Electrochemical Hydrogen Evolution at Ordered Mo₇Ni₇

Ni-Mo alloys containing up to ∼15 mol % Mo are excellent non-noble electrocatalysts for the hydrogen evolution reaction (HER) in alkaline aqueous electrolytes. To date, studies have not addressed the details of HER activity of ordered Ni-Mo intermetallic compounds, which can contain a significantly larger fraction of Mo (up to 50 mol %) than can be accessed through high-temperature alloying. Here, we present a straightforward and facile synthesis of three phase-pure electrocatalyst powders using a precipitation-reduction approach: ordered Mo₇Ni₇, disordered Ni_0.92Mo_0.08, and pure Ni. The Ni_0.92Mo_0.08 alloy exhibited a nearly 10-fold higher mass-specific HER activity than either pure Ni or Mo₇Ni₇, where much of the difference could be attributed to relative surface area. Therefore, we attempted to quantify and account for differences in surface areas using electron microscopy, impedance spectroscopy, and gas adsorption measurements. These data suggest that Ni-Mo alloys and intermetallic compounds exhibit substantial pseudocapacitance at potentials near the onset of hydrogen evolution, which can cause impedance spectroscopy to overestimate the interfacial capacitance, and thus the electrochemically active surface area, of these materials. From these observations, we postulate Mo redox activity as the chemical basis for the observed pseudocapacitance of Ni-Mo composites. Furthermore, using gas adsorption measurements, rather than capacitance, to estimate active surface area, we find that ordered Mo₇Ni₇ is more intrinsically active than the Ni_0.92Mo_0.08 alloy, implying that Mo₇Ni₇ intermetallics with high surface area will also give higher mass-specific activities than alloys with comparable roughness. (Graph Presented).