Computational insights into spin-polarized density functional theory applied to actinide-based perovskites XBkO₃ (X = Sr, Ra, Pb)

The exploration of perovskite compounds incorporating actinide and divalent elements reveals remarkable characteristics. Focusing on PbBkO3, RaBkO3, and SrBkO3, these materials were studied using density functional theory (DFT) via the CASTEP code to analyze their electronic, optical, and mechanical properties. The results show semiconductor behavior, with respective band gaps of 1.320 eV for PbBkO3, 3.415 eV for RaBkO3, and 2.775 eV for SrBkO3. Additionally, the elastic constants Cij, bulk modulus B, elasticity modulus G, Young's modulus Y, and Poisson's ratio v were optimized, highlighting anisotropic behavior. The mechanical stability of the compounds meets Born's criteria, and RaBkO3 stands out with a stable lattice dynamic, as demonstrated by phonon dispersion curves in the Pm-3 m space group. The optical properties of these materials indicate they are excellent absorbers of incident radiation, suggesting their potential for applications in magnetic sensors due to their anisotropic magnetic behavior, as well as for capturing solar radiation in the ultraviolet range.

Ni-Doped Pr0.7Ba0.3MnO3-δ Cathodes for Enhancing Electrolysis of CO2 in Solid Oxide Electrolytic Cells

Solid Oxide Electrolysis Cells (SOECs) can electro-reduce carbon dioxide to carbon monoxide, which not only effectively utilizes greenhouse gases, but also converts excess electrical energy into chemical energy. Perovskite-based oxides with exsolved metal nanoparticles are promising cathode materials for direct electrocatalytic reduction of CO2 through SOECs, and have thus received increasing attention. In this work, we doped Pr0.7Ba0.3MnO3-δ at the B site, and after reduction treatment, metal nanoparticles exsolved and precipitated on the surface of the cathode material, thereby establishing a stable metal-oxide interface structure and significantly improving the electrocatalytic activity of the SOEC cathode materials. Through research, among the Pr0.7Ba0.3Mn1-xNixO3-δ (PBMNx = 0-1) cathode materials, it has been found that the Pr0.7Ba0.3Mn0.9Ni0.1O3-δ (PBMN0.1) electrode material exhibits greater catalytic activity, with a CO yield of 5.36 mL min-1 cm-2 and a Faraday current efficiency of ~99%. After 100 h of long-term testing, the current can still remain stable and there is no significant change in performance. Therefore, the design of this interface has increasing potential for development.

Development of In Situ Methods for Preparing La-Mn-Co-Based Compounds over Carbon Xerogel for Oxygen Reduction Reaction in an Alkaline Medium

Metal oxides containing La, Mn, and Co cations can catalyze oxygen reduction reactions (ORRs) in electrochemical processes. However, these materials require carbon support and optimal interactions between both compounds to be active. In this work, two approaches to prepare composites of La-Mn-Co-based compounds over carbon xerogel were developed. Using sol-gel methods, either the metal-based material was deposited on the existing carbon xerogel or vice versa. The metal oxide selected was the LaMn0.7Co0.3O3 perovskite, which has good catalytic behavior and selectivity towards direct ORRs. All the as-prepared composites were tested for ORRs in alkaline liquid electrolytes and characterized by diverse physicochemical techniques such as XRD, XPS, SEM, or N2 adsorption. Although the perovskite structure either decomposed or failed to form using those in situ methods, the materials exhibited great catalytic activity, which can be ascribed to the strengthening of the interactions between oxides and the carbon support via C-O-M covalent bonds and to the formation of new active sites such as the MnO/Co heterointerfaces. Moreover, Co-Nx-C species are formed during the synthesis of the metal compounds over the carbon xerogel. These species possess a strong catalytic activity towards ORR. Therefore, the composites formed by synthesizing metal compounds over the carbon xerogel exhibit the best performance in the ORR, which can be ascribed to the presence of the MnO/Co heterointerfaces and Co-Nx-C species and the strong interactions between both compounds. Moreover, the small nanoparticle size leads to a higher number of active sites available for the reaction.

Yttrium-based Double Perovskite Nanorods for Electrocatalysis

Herein, we investigate the effect of the chemical composition of double perovskite nanorods on their versatile electrocatalytic activity not only as supports for the oxidation of small organic molecules but also as catalysts for the oxygen evolution reaction. Specifically, Y₂CoMnO₆ and Y₂NiMnO₆ nanorods with average diameters of 300 nm were prepared by a two-step hydrothermal method, in which the individual effects of synthetic parameters, such as the pH, annealing temperature, and precursor ratios on both the composition and morphology, were systematically investigated. When used as supports for Pt nanoparticles, Y₂CoMnO₆/Pt catalysts exhibited an electrocatalytic activity for the methanol oxidation reaction, which is 2.1 and 1.3 times higher than that measured for commercial Pt/C and Y₂NiMnO₆/Pt, respectively. Similarly, the Co-based catalyst support material displayed an ethanol oxidation activity, which is 2.3 times higher than both Pt/C and Y₂NiMnO₆/Pt. This clear enhancement in the activity for Y₂CoMnO₆ can largely be attributed to strong metal-support interactions, as evidenced by a downshift in the binding energy of the Pt 4f bands, measured by X-ray photoelectron spectroscopy (XPS), which is often correlated not only with a downshift in the d-band center but also to a decreased adsorption of poisoning adsorbates. Moreover, when used as catalysts for the oxygen evolution reaction, Y₂CoMnO₆ displayed a much greater activity as compared with Y₂NiMnO₆. This behavior can largely be attributed not only to a preponderance of comparatively more favorable oxidation states and electronic configurations but also to the formation of an active layer on the surface of the Y₂CoMnO₆ catalyst, which collectively gives rise to improved performance metrics and greater stability as compared with both IrO₂ and Y₂NiMnO₆. Overall, these results highlight the importance of both the chemical composition and the electronic structure of double perovskites, especially when utilized in multifunctional roles as either supports or catalysts.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Activation Strategies of Perovskite-Type Structure for Applications in Oxygen-Related Electrocatalysts

The oxygen-related electrochemical process, including the oxygen evolution reaction and oxygen reduction reaction, is usually a kinetically sluggish reaction and thus dominates the whole efficiency of energy storage and conversion devices. Owing to the dominant role of the oxygen-related electrochemical process in the development of electrochemical energy, an abundance of oxygen-related electrocatalysts is discovered. Among them, perovskite-type materials with flexible crystal and electronic structures have been researched for a long time. However, most perovskite materials still show low intrinsic activity, which highlights the importance of activation strategies for perovskite-type structures to improve their intrinsic activity. In this review, the recent progress of the activation strategies for perovskite-type structures is summarized and their related applications in oxygen-related electrocatalysis reactions, including electrochemistry water splitting, metal-air batteries, and solid oxide fuel cells are discussed. Furthermore, the existing challenges and the future perspectives for the designing of ideal perovskite-type structure catalysts are proposed and discussed.

Recent advances in perovskite oxides as electrode materials for supercapacitors

Owing to the high power density and ultralong cycle life, supercapacitors represent an alternative to electrochemical batteries in energy storage applications. However, the relatively low energy density is the main challenge for supercapacitors in the current drive to push the entire technology forward to meet the benchmark requirements for commercialization. To effectively solve this issue, it is crucial to develop electrode materials with excellent electrochemical performance since the electrode used is closely related to the specific capacitance and energy density of supercapacitors. With the unique structure, compositional flexibility, and inherent oxygen vacancy, perovskite oxides have attracted wide attention as promising electrode materials for supercapacitors. In this review, we summarize the recent advances in perovskite oxides as electrode materials for supercapacitors. Firstly, the structures and compositions of perovskite oxides are critically reviewed. Following this, the progress in various perovskite oxides, including single perovskite and derivative perovskite oxides, is depicted, focusing on their electrochemical performance. Furthermore, several optimization strategies (i.e., modulating the stoichiometry of the anion or cation, A-site doping, B-site doping, and constructing composites) to improve their electrochemical performance are also discussed. Finally, the significant challenges facing the advancement of perovskite oxide electrodes for supercapacitor applications and future outlook are proposed.

The Effects of Sr Content on the Performance of Nd1-xSrxCoO3-δ Air-Electrode Materials for Intermediate Temperature Solid Oxide Fuel Cells under Operational Conditions

The potential of the perovskite system Nd_1-xSr_xCoO_3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm^-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd_1/3Sr_2/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ present lower ASR values (0.10 Ω·cm² at 973 K in air) than composites prepared with Nd_2/3Sr_1/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ (0.34 Ω·cm²). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

Magnetic Fe3O4-encapsulated VAN@MIL-101(Fe) with mixed-valence sites and mesoporous structures as efficient bifunctional water splitting photocatalysts

Fe3O4/VAN@MIL-101(Fe) with both mesoporous and mixed-valence Fe3+/Fe2+ structures was controllably synthesized in the synthesis of MIL-101(Fe), and it was used as a bifunctional photocatalyst in both oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs) of photocatalytic water splitting. By the reduction of auxiliary ligand vanillin (VAN) and the introduction of Fe3O4, the mixed-valence Fe3+/Fe2+ structure in Fe3O4/VAN@MIL-101(Fe) was obtained, which improves the band gap of the Fe3+ reactive active center and increases the separation efficiency of photogenerated carriers. Owing to the partial difference in the structure between VAN and ligand terephthalic acid (H2BDC), hierarchical porous and vacant structures were effectively improved in Fe3O4/VAN@MIL-101(Fe), which can induce more active sites to adsorb more water molecules and shorten the electron-hole migration distance to improve the transfer efficiency of photogenerated carriers. Therefore, Fe3O4/VAN@MIL-101(Fe) presents excellent photocatalytic activities for improving the O2 and H2 production rate up to 360 000 μmol g-1 h-1 and 584 μmol g-1 h-1, respectively. Meanwhile, Fe3O4/VAN@MIL-101(Fe) maintains the excellent catalytic activity in OERs and HERs after recycling for 5 times. Moreover, the introduction of magnetic Fe3O4 nanoplates into Fe3O4/VAN@MIL-101(Fe) can make it easily recyclable by magnetic separation, which can maximize its performance.

Metal-Organic Frameworks for Hydrogen Energy Applications: Advances and Challenges

Hydrogen is in limelight as an environmental benign alternative to fossil fuels from few decades. To bring the concept of hydrogen economy from academic labs to real world certain challenges need to be addressed in the areas of hydrogen production, storage, and its use in fuel cells. Crystalline metal-organic frameworks (MOFs) with unprecedented surface areas are considered as potential materials for addressing the challenges in each of these three areas. MOFs combine the diverse chemistry of molecular linkers with their ability to coordinate to metal ions and clusters. The unabated flurry of research using MOFs in the context of hydrogen energy related activities in the past decade demonstrates the versatility of this class of materials. In the present review, we discuss major strategical advances that have taken place in the field of "hydrogen economy and MOFs" and point out issues requiring further attention.

Plasma engraved Bi0.1(Ba0.5Sr0.5)0.9Co0.8Fe0.2O3-δ perovskite for highly active and durable oxygen evolution

The development of highly active and cost-effective catalysts based on noble metal free oxygen electro-catalysis is critical to energy storage and conversion devices. Herein, we highlight a plasma-treated Bi0.1(Ba0.5Sr0.5)0.9Co0.8Fe0.2O3-δ perovskite (denoted as P-Bi0.1BSCF) as a promising catalyst for oxygen evolution reaction (OER) in alkaline media. H2/Ar plasma engraving could significantly increase electrochemically active O22-/O- concentration and tune the electronic structure of Co ions efficiently, and consequently tailor the intrinsic electrocatalytic ability for OER. Of note, P-Bi0.1BSCF, with unique crystalline core/amorphous shell structure, exhibits an enhanced intrinsic OER activity and higher stability than the noble metal IrO2 catalyst, which outperforms most of the reported perovskite catalysts. The present work provides new insights into exploring efficient catalysts for OER, and it suggests that, in addition to the extensively applied for surface treatment of various catalysts such as carbons and metal oxides, the plasma engraved perovskite materials also exhibits great potential as precious metal-free catalysts.

High-Energy-Density Metal-Oxygen Batteries: Lithium-Oxygen Batteries vs Sodium-Oxygen Batteries

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium-oxygen (Na-O₂ ) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na-O₂ battery is lower than that of the lithium-oxygen (Li-O₂ ) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium-oxygen and sodium-oxygen cells. An in-depth understanding of the differences and similarities between Li-O₂ and Na-O₂ battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal-oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium-oxygen and sodium-oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.

Defect Chemistry, Electrical Properties, and Evaluation of New Oxides Sr2 CoNb1-x Tix O6-δ (0≤x≤1) as Cathode Materials for Solid Oxide Fuel Cells

The perovskite series Sr₂ CoNb_1-x Ti_x O_6-δ (0≤x≤1) was investigated in the full compositional range to assess its potential as cathode material for solid oxide fuel cell (SOFC). The variation of transport properties and thus, the area specific resistances (ASR) are explained by a detailed investigation of the defect chemistry. Increasing the titanium content from x=0-1 produces both oxidation of Co³⁺ to Co⁴⁺ (from 0 up to 40 %) and oxygen vacancies (from 6.0 to 5.7 oxygen atom/formula unit), although each charge compensation mechanism predominates in different compositional ranges. Neutron diffraction reveals that samples with high Ti-contents lose a significant amount of oxygen upon heating above 600 K. Oxygen is partially recovered upon cooling as the oxygen release and uptake show noticeably different kinetics. The complex defect chemistry of these compounds, together with the compositional changes upon heating/cooling cycles and atmospheres, produce a complicated behavior of electrical conductivity. Cathodes containing Sr₂ CoTiO_6-δ display low ASR values, 0,13 Ω cm² at 973 K, comparable to those of the best compounds reported so far, being a very promising cathode material for SOFC.

Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

Oxygen reduction is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in fuel cells and metal-air batteries. In this review, the prospect of perovskite oxides for the oxygen reduction reaction (ORR) in alkaline media is reviewed with respect to fundamental insight into activity and possible mechanisms. For gaining these insights, special emphasis is placed on highly crystalline perovskite films that have only recently become available for electrochemical interrogation. The prospects for applications are evaluated based on recent progress in the synthesis of perovskite nanoparticles. The review concludes with the current understanding of oxygen reduction on perovskite oxides and a perspective on opportunities for future fundamental and applied research.

Recent Progress in Metal-Organic Frameworks for Applications in Electrocatalytic and Photocatalytic Water Splitting

The development of clean and renewable energy materials as alternatives to fossil fuels is foreseen as a potential solution to the crucial problems of environmental pollution and energy shortages. Hydrogen is an ideal energy material for the future, and water splitting using solar/electrical energy is one way to generate hydrogen. Metal-organic frameworks (MOFs) are a class of porous materials with unique properties that have received rapidly growing attention in recent years for applications in water splitting due to their remarkable design flexibility, ultra-large surface-to-volume ratios and tunable pore channels. This review focuses on recent progress in the application of MOFs in electrocatalytic and photocatalytic water splitting for hydrogen generation, including both oxygen and hydrogen evolution. It starts with the fundamentals of electrocatalytic and photocatalytic water splitting and the related factors to determine the catalytic activity. The recent progress in the exploitation of MOFs for water splitting is then summarized, and strategies for designing MOF-based catalysts for electrocatalytic and photocatalytic water splitting are presented. Finally, major challenges in the field of water splitting are highlighted, and some perspectives of MOF-based catalysts for water splitting are proposed.

Perovskite/Carbon Composites: Applications in Oxygen Electrocatalysis

Oxygen electrocatalysis, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), plays an extremely important role in oxygen-based renewable-energy technologies such as rechargeable metal-air batteries, regenerative fuel cells and water splitting. Perovskite oxides have recently attracted increasing interest and hold great promise as efficient ORR and OER catalysts to replace noble-metal-based catalysts, owing to their high intrinsic catalytic activity, abundant variety, low cost, and rich resources. The introduction of perovskite-carbon interfaces by forming perovskite/carbon composites may bring a synergistic effect between the two phases, thus benefiting the oxygen electrocatalysis. This review provides a comprehensive overview of recent advances in perovskite/carbon composites for oxygen electrocatalysis in alkaline media, aiming to provide insights into the key parameters that influence the ORR/OER performance of the composites, including the physical/chemical properties and morphologies of the perovskites, the multiple roles of carbon, the synthetic method and the synergistic effect. A special emphasis is placed on the origin of the synergistic effect associated with the interfacial interaction between the perovskite and the carbon phases for enhanced ORR/OER performance. Finally, the existing challenges and the future directions for the synthesis and development of more efficient oxygen catalysts based on perovskite/carbon composites are proposed.

Enhancing Electrode Performance by Exsolved Nanoparticles: A Superior Cobalt-Free Perovskite Electrocatalyst for Solid Oxide Fuel Cells

The successful development of low-cost, durable electrocatalysts for oxygen reduction reaction (ORR) at intermediate temperatures is critical for broad commercialization of solid oxide fuel cells. Here, we report our findings in design, fabrication, and characterization of a cobalt-free SrFe_0.85Ti_0.1Ni_0.05O_3-δ cathode decorated with NiO nanoparticles. Exsolved from and well bonded to the parent electrode under well-controlled conditions, the NiO nanoparticles uniformly distributed on the surface of the parent electrode greatly enhance cathode performance, demonstrating ORR activity better than that of the benchmark cobalt-based Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ. Further, a process for regeneration of the NiO nanoparticles was also developed to mitigate potential performance degradation due to coarsening of NiO particles under practical operating conditions. As a general approach, this exsolution-dissolution of electrocatalytically active nanoparticles on an electrode surface may be applicable to the development of other high-performance cobalt-free cathodes for fuel cells and other electrochemical systems.