Hydrogen is the key to a climate-friendly or even climate-neutral industry from steel production to mobility. For the latter, polymer electrolyte membrane fuel cells are the choice because of the good efficiency, long travel distances and fast refuelling cycles. Major obstacle are the high production costs and the limited operational life time which, among other things, depends on the stability of metallic bipolar plates (BPP) in the acidic fuel cell environment. Austenitic stainless steel (SS316L) exhibits a promising combination of properties such good mechanical properties such as formability and strength, rendering it as a suitable material for bipolar plates. Because of the native oxide layer, the corrosion resistance is relatively good, but not sufficient. In addition to this, the oxide layer leads to a high electrical resistance resulting in performance losses. Several coating variants were and are under investigation to prevent corrosion and increase the electrical conductivity. Especially, carbon-based coatings show excellent properties, but are often deposited by magnetron sputtering which has a limited deposition rate.

In this work, the effect of the process pressure on the structure as well as electrochemical and electrical properties of carbon-based coating, deposited by cathodic arc evaporation, on SS316L substrates is investigated. The structure is examined by Raman spectroscopy and electron microscopy. Potentiodynamic polarization tests and interfacial contact resistance measurements were conducted to evaluate the corrosion resistance as well as electrical conductivity. It is found that the best coating variant has a graphite-like structure with a very low contact resistance of 1.5 mΩcm2 as well as good corrosion resistance with a corrosion current density below 10-6 Acm-2. Additionally, the microstructure changes with the process pressure, e.g. at 0.1 Pa the structure is graphite-like. The resulting microstructure depending on pressure has a great influence on the corrosion and electrical resistance.

References:

D. James et al., “Bipolar Plate Cost and Issues at High Production Rate”, DOE Workshop on Research and Development Needs for Bipolar Plates for PEM Fuel Cell Technologies, 2017

Z. Xu, D. Qiu, P. Yi, L. Peng, and X. Lai, “Towards mass applications: A review on the challenges and developments in metallic bipolar plates for PEMFC,” Progress in Natural Science: Materials International, vol. 30, pp. 815–824, 2020.