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.