John A. Kilner

Optimisation of composite cathodes for intermediate temperature SOFC applications

Abstract The electrochemical properties of the interfaces between porous composites of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ /Ce 0.9 Gd 0.1 O 2− δ cathodes and Ce 0.9 Gd 0.1 O 2− δ electrolytes have been investigated at intermediate temperatures (500–700°C) using AC impedance spectroscopy. Results indicate that the electrochemical properties of these composites are quite sensitive to the composition and the microstructure of the electrode. The optimum Ce 0 9 Gd 0.1 O 2− δ addition (36% by volume) to La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ resulted in four times lower area specific resistivity, which classify this composite as a promising material for solid oxide fuel cells based on Ce 0 9 Gd 0.1 O 2− δ electrolytes. The observed high performance of the composite electrodes at this composition is consistent with the effective medium percolation theory which predicts the ambipolar transport behaviour of composite mixed ionic-electronic conductors as a function of the volume fraction of each of the randomly-distributed constituent phases. Quantitatively, a slight discrepancy between measurements and theory was observed. This is believed to be due to the fact that the overall performance of a porous electrode is not only determined by the mixed conducting transport properties in the solid phase of the electrode, but also by the inherent catalytic property of the triple phase boundary, and by the gas transport to, or away, from the triple phase boundary.

Oxygen diffusion and surface exchange in La2−xSrxNiO4+δ

Abstract The compounds La 2− x Sr x NiO 4+ δ , x =0, 0.1, have been prepared with an oxygen excess of up to δ =0.24. The oxygen tracer diffusion coefficient and surface exchange coefficient of the materials have been determined by the isotope exchange depth profile method (IEDP). La 2 NiO 4+ δ was found to have an oxygen diffusivity higher than that of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) and one order of magnitude lower than the best perovskite oxide ion conductor La 0.3 Sr 0.7 CoO 3 (LSC). The fast oxide ion diffusion of La 2 NiO 4+ δ combined with its thermal stability indicate that this material would be a good candidate for use in ceramic oxygen generators (COGs) and solid oxide fuel cells (SOFCs). Furthermore, optimisation by a combination of donor and/or acceptor doping should improve the properties reported here.

Oxygen transport in La1−xSrxMn1−yCoyO3±δ perovskites: Part I. Oxygen tracer diffusion

Oxygen tracer diffusion coefficients of selected compositions in the La1−xSrxMn1−yCoyO3±δ perovskite oxide system have been measured by means of the Isotopic Exchange Depth Profile method (IEDP). 18O/16O exchange anneals were performed at various temperatures between 600°C and 1000°C (PO2∼1 atm), and the subsequent 18O diffusion profiles determined by Secondary Ion Mass Spectrometry (SIMS). It was seen that, in general, the oxygen tracer diffusivity of these materials increases significantly with strontium site fraction, x, and cobalt site fraction, y. This was attributed to variations in the site fraction of oxygen vacancies, with approximately constant isothermal vacancy diffusivities. The activation enthalpies for oxygen tracer diffusion were also found to be strongly dependent upon composition.

A study of oxygen ion conductivity in doped non-stoichiometric oxides

Abstract In doped non-stoichiometric oxides that show oxygen ion conductivity, association commonly takes place between the dopant cations and the compensating oxygen vacancies. The activation energy thus comprises two parts, a migration enthalpy and an association enthalpy. We have determined the effects of structure and host cation type on the migration enthalpies, and the effect of dopant cation size on the association enthalpies. This we did by a variety of methods including theoretical calculations and experiment. We have further reviewed the literature in order to verify our calculations and we conclude that size terms in the association enthalpies are the most important factor in the determination of the magnitude of oxygen ion conduction.

Oxygen transport in selected nonstoichiometric perovskite-structure oxides

New results on oxygen self-diffusion (D∗), surface exchange coefficient (k) and activation energy (Ea) of oxygen self-diffusion for chosen compositions of manganite and cobaltite perovskites illustrate the effect of doping in the A and B sites of the ABO3 structure. The electrical conductivity (σT) was measured for the manganite group and permeability (J) was determined for the cobaltite perovskites. D∗ increases with increasing x in La1−x(Sr,Ca)x(Mn,Co)O3−δ due to formation of new oxyge vacancies by introducing metals of lower valency (M2+) into the A3+ sites. A substitution of M2+ for B3+ or a reduction of the metal in the B3+ site to a lower positive valency also increases D∗ . D∗ cobaltites is significantly higher than that of the manganites (by 4–6 orders of magnitude), however, the potentially high oxygen fluxes that would be allowed through the materials by the high D∗ values seem to be limited by the surface exchange kinetics. Ea-values of the manganites are considerably higher than those the cobaltites. In general, the electrical conductivity, σT, decreased on doping the B site of the manganites with Co and Ni. However, whilst the pure manganite material exhibits a metallic type of conduction (i.e. σT decreased with increasing T), the conduction mechanism in the Co-doped and Ni-doped manganites changed to a localized hopping of charge carriers between the Mn3+ and Mn4+ sites (σT increases with increasing T).

Electrochemical Characterization of La0.6Sr0.4Co0.2Fe0.8 O 3 Cathodes for Intermediate-Temperature SOFCs

The electrochemical properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) have been assessed for its application as a cathode in intermediate-temperature solid oxide fuel cells. van der Pauw dc conductivity, two-electrode impedance, and three-electrode measurements were carried out to investigate the kinetics of the oxygen reduction reaction at various temperatures, oxygen partial pressures, and polarization values. A change in cathode behavior at temperatures around 600°C was observed. This is interpreted in terms of LSCF behaving as a mixed ionic electronic conductor at temperatures above around 600°C, oxygen reduction being stimulated by the formation of oxygen vacancies with increasing cathode overpotential. However, at temperatures below 600°C the contribution of mixed conductivity is low, and cathode behavior can then be interpreted in terms of the classical triple-phase-boundary model.

Layered perovskites as promising cathodes for intermediate temperature solid oxide fuel cells

The suitability of GdBaCo2O5+δ as a cathode material for intermediate temperature solid oxide fuel cells has been evaluated. The 18O/16O isotope exchange depth profile (IEDP) method has been used to obtain the oxygen surface exchange and oxygen tracer diffusion coefficients yielding optimum values for applicability in fuel cells (k* = 2.8 × 10−7 cm s−1 and D* = 4.8 × 10−10 cm2 s−1 at 575 °C) especially in terms of low activation energies (EAk = 0.81(4) and EAD = 0.60(4) eV). The same material has been characterized electrically as a part of a symmetrical electrochemical system (GdBaCo2O5+δ/Ce0.9Gd0.1O2−x/GdBaCo2O5+δ), by means of impedance spectroscopy measurements, corroborating an excellent performance in the classical intermediate temperature range for solid oxide fuel cells (500–700 °C). An area specific resistance (electrode–electrolyte interface) of 0.25 Ω cm2 at 625 °C was achieved for a cell processing temperature of 975 °C. Finally, layered perovskites are presented as a promising new family of materials for cathode use in solid oxide fuel cells at low temperatures.

Fast oxygen transport in acceptor doped oxides

Abstract The basic expressions governing the ionic conductivity of fast ion conductors are examined. Acceptor doped oxides displaying fast ion conduction are taken as an example. For selected materials, the role of dopant–vacancy interactions in influencing the concentration of mobile vacancies is assessed. Examination of experimental data and the results from atomistic lattice simulations lead to the following conclusions. For acceptor dopants where the effective charge of the substitutional ion is −1, a minimum is seen in the concentration dependence of the activation energy for oxygen ion conduction. This minimum is a characteristic feature and can be used as an indicator of dopant–vacancy interactions. The activation energy for conduction is also dependent upon the size of the dopant, through a size dependence of the association enthalpy of the dopant–vacancy pairs. The activation energy is minimised when the dopant is close to the size of the host cation. A particular example of materials where this is optimised is the Sr′ La substitution found in many of the mixed conducting perovskites, which may go part way in explaining the very high diffusivities of oxygen found for these materials.

Oxygen transport in La0.6Sr0.4Co0.2Fe0.8O3-δ

Oxygen transport and oxygen surface reaction measurements have been obtained for La0.6Sr0.4Co0.2Fe0.8O3−δ using the conductivity relaxation technique. These measurements are compared with tracer diffusion coefficients and surface exchange coefficients obtained for the same material. The change in oxygen stoichiometry of this material with temperature and oxygen partial pressure has been demonstrated. The values derived from these experiments have been used to calculate the theoretical oxygen fluxes and close agreement has been found with actual oxygen fluxes obtained from permeation experiments through a dense La0.6Sr0.4Co0.2Fe0.8O3−δ sample using a modified Wagner model.

Advances in layered oxide cathodes for intermediate temperature solid oxide fuel cells

In the context of solid oxide fuel cells (SOFCs) applications, mixed-ionic electronic conductors offer significant advantages over conventional cathodes especially in the intermediate-to-low range of temperatures where the performance of the cathode is of fundamental importance. An increasing number of layered oxide materials have been found to present excellent properties as mixed ionic-electronic conductors. Therefore, considerable efforts have been recently devoted to better understand and evaluate layered ordered structures. This article highlights the most important advances in this topic concentrating on both structural aspects and impact in cathode performance for SOFCs applications.