Reversible alkaline hydrogen evolution and oxidation reactions using Ni–Mo catalysts supported on carbon

Abstract

Coupling water electrolysis and fuel cell energy conversion is an attractive strategy for long-duration energy storage. This mode of operation depends on the ability to reversibly catalyze hydrogen evolution and oxidation, 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 the ability to catalyze reversible hydrogen evolution and oxidation. For the hydrogen evolution reaction, we observed mass-specific activities exceeding 80 mA mg⁻¹ at 100 mV overpotential, and measurements using hydroxide exchange membrane electrode assemblies yielded full cell voltages that were only ∼100 mV larger for Ni–Mo/oC cathodes than for Pt–Ru/C cathodes at current densities exceeding 1 A cm⁻². For hydrogen oxidation, Ni–Mo/oC films required <50 mV overpotential to achieve half the maximum anodic current density, but activity at larger overpotentials 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.