Thin Niobium and Niobium Nitride PVD Coatings on AISI 304 Stainless Steel as Bipolar Plates for PEMFCs

In this paper, Nb, NbN, and Nb/NbN thin films were successfully deposited on AISI 304 stainless steel (304 SS) as the bipolar plate (BPP) for proton-exchange membrane fuel cell (PEMFC) by employing a radio-frequency (RF) magnetron sputtering system. Corrosion assessments in simulated PEMFC operating conditions (1 M H2SO4 + 2 mg/kg HF, 70 °C) revealed that the Nb and NbN coatings significantly improved the corrosion resistance of the 304 SS substrates. The Nb and NbN deposited samples at 350 °C exhibited superior corrosion resistance compared to those coated at 25 °C. Potentiostatic tests were also performed at the constant potentials of +0.644 and −0.056 V vs. Ag/AgCl to simulate the cathodic and anodic PEMFC conditions, respectively. The minimum current densities were recorded for the Nb coating in both anodic and cathodic conditions. Compared with the 304 SS substrate, all coatings showed lower interfacial contact resistance (ICR) and higher hydrophobicity. Among the tested coatings, the Nb coating exhibited the smallest ICR (9 mΩ·cm2 at 140 N/cm2). The results of this investigation revealed that the Nb and NbN coatings deposited by RF magnetron sputtering on 304 SS can be regarded as promising candidates for BPPs in PEMFCs.