Common Carbon Felt Electrode Modification Methods

 

At present, the main methods to reduce the electrochemical polarization, concentration polarization and ohmic polarization of carbon felt are: adding surface active functional groups. Skyllas-Kazacos et al. first used hot air and hot acid to treat carbon felt, and studied its surface oxygen-containing functional groups, proposing that -0H and C=0 functional groups are the main reasons for improving the electrochemical activity of carbon felt.

 

Sim et al. pointed out that the presence of -0H functional groups on the surface of carbon felt makes the oxygen transfer process easier than direct transfer from H20, accelerating the electron transfer reaction.

 

Zhang et al. believed that COOH functional groups on the surface of carbon felt are easier to provide H+ than -0H functional groups, making it easier for electrolyte to adsorb on the surface of carbon felt and become active centers.

 

Since then, a large number of studies have been related to methods of introducing oxygen-containing functional groups on the surface of carbon felt, such as electrochemical oxidation, plasma treatment, γ-irradiation, Hummels method, and microwave etching.

 

The principle of the above method is to directly contact the oxidizing medium (such as gas or oxidizing solution) with the surface of the carbon fiber, so that the unsaturated carbon in the carbon fiber is oxidized, and then oxygen-containing functional groups such as -0H, C=0 and -COOH are introduced into the carbon fiber structure.

 

Due to the consumption of unsaturated carbon, holes may also be formed on the surface of the carbon fiber, increasing the specific surface area of ​​the carbon felt. Different oxidizing media cause differences in the surface morphology of the carbon fiber, and the cause can be attributed to the different degrees of oxidation reaction between the oxidizing medium and the carbon fiber. The surface morphology of the carbon fiber after partial oxidation is shown in the figure

 

 

Oxidizing gases include oxygen (air, oxygen plasma), water vapor, and CO2. The above gases can react with the unsaturated carbon in carbon fiber under heating conditions to produce the following chemical reactions: Oxidizing solutions mainly include concentrated HNO3, H2SO4, H2O2, concentrated HN3 + concentrated H2SO4, etc.

 

 

Oxidizing solutions can directly oxidize unsaturated carbon in the carbon fiber structure, producing oxygen-containing functional groups on its surface; they can also use an external electric field and the conductivity of carbon fiber to make the oxygen-containing anions in the solution move to the carbon fiber under the action of the electric field, discharge on its surface to generate new ecological oxygen, and then oxidize the graphite felt.

 

Yue et al. also further analyzed the mechanism of H2SO4 oxidation of carbon fiber. They believe that H2SO4 molecules will “attack” defective positions in carbon fiber, such as unsaturated olefins, conjugated double bonds and benzene rings, thereby forming C-OSO2OH. On the one hand, C-0SO2OH forms -OH after contacting water; on the other hand, C-OSO2OH. It can protect C=C from damage, so that the number of -OH increases while the number of C=0 decreases.

 

In addition, because carbon felt has a graphite-like microcrystalline structure, the hanging position of oxygen-containing functional groups on the carbon felt structure is mainly the carbon network plane and edge surface. The results show that the edge oxygen functional groups make the edge position of carbon felt have a higher energy density at the Fermi level, thereby reducing the restriction on the electrode reaction and improving the electrode reaction activity.

 

In order to form more edge oxygen functional groups, Mamiyama et al. coated a metal-containing carbon film on the surface of carbon fiber and then thermally oxidized it. They found that the above-mentioned fine etching made the surface of carbon fiber rich in edge oxygen functional groups, which enhanced the activity of positive and negative electrode reactions.

 

Although the oxygen-containing functional groups on the surface of carbon felt can provide active sites for electrode reactions, the conductivity and strength of carbon felt will also decrease, resulting in increased ohmic polarization loss of the battery.

 

DiBlasi et al. studied the effect of functional groups on the electrochemical properties of several carbonaceous electrodes and found that about 4% to 5% of oxygen-containing substances made the electrodes show good electrochemical properties and appropriate conductivity.

 

At the same time, Langner et al. also found that when the oxygen content on the surface of carbon felt was about 5%, the activity of the negative reaction of vanadium battery increased significantly.

 

When the value is lower than this value, the reaction mechanism changes from the outer layer to the inner layer due to the increase of polar functional groups on the fiber surface, resulting in a significant decrease in the reducibility of V3-.

 

 

In order to avoid the negative impact of oxygen-containing functional groups on the conductivity of carbon felt, nitrogen-containing functional groups have attracted widespread attention from scholars. On the one hand, the five valence electrons of nitrogen atoms can transfer additional charges to the carbon network plane of carbon felt, thereby reducing the effect of nitrogen doping on the conductivity of carbon felt; on the other hand, due to the high negative charge density and strong electronegativity of nitrogen-containing functional groups, it is conducive to the adsorption and coordination of electrolytes on negatively charged N atoms, and has high electrocatalytic activity.

 

Wu et al. immersed carbon felt in an autoclave filled with ammonia water. After being treated at 180°C for 15h, the surface morphology of carbon felt did not change much. However, the N element content increased by 1.56%. The increased N element may exist in the form of amides, aromatic amines, lactams, etc., which improved the battery performance.

 

Kim et al. further explained this result: on the one hand, the lone pair of electrons on the nitrogen-containing functional group increases the basicity and conductivity of the carbon felt; on the other hand, according to the density functional theory calculation results, the C atoms adjacent to the doped N atoms have a higher positive charge density, which offsets the strong electron affinity of the N atoms, so the positively charged C atoms behave as active centers.

 

At the same time, nitrogen doping also helps to increase the activity of oxygen-containing functional groups. Huang et al. used a plasma of a NH3-02 mixed gas to treat carbon felt, which significantly increased the number of C=0 functional groups and pyridinic nitrogen on the surface of the carbon felt, thereby improving the performance of the battery, as shown in the figure.

 

Kim et al. used a NH3-02 mixed gas to treat the carbon felt, and the N atoms in the carbon felt increased by 3.6% in the form of pyridinic nitrogen and pyrrolic nitrogen, and the O atoms in the form of carbonyl and hydroxyl groups increased by 8.7%, effectively improving the activity of the carbon felt electrode.

 

Introducing surface catalysts The catalysts are introduced into the surface of carbon felt by impregnation, reduction or electrochemical deposition, so as to catalyze electrode reactions and reduce electrochemical polarization losses.

 

There are three main types of surface catalysts: Metals: Although metal electrodes are rarely used in flow batteries due to corrosion, dissolution and weight problems, some metals can be used as catalysts and become composite electrode materials with carbon felt by impregnation, electrochemical deposition or coating.

 

It should be noted that the flow battery system has a certain selectivity for metal components, such as vanadium is not reversible on Au; Pt will form a passivation film on the surface during the oxidation of Ce3+; Cr3+/Cr2+ has good reversibility on lead-tungsten-rhenium alloy.

 

Therefore, in 1988, Skyllas-Kazacos et al. concluded that the metal catalysts that can be used in vanadium batteries are: Au, Mn, Ir, Ru, Os, Re, Rh, Sb, Te, Pb, Ag can be used to improve the kinetics of the positive electrode reaction: Pb, Bi, Tl, Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca, Mg can be used to inhibit the negative electrode hydrogen evolution reaction. Although the above metal catalysts have excellent chemical stability and conductivity, the cost of precious metal Bi is not suitable for large-scale energy storage.

 

The article proposes to deposit a small amount of non-precious metal Bi or Sb on the surface of carbon fiber to catalyze the electrode reaction and thus reduce the small electrochemical polarization loss. Among them, Shen et al. proposed to add a small amount of soluble SbCl4 to the negative electrode electrolyte of the vanadium battery.

 

During the battery charging process, Sb particles are simultaneously reduced and deposited on the surface of the carbon felt electrode, avoiding the complicated pretreatment process. Sb particles have a significant catalytic effect, which improves the energy efficiency of the vanadium battery, and the energy efficiency value has no obvious decay in 53 cycles, indicating that the Sb/carbon felt composite electrode has good stability in repeated cycles.

 

Metal Oxides: In recent years, some researchers have proposed to use low-cost metal oxides as catalysts for flow battery electrode materials, such as Mn3O4, Nb2O5, CeO2, PbO2, etc. The main way to combine metal oxides with carbon felt is to synthesize oxides on the surface of carbon fibers by hydrothermal method.

 

In order to enhance the bonding force between metal oxides and carbon fibers, the composite battery can be The electrode was treated at 500-600° for several hours in an Ar or N2 atmosphere to prevent the side effects of the functional groups on the surface of the carbon felt. In order to further increase the loading of Nb2O5 on the surface of the carbon felt and reduce the agglomeration of nano-ions, Li et al. added a small amount of ammonium metatungstate to the precursor solution and kept the atomic ratio of W to Nb at 1:10.

 

The results show that the catalytic effect of Nb2O5 increased the energy efficiency of the flow battery by 7.6%, and the introduction of W promoted the precipitation of active substances, which further increased the battery energy efficiency by 3.1%. Zhou et al. explained the catalytic principle of Zr02: XPS results show that the ZrO2 nanoparticles synthesized on the surface of carbon fibers absorb a certain amount of -OH functional groups, and together with the OH functional groups on the surface of carbon fibers, they act as active centers to catalyze the oxidation reaction of the electrolyte.

 

 

Carbon Materials: Carbon materials such as carbon nanotubes, graphite oxide, and activated carbon have high specific surface area, good chemical stability and conductivity, and are widely used in many fields.

 

Most of the research on flow batteries reported so far has proved the feasibility of the above materials as electrode materials through simple electrochemical experiments.

 

From the perspective of practical application, this is far from enough, and there are still many obstacles and challenges to be overcome. However, combining them with carbon felt to form a composite electrode material has great practical value. Chuanwei Yan’s research group proposed to combine functionalized SWCNT or MWCNT with carbon felt as an electrode material for vanadium batteries.

 

The specific method is as follows: At room temperature, SWCNT or MWCNT is placed in a mixed solution of H2S04+HNQ3, refluxed for 6 hours, then washed with deionized water to pH 7, and then the carbon felt is immersed in the functionalized carbon nanotube suspension by ultrasound until all the carbon nanotube suspension is adsorbed into the carbon felt, and finally dried at 80°C. The results show that the surface of functionalized carbon nanotubes contains hydroxyl and carboxyl functional groups, which can promote the redox reaction of +5/+4 vanadium electrode pairs and enhance reversibility, and the carboxyl functional group is more effective than the hydroxyl functional group.

 

Munaiah et al. coated single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) on the surface of carbon felt electrodes by impregnation method and used them as electrode materials for zinc-bromine batteries.

 

They believe that it may be because SWCNT and MWCNT have a large number of reference planes and edge planes, respectively, that the composite electrode exhibits significant electrochemical behavior in terms of peak current density and peak separation, and the effect of SWCNT is better than that of MWCNT.

 

The role of the composite electrode directly affects the performance of the zinc-bromine battery. The battery has good charge and discharge cycle capacity and reversibility, and the composite structure of carbon nanotubes and carbon felt shows good structural stability and durability.

 

Wang et al. used chemical vapor deposition (CVD) to prepare carbon nanotube/carbon felt composite electrodes, and in-situ nitrogen doping of carbon nanotubes was performed by adding ethylenediamine. The study found that the energy efficiency of vanadium batteries using nitrogen-doped carbon nanotube/carbon felt composite electrodes has been greatly improved.