Role of oxygen on functional properties of TiOxNy coating for polymer electrolyte membrane fuel cell’s bipolar plate

• Oxygen addition enhances TiN chemical stability at high electrochemical potentials • Oxygen reduces roughness and defects, improving protection and lowering activity • Oxygen content below 5 at% is key to maintaining low interfacial resistance Titanium oxynitride (TiON) coatings were synthesized via reactive magnetron sputtering to functionalize the surface of metallic bipolar plates in polymer electrolyte membrane fuel cells (PEMFCs), with the dual objective of ensuring both durability and electrical conductivity. The incorporation of oxygen into titanium nitride (TiN) was identified as a critical factor in enhancing the chemical stability of the material, particularly at elevated potentials (>1.2 V/NHE), due to the greater stability of iono-covalent bonds—characterized by partial electron sharing and separation—compared to metallic bonds. Beyond influencing the chemical properties, oxygen addition also altered the coating morphology, leading to larger columnar structures and smoother, less reactive surfaces. Ex-situ potentiodynamic polarization and interfacial contact resistance (ICR) measurements were performed to determine the optimal oxygen content, identified as 5 at.%, which simultaneously maintains a low ICR (<10 mΩ·cm²) suitable for practical applications, while significantly improving corrosion resistance relative to pure TiN coatings. This comprehensive study highlights the intricate interplay between microstructural and chemical factors, providing valuable insights for the development of advanced coatings specifically designed for PEMFC applications.