Role of oxide layer on corrosion resistance and surface conductivity of titanium bipolar plates for proton exchange membrane fuel cell

Heat-assisted forming (either hot stamping or multi-step forming with intermediate annealing) has been demonstrated as an effective novel forming process of titanium bipolar plate (Ti-BPP) with fine flow channels and high geometrical accuracy, where an oxide layer can be introduced to Ti-BPPs due to thermal oxidation. Although low-cost coating (e.g., amorphous carbon coating) is normally applied to Ti-BPPs in order to increase their corrosion resistance and surface conductivity, the impact and mechanism of oxide layer on the corrosion resistance and surface conductivity of coated Ti-BPPs remain unclear. This work finds that the oxide layer introduced by thermal oxidation enhances the corrosion resistance of coated Ti-BPPs (cathodic corrosion current density decreased from 1.54 to 0.54 mu A/cm(2)) without reducing surface conductivity. The oxide layer thickens from <10 to 100-150 nm, with its primary composition changing from TiO2 to Ti3O during thermal oxidation and coating process. The presence of Ti3O enhances the resistance of the charge transfer and point defect migration of Ti-BPPs while moderating the capacitive behavior of amorphous carbon coating. Maintaining surface conductivity relies on two key mechanisms: the reduction of concentrated and film resistance by amorphous carbon coating, and the transformation of oxide layer's composition.