Self-passivating carbon film as bipolar plate protective coating in polymer electrolyte membrane fuel cell

The effects of W doping on the microstructure, ICR, and corrosion resistance of carbon films are systematically investigated. The W-doped carbon film has a compact structure and the surface topography changes slightly with W concentration. W exists in mainly the metallic state when the concentration is small, but amorphous WC is formed in the C_W2(A) and C_W3(A) samples. The ICR at a typical compaction force of 150 N cm^-2 increases marginally in the range of 6.25-7.21 m Ω-cm² as the W concentration is varied. The carbon films with smaller W concentrations have better corrosion resistance and even the self-passivating ability. During pulse polarization, the bare SS316L shows a very large current density of 150 µA cm^-2 due to breakdown of the passive film, but C_W1(A) shows a stable and low current density of about 0.6 µA cm^-2 due to the good self-passivating ability. The self-passivating ability originates from oxidation of metallic state Win the carbon film and so a proper W concentration can yield the desirable self-passivating effect.