Study On The Effect And The Mechanism Of Carbohydrate Additives On The Capacity Decay For Vanadium Redox Flow Battery

Vanadium battery energy storage systems have been widely studied for their long cycle life and environmental friendliness,and have good prospects for development.However,the capacity degradation caused by vanadium ion migration across the membrane and internal polarization during the operation of vanadium batteries still restricts their commercialization.In this paper,we investigate the factors influencing capacity degradation by adding carbohydrate additives to the cathode electrolyte to suppress capacity degradation.Vanadium ions across the film and the internal polarization of the cell jointly affect the capacity decay of the battery.The capacity decay of the battery is first reduced by optimizing the electrolyte components,current density and electrolyte flow rate before adding carbohydrate additives.For a single cell with an electrode area of 64 cm~2,the maximum total discharge capacity and capacity retention are achieved at a vanadium concentration of 1.8 mol/L,a sulfuric acid concentration of 3.0 mol/L,a current density of 80 m A/cm~2 and an electrolyte flow rate of 60 m L/min.By adding carbohydrate additives to the negative electrolyte,the capacity degradation of the battery can be suppressed and the performance of the negative electrolyte can be improved.Comparing the effects of several sugar additives on the capacity and performance of the battery,it can be found that the effects areα-lactose monohydrate>sucrose>D(+)-xylose>glucose in descending order.The initial discharge capacity of the negative electrolyte withα-lactose monohydrate is increased by3.3%,and the dischargecapacity retention rate of the battery is increased from 65.49%to80.03%after 50 cycles,and the electrochemical activity and reversibility of the electrolyte are also improved.The mechanisms of action of theα-lactose monohydrate are investigated using vanadium ion diffusion experiments,UV-vis,Raman and FTIR analyses,which shows that the addition of theα-lactose monohydrate to the negative electrolyte provides additional osmotic pressure to reduce the osmotic pressure difference between the two electrolytes during cycling,which reduces the migration of vanadium ions across the membrane and slows down the capacity decay.The hydroxyl functional group carried byα-lactose monohydrate can form a stable V-O structure with vanadium ions and can be adsorbed on the electrode surface to provide more reactive sites for electrochemical reactions.