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

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2022-05, Vol.435, p.134902, Article 134902
• Main Authors: Mara Ikhsan, Muhammad, Abbas, Saleem, Do, Xuan Huy, Choi, Seung-Young, Azizi, Kobra, Hjuler, Hans Aage, Jang, Jong Hyun, Ha, Heung Yong, Henkensmeier, Dirk
• Format: Article
• Language: English
• Subjects: interchain spacing; Polybenzimidazole; Pre-treatment; Redox flow battery; Sulfuric acid
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2022.134902

[Display omitted] •meta-PBI membrane was pretreated with 10 M H2SO4.•Membrane swelled 55% in thickness, resulting in a more open structure.•Conductivity in H2SO4 increased 4 times to 9 mS/cm (6.5 mS/cm in VRFB electrolyte).•In VRFB, coulomb, voltage and energy efficiency were higher than for Nafion 212.•EE reached 89.6% @ 80 mA cm−2, and all efficiencies were stable over 135 cycles. 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.