Phosphoric acid pre-treatment to tailor polybenzimidazole membranes for vanadium redox flow batteries

• Published in: Journal of membrane science 2023-02, Vol.668, p.121233, Article 121233
• Main Authors: Maurya, Sandip, Diaz Abad, Sergio, Park, Eun Joo, Ramaiyan, Kannan, Kim, Yu Seung, Davis, Benjamin L., Mukundan, Rangachary
• Format: Article
• Language: English
• Subjects: electric potential difference; phosphoric acid; species; sulfuric acid; vanadium
• ISSN: 0376-7388; 1873-3123
• DOI: 10.1016/j.memsci.2022.121233

Vanadium redox flow batteries (VRFBs) use ion-selective membranes for transporting ionic species while separating the positive and negative electrolytes. In this paper, we report phosphoric acid doped polybenzimidazole (PBI) membranes that yield high ionic selectivity and conductivity in VRFBs. The phosphoric acid pre-treatment swells the PBI matrix irreversibly and increases its sulfuric acid doping in VRFB electrolytes. The pre-treated membranes show comparable area resistance with Nafion-212 with better selectivity towards vanadium ions. A low resistance was obtained with a reduced membrane thickness, which can reduce the overall cost of materials. The VRFB using an optimized phosphoric acid pre-treated PBI membrane demonstrates coulombic, voltage, and energy efficiencies of 99.7, 90.3, and 90.0%, respectively, at 40 mA cm−2 while achieving ∼40% higher discharge capacity compared to Nafion-212. Furthermore, the VRFB cell shows excellent cycling stability, i.e., only 6.8% or 0.068% per cycle capacity decay for 100 cycles while maintaining coulombic efficiency at >99.9% (98.5% coulombic efficiency for Nafion-212 with a capacity decay of 0.42% per cycle) proving the effectiveness of pre-treatment of phosphoric acid on PBI membranes. [Display omitted] •Properties of polybenzimidazole membranes were tailored by phosphoric acid pretreatment for VRFB applications.•Pretreated polybenzimidazole membranes showed comparable power densities to Nafion-212 in VRFBs.•Significantly high CE of 99.7% and EE of 90% were achieved at 40 mA cm−2.•Pretreated membrane showed ∼40% higher discharge capacity and 6 times higher capacity retention than Nafion-212.