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The polymer fuel cell electrode (PFCE), consisting of platinum particles or platinum alloy particles supported on carbon particles and proton‐conducting ionomer binder, is one of the most important components in the fuel cell. The three‐dimensional (3‐D) imaging technique is important for the precise characterization of the carbon‐supported catalysts and the electrode. In this study, the Pt particles as the catalysts supported on different kinds of carbon particles were examined by transmission electron microtomography (TEMT) to obtain the first 3‐D structure of the carbon‐supported catalyst and to then evaluate the significant parameters that should allow characterization of the catalyst, i.e., the Pt nanoparticles supported on high‐surface area carbons. To obtain quantitative 3‐D images of the catalysts that contain extremely small Pt particles on the order of a few nanometers in diameter, a special sample preparation technique was developed for the TEMT experiments. As a result, the spatial resolution of ca 0.5 nm was achieved for the first time. On the basis of such very quantitative 3‐D images, a series of 3‐D image analysis algorithms were developed and used to evaluate significant structural parameters, e.g., the size and spatial distribution of the Pt particles on the carbon substrate, the “degree of embeddedness” of Pt into the carbon support, and the effective surface area of the catalyst per unit volume. These morphological parameters are useful in characterizing fuel cell catalysts and may be significant in studying the degradation mechanisms of catalysts during operation if they are examined before and after the operation.

The effect for oxygen reduction characteristics of O-functionalities formed on the edge planelike defects at the open ends of multiwalled carbon nanotubes ͑MWCNTs͒ and at the hole defects on the tube walls was investigated by using the rotating disk electrode technique. The onset potential of the electrochemical reduction of oxygen for MWCNTs shifts to the positive direction of 0.19 V, while the O-functionalities were removed from the defects sites. These results suggest that the control of functional groups is important for the control of the oxygen reduction reaction catalyst activity.In recent years, carbon nanotubes ͑CNTs͒ have attracted a great deal of attention as a new type of electrocatalyst material owing to their unique structural, electronic, chemical, and mechanical properties. 1 So, numerous studies have been made to elucidate the electrochemistry of CNTs, including the oxygen reduction reaction ͑ORR͒ characteristics in both acid 2-5 and alkaline, 6-11 because of the possibility of using them as support materials for various catalysts and catalyst materials themselves.Britto et al. 2 confirmed by cyclic voltammograms ͑CVs͒ that the multiwalled carbon nanotube ͑MWCNT͒ past electrode demonstrate a well-defined peak at Ϫ0.31 V vs saturated calomel electrode ͑SCE͒ for ORR in H 2 SO 4 ͑pH = 2͒ compared with graphite powder and the potential of using nanotubes in electrodic applications such as fuel cells. Also, the results of the density functional theory calculations indicated that the pentagon and heptagon sites could act as favorable sites for the electron transfer for ORR. However, the effect of the O-functionalities formed on the edge planelike defects at the open ends of the MWCNTs and at the hole defects on the tube walls for the oxygen reduction characteristics has not been unambiguously identified yet in acid media because the defect sites on the surface of the MWCNTs introduced by the oxidation process are decorated with the O-functionalities.The aim of this work is to investigate the effect of the O-functionalities on the edge planelike defects and hole defects on MWCNTs for the oxygen reduction characteristics by a comparison of the ORR characteristics for MWCNTs before and after the Ar treatment. ExperimentalMWCNTs with a hollow structure ͑Nano Laboratory, purity Ͼ 95%, diameter: 30 Ϯ 15 nm, length: 1-5 m͒, which have iron and sulfur as residuals, 12 were treated by the following oxidation procedure: MWCNTs were heated at 500°C in Ar to remove amorphous carbon, and were refluxed in a mixture of 2 M sulfuric acid and concentrated nitric acid ͑1:1, v/v͒ at 120°C for 4 h to form the O-functionalities on MWCNTs and to remove the impurity in MWCNTs. Then, the O-functionalities of the acid-treated MWCNTs were removed by heating them in Ar at 900°C for 1 h. For brevity, acid-treated MWCNTs as MWCNT-A and acid-treated MWCNTs, followed by Ar treatment MWCNTs, as MWCNT-B were indicated hereafter. It was examined by inductively coupled plasma-mass spectrometry ͑ICP-MS͒ that the iron content for the untr...

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