Mechanisms of growth, properties and degradation of amorphous carbon films by closed field unbalanced magnetron sputtering on stainless steel bipolar plates for PEMFCs

Stainless steel bipolar plates possess good manufacturability, low costs, but inadequate interfacial conductivity and corrosion resistance in proton exchange membrane fuel cells (PEMFCs). Amorphous carbon films have been deposited on stainless steel as a function of bias voltage through closed field unbalanced magnetron sputtering ion plating system (CFUMSIP) to enhance the interfacial conductivity and corrosion resistance. Surface and cross-section morphologies, hybridization, interfacial conductivity, corrosion resistance and degradation mechanisms of a-C films were systemically investigated and results are very sensitive to the substrate bias voltage. The compactness and hybridization sp2/sp3 ratio have parabolic relation with the substrate bias voltage and the a-C film deposited with 120 V has the densest cross structure and maximum sp3 percentage. Various electrochemical corrosion tests in the simulated PEMFCs cathode environment confirm the fact corrosion resistance is closely related to the film compactness and hybridization. Then a-C film prepared at bias voltage 120 V has the lowest corrosion current density. The initial interfacial contact resistance (ICR) of a-C is a combination result of sp2 percentage and film compactness and the samples deposited with bias voltage 60 V and 300 V have much lower ICR values than DOE target. Afterwards, the ICR increase mechanism of a-C film after electrochemical corrosion tests is illustrated through Raman and XPS detection, and the results reveal that increased oxygen content adsorbed on the film surface contributes to the increase of ICR value.