Research On Nitrogen And Phosphorus Co-doped Graphite Felt Electrodes For Vanadium Flow Battery

Vanadium flow batteries?herein after abbreviated as VFB?have the advantages of long cycle life and high safety performances.In recent years,they have attracted wide attention in the field of large-scale energy storage.The electrode is one of the key materials in the vanadium battery.The graphite felt electrode?GF?commonly used in VFBs has the disadvantages of poor hydrophilicity and weak activity.Therefore,the development of electrode materials with outstanding activity and excellent hydrophilicity can improve energy conversion efficiency and power density of VFB,resulting in promoting the commercial application of VFB.In this thesis,nitrogen and phosphorous co-doping method was adopted to modify the GF,and the highly active PNGF-800 and P-C₃N₄@GF composite electrodes were prepared.The preparation processes of the composite electrode were optimized and the catalytic mechanism of the experiment is proposed.A highly active PNGF-800 composite electrode was prepared by nitrogen and phosphorus co-doped GF.The electrochemical performance,battery's rate performance and long cycle life of the PNGF-800 composite electrode were tested,indicating that the electrochemical performance of PNGF-800 is significantly improved compared to GF.The hydrothermal temperature,precursor solution and ratio of nitrogen and phosphorus sources for preparation of PNGF-800 composite electrode were systematically analyzed to determine the optimal reaction conditions.While the mass ratio of nitrogen and phosphorus sources is 1:2,the hydrothermal reaction temperature and calcination temperature is 200 ^oC and 800 ^oC,respectively,the prepared PNGF-800 possesses the best electrocatalytic activity.Furthermore,employing the low-cost and environmentally friendly melamine and1-hydroxyethylidene-1,1-diphosphonic acid?HEDP?as nitrogen and phosphorus sources,respectively,and calcined at 550 ^oC,then phosphorus-doped carbon nitride modified GF?P-C₃N₄@GF?with excellent hydrophilicity and high activity was successfully prepared,which significantly improves the energy efficiency and power density of VFB.The P-C₃N₄@GF electrode was characterized by SEM,EDS,XRD and FT-IR techniques,and the active material on the surface of GF is confirmed to be P-C₃N₄.Electrode wettability and contact angle tests were performed on the P-C₃N₄@GF electrode,demonstrating excellent electrolyte wettability of P-C₃N₄@GF.The cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?tests of the composite electrode were conducted,signifying P-C₃N₄@GF electrode possesses excellent electrochemical activity and reversibility.To further evaluate the electrocatalytic performance of P-C₃N₄@GF electrode,the rate performances,long cycle life and polarization resistance of VFB assembled with P-C₃N₄@GF were carried out.At a current density of 150 mA cm^-2,the energy efficiency?EE?of VFB with P-C₃N₄@GF is increased by 9.4%compared with GF,4.0%higher than that of TGF.In addition,P-C₃N₄@GF can operate stably at high current density 300 mA cm^-2 and the EE value of VFB is 57.0%.To assess the durability of the P-C₃N₄@GF electrode,polarization curves and long life tests were experimented.The maximum power density of the composite electrode is 616.2 mW cm^-2,which is 13.3%higher than that of GF.Moreover,there is no significant decay of efficiencies under 1500 cycles at 150mA cm^-2.In summary,the thesis systematically studied the nitrogen and phosphorus co-doped GF of VFB.The prepared PNGF-800 and P-C₃N₄@GF electrodes exhibit excellent electrocatalytic activity,which broadens the research field of electrocatalyst of GFs in VFBs.