Niobium and tantalum nitride-based coatings for improved corrosion resistance and conductivity of titanium-based bipolar plates for PEMFCs

Titanium (Ti) is considered as one of the promising substrates for bipolar plates (BPPs) used in proton exchange membrane fuel cells (PEMFCs) due to its resistance to corrosion and low weight. However, its surface undergoes rapid passivation, forming a stable oxide layer that results in high interfacial contact resistance (ICR) with its adjacent components, ultimately degrading cell performance. In this study, niobium nitride (NbN) and tantalum nitride (TaN) coatings were deposited on Ti substrates using magnetron sputtering to suppress passivation and improve conductivity. A thin titanium nitride (TiN) interlayer is pre-deposited to enhance adhesion and prevent delamination. Under harsh accelerated corrosive conditions, TaN coating reduced the corrosion current density and lowered the ICR, fulfilling the 2025 U.S. Department of Energy (DOE) targets for BPPs. Chronoamperometry tests were conducted under conditions simulating PEMFC cathode-side exposure, and at an elevated potential simulating start-up/shut-down and fuel-starvation conditions demonstrated that TaN remained stable and exhibited greater resistance to degradation under both conditions, whereas NbN showed more pronounced degradation. The superior performance of TaN compared to NbN was attributed to its lower electron donor density, as indicated by Mott–Schottky analysis, which reflected the presence of a more stable and less reactive passive film. This finding underscored the critical role of nitride chemistry and electronic structure in governing corrosion and interfacial properties. Overall, the findings established TaN as an effective coating for potential usage to increase the performance, durability, and conductivity of Ti-based BPPs for PEMFCs.