Steric-Dominated Intermediate Stabilization by Organic Cations Enables Highly Selective CO₂ Electroreduction

Precisely modulating the binding energies of intermediates through cationic engineering remains a pivotal challenge in controlling reaction pathways and improving selectivity for electrocatalytic CO₂ reduction reaction (CO₂RR). Although alkali cations are widely recognized as stabilizing intermediates via electrostatic interactions, this study proposes a steric-effect-dominated strategy using organic quaternary ammonium cations (C_nTA⁺) to flexibly tune the adsorption of intermediates. Through in-situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS), we elucidate that C_nTA⁺ cations with longer alkyl chains substitute K⁺ more efficiently within the electrical double layer, exhibiting higher affinity for the Cu surface. This steric dominance significantly enhances the adsorption of key intermediates, steering the pathway toward formic acid (HCOOH) production. An improved Faradaic efficiency (FE) up to 90% for HCOOH was achieved using octadecyl trimethyl ammonium (C₁₈TA⁺) cations. This molecular engineering strategy provides a route to flexibly tune the adsorption of intermediates to improve the performance of CO₂RR.