Search

Ceramic SOFC anodes with nano scaled electrocatalyst particles are of current interest for low temperature application of SOCs and to increase robustness in terms of redox stability and coking tolerance. Strontium titanates are a materials class that is compatible with the state-of the art electrolyte yttria stabilized zirconia and possesses sufficient electronic conductivity under reducing conditions that make them attractive for SOC fuel electrodes. However, their electrocatalytic activity is rather poor and nano scaled electrocatalyst particles are infiltrated into the porous strontium titanate to achieve technically relevant performance. Various electroactive materials have been explored by infiltration into Strontium titanates. Ni, Co and Fe with and without ceria-gadolinia (CGO) have been infiltrated as metal particles into two different Strontium titanates, A-site deficient La-Sr-C-titanate (LSCT) modified with different elements (Cr, Mn, Zn) on the B-site and Niobium modified Strontium-titanate (STN). Zn substitution showed a positive effect on the pure LST electrodes and electrode area specific resistances down to 0.2 Ω∙cm2 have been achieved for LST-Zn electrodes infiltrated with Ni-CGO. The influence of backbone vs. electrocatalyst on the electrode performance will be presented in this contribution.

A group of experts from European research organisations and industry have assessed the state of the art and future needs for materials' R&D for hydrogen and fuel cell technologies. The work was performed as input to the European Commission's roadmapping exercise on materials for the European Strategic Energy TechnologyPlan. The report summarises the results, including key targets identified for medium term (2020/2030) and long term (2050) timescales.

The thermally activated proton diffusion in BaZr0.9Y0.1O3-�� was studied with electrochemical impedance spectroscopy (IS) and quasi-elastic neutron scattering (QENS) in the temperature range from 300 K to 900 K. The diffusivities for the bulk material and the grain boundaries as obtained by IS obey an Arrhenius law with activation energies of 0.46 eV and 1.21 eV, respectively. The activation energies obtained by IS for the bulk are 0.26 eV above 700 K and 0.46 eV, below 700 K. The total diffusivity as obtained by IS is by one order of magnitude lower than the microscopic diffusivity as obtained by QENS. The activation energies obtained by QENS are 0.13 eV above 700 K and 0.04 eV, below 700 K. At about 700 K, the diffusion constants for IS and QENS have a remarkable crossover, suggesting two processes with different activation energies.

Partial substitution of Fe by Ta in the hole-doped La0.5Sr0.5FeO3 decreases the electric conductivity by up to three orders of magnitude. This decrease is in immediate correlation with a decrease of the electron hole concentration and a shift of the spectral weight within the O(1s)-Fe(3d) mixed states from eg bands near the Fermi energy to t2g bands. Corresponding differences in the Fe(2p) and O(1s) X-ray absorption spectra reveals formation of states with increased covalency in the initial state and states which contain O(2p) character, and that hole states are responsible for transport changes in the material. The intensity ratio of eg and t2g bands appears to be a spectral indicator for hole formation., Comment: PACS numbers: 71.27.+a, 71.30.+h, 71.38.-k, 72.10.Fk, 78.70.Dm

Journal of Electroceramics, 22 (1-3, Special Issue), ISSN:1385-3449, ISSN:1573-8663

not Available.

The influence of partial substitution of Fe with Ti on the oxygen transport properties of La1- xSrxFeO3 membranes was investigated in view of their application for oxygen separation. Samples of composition La0.5Sr0.5 Fe1-yTiyO3-δ (y = 0, 0.2) were prepared and their oxygen transport properties characterised by potential-step relaxation and by oxygen permeation measurement in an air/argon gradient. With the first technique, chemical diffusion (D) and surface exchange (kS) coefficients were obtained by fitting of the current relaxation data to a single expression valid over the complete time range. The Ti- substituted composition gave slightly larger values of D and kS. The trend was opposite for the measured oxygen permeation flux. In the latter experience, ordering of oxygen vacancies was observed at lower temperature, reducing significantly the performance of the material.

The aim in this project was the optimization of LaFeO3-based perovskite materials for oxygen separation membranes (OSM). The work was carried out by the Laboratory for High Performance Ceramics, Empa in collaboration with the Laboratory for Industrial Energy Systems, LENI, EPFL and the Ceramics Laboratory, LC, EPFL. The ceramic material of interest is a mixed conductor for oxygen ions and electrons at high temperature (800°-900°C) and therefore can separate oxygen from nitrogen with 100% selectivity when exposed to an oxygen partial pressure gradient (e.g. air vs. natural gas). In this capacity, such materials are attractive for the application of partial oxidation (POX) of natural gas (NG) to synthesis gas in an important and growing business. The crucial point in this developing technology is the chemical and mechanical stability of the membrane under the operating conditions (high temperature, PO2 gradient). Because of thermal and chemical expansion, large stress can be induced in the material, which can lead to fracturing. Additional destabilizing processes such as kinetic demixing (elemental segregation under influence of the oxygen gradient) or slow phase transformation can lead to decreased performance with time. The overall goal in this Ph.D. thesis was to address these stability problems from a combined experimental and theoretical point of view in order to develop a more detailed understanding and strategy for improving the materials properties. Experimentally, the ceramic membrane materials were synthesized with different B-site substitutions with the aim of decreasing chemical expansion. The focus lied on LaFeO3 materials substituted with Sr on A-site. Detailed characterizations of the most promising compositions were done including structural, transport, and mechanical properties.

Mixed ionic electronic conducting perovskite materials have been receiving considerable attention for the application of oxygen separation membranes. These membranes have potential to be integrated in industrial processes that require pure oxygen and to provide advantages considering economical and environmental aspects. The used perovskite materials can accommodate a high concentration of disordered oxygen vacancies and provide high oxygen permeation flux in the presence of an oxygen partial pressure gradient. However, the materials with high oxygen flux are observed to have low chemical and mechanical stability, originating from oxygen vacancy formation and reduction of the B-site transition metal ion. Based on the fact that the stability of the materials at reducing pO2 is dependent on the redox stability of the B-site cation, a partial substitution of the B-site transition metal with a more stable cation is considered in this thesis to improve the stability of the base perovskite oxide. The approach used for the identification of the suitable material based on the host composition La0.5Sr0.5FeO3-δ (LSF) was to first explore the B-site substituting elements in terms of their effect on the thermal expansion and the isothermal expansion measured during the atmosphere change from air to argon. The elements Al, Cr, Zr, Ga, Ti, Sn, Ta, In, V, and Mg were used for 10 or 20% substitution of Fe. The isothermal expansion of the host material was decreased by substitution, which was attributed to decreased amount of oxygen vacancy formation as it was evidenced later by TGA of chosen compositions. As a result, the compositions substituted with higher valence ions (particularly, 20% Ti4+, LSFTi2, and 10% Ta5+, LSFTa1) were identified as the least expanding materials in isothermal conditions. The effect of substitution was further characterized extensively including structural, mechanical, and oxygen transport properties. The oxygen vacancy (VO••) concentration changes induced by substitution of Fe were reflected in unit cell sizes of heat-treated samples under argon atmosphere. The increase in the unit cell size was attributed to the isothermal expansion due to B-site reduction and lowered to half in case of substituted LSFTi2 and LSFTa1 compared to the host material LSF. Similar trends were observed for interrelated properties such as the coefficient of thermal expansion, isothermal expansion, and the actual amount of oxygen vacancies formed under argon at 900°C, all of which were higher for LSF. The mechanical properties of LSF, LSFTi2, and LSFTa1 were characterized at room temperature. The Young's modulus and the bending strength of the materials were in the same range (141-147 GPa and ∼120 MPa, respectively) while the fracture toughness of the LSF sample was improved by Ti and especially Ta substitution. The fractography of the samples provided evidence that the several LSF samples showed extended cracks possibly related to residual stresses forming during cooling, due to non-equilibrated oxygen stoichiometry across the sample. The potential step measurements showed that the chemical diffusion and the surface exchange coefficients measured during oxidation of the samples were higher than the ones measured during reduction of the samples. The tendency was reversed at lowered pO2 along with the decrease of both coefficients. The influence of B-site substitution on both coefficients was not substantial. The oxygen permeation flux of LSF, LSFTi2, and LSFTa1 was measured under air/argon gradient on planar membranes. The permeation rates of the membranes with thicknesses close to 1 mm were considered to be limited by surface exchange kinetics at temperatures above 875°C. The permeation rate of LSF was decreased (from 0.2 µmolcm-2s-1) to its third by Ti and Ta additions. The permeation rate of Ti and Ta-substituted samples showed a drop around 875°C, keeping a similar activation energy at lower and higher temperature regions. Oxygen vacancy ordering in the perovskite structure was given as a possible explanation, which provided explanation for the slow equilibrium in the lower temperature region. Tubular membranes of LSF and LSFTi2 (0.47 mm and 0.36 mm thick, respectively) and a planar LSFTa1 membrane were characterized using an air/(Ar-CH4) gradient. The measurements conducted using pure Ar on the lean-side provided the possibility to compare the thickness dependence of the permeation. In agreement with the previous observations, the permeation of the materials at and above 900°C was independent of the thickness, therefore, surface exchange limited. Stability under reducing atmospheres was increased by substitution. The LSF membrane failed shortly after the introduction of CH4 to the system, while LSFTi2 survived 5% CH4 and its oxygen permeation was improved substantially by a factor of 14. LSFTa1 membrane was measured with pure CH4 and the permeation was increased by a factor of 9, reaching 0.7 µmolcm-2s-1 at 900°C. The membrane operated stably over 2000h with pure CH4, 1000h. A slow degradation of 3.7% per 1000h was observed at 1000°C, most probably due to cation migration. The compositions identified as a result of B-site substitution screening were effective in improving the stability of the materials without extensive loss of the oxygen permeation rate. Ta-substituted material was shown to be promising with its long-term stability as a partial oxidation membrane.