Unraveling the mechanism of hydrogen evolution reaction on cobalt compound electrocatalysts

Highlights: • Co compounds (CoP, CoS and Co₃O₄) with similar morphology and uniform size are prepared. • The reaction mechanism and HER pathways of Co compounds are explored. • Origin of the large difference in HER activity of the Co compounds is revealed. • Effective descriptor ε_p-ε_d of Co compounds for HER activity is predicted. While many metal compounds have been widely studied as electrocatalysts for hydrogen evolution reaction (HER), the mechanism of HER on these materials is still not well understood nor why one compound is more electrocatalytically active than another. Here, we report our findings in synthesis and characterization of several Co compounds (CoP, CoS and Co₃O₄) with similar morphology and uniform size but dramatically different HER activities. Electrochemical measurements indicate that CoP has the best HER activity, achieving an overpotential of 80 mV at 10 mA cm⁻², which is much smaller than those for CoS and Co₃O₄. DFT-based computations suggest that electron delocalization resulting from the p-d orbits coupling at the surfaces facilitates HER charge transfer kinetics and an optimal balance between the surface adsorption of H atom and desorption of H₂. Both experimental and computational analyses reveal that the HER activity of the Co compounds is correlated closely with the energy gap between the anion p-band and the Co d-band centers, which can be used as an effective descriptor for HER activity. This work offers the scientific basis for rational design of more efficient metal compound electrocatalysts with high HER activity.