The technological properties of graphite are mainly determined by its crystalline form. Crystalline morphology of different graphite minerals, with different industrial value and purpose. Industry, according to different crystalline morphology, the natural graphite into three categories.

　　1.dense crystalline graphite

　　Dense crystalline graphite is also called bulk graphite. Such graphite crystal clear crystal visible. Particle diameter greater than 0.1 mm. The crystal arrangement is disordered and the structure is compact and massive. This: the characteristics of graphite is high grade, general carbon content is 60 ~ 65%, sometimes up to 80 ~ 98%, but plasticity and smooth than that of flake graphite.

　　2.flake graphite

　　Graphite crystals are in the form of a scale; this is made of high strength under high pressure. This kind of graphite ore is characterized by high grade, generally in the 2 to 3%, or 100 to 25%. Is one of the best ore floatability in nature, which can take over HISTEP grade graphite concentrate. This type of graphite can be floating, lubricating, and more malleable than other types of graphite; therefore, it is the largest industrial value.

　　3.aphanitic graphite

　　Hidden quality graphite also known as amorphous or soil shaped graphite, the graphite crystal diameter, generally less than 1 micron, is a collection of microcrystalline graphite, only under the electron microscopy can be seen in the crystal. The characteristics of this kind of graphite is that the surface is ground, lack of luster, and poor lubrication. Higher grade. General 60 ~ 80%. A few more than 90%. Ore optional sex is poor.

      This way has an advantage of stable oxidation reaction and easy control,the graphite felt after electrochemistry treatment shows that the voltage efficiency can be reach 85%,and the current efficiency can reach 95%,and the circle property is very stable.after the analysis of XPS and FT-IR,the results show that the effect of electrochemistry treatment mainly increase the quality of the O/C rate on the surface of graphite felt,especially to the number of COOH.the results of SEM and (BET)/N2 show that the electrochemistry treatment can increase the surface area of the graphite felt,and increase the reaction area of the electrode.the electrochemistry treatment reduce the electric resistance of the graphite felt,the reason is mainly from the increasing of the COOH number on the surface of the graphite felt.

　　The nitrogen treatment is make the graphite felt under the 180℃ situation of heating in water bath.after the experiment,the result shows that the best treatment time is about 15h,and.use the graphite felt after treatment as electrode in the vanadium redox flow battery,it can charge and discharge in the range of 20~60 mA/cm2.when the current density reach the 20 mA/cm2,the voltage efficiency can reach 83.7%,and the electric energy efficiency can reach 85.3%.

　　Through the electrochemistry research and test,the result shows the reason why it can improve the activity of the graphite felt:the nitrogen containing functional groups on the surface of the carbon fiber increased,the functional groups improve the acid base property of the functional groups and the charge density of the carbon atom,reduce the electric resistance,so the activity of the graphite felt is been improved,and increase the efficiency of the vanadium redox flow battery.

Carbon carbon composite material is made of reinforcing carbon fiber and carbon matrix,the C/C composite material need presoma decompose into matrix carbon,the property of the carbon decide that it can not be made by the inorganic material and metal matrix composite material.

　　The preparation method mainly divided into three steps: prefabricate the carbon fiber embryo then increase the carbon content and density of the matrix and the last one is high temperature heating(graphitization).

　　1、According to the different requirement and different production,the way to make the carbon fiber embryo mainly have:

　　1) Short fiber molding,

　　2) Long fiber weaving and lamination,

　　3) Multidimensional weaving or puncturing.

　　4) Novoltex 3D fine woven fabric.

　　2、There are three methods to increase the carbon content and density of the matrix:

　　1) Liquid phase method

　　thermosetting resin or thermoplasticity pitch impregnation and carbonization method.

　　2) Chemical vapor phase method

　　Mainly refers to Chemical Vapor Infiltration(CVI)

　　3) Combination of the two methods.

　　3、high temperature treatment

　　The temperature of carbon carbon composite material usually range from 2400℃~3000℃,the purpose is to promote the formation of graphite structure,so it also called the graphitization.after the graphitization,the thermal conductivity of carbon carbon composite material can increase from 10W/(m.k) to 120W/(m.k),but at the same time ,the mechanical property will reduce about 10%.

　　Graphite is a carbonaceous element mineral crystals, crystal lattice of its hexagonal layered structure, shown in Figure 1 - 1. The distance between each grid is 3.40, and the spacing of carbon atoms in the same network layer is 1 42A. Is the six party system, layered with complete cleavage. The cleavage surface is mainly composed by molecular bonding, weak on the molecular attraction, so the natural floatability is very good.

　　Graphite soft, dark gray, greasy feeling, can pollute paper. Hardness of 1 to 2, along the vertical direction along with the increase of impurities can be increased from 3 to 5. Specific gravity is 1.9 to 2.3. Under the condition of isolated oxygen, its melting point is above 3000, and it is one of the most heat resistant minerals.

　　Pure graphite in nature is not, which often contains A1203, Fe0, Si02, CaO, P2O5, Cu0 and other impurities. These impurities often appear in the form of quartz, pyrite, carbonate and other minerals. In addition, there are water, asphalt, CO2, H2, N2, CH4 and other gas parts. Therefore, the analysis of graphite, in addition to the determination of fixed carbon content, it must also determine the content of volatile and ash content.

　　Acid treatment is an oxidation process to graphite felt. Usually using concentrated sulfuric acid, nitric acid, potassium dichromate to modify the graphite felt.put the graphite felt into concentrated sulfuric acid and under boiling situation for 5h,then make it as electrode material for vanadium redox flow battery,it could make the energy efficiency increase to about 90%.the result of testing shows that after the treatment of concentrated sulfuric acid,it can reduce the electric resistance of the electrode material,and the electrochemical property of the graphite felt can increase with the concentration of the sulfuric acid.the number of the active group on the surface of graphite felt have an obvious increase.

　　Some researchs showed that the graphite felt under the mixed acid and boiling situation can increase the hydrophily.make a comparison between the activation graphite felt and non activation graphite felt,the activation one can be soaked soon,but the non activation graphite felt can not be soaked lasting for many days.

　　After observation with the SEM,we could find that there are several little partical adhere the surface of the graphite felt,and after acid treatment,the partical disappeared,and the surface become smooth.

　　After the acid treatment to the carbon/graphite felt,the number of O atoms increase.the corrosion effect of the acid obviously increase the specific surface area.it found that,the graphite felt electric resistance of mixed acid is higher than only sulfuric acid.

　　There are a lot of ways to activate the graphite felt,but few articals to describe the difference and influence between the different ways.

       The graphite felt used in the vanadium redox flow battery need a treatment to make it have a larger specific surface area,more oxygen functional groups and lower electronic resistance,by these ways to improve the electrochemical property.

　　The main ways to improve the graphite felt include:heat treatment,acid treatment,electrochemistry treatment etc.In addition to those single method,it also has comprehensive treatment of the graphite felt.

　　To the Pan base graphite felt,after 5h of concentrated sulfuric acid environment,and then make it in the 450℃ environment for 2h,the results show that it could increase the　electrochemical property of the electrode material in the V4+ solution.by the testing of SEM,the impurity on the surface of the graphite felt is removed,and the number of the surface active group obviously increased.

　　Using chloro-iridic aicd treat the graphite felt which is treated with heating and acid treatment,the graphite felt show a good electrochemical property and reversibility.It can make the vanadium battery pack more efficient.the main reason mostly is acid treatment after heating can further improve the electrochemical property.

　　Except for the treatment above,there are also surface metallization, ammoniation etc.　There are all kinds of ways to treat the graphite felt and to make it more activity,but the way is still need to improve,if the graphite felt could be used for commercial process,it must has a high property and low cost,so we still need to find out a way to achieve this purpose.The modification method of graphite felt is a major development direction of the electrode material in the vanadium redox flow battery.

　　1、Graphite electrodes can be used As refractory material: graphite and its products have properties of high - strength, high temperature resistance, mainly used to produce graphite crucible in the metallurgical industry, in steelmaking in graphite is usually used for the protection of steel ingot agent, metallurgical furnace lining.

　　2、Graphite electrodes can be used as the Conductive material: in the electrical industry as manufacturing electrode, carbon brush, carbon rod, carbon nanotubes, mercury is the cathode, graphite gaskets, telephone parts, TV picture tube coating.

　　3、Graphite electrodes can be used as Wear resistant lubricating material: graphite is often used as a lubricant in the mechanical industry. Lubricating oil is often not used in high speed, high temperature and high pressure, while the graphite wear resistant material can be used in the temperature of the 200~2000 (a) at a very high speed, without lubricating oil. Many devices for conveying corrosive media are widely used in the piston cup, sealing rings and bearings, which do not need to be added to the lubricating oil. Shi Moru is one of many metal processing (drawing, drawing) the good lubricant.

　　4、Graphite Electroded material has good chemical stability. After special processing of graphite, with corrosion resistance, good thermal conductivity, low permeability characteristics, which is widely used in the manufacturing of heat exchanger, reaction tank, condenser, the burning tower, absorption tower, a cooler, a heater, a filter and pump equipment. Widely used in petrochemical, wet metallurgy, acid alkali production, synthetic fiber, paper and other industrial sectors, can save a lot of metal materials.

　　5、 For casting, foundry, molding and high temperature metallurgical material: due to graphite thermal expansion coefficient is small, and the capability of rapid quench heat changes, as the mold of the glass, the use of graphite black metal by casting, precise size, smooth surface and high rate of finished products, processed or minor processing can be used, so we can save a lot of metal. Production of hard alloy and other powder metallurgy process, usually made of graphite materials and sintered in the use of the porcelain boat. The crystal growth crucible, the area of the refining container, the support fixture, the induction heater and so on are all made of high purity graphite. Graphite can also be used for vacuum smelting of graphite heat insulation board and base, high temperature resistance furnace tube, rod, plate, grid and other components..

       Among all the electrode materials used in the vanadium battery,the carbon/graphite felt comprehensive performance is the best,but it still need to do some activation treatment. The commonly used method:heat treatment,acid treatment, electrochemistry treatment etc.

　　The heat treatment of graphite felt is to put it into the heating equipment, like muffle furnace,to make it oxidation around the surface.

　　Some scholars have found that if put the un-modified graphite felt into the H2SO4 solution,the surface of the graphite felt is hydrophobic,and it need about 48h to be complete wetting.under 200℃ heating environment,and after that,the wetting time can be reduce to about 30h,and with the heating temperature increasing,the graphite felt is easier to be wetting.

　　Under the heating temperature at 400℃ in the air environment for 30h,then make the graphite felt used in the vanadium redox flow battery,we could found that,the heating treatment can apparently improve the electrochemistry property of the graphite felt,and can make the energy conversion efficiency of the battery increase from about 70% to 89%.

　　We could also have a comparation between the modified graphite felt and the un-modifified ones,we could found that the absorption rate of un-modified graphite felt is mearly about 3.2g/g,but the modifified ones,under 400 ℃ environment for 2h,can reach about 10g/g absorption rate.

　　The reason that heating treatment can improve the graphite felt activation is mainly the heating treatment can improve the directional arrangement of the graphitization,and make the electric resistance come down apparently.

　　after the experiment and testing,an appropriate condition can make the graphite felt with a minimum electric resistance,make it used in the vanadium battery pack,during the charging and discharging process,the coulomb efficiency can reach more than 90%.

　　Flexible graphite sheet is light weight and easy to make,the price is much lower.it’s a suitable material to set fluid.this rechargeable battery,with a polyaniline anode and zinc negative pole,and a graphite sheet set fluid,has a reasonable price,large specific area,light weight,easy to use and no pollution to environment.

　　Because of the almost same structure between flexible graphite sheet and natural graphite sheet,so the flexible graphite sheet has a similar property with natural graphite.however,as to the different processing from natural graphite ,so the flexible graphite sheet has some special properties that natural one do not have,such like :high temperature resistance,radiation resistance,chemical erosion resistance,lubrication,sealing,mechanical,easy to mold,etc.Graphite resource is rich in our country,so the development and search of graphite material has a very important significance.

　　Flexible graphite material is made of natural flake graphite ,flake graphite and intercalation agent will have a reaction,and to be the expandable graphite ,after high temperature pressing ,the graphite sheet will be done.

　　The flexible graphite could be used not only in the sealing but fuel cell industry.flexible graphite has a good thermal conductivity and airtightness,so it can meet the requirement of fuel cell industry.

　　Flexible graphite sheet was invented in the late last century,at first,it’s used to solve the problem about the atomic energy valve leakage,the flexible graphite sheet is most suitable for this using,no cotton of the automotive industry promotes the development of graphite sheet industry,the flexible graphite sheet solved the problem of cotton material that leakage less than half one year,and reduce the cost.the prospects of flexible graphite material will be more and more broad.

　　Vanadium redox flow battery is one of the most development prospect of technology because of the long cycle life, high capacity, flexible design, safe and reliable. Among many carbon materials,the carbon/graphite felt has the most application prospect.

　　Campared with the metal material,the graphite felt has many advantage such like: large specific surface area,good flexibility,easy to machine and install and the price is low,so it’s more suitable to be as electrode material in the vanadium redox flow battery.

　　Now the problem is how to improve the electrochemical activity of graphite felt.The research and development mainly bifurcated graphite felt modification and the preparation of Graphite felt composite electrode.

　　1) the modification of graphite felt is to improve the property through modify the surface physical structure(specific surface area, surface morphology)and chemical structure(surface functional group).

　　It can be devided into two types: oxygen functional groups and nitrogen functional groups.

　　Use nitrate to treat the carbon/graphite felt,carbon fiber,carbon sheet and graphite rod etc,the result showed that to the graphite/carbon felt and carbon fiber,the content of oxygen has a large increase,however, to the carbon sheet and graphite rod,the content of oxygen has a decrease.

　　The high content of oxygen does not mean high activity,some researchers believe that appropriate content of oxygen(4.0%~5.0%)is good to the reaction of vanadium battery.

　　Nitrogen element can change the energy level structure of carbon/graphite material,and to make it with high electrochemical activity.Compared with the oxygen functional groups,it can improve not only electrical conductivity of the material but the hydrophily.

　　2) the preparation of Graphite felt composite electrode is to increase the electrochemical activity through complex catalysis on the surface of the graphite felt.

　　The catalyst includes carbon nano-tube, grapheme and metal base(Iridium, Bismuth, Platinum alloy, Tungsten oxide etc.)