Covalently Anchored Cationic Groups Tailor Electric Double Layer for Supporting-Electrolyte-Free CO2 Reduction in Acidic Media.

Electrochemical CO2 reduction reaction (CO2RR) in acidic media without supporting electrolytes-including dissolved alkali salts or cationic polyelectrolytes coated on the catalyst surface-remains highly challenging. A promising strategy is to covalently anchor cationic groups near catalytic sites, generating local electric fields to stabilize polar intermediates. Here, we employed carbon nanotube (CNT)-immobilized cobalt tetraphenylporphyrin (CoTPP) derivatives as model catalysts, functionalized with quaternary ammonium groups at distinct positions of the phenyl rings. Using vibrational Stark and Tafel analyses, we probed how these cationic groups shape the electric double-layer and CO2RR performance. When anchored at ortho-sites, the cationic groups induced a strong local electric field near the Co site, achieving a Faradaic efficiency (FE) up to 94% in pure acidic electrolyte. These cationic groups at the ortho-sites demonstrated greater ability to promote acidic CO2RR compared to alkali cations or cationic polyelectrolyte modification layers. This enhancement is attributed to the covalent anchoring approach, which enables the creation of a higher cation density around the catalytic site, thereby more substantially increasing the local electric field strength.