Yi-Lin Liu

Detailed Characterization of Anode-Supported SOFCs by Impedance Spectroscopy

Anode-supported thin electrolyte cells are studied by electrochemical impedance spectroscopy (EIS). The aim is to describe how the losses of this type of cells are distributed at low current density (around open-circuit voltage) as a function of temperature. An equivalent circuit consisting of an inductance, a serial resistance (R s ), and five arcs to describe the polarization resistance is suggested. This equivalent circuit is based on previous studies of single electrodes in three-electrode and two-electrode symmetric cell setups. The equivalent circuit components have been assigned to the electrode processes, and the assignments were verified by extensive full cell studies in which the partial pressure of reactant gases on both the electrodes as well as temperature was systematically varied with the aim to identify frequency regions which are dominated by an electrode specific process. Furthermore, the model is applied on a good performing cell with area specific resistance (ASR) = 0.15 Ω cm 2 at 850°C and a poor performing cell with ASR = 0.29 Ω cm 2 at the same temperature. Both cells were fabricated using nominally the same procedure. The EIS analysis indicated that the difference in performance originates from microstructural differences on the cathode. This is further supported by the observation of large differences in the cathode microstructure by scanning electron microscope.

Degradation of Anode Supported SOFCs as a Function of Temperature and Current Load

The degradation behavior of anode supported solid oxide fuel cells (SOFCs) was investigated as a function of operating temperature and current density. Degradation rates were defined and shown to be mainly dependent on the cell polarization. The combination of a detailed evaluation of electrochemical properties by impedance spectroscopy, in particular, and post-test microscopy revealed that cathode degradation was the dominant contribution to degradation at higher current densities and lower temperatures. The anode was found to contribute more to degradation at higher temperatures. Generally, the degradation rates obtained were lower at higher operating temperatures, even at higher current densities. A degradation rate as low as 2%/1000 h was observed at 1.7 A/cm 2 and 950°C over an operating period of 1500 h.

Effects of impurities on microstructure in Ni/YSZ–YSZ half-cells for SOFC

Long-term properties of Ni/yttria-stabilized zirconia (YSZ) cermet anode+YSZ electrolyte pellet (d=7.4 mm) half-cells were evaluated experimentally. Two commercial NiO powders containing different levels of impurities were used for the anodes. The durability was evaluated at temperatures of 850 °C over 1500–1800 h in H2 with 1–3% H2O under an anodic load of 300 mA cm−2. The anodes containing, among others, SiO2 and Na2O at a concentration level of hundreds ppm degrade faster (within a period of 150–400 h) than those with a few tens ppm of SiO2. The impurity phase is characterized as a type of sodium silicate glass phase and is found to segregate and accumulate at the anode/electrolyte interface causing serious damage to the YSZ electrolyte in the vicinity of the interface. In the center region of the circular pellet, the YSZ grains are separated by silicate glass. The distribution of these impurities and the extent of structural damage are qualitatively compared to the profile of the H2O concentration.

Recrystallization Microstructure in Cold-Rolled Aluminum Composites Reinforced by Silicon Carbide Whiskers

Recrystallization behavior has been studied in 50 and 90 pct cold-rolled, silicon carbide whisker-reinforced aluminum composites containing fine aluminum-oxide particles. The micro-structure in the cold-worked state, in the early stage of recrystallization, and after the completion of recrystallization was examined by transmission and scanning electron microscopy and light microscopy. Recrystallization kinetics were studied by microhardness measurement. In the cold-worked state, it was found that the presence of SiC whiskers reduced the cell size, increased the hardness, and altered the distribution of dislocations. During recrystallization, the SiC whis-kers, often present in groups, showed a strong tendency to stimulate nucleation, increasing the number of nuclei and lowering the recrystallization temperature. The recrystallization kinetics in the composite were accelerated; however, the grain refinement effect of SiC was limited, apparently due to the presence of the fine aluminum-oxide particles. The structural observation and kinetics have been discussed and related to results from previous studies of dispersion-strengthened aluminum/aluminum-oxide materials and aluminum of commercial purity.

Assessment of the Cathode Contribution to the Degradation of Anode-Supported Solid Oxide Fuel Cells

The degradation of anode-supported cells was studied over 1500 h as a function of cell polarization either in air or oxygen on the cathode side. Based on impedance analysis, contributions of the anode and cathode to the increase of total resistance were assigned. Accordingly, the degradation rates of the cathode were strongly dependent on the pO 2 . Microstructural analysis of the cathode/electrolyte interface carried out after removal of the cathode showed craters on the electrolyte surface where the lanthanum strontium manganite (LSM) particles had been located. The changes of shape and size of these craters observed after testing correlated with the cell voltage degradation rates. The results can be interpreted in terms of element redistribution at the cathode/ electrolyte interface and formation of foreign phases giving rise to a weakening of local contact points of the LSM cathode and yttria-stabilized zirconia electrolyte and consequently a reduced three-phase boundary length.

Effect of Humidity in Air on Performance and Long-Term Durability of SOFCs

Anode-supported solid oxide fuel cells (SOFCs) based on Ni-yttria-stabilized zirconia (YSZ) anodes, YSZ electrolytes, and lanthanum strontium manganite (LSM)-YSZ cathodes were studied with respect to durability in humid air (∼4%) typically over 1500 h. Operating temperature and current density were varied between 750 and 850°C and 0.25-0.75 A/cm 2 , respectively. The introduction of humidity affected the cell voltage under polarization of the cell, and this effect was (at least partly) reversible upon switching off the humidity. Generally, the studied cells were operated in humid air under technologically relevant conditions over more than 1500 h. Improvements at the cathode/electrolyte interface made it possible to obtain highly stable cells, which can be operated under high current density and at 750°C in humid air, conditions that cause significant cell voltage degradation in dry air on cells with LSM/YSZ-based cathodes.

Nanostructured Lanthanum Manganate Composite Cathode

Anode-supported cells were fabricated with optimized cathodes showing high power density of 1.2 W/cm2 at 800°C under a cell voltage of 0.7 V and an active area of 4 4 cm. A microstructure study was performed on such cell using a field-emission gun scanning electron microscope, which revealed that the La1−xSrx yMnO3± LSM composite cathodes consist of a network of homogenously distributed LSM, yttria-stabilized zirconia YSZ , and pores. The individual grain size of LSM or YSZ is approximately 100 nm. The degree of contact between cathode and electrolyte is 39% on average.

Recovery and recrystallization in cold-rolled Al-SiCW composites

Recrystallization behavior has been studied in 50, 70, and 90 pct cold-rolled silicon carbide whisker-reinforced aluminum composites containing fine aluminum-oxide particles. The distribution of aluminum-oxide particles in the composites was not uniform. Macrohardness measurement, transmission electron microscopy (TEM), and light microscopy were used in the investigation. It was found that in regions with low aluminum oxide content, the introduction of SiC whiskers resulted in recovery reactions during and after cold rolling and in an increase in the growth rate of subgrains during annealing. These recovery reactions were enhanced by an increase in the degree of deformation. The number density of nuclei varied between areas with different contents of aluminum-oxide particles. The impingement of nuclei and recovery reactions limited the growth of nuclei, resulting in a process where recrystallization (growth of nuclei by high-angle grain boundary migration) and extended recovery (growth of subgrains) took place simultaneously. The relative amounts of recrystallization and extended recovery that occur simultaneously affect the recrystallization kinetics as well as the grain size distribution and texture after recrystallization.

Effect of Whiskers and Small Particles on the Deformation and Recrystallization Texture of Aluminium

The effect of small Al2O3 particles (d < 0.1 μm) and of a mixture of Al2O3 particles and SiC-whiskers on the cold-rolling and on the recrystallization texture of aluminium has been studied by neutron diffraction. It has been observed that the strength and the relative intensity of the main rolling components in the deformation texture are affected both by small particles and by introduction of SiC whiskers. In the recrystallized state very different textures have been found ranging from a weak cube texture to a strong {100} (N-texture). The measured textures are related to the size of the recrystallized grains in a discussion of the effect of small particles and whiskers on deformation microstructures and nucleation and growth processes.

Analysis of the LSM/YSZ interface on microand nano-scale by SEM, FIB/SEM and (S)TEM

8 mol% Y2O3-stabilized ZrO2 (8YSZ) and lanthanum strontium manganite (LSM) are well developed materials for electrolyte and cathode, respectively, in solid oxide fuel cells (SOFCs). Thermodynamically the 8YSZ/LSM interface is not stable and solid state reactions will result in formation of zirconates (La2Zr2O7, SrZrO3) which are electrically insulating phases. For cells operated at 1000°C, zirconates were found covering large areas of the interface: this was readily characterized by XRD, SEM, TEM and identified as a degradation mechanism in SOFC [1,2]. However, the newly developed SOFCs are operated at much lower temperatures (700–800°C). Due to the slow reaction kinetics, the zirconate formation is localized and its growth is limited. In this case, identification of microstructural degradation has become a great challenge. This paper presents how such an interface has been studied by FEGSEM, FIB and (S)TEM combined with EDS from the micro- to nanometre scales.