Engineering the Ionomer-Mediated Microenvironment in CO2 Electrolysis: Effects on Efficiency and Degradation

Abstract As CO2 electrolysis advances toward industrially relevant zero-gap architectures, reaction microenvironments consisting of polymeric ionomers have emerged as critical yet underappreciated components governing device performance. Beyond their conventional role as ion conductors and binders, ionomers actively define the catalyst microenvironment by regulating the local pH, cation distribution, interfacial water structure, and CO2 transport. This itself imparts a noticeable effect on activity, selectivity, and stability. Significant progress has been made in using ionomer chemistry for catalyst microenvironment engineering. Therefore, the chemical and electrochemical stability under realistic operating conditions-characterized by high current densities, extreme local alkalinity, reactive intermediates, and strong electric fields-needs to be well understood. In this perspective, we establish a unified framework that connects ionomer-driven microenvironment modulation with degradation pathways in CO2 electrolyzers. We critically analyze how ionomer structure, hydrophobicity, and interfacial interactions govern both catalytic performance and long-term durability. The focus would be on highlighting fundamental trade-offs between activity enhancement and chemical stability. Furthermore, we discuss the emerging needs for standardized accelerated stress testing (AST) protocols and propose strategies to systematically probe microenvironment-driven degradation mechanisms. By integrating insights from mature technologies like fuel cells and water electrolyzers and recent advances in CO2 electroreduction, this work positions ionomer design as a central lever for achieving durable, high-performance CO2 electrolyzers.