Stephen J. Skinner

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.

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.

Recent advances in Perovskite-type materials for solid oxide fuel cell cathodes

Abstract Perovskite materials, particularly of the LaMO3 type, where M is typically a transition metal, have in increasing numbers been found to possess mixed ionic–electronic conductivity. For use as cathodes in fuel cells the presence of mixed ionic–electronic conductivity in oxides has been found to be highly beneficial. Therefore, considerable efforts have been directed towards the development of this type of cathode. The performance of the cathode material is of fundamental importance to the operation of these devices. In recent years a significant amount of time has been expended on the development of these perovskite materials, identifying new mixed conductors and improving the operational performance of existing materials through development of improved cell designs. This article concentrates on the advances in performance and the strategies used to achieve this and, in particular, focuses on developments within the last 10 years.

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.

Oxygen ion conductors

Abstract Ionic conductors have always provided a fascinating interdisciplinary field of study ever since their discovery by Faraday at the Royal Institution in London over 200 years ago. More recently, and particularly in the past decade, the pace of research has been rapid, driven by the requirements for new clean energy sources, sensors, and high energy density batteries. A very interesting subgroup of this class of materials are the oxides that display oxygen ion conductivity. As well as the intrinsic interest in these materials, there has been a continued drive for their development because of the promise of important technological devices such as the solid oxide fuel cell (SOFC), oxygen separation membranes, and membranes for the conversion of methane to syngas 1 . All of these devices offer the potential of enormous commercial and ecological benefits provided suitable high performance materials can be developed. In this article we will review the materials currently under development for application in such devices with particular reference to some of the newly discovered oxide ion conductors.

Solid oxide fuel cells—a challenge for materials chemists?

Solid oxide fuel cells (SOFCs) are complex electrochemical devices that offer significant advantages over conventional power generation technologies. Many of these advantages surround the environmental impact of energy generation and in particular the efficiency of power production coupled with the potential range of fuel sources that can be foreseen. Despite these advantages there remain a number of challenges that may delay the full commercialisation of the solid oxide fuel cell. Several of these surround the materials selection, function and interactions with other cell components. It is the intention of this article to highlight the contribution that materials chemistry has made to the development of SOFCs and the future progress that is dependent on advances in materials chemistry.

Anisotropic oxygen diffusion properties in epitaxial thin films of La2NiO4+δ

We report on the development and validation of a new methodology for the determination of anisotropic tracer diffusion and surface exchange coefficients of high quality epitaxial thin films in the two perpendicular directions (transverse and longitudinal), by the isotopic exchange technique. Measurements were performed on c-axis oriented La2NiO4+δ films grown on SrTiO3 (100) and NdGaO3 (110) by pulsed injection metal organic chemical vapour deposition (PIMOCVD), with different thicknesses ranging from 33 to 370 nm. The effect that the strain induced by the film–substrate mismatch has on the oxygen diffusion through the film was evaluated. Both tracer diffusion coefficients, along the c-axis and along the ab plane, were found to increase with film thickness, i.e., as the stress of the film decreases, while the thickness seems to have no effect on the tracer surface exchange coefficient. Best fits were obtained when considering the thickest films composed by two regions with different c-axis tracer diffusion coefficient values, a higher and constant D* close to the film surface and a variable decreasing D* closer to the substrate. As expected, the tracer diffusion and surface exchange coefficients are thermally activated and are approximately two orders of magnitude higher along the ab plane than along the c-axis. The low activation energies of D* compared with bulk values for both directions at low temperatures seem to confirm the contribution of a vacancy mechanism to the ionic conduction.

Recent advances in perovskite-type materials for SOFC cathodes

Perovskite materials — particularly of the LaM03 type, where M is typically a transition metal — have in increasing numbers been found to posse

Absence of Ni on the outer surface of Sr doped La2NiO4 single crystals

A combination of surface sensitive techniques was used to determine the surface structure and chemistry of La2−xSrxNiO4+δ. These measurements unequivocally showed that Ni is not present in the outermost atomic layer, suggesting that the accepted model with the B-site cations exposed to the environment is incorrect.

Oxygen transfer in BIMEVOX materials

Two steps govern the oxygen transport in ceramic oxide ion conductors: (i) the oxygen exchange at the surface of the material and (ii) the oxygen diffusion through the material. The 18 O/ 16 O Isotope Exchange Depth Profile technique (IEDP) was applied to BIMEVOX materials to characterise the oxygen transfer in these ceramics. The isotope concentration profiles, obtained by secondary ion mass spectrometry (SIMS), revealed that the equilibrium exchange kinetics in BIMEVOXes under nominally dry oxygen are dominated by a relatively slow surface exchange step. This produces deep penetration profiles with very low isotopic concentrations close to the natural isotopic background. The oxygen surface exchange coefficient in these materials is of the same orderofmagnitudeasintheclassicaloxideelectrolytes,ceriagadolinia(CGO)andyttria-stabilisedzirconia(YSZ).Thetransferof oxygen from water is far easier, which makes the accurate determination of the true coefficient of exchange of the molecular oxygen for these materials more complicated and is probably dominated by the presence of residual water. D 2003 Published by Elsevier Science B.V.