Study On The Positive Electrolyte And Modification Of Electrode For Vanadium Flow Battery

All vanadium redox flow battery (VRFB) has been attracted much attention for researchers and considered to be one of the most widely applied large scale energy storage systems, due to its low cost, fast response, long cycle life and deep charge-discharge ability compared to the other energy conservation technologies. However, the low electrochemical activity of graphite felt electrode and low energy density of electrolyte limit the wider application. In this study, we mainly focused on the preparation of vanadium electrolyte, the modification of graphite felt electrode and how to improve the energy density of vanadium battery. The main points in this research can be summarized as following.Positive electrolyte of VRFB is prepared via chemical reduction method. Sulfuric acid and V2O5is mixed and heated to be110℃, then economic and environmental glycerol/glucose employed as the reductants is added for reducing V2O5to be V(IV) electrolyte. Kinetic viscosity test shows that the viscosity of2.0M V(IV)/3M H2SO4electrolyte prepared via chemical reduction or electrolysis method is between5.1-5.3mm2s-1; Thermal stability test exhibits that the thermal stability of electrolyte prepared via chemical reduction method is superior to the one from electrolysis method. Cyclic voltammetry and charge-discharge test confirm that the electrochemical activity of the two is excellent, and is comparable to each other, so the electrolyte prepared via chemical reduction method is practical valuable.Anionic surfactant (Sodium Lauryl Ether Sulfate, SLES; Ammonium Lauryl Ether Sulfate, ALES) is selected as additives of positive electrolyte for vanadium battery, and its effects on the conductivity, viscosity and electrochemical performance of electrolyte are investigated. The conductivity and viscosity can be increased by the addition of both SLES and ALES; The amount of precipitation is obviously decreased by SLES, however, ALES can accelerate the precipitation of V(V) ion; Cyclic voltammetry and charge-discharge test show that the reversibility of electrode reaction is improved by the addition of SLES, and the capacity and cell efficiency are also enhanced. The energy efficiency is increased from76.2%of pristine to79.4%of with SLES.The redox couple of Mn(Ⅱ)/Mn(Ⅲ) is successfully introduced into positive electrolyte of VRFB and a V/Mn hybrid redox flow battery is obtained. The effect of sulfuric acid concentration on the kinetic viscosity, conductivity and electrochemical performance is investigated. The results show that the performance of electrolyte is achieve to be the best when sulfuric acid concentration is4M, which is selected as the concentration of cell test. Cell test shows that the cell efficiency of V/Mn hybrid redox flow battery is stale and retains at～80%, and its discharge capacity is slightly lower than that of all vanadium redox system but the discharge energy is higher. The energy density of fist cycle is20.8Wh L-1, which is25.6%higher than16.7Wh L-1obtained from VRFB, and the average The energy density of40cycles is18.8Wh L-1, which is also22.1%increased compared with VRFB of15.4Wh L-1.An environmental, economic and highly effective method for carbon fiber hydroxylated-functionalization based on Fenton’s reagent treatment is used to improve the electrochemical activity of graphite felt (GF) as the positive electrode for all vanadium redox flow battery (VRFB). The effect of H2O2content in Fenton’s reagent on the structure and electrochemical properties of GF is investigated. The scanning electron microscope (SEM) indicates that the surface of the treated GF is etched increasingly with the content of H2O2. The Fourier transformation infrared spectroscopy (FTIR) shows that the peak intensity of hydroxyl groups on the treated felt is increased with the H2O2concentration, which is further verified by X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show the treated sample exhibits a higher electrochemical activity. A VRFB with the treated GF as electrodes exhibits higher energy efficiency (74.2%) than that with the untreated GF (67.7%) at60mA cm-2.