External‐Field‐Enhanced Helmholtz‐Layer Local Charge Density Enables C─C Coupling in Pure‐H ₂ O‐Fed CO ₂ Electroreduction To C ₂₊ Products

Electrochemical CO 2 reduction (ECO 2 R) is a promising decarbonization technology but is limited by the trade‐off between catalytic performance and system stability. Here, we present an external‐field‐assisted strategy to enhance the local charge density of the Helmholtz layer, thereby promoting C─C coupling in a pure‐H 2 O‐fed ECO 2 R system. By introducing a cationic organic ionomer (QAS) onto the Cu 2 O surface, an interfacial external field is established, which amplifies Helmholtz‐layer charge density, suppresses hydronium accumulation and the hydrogen evolution reaction (HER), and accelerates ECO 2 R kinetics. The optimized Cu 2 O/QAS electrode delivers a C 2+ Faradaic efficiency (FE) of ∼85% at 400 mA cm −2 in an alkaline flow cell, with a C 2+ /C 1 ratio of ∼6.8, representing a 3.4‐fold enhancement over pristine Cu 2 O. Notably, a high C 2+ FE of ∼60% is retained even in acidic flow cells. To meet industrial requirements, a pure‐H 2 O‐fed membrane‐electrode‐assembly (MEA) cell is constructed, achieving ∼62% C 2+ FE at 300 mA cm −2 and ∼4 V. Moreover, the scaled‐up MEA system demonstrates stable operation for over 100 h at 45 A and ∼176 W. In situ electrochemical analyses, operando spectroscopy, and theoretical calculations reveal that enhanced Helmholtz‐layer charge density stabilizes C─C coupling intermediates and lowers the thermodynamic barrier, enabling high C 2+ selectivity and activity.