Stabilizing Highly Sulfonated Poly(ether ether ketone) Membranes with Polybenzimidazole for Zinc−Bromine Redox Flow Batteries

Zinc−bromine redox flow batteries (ZBRFBs) suffer from membrane degradation under bromine-rich operating conditions. Highly sulfonated poly(ether ether ketone) (SPEEK) at DS ≈ 75–79% offers superior ionic conductivity but undergoes excessive swelling and mechanical failure in ZBRFB electrolytes. Here, we report the first systematic application of Na+-pretreated SPEEK/polybenzimidazole (PBI) blend membranes to ZBRFBs, where Na+-ion exchange pretreatment enables homogeneous blending of these otherwise incompatible polymers. The optimized SPEEK/PBI 2.5 wt % membrane achieves a 3-fold reduction in Br2 permeability and improves energy efficiency from 83.6% to 88.4%, retaining 94.22% discharge capacity after 280 cycles versus 54% at failure for pristine SPEEK at cycle 85. Postmortem analysis confirms reduced desulfonation and improved morphological integrity, consistent with the acid−base network reinforcement by PBI. These results demonstrate that controlled PBI blending effectively extends the usable DS range of SPEEK in harsh bromine-containing electrolytes.