The performance of a single-chamber solid oxide fuel cell was studied using a ceria-based solid electrolyte at temperatures below 773 kelvin. Electromotive forces of approximately 900 millivolts were generated from the cell in a flowing mixture of ethane or propane and air, where the solid electrolyte functioned as a purely ionic conductor. The electrode-reaction resistance was negligibly small in the total internal resistances of the cell. The resulting peak power density reached 403 and 101 milliwatts per square centimeter at 773 and 623 kelvin, respectively.

No abstract

A systematic study of LiFePO 4 with cyclic voltammetry ͑CV͒ 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 ϫ 10 −14 and 1.4 ϫ 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 carbonateϭ3:7 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.

Although all-solid-state lithium-ion batteries (ALIBs) have been believed as the ultimate safe battery, their true character has been an enigma so far. In this paper, we developed an all-inclusive-microcell (AIM) for differential scanning calorimetry (DSC) analysis to clarify the degree of safety (DOS) of ALIBs. Here AIM possesses all the battery components to work as a battery by itself, and DOS is determined by the total heat generation ratio (ΔH) of ALIB compared with the conventional LIB. When DOS = 100%, the safety of ALIB is exactly the same as that of LIB; when DOS = 0%, ALIB reaches the ultimate safety. We investigated two types of LIB-AIM and three types of ALIB-AIM. Surprisingly, all the ALIBs exhibit one or two exothermic peaks above 250 °C with 20-30% of DOS. The exothermic peak is attributed to the reaction between the released oxygen from the positive electrode and the Li metal in the negative electrode. Hence, ALIBs are found to be flammable as in the case of LIBs. We also attempted to improve the safety of ALIBs and succeeded in decreasing the DOS down to ∼16% by incorporating Ketjenblack into the positive electrode as an oxygen scavenger. Based on ΔH as a function of voltage window, a safety map for LIBs and ALIBs is proposed.

There is worldwide interest in the development and commercialization of fuel cells for vehicles and portable electric devices. It is a presently accepted notion that polymer electrolyte fuel cells (PEFCs) are the only devices capable of operating at low temperatures. We believe, however, that PEFCs are not perfect from a practical point of view because they require hydrogen as the fuel, which is impractical in terms of storage and handling. An external reformer, therefore, must be used to convert more viable alcohols and hydrocarbons into hydrogen, thereby defeating their portability. There have been recent successes with solid oxide fuel cells (SOFCs) which perform well between 500 and 700ЊC directly using alcohols and hydrocarbons as the fuels. 1-3 A further reduction in the operating temperature of internal-reforming SOFCs and an enhancement of their thermal-and mechanical-shock resistance would make this technology a promising alternative to PEFCs.A novel type of fuel cell, which is distinguished from conventional fuel cells in design and principle, has been proposed by many researchers. 4-8 This fuel cell consists of only one gas chamber, where both the anode and the cathode are exposed to the same mixture of fuel and air. We will use a single-chamber fuel cell (SCFC) as a notation for this type of fuel cell. Because there is no need to separate the supply of fuel and air, it is more thermally and mechanically shock resistant than conventional fuel cells. We have recently succeeded in applying this cell design to an SOFC constructed from an yttria-stabilized zirconia (YSZ) electrolyte with a Ni-based anode and a strontium-doped lanthanum manganite (LSM) cathode. 9 This SCFC exhibits high power density in a flowing mixture of methane and air, but it must operate at the high temperature of 950ЊC in order to achieve sufficient ionic conduction in the solid electrolyte.The operation of SOFCs at reduced temperatures causes excessive ohmic and polarization losses in the cell. Thus, it is necessary to use a highly conductive electrolyte together with a highly active anode and cathode. Lanthanum gallate-10-14 or ceria-based 15-18 oxides would be promising electrolytes because of their much higher ionic conductivities than that of YSZ. In addition, Ni-ceria cermets 19 and Co-based perovskite oxides, 20,21 which exhibit mixed ionic and electronic conduction under each of their respective operating conditions, have been generally regarded as suitable anodes and cathodes, respectively, at reduced temperatures.In this study, we demonstrate that it is possible to operate a thermally and mechanically shock-resistant SOFC at reduced temperatures by combining the advantages of using a highly conductive electrolyte with the single-chamber cell design. We also show that ethane, propane, and liquefied petroleum gas (LPG) can be successfully used as the fuels in the present SCFC, especially at operating temperatures below 550ЊC. Figure 1a shows an SCFC constructed for fuel-cell tests at reduced temperatures. ExperimentalThe perf...

No abstract

No abstract

LiFePnormalO4 electrodes with thicknesses from 15to120μm 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 25C and still give a capacity of 70mAh∕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 LiFePnormalO4 electrodes gives a maximum capacity of 1050mAh at 1-C rate for a 60-μm electrode. This corresponds to a volumetric and gravimetric energy density of 214Wh∕L and 96.5Wh∕kg , respectively. The effective Li diffusivity in the active material is estimated to be of the order of 10−13cm2∕s .