Reaction microenvironment control in membrane electrode assemblies for CO2 electrolysis

CO₂ electrolysis is an emerging and promising carbon neutrality technology, but currently suffers from challenging selectivity issues at industrially relevant reaction rates. Selectivity control in CO₂ electrolysis relies on the molecular understanding and manipulation of multiple parallel reaction pathways that are equally governed by catalytically active sites and the reaction microenvironments in their vicinity. In this perspective, we summarize and discuss the latest achievements in reaction microenvironment control for active, selective, energy- and carbon-efficient CO₂ electrolysis, with particular attention being paid to that in membrane electrode assembly electrolyzers operating at industrial current densities (≥200 mA cm⁻²). The effects and underlying catalytic mechanisms of reaction microenvironments tailored by functional organic molecules/polymers and reactant feed compositions on the activity and selectivity of CO₂ electrolysis are discussed using selected examples. The efforts made to tailor acidic reaction microenvironments by controlling the transport of reactive species for carbon-efficient CO₂ electrolysis are also exemplified. Finally, we illustrate current challenges and future opportunities in the mechanistic understanding and rational design of reaction microenvironments for improving CO₂ electrolysis performance.