Min Liu, Yuke Li, Jianan Erick Huang, Xingyuan Chu, Qian Sun, Feng Li, Yurou Celine Xiao, Mengyang Fan, Chengqian Wu, Zhizheng Wu, Qiyou Wang, Jiexin Zhu, Zunmin Guo, Xiaodong Li, Sekar Wibawa, Panagiotis Papangelakis, Loann Bonnenfant, Hyun Sik Moon, Cao‐Thang Dinh, Rui Kai Miao, Edward H. Sargent, David Sinton
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.