Niobium doped amorphous carbon film on metallic bipolar plates for PEMFCs: First principle calculation, microstructure and performance

Metallic bipolar plates (BPPs) are a promising candidate to replace conventional graphite BPPs due to higher power density and lower cost in proton exchange membrane fuel cells (PEMFCs). Surface coating is essential to enhance interfacial conductivity and corrosion resistance. Amorphous carbon (a-C) films have attracted broad attention from both academia and industry. This study incorporated Niobium (Nb) into a-C to further enhance its performance. First principle calculation was introduced to investigate the evolution mechanism of bond and phase by Nb doping and instruct the coating design. Then, a series of Nb-doped a-C samples were deposited by closed field unbalanced magnetron sputtering ion plating (CFUBMSIP) method. The microstructure was systematically characterized by SEM, XRD, Raman spectrum, and XPS. Interfacial contact resistance (ICR) and electrochemical corrosion were also tested to evaluate the effect of Nb doping. A higher C-sp(2)/C-sp(3) ratio was observed in a-C film with moderate Nb doping. As a result, the ICR decreased to 1.22 m Omega.cm(2) from initial value of 4.41 m Omega.cm(2). Besides, the doped Nb also refined grain size and increased the film compactness, which was beneficial for corrosion resistance. Rct, which reflects the relative anticorrosive property, was increased from 1.8 x 10(6) Omega.cm(2) to 3.29 x 10(6) Omega.cm(2). Moreover, the current density in 0.84 V (vs. SHE) potentiodynamic polarization is 3.59 x 10(-7) A/cm(2), lower than 4.59 x 10(-7) A/cm(2) of a-C films. Besides, the current density in 0.84 V (vs. SHE) potentiostatic polarization shows the same tendency. The enhanced performance of Nb-doped a-C coatings would advance the commercialization of metallic BPPs for PEMFCs. (C) 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.