| Due to the gradual depletion of fossil energy and the corresponding worse environmental pollution, the new energy, such as the solar and wind power, should be researched and developed to optimize energy structure. However, the new energy are intermittent and discontinuit, therefore, large-scale energy storage systems are needed to make it used properly. Vanadium redox flow battery (VRFB) has many advantages such as green safety, high energy efficiency, long life, deep charge-discharge ability, easily-designed capacity, big freedom of selecting location. It is considered as one of the most promising energy storage systems. The energy storage process of VRFB is achieved by the electrochemical reaction of vanadium ions on the surface of electrode. Therefore, it is very important to enhance the activity of electrolyte and electrode for enhancing the performance of energy storage. In this paper, the research work was focused on the preparation and properties of high-activity and high-stability electrolyte and high-activity graphite felt. The main points in this research can be summarized as following.(1) Effect of In3+on electrochemical performance of positive electrolyte was investigated. The results indicate that In3+can enhance the electrochemical performance of electrolyte, and the electrolyte shows the best electrochemical performance when the concentration of In3+was10mmol·L-1. The standard rate constant of electrolyte with10mmol·L-1In3+is6.53×10-5cm·s-1,42%higher than that of the pristine electrolyte. The average energy efficiency of the cell employing the electrolyte with In3+can reach84.7%,1.9%higher than that of the pristine electrolyte.(2) The organics containing-NH2,-SO3H, methanesulfonic acid (MSA) and aminomethylsulfonic acid (AMSA) were used as the additives of positive electrolyte. The introduction of additives can enhance the thermal stability of electrolyte. The results of cyclic voltammetry and Tafel polarization curves indicate that the additives can enhance kinetics of liquid mass transfer step and electron transfer step, and the AMSA has the best effect. The diffusion coefficient VO2+in electrolyte with1%AMSA increases to0.95×10-6cm2·s-1from0.75×10-6cm2·s-1, and rate constant of VO2+/VO2+increases to3.16×10-6cm·s-1from2.41×10-6cm·s-1. The cell employing electrolyte with AMSA shows the best charge-discharge performance, and the average energy efficiency of the cell with AMSA is1.6%higher than that of the pristine cell. The electrolyte adsorption tests and XPS measurements indicate that the enhancement of electrochemical performance is due to that additives can increase the wetting ability of electrolyte, moreover, the additive can be adsorbed on the electrode to increase the electrochemical active site on the surface of graphite felt.(3) The sulfamic acid as the supporting electrolyte of VO2+/VO2+redox couple was investigated. The chemical reduction method and electrolysis of electrolyte preparation were compared, and the electrolyte prepared by electrolysis shows the better electrochemical activity. The kinetics of liquid mass transfer and electron transfer are higher than that of the sulfuric acid. The oxidation of VO2+has similar kinetics with the reduction of VO2+. Adding NH4+can further increase the electrochemical performance and electrical conductivity. The energy efficiency of the cell with sulfamic acid as the supporting electrolyte of VO2+/VO2+reodx couple can reach75.87%. When the concentration of NH4+are1.0mol·L-1and3.0mol·L-1, respectively, the average energy efficiencies for50cycles are79.54%and84.57%, respectively,3.66%and8.69%higher than that of the pristine cell.(4) N doping of graphite felt is achieved by heat-treating at the atmosphere of NH3. There is no difference of mechanical property before and after heat-treating. The electrochemical activity and kinetics of positive and negative redox couples were enhanced by the heat-treating at the atmosphere of MH3by the cyclic voltammetry and Tafel polarization curves. The results of IR, XPS and electrolyte adsorption measurements indicate that the enhancement of electrochemical performance of graphite felt treated by heating at the atmosphere of NH3is due to that heating at the atmosphere of NH3can introduce the nitrogenous groups on the surface of graphite felt, which can be employed as active sites to enhance the electrochemical activity and reduce the polarization, moreover, the wetting ability of electrolyte on the electrode also can be enhance. Compared with the reaction at600℃(NGF-600), the stronger N doping reaction at900℃(NGF-900) can introduce more nitrogenous groups, therefore, the bigger enhancement of electrochemical performance of graphite felt treated at900℃was got. The nitrogen content on the surface of NGF-900was increased from1.70%to5.54%. The energy efficiencies of the cells assembled with NGF-600and NGF-900as electrode are5.44%and4.08%higher than that of the pristine cell, respectively. |
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