Investigation of processing and structural properties of cathodic arc deposited carbon-based coatings for bipolar plates

With the urgent need for renewable energy and clean mobility solutions, polymer electrolyte membrane fuel cells (PEMFCs) are a crucial technology. The performance and durability of a PEMFC are heavily dependent on the metallic bipolar plate which requires a protective and conductive coating to prevent corrosion and provide high electrical conductivity. The aim of this thesis is the development and investigation of carbon-based coatings deposited via cathodic arc evaporation on SS316L substrates to improve the electrochemical properties and electrical conductivity for metallic bipolar plates. Cathodic arc evaporation offers distinct advantages in terms of high deposition rate and elevated ionization enabling rapid processing and precise tuning of coating properties. This capability allows for the development of coatings with enhanced performance characteristics important for metallic bipolar plates. First, metal ion sputtering (MIS) for surface preparation is studied. The process duration was varied from 0 to 60 min revealing that a 15-minute MIS duration beneficial for a high corrosion and low contact resistance. Following this, the impact of the chromium interlayer thickness is investigated, showing its role in enhancing corrosion resistance and further reducing ICR. Central to this thesis is the systematic variation of the process parameters for the carbon top layer deposition, including bias voltage, process temperature and pressure as well as deposition time. Here, the focus is on the individual impact of the process parameters on the microstructure and resulting properties. The results indicate that a graphitic structure is crucial for achieving suitable bipolar-plate related properties. Additionally, the optimal deposition time is established to achieve a sufficient thickness without compromising coating properties. Based on the results, a parameter combination of -600 V bias voltage, 200 °C process temperature, 0.1 Pa process pressure, and a deposition time of 2 x 60 s is preferred to further optimize the carbon-based coating. In conclusion, the combination of optimized parameters leads to a significant improvement of the functionality of SS16L as bipolar plate material via a more efficient and fast deposition process.