15, 25, and 35 μm thick meta-polybenzimidazole (PBI) membranes are doped with H2SO4 and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H2SO4, PBI absorbs about 2 mol of H2SO4 per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, meta-PBI shows a negligible crossover of V3+ and V4+ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBI's nanosized pores. It turns out that charge efficiency (CE) of VRFBs using meta-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick meta-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin meta-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H2SO4-doped meta-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 μm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm-2.
Bibliographical noteFunding Information:
This work was supported by the German−Korean joint SME R&D projects program of MOTIE/KIAT and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20164030201060). We thank Robert Jan Sedelmayer for preparation of some of the membranes.
© 2017 American Chemical Society.
- coulombic efficiency
- electrostatic energy barrier
- meta-polybenzimidazole-based membranes
- vanadium crossover
- vanadium redox flow batteries
ASJC Scopus subject areas
- General Materials Science