Research Of High Performance Electrode Materials On All Vanadium Redox Flow Battery

All Vanadium Redox Flow Battery(VRFB for shot)has arouse wide concern as it possesses advanced characteristics such as long cyclic life,high energy efficient and environmental friendly.It was considered to have the most practical prospect for one of large-scale high efficient energy storage devices.Electrode is one of the major critical component in VRFB,therefore,it is very important to study the electrode materials.Carbon nanotubes(CNTs)have been used in various fields such as catalysis of redox reactions,electrochemical capacitors,electrochemical sensors,etc.,due to their unique physical and chemical properties.However,few studies are about the CNTs used as active materials for the VRFB electrodes.In this paper,based on the review of research and development of VRFB and its electrode materials,MWCNTs and functional MWCNTs were used as electrodes in VRFB and their electrochemical performance were investigated by different material characterization methods and electrochemical techniques,such as SEM,XRD,BET,XPS,cyclic voltammetry and electrochemical impedance spectroscopy etc..In the third chapter of this dissertation,the composite electrodes containing different ratios of graphite powder(GP)and multi-walled carbon nanotube(MWCNT)were prepared using polytetrafluoroethylene(PTFE)as binding agent,based on the preparation methods of gas diffusion electrodes in fuel cells and the studies of research group on PTFE bonded carbon-based porous electrode.The electrochemical performances of GP-MWCNT composite electrodes and the catalytic activity of MWCNT VRFB were characterized by chronoamperometry,cyclic voltammetry and electrochemical impedance spectroscopy.In addition,the stability of PTFE bonded MWCNTs were tested in fuel cells with serpentine flow field.The experimental results showed that it was beneficial for both positive and negative electrode reactions of VRFB to increase the content of MWCNT in the composite electrode,and the best electrochemical performance was obtained with the pure MWCNT electrode,the reaction process of which are controlled by the diffusion.A VRFB was constructed using pure MWCNT electrodes as positive and negative electrodes.The preliminary results,obtained by charge/discharge test at a current density of 30 mA/cm2,illustrated that the VRFB has good stability and electrochemical performance.The current efficiency,voltage efficiency and energy efficiency were 96%,87%and 84%,respectively.In the fourth chapter,MWCNTs were disposed in the solution of V(H2SO4):V(HNO3)is equal to 4:1.The structure and surface chemical property of processed MWCNTs were investigated by SEM,XRD,XPS and their electrochemical performance applied in VRFB were studied carefully.It was found that the processed MWCNTs is beneficial for the VO2+/VO2+ redox couples,but it is unfavourable for the V2+/V3+ redox reactions.This can ascribe to the large amount oxygen functional group which increase the hydrophily of electrodes in the surface of electrodes.The oxygen functional group speed up the VO2+/VO2+ redox reactions and hydrogen evolution reaction,the hydrogen evolution reaction restrain the V2+/V3+ redox reactions in some degree.Carbon paper loading processed MWCNTs electrodes were used as positive and negative electrodes in VRFB.At three current densities of 20 mA/cm2,40 mA/cm2 and 50 mA/cm2,the charge-discharge test illustrated that the VRFB has good stability and electrochemical performance and the discharge voltage is higher than the untreated MWCNTs.When the current density is 40 mA/cm2,the current efficiency,voltage efficiency and energy efficiency were 98%,71%and 70%,respectively.In the fifth chapter,the electrochemical performance of electrochemically oxidized MWCNTs electrodes and electrodes with different oxidation time applied in VRFB were studied.The results showed that the electrochemical activity of electrochemically oxidized MWCNTs electrodes were enhanced firstly and became weaken with prolong the oxidation time.This indicated that right amount oxygen functional group benefit both VO2+/V02+ and V2+/V3+ redox reactions and too much oxygen functional group are harmful for their reactions.MWCNTs electrodes with 20min oxidation time were used as positive and negative electrodes in VRFB.At four current densities of 10 mA/cm2,20 mA/cm2,30 mA/cm2 and 40 mA/cm,the charge-discharge test illustrated that the VRPB has good stability.When the current density is 40 mA/cm2,the current efficiency,voltage efficiency and energy efficiency were 98%,81%and 80%,respectively.