Chain-Length-Tunable Hydrophobic Poly(ionic liquid) Modulates the Microenvironment at Copper-Based Electrodes for Selective CO2 Electroreduction to Ethylene

Abstract By regulating the microenvironment at the catalyst interface, the mass transfer processes and the behavior of reaction intermediates in the electrochemical reduction of carbon dioxide (CO2RR) can be effectively optimized. However, achieving a balance between interfacial water management and reactant accessibility remains a challenge during the generation of C2 products catalyzed by copper-based catalysts. In this work, we employ an in situ polymerization approach to construct functional layers of hydrophobic poly(ionic liquid) (HPIL) with tunable alkyl chain lengths (C8, C12, C16) on the surface of CuO nanosheets (CuO NSs) to regulate their interfacial properties. The longer alkyl chain (C16) facilitates the formation of a dense superhydrophobic structure that, together with the cationic imidazolium groups, significantly suppresses interfacial water activity while retaining CO2-philic character, thereby creating a local reaction microenvironment conducive to C–C coupling. In conventional H-type electrolysis cells, the optimized Cu@P(VIM-16A-DVB) catalyst achieved an ethylene Faraday efficiency of 36% (compared to only 18% for the bare copper catalyst), while the hydrogen evolution reaction (HER) decreased to 34% (compared to 47% for the bare copper catalyst). This reduction was accompanied by a decrease in the Faraday efficiency of C1 products (carbon monoxide and formic acid). This indicates a shift in the system from multiple coexisting products toward a more concentrated ethylene-dominant pathway. Notably, we have found that excessively long alkyl chains induce overly dense hydrophobic structures that impair reactant accessibility and increase mass transfer resistance. This work establishes an interfacial design principle: the poly(ionic liquid) functional layer requires tailored wettability to guide the copper-based CO2RR toward C2 product formation. In summary, precisely engineering the local reaction microenvironment by tuning the alkyl chain length of the HPIL functional layer represents an effective strategy, offering universal insights for suppressing side reactions in aqueous electrolytes.