Coatings for metallic bipolar plates in high-temperature polymer electrolyte fuel cells

At present, High-temperature polymer electrolyte fuel cells (HT-PEFCs) are usually constructed using graphite bipolar plates (BPPs). Although continuous efforts have been made to increase their properties and reduce their volume, weight, and cost, graphite BPPs are still far from the requirements of the U.S. Department of Energy (DOE). However, metallic BPPs produced from thin metal foils are promising candidates for reaching DOE targets. The high temperature (ca. 120 °C-180 °C) and the use of phosphoric acid as electrolyte within the HT-PEFC provide a significantly aggressive environment for the components of HT-PEFC, and especially in metallic BPPs. In this thesis, stainless steel 316L (SS316L) is chosen as the substrate for the metallic BPPs, due to its reasonable degree of corrosion resistance and low cost. The corrosion properties of SS316L and the influence of gases on the corrosion behavior of SS316L in the simulated HT-PEFC environment were thoroughly examined. It is extremely difficult for bare metallic materials to reach the acceptable level of corrosion rates of HT-PEFC. Therefore, a protective coating is mandatory for the application of metallic BPPs in HT-PEFC. Based on the initial results by which a type of Cr-rich layer formed in the simulated cathodic environment of HT-PEFC was found able to tolerate the aggressive environment of hot phosphoric acid under an O2 purged atmosphere, a bilayer CrN/Cr coating was chosen as the coating research object for this work. This work investigated the corrosion behavior of bilayer CrN/Cr-coated SS316L BPPs in the simulated environment of HT-PEFC, and its self-healing ability when artificially induced defects appeared on its surface. Finally, the performances and durability of the CrN/Cr-coated SS316L BPPs were validated in a real HT-PEFC.A series of experiments revealed that CrN/Cr-coated SS316L demonstrates superior performance in the HT-PEFC environment, even including a degree of self-healing ability when defects appeared on its surface in an O2 purged atmosphere. Therefore, this work has demonstrated that bilayer CrN/Cr-coated SS316L is a promising candidate as a low-cost metallic BPP in HT-PEFCs