Electrode is one of the key materials of the liquid flow battery, the main function is to provide the electrochemical reaction place, and it has a certain catalytic effect. Polyacrylonitrile (PAN) based graphite felt by numerous graphite fibers, the real surface area is far greater than the geometric surface area, good electrical conductivity, graphite fiber is small, the number of unsaturated carbon atoms in the fiber surface edge and the edges and corners, widely used as electrode materials for liquid flow.

　　Wind energy, solar energy, tidal energy and other green pollution-free renewable energy development and utilization, so that high efficiency, long life, low cost of large-scale efficient storage battery energy storage equipment is particularly important.

　　The active specific surface area of graphite felt is small, the ability is poor, and its surface activity is lower, and its practical application is limited, so it is necessary to carry out surface modification to meet the requirements of good electrode materials.

　　The main method is: deposition, air oxidation, liquid phase oxidation, plasma oxidation, electrochemical oxidation, carbon nanotube modified method, which is simple in the air oxidation process, oxidation time and oxidation temperature is easy to control, easy to realize industrialization.

　　The optimum heat treatment of graphite felt oxidized in the air temperature is 550 DEG C; 550 degrees of heat treatment after 5 h, the pits in the surface of graphite fiber increased significantly enhance the number of surface oxygen containing functional increased; electrode performance can be improved, the current efficiency of battery voltage efficiency reached 94.60%, reached 85.36%, energy efficiency reached 80.75%; charge discharge resistance reduce battery.

　　To improve the performance of the electrode due to air oxidation, fiber surface area increased, the number of surface active nodes is increased; in addition, the increase of fiber surface oxygen, reduces the charge transfer resistance of electrochemical reaction.

　　Carbon felt in the vacuum or inert atmosphere after more than 2000 degrees under high temperature treatment for graphite felt, carbon content than carbon felt, up to more than 99%. At the end of 60s the world has a graphite felt good supply. Because of the different selection of raw graphite felt for pitch based and PAN based graphite felt and viscose based graphite felt three.

　　The Japan Kureha chemical as a representative of the asphalt in the insulation industry mainstream, Europe and the United States insulation felt basically to adhesives based, and China is most clear polypropylene based as raw material. Process of polyacrylonitrile based carbon felt or rayon based carbon felt cut into the desired size, rolled into a cylindrical shape is arranged in a graphite material container, graphite container is placed in a high temperature furnace (high temperature furnace graphite tube furnace, medium frequency, high-frequency induction furnace or other forms of heating furnace), vacuum pumping or through a high purity inert gas protection, to 100 to 300 DEG / h of the heating rate of heating to 22 to 25 DEG C; and naturally cooling to 100 DEG C to obtain.

　　PAN based graphite felt than viscose base graphite felt strong and strong antioxidant capacity, but poor flexibility, volume density, good insulation properties. They have the advantages of high purity, high temperature resistance, corrosion resistance, non melting and so on, but also can be flexible, can be folded, cut and can be used with graphite yarn stitching and so on. Graphite felt is mainly used as a single crystal silicon smelting furnace insulation, thermal insulation materials. In the chemical industry can be used as a filter material of high purity corrosive chemical reagents. Under the condition of non oxidizing atmosphere, the temperature of graphite felt is up to 3000.

The graphite felt used for vanadium redox flow battery need to be modified,to improve the chemical activity,and find out the influence factors in graphite felt electrode,we could use different treatments on the graphite felt electrode material. Below treatments are heating treatment and acid treatment,we use polarization curve, cyclic voltammetry and AC impedance to describe the electrochemical properties of the graphite felt electrode,and describe the differences between the chemical treatment before and after,and we need to use XPS,SEM and

　　There are different ways to treat the graphite felt electrode,such as : heating treatment,acid treatment, electrochemical treatment,etc.

　　Put the PAN graphite felt on the crucible and put it into the muffle furnace,under the 500 C°,after 0.5 h calcinations,make it natural cooling;and on the other hand,put the graphite felt into 85 C°concentrated acid,and after 4 hours immersion,then use the distilled water to make it clean.

　　After polarization curve analysis,we could find that the corrosion current of untreat graphite felt electrode is highest,and the polarization resistance of it is the lowest. As for the heating treatment and acid treatment graphite felts,they are almost the same,and two times higher than the untreat one. The result show that the corrosion resistance of treated graphite felt is better than the untreated ones,and the acid treatment graphite felt is the best of all.

　　After cyclic voltammetry,the result shows that the reactive activity and reversibility of the acid treatment is better than the heating treatment and the untreated graphite felts. The SEM result shows that,the heating treatment can make cause surface damage of the carbon fiber,so it will make the resistance more higher,however,after acid treatment,the acid can make the carbon fiber surface smoothly,it will also make the resistance increase. Although the resistance will increase after acid and heating treatment,but it will not have any influence on the reactivity and reversibility.

      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.

　　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.

       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%.

　　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.)

The property of graphite felt used in redox flow battery has a significance to the widely use of redox flow battery.and the Pan base material is the first choice to make the graphite/carbon felt because of the competitive price and good conductivity, chemical stability etc.

　　The electrode material is core component of the redox flow battery,the stability of the electrode material can decide the service time of the battery.now the PAN graphite/carbon felt is widely used as a commercial.

　　The PAN graphite/carbon felt still has a disadvantage,the chemical activity and hydrophilia is not enough,so it need a chemical modification before used in the battery.

　　The main way to modify the graphite/carbon felt as below:

　　1、Oxidation style

　　And it devides into air oxidation style,liquid oxidation style and the electrochemical oxidation.

　　The main purpose is to increase the number of oxygen containing functional groups,such as

　　—COH、—C=O、—COOH and —COOR etc.it can help increase the hydrophilia of the redox flow battery to enlarge the energy efficiency.

　　Currently used for commercial production way is air oxidation style,and it used for Vanadium redox flow battery and Fe/ Cr redox flow battery.The reaction conditions of liquid oxidation style is more calm,and the number of oxygen containing functional groups is more than the air oxidation style,but it need strongly acidic oxidant to modify the graphite/carbon felt,it has a badly corrosion to the equipments,so it’s not suitable for industrial production.

　　2、Other ways

　　Besides of the oxidation style,it also can use Oxygen plasma, γ ray method, hydrothermal method Surface deposition etc.

　　Redox flow battery is a novel kind of secondary batteries to achieve large-scale renewable energy application,and the research and the development on the graphite/carbon felt modification has an important significance.

　　Used in the vanadium redox flow battery,is one of the most useful application of carbon felt/graphite felt.If other materials are used in the vanadium redox flow battery,something will happen,like: the electric current density of vanadium redox flow battery is very low,only about 20mA/cm2 or even lower,but the vanadium redox flow battery with carbon felt/graphite felt can be about 80mA/cm2,and at the same time,the service life of the carbon plate used as bipolar plate is very short and it will lead to the service life of vanadium redox flow battery is very short.

　　So the vanadium battery system must use the carbon felt/graphite felt as electrode materials.

　　The electrode material which is used in the vadadium system divided into two types,one is graphite felt,and another one is carbon felt.To the graphite felt,the baking temperature is higher,and the degree of graphitization is also higher,accordingly,the graphite felt has a higher cost and price than carbon felt.

　　The felt used for electrode material in the vanadium battery mainly divided into PAN base and RAYON base,the rayon ones are more expensive than the PAN ones,but the property of vanadium battery depends on the design of the battery system.

　　Among so many carbon electrode materials ,the graphite felt holds greatest promise. The structure cross section as below:

　　PAN base graphite felt is the first choice to be used in the vanadium redox flow battery,but it still has some disadvantage such as the hydrophilic property can not meet the battery requirement,and the chemical activity can not make it ether. So the research and development of PAN carbon/graphite felt is more and more important.