Polyelectrolyte Additive‐Modulated Interfacial Microenvironment Boosting CO₂ Electrolysis in Acid

Acidic CO₂ electrolysis offers a promising strategy to achieve high carbon utilization and high energy efficiency. However, challenges still remain in suppressing the competitive hydrogen evolution reaction (HER) and improving product selectivity. Although high concentrations of potassium ions (K⁺) can suppress HER and accelerate CO₂ reduction, they still inevitably suffer from salt precipitation problems. In this study, we demonstrate that the sulfonate-based polyelectrolyte, polystyrene sulfonate (PSS), enables to reconstruct the electrode-electrolyte interface to significantly enhance the acidic CO₂ electrolysis. Mechanistic studies reveal that PSS induces high local K⁺ concentrations through the electrostatic interaction between PSS anions and K⁺. In situ spectroscopy reveals that PSS reshapes the interfacial hydrogen-bond (H-bond) network, which is attributed to the H-bonds between PSS anions and hydrated proton, as well as the steric hindrance of the additive molecules. This greatly weakens proton transfer kinetics and leads to the suppression of undesirable HER. As a result, a Faradaic efficiency of 93.9 % for CO can be achieved at 250 mA cm^-2, simultaneous with a high single-pass carbon efficiency of 72.2 % on commercial Ag catalysts in acid. This study highlights the important role of the electrode-electrolyte interface induced by polyelectrolyte additives in promoting electrocatalytic reactions.