External-Field-Enhanced Helmholtz-Layer Local Charge Density Enables C─C Coupling in Pure-H₂O-Fed CO₂...

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.