEMs to overcome material stability challenges in advanced electrochemical systems.

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What is Next in Anion-Exchange Membrane Water Electrolyzers? Bottlenecks, Benefits, and Future

Authors:
C Santoro, A Lavacchi, P Mustarelli, V Di Noto, L Elbaz, D R Dekel, F Jaouen
Journal:ChemSusChem 15, e202200027, 2022

Authors: Carlo Santoro, Alessandro Lavacchi, Piercarlo Mustarelli, Vito Di Noto, Lior Elbaz, Dario R. Dekel, and Frédéric Jaouen

As highlighted by the recent roadmaps from the European Union and the United States, water electrolysis is the most valuable high-intensity technology for producing green hydrogen. Currently, two commercial low-temperature water electrolyzer technologies exist: alkaline water electrolyzer (A-WE) and proton-exchange membrane water electrolyzer (PEM-WE). However, both have major drawbacks. A-WE shows low productivity and efficiency, while PEM-WE uses a significant amount of critical raw materials. Lately, the use of anion-exchange membrane water electrolyzers (AEM-WE) has been proposed to overcome the limitations of the current commercial systems. AEM-WE could become the cornerstone to achieve an intense, safe, and resilient green hydrogen production to fulfill the hydrogen targets to achieve the 2050 decarbonization goals. Here, the status of AEM-WE development is discussed, with a focus on the most critical aspects for research and highlighting the potential routes for overcoming the remaining issues. The Review closes with the future perspective on the AEM-WE research indicating the targets to be achieved.

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Magnetic-field-oriented mixed-valence-stabilized ferrocenium anion-exchange membranes for fuel cells

Authors:
X Liu , N Xie, J Xue, M Li, C Zheng, J Zhang , Y Qin, Y Yin, D R Dekel, M D Guiver
Journal:Nature Energy 7, 329–339, 2022

Authors: Xin Liu , Na Xie, Jiandang Xue, Mengyuan Li, Chenyang Zheng, Junfeng Zhang , Yanzhou Qin, Yan Yin, Dario R. Dekel and Michael D. Guiver

Through-plane (TP) conducting pathways in anion-exchange membranes (AEMs) are desirable for AEM fuel cells as they serve as short and efficient routes for hydroxide ion transport between electrodes, improving power output. Electric and magnetic fields have previously been used to create TP-oriented structures in AEMs, but with modest performance gains. Here we use paramagnetic ferrocenium polymers to prepare TP-oriented AEMs under a magnetic field. The magnetic field induces a mixed-valence state, which effectuates higher anion dissociation and enhanced alkali/redox stability. Our AEMs display a promising TP hydroxide conductivity of ~160 mS cm⁻¹ at 95 °C in water, and no appreciable hydroxide conductivity loss over 4,320 h at 95 °C in alkali. The assembled fuel cells achieve a power output of 737 mW cm⁻² at 80 °C and 80% relative humidity, and a durability of 3.9% voltage loss and 2.2% high-frequency resistance increase over 500 h at 500 mA cm⁻², 120 °C and 40% relative humidity.

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Designing the feasible membrane systems for CO2 removal from Air-fed Anion-Exchange membrane fuel cells

Authors:
Z Liang, F Yang, Y Li, J Tang, D R Dekel, X He
Journal:Sep. Purification Tech. 289, 120713, 2022

Authors: Zhicong Liang, Feng Yang, Yang Li, Jiali Tang, Dario R. Dekel, Xuezhong He

Highly CO₂ permeable membranes with good selectivity are ideal candidates for CO₂ separation_. Herein, we, for the first time, designed different polymeric membrane systems for CO₂ removal from the air-fed anion-exchange membrane fuel cells (AEMFCs) by UniSim simulation under different operating conditions. The results indicate that the operations with higher feed pressure and permeate vacuum degree reduce the required membrane areas but increase the power demands. In addition, the single-stage facilitated transport membrane system (CO₂ permeance of 3000 GPU) with an area of<10 m² is feasible to reach a low CO₂ content of < 5 ppm for automotive AEMFCs with an air flow rate of 30 Nm³/h at 87 °C. It was also found that pursuing an extremely high CO₂ removal ratio dramatically increases the required membrane area and the O₂ loss, and sweep gas is more applicable for automotive vehicles. Nevertheless, highly permeable membranes should be further developed to enhance their competitiveness for CO₂ removal from the air-fed AEMFCs.

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Non-Monotonic Temperature Dependence of Hydroxide Ion Diffusion in Anion Exchange Membranes

Authors:
T Zelovich, L Vogt-Maranto, C Simari, I Nicotera, M A Hickner, S J Paddison, C Bae, D R Dekel, M E Tuckerman
Journal:Chem. Mater. 34, 2133-2145, 2022

Authors: Tamar Zelovich, Leslie Vogt-Maranto, Cataldo Simari, Isabella Nicotera, Michael A. Hickner, Stephen J. Paddison, Chulsung Bae, Dario R. Dekel, and Mark E. Tuckerman

Recent studies suggest that operating anion exchange membrane (AEM) fuel cells at high temperatures has enormous technological potential. However, obtaining a fundamental understanding of the effect of temperature on hydroxide conductivity and membrane stability remains a key hurdle to realizing the full potential of high-temperature AEM fuel cells. In this work, we present a combined theoretical and experimental study to explore the effect of temperature on hydroxide ion and water diffusivities in AEMs. Both fully atomistic ab initio molecular dynamics simulations and ¹H pulsed field gradient NMR measurements confirm that the OH^– diffusion changes non-monotonically with increasing temperature. Specifically, the D_OH^– versus T curve exhibits a region in which dD_OH^–/dT < 0, indicating the presence of a kink in the curve, which we refer to as a “diffusion kink”. The simulations show that the underlying causes of this behavior vary with the hydration level. Furthermore, we were able to rationalize the conditions underlying this counterintuitive behavior and to suggest ways to identify the optimal operating temperature for each model AEM system. We expect that the discovery of this unusual temperature dependence of the diffusivity will play an important role in the design of new, stable, and highly conductive AEM-based devices such as electrolyzers, redox flow batteries, and fuel cells.

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Measuring the alkaline stability of anion-exchange membranes

Authors:
S Haj-Bsoul, J R Varcoe, D R Dekel
Journal:J. Electroanal. Chem. 908, 116112, 2022

Authors: Saja Haj-Bsoul, John R. Varcoe, Dario R. Dekel

One significant barrier in developing durable, robust anion-exchange membranes (AEMs) for liquid-electrolyte-free fuel cells (AEMFCs) and water electrolyzers (AEMWEs) is their limited chemical stability to alkali. To measure the alkaline stability of AEMs, ex-situ tests are commonly used where the AEMs are immersed for long durations in aqueous alkali solutions. However, such tests do not adequately simulate the liquid-electrolyte-free environment of AEMFCs and AEMWEs, as the hydration and alkaline conditions do not always mimic actual operando conditions, yielding misleading and inaccurate indications of degradation rates for relatively low hydration conditions. We recently reported a unique ex-situ method which determines the alkaline stability of AEMs under conditions that mimic in-situ operating environments. In this study, we apply this technique to determine the alkaline stability of several AEMs containing different functional group and backbone chemistries. The alkaline stability of HDPE-based radiation-grafted (RG)-AEMs containing different functional group chemistries follows the trend: TMA ≥ MPY ∼ MPIP ≫ DEMA > TEA. Radiation-grafted AEMs (and a non-radiation-grafted PPO benchmark) containing different backbones and the same stable TMA group follows the stability order: ETFE ≥ LDPE > HDPE ≫ PPO. This technique is recommended for ex-situ testing of the alkaline stability of AEMs for both AEMFC and AEMWE applications.

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Elucidating the role of anion-exchange ionomer conductivity within the cathode catalytic layer of anion-exchange membrane fuel cells

Authors:
K Yassin, I G Rasin, S Brandon, D R Dekel
Journal:J. Power Sources 524, 231083, 2022

Authors: Karam Yassin, Igal G. Rasin, Simon Brandon, Dario R. Dekel

The anion-exchange ionomer (AEI) is a crucial component of anion-exchange membrane fuel cells (AEMFCs). In this study, computational analysis is employed to study the unexplored and critical effect of AEI hydroxide conductivity, within the cathode electrode, on AEMFC performance and its stability. The cathode is of particular importance due to its tendency to dry-out in a manner that may impact ionomer conductivity during AEMFC operation. Our modeling results clearly show that enhanced AEI hydroxide conductivity, within the cathode, significantly increases AEMFC performance. Less intuitive is its positive impact on cell stability. Superior conductivity is associated with high catalyst utilization and a reduced potential drop across the cathode. This enhances hydration levels in the cathode resulting in slower degradation kinetics. While the cathode reaction kinetics is considered as a major factor restricting cell performance, the transport of water and hydroxide through the cathode catalyst layer is extremely critical to ensure long-term AEMFC performance stability. In addition, a two-step degradation mechanism is observed where initially the voltage loss is controlled by the cathode AEI degradation, and only later membrane degradation becomes dominant. These insights are critical for understanding cell operation and for the achievement of further progress in AEMFC performance stability.

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Recent developments in Pd-CeO2 nano-composite electrocatalysts for anodic reactions in anion exchange membrane fuel cells

Authors:
H A Miller, M Bellini, D R Dekel, F Vizza
Journal:Electrochemistry Communications 135, 107219, February 2022

Authors: Hamish A. Miller, Marco Bellini, Dario R. Dekel, Francesco Vizza

In 2016, for the first time a polymer electrolyte fuel cell free of Pt electrocatalysts was shown to deliver more than 0.5 W cm⁻² of peak power density from H₂ and air (CO₂ free). This was achieved with a silver-based oxygen reduction (ORR) cathode and a Pd-CeO₂ hydrogen oxidation reaction (HOR) anodic electrocatalyst. The poor kinetics of the HOR under alkaline conditions is a considerable challenge to Anion Exchange Membrane Fuel Cell (AEMFC) development as high Pt loadings are still required to achieve reasonable performance. Previously, the ameliorative combination of Pd and CeO₂ nanocomposites has been exploited mostly in heterogeneous catalysis where the positive interaction is well documented. Carbon supported Pd-CeO₂ HOR catalysts have now been prepared by different synthetic techniques and employed in AEMFCs as alternative to Pt and PtRu standards. Important research has also been recently reported, delving into the origin of the HOR enhancement on Pd-CeO₂. Such work has highlighted the importance of the bifunctional mechanism of the HOR at high pHs. Carefully prepared nano-structures of Pd and CeO₂ that promote the formation of the Pd-O-Ce interface provide optimal binding of both H_ad and OH_ad species, aspects which are crucial for enhanced HOR kinetics. This review paper discusses the recent advances in Pd-CeO₂ electrocatalysts for AEMFC anodes.

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Alkaline Stability of Low Oxophilicity Metallopolymer Anion-Exchange Membranes

Authors:
K Aggarwal, S Bsoul, J C Douglin, S Li, D R Dekel, C E Diesendruck
Journal:Chemistry A European Jornal – 2022

Authors: Kanika Aggarwal, Saja Bsoul, John C. Douglin, Songlin Li, Dario R. Dekel, Charles E. Diesendruck

Anion-exchange membrane fuel cells (AEMFCs) are promising energy conversion devices due to their high efficiency. Nonetheless, AEMFC operation time is currently limited by the low chemical stability of their polymeric anion-exchange membranes. In recent years, metallopolymers, where the metal centers assume the ion transport function, have been proposed as a chemically stable alternative. Here we present a systematic study using a polymer backbone with side-chain N-heterocyclic carbene (NHC) ligands complexed to various metals with low oxophilicity, such as copper, zinc, nickel, and gold. The golden metallopolymer, using the metal with the lowest oxophilicity, demonstrates exceptional alkaline stability, far superior to state-of-the-art quaternary ammonium cations, as well as good in situ AEMFC results. These results demonstrate that judiciously designed metallopolymers may be superior to purely organic membranes and provides a scientific base for further developments in the field.

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Effect of LDH platelets on the transport properties and carbonation of anion exchange membranes

Authors:
C Simari, E Lufrano, M H Ur Rehmana, A Zhegur-Khais, S Haj-Bsoul, D R Dekel, I Nicotera
Journal:Electrochim. Acta 403, 139713, 2022

Authors: Cataldo Simari, Ernestino Lufrano, Muhammad Habib Ur Rehmana, Avital Zhegur-Khais, Saja Haj-Bsoul, Dario R. Dekel, Isabella Nicotera

We report on an extensive study on nanocomposite Anion Exchange Membranes (AEMs) based on tetramethylammonium Polysulfone ionomer and Layered Double Hydroxide (LDH) as nanofiller. The AEMs were investigated in both OH⁻ and $H C O_{3}^{-}$ forms, comparing swelling capacity and transport properties. Ionic conductivity measurements were performed both by Electrochemical Impedance Spectroscopy and Ziv and Dekel’s method, while the water and ions mobility by H Pulse Field Gradient (PFG) NMR spectroscopy.

One of the most serious problems to be addressed in AEMs fuel cell technology is that of the significant loss of performance when CO₂ is present in the reaction oxidant gas (e.g., air) due to the phenomenon of carbonation. In this work, the carbonation kinetics of AEMs and the effect of LDH filler were addressed through C NMR spectroscopy (including diffusometry and relaxometry measurements). The presence of LDH platelets in the AEM significantly reduces the conversion rate of hydroxide groups. The interaction between the polymer chains and the anionic clay lamellae creates a suitable network that benefits the membrane’s ionic conductivity, its mechanical properties (DMA tests), and finally reduces the rate of alkaline degradation.

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High-performance radiation grafted anion-exchange membranes for fuel cell applications: Effects of irradiation conditions on ETFE-based membranes properties

Authors:
A L G Biancolli, S Bsoul-Haj, J C Douglin, A S Barbosa, R R de Sousa Jr, O Rodrigues Jr, A J C Lanfredi, D R Dekel, E I Santiago
Journal:Journal of Membrane Science, 119879, 2022

Authors: Ana Laura G. Biancolli, Saja Bsoul-Haj, John C. Douglin, Andrey S. Barbosa, Rog´erio R. de Sousa Jr., Orlando Rodrigues Jr., Alexandre J.C. Lanfredi, Dario R. Dekel, Elisabete I. Santiago

Anion-exchange membrane fuel cells (AEMFCs) are rapidly gaining visibility in the clean energy research field due to their high power output and potential to significantly reduce materials costs. However, despite the high performances obtained, in-operando stability still presents a major obstacle for this technology. The durability issues are usually attributed to the core component of the AEMFCs – the anion-exchange membrane (AEM). An easy and simple way to produce these AEMs is through radiation grafting. Radiation-induced processes involve changes in the intrinsic properties of the polymer that can promote both crosslinking and chain scissioning, which may directly affect the mechanical properties and durability of AEMs. This study presents a comprehensive report of the effects of irradiation on the final properties of electron-beam grafted ETFE-AEMs. The results strongly suggest that low absorbed doses (<40 kGy) and an inert atmosphere (N₂) should be used during the irradiation process in order to obtain better backbone stability and, consequently, AEMFC durability, considering ETFE-based AEMs.

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Surface Acoustic Wave Mitigation of Precipitate Deposition on a Solid Surface – An Active Self-Cleaning Strategy

Authors:
Y Li, D R Dekel, O Manor
Journal:ACS Appl. Mater. Interfaces 2021, 13, 59471−59477

Authors: Yifan Li, Dario R. Dekel, and Ofer Manor

We demonstrate the application of a 20 MHz frequency surface acoustic wave (SAW) in a solid substrate to render its surface “self-cleaning”, redirecting the deposition of precipitating mass onto a nearby inert substrate. In our experiment, we confine a solution of poly(methyl methacrylate) polymer and a volatile toluene solvent between two substrates, lithium niobate and glass, at close proximity. We render the glass surface low energy by employing hydrophobic coating. In the absence of SAW excitation, we observe that the evaporation of the solvent yields polymer coating on the higher energy lithium niobate surface, while the glass surface is mostly devoid of polymer deposits. The application of a propagating SAW in the lithium niobate substrate mitigates the deposition of the polymer on its surface. As a response, we observe an increase in the deposition of the polymer precipitates on glass. Above a SAW power threshold, the polymer appears to deposit solely on glass, leaving the surface of the lithium niobate substrate devoid of polymer mass.

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Understanding how single-atom site density drives the performance and durability of PGM-free FeeNeC cathodes in anion exchange membrane fuel cells

Authors:
H Adabi, P Giovanni Santori, A Shakouri, X Peng, K Yassin, I G Rasin, S Brandon, D R Dekel, N Ul Hassan, M-T Sougrati, A Zitolo, J R Varcoe, J R Regalbuto, F Jaouen, W E Mustain
Journal:Materials Today Advances, December, 100179, 2021

Authors: Horie Adabi, Pietro Giovanni Santori, Abolfazl Shakouri, Xiong Peng, Karam Yassin, Igal G. Rasin, Simon Brandon, Dario R. Dekel, Noor Ul Hassan, Moulay-Tahar Sougrati, Andrea Zitolo, John R. Varcoe, John R. Regalbuto, Frederic Jaouen, William E. Mustain

One of the most important needs for the future of low-cost fuel cells is the development of highly active platinum group metal (PGM)-free catalysts. For the oxygen reduction reaction, Fe–N–C materials have been widely studied in both acid and alkaline media. However, reported catalysts in the literature show quite different intrinsic activity and in-cell performance, despite similar synthesis routes and precursors. Here, two types of Fe–N–C are prepared from the same precursor and procedure – the main difference is how the precursor was handled prior to use. It is shown that in one case Fe overwhelmingly existed as highly active single-metal atoms in FeN₄ coordination (preferred), while in the other case large Fe particles coexisting with few single metal atoms were obtained. As a result, there were drastic differences in the catalyst structure, activity, and especially in their performance in an operating anion exchange membrane fuel cell (AEMFC). Additionally, it is shown that catalyst layers created from single-atom-dominated Fe–N–C can have excellent performance and durability in an AEMFC using H₂/O₂ reacting gases, achieving a peak power density of 1.8 W cm⁻² – comparable to similar AEMFCs with a Pt/C cathode – and being able to operate stably for more than 100 h. Finally, the Fe–N–C cathode was paired with a low-loading PtRu/C anode electrode to create AEMFCs (on H₂/O₂) with a total PGM loading of only 0.135 mg cm⁻² (0.090 mg_Pt cm⁻²) that was able to achieve a very high specific power of 8.4 W mg_PGM⁻¹ (12.6 W mg_Pt⁻¹).

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Bifunctional Oxygen Electrocatalysis on Mixed Metal Phthalocyanine-Modified Carbon Nanotubes Prepared via Pyrolysis

Authors:
Y Kumar, E Kibena-Põldsepp, J Kozlova, M Rähn, A Treshchalov, A Kikas, V Kisand, J Aruväli, A Tamm, J C Douglin, S J Folkman, I Gelmetti, F A Garcés-Pineda, J R Galán-Mascarós, D R Dekel, K Tammeveski
Journal:ACS Appl. Mater. Interfaces 2021, 13, 35, 41507–41516

Authors: Yogesh Kumar, Elo Kibena-Põldsepp, Jekaterina Kozlova, Mihkel Rähn, Alexey Treshchalov, Arvo Kikas, Vambola Kisand, Jaan Aruväli, Aile Tamm, John C. Douglin, Scott J. Folkman, Ilario Gelmetti, Felipe A. Garcés-Pineda, José Ramón Galán-Mascarós, Dario R. Dekel, and Kaido Tammeveski

Non-precious-metal catalysts are promising alternatives for Pt-based cathode materials in low-temperature fuel cells, which is of great environmental importance. Here, we have investigated the bifunctional electrocatalytic activity toward the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) of mixed metal (FeNi; FeMn; FeCo) phthalocyanine-modified multiwalled carbon nanotubes (MWCNTs) prepared by a simple pyrolysis method. Among the bimetallic catalysts containing nitrogen derived from corresponding metal phthalocyanines, we report the excellent ORR activity of FeCoN-MWCNT and FeMnN-MWCNT catalysts with the ORR onset potential of 0.93 V and FeNiN-MWCNT catalyst for the OER having E_OER = 1.58 V at 10 mA cm^–2. The surface morphology, structure, and elemental composition of the prepared catalysts were examined with scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The FeCoN-MWCNT and FeMnN-MWCNT catalysts were prepared as cathodes and tested in anion-exchange membrane fuel cells (AEMFCs). Both catalysts displayed remarkable AEMFC performance with a peak power density as high as 692 mW cm^–2 for FeCoN-MWCNT.

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Multi-scale study on bifunctional Co/Fe–N–C cathode catalyst layers with high active site density for the oxygen reduction reaction

Authors:
W Zhu, Y Pei, J C Douglin, J Zhang, H Zhao, J Xue, Q Wang, R Li, Y Qin, Y Yin, D R Dekel, M D Guiver
Journal:Applied Catalysis B: Environmental, 2021

Authors: Yabiao Pei, John C. Douglin, Junfeng Zhang, Haoyang Zhao, Jiandang Xue, Qingfa Wang, Ran Li, Yanzhou Qin, Yan Yin, Dario R. Dekel, Michael D. Guiver

Recently, much work has been devoted to designing catalysts with high porosity and efficient active sites. Although very promising results are achieved using Co/Fe–N–C catalysts based on rotating disk electrode (RDE) tests, actual fuel cell performance is below expectations, probably due to insufficient understanding of the catalyst layer (CL). Therefore, catalyst design should be considered holistically by taking into account CL performance, not only intrinsic activity. Here, Co/Fe–N–C with highly dispersed CoFe nanoalloy in the carbon network is obtained by careful design of Co/Fe-ZIF precursor, resulting in a high oxygen reduction reaction (ORR) site density with good stability. Concerning RDE test in the kinetic region and single cell test (SCT) with complex influence factors, the half-cell test (HCT) is introduced to more accurately evaluate the quality of the Co/Fe–CL. Multi-scale measurements (RDE, HCT and SCT) in different current density ranges allows targeting the key CL influence factors for fuel cell performance.

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A surprising relation between operating temperature and stability of anion exchange membrane fuel cells

Authors:
K Yassin, I G Rasin, S Willdorf-Cohen, C E Diesendruck, S Brandon, D R Dekel
Journal:Journal of Power Sources Advances, 2021, 100066

Authors: Karam Yassin, Igal G. Rasin, Sapir Willdorf-Cohen, Charles E. Diesendruck, Simon Brandon, Dario R. Dekel

Anion-exchange membrane fuel cells (AEMFCs) show substantially enhanced (initial) performance and efficiency with the increase of operational temperature (where typical values are below 80 °C). This is directly due to the increase in reaction and mass transfer rates with temperature. Common sense suggests however that the increase of ionomeric material chemical degradation kinetics with temperature is likely to offset the above mentioned gain in performance and efficiency. In this computational study we investigate the combined effect of a high operating temperature, up to 120 °C, on the performance and stability of AEMFCs. Our modeling results demonstrate the expected positive impact of operating temperature on AEMFC performance. More interestingly, under certain conditions, AEMFC performance stability is surprisingly enhanced as temperature increases. While increasing cell temperature enhances degradation kinetics, it simultaneously improves water diffusivity through the membrane, resulting in higher hydration levels at the cathode. This, in turn, encourages a decrease in ionomer chemical degradation which depends on the hydration as well as on temperature, leading to a significant increase in AEMFC performance stability and, therefore, in its lifetime. These findings predict the possible advantage (and importance), in terms of performance and durability, of developing high-temperature AEMFCs for automotive and other applications.

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Metal nanoparticles entrapped in metal matrices

Authors:
D Pinsky, N Ralbag, R K Singh, M Mann-Lahav, G E Shter, D R Dekel, G S Grader, D Avnir
Journal:Nanoscale Adv., 2021, 3, 4597-4612

Authors: Dina Pinsky, Noam Ralbag, Ramesh Kumar Singh, Meirav Mann-Lahav, Gennady E. Shter, Dario R. Dekel, Gideon S. Grader and David Avnir

We developed synthetic methods for the doping of metals (M) with metallic nanoparticles (NPs). To the best of our knowledge – unlike oxides, polymers and carbon-based supports – metals were not used so far as supporting matrices for metallic NPs. The composites (denoted M1-NPs@M2) comprise two separate phases: the metallic NPs (the dopant) and the entrapping 3D porous metallic matrix, within which the NPs are intimately held and well dispersed. Two different general synthetic strategies were developed, each resulting in a group of materials with characteristic structure and properties. The first strategy uses pre-prepared NPs and these are entrapped during reductive formation of the metallic matrix from its cation. The second strategy is in situ growth of the doped metallic NPs within the pre-prepared entrapping metallic matrix. These two methods were developed for two types of entrapping metallic matrices with different morphologies: porous aggregated metallic matrices and metallic foams. The leading case in this study was the use of Pt as the NP dopant and Ag as the entrapping matrix, using all of the four combinations – entrapment or growth within aggregated Ag or Ag foam matrices. Full physical and chemical properties analysis of these novel types of materials was carried out, using a wide variety of analytical methods. The generality of the methods developed for these bi-metallic composites was investigated and demonstrated on additional metallic pairs: Au NPs within Ag matrices, Pd NPs within Ni matrices and Ir-NPs within a Rh matrix. As the main application of metallic NPs is in catalysis, the catalytic activity of M1-NPs@M2 is demonstrated successfully for entrapped Pt within Ag for reductive catalytic reactions, and for Pd within Ni for the electrocatalytic hydrogen oxidation reaction.

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A high-temperature anion-exchange membrane fuel cell with a critical raw material-free cathode

Authors:
J C Douglin, R K Singh, S Haj-Bsoul, S Li, J Biemolt, N Yan, J R Varcoe, G Rothenberg, D R Dekel
Journal:Chem. Eng. J. Adv. 8, 100153, 2021

Authors: John C. Douglin, Ramesh K. Singh, Saja Haj-Bsoul, Songlin Li, Jasper Biemolt, Ning Yan, John R. Varcoe, Gadi Rothenberg, Dario R. Dekel

We present a first high-temperature anion-exchange membrane fuel cell (HT-AEMFC, operating at 105 °C) based on a critical raw material (CRM)-free cathode catalyst; at the same temperature, the anion-exchange membrane (AEM) has a high ex-situ hydroxide conductivity value of 201 mS cm⁻¹. Our HT-AEMFC, containing a highly active nitrogen-doped carbon (N-doped-C) cathode catalyst, also features low polarization resistances, high catalytic activity and stability, with retention of 81% of the catalyst layer capacitance after an initial 10 h longevity test, and delivery of a peak power of 1.14 W cm⁻². This is one of the highest power densities reported for an AEMFC containing a CRM-free cathode. This work shows the potential of the new field of HT-AEMFCs, opening opportunities for developing and using novel CRM-free catalysts that are highly active at these high operating temperatures.

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Ligand Valency Effects on the Alkaline Stability of Metallopolymer Anion-Exchange Membranes

Authors:
K Aggarwal, S Bsoul, S Li, D R Dekel, C E Diesendruck
Journal:Macromolecular Rapid Communications, 2021

Authors: Kanika Aggarwal, Saja Bsoul, Songlin Li, Dario R. Dekel, and Charles E. Diesendruck

Long-term stability is a key requirement for anion-exchange membranes (AEMs) for alkaline fuel cells and electrolyzers that is yet to be fulfilled. Different cationic chemistries are being exploited to reach such a goal, and metallopolymers present the unique advantage of chemical stability towards strong nucleophiles as compared to organic cations. Yet, the few metallopolymers tested in strongly alkaline conditions or even in fuel cells still degrade. Therefore, fundamental studies can be advantageous in directing future developments towards this goal. Here, a systematic study of the effect of ligand valency is presented, using nickel-based metallopolymers on polynorbornene backbones, functionalized with multidentate pyridine ligands. Metallopolymers using a single ligand type as well as all the possible mixtures are prepared and their relative stability towards aggressive alkaline conditions compared. Metallopolymer in which nickel ions are hexacoordinated with two tridentate ligands demonstrates superior stability. More importantly, by comparing all the metallopolymers’ stability, the reason behind such relative stability provides design parameters for novel metallopolymer AEMs.

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Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells

Authors:
N Zion, J C Douglin, D A Cullen, P Zelenay, D R Dekel, L Elbaz
Journal:Advanced Functional Materials, 2021

Authors: Noam Zion, John C. Douglin, David A. Cullen, Piotr Zelenay, Dario R. Dekel, and Lior Elbaz

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction have shown high oxygen reduction reaction activity in alkaline media. In order to further increase the power density of anion-exchange membrane fuel cells (AEMFCs), PGM-free catalysts need to have a high site density to reach high current densities. Herein, synthesis, characterization, and utilization of heat-treated iron porphyrin aerogels are reported as cathode catalysts in AEMFCs. The heat treatment effect is thoroughly studied and characterized using several techniques, and the best performing aerogel is studied in AEMFC, showing excellent performance, reaching a peak power density of 580 mW cm⁻² and a limiting current density of as high as 2.0 A cm⁻², which can be considered the state-of-the-art for PGM-free based AEMFCs.

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Effect of the Synthetic Method on the Properties of Ni-Based Hydrogen Oxidation Catalysts

Authors:
E S Davydova, M Manikandan, D R Dekel, S Sunde
Journal:ACS Appl. Energy Mater. 4, 3404−3423, 2021

Authors: Elena S. Davydova, Maidhily Manikandan, Dario R. Dekel, and Svein Sunde

The latest progress in alkaline anion-exchange membranes has led to the expectation that less costly catalysts than those of the platinum-group metals may be used in anion-exchange membrane fuel cell devices. In this work, we compare structural properties and the catalytic activity for the hydrogen-oxidation reaction (HOR) for carbon-supported nanoparticles of Ni, Ni₃Co, Ni₃Cu, and Ni₃Fe, synthesized by chemical and solvothermal reduction of metal precursors. The catalysts are well dispersed on the carbon support, with particle diameter in the order of 10 nm, and covered by a layer of oxides and hydroxides. The activity for the HOR was assessed by voltammetry in hydrogen-saturated aqueous solutions of 0.1 mol dm^–1 KOH. A substantial activation by potential cycling of the pristine catalysts synthesized by solvothermal reduction is necessary before these become active for the HOR; in situ Raman spectroscopy shows that after activation the surface of the Ni/C, Ni₃Fe, and Ni₃Co catalysts is fully reduced at 0 V, whereas the surface of the Ni₃Cu catalyst is not. The activation procedure had a smaller but negative impact on the catalysts synthesized by chemical reduction. After activation, the exchange-current densities normalized with respect to the ECSA (electrochemically active surface area) were approximately independent of composition but relatively high compared to catalysts of larger particle diameter.

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Crosslinked quaternary phosphonium-functionalized poly(ether ether ketone) polymer-based anion-exchange membranes

Authors:
M Kumari, J C Douglin, D R Dekel
Journal: J. Membrane Science 626, 119167, 2021

Authors: Mamta Kumari, John C. Douglin, Dario R. Dekel

A series of mechanically robust and highly conducting crosslinked anion-exchange membranes are synthesized by using quaternary phosphonium-functionalized poly(ether ether ketone) (QPPEEK) as base polymer and poly(ethylene glycol) (PEG) as the crosslinker. The crosslinked membrane containing both rigid and flexible polymer constituents (QPPEEK-PEG) revealed better flexibility and mechanical strength, and higher conductivity than the pristine QPPEEK membrane. The mechanical properties and ionic conductivity of the crosslinked membranes are tuned by the mass fractions of the QPPEEK and PEG components. Among the different QPPEEK and PEG compositions, the QPPEEK-PEG 20 (QPPEEK:PEG 80:20) membrane has the best properties, reaching a hydroxide conductivity of 102 mS cm⁻¹ at 80 ᵒC, a tensile strength of 13.7 MPa, and 48% of elongation. The membrane is chemically stable in alkaline environment, retaining 85% of its initial conductivity and mechanical strength after being exposed to 1 M KOH at 80 °C for 400 h. Finally, in the operando anion-exchange membrane fuel cell the QPPEEK-PEG 20 crosslinked membrane performs well, displaying an open-circuit voltage of 1.05 V and a maximum power density of 154 mW cm⁻² measured at 0.55 V, showing the potential of PEEK-based membranes for fuel cell applications.

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An Anion-Exchange Membrane Fuel Cell Containing Only Abundant and Affordable Materials

Authors:
J Biemolt, J C Douglin, R K Singh, E S Davydova, N Yan, G Rothenberg, D R Dekel
Journal:Energy Technology 9, 2000909, 2021

Authors: Jasper Biemolt, John C. Douglin, Ramesh K. Singh, Elena S. Davydova, Ning Yan, Gadi Rothenberg, and Dario R. Dekel

Herein, a unique anion‐exchange membrane fuel cell (AEMFC) containing only affordable and abundant materials is presented: NiFe hydrogen oxidation reaction (HOR) and nitrogen‐doped carbon oxygen reduction reaction (ORR) electrocatalysts. AEMFCs are an attractive alternative to proton‐exchange membrane fuel cells. They can run under alkaline conditions, allowing the use of platinum group metal (PGM)‐free electrocatalysts. Yet, the same alkaline conditions incur an overpotential loss in ORR and also slow the HOR. This can be solved by using PGM electrodes, but then the original advantage disappears. In contrast, the fuel cell is free of both PGMs and critical raw materials (CRMs). Electrochemical studies confirmed that the catalysts are highly active in both HOR and ORR in an alkaline electrolyte. The morphology, composition, and chemical states of the electrocatalysts are characterized by different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), and electron energy loss spectroscopy (EELS). Then, the electrocatalysts’ performance is tested in a fuel cell device. The cell gives a maximum power density of 56 mW cm⁻² and a limiting current density of 220 mA cm⁻². These results are among the best CRM‐free anion‐exchange membrane fuel cells reported to date.

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Atomistic Insights into the Hydrogen Oxidation Reaction of Palladium-Ceria Bifunctional Catalysts for Anion-Exchange Membrane Fuel Cells

Authors:
S Sahoo, D R Dekel, R Maric, S P Alpay
Journal:ACS Catal., 11, 2561−2571, 2021

Authors: Sanjubala Sahoo, Dario R. Dekel, Radenka Maric, S. Pamir Alpay

Hydrogen oxidation reaction (HOR) is one of the critical processes in clean and sustainable energy conversion devices such as anion-exchange membrane fuel cells (AEMFCs). There is significant interest in the design of highly active anode catalysts for such fuel cells. Here, we present the results of an ab initio study that explores the mechanism of HOR for palladium-ceria anode catalysts. This combination of materials has been shown to display excellent HOR performance experimentally. We use density functional theory with exchange–correlation functionals described by the generalized gradient approximation and the necessary Hubbard corrections. This allows us to accurately capture the electronic structure and the associated functional properties of all the components of the catalyst. The computations are carried out for multiple palladium concentrations on ceria surfaces. The reaction pathway for HOR is investigated via the Tafel reaction for the dissociation of hydrogen molecules and Volmer reaction for the formation of water molecules. Our findings show that palladium-ceria bifunctional systems have improved HOR activity compared to their individual components. Specifically, an enhanced catalytic activity is predicted for 10 at. % (7 wt %) palladium on ceria. We explain this behavior using multiple activity descriptors including hydrogen, OH, and H₂O binding energies, and hybridization and charge transfer between the catalyst, the substrate, and adsorbents. The results suggest that the high HOR activity can be attributed to the delicate balance between the H and OH interactions with the palladium-ceria support as well as the interaction between the individual components that make up the heterostructure. The detailed ab initio analysis provides invaluable insights toward electronic, atomistic, and molecular mechanisms of HOR and paves the way for the development of catalysts that use significantly reduced amounts of precious metals.

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Transition-Metal- and Nitrogen-Doped Carbide-Derived Carbon/ Carbon Nanotube Composites as Cathode Catalysts for Anion-Exchange Membrane Fuel Cells

Authors:
J Lilloja, E Kibena-Põldsepp, A Sarapuu, J C Douglin, M Kaärik, J Kozlova, P Paiste, A Kikas, J Aruvali, J Leis, V Sammelselg, D R Dekel, K Tammeveski
Journal:ACS Catal., 11, 1920–1931, 2021

Authors: Jaana Lilloja, Elo Kibena-Põldsepp, Ave Sarapuu, John C. Douglin, Maike Kaärik, Jekaterina Kozlova, Paärn Paiste, Arvo Kikas, Jaan Aruvali, Jaan Leis, Vaino Sammelselg, Dario R. Dekel, Kaido Tammeveski

Transition-metal- and nitrogen-codoped carbide-derived carbon/carbon nanotube composites (M-N-CDC/CNT) have been prepared, characterized, and used as cathode catalysts in anion-exchange membrane fuel cells (AEMFCs). As transition metals, cobalt, iron, and a combination of both have been investigated. Metal and nitrogen are doped through a simple high-temperature pyrolysis technique with 1,10-phenanthroline as the N precursor. The physicochemical characterization shows the success of metal and nitrogen doping as well as very similar morphologies and textural properties of all three composite materials. The initial assessment of the oxygen reduction reaction (ORR) activity, employing the rotating ring–disk electrode method, indicates that the M-N-CDC/CNT catalysts exhibit a very good electrocatalytic performance in alkaline media. We find that the formation of HO₂^– species in the ORR catalysts depends on the specific metal composition (Co, Fe, or CoFe). All three materials show excellent stability with a negligible decline in their performance after 10000 consecutive potential cycles. The very good performance of the M-N-CDC/CNT catalyst materials is attributed to the presence of M-N_x and pyridinic-N moieties as well as both micro- and mesoporous structures. Finally, the catalysts exhibit excellent performance in in situ tests in H₂/O₂ AEMFCs, with the CoFe-N-CDC/CNT reaching a current density close to 500 mA cm^–2 at 0.75 V and a peak power density (P_max) exceeding 1 W cm^–2. Additional tests show that P_max reaches 0.8 W cm^–2 in an H₂/CO₂-free air system and that the CoFe-N-CDC/CNT material exhibits good stability under both AEMFC operating conditions.

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Electroreduction of oxygen on cobalt phthalocyanine-modified carbide-derived carbon/carbon nanotube composite catalysts

Authors:
R Praats, M Käärik, A Kikas, V Kisand, J Aruväli, P Paiste, M Merisalu, A Sarapuu, J Leis, V Sammelselg, J C Douglin, D R Dekel, K Tammeveski
Journal:Journal of Solid State Electrochemistry 25, 57–71, 2021

Authors: Reio Praats, Maike Käärik, Arvo Kikas, Vambola Kisand, Jaan Aruväli, Päärn Paiste, Maido Merisalu, Ave Sarapuu, Jaan Leis, Väino Sammelselg, John C. Douglin, Dario R. Dekel, Kaido Tammeveski

In this work, composite materials based on carbide-derived carbon (CDC) and carbon nanotubes (CNT) modified with Co phthalocyanine (CoPc) were employed as electrocatalysts towards the oxygen reduction reaction (ORR) in both alkaline and acid media. Two different CDCs derived from titanium carbide and silicon carbide were used and the CDC-to-CNT ratio was varied in the composite materials. The final catalysts were obtained after pyrolysis at 800 °C. The catalyst materials were characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and microwave plasma atomic emission spectroscopy. The ORR measurements were performed using the rotating disk electrode (RDE) method. The RDE results revealed that the composite catalysts with higher CNT content possessed higher ORR electrocatalytic activity. The catalyst showing the highest activity in RDE tests was selected as a cathode material and tested in an anion exchange membrane fuel cell (AEMFC). An excellent AEMFC performance was obtained, with a peak power density of 473 mW cm⁻².

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Carbide-Supported PtRu Catalysts for Hydrogen Oxidation Reaction in Alkaline Electrolyte

Authors:
E R Hamo, R K Singh, J C Douglin, S Chen, M Ben Hassine, E Carbo-Argibay, S Lu, H Wang, P J Ferreira, B A Rosen, D R Dekel
Journal:ACS Catal., 11, 932−947, 2021

Authors: Eliran R. Hamo, Ramesh K. Singh, John C. Douglin, Sian Chen, Mohamed Ben Hassine, Enrique Carbo-Argibay, Shanfu Lu, Haining Wang, Paulo J. Ferreira, Brian A. Rosen, Dario R. Dekel

Owing to the sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline electrolyte, it is considered a limiting reaction for the development of anion-exchange membrane fuel cell (AEMFC) technology. Studies of alkaline HOR catalysis mainly focus on carbon-supported nanoparticles, which have weak metal–support interactions. In this contribution, we present a unique support based on transition metal carbides (TMCs = Mo₂C, Mo₂C–TaC, and Mo₂C–W₂C) for the HOR. PtRu nanoparticles are deposited onto the TMC supports and are characterized by a variety of analytical techniques. The major findings are (i) experimental and theoretical evidence for strong-metal support interaction by both X-ray absorption near-edge structure and density functional theory, (ii) the kinetic current density (j_k,s) @25 mV of PtRu/Mo₂C–TaC catalyst are 1.65 and 1.50 times higher than that of PtRu/Mo₂C and PtRu/Mo₂C–W₂C, respectively, and (iii) enhanced “tethering” of PtRu nanoparticles on TMC supports. Furthermore, the AEMFC based on the PtRu/Mo₂C–TaC anode exhibited a peak power density of 1.2 W cm^–2 @70 °C, opening the doors for the development of advanced catalysts based on engineering support materials.

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Electrospun anion conducting ionomer fibers – effect of humidity on final properties

Authors:
M Halabi, M Mann-Lahav, V Beilin, G E Shter, O Elishav, G S Grader, D R Dekel
Journal:Polymers 12(5), 1020, 2020

Authors: Manar Halabi, Meirav Mann-Lahav, Vadim Beilin, Gennady E. Shter, Oren Elishav, Gideon S. Grader, and Dario R. Dekel

Anion-conducting ionomer-based nanofibers mats are prepared by electrospinning (ES) technique. Depending on the relative humidity (RH) during the ES process (RH_ES), ionomer nanofibers with different morphologies are obtained. The effect of relative humidity on the ionomer nanofibers morphology, ionic conductivity, and water uptake (WU) is studied. A branching effect in the ES fibers found to occur mostly at RH_ES < 30% is discussed. The anion conductivity and WU of the ionomer electrospun mats prepared at the lowest RH_ES are found to be higher than in those prepared at higher RH_ES. This effect can be ascribed to the large diameter of the ionomer fibers, which have a higher WU. Understanding the effect of RH during the ES process on ionomer-based fibers’ properties is critical for the preparation of electrospun fiber mats for specific applications, such as electrochemical devices.

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Increasing the Alkaline Stability of N,N-Diaryl Carbazolium Salts Using Substituent Electronic Effects

Authors:
N Gjineci, S Aharonovich, D R Dekel, C E Diesendruck
Journal:ACS Appl. Mater. Interfaces, 12, 44, 49617–49625, 2020

Authors: Nansi Gjineci, Sinai Aharonovich, Dario R. Dekel, Charles E. Diesendruck

Anion-exchange membrane fuel cells (AEMFCs) have attracted the attention of the scientific community during the past years, mostly because of the potential for eliminating the need for using costly platinum catalysts in the cells. However, the broad commercialization of AEMFCs is hampered by the low chemical stability of the cationic functional groups in the anion-conducting membranes required for the transportation of hydroxide ions in the cell. Improving the stability of these groups is directly connected with the ability to recognize the different mechanisms of the OH^– attack. In this work, we have synthesized eight different carbazolium cationic model molecules and investigated their alkaline stability as a function of their electronic substituent properties. Given that N,N-diaryl carbazolium salts decompose through a single-electron-transfer mechanism, the change in carbazolium electron density leads to a very significant impact on their chemical stability. Substituents with very negative Hammett parameters demonstrate unparalleled stability toward dry hydroxide. This study provides guidelines for a different approach to develop stable quaternary ammonium salts for AEMFCs, making use of the unique parameters of this decomposition mechanism.

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Water Content and Ionic Conductivity of Thin Films of Different Anionic Forms of Anion Conducting Ionomers

Authors:
U N Shrivastava, A Zhegur-Khais, M Bass, S Willdorf-Cohen, V Freger, D R Dekel, K Karan
Journal:Phys. Chem. C, 124, 43, 23469–23478, 2020

Authors: Udit N. Shrivastava, Avital Zhegur-Khais, Maria Bass, Sapir Willdorf-Cohen, Viatcheslav Freger, Dario R. Dekel, Kunal Karan

Typically, in polymer electrolyte-based electrochemical devices such as electrolyzers and fuel cells, ionomers in the catalyst layers are present as ultrathin films coating the electrochemically active component. Acidic ionomer thin films have been extensively characterized over the past decade, yet there are few reports on the alkaline ionomer thin films. Here, we present a study on anion-exchange ionomers; specifically, we investigate the water content and conductivity of fluoride, bromide, and carbonate forms of 50 nm thick FAA3 and PPO ionomer thin films at 30 °C and 0–90% RH. A thermodynamic analysis was performed to compute the Gibbs free energy of anionic interaction with water to discuss the impact of anion type on the anionic mobility. Structural analysis using GISAXS was performed on the anion-exchange ionomer thin films. Furthermore, conductivity and water content relationships between FAA3 thin films and membranes and between FAA3 and PPO thin films were compared and discussed in terms of structure and ion clustering.

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Improved Hydrogen Oxidation Reaction Activity and Stability of Buried Metal-Oxide Electrocatalyst Interfaces

Authors:
F D Speck, F S M Ali, M T Y Paul, R K Singh, T Böhm, A Hofer, O Kasian, S Thiele, J Bachmann, D R Dekel, T Kallio, S Cherevko
Journal:Chem. Mater., 32, 18, 7716–7724, 2020

Authors: Florian D. Speck, Farhan S. M. Ali, Michael T. Y. Paul, Ramesh K. Singh, Thomas Böhm, André Hofer, Olga Kasian, Simon Thiele, Julien Bachmann, Dario R. Dekel, Tanja Kallio, Serhiy Cherevko

Various bifunctional metal-oxide composites have recently been proposed as advanced hydrogen oxidation reaction (HOR) electrocatalysts for anion-exchange membrane fuel cells (AEMFCs). It is postulated that metal and oxide are active sites for the adsorption of hydrogen/proton and hydroxide ions, respectively. Of particular interest are the so-called buried interfaces. To investigate processes governing activity and stability at such interfaces, we prepare model Pd and Pt electrocatalysts which are fully covered by thin CeO_x films. We investigate how oxide thickness influences HOR activity and dissolution stability of the electrocatalysts. It is found that materials behave very differently and that only Pd exhibits an enhanced HOR activity, while both oxide-protected metals are more stable toward dissolution. A 10-fold decrease in dissolution and 15-fold increase in HOR exchange current density are demonstrated for the optimized Pd/CeO_x composites in comparison to pure Pd. We assess the mechanism of the electrocatalytic improvement as well as the role of the protective oxide films in such systems through advanced electrochemical and physical analysis. It is highlighted that a uniform, semipermeable oxide layer with a maximized electrocatalyst–oxide interface is crucial to form HOR catalysts with improved activity and stability.

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Are Radicals Formed During Anion-Exchange Membrane Fuel Cell Operation?

Authors:
S Wierzbicki, J C Douglin, A Kostuch, D R Dekel, K Kruczała
Journal:J. Phys. Chem. Lett., 11, 7630−7636, 2020

Authors: Szymon Wierzbicki, John C. Douglin, Aldona Kostuch, Dario R. Dekel, Krzysztof Kruczała

In this paper we present a study on stable radicals and short-lived species generated in anion-exchange membrane (AEM) fuel cells (AEMFCs) during operation. The in situ measurements are performed with a micro-AEMFC inserted into a resonator of an electron paramagnetic resonance (EPR) spectrometer, which enables separate monitoring of radicals formed on the anode and cathode sides. The creation of radicals is monitored by the EPR spin trapping technique. For the ﬁrst time, we clearly show the formation and presence of stable radicals in AEMs during and after long-term AEMFC operation. The main detected adducts during the operation of the micro-AEMFC are DMPO-OOH and DMPO-OH on the cathode side, and DMPO-H on the anode side. These results indicate that oxidative degradation involving radical reactions has to be taken into account when stability of AEMFCs is investigated.

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A high-temperature anion-exchange membrane fuel cell

Authors:
J C Douglin, J R Varcoe, D R Dekel
Journal:J. Power Sources Advances, 2020

Authors: John C. Douglin, John Varcoe, Dario R. Dekel

In the past few years, developments in anion exchange membranes (AEMs) have led to a significant increase in hydroxide conductivities, ultimately yielding striking improvements in the performance of anion exchange membrane fuel cells (AEMFCs) at low operating temperatures, usually at 40–80 °C. Aside from these remarkable achievements, the literature is void of any work on AEMFCs operated at temperatures above 100 °C, despite the consensus from various models remarking that working at higher cell temperatures may lead to many significant advantages. In this work, we present the first high-temperature AEMFC (HT-AEMFC) tested at 110 °C. The HT-AEMFC exhibits high performance, with a peak power density of 2.1 W cm⁻² and a current density of as high as 574 mA cm⁻² measured at 0.8 V. This initial work represents a significant landmark for the research and development of the fuel cell technology, opening a wide door for a new field of research we call hereafter, HT-AEMFCs.

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Synthesis of CeOx-decorated Pd/C catalysts by controlled surface reactions for hydrogen oxidation in anion exchange membrane fuel cells

Authors:
R K Singh, E S Davydova, J Douglin, A O Godoy, H Tan, M Bellini, B J Allen, J Jankovic, H A Miller, A C Alba-Rubio, D R Dekel
Journal: Advanced Functional Materials, 2002087, 2020

Authors: Ramesh K. Singh, Elena S. Davydova, John Douglin, Andres O. Godoy, Haiyan Tan, Marco Bellini, Bryan J. Allen, Jasna Jankovic, Hamish A. Miller, Ana C. Alba‐Rubio, Dario R. Dekel

Due to the sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline electrolytes, the development of more efficient HOR catalysts is essential for the next generation of anion‐exchange membrane fuel cells (AEMFCs). In this work, CeO_x is selectively deposited onto carbon‐supported Pd nanoparticles by controlled surface reactions, aiming to enhance the homogenous distribution of CeO_x and its preferential attachment to Pd nanoparticles, to achieve highly active CeO_x‐Pd/C catalysts. The catalysts are characterized by inductively coupled plasma–atomic emission spectroscopy, X‐ray diffraction, high‐resolution transmission electron microscopy, scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, and X‐ray photoelectron spectroscopy to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation, and surface chemical states, respectively. The intimate contact between Pd and CeO_x is shown through high‐resolution STEM maps. The oxophilic nature of CeO_x and its effect on Pd are probed by CO stripping. The interfacial contact area between CeO_x and Pd nanoparticles is calculated for the first time and correlated to the electrochemical performance of the CeO_x‐Pd/C catalysts. Highest recorded HOR specific exchange current (51.5 mA mg⁻¹_Pd) and H₂–O₂ AEMFC performance (peak power density of 1,169 mW cm⁻² mg_Pd⁻¹) are obtained with a CeO_x‐Pd/C catalyst with Ce_0.38/Pd bulk atomic ratio.

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Measuring the true hydroxide conductivity of anion exchange membranes

Authors:
A Zhegur-Khais, F Kubannek, U Krewer, D R Dekel
Journal:Journal of Membrane Science, 612, 118461, 2020

Authors: Avital Zhegur-Khais, Fabian Kubannek, Ulrike Krewer, Dario R. Dekel

In this study, a CO₂-in-situ purging technique was used to measure the true OH‾ conductivity of several anion exchange membranes (AEMs). During this process, membranes are in-situ de-carbonated allowing the AEMs to reach their full OH‾ form, and therefore to measure their highest (true) OH‾ conductivity. The de-carbonation process in all the studied AEMs was also investigated. The time constant of the de-carbonation process τ was calculated and related to the membrane properties as well as to the de-carbonation dynamics. The time constant of the de-carbonation process was found to decrease with increasing current densities and decreasing the IEC of the membranes. This work provides unique and important data crucial to increasing the understanding of the main factors that may mitigate the de-carbonation process in AEMs to allow AEM fuel cells to be operated with ambient air.

WeChat (Paper highlights in Chinese language) https://mp.weixin.qq.com/s/xV6pyxrmhAz-ce_dot5BMQ

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The critical importance of ionomers on the electrochemical activity of platinum and platinum-free catalysts for anion-exchange membrane fuel cells

Authors:
P G Santori, A N Mondal, D R Dekel, F Jaouen
Journal:Sustainable Energy Fuels, 4, 3300, 2020

Authors: Pietro G. Santori, Abhishek N. Mondal, Dario R. Dekel, Frédéric Jaouen

Anion-exchange membrane fuel cells (AEMFCs) show remarkable and rapid progress in performance, significantly increasing the relevance for research on electrocatalysis of the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) for this technology. Since much of the recent progress in AEMFC performance can be tied to the improved interface between anion-exchange ionomers (AEIs) and catalysts, this topic deserves specific attention. This work reports the ORR and HOR activity measured in rotating disk electrodes for several ionomer–catalyst combinations, involving five different AEIs and Nafion® and four ORR and HOR catalysts selected from the best-in-class PGM-based and PGM-free catalytic materials. The results show little impact of the ionomers on the ORR and HOR activity of Pt/C and PtRu/C catalysts, respectively; however, the choice of the AEI has critical importance on the ORR activity of Fe–N–C and significant effect on the HOR activity of Pd–CeO₂/C.

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Unexpected hydroxide ion structure and properties at low hydration

Authors:
I Zadok, H Long, B Pivovar, A Roznowska, A Michalak, D R Dekel, S Srebnik
Journal:Molecular Liquids, 313, 113485, 2020

Authors: Israel Zadok, Hai Long, Bryan Pivovar, Aleksandra Roznowska, Artur Michalak, Dario R. Dekel, Simcha Srebnik

Understanding the behavior of hydroxide ions in aqueous and non-aqueous media is fundamental to many chemical, biological, and electrochemical processes. Research has primarily focused on a single fully solvated hydroxide ion, either as an isolated cluster or in bulk. This work presents the first computational study to consider hydroxide under low hydration levels in detail, where the anion may not be fully solvated. Under such conditions, we find that the anions are predominantly present as unique water-bridged hydroxide pair complexes, distinct from previously reported structures under fully hydrated conditions. Although similar hydroxide pair structures were previously reported, we analyze these structures for the first time in the disordered liquid state where they are found to be unusually stable in the presence of bulky quaternary ammonium cations. Our findings help explain the unusual diffusion behavior as well as the higher reactivity of hydroxide anions observed under low hydration conditions.

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The Reaction Mechanism Between Tetraarylammonium Salts and Hydroxide

Authors:
N Gjineci, S Aharonovich, S Willdorf-Cohen, D R Dekel, C E Diesendruck
Journal:European Journal of Organic Chemistry, 2020

Authors: Nansi Gjineci, Sinai Aharonovich, Sapir Willdorf‐Cohen, Dario R. Dekel, Charles E. Diesendruck

The reaction between different N,N ‐diarylcarbazolium salts and hydroxide is investigated to understand the reaction mechanism. The regioselectivity of the reaction and an unexpected H/D exchange indicate that the reaction does not follow the expected S_NAr mechanism, but instead proceeds through a radical pathway.

The mechanism of the reaction between tetraaryl ammonium salts and hydroxide is studied experimentally for different N,N-diaryl carbazolium salts. The N,N-diarylcarbazolium salts are designed, synthesized, characterized, and reacted with hydroxide under different conditions. The products of the reactions were directly characterized or isolated when possible and, using different substituents, the reaction mechanisms were compared. An unexpected H/D exchange observed in these salts helped to discard the classical SNAr mechanisms, supporting instead a radical mechanism initiated by a singleelectron transfer from the hydroxide. By understanding the preferred reaction pathways, better quaternary ammonium salts can be designed to withstand aggressive alkaline environments, critical for many practical applications such as anion-exchange membrane fuel cells.

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Quantifying the critical effect of water diffusivity in anion exchange membranes for fuel cell applications

Authors:
K Yassin, I G Rasin, S Brandon, D R Dekel
Journal:Journal of Membrane Science, 2020

Authors: Karam Yassin, Igal G. Rasin, Simon Brandon, Dario R. Dekel

During the past decade, one of the main goals of research and development of anion exchange membrane fuel cells (AEMFCs), was to increase the hydroxide conductivity of the anion exchange membranes (AEMs); this goal is based on the obvious and known impact of AEM conductivity on AEMFC performance (including efficiency). We propose a paradigm shift according to which a main AEMFC research goal should be to increase membrane water diffusivity. This is a result of detailed and quantitative computational analyses of AEMFC performance and its stability, presented in this manuscript. Our modeling results clearly show that, while improved AEM hydroxide conductivity is truly important for the achievement of high cell performance, enhanced water diffusivity through the membrane is extremely critical to ensure long-term AEMFC performance stability, as required by practical automotive and other applications. Superior water diffusivity, which is imperative for increasing water transport from the anode towards the cathode, provides improved levels of hydration. This has a favorable impact on performance but, more importantly, it promotes a reduction in ionomer chemical degradation and as a result leads to a significant improvement in AEMFC performance-stability and (therefore) in its lifetime.

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Ceria Entrapped Palladium Novel Composites for Hydrogen Oxidation Reaction in Alkaline Medium

Authors:
N Ralbag, E S Davydova, M Mann-Lahav, P Cong, J He, A M Beale, G S Grader, D Avnir, D R Dekel
Journal:Journal of The Electrochemical Society, 2020

Authors: Noam Ralbag, Elena S. Davydova, Meirav Mann-Lahav, Peixi Cong, Jin He, Andrew M. Beale, Gideon S. Grader, David Avnir, Dario R. Dekel

A new heterogeneous catalyst for hydrogen oxidation reaction (HOR), metallic palladium within which nanoparticles of ceria are entrapped, CeO₂@Pd, is described. Its preparation is based on a new materials methodology of molecular doping of metals. The metallic matrix, which encages the nanoparticles, is prepared in foam architecture, to ensure easy molecular diffusion. Characterization of the structural properties of the CeO₂@Pd composite using SEM, STEM, TEM, XRD, EXAFS and nitrogen adsorption reveals its morphological architecture, which leads to improved catalytic activity. In-situ electrochemical and H₂ temperature-programmed reduction (H₂-TPR) spectra provide direct experimental evidence of the weakening of Pd‒H bond in the CeO₂@Pd composites, relative to pure (undoped) Pd catalysts. Gas diffusion electrodes based on the entrapped CeO₂@Pd catalysts demonstrated one order of magnitude higher activity than pure Pd analog in the HOR reaction in an alkaline medium.

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Changes of Anion Exchange Membrane Properties During Chemical Degradation

Authors:
A Zhegur, N Gjineci, S Willdorf-Cohen, A N Mondal, C E Diesendruck, N Gavish, D R Dekel
Journal:ACS Applied Polymer Materials, 2, 2, 360-367, 2020

Authors: Avital Zhegur, Nansi Gjineci, Sapir Willdorf-Cohen, Abhishek N. Mondal, Charles E. Diesendruck, Nir Gavish, Dario R. Dekel

Water content plays a major role in the properties of anionexchange membranes (AEMs) and, therefore, in the AEM fuel cell (AEMFC) performance and performance stability. Characterization of AEMs during membrane degradation is critical in order to understand the membrane behavior during fuel cell operation time. In spite of its importance, the relationship between diﬀerent membrane properties during chemical degradation has yet to be investigated. In this study, we measure the changes in AEM propertiesin particular, ion exchange capacity (IEC), water uptake (WU) and conductivityduring membrane chemical degradation in alkaline medium. To the best of our knowledge, this is the ﬁrst time this kind of data has been investigated. While all the properties change during AEM degradation, results indicate that membrane WU decreases linearly with decreasing IEC; however, AEM ion-conductivity shows a sigmoidal-like shape relationship with IEC. We introduce a simple model to explain these eﬀects.

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Platinum and Platinum Group Metal-Free Catalysts for Anion Exchange Membrane Fuel Cells

Authors:
J R Tolchard, J Svendby, M Manikandan, H A Miller, S Sunde, H Yang, D R Dekel, A O Barnett
Journal:Energies, 13, 1-21, 2020

Authors : Julian Richard Tolchard, Jørgen Svendby, Maidhily Manikandan, Hamish A Miller, Svein Sunde, Hsiharng Yang, Dario R Dekel, Alejandro Oyarce Barnett

The development of active hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) catalysts for use in anion exchange membrane fuel cells (AEMFCs), which are free from platinum group metals (PGMs), is expected to bring this technology one step closer to commercial applications. This paper reports our recent progress developing HOR Pt-free and PGM-free catalysts (Pd/CeO 2 and NiCo/C, respectively), and ORR PGM-free Co 3 O 4 for AEMFCs. The catalysts were prepared by different synthesis techniques and characterized by both physical-chemical and electrochemical methods. A hydrothermally synthesized Co 3 O 4 + C composite ORR catalyst used in combination with Pt/C as HOR catalyst shows good H 2 /O 2 AEMFC performance (peak power density of ~388 mW cm −2 ), while the same catalyst coupled with our flame spray pyrolysis synthesised Pd/CeO 2 anode catalysts reaches peak power densities of ~309 mW cm −2 . Changing the anode to nanostructured NiCo/C catalyst, the performance is significantly reduced. This study confirms previous conclusions, that is indeed possible to develop high performing AEMFCs free from Pt; however, the challenge to achieve completely PGM-free AEMFCs still remains.

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Practical ex-Situ Technique To Measure the Chemical Stability of Anion-Exchange Membranes under Conditions Simulating the Fuel Cell Environment

Authors:
J Müller, A Zhegur, U Krewer, J R Varcoe, D R Dekel
Journal:ACS Materials Letters, 2, 168-173, 2020

Authors: Jasmin Muller, Avital Zhegur, Ulrike Krewer, John R Varcoe, Dario R Dekel

Anion-exchange membrane (AEM) degradation during fuel cell operation represents the main challenge that hampers the implementation of AEM fuel cells (AEMFCs). Reported degradation values of AEMs are difficult to reproduce as no standard methods are used. The present use of different techniques based on exposure of membranes to aqueous KOH solutions under different conditions and measuring different outputs during time does not allow for a reliable and meaningful comparison of reported degradation data of different AEMs. In this study, we present a practical and reproducible ex-situ technique to measure AEM degradation in conditions that mimic an operando fuel cell environment. In this novel technique, we measure the change of the true hydroxide conductivity of the AEM over time, while exposing it to different relative humidity conditions. The technique does not make use of liquid alkaline solution, thus simulating real fuel cell conditions and providing a good baseline for comparative degradation studies.

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Self-crosslinked blend alkaline anion exchange membranes with bi-continuous phase separated morphology to enhance ion conductivity

Authors:
T Huang, G He, J Xue, O Otoo, X He, H Jiang, J Zhang, Y Yin, Z Jiang, J C Douglin, D R Dekel, M D Guiver
Journal:Journal of Membrane Science , 117769, 2020

Authors : Tong Huang, Guangwei He, Jiandang Xue, Obed Otoo, Xueyi He, Haifei Jiang, Junfeng Zhang, Yan Yin, Zhongyi Jiang, John C Douglin, Dario R Dekel, Michael D Guiver

Combining two facile methods, spinodal blending and self-crosslinking by chloromethyl groups are jointly utilized in fabricating alkaline anion exchange membranes (AEMs). Highly and moderately chloromethylated poly(ether ether ketone)s and sub-equimolar (to chloromethyl) amounts of 1-methylimidazole are mixed and reacted to form blend AEMs. Nanoscale bi-continuous phase separated morphologies are obtained due to spinodal decomposition. Subsequent heat treatment triggers self-crosslinking of the AEMs arising from residual chloromethyl groups in membranes. Compared with unblended and uncrosslinked AEMs having similar ion exchange capacity (IEC), the ion conductivity of the present AEMs is increased by 45.4% in water and 113% in 95% RH at 60 °C. Dimensional and alkaline stabilities of AEMs are also enhanced by the self-crosslinking, i.e., the swelling ratio decreases by 50.3% at 60 °C and the residual conductivity increases by 26.3% after alkaline treatment in 1 M NaOH at 60 °C. Interestingly, individual spinodal blending or self-crosslinking are found ineffective in overcoming the trade-off between AEM conductivity and stability, while a combination of both methods leads to a simultaneous improvement in conductivity and membrane stability.

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Effect of Ammonium Cations on the Diffusivity and Structure of Hydroxide Ions in Low Hydration Media

Authors:
I Zadok, D R Dekel, S Srebnik
Journal:The Journal of Physical Chemistry C, 123, 45, 27355-27362, 2019

Authors : Israel Zadok, Dario R Dekel, Simcha Srebnik

Anion exchange membrane (AEM) fuel cells are an attractive alternative technology to the acidic proton exchange membrane-based fuel cells. Conduction of hydroxide ions in AEMs creates an alkaline operating environment that allows using platinum-free catalysts, while still maintaining the performance needed for commercial application (e.g., the automotive industry). However, this technology is very sensitive to the behavior of hydroxide ions under low hydration conditions because of the consumption of water near the cathode. We use molecular dynamics simulation to investigate the behavior of two model quaternary ammonium cations used in AEM technologies at low hydration states in the presence of hydroxide anions. Both systems show the existence of an interesting ion complex—water-bridged hydroxide pair—that surprisingly involves two hydroxide anions in close proximity that is found to be highly stable. We use these new insights to explain the observed change in diffusivity of hydroxide and water from high hydration to low hydration regimes. The prevalence of these structures at different levels of hydration also explains the difference in diffusivity observed between the two studied cations. We believe that this hydroxide pair complex is key to understanding and controlling performance and stability in AEMs and in similar electrolyte systems.

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Composite Materials with Combined Electronic and Ionic Properties

Authors:
N Ralbag, M Mann-Lahav, E S Davydova, U Ash, R Galed, M Handl, R Hiesgen, E Magliocca, W Mustain, J He, P Cong, A M Beale, G S Grader, D Avnir, D R Dekel
Journal:Matter, 1, 4, 959-975 , 2019

Authors: Noam Ralbag, Meirav Mann-Lahav, Elena S Davydova, Uri Ash, Reuven Galed, Michael Handl, Renate Hiesgen, Emanuele Magliocca, William Mustain, Jin He, Peixi Cong, Andrew M Beale, Gideon S Grader, David Avnir, Dario R Dekel

In this work, we develop a new type of composite material that combines both electrocatalytic and ionic properties, by doping a silver metal catalyst with an anion-conducting ionomer at the molecular level. We show that ionomer entrapment into the silver metallic structure is possible, imparting unique properties to the catalytic character of the metallic silver. The novel composite material is tested as the cathode electrode of fuel cells, showing significant improvement in cell performance as compared with the undoped counterpart. This new type of material may then replace the current design of electrodes in advanced fuel cells or other electrochemical devices. The possibility to merge different properties into one composite material by molecular entrapment in metals can open the way to new materials, leading to unexplored fields and applications.

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Hydroxide Ion Diffusion in Anion-Exchange Membranes at Low Hydration: Insights from Ab Initio Molecular Dynamics

Authors:
T Zelovich, L Vogt-Maranto, M A Hickner, S J Paddison, C Bae, D R Dekel, M E Tuckerman
Journal:Chemistry of Materials, 31, 15, 5778-5787, 2019

Authors : Tamar Zelovich, Leslie Vogt-Maranto, Michael A Hickner, Stephen J Paddison, Chulsung Bae,

Dario R Dekel, Mark E Tuckerman

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Palladium-ceria catalysts with enhanced alkaline hydrogen oxidation activity for anion exchange membrane fuel cells

Authors:
M Bellini, M V Pagliaro, A Lenarda, P Fornasiero, M Marelli, C Evangelisti, M Innocenti, Q Jia, S Mukerjee, J Jankovic, L Wang, J R Varcoe, C B Krishnamurthy, I Grinberg, E Davydova, D R Dekel, H A Miller, F Vizza
Journal:ACS Applied Energy Materials, 2, 7, 4999-5008, 2019

Authors: Marco Bellini, Maria Vincenza Pagliaro, Anna Lenarda, Paolo Fornasiero, Marcello Marelli, Claudio Evangelisti, Massimo Innocenti, Qingying Jia, Sanjeev Mukerjee, Jasna Jankovic, Lianqin Wang, John Robert Varcoe, Chethana Bhadravathi Krishnamurthy, Ilya Grinberg, Elena Davydova, Dario R Dekel, Hamish Andrew Miller, Francesco Vizza

Anion exchange membrane fuel cells (AEMFCs) offer several important advantages with respect to proton exchange membrane fuel cells, including the possibility of avoiding the use of platinum catalysts to help overcome the high cost of fuel cell systems. Despite such potential benefits, the slow kinetics of the hydrogen oxidation reaction (HOR) in alkaline media and limitations in performance stability (because of the degradation of the anion conducting polymer electrolyte components) have generally impeded AEMFC development. Replacing Pt with an active but more sustainable HOR catalyst is a key objective. Herein, we report the synthesis of a Pd–CeO₂/C catalyst with engineered Pd-to-CeO₂ interfacial contact. The optimized Pd–CeO₂ interfacial contact affords an increased HOR activity leading to >1.4 W cm^–2 peak power densities in AEMFC tests. This is the only Pt-free HOR catalyst yet reported that matches state-of-the-art AEMFC power performances (>1 W cm^–2). Density functional theory calculations suggest that the exceptional HOR activity is attributable to a weakening of the hydrogen binding energy through the interaction of Pd atoms with the oxygen atoms of CeO₂. This interaction is facilitated by a structure that consists of oxidized Pd atoms coordinated by four CeO₂ oxygen atoms, confirmed by X-ray absorption spectroscopy.

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Effect of Carbonate Anions on Quaternary Ammonium-Hydroxide Interaction

Authors:
S Srebnik, S Pusara, D R Dekel
Journal:The Journal of Physical Chemistry C 123, 26, 15956-15962, 2019

Authors: Simcha Srebnik, Srdjan Pusara, Dario R Dekel

Currently, there are two main challenges in state-of-the-art anion-exchange membrane fuel cells (AEMFCs)—first, cation degradation in the presence of hydroxide anions; second, carbonation process during AEMFC operation. Both degradation and carbonation processes lead to a significant decrease in the ionic conductivity of the anion exchange membranes (AEMs), and, in turn, in the AEMFC performance. In this work, we use molecular dynamics simulations to bring first insights into the contributing factors that lead to changes in the degradation of quaternary ammonium cations due to the presence of carbonate anions. Focusing on low hydration levels, we explore the behavior of benzyltrimethylammonium cation (BTMA⁺) in the presence of a mixture of hydroxide and carbonate anions at different water:cation ratios. Water is shown to have a stronger affinity toward carbonate than hydroxide. Thus, the introduction of carbonate anions effectively lowers the concentration of free hydroxide anions and thereby decreases the conductivity of the AEM. Lower hydration of the hydroxide anion, in turn, leads to higher coordination of hydroxide compared with carbonate around BTMA⁺, hence increasing the probability of degradation of the cation. Nonetheless, carbonate competes with hydroxide in its interaction with cation, leading to approximately 20% reduction in hydroxide coordination around the BTMA⁺ when carbonate is present. We examine in detail these two competing factors—steric shielding of BTMA⁺ by carbonate and effectively lower hydration of the hydroxide—which are critical for understanding the effect of carbonate on the stability of quaternary ammonium cations.

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Stability limits of Ni-based hydrogen oxidation electrocatalysts for anion exchange membrane fuel cells

Authors:
E Davydova, F D Speck, M TY Paul, D R Dekel, S Cherevko
Journal:ACS Catalysis 9, 8, 6837-6845, 2019

Authors: Elena Davydova, Florian D Speck, Michael TY Paul, Dario R Dekel, Serhiy Cherevko

Among the non-noble-metal electrocatalysts for the hydrogen oxidation reaction (HOR) in anion exchange membrane fuel cells (AEMFCs), Ni-based nanoparticles have shown the highest reported activities. In this work, we investigated the chemical and electrochemical stability of representative Ni-based electrocatalysts. For this, carbon-supported monometallic Ni and bimetallic Ni₃M (M = Co, Fe, Cu, Mo) nanoparticles were synthesized and tested using a set of complementary techniques. It was found that Mo suffers from intense dissolution due to thermodynamic instability. Cu was stable below 0.4 V_RHE, though it undergoes noticeable electrochemical transient dissolution if the potential range is extended to 0.5 V_RHE and higher. However, Ni, Co, and Fe showed negligible dissolution up to 0.7 V_RHE. Despite the absence of dissolution, all catalysts lose their HOR activity if they are cycled to these high potentials. Physicochemical characterization of the aged catalysts revealed full oxidation of the metal nanoparticles, which could be responsible for the performance deterioration. Although our results demonstrate that, besides Ni₃Mo, all studied materials show high stability under operating potentials of AEMFCs, if fuel starvation in AEMFCs results in high anodic potentials, cell activation and operation strategies are needed to prevent the passivation of the catalysts. These results present critical insights toward the design and development of affordable Ni-based electrocatalysts for AEMFCs as well as provide a better understanding of the operation strategies for the stability of AEMFCs.

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Poly (bis-arylimidazoliums) possessing high hydroxide ion exchange capacity and high alkaline stability

Authors:
J Fan, S Willdorf-Cohen, E M Schibli, Z Paula, W Li, T JG Skalski, A T Sergeenko, A Hohenadel, B J Frisken, E Magliocca, W E Mustain, C E Diesendruck, D R Dekel, S Holdcroft
Journal:Nature Communications 10, (1), 2306, 2019

Authors :Jiantao Fan, Sapir Willdorf-Cohen, Eric M Schibli, Zoe Paula, Wei Li, Thomas JG Skalski, Ania Tersakian Sergeenko, Amelia Hohenadel, Barbara J Frisken, Emanuele Magliocca, William E Mustain, Charles E Diesendruck, Dario R Dekel, Steven Holdcroft

Solid polymer electrolyte electrochemical energy conversion devices that operate under highly alkaline conditions afford faster reaction kinetics and the deployment of inexpensive electrocatalysts compared with their acidic counterparts. The hydroxide anion exchange polymer is a key component of any solid polymer electrolyte device that operates under alkaline conditions. However, durable hydroxide-conducting polymer electrolytes in highly caustic media have proved elusive, because polymers bearing cations are inherently unstable under highly caustic conditions. Here we report a systematic investigation of novel arylimidazolium and bis-arylimidazolium compounds that lead to the rationale design of robust, sterically protected poly(arylimidazolium) hydroxide anion exchange polymers that possess a combination of high ion-exchange capacity and exceptional stability.

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Effect of CO2 on the properties of anion exchange membranes for fuel cell applications

Authors:
N Ziv, A N Mondal, T Weissbach, S Holdcroft, D R Dekel
Journal:Journal of Membrane Science 420, 118-123, 2019

Authors : Noga Ziv, Abhishek N Mondal, Thomas Weissbach, Steven Holdcroft, Dario R Dekel

In this study the effect of CO₂, HCO₃‾ and CO₃²²‾ on the ionic conductivity and water uptake properties of anion exchange membranes (AEMs) was investigated in order to better understand the detrimental effect of ambient air feed on the performance of AEM fuel cells. Three types of AEMs were examined, including Poly(hexamethyl-pp-terphenyl benzimidazolium) (HMT-PMBI), Fumatech^® FAA-3, and poly(phenylene oxide) functionalized with imidazole (PPO-Im). The effect of temperature and humidity on AEM properties in their different anion forms was studied, including both steady state and dynamic measurements. In addition, the response to changes in CO₂ concentration and to application of ex-situ electric current was examined. Results showed that an increase in humidity leads to an increase in water content and an increase in conductivity of the AEMs, regardless the anion type. It was found that both temperature and relative humidity improve conductivity in carbonated forms, however relative humidity has the most significant impact. The carbonation process in 400 ppm CO₂ is slightly quicker in AEMs with low conductivity, lasting ca. 40 min; however it was shown that a reverse process can be achieved by applying an electric current through the AEMs. An increase by 2–10 fold in conductivity is obtained using this method, which is analogous to the changes observed during operation of the fuel cell. This work provides important data that needs to be taken into account in future work in order to ultimately mitigate the carbonation effects and improve the performance of AEM fuel cells running with ambient air.

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Anion Exchange Membranes with Low Hydration Conditions from an Ab Initio Molecular Dynamics Perspective

Authors:
T Zelovich, L Vogt, D R Dekel, M Hickner, C Bae, S J Paddison, M E Tuckerman
Journal:Meeting Abstracts 40, 1957-1957, 2019

Authors : Tamar Zelovich, Leslie Vogt, Dario R Dekel, Michael Hickner, Chulsung Bae, Stephen J Paddison,

Mark E Tuckerman

Regardless of the initial hydration level, a typical Anion Exchange Membranes (AEMs) fuel cell device would operate ultimately under extremely low hydration values. It is therefore vital to fully explore the hydroxide diffusion in AEMs under such conditions. In this work, we present fully atomistic ab initio molecular dynamics (AIMD) simulations to obtain a molecular-level understanding of the hydroxide solvation complexes and diffusion mechanism in low hydration AEMs. By changing the polymer electrolyte cation spacing and the hydration values we create three different AEMs models. We find that under extreme low hydration conditions, the water molecules attain a non-uniform distribution. We further see that the unique water distribution results in distinct hydroxide diffusion mechanisms varies from a stationary behavior, a vehicular diffusion, and a mixture of structural and vehicular diffusion, depending on the existence of a water oxygen at the hydroxide second solvation shell. Therefore, we find the water density to be a better descriptor than hydration values when exploring AEM under low hydration condition. Furthermore, as the hydroxide transport is essentially different compared to bulk solution, we provide an idealized hydroxide diffusion mechanism for the three diffusion categories. We believe the results presented in this study enable us to define the terms required for achieving high hydroxide conductivity in high-performance AEM fuel cell devices under extremely low hydration conditions.

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Predicting performance stability of anion exchange membrane fuel cells

Authors:
D R Dekel, I G Rasin, S Brandon
Journal:Journal of Power Sources 420, 118-123, 2019

Authors: Dario R Dekel, Igal G Rasin, Simon Brandon

Anion-exchange membrane fuel cells (AEMFCs) are attracting increasing attention worldwide mainly due to this technology’s potential to considerably reduce fuel cell device costs. However, their development and implementation is significantly handicapped by the membrane and ionomer’s decomposition during cell operation. In this study we propose and apply a unique one-dimensional model capable of predicting, for the first time, the performance stability of AEMFCs. The model accounts for the ionomeric material degradation and its relationship with local hydration, which depends on cell material properties, design parameters and operating conditions. Using this model, we successfully demonstrate the strong impact of operating current density and membrane characteristics on the performance stability of a representative cell. The predicted cell stability provides critical insights for the design and development of highly stable AEMFCs. By using membranes with achievable targeted properties, the model predicts an AEMFC life-time higher than 5000 h, suitable for automotive applications.

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Palladium-ceria nanocatalyst for hydrogen oxidation in alkaline media: Optimization of the Pd–CeO2 interface

Authors:
H Yu, E S Davydova, U Ash, H A Miller, L Bonville, D R Dekel, R Maric
Journal:Nano Energy 57, 820-826, 2019

Authors: Haoran Yu, Elena S Davydova, Uri Ash, Hamish A Miller, Leonard Bonville, Dario R Dekel, Radenka Maric

The development of Pt-free catalyst for anion exchange membrane fuel cells is limited by the sluggish hydrogen oxidation reaction (HOR) at the anode. Previously, the use of CeO₂ as a catalyst promoter facilitated drastic ennoblement of Pd for the HOR kinetics in base media. However, further optimization and understanding of the Pd–CeO₂interaction, surface properties, and their influence on HOR are still needed. In this work, three types of Pd–CeO₂/C catalysts are synthesized by a flame-based process, where the Pd–CeO₂ interface and the HOR activity are improved as compared to catalysts prepared by wet-chemistry processes. The correlation between the Pd–CeO₂ interaction and the HOR activity is established through comparisons of three types of Pd–CeO₂/C synthesized catalysts using electrochemical techniques and X-ray photoelectron spectroscopy.

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Electrospun Ionomeric Fibers with Anion Conducting Properties

Authors:
M Mann‐Lahav, M Halabi, G E Shter, V Beilin, M Balaish, Y Ein‐Eli, D R Dekel, G S Grader
Journal:Advanced Functional Materials, 1901733, 2019

Electrospun Ionomeric Fibers with Anion Conducting Properties

Authors : Meirav Mann‐Lahav, Manar Halabi, Gennady E Shter, Vadim Beilin, Moran Balaish, Yair Ein‐Eli, Dario R Dekel, Gideon S Grader

Anion conductive nanofiber mats from FAA‐3 ionomers are obtained by electrospinning. Depending on the solvent used in the precursor solution, nanofibers with either nonhollow cylindrical or flat ribbon‐like cross‐sections are prepared. The anion conductivity and water uptake of the ionomeric nanofiber mats are measured as a function of the relative humidity in the 10–90% range and compared to that of a solid membrane cast from the same ionomer. In addition, the anion conductivity of an isolated single fiber of the ionomer is measured for the first time. The anion conductivity of the electrospun single fiber is found to be higher than that of the mats, which is, in turn, one order of magnitude higher than that of the solid ionomer membrane. The higher conductivity of the mats relative to the solid membrane (in both in‐plane and through‐plane directions) is found to be related to the variation in water uptake, which stems from the morphological distinctions. These results increase the understanding of the electrospinning process of ionomers, toward the development and design of new anion conductive ionomer fibers, useful for high performance electrochemical devices.

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Unexpected water-hydroxide ion structure and diffusion behavior in low hydration media

Authors:
I Zadok, D R Dekel, S Srebnik
Journal:arXiv preprint arXiv: 1812.06961, 2018

Authors : Israel Zadok, Dario R Dekel, Simcha Srebnik

Hydroxide ion transport and structure in aqueous media is fundamental to many chemical and biological processes. Research on hydroxide behavior has primarily focused on a single fully solvated hydroxide, either as an isolated cluster or in the bulk. This work presents the first computational study to consider a medium of low hydration levels where the hydroxide ion is microsolvated. Under such conditions, hydroxide ions are shown to be predominantly present as unique water-bridged double-hydroxide charged clusters, distinct from previously reported structures under hydrated conditions. Although layered double hydroxides were reported in the crystalline state, this is the first time to be seen in the disordered liquid state. These newly observed double-hydroxide structures presumably disrupt the hydrogen bonded network required for structural diffusion of hydroxide ions through water. These ion complexes have a higher ionic strength which may explain the unexpected diffusion behavior in comparison to the single hydroxide-water complex.

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Unravelling mysteries of hydrogen electrooxidation in anion exchange membrane fuel cells

Authors:
D R Dekel
Journal:Current Opinion in Electrochemistry 12, 182-188, 2018

Author: Dario R Dekel

Anion exchange membrane fuel cells (AEMFCs) can potentially revolutionize the energy generation market; however, to be seriously considered as a real alternative to the mainstream fuel cell technology, complete removal of previous metal electrocatalysts needs to be achieved. While in cathode electrodes platinum can be easily substituted, the electrochemical hydrogen oxidation reaction (HOR) in the AEMFC anodes currently involves prohibitive overpotential losses, making the removal of platinum extremely challenging. Understanding the HOR in AEMFCs will facilitate the path to overcome the challenge and finally develop and demonstrate platinum-free high-performance AEMFC devices.

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Hydrogen Oxidation on Ni-Based Electrocatalysts: The Effect of Metal Doping

Authors:
E S Davydova, J Zaffran, K Dhaka, M Toroker, D R Dekel
Journal:Catalysts 8, (10), 454, 2018

Authors : Elena S Davydova, Jérémie Zaffran, Kapil Dhaka, Maytal Toroker, Dario Dekel

Carbon supported nanoparticles of monometallic Ni catalyst and binary Ni-Transition Metal (Ni-TM/C) electrocatalytic composites were synthesized via the chemical reduction method, where TM stands for the doping elements Fe, Co, and Cu. The chemical composition, structure and morphology of the Ni-TM/C materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS). The electrochemical properties towards hydrogen oxidation reaction in alkaline medium were studied using the rotating disc electrode and cycling voltammetry methods. A significant role of the TM dopants in the promotion of the hydrogen electrooxidation kinetics of the binary Ni-TM/C materials was revealed. A record-high in exchange current density value of 0.060 mA cm 2 Ni was measured for Ni 3 Fe 1/C, whereas the monometallic Ni/C counterpart has only shown 0.039 mA cm 2 Ni. In order to predict the feasibility of the electrocatalysts for hydrogen chemisorption, density functional theory was applied to calculate the hydrogen binding energy and hydroxide binding energy values for bare Ni and Ni 3 TM 1.

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Surface Adsorption Affects the Performance of Alkaline Anion-Exchange Membrane Fuel Cells

Authors:
S Maurya, J H Dumont, C N Villarrubia, I Matanovic, D Li, Y S Kim, S Noh, J Han, C Bae, H A Miller, C H Fujimoto, D R Dekel
Journal:ACS Catalysis 8, 9429-9439, 2018

Authors: S Maurya, J H Dumont, C N Villarrubia, I Matanovic, Dongguo Li, Y S Kim, S Noh, J Han, C Bae, H A Miller, C H Fujimoto, Dario R Dekel

Material interactions at the polymer electrolytes–catalyst interface play a significant role in the catalytic efficiency of alkaline anion-exchange membrane fuel cells (AEMFCs). In this work, the surface adsorption behaviors of the cation–hydroxide–water and phenyl groups of polymer electrolytes on Pd- and Pt-based catalysts are investigated using two Pd-based hydrogen oxidation catalysts—Pd/C and Pd/C-CeO₂—and two Pt-based catalysts—Pt/C and Pt-Ru/C. The rotating disk electrode study and complementary density functional theory calculations indicate that relatively low coadsorption of cation–hydroxide–water of the Pd-based catalysts enhances the hydrogen oxidation activity, yet substantial hydrogenation of the surface adsorbed phenyl groups reduces the hydrogen oxidation activity. The adsorption-driven interfacial behaviors of the Pd- and Pt-based catalysts correlate well with the AEMFC performance and short-term stability. This study gives insight into the potential use of non-Pt hydrogen oxidation reaction catalysts that have different surface adsorption characteristics in advanced AEMFCs.

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Water–a key parameter in the stability of anion exchange membrane fuel cells

Authors:
C E Diesendruck, D R Dekel
Journal:Current Opinion in Electrochemistry 9, 173-178, 2018

Authors : Charles E Diesendruck, Dario R Dekel

Anion exchange membrane fuel cells can potentially revolutionize energy storage and delivery; however, their commercial development is hampered by the chemical decomposition of the anion exchange membranes during operation. The hydroxide anions, while transported from the cathode to the anode, attack the positively charged functional groups in the polymer membrane, neutralizing it and suppressing its anion-conducting capability. In recent years, several new quaternary ammonium salts have been proposed to address this challenge, but while they perform well in ex-situ chemical studies, their performance is very limited in real fuel cell studies. While cation chemistry dictates the intrinsic chemical stability of the anion conducting ionomeric materials, it was recently shown that chemical degradation is significantly influenced by the hydration level at which the fuel cell operates. Understanding the principles governing the chemical degradation under fuel cell operation, and its critical relationship with the hydration levels in the operating fuel cell electrodes will facilitate the path to overcome the challenge and finally develop and demonstrate highly stable AEMFC devices.

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An Effective Synthesis of N, N-Diphenyl Carbazolium Salts

Authors:
S Aharonovich, N Gjineci, D R Dekel, C E Diesendruck
Journal:Synlett 29 (10), 1314-1318, 2018

Authors : Sinai Aharonovich, Nansi Gjineci, Dario R Dekel, Charles E Diesendruck

Tetraaryl ammonium salts are a synthetic challenge, since there is no general method for the arylation of triaryl amines. Contrary to other quaternary ammonium salts, tetraaryl ammonium salts should be very chemically stable. The ipso carbons are not very electrophilic, since the positive charge is distributed throughout the pi systems and they have no acidic β hydrogens. Here we demonstrate a simple approach to N,N-diphenyl carbazolium salts using only three synthetic steps, allowing for an easy production of these salts in large amounts and in a relatively short time. In addition, we study the Cu(I) catalyzed multi-arylation of 2,2’-diaminobiphenyl, focusing on the regioselectivity of each step. Finally, we characterize, for the first time, the solid state structure of a tetraaryl ammonium salt.

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Electrocatalysts for hydrogen oxidation reaction in alkaline electrolytes

Authors:
E S Davydova, S Mukerjee, F Jaouen, D R Dekel
Journal:ACS Catalysis 8 (7), 6665-6690, 2018

Authors: Elena S Davydova, Sanjeev Mukerjee, Frederic Jaouen, Dario R Dekel

In the past 5 years, advances in anion-conductive membranes have opened the door for the development of advanced anion-exchange membrane fuel cells (AEMFCs) as the next generation of affordable fuel cells. Several recent works have shown that AEMFCs currently achieve nearly identical beginning-of-life performance as state-of-the-art proton exchange membrane fuel cells. However, until now, these high AEMFC performances have been reached with platinum-group metal (PGM)-based anode and cathode catalysts. In order to fulfill the potential of AEMFCs, such catalysts should in the near future be free of PGMs and, eventually, free of critical raw materials. Although great progress has been achieved in the development of PGM-free catalysts for the oxygen reduction reaction in basic media, significantly less attention has been paid to the catalysis of the hydrogen oxidation reaction (HOR). The much lower HOR activity of Pt in basic media compared with that in acid was itself revealed only relatively recently. While several PGM-based composite materials have shown improved HOR activity in basic media, the HOR kinetics remains slower than necessary for an ideal nonpolarizable electrode. In addition, attempts to move away from PGMs have hitherto resulted in high anode overpotentials, significantly reducing the performance of PGM-free AEMFCs. This would be a major barrier for the large-scale deployment of this technology once the other technological hurdles (e.g., membrane stability) have been overcome. A fundamental understanding of the HOR mechanism in basic media and of the main energy barriers needs to be firmly established to overcome this challenge. This review presents the current understanding of the HOR electrocatalysis in basic media and critically discusses the most recent material approaches. Promising future research directions in the development of the HOR electrocatalysts for alkaline electrolytes are also outlined.

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Molecular simulation of quaternary ammonium solutions at low hydration levels

Authors:
S Pusara, S Srebnik, D R Dekel
Journal:The Journal of Physical Chemistry C 122, (21), 11204-11213, 2018

Authors : Srdjan Pusara, Simcha Srebnik, Dario R Dekel

Much research has focused on the stability of substituted ammonium salts in anion-exchange membranes (AEMs). While cation chemistry dictates AEM stability, chemical degradation has been recently shown to be significantly influenced by the hydration level at which the AEM operates. At low hydration, it is now known that almost every quaternary ammonium may suffer significant decomposition. In this work, we use molecular dynamics simulations to explore the behavior of three common quaternary ammonium cations with stoichiometric hydroxide concentration and at very low hydration. We find that water preferentially solvates hydroxide anions and hence when water is present in sufficient amount (more than four water molecules per ion pair), stability of the cations is expected to significantly improve. However, lower amounts of water result in the formation of isolated molecular clusters and ammonium hydroxide pairing that lead to degradation of the cation. The composition and size of the water–hydroxide–cation clusters that form is shown to be significantly affected by the cation chemistry. Implications of the observed behavior are discussed in view of recent experimental results on the differences in stability of these cations. This study highlights, for the first time, the crucial importance of studying hydroxide–water–cation interactions at low hydration levels.

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Water uptake study of anion exchange membranes

Authors:
Y Zheng, U Ash, R P Pandey, A G Ozioko, J Ponce-González, M Handl, T Weissbach, J R Varcoe, S Holdcroft, M W Liberatore, R Hiesgen, D R Dekel
Journal:Macromolecules 51, (9), 3264-3278, 2018

Authors: Yiwei Zheng, Uri Ash, Ravi P Pandey, Amobi G Ozioko, Julia Ponce-González, Michael Handl, Thomas Weissbach, John R Varcoe, Steven Holdcroft, Matthew W Liberatore, Renate Hiesgen, Dario R Dekel

Anion exchange membrane fuel cells (AEMFCs) have attracted extensive attention in the recent years, primarily due to the distinct advantage potentials they have over the mainstream proton exchange membrane fuel cells. The anion exchange membrane (AEM) is the key component of AEMFC systems. Because of the unique characteristics of water management in AEMFCs, understanding the water mobility through AEMs is key for this technology, as it significantly affects (and limits) overall cell performances. This work presents a study of the equilibrium state and kinetics of water uptake (WU) for AEMs exposed to vapor source H₂O. We investigate different AEMs that exhibit diverse water uptake behaviors. AEMs containing different backbones (fluorinated and hydrocarbon-based backbones) and different functional groups (various cations as part of the backbone or as pendant groups) were studied. Equilibrium WU isotherms are measured and fitted by the Park model. The influence of relative humidity and temperature is also studied for both equilibrium and dynamic WU. A characteristic time constant is used to describe WU kinetics during the H₂O sorption process. To the best of our knowledge, this is the first time that WU kinetics has been thoroughly investigated on AEMs containing different backbones and cationic functional groups. The method and analysis described in this work provide critical insights to assist with the design of the next-generation anion conducting polymer electrolytes and membranes for use in advanced high-performance AEMFCs.

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Investigation of Pd and Pd-CeO2 Based Carbon-Supported Electrocatalysts for the Electrooxidation of Borohydride in Direct Borohydride Fuel Cell

Authors:
C Lafforgue, M Chatenet, R W Atkinson, K Swider-Lyons, H Miller, D R Dekel
Journal:Meeting Abstracts 30, 1727-1727, 2018

Authors : Clémence Lafforgue, Marian Chatenet, Robert W Atkinson, Karen Swider-Lyons, Hamish Miller,

Dario R Dekel

Direct borohydride fuel cells (DBFC) are considered high-energy density generators. The ideal case for the anodic reaction of a DBFC is the direct and complete borohydride oxidation reaction (BOR) that releases 8 electrons. However, the BOR is a very complex reaction that involves numerous intermediate species and suffers from competition with the heterogeneous hydrolysis of BH₄^– that leads to molecular hydrogen, making full utilization of the fuel challenging [1]. Despite many studies of the BOR on several electrocatalysts (especially Au and Pt), this reaction is still poorly understood on most practical electrocatalysts and in relevant conditions to DBFC operation, and no ideal electrocatalyst fulfilling both high activity and high faradaic efficiency at low potential has been isolated, yet.

In this study, the BOR is investigated on Pd-based carbon-supported electrocatalysts. A set of four Pd/C electrocatalysts have been synthesized using Vulcan XC72 as substrate and loaded at 22, 33, 44 and 53 _wt% Pd. These materials have been thoroughly characterized in terms of BOR activity in a rotating disk electrode setup. The experiments were performed for three NaBH₄ concentrations (5, 50 and 500 mM) at 60°C. The electrochemical characterizations of Figure 1 (A) reveal that the onset potential (E_i=0) (see Figure 1 (B)) is lower than 0 V vs. RHE on all the Pd electrocatalysts, meaning that BH₄^– anions can be valorized directly, albeit with slow kinetics. In contrast, Pt/C first decomposes BH₄^– and utilizes hydrogen species, above 0 V vs. RHE. However, the kinetic is very slow. Similar experiments have been performed with advanced Pd electrocatalysts with a composite support made of Vulcan XC72 carbon and cerium oxide, which enhance hydrogen oxidation reaction (HOR) kinetics in alkaline media [2].

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Durability of Platinum-Based Carbon-Supported Electrocatalysts in Liquid Versus Solid Polymer Alkaline Electrolytes

Authors:
C Lafforgue, L Dubau, F Maillard, D R Dekel, M Chatenet
Journal:Meeting Abstracts 30, 1778-1778, 2018

Authors : Clémence Lafforgue, Laetitia Dubau, Frederic Maillard, Dario R Dekel, Marian Chatenet

Alkaline fuel cells (AFC) are competitors to proton-exchange membrane fuel cells for stationary applications [1]. Because many metals and metal oxides are stable at high pH [2], one may think that AFC electrocatalysts will be more stable in operation than in acidic medium. However, this was proven wrong for carbon-supported Pt and Pd nanoparticles (NPs) aged in liquid alkaline environment: these undergo severe electrochemical surface area losses even for a very mild potential cycling procedure in 0.1 M NaOH [3, 4]. Identical-location transmission electron microscopy (ILTEM) experiments revealed pronounced detachment of the Pt (and Pd) NPs from the carbon support, but minor degradation phenomena for the metal NPs and the carbon support itself. Additional experiments performed in various alkaline electrolytes (LiOH, NaOH, KOH, CsOH) coupled with in situ Fourier-transform infrared spectroscopy enabled to link this detachment to the formation of solid carbonates at the interface between the Pt (Pd) NPs and the carbon support, because the metal nanoparticles assist the local corrosion of the carbon support (firstly into CO₂, then CO₃^2-anions and finally into M₂CO₃, M = Li, Na, K or Cs).

Figure 1 demonstrates that the loss of Pt NPs is greatly decreased when the alkaline electrolyte is an anion-exchange membrane. In that case, the formation of solid carbonates is severely depreciated, because the counter-cation of the OH^– species are immobilized on the polymer backbone and can therefore not precipitate (as Na₂CO₃ does in NaOH aqueous electrolyte). Although the degradation of the Pt/C NPs is minored when the materials are aged in interface with an AEM, it is not suppressed: Ostwald ripening and Pt redeposition are observed, i.e. the mechanisms of degradation differ in solid versus liquid alkaline environment. Such differences between the fate of Pt-based electrocatalysts were also demonstrated for solid versus liquid acidic electrolytes [5, 6].

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The Effect of Ambient Carbon Dioxide on Anion‐Exchange Membrane Fuel Cells

Authors:
N Ziv, W E Mustain, D R Dekel
Journal:ChemSusChem 11 (7), 1136-1150, 2018

Authors : Noga Ziv, William E Mustain, Dario R Dekel

Over the past 10 years, there has been a surge of interest in anion‐exchange membrane fuel cells (AEMFCs) as a potentially lower cost alternative to proton‐exchange membrane fuel cells (PEMFCs). Recent work has shown that AEMFCs achieve nearly identical performance to that of state‐of‐the‐art PEMFCs; however, much of that data has been collected while feeding CO₂‐free air or pure oxygen to the cathode. Usually, removing CO₂ from the oxidant is done to avoid the detrimental effect of CO₂ on AEMFC performance, through carbonation, whereby CO₂ reacts with the OH⁻ anions to form HCO₃⁻ and CO₃²⁻. In spite of the crucial importance of this topic for the future development and commercialization of AEMFCs, unfortunately there have been very few investigations devoted to this phenomenon and its effects. Much of the data available is widely spread out and there currently does not exist a resource that researchers in the field, or those looking to enter the field, can use as a reference text that explains the complex influence of CO₂ and HCO₃⁻/CO₃²⁻ on all aspects of AEMFC performance. The purpose of this Review is to summarize the experimental and theoretical work reported to date on the effect of ambient CO₂ on AEMFCs. This systematic Review aims to create a single comprehensive account of what is known regarding how CO₂ behaves in AEMFCs, to date, as well as identify the most important areas for future work in this field.

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A practical method for measuring the true hydroxide conductivity of anion exchange membranes

Authors:
N Ziv, D R Dekel
Journal:Electrochemistry Communications 88, 109-113, 2018

Authors: Noga Ziv, Dario R Dekel

Hydroxide ions in anion exchange membranes (AEMs) are quickly exchanged for larger and less mobile anions (CO₃²⁻ and HCO₃⁻) when the membrane is exposed to ambient air. Therefore, reported conductivity values of AEMs in hydroxide form are difficult to reproduce, and existing conductivity measurement techniques are not always reliable. Up to now, comparison of reported data for the hydroxide conductivity of different membranes has not been possible because tests have been performed not just with different anions, but also under different conditions and using different methods. In this work we present a practical and reproducible ex situ method for measuring the true value of the hydroxide conductivity of AEMs.

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Impact of carbonation processes in anion exchange membrane fuel cells

Authors:
U Krewer, C Weinzierl, N Ziv, D R Dekel
Journal:Electrochimica Acta 263, 433-446, 2018

Authors : Ulrike Krewer, Christine Weinzierl, Noga Ziv, Dario R Dekel

Alkaline anion exchange membrane fuel cell (AEMFC) is a promising technology to replace precious metals used today as fuel cell catalysts. However, AEMFC does not yet demonstrate high performance when running on ambient air where they are exposed to CO₂. The resulting carbonation reaction reduces membrane conductivity. This paper analyses and quantifies the effect of CO₂ from ambient air on the concentration profiles in the membrane and the anode and, thus, assesses the CO₂ impact on fuel cell performance. The physico-chemical model contains chemical and electrochemical reactions, liquid-gas phase equilibria as well as the transport processes in the cell. Results imply that a significant part of fed CO₂ is absorbed in the cathode and is transported as carbonate ions to the anode. Concentration profiles in the membrane reveal an enrichment zone of CO₂ in the membrane close to the anode, negligible $HC O_{3}^{-}$ and a wide distribution of $C O_{3}^{2 -}$ across the membrane. The carbonate distribution affects overall anion exchange membrane conductivity. For practical relevant current densities of $i > 500 mA {cm}^{- 2}$ and typical excess ratios of 1.5 for the hydrogen feed, less than 10% of the anions in the membrane are $C O_{3}^{2 -}$ . We show that while increasing cell temperature has an ambiguous effect on the carbonation process and on the total effect of CO₂ on the cell, current density has a significant effect. The impact of CO₂ on AEMFC performance can be significantly decreased when operating the cell at high current densities above $1000 mA {cm}^{- 2}$ .

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Anion exchange membrane fuel cells: Current status and remaining challenges

Authors:
S Gottesfeld, D R Dekel, M Page, C Bae, Y Yan, P Zelenay, YS Kim
Journal:Journal of Power Sources 375, 170-184, 2018

Authors : Shimshon Gottesfeld, Dario R Dekel, Miles Page, Chulsung Bae, Yushan Yan, Piotr Zelenay, YS Kim

The anion exchange membrane fuel cell (AEMFC) is an attractive alternative to acidic proton exchange membrane fuel cells, which to date have required platinum-based catalysts, as well as acid-tolerant stack hardware. The AEMFC could use non-platinum-group metal catalysts and less expensive metal hardware thanks to the high pH of the electrolyte. Over the last decade, substantial progress has been made in improving the performance and durability of the AEMFC through the development of new materials and the optimization of system design and operation conditions. In this perspective article, we describe the current status of AEMFCs as having reached beginning of life performance very close to that of PEMFCs when using ultra-low loadings of Pt, while advancing towards operation on non-platinum-group metal catalysts alone. In the latter sections, we identify the remaining technical challenges, which require further research and development, focusing on the materials and operational factors that critically impact AEMFC performance and/or durability. These perspectives may provide useful insights for the development of next-generation of AEMFCs.

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The critical relation between chemical stability of cations and water in anion exchange membrane fuel cells environment

Authors:
D R Dekel, S Willdorf, U Ash, M Amar, S Pusara, S Dhara, S Srebnik, C E Diesendruck
Journal:Journal of Power Sources 375, 351-360, 2018

Authors : Dario R Dekel, Sapir Willdorf, Uri Ash, Michal Amar, Srdjan Pusara, Shubhendu Dhara, Simcha Srebnik, Charles E Diesendruck

Anion exchange membrane fuel cells can potentially revolutionize energy storage and delivery; however, their commercial development is hampered by a significant technological impedance: the chemical decomposition of the anion exchange membranes during operation. The hydroxide anions, while transported from the cathode to the anode, attack the positively charged functional groups in the polymer membrane, neutralizing it and suppressing its anion-conducting capability. In recent years, several new quaternary ammonium salts have been proposed to address this challenge, but while they perform well in ex-situ chemical studies, their performance is very limited in real fuel cell studies. Here, we use experimental work, corroborated by molecular dynamics modeling to show that water concentration in the environment of the hydroxide anion, as well as temperature, significantly impact its reactivity. We compare different quaternary ammonium salts that have been previously studied and test their stabilities in the presence of relatively low hydroxide concentration in the presence of different amounts of solvating water molecules, as well as different temperatures. Remarkably, with the right amount of water and at low enough temperatures, even quaternary ammonium salts which are considered “unstable”, present significantly improved lifetime.

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Review of cell performance in anion exchange membrane fuel cells

Authors:
D R Dekel
Journal:Journal of Power Sources 375, 158-169, 2018

Author: Dario R Dekel

Anion exchange membrane fuel cells (AEMFCs) have recently received increasing attention since in principle they allow for the use of non-precious metal catalysts, which dramatically reduces the cost per kilowatt of power in fuel cell devices. Until not long ago, the main barrier in the development of AEMFCs was the availability of highly conductive anion exchange membranes (AEMs); however, improvements on this front in the past decade show that newly developed AEMs have already reached high levels of conductivity, leading to satisfactory cell performance. In recent years, a growing number of research studies have reported AEMFC performance results. In the last three years, new records in performance were achieved. Most of the literature reporting cell performance is based on hydrogen-AEMFCs, although an increasing number of studies have also reported the use of fuels others than hydrogen – such as alcohols, non-alcohol C-based fuels, as well as N-based fuels. This article reviews the cell performance and performance stability achieved in AEMFCs through the years since the first reports in the early 2000s.

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Steady state and transient simulation of anion exchange membrane fuel cells

Authors:
D R Dekel, I G Rasin, M Page, S Brandon
Journal:Journal of Power Sources 375, 191-204, 2018

Authors : Dario R Dekel, Igal G Rasin, Miles Page, Simon Brandon

We present a new model for anion exchange membrane fuel cells. Validation against experimental polarization curve data is obtained for current densities ranging from zero to above 2 A cm⁻². Experimental transient data is also successfully reproduced. The model is very flexible and can be used to explore the system’s sensitivity to a wide range of material properties, cell design specifications, and operating parameters. We demonstrate the impact of gas inlet relative humidity (RH), operating current density, ionomer loading and ionomer ion exchange capacity (IEC) values on cell performance. In agreement with the literature, high air RH levels are shown to improve cell performance. At high current densities (>1 A cm⁻²) this effect is observed to be especially significant. Simulated hydration number distributions across the cell reveal the related critical dependence of cathode hydration on air RH and current density values. When exploring catalyst layer design, optimal intermediate ionomer loading values are demonstrated. The benefits of asymmetric (cathode versus anode) electrode design are revealed, showing enhanced performance using higher cathode IEC levels. Finally, electrochemical reaction profiles across the electrodes uncover inhomogeneous catalyst utilization. Specifically, at high current densities the cathodic reaction is confined to a narrow region near the membrane.

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Accelerated Stress Test of Pt/C Nanoparticles in an Interface with an Anion-Exchange Membrane—An Identical-Location Transmission Electron Microscopy Study

Authors:
C Lafforgue, M Chatenet, L Dubau, D R Dekel
Journal:ACS Catalysis 8 (2), 1278-1286, 2018

Authors : Clémence Lafforgue, Marian Chatenet, Laetitia Dubau, Dario R Dekel

The durability of a state-of-the-art Pt/C electrocatalyst was assessed by accelerated stress test (AST) procedures conducted in liquid alkaline electrolyte (0.1 M NaOH) and in solid anion exchange polymer electrolyte using a “dry cell”, i.e. in absence of liquid electrolyte. In a liquid environment, the positive and negative vertex potential values have a great influence on the extent and on the magnitude of the degradations: the loss of electrochemical surface area observed for a wide potential range (0.1 < E < 1.23 V vs RHE) is ascribed to detachment of the Pt nanoparticles from their support. The Pt nanoparticles assist the local corrosion of the carbon support material, eventually yielding solid alkali–metal carbonates that mechanically expel them from their support. Such corrosion is linked to the propensity of the Pt nanoparticles to (i) accept carbon surface groups (CO_ad-like species) when their surface is free of oxides (“reduced” metal state, for E < 0.6 V vs RHE) and then to (ii) electro-oxidize the CO_ad species into CO₂ in the well-known Langmuir–Hinshelwood CO-stripping reaction, possible if OH_ad species do form (“oxidized” metal state, for E > 0.6 V vs RHE). As a result, when the AST is performed between 0.1 < E < 0.6 V vs RHE or between 0.6 < E < 1.23 V vs RHE, i.e. when the Pt nanoparticles are either mostly reduced or oxidized, respectively, the degradation processes at stake are less intense and different: Ostwald ripening proceeds in the former case and Pt nanoparticle agglomeration in the latter. In contrast to the case of liquid electrolyte, when the most severe AST (0.1 < E < 1.23 V vs RHE) is performed in the dry cell, the magnitude and main mechanisms of degradation significantly change. Because there is no excess water to dissolve the Pt^z+ species formed by corrosion of the Pt nanoparticles, 3D Ostwald ripening and local redeposition on existing particles become more likely: the anion-exchange ionomer better traps the Pt^z+ species and prevent their diffusion away from the active layer. In addition, the absence of free alkali metal cation avoids the precipitation of solid carbonates, and therefore, the detachment of the Pt nanoparticles from their support is not favored. This shows that the degradation processes of a given electrocatalyst not only depends on its nature but also on the vertex potential values scanned in the AST and, importantly, on the nature of the electrolyte medium investigated. Finally, the very dramatic degradations experienced in liquid electrolyte for Pt/C nanoparticles are somewhat mitigated in solid alkaline electrolyte, which harbors hope to develop durable AEM-based fuel cells and electrolyzers.

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Beyond 1.0 W cm−2 Performance without Platinum: The Beginning of a New Era in Anion Exchange Membrane Fuel Cells

Authors:
T J Omasta, X Peng, H A Miller, F Vizza, L Wang, J R Varcoe, D R Dekel, W E Mustain
Journal:Journal of The Electrochemical Society 165 (15), J3039-J3044, 2018

Authors : Travis J Omasta, Xiong Peng, Hamish A Miller, Francesco Vizza, Lianqin Wang, John R Varcoe, Dario R Dekel, William E Mustain

This work reports a high power, stable, completely Pt-free anion exchange membrane fuel cell (AEMFC) comprised of highly active catalysts – Pd-CeO₂/C at the anode and PdCu/C alloy at the cathode for the hydrogen oxidation and oxygen reduction reactions, respectively. The resulting AEMFC shows outstanding performance, reaching a peak power density of 1 W cm⁻², twice the value of the best performance for Pt-free cells reported in the literature to date. The AEMFC also shows a low voltage degradation rate when operated continuously for more than 100 h at a constant 0.5 A cm⁻², with a voltage degradation rate of only 2.5 mV h^-1, which is excellent when compared to nearly all of the AEMFCs reported in the literature to date. This combination of high performance and high stability in the absence of Pt-based catalysts represents a significant landmark in the progress of the AEMFC technology.

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Chemical stability of poly(phenylene oxide)-based ionomers in an anion exchange-membrane fuel cell environment

Authors:
S Willdorf-Cohen, A N Mondal, D R Dekel, C E Diesendruck
Journal:Journal of Materials Chemistry A 6 (44), 22234-22239, 2018

Authors : Sapir Willdorf-Cohen, Abhishek N Mondal, Dario R Dekel, Charles E Diesendruck

In recent years, intense research interest has been focused towards the development of anion exchange membrane fuel cells (AEMFCs) due to their potential to circumvent the need for expensive platinum catalysts, tackling the high cost that impedes mass commercialization of fuel cells. However, AEMFCs are not yet practical due to the low chemical stability of the quaternary ammonium (QA) cationic groups during cell operation. Several functionalized polymers for anion exchange membranes (AEMs), including substituted poly(phenylene oxide) (PPO), have been proposed as suitable ionomeric materials, as they present good stability in strong alkaline solutions. However, while they perform well in ex situ stability tests in aqueous solutions, they still present limited performance during AEMFC operation. As the current density in the fuel cell increases, more water is consumed at the cathode side, reducing the hydration level and, in turn, increasing the nucleophilicity of OH⁻ and its capability to attack the QA groups. Here, using our recently reported ex situ stability protocol that simulates the low-hydration environment of an AEMFC during operation, the alkaline stability of PPO-based anion exchange ionomers is measured and compared. Good agreement with previously studied QA molecules tested using the same protocol was found. Yet, the degradation processes in these ionomers are further accelerated compared to the small QA molecules as a consequence of the lower polarity of the polymer environment, which further increases the hydroxide reactivity. This study demonstrates the competence of this new ex situ stability protocol to test not only QA molecules, but also ionomers and membranes, showing alkaline stability results that are comparable to those obtained in real AEMFC tests.

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Ni-Based Hydrogen Oxidation Reaction Electrocatalysts for Alkaline Anion-Exchange Membrane Fuel Cells: Systematic Study on the Doping Effect

Authors:
E S Davydova, D R Dekel
Journal:Meeting Abstracts 36, 1635-1635, 2017

Authors : Elena S Davydova, Dario R Dekel

Recent developments in hydroxide conducting anion exchange membranes have contributed to the growing interest in alkaline anion exchange membrane fuel cells (AEMFC) which has the potential to be the next fuel cell generation as in principle this technology allows to replace platinum-based catalysts for affordable metal catalysts. Until now, there are a few research studies describing initial development of hydrogen fueled AEMFC [1], and practically no work done on completely platinum group metal (PGM)-free catalysts for AEMFC. Though some progress was made in the development of low cost electrocatalysts for oxygen reduction reaction (ORR) in base media [2], much less attention was paid to the electrocatalysis for hydrogen oxidation reaction (HOR) in alkaline solutions. Attempts to move away from the use of PGMs results in prohibitively large voltage losses on the anode in the base media (Fig. 1).

This work provides an extensive literature review on both theoretical and experimental advances in HOR electrocatalysts for AEMFCs. In addition, this work provides new insights into new Ni-based electrocatalysts for the HOR in alkaline medium that are being developed in our group at the Technion, including RDE, XRD, XPS, TPD/TPR/TPO, SEM results and some aspects of DFT calculations.

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A model based analysis of alkaline membrane fuel cells

Authors:
I G Rasin, M Page, D R Dekel, S Brandon
Journal:ECS Transactions 80 (8), 1051-1057, 2017

Authors : Igal G Rasin, Miles Page, Dario R Dekel, Simon Brandon

The implementation of reduced-cost catalyst materials in Anion Exchange Membrane Fuel Cells (AEMFCs) renders them an attractive alternative to Proton Exchange Membrane Fuel Cells (PEMFCs). An important challenge existing in the development of AEMFCs involves water management, which is especially problematic since in these systems (as compared to PEMFCs) water is produced in the anode and consumed in the cathode. We present a model-based analysis of AEMFC performance with an emphasis on water management. Using this approach it is possible to elucidate the relation between reduced performance and issues of hydration throughout the Membrane Electrode Assembly (MEA). We provide a brief overview of the model, validate our results against published experimental data and present results related to the impact of relative humidity, as well as the depletion of water vapor in the air flow stream, on system performance.

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Effect of Water on the Stability of Quaternary Ammonium Groups for Anion Exchange Membrane Fuel Cell Applications

Authors:
D R Dekel, M Amar, S Willdorf, M Kosa, S Dhara, C E Diesendruck
Journal:Chemistry of Materials 29 (10), 4425-4431, 2017

Authors: Dario R Dekel, Michal Amar, Sapir Willdorf, Monica Kosa, Shubhendu Dhara, Charles E Diesendruck

Here we present a novel methodology to measure the alkaline stability of anion conducting polymers to be used as anion exchange membranes and anion exchange ionomers for fuel cells. The new ex situ technique simulates the environment of an anion exchange membrane fuel cell (AEMFC) during operation, where nucleophilic and basic OH^– species in the absence, or with a scarce amount of water, attack the functional groups of the ionic polymer. Using this technique, we clearly show the critical effect of water molecules on the alkaline stability of quaternary ammonium (QA) cations commonly used as functional groups in AEMFCs. The results show that as the water content is reduced, the QA cations are more rapidly degraded in the presence of OH^– at room temperature. With an increasing number of water molecules solvating the hydroxide, its nucleophilicity and basicity are hindered, and the QA degradation is significantly slowed. These results indicate that the currently used aqueous alkali ex situ tests to measure anion exchange membrane (AEM) stability may lead to false positive stability results where anion conducting polymers may appear more alkali stable than they really are.

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Highly active nanostructured palladium-ceria electrocatalysts for the hydrogen oxidation reaction in alkaline medium

Authors:
H A Miller, F Vizza, M Marelli, A Zadick, L Dubau, M Chatenet, S Geiger, S Cherevko, H Doan, R K Pavlicek, S Mukerjee, D R Dekel
Journal:Nano Energy 33, 293-305, 2017

Authors : Hamish A Miller, Francesco Vizza, Marcello Marelli, Anicet Zadick, Laetitia Dubau, Marian Chatenet, Simon Geiger, Serhiy Cherevko, Huong Doan, Ryan K Pavlicek, Sanjeev Mukerjee, Dario R Dekel

We report an interesting new class of bifunctional electrocatalysts, Pd/C-CeO₂, with excellent activity and stability for the hydrogen oxidation reaction (HOR) under alkaline conditions. The unique structure of palladium deposited onto a mixed support of Vulcan XC-72 carbon and CeO₂ consists of Pd metal preferable deposited on the ceria regions of the catalyst. The CeO₂-Pd interaction leads to enhanced HOR kinetics and increased stability. Here we compare catalysts with three different Pd loadings and show that the 10 wt% Pd sample has optimized activity. Hydrogen pumping and fuel cell experiments based on this catalyst show higher activities as compared to a Pd/C sample without ceria. Metal dissolution tests and identical location transmission microscopy experiments show that the catalyst stability under harsh potential cycling experiments in alkaline medium is significantly improved as compared to Pd/C, making this material one of the best options for use as highly active and highly stable electrocatalysts for the HOR in anion exchange membrane fuel cells.

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Bicarbonate and chloride anion transport in anion exchange membranes

Authors:
A Amel, N Gavish, L Zhu, D R Dekel, M A Hickner, Y Ein-Eli
Journal:Journal of Membrane Science 514, 125-134, 2016

Authors : Alina Amel, Nir Gavish, Liang Zhu, Dario R Dekel, Michael A Hickner, Yair Ein-Eli

Quaternary ammonium poly(sulfone) based anion exchange membrane (AEM) in Cl⁻ and HCO₃⁻forms were characterized chemically and morphologically. It was found that the surface of the membrane in both of the forms has highly connective island-like structure, where the diameters of the hydrophilic regions are approximately 5–20 nm. Thermal gravimetric analysis of the membrane in the HCO₃⁻ form presented lower decomposition temperatures for the backbone and the side chains, than the membrane in the Cl⁻ form. In addition, the AEM in its HCO₃⁻ form showed higher water uptake values than in its Cl⁻ form across the temperature range of 25–80 °C. Conductivity experiment measured at same temperatures in both AEM forms showed higher results for Cl⁻ form than for HCO₃⁻ form. A computational model was developed in order to understand the conductivity mechanism and the relevant parameters that limit ion transport in these materials. Together with the experimental results, it was found that only 40% of the ions are free for ionic conductivity, while 60% of the ions are bound to the cationic groups, therefore unavailable to participate in the conduction process.

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A Pd/C‐CeO2 Anode Catalyst for High‐Performance Platinum‐Free Anion Exchange Membrane Fuel Cells

Authors:
H A Miller, A Lavacchi, F Vizza, M Marelli, F Di Benedetto, F D'Acapito, Y Paska, M Page, D R Dekel
Journal:Angewandte Chemie International Edition 55 (20), 6004-6007, 2016

Authors : Hamish A Miller, Alessandro Lavacchi, Francesco Vizza, Marcello Marelli, Francesco Di Benedetto, Francesco D’Acapito, Yair Paska, Miles Page, Dario R Dekel

One of the biggest obstacles to the dissemination of fuel cells is their cost, a large part of which is due to platinum (Pt) electrocatalysts. Complete removal of Pt is a difficult if not impossible task for proton exchange membrane fuel cells (PEM‐FCs). The anion exchange membrane fuel cell (AEM‐FC) has long been proposed as a solution as non‐Pt metals may be employed. Despite this, few examples of Pt‐free AEM‐FCs have been demonstrated with modest power output. The main obstacle preventing the realization of a high power density Pt‐free AEM‐FC is sluggish hydrogen oxidation (HOR) kinetics of the anode catalyst. Here we describe a Pt‐free AEM‐FC that employs a mixed carbon‐CeO₂ supported palladium (Pd) anode catalyst that exhibits enhanced kinetics for the HOR. AEM‐FC tests run on dry H₂ and pure air show peak power densities of more than 500 mW cm⁻².

Recent analyses have shown that among PEM‐FC components around 45 % of the cost comes from the platinum (Pt) electrocatalyst.1 Therefore, a complete removal of Pt from fuel cells and replacement with metals that are less expensive and more abundant in nature is crucial to make this technology an affordable solution for automotive as well as other large scale applications. As an alternative to PEM‐FCs that operate under corrosive acidic conditions the use of alkaline anion exchange membrane fuel cells (AEM‐FC) may reduce costs by avoiding the use of platinum.2

In the AEM‐FC cathode, non‐noble metals can readily replace Pt.3 Varcoe et al. have recently comprehensively reviewed the range of AEMs and ionomers developed for AEM‐FCs.4 Less attention has been paid to the anode catalyst for the hydrogen oxidation reaction (HOR).2e, 5 In contrast to PEM‐FCs, HOR kinetics are quite slow in alkaline media. Indeed, the HOR activity on noble metals (Pt, Pd and Ir) decreases by a factor of ca. 100 when switching from low to high pH.6 Some work has been done on developing non‐Pt HOR catalysts7 but very few reports of complete AEM‐FCs are available. Zhuang and co‐workers with NiCr and NiW anode catalysts have demonstrated Pt‐free H₂/O₂ AEM‐FCs generating around 50 mW cm⁻² peak power.5c, 8 Recently, with a PdNi anode catalyst 400 mW cm⁻² was obtained by Alesker et al.9 The main obstacle is the challenge of overcoming poor HOR kinetics in alkaline media.

In this work, we present a nanoparticle (NP) Pd HOR catalyst with a composite support made of Vulcan XC‐72 carbon and CeO₂ (C‐CeO₂) which exhibits enhanced HOR kinetics in alkaline media. Ceria (CeO₂) was used, as it is one of the most oxygen deficient compounds, known for rapid its saturation with OH⁻ ions in alkaline media10 and spillover of OH⁻ to supported metal nanoparticles.10b We have found in previous studies that a mixed ceria‐carbon support enhances the activity of Pd anodes in direct ethanol fuel cells (DEFC) by promoting the transfer of OH⁻ to form active Pd^IOH_ads species.11

The mixed support contains 50 wt % CeO₂ and 50 wt % Vulcan XC‐72 carbon. Pd (10 wt %) was deposited by chemical deposition and reduction (see Supporting Information (SI) for synthesis details). The fuel cell anode was prepared using either the new composite Pd/C‐CeO₂ catalyst or a homemade reference Pd/C catalyst (C=Vulcan XC‐72, 10 wt % Pd).12 The anode Pd loading was 0.3 mg cm⁻². Silver (Ag) was used as a cathode catalyst with a loading of 3.0 mg_Ag cm⁻². Membrane electrode assemblies (MEAs)13 with an active area of 5 cm² were tested in AEM‐FC single cells (see SI for complete description).13b, 14

Figure 1 shows the cell performance at 73 °C with air (<10 ppm CO₂) at the cathode (1.0 slm, 1.0 barg, dew point 73 °C) and dry H₂ (0.2 slm, 3.0 barg, 25 °C) fed to the anode.

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Palladium/nickel bifunctional electrocatalyst for hydrogen oxidation reaction in alkaline membrane fuel cell

Authors:
M Alesker, M Page, M Shviro, Y Paska, G Gershinsky, D R Dekel, D Zitoun
Journal:Journal of Power Sources 304, 332-339, 2016

Authors : Maria Alesker, Miles Page, Meital Shviro, Yair Paska, Gregory Gershinsky, Dario R Dekel, David Zitoun

Investigation of the hydrogen oxidation reaction (HOR) in alkaline media has been pursued in the past few years side by side with the development of alkaline membrane fuel cells (AMFCs), also called anion exchange membrane fuel cells (AEM-FCs). In this communication, we present the synthesis, electrochemistry and AMFC test of a platinum-free HOR catalyst. The anode catalyst is prepared by growing palladium nanoparticles onto nanoparticles of an oxophilic metal (nickel), resulting in nano-dispersed, interconnected crystalline phases of Ni and Pd. When used in the anode of a hydrogen/air AMFC, such Pd/Ni catalyst exhibits high HOR activity, resulting in record high performance for a platinum-free AMFC (0.4 A cm⁻² at 0.6 V vs RHE). The enhancement of HOR catalytic activity vs. that observed at Pd (or Ni) alone is revealed directly in rotating disc electrode tests of this Pd/Ni catalyst that shows a significant negative shift (200 mV) of the onset potential for the HOR current vs. the case of Pd.

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A simulator for system-level analysis of heat transfer and phase-change in thermal batteries II. multiple-cell simulations

Authors:
N Haimovich, D R Dekel, S Brandon
Journal:Journal of The Electrochemical Society 162 (3), A350-A362, 2015

Authors : Nir Haimovich, Dario R Dekel, Simon Brandon

We present the application of our thermal battery system-level simulator [J. Electrochem Soc., 156, A442 (2009)] in novel multiple-cell thermal analyses. Several model batteries are chosen to demonstrate the simulator’s versatility and robustness in developing advanced thermal battery designs. The heat transfer phase-change model and supporting mass balance are modified to improve model consistency. Simulation results are presented from several case-studies covering different battery structures and operating conditions, including low and high current densities, different number of electro-active cells, various internal and external battery geometrical details, the use of salt buffers, external flanges, and inhomogeneous initial conditions in the battery stack. These results show the simulator to be a highly user-friendly and powerful tool for development of complex thermal batteries. Investigation of heat of reactions and joule heating effects unfold insights regarding dominant processes in multiple-cell batteries. Furthermore, these detailed analyses emphasize the need to track solidification dynamics at the sub-cell level and, at the same time, show that thermal battery design should include a significant ingredient of multiple-cell analysis. Altogether, this study presents a first-in-its-kind portable simulator with great flexibility and a capability for supporting the analysis and development of most thermal battery structures and designs.

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Characterization and chemical stability of anion exchange membranes cross-linked with polar electron-donating linkers

Authors:
A Amel, S B Smedley, D R Dekel, M A Hickner, Y Ein-Eli
Journal:Journal of The Electrochemical Society 162 (9), F1047-F1055, 2015

Authors : Alina Amel, Sarah B Smedley, Dario R Dekel, Michael A Hickner, Yair Ein-Eli

The effect of the cross-linker chemical structure on the properties and chemical stability of anion exchange membrane is the focus of this study. Two different cross-linkers were investigated, one with linear hexyl chain between crosslinking sites, and the other, ether in the center of the alkyl linker. These two cross-linkers have a fundamental difference in their polarity and hydrophilicity. The ether-containing cross-linker is more polar and therefore will improve membrane’s water uptake and conductivity. Swelling and conductivity measurements were performed at various temperatures for both types of samples. While water uptake and conductivity were found to be higher for the ether-based cross-linker, degradation measurements indicated that the membrane that is cross-linked with electron-rich linker degraded in hydroxide faster than the alkyl linker at temperature of 60°C.

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Alkaline Membrane Fuel Cells, Membranes

Authors:
D R Dekel
Journal:Encyclopedia of Applied Electrochemistry 33-45, 2014

Authors : Dario R Dekel

Activated carbon is a porous carbon material with developed nano-sized pores and a high specific surface area (> 1,000 m2g À1). Nowadays, the technical term “nanoporous carbon” is very often used to mean “activated carbon,” but the activated carbon should be strictly defined as a porous carbon prepared by an activation process consisting of a gasification reaction to form the developed nano-sized pore structure in the carbon matrix. The activated carbons have been widely utilized as industrial materials, for example, an adsorbent, decolorizing agent, deodorant, and catalyst. The details of the preparation method, pore structure, and applications of activated carbons have been described [1]. The most significant electrochemical application of activated carbons is use as an electrode active material for an electric double-layer capacitor (EDLC). This article reviews the use of activated carbons in an EDLC.

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Anion-exchange membranes in electrochemical energy systems

Authors:
J R Varcoe, P Atanassov, D R Dekel, A M Herring, M A Hickner, P A Kohl, A R Kucernak, W E Mustain, K Nijmeijer, K Scott, T Xu, L Zhuang
Journal:Energy & environmental science 7 (10), 3135-3191, 2014

Authors : John R Varcoe, Plamen Atanassov, Dario R Dekel, Andrew M Herring, Michael A Hickner, Paul A Kohl, Anthony R Kucernak, William E Mustain, Kitty Nijmeijer, Keith Scott, Tongwen Xu, Lin Zhuang

This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technological and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the references that the authors deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are included.

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Alkaline membrane fuel cell (AMFC) materials and system improvement-state-of-the-art

Authors:
D R Dekel
Journal:ECS Transactions 50 (2), 2051-2052, 2013

Authors : Dario R Dekel

In the last few years, developmental work on anion conductive membranes for AMFC has significantly increased and several polymer chemistries have already been tested at very small AMFC scales. On the other hand, scarce work is being done on anion conductive ionomers, and just a very few groups are working on electrocatalysts for the electrodes of this technology. From the limited materials developed for this technology, peak power density records of almost 200mW/cm2 were already achieved. Non-published data suggests that the actual power densities that can be achieved with today’s state-of-the-art anion conductive polymers are even much higher. With the latest and most advanced developed materials, AMFC stacks providing 2kW power output were already developed. During this presentation, the state of the art on AMFCs will be reviewed, and current challenges in the development of new materials will be briefly discussed.

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A simulator for system-level analysis of heat transfer and phase change in thermal batteries I. computational approach and single-cell calculations

Authors:
N Haimovich, D R Dekel, S Brandon
Journal:Journal of The Electrochemical Society 156 (6), A442-A453, 2009

Authors : Nir Haimovich, Dario R Dekel, Simon Brandon

We present a complete and detailed thermal simulator designed for the computational analysis of thermal batteries from the level of a single cell up to that of the entire system. Our simulator is based on a comprehensive transient and two-dimensional (axisymmetric) mathematical heat-transfer model, with significant flexibility in the geometrical modeling and the materials used. The model accounts for different aspects of heat transfer, including conduction, joule heating, heat of reactions, and latent heat of fusion associated with electrolyte phase change (salt solidification). It is supported by a simplified mass balance involving the current drawn from the battery and accounting for the mass-transfer resistance of each of the cell’s components. Results presented include model verification-and-validation calculations as well as single-cell thermal battery simulations performed under realistic operating conditions. The latter reveal the significance of the phase-change process to heat transfer and thus to the prediction of its operation time. Solidification dynamics are found to be different in each of the cell’s components, emphasizing the necessity of accounting for details at the subcell level. Additional results uncover the effect of heat of reactions as well as joule heating on single-cell battery thermal behavior.

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A new lithium anode composite for thermal batteries

Authors:
D R Dekel
Journal:ETDEWEB 164-165, 2003

Authors : Dario R Dekel

As a rule, thermal batteries consist of a series or series-parallel arrays of cells. Each cell comprises an anode, electrolyte-separator, cathode, and a pyrotechnic heat source. Activation of the cell occurs when sufficient heat is applied to melt the electrolyte. Lithium alloys and immobilized molten lithium are typically used as anodes. Molten lithium anodes possess the advantage of higher capacity and faster kinetics. One disadvantage is the formation of lithium nitride due to nitrogen attack, which results in serious degradation of the lifetime of thermal batteries. The author proposes a new lithium anode composite for thermal batteries, consisting of a new composition of Li-Al-Fe (lithium, aluminium, iron). Good electrical characteristics are exhibited by these new anodes, similar to those displayed by conventional Li-Fe anodes. There is no indication that the anodes are being nitrided during normal anode production or during battery aging. 1 tab.

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Effect of solvent properties on permeate flow through nanofiltration membranes: Part II. Transport model

Authors:
D R Dekel, D Hasson, R Semiat
Journal:Journal of Membrane Science 166 (1), 63-69, 2000

Authors : Dario R Dekel, David Hasson, Raphael Semiat

In Part I of this work, permeation flows of a large number of solvents were measured and found to exhibit a wide spread in permeate flux levels. The flux of both pure and mixed solvents was mainly affected by surface tension and viscosity.

This paper presents a transport model describing solvent–membrane interactions, governed by viscous and surface forces. The model relates the flux of a solvent mixture with easily measurable solvent and membrane properties (surface tension, viscosity and membrane hydrophobicity).

Extensive flux measurements of mixed solvents belonging to several chemical families were well correlated by the model using the following experimentally determined parameters. Membrane properties were characterized by two solvent independent coefficients f₁ and f₂, while, with minor exceptions, each solvent mixture was characterized by a specific coefficient φ.

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Effect of solvent properties on permeate flow through nanofiltration membranes: Part I. Investigation of parameters affecting solvent flux

Authors:
D R Machado (Dekel), D Hasson, R Semiat
Journal:Journal of Membrane Science 163 (1), 93-102 1999

Effect of solvent properties on permeate flow through nanofiltration membranes: Part I. Investigation of parameters affecting solvent flux

Authors : Dario R. Machado (Dekel), David Hasson, Raphael Semiat

The objective of the present study was to characterize transport properties of solvents permeating through solvent resistant nanofiltration membranes that have only recently become available. Permeation flows of a number of solvents of different chemical families were measured in a batch cell. The solvents studied were alcohols, paraffins, ketones, acetates, and water, as well as their binary mixtures. The experimental data revealed a marked variation in the level of permeate flux among the various solvents. For instance, permeation flow of pentane was about 60-times faster compared to that of water while permeation flow of ethanol was about 10-times faster than that of water. The dependence of solution flux on the fractional composition of the solvents in the mixture was found to be highly non-linear. The flux of either pure or mixed solvents was mainly affected by surface tension and viscosity of the solvents. The flux of paraffins mixtures in particular was affected by the dielectric constant also.

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Spray pyrolysis of YBCO precursors

Authors:
G S Grader, D R Dekel, R Semiat
Journal:J. Mater. Res. 9 (10), 2490-2500, 1994

Authors : Gideon S Grader, Dario R Dekel, Raphael Semiat

Acetate, nitrate, and oxalate precursors for YBCO have been spray pyrolyzed under different conditions. Shelled and nonhollow microparticles were obtained from acetate and nitrate precursors, while nonhollow agglomerates were obtained from the oxalate suspension. At low furnace temperatures, the temperature and residence time of the particles were insufficient for complete decomposition of the precursors leading to Cu2O and Cu metal in the product. At 900 °C and above, reduced forms of CuO were not detected by x-ray measurements, and up to ∼60 wt.% YBCO was obtained. An approximate model predicting the particle and gas temperatures along the reactor under different operating conditions was developed. The model demonstrates that under the experimental conditions used here, the absorbed radiation heat by the particles from the furnace walls is significant in heating the gas. The gas and the particle temperatures are fairly close due to the effective heat transfer to the particles. At furnace temperatures of 700 °C, the maximum predicted particle temperature is about 500 °C (for ∼1 s). This explains the incomplete reactions obtained under these conditions. Above 900 °C the reactions are predicted to be complete within the first half of the furnace, leaving sufficient residence time for partial conversion into YBCO. Finally, an approximate expression predicting the relative contribution to the gas heating by the walls and the aerosol has been developed.

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