Tatsuya Hatanaka

Cell performances of direct methanol fuel cells with grafted membranes

Cell performances were evaluated with grafted polymer membranes as an electrolyte for a direct methanol fuel cell (DMFC). The membranes were prepared using a poly(ethylene-tetrafluoroethylene), or ETFE, film. The base polymer film was added to sulfonic groups using γ-radiation activated grafting technique as ion-exchange groups. These membranes had more suitable properties for DMFCs, i.e. higher electric conductivity and lower methanol permeability than perfluorinated ionomer membrane (Nafion). Nevertheless, the cell performance with the grafted membrane was inferior to that with Nafion. The analysis of electrode potentials vs. reversible hydrogen electrode showed larger activation overpotential for both the electrodes on the grafted membranes. We concluded that this is due to poor bonding of the catalyst layers to the grafted membranes.

Measurement of Flooding in Gas Diffusion Layers of Polymer Electrolyte Fuel Cells with Conventional Flow Field

The objective of this work is to clarify the location and magnitude of flooding in polymer electrolyte fuel cells with a conventional flow field experimentally and by a numerical calculation. In the experiment, a newly developed cell with a conventional, interdigitated-switchable gas-flow field was used and the pressure drop between the inlet and outlet of the cathode was measured with the interdigitated flow field after the cell was operated with the conventional flow field. Significant pressure drop was observed after high current densities were flown; the pressure drop indicates the flooding level in a gas diffusion layer (GDL) during the conventional flow field operation. The cell performance and the flooding behavior depended significantly on wetting properties of catalyst layers and GDLs. In the simulation, liquid water distribution in the cathode GDL was predicted using a two-phase model, and validated by comparing the pressure drop measured and calculated using a gas-flow model. The simulation results agreed well with experimental data at high humidity condition and showed that a large amount of liquid water exists in the cathode GDL at high current densities.

First principles study of sulfuric acid anion adsorption on a Pt(111) electrode

A first principles theory combined with a continuum electrolyte theory is applied to adsorption of sulfuric acid anions on Pt(111) in 0.1 M H(2)SO(4) solution. The theoretical free energy diagram indicates that sulfuric acid anions adsorb as bisulfate in the potential range of 0.41 < U ≤ 0.48 V (RHE) and as sulfate in 0.48 V (RHE) < U. This diagram also indicates that sulfate inhibits formations of surface oxide and hydroxide. Charge analysis shows that the total charge transferred for the formation of the full coverage sulfate adlayer is 90 μC cm(-2), and that the electrosorption valency value is -0.45 to -0.95 in 0.41 < U ≤ 0.48 V (RHE) and -1.75 to -1.85 in U > 0.48 V (RHE) in good agreement with experiments reported in the literature. Vibration analysis indicates that the vibration frequencies observed experimentally at 1250 and 950 cm(-1) can be assigned, respectively, to the S-O (uncoordinated) and symmetric S-O stretching modes for sulfate, and that the higher frequency mode has a larger potential-dependence (58 cm(-1) V(-1)) than the lower one.

Catalytic Activity of Pt/TaB2(0001) for the Oxygen Reduction Reaction

Proton-exchange-membrane fuel cells (PEMFCs) are a promising power source for automobiles. For their wide application, however, there still remain several problems. 2] One problem is the limited mass activity (reaction rate per mass) of cathode electrocatalysts for the oxygen reduction reaction (ORR). Bulk Pt has a high specific activity (reaction rate per surface area), and the specific activity can be further increased by alloying the subsurfaces with several nonprecious metals, such as Fe, Co, Ni, Cu, Sc, or Y, or by replacing subsurfaces with Pd. However, the specific areas (surface area per mass of the precious metal) of bulk materials are small, and therefore, the mass activities (specific activity multiplied by specific area) are also small. To increase the mass activity, the specific surface area should be increased by decreasing the catalyst size to the nanometer scale. Although Pt nanoparticles supported on carbon (Pt/C) are used practically in PEMFCs, the mass activity is not sufficiently high because the decrease in the size of the catalyst leads to a decrease in the specific activity as a result of the so-called particle-size effect. 9, 10] To avoid the particlesize effect, the specific surface area must be increased while maintaining the extended bulklike surface morphology. This new approach was employed by the company 3M in the development of nanostructured thin-film (NSTF) catalysts, in which Pt films with a thickness of a few tens of nanometers are deposited on organic nanostructured whiskerlike supports. The discovery of these new electrocatalysts inspired a number of studies on the fabrication of electrocatalysts with an extended Pt surface and high specific surface area with the aim of further increasing the mass activity. Herein, we show that a high mass activity of 1890 Ag , which is six times as high as that of Pt/C (299 Ag ), can be attained by the use of an epitaxial Pt thin film with a thickness of 1.5 nm on a TaB2(0001) single-crystal substrate. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Pt/TaB2 structure at the 10 10 TaB2 incidence and the 2 1 10 TaB2 incidence are shown in Figure 1. TaB2(0001) was selected because of its strong bonding with Pt, as shown by our DFT calculations, which indicated a cohesive energy of the Pt monolayer on TaB2(0001) terminated with Ta of 6.47 eV, which is much larger than that of a Pt monolayer on a graphene sheet (3.76 eV) or on Pt(111) (5.06 eV). We deposited Pt on the cleaned TaB2(0001) substrate and carried out CO annealing to obtain a flat and uniform Pt surface. Figure 2a shows the cyclic voltammogram (CV) recorded during CO annealing. In the anodic scan of the first cycle, the oxidation current appeared at approximately 0.5 V and then gradually increased (preignition potential region). This oxidation current disappeared in following cycles. The oxidation current in the preignition region is due to CO oxidation at the sites of Pt adatoms and adislands; thus, the disappearance of this current suggests the elimination of the Pt adatoms and adislands. Figure 2b shows the voltammogram for CO stripping in an argon-purged solution. The electrochemical surface area (ECSA) was estimated from the charge of 420 mC cm 2 required for CO oxidation to be 0.21 cm, which corresponds to a roughness factor (ECSA/ geometrical surface area) of 1.06. The ORR activity of the Pt/ TaB2(0001) alloy was evaluated by linear sweep voltammetry with a rotating disk electrode (measurement at 1600 rpm) under oxygen-saturated conditions (Figure 2c). The current on Pt/TaB2(0001) was corrected to compensate for the geometrical conditions of the working electrode (see Figure S1 in the Supporting Information for details). The specific activity of Pt/TaB2(0001) (kinetic current at 0.9 V) was 4961 mAcm , which is more than twice that observed for polycrystalline Pt (1400 mAcm ) and Pt(111) (1867 mAcm ). The mass activity of Pt/TaB2(0001) was 1890 Ag , which is almost six times that of Pt/C (299 Ag ). The CVs recorded under argon-saturated conditions are shown in Figure 2d. The shape of the CV of Pt/ TaB2(0001) is more similar to that of polycrystalline Pt than to Figure 1. HAADF-STEM images of the epitaxial Pt thin film on the TaB2(0001) single-crystal substrate as observed for: a) 10 10 1⁄2 TaB2 incidence; b) 2 1 10 1⁄2 TaB2 incidence.

Dielectric properties of Pb(ZrxTi1−x)O3 single crystals including monoclinic zirconia

Pb(Zr x Ti 1-x )O 3 single crystals including monoclinic zirconia were grown from Pb(Zr 0.5 Ti 0.5 )O 3 ceramics in a PbO-KF-PbCl 2 flux. The composition of the crystals was determined as Pb(Zr 0.3 Ti 0.7 )O 3 and the crystals included 8 vol% zirconia particles with a size of ∼5 μm. The dielectric constants showed a maximun around 415°C although the expected phase transition point (T c ) is about 440°C. The shift of T c was discussed in terms of the phenomenological Landau theory and explained by the existence of the internal compressive stress caused by the volume expansion of ZrO 2 .

Observation of domain structures in tetragonal Pb(ZrxTi1-x)O3 single crystals by chemical etching method

Domain structures in tetragonal Pb(ZrxTi1-x)O3 (PZT) single crystals etched by HF+HCl solution were observed by means of a scanning electron microscope. The relationship between the etch patterns of pseudocubic {100} faces and spontaneous polarization vectors was examined using micro-Raman spectroscopy and electron channeling patterns. Both the a-face and the positive c-face are quickly etched, but the former results in a rough surface while the latter results in a smooth one. The negative c-face is rather slowly etched. Another type of etch pattern with a wedge-shaped section was observed; this is usually observed in PZT ceramics but has not been reported in BaTiO3. This pattern can be explained by the difference in etch rates between various {100} faces and the peculiar domain structures which consist of a single domain region with narrow or wedge-shaped 90° domains. It is assumed that such a domain structure is, although unstable under electrical stress, stable under mechanical stress.

Degradation of Perfluorinated Membranes Having Intentionally Formed Pt-Band

INTRODUCTION It has been well known that perfluorinated membranes suffer serious degradation in the PEFC environment in a short period of time. This degradation seems to be caused by chemically aggressive species like hydroxy radical generated by reactions involving the cross-leaked gases . The detail is, however, yet to be fully understood, especially on the role of precipitated Pt particles inside the membrane, namely Pt-band. Some studies showed that the radicals are generated on Pt-band and the membrane degradation is initiated there . Other studies, however, showed that Pt-band is nothing to do with the degradation 3 or even that it decomposes radicals and suppresses the degradation. In this study, open-circuit durability tests were carried out for intentionally Ptprecipitated membranes with and without electrodes to clarify the roles of Pt-band and the electrodes.

Wetting Induced Oxidation of Pt-based Nano Catalysts Revealed by In Situ High Energy Resolution X-ray Absorption Spectroscopy

In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption <O2 adsorption <O2 + H2O coadsorption) suggests that cooperative behavior between O2 and H2O adsorptions is responsible for the overpotential induced by hydrated species in fuel cells. From the alloying and particle size effects, it is found that both strength of O2/H2O adsorption and their cooperative effect upon coadsorption are responsible for the specific activity of Pt catalysts.

Analysis of Electronic State of Pt Small Particles on Carbon Substrate by sXPS

INTRODUCTION For wide commercialization of fuel cell technologies, reduction in the required amount of platinum is one of the top-prioritized issues. The catalytic activity toward oxygen reduction reaction of platinum, which is the element having the highest activity as a single metal, is known to be dependent on kinds of neighboring elements: i.e. alloy etc., and particle size. This dependency and catalytic activities of modified platinum should stem from the electronic state of platinum particles because catalytic reactions proceed by the electronic interaction among catalysts and reactants. Nevertheless, actual measurement of realistic platinum particles has not been carried out to clarify the relation between the catalytic activity and electronic state of Pt. In this communication, the electronic state of surface platinum atoms on carbon substrate was measured by sXPS (soft X-ray Photoelectron Spectroscopy) and calculated by DFT as the first steps of the discussion on the electronic state and catalytic activity.