Failure of Au-coated metallic bipolar plates for fuel cell in a 3-kW stack under the new European driving cycle

The studies on coating materials and their failure modes are significant for promoting the commercialization of metallic bipolar plates (BPPs) in proton exchange membrane fuel cells. Here in this work, the influence of simulative operation condition in automobile on the degradation of gold (Au)-coated 316 L BPPs is systematically investigated using New European Driving Cycle (NEDC) in a 3-kW stack for 5000 h. The original stack showed 36% decay after 5000 h while negligible change on ohmic resistance (3-5 m omega) is surprisingly found, indicating the stable interface between aged BPPs and membrane electrode assembly (MEA). However, 15% voltage loss is found in a reassembled single cell using aged BPP and fresh MEA, indicating the degradation of BPPs indeed. Further short stack matching testing and microscopic analysis reveal the well matching state and unobstructed conductivity between BPPs and MEA in the original stack, while it would be broken during reassembly (the high-frequency resistance has increased by about 2 times). The postmortem analysis on degraded BPPs indicates the failure mode of coating including the aggregation of the Au, the growth of oxides and gradual coverage of Au. The failure of coating not only results in the increase of interfacial contact resistance (ICR) from 5.4 m omega cm2 to 137.8 m omega cm2 and decrease of contact angle from 63.5 degrees to 0 degrees in some severely corroded region, but also has significant impact on the mass transport due to increased surface roughness. Especially, the formed passive layer is found to be dissolved at high potential, leading to the ion precipitation risk. These results provide guidance for optimizing coating materials, operation conditions, and assembly procedures in the future.