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 (GC-DFT) calculations, we elucidate the reaction kinetics of 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 the conventional thermodynamic descriptor ΔGH* (hydrogen binding energy): both competing surface phases that emerge close to the equilibrium potential meet the ΔGH*~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 easily reduced to H2O. Consequently, the surface Mo atoms, exposed from the rapid reduction of the surface *O species, serve as the actual active site catalyzing HER via the Volmer-Heyrovsky mechanism, as confirmed by the experimental studies. Our findings highlight the overlooked role of electrostatic repulsion in HER kinetics, a factor not captured by the thermodynamic descriptor ΔGH*. This work provides new insights into the HER mechanism and emphasizes the importance of kinetic investigation for a comprehensive understanding of HER.