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 PdIOHads 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 mgAg 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.
