CuFe2O4/CuO Bilayer Protective Coating Deposited
CuFe2O4/CuO bilayer protective coating deposited onto SUS430 interconnects by ultrasonic sprayed for solid oxide fuel cells
In the field of clean energy, solid oxide fuel cells (SOFCs) are a power generation device with great development potential. They can directly convert the chemical energy in fuels such as natural gas and hydrogen into electrical energy, with an energy conversion efficiency far exceeding that of traditional thermal power generation. Furthermore, their operation is low-pollution and low-noise, making them suitable for large-scale centralized power supply scenarios as well as distributed power sources serving industrial and civilian sectors. They are currently one of the key directions in new energy technology research and development.
A single fuel cell generates a relatively low voltage. To meet actual power demand, a large number of individual cells need to be connected in series and parallel to assemble into a fuel cell stack. The metal components connecting the individual cells are the core components ensuring the stable operation of the stack. Currently, ferritic stainless steel is often used for these metal components due to its high mechanical strength and low cost, making it suitable for the assembly requirements of fuel cells. However, this material exposes two fatal defects in the high-temperature oxidation environment of fuel cells (around 800℃), severely shortening the overall lifespan of the battery.
First is the problem of chromium poisoning from volatilization. A protective oxide film naturally forms on the surface of stainless steel. In a high-temperature, oxygen-rich environment, this chromium oxide layer further reacts, generating volatile gaseous chromium compounds. These gaseous substances migrate to the cathode area of the battery with the airflow, continuously depositing and accumulating, gradually reducing the electrode’s reactivity, increasing the battery’s internal resistance, and ultimately causing a continuous decline in power generation efficiency. Secondly, the metal substrate is constantly exposed to a high-temperature oxidizing atmosphere, causing the oxide layer to thicken continuously. This not only increases the contact resistance between components, but also, due to the difference in thermal expansion characteristics between the oxide layer and the stainless steel substrate, the oxide layer is prone to peeling and cracking after repeated thermal cycles, further accelerating battery performance degradation. In addition, the bidirectional migration of matter—oxygen penetrating inward and elements diffusing outward from the metal interior—also continuously undermines the structural stability of the components. Therefore, creating a high-performance protective coating on the surface of the metal connectors is crucial to solving these problems.
To balance conductivity and barrier properties, researchers have turned their attention to composite oxide coating materials. Spinel oxides, with their strong conductivity and strong adhesion to the metal substrate, are often used as the main body of protective coatings, capable of blocking chromium leakage and oxygen intrusion to a certain extent. Among many candidate materials, copper-iron composite spinel materials are abundant, cost-effective, have a high coefficient of thermal expansion matching with stainless steel, and exhibit excellent conductivity, making them ideal inner coating materials. While copper oxide has slightly lower electrical conductivity, it offers excellent barrier properties against the diffusion of elements like chromium and manganese. Combining the advantages of both materials, the industry has proposed a dual-layer composite coating design: an inner layer using copper-iron spinel with excellent conductivity to ensure the component’s conductivity and bonding strength; and an outer layer of copper oxide acting as a dense diffusion barrier, maximizing the protective effect through this dual action.
Traditional coating processes have significant limitations in their preparation methods. Magnetron sputtering equipment is expensive and slow, making it difficult to handle large-scale processing of large-area, complex-shaped components. While electroplating offers high deposition efficiency, it’s challenging to precisely control the proportions of various metal elements within the coating, potentially affecting its overall performance. In contrast, spraying processes are simple to operate, offer mild preparation conditions, high raw material utilization, and allow for flexible control of the coating composition. Ultrasonic spraying technology stands out, relying on high-frequency vibration to disperse the slurry into uniformly sized ultrafine droplets. This excellent droplet dispersion allows for the formation of a uniform, dense coating on the component surface, perfectly suited to the requirements of dual-layer composite coatings.
Utilizing ultrasonic spraying technology, researchers sequentially fabricated a copper-iron spinel inner layer and a copper oxide outer layer on the surface of stainless steel components, forming a complete double-layer protective system. Subsequent long-term high-temperature oxidation tests fully verified the practical value of this coating. After 1000 hours of continuous operation at 800℃, the oxide layer on the unprotected stainless steel surface continuously thickened, while the oxide layer thickness of the sample with the double coating remained at only about 3 micrometers. Data shows that this coating can significantly reduce the oxidation rate of stainless steel, achieving a protective efficiency of 86.6%, effectively curbing the problem of continuous oxide layer growth.
Conductivity is a core indicator of fuel cell connectors, and the test results were equally impressive. After 100 hours of high-temperature operation, the surface resistivity of the coated component was only 2.4 milliohms per square centimeter; even after a cumulative operating time of 1000 hours, the resistance value remained below 7.6 milliohms per square centimeter, maintaining excellent conductivity throughout. Electrochemical testing further demonstrated that the dual-layer coating effectively blocks chromium migration, significantly mitigating the destructive effects of chromium poisoning on the cathode reaction, stabilizing the electrode interface, and ensuring long-term stability of the battery’s electrochemical performance. Simultaneously, the coating provides complete overall coverage and exhibits strong adhesion to the substrate, showing no peeling or flaking even after thousands of hours of high-temperature testing, demonstrating excellent structural stability.
Compared to a single-coating structure, this dual-layer design achieves functional complementarity: the inner layer strengthens the conductive foundation, while the outer layer enhances barrier capabilities, synergistically addressing multiple challenges such as high-temperature oxidation of metal components, chromium volatilization, and elemental interdiffusion. Furthermore, the ultrasonic spraying process offers flexibility and strong scalability, overcoming the limitations of high cost and poor adaptability of traditional coating technologies, paving the way for the industrialization of this type of protective coating.
This research provides a novel solution for the long-term protection of metal components in high-temperature fuel cells. In the future, with continuous optimization of coating formulations and spraying processes, this type of dual-layer protective coating is expected to further improve overall performance, driving the development of solid oxide fuel cells towards longer lifespan, higher stability, and lower cost, helping this clean energy device to achieve large-scale commercialization more quickly and play a greater role in the energy transition process.
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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