Electrospun carbon nanofiber-supported Pt-Pd alloy composites for oxygen reduction
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Carbon nanofiber-supported Pt–Pd alloy composites were prepared by co-electrodepositing Pt–Pd alloy nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt–Pd alloy nanoparticles were controlled by the surface treatment of carbon nanofibers and the electrodeposition duration time. Scanning electron microscopy/energy dispersive spectrometer (SEM)/(EDS) and x-ray photoelectron spectroscopy (XPS) were used to study the composition of Pt–Pd alloy on the composites, and the co-electrodeposition mechanism of Pt–Pd alloy was investigated. The resultant Pt–Pd/carbon nanofiber composites were characterized by running cyclic voltammograms in oxygen-saturated 0.1 M HClO4 at 25 C to study their electrocatalytic ability to reduce oxygen. Results show that Pt–Pd/carbon nanofiber composites possess good performance in the electrocatalytic reduction of oxygen. Among all Pt–Pd/carbon nanofibers prepared, the nanofiber composite with a Pt–Pd loading of 0.90 mg/cm2 has the highest electrocatalytic activity by catalyst mass.
I. INTRODUCTION
Because of high-energy demands, fossil fuel depletions, and environmental pollution throughout the world, considerable efforts have been dedicated to developing fuel cells with good performance.1–5 However, the commercialization of fuel cells is still hindered by certain problems, including poor kinetics of both the anodic and cathodic reactions and high cost of Pt-based electrocatalysts.6–11 At the cathode, the oxygen reduction reaction is kinetically limited by endothermic and irreversible reactions and “mixed potential effect,” which are caused by the competitive reactions between the adsorption of oxygen molecules on the catalyst surface and the formation of oxide and hydroxide intermediates. For instance, the overpotential losses of oxygen reduction on Pt, a commonly used fuel cell electrocatalyst, amount to 0.3–0.4 V.10,12 To improve the catalytic ability to reduce oxygen, a second element or third is often alloyed into Pt. Fundamentally, the second or third element can alter the interatomic spacing of Pt and increase the Pt 5d orbital vacancies, thereby enhancing the efficiency of the oxygen reduction.13–16 So far, many second or third elements (such as Pd) have been alloyed into Pt-based catalysts for the oxygen reduction, and most of them used carbon particles as the catalyst support.17–22 However, the optimized efficiency of carbon particle-
II. EXPERIMENTAL A. Chemicals and reagents
Polyacrylonitrile (PAN), N,N-dimethylformamide (DMF), chloroplatinic acid hydrate (H2PtCl6H2O), ammonium hexachloropalladate [(NH4)2PdCl6], sulfuric acid
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0163 J. Mater. Res., Vol. 25, No. 7, Jul 2010
http://journals.cambridge.org
supported catalysts is relatively hard to attain because of the specific physical and chemical properties of carbon particles and their high ohmic resistance and mass transport limitation during fuel cell operation.23 Nanostructured carbons with
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