Efficient Acidic CO 2 Electrolysis with Suppressed Crossover in a Separator-Based Membrane Electrode Assembly

Performing acidic electrochemical CO2 reduction (CO2R) in flow cells suppresses CO2 crossover but requires thick catholyte layers that impose large ohmic losses. Removing the catholyte, however, shifts selectivity toward H2 due to excessive proton (H+) transport through cation exchange membranes (CEM). Here, we present a zero-gap membrane electrode assembly (MEA) incorporating an ∼100 μm electrolyte-filled hydrophilic porous separator. The separator uniquely enables zero-gap operation by regulating coupled H+ and K+ transport; however, at high potassium ion (K+) concentrations, stronger ion pairing between K+ and (bi)carbonates suppresses their protonation at the cathode, thereby increasing CO2 crossover. In contrast, a higher H+ to K+ ratio (2.4 M/0.2 M) establishes an H+-enriched yet K+-stabilized interface that promotes C2+ production while suppressing crossover. Controlling electrolyte permeance (∼1.5-3 mL h-1 cm-2) further limits (bi)carbonate electromigration. Using this approach, we reduce CO2 crossover to 0.19 sccm A-1 (∼5% of the CO2 converted to products), while achieving 75% multicarbon (C2+) Faradaic efficiency and 24% energy efficiency at 3.5 V (300 mA cm-2) using a 7,7,8,8-tetracyanoquinodimethane-modified copper oxide catalyst. This work demonstrates efficient acidic CO2 electrolysis with suppressed crossover using a separator-based MEA with copper catalysts.