Performance Test And Mesoscopic Mass Transfer Analysis Of MWCNTs Modified Electrode For All-Vanadium Liquid Flow Batteries

All-vanadium flow battery is a fast-growing new energy storage device.As a new type of energy storage device,the all-vanadium flow battery has attracted much attention because it does not involve solid-state reaction,easy change of energy storage capacity,safe and reliable system operation,small environmental load during the whole life cycle,and small self-discharge.At present,the development of all-vanadium flow battery is mainly restricted by three factors:electrode,diaphragm and electrolyte.The electrode material of the all-vanadium flow battery is mainly graphite felt.However,graphite felt has the disadvantages of poor hydrophilicity,low reversibility of electrochemical reaction and low electrochemical activity,which is not conducive to the progress of mass transfer process and electrochemical reaction in the electrode.Therefore,it is necessary to modify the graphite felt electrode to improve the hydrophilicity,electrochemical activity and reversibility of the electrochemical reaction of the graphite felt.In addition,the mass transfer performance in the electrode plays an important role in the performance of the battery.The good mass transfer performance of the electrode can effectively reduce the resistance of reactants and products in the electrode,thus reducing the influence of concentration polarization on the performance of all vanadium flow battery.Based on reading a large amount of literature,this paper is based on the basic theories and research methods of electrochemistry and thermodynamics.The effect of electrode modification on the overall performance of electrodes and batteries is studied by experiments.By simulating the mass transfer process in the positive and negative electrodes,the factors affecting mass transfer in the electrodes are analyzed.In the experiment,the graphite felt electrode was modified by multi-walled carbon nanotubes(MWCNTs).The surface morphology and specific surface area of the modified electrode were observed by scanning electron microscopy(SEM)and specific surface area measurement.FTIR and X-ray photoelectron spectroscopy(XPS)were used to analyze the changes of the bonding state of the groups and elements on the surface of the modified electrode.The changes of resistivity and vanadium adsorption ability before and after modification were compared to conductivity test and adsorption test.Cyclic voltammetry was used to analyze the changes of overpotential and electrochemical reversibility before and after modification.The changes of ohmic impedance,charge transfer impendance and Warburg impedance before and after electrode modification were compared to AC impedance measurement.Finally,the charge-discharge performance of the self-built liquid-flow battery was tested,and the overall performance of all-vanadium liquid-flow battery before and after electrode modification was compared to analyzing Coulomb efficiency,voltage efficiency and energy efficiency.The results showed that electrical conductivity,electrochemical reversibility and electrochemical activity of graphite felt electrode were significantly improved after modified by multi-walled carbon nanotubes.The application of modified graphite felt as positive and negative electrodes of the battery can significantly improve the efficiency and capacity of the battery.In order to reduce the concentration polarization caused by mass transfer factors,a mass transfer model for positive and negative electrodes of all-vanadium liquid flow battery was established by Material Studio software.The mass transfer process in the electrodes was simulated and analyzed by coarse-grained molecular dynamics.The mass transfer process of positive and negative electrodes of all vanadium liquid flow battery was studied by static performance analysis(radial distribution function)and dynamic performance analysis(diffusion coefficient).The effects of different temperatures,electrode porosity and solution concentration on mass transfer in positive and negative electrodes were analyzed.The results show that the increase of temperature is beneficial to mass transfer and diffusion.In the negative vanadium solution,the porosity of porous electrodes plays a dominant role in mass transfer,and the increase of porosity is beneficial to diffusion.The concentration in the positive vanadium solution plays a dominant role in the mass transfer process.Reducing the concentration of vanadium solution is conducive to the occurrence of mass transfer diffusion.