Impact of Film Thickness on Defects and the Graphitization of Nanothin Carbon Coatings Used for Metallic Bipolar Plates in Proton Exchange Membrane Fuel Cells

Metallic bipolar plates (BPPs) are considered promising alternatives to traditional graphite BPPs used in proton exchange membrane fuel cells (PEMFCs). Major auto companies, such as Toyota, GM, Ford, and BMW, are focusing on the development of metallic BPPs. Amorphous carbon (a-C) coating are widely known to be effective at enhancing the performance of metallic BPPs. However, a-C coatings prepared by sputtering are mostly micrometers thick, which can render mass production difficult due to their low deposition rates. In this study, we investigate effects of thickness on the formation of defects and the graphitization of nanothin a-C layers deposited by magnetron sputtering from scanning electron microscope (SEM) and transmission electron microscope (TEM) observations, internal stress measurements, X-ray diffractometer (XRD) data, Raman spectra, and X-ray photoelectron spectroscopy (XPS). Furthermore, corrosion and interfacial contact resistance (ICR) test results show that an approximately 69 nm a-C layer, with a deposition time of only 15 min, can meet ex situ technical targets of US Department of Energy. As the thickness of a-C layers increases, vacancy-like defects become more pronounced, which is accompanied by stress relaxation. Furthermore, the larger the graphite-like clusters, the more sp2-hybridization carbon atoms found in loose a-C films. The good properties of nanothin a-C layers are attributed to their limited defects and proper graphitization.