Theoretical Study of Transition-Metal-Modified Mo₂ CO₂ MXene as a Catalyst for the Hydrogen Evolution Reaction

The 2D MXenes have attracted great recent attention as the electrocatalytic materials for hydrogen evolution reaction (HER). However, the activity and the modification strategy of the catalytic properties have not been firmly established yet. In this study, we performed density functional theory (DFT) calculations to investigate the stability and HER performance of functionalized Mo₂ C MXene. The Pourbaix diagram indicates the fully oxidized surface is the most stable state. The oxidized Mo₂ CO₂ is electrically conductive, yet the surface HER activity is unsatisfactory owing to the strong first H adsorption. The doping of transition metals (TM) into the Mo lattice, however, leads to much more enhanced H adsorption and deteriorates the activity. Alternatively, the H binding can be effectively weakened and flexibly tuned by anchoring the TM atoms over the surface with appropriate coverage, and Mn/Fe decoration at 12.5 % ML (monolayer) coverage is identified as the promising candidates with close to zero Gibbs free energy of H adsorption (ΔG_H* ) for the first H adsorption. The weakening effect arises from charge transfer from TM to surface O, resulting in increased occupancy and weakened O-H bonds. Furthermore, contrary to the weakening effect, the tensile strain leads to enhanced O-H binding by the up-shifted Op electronic states, which can further modulate the HER performance of TM-modified Mo₂ CO₂ . The synergistic effect between TM modification and strain engineering offers beneficial advantages for the realization of efficient electrochemical HER, which can be applied to other MXenes for electronic and catalytic applications.