Fluoride ions regulate the disproportionation rate of Nb(IV) to produce high-quality Nbx(C,N)y coating for PEMFC bipolar plates

Coatings deposited via the disproportionation reaction of multivalent transition-metal ions in molten salts offer advantages of simple apparatus, straightforward processing, and the formation of metallurgically bonded interfaces with the substrate. In this study, high-quality niobium carbide and nitride coatings were fabricated for proton exchange membrane fuel cell (PEMFC) bipolar plates via disproportionation of Nb(IV) in molten salts. The effect of fluoride ions in the molten salt medium on the composition, microstructure, and properties of coatings deposited on austenitic stainless steel substrates was investigated. Results show that the addition of fluoride ions to NaCl-KCl molten salt effectively modulates the disproportionation rate of Nb(IV), enabling the formation of uniform and dense Nb x (C,N) y coatings on 304 SS. This regulation prevents excessive disproportionation of Nb(IV) and the consequent accumulation of niobium particles, which would otherwise accelerate the corrosion of the coating under galvanic conditions. Regular electrochemical tests demonstrated that the Nb x (C,N) y coating significantly improved the corrosion resistance of 304 SS. Moreover, even under an elevated potential of 1.0–1.6 V vs. SHE, the corrosion current density stayed below the target set of United States Department of Energy (1 μA·cm −2 ), without significant fluctuations. This demonstrates that the coating possesses outstanding chemical stability and long-term corrosion resistance.