Porous-Nafion/PBI composite membranes and Nafion/PBI blend membranes for vanadium redox flow batteries

•A trilayer membrane of porous Nafion, an adhesion promoting layer and PBI.•The layers are applied on porous Nafion by spray coating.•Polybenzimidazole (PBI) reduces crossover of vanadium ions in flow batteries.•A blend of Nafion and PBI promotes adhesion but increases ionic resistance.•The closed pores in Nafion are not easily filled with acid and increase resistance.