David Parfitt

Oxygen diffusion in solid oxide fuel cell cathode and electrolyte materials: mechanistic insights from atomistic simulations

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.

Anisotropic oxygen diffusion in tetragonal La2NiO4+δ: molecular dynamics calculations

Molecular dynamics simulations, used in conjunction with a set of Born model potentials, have been employed to study oxygen transport in tetragonal La2NiO4+δ. We predict an interstitialcy mechanism with an activation energy of migration of 0.51 eV in the temperature range 800–1100 K. The simulations are consistent with the most recent experiments. The prevalence of oxygen diffusion in the a–b plane accounts for the anisotropy observed in measurements of diffusivity in tetragonal La2NiO4+δ.

Oxygen ion diffusion in cation ordered/disordered GdBaCo2O5+δ

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 GdBaCo2O5+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 GdBaCo2O5.5 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 GdBaCo2O5+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.

Molecular dynamics study of oxygen diffusion in Pr2NiO4+δ

Oxygen transport in tetragonal Pr(2)NiO(4+delta) has been investigated using molecular dynamics simulations in conjunction with a set of Born model potentials. Oxygen diffusion in Pr(2)NiO(4+delta) is highly anisotropic, occurring almost entirely via an interstitialcy mechanism in the a-b plane. The calculated oxygen diffusivity has a weak dependence upon the concentration of oxygen interstitials, in agreement with experimental observations. In the temperature range 800-1500 K, the activation energy for migration varied between 0.49 and 0.64 eV depending upon the degree of hyperstoichiometry. The present results are compared to previous work on oxygen self-diffusion in related K(2)NiF(4) structure materials.

Strain fields and line energies of dislocations in uranium dioxide

Computer simulations are used to investigate the stability of typical dislocations in uranium dioxide. We explain in detail the methods used to produce the dislocation configurations and calculate the line energy and Peierls barrier for pure edge and screw dislocations with the shortest Burgers vector ½⟨110⟩. The easiest slip system is found to be the {100}⟨110⟩ system for stoichiometric UO(2), in agreement with experimental observations. We also examine the different strain fields associated with these line defects and the close agreement between the strain field predicted by atomic scale models and the application of elastic theory. Molecular dynamics simulations are used to investigate the processes of slip that may occur for the three different edge dislocation geometries and nudged elastic band calculations are used to establish a value for the Peierls barrier, showing the possible utility of the method in investigating both thermodynamic average behaviour and dynamic processes such as creep and plastic deformation.

Mechanisms of nonstoichiometry in HfN1−x

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1−x, 0≤×≤0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater.

Thermodynamic calculations of oxygen self-diffusion in mixed-oxide nuclear fuels

Mixed-oxide fuels containing uranium with thorium and/or plutonium may play an important part in future nuclear fuel cycles. There are, however, significantly less data available for these materials than conventional uranium dioxide fuel. In the present study, we employ molecular dynamics calculations to simulate the elastic properties and thermal expansivity of a range of mixed oxide compositions. These are then used to support equations of state and oxygen self-diffusion models to provide a self-consistent prediction of the behaviour of these mixed oxide fuels at arbitrary compositions.

Characterisation of interfacial segregation to Cu-enriched precipitates in two thermally aged reactor pressure vessel steel welds.

To understand the contribution of long term thermal ageing to Reactor Pressure Vessel (RPV) embrittlement two high Cu steel welds with different Ni contents were thermally aged for times up to 100,000 h at 330 °C and 365 °C. Microstructural characterisation using Atom Probe Tomography was performed. Thermal ageing produced a high number density of nano-scale Cu-enriched precipitates. The precipitate-matrix interfaces were enriched in Ni, Mn and Si. The characterisation of these interfaces using a double cluster search approach is the subject of this work. The interface region around thermally-induced precipitates was found to be wider in steels with higher bulk Ni contents and where precipitates had larger core radii. The effect of ageing temperature on interface width was small when comparing precipitates of equal core radius. The narrower interface width in the lower Ni steels is reflected in the composition of the interface, which has a lower Ni content than in the higher Ni material. The reduction in interfacial energy due to the segregation of Ni, Mn and Si has been calculated and shows enhanced reductions in interfacial energy with increasing precipitate size, but no obvious effect of temperature.

Optimizing oxygen diffusion in cathode materials for solid oxide fuel cells

Recent investigations have revealed that the Ruddlesden–Popper series (An+1BnO3n+1) and the layered perovskite LnBaCo2O5+δ (Ln = rare-earth cations) are promising as cathodes for intermediate temperature solid oxide fuel cells. For these to be economical the oxygen diffusion must be maximized. Based on atomistic simulations, we propose strategies for optimizing oxygen diffusion in these materials by modifying the oxygen stoichiometry, the composition and cation disorder. The present investigation is focused on La2CoO4+δ and GdBaCo2O5+δ and the results are discussed in view of recent experimental and theoretical studies.

Diffusion in energy materials: Governing dynamics from atomistic modelling

Understanding diffusion in energy materials is critical to optimising the performance of solid oxide fuel cells (SOFCs) and batteries both of which are of great technological interest as they offer high efficiency for cleaner energy conversion and storage. In the present review, we highlight the insights offered by atomistic modelling of the ionic diffusion mechanisms in SOFCs and batteries and how the growing predictive capability of high-throughput modelling, together with our new ability to control compositions and microstructures, will produce advanced materials that are designed rather than chosen for a given application. The first part of the review focuses on the oxygen diffusion mechanisms in cathode and electrolyte materials for SOFCs and in particular, doped ceria and perovskite-related phases with anisotropic structures. The second part focuses on disordered oxides and two-dimensional materials as these are very promising systems for battery applications.