If the negative charge is spherically symmetric, then the lifting of the d electron energies will be uniform. This results in a nonuniform energy splitting of the d electrons. As the A-site cation exhibits a higher energy state, it does not explicitly contribute to the valence or conduction bands near the Fermi level. These states are usually unoccupied and for this reason, the A-site cation is generally overlooked in band structure analysis. Due to the mixed ionic covalent nature of the BO6 octahedra in perovskites, the metal d orbitals and oxygen p orbitals may hybridize and form continuous bands.
Furthermore, the covalent mixing from the p-d electron overlap provides electron pathways between the B-site cation and O anion, which is primary for the material electronic conductivity. Therefore, the enhancement of the p-d hybridization is key and methods include, increasing the metal cation electronegativity and d orbital occupation. In the periodic table, the number of d electrons and the electronegativity increase from left to right across a row. For perovskites with early transition metal as B site, e. It was also suggested that, by increasing the covalency between the B-site transition-metal 3d and oxygen 2p orbitals, an even higher ORR activity for perovskite-type catalysts could be achieved.
Later, they continued this line of research of ORR descriptors and applied the same principle to predict OER activity.
The group demonstrated that the OER activity follows similar design principles for most perovskite oxide materials. This work undertook detailed characterization of the perovskite oxides studied, using RRDE and galvanostatic measurement, and suggested an exceptionally high OER activity for the Ba0.
Surprisingly, the measured intrinsic OER activity of Ba0. Therefore, the reactive states near the Fermi level could be a good descriptor. Previous work has suggested the transition-metal dband center, for metal catalysts, as a descriptor to correlate with the catalytic performance. However, for the DFT calculations involving transition-metal oxides, identifying an accurate method to account for the localized d-orbital electrons is often a challenge.
As a result, in several studies the U parameter is not included.
Influence of the surface on the equilibration kinetics of nonstoichiometric oxides
Chemical Reviews Review the Morgan group. Calle-Vallejo et al. The investigation determined a correlation between the adsorption energies of the reaction intermediates and the outer electrons, following a simple linear trend, as shown in Figure 24b. In addition, the existing descriptors that are related to band structures have primarily used the d-band of the transition metal or the O p-band for OER of perovskites. It may be interesting to investigate how these two bands interact within perovskites from DFT calculations and how the interaction may be correlated to the activities.
Recently, another descriptor was proposed by Deml et al. Therefore, both parameters drive the oxygen vacancy formation energy. Although accurate, the linearity of this descriptor was limited to materials from a single crystal structure group, in this case perovskites. Therefore, the group recently extended this study and developed a new descriptor model of single functional form, for 45 binary and ternary oxide groups including perovskites. The main development in the model was the inclusion of the midgap energy relative to the O 2p band center and the atomic electronegativities.
Furthermore, as discussed previously, material properties are strongly dependent on fabrication method and environment. For example, Co3O4 was predicted to be more active than RuO2 from computations, whereas experimental observations suggested the opposite. These aspects, however, are not considered in computations. The main concept in the drawing is according to ref This suggestion is reasonable given the excellent catalytic ability of transition-metal cations for the decomposition of hydrogen peroxides.
The direct 4-electron oxygen reduction mechanism has been associated with the catalyst sites of perovskite oxides only, even if carbon is added to the electrodes during the preparation. The ratedetermining step of this mechanism has been suggested as either the surface hydroxide displacement 1 or the surface hydroxide regeneration 4. Despite numerous studies on the OER, mechanistic models of the reaction steps based on atomic-level experiments are scarce. The multiple OER paths suggested for electrode kinetics in alkaline solution were summarized in a recent review.
A free electron is generated as a result of this reaction, increasing the valence state of the B-site transition metal. Proposed ORR pathways in alkaline solution. This technique has been employed in many studies to identify the preferred pathway for material development. Detailed information about the RRDE technique and the usage in the evaluation of oxygen catalysts can be found in a recent book.
Step 2 peroxide formation and step 3 extraction of proton are generally considered to be the rate-limiting steps in the overall OER process. Another plausible pathway for OER involves surface lattice hydroxide species. Considering the contribution of surface lattice oxygen, another 4-step OER mechanism was proposed based on the original work of Bockris et al. In step 1, hydroxide radicals form on the surface. These two steps are similar to step 1 of Figure 27b. Recently, Hardin et al.
Furthermore, LaNi0. Standard free energy diagrams for an ideal catalyst a and c and LaMnO3 b and d.
Adapted with permission from ref However, for LaMnO3 the standard free energy of step 3 is much higher than the rest of the steps, indicating the formation of peroxide as the PDS. The ORR proceeds in the reverse direction, i. For the ideal catalyst, the ORR is spontaneous at the Nernst potential. The extension of this method to other cation-sensitive surfaces, such as Ba0. DFT calculations. The highest reaction free energy can therefore be indicative of the ratedetermining step, which is also referred to as the PDS. For the ideal catalyst, all reaction steps have an equal free enecrgy of 1.
This trend was rationalized to the strain induced changes to the Bsite and oxygen overlap, and consequent B-site d-band properties. Recently, several new methods to model the solvent media have been introduced, such as the water bilayer approach. This method enables the tuning of the electrode potential by introducing additional solvated protons into the water layer and excess electrons on the electrode surface. Among the oxygen nonstoichiometric perovskite-type oxides, Co-based catalysts are generally highly active yet unstable, especially in severe conditions, e.
Conversely, Febased and Cr-based catalysts are stable yet not very active. Furthermore, Mn-based and Ni-based catalysts perform better as practical oxygen catalysts with rather high activity and good stability. Usually, a balance between electrocatalytic activities and chemical stabilities may be reached through the rational design of catalysts, yet still far below the expectation. Therefore, the development of methods to enlarge the active sites is key. The readers are referred to a critical review on the crystallization of ABO3 perovskites under hydrothermal or solvothermal conditions.
Pyrochlore-Type Oxygen Catalysts One drawback of pyrochlore-type oxides is the employment of toxic Pb or expensive Ru in some compositions. Therefore, replacing them with less toxic and less expensive elements, e. Increasing the concentration of anion or cation defects is a promising method to enhance both the electronic and ionic conductivity. Other directions, including partial substitution of cations, innovative synthesis, or adopting hybrids, may also be feasible to further develop these materials.
Carbon support enables dispersion and has several additional advantages, including lightweight, good electronic conduction, low cost, and a porous structure. Furthermore, an additional disadvantage of carbon in nonaqueous LABs is that it is reactive with Li2O2 and produces an interfacial layer of Li2CO3 between carbon and 9. Li2O2 on the charge oxidation cycle. Developing electrode architecture with carbon- and binderfree methods is also a feasible way. The application of solid electrolytes is, however, currently limited by their relatively low conductivities. The recently developed Li1. However, several factors still remain challenging for their application, including the chemical stability with electrode, phase, and electrochemical stability of these materials.
System Despite a high energy density, MABs have a relatively low power density.
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- Interaction of small and extended defects in nonstoichiometric oxides!
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Besides, cations in the alkaline solution, e. In addition, for the aprotic nonaqueous LABs, additional challenges are also required to improve energy storage and prevent Li2O2 from clogging and surface growth, and for better mechanistic understanding of the entire system.
Materials Design with Reliable Measurement Until now, the discovery of the majority of the successful oxygen catalysts has been via an Edisonian approach.
Nonstoichiometric Oxides - 1st Edition
Despite recent work and progress in theoretical and computational methods to rationally design nonstoichiometric oxide catalysts, few novel catalysts have been suggested. Instead, most recently reported high-performance catalysts e. The main progress in recent developed materials is attributed to the advanced synthesis and characterizations. AK DOI: Zarah Medina Baiyee has worked in the UK photovoltaic and renewable energy industry for 3 years, subsequent to completing an M.
Since returning to academia, she has worked on density function theory calculations for oxygen catalyst electrode materials, as part of Prof. She has been awarded the Hong Kong Ph. Fellowship to support her work. Zongping Shao earned his Ph. Currently, he is the dean of College of Energy. His research interests include lithium-related batteries, supercapacitors, solid oxide fuel cells, oxygen permeable membranes, and low-temperature fuel cells.
He has published over journal papers with more than citations in total. Currently, Prof. Shao also directs a research group at Curtin University, Australia. These catalysts are characterized to be low cost and earth-abundant, as well as possess relatively high activity and stability under operation conditions. It is expected that these catalysts will be essential to the future development of multiple technologies. For practical large-scale commercialization, the development of even more active and stable oxygen catalysts is essential and will be the central topic of advanced research.
He received his Ph. Prior to that he obtained a dual M. Biographies Dengjie Chen was born in in Zhejiang, China. He received his B.
Zongping Shao. His current research interests include molecular dynamics simulations and density functional theory calculations applied to oxide materials for fuel cells. AL DOI: Pure Appl. Energy Mater. A: Chem. Nanomaterials for Renewable Energy Production and Storage. Nanomaterials for Rechargeable Lithium Batteries. Lithium Batteries: Status, Prospects and Future.
Advanced Materials for Energy Storage. Nanomaterials for Energy Conversion and Storage. Metal-Air Technology. Alkaline Membrane Fuel Cells. In Fuel Cells; Kreuer, K. AM DOI: Chemical Reviews Review 44 Zhou, Z. Oxygen Evolution on Semiconducting Oxides. Electrochemistry of Oxygen; Interscience Publishers: Electrocatalysis in Fuel Cells; Springer: Energy Environ. ACS Appl. Nano Lett. Nano Res.
AN DOI: Chemical Reviews Review Pettersson, J. Synthesis of Perovskite-Based Porous La0.
Nonstoichiometric Oxides (Materials science and technology)
Low-Cost Oxygen Electrode Material. Oxygen-Reduction Catalysts: Picking Perovskites. Lithium-Air Batteries: Something from Nothing. The Electrocatalysis of Oxygen Evolution on Perovskites. Electrocatalysis by Oxides - Attempt at a Unifying Approach. Interfacial Electrochem. Organohalide Lead Perovskites for Photovoltaic Applications. Die Gesetze Der Krystallochemie. Chemical Structures and Performance of Perovskite Oxides. B: Condens. One-Dimensional Nanostructures of Ferroelectric Perovskites. Lead-Free Piezoceramics. Lithium-Air and Lithium-Sulfur Batteries. MRS Bull.
AO DOI: Chemical Reviews Review Maignan, A. Electrochemical Study of Ba0. A Novel Bifunctional Catalyst of Ba0. Perovskite Sr0. Hierarchical Mesoporous Perovskite La0. Investigation of the Permeation Behavior and Stability of a Ba0. Assessment of Ba0. ReEvaluation of Ba0. Progress in Understanding and Development of Ba0.
Composite Electrode Boosts the Activity of Ba0. ACS Catal.
New Ba0. Solid State Electrochem. Nonstoichiometry of Sintered Oxide Ca0. AP DOI: Chemical Reviews Review Velraj, S. Solid State Chem. Advances in Designing Perovskite Catalysts. Solid State Mater. Synthesis of La0. Synthesis and Characterization of La0. Thermal Treatment of La0. Comparison of Ln0. Matter Mater. Electrocatalytic Activities of La0. ECS Trans. Articles by Shenoy, V. Search for related content. PubMed Articles by Er, D.
Latest Articles , 3. Alert me to new issues of J. Based on the relationship between the cohesive enthalpy and crystal composition, a new method of determination of enthalpy of point defects formation has been proposed. The obtained results remain in agreement with experimentally observed negative values of Delta Hdef corresponding to the formation of either cation vacancies or defect complexes in Fe1-yO and Mn1-yO, and positive values of enthalpy, Delta HVO , in the case of formation of oxygen vacancies in metal oxides such as M2O3-x and MO2-x.
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Kofstad P. Wiley, New York, Kroger F. Mrowec S. Smyth D. Rao C. Fender B. Eyring and M. Vellet P. France , Ser.