Preparation And Performance Of Polybenzimidazole/Polymeric Ionic Liquid Composite Membrane For Vanadium Redox Flow Batteries

Vanadium redox flow battery（VRFB）is a large-scale energy storage technology that utilizes changes in the valence state of vanadium ions in the electrolyte to achieve mutual conversion between electrical and chemical energy.Researchers have paid a lot of attention to it because of its benefits,which include a long lifespan,a high level of safety,and an inexpensive price.Among them,ion exchange membranes are an important component of batteries,providing ion transport channels,forming battery circuits,and preventing the penetration and mixing of vanadium ions in the positive and negative electrodes.The performance of the membrane largely determines the performance of the battery.Commercial Nafion membranes function quite well in batteries,but their severe vanadium permeability and high cost substantially limit their application in VRFB.Finding alternatives to Nafion membranes are currently a prominent topic in research.Polybenzimidazole（PBI）proton exchange membranes offer a lot of potential applications in VRFB,but their proton conductivity still needs to be improved.In order to solve the problem of low proton conductivity,amino polybenzimidazole（Am PBI-MOE）with flexible hydrophilic side chains was synthesized by introducing amino groups and grafting 1-bromo-2-（2-methoxyethoxy）ethane in PBI.The Am PBI-MOE-PIL membrane performed admirably as a result of the addition of polymeric ionic liquids to the matrix and the creation of a hydrogen bonding cross-linking network.The Am PBI-MOE-PIL-5membrane not only exhibits a vanadium ions permeability of 0.88×10^-9 cm² min^-1,which is much lower than Nafion117(6.07×10^-8 cm² min^-1),but also shows a very excellent blocking ability for vanadium ion.Compared with the Nafion117 membrane,Am PBI-MOE-PIL-5composite membrane exhibits better energy efficiency at a current density of 40-120 m A cm^-2,up to 82.87%.Building on the previous work,in order to enhance the ion selectivity,and retain good chemical stability,A series of grafted ether-bonded polybenzimidazole composite films containing polymeric ionic liquids were also prepared.Then,alkylation method was used to branch the benzotriazole side link onto the ether-bonded polybenzimidazole（OPBI）.Three nitrogen sites in benzotriazole assist to increase proton conductivity and low area resistance.At the same time,a double bond type ionic liquid（trimethyl-pentenyl ammonia bromide[TPAm]Br）was synthesized,and the polymeric ionic liquid was formed through in-situ free radical polymerization in an OPBI matrix to construct a hydrogen bond cross-linking network,achieving higher ion selectivity and high vanadium resistance.The vanadium permeability of OPBI-BTZ₁₀-PIL₁₀is as low as 1.51×10^-9 cm² min^-1.Its ion selectivity is up to 2.45×10⁶S min cm^-3.Compared with Nafion117,the OPBI-BTZ₁₀-PIL₁₀ film exhibits excellent energy efficiency at 60 m A cm^-2,up to 86.2%.The battery with OPBI-BTZ₁₀-PIL₁₀composite membrane demonstrated good and steady battery efficiency after 200 cycles,and the oxidation stability of the membrane material has also been significantly enhanced.