How Buffer Ions Shape Hydrogen Evolution at Non-extreme pH: Operando Insights into Cu-Mo Electrocatalyst Surface...

Non-extreme pH conditions enable the use of abundant elements, attracting attention for reducing electrolyzer costs. However, electrolyte engineering─specifically, optimizing buffer ion identities and concentrations─is crucial for achieving high-efficiency water electrolysis. This study focuses on the HER in a carbonate electrolyte at pH 10.5, where the participation of bicarbonate anions (HCO₃^-) in the process is essential. The overpotentials of the HER on copper-molybdenum oxide (MoO_x(Cu)) electrocatalyst were uniquely reduced by adding phosphate anions to the K-carbonate electrolyte from the low current density region to -1 A cm^-2. Electrochemical tests suggested that near-surface electrolyte cations have a significant role in facilitating the HER, which is favorable with lower amounts of hydrated cations (i.e., K⁺ and Cs⁺). HER improvement was not observed with borate and sulfate anions, highlighting the crucial role of the phosphate anion. The enhancement of HER by the phosphate additive was not observed in the absence of Cu, suggesting unique interactions among Cu, phosphate, and weakly hydrated cations. The presence of phosphate increases the double-layer capacitance, likely by inducing positive surface reconstruction for HER through Cu-phosphate interaction. Operando Raman, infrared (IR), and X-ray absorption spectroscopies consistently suggested the presence of phosphate anion adsorbates on the MoO_x(Cu) surface, which in turn attract more cations and reactant HCO₃^- to the reaction interface and boost the HER using HCO₃^- as the proton source. Density functional theory (DFT) calculations indicate the strong adsorption of phosphate species on Cu, which is associated with enhanced interaction of K⁺ with the phosphate-modified surface. These findings provide insight into electrolyte engineering, highlighting how interfacial cation-anion organization driven by adsorption may influence the rate of electrochemical reactions involving ionic reactants.