Water Solubility in Ceria and Zirconia Doped With Rare Earth Oxides

138

163

Composites of La 0.8 Sr 0.2 MnO 3 ͑LSM͒ and yttria-stabilized zirconia ͑YSZ͒ were prepared by impregnating porous YSZ with salts of Sr, La, and Mn to 40 wt % LSM. The effect of calcination temperature on cathode performance at 973 K was then studied using both symmetric cells and solid oxide fuel cells ͑SOFCs͒. Following calcination at 1123 K, the LSM in the composite remained porous. In symmetric-cell tests, LSM͑1123͒-YSZ electrodes exhibited a low loss, 0.5 ⍀ cm 2 , which was unchanged by polarization; however, in fuel cell tests, the performance was poor at current densities above 0.2 A/cm 2 . Following calcination at 1323 or 1523 K, LSM formed a dense layer coating on the YSZ. In symmetric-cell tests, the composite exhibited an ASR greater than 4 ⍀ cm 2 , but this decreased, reversibly, upon cathodic polarization or upon heating in humidified H 2 at 973 K. In fuel cell tests, the performance of composites calcined at 1323 K was superior to ones at 1523 K, primarily because the ohmic resistance of the cell was 0.25 ⍀ cm 2 higher after 1523 K, probably due to solid-state reactions with the electrolyte. The optimal conditions for forming impregnated LSM-YSZ cathodes and the reasons for the observed hysteresis in cathode polarization are discussed based on these results.

Section: Resultsmentioning

confidence: 99%

Characterization of LSM-YSZ Composites Prepared by Impregnation Methods

Huang

Vohs

Gorte

2005

138

163

“…With regard to item ͑i͒, we have reported the change of 18 O diffusion profiles with preannealing temperatures under the nonpolarized condition. 32 The preannealing processes cause the microstructure change and Mn diffusion from LaMnO 3 into YSZ, 14,32 which affects the 18 O diffusion paths at the interface of LaMnO 3 /YSZ ͑although we have adopted the samples at the optimum preannealing temperature͒. With regard to item ͑ii͒, the cell temperature has been carefully controlled in the focused infrared furnace.…”

Section: Resultsmentioning

confidence: 99%

“…19 Secondary ion mass spectrometry analysis ͑SIMS analysis͒ was applied to determine the active sites for 18 O incorporation at the LaMnO 3 mesh/YSZ interface. [20][21][22] Figure 1 shows an example of our SIMS analysis for the mesh-shaped LaMnO 3 on YSZ ͑Fig. 1a͒ and schematic diagram for oxygen transports at the interface ͑Fig.…”

mentioning

confidence: 99%

“…20 In our previous study, the effect of the cathodic polarization has been clarified with respect to the distributions of active sites for oxygen incorporation and the diffusion paths of oxide ions inside the YSZ electrolyte. 21 However, the effect of cathodic polarization on the 18 O diffusion profiles in LaMnO 3 has not been clarified thoroughly. 22 The aim of this study is, therefore, the investigation of oxygen diffusion in LaMnO 3 under cathodic polarization.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Oxygen Transport at the LaMnO[sub 3] Film/Yttria-Stabilized Zirconia Interface under Different Cathodic Overpotentials by Secondary Ion Mass Spectrometry

Horita

Yamaji

Sakai

et al. 2001

Oxygen transport under different cathodic overpotentials was investigated at the LaMnO 3 film/Y 2 O 3 stabilized ZrO 2 interface. A stable isotope oxygen ( 18 O) was adopted as a diffusion marker at 1073 K under cathodic polarization, and the quenched samples were analyzed by secondary ion mass spectrometry. The 18 O diffusion profiles indicate the higher diffusivity of 18 O in LaMnO 3 film with the higher cathodic overpotentials. The introduction of oxygen vacancies by the cathodic polarization can change the diffusion profiles of 18 O in the LaMnO 3 film. The diffusion profiles of oxygen in LaMnO 3 film under the cathodic polarization were calculated from the literature data of oxygen content in LaMnO 3 as a first estimation. The calculated diffusion profiles showed a similar trend with the measured ones, but different concentration in the LaMnO 3 film under the higher polarization.Doped LaMnO 3 has been considered as a cathode for solid oxide fuel cells ͑SOFCs͒ because of its relatively high electronic conductivity and high catalytic activity for oxygen reduction. 1 Around the interface of O 2 ͑gas͒/LaMnO 3 /yttria-stabilized zirconia ͑YSZ͒ threephase boundary ͑TPB͒, the oxygen reduction generates oxide ions that are transported into the electrolyte. The cathodic overpotentials are related to the resistance for oxygen reduction around the TPB, and they must be minimized for SOFCs with a high performance. To reduce the cathodic reaction resistance, clarification of the reaction mechanism around the TPB is necessary. To date a number of reports have been made about the transport of oxygen and the model for the oxygen reduction at the interface. 2-18 However, there seems to be some contradictions among the authors. Some authors suggested the contribution of oxide ion diffusion in the LaMnO 3 bulk is due to the cathode reaction under the higher cathodic overpotential. 6,7,12 On the other hand, some authors suggested that the surface diffusion of adsorption oxygen on LaMnO 3 as the main reaction paths under the cathodic polarization. 3,4,[8][9][10] This difference may be partly because the difficulty in understanding the oxygen transports phenomena in the solid materials by electrochemical methods. The electrochemical methods, such as dc polarization and ac impedance, are very sensitive to the movements of electrons and the change of the electrical paths. On the other hand, it is desirable to analyze the movement of oxygen or oxide ions in the solid materials, such as surface oxygen exchange rates and diffusion coefficients. In that sense, the conventional electrochemical methods can be regarded as indirect observation for the oxygen movements.In order to overcome the difficulty in understanding the oxygen transports in solid materials, we have examined a direct observation of oxygen movements by adopting a stable isotope oxygen ( 18 O) as a diffusion marker, based on previous work. 19 Secondary ion mass spectrometry analysis ͑SIMS analysis͒ was applied to determine the active sites for 18 O incorporation at the La...

“…20 Oxygen exchange experiments indicate that the reaction zone is close to the TPB or spreads at the interface between LSM and YSZ. 21 Adler et al suggest that the extension of the reaction zone on the free electrode surface is small for LSM. 22 Altogether this indicates that the extension of the reaction zone on LSM or YSZ is relatively small and therefore cannot solely account for the observed activation.…”

Section: Literature Surveymentioning

confidence: 99%

Impedance of Solid Oxide Fuel Cell LSM/YSZ Composite Cathodes

Jørgensen

Mogensen

2001

536

441

A number of lanthanum strontium manganate/yttria-stabilized zirconia ͑LSM/YSZ͒ composite electrodes are produced with varying composition and processing parameters. The composites are investigated using impedance spectroscopy. General trends related to the oxygen reduction process are extracted from the impedance data. Literature concerning kinetic studies of LSM/YSZ electrodes and related systems is reviewed and compared to new experimental data. From this it is found that at least five processes affect the impedance. Going from high to low frequency, these processes are ͑i͒, ͑ii͒ two geometry-related contributions interpreted as transport across LSM/YSZ interfaces and through the YSZ of the composite, ͑iii͒ a process reflecting competitive reaction steps such as bond breaking and surface diffusion, (iv) gas diffusion in a stagnant gas layer above the electrode structure, and (v) an activation process ͑inductive͒ presumably located at the triple phase boundary of electrode, electrolyte, and gas phase.Lanthanum strontium manganate ͑LSM͒ is the classical cathode material for solid oxide fuel cells ͑SOFCs͒. 1 Addition of the electrolyte material yttria-stabilized zirconia ͑YSZ͒ to the cathode may improve the performance considerably compared to a simple porous LSM electrode. 2-3 To optimize the performance of such a composite electrode, it is important to gain knowledge about the oxygen reduction mechanism. Electrochemical impedance spectroscopy is used for this purpose, as it seems to be an efficient tool. The impedance of a composite cathode is complex. In a Nyquist plot, the impedance is composed of a number of more or less overlapping arcs each reflecting a physical or chemical process. Usually, it cannot be determined from a single impedance spectrum, how many physically relevant processes the impedance includes. Further, the impedance is affected by many features, which are not all known. These features are not all directly related to the oxygen reduction process. Therefore, it is necessary to perform many experiments varying parameters like temperature, oxygen partial pressure, overpotential and electrode geometry in order to interpret the impedance data using a simple physical or chemical model.The present paper attempts to summarize the knowledge about the O 2 /LSM/YSZ composite electrode gained through a large number of experiments performed on this system. Examples showing how the impedance is affected by processing and testing are given. Literature concerning kinetic studies of related electrode systems is reviewed. Some general trends related to the oxygen reduction mechanism are extracted from the presented impedance data and discussed in relation to literature. ExperimentalSample preparation.-Electrolyte substrates are produced from YSZ with 8 mol % Y 2 O 3 ͑TZ8Y, Tosoh Corporation, Japan͒. Threeelectrode electrochemical test cells are made using pellets, which are pressed and machined to the shape shown in Fig. 1a before sintering at 1600°C. The area of the working electrode is approximately 0.4 cm ...