Roles of copper(I) in water-promoted CO2 electrolysis to multi-carbon compounds.

The membrane electrode assembly (MEA) is promising for practical applications of the electrocatalytic CO₂ reduction reaction (CO₂RR) to multi-carbon (C₂₊) compounds. Water management is crucial in the MEA electrolyser without catholyte, but few studies have clarified whether the co-feeding water in cathode can enhance C₂₊ formation. Here, we report our discovery of pivotal roles of a suitable nanocomposite electrocatalyst with abundant Cu₂O-Cu⁰ interfaces in accomplishing water-promoting effect on C₂₊ formation, achieving a current density of 1.0 A cm^-2 and a 19% single-pass C₂₊ yield at 80% C₂₊ Faradaic efficiency in MEA. The operando characterizations confirm the co-existence of Cu⁺ with Cu⁰ during CO₂RR at ampere-level current densities. Our studies reveal that Cu⁺ works for water activation and aids C‒C coupling by enhancing formations of adsorbed CO and CHO species. This work offers a strategy to boost CO₂RR to C₂₊ compounds in industrial-relevant MEA by combining water management and electrocatalyst design.