Geometry-Enabled Hydrogen Bonding Alignment Dictates CO 2 Electroreduction Kinetics on Gold Facets

Electrocatalytic carbon dioxide reduction (CO2RR) presents a compelling strategy for simultaneously storing renewable electrical energy and valorizing emitted CO2. Surface crystallographic orientation is a common yet highly effective factor influencing CO2RR catalytic activity. The mechanism has been primarily rationalized in terms of surface−intermediate interactions, whereas the facet-dependent modulation of the electric double layers has remained largely unexplored. In this work, we broaden the mechanistic origin of facet effects to encompass the regulation of surface−adsorbate−interfacial water interactions, using the CO2-to-CO conversion on Au(110) and Au(111) as a model system. Constant-potential ab initio molecular dynamics simulations, combined with a slow-growth method, reveal that facet-dependent reactivity originates from distinct adsorbate geometries enabled by surface structure. These geometries, in turn, dictate the orientation of hydrogen bonding with interfacial water, thereby altering the activation barriers of elementary steps. The pronounced kinetic difference between Au(110) and Au(111) occurs in the *COOH hydrogenation step. On the more open Au(110) facet, the −OH moiety in *COOH possesses greater configurational freedom, allowing it to form hydrogen bonds with interfacial water at an angle analogous to the intrinsic bond angle of water molecules. This geometric compatibility enables a more seamless progression toward the transition state, requiring minimal structural rearrangement and thereby facilitating the hydrogenation step. Together, these findings establish an atomic-level framework where the synergistic coupling among facet structure, adsorbate geometry, and interfacial water organization collectively governs CO2RR activity.