Abstract
<jats:p> Electrochemical CO <jats:sub>2</jats:sub> reduction in gas-fed, zero-gap electrolyzers typically relies on alkali cations to enhance selectivity, stabilize key intermediates, and structure the electric double layer (EDL). However, supplying cations via liquid electrolytes can lead to instability due to flooding and salt precipitation. Here, we investigate whether fixed-charge ionomers in the cathode catalyst layer can replicate these beneficial effects in an electrolyte-free configuration. </jats:p> <jats:p>Using a fully gas-fed cell, we control the cathode ionic environment through initial membrane equilibration with a salt solution. Even in the absence of continuous cation feed, performance remains strongly dependent on cation presence: trace potassium levels introduced by pretreatment measurably influence selectivity and overpotential, confirming that cations accumulate at the cathode and shape the EDL.</jats:p> <jats:p> Varying ionomer nature, high-IEC ionomers (3.55 meq g <jats:sup>–1</jats:sup> ) significantly improve durability, reduce cathode overpotential, and increase pseudocapacitance by an order of magnitude compared to low-IEC systems. These results indicate that, in electrolyte-free configurations, the ionomer in the catalyst layer plays a crucial role in shaping the interfacial environment and controlling reaction kinetics. This provides a foundation for designing durable, electrolyte-free CO <jats:sub>2</jats:sub> electrolysis devices using fixed-charge catalyst layers. </jats:p>