Decoding carbonation-driven voltage losses in anion exchange membrane fuel cells: Anodic CO32− enrichment and interfacial double-layer restructuring

Technologies based on anion exchange membranes (AEMs), including fuel cells (AEMFCs) and water electrolysis (AEMWE), offer strong potential due to their use of non-precious metal catalysts and fluorine-free polyelectrolytes. However, CO 2 in cathode air causes severe voltage loss (“carbonation”), restricting large-scale AEMFC deployment. Using multiple operando techniques, this study elucidates the carbonation-induced degradation mechanism. Combined operando electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis reveal that carbonation elevates mass transport polarization resistance by over two orders of magnitude, dominating voltage loss. Operando EIS and anode CO 2 outlet monitoring show this effect arises not from reduced membrane conductivity, but from CO 3 2− enrichment in the anode catalyst layer ionomer via the CO 2 –CO 3 2- equilibrium. AEMFC operation follows mass conservation: CO 3 2− and OH − migrate simultaneously to the anode and participate in the hydrogen oxidation reaction in ratios determined at the cathode. In-situ surface-enhanced Raman spectroscopy (SERS) and electrochemical quartz crystal microbalance (EQCM) reveal carbonation forms a thickened electric double layer (EDL) at the electrode/polyelectrolyte interface, weakening interfacial electric fields and hindering CO 3 2− transport. Modulating anode ionomer ion exchange capacity (IEC) reduces both mass transport resistance and voltage loss, experimentally validating the mechanism and advancing understanding of EDL dynamics in AEMFCs.