The chemical industry makes extensive use of hydrogen (H₂) as feedstock, obtained from fossil fuels or coal as primary source. For environmental sustainability of H₂ production, these sources need to be replaced with approaches based on renewables, such as solar-driven water electrolysis. Alkaline anion exchange membrane (AAEM) electrolyzers are attractive for cost-effective hydrogen production. They facilitate the use of non-precious catalysts and ancillary components, which offer significant reduction in capital cost. AAEM electrolyzers have the potential to meet the $2/kg cost target for H₂ production set by the US Department of Energy. Despite this promise, considerable work remains to develop efficient and stable catalysts for use under alkaline conditions. In this context, our focus is on Ni–Mo composites, which have been studied and deployed as alkaline H₂ evolution catalysts, but the associated reaction mechanism remains relatively less explored.

We have clarified the composition of Ni–Mo alloys by showing that they consist of a core@shell structure where the metallic core is Ni rich and the shell is Mo-rich oxides. We directly observed the evolution of this core-shell morphology using in situ environmental transmission electron microscopy (ETEM). When combined with hydrogen evolution reaction (HER) activity measurements, the data show that the most competent Ni–Mo catalyst requires intimate contact between a metallic, Ni-rich alloy phase and the Mo-rich suboxide phase. We have also determined that unsupported Ni–Mo catalysts suffer from electronic transport limitations, which we have successfully mitigated via carbon supports. 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. The focus of the present study is to elucidate the activity and stability limits of carbon-supported Ni–Mo composites for the alkaline HOR.

(a) Cyclic voltammograms depicting HER activity. (b) Chronoamperometry derived voltammograms depicting HOR activity of Ni–Mo catalysts with and without carbon along with Pt/C catalyst in 0.1 M KOH at 1600 rpm under hydrogen purge. The inset in (a) shows TEM image of Ni–Mo/C catalyst.

Stephen House

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