Zyun Siroma

Platinum dissolution and deposition in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling

The behavior of platinum dissolution and deposition in the polymer electrolyte membrane of a membrane-electrode-assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using potential cycling experiment and high-resolution transmission electron microscopy (HRTEM). The electrochemically active surface area decreased depending on the cycle number and the upper potential limit. Platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. Platinum deposition was accelerated by the presence of hydrogen transported through the membrane from an anode compartment. Platinum was transported across the membrane and deposited on the anode layer in the absence of hydrogen in the anode compartment. This deposition was also affected by the presence of oxygen in the cathode compartment.

Stability of Corrosion-Resistant Magnéli-Phase Ti4O7-Supported PEMFC Catalysts at High Potentials

Substoichiometric titanium oxide (Ti 4 O 7 )-supported Pt catalysts were prepared and their electrochemical properties, particularly the effects of high-potential conditions on the activity and stability of Pt/Ti 4 O 7 catalysts, were compared to those of Pt/C catalyst. Polarization measurements using membrane electrode assemblies revealed that the Pt/Ti 4 O 7 cathode shows a similar activity for the oxygen reduction reaction as Pt/C catalyst at 80°C. A high-potential holding test (1 h holding at 1.0-1.5 V vs anode) demonstrated that the Pt/Ti 4 O 7 catalyst is quite stable against high potential up to 1.5 V. A single cell using a Pt/Ti 4 0 7 cathode was operated at 80°C, and voltage stability for >350 h with H 2 /O 2 was also demonstrated.

Electrochemical scanning tunneling microscopy analysis of the surface reactions on graphite basal plane in ethylene carbonate-based solvents and propylene carbonate

Abstract In order to elucidate the mechanism of surface film formation on graphite negative electrodes of rechargeable lithium-ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite were observed in a few electrolyte solutions under polarization by electrochemical scanning tunneling microscopy. In 1 M LiClO4/ethylene carbonate (EC) + diethyl carbonate, a hill-like structure of ∼ 1 nm height appeared on the surface of highly oriented pyrolytic graphite at 0.95 V versus Li/Li+, and then changed at 0.75 V to irregular shaped blister-like features with a maximum height of ∼ 20 nm. In 1 M LiClO4/EC + dimethoxyethane, hemispherical blisters of ∼ 20 nm height appeared at 0.90 V. These morphology changes, hill and blister formation, were attributed to the intercalation of solvated Li− ions into graphite interlayers and to the accumulation of its decomposed products, respectively. On the other hand, only rapid exfoliation and rupturing of graphite layers were observed in 1 M LiClO4/propylene carbonate (PC), which was considered to be responsible for ceaseless solvent decomposition when graphite electrodes are charged in PC-based solutions. From the observed topographical changes, it was concluded that the intercalation of solvated Li+ ions is a necessary step for stable surface film formation on graphite.

Platinum and molybdenum oxide deposited carbon electrocatalyst for oxidation of hydrogen containing carbon monoxide

Carbon-supported Pt/MoOx catalysts for use in PEFC anodes were prepared and their catalytic activity for the oxidation of CO-contaminated H2 was examined based on the fuel cell performance in PEFC single cell arrangements. Based on the XRD pattern and XPS measurements of the prepared Pt/MoOx/C catalysts, it was found that the deposited MoOx exists as an amorphous oxide phase. The MoOx phase shows a redox peak at around 0.45 V, which was revealed by the cyclic voltammogram of the Pt/MoOx/C in sulfuric acid solution. The PEFC performance of the cell with Pt/MoOx/C was improved under 100 ppm CO-contaminated H2 conditions compared to the Pt/C catalyst, and was almost comparable to the PtRu(1:1)/C catalyst.

Preparation of platinum–ruthenium onto solid polymer electrolyte membrane and the application to a DMFC anode

The ‘impregnation–reduction method’ has been investigated as a tool for the preparation of a direct methanol fuel cell (DMFC) anode. In this method, PtRu electrocatalysts were directly bonded onto a polymer electrolyte membrane by the chemical reduction of a mixture of Pt and Ru complexes impregnated in the membrane. The deposited PtRu particles were embedded in the 3–4 μm region of the membrane surface to form a porous and hydrophilic layer. The PtRu layers turned out to be applicable to the DMFC anode, despite their small active surface areas compared to PtRu nanoparticles used in the conventional method. Approximately, 3 mg cm−2 of the PtRu layer exhibited better catalyst utilization and facilitated the release of evolving CO2. This preparation technique is attractive for the application of various solid polymer electrolyte materials with low heat-resistance or various shapes, etc.

Characteristics of a Platinum Black Catalyst Layer with Regard to Platinum Dissolution Phenomena in a Membrane Electrode Assembly

The nature of platinum dissolution and precipitation in a polymer electrolyte membrane of a membrane electrode assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using a potential holding experiment at 1.0 V vs a reversible hydrogen electrode and high-resolution transmission electron microscopy. The electrochemically active surface area decreased depending on the holding time, and platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. However, platinum dissolution and deposition out of the catalyst layer were greatly reduced when a platinum black electrode was used. In the experiment using a double-layered catalyst layer, platinum redeposited not on the carbon black surface but rather on the platinum black surface.

Enhanced CO-Tolerance of Carbon-Supported Platinum and Molybdenum Oxide Anode Catalyst

The activity of carbon-supported Pt/MoO x (Pt/MOO x /C) toward oxidation of 100 ppm CO in H 2 was investigated using a rotating disk electrode in 0.5 M H 2 SO 4 solution and by anodic polarization behavior in a single cell configuration. CO-stripping voltammetry of the Pt/MoO x /C catalyst showed a higher stripping peak potential compared to PtRu/C; however, the onset potential of the potentiodynamic oxidation of the CO/H 2 mixture was somewhat smaller than that of PtRu/C. Anodic polarization of a Pt/MoO x /C anode in a CO/H 2 mixture decreased with decreasing fuel flow rate and with decreasing thickness of the Nafion membrane, which indicated that part of the improved CO tolerance of the Pt/MoO x /C anode was due to O 2 that permeated from the cathode. This is supported by the anodic polarization of Pt/MoO x /C increasing and becoming independent of the fuel flow rate when the cathode gas was changed from Or to H 2 . Analyses of CO concentration in the exhaust gas from the anode side showed that part of the CO in the fuel gas stream was oxidized by electrochemical and nonelectrochemical processes. The CO tolerance of the Pt/MoO x /C anode was attributed to the intrinsic CO tolerance of the catalyst and nonelectrochemical CO oxidation by the permeated O 2 .

Anisotropic Conductivity Over In-Plane and Thickness Directions in Nafion-117

Proton conductivity of Nation-117 membranes in the direction of thickness was measured in 30°C, 30-90% relative humidity, by means of 2- and 4-probe methods in thickness direction developed in this group. Heavy conductivity anisotropy of σ in-plane /σ thickness = 2.5-5 was found over the in-plane and thickness directions for membranes pretreated by hot-pressing at high temperature of 150°C and pressures of 1200 and 600 kgf/cm 2 , respectively. On the other hand, no conductivity fade was found for membrane pretreated at 0 kgf/cm 2 in the thickness direction compared to that in the plane direction. Small angle X-ray scattering measurements revealed ion domains flattened along the in-plane direction and ordered along the thickness direction. The origin of the persistent conductivity anisotropy is believed to come from the maximized potential energy barrier for ion transport in interaggregate along the thinned thickness direction.

Proton conductivity along interface in thin cast film of Nafion

Using a newly developed substrate with microelectrodes for the four-point method, the proton conductivity of a cast thin film of the Nafion® along the lateral (parallel to the interface) direction in a humidified atmosphere was measured. It was found to be much smaller than that of the as-received Nafion membrane. This difference should affect the simulation to design the gas diffusion electrodes for proton exchange membrane fuel cells (PEMFCs), because the ionic conductivity is one of the fundamental data for it.

Ionic Conduction in Lithium Ion Battery Composite Electrode Governs Cross-sectional Reaction Distribution

Composite electrodes containing active materials, carbon and binder are widely used in lithium-ion batteries. Since the electrode reaction occurs preferentially in regions with lower resistance, reaction distribution can be happened within composite electrodes. We investigate the relationship between the reaction distribution with depth direction and electronic/ionic conductivity in composite electrodes with changing electrode porosities. Two dimensional X-ray absorption spectroscopy shows that the reaction distribution is happened in lower porosity electrodes. Our developed 6-probe method can measure electronic/ionic conductivity in composite electrodes. The ionic conductivity is decreased for lower porosity electrodes, which governs the reaction distribution of composite electrodes and their performances.