Hiroshi Senoh

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.

“Hybrid hydrogen storage vessel”, a novel high-pressure hydrogen storage vessel combined with hydrogen storage material

Abstract Potential of a novel hydrogen storage vessel, “hybrid hydrogen storage vessel”, combining an aluminum–carbon fiber reinforced plastic (Al–CFRP) composite vessel and hydrogen storage alloy, is reported through calculation of the weight and volume of the hydrogen storage system for 5 kg of hydrogen. Evaluation of this system showed that the concept of the hybrid hydrogen storage vessel allowed us to realize a hydrogen storage system advantageous in both gravimetric and volumetric hydrogen density compared with conventional hydrogen storage techniques. The hybrid vessel requires a hydrogen storage alloy with a higher volumetric hydrogen density as well as a higher gravimetric density, and with a higher equilibrium hydrogen pressure than the hydrogen storage alloys which have been used for conventional hydrogen storage vessels.

Electrochemical characterization of MmNi4.0−xMn0.75Al0.25Cox electrodes as a function of cobalt content

Abstract Hydrogen storage alloys based on misch metal (Mm) are commercially used as negative electrode materials for the nickel–hydride battery. In general, cobalt is contained in the alloys to guarantee the long cycle life of the negative electrode. Consequently, cobalt is an essential element in the hydrogen storage alloy. In order to elucidate the function of cobalt in the charge and discharge processes, the electrochemical feature of the negative electrode as well as the diffusion behavior of hydrogen in the alloy were systematically examined for the MmNi 4.0− x Mn 0.75 Al 0.25 Co x (0≤ x ≤0.6) alloys as a function of the cobalt content. The results of electrochemical measurements are interpreted from the viewpoint of the changes in unit cell volume of the alloys that strongly influences the stability of the hydride.

All-Solid-State Lithium Secondary Battery with Li2S – C Composite Positive Electrode Prepared by Spark-Plasma-Sintering Process

Electrochemically active lithium sulfide-carbon (Li 2 S-C) composite positive electrodes, prepared by the spark plasma sintering process, were applied to all-solid-state lithium secondary batteries with a Li 3 PO 4 -Li 2 S-SiS 2 glass electrolyte. The electrochemical tests demonstrated that In/Li 2 S-C cells showed the initial charge and discharge capacities of ca. 1010 and 920 mAh g -1 -Li 2 S, respectively, which showed higher discharge capacity and coulombic efficiency (ca. 91%) than the Li/Li 2 S-C cells with nonaqueous liquid electrolytes (ca. 200-380 mAh g -1 -Li 2 S and ca. 27%, respectively). The ex situ S K-edge X-ray absorption fine structure measurements suggested the appearance and disappearance of elemental sulfur in the positive electrodes after charging and discharging, respectively, indicating that the ideal electrochemical reaction Li 2 S ⇔ 2Li + S proceeded in the present all-solid-state cells. Such ideal electrochemical reaction, due probably to the suppression of the dissolution of Li 2 S in the form of polysulfides into the electrolytes, would result in higher coulombic efficiency and discharge capacity as compared with those of the liquid-electrolyte cells.

Indigo carmine: An organic crystal as a positive-electrode material for rechargeable sodium batteries

Using sodium, instead of lithium, in rechargeable batteries is a way to circumvent the lithiums resource problem. The challenge is to find an electrode material that can reversibly undergo redox reactions in a sodium-electrolyte at the desired electrochemical potential. We proved that indigo carmine (IC, 5,5′-indigodisulfonic acid sodium salt) can work as a positive-electrode material in not only a lithium-, but also a sodium-electrolyte. The discharge capacity of the IC-electrode was ~100 mAh g−1 with a good cycle stability in either the Na or Li electrolyte, in which the average voltage was 1.8 V vs. Na+/Na and 2.2 V vs. Li+/Li, respectively. Two Na ions per IC are stored in the electrode during the discharge, testifying to the two-electron redox reaction. An X-ray diffraction analysis revealed a layer structure for the IC powder and the DFT calculation suggested the formation of a band-like structure in the crystal.

Charge efficiency of misch metal-based hydrogen storage alloy electrodes at relatively low temperatures

Abstract Charge efficiency of Mm (misch metal)-based hydrogen storage alloy (MmNi3.6Mn0.4Al0.3Co0.7) electrode was evaluated in the temperature range of 25 to −40°C. The charge efficiency obviously decreased less than 0°C. From the rate of hydrogen absorption in solid–gas phase, exchange current density and charge-transfer resistance measured at various temperatures, it was inferred that the rate-determining step in the charge process of the negative electrode at relatively low temperatures was the charge-transfer reaction on the electrode surface. The addition of Mo to the alloy was effective for improving the electrocatalytic activity, leading to the improvement of the charge efficiency at relatively low temperatures.

Organic Positive-Electrode Materials Based on Dialkoxybenzoquinone Derivatives for Use in Rechargeable Lithium Batteries

The performance of 2,5-dialkoxy-1,4-benzoquinone derivatives as positive-electrode active materials for use in rechargeable lithium batteries was investigated. These derivatives showed stepwise two-electron redox behaviors in cyclic voltammetry, and also exhibited high initial discharge capacities of more than 200 mAh g in charge/discharge tests. In particular, a positive-electrode that used a methoxy derivative showed the largest discharge capacity (close to 300 mAh g) among the dialkoxy derivatives tested. These discharge curves imply benzoquinone-based twoelectron redox behavior. Theoretical quantum calculations based on the density functional theory (DFT) were also performed to discuss the electrochemical properties of these materials, and supported the experimental results.

Electrochemical and crystallographic characterization of Co-free hydrogen storage alloys for use in nickel–metal hydride batteries

Abstract Electrochemical and crystallographic characterization of Co-free hydrogen storage alloys containing Si and/or Fe was carried out. In the charge–discharge cycle test, the maximum discharge capacity of a Co-free MmNi 4.3 Mn 0.4 Al 0.3 (Mm=misch metal) alloy electrode was decreased by the partial substitution of Si and/or Fe for Ni, but the cycle performance was improved. The MmNi 3.6 Mn 0.4 Al 0.3 Si 0.1 Fe 0.6 electrode showed the most excellent cycle durability in this study. It was found by SEM, EPMA and XRD that Fe and Si contained in these alloys more or less suppressed the dissolution of Al and the lattice expansion with hydrogen absorption, leading to the long cycle life. This effect was strengthened by the simultaneous partial substitution of Si and Fe for Ni in the Co-free alloy.

Relationship Between Equilibrium Hydrogen Pressure and Exchange Current for the Hydrogen Electrode-Reaction at Mmni(3.9-X)Mn(0.4)A1(X)Co(0.7) Alloy Electrodes

We present a theoretical relationship between equilibrium hydrogen pressure and exchange current for the hydrogen electrode reaction which considers the degree of hydrogen coverage at the electrode surface. Electrochemical measurements at MmNi 3.9-x Mn 0.4 Al x Co 0.7 (0 ≤ x ≤ 0.8) electrodes were performed to prove the theoretical model. The equilibrium hydrogen pressures were analyzed from electrochemical pressure-composition isotherms, and the exchange currents were determined by linear polarization measurements. Fitting the experimental data to the theoretical model indicated that the rate constants for the charge-transfer reaction as well as the charge-transfer coefficient were influenced by the partial substitution of nickel by aluminum. Also, the exchange current passed through a maximum with decreasing equilibrium hydrogen pressure, i.e., with increasing aluminum content, indicating that it is possible to design new materials which combine high electrocatalytic activity with an appropriate equilibrium hydrogen pressure. Because of its high energy density and high power density, MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 was found to be the most appropriate composition.

Ab Initio Simulations of Li/Pyrite- MS2 ( M = Fe , Ni ) Battery Cells

An electrochemical energy profile on the charge-discharge cycle model of Li/pyrite-MS 2 (M = Fe, Ni) secondary battery cells was simulated using density functional theory. For a full-discharge reaction of MS 2 under enough Li-ion concentration, the calculated results indicate that the final products are Li 2 S and M metal via the intermediate compound of commonly suggested Li 2 MS 2 (unknown for M = Ni), which is the intermediate product that continues the self-decomposition as Li 2 MS 2 → MS + Li 2 S. For a full-charge reaction Li 2 S + M metal at the cathode, the reproduction of the initial pyrite FeS 2 is a more favorable scheme than the production of other iron sulfides (FeS and Fe 3 S 4 ), indicating the capability of the reversible charge-discharge cycle of Li/FeS 2 cell. However, for Li/NiS 2 , there is difficulty in the reproduction of the initial pyrite NiS 2 due to a closer formation enthalpy between NiS 2 and the other nickel sulfides (NiS, Ni 3 S 2 , and Ni 3 S 4 ). This is one of the reasons that the Li/FeS 2 cell shows an experimentally better charge-discharge cycle performance than Li/NiS 2 . The temperature dependence of the open-circuit voltage (OCV) was also estimated using the Nernsts equation, namely, the change in the Gibbs free energy derived from the enthalpy and entropy obtained by phonon calculations for the crystal lattices by applying the density functional perturbation theory. There is little temperature dependence of the OCV in the temperature range 0-100°C, and the maximum OCVs at the thermodynamic standard state (1.67 V for Li/FeS 2 and 1.84 V for Li/NiS 2 ) are consistent with the experimental results.