Reversible Alkaline Hydrogen Evolution and Oxidation Reactions Using Ni–Mo Catalysts Supported on Carbon

Unitized regenerative fuel cells based on hydroxide exchange membranes are attractive for long duration energy storage. This mode of operation depends on the ability to catalyze hydrogen evolution and oxidation reversibly, and ideally using nonprecious catalyst materials. Here we report the synthesis of Ni–Mo catalyst composites supported on oxidized Vulcan carbon (Ni–Mo/oC) and demonstrate their performance for reversible hydrogen evolution and oxidation. For the hydrogen evolution reaction, we observed mass-specific activities exceeding 80 mA/mg at 100 mV overpotential, and additional measurements using hydroxide exchange membrane electrode assemblies yielded full cell voltages that were only ~100 mV larger for Ni–Mo/oC cathodes compared to Pt–Ru/C at current densities exceeding 1 A/cm2. For hydrogen oxidation, Ni–Mo/oC films required <50 mV overpotential to achieve half the maximum anodic current density, but activity was limited by internal mass transfer and oxidative instability. Nonetheless, estimates of the mass-specific exchange current for Ni–Mo/oC from micropolarization measurements showed its hydrogen evolution/oxidation activity is within 1 order of magnitude of commercial Pt/C. Density functional theory calculations helped shed light on the high activity of Ni–Mo composites, where the addition of Mo leads to surface sites with weaker H-binding energies than pure Ni. These calculations further suggest that increasing the Mo content in the subsurface of the catalyst would result in still higher activity, but oxidative instability remains a significant impediment to high performance for hydrogen oxidation.