Nikolai Trofimenko

Transition metal doped lanthanum gallates

Abstract La0.9Sr0.1Ga0.8Mg0.2O3−x+δ (LSGM) was additionally doped by y=0.1–0.3 of the transition metals M=Cr, Mn, Fe, Co to form La0.9Sr0.1(Ga1−yMy)0.8Mg0.2O3−x+δ. After heating in air at 1400°C single phase materials of the cubic perovskite-type are obtained. Only the addition of Co y>0.1 in oxidizing conditions (air) leads to a hexagonal structure. The dopants occupied the Ga-site of the perovskite structure. The oxygen stoichiometry range δ between the reduced and oxidized state of the mixed oxides were determined by solid electrolyte coulometry. The δ-values increase with increasing transition metal doping and in the row Cr, Mn, Fe, Co. An approach is given for the calculation of mean ionic radii, Goldschmidt tolerance factors and free volumes of mixed and nonstoichiometric oxides taking into account the real mole fractions and oxidation states of the cations. At the dopant concentration y=0.1 the lattice parameter a increases from Co to Cr according to the mean ionic radii. The electrical conductivity of La0.9Sr0.1(Ga0.9M0.1)0.8Mg0.2O3−x+δ (M=Fe, Co) is of the ionic type. No break in the activation energy is observed, in opposition to the behaviour of LSGM near 600°C. The A-substoichiometric La0.85Sr0.1(Ga0.9Co0.1)0.8Mg0.2O3−x+δ, has the highest ionic conductivity, at 600°C twice as high as LSGM. At M-fractions y>0.1, the conductivity increases due to additional p-type conduction.

Composition, structure and transport properties of perovskite-type oxides

Abstract Perovskite-type oxides La 1− a A a M 1− b B b O 3− x with A=Sr 2+ , Ln 3+ , Ce 4+ , M=Fe, Co, Ga and B=Co, Fe, Mg were prepared in the concentration range a =0.1–1 mol and b =0.1–0.5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of p O 2 in the range 10 5 > p O 2 >10 −14 Pa and temperature (500–1000°C) was measured on ceramic shapes by a dc four-point technique in combination with solid electrolyte coulometry. The ionic part of the conductivity in mixed conductors was determined by oxygen permeation measurements. The II–III perovskites Sr(Co,Fe)O 3− x in their stabilized form are excellent mixed conductors (maximum 500 S cm −1 at 400°C) and have an up to two orders of magnitude higher oxygen ionic conductivity than the preferred III–III perovskite La(Sr)Mn(Co)O 3− x . The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O 3− x could be increased by doping with 0.1 mol Co. By application of higher Co or Fe doping concentrations the lanthanum gallate becomes a mixed conductor.

Co-doped LSGM: composition–structure–conductivity relations

La0.9Sr0.1Ga0.8Mg0.2O3−x (LSGM) was additionally doped with 0.1 to 0.3 mol Co. Powders and pellets were prepared by a solid-state reaction in air. X-ray diffraction shows a cubic structure for y≤0.1 mol Co and an hexagonal structure for y≥0.2 mol Co. At pO2<10−6 Pa the structure of all compositions changed to cubic, independent of Co content. Oxygen stoichiometries of the reduced and oxidized states of the mixed oxides were determined by solid electrolyte coulometry. The Co2+/Co4+ concentrations were calculated from oxygen stoichiometries. At y=0.1 mol Co is stabilized at the Ga site as Co2+ up to high pO2. The electrical conductivity of La0.9Sr0.1(Ga0.9Co0.1)0.8Mg0.2O3−x+δ shows a wide ionic domain. With increasing y the conductivity increases due to additional p-type conduction. A defect model including electron holes and oxygen vacancies is discussed.

Estimation of effective ionic radii in highly defective perovskite-type oxides from experimental data

Abstract Effective ionic radii in crystals with higher concentrations of defects may considerably differ from tabulated values. For a number of perovskite-type oxides A 1− a A′ a B 1− b B′ b O 3± x (A, A′=rare earth, earth alkaline, B, B′=Al, Ga, In, Zr, Ce, Cr, Mn, Fe, Co, Mg) a calculation mode for average ionic radii of each sub-lattice is proposed on the basis of experimentally determined oxygen stoichiometries (vacancy concentrations) and unit cell volumes. The effect of the vacancies on the lattice expansion is considered. A two-dimensional radius diagram combined with Goldschmidt’s tolerance factors resulting from the effective radii represents the structure modifications of non-defective and cation vacant perovskite-type oxides. For anion vacant perovskite-type oxides a three-dimensional diagram with the axes r A – r B – r O was constructed. For more than 50 compositions of oxides the specific free volumes of the unit cell are correlated with the t -factors calculated from the effective ionic radii. Independently on structural modifications of the perovskite-type and of the defect-type a linear relation between V f,s and t was found. The approaching character of the calculation scheme is discussed, and the results are evaluated with regard to the gradually improved calculation modes of t -factors.

Oxygen non-stoichiometry and electrical conductivity of the binary strontium cobalt oxide SrCoOx

Abstract Strontium cobaltite was investigated using solid-electrolyte coulometry and resistivity measurements in the temperature range 20–1050°C and oxygen partial pressures 0.5–400 Pa. Two observed oxygen desorption/sorption maxima within the temperature range 500–950°C correlate with phase transitions of this compound as reported in the literature. An additional oxygen desorption/sorption maximum was found at a temperature of 965–1000°C, which is explained an order-disorder transition of the cubic high-temperature phase. The dependencies of equilibrium values of oxygen content as well as specific resistivity on temperature and oxygen partial pressure were founded for the cubic phase.

Influence of Electrode Design and Contacting Layers on Performance of Electrolyte Supported SOFC/SOEC Single Cells

The solid oxide cell is a basis for highly efficient and reversible electrochemical energy conversion. A single cell based on a planar electrolyte substrate as support (ESC) is often utilized for SOFC/SOEC stack manufacturing and fulfills necessary requirements for application in small, medium and large scale fuel cell and electrolysis systems. Thickness of the electrolyte substrate, and its ionic conductivity limits the power density of the ESC. To improve the performance of this cell type in SOFC/SOEC mode, alternative fuel electrodes, on the basis of Ni/CGO as well as electrolytes with reduced thickness, have been applied. Furthermore, different interlayers on the air side have been tested to avoid the electrode delamination and to reduce the cell degradation in electrolysis mode. Finally, the influence of the contacting layer on cell performance, especially for cells with an ultrathin electrolyte and thin electrode layers, has been investigated. It has been found that Ni/CGO outperform traditional Ni/8YSZ electrodes and the introduction of a ScSZ interlayer substantially reduces the degradation rate of ESC in electrolysis mode. Furthermore, it was demonstrated that, for thin electrodes, the application of contacting layers with good conductivity and adhesion to current collectors improves performance significantly.

Long-term and Redox Stability of Electrolyte Supported Solid Oxide Fuel Cells Under Various Operating Conditions

The electrolyte supported cells satisfy the requirements for the application in the solid oxide fuel cell such as mechanical stability, long-term stability during operation at temperatures <1000{degree sign}C, thermal cycling and very low degradation during repeated anode reduction/oxidation cycles. Nevertheless an enhancement of the power density at T=800-850{degree sign}C should be achieved to increase the power density level towards anode supported cells. The electrolyte supported solid oxide fuel cells on dense 8YSZ or 10Sc1CeSZ tapes (50x50x0.150 mm) with screen printed nickel oxide and yttria stabilized zirconia cermet anode (NiO/YSZ) and lanthanum strontium manganite and yttria stabilized zirconia composite cathode (uLSM/YSZ) were sintered in co-firing. The long-term tests (over 1000 hours) were carried out at 850{degree sign}C at constant load of 550 mA/cm2 (H2:H2O:N2=40:5:55, fuel utilization uf=50%) for 8YSZ based MEA and 650 mA/cm2 (H2:H2O=50:50, uf=23%) for 10Sc1CeSZ based MEA. During the redox cycle the cells were unloaded and fully oxidized by air for 120-180 min. Up to 10 redox cycles were performed. The field emission scanning electron microscopy (FESEM) was used to characterize the microstructural changes that occurred after long-term and redox cycles experiments. Changes of polarization resistance of the cells during the experiments were analyzed by impedance spectroscopy.

Long-Term Stability of Composite Cathode at High Current Densities

The durability tests were carried out on substrate supported cells based on yttria and scandia stabilized zirconia with conventional uLSM/YSZ and Ni/YSZ electrodes. The cells were operated at 850{degree sign}C in air/H2:H2O (1:1) for more than 1000h at current densities >450 mA/cm2. The reduction of the cathode polarization resistance is observed from the analysis of the impedance spectra. Microstructure investigations of the composite cathode using FESEM and TEM were made. It was found that the composite cathode undergoes morphology changes during the first 200-500h of the operation creating some nano-porosity at the uLSM/YSZ interface. Furthermore it was found that the change of the microstructure is strongly affected by the magnitude of the current density. The depth of the electrochemically structured zone as well as the change of the uLSM-composition at the uLSM/YSZ-interface was investigated using transmission electron microscopy.

Electrochemical and Microstructural Characterization of the Solid Oxide Fuel Cell Anode Prepared by Co-precipitation

Nickel oxide and yttria stabilised zirconia (NiO/YSZ) cermets have been prepared using a chemical co-precipitation of hydroxides. The influence of different factors on the microstructure, mechanical and electrochemical properties of the anode were investigated. The electrochemical tests were carried out on dense 8YSZ and 10Sc1CeSZ tapes (50x50x0.150 mm, Kerafol GmbH) with symmetrically screen-printed cathodes and anodes with a lateral dimension of 40x40 mm. The cells were sintered in co-firing. The anodes were characterized by impedance spectroscopy at open circuit potential and under current load at temperatures of 850-950{degree sign}C in hydrogen/steam atmosphere (H2:H2O=1:1). The electrode polarization resistance (Rp) of the spectra was identified and the influences of operating temperature (850{degree sign}C, 900{degree sign}C and 950{degree sign}C), current load and pH2O on impedance spectra of anode were discussed. The long-term stability of the cell with an anode prepared by co-precipitation was carried out over 1000 hours under current density of 430-450 mA/cm2@0.7V at temperature 850{degree sign}C.