Introduction 9870 1.1. Background 9870 1.2. Oxygen Catalysts 9871 1.3. Nonstoichiometric Oxides 9872 2. ABO 3-δ Perovskite-Type Catalysts 9873 2.1. General Introduction 9873 2.2. Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ Oxygen Catalysts 9873 2.3. Effects of Cations 9876 2.3.1. Effects of A-Site Cations 9876 2.3.2. Effects of B-Site Cations 9877 2.3.3. Effects of A′-Site Cations 9878 2.3.4. Effects of B′-Site Cations 9879 2.4. Extrinsic Strategies for Enhancing Performance and Stability 9880 2.4.1. Controlling Phase Structure and Surface Activities 9880 2.4.2. Nanostructured Materials 9881 2.4.3. Crystallographic Orientations 9884 2.4.4. Amorphous Phases 9886 2.4.5. Composites 9886 2.5. Summary 9886 3. Layered-Perovskite-Type Catalysts 9887 3.1. Ruddlesden−Popper Phases 9887 3.2. Double Perovskites 9887 3.3. Summary 9889 4. Pyrochlore-Type Catalysts 9890 4.1. Structure and Properties 9890 4.2. Summary 9891 5. Other Typical Nonstoichiometric Oxides 9891 5.1. MnO x Oxygen Catalysts 9891 5.2. Ceria-Related Oxygen Catalysts 9892 5.3. NiO x -Based Oxygen Catalysts 9893 5.4. Delithiated LiCoO 2 −Related Oxygen Catalysts 9894 5.5. Summary 9894 6. Benchmarking Oxygen Catalysts 9894 6.1. Overview of the State-of-the-Art Oxygen Catalysts 9894 6.2. ORR/OER Bifunctional Oxygen Catalysts 9895 7. Design Principles 9897 7.1. Descriptors from Molecular Orbital Theory 9897 7.2. Descriptors from DFT Calculations 9899 8. Mechanistic Understanding 9900 9. Summaries and Research Directions 9904 9.1. Perovskite-Type Oxygen Catalysts 9904 9.2. Pyrochlore-Type Oxygen Catalysts 9904 9.3. MnO x , CeO x , and NiO x Related Oxygen Catalysts 9904 9.4. Catalyst Support 9904 9.5. Electrode Architectures 9905 9.6. Solid Electrolytes/Separators/Protectors 9905 9.7. System 9905 9.8. Advanced Characterizations 9905 9.9. Transferring Current Materials 9905 9.10.

Lithium-rich anti-perovskites (LiRAPs) are a promising family of solid electrolytes, which exhibit ionic conductivities above 10(-3) S cm(-1) at room temperature, among the highest reported values to date. In this work, we investigate the defect chemistry and the associated lithium transport in Li3OCl, a prototypical LiRAP, using ab initio density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations. We studied three types of charge neutral defect pairs, namely the LiCl Schottky pair, the Li2O Schottky pair, and the Li interstitial with a substitutional defect of O on the Cl site. Among them the LiCl Schottky pair has the lowest binding energy and is the most energetically favorable for diffusion as computed by DFT. This is confirmed by classical MD simulations, where the computed Li ion diffusion coefficients for LiCl Schottky systems are significantly higher than those for the other two defects considered and the activation energy in LiCl deficient Li3OCl is comparable to experimental values. The high conductivities and low activation energies of LiCl Schottky systems are explained by the low energy pathways of Li between the Cl vacancies. We propose that Li vacancy hopping is the main diffusion mechanism in highly conductive Li3OCl.

Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction reaction. Pristine SrCoO(3-δ) is a widely used parent oxide for the development of highly active mixed conductors. Doping a small amount of redox-inactive cation into the B site (Co site) of SrCoO(3-δ) has been applied as an effective way to improve physicochemical properties and electrochemical performance. Most findings however are obtained only from experimental observations, and no universal guidelines have been proposed. In this article, combined experimental and theoretical studies are conducted to obtain fundamental understanding of the effect of B-site doping concentration with redox-inactive cation (Sc) on the properties and performance of the perovskite oxides. The phase structure, electronic conductivity, defect chemistry, oxygen reduction kinetics, oxygen ion transport, and electrochemical reactivity are experimentally characterized. In-depth analysis of doping level effect is also undertaken by first-principles calculations. Among the compositions, SrCo0.95Sc0.05O(3-δ) shows the best oxygen kinetics and corresponds to the minimum fraction of Sc for stabilization of the oxygen-vacancy-disordered structure. The results strongly support that B-site doping of SrCoO(3-δ) with a small amount of redox-inactive cation is an effective strategy toward the development of highly active mixed conducting perovskites for efficient solid oxide fuel cells and oxygen transport membranes.