Controlling Bicontinuous Polyelectrolyte Complexation for Membrane Selectivity: Redox‐Mediated Electrochemical Separation of Volatile Fatty Acids

Abstract Fermentative volatile fatty acid (VFA) production is a sustainable approach for waste valorization. However, selective product recovery remains challenging due to the range of VFAs produced and their dilute concentrations, requiring energy‐intensive purification. Membrane‐based electrochemical separations comprise an energy‐efficient and continuous platform for small molecule separations. At the same time, there is a lack of suitable ion‐exchange membranes for separating between structurally similar organic acids. Here, bicontinuous polyelectrolyte complex (PEC)‐layered nanofiltration membranes are designed for the selective recovery of VFAs using redox‐mediated electrodialysis. Hydrophobic modification of polyelectrolytes via aza‐Michael addition precisely tunes the complexation‐induced phase separation behaviors and the assembled nanostructures. Surface‐confined layer‐by‐layer complexation generates a nanoscale bicontinuous PEC active layer with tailored surface properties that is inaccessible through bulk complexation. Redox‐mediated electrodialysis using the nanostructured membrane exhibits enhancement of both ion permeability and selectivity toward VFAs, with notable reduction of energy consumption by up to 80% compared to conventional electrodialysis. Treatment of synthetic and cow manure fermentation effluents showcases 2 to 4‐fold enrichment of VFAs and simultaneous removal of co‐existing organic acids, with an energy consumption as low as 1.5 kWh kg −1 . These findings advance the understanding of interfacial complexation‐induced phase separation of polyelectrolytes and the development of next‐generation nanostructured membranes for multicomponent separations.