Ambipolar Ion Transport Membranes Enable Stable Noble-Metal-Free CO2 Electrolysis in Neutral Media

Abstract

Carbon dioxide (CO₂) electrolysis traditionally relies on monopolar ion-exchange membranes, including anion exchange membranes (AEM), cation exchange membranes (CEM), and a layered juxtaposition of two such monopolar membranes (bipolar membrane, BPM). The monopolar approach has worked well in water electrolysis in alkaline and acidic environments; however, in CO₂ electrolysis, alkaline conditions promote carbonate formation, reducing stability, while acidic media favor competing hydrogen evolution reaction. Here, ambipolar ion transport membranes (AITMs) are reported in CO₂ electrolysis. AITMs facilitate the free movement of ions and water between the cathode and anode, enabling stable CO₂ electrolysis with highly concentrated electrolytes. This results in low operating cell voltages and compatibility with Earth-abundant anodes in pH-neutral electrolytes. Using a membrane electrode assembly cell, the electrochemical CO₂ reduction is demonstrated in neutral media with a benchmark copper-sputtered polytetrafluoroethylene cathode and a nickel anode, achieving a high Faradaic efficiency of 75% for C₂ products. This system operates at a current density of 110 mA cm⁻² and a cell voltage of 3.15 V, maintaining a stable performance with a total operation exceeding 950 h. This work provides a pathway for stable CO₂ conversion at relatively high current densities using Earth-abundant materials.