| With the advocacy of green economy in modern society,the exploitation of renewable clean energy become the development tendency of the future energy.However,the renewable clean energies influenced by geographical and climate,such as solar and wind energies,display inherent instability and intermittent problem,which needs large-scale energy storage system to provide the stable energy output.Among these,vanadium redox flow batteries have been widely used for the electrochemical energy storage due to the advantages of flexible design,low cost,long cycling life and environmentally friendly.As the redox reaction zones,the electrode materials largely restrict the rate capability and cycle life of vanadium redox flow batteries due to the poor electrocatalytic activity and ion diffusion dynamics.Based on this background,this paper construct a series of multilevel composite structures base on molecular structure design,which optimizes the mixed transport behavior of the electron and ion in the electrode materials,achieving durable cycle life and high-rate performance toward vanadium redox flow batteries.Based on above research purposes,the highly electrochemically active carbon layer was formed on the graphite fibers via the cross-linked polymerization and high temperature carbonization of the polyvinyl alcohol and potassium borate.The electrode reaction process were analyzed by the cyclic voltammetry test and electrochemical impedance spectroscopy,and the results show the composite electrode possesses higher reversibility,lower polarization effect and charge transfer resistance.In addition,through the calculation of density functional theory,the boron dopants can effectively enhance the electrochemical activity of the reactive sites.At the current density of 250 mA cm-2,compared with the pristine electrode,the energy efficiency of the modified electrode have been increased by5.2%.In addition,the modified electrode still retains a discharge capacity of 5.4 Ah L-1 at the current density of 350 mA cm-2,while the pristine electrode break down at the current density of 300 mA cm-2.On the basis of this research,the pyrrole and phytic acid was used as linear polymer monomer and cross-linking agent,respectively.Through low temperature polymerization,hydrothermal reaction and high temperature sintering,the graphite fibers was coated two-dimensional conductive network,which effectively enhances the interfacial area between electrode and electrolyte to increase the amount of reactive sites.Meanwhile,the phosphorus and oxygen functional group co-doping can promote the electrochemical activity of the reactive sites.The results of charge-discharge test showed the discharge capacity still reaches 5.0 Ah L-1 at current density of 400 mA cm-2,indicating excellent rate capability.The overpotential of composite electrode has been decreased by 44.8%,and the battery with high energy efficiency steadily run for 200 cycles.Finally,the graphene oxide and phytic acid were used as plane frame and cross-linking agent,through the hydrothermal dehydration reaction and vacuum freeze drying,the three-dimensional hybrid transport network for electrons/ions is constructed in graphite fibers.The electrochemical performance displays lower charge transfer resistance,high reaction reversibility,faster electron and ion conduction behavior.At current density of 350 mA cm-2,the VRFB assembled modified composite electrodes still reach a high discharge capacity of 10 Ah L-1,and the battery can steadily operate for 1000 cycles at 200mA cm-2 without obvious depression of energy efficiency,indicating excellent rate performance and high-rate performance. |
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