Effects of phase transformation on the corrosion resistance and conductivity of NbN coatings for metal bipolar plates

Metal bipolar plates are the core components of proton exchange membrane fuel cells (PEMFC), which require excellent corrosion resistance and electrical conductivity. Coating technology is often employed to modify the surface to satisfy the requirement. In this paper, the phase structure, elemental composition and microscopic morphology of the NbN coating can be changed by adjusting the sputtering power. Under the low sputtering power, the NbN coating is a δ-NbN face-centered cubic structure, with an N content of 43.92 at.% and an Nb content of 56.08 at.%. The electrochemical corrosion test shows that the corrosion resistance of the coating shows an increasing trend with the increase of sputtering power. The β-Nb₂N hexagonal coating generated at a higher sputtering power has a better corrosion resistance of 2.805 × 10⁻⁷ A cm⁻². After the 8-h potentiostatic polarization test, the corrosion current density was maintained at 10⁻⁷ A cm⁻², which satisfy the requirement of the bipolar plate. The results of interfacial contact resistance and energy-state density show that the covalent properties of the β-Nb₂N hexagonal structure are stronger, and the conductivity of the δ-NbN face-centered cubic structure is better.