Microstructure and corrosion properties of N–Cr co-doped amorphous carbon coatings for proton exchange membrane fuel cell bipolar plates

Nitrogen and chromium co-doped amorphous carbon coatings were deposited on 304 stainless steel substrates using magnetron sputtering combined with pulsed cathodic arc techniques. The influence of chromium target power on the microstructure, corrosion behavior, and electrical and mechanical properties of the coatings was systematically investigated using scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, electrochemical measurements, Hall effect measurements, and scratch testing. The results indicate that chromium doping markedly alters the bonding structure of the coatings and facilitates the formation of stable Cr–C and Cr–N ceramic phases. With increasing chromium content, the size and structural ordering of the sp2 clusters in the coatings initially increase and then decrease. At 350 W, the coating exhibits the densest structure and the highest degree of graphitization, with a corrosion potential of 0.238 V, a low corrosion current density of 1.5 × 10−9 A cm−2, and a low resistivity of 6.865 × 10−3 Ω m. The enhanced coating performance is attributed to the synergistic effects of structural ordering, the formation of stable ceramic phases, and coating densification, which collectively inhibit the penetration of corrosive species and facilitate electron transport. The results demonstrate that N–Cr co-doped amorphous carbon coatings provide a cost-effective and high-performance solution for stainless steel bipolar plates, thereby facilitating the commercialization of proton exchange membrane fuel cells.