Coating a layer of conductive corrosion-resistant coating on the titanium surface can effectively avoid the formation of oxide film on the surface of the titanium bipolar plate and meet the performance requirements of the electrode plate. In addition to corrosion resistance and excellent electrical conductivity, the coating also needs to have good bonding strength with the substrate. At the same time, since the temperature of the PEMFC will change between room temperature and 80 °C, the coating and the substrate material need to have similar thermal expansion coefficients. In order to avoid delamination and cracking of the coating during the temperature change process, the protection of the material will be lost.
Commonly used coatings are mainly divided into 2 categories, namely metal-based coatings (precious metals, metal carbon/nitride) and carbon-based coatings (graphite, conductive polymers, amorphous carbon, etc.).
Performance parameters of titanium bipolar plates with different coatings
As an important part of hydrogen fuel cells, bipolar plates play a decisive role in cell performance, cost and durability. The two important issues currently restricting the commercialization of hydrogen fuel cells are cost and durability, and the cost of bipolar plates is determined to a certain extent by the electrode material, flow field processing and electrode coating preparation process.
Graphite and carbon-based composite materials can no longer meet the requirements of hydrogen fuel cells in terms of performance, and metal materials have now become the mainstream materials for hydrogen fuel cell bipolar plates. In addition, high power has always been the pursuit of hydrogen fuel cells. Titanium and titanium alloys in metal materials have low density and high specific strength, and have excellent corrosion resistance in hydrogen fuel cells, which can significantly reduce the weight and volume of bipolar plates. The mass specific power and volume specific power of the battery are significantly improved, and the corrosion products generated by titanium and titanium alloys during long-term service operation are less toxic to proton exchange modes and catalysts, which is conducive to improving the stability and durability of battery operation.
The metal carbon/nitride and amorphous carbon coatings prepared on the surface of titanium bipolar plates have excellent comprehensive properties and have high research and application value. However, these coatings are prone to pinhole defects, so the main goal of the current research is to improve Coating compactness, film-base bond strength and coating surface conductivity. In addition, the coating should have good hydrophobicity to facilitate the discharge of the water produced by the reaction.
To meet these comprehensive properties, higher requirements are placed on the structural design and organizational composition of the coating. The composite and nano-structure of the coating structure can improve the density, corrosion resistance and electrical conductivity of the coating to a certain extent, and enhance the service stability and reliability of the titanium plate, which is the main direction of future development.