Overlooked Role of Electrostatic Interactions in HER Kinetics on MXenes: Beyond the Conventional Descriptor ΔG ∼ 0 to Identify the Real Active Site.

Understanding the atomic-level mechanism of the hydrogen evolution reaction (HER) on MXene materials is crucial for developing affordable HER catalysts, while their complex surface terminations present a substantial challenge. Herein, employing constant-potential grand canonical density functional theory calculations, we elucidate the reaction kinetics of the HER on MXenes with various surface terminations by taking experimentally reported Mo₂C as a prototype. We observe a contradictory scenario on Mo₂C MXene when using conventional thermodynamic descriptor ΔG_H* (hydrogen binding energy). Both competing surface phases that emerge close to the equilibrium potential meet the ΔG_H* ∼ 0 criterion, while they exhibit distinctly different reaction kinetics. Contrary to previous studies that identified surface *O species as active sites, our research reveals that these *O sites are kinetically inert for producing H₂ but are easily reduced to H₂O. Consequently, the surface Mo atoms, exposed from the rapid reduction of the surface *O species, serve as the actual active sites catalyzing the HER via the Volmer-Heyrovsky mechanism, as confirmed by experimental studies. Our findings highlight the overlooked role of electrostatic repulsion in HER kinetics, a factor not captured by thermodynamic descriptor ΔG_H*. This work provides new insights into the HER mechanism and emphasizes the importance of kinetic investigations for a comprehensive understanding of the HER.