Dario R. Dekel

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.

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.

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.

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.

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.

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

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

Author: Dario R Dekel

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.

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.

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.

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.

Authors: Noga Ziv, Dario R Dekel