Carbon Supported Ni-Mo Catalysts for Reversible Alkaline Hydrogen Electrochemistry

Abstract

Alkaline anion exchange membrane (AAEM) electrolyzers are attractive for cost-effective production of hydrogen using carbon-free electricity as the energy input. These devices facilitate the use of non-precious metal catalysts and ancillary components, which together offer significant reduction in capital cost. When combined with the low and still rapidly decreasing cost of renewable electricity, AAEM electrolyzers have the potential to meet the cost target of $2/kg for hydrogen production set by the US Department of Energy. Despite this promise, considerable work remains to develop efficient and stable catalysts and membranes for use under alkaline operating conditions. In this context, non-precious Ni-based catalysts are among the most promising for use as hydrogen-evolving cathodes. Our focus is on Ni-Mo composites, which have been studied and deployed for decades as alkaline hydrogen evolution catalysts, but the associated reaction mechanism remains relatively less explored.We recently showed that Ni-Mo catalysts comprise a Ni-rich metallic core surrounded by Mo-rich oxide shell, where the electrically resistive shell limits the overall catalytic activity in unsupported thin films. By contrast, incorporation of conductive carbon black supports significantly enhances catalyst mass activity toward the alkaline hydrogen evolution reaction (HER) without measurably altering the composition and structure of the active Ni-Mo composite. More recently, we have directly observed the evolution of the core-shell architecture in Ni-Mo catalysts using in situ environmental transmission electron microscopy (ETEM). These measurements suggest that thermal reduction of NiMoO4 precursors proceeds in at least two sequential events. When combined with HER catalytic activity measurements, the ETEM data show that the most competent Ni-Mo catalyst requires intimate contact between a metallic, Ni-rich face-centered cubic alloy phase and a Mo-rich suboxide phase. Carbon supported Ni-Mo catalyst composites were also found to be active for the alkaline hydrogen oxidation reaction (HOR), which lends to their potential use in unitized reversible fuel cell systems. Preliminary results suggest that the Ni-Mo mass loading relative to the carbon support has a stronger influence of its HOR activity than for the HER, suggesting that mass transfer plays a significant role in the practical HOR activity.