Study On Key Materials Of Organic Redox Flow Batteries And Their Electrochemical Properties

Organic compounds are inexpensive and readily available,and can be modified with functional group to change the solubility,stability,and electrode potential difference between couples.The study of organic molecules as active materials for the redox flow batteries is in line with the requirement to develop low carbon-emission and sustainable strategies.However,to further develop organic redox flow batteries,the effective discharge capacity?compared to the theoretical discharge capacity?and energy efficiency need to be improved.This thesis focuses on the study of graphite felt electrodes to improve the performance of the organic redox flow battery through enhancing their electrochemical activity and thus promoting the reaction rate of organic molecules on the surface of electrode.The effect and generality of graphite felt electrodes in our study have been verified in two different organic electrolyte systems.As one of the key materials of organic redox flow battery,the electrode supplies location for organic active molecules to undergo redox reaction.In our study,the graphite felt electrode was treated by three different methods,and modified by manganese dioxide?MnO₂?with two different methods.The cyclic voltammetry?CV?test and the charge-discharge cycle test confirms that the graphite felt treated by mixed acid(V_H2SO4:V_HNO3=3:1)in the ultrasonic condition or manganese dioxide modification can only slightly improve electrochemical properties of the graphite felt.Combination of the treatment and modification shows the synergistic effect and makes the electrochemical activity of graphite felt?GF-T?greatly improved,the mass transfer rate accelerated,and the active reaction sites increased.According to Randles-Sevcik equation,the electrochemical surface area is increased by a factor of 1.3.The treatment process was optimized in terms of ultrasonic treatment time as the only variable,and the optimal ultrasonic time was 10 min.The properties of electrolyte is critical to the performance of organic redox flow batteries.At present,the research of electrolyte mainly focuses on the design,synthesis,screening and matching of the active molecular couples.This thesis researches two electrolyte systems,i.e.,anthraquinone-2-sulfonic acid/1,2-dihydroxybenzene-3,5-disulphonic acid?AQS/BQDS?and 2,5-dihydroxy-1,4 benzoquinone/potassium ferrocyanide?DHBQ/K₄Fe?CN?₆?.In the case of acidic AQS/BQDS system,the concentration of the active molecule and the supporting electrolyte were varied to investigate the performance of organic redox flow battery.The energy efficiency of 0.5M active molecular electrolyte flow battery is 5.01%higher when compared with 0.2 M electrolyte.The concentration of the supporting electrolyte is steadily proportional to the discharge capacity,and to some extent,it can also improve energy efficiency.The flow battery assembled with GF-T graphite felt have a capacity increase of 9%and 12%at the current density of 100 mA/cm² and 160 mA/cm²,respectively,and the energy efficiency is increased by 10%compared with the cell using the pristine graphite felt.In the alkaline DHBQ/K₄Fe?CN?₆ system,the GF-T electrode significantly enhances the electrochemical activity and mass transfer rate of the DHBQ side,and the discharge capacity of the assembled battery is increased by 3.6 times and the energy efficiency is increased by10.28%.Long term charge and discharge cycle at a current density of 100 mA/cm² shows that when the use of the GF-T electrode only on the negative electrode is compared with GF-T on both sides,the average coulombic efficiency and energy efficiency difference are only less than 0.09%,0.4%.These results confirms that the GF-T electrode only promotes the electrochemical reaction of quinone/hydroquinone,and the mechanism of catalysis is the intercalation/deintercalation of protons in thin layer of MnO₂,which accelerates the transfer of protons and electrons,and promotes the the reaction kinetics of the entire redox reaction of the full cell.