Zn and Sc doped Perovskites and Nano composite structure for the SOFC cathode

Abstract

Solid oxide fuel cells (SOFCs) are an attractive energy converting system having a high efficiency, fuel flexibility and cheap ceramic cell components. Normally, the activation of oxygen in cathode of ceramic material is more difficult than that of hydrogen in the anode of metal cermets so that the oxygen reduction in cathode is acting as the bottle-neck of cell performance. In this thesis, therefore, studies were carried out for the cathode materials in order to improve both intrinsic property by synthesizing new materials and extrinsic properties by introducing composite cathodes with nano structure. Zinc-doped barium strontium cobalt ferrite (Ba0.5Sr0.5Co0.2-xZnxFe0.8O3-δ (BSCZF), x = 0, 0.05, 0.1, 0.15, 0.2) powders having various zinc contents were prepared by using ethylenediamine tetraacetic acid (EDTA)-citrate method, and followed by repeated ball-milling and calcining. The synthesized samples were evaluated by means of XRD, H2-TPR, SEM and electrochemical test for aiming at using as cathode materials for solid oxide fuel cells at intermediate temperatures (IT-SOFCs). As the zinc doping (x) was increased from zero to 0.2 (substitution for cobalt from zero to 100%), the lowest doping of 0.05 (BSCZF05) showed the highest electrical conductivity of 30.7 Scm-1 at 500℃ and the lowest decrease of electrochemical performance upon increasing the sintering temperature from 950℃ to 1,000℃, due to increased thermal stability and decreased reduction of Co4+ without a loss in the electrical conductivity. The polarization resistances of BSCZF05 calcined at 950℃ were 0.15 Ωcm2, 0.28 Ωcm2 and 0.59 Ωcm2 at 700℃, 650℃ and 600℃, respectively. It was further decreased about 30% by the incorporation of Sm0.2Ce0.8O2-δ (SDC) electrolyte particles and by increasing the sintering temperature to 1,000℃. Using a composite cathode of BSCZF05 mixed with 30wt.% of SDC, button cells of Ni-SDC support having 30μm of SDC dense membrane yielded a maximum power density of 605 mWcm-2 at 700℃. It is therefore concluded that zinc doping increases the thermal stability and interrupts the reduction of Co4+ of active ions.Scandium was also used as a doping element to improve cathode performance of La0.6Sr0.4FeO3. LSFSc having various zinc contents (La0.6Sr0.4Fe1.0-xScxO3-δ, x = 0, 0.05, 0.1, 0.2) were prepared by the EDTA-citrate method. Prepared LSFSc powders having well-indexed crystalline peaks were investigated for the electrical conductivity, polarization resistance, microstructure, reaction order and electrochemical properties. When scandium substituted 5mol% of iron in B-site, it showed the best performances toward cathode ORR due to the increase of oxygen vacancy concentration. When it was further incorporated with 50wt% of YSZ particles, its Rp values decreased 70% and MPD also increased 40%. It is concluded that the minor doping of elements having constant oxidation state such as Zn2+ and Sc3+ results in positive effects on the electrochemical properties of SOFC cathodes.Various synthetic methods were also developed for LSM-YSZ composite cathodes. Nano-sized (50 nm) lanthanum strontium manganite (La0.8Sr0.2MnO3, LSM) particles were deposited on yttria-stablized zirconia (8YSZ) by synthesizing LSM particles in situ in an YSZ-dispersed solution. As the LSM content was decreased from 80 wt% to 25 wt%, 50 wt% powder showed the best microstructure and phase connectivity. This composite, when used as cathode in a button cell, also exhibited the highest power density of 791 mWcm-2 at 800℃ and the lowest values of the cathode Rp and high frequency arc (0.315 Ωcm2 and 0.120 Ωcm2, respectively). Initially, the low frequency arc showed a rapid decrease as the LSM content was reduced from 80 wt% to 60 wt%, after which, with further decreases in the LSM content, an abrupt drop at 50 wt% LSM content was followed by a slow decrease in the low frequency arc. From the observed results, it is suggested that the high frequency arc is related to the charge transfer and the low frequency arc to the site density of the triple phase boundary (TPB). A new parameter, the charge transfer efficiency of the TPB site, is defined and used to further explain the effect of LSM content on YSZ in the observed results. A composite cathode was prepared by introducing electrolyte material such as yttria-stablized zirconia (YSZ), gadolinium doped ceria (GDC) and samarium doped ceria (SDC) (all are 150 nm of size) to the LSM by in-situ technique. As the sintering temperature increased, ceria based composite cathodes were sintered in a lesser degree than zirconia based composite cathodes and exhibited a better cathodic polarization resistance. Especially, LSM-GDC cathode sintered at 1050℃ showed the lowest Rp value of 0.208 Ωcm2 at 800℃. The button cell test of LSM-GDC composite showed the highest power density of 1066 mWcm-2 at 800℃. The activation energy calculated from Arrhenius plots of LSM-GDC was decreased as the sintering temperature decreased. On the contrary, Ea of LSM-YSZ cathode was increased due to the lower ionic conductivity than ceria based electrolyte and forming a secondary phase by the reaction between LSM and YSZ. Observed results of impedance spectra suggests that the high-frequency arc is related to charge transfer and low-frequency arc to the site density of triple-phase boundary (TPB) and diffusion of gas molecules in sintered layer. According the combination of electrolyte materials, the optimum conditions for the manufacturing electrode are differently appeared.Another investigation conducted was the effect of synthesis method and condition on the electrochemical and structural properties. Nano-structure of LSM-YSZ composite cathodes could be prepared by using in-situ techniques in which are synthesizing and deposing of LSM particles in YSZ dispersion solution. Wet chemistry methods such as cellulose-GNP or precipitation yielded composite cathodes with a large surface area. Their polarization resistances were smaller than those prepared by solid state methods. But cathodes derived by wet chemistry methods were weak for the high temperature sintering. Another method is a synthesis of meso-structured LSM-YSZ composite cathodes by using silica hard template method. That cathode having nano contacts shows the superior performances than a simple cathode. The conditions for the synthesis and electrochemical and structural analysis are investigated. These materials showed superior performances than conventional cathodes.