Biphasic anion-exchange nanofibers enable bipolar junction engineering for enhanced electrocatalytic CO2 conversion in acidic media†

Driven by renewable energies, electrocatalytic CO₂ reduction (eCO₂R) in acidic media using membrane electrode assemblies (MEAs) has emerged as a highly promising approach for large-scale CO₂ utilization with economic viability. Nevertheless, the practical implementation faces significant challenges, including competing hydrogen evolution reaction, salt precipitation, and water flooding, which collectively undermine the long-term faradaic efficiency and operational durability. In this work, we develop an innovative asymmetric porous bipolar membrane (BPM) architecture by integrating electrospun anion-exchange nanofibers with a planar cation-exchange membrane, and configure it in the forward-bias mode (f-BPM) within MEAs to enable efficient acidic eCO₂R. The biphasic anion-exchange nanofibers, comprising polycationic piperidinium copolymer and hydrophobic polyvinylidene difluoride, are engineered to simultaneously optimize ion conductivity, membrane swelling, and mechanical integrity, thereby effectively regulating cation migration, electrochemical impedance, and water and gas transport properties. The optimized f-BPM configuration demonstrates exceptional performance, maintaining stable operation for 325 hours in acidic conditions, while achieving an average CO faradaic efficiency of 88% and a remarkable single-pass CO₂ conversion efficiency of 67% at a current density of 300 mA cm⁻² with a CO₂ flow rate of 15 sccm. Furthermore, the scalability of this technology is successfully demonstrated through the fabrication of a larger 5 × 5 cm² f-BPM, showcasing a stable operation over 110 hours with an energy efficiency of 34.2%. This breakthrough represents a significant advancement in acidic MEA technology, marking a crucial step toward industrial-scale implementation of eCO₂R.