The electrocatalytic CO₂ reduction reaction (CO₂RR) using renewable electricity to produce multi-carbon (C₂₊) compounds, in particular ethylene and ethanol, which are widely used in the current chemical and energy industries, is one of the most promising routes to recycle CO₂ and reduce carbon emission.  Among different electrolysers, the zero-gap membrane electrode assembly (MEA), which has been successfully employed for the polymer electrolyte membrane-based water electrolysis and fuel cells, holds great potential as an energy-efficient and scalable device for CO₂ electrolysis. In the MEA electrolyser for CO₂RR, the cathode catalyst is directly pressed onto a membrane without a catholyte, but the lack of the catholyte solution makes the management of water, which is the proton source and is essential to CO₂RR, in the MEA electrolyser more crucial than in the conventional flow cell. In MEA, the H₂O molecules needed for the cathode reaction usually comes from the anolyte by permeation through the membrane and the problem of H₂O deficiency may occur at a high current density, worsening the CO₂RR performance. However, only very few studies have been devoted to H₂O management for MEA-based CO₂RR to C₂₊ compounds. The cathode catalyst is expected to participate in the activation of H₂O co-fed with CO₂, but the role of the electrocatalyst in H₂O management in CO₂RR is under explored.

Actually, the lack of catholyte as well as useful insights for the design of cathode catalyst for efficient water management has resulted in limited C₂₊ formation performances of the MEA-based CO₂RR. The current density to attain a C₂₊ Faradaic efficiency of ≥ 80% is usually lower than 400 mA cm^-2, regardless of using a cathode-dry or H₂O vapour-feeding MEA system (Supplementary Table 1).  It remains challenging to achieve simultaneously a high current density and a high C₂₊ FE in the full-cell MEA system.

Here, we report our discovery of a suitable electrocatalyst in fulfilling the role of co-feeding H₂O in enhancing formation of C₂₊ compounds during CO₂RR in the MEA system. We demonstrate that the co-feeding of H₂O has no effect on C₂₊ formation over a Cu⁰ nanoparticle catalyst, but a nanocomposite containing abundant Cu₂O−Cu⁰ interfaces shows significantly positive effect of co-feeding H₂O on CO₂RR to C₂₊ compounds. Under an optimized H₂O pressure, we achieve a current density of 1.0 A cm⁻² at 80% FE of C₂₊ compounds (mainly C₂H₄ and C₂H₅OH) in the full-cell MEA electrolyser. The molar carbon-based selectivity and yield of C₂₊ compounds (C_2-3 olefins and oxygenates) reach 83% and 19%, respectively, better than those attained in thermocatalytic systems for CO₂ hydrogenation under harsher reaction conditions. Our operando spectroscopic characterizations confirm the presence of Cu⁺ in the nanocomposite during CO₂RR operated at an ampere-level current density in MEA. We propose an in situ re-oxidation mechanism for the formation of Cu⁺ sites. The catalyst with Cu⁺ sites promotes the activation of H₂O, resulting in enhanced formation of adsorbed CO and CHO intermediates for C−C coupling, and thus showed significant promoting effect of co-feeding H₂O for C₂₊ formation. This work paves the way for constructing an efficient MEA system for CO₂RR to C₂₊ compounds.