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 ?degrees C in 10M SA) increases its thickness during the treatment from 10 to 17 mu m, 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 m omega cm(2) , which is 3.3 and 1.7 times better than for 10 mu m thick standard PBI (13 mu m thick in 2 M SA) and 54 mu m thick Nafion 212 membranes, respectively. The selectivity is 458x10(4) 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 mu m thick 25C_10M PBI membrane showed an energy efficiency of 89.6% at 80 mA cm(-2).