Cost-effective hydrogen oxidation reaction catalysts for hydroxide exchange membrane fuel cells

Fuel cells are clean, efficient energy conversion devices that produce electricity from chemical energy stored within fuels. The development of fuel cells has significantly progressed over the past decades. Specifically, polymer electrolyte fuel cells, which are representative of proton exchange membrane fuel cells (PEMFCs), exhibit high efficiency, high power density, and quick start-up times. However, the high cost of PEMFCs, partially from the Pt-based catalysts they employ, hinders their diverse applicability. Hydroxide exchange membrane fuel cells (HEMFCs), which are also known as alkaline polymer electrolyte fuel cells (APEFCs), alkaline anion-exchange membrane fuel cells (AAEMFCs), anion exchange membrane fuel cells (AEMFCs), or alkaline membrane fuel cells (AMFCs), have attracted much attention because of their capability to use non-Pt electrocatalysts and inexpensive bipolar plates. The HEMFCs are structurally similar to PEMFCs but they use a polymer electrolyte that conducts hydroxide ions, thus providing an alkaline environment. However, the relatively sluggish kinetics of the hydrogen oxidation reaction (HOR) inhibit the practical application of HEMFCs. The anode catalyst loading needed for HEMFCs to achieve high cell performance is larger than that required for other fuel cells, which substantially increases the cost of HEMFCs. Therefore, low-cost, highly active, and stable HOR catalysts in the alkaline condition are greatly desired. Here, we review the recent achievements in developing such HOR catalysts. First, plausible HOR mechanisms are explored and HOR activity descriptors are summarized. The HOR processes are mainly controlled by the binding energy between hydrogen and the catalysts, but they may also be influenced by OH adsorption, interfacial water adsorption, and the potential of zero (free) charge. Next, experimental methods used to elevate HOR activities are introduced, followed by HOR catalysts reported in the literature, including Pt-, Ir-, Pd-, Ru-, and Ni-based catalysts, among others. HEMFC performances when employing various anode catalysts are then summarized, where HOR catalysts with platinum-group metals exhibited the highest HEMFC performance. Although the Ni-based HOR catalyst activity was higher than those of other non-precious metal-based catalysts, they showed unsatisfactory performance in HEMFCs. We further analyzed HEMFC performances while considering anode catalyst cost, where we found that this cost can be reduced by using recently developed, non-Pt HOR catalysts, especially Ru-based catalysts. In fact, an HEMFC using a Ru-based HOR catalyst showed an anode catalyst cost-based performance similar to that of PEMFCs, making the HEMFC promising for use in practical applications. Finally, we proposed routes for developing future HOR catalysts for HEMFCs.