V.V. Kharton

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.

Ionic transport in oxygen-hyperstoichiometric phases with K2NiF4-type structure

Abstract Results on oxygen permeation through dense ceramics of La2−xSrxNi1−y−zFeyCuzO4+δ (x=0–0.10; y=0.02–0.10; z=0–0.10), LaPrNi0.9Fe0.1O4+δ, La2Cu1−xCoxO4+δ (x=0.02–0.30) and Ln2CuO4+δ (Ln=Pr, Nd) at 973–1223 K suggest two significant contributions to the ionic conductivity of the oxygen-hyperstoichiometric phases with K2NiF4-type structure. The relative role of the first of them, oxygen interstitial migration in the rock-salt-type layers of the K2NiF4-like lattice, increases with increasing temperature; the role of oxygen vacancy diffusion in the perovskite layers increases when temperature decreases. This behavior was attributed to the lower activation energy for ionic conduction via the vacancy diffusion mechanism. The oxygen permeability of the title materials was found to be limited by both bulk ionic conductivity and surface exchange rates and may thus be enhanced by catalytically active layers, including Pt, Ag and praseodymium oxide, deposited on the membrane surface. Oxygen permeability of K2NiF4-type phases exhibiting maximum ionic transport, such as La2Ni0.98Fe0.02O4+δ, La2Ni0.88Fe0.02Cu0.10O4+δ and La2Cu0.90Co0.10O4+δ, is about one order of magnitude lower than that of most permeable perovskite-type materials. Decreasing radii of the rare-earth cations in the A-sublattice of cuprates and nickelates leads to a dramatic decrease in ionic transport, similar to perovskite oxides. Thermal expansion coefficients of the title materials vary in the range (10.1–13.4)×10−6 K−1.

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 transport in Ce0.8Gd0.2O2−δ-based composite membranes

Abstract Gadolinia-doped ceria electrolyte Ce 0.8 Gd 0.2 O 2− δ (CGO) and perovskite-type mixed conductor La 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3− δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970–1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La 0.7 Sr 0.3 MnO 3− δ /Ce 0.8 Gd 0.2 O 2− δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase.

Oxygen Permeability of Ce0.8Gd0.2 O 2 − δ ‐ La0.7Sr0.3MnO3 − δ Composite Membranes

(CGO) and (LSM) possess similar thermal expansion coefficients and were thus combined in dual‐phase membranes for oxygen separation. Studies of oxygen permeation through CGO‐LSM composite ceramics, containing similar volume fractions of the phases, showed that the oxygen transfer is limited by the bulk ionic conductivity. The oxygen conduction in the composites depends strongly on processing conditions, decreasing with interdiffusion of the phase components. Blocking oxygen ionic conduction is assumed to be due to formation of layers with low ionic conductivity at the CGO grain boundaries, caused by diffusion of lanthanum and strontium into CGO. The permeation fluxes through CGO‐LSM membranes at high feed‐side oxygen pressures (1–50 atm) exhibit Wagner‐type behavior and exceed significantly the oxygen permeability at lower oxygen pressures.

Materials of high-temperature electrochemical oxygen membranes

Oxygen permeability of Ln1-xMxCoO3-δ (Ln = La, Pr, Nd; M = Sr, Ca, Bi, Pb; x = 0–0.9) and SrCo1-xMexO3-δ (Me = Cr, Mn, Fe, Ni, Cu; x = 0–0.5) perovskite-like oxide ceramics, which are promising materials for high-temperature electrochemical oxygen membranes where matter is transferred owing to conjugate transport of oxide ions O2− and electrons through a gas-tight ceramic material, has been investigated. Dependencies of the density of the molecular oxygen flow passing throuth the membrane on the chemical potential gradient of O2 in the gas phase and temperature have been analyzed. Physicochemical models of such dependencies are proposed. It is shown that complex oxides SrCo1-xFexO3-δ (x = 0.2–0.35) and La1-xSrxCoO3-δ (x = 0.65–0.75) having the highest oxide ionic conductivity can be used as materials for electrochemical oxygen membranes.

The effect of cobalt oxide sintering aid on electronic transport in Ce0.80Gd0.20O2−δ electrolyte

Abstract Additions of 2 mol% CoO 1.333 into gadolinia-doped ceria (CGO) solid electrolyte considerably improve sinterability and make it possible to obtain Ce 0.8 Gd 0.2 O 2− δ ceramics with 95–99% density at 1173–1373 K. The effect of cobalt oxide on the total electrical conductivity in air is negligible if the sintering is performed at 1173 K, although p-type electronic conduction measured at 900–1200 K increases with doping by 10–30 times. When increasing the sintering temperature up to 1773 K, grain growth in Co-containing CGO ceramics is accompanied with a decrease in both ionic and electron-hole transport. The oxygen ion transference numbers under oxygen/air gradient vary in the range 0.89–0.99. The n-type conductivity measured by the ion-blocking technique is lower for Co-containing materials than for undoped CGO, suggesting that the electrolytic domain can, to some extent, be enlarged by cobalt oxide additions. The relative role of both p- and n-type electronic contributions to the total conductivity of CGO increases with increasing temperature. The results show that Co-doped materials can still be used as solid electrolyte for intermediate-temperature electrochemical applications, when the operation temperature is 770–970 K.

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

Abstract The maximum solid solubility of gallium in the perovskite-type La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.40–0.80; y =0–0.60) was found to vary in the approximate range y =0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O 3− δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10 −6 K −1 at 300–800 K and (19.3–26.7)×10 −6 K −1 at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.7–0.8; y =0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La 1− x Sr x Fe 1− y Ga y O 3− δ increases with strontium content up to x =0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.

Ionic conductivity of La(Sr)Ga(Mg,M)O3−δ (M=Ti, Cr, Fe, Co, Ni): effects of transition metal dopants

Abstract Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1−zM2O3−δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa1−y−zMgyMzO3−δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa1−zNizO3−δ (z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10−6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, EA, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, EA tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni): I. Oxygen transport in perovskites LaCoO3–LaGaO3

Abstract The formation of a continuous series of solid solutions with a rhombohedrally-distorted perovskite type structure has been found in the pseudobinary oxide system LaCoO 3 –LaGaO 3 . Substitution of gallium with cobalt in the lanthanum gallate results in blocking oxygen ionic transport and increasing electronic conductivity. The activation energy of the electrical conductivity of the LaGa 1− x Co x O 3 ceramics ( x =0.2–0.6) is in the range 57–65 kJ mol −1 . The thermal expansion coefficients increase regularly with the cobalt content and lie in the range from 11.2×10 −6 to 22.3×10 −6 K −1 . Doping LaCoO 3 with gallium has been ascertained to lead to lower electronic conductivity and oxygen permeability. The results of electrical conductivity and electron paramagnetic resonance (EPR) studies suggest that introduction of gallium into the cobalt sublattice leads to insulating cobalt ions.