Nørskov and collaborators proposed a simple kinetic model to explain the volcano relation for the hydrogen evolution reaction on transition metal surfaces in such that j_0= k_0 f(ΔG_H) where j_0...

Alkaline water electrolysis offers the use of low-cost active materials and ancillary components, making it attractive for hydrogen production from renewables. Nevertheless, the practical performance of nonprecious electrocatalysts for alkaline hydrogen evolution still lags behind platinum-group metals. This disparity motivates work to understand how the solid-state chemistry of nonprecious transition metal alloys influences their activity toward alkaline hydrogen evolution. To this end, we have clarified the composition, chemical structure, and morphology of a previously reported Ni–Mo nanopowder electrocatalyst. The as-synthesized catalyst is mixed phase, comprising crystalline Ni-rich alloy nanoparticles embedded in a Mo-rich oxide matrix, and exhibits low activity toward hydrogen evolution. Its activity markedly increases upon activation by postdeposition reductive annealing or by including carbon black as a catalyst support. These results are consistent with a physical picture in which activity is limited not by kinetics but by electrical resistivity arising from thin oxide layers at the interfaces between the Ni–Mo alloy nanoparticles. Additional efforts to optimize the dispersion on carbon black supports resulted in mass activities exceeding 60 mA/mg (on the basis of Ni–Mo mass) at 100 mV overpotential. This was over 5-fold greater than we observed for activation by hydrogen annealing, and we postulate that it still represents a lower-bound estimate of the true activity of this catalyst.

We used in situ environmental transmission electron microscopy to image the formation of a Ni−Mo composite nanocatalyst via thermal reduction of NiMoO 4 nanorods. Two clear structural changes were observed as the temperature was increased from 25 to 500 °C in the presence of H 2 (g): the first involved nucleation of nanoscale Ni-rich particles and the second involved general collapse of the remaining oxide phase along with substantial coarsening of the alloy particles. The activity of the catalyst toward alkaline hydrogen evolution was found to reach a maximum in a narrow range of reduction temperatures from 375 to 425 °C. This resulted in the formation of a mixed-phase product comprising sub-10 nm Ni 0.9 Mo 0.1 particles embedded in a porous Mo-rich oxide matrix. Thus, the most active Ni−Mo catalyst apparently requires intimate contact between the alloy component and the oxide phase, lending support to a catalytic mechanism involving metallic and oxidized surface sites.

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We have studied the composition and morphology of Ni-Mo alloys. These alloys consist of a Ni-rich core surrounded by Mo-rich oxide layer. The HER activity of Ni-Mo alloys was seen to be limited by interfacial resistance rather than kinetic and solution transport. Vulcan carbon, a conductive support mitigate the resistive limitations by providing conductive percolation networks.