Pt-Ni/WC Alloy Nanorods Arrays as ORR Catalyst for PEM Fuel Cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) among the other types of fuel cells technology are attractive power sources especially for electric vehicle applications. Even though there is ongoing progress and plausible prospects of PEMFCs, there are several challenges that need to be overcome to make them successfully and economically viable. One of the most important concerns in a PEMFC is the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) that requires an expensive platinum (Pt) catalyst with high weight loadings (~0.3 mg/cm2) to meet the power requirements, which is mostly responsible for the high cost of PEMFCs. Therefore, reduction in the major cost of the system can be achieved by increasing the ORR kinetics of platinum-based catalyst and by more efficient utilization of the Pt component. In addition, carbon support used in conventional Pt/C catalyst present problems including oxidation of carbon, formation of peroxide species causing degradation of polymeric membrane, and separation from the ionomer over time leading to loss of catalyst effectiveness. For this purpose, we developed vertically aligned nanorod arrays of tungsten carbide (WC) as a support and coated them with platinum-nickel (Pt-Ni) alloy shell. A glancing angle deposition (GLAD) technique was used to fabricate the WC nanorod arrays and the shell comprised of different atomic ratios of Pt:Ni prepared by a small angel deposition (SAD) technique for conformal thin film coating. We utilized a multi-source sputter deposition unit that is capable of performing GLAD and SAD in the same chamber. In our design, WC nanorods offer advantages including higher electron conductivity, thermal and chemically stability, compared to the conventional carbon support. In addition, GLAD WC nanorods are coated with SAD Pt-Ni thin film in the same deposition unit without breaking the vacuum allowing to minimize surface oxidation resulting in decreased contact resistance between Pt-Ni and WC. Moreover, Pt-Ni alloy shell can achieve superior ORR activity due to expected electronic and geometric effects. Pt-Ni/WC nanorods were deposited on glassy carbon electrodes and as well as on silicon substrates for evaluation of their electrocatalytic ORR activity and physical properties. Cyclic voltammetry and rotating disk electrode experiments were performed in a 0.1 M HClO4 electrolyte at room temperature to characterize the ORR activity and activity stability of Pt-Ni/WC nanorods. Scanning electron microscopy and X-ray diffraction methods were utilized to study morphology and crystallographic properties, respectively.