Study On The Electrolyte Of All-vanadium Redox Flow Battery

All-vanadium redox flow battery is a new type energy storage battery technology which based on the VO2+/VO2+ and V2+/V3+ pairs, for energy storage in the electrolyte. If vanadium battery is compared with conventional batteries, vanadium battery has the characteristics such as quick, high charge and discharge capacity, low self-discharge, simple battery structure. How to get high specific energy, stable vanadium battery electrolyte is one of the key issues.As China’s Renewable Energy Law formally implemented, the state of wind, solar and other new energy is attached great importance. Development of renewable energy requires large capacity storage technologies with matching, combined with advantages that China has rich vanadium resources, for the vanadium battery energy storage system establishment of large-scale, low cost, and widely used, which has provided a broad space for development. Vanadium battery positive and negative electrolyte basic components is different valences vanadium, sulfuric acid and water, it is different from other secondary batteries, vanadium battery electrolyte is not only the conductive medium, but also the electrical activity material of the energy storage, it is the core of the vanadium battery energy conversion and storage, so the vanadium battery charge and discharge efficiency, operating life and the energy density and other key performance properties are closely related with the electrolyte, especially with the thermodynamic properties of the electrolyte. The main content of this research are:(1) The stability of vanadium battery electrolyte was studied by adding different additives in the electrolyte, potentiometric titration and ultraviolet quantitative analysis, it was discussed vanadium additive reduction of battery electrolyte. The results showed that, the higher the temperature of vanadium battery anode electrolyte is, the more unstable vanadium solution is; under the same temperature, the higher pentavalent vanadium solution concentration is, the poorer the solution stability changes; adding a small amount, a low concentration of additives in the electrolyte, it can significantly improve the stability of the electrolyte; sulfuric acid concentration constants at 2mol/L, when vanadium solution concentration is 1.5mol/L, acid sodium, ammonium oxalate stabilizing effect played better in the five additives, when vanadium solution concentration is 1.8mol/L, it is the more suitable choice of urea and CTAB at 40℃ temperature, and suitable choice oxalate at 50℃ temperature; after adding the sodium oxalate and ammonium oxalate in the different concentrations and temperatures in the UV quantitative analysis, the V4+ contents are higher, ammonium oxalate and sodium oxalate plays a role in inhibition of precipitation to the V5+ stability.(2) It is studied on additives to the electrochemical reaction reversibility and reactivity of performance of the electrolyte through measuring cyclic voltammetry and electrochemical impedance spectroscopy by the electrochemical workstation. The results showed that:the electrode reaction activity has been greatly improved by adding different concentrations of additives in the vanadium electrolyte, the reversibility of the reaction also increased, especially anodic peak currents and cathodic peak currents of sodium oxalate and ammonium oxalate are larger; 1.5 mol/L vanadium solution have the better reversibility and electrochemical activity than 1.8 mol/L vanadium solution; the electrochemical activity of the electrolyte has improved after adding additives, and electrochemical impedance is reduced after adding sodium oxalate and ammonium oxalate; the electrochemical reaction resistance increases after adding 3% CTAB or 2% urea.(3) The dissociation constant of the very dilute water-vanadyl sulfate binary solution system using the conductance method, the dissociation constant of vanadium ions and the activity of vanadium ion in the vanadium battery electrolyte is directly related to battery performance, but also the dissociation constant is related to temperature, but to be independent of concentration, studying its variation with the temperature and composition, then studying the relevant thermodynamic properties of vanadium battery, which provides some theoretical basis for further optimize the vanadium battery electrolyte. The results showed that:a very dilute aqueous solution of vanadyl sulfate conductivity k value increases with the increase of the concentration, decreased with increasing temperature; limiting molar conductivity Aq increases with the temperature increasing; it was estimated for vanadyl sulfate ions on the degree of dissociation a, the activity coefficient y, and real solution ionic strength I applying improved Ostwald dilution laws and improved Davies equation; the data processing was to be done using Fouss and Shedlovsky different methods, the data of the dissociation constant, vanadyl ion transference number, and vanadyl ion radius can be as a reference, the degree of dissociation a decreases with the concentration and temperature increases; it is calculated on thermodynamic functions of vanadyl sulfate ions at different temperatures on the dissociation, such as △G0, △H0, △S0, △CP0, and gained the empirical equations of the dissociation constant Kd and the temperatures, etc., it can be used as thermodynamics data reference for studying vanadyl sulfate solution, and provides a theoretical basis for studying ternary and quaternary solution system.