Ultrasonic Spraying Graphite Felt Electrode
Graphite felt electrode for all-vanadium liquid flow battery: performance optimization and ultrasonic spraying application
Graphite felt electrode plays a key role in the core link of energy conversion of all-vanadium liquid flow battery. Graphite felt is composed of carbon fiber, and its appearance is similar to thick felt. It has a unique three-dimensional mesh pore structure inside, which provides a key reaction site for vanadium ions in the electrolyte. During the battery charging process, the tetravalent vanadium ions in the positive electrode capture electrons and transform into a pentavalent form, while the trivalent vanadium ions in the negative electrode release electrons and become a divalent form. During this period, graphite felt must not only achieve rapid electron transfer, but also ensure that ion exchange is efficient.
Analysis of graphite felt electrode characteristics
The advantages of graphite felt electrodes are prominently reflected in their pore structure. Unlike ordinary metal electrodes with smooth surfaces and limited reaction areas, the pore surface area of each gram of graphite felt can reach more than 10 square meters. Figuratively speaking, a reaction surface the size of a desk is delicately folded into a space the size of a fingernail. Such a rich pore structure creates more contact opportunities for vanadium ions, which can increase the reaction rate by three to five times. In addition, the material has excellent resistance to strong acid corrosion. Even in an electrolyte with a sulfuric acid concentration of more than 3 mol/L, it can still maintain a porosity of more than 90% after working continuously for 5,000 hours.
However, in practical applications, graphite felt electrodes also expose obvious pain points. The original graphite felt surface has insufficient active sites, just like a newly bought iron pot needs to be opened to maintain the oil film. The carbon atoms on the surface of the untreated electrode are arranged too regularly, making it difficult for vanadium ions to adhere. In a comparative experiment, the current density of the electrodes before and after treatment at a voltage of 1.5V differed by as much as two times. In addition, during long-term cycling, some carbon fibers will be oxidized and etched, and the originally uniform pores will gradually evolve into interconnected macropores, which will lead to uneven distribution of the electrolyte. This phenomenon begins to appear after 200 charge and discharge cycles.
Performance improvement strategy
To improve the performance of graphite felt electrodes, it is mainly carried out from three dimensions. Surface modification can be compared to putting “staggered armor” on graphite felt. Acid treatment can form nano-scale pits on the fiber surface, and high-temperature oxidation will form oxygen-containing functional groups. For example, after a research team treated with concentrated nitric acid, the polarization voltage of the electrode at 0.8mA/cm² was reduced by 40%. The composite coating technology is more ingenious. One team grew carbon nanotubes on the fiber surface, like hanging lanterns on branches, and the effective area increased fivefold; another group of researchers coated a titanium dioxide layer, like applying a scratch-resistant film to the electrode, and the cycle stability increased by 70%. In terms of structural design, changing the traditional paving method and cutting the graphite felt into a staggered wave shape can reduce the flow resistance of the electrolyte by 30%.
In the production process, many details are crucial. When the diameter of the carbon fiber is controlled at 7-9 microns, the overall performance of the electrode is the best. If it is too thin, the mechanical strength is insufficient, and if it is too thick, the effective contact points will be reduced. The heat treatment temperature needs to be controlled in stages, with 800℃ pre-carbonization to remove impurities and 1200℃ graphitization to form a conductive network. Once the temperature deviation exceeds 50℃, the conductivity fluctuation will exceed 15%. The application scenario has a direct impact on material selection. Grid-level energy storage pays more attention to cycle life and will use electrodes that have been treated with high-temperature graphitization. Although the cost increases by 20%, the service life can be extended to 15 years. Mobile emergency power supplies focus on power density and use plasma-treated ultra-thin graphite felt. The thickness is halved while the power output is doubled. In low-temperature environments such as the polar regions, special treatment is required. The modified electrode doped with nitrogen can still maintain 80% capacity at -30℃, while the capacity of the unmodified electrode has decayed to 45% at this time.
Application of ultrasonic spraying in graphite felt electrodes
In the preparation process of graphite felt electrodes for all-vanadium liquid flow batteries, Cheersonic’s ultrasonic spraying technology shows unique value. When preparing electrodes, ultrasonic spraying can accurately and evenly spray various materials used for surface modification or composite coating, such as solutions for acid treatment, precursors for growing carbon nanotubes, or raw materials for coating titanium dioxide, on the surface of graphite felt. Using the high-frequency vibration of ultrasound, the liquid is atomized into extremely small particles, thereby achieving precise coating on the surface of the graphite felt electrode.
Significant advantages of ultrasonic spraying
The advantages of ultrasonic spraying are very significant. First, its atomized particles are extremely small, which can achieve extremely uniform coating distribution. Compared with traditional spraying methods, it greatly reduces the thickness deviation of the coating, ensures the high consistency of the performance of the graphite felt electrode, makes the reaction activity of the electrode in different areas more balanced, and improves the stability of the overall performance of the battery. Second, this technology can be sprayed at a lower temperature, effectively avoiding the damage of high temperature to the graphite felt electrode material and active substances, and can maintain the original performance of the electrode material intact, ensuring the stability and reliability of the graphite felt electrode in subsequent use. Third, the utilization rate of the coating during ultrasonic spraying is high, which can greatly reduce the waste of raw materials. From the perspective of cost control, this is of great significance for the large-scale production of all-vanadium liquid flow battery graphite felt electrodes, and can effectively reduce production costs. In addition, Cheersonic’s ultrasonic spraying equipment has a high degree of automation and stability, which can meet the needs of large-scale production, significantly improve production efficiency, and provide strong support for the industrialization development of all-vanadium liquid flow batteries.
Technical bottlenecks and future prospects
At present, the technical bottleneck of all-vanadium liquid flow battery graphite felt electrodes is concentrated on the balance between durability and cost. For example, the platinum-doped graphite felt prepared in the laboratory has excellent performance, but its material cost is ten times that of conventional products. The acid treatment process will produce nitrogen oxide waste gas, and the treatment cost accounts for 12% of the total production cost. Some companies have tried to use laser etching instead of chemical treatment, and the processing time of a single piece has been shortened from 3 hours to 20 minutes, but the equipment investment needs to increase by two million yuan.
Looking to the future, new composite materials may become the focus of development. Graphene composite felt has achieved an ultra-high current density of 150mA/cm² in the laboratory, which is five times that of traditional materials. Biomass-derived carbon fibers have also begun to enter people’s field of vision. The cost of electrodes made from bamboo fibers has been reduced by 40%, but the conductivity needs to be further improved. Three-dimensional weaving technology is being tested to make graphite felt into a honeycomb structure, and the reaction area per unit volume is expected to increase three times. With the continuous innovation of technology, especially the widespread application of advanced processes such as Cheersonic ultrasonic spraying technology, all-vanadium liquid flow battery graphite felt electrodes are expected to achieve greater breakthroughs in performance improvement and cost control, thereby promoting the wider application of all-vanadium liquid flow batteries in more fields.
About Cheersonic
Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.
Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.
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