Mass Transfer Characteristics Of Membrane For All Vanadium Flow Battery

All-Vanadium Redox Flow Battery(short for VRB)is a kind of energy storage device which has developed rapidly in recent years.Because of its high safety and economy,and environmental friendliness,VRB is widely used as a key support technology to solve the large-scale access of renewable energy,the peak load of traditional power system,and the load balance of distributed regional energy system.However,due to the low energy density of the vanadium redox flow battery and the narrow working temperature range,the development of the vanadium redox flow battery has been greatly limited.Improving the mass transfer process and the electrochemical reaction process is the key to improve the battery energy density.Based on the thermodynamics and kinetics theory of VRB,the VRB testing platform was built and the performance test was studied in this thesis.The influence of various factors on the performance of VRB and the analysis of the mass transfer characteristics of the membrane were mainly discussed.In the experiment,the flow field plate of the vanadium redox flow battery was fabricated,and the battery cell was assembled.The influence of operating parameters such as electrolyte flow rate,electrolyte concentration and current density on the performance of all-vanadium redox flow battery were tested by means of flow cell test device.The charge/discharge characteristic curve and AC impedance spectrum were obtained,and the equivalent impedance of all-vanadium redox flow battery was analyzed.The influence of each factor on ohm impedance,Faraday impedance,coulomb efficiency,voltage efficiency and energy efficiency were summarized.The results showed that increasing the concentration of electrolyte and the charge/discharge current density can shorten the charging/discharging time and improve the efficiency of the battery.With the current density increasing the Faraday impedance of the cell could be reduced.Increasing the concentration of sulfuric acid in a certain range in the electrolyte may be appropriate to improve the battery voltage efficiency and energy efficiency.The positive Faraday impedance of the vanadium redox flow battery is larger than the negative Faraday impedance.Since the mass transfer process has an important effect on the performance of the vanadium redox flow battery,in order to analyze the transport law and influence factors of the effluent in the proton exchange membrane,a Nafion117 chain and a three-dimensional periodic amorphous cell model based on the theory of dissipative particle dynamics have been established from mesoscopic view.During the whole process of model building and calculation,the DPD force field is used to calculate the force between mesoscopic beads using the truncation radius method.In the process of dissipative particle dynamics calculation,the Newton equation of motion is solved by Verlet algorithm.The radial distribution function and diffusion coefficient were analyzed according to the calculated results.The effects of temperature and water content on the mesostructure and mass transfer process of proton exchange membrane were studied.The results showed that the interconnection water channels formed in the proton exchange membrane,hydrophilic phases formed around the sulfonic acid groups,and hydrophobic phases surrounded the fluorine atom.The number of water molecules coordinated(coordination number)around the sulfonic acid groups increases with the increasing of the water content at a certain temperature.In the proton exchange membrane,water is a carrier of proton transfer.The increase of both the temperature and the water content could be beneficial to proton transfer in the Nafion117 membrane.