Preparation And Properties Of Electrolyte For All-vanadium Redox Flow Batteries

All-vanadium redox flow battery（VFB）is an electrochemical energy storage technology with high safety,excellent performance and broad application prospects.As one of the key components of VFB,vanadium electrolyte has a significant impact on battery performance and working life.At present,a mixed solution of equal amounts of V⁴⁺and V³⁺is commonly used as the electrolyte for VFB,and its preparation process is subject to the slow kinetics of the reduction reaction of V⁴⁺to V³⁺,resulting in high energy consumption and aggravating the production cost of vanadium electrolyte.In this paper,a highly efficient V⁴⁺reduction catalyst developed,an electrolyzer for preparing vanadium electrolyte with low energy consumption constructed,a bifunctional primary battery for power generation and electrosynthesis of vanadium electrolyte designed and the performance of the prepared electrolyte was studied.The main results and conclusions are as follows:（1）Aiming at the slow kinetics of the reduction reaction of V⁴⁺to V³⁺,the preparation method of highly active V⁴⁺reduction catalyst was explored and a low-energy-consumption electrolyzer for preparing vanadium electrolyte was constructed in this paper.and electrochemical tests such as CV,EIS,i-t,and Tafel showed that PTSA@ZIF-8-Fe-C could efficiently catalyze the reduction of V⁴⁺to V³⁺.Compared with carbon felt,the catalyst improves the exchange current density of the reduction reaction of V⁴⁺to V³⁺by more than 60 times.When it is applied to the electrolyzer where V⁴⁺is reduced to V³⁺,the electrolysis voltage is reduced by 0.41 V at 30 m A/cm²,and the energy consumption is reduced by 22.5%.The prepared and commercial vanadium electrolyte were applied to VFB,and the constant current charge-discharge test and cycle stability test results showed that compared with the VFB of commercial vanadium electrolyte,the energy efficiency（EE）of VFB based on self-made vanadium electrolyte was improved by 2.2%at 100 m A/cm²,and the discharge capacity decay rate per cycle was decreased by 0.092%.（2）To further reduce energy consumption,a bifunctional primary battery system of power generation and electrosynthesis of vanadium electrolyte was constructed.Using LSV test,the oxidation potentials and activities of three organic small molecules,HCOOH,CH₃OH,and C₂H₂O₄,were obtained.Combined with the potential difference between them and V⁴⁺reduction to V³⁺,HCOOH was screened out as the anode fuel of the primary cell.The open circuit voltage of the primary cell can reach 0.375 V,and at 1.75 m A/cm²,the peak power reaches 0.276 m W/cm².The primary battery can continuously prepare vanadium electrolyte at 2.5 m A/cm²,and the initial performance is still maintained after four batches of electrolyte are prepared,indicating that the battery has good stability.The constant current charge-discharge and cycling stability tests of the prepared and commercial vanadium electrolyte showed that the efficiency of the VFB based on the self-made vanadium electrolyte is comparable to that of the commercial electrolyte,with an energy efficiency of80%at 100 m A/cm²,the discharge capacity retention rate increased by 12.56%after 120 cycles.