Nitride films as protective layers for metallic bipolar plates of polymer electrolyte membrane fuel cell stacks

The idea of using nitride films coated by sputtering Nb and Cr, using N₂ as a reaction gas, as protective layers for metallic bipolar plates (BP) of polymer electrolyte membrane fuel cell (PEMFC) stacks, was explored by experiments. Specimens were fabricated from austenite 304 stainless steels, which are frequently used as bipolar plate materials for fuel cells due to their good corrosion resistance and high strength at elevated temperature. The results of XRD analysis and Gaussian function analysis of the coated films suggest that NbN or NbN/NbCrN films were induced depending on the process parameters. The NbN/NbCrN multiphase films were induced at high Cr target powers and low gas ratios among the process parameters selected in this investigation. The result of ICR measurement of the films suggests that the effect of the Cr target power, i.e. the effect of the Cr amount in the film, on the ICR of the films is not significant, while the effect of the gas ratio on the ICR of the films is noticeable. For the films deposited at different gas ratios, the ICR of the film generally decreases as the gas ratio increases. In general, all the 304 SS specimens coated by the NbN single phase or NbN/NbCrN multiphase films at the given process parameters showed significantly improved corrosion resistance in comparison with the bare 304 SS. Among NbN single phase and NbN/NbCrN multiphase films, the performance of NbN/NbCrN multiphase films was more stable. Depending on the process parameters, the polarization curves of the specimens coated with NbN films showed rapid increase of the current density due to the pitting. Therefore, as corrosion resistance coating for metallic BP of PEMFC stacks, NbN/NbCrN multiphase film may be preferred to NbN single phase films.