Aryl‐Sulfonate‐Functionalized V 2 CT x /SPEEK Membranes Regulate Cation and Water Transport in Zinc‐Ferricyanide Flow Batteries

Zinc‐ferricyanide flow batteries offer low‐cost energy storage, but osmotic water redistribution and the conductivity‐selectivity trade‐off of sulfonated poly(ether ether ketone) (SPEEK) membranes limit operation at high electrolyte concentrations. Here, an aryl‐sulfonate‐functionalized V 2 CT x /SPEEK interface is engineered to promote cation conduction while limiting ferricyanide crossover and electrolyte redistribution. Tetramethylammonium hydroxide‐assisted redispersion provides thin V 2 C‐SO 3 nanosheets for incorporation into SPEEK. Compared with the unfunctionalized V 2 CT x /SPEEK control, the resulting membrane exhibits higher ionic conductivity, lower swelling and ferricyanide permeability, and reduced electrolyte redistribution during cycling. With a 0.8 M K 3 [Fe(CN) 6 ] catholyte, the optimized membrane retains 81.4% of its initial capacity after 390 cycles and delivers an energy efficiency of 88.9% at cycle 390. At 0.1 M, the cell operates for 7,000 cycles with a capacity‐decay rate of 0.0013% per cycle. The peak power density increases from 691.3 mW cm − 2 for pristine SPEEK to 822.9 mW cm − 2 . These results demonstrate that aryl‐sulfonate functionalization improves the balance between cation transport, dimensional stability, and electrolyte retention in zinc‐ferricyanide flow batteries.