Takao Inoue

Study of LiFePO4 by Cyclic Voltammetry

A systematic study of LiFePO 4 with cyclic voltammetry (CV) was conducted using thin electrodes with a loading of 4 mg/cm 2 . Peak current of the CV profile was proportional to the square root of scan rate under 0.2 mV/s. Results were analyzed using a reversible reaction model with a resistive behavior. This resistance was consistent with other resistances obtained from electrochemical impedance spectroscopy and charge-discharge curves. Apparent Li diffusion constants of 2.2 ×10 -14 and 1.4 X 10 -14 cm 2 /s were obtained at 25°C for charging and discharging LiFePO 4 electrodes in 1 M LiPF 6 ethylene carbonate/diethyl carbonate=3:7 by volume, respectively. Activation energies of the apparent diffusion constants and electrode resistance are about 0.4 eV. These parameters are good indicators for assessing the effectiveness of material modifications such as surface coating and doping.

Single‐Chamber Solid Oxide Fuel Cells at Intermediate Temperatures with Various Hydrocarbon‐Air Mixtures

The performance of a single-chamber solid oxide fuel cell (SOFC) was studied between 350 and 900°C in flowing mixtures of methane, ethane, propane, or liquefied petroleum gas and air with a fuellair volume ratio of one, where their oxidation proceeded safely without explosion. Among all tested electrode materials, Ni-Ce 0.8 Sm 0.2 O 1.9 cermet and Sm 0.5 Sr 0.5 CoO 3 oxide functioned best as the anode and cathode, respectively, in various gas mixtures. A cell constructed from a La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 electrolyte with the two electrodes generated >900 mV in a methane-air mixture between 600 and 800°C and in an ethane-air mixture between 450 and 650°C. A small electrode reaction resistance resulted in increasing power density with decreasing electrolyte thickness. The peak power density at 450°C increased from 34 to 101 mW cm -2 with decreasing electrolyte thickness from 0.50 to 0.18 mm. The working mechanism of the single-chamber SOFC at different temperatures was also studied by measuring the catalytic activities of the two electrodes for partial oxidation of the hydrocarbons.

An Investigation of Capacity Fading of Manganese Spinels Stored at Elevated Temperature

The storage characteristics of a manganese spinel at various discharge depths and 80°C were examined in 1 M LiPF 6 ethylene carbonate/dimethyl carbonate (1:2 by volume) electrolyte. The quantities of Mn dissolution and discharge-capacity loss were measured after the cathode at each discharge depth had been exposed to the electrolyte at 80°C. The quantities of dissolved manganese in the solution were less than 1.2% of the total manganese in all cathodes examined. Little capacity fading (3%) was found in the fully charged cathode, but a 59% capacity loss was observed in the fully discharged cathode. Correlations of the capacity loss with the X-ray diffraction peak widths were found, and the amount of capacity loss increased with broadening peak width. On the other hand, no correlation between the amount of Mn dissolution and the capacity loss was found. From these results, we propose a mechanism of the capacity fading of the spinel LiMn 2 O 4 stored at elevated temperatures as follows: lattice defects in the spinel due to Mn dissolution cause disordered crystal structures and as a result, the Li insertion-extraction paths are blocked, leading to capacity fading.

A causal study of the capacity fading of Li1.01Mn1.99O4 cathode at 80°C, and the suppressing substances of its fading

Abstract Discharge capacities of the Li1.01Mn1.99O4 cathode and Mn content dissolved from the cathode were examined on the addition of some chemicals, as the storage temperatures and time were changed. On the addition of 1000xa0ppm of H2O to 1.0xa0M LiPF6 solution in EC/DMC (1:2 in volume), 41% of capacity of the cathode was lost after 24xa0h storage at 80°C, in contrast to 5% loss in the case of no additive. Water would be responsible for causing the capacity fading of the cathode under such condition as elevated temperatures. In the EC/DMC solution without LiPF6, on the contrary, no capacity fading was observed with H2O additive at 80°C storage, while the CF3SO3H addition resulted in 40% of capacity fading. These results show that plausible acid derived from the reaction of LiPF6 with H2O causes the capacity fading on the lithium manganese oxide at 80°C, relatively high storage temperature. On the other hand, addition of 500–1000xa0ppm of (CH3)3SiNHSi(CH3)3 resulted in less capacity fading of the cathode and made Mn dissolution decrease drastically. Therefore, dehydration and acid-neutralization in the electrolyte solution would restrain the capacity fading of the lithium manganese oxide cathode during high temperature storage, and we confirmed it in some experiments.

A Solid Oxide Fuel Cell Using an Exothermic Reaction as the Heat Source

Performance of a single-chamber solid oxide fuel cell was evaluated using a 0.15 mm thick Sm-doped ceria (SDC) electrolyte together with a 30 wt % SDC-Ni anode and a Sm 0.5 Sr 0.5 CoO 3 cathode at heating temperatures below 500°C in a flowing mixture of butane and air. A large quantity of reaction heat, which was evolved by the partial oxidation of butane by oxygen at the anode, caused a temperature rise of more than 100°C at the anode, followed by thermal conduction to the cathode through the electrolyte. Simultaneously, the cell generated a large electromotive force of ca. 900 mV between the two electrodes. The resulting peak power density reached 245, 180, 105, and 38 mW cm -2 at heating temperatures of 450, 400, 350, and 300°C, respectivcly. The comparison of the butane fuel with the other hydrocarbon fuels showed that the fuel cell performance became enhanced, especially at reducing temperatures, as the carbon number of the hydrocarbon increased, and the chain structure was branched

Effect of Electrode Parameters on LiFePO4 Cathodes

LiFePO 4 electrodes with thicknesses from 15 to 120 μm were coated on Al current collectors. The electrochemical characteristics of these electrodes depend strongly on film thickness, with the largest rate capability for the thinnest film-a 15-μm electrode can be discharged at a current rate of 25 C and still give a capacity of 70 mAh/g. This shows great promise for high-power applications such as hybrid electrical vehicles. Increasing the amount of carbon in the electrode, decreasing the packing density, or using an electrolyte with lower viscosity and higher ionic conductivity improved the rate performance. This suggests that the thickness effect is caused by a larger electrode resistance and a slower Li-ion conduction through the electrolyte for thicker films. Electrode thickness in turn affects the energy density of a battery, because the percentage of inactive materials increases with decreasing film thickness. An energy density prospect for a 18650-type battery with these LiFePO 4 electrodes gives a maximum capacity of 1050 mAh at 1-C rate for a 60-μm electrode. This corresponds to a volumetric and gravimetric energy density of 214 Wh/L and 96.5 Wh/kg, respectively. The effective Li diffusivity in the active material is estimated to be of the order of 10 -13 cm 2 /s.

Electrochemical reduction of NO by alternating current electrolysis using yttria-stabilized zirconia as the solid electrolyte: Part I. Characterizations of alternating current electrolysis of NO

Abstract The electrochemical promotion of the decomposition of NO in the presence of excess O2 was carried out by applying AC voltages lower than 6 V at frequencies from 0.01 to 103 Hz to a YSZ cell having two same-metal electrodes, Au, Pd, Pt or Rh. The Pd and Pt electrodes were relatively effective for this reaction. This could be explained by their electrochemical oxygen-pumping properties and catalytic activities for the reduction of NO. The promotional effect was the most enhanced at a frequency of 0.1 Hz and became larger as the concentration of NO increased or the concentration of O2 decreased. Most of the product was N2, although a small amount of N2O was observed at low conversion of NO. The comparison of AC electrolysis with DC electrolysis showed that the former had a somewhat lower current efficiency for the decomposition of NO than the latter at lower than 3 V, above which the opposite result was observed. In addition, DC electrolysis brought about a significant deterioration of the cell, but AC electrolysis did not result in such a deterioration even at a high voltage of 4 V.

Electrochemical reduction of NO by alternating current electrolysis using yttria-stabilized zirconia as the solid electrolyte: Part II. Modification of Pd electrode by coating with Rh

Abstract The electrochemical promotion of the decomposition of NO by applying an AC voltage to a YSZ cell having two Pd electrodes has been modified by calcining the Pd electrode at 1450°C and then coating the obtained Pd electrode with Rh metal. The calcination of the Pd electrode at 1450°C depressed the oxidation of Pd to PdO at temperatures lower than 800°C. The coating of the Pd electrode with Rh increased the catalytic activity for the decomposition of NO at temperatures lower than 800°C. As a result, these modifications enhanced the current efficiency for the decomposition of NO at low temperatures from 650 to 450°C and at high concentrations of O2 from 2 to 13%. No negative effect on the decomposition of NO was observed in the presence of 5% water vapor and 5% CO2, while a positive effect on this reaction was found for the presence of 500 ppm CH4, C3H8 and C5H12. Based on the measurements of the selectivity for N2 over N2O and the decomposition of N2O, it was concluded that NO is reduced to N2 via intermediate N2O, especially at low temperatures.

Selective Catalytic Reduction of Nitric Oxide by Ethane Using Solid Oxide Membranes

The Pt-catalyzed reduction of nitric oxide (NOx) using ethane as a reductant can be promoted by the application of positive potentials to the cell NO,C 2 H 6 , O 2 . Pt or (Pt + M x O y )|YSZ|Pt, air between 600 and 700°C. At an O 2 concentration of 0% in the reactant gas stream, almost all of the oxygen species transported to the Pt catalyst through the yttria-stabilized zirconia is consumed in the reaction with NO and ethane, which cannot proceed by the supply of gaseous O 2 in an amount equimolar to the transported oxygen species under open-circuit conditions. At O 2 concentrations of 2-12% in the reactant gas stream, the oxygen species transported to the Pt catalyst shows a certain promotional effect on the reduction of NO by ethane. The promotional effect is further enhanced by the addition of 15 wt % Mn 2 O 3 or 15 wt % MoO 3 to the Pt catalyst: Both reaction rates at potentials of 2 V are ca. four times those under open-circuit conditions, although the reaction over the MoO 3 -containing Pt catalyst is accelerated by the application of not positive but negative potentials to the cell. The mechanism of the promotional effect is discussed on the basis of the overall kinetic studies.