Ionomer-directed reaction pathway for CO₂ electroreduction

Electrochemical CO₂ reduction reaction (CO₂RR) is a sustainable method of converting carbon emissions into valuable fuels and chemicals; however, its selectivity and stability are governed by the catalyst itself and the local microenvironment within the catalyst layer. Ionomers, once considered to be passive binders, have emerged as key regulators of this microenvironment. However, the ionomer content in the catalyst layer is very low, because of which its effect on the CO₂RR performance is often ignored. This review aims to address this gap. First, the structural diversity of proton- and anion-exchange ionomers is discussed, and their distinct physicochemical properties, which are critical for shaping ion transport and interfacial reactions, are described. Next, the effects of solvent-controlled dispersion and film formation on the spatial distribution and morphology of ionomers in the catalyst layers and the subsequent effects on mass transport and local hydration are discussed. Based on this foundation, the control of CO₂RR pathways via the ionomer configuration is discussed, specifically considering the coupled effects on reactant diffusion, intermediate adsorption energetics, and interfacial hydrogen bond networks. Finally, the current challenges and future opportunities are summarized, with an emphasis on nanoscale characterization, ionomer durability, and the co-design of catalysts and ionomers. By elucidating the molecular-level connections between ionomer structure, dispersion, and catalytic function, this review establishes design principles that can guide the development of next-generation electrodes and electrolyzers for efficient, selective, and durable CO₂ conversion. Moreover, as demonstrated in this review, these principles are broadly applicable to a wide range of other electrocatalytic reactions.