Interfacial Cations Modulate CO2 Reduction Selectivity by Shaping Hydrogen-Bond Networks and Water Orientation.

Electrochemical CO2 reduction (ECR) is highly sensitive to the electrode-electrolyte interfacial microenvironment (EEIM). However, how the hydration structures of alkali-metal cations reorganize EEIM and thereby influence the energetics of ECR and hydrogen evolution reaction (HER) related elementary steps remains unclear. Here, explicit-solvent ab initio molecular dynamics simulations were used to elucidate the cation-dependent restructuring of interfacial water. Specifically, cation-specific hydration governs the spatial distribution, hydrogen-bond connectivity, and orientation of interfacial water. Strongly hydrated Li+ maintains a highly connected hydrogen-bond network, lowers the activation barrier for Volmer water activation, and facilitates the removal of the generated OH- from the interface. In contrast, weakly hydrated K+ creates a broader water-depletion region near the electrode, disrupts hydrogen-bond connectivity, and promotes a more pronounced H-down orientation of interfacial water. This reconstructed interface increases the accessibility of water hydrogen atoms to adsorbed *CO2, thereby creating an EEIM that favors ECR. Moreover, weakly hydrogen-bonded, K+ coordinated water provides a more energetically favorable proton-donation environment than water embedded in a strongly connected hydrogen-bond network, thereby lowering the activation barrier for *COOH formation. These findings establish a molecular-level link among cation hydration strength, interfacial water restructuring, and reaction selectivity, providing a mechanistic basis for tuning electrochemical interfacial microenvironments through cation engineering.