Research On The Nitrogen-doped Carbon Based Electrode For Vanadium Flow Battery

Recently,vanadium flow battery(VFB)has become one of the most promising largescale energy storage systems because of the advantage in large capacity,long cycle life,security and environment friendly.Electrode is one of the key materials for VFBs,and the performance of electrode materials directly determine the energy conversion efficiency and power density.Currently,graphite felt(GF)with three-dimensional(3D)porous structure is the most widely used electrode materials in VFB due to the good conductivity and high corrosion resistance.However,the low electrochemical activity and poor hydrophilicity on GF restrict the performance of VFB.In this paper,we describe a simple method to obtain the highly active nitrogen-doped carbon nanospheres modified graphite felt(NCS/GF)composite electrode.Furthermore,the electrochemical activity of NCS/GF is investigated by the improved electrode evaluation method.Firstly,we have improved the three-electrode device used in CV and EIS tests to make the results more reliable.The influence of the area and thickness of GF on CV testing are analyzed and the volume current density is proposed as the main evaluation indicator.Furthermore,the effect of side reactions is studied by changing the voltage of electrochemical test.The results show that the oxygen evolution reaction and hydrogen evolution reaction,which may destroy the electrode and affect the electrochemical test results,will be occurred on electrode surface when the voltage range is too large in positive and negative CV tests.Moreover,if the cut-off voltage is too high in charge process,the side reactions will occurred on positive and negative half-cells and accelerate the decay of battery capacity.Consequently,the voltage ranges used in positive and negative CV tests are 0.5~1.2 V and-0.7~-0.3 V,respectively,and the voltage of 1.6 V is selected as the upper limit voltage to reduce the capacity degradation caused by side reactions in charge process of single cell test.The NCS/GF is prepared by a facile carbonization process after the self-polymerization of dopamine on GF.The dopamine is used as carbon and nitrogen sources.The physical and electrochemical characterization are performed to explore the catalytic mechanism of N-Doped on redox reactions of vanadium ions.The results show that the electrochemical reversibility and electrolyte wettability of NCS/GF are obviously higher than GF.According to XPS analysis,a mount of N-containing functional groups are introduced into composite electrode after N-doped,which may catalytic vanadium ion reaction as active sites.In addition,the introduced C-N polar bonds may improve the hydrophilic and increase the defects and vacancies on electrode surface to promote the ion migration.Cycling performance of NCS/GF is evaluated by single cell tests in a broad current densities range of 50~300 m A cm-2.The results indicate that the energy efficiency(EE)and capacity retention of NCS/GF are much higher than that of GF.Moreover,NCS/GF exhibits an EE value of 53% even at the current density of 300 m A cm-2,which is rarely reported in previous works.Long-term cycle life test is performed at a high current density of 150 m A cm-2 to evaluated the durability of NCS/GF in acidic conditions.When the discharge capacity retention fall to 50%,the NCS/GF-cell is run for 343 cycles and the EE is stable at 69.4%.The values of GF-cell are 189 cycles and 60.3%,respectively,which are much lower than NCS/GF-cell.More importantly,the NCS/GF exhibits outstanding adaptability and superior stability at a temperature range of-15~50?.For instance,the EE of NCS/GF-cell(73.2%)is much higher than that of GF-cell(61.6%)at the lowest temperature of-15?.