Aleksey A. Yaremchenko

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.

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. II. Perovskite-related oxides

Abstract The review is devoted to the analysis of experimental results on electrochemical and physicochemical properties of the perovskite-related oxide phases obtained at scientific centers of the former Soviet Union. The main attention is focused on oxides with high electronic conductivity, which are potentially useful as electrodes for high-temperature electrochemical cells with oxygen-ion conducting solid electrolytes and interconnectors of solid oxide fuel cells, and on mixed ionic-electronic conductors for oxygen separation membranes. Along with perovskite-like solid solutions based on LnMO3−δ (Ln is a rare-earth element, M = Cr, Mn, Fe, Co, Ni) and SrCoO3−δ, properties of the oxide phases Ln2MO4±δ (M = Cu, Ni, Co) with the K2NiF4-type structure are briefly reviewed.

Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes

The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5-y Fe y O 26±δ (y = 0-1.5), La 10-x Si 6-y Fe y O 26±δ (x = 0-0.77; y = 1-2), and La 7-x Sr 3 Si 6 O 26-δ (x = 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La 9.83 Si 4.5 Al 1.5-y Fe v O 26+δ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4+ fraction, which was evaluated by Mossbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe)O 26+δ apatites stabilize the Fe 3+ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O 2 ) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) X 10 -6 K -1 at 300-1250 K.

p-Type electronic conductivity, oxygen permeability and stability of La2Ni0.9Co0.1O4+δElectronic supplementary infromation (ESI) available: further experimental data for the oxygen permeability, total conductivity and Seebeck coefficient of La2Ni0.9Co0.1O4+δ. See http://www.rsc.org/suppdata/jm/b3/b300357d/

The oxygen permeability, total conductivity and Seebeck coefficient of La2Ni0.9Co0.1O4+δ were studied in the oxygen partial pressure range of 10−16 Pa to 50 kPa at 973–1223 K. The conductivity of La2Ni0.9Co0.1O4+δ is predominantly p-type electronic within the whole p(O2) range in which the K2NiF4-type structure exists. Thermally-activated mobility, the values of which are 0.02–0.08 cm2 V−1 s−1, and the p(O2) dependencies of electron-hole transport suggest a small-polaron conduction mechanism. Oxygen permeability of dense La2Ni0.9Co0.1O4+δ membranes, with an apparent activation energy of 192 kJ mol−1 in oxidising conditions, is limited by both bulk ionic conductivity and the surface exchange rate. The role of surface processes in limiting permeation is also significant under air/H2–H2O gradients and increases with decreasing temperature. The stability boundary of the La2Ni0.9Co0.1O4+δ phase at low oxygen pressures is similar to that of undoped lanthanum nickelate, which allows stable operation of nickelate membranes under high oxygen chemical potential gradients, such as air/10% H2–90% N2, at 973 K. At temperatures above 1000 K, the decomposition products form blocking layers on the membrane surface causing degradation of the membrane performance with time. The average thermal expansion coefficient of La2Ni0.9Co0.1O4+δ ceramics, calculated from dilatometric data in air, is 12.8 × 10−6 K−1 at 400–1265 K.

Stability of δ-Bi2O3-based solid electrolytes

Abstract The fluorite-type oxygen-ion conducting solid solutions (Bi 1−x Zr x ) 0.85 Y 0.15 O 1.5+δ (x = 0.05 and 0.07) and (Bi 0.95 Nb 0.05 ) 0.85 Ho 0.15 O 1.5+δ partially decompose at temperatures below 900 K, forming several phases isostructural to δ- and γ-Bi 2 O 3 . Formation of the bcc γ-Bi 2 O 3 -type phases at 770 K leads to a sharp decrease in conductivity. Interaction of the Bi 2 O 3 -based solid electrolytes with silver electrodes was observed at temperatures above 850 K, resulting in increasing resistance of the electrochemical cells. Electrodes of platinum and perovskite-type La 0.7 Sr 0.3 CoO 3−δ were ascertained to exhibit sufficient stability when in contact with the bismuth oxide-based electrolytes in the temperature range of 870–1000 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...

Physicochemical and Transport Properties of Bicuvox-Based Ceramics

Polycrystalline Bi2-xLaxV0.90Cu0.10O5.5-α (x = 0, 0.10 and 0.20) and Bi1.90Pr0.10V0.90Cu0.10O5.5-α were prepared by the standard ceramic synthesis technique. Electrical conductivity of the Bi1.90La0.10V0.90Cu0.10O5.5-α solid solution at temperatures above 500 K is lower in comparison with undoped BICUVOX.10, whereas transport properties of these materials at 370–450 K are close to each other. Doping Bi2V0.90Cu0.10O5.5-α with praseodymium was found to result in segregating secondary phases and decreasing conductivity and thermal expansion of the ceramics. Oxygen ion transference numbers of the oxides with moderate rare-earth dopant content (x ≤ 0.10) vary in the range of 0.90–0.99 at 780–910 K, decreasing with increasing temperature. Thermal expansion coefficients of Bi2-xLnxV0.90Cu0.10O5.5-α ceramics were calculated from the dilatometric data to be (16.1–18.0) × 10-6K-1 at 730–1050 K.

Oxygen ionic and electronic conductivity of La-doped BIMEVOX

Abstract Doping by either lanthanum or nickel has been found to result in decreasing ionic conductivity and thermal expansion of the Bi 2− x La x V 1− y − z Cu y Ni z O 5.5−δ ( x =0.10–0.20; y =0–0.10; z =0–0.20) ceramics. The infrared absorption spectroscopic data on Bi 2− x La x V 1− y − z Cu y Ni z O 5.5−δ have been shown to correlate with the conductivity and crystal structure of the oxides. The thermal expansion coefficients of the ceramics are (13.5–15.5)×10 −6 K −1 at temperatures of 300–570 K and (14.8–18.6)×10 −6 K −1 at 580–1050 K. Ion transference numbers of Bi 2− x La x V 0.90 Cu 0.10 O 5.5−δ ( x =0.10–0.20) have been determined by Faradaic efficiency measurements at 720–800 K to vary in the range from 0.65 to 0.95. Doping by lanthanum results in increasing electronic conductivity. A new experimental arrangement to study Faradaic efficiency of oxygen ionic conductors has been suggested and verified.

Ionic transport in SrCo0.85Ti0.15o3−δ ceramics at high oxygen pressures

Abstract Oxygen permeation through dense ceramic membranes of the SrCo 0.85 Ti 0.15 O 3−δ cubic perovskite solid solution was studied in the range of the membrane feed-side oxygen partial pressures (p 2 ) 0.21 to 100 atm at temperatures of 850–1040 K. The dependence of the oxygen fluxes on membrane thickness suggests that bulk ionic conductivity is the permeation-determining factor at high oxygen pressures (p 2 > 1 atm). Increasing the oxygen pressure up to 80–100 atm was found to result in a drastic increase of the permeation flux, accompanied by a partial decomposition of the cubic perovskite phase of SrCo 0.85 Ti 0.15 O 3−δ , forming two isostructural perovskite-type phases. No phase changes were observed after testing ceramics in the oxygen pressure range of 1–60 atm. The high oxygen permeation fluxes obtained demonstrate the applicability of SrCo 0.85 Ti 0.15 O 3−δ membranes at high oxygen pressures.

High-temperature characterization of oxygen-deficient K2NiF4-type Nd2−xSrxNiO4−δ (x = 1.0–1.6) for potential SOFC/SOEC applications

Previously unexplored oxygen-deficient Ruddlesden–Popper Nd2−xSrxNiO4−δ (x = 1.0–1.6) nickelates were evaluated for potential use as oxygen electrode materials for solid oxide fuel and electrolysis cells, with emphasis on structural stability, oxygen nonstoichiometry, dimensional changes, and electrical properties. Nd2−xSrxNiO4−δ ceramics possess the K2NiF4-type tetragonal structure under oxidizing conditions at 25–1000 °C. Acceptor-type substitution by strontium is compensated by the generation of electron–holes and oxygen vacancies. Oxygen deficiency increases with temperature and strontium doping reaching ∼1/8 of oxygen sites for x = 1.6 at 1000 °C in air. Strongly anisotropic expansion of the tetragonal lattice on heating correlated with oxygen nonstoichiometry changes results in an anomalous dilatometric behavior of Nd2−xSrxNiO4−δ ceramics under oxidizing conditions. Moderate thermal expansion coefficients, (11–14) × 10−6 K−1, ensure however thermomechanical compatibility with common solid electrolytes. Reduction in inert atmosphere induces oxygen vacancy ordering accompanied by a contraction of the lattice and a decrease of its symmetry to orthorhombic. Nd2−xSrxNiO4−δ ceramics exhibit a p-type metallic-like electrical conductivity at 500–1000 °C under oxidizing conditions, with the highest conductivity (290 S cm−1 at 900 °C in air) observed for x = 1.2. The high level of oxygen deficiency in Sr-rich Nd2−xSrxNiO4−δ implies enhanced mixed ionic–electronic transport favorable for electrode applications.