Andrei V. Kovalevsky

Ceria-based materials for solid oxide fuel cells

This paper is focused on the comparative analysis of data on electronic and ionic conduction in gadolinia-doped ceria (CGO) ceramics as well as on the electrochemical properties of various oxide electrodes in contact with ceria-based solid electrolytes. Properties of electrode materials, having thermal expansion compatible with that of doped ceria, are briefly reviewed. At temperatures below 1000 K, Ce0.90Gd0.10O2−δ (CGO10) was found to possess a better stability at reduced oxygen pressures than Ce0.80Gd0.20O2−δ (CGO20). Incorporation of small amounts of praseodymium oxide into Ce0.80Gd0.20O2−δ leads to a slight improvement of the stability of CGO20 at intermediate temperatures, but the difference between electrolytic domain boundaries of the Pr-doped material and CGO10 is insignificant. Since interaction of ceria-based ceramis with electrode materials, such as lanthanum-strontium manganites, may result in the formation of low-conductive layers at the electrode/electrolyte interface, optimization of electrode fabrication conditions is needed. A good electrochemical activity in contact with CGO20 electrolyte was pointed out for electrodes of perovskite-type La0.8Sr0.2Fe0.8Co0.2O3−δ and LaFe0.5Ni0.5O3−δ, and LaCoO3−δ/La2Zr2O7 composites; surface modification of the electrode layers with praseodymium oxide results in considerable decrease of cathodic overpotentials. Using highly-dispersed ceria for the activation of SOFC anodes significantly improves the fuel cell performance.

Oxygen ionic conductivity of Ti-containing strontium ferrite

Abstract The total electrical conductivity, oxygen permeability, Faradaic efficiency and thermal expansion of perovskite-type Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ were studied. The oxygen permeation through strontium ferrite–titanate ceramics was found to be limited by both bulk ionic conduction and surface exchange rates, similar to other Fe- and Ti-containing perovskites. Increasing iron content in the system Sr 0.97 (Ti,Fe)O 3− δ leads to a considerable increase in oxygen ionic and electronic conductivities, and thermal expansion. The ion transference numbers of Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ were determined by the Faradaic efficiency measurements, and estimated from the oxygen permeability data. Typical values of the transference numbers vary in the range from 4×10 −3 to 1.5×10 −2 at 973–1223 K, decreasing with reducing temperature. The oxygen ionic conductivity of Sr 0.97 Fe 0.80 Ti 0.20 O 3− δ is close to the conductivity of the Zr 0.92 Y 0.08 O 1.96 solid electrolyte. The average thermal expansion coefficient of strontium ferrite–titanate, calculated from dilatometric data, is (13.8±0.1)×10 −6 K −1 at 300–780 K and (27.0±0.4)×10 −6 K −1 at 780–1040 K.

Enhancement of thermoelectric performance in strontium titanate by praseodymium substitution

In order to identify the effects of Pr additions on thermoelectric properties of strontium titanate, crystal structure, electrical and thermal conductivity, and Seebeck coefficient of Sr1−xPrxTiO3 (x = 0.02–0.30) materials were studied at 400 < T < 1180 K under highly reducing atmosphere. The mechanism of electronic transport was found to be similar up to 10% of praseodymium content, where generation of the charge carriers upon substitution resulted in significant increase of the electrical conductivity, moderate decrease in Seebeck coefficient, and general improvement of the power factor. Formation of point defects in the course of substitution led to suppression of the lattice thermal conductivity, whilst the contribution from electronic component was increasing with carrier concentration. Possible formation of layered structures and growing distortion of the perovskite lattice resulted in relatively low thermoelectric performance for Sr0.80Pr0.20TiO3 and Sr0.70Pr0.30TiO3. The maximum dimensionless figu...

Oxygen ion conductivity of hexagonal La2W1.25O6.75

Abstract Mixed oxygen ionic and electronic conduction was found for hexagonal lanthanum–tungsten oxide, La 2 W 1.25 O 6.75 , at temperatures of 900–1450 K. In atmospheric air, the partial ionic conductivity was established to prevail electronic conductivity and to be 1.3×10 −4 S/cm at 1170 K. Ion transference numbers were determined by Faradaic efficiency measurements to be 0.92±0.01 in air at 1170−1300 K. Decreasing oxygen partial pressure leads to increasing electronic conductivity due to leaving oxygen from the crystal lattice of La 2 W 1.25 O 6.75 . The activation energy for total electrical conductivity of the lanthanum–tungsten oxide in air is 113±2 kJ/mol. Mean thermal expansion coefficient of the La 2 W 1.25 O 6.75 ceramics in the temperature range of 300 to 1100 K was calculated from the results of dilatometric studies to be (10.9±0.3)×10 −6 K −1 .

Testing tubular solid oxide fuel cells in nonsteady-state conditions

Abstract Fabrication of tubular-type solid oxide fuel cells (SOFCs) with yttria-stabilized zirconia electrolyte, cathodes and current collectors of lanthanum–strontium manganite (LSM) is described. Particular emphasis is given to the techniques of producing LSM tubes by the isostatic pressing method, preparing oxide electrodes via cellulose precursor decomposition, and activation of SOFC electrodes by applying dispersed catalysts onto their surface. Coating nickel-cermet anodes with dispersed ceria and depositing praseodymium oxide onto manganite cathode surface was found to result in improving SOFC performance. Testing single cells with externally switched pulse load demonstrated a possibility to optimize the SOFC operating mode at a given resistance of the closing circuit by variation of the pulse period-to-pulse duration ratio of the pulses which open the circuit. No effect of the pulse load frequency on SOFC performance was observed in the frequency range from 2 Hz to 50 kHz. The results of testing SOFCs in nonsteady-state conditions suggest applicability of the externally switched pulse load to match resistances of single cells in the SOFC stacks.

Oxygen nonstoichiometry of Bi2V0.9Cu0.1O5.5-δ solid electrolyte by coulometric titration technique

Abstract Oxygen deficiency of Bi 2 V 0.90 Cu 0.10 O 5.5− δ (BICUVOX.10) solid electrolyte was studied by the coulometric titration technique and thermogravimetric analysis at oxygen partial pressures from 1×10 −7 to 0.5 atm (atmospheric air) in the temperature range 650–1050 K. Within the phase stability domain, the nonstoichiometry ( δ ) varies in the narrow range from 0.150 to 0.155. Increasing oxygen deficiency leads to a greater n -type electronic conductivity, which can be described by common models for other solid electrolytes. The partial molar enthalpy and entropy for oxygen incorporation into Bi 2 V 0.9 Cu 0.1 O 5.5− δ lattice linearly decrease with increasing δ . Further reduction of the oxygen partial pressure results in decomposition of Bi 2 V 0.90 Cu 0.10 O 5.5− δ , forming a mixture of an Aurivillius-type phase and binary metal oxides, which is accompanied with decreasing ionic conductivity. The results of the coulometric titration and ion transference number measurements suggest that BICUVOX.10 ceramics can be used as electrolyte only at atmospheric or higher oxygen pressures, preferably at temperatures below 900–950 K.

Designing strontium titanate-based thermoelectrics: insight into defect chemistry mechanisms

Driven by a need to develop low-cost and thermally stable materials for thermoelectric applications, donor-substituted strontium titanate is considered as a promising alternative to traditional thermoelectrics. The complex defect chemistry of SrTiO3-based materials imposes various limitations on identifying the relevant effects exerted on the electronic band structure and heat transfer, being a subject of debate and intensive research. Based on combined XRD, SEM/EDS, HRTEM, XPS, and TGA studies and measurements of thermoelectric properties, this work uncovers the particular role of various structural defects in electrical and thermal transport in Sr1±yTi0.9Nb0.1O3±δ, selected as a model system. Introduction of A-site cation vacancies provides a synergistic effect of combining fast charge transport in the perovskite lattice and suppressing the thermal conductivity mostly due to simultaneous generation of oxygen vacancies. The presence of oxygen vacancies promotes more efficient phonon scattering compared to Ruddlesden–Popper-type layers. These findings provide a link between structural and thermoelectric properties, offering further prospects for seeking highly performing SrTiO3-based thermoelectrics by tailoring the defect chemistry mechanisms.

Formation of Mgx Nby Ox+y through the Mechanochemical Reaction of MgH2 and Nb2O5, and Its Effect on the Hydrogen-Storage Behavior of MgH2.

The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. To clarify the chemical interaction between MgH2 and Nb2 O5 , the mechanochemical reaction products of a composite mixture of MgH2 +0.167 Nb2 O5 was monitored at different time intervals (2, 5, 15, 30, and 45 min, as well as 1, 2, 5, 10, 15, 20, 25, and 30 h). The study confirms the formation of catalytically active Nb-doped MgO nanoparticles (typically Mgx Nby Ox+y , with a crystallite size of 4-8 nm) by transforming reactants through an intermediate phase typified by Mgm-x Nb2n-y O5n-(x+y) . The initially formed Mgx Nby Ox+y product is shown to be Nb rich, with the concentration of Mg increasing upon increasing milling time. The nanoscale end-product Mgx Nby Ox+y closely resembles the crystallographic features of MgO, but with at least a 1-4 % higher unit cell volume. Unlike MgO, which is known to passivate the surfaces in MgH2 system, the Nb-dissolved MgO effectively mediates the Mg-H2 sorption reaction in the system. We believe that this observation will lead to new developments in the area of catalysis for metal-gas interactions.

Phase interaction and oxygen transport in oxide composite materials

Abstract The oxygen permeability of oxide composite membranes containing similar volume fractions of the components, including (La0.9 Sr0.1)0.98 Ga0.8 Mg0.2 O3-δ(LSGM)–La0.8 Sr0.2Fe0.8Co0.2O3-δ (LSFC), LSGM–La2Ni0.8Cu0.2O4+δ (LNC), SrCoO3-δ–Sr2Fe3O6.5 ±δ, Ce0.8Gd0.2O2-δ (CGO)–LSFC and CGO–La0.7Sr0.3MnO3-δ (LSM), was studied at 973–1223 K. In most cases, oxygen transport is substantially affected by component interaction, decreasing ionic conductivity due to cation interdiffusion, and formation of intermediate phases and/or blocking layers at grain boundaries. This interaction is maximised in systems where the phase components have similar structure and thus may form continuous solid solutions, for example LSGM–LSFC, or intermediate compounds such as Roddlesden–Popper phases in LSGM–LNC composites. The results show that, in addition to knowledge of the transport properties and volume fractions of percolating phases, analysis of ionic conduction in oxide composite materials requires assessment of phase interaction and grain boundary processes.

Dehydrogenation Properties of Magnesium Hydride Loaded with Fe, Fe−C, and Fe−Mg Additives

This study highlights that Fe additives offer better catalytic properties than carbon, Fe-C (iron carbide/carbon composites), and Fe-Mg (Mg2 FeH6 ) additives for the low-temperature dehydrogenation of magnesium hydride. The in situ X-ray diffraction measurements prove the formation of a Mg2 FeH6 phase in iron additive loaded MgH2 . Nonetheless, differential scanning calorimetry data suggest that this Mg2 FeH6 phase does not have any influence on dehydrogenation properties of MgH2 . On the other hand, the composite system Mg2 FeH6 /MgH2 shows significantly improved dehydrogenation properties even in absence of further additives. It is suggested that the improved system performance of Fe loaded MgH2 is attributed to restrictions on crystal growth of MgH2 and the catalytic behavior of Fe nanoparticles, rather than any intrinsic catalytic properties offered by the formed mixed metal phase Mg2 FeH6 .