Structure Design And Performance Study Of Key Materials In Neutral Aqueous Organic Redox Flow Battery

The capturing and utilizing of clean and renewable energy resources requires energy storage technology as an essential support.The flow battery,a large-capacity electrochemical energy storage technology,is the priority option to enable the grid-scale application of renewable energy.Traditional flow batteries are based on the transition metal electroactive material,which suffer from high cost and serious corrosiveness of the electrolytes,impeding further deployment.Aqueous organic flow battery is a newly developed system,which utilizes organic redox-active materials as electrolyte,possessing advantages of low cost and controllable performances.Specially,the neutral aqueous organic flow battery,which operates at mild conditions,lowing the requirements on the equipment and raising the safety factor,is a promising candidate for practical application.Currently,the reported neutral aqueous organic flow batteries employ positive electrolytes based on 2,2,6,6-tetramethylpiperidine nitrogen oxide(TEMPO)derivatives,whereas their poor structural stability fails to meet the cycling life-time requirements in practical application.One of the major challenges is to develop TEMPO derivatives with adequate structural stability.On the other hand,the developed negative electrolytes mainly based on viologen derivatives,and various derivative structure have been developed.However,how the structure of viologen derivatives affects their electrochemical properties and flow battery performances remains unclear,lacking basic theoretical guidance for next generation of viologen-based electrolyte.Additionally,the ion exchange membrane adopted in this flow battery system is mainly based on commercial ion exchange membranes,while the cost needs to be further decreased for practical application.In this context,the main contents in this work are summarized as follows:(1)Taking the typical hydroxyl-modified viologen derivative as an example,a series of derivatives were designed and synthesized by adjusting the alkyl chain length between the water-soluble hydroxyl group and the viologen core.Their electrochemical properties and performances in the flow battery are characterized and tested systematically.Combined with simulation calculations,the molecular frontier orbit,spatial configuration,and surface electrostatic potential of different viologen derivatives are studied to understand their redox behaviors,structural stability and performance in flow battery,and thus elaborate the relationship of structure-property-performance of viologen derivatives.More importantly,this provides basic theoretical guidance for the structural design of viologen derivatives.(2)Using the commercial 4-OH-TEMPO as the raw material,a positively charged side chain is imported into TEMPO via a two-step reaction,and a modified TEMPO derivative(TMAP-TEMPO)was synthesized.On the one hand,the introduction of charged side chain groups increases the polarity of TMAP-TEMPO,improving the solubility of TMAP-TEMPO in water up to 4.62M;on the other hand,the structural degradation caused by intermolecular interactions is alleviated effectively by the electrostatic repulsion and steric hindrance effect between TMAP-TEMPO molecules.This effect was verified by an unbalanced symmetric flow battery test of TMAP-TEMPO.In the full battery test,the cycling stability of the flow battery employing TMAP-TEMPO is improved by nearly an order of magnitude(the capacity loss rate of 0.026%per hour)compared to the that employing 4-OH-TEMPO(the capacity loss rate per hour of 0.22%per hour),and even at higher electrolyte concentration(0.5M and 1.5M)this high cycling stability is maintained.(3)Based on poly(2,6-dimethyl-1,4-phenylene oxide(PPO),through simple functional modification,the positively charged quaternary ammonium functional group is introduced to prepare an anion selective membrane(QPPO).This modification is simple and ready to be scalable.Compared with other commercial ion exchange membranes,the production cost of QPPO membrane is largely decreased.The as-prepared QPPO anion exchange membrane is utilized in the neutral TMAP-TEMPO/BTMAP-Vi system.The tests indicate that the peak power density of flow battery utilizing QPPO membrane is up to 95.34mW cm-2,and the capacity retention rate during the cycle is 99.943%per hour.It should be noted that the energy efficiency of this battery is as high as 81%,higher than most reported flow battery based on TEMPO and viologen derivatives with other membrane.