Langmuir 2007, 23, 5279-5282
5279
Synergistic Assembly of Dendrimer-Templated Platinum Catalysts on Nitrogen-Doped Carbon Nanotube Electrodes for Oxygen Reduction Ganesh Vijayaraghavan and Keith J. Stevenson* Department of Chemistry and Biochemistry, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed December 23, 2006. In Final Form: March 21, 2007 A study of the synergistic tuning of nitrogen-doped carbon nanotubes (NCNTs) as support- and size-monodisperse platinum nanoparticles templated from G4-NH2 dendrimers (Pt-DEN’s) as catalysts targeted toward oxygen reduction is reported. UV-vis spectroscopy, adsorption isotherms, TGA, TEM, and voltammetry were used to characterize the loading and activity of Pt-DENs immobilized on CNT and NCNT supports. The facile uptake of Pt-DENs was found to be influenced by the number of edge plane sites on the NCNT support with higher adsorption rates observed for NCNTs with increased nitrogen content. Pt-DEN/NCNT composites exhibit high activity with a mass-transportlimited current density and mass activity of 2.3 mA cm-2 and 0.05 mA g-1, respectively, for the oxygen reduction reaction (ORR).
High-surface-area carbons such as Ketjenblack and Vulcan carbon used as supports for electrocatalysis applications are beneficial in terms of providing electronic conductivity and a high dispersion of metal catalysts.1,2 Unfortunately, the multitude of preparation strategies for carbon-supported catalysts makes it difficult to understand the role of the carbon support on electrocatalysis including the degree of catalyst utilization, the promotion of catalyst-support interactions, and the stability of the catalyst toward dissolution, agglomeration, and other degradation processes. Moreover, carbon supports are typically prepared via aggressive processes for activation including refluxing in concentrated acids3 (HNO3, H2SO4, HCN) or strong oxidizing agents4 (H2O2, KMnO4) to create surface functionalities (carbonyl, carboxylate, ester-like oxygen, alcohol) to facilitate more efficient anchoring and loading of the metal catalyst via impregnation,5 coprecipitation,6 microemulsion,7 or sonochemical8 methods. The activation methods often significantly degrade the preferred structural and compositional properties of both the carbon support and active metal catalyst and have typical drawbacks of large average catalyst size, broad size distribution, and poor reproducibility.9 Of vital importance is the need to improve catalyst dispersion and utilization significantly (typically
