Tuned interfacial hydrogen bonds for enhanced H₂ electrocatalysis kinetics on Ti₂C MXenes

The kinetics of proton-coupled electron transfer (PCET) in electrocatalytic hydrogen evolution reactions (HER) are profoundly influenced by the hydrogen-bond (H-bond) network structure within the electric double layer. Here, we elucidate the role of surface terminations in modulating PCET kinetics on Ti₂CT₂ (T = O, OH) MXenes under acidic conditions, employing our in-house developed constant-potential constrained molecular dynamics approach. Our simulations reveal that mixed O-/OH-terminated Ti₂C MXenes exhibit significantly enhanced HER kinetics compared to pure O-terminated surfaces. By analyzing the interfacial water structure, we demonstrate that OH terminations shorten the basal water-MXene distance and reorganize the electric double layer, transforming fragmented H-bond networks into continuous ones that facilitate efficient proton transfer. This structural arrangement facilitates hydronium ion (H₃O⁺) dehydration while stabilizing critical reaction intermediates, particularly Zundel cations (H₅ $O_2^{+}$ ), thereby reducing the activation energy barriers for both the Volmer and Heyrovsky steps. Crucially, we establish a mechanistic link between H-bond connectivity and PCET kinetics - a framework extending beyond the conventional ΔG_H descriptor. Moreover, our methodology demonstrates how dynamic interfacial processes can be rigorously captured, paving the way for more accurate simulations of electrified interfaces.