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

Performing acidic electrochemical CO 2 reduction (CO 2 R) in flow cells suppresses CO 2 crossover but requires thick catholyte layers that impose large ohmic losses. Removing the catholyte, however, shifts selectivity toward H 2 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 CO 2 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 C 2+ 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 CO 2 crossover to 0.19 sccm A –1 (∼5% of the CO 2 converted to products), while achieving 75% multicarbon (C 2+ ) 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 CO 2 electrolysis with suppressed crossover using a separator-based MEA with copper catalysts.