AbstractElectrocatalytic CO₂ reduction at near-ambient temperatures requires a complex inventory of protons, hydroxyls, carbonate ions and alkali-metal ions at the cathode and anode to be managed, necessitating the use of ion-selective membranes to regulate pH. Anion-exchange membranes provide an alkaline environment, allowing CO₂ reduction at low cell voltages and suppression of hydrogen evolution while maintaining high conversion efficiencies. However, the local alkaline conditions and the presence of alkali cations lead to problematic carbonate formation and even precipitation. Here we report a pure-water-fed (alkali-cation-free) membrane–electrode–assembly system for CO₂ reduction to ethylene by integrating an anion-exchange membrane and a proton-exchange membrane at the cathode and anode side, respectively, under forward bias. This system effectively suppresses carbonate formation and prevents salt precipitation. A scaled-up electrolyser stack achieved over 1,000 h stability without CO₂ and electrolyte losses and with 50% Faradaic efficiency towards ethylene at a total current of 10 A.