Mass Transfer In Electrode Of All-Vanadium Redox Flow Battery And Experiments On Its Performance

All-Vanadium redox flow batteries(VRB)is a massive innovative energy storage batteries with advantages of long service life,high efficiency,large capacity which can be widely used in the fields of solar power,wind power,UPS system,remote power supply application,island application,etc.So far,VRB can not be applied widely because of low energy density and high requirement for the ambient temperature.Improving polarization and electrochemical reaction process are the ways of enhancing battery energy density;improving mass transfer process can control the battery temperature rise of its working process effectively.The VRB charging and discharging characteristics and electrode mass transfer rules were studied on the basis of previous researches by experiment and numerical simulation.The development history and research situation are summarized in this thesis.The structure and functions of all vanadium flow batteries were overviewed,and the reaction principle of vanadium battery on the electrode was expounded,then the various components of VRB about the research process and the research status of heat mass transfer based on reading a amount of literatures about flow batteries are expounded emphatically.The basic theories of electrochemical thermodynamics and kinetics of the electrode process were introduced in this thesis during the VRB researching.The VRB performance test platform including the charger,piping system,electronic load,the electrolyte storage tank,micro flow pump,battery monitoring system,computer management system and software system was constructed.All-vanadium flow batteries fabricated including flow field plate,graphite felt diffusion layer,proton exchange membrane,current collection board,plywood,etc.The experimental procedures and notes were obtained by debugging the test platform.Charging and discharging characteristics,ac impedance characteristics were tested by utilizing test platform,and the influences of current density,electrolyte concentration and electrolyte flow rate were analyzed on voltage,coulomb and energy efficiency of VRB.All-vanadium flow battery circuit inside the battery components was acquired.All vanadium flow battery internal impedance circuitry was drawn,the impacts of the current density,electrolyte concentration and the electrolyte flow on the impedance of circuit elements were discussed base on internal impedance diagram and equivalent circuit method.It was found that charge and discharge time can be shorten and the efficiency was improved by increasing current density and electrolyte concentration;positive Faraday impedance was greater than negative Faraday impedance;positive and negative Faraday impedance was lower with the increase of current density,but ohm impedance was almost the same;ohm and Faraday impedance was increased with increase of electrolyte concentration.A three-dimensional single phase model was established to simulate mass transfer mechanism and material distribution in the electrode of the VRB,based on fluid dynamics,mass transfer and electrochemical principle,according to the all-vanadium flow batteries in mass transfer and the need of geometric model.The equations of mass conservation,momentum conservation equation,energy conservation equation,composition equation,proton exchange membrane mass transfer conditions equations,electrode electrochemical equation,and the boundary condition of model calculation through the model are coupled.The VRB in the positive and negative mass transfer regularity and distribution of various of components were simulated and analyzed by established model,and the impact of electrolyte flow rate,concentration of electrolyte and battery temperature on mass transfer process and the distribution of Vo2+,VO2+,V3+,V2+ are analyzed.It was founded that the porosity of graphite felt plays a key role of vanadium ion distribution in the electrode and the mass transfer.Temperature affects the mass transfer process by the influence of vanadium ion viscosity.