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.

Solid oxide fuel cells are of technological interest as they offer high efficiency for energy conversion in a clean way. Understanding fundamental aspects of oxygen self-diffusion in solid state ionic systems is important for the discovery of next-generation electrolyte and cathode material compositions and microstructures that can enable the operation of SOFCs at lower temperatures more efficiently, durably, and economically. In the present perspective article, we illustrate the important role of modelling and simulations in providing direct atomic scale insights on the oxygen ion transport mechanisms and conduction properties in the cathode and electrolyte materials, and in accelerating the progress from old materials to new concepts. We first summarize the ionic transport mechanisms in the traditional cathode and electrolyte materials which have been widely studied. We then pay our attention to the non-traditional materials and their oxygen transport paths from recent studies, focusing on structural and transportanisotropy and lattice dynamics. Lastly, we highlight the new developments in the potential to increase the ionic conductivity of the traditional materials through external mechanical stimuli, bringing about the mechano-chemical coupling to drive fast ionic transport.

The suitability of GdBaCo₂O_5+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 (E_A ^k = 0.81(4) and E_A ^D = 0.60(4) eV). The same material has been characterized electrically as a part of a symmetrical electrochemical system (GdBaCo₂O_5+δ/Ce_0.9Gd_0.1O_2−x/GdBaCo₂O_5+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 regarding the implementation of 3D printing for solid oxide fuel cells, batteries, solar energy and environmental applications are reviewed.

Lowering the operating temperature of solid oxide fuel cells (SOFCs) to the intermediate range (500–700 ºC) has become one of the main SOFC research goals. High operating temperatures put numerous requirements on materials selection and on secondary units, limiting the commercial development of SOFCs. The present review first focuses on the main effects of reducing the operating temperature in terms of materials stability, thermo-mechanical mismatch, thermal management and efficiency. After a brief survey of the state-of-the-art materials for SOFCs, attention is focused on emerging oxide-ionic conductors with high conductivity in the intermediate range of temperatures with an introductory section on materials technology for reducing the electrolyte thickness. Finally, recent advances in cathode materials based on layered mixed ionic-electronic conductors are highlighted because the decreasing temperature converts the cathode into the major source of electrical losses for the whole SOFC system. It is concluded that the introduction of alternative materials that would enable solid oxide fuel cells to operate in the intermediate range of temperatures would have a major impact on the commercialization of fuel cell technology.

We use molecular dynamics in conjunction with an established set of Born model potentials to examine the oxygen ion diffusion mechanism in the double perovskite GdBaCo₂O_{5 +}δ. We predict that the mechanism of oxygen diffusion is highly anisotropic diffusion occurring only in the Gd–O and adjacent Co–O layers. For GdBaCo₂O_5.5 the activation energy of oxygen diffusion is 0.5 eV. We investigate the effect that cation disorder of the Gd–Ba sublattice has upon the diffusivity, the anisotropy and the diffusion mechanism in GdBaCo₂O_{5 +}δ, which is a model system for double perovskites and other layered compounds. Cation disorder results in a reduction of the oxygen diffusivity and the appearance of diffusion along the c-axis of the material. Oxygen diffusion becomes effectively isotropic when the cation sublattice is disordered.