Abstract
Tantalum nitride (TaNx) coatings deposited by High Power Pulsed Magnetron Sputtering (HPPMS) technology at different N2-to-Ar ratios; i.e. 0, 0.25, 0.625 and 1, corresponding to different N/Ta atomic ratio in the film, were investigated as suitable candidates to improve SS316L bipolar plate's performance and durability for polymer electrolyte membrane fuel cells (PEMFC). The corrosion resistance of TaNx coatings was evaluated by potentiodynamic and potentiostatic tests carried out under different cathodic potentials (0.8 VSHE, 1 VSHE, and 1.4 VSHE), electrolyte acidities (pH 3 and pH 6) and test durations (5 and 180 min) in order to mimic real fuel cell operation conditions. Corrosion currents observed for all TaNx coatings were relatively low (1–15 μA cm−2) regardless of N/Ta atomic ratio and applied variable testing parameters. However, considerable differences in Interfacial Contact Resistance (ICR) values were observed after polarization, depending on tested coating material and conditions. The ICR increased with increasing applied potential, electrolyte pH and test duration for the substrate and all TaNx coatings. Ta coated SS316L exhibited lower ICR (42–82 mΩ cm2) values than the uncoated SS316L (47–278 mΩ cm2) at potentials higher than 1 VSHE. A significant rise in ICR was detected for all nitride TaN films after 180 min of polarization at 1.4 VSHE in pH 3, showing ICR values from 362 to 538 mΩ cm2, depending on N2-to-Ar ratio. Ta coated SS316L polarized at 0.8 VSHE showed low ICR values, around 25–37 mΩ cm2. Auger electron spectroscopy (AES) was performed before and after polarization to investigate barrier oxide film formation kinetics. AES study revealed the growth of different composition and thickness oxide layers for each TaNx coating, exposing the great importance of coating composition on subsequent type of oxide formation. Barrier oxide layer characteristics have been found to dominate the ICR response of TaNx films after polarization.
