Construction And Properties Of Carbon Fiber/Metal Oxide Composite Electrode For Vanadium Flow Battery

Vanadium redox flow battery（VRFB）has good development prospects due to its advantages of flexible design,high efficiency and fast response speed.However,carbon-based electrode has problems of poor hydrophilicity and insufficient active sites,resulting in poor catalytic activity for vanadium redox reactions,which limits the energy storage performance of VRFB.Metal oxide catalysts have merits of high catalytic activity,easy regulation,good stability and low cost,which can improve the electrochemical activity of electrode.To address the problems of carbon-based electrodes,regulation of the intrinsic properties and loading mode of metal oxides and construction of high-performance carbon-based material/metal oxide electrodes,thereby improving the performance of VRFB.Based on the high specific area and electrical conductivity of CNTs and high activity and stability of SnO₂,the composite catalyst of SnO₂-CNTs was prepared by a sol-gel method.SnO₂-CNTs shows excellent catalytic activity for both V³⁺/V²⁺and VO²⁺/VO₂⁺redox reactions due to the enhanced mass transfer,charge transfer and electron transport rate of vanadium redox reactions.Compared with pristine cell,the cell with SnO₂/CNTs can increase the discharge capacity and energy efficiency by 20.7 m A·h and 5.6%at 150 m A·cm^-2,respectively.F doped SnO₂nanoparticles were prepared by sol-gel method.The replacement of O^2-by F^-in the SnO₂structure can improve the conductivity of SnO₂and increase its oxygen vacancies.The influence of F doping ratio on the physical properties and chemical structure of SnO₂was systematically studied,and the effect of F doping SnO₂on the electrochemical properties of VO²⁺/VO₂⁺and V³⁺/V²⁺was analyzed.F doping increases the electrochemical activity of SnO₂by increasing the active site,promoting vanadium ion adsorption and accelerating electron transport.SnO₂with doping mass ratio of 10%（FTO-10）shows the best electrochemical performance.FTO-10 was used as a bifunctional catalyst to modify graphite felt electrode.At 150 m A·cm^-2,discharge capacity of the cell modifed cell with FTO-10 is increased to 72.0 m A·h,which is 2.4times than pristine cell.Sb-doped SnO₂nanoparticles were made by chemical coprecipitation way,and the effects of the doping ratio of Sb on the structure and electrochemical properties of SnO₂were explored.With the increase of doping ratio,the size of SnO₂nanoparticles is decreased,crystallinity is reduced and electrical conductivity is imrpoved,which can add the active sites and improve electron transfer rate of reaction.Sb-doped SnO₂nanoparticles with the doping ratio of 5%（SnO₂/Sb-5%）shows the best electrocatalytic activity.When SnO₂/Sb-5%was loaded on graphite felt as modifed electrode,the cycling stability and rate performance of the modifed cell are improved,the capacity retention rate is raised by 8%.At 150 m A·cm^-2,the energy efficiency of modified cell is raised by9.2%.Sn and Sb were loaded on the carbon paper（CP）by electrodeposition,and Sb doped SnO₂modified CP electrode（CP-SnO₂/Sb）was made by further oxidation.SnO₂/Sb nanoparticles are dispersed evenly on CP to form more active sites.Meanwhile,SnO₂/Sb has high conductivity and good hydrophilicity,which can promote the kinetics and mass transfer processes of vanadium redox reactions.When the cell used CP-SnO₂/Sb as electrode,at 150 m A·cm^-2,the energy and voltage efficiency of the cell are increased by8.1%and 9.1%,respectively.Moreover,the polarization of modifed cell is reduced.Based on the oxidizing properties of potassium permanganate and the reducing properties of carbon paper substrate,a simple immersion was used to obtain in-situ grown binder-free Mn O₂nanosheets composite electrode.The electrochemical performance of the composite electrode depends on the loading and distribution of Mn O₂on CP,which is controlled by p H of solution,concentration of KMn O₄and deposition time.After the CP was treated with 0.05 M KMn O₄+0.5 M H₂SO₄solution for 30 min,Mn O₂nanosheets were uniformly coated on CP.The composite electrode shows the best electrocatalytic property for V³⁺/V²⁺redox reaction.Mn O₂nanosheets raise active sites,hydrophilicity of the electrode,thereby enhancing charge transfer and diffusion processes of redox reaction.Using composite electrode as negative electrode can improve the rate performance and cycle stability of the cell.Compared with pristine cell,at 100 m A·cm^-2,the energy efficiency of the cell with composite electode is increased by 6%.After 50 cycles,the capacity retention rate of the modied cell is increased by 7.8%.Figure 116;Table 14;Reference 205...