Electroosmotic flow steers neutral products and enables concentrated ethanol electroproduction from CO2

•Electrolyzer enables stable CO2 electroproduction of highly concentrated ethanol •<1% ethanol crossover loss to the anode side •Ethanol production with concentration >13 wt %, comparable with bioethanol production •Continuous ethanol production over 80 h at a current density of 200 mA/cm2 Electrochemical reduction of carbon dioxide (CO2RR) converts intermittent renewable energy into high energy density fuels, such as ethanol. Membrane electrode assembly (MEA) electrolyzers are particularly well-suited for CO2-to-ethanol conversion in view of their low ohmic resistance and high stability. However, over 75% of the ethanol produced at the cathode migrates through the membrane where it is diluted by the anolyte and may be oxidized. The ethanol concentration that results is two orders of magnitude below the 10 wt % standard set by the incumbent industrial process, fermentation. Here, we reverse the direction of ion and electroosmotic transport by means of a porous proton exchange layer, thereby blocking both the convective and diffusive routes of ethanol loss. With this strategy, we eliminate ethanol crossover to the anode (< 1%) and achieve an ethanol concentration of 13.1 wt % directly from the cathode outlet. Electrochemical reduction of carbon dioxide (CO2RR) converts intermittent renewable energy into high energy density fuels, such as ethanol. Membrane electrode assembly (MEA) electrolyzers are particularly well-suited for CO2-to-ethanol conversion in view of their low ohmic resistance and high stability. However, over 75% of the ethanol produced at the cathode migrates through the membrane where it is diluted by the anolyte and may be oxidized. The ethanol concentration that results is two orders of magnitude below the 10 wt % standard set by the incumbent industrial process, fermentation. Here, we reverse the direction of ion and electroosmotic transport by means of a porous proton exchange layer, thereby blocking both the convective and diffusive routes of ethanol loss. With this strategy, we eliminate ethanol crossover to the anode (< 1%) and achieve an ethanol concentration of 13.1 wt % directly from the cathode outlet.