Study On Modification Of Electrode Materials For All Vanadium Redox Flow Batteries

Because of their outstanding features such as design flexibility,high efficiency,long cycle life,high safety and environmental friendliness,all-vanadium redox flow battery?VRFB?is considered as one of the most effective energy storage batteries.It can be used as auxiliary power supply in the fields of power grid peak regulation,renewable energy generation and military power.In the past years,much progress has been made on the VRFB related technologies.However,there are still some technical obstacles to overcome for the large-scale commercial application of VRFBs.At present,graphite felt?GF?has become the most widely used electrode material in the VRFB due to its good conductivity,high stability and three-dimensional network structure.However,the poor electrochemical activity and low specific surface area of the pristine graphite felt?P-GF?limit the comprehensive performance of VRFB.In this work,surface etching and loading bismuth particles are attempted to improve the surface area and electrochemical catalytic activity of the GFs.This research would provide the necessary experimental basis for high performance of VRFB.In the third chapter,the influence of time and temperature for the thermal treatment in the air atmosphere on the surface morphology,hydrophilicity and electrochemical properties of GF was studied.The experimental results show that the oxidized graphite felt?O-GF?obtained by thermal treatment for 3 hours at 500°C presents the best comprehensive properties.Moreover,the effect of different thermal treatment atmospheres on the surface morphology and electrochemical properties of GF were investigated to elucidate the mechanism of thermal treatment.Although the hydrophilicity of GF can be improved after thermal treating in air atmosphere,it has limited effect on increasing the electrochemical active area due to the small specific surface area of P-GF.Therefore,we attempted to introduce manganese oxide onto the GF surface by impregnation-calcination method in the fourth chapter,and then the roughened graphite felt?R-GF?with larger specific surface area was obtained using manganese oxide as etching agent,which could provide more reaction sites for the electrode reaction.The experimental results show that porous structure can be observed on the surface of R-GF fibers,and the specific surface area of R-GF is nearly 6 times larger than that of P-GF.At the same time,abundant oxygen-containing functional groups are introduced onto the surface of R-GF during the etching process,which not only improves the wettability of the electrode,but also provides abundant reactive sites for vanadium redox reaction.Then,the VRFBs were assembled with P-GF,O-GF and R-GF as electrodes,respectively.The charge-discharge tests were carried out at the current density of 80 mA·cm^-2.The initial discharge capacity of VRFB using R-GF is up to 36.8 A·h·L^-1,which is 91.5%of the theoretical capacity.In addition,the energy efficiency of VRFB using R-GF is79.5%,which is 6.5%higher than that of P-GF and 1.9%higher than that of O-GF.The capacity retention of VRFB using R-GF reaches 77.9%which is much higher than that of O-GF?69.4%?.This means that the VRFB using R-GF has excellent stability during 100 charge/discharge cycles.Using R-GF as electrode material is an effective way to improve the performance of VRFB,but the results of CV tests show that the reversibility of negative redox reaction on R-GF is relatively poor.In the fifth chapter,Bi/GF was prepared by solvothermal method.The electrochemical properties of Bi/GF electrode in negative electrolyte were studied by cyclic voltammetry and electrochemical impedance spectroscopy.The results show that the introduction of bismuth particles can not only inhibit the hydrogen evolution reaction at the negative electrode,but also accelerate the redox reaction rate of the V²⁺/V³⁺pair.Two asymmetric VRFBs were assembled with GF and Bi/GF as anode materials and R-GF as cathode materials.Charge/discharge tests were carried out at the current density of 80 mA·cm^-2.The results show that the VRFB using Bi/GF as negative electrode has better electrochemical performance than that using GF as negative electrode.The energy efficiency of the VRFB using Bi/GF as negative electrode reaches 81.3%,which is3.5%higher than that using GF as negative electrode.