Carbon Electrode Materials for Flow Batteries – High “Felt” Foresight, Integrated “Liquid”

 

1. Background of Flow Battery

 

Flow battery is a battery technology in which active materials exist in liquid electrolytes. It is generally composed of a stack unit, an electrolyte, an electrolyte storage and supply unit, and a management and control unit.

 

It uses the change in the redox state of active materials in the solution on both sides of the positive and negative electrodes to achieve charging and discharging. The electrolyte is pumped into the stack, and a redox reaction is carried out at the electrode. After the reaction, the active material flows back to the external storage tank with the electrolyte. Between the anode and the cathode is a diaphragm that selectively allows the supporting electrolyte to pass through to maintain electrolyte balance.

 

 

As a representative of aqueous solutions, flow batteries can better solve the problem of wind and solar power abandonment caused by excessive wind and solar power generation with their excellent safety, high energy storage capacity, long cycle life and low cost, and are well adapted to the discontinuous and intermittent characteristics of solar power generation and wind power generation.

 

Among flow battery technologies, all-vanadium flow battery technology has developed the most maturely under the continuous technological innovation in recent years. At present, all-vanadium flow battery energy storage stations have also entered the stage of large-scale commercial operation.

 

2. Concept of Flow Battery Electrodes

 

(I) Definition of Electrodes

 

The flow battery system is mainly composed of a stack, an electrolyte and other system control parts. The electrolyte acts as the positive and negative active substances. The stack acts as the reaction site of electricity. The battery power and capacity are independent of each other. The power is determined by the specifications and quantity of the stack, and the capacity is determined by the concentration and volume of the electrolyte.

 

 

As the core material cost of the system, the battery stack accounts for 31%, and its core components are electrodes, diaphragms and bipolar plates. Among them, the electrode is the place of electrochemical reaction of battery system charging and discharging. Its electrochemical activity directly determines the charging and discharging efficiency of the battery stack and the system. As the core component of the flow battery, it can improve its operating power density and reduce the energy loss caused by the charge and discharge cycle.

 

(II) Industrial Chain Position

 

The difference in the technical route of the flow battery mainly refers to the difference in the main components of the electrolyte solution. However, the industrial chain links of different technical routes are basically similar. It is generally divided into three major links: upstream raw materials and battery stack materials; midstream battery stack assembly, control system, and other equipment; downstream assembly, project development and construction. Among them, the core components of the battery stack are electrodes, diaphragms and bipolar plates, as well as current collectors, liquid flow frames, etc.

 

 

(III) Electrode Function

The physicochemical properties of electrode materials have an important influence on liquid flow batteries:

1) The conductivity and catalytic performance of the electrode directly affect the polarization state and current density of the battery, and thus affect the energy efficiency;

2) The physicochemical stability of the electrode material directly affects the overall working stability and actual life of the battery. Therefore, excellent electrode materials must have the following characteristics:

 

(1) Excellent conductivity: The excellent electrical properties corresponding to high conductivity have a great influence on the overall operating efficiency and power output of the battery system. In liquid flow batteries, the resistance of the electrode material used should be as small as possible to reduce its ohmic polarization during the reaction process and improve the overall battery system. Efficiency;

(2) Outstanding mechanical properties: High mechanical strength is conducive to achieving good catalyst loading and ensuring the structural stability of the flow battery during operation to avoid the collapse of the internal structure of the battery, which in turn leads to the collapse of the battery system;

(3) Good structural characteristics: Stable and good electrode material structure will help the effective contact between the reactive substances and the catalyst loaded on the electrode, and promote the efficient redox reaction in the electrolyte;

(4) Cost advantage and environmentally friendly characteristics: On the basis of meeting the conductivity, mechanical properties and structural characteristics, the electrode cost should be reduced as much as possible to reduce the impact on the environment, so as to achieve the large-scale application of flow batteries.

 

(IV) Product Categories

Electrode materials are divided into metal electrodes, carbon electrodes and composite electrodes. In the early days, metal electrode materials were used, such as single metals such as gold, lead, and titanium, as well as alloy materials such as titanium-based platinum and titanium-based iridium oxide. However, metal electrode materials have many defects, poor electrochemical performance or high cost.

 

Later, carbon electrode materials were used, such as graphite, glassy carbon, carbon felt, graphite felt, carbon cloth and carbon fiber. Such carbon materials have good chemical stability, good conductivity, easy preparation and low cost. Studies have found that glassy carbon electrodes have poor reversibility; graphite and carbon cloth electrodes are easily etched and lost during the charging and discharging process, and these materials have a small specific surface area, resulting in a large internal resistance of the battery, making it difficult to charge and discharge with a large current; although the carbon paper electrode has a large specific surface area and good stability, it has poor hydrophilicity and low electrochemical activity.

 

At present, the most widely used electrode materials are carbon felt or graphite felt, both of which belong to carbon fiber textile materials.

 

Carbon felt refers to carbon fiber obtained by carbonization at 1000℃, and graphite felt is graphite fiber felt obtained by heating carbon fiber felt to 2000℃ in an oxygen-free environment.

 

Carbon felt and graphite felt are porous felt-like fiber materials. Both of them have become the main materials of electrode materials due to their extensive three-dimensional network structure, high specific surface area, good conductivity and electrochemical stability, but their surfaces need to be modified.

 

 

Graphite Felt Illustration and SEM Micromorphology

 

Depending on the substrate, graphite felt can be divided into three types: polyacrylonitrile-based (PAN-based) graphite felt, asphalt-based graphite felt, and viscose-based graphite felt. Polyacrylonitrile-based graphite felt refers to graphite felt made by chopped, oxidized, carbonized, and graphitized polyacrylonitrile (PAN) fibers as the substrate. Compared with other graphite felts, polyacrylonitrile-based graphite felt has the advantages of strong antioxidant ability, good thermal insulation performance, high strength, and high temperature resistance. It has a wide range of applications and is the mainstream product in the graphite felt market.

 

(V) Preparation Process

The current commercial preparation method is as follows:

1) PAN-based pre-oxidized fibers are woven and processed into flat felts with specific thickness requirements and square meter weight and good uniformity on high-end non-woven needle punching equipment as raw material felt.

2) Catalyst coating: At the entrance of the continuous carbonization graphitization furnace, the oxidized silk industrial flat felt is first evenly coated with sintering catalyst using specific equipment.

3) High temperature sintering: The flat felt continuously enters the continuous carbonization graphitization furnace, and undergoes carbonization, deposition, and graphitization treatment to obtain a graphite felt intermediate product with a large amount of carbon nanotubes deposited on the surface.

4) Surface treatment: The graphite carbon fiber intermediate product continuously enters the activation furnace, and the graphite carbon fiber intermediate product with a large amount of carbon nanotubes deposited on the surface is subjected to carbonylation treatment to increase the electrochemical activity of the electrode graphite felt.

 

 

III. Technical Difficulties and Development

 

(1) Process Preparation

 

Graphite felt electrode itself has certain catalytic activity, but the catalytic activity is limited and will produce a large electrochemical polarization impedance. Therefore, for liquid flow batteries, especially for all-vanadium liquid flow batteries operating at higher current density, it is very necessary to modify the electrode material to improve the electrocatalytic activity and electrochemical reversibility. The usual treatment methods include acid treatment, air heat treatment, electrochemical treatment and other methods for modification. The electrochemical activity of graphite felt electrode is improved by oxidation modification.

 

Among them, Li Aikui and other inventors proposed a modification treatment method for graphite felt electrode for all-vanadium liquid flow battery, which uses oxidation method and ammonia method to treat and modify the surface of graphite felt. The specific method is to introduce oxygen to oxidize the graphite felt under heating conditions; then use ammonia method to ammoniate the graphite felt, introduce ammonia water to ammoniate the graphite felt under heating and high pressure conditions to achieve modification. Its advantages are simple operation, no use of hazardous chemicals such as strong acids and alkalis, and removal of impurities such as amorphous carbon on the surface of graphite felt without damaging the surface of graphite felt. It also increases the number of nitrogen-containing functional groups on the surface of graphite felt, enhances the ability to adsorb vanadium ions, and improves its electrochemical performance.

 

(2) Process aAssembly End

In the assembly method of graphite felt, usually the whole graphite felt is stacked with the diaphragm and the graphite bipolar plate in sequence, and the whole is pressurized during assembly. While ensuring the seal, the graphite felt is compressed to reduce the contact resistance between the graphite felt and the graphite bipolar plate. However, this method may have the problem of excessive compression affecting the flow rate of the electrolyte, inconsistent compression rate leading to overcharging, and leakage under sealed high-pressure environment.

 

Therefore, inventors such as Zu Ge proposed a graphite felt structure for liquid flow battery. Its technical concept is to guide the electrolyte. It designs straight parallel inlet/outlet flow channels and several serpentine branch flow channels, which does not involve the problems caused by compression of graphite felt during installation. Its advantages can reduce contact resistance and improve battery conversion efficiency; at the same time, it can reduce the internal pressure of the battery stack, ensure sealing, ensure the flow rate of electrolyte, avoid the battery from being scrapped due to local overcharging, and thus extend the life of the battery stack.

 

1. Market Size

According to statistics from China Business Network, the market size of all-vanadium liquid flow batteries in 2022 is about 1.18 billion yuan, and it is expected that the market size of all-vanadium liquid flow batteries in my country will increase to 8.17 billion yuan in 2025, with a compound annual growth rate of 163.3% from 2021 to 2025.

 

 

According to data from Gaogong Industry Research Institute, by the end of 2021, my country’s new energy storage installed capacity was 3.81GW (including 3.27GW of electrochemical energy storage and 2MW of flow battery energy storage); it is expected that the new energy storage installed capacity will reach more than 30GW in 2025, and the flow battery is expected to be more than 3GW.

 

According to the 1500m² graphite felt electrode required for a 1MW flow battery stack, the demand for graphite felt electrodes for 3GW flow battery installed capacity is 4.5 million m². According to the general selling price of flow battery electrode felt of 330 yuan/square meter (the average price in 2022 including tax), the market capacity of all-vanadium flow battery electrode materials in my country in 2025 is about 2.227 billion yuan.

 

[Note] According to the Environmental Impact Report of the Technical Transformation and Expansion Project of the Intelligent Production Line of Leshan Weilide All-vanadium Flow Battery Energy Storage System, the battery stack of 1MW all-vanadium flow battery corresponds to 1450 m² of graphite felt/carbon felt, while the iron-chromium flow battery, all-iron flow battery and organic system flow battery have lower charge and discharge current density than all-vanadium battery, and the consumption of various materials increases. The average consumption of graphite felt electrodes per 1MW is about 1500 m².

 

1. Industry Development Trend

The carbon electrode material industry has high requirements for technology, which is reflected in the design of production equipment, the selection of raw materials, and the precise control of process conditions, temperature and processing time in carbonization and graphitization. Through innovative design of production equipment, improvement and optimization of production processes and other technical innovation methods, the core competitiveness of carbon electrode material companies is to reduce production costs while improving product performance.

 

In addition, the flow battery carbon electrode material industry is a capital-intensive industry. On the one hand, the production of its materials involves multiple processes such as weaving, acupuncture, pre-oxidation, carbonization, graphitization, and activation. There are many corresponding production equipment, and the production site occupancy and fixed asset investment are large; on the other hand, the company’s downstream customers are mainly liquid flow battery stack system companies. The industry is in its early stages, and customers also have a greater say in terms such as credit period, and there are certain capital barriers.

 

At present, the main players in the domestic liquid flow battery carbon electrode are companies with general functional carbon-based material research and development and production capabilities; the reason is the commonality of some production process equipment. Its production process and production equipment are basically the same as PAN-based graphite soft felt. Only the activation process and corresponding equipment need to be added, and the product extension and reaction speed are faster. As for the preparation of high-end incoming materials, there is still room for improvement in domestic PAN-based carbon fiber, Japan’s Toray and Kureha, the United States Amoco and Germany’s SGL.

 

In summary, the widespread commercialization of liquid flow batteries requires improving power density through electrode modification, reducing material consumption and stacking size. Therefore, the preparation of carbon electrodes with high electrochemical activity, high battery kinetic reversibility, high wettability and high stability is undoubtedly one of the key factors to improve the working efficiency of flow batteries. With the support of national policies, electrodes for all-vanadium flow batteries will continue to achieve further development and breakthroughs, and the significant reduction in the cost of the battery system will help the further development and commercial application of all-vanadium flow batteries in the field of electrochemical energy storage.

 

Appendix: Major domestic carbon electrode companies for flow batteries