Alkaline anion exchange membrane electrolyzers are attractive for their potential to generate cost-effective hydrogen from renewable electricity sources. This has driven renewed research interest in catalysts for alkaline water electrolysis, including Ni–Mo alloys for the hydrogen evolution reaction (HER). Although decades of research on Ni–Mo alloys have shown these materials to be among the most active non-precious HER catalysts, the precise composition and structure of Ni–Mo (let alone the atomic structure of its active site or sites) remains unclear. Thus, it is also unclear why Ni–Mo catalysts exhibit improved HER activity compared to Ni or Mo alone. To address this knowledge gap, we have worked to further elucidate composition-structure-activity relationships in a previously reported Ni–Mo alloy nanopowder catalyst. We synthesized Ni–Mo nanopowders via thermal reduction of homogeneous mixed oxide precursors containing several mole ratios of Ni to Mo. Electron microscopy and X-ray diffraction measurements showed that the as-synthesized catalysts consists of Ni-rich alloy nanoparticles (whose empirical composition is Ni9Mo) embedded in a nearly amorphous Mo-rich oxide matrix. This structure apparently results from preferential reduction of the Ni component, which then proceeds to incorporate Mo near its solubility limit in the Ni FCC crystal structure while excluding any remaining Mo. The residual Mo oxide also appears to play a structural role by preventing the sintering of the alloy nanoparticles under thermal reduction conditions, but it remains unclear whether the oxide component is integral to the activity of the catalyst. Importantly, we also obtained compelling evidence that the catalytic activity of these Ni–Mo nanopowders is limited not by electron-transfer kinetics or solution-phase mass transfer, but instead by electrical resistivity within the catalyst film that results from the need for electrons to travel through many oxide layers between nanoparticles. We have successfully mitigated, but not eliminated, these resistivity limitations by incorporating large quantities of carbon black as a conductive additive. Thus, we conclude that the maximum activity we have observed from Ni–Mo is in fact a lower bound on its true catalytic activity, and that similar resistivity limitations may be present in other non-precious electrocatalysts whose surfaces are susceptible to oxidation in air or water.

New insights regarding composition and practical limitations of Ni–Mo nanopowder hydrogen evolving electrocatalysts

2019 Spring National Meeting

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