Polybenzimidazole membranes for vanadium redox flow batteries: Effect of sulfuric acid doping conditions

Polybenzimidazole (PBI) has been considered as promising membrane material for all-vanadium redox flow batteries (VRFBs) due to its compact morphology that can hinder vanadium crossover. However, its 2–4 mS cm⁻¹ proton conductivity remains a challenge to achieve high energy efficiency. Recently developed PBI membranes showed conductivity up to 18 mS cm⁻¹ by pre-treatment with phosphoric acid (PA) and up to 56 mS cm⁻¹ with KOH. However, since the operation of VRFB uses sulfuric acid (SA), pre-treatment with different chemicals generates chemical wastes. Here we investigate the effects of pre-treaments with SA at various concentrations and temperatures. The optimized membrane (25C_10M, pretreated at 25 °C in 10M SA) increases its thickness during the treatment from 10 to 17 µm, and shows an improved conductivity in 2 M SA of 9.1 mS cm⁻¹. In V⁴⁺ containing electrolyte, the area specific resistance was 262 mΩ cm², which is 3.3 and 1.7 times better than for 10 µm thick standard PBI (13 µm thick in 2 M SA) and 54 µm thick Nafion 212 membranes, respectively. The selectivity is 458x10⁴ S min cm⁻³, 7, 30, and 29 times better than for PA, KOH pre-swelling, and Nafion 212 membranes, respectively. A VRFB performance test with a 17 µm thick 25C_10M PBI membrane showed an energy efficiency of 89.6% at 80 mA cm⁻².