Surface-immobilized cross-linking tetraalkylammonium cations networks mitigate hydrogen evolution for pure acidic CO2 reduction in proton-exchange membrane electrolyzers

The scaling-up of electrochemical CO₂ reduction requires circumventing the CO₂ loss as carbonates under alkaline conditions. Zero-gap MEA cell configurations with a proton exchange membrane represent an alternative solution in a pure acidic system, but the catalyst layer in direct contact with the hydrated proton environment usually leads to H₂ evolution dominating. Herein, we show that polydimethyldiallyl-ammonium-chloride-coated Ag (Ag@PDDA) electrode exhibits outstanding performance with a FE of 86 %, a single-pass conversion of 72 %, and a stability of 28 h for CO production in pure-acid MEA compared with ammonium poly(N-methyl-piperidine-co-pterphenyl) decorated Ag (Ag/QAPPT) and cetyltrimethylammonium bromide decorated Ag (Ag/CTAB). The in situ ATR-SEIRAS reveal that PDDA creates a positive charge-rich protective outer layer and an N-rich hybrid inner layer, which not only suppresses the migration of H⁺ during the electrolysis process and blocks the direct contact between H₂O and Ag catalyst, but also promotes the generation from CO₂ to *COOH in a pure-acid system. This work highlights the importance of polyelectrolyte engineering in regulating the electrocatalytic interface and accelerates the development of proton exchange membrane CO₂ electrolysis.