Anke Hagen

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.

Generation of active sites for ethane aromatization in ZSM-5 zeolites by a solid-state reaction of zinc metal with bronsted acid sites of the zeolite

A zinc-modified ZSM-5 catalyst was prepared by a solid-state reaction between zinc dust and H-ZSM-5. The resulting catalyst is as active for the aromatization of ethane as zinc-modified ZSM-5 prepared by impregnation.

The performance of Ti-MCM-41 in aqueous media and after mechanical treatment studied by in situ XANES, UV/Vis and test reactions

Abstract The influence of water on the epoxidation of cyclohexene with H 2 O 2 and tert -butyl hydroperoxide (tbhp) on Ti-MCM-41 molecular sieves prepared by post-synthetic modification of the support with titanium alkoxides was investigated. The catalytic performance depends on the hydrophilicity/hydrophobicity of the MCM-41 surface, but also on the interactions between titanium centers and oxidants. Using XANES, a higher coordination number and an irreversible transformation of isolated into oxide-like titanium species was observed when the reaction was performed with H 2 O 2 compared to tbhp. The mechanical treatment of Ti-MCM-41 in a ball mill or press did not influence the local environment around the titanium centers, whereas the MCM-41 structure was damaged causing a decrease of the conversion of cyclohexene.

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.

Defect Chemistry and Thermomechanical Properties of Ce0.8Pr x Tb0.2 − x O2 − δ

The oxygen nonstoichiometry (δ) of Ce 0.8 Pr x Tb 0.2-x O 2-δ (x = 0, 0.05, 0.10, 0.15, 0.20) was measured as a function of P o2 at temperatures between 600 and 900°C by coulometric titration and thermogravimetry. A nonideal solution model, allowing for a linear δ dependence of the partial molar enthalpy of reduction in the dopants, could successfully reproduce the experimentally determined oxygen nonstoichiometry. X-ray absorption near-edge spectroscopy measurements were performed at the Ce/Pr/Tb L3 and L2 edges. The valence state of each dopant was affected by the presence of the co-dopant. The redox properties strongly depended on the lattice strain energy and the mean metal-oxygen bond strength. The thermal and chemical expansion coefficients were determined by dilatometry. The strongly nonlinear behavior of the thermal expansion coefficient originated from the chemical strain due to increasing oxygen nonstoichiometry with increasing temperature.

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.

Conversion of Ethane into Aromatic Compounds on Zsm-5 Zeolites Modified by Zinc

Abstract H-ZSM-5 zeolites modified by zinc exhibit a high activity for the conversion of ethane into aromatics, even at the relatively low temperature of 773 K. The activity and selectivity of these catalysts are related to the zinc species introduced via various methods but, surprisingly, not to the acidic properties.

In situ XANES study on TiO2–SiO2 aerogels and flame made materials

The structures of TiO2–SiO2 based aerogels and flame made particles, used as epoxidation catalysts, are compared by X-ray absorption near edge structure (XANES) spectroscopy and in situ XANES during dehydration of the calcined catalyst samples. The results are interpreted in terms of the titanium species present in the catalyst and the catalytic performance in the test reaction, the epoxidation of 2-cyclohexene-1-ol by tert-butylhydroperoxide (THBP) at 90 °C. XANES shows that tetrahedral titanium sites are the favoured sites at low TiO2 concentrations (1–3.2 wt.% in this study). The catalytic performance (turnover frequency, selectivity) of such aerogels or flame made materials with low TiO2 content is much better than that of corresponding samples with high TiO2 content. In aerogels, regardless of the TiO2 content, the amount of titanium with tetrahedral coordination increases significantly after dehydration at elevated temperature. In contrast, the pre-edge height and its position in the XANES spectra of the flame made catalyst samples is comparable, even without dehydration, to that of TS-1, which contains titanium in tetrahedral coordination. Thus the extent of tetrahedral Ti is intrinsically high in flame made catalysts and increases only slightly during the drying process. This behaviour can be attributed to the hydrophobic character of the flame made materials and ameliorates the catalytic performance, i.e. the selectivity.

Synthesis of iron-containing MFI type zeolites and its application to the conversion of ethane into aromatic compounds

Abstract Iron was incorporated into H[Al]-MFI by synthesis. The state of iron was characterized by e.s.r. and Mossbauer spectroscopies. Because of the location of iron species inside the channel system this zeolite showed a higher activity in the aromatization of ethane at 823 K than an iron-impregnated H[Al]-MFI.