Browsing by Author "Wang, Lei"

Now showing 1 - 3 of 3

Open Access
Ba1-xSrxCoyFe1-yO3-delta SOFC cathode materials : bulk properties, kinetics and mechanism of oxygen reduction
(2009) Wang, Lei; Maier, Joachim (Prof. Dr.)
This work is mainly concerned with the mixed conducting perovskite solid solution materials family Ba1-xSrxCoyFe1-yO3-delta (BSCF) which is discussed as solid oxide fuel cell (SOFC) cathode material. The aim is to get an improved understanding of the complex oxygen reduction reaction on such oxides in general, and in particular for the application as catalytically active cathode in SOFC. As a SOFC cathode candidate, the stability of BSCFs with regard to the application was first studied on powders synthesized from the metal nitrates by the glycine nitrate process. With respect to the stability towards electrolyte materials, at 750 °C (Ba0.5Sr0.5)1.04Co0.8Fe0.2O3-delta (BSCF5080) already reacts with 8 mol % Y2O3-ZrO2 (YSZ). This undesired reactivity is more pronounced than that of La0.6Sr0.4Co0.8Fe0.2O3-delta and YSZ. At 850 °C, BSCF5080 even starts to react with Ce0.9Ge0.1O2-delta (CGO). The increased reactivity of BSCF5080 may mainly come from the higher A-/B- cation size mismatch and therefore BSCFs with lower Ba content or Ba-deficiency may show higher stability with the electrolytes. Concerning the stability of BSCFs under CO2-containing atmosphere, a high Ba content is detrimental. Even Ba-deficiency can not effectively inhibit the formation of carbonates. As a result, Ba-containing perovskite oxide cathodes require highly purified oxidizing atmosphere which is not very practical. Some of the BSCF compositions with cubic perovskite structure from the synthesis slowly transform to noncubic phases at intermediate temperatures. For the investigated compositions, no obvious relationship between the cubic perovskite stability at intermediate temperatures and the Goldschmidt tolerance factor could be established. However, BSCF with lower Co content shows higher long-term phase stability, therefore Co/Fe ratio may play an important role. Co changes its oxidation state more easily and the ionic radius change upon the change of the oxidation state facilitates the formation of face-sharing octahedra with shorter B-O bond length and the resulting noncubic perovskites. The oxygen reduction kinetics and mechanism was studied on geometrically well defined microelectrodes. 100 nm thick dense thin films of the respective compositions were deposited by pulsed laser deposition (PLD) on (100)-oriented YSZ single crystals. X-ray diffraction (XRD) shows the films maintain high phase purity and exhibit a highly textured structure which depends on the film thickness, the exact cation composition and the substrate. Scanning electron microscopy (SEM) measurements confirm that the thin films prepared under the present deposition conditions are dense films with columnar growth. The thin films were subsequently patterned into circular microelectrodes with 20 to 100 µm diameter by photolithography. Silver paste and foil were attached to the back side of the sample as counter electrode. Impedance spectra were recorded on these microelectrodes and the temperature, oxygen partial pressure and dc bias dependence was studied. It is found that the oxygen reduction on these BSCF microelectrodes proceeds through the "bulk path", i. e., incorporation of oxygen into the electrode on the whole electrode surface, and subsequent oxygen ions diffusion through the electrode bulk. The rate of oxygen incorporation is limited by the oxygen surface reaction rate. Based on the equivalent circuit developed from previous works, the surface resistance Rs corresponding to the oxygen surface incorporation reaction can be quantitatively compared for different cation compositions in BSCF. The temperature dependence of Rs is similar for the six BSCF compositions studied and the activation energies are in the range of 1.3 to 1.8 eV. In the oxygen partial pressure P(O2) dependence, all compositions have an exponent between -0.5 and -1 in the logRs-logP(O2) plot indicating the reaction order of the oxygen molecules is 1. Both cathodic and anodic dc bias decrease Rs. Applied cathodic bias values up to 400 mV do not introduce obvious irreversible changes on a less degraded ("fresh") sample while a cathodic bias of 300 mV already obviously decreases Rs and introduces irreversible changes on a more degraded ("aged") sample. Due to the difficulty in directly detecting the coverage and nature of the intermediate oxygen species on the surface of the cathodes under SOFC operation conditions, correlations between the oxygen incorporation reaction rate kq calculated from Rs and various bulk materials properties were studied and used to supply information on the detailed oxygen reduction mechanism. There is no straightforward correlation between kq and the lattice constant, the electronic conductivity, the Goldschmidt tolerance factor or the oxidation enthalpy. However, kq increases nonlinearly with increasing oxygen vacancy concentration cVö which indicates that the oxygen vacancies are involved in the rate-determining step of the oxygen incorporation reaction. The nonlinear increase of kq with increasing cVö was assumed to be caused by the increased oxygen vacancy mobility. To confirm this assumption, acquiring reliable oxygen vacancy diffusion coefficient data is necessary. Due to the relatively low density of sintered pellets and the complexity in the experiments, oxygen vacancy diffusion coefficients from conductivity relaxation experiments for BSCF5080 were not accurate enough. However, reliable oxygen vacancy diffusion coefficients were obtained on 250 nm dense BSCF thin films deposited by PLD on (100)-oriented MgO single crystals. A gas-tight gold cover layer was deposited on top of the BSCF film by evaporation. In order to enable oxygen isotope exchange between BSCF and the atmosphere, a cut about 30 µm wide with relatively sharp edges was created through the gold and BSCF layers with an automatic dicing saw. Isotope exchange was carried out at different temperatures and later on the 18O concentration profile in the quenched samples was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The oxygen vacancy diffusion coefficients DVö calculated from the extracted oxygen tracer diffusion coefficient D* for BSCF5080 are significantly higher than that of (La,Sr)(Mn,Fe,Co)O3-delta perovskites. The activation energy Ea of DVö for BSCF5080 is only 0.47 (+/- 0.04) eV which is much lower than that of Ba0.5Sr0.5FeO3-delta (BSF, 1.1 (+/- 0.1) eV), SrFeO3-delta (SF, 0.9 (+/- 0.1) eV) and (La,Sr)(Mn,Fe,Co)O3-delta (~ 0.9 eV). The high DVö and low Ea is thought to be due to low cation charge (A2+, fraction of Co2+) and high polarizability (Ba2+, Co2+). With DVö calculated from D*, the correlation between kq and DVö is confirmed. While the increase of kq from (La,Sr)(Mn)O3+/-delta (LSM) to (La,Sr)(Co,Fe)O3-delta (LSCF) is mainly due to the increase of the oxygen vacancy concentration, the increase of kq from LSCF/SF to Ba0.5Sr0.5Co0.8Fe0.2O3-delta mainly comes from the accelerated (surface) oxygen vacancy diffusion. Based on the experimental observations and the conclusions from DFT calculations on LaMnO3 slabs, the rate-determining step of the oxygen incorporation reaction on BSCF is proposed to be the diffusion of an oxygen vacancy towards an adsorbed O2- resulting in an O2(2-) adsorbed at the vacancy position, the dissociation of which is fast. There are intrinsic problems of BSCF when it is applied as SOFC cathode, such as the undesired reactivity with the electrolyte, the carbonate formation under CO2-containing atmosphere and the long-term phase transformation. It is almost impossible to solve them only by varying the cation composition within this material family. Therefore BSCF may not be applicable as a SOFC cathode. Nevertheless, based on the proposed oxygen incorporation mechanism for BSCF, the combination of high oxygen vacancy concentration and mobility is an adequate criterion for selecting other SOFC cathode candidates. In BSCF, the high oxygen vacancy concentration is due to the fact that the 4+ oxidation state of B-site cations is not so stable, and therefore the 2+ oxidation state at A-site is mainly compensated by the formation of oxygen vacancies. To avoid the problems introduced by Ba and maintain the average oxidation state of A-site to be 2+, perovskites with a combination of 1+ alkali metals and 3+ rare earth metals at A-site were studied. The solubility of Ag in (La,Ag)Co0.4Fe0.6O3-delta perovskites is rather limited. For K-containing (La,K)Co0.4Fe0.6O3-delta and (Nd,K)Co0.4Fe0.6O3-delta perovskites, it is difficult to get phase pure powders. Rs from impedance spectroscopy studies of the material with a nominal composition La0.5Co0.4Fe0.6O3-delta and a predominant perovskite phase is one order of magnitude higher than LSCF and this is most probably due to the phase impurities. Based on the mechanism suggested in this thesis, fast oxygen exchange materials should have a high oxygen vacancy mobility. This motivates to investigate perovskites with highly polarizable A-cations. In this respect, Bi3+ instead of Ba2+ would be an interesting candidate, which also might help to reduce the carbonate formation problem.
Open Access
Optimierung der Genexpression in Escherichia coli und Thermus thermophilus
(2017) Wang, Lei; Mattes, Ralf (Prof. Dr.)
Open Access
PRMT6 activates cyclin D1 expression in conjunction with the transcription factor LEF1
(2021) Schneider, Lucas; Herkt, Stefanie; Wang, Lei; Feld, Christine; Wesely, Josephine; Kuvardina, Olga N.; Meyer, Annekarin; Oellerich, Thomas; Häupl, Björn; Seifried, Erhard; Bonig, Halvard; Lausen, Joern
The establishment of cell type specific gene expression by transcription factors and their epigenetic cofactors is central for cell fate decisions. Protein arginine methyltransferase 6 (PRMT6) is an epigenetic regulator of gene expression mainly through methylating arginines at histone H3. This way it influences cellular differentiation and proliferation. PRMT6 lacks DNA-binding capability but is recruited by transcription factors to regulate gene expression. However, currently only a limited number of transcription factors have been identified, which facilitate recruitment of PRMT6 to key cell cycle related target genes. Here, we show that LEF1 contributes to the recruitment of PRMT6 to the central cell cycle regulator CCND1 (Cyclin D1). We identified LEF1 as an interaction partner of PRMT6. Knockdown of LEF1 or PRMT6 reduces CCND1 expression. This is in line with our observation that knockdown of PRMT6 increases the number of cells in G1 phase of the cell cycle and decreases proliferation. These results improve the understanding of PRMT6 activity in cell cycle regulation. We expect that these insights will foster the rational development and usage of specific PRMT6 inhibitors for cancer therapy.