The Charge State of Adsorbed H*: A Missing Kinetic Descriptor for Hydrogen Evolution Reaction

The free energy of hydrogen adsorption (ΔGH*) is widely used as a descriptor for the hydrogen evolution reaction (HER), yet its exclusive focus on thermodynamics may fail to capture reaction kinetics, thereby limiting its predictive power. Here, taking the experimentally debated active sites for HER on the MoS2 basal plane─sulfur vacancies versus oxygen-substituted sulfur sites─as a model system, we reveal through grand canonical density functional theory (GC-DFT) calculations that the charge state of adsorbed H* (ΔQH*) is a key factor governing HER kinetics. Systematic validation across diverse catalysts further confirms the generality of ΔQH* as a descriptor of HER kinetics. Guided by ΔQH*, we unveil an H2 formation pathway on MoS2 edges that involves dynamic charge switching during H* diffusion, which helps to reconcile theoretical predictions with experimental observations. By integrating ΔQH* with ΔGH*, we establish an effective dual-descriptor strategy for reliable catalyst screening, which identifies graphene-supported PtC3 and OsC4 single-atom catalysts as promising HER candidates. This work introduces ΔQH* as a missing kinetic descriptor for the HER, providing a robust principle for the rational design of HER catalysts.