Yu Morimoto

Comparison of methanol oxidations on Pt, Pt/Ru and Pt/Sn electrodes

Abstract Electrochemical oxidations of methanol were compared on smooth or high area platinum, platinum|tin and platinum|ruthenium electrodes. On smooth platinum, while ruthenium promoted the catalytic activities steadily according to the coverage, tin showed an enhancing effect only for a short period of time because of the dissolution of tin from the surface. On high area platinum, tin seemed much stabler and exhibited long-lasting enhancing effects as well as ruthenium. The reasons for the inconsistent effects of tin among past studies are discussed.

CO oxidation on smooth and high area Pt, Pt-Ru and Pt-Sn electrodes

The electrochemical oxidation of COad was investigated on smooth and high area platinum, platinum—tin and platinum—ruthenium electrodes. On high area platinum, COad was classified into two types by their oxidation potentials: an easily oxidized one and one which is hard to oxidize. Pt-Ru was found to promote the oxidation of the latter, whereas Pt-Sn did the same for the former. The ternary system, Pt-Ru-Sn, showed even higher catalytic activity for COad oxidation than the binary systems.

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.

Analysis of Oxygen Dissolution Rate from Gas Phase into Nafion Surface and Development of an Agglomerate Model

To clarify the rate determining process for the oxygen transport in a catalyst layer of PEFCs, an oxygen dissolution resistance from gas phase into Nafion surface was measured. The oxygen dissolution resistance, which was estimated form the relationship between Nafion thickness on Pt electrode and the diffusion-limited current density on the electrode, was found to account for a large part of the total oxygen transport loss. By introducing the oxygen dissolution resistance, an agglomerate model was able to predict I-V performance of a PEFC using a reasonable size of agglomerate.

Relative Humidity Dependence of Pt Utilization in Polymer Electrolyte Fuel Cell Electrodes: Effects of Electrode Thickness, Ionomer-to-Carbon Ratio, Ionomer Equivalent Weight, and Carbon Support

Pt utilization, i.e., the ratio of the electrochemically active Pt surface area of an MEA to the total Pt surface area of the catalyst, was analyzed at 20-100% RH, and 40-80°C by a newly developed technique using CO as a probe. The Pt utilization significantly decreased with the decreasing relative humidity (RH) for a Pt/C catalyst in which the Pt particles are supported not only on the outer surface, but also within the pores of the carbon particles, while it remained high even at a low RH for the samples with Pt particles supported only on the outer surface of the carbon particles. We presume that this is because the Pt particles within the pores are not in contact with the ionomer therefore, the protons cannot access them at a low RH.

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.

Activities and Stabilities of Au-Modified Stepped-Pt Single-Crystal Electrodes as Model Cathode Catalysts in Polymer Electrolyte Fuel Cells

The purpose of this study is to test the concept of protecting vulnerable sites on cathode catalysts in polymer electrolyte fuel cells. Pt single-crystal surfaces were modified by depositing Au atoms selectively on (100) step sites and their electrocatalytic activities for oxygen reduction reaction (ORR) and stabilities against potential cycles were examined. The ORR activities were raised by 70% by the Au modifications, and this rise in the activity was ascribed to enhanced local ORR activities on Pt(111) terraces by the surface Au atoms. The Au modifications also stabilized the Pt surfaces against potential cycles by protecting the low-coordinated (100) step sites from surface reorganizations. Thus, the surface modification by selective Au depositions on vulnerable sites is a promising method to enhance both the ORR activity and durability of the catalysts.

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.

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.