15, 25, and 35 μm thick meta-polybenzimidazole (PBI) membranes are doped with HSO and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M HSO, PBI absorbs about 2 mol of HSO 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 V and V 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, HSO-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.

When hydroxamic acid functionalized carbon nanotube (CNT) catalyst is adopted for VO2+/VO2+ reaction, VRFB performance is best. It is due to the chelation ability of hydroxamic acid. With the role as chelating agent, hydroxamic acid properly interacts with VO2+ and VO2+, promoting their redox reactions.

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