Unveiling the Cation Dependence in Alkaline Hydrogen Evolution by Differently-Charged Ruthenium/Molybdenum Sulfide Hybrids

Abstract

The sluggish kinetics of hydrogen evolution reaction (HER) via water reduction limits the efficiency of alkaline water electrolysis. The HER kinetics is not only intimately related to the catalyst surface structure but also relevant to the cation identity of the electrolyte. The cation dependence also relies on the surface electronic structure and applied potential, but this interrelated effect and its underlying mechanism awaits elucidation. Herein, differently-charged molybdenum sulfide (MoS_x) cluster supports ([Mo₃S₁₃]²⁻ and [Mo₃S₇]⁴⁺) are utilized to hybridize with the identical metallic Ru centers. The specific electrostatic interaction between MoS_x clusters and Ru precursors induces different Ru valences of the hybrids, with a higher valence state for Ru/Mo₃S₁₃ endowing a higher activity. The Ru/Mo₃S₁₃ and Ru/Mo₃S₇ exhibited drastically-different cation dependence, in which the charged support determines the local accumulation of cations and resulting water structures. The more negatively-charged Mo₃S₁₃ support induces the facile accumulation of cations, especially for less-hydrated K⁺ cations. The water activation capability by Ru valences and cation accumulation from the support effect in-together determine the cation-dependent alkaline HER activity. This work not only enriches the understanding about the cation-dependent HER mechanism but also shines a light on the rational optimization strategy of electrode/electrolyte interfaces.