Decoupling Product Selectivity in Electrocatalytic CO ₂ Reduction by Steering the Interfacial Water Structure

Achieving precise control over reaction pathways in the electrochemical CO₂ reduction reaction (CO₂RR) is a central challenge. Silver, for instance, is widely recognized for its high selectivity toward carbon monoxide (CO). Here, we demonstrate a strategy to steer the selectivity of Ag away from CO and toward formic acid (HCOOH) by engineering the nanoscale structure of water at the electrode-electrolyte interface. Using a polymeric cation, poly(diallyldimethylammonium chloride) (PDDA), in an alkali-metal-cation-free, strongly acidic electrolyte, we create a hydrophobic interfacial environment that promotes weakly hydrogen-bonded, "free-like" water (<i>f</i>-H₂O). Using operando spectroscopy and isotope labeling, we establish a direct, quantitative correlation between the abundance of <i>f</i>-H₂O and HCOOH selectivity. Electrochemical analyses and theoretical simulations using density functional theory and ab initio molecular dynamics suggest that the <i>f</i>-H₂O-rich environment opens a distinct mechanistic channel for HCOOH formation via a direct, energetically favorable *H + CO₂ hydrogenation reaction, a pathway disfavored in conventional alkali-metal-cation-based electrolytes where strongly hydrogen-bonded water (<i>h</i>-H₂O) facilitates the *COOH pathway to CO. These findings highlight that the tuning of interfacial water structure is powerful in overriding the intrinsic selectivity of a catalyst and rationally directing CO₂RR pathways.