Ching-ju Wen

Reaction model of dense Sm0.5Sr0.5CoO3 as SOFC cathode

Overpotential and AC impedance spectra were measured to study the reaction model of dense Sm0.5Sr0.5CoO3 (SSC) as SOFC cathode. From the PO2 dependence of interfacial conductivity, the rate determining step of dense SSC electrode was adsorption and desorption processes at the surface of the electrode. Rate constants of adsorption and desorption were calculated from the interfacial conductivity of the dense electrode to be 3×10−5 mol cm−2 s−1 atm−1 and 2×10−8 mol cm−2 s−1, respectively. These were approximately one order of magnitude larger than the corresponding values calculated for La0.6Sr0.4CoO3. These rate constants can elucidate the overpotentials of porous electrodes.

Protonic conduction of CsH2PO4 and its composite with silica in dry and humid atmospheres

Abstract Conductivity measurements of polycrystalline CsH 2 PO 4 and CsH 2 PO 4 /silica composite were carried out in the temperature range from 150 to 250 °C under a humid condition. A steep change in the conductivity of polycrystalline CsH 2 PO 4 due to a phase transition from a low conductive phase to a high conductive phase (superionic conduction phase) occurs reversibly at around 230 °C in the humid condition. It is found that the conductivity of CsH 2 PO 4 /silica composite is influenced by hydrophilicity and specific surface area of silica. The hydrophilic silica accelerates the conductivity of CsH 2 PO 4 /silica composite at the temperature nearly below the phase transition. In the composite system, impedance analysis showed two semicircles. It is considered that the high frequency semicircle is related to protonic conduction including bulk transfer and interface transfer between the ionic salt and silica, and the low frequency semicircle is possibly associated with grain boundaries. The accelerations of the conductivities can be induced by point defects generated in the ionic salt near the interface between ionic salt and silica.

Object-based modeling of SOFC system: dynamic behavior of micro-tube SOFC

A simulation model for a tubular solid oxide fuel cell (SOFC) was developed by the object-based approach to calculate the current distribution, gas concentration distribution, and temperature distribution at the steady states and transient operation states. The transient electrical and temperature response to a load change was simulated for the both cells with the diameter of 22 mm (standard cell) and the diameter of 2.4 mm (micro-tube cell). The time required to reach the new steady state as the operating voltage was changed from 0.7 to 0.5 V for the micro-tube cell was found to be 15 s, which is much shorter than that of the standard cell.

Cathodic reaction mechanism of dense La0.6Sr0.4CoO3 and La0.81Sr0.09MnO3 electrodes for solid oxide fuel cells

Abstract Dense La 0.6 Sr 0.4 CoO 3 (LSC) and La 0.81 Sr 0.09 MnO 3 (LSM) were prepared by laser ablation on samaria-doped ceria (SDC) and yttria-stabilized zirconia (YSZ) substrates, respectively. The interfacial conductivity of the dense LSM electrodes was independent of the oxygen partial pressure and inversely proportional to the thickness of the LSM film. On the other hand, the interfacial conductivity of the dense LSC electrode was proportional to the square root of the oxygen partial pressure, which indicates the sensitivity of the surface reaction to the overpotential. To determine the origin of the overpotential, we constructed a surface-reaction model. Good agreement was found between our model and experimental results.

Investigation of the interaction between NiO and yttria-stabilized zirconia (YSZ) in the NiO/YSZ composite by temperature-programmed reduction technique

Temperature-programmed reduction (TPR) was performed to investigate the reducibility of the NiO/YSZ composite that is related to the anode of solid oxide fuel cells (SOFCs). The TPR profiles were found to be affected by preparation method, composition and pretreatment conditions. The calcination temperature strongly affected the reduction behavior; the TPR profile of the sample calcined at 1723 K shows a high temperature reduction peak that was not found in the TPR profile of the sample calcined at 1473 K. Such a difference in the TPR profiles can be attributed to the presence of distinct states of NiO species, which are due to the difference of interactions between NiO and YSZ. Five distinct states of NiO species are shown to be present in the NiO/YSZ composite.

Electrical conductivity of a pure C60 single crystal

The temperature dependence of the electrical conductivity of a C60 single crystal is presented in this letter. The single‐crystal samples free from solvent contamination were grown up to a size of millimeters order by sublimation of C60 powder with oscillation of the crystal temperature. The electrical conductivity of the single crystal was measured at temperatures between 250 and 295 K. The sharp decrease of the electrical conductivity with temperature around 256 K was observed in association with the phase transition of C60.

Low Overvoltage Mechanism of High Ionic Conducting Cathode for Solid Oxide Fuel Cell

Electrochemical characteristics of three types of La 1-x Sr x CoO 3 (LSC) and La 1-x Sr x MnO 3 (LSM) electrodes, (dense, porous, and porous/dense double layer) were measured. The dense layers of LSC and LSM were prepared by a laser ablation method and the porous layers were prepared by a tape-casting method. Electrochemical measurements (ac impedance and dc polarization measurements) revealed that surface area is an important factor for the reaction rate on the LSC electrode, while the electrode/electrolyte boundary structure is important on the LSM electrode. These results indicate that high performance electrodes can be obtained using a high ionic conductor electrode with a large surface area.

Direct alcohol fuel cell-relation between the cell performance and the adsorption of intermediate originating in the catalyst-fuel combinations

Abstract Since a direct alcohol fuel cell (DAFC) can be downsized and can operate at room temperature, it is expected as a power source for mobile applications. On the other hand, cost reduction is also expected, because Pt is used in the electrocatalyst. In this study, the use of base metals for Pt was investigated by combined with the 2-propanol fuel that had an advantage compared to methanol in terms of oxidability. The various base metal catalysts were compared to the Pt catalyst as an anode electrocatalyst in the DAFC performance. The Ni catalyst showed the highest open circuit voltage ( V oc ) among the base metal catalysts. The V oc reflected the hydrogen abstraction activity of the metal species. The Ni catalyst was a match for the Pt catalyst in the V oc . However, the Ni catalyst could not reach to the Pt catalyst in the cell performance. The relation between the catalyst and the fuel was investigated in both the Ni and the Pt catalyst. The suitable catalytic performance was observed depending on each fuel. The Ni catalyst was suitable for the 2-propanol, while the Pt catalyst was suitable for the methanol fuel. The stripping voltammetry indicated that the DAFC showed better performance as the adsorbate could be removed easily and the easiness depended on the fuel-catalyst combinations. Considering the catalyst cost and the possibility of the improvement in the catalyst, Ni is the potential candidate for the anode catalyst by combined with the 2-propanol fuel in the DAFC for the mobile applications.

La0.6Ba0.4CoO3 as a Cathode Material for Solid Oxide Fuel Cells Using a BaCeO3 Electrolyte

The cathodic reaction mechanism of a solid oxide fuel cell (SOFC) was investigated for an electrode-electrolyte system of La{sub 0.6}Ba{sub 0.4}CoO{sub 3} (LBC)-BaCeO{sub 3}, under both the O{sup 2{minus}} conducting and H{sup +}/O{sup 2{minus}} mixed-ionic conducting conditions. AC impedance measurements were carried out, and the electrode interfacial conductivities were calculated. The experimental results revealed that the processes dominating the electrode resistance for the O{sup 2{minus}} conducting and the H{sup +}/O{sup 2{minus}} mixed-ionic conducting conditions are different. It was also found that the process dominating the electrode resistance changes at 700 C under H{sup +}/O{sup 2{minus}} mixed-ionic conducting conditions. The process dominating the electrode resistance above 700 C is postulated to be the reaction O{sub LBC} + H{sub LBC} {r{underscore}arrow} OH{sub LBC}. LBC showed high-performance cathode characteristics for a SOFC using BaCeO{sub 3} electrolyte.

Protonic Conduction and Impedance Analysis in CsHSO4 / SiO2 Composite Systems

Proton conductivity measurement and ac impedance analysis of CsHSO 4 /SiO 2 composites where carried out using two different methods to prepare sample powders: mechanical mixing and evaporation to dryness. At temperatures below the superprotonic phase transition temperature of CsHSO 4 , ca. 140°C, the conductivity of the composite prepared by the evaporation to dryness method was larger by 4 orders of magnitude than that of pure CsHSO 4 , and larger by 2 orders of magnitude than that of the composite prepared by mechanical mixing. The pore size of SiO 2 particles (2-16 nm) influenced the conductivity, with the conductivity becoming higher as the pore size became larger. In the impedance spectra, two semicircles appeared in the composite system and were deconstructed to study proton conduction processes. Impedance analysis suggests that the formation of an interface between CsHSO 4 and SiO 2 and the high dispersion of SiO 2 particles result in the enhancement of conductivity. X-ray diffraction and differential thermal analysis indicate that structural disorder in the interface would induce a significant enhancement of conductivity even at temperatures below the phase transition temperature.