Probing the Effect of Spectator Anions on Chloride Adsorption for Selective Oxygen Evolution Reaction over Ni Catalysts

Water electrolysis is a promising route to green hydrogen production, but its operation in chloride‐containing electrolytes (e.g., seawater) is hindered by the competing chlorine evolution reaction (CER) that lowers oxygen evolution reaction (OER) selectivity and accelerates catalyst degradation. Here, we use a combination of electrochemical quartz crystal microbalance (EQCM) and operando Surface‐Enhanced Raman spectroscopy (SERS) to directly probe chloride adsorption on Ni‐based catalysts. Our study reveals that chloride ions (Cl − ) adsorb on the Ni surface even at low potentials where Ni(OH) 2 is the predominant phase, and that this adsorption intensifies on high‐valent Ni OOH during OER, leading to hypochlorite (OCl − ) formation significantly reduce OER selectivity and catalyst stability. Importantly, introducing spectator anions such as CO 3 2− , SO 4 2− , or NO 3 − suppresses Cl − adsorption. Among these, CO 3 2− binds strongly to Ni sites and inhibits both Cl − and OH − adsorption, whereas SO 4 2− and NO 3 − , with their weaker binding, preferentially block Cl − while still allowing OH − adsorption. As a result, OCl − generation is dramatically decreased even under locally acidic conditions caused by high OER current densities, thereby enhancing catalyst activity and stability by selectively favoring OER over CER. This study highlights the utility of combined EQCM–SERS analysis to unravel interfacial adsorption processes in complex electrolytes like seawater splitting and provides new insights into leveraging adsorption preferences of spectator anions.