Takaaki Mizukami

Platinum–Phosphorus Nanoparticles on Carbon Supports for Oxygen-Reduction Catalysts

Highly dispersed platinum nanoparticles on carbon supports were synthesized by electrochemical reducing platinum ions in an aqueous solution containing hypophosphite H3PO2 . Adding H3PO2 during the synthesis of the catalyst was effective for reducing platinum particle size, and the platinum particles with a mean size of 2.0–2.3 nm were obtained at a high platinum loading amount of over 50 wt %. The oxygen-reduction activity of the catalysts that added H3PO2 was higher than that of the catalyst that did not add H3PO2, which was due to the large surface area of the platinum in the former catalyst. According to the results of scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analysis, the phosphorus in the catalysts bonded with the surface of platinum particles as an oxide. The growth suppression of platinum particles was therefore attributed to the existence of a phosphorus oxide on the surface of platinum particles. But, adding H3PO2 excessively reduced the platinum surface area.

NiO Cathode Dissolution and Ni Precipitation in Li/Na Molten Carbonate Fuel Cells: Distribution of Ni Particles in the Matrix

Due to the dissolution of the lithiated nickel oxide cathode, the life expectancy of a molten carbonate fuel cell is reduced. The use of a Li/Na carbonate electrolyte is expected to lead to a higher voltage and a longer life expectancy due to its higher ionic conductivity and its lower nickel oxide cathode solubility. Using the Li/Na electrolyte, single cells have been tested to evaluate their performance and their life expectancy. Empirical equations for these cells have been presented to determine the temperature, the CO 2 partial pressure in the cathode gas, and the matrix thickness. The results prove that the life expectancy of LilNa cells is reduced by nickel short-circuiting in comparison to Li/K cells, for which the life expectancy is many times longer. The dependence of the nickel-containing particle distribution in the matrix on the temperature has been evaluated using an image processing method. At 973 K, most of the particle distribution moves toward the anode more rapidly than at 873 K, because the rate of particle growth is lower at the higher temperature, and the particles move toward the anode due to the convection of the molten carbonate in the matrix. The initiation time for nickel short-circuiting was derived from the results of this study to explain the relationship between the shorting conductance and the volume of nickel-containing materials in the matrix porosity. Moreover, the results show that the predominant element contributing to short-circuiting is the nickel oxide, and not the metal.

Microstructure of Platinum-Carbon Agglomerates with Hydrocarbon-Based Binder and Its Effect on the Cathode Performance of PEFC

The relationship between the microstructure of platinum-carbon (Pt/C) agglomerates and cathode performance was investigated for membrane-electrode-assemblies (MEAs) with a hydrocarbon-based (HC) binder and a poly (perfluorosulfonic acid) (PFSA) binder. The MEA with an HC binder exhibited a higher gas diffusion resistance than that with the PFSA binder. SEM, TEM, and pore size distribution measurements showed that the HC binder was likely to cover a larger area of the carbon support surface compared with the PFSA binder, and that a large amount of the HC binder easily penetrated the primary pores inside the Pt/C agglomerates, which decreased the volume of the pores. It seems probable that the HC binder in the primary pores blocked the oxygen diffusion to the cathode catalyst. Based on the above consideration, we focused on increasing the primary pore volume. Consequently, the volume was doubled and the gas diffusion resistance at 0.25 A/cm 2 was successfully reduced from 1600 to 410 mΩ·cm 2 .

Effect of Surface Composition of Platinum-Ruthenium Nanoparticles on Methanol Oxidation Activity

Platinum-ruthenium (PtRu) nanoparticles on carbon supports were synthesized by electroless plating. Methanol oxidation reaction (MOR) activity of the PtRu nanoparticles was investigated focusing on their bulk and surface compositions. It was demonstrated that the MOR activity did not correlate with the bulk composition but with the surface composition. In the case of PtRu nanoparticles, the maximum MOR activity was observed at a surface composition of around Pt50Ru50 (at. %). Introduction PtRu nanoparticles on carbon supports are widely used as an anode catalyst for direct methanol fuel cells due to its higher MOR activity relative to a Pt catalyst. Since the MOR occurs on the surface of the catalyst, it is essential to optimize the surface composition of the PtRu catalyst. The correlation between the MOR activity and the surface composition has been reported using PtRu bulk-plate as a model catalyst [1-3]. In this report, the correlation was investigated using PtRu nanoparticles. Experimental The PtRu nanoparticles on carbon supports were synthesized by electroless plating [4]. The bulk and surface compositons of the PtRu nanoparticles were evaluated with an inductively coupled plasma spectrometer and a copper under potential deposition/copper stripping technique [3], respectively. The crystallographic structure of the PtRu nanoparticles was analyzed by X-ray diffraction (XRD). The MOR activity of the PtRu nanoparticles was measured by linear sweep voltammetry using a rotating disk electrode in a 0.5 mol/l aqueous solution containing a 1.0 mol/l methanol at 308 K. Results and Discussions The lattice constant of the PtRu nanoparticles decreased with the increase of the bulk Ru composition (Fig. 1). The lattice constant was consistent with the value of the PtRu bulk-alloy, indicating the PtRu nanoparticles formed the solid-solution alloy. The correlation between the bulk and surface compositions of the PtRu nanoparticles is shown in Fig. 2. It is clear that the bulk composition was not the same as the surface one. Moreover, it was found that the MOR activity of the PtRu nanoparticles strongly depends on the surface composition as demonstrated in Fig. 3. The maximum MOR activity was observed at a surface composition of around Pt50Ru50. In the case of the bulk- plate PtRu catalysts, the highest MOR activity was observed with the surface composition of Pt80Ru20 to Pt60Ru40 [1-3]. The mobility of chemisorbed CO is reported to be lower on the PtRu nanoparticles than on the PtRu bulk-plate [5-6], which required the higher Ru surface composition in the PtRu nanoparticles to oxidize methanol smoothly. References [1] H.A. Gasteiger, et al., J. Electrochem. Soc., 141, 1795 (1994) [2] T. Iwasita, et al., Langmuir, 16, 522 (2000) [3] C.L. Green, et al., J. Phys. Chem. B, 106, 11446 (2002) [4] S. Suzuki, et al., J. Electrochem. Soc., 156, B27 (2009) [5] F. Maillard, et al., Faraday Discuss., 125, 357 (2004) [6] P.K. Babu, et al., Electrochimica Acta, 53, 6672 (2008) Figure 2. Bulk and surface Ru compositions of PtRu nanoparticles. Figure 3. Correlation of MOR activity (0.5 V vs. RHE) of PtRu nanoparticles with surface Ru composition. 20 30 40 50 60 70 80 20 30 40 50 60 70 80 Su rf ac e R u co m po sit io n / at . % Bulk Ru composition / at. % 0 20 40 60 80 100 120 20 30 40 50 60 70 80 M et ha no l o xi da tio n cu rr en t / μ A cm -2 -P tR u Surface Ru composition / at. % 0.382 0.384 0.386 0.388 0.390 0.392 0.394 0 20 40 60 80 100 La tti ce co ns ta nt / n m Bulk Ru composition / at. % Abstract #875, 218th ECS Meeting,

Effect of Pore Size Distribution on Cathode Performance of Membrane-Electrode Assembly with a Hydrocarbon-Based Binder

The relationship between pore size distribution and cathode performance was investigated for membrane-electrode-assemblies (MEAs) with a hydrocarbon-based (HC) binder and a poly (perfluorosulfonic acids) (PFSA) binder. The MEA with an HC binder exhibited a higher gas diffusion resistance than that with the PFSA binder. The pore size distribution measurement revealed that the HC binder was likely to cover a larger area of the carbon support surface compared with a PFSA binder, and that a large amount of the HC binder easily penetrated the primary pores inside Pt/C agglomerates, which decreased the volume of the pores. Conceivably, the HC binder in primary pores blocked the oxygen diffusion to the cathode catalyst. Based on the above consideration, we focused on increasing the primary pore volume. Consequently, the volume was doubled, and therefore, the gas diffusion resistance at 0.25 A cm-2 was successfully reduced from 1600 to 410 mΩ cm2.