Theoretical insights into Ru single-atom modulation of OER activity on Ni-LDHs: roles of hydroxyl coordination and interfacial water.

Single-atom (SA) electrocatalysts anchored on layered double hydroxides (LDHs) can deliver outstanding oxygen evolution reaction (OER) activity in water splitting. Nevertheless, the evolution of LDH-supported single atoms and their associated interfacial microenvironments remains incompletely understood from both thermodynamic and kinetic perspectives. Herein, Ru@SA supported on Ni-LDHs is selected as a representative model system to address the aforementioned issues. Under vacuum conditions, increasing the number of coordinated *OH ligands elevates the valence state of Ru@SA and concomitantly enhances the OER activity. The Ru-2OH configuration is identified as optimal, as it achieves a comparatively high oxidation state while avoiding excessive steric congestion. Under aqueous conditions, *H species generated via H2O dissociation coordinate to Ru@SA, thereby inducing reconstruction of the local microenvironment and proposing a plausible pathway for *OOH formation. Specifically, *O cannot be stably sustained in solution, and *OH reacts with interfacial H2O to directly form *OOH via four consecutive stages: (i) departure of H2O from the interfacial water layer and formation of an O-H-O motif with *OH, (ii) reorientation of the H2O molecule, (iii) O-O coupling, and (iv) removal of two protons. In addition, *OH coordination facilitates the reorientation of interfacial water from an H-up to an H-down configuration, thereby promoting *OOH formation. In summary, rational control over the number of coordinated *OH ligands enables modulation of the interfacial water by attenuating the hydrogen bond network and lowering proton-transfer barriers, thereby accelerating the reaction kinetics.