Preparation And Characterization Of Nafion Modified Ion Exchange Membrane For Vanadium Redox Flow Battery

As a clean and efficient power device for large scale energy storage and residential power sources,all vanadium redox flow battery?VRB?is receiving widespread attention for its potential in electrochemical energy storage because of its advantages of high energy efficiency and capacity,flexible design,long cycle life,low operation cost,fast response time and electrolyte recycling.It is a key part of VRB for the ion exchange membranes which plays an important role in transferring proton and isolating from the electrolyte to prevent crossover of the electrolyte.Perfluorosulfonic acid?PFSA?membranes like Nafion produced by DuPont is widely used in all vanadium redox flow battery because of its good chemical,mechanical,and thermal stability,as well as its relatively high proton conductivity in hydrated state.Although this membrane has many advantages as an ion exchange membrane,the battery suffers dramatic declining of columbic efficiency and increase of the self-discharge in battery application for a long term due to high vanadium ions crossover.Thus,reducing vanadium ions crossover and improving ion selectivity are important issues for developing durable IEMs for VRB.The Nafion-SiO₂ nanoparticles were synthesized by the self-assembled synthetic route having more uniform distribution and uniform particle size.The particle size can be determined by the concentration of the additive adjustment.The best Nafion/SiO₂ composite membrane with different silica content?0%,3%,5%,10%,15%?were synthesized and used in vanadium redox flow batteries?VRB?to reduce the vanadium ion permeability of Nafion membranes.To help the proton conductivity of composite membranes Annealing the Nafion/SiO₂ composite membrane was conducted under the protection of alkali metal ions Na⁺at various temperatures of240,270 and 300°C,respectively.The influence of annealing of Nafion/SiO₂composite membrane of a vanadium redox flow battery?VRB?on membrane performance was investigated.The basic properties of the membranes such as conductivity,vanadium ion permeability and so on,as well as the performance of single cell tested with pure Nafion and heat-treated 5%@Nafion/SiO₂ composite membranes were tested.The annealed composite membrane possessed both a lower vanadium permeability and better selectivity coefficient than pure Nafion membrane.Simultaneously,the results show that the high temperature heat-treated composite membrane has a lower vanadium ion permeability than pure Nafion membrane and the effect of the added silica on the conductivity of the membrane is no significant compared with the high temperature on the conductivity.Annealing temperature at270°C could produce highest proton conductivity of the Nafion/SiO₂ membrane.And when the content of silica is 5%,the heat-treated composite membrane has better ion selectivity.The results of single cell test on the 5%high temperature heat-treated composite membrane showed that the composite membrane had better battery performance than the pure Nafion membrane.The 5%@Nafion/SiO₂@270?composite membrane showed good performance at 65mAcm^-22 current density and the Coulomb efficiency and energy efficiency were 93.5%and 83.9%,respectively.The self-discharge properties of pure Nafion and 5%@Nafion/SiO₂@240?composite membranes also were tested.The experimental results show that self-discharge of5%@Nafion/SiO₂@240?composite membrane is smaller than pure Nafion membrane.Combined with experimental results,Nafion/SiO₂ nanocomposite membranes with high temperature heat treatment can effectively improve the properties of Nafion membrane both reduced the vanadium ion permeability of the membrane and helped the conductivity of the membrane.The experimental results will contribute to the Nafion membrane in vanadium flow battery system applications.