Flow Battery with Remarkably Stable Performance at High Current Density: Development of A Nonfluorinated Separator with Concurrent Rejection and Conductivity

Redox flow batteries show promise for large-scale grid stabilisation. Of these, organic redox flow batteries (ORFBs) harbour the potential for sustainable and economic operation due to the materials deployed. Their long-term operation requires exquisite transport control of species across the cell, with movement of cations key for high current density, and anionic rejection needed for cycling stability. Nafion, although promising as a commercial separator, faces cost and sustainability limitations due to its fluorinated nature and per- and polyfluroralkyl substances (PFAS) generation. Here, we report the tailored combination of a hydrophilic mixed-matrix membrane, SPEEK-SX, with sulphonated polydichloroxylene (Sp-DCX) as the additive and sulphonated poly(ether ether ketone) (SPEEK) as the matrix. Compared to Nafion-212, the dense aromatic backbone of SPEEK efficiently rejected the crossover of electrolytes, with sulfonate groups housed within Sp-DCX micropores increasing Na⁺ mobility. SPEEK-SX2 exhibited 190 times higher Na⁺/ Fe(CN)₆ ^4- selectivity and 6 times higher Na⁺/ 2,6-DHAQ^2- selectivity compared to Nafion-212. This enabled stable operation for 600 cycles at a high current density of 160 mA cm^-2 with only 0.00935% per cycle capacity decay. In contrast, the SPEEK membrane exhibited 0.07% per cycle decay, whereas Nafion-212 failed to run at this high current density.