Study On Modification And Performance Of Anode Electrode For Vanadium Redox Flow Battery

Vanadium redox flow battery（VRFB）is regarded as the most promising large-scale energy storage system for development and application because of its adjustable capacity and power,long cycle life,fast response time,and high safety.As the most commonly used electrode material for VRFB,carbon felt（CF）provides the site for V³⁺/V²⁺and VO₂⁺/VO²⁺redox reactions and directly affects the power density of VRFB.When the pristine-CF is used as the anode electrode of VRFB,its poor electrical conductivity,poor hydrophilicity and serious hydrogen evolution limit the further application of the CF.In this paper,to solve the above problems,H_xMo O₃,H_xWO₃,Bi₂Mo O₆,Bi PO₄ were loaded on the surface of pristine-CF to improve the electrochemical reactivity of VRFB negative electrode.For the problem of poor conductivity of the commonly used negative catalysts Mo O₃ and WO₃,hydrazine hydrate was used to reduce Mo O₃ and WO₃ by hydrogen insertion in acidic solution to generate H_xMo O₃and H_xWO₃.The conductivities of Mo O₃ and WO₃ were 25.59 S cm^-2 and30.02 S cm^-2,respectively,while those of H_xMo O₃ and H_xWO₃ were 64.27S cm^-2 and 64.47 S cm^-2,respectively.The significant increase in electrical conductivity leads to an increase in the electron transfer rate of the V³⁺/V²⁺reaction on the surface of the CF,and thus the CF exhibits higher electrochemical activity.H_xMo O₃-VRFB and H_xWO₃-VRFB show smaller charge/discharge resistance than pristine-VRFB,thus exhibiting better rate performance,while at the high current density(200 m A cm^-2),the energy efficiency of H_xMo O₃-VRFB and H_xWO₃-VRFB is as high as 65.65%and68.27%,respectively,which is 6.24%and 8.86%higher than that of pristine-VRFB（59.41%）.However,the improvement in conductivity did not solve the problem of hydrogen evolution,which were particularly serious on the surface of the H_xMo O₃-CF.The problem of serious hydrogen evolution in CF is solved by loading Bi₂Mo O₆ as a bimetallic oxide on the surface of CF.Some of the bimetals of Bi₂Mo O₆ are reduced in the first cycle of CV,and Bi can react with H⁺in the electrolyte to form intermediate Bi H_x to weaken the occurrence of hydrogen evolution,while the low valent state of Mo is beneficial to the catalyst conductivity.Therefore,the Bi₂Mo O₆-CF exhibits higher electrochemical activity.The Bi₂Mo O₆-VRFB exhibits a higher power density(563 m W cm^-2)than the pristine-VRFB(404 m W cm^-2).The increase in power density is important for the reduction of VRFB size and large-scale applications.However,the loading of Bi₂Mo O₆ did not solve the problem of poor hydrophilicity of CF.Bi PO₄ was synthesized by hydrothermal method and loaded on the surface of CF.The strong electronegativity of P in PO₄^3-enhances the adsorption ability of the electrode to vanadium ions in the electrolyte,thus improving the wettability of the electrode,while Bi can effectively inhibit hydrogen evolution.As a result,the electrochemical activity of the CF was significantly improved.With the Bi PO₄ modified,The Bi PO₄-VRFB exhibited better rate performance and long cycle stability than the pristine-VRFB.The average decay rate of Bi PO₄-VRFB discharge capacity over 300 cycles in 150 m A cm^-2 was 1.82 m A h,which was significantly lower compared to the pristine-VRFB（2.47 m A h）.