Ab Initio Investigation of Synergistic Solvation and Cation Effects in Mo₂C MXene for Enhanced C–C Coupling in CO₂ Electroreduction

The electrochemical CO2 reduction reaction (CO2RR) to multicarbon (C2+) products is strongly influenced by the electrolyte–catalyst interface, yet the atomistic mechanisms underlying solvent–cation synergies remain poorly understood. Herein, we employ constrained ab initio molecular dynamics (AIMD) and slow-growth sampling to unravel the cooperative role of interfacial water molecules and alkali cations in promoting C–C coupling on Mo2C MXene surfaces. Our results underscore a remarkable enhancement in *CO-*CO dimerization kinetics under solvated conditions compared to that under vacuum, attributed to the stabilization of key transition states and *OCCO intermediates via specific hydrogen-bonding and electrostatic interactions. Furthermore, the solvation microenvironment promotes charge retention at the adsorbate–catalyst interface, effectively suppressing competing hydrogen evolution. Most notably, we identify that larger cations (Na+, Cs+) induce localized hydrophobicity and reduce the C–C coupling barrier by up to 0.23 eV, with Cs+ exhibiting the greatest enhancement by maintaining an optimal separation distance of 3–4.25 Å from the reaction intermediates. This work provides fundamental insights into electrolyte engineering strategies for steering the selective CO2RR toward C2+ products.