Switching CO ₂ electroreduction selectivity between CO and HCOOH on poly(ionic liquid)–Ag hybrids

The electrocatalytic conversion of CO₂ into CO and HCOOH is highly attractive due to its potentially high energy efficiency. In this study, selectivity switching from CO to HCOOH on Ag sites was achieved by finely fabricating poly(ionic liquid)-silver (PIL-Ag) hybrids containing alkyl chains. The length of the alkyl chain influenced the structure of Ag sites with different intrinsic activities of *COOH and *H (and thereby *OCHO) formation, as well as the mass transfer behavior of CO₂ and H₂O. During acidic CO₂RR at -300 mA cm^-2, Ag@PIL-C(4)-0.5 demonstrated a high faradaic efficiency ratio of HCOOH to CO (FE_HCOOH/FE_CO = 1.45), while Ag@PIL-C(6)-0.5 favored the production of CO. By contrast, Ag@PIL-C(12)-0.5, which possessed low activities and poor mass transfer, exhibited a high apparent FE_HCOOH/FE_CO ratio of 1.35. Experimental and mechanistic studies demonstrated that the higher enrichment of *H versus *CO₂ intermediates at the Ag sites is beneficial for *OCHO formation. In light of this point, the FE_HCOOH/FE_CO ratio increased from 0.15 to 0.87 with a low FE_{H₂} (<5.0%) on Ag@PIL-C(6)-2.0, which exhibited high activity and good mass transfer of CO₂ upon increasing the current density from -100 mA cm^-2 to -600 mA cm^-2.