Plasma-Enhanced Multilayer Coatings on Metal Bipolar Plates for Corrosion Resistance and High Electrical Conductivity

Metal bipolar plates are central to lowering the cost and weight of proton exchange membrane fuel cell (PEMFC) stacks, yet their long-term use is limited by aggressive corrosion in the acidic, humid, low-pH cathode environment. Multilayer protective coatings combining adhesion-promoting metal interlayers with chemically inert graphitic carbon are widely regarded as a promising solution, but they have historically been deposited slowly and from precursors such as titanium tetrachloride that are difficult to scale safely. Here, we report the design and operation of a showerhead plasma-enhanced chemical vapor deposition (PECVD) reactor that deposits Ti/TiC _x /amorphous-carbon (a-C) multilayer coatings on 304 stainless steel using the comparatively safe titanium isopropoxide (TiPO) precursor in combination with a hydrogen plasma. Both continuous radio frequency (RF) and nanosecond-pulsed glow discharges were tested at 10 W and at substrate temperatures up to 300 °C. Cross-sectional scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Raman scattering confirm uniform, pinhole-free multilayers in which Ti is reduced from TiPO by atomic hydrogen and the carbon top layer is sp²-rich and graphitic. Carbon deposition rates of 200-237 nm min^-1 were obtained, matching or exceeding the best values reported in the literature. A coupled plasma kinetics and showerhead transport model rationalizes these rates, the distinct microstructures obtained under continuous and nanosecond-pulsed excitation, and the measured coating uniformity. The best carbon coatings reach interfacial contact resistance (ICR) values of 5.4-13.5 mΩ·cm² at 1.10 MPa, with the lowest well below the 2025 U.S. Department of Energy (DOE) target of 10 mΩ·cm². The coatings showed no detectable Raman or morphological degradation after accelerated thermal cycling between 30 and 100 °C in pH 3 H₂SO₄, demonstrating their potential as a fast, safe, and scalable coating route for next-generation PEMFC bipolar plates.