Spectator Anion Accelerates Electrocatalytic C–H Dissociation by Modulating the Local Hydrogen Bonding Interaction

Solvent dynamics at solid–liquid interfaces are closely coupled with the configuration and reactivity of electrocatalytic intermediates, endowing tunable activity and pathway selectivity to electrochemical processes. Yet, direct experimental evidence linking interfacial solvent structures to intermediate configurations remains scarce. Propylene oxidation, a key reaction in both thermal and electrocatalysis, offers an ideal platform to probe such solvent-mediated effects. Here, we combine electrochemical measurements, in situ infrared spectroscopy, and ab initio molecular dynamics simulations to investigate propylene oxidation on atomically flat Au single-crystal surfaces. Spectroscopic analysis directly reveals that hydrogen-bond networks of interfacial water regulate the configuration of reaction intermediates, which, in turn, modulates C–H bond activation kinetics. Notably, we identify an anomalous anion effect, in which specifically adsorbed sulfate on Au(111) promotes rather than inhibits electrocatalytic oxidation, challenging the conventional view of anions as passive spectators and suggesting innovative strategies for interface engineering in aqueous electrocatalysis.