Graphene-based composite coatings can be applied on metallic bipolar plates (BPs) of PEMFC for corrosion protection and electron conduction, benefited from the high electrical conductivity, chemical stability and physical barrier properties of graphene. To reach the high electrical conductivity requirement, the composite coatings shall have high graphene content, which brings challenges including dispersion, processability and interface compatibility. In this work, polyisocyanate was grafted on graphene surfaces to enhance the compatibility with polyurethane (PU). Carbon black was introduced as conductive bridging, which filled in defects of graphene/PU coatings. The corrosion current density of the ternary composite coatings with 60 % functionalized graphene, 30 % PU and 10 % carbon black was 0.43 μA·cm−2 in H2SO4 solution of pH 3 at 80 °C, while the interfacial contact resistance was 6.75 mΩ·cm2 and the in-plane conductivity was 137.4 S·cm−1. The composite coatings provided an efficient and cost-effective solution for up-scaling production of metallic BPs.