(Titanium,chromium) nitride coatings for bipolar plate of polymer electrolyte membrane fuel cell

(Titanium, chromium) nitride (Ti,Cr)N] coatings are synthesized on a 316L stainless-steel substrate by inductively-coupled, plasma-assisted, reactive direct current magnetron sputtering. The chemical and electrical properties of the coating are investigated from the viewpoint of it application to bipolar plates. Nanocrystallized Cr–Ti films are formed in the absence of nitrogen gas, while a hexagonal β-(Ti,Cr)₂N phase is observed at N₂ = 1.2 sccm. Well-crystallized (Ti,Cr)N films are obtained at N₂ > 2.0 sccm. The corrosion resistance of the coating is examined by potentiodynamic and potentiostatic tests in 0.05 M H₂SO₄ + 0.2 ppm HF solution at 80 °C, which simulates the operation conditions of a polymer electrolyte membrane fuel cell. The Davies method is used to measure the interfacial contact resistance between the sample and carbon paper. The (Ti,Cr)N coating exhibits the highest corrosion potential and lowest current density. In a cathode environment, the corrosion potential and current density are 0.33 V (vs. SCE) and <5 × 10⁻⁷ A cm⁻² (at 0.6 V), respectively. In an anode environment the corresponding values are 0.16 V and <−5 × 10⁻⁸ A cm⁻² at −0.1 V. The (Ti,Cr)N coatings exhibit excellent stability during potentiostatic polarization tests in both anode and cathode environments. The interfacial contact resistance decreases with deposition of the (Ti,Cr)N film, and a minimum value of 4.5 mΩ cm² is obtained at a compaction force of 150 N cm⁻², which indicates that the formation of oxide films can be successfully prevented by the (Ti,Cr)N film. Analysis with Auger electron spectroscopy reveals that the oxygen content at the surface decreases with increase in the nitrogen content.