External-Field-Enhanced Helmholtz-Layer Local Charge Density Enables C─C Coupling in Pure-H2O-Fed CO2 Electroreduction To C2+ Products.

Electrochemical CO2 reduction (ECO2R) 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-H2O-fed ECO2R system. By introducing a cationic organic ionomer (QAS) onto the Cu2O surface, an interfacial external field is established, which amplifies Helmholtz-layer charge density, suppresses hydronium accumulation and the hydrogen evolution reaction (HER), and accelerates ECO2R kinetics. The optimized Cu2O/QAS electrode delivers a C2+ Faradaic efficiency (FE) of ∼85% at 400 mA cm-2 in an alkaline flow cell, with a C2+/C1 ratio of ∼6.8, representing a 3.4-fold enhancement over pristine Cu2O. Notably, a high C2+ FE of ∼60% is retained even in acidic flow cells. To meet industrial requirements, a pure-H2O-fed membrane-electrode-assembly (MEA) cell is constructed, achieving ∼62% C2+ 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 C2+ selectivity and activity.