Water-in-Supercritical CO₂ Microemulsion for Synthesis of Carbon-Nanotube-Supported Pt Electrocatalyst for the Oxygen Reduction Reaction

By Shimizu, Kenichi; Cheng, I. Francis; Wang, Joanna S.; Yen, Clive H.; Yoon, Byunghoon & Wai, Chien M.
Published in Energy & Fuels NULL 2008

Abstract

Four electrocatalysts, including a commercial Pt on carbon black (Pt-CB), were compared for performance in the reduction of oxygen. Three of the catalysts were prepared on the basis of the deposition of Pt onto carbon nanotubes using (i) water-in-supercritical CO₂ microemulsion (Pt-CNT SCME), (ii) direct supercritical CO₂ fluid deposition (Pt-CNT SC), and (iii) water-in-hexane microemulsion (Pt-CNT ME). Cyclic voltammetric studies yielded an electrochemically active surface area for Pt-CNT SCME at 31.1 m²/g, which was the largest among all electrocatalysts tested in this work. Hydrodynamic polarization curves for oxygen reduction exhibited that the cell potential of the Pt-CNT SCME catalyst was over 350 mV more positive than the commercial Pt-CB system at 10 A/g of Pt. In chronoamperometric analyses, Pt-CNT SCME catalyst (6.6 x 10³ A/g of Pt) generated 2.5 times more specific activity at 30 s than Pt-CNT ME (2.6 x 10³ A/g of Pt) and 5 times more than the commercial Pt-CB (1.3 x 10 A/g of Pt). Tafel analysis indicated the exchange current density of 7.87 µA/cm² for Pt-CNT SCME that was significantly higher than the commercial Pt-CB (1.37 µA/cm²).

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