Charge-Density-Dependent Ionomer Effects in Pure-Water CO2 Electrolysis

Pure-water membrane electrode assemblies (MEAs) represent a promising strategy to eliminate (bi)carbonate salt precipitation for electrochemical CO2 reduction (CO2R). However, the absence of alkali metal cations fundamentally restricts CO2 activation and intermediate stabilization, thereby reducing catalytic performance. Here, we overcome this limitation by identifying the ionomer charge density as a key molecular-level parameter associated with pure-water CO2R. By comparing anion-exchange ionomers with varying charge characteristics, we reveal that high-charge-density ionomer (HCDI) introduces a favorable catalyst–membrane interfacial environment, enabling superior macroscopic CO2R performance. HCDI-enabled MEAs achieve >99% CO Faradaic efficiency at 100 mA cm–2 and sustain ∼92% at 1000 mA cm–2. In situ diagnosis and electrochemical characterization reveal that the HCDI-functionalized interface produces a substantial *CO signal, while concurrently suppressing hydrogen evolution. By decoupling cation effects from liquid electrolytes, these findings support interfacial ionomer charge density as a useful comparative parameter for pure-water CO2 MEA electrolyzers.