Electronic Supplementary Materials including models and fitting of the electrochemical impedance spectroscopy, XRD, XPS, EDX spectra, and polarization curves. from Enhanced hydrogen evolution reaction on hybrids of cobalt phosphide and molybdenum phosphide

Production of hydrogen from water electrolysis has stimulated the search of sustainable electrocatalysts as possible alternatives. Recently, cobalt phosphide (CoP) and molybdenum phosphide (MoP) received great attention due to their superior catalytic activity and stability towards the hydrogen evolution reaction (HER) which rivals platinum catalysts. In this study, we synthesize and study a series of catalysts based on hybrids of CoP and MoP with different Co/Mo ratio. The HER activity shows a volcano shape and reaches a maximum for Co/Mo = 1. Tafel analysis indicates a change in the dominating step of Volmer–Hyrovský mechanism. Interestingly, X-ray diffraction patterns confirmed a major ternary interstitial hexagonal CoMoP<sub>2</sub> crystal phase is formed which enhances the electrochemical activity.