Carbon Coatings for Bipolar Plates in Proton Exchange Membrane Fuel Cells : Microstructure, Functional Properties, and Fuel Cell Requirements

Amorphous carbon coatings show great promise for metallic bipolar plates in proton exchange membrane fuel cells. Due to the diverse bonds of carbon, the thin films can achieve a wide range of properties, including excellent electrical conductivity and corrosion resistance. In this study, different sets of carbon coatings were synthesized on 316L stainless steel sheets using direct current magnetron sputtering. This thesis investigated the effect of different sputtering parameters, carbon bonding characteristics, and film thicknesses on the functional properties and production feasibility. The samples were characterized by scanning electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy, and X-ray reflectivity. The performance of the coatings was evaluated through electrochemical corrosion tests as well as interfacial contact resistance measurements before and after the corrosion tests. The results showed that the amorphous carbon coatings significantly improve the performance of the bare 316L stainless steel substrate. The SEM images revealed a relatively compact and dense microstructure with island growth and noticeable surface roughness. Raman spectra demonstrated clear correlations between the carbon bonding characteristics and the interfacial contact resistance. The results showed that the larger the ID/IG-ratio, the lower the ICR value. The overall best performing sample achieved an initial interfacial contact resistance of 4.68 mΩ cm2 and a current density of 0.305 µA/cm2 in the potentiostatic polarization (at +0.6 V vs Ag/AgCl for 24 h). The values met the industrial requirements for bipolar plates, defined by the US Department of Energy. However, the coating failed to satisfy the criteria in the potentiodynamic polarization, exhibiting a current density of 2.975 µA/cm2 at +0.6 V (vs Ag/AgCl). Furthermore, the ICR significantly increased after the corrosion tests to a value of 25.97 mΩ cm2. Analysis by SEM and EDX after corrosion tests revealed that the thin film underwent structural changes in the surface morphology and elemental composition. Thus, future investigations are needed to densify the carbon coating microstructure and enhance the anti-corrosive properties.