Development of a robust ion-selective membrane from sulfonated cellulose nanofibers for zinc–iodine redox flow batteries

Electrochemical cells play a critical role across various sectors, enabling technologies in renewable energy generation and storage, environmental monitoring, and management. Among different options, redox flow batteries (RFBs) are gaining importance for large-scale energy storage applications due mainly to their decoupled power and energy outputs associated with easily scalable design. As part of most acidic and neutral RFBs, proton exchange membranes (PEMs) are essential for transport of cations, preventing the mixing of anolyte and catholyte while still allowing high ion conductivity. However, many commercial PEMs are made of poly and perfluoroalkyl substances (PFAS) and besides being expensive and petroleum feedstock-derived, they raise significant environmental concerns. In this work, we developed high-efficiency sulfonated nanocellulose membranes (S-CNF) incorporating layer-by-layer assembly silanization modification. The S-CNF membranes exhibited excellent thermal-oxidative stability (up to 250 °C), dimensional stability, and mechanical strength (Young’s modulus reaching 1.4 GPa and storage modulus exceeding 1.1 GPa). Additionally, they demonstrated high moisture-uptake (absorbing up to 90% water after 48 h). A zinc-iodine redox flow battery (ZIRFB) assembled with S-CNF membranes achieved an average coulombic, voltage and energy efficiencies of 98%, 66%, and 65%, respectively, at a current density of 20 mA cm -2 —comparable to commercially available RFBs. Overall, the S-CNF membranes developed in this study offer a bio-based, simple, cost-effective, and durable alternative to conventional PEMs, with promising scalability for energy storage applications in ZIRFBs. • The S-CNF membranes have homogenous, dense, and pore-free structures. • All the S-CNF membranes demonstrated higher thermal and mechanical stability. • The S-CNF membrane exhibited comparable battery efficiency to Nafion 212.