This study discusses the synthesis and preparation of ionically cross-linked acid-base blend membranes based on phosphonated poly(pentafluorostyrene) and poly(benzimidazole) for vanadium redox flow battery application (VRFB). Poly(pentafluorostyrene) was obtained by emulsion polymerization and reacted with tris(trimethylsilyphosphite) to obtain highly phosphonated poly(pentafluorostyrene) (PWN). For obtaining chemically and mechanically stable membrane PWN was blended with poly(benzimidazole) (F6PBI) in different weight ratios. The blend membranes showed high resistance to heat (above 400 degrees C) and the harsh conditions of the VRFB (highly acid and oxidizing conditions). Among the blends, the membrane consisting of PWN/F6PBI (9/1) weight ratio was identified as the most suitable separator due to its high ion-exchange capacity and conductivity. Additionally, this membrane showed the highest Coulomb efficiency (99%) proving its excellent separation ability for VRFB electrolytes. A more detailed study of this blend membrane revealed low self-discharge rate of about 1.6 mV h(-1) being almost an order of magnitude lower than those of Nafion (R) 212 (10.7 mV h(-1)) and lasting above 120 h to the voltage drop. Additionally, the membrane showed no lost of capacity until 600 charge-discharge cycles and only 30% after the 1600 cycles. Thus, in this study, we were able to show for the first time the potentials of a phosphonic acid based electrolyte separator for the application in VRFB.