| The all-vanadium flow battery?VRFB?is a large-scale energy storage device with long life,high efficiency and high safety.As an important component of VRFB,ion exchange membranes have impact on battery performance.At present,Nafion series membranes have the problem of high price and serious vanadium penetration.Nonfluorinated membrane with immobilized positively charged groups appeared to be a good option in recent years due to their advantages of low price and vanadium ion exclusion,but the proton conductivity has been greatly limited.Based on the above,a quaternary ammonium and hydroxyl bifunctionalized polybenzimidazole anion exchange membrane was designed.The positively charged quaternary ammonium group repels vanadium ions through the Donnan effect,which improves the coulombic efficiency of the battery.In addition,quaternary ammonium groups can conduct protons in combination with water molecules,and hydroxyl groups form hydrogen bonds to assist in the conduction of protons.Therefore,the obtained exchange membrane has both high ion selectivity and proton conductivity.Polybenzimidazole anion exchange membranes with different degrees of functionalization were prepared.With the increase of quaternary ammonium groups and hydroxyl groups,the water uptake rate increased and the area resistance decreased from 0.9 ? cm2 to 0.31 ? cm2.At 200 mA cm-2,the voltage efficiency?VE?of the membrane with a degree of substitution of 112%?PBI-GTA-112%?still higher than 80%,better than Nafion 212?77%?.The slight increase in swelling ratio did not lead to an increase in vanadium penetration.The vanadium permeability of the PBI-GTA-112% membrane was still 3.9×10-9 cm2 s-1,which was still much lower than that of the Nafion 212 membrane(99.6×10-9 cm2 s-1).At 40-200 mA cm-2,coulombic efficiency?CE?is always higher than 96% better than Nafion212?78%?.And after 500 charge and discharge cycles,at 120 mA cm-2,EE was still constant at 80%.In order to improve the vanadium resistance of the membrane,SO42-/ZrO2/WO3 solid acid was doped into the PBI-GTA-112% membrane to reduce the hydrophilic channel,which better prevented the transport of vanadium ions.Sulfonic acid groups facilitate proton conduction and improve CE without sacrificing VE.The membrane with 1% doping?PG112-SWZ-1%?has better vanadium resistance,and the vanadium permeability is 2.3×10-9 cm2 s-1 lower than PBI-GTA-112%.At 40-200 mA cm-2,CE is always higher than 98%.The discharge time of PG112-SWZ-1% membrane was up to 230 hours,which were twice that of PBI-GTA-112%?88h?and five times that of Nafion 212?35h?. |
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