Electrodes, as one of the key materials of the core components of flow battery stacks, provide a reaction site for the oxidation-reduction reactions during charging and discharging, and also offer a pathway for the transportation of internal active substances. The performance of electrode materials directly affects the rate of electrochemical reactions, the internal resistance of the battery, and the electrolyte transportation process. Therefore, structural design and surface property modification of electrode materials are of great significance for improving the power density, operation efficiency, and service life of batteries, as well as reducing system costs.
At present, the commonly used electrodes for VRFB (Vanadium Redox Flow Battery) are graphite felt (GF) or carbon felt materials (CF). These materials have advantages such as low cost, good electrical conductivity, large specific surface area, and good chemical stability. However, they also have problems such as incompatibility of solid-liquid interfaces, few active sites, and large mass transfer resistance. Most previous studies have focused on modifying the chemical properties of electrode materials through methods such as thermal treatment, electrochemical/chemical treatment, and functional material modification, thereby reducing battery polarization and enhancing battery performance. However, research on the impact of electrode structural design on performance has been relatively less concerned.
Electrode materials are divided into metal electrodes, carbon electrodes, and composite electrodes. In the early days, metal electrode materials were used, such as elemental metals like gold, lead, and titanium, as well as alloy materials like titanium-based platinum and titanium-based iridium oxide. However, metal electrode materials have many defects, such as insufficient electrochemical performance or high cost. Later, carbon electrode materials were used instead. For example, graphite, glassy carbon, carbon felt, graphite felt, carbon cloth, and carbon fibers, etc. These carbon materials have good chemical stability, good electrical conductivity, are easy to prepare, and are low in cost. It has been found that glassy carbon electrodes have poor reversibility, graphite and carbon cloth electrodes are easily eroded during charging and discharging, and the specific surface area of these materials is small, resulting in a large internal resistance of the battery and difficulty in high-current charging and discharging. Although carbon paper electrodes have a large specific surface area and good stability, their hydrophilicity is poor and their electrochemical activity is not high. At present, the most widely used electrode materials are carbon felt or graphite felt, both of which are carbon fiber textile materials.
The commercial preparation methods of carbon felt currently include: 1. Using PAN-based pre-oxidized fibers, they are woven into a flat felt with specific thickness, weight per square meter, and uniformity requirements on high-end non-woven needle-punching equipment as the raw felt; 2. Catalyst coating: at the entrance of the continuous carbonization and graphitization furnace, the oxidized industrial flat felt is first evenly coated with a sintering catalyst using specific equipment; 3. High-temperature sintering: the flat felt continuously enters the continuous carbonization and graphitization furnace, and after carbonization, deposition, and graphitization treatment, a graphite felt intermediate product with a large amount of carbon nanotubes deposited on the surface is obtained; 4. Surface treatment: the graphite carbon fiber intermediate product continuously enters the activation furnace, and the surface of the graphite carbon fiber intermediate product with a large amount of carbon nanotubes deposited is subjected to carbonylation treatment to increase the electrochemical activity of the electrode graphite felt.
Graphite felt electrodes themselves have certain catalytic activity, but the catalytic activity is limited, resulting in a large electrochemical polarization impedance. Therefore, for flow batteries, especially for all-vanadium flow batteries operating at higher current densities, it is very necessary to modify the electrode materials to improve their electrocatalytic activity and electrochemical reversibility. Among them, ZH Energy Storage has independently developed a catalyst-loaded electrode with intellectual property rights. In the technology and process of loading catalyst particles on graphite felt electrodes, by adding high-reactivity catalytic substances to the surface of graphite felt electrodes, the efficiency of vanadium batteries can be increased by 3-5 percentage points, and the utilization rate of electrolyte can be increased by 5%. From the perspective of improving efficiency and reducing costs, the EPC cost of the all-vanadium flow battery system can also be reduced by about 5%. Preparing electrodes with high electrochemical activity, high battery kinetic reversibility, high wettability, and high stability is undoubtedly one of the key factors for improving the operation efficiency of flow batteries.
With the support of national policies, the electrodes for flow batteries will continue to achieve further development and breakthroughs. The significant cost reduction brought to the battery system will help the further development and commercial application of flow batteries in the field of electrochemical energy storage.
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