Study On Composite Doping And Carbon Microsphere Loading Of Graphite Felt Electrodes For All-Vanadium Redox Flow Batteries

The adoption of safe and efficient electrochemical energy storage technology holds great significance in accelerating energy transition and carbon reduction efforts.Among the various large-scale energy storage technologies,the all-vanadium flow battery,also known as the vanadium battery,has garnered significant interest due to its favorable features,including high capacity,superior energy efficiency,long lifespan,and impeccable safety.The electrochemical energy storage reaction of the flow battery occurs within the electrode,with the electrode’s performance acting as the key determinant of the vanadium battery’s energy conversion efficiency.Despite the graphite felt（GF）electrode’s ability to meet the basic requirements of the flow battery through its high conductivity and corrosion resistance,the inherent limitations of GF,such as low electrolyte wettability and weak electrochemical activity,hinder the construction of high-performance vanadium batteries.Thus,there is immense value in maintaining GF’s essential properties while enhancing its electrochemical activity.With this in mind,this study designed a modification process by non-metallic element doping of oxygen and nitrogen in graphite fibers of GF and surface loading of heterogeneous carbon microspheres,and investigated the structural changes of GF materials,the influence of modification process on their electrochemical performance,and the activation mechanism.The main research contents are as follows:（1）Surface micromachining with K₂Fe O₄strong oxidant and N doping in low-temperature NH₃atmosphere to form oxygen and nitrogen functionalized GF as electrode material.GF original felt was immersed in K₂Fe O₄ solutions of different concentrations,followed by hydrothermal treatment and then NH₃ or Ar atmosphere at 700℃.The changes in the material structure were analyzed by SEM,XPS,Raman,XRD,etc.The performance of the modified GF as an electrode was tested using a self-made vanadium battery test bench.K₂Fe O₄ decomposes into KOH and iron oxide,which is loaded on the surface of the graphite felt,and the high-temperature treatment process realizes controlled activation and etching of the micro-areas on the surface of the graphite fibers,and promotes N doping at low temperature.The etching effect was best when the concentration of K₂Fe O₄ solution was 0.1 mol·L^-1.After optimization treatment,the surface of the fibers of GF became rougher and rich in oxygen and nitrogen functional groups,with good electrolyte wettability,providing more active sites for vanadium ion redox reaction and promoting the conversion of VO²⁺/VO₂⁺.After 50 cycles of the vanadium battery with the optimized electrode at a current density of 80 m A·cm^-2,the capacity retention rate was as high as 80.8%,far better than that of GF batteries at 48.4%,demonstrating better cycle stability.（2）The preparation process of carbon microspheres from soluble starch sources using a secondary hydrothermal method,and the doping of graphene oxide（GO）in the carbon microspheres using a primary hydrothermal method,were studied.The prepared heterogeneous carbon microspheres were then loaded onto GF as electrode materials.When preparing carbon microspheres using the secondary hydrothermal method,a primary hydrothermal time of 3hours resulted in amorphous carbon microspheres with a diameter of about 4.5μm,with a well-concentrated particle size and good dispersion.A short primary hydrothermal time resulted in insufficient nucleation,while a long time increased nucleation,which could reduce the diameter of the carbon microspheres.However,when they grew again in the secondary process,cross-linking and aggregation occurred,resulting in irregular shapes and a large particle size distribution.The well-dispersed GO introduced into the controllable starch solution inhibited the nucleation and growth of carbon microspheres,with the optimal amount being 0.3%,forming a large number of～2μm carbon microspheres.The electrochemical performance test of the graphite felt electrode loaded with secondary hydrothermal carbon microspheres and primary hydrothermal graphene oxide-doped carbon microspheres showed that the polarization of oxidation-reduction was significantly reduced,especially after the carbon microspheres were doped with graphene oxide,which further increased conductivity,formed a larger active contact area,and enhanced the adsorption capacity of vanadium ions,demonstrating superior electrocatalytic activity.Among them,the doped r GO carbon microspheres formed with a 0.3%GO addition rate and loaded on GF achieved a discharge capacity of 1.098A·h at a current density of 80 m A·cm^-2,which was 0.302 A·h higher than the GF battery.After 50 charge and discharge cycles,the capacitance retention rate of the vanadium battery was 86.3%,which was a 37.5%improvement over the GF battery.