Key role of electron accessibility at the noble metal-free catalytic interface in hydrogen evolution reaction

The reactant concentration at the catalytic interface holds the key to the activity of electrocatalytic hydrogen evolution reaction (HER), mainly referring to the capacity of adsorbing hydrogen and electron accessibility. With hydrogen adsorption free energy (ΔG_H) as a reactivity descriptor, the volcano curve based on Sabatier principle is established to evaluate the hydrogen evolution activity of catalysts. However, the role of electron as reactant received insufficient attention, especially for noble metal-free compound catalysts with poor conductivity, leading to cognitive gap between electronic conductivity and apparent catalytic activity. Herein we successfully construct a series of catalyst models with gradient conductivities by regulating molybdenum disulfide (MoS₂) electronic bandgap via a simple solvothermal method. We demonstrate that the conductivity of catalysts greatly affects the overall catalytic activity. We further elucidate the key role of intrinsic conductivity of catalyst towards water electrolysis, mainly concentrating on the electron transport from electrode to catalyst, the electron accumulation process at the catalyst layer, and the charge transfer progress from catalyst to reactant. Theoretical and experimental evidence demonstrates that, with the enhancement in electron accessibility at the catalytic interface, the dominant parameter governing overall HER activity gradually converts from electron accessibility to combination of electron accessibility and hydrogen adsorbing energy. Our results provide the insight from various perspective for developing noble metal-free catalysts in electrocatalysis beyond HER.