About the basic principle of flow battery

1. Background of Flow Batteries

Liquid 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 the active materials in the solution on both sides of the positive and negative electrodes to achieve charging and discharging. The electrolyte is transported to the inside of the stack by a pump, and a redox reaction is carried out at the electrodes. After the reaction, the active materials flow 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 effectively solve the problem of wind and solar power abandonment caused by excess wind and solar power generation due to 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 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, and all-vanadium flow battery energy storage stations have now entered the stage of large-scale commercial operation.

  2. Concept of flow battery electrodes

1. Definition of electrode

The liquid flow battery system is mainly composed of a battery stack, electrolyte and other system control parts. The electrolyte acts as the positive and negative active substances. The battery 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 battery stack, and the capacity is determined by the concentration and volume of the electrolyte.

II) Position in the industry chain

The difference in the technical routes of flow batteries 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. Generally, they are 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.

Electrode materials are located at the upstream of the industry chain, and usually use carbon felt or graphite felt materials. This type of carbon fiber electrode has a large specific surface area, good chemical stability and conductivity, but it is prone to oxidation and shedding during long-term use, so it needs to be modified, including material intrinsic treatment, metallization treatment and oxidation treatment, or made into a composite material with an inert polymer matrix.

 (III) Electrode function

The physicochemical properties of electrode materials have an important impact on liquid flow batteries: 1) The conductivity and catalytic performance of the electrode directly affect the polarization state and current density of the battery, which in turn affects 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 electrical conductivity: The excellent electrical performance corresponding to high conductivity has a great impact 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 the degree of ohmic polarization during the reaction and improve the overall efficiency of the battery system.

(2) Outstanding mechanical properties: High mechanical strength is conducive to achieving good catalyst loading and ensuring that the structural stability of the flow battery is maintained 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 properties: A stable and good electrode material structure will help the reactive substances to effectively contact with the catalysts loaded on the electrode, thus promoting the efficient redox reaction in the electrolyte;

(4) Cost advantage and environmentally friendly characteristics: On the basis of meeting the requirements of conductivity, mechanical properties and structural characteristics, the electrode cost should be reduced as much as possible and the impact on the environment should be reduced to achieve large-scale application of liquid 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 too high cost. Later, carbon electrode materials such as graphite, glassy carbon, carbon felt, graphite felt, carbon cloth and carbon fiber were used instead. 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 the specific surface area of these materials is small, resulting in a large internal resistance of the battery, making it difficult to charge and discharge with large currents; although carbon paper electrodes have a large specific surface area and good stability, they have poor hydrophilicity and low electrochemical activity. At present, the most widely used electrode materials are carbon felt or graphite felt, both of which are carbon fiber textile materials.

Carbon felt refers to carbon fiber obtained by carbonizing at 1000°C, while graphite felt refers to graphite fiber felt obtained by heating carbon fiber felt to 2000°C in an oxygen-free environment. Carbon felt and graphite felt are porous felt-like fiber materials. Both of them have become the main materials for electrode materials due to their extensive three-dimensional network structure, high specific surface area, good electrical conductivity and electrochemical stability, but their surfaces need to be modified.

According to the different substrates, 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 from polyacrylonitrile (PAN) fiber as the substrate through processes such as chopped, oxidized, carbonized, and graphitized. 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 on high-end non-woven needle punching equipment into flat felts with specific thickness, square meter weight and good uniformity as raw material felts.

2) Catalyst coating: At the entrance of the continuous carbonization graphitization furnace, the oxidized silk industrial flat felt is first evenly coated with sintered 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 is continuously fed into 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.