Effects of Gradient Bias on Amorphous Carbon Film for Bipolar Plates

Amorphous carbon (a-C) films were deposited at constant and gradient substrate bias using magnetron sputtering. The influence of applied substrate bias on the microstructure, chemical bonding, corrosion resistance, and electrical conductivity of a-C films was investigated. The a-C film obtained at a gradient substrate bias of 120 V to 60 V (V120-60) shows smooth surface morphology, dense multilayer, and fine columnar structures, similar to the samples prepared at a constant bias of 120 V (V120). AFM results indicate that V120-60 has the lowest surface roughness (Ra = 1.79 nm). The current density of V120-60 is 0.664 × 10⁻⁶ A/cm² at the applied 0.6 V_SCE after potentiodynamic polarization testing, which is almost half that of V60 (1.13 × 10⁻⁶ A/cm²). The initial interfacial contact resistance of V120-60 is 12.22 mΩ·cm², which is about half of that of V120 (21.80 mΩ·cm²) and close to that of V60 (10.54 mΩ·cm²). It is found that the proper balance between the corrosion resistance and interfacial conductivity was achieved in V120-60 due to the increasing sp²-C/sp³-C ratio from substrate to surface. Therefore, the present strategy with decreasing gradient substrate bias is expected to be a practical improvement process for modifying the properties of a-C films on bipolar plates. This research enhances the performance and lifespan of energy storage and conversion devices, such as fuel cells and supercapacitors.