The Effects Of Modification Of Key Materials Of Vanadium Redox Flow Battery On Catalytic Activity Of Positive Couple And Battery Performance

Vanadium redox flow battery (VRFB) is secondary energy storage system, which utilize different valence states of vanadium ion redox reaction to complete energy storage and conversion. VRFB is the ideal power form for the mass storage of mind energy, solar energy and other new energy, with the fast and large current charge and discharge, low self-discharge rate long cycle life and power and capacity adjustable.Carbon felts (CFs) are used as electrode for VRB. The VRB employs the VO2+/VO2+and V2+/V3+redox couples in each positive and negative half-cell electrolytes, with sulfuric acid as supporting electrolyte. During the process of charge and discharge, the active substances react in each positive and negative half-cell. The velocity and efficiency of battery charge and discharge depend on reaction activity of active substances on positive and negative electrode. There are two ways to improve the activity of the electrode, one is elevation of the catalytic activity of the electrode, another is that substances with catalytic activity add to solution. In this paper, carbon felts and electrolyte, as key materials of VRB, are modified to improve positive catalytic activity, and charge and discharge performance of VRB will be improved.This article firstly use three typical chemical method to modify PAN-based graphite felt electrode material, including concentrated sulfuric acid immersion method, the Prussian blue film electrodeposition and ammonium bicarbonate by electrochemical oxidation. Electrode reaction of the vanadium ions was studied by cyclic voltammetry(CV) method. The valves of oxidation peak and reduction peak are about1.5V and0.75V, respectively, and this corresponds to the VO2+/VO2+redox reaction. The peak current density and symmetry of CV curve are increased by three methods. The results indicate that the velocity and reversibility of reaction of modified electrode are increased, and the transfer of vanadium ion is speeded up. Quantitative analysis of the oxygen functional group on graphite felt before and after modification are taken by potential titration. The surface oxygen functional group-COOH,-COOR on graphite felt after modification obviously increase. The best method is ammonium bicarbonate electrochemical oxidation, and the content of functional group increase377.19%. The increase of the oxygen functional groups reduce the electrode resistivity to improve the compatibility of the active material and electrode interface.Voltage efficiency and energy efficiency are86.45%and79.94%in Charge and discharge experiments.The influences of Electrochemical oxidation method to modify the carbon felts (CFs) electrode in different weak acid (citric acid, oxalic acid and ethylene diamine tetraacetic acid) solutions on VRB performance are investigated. The results indicate that the carbon fiber in CFs electrochemical oxidized in the week acids solutions is not etched obviously. The physical performance of CFs is not influenced, and the electrochemical activity of the CFs is enhanced. The results of CV show that the CFs treated in EDTA solution for1h exhibit the best electrochemical performances, and this is also consistent with the results of battery performance test. The performance improvement is mainly due to the removal carbon impurities (such as the residual of amorphous carbon or carbon black after carbonization) and elevation of ratio of surface oxygen and carbon of the original CFs.The electrochemical methods (cyclic voltammetry, electrochemical impedance spectroscopy) are used to study on the reaction mechanism and kinetic parameters of the positive electrode. The result suggests that electrode process of positive electrode is quasi-reversible and there is a postpositive chemical process before electron transferring of VO2+. The effects of additive of SCN-on dynamic characters of electrochemical reaction of VO2+/VC>2+and battery performance are studied. The results showed that SCN-could improve the oxidative peak current value of positive couple VO2+/VO2+, but had little influence on the reductive peak. The total coulomb during the charge and discharge process increased after the introduction of KCNS, which mean that the usage efficiency of active species in Ihe VRB solution was improved. The standard velocity constant ko and the diffused coefficient Do of VO2+/VO2+redox couple are increased, which mean that the introduction of KCNS is helpul for electrochemical process of positive reaction and the standard rate constants ko was also improved with KCNS concentration increase.The effects of purity ions Fe2+,Cu+and Al3+in electrolyte on positive reaction mechanism are studied. The valves of electrochemical impedance spectroscopy (EIS) show a trend of decreases and then increases after introduction of three ions, which mean that little Fe2+,Cu2+and Al3+will not conducive to electrode reaction, but the effects will decrease after critical amount. The standard velocity constant ko and diffuse coefficient Do of electrode after introduction of three ions are calculated,and the rules of standard velocity constant ko vs concentration of additive are discussed.