Preparation And Properties Of Ion Exchange Membrane For All Vanadium Redox Flow Battery

In recent years, the perils of fossil fuel depletion, the increasingly polluted environment, and the growing sense to balance economic development and environmental protection compel us to develop renewable and environmentally sustainable energy sources, such as wind energy, solar energy, etc. However, these energies have instability and discontinuous interrupt due to the climate change. Therefore, energy storage technologies have attracted much scientific and public interest. Compared to other energy storage devices, all vanadium redox flow battery (VRB) is widely considered as a promising energy storage device for large-scale energy storage due to its fast response, standalone modular design, no geographical requirement, high efficiency, environmentally friendly, deep charge-discharge ability.As a key component in VRB, the ion exchange membranes (IEMs) not only separate the electrolyte, but also conduct ions to form charge-discharge circuit. Despite its compelling merits, commercialization of VRBs is still hampered by technical and economic barriers (e.g., poor long-term operation stability and high cost), and both of these challenges are largely associated with IEMs. Until now, commercial cation exchange membranes, like Dupont’s Nafion(?), have been widely used as the state-of-the-art IEMs for VRB. However, the extremely high cost and relatively high vanadium ion permeability have limited the commercialization of VRB. To reduce the permeability of vanadium ion through the membrane, researchers have pay attention to anion exchange membranes (AEMs) as they exhibit lower vanadium permeability due to the Donnan exclusion effect. However, proton conduction is also hindered. Ion mobility of H+is4.76(relative to K+in infinitely dilute solution at25℃(K+=1)), higher than other ions in the electrolyte. Once proton conduction hindered, charge-discharge circuit will be impacted negatively, even other ions (HSO4", SO42") conduct. In order to balance vanadium ion permeability and proton selectively, the research work focus on the preparation and properties of the membrane with high ionic conductivity, good ionic selectivity, long life, and low cost. The main points based on this research can be summarized as follows.(1) Novel Methylthiazole (MTz) functionalized anion exchange membranes (AEMs) based on poly (2,6-dimethyl-1,4-phenylene oxide)(PPO) are synthesized and characterized for VRB. Such MTz-PPO AEMs show favorable VRB-related performance, including low vanadium ion permeability, high ion exchange capacity, high energy efficiency. The energy efficiency of cell with the optimized membrane is64.62%at50mA cm2, although not up to the level of Nation117, it still meets the requirement of VRB application.(2) Surface quaternized cation exchange membranes(QSP) based on semi-interpenetrating network (sIPN) are prepared for VRB. This strategy involves first the blend of (Dimethyl aminomethyl) styrene (DMAS), Poly(vinylidene difluoride)(PVDF), sulfonate Poly (2,6-dimethyl-1,4-phenylene oxide)(SPPO), and divinylbenzene (DVB), followed by in situ polymerization to get flexible base membranes, which were immersed in methyl iodide (CH3I) for an accurately controlling time. The resulting surface quatemized cation exchange membranes based on sIPN were obtained. Quaternary ammonium groups in the surface of the membrane can decrease vanadium ion permeability, In the system, tertiary amine groups serve as proton acceptor and sulfonic acid groups serve as proton donator. Tertiary amines distributed uniformity in sIPN will coordinate with sulfonic acid groups to form the proton conduction pathway. So as to promote the proton conductivity.The single cell test shows the energy efficiency of the cell with QSP membrane is higher than that with Nation117membrane. Considering the significantly lower cost, simple preparation process, and the excellent cell performance, the QSP membrane thus offers a promising solution to the ongoing search for a Nation substitute in VRB technology.(3) Novel solvent-free route for quaternized membranes bearing zwitterionic groups have been prepared by employing an in situ polymerization strategy. The method is environmentally-friendly and different from traditional methods using organic solvents as reaction media. Zwitterionic groups enhance the chain packing density greatly, and consequently improve membrane stability and decrease the vanadium ion permeability by chain-chain interaction. Comparing the commercial Nafion117with the optimized membrane, the vanadium ion permeability sharply decreased from10.80×10"5cm min-1to0.21×10-5cm min’1while the energy efficiency increased from68.3%to73.4%at50mA cm-2. In addition, energy efficiencies of the cycle performance show no decrease, which indicates good stability of the membrane. The present quaternized membrane with zwitterionic groups shows good characteristics for application in VRBs.