Origin of Hydrogen Incorporated into Ethylene during Electrochemical CO₂ Reduction in Membrane Electrode Assembly

A catholyte-free membrane electrode assembly (MEA) has been proposed for practical application in the electrochemical CO₂ reduction reaction (eCO₂RR), and water management becomes critical in its catalyst-membrane interface. We investigate roles of the water supply within the MEA for ethylene production by utilizing deuterium-labeled water. The protons of ethylene originated mainly from the anolyte not the humidified water through the cathode, indicating that dominant water flux from the anolyte acts as a major proton supplier for the eCO₂RR. Meanwhile, humidification of CO₂ is still important in the Faradaic efficiency and current density because it affects the water activity at the catalyst junction, supported by multiphysics simulations. At low cell potentials, the eCO₂RR dominates and is kinetically controlled, and the mass transport of CO₂ and water limits its performance as the potential increases. This understanding of the water kinetics and transportation provides valuable insights into the design of active MEAs.