Poly(Ionic Liquid)-Engineered Interfacial Water Reorganization for Selective Acidic CO2-to-Formic Acid Electroreduction

Acidic CO2 electroreduction offers a promising route to improve carbon utilization compared with alkaline or neutral systems. However, maintaining high product selectivity at industrially relevant current densities remains challenging because local CO2 depletion and excessive proton availability intensify the competing hydrogen evolution reaction (HER). Herein, we develop a bismuth sulfide (Bi2S3) catalyst modified with a long-chain imidazolium poly(ionic liquid) layer (PIL@BS) for electrocatalytic CO2-to-formic acid conversion at pH 2. In a flow cell containing 3 M KCl, PIL@BS catalyst delivers a high Faradaic efficiency for formic acid (FEFA = 98.4%) at a current density exceeding 300 mA cm–2 and maintains over 80% FEFA after 120 h of continuous operation. Even at low 15 vol % CO2 concentration typical of industrial flue gas, PIL@BS retains higher FA selectivity than unmodified BS, underscoring the effectiveness of PIL in suppressing HER under CO2-limited conditions. Structural characterizations and electrochemical analyses reveal that the PIL backbone creates a hydrophobic interface while preserving CO2 affinity and high ionic conductivity, effectively mitigating the flooding issues and enhancing the local availability of CO2. Operando spectroscopies further show that PIL-regulated interface facilitates access to weakly hydrogen-bonded water. Such interfacial water reorganization disrupts the extended hydrogen-bond network, attenuating the continuous proton-relay pathways that promote the competing HER while maintaining localized proton transfer for CO2 reduction. Combined with theoretical calculations, these findings indicate that the PIL-modified microenvironment enables a direct and energetically favorable hydrogenation pathway toward FA formation. This work highlights the PIL-engineered interfacial microenvironment as a key factor for overriding the intrinsic selectivity of a catalyst and enabling practical CO2 valorization.