Energetics of Nanoparticle Exsolution from Perovskite Oxides

A Single Model for the Thermodynamics and Kinetics of Metal Exsolution from Perovskite Oxides

Exsolution has emerged as an outstanding route for producing oxide-supported metal nanoparticles. For ABO3-perovskite oxides, various late transition-metal cations can be substituted into the lattice under oxidizing conditions and exsolved as metal nanoparticles after reduction. A consistent and comprehensive description of the point-defect thermodynamics and kinetics of this phenomenon is lacking, however. Herein, supported by hybrid density-functional-theory calculations, we propose a single model that explains diverse experimental observations, such as why substituent transition-metal cations (but not host cations) exsolve from perovskite oxides upon reduction; why different substituent transition-metal cations exsolve under different conditions; why the metal nanoparticles are embedded in the surface; why exsolution occurs surprisingly rapidly at relatively low temperatures; and why the reincorporation of exsolved species involves far longer times and much higher temperatures. Our model's foundation is that the substituent transition-metal cations are reduced to neutral species within the perovskite lattice as the Fermi level is shifted upward within the bandgap upon sample reduction. The calculations also indicate unconventional influences of oxygen vacancies and A-site vacancies. Our model thus provides a fundamental basis for improving existing, and creating new, exsolution-generated catalysts.

Space charge governs the kinetics of metal exsolution

Nanostructured composite electrode materials play a major role in the fields of catalysis and electrochemistry. The self-assembly of metallic nanoparticles on oxide supports via metal exsolution relies on the transport of reducible dopants towards the perovskite surface to provide accessible catalytic centres at the solid-gas interface. At surfaces and interfaces, however, strong electrostatic gradients and space charges typically control the properties of oxides. Here we reveal that the nature of the surface-dopant interaction is the main determining factor for the exsolution kinetics of nickel in SrTi0.9Nb0.05Ni0.05O3-δ. The electrostatic interaction of dopants with surface space charge regions forming upon thermal oxidation results in strong surface passivation, which manifests in a retarded exsolution response. We furthermore demonstrate the controllability of the exsolution response via engineering of the perovskite surface chemistry. Our findings indicate that tailoring the electrostatic gradients at the perovskite surface is an essential step to improve exsolution-type materials in catalytic converters.

Metal exsolution from perovskite-based anodes in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are highly efficient and environmentally friendly devices for converting fuel into electrical energy. In this regard, metal nanoparticles (NPs) loaded onto the anode oxide play a crucial role due to their exceptional catalytic activity. NPs synthesized through exsolution exhibit excellent dispersion and stability, garnering significant attention for comprehending the exsolution process mechanism and consequently improving synthesis effectiveness. This review presents recent advancements in the exsolution process, focusing on the influence of oxygen vacancies, A-site defects, lattice strain, and phase transformation on the variation of the octahedral crystal field in perovskites. Moreover, we offer insights into future research directions to further enhance our understanding of the mechanism and achieve significant exsolution of NPs on perovskites.

Nanoparticle Exsolution on Perovskite Oxides: Insights into Mechanism, Characteristics and Novel Strategies

Supported nanoparticles have attracted considerable attention as a promising catalyst for achieving unique properties in numerous applications, including fuel cells, chemical conversion, and batteries. Nanocatalysts demonstrate high activity by expanding the number of active sites, but they also intensify deactivation issues, such as agglomeration and poisoning, simultaneously. Exsolution for bottom-up synthesis of supported nanoparticles has emerged as a breakthrough technique to overcome limitations associated with conventional nanomaterials. Nanoparticles are uniformly exsolved from perovskite oxide supports and socketed into the oxide support by a one-step reduction process. Their uniformity and stability, resulting from the socketed structure, play a crucial role in the development of novel nanocatalysts. Recently, tremendous research efforts have been dedicated to further controlling exsolution particles. To effectively address exsolution at a more precise level, understanding the underlying mechanism is essential. This review presents a comprehensive overview of the exsolution mechanism, with a focus on its driving force, processes, properties, and synergetic strategies, as well as new pathways for optimizing nanocatalysts in diverse applications.

Influence of Sr-Site Deficiency, Ca/Ba/La Doping on the Exsolution of Ni from SrTiO3

Cermet catalysts formed via exsolution of metal nanoparticles from perovskites promise to perform better in electro- and thermochemical applications than those synthesized by conventional wet-chemical approaches. However, a shortage of robust material design principles still stands in the way of widespread commercial adoption of exsolution. Working with Ni-doped SrTiO₃ solid solutions, we investigated how the introduction of Sr deficiency as well as Ca, Ba, and La doping on the Sr site changed the size and surface density of exsolved Ni nanoparticles. We carried out exsolution on 11 different compositions under fixed conditions. We elucidated the effect of A-site defect size/valence on nanoparticle density and size as well as the effect of composition on nanoparticle immersion and ceramic microstructure. Based on our experimental results, we developed a model that quantitatively predicted a composition's exsolution properties using density functional theory calculations. The model and calculations provide insights into the exsolution mechanism and can be used to find new compositions with high exsolution nanoparticle density.

Exsolution Modeling and Control to Improve the Catalytic Activity of Nanostructured Electrodes

In situ exsolution for nanoscale electrode design has attracted considerable attention because of its promising activity and high stability. However, fundamental research on the mechanisms underlying particle growth remains insufficient. Herein, cation-diffusion-determined exsolution is presented using an analytical model based on classical nucleation and diffusion. In the designed perovskite system, the exsolution trend for particle growth is consistent with this diffusion model, which strongly depends on the initial cation concentration and reduction conditions. Based on the experimental and theoretical results, a highly Ni-doped anode and an electrochemical switching technique are employed to promote exsolution and overcome growth limitations. The optimal cell exhibits an outstanding maximum power density of 1.7 W cm^-2 at 900 °C and shows no evident degradation when operating at 800 °C for 240 h under wet H₂ . This study provides crucial insights into the developing and tuning of heterogeneous catalysts for energy-conversion applications.

Real-time insight into the multistage mechanism of nanoparticle exsolution from a perovskite host surface

In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving forces, leading to transformative advances in stability, activity, and efficiency over deposition techniques, and resulting in a wide range of new opportunities for catalytic, energy and net-zero-related technologies. However, the mechanism of exsolved nanoparticle nucleation and perovskite structural evolution, has, to date, remained unclear. Herein, we shed light on this elusive process by following in real time Ir nanoparticle emergence from a SrTiO₃ host oxide lattice, using in situ high-resolution electron microscopy in combination with computational simulations and machine learning analytics. We show that nucleation occurs via atom clustering, in tandem with host evolution, revealing the participation of surface defects and host lattice restructuring in trapping Ir atoms to initiate nanoparticle formation and growth. These insights provide a theoretical platform and practical recommendations to further the development of highly functional and broadly applicable exsolvable materials.

Exsolution on perovskite oxides: morphology and anchorage of nanoparticles

Perovskites are very promising materials for a wide range of applications (such as catalysis, solid oxide fuel cells…) due to beneficial general properties (e.g. stability at high temperatures) and tunability - doping both A- and B-site cations opens the path to a materials design approach that allows specific properties to be finely tuned towards applications. A major asset of perovskites is the ability to form nanoparticles on the surface under certain conditions in a process called "exsolution". Exsolution leads to the decoration of the material's surface with finely dispersed nanoparticles (which can be metallic or oxidic - depending on the experimental conditions) made from B-site cations of the perovskite lattice (here, doping comes into play, as B-site doping allows control over the constitution of the nanoparticles). In fact, the ability to undergo exsolution is one of the main reasons that perovskites are currently a hot topic of intensive research in catalysis and related fields. Exsolution on perovskites has been heavily researched in the last couple of years: various potential catalysts have been tested with different reactions, the oxide backbone materials and the exsolved nanoparticles have been investigated with a multitude of different methods, and the effect of different exsolution parameters on the resulting nanoparticles has been studied. Despite all this, to our knowledge no comprehensive effort was made so far to evaluate these studies with respect to the effect that the exsolution conditions have on anchorage and morphology of the nanoparticles. Therefore, this highlight aims to provide an overview of nanoparticles exsolved from oxide-based perovskites with a focus on the conditions leading to nanoparticle exsolution.

Dynamic Surface Evolution of Metal Oxides for Autonomous Adaptation to Catalytic Reaction Environments

Metal oxides possessing distinctive physical/chemical properties due to different crystal structures and stoichiometries play a pivotal role in numerous current technologies, especially heterogeneous catalysis for production/conversion of high-valued chemicals and energy. To date, many researchers have investigated the effect of the structure and composition of these materials on their reactivity to various chemical and electrochemical reactions. However, metal oxide surfaces evolve from their initial form under dynamic reaction conditions due to the autonomous behaviors of the constituent atoms to adapt to the surrounding environment. Such nanoscale surface phenomena complicate reaction mechanisms and material properties, interrupting the clarification of the origin of functionality variations in reaction environments. In this review, the current findings on the spontaneous surface reorganization of metal oxides during reactions are categorized into three types: 1) the appearance of nano-sized second phase from oxides, 2) the (partial) encapsulation of oxide atoms toward supported metal surfaces, and 3) the oxide surface reconstruction with selective cation leaching in aqueous solution. Then their effects on each reaction are summarized in terms of activity and stability, providing novel insight for those who design metal-oxide-based catalytic materials.

Anti-phase boundary accelerated exsolution of nanoparticles in non-stoichiometric perovskite thin films

Exsolution of excess transition metal cations from a non-stoichiometric perovskite oxide has sparked interest as a facile route for the formation of stable nanoparticles on the oxide surface. However, the atomic-scale mechanism of this nanoparticle formation remains largely unknown. The present in situ scanning transmission electron microscopy combined with density functional theory calculation revealed that the anti-phase boundaries (APBs) characterized by the a/2 < 011> type lattice displacement accommodate the excess B-site cation (Ni) through the edge-sharing of BO₆ octahedra in a non-stoichiometric ABO₃ perovskite oxide (La_0.2Sr_0.7Ni_0.1Ti_0.9O_3-δ) and provide the fast diffusion pathways for nanoparticle formation by exsolution. Moreover, the APBs further promote the outward diffusion of the excess Ni toward the surface as the segregation energy of Ni is lower at the APB/surface intersection. The formation of nanoparticles occurs through the two-step crystallization mechanism, i.e., the nucleation of an amorphous phase followed by crystallization, and via reactive wetting on the oxide support, which facilitates the formation of a stable triple junction and coherent interface, leading to the distinct socketing of nanoparticles to the oxide support. The atomic-scale mechanism unveiled in this study can provide insights into the design of highly stable nanostructures.

Exsolution of transition metal cations from non-stoichiometric perovskites offer a route for the formation of stable nanoparticles on the surface. Here authors present an anti-phase boundaries-accelerated exsolution and two-step crystallisation of nanoparticles in non-stoichiometric perovskite thin films.

Low-Temperature Exsolution of Ni-Ru Bimetallic Nanoparticles from A-Site Deficient Double Perovskites

Exsolution of stable metallic nanoparticles for use as efficient electrocatalysts has been of increasing interest for a range of energy technologies. Typically, exsolved nanoparticles show higher thermal and coarsening stability compared to conventionally deposited catalysts. Here, A-site deficient double perovskite oxides, La_{2- x} NiRuO_{6- δ} (x = 0.1 and 0.15), are designed and subjected to low-temperature reduction leading to exsolution. The reduced double perovskite materials are shown to exsolve nanoparticles of 2-6 nm diameter during the reduction in the low-temperature range of 350-450 °C. The nanoparticle sizes are found to increase after reduction at the higher temperature (450 °C), suggesting diffusion-limited particle growth. Interestingly, both nickel and ruthenium are co-exsolved during the reduction process. The formation of bimetallic nanoparticles at such low temperatures is rare. From the in situ impedance spectroscopy measurements of the double perovskite electrode layers, the onset of the exsolution process is found to be within the first few minutes of the reduction reaction. In addition, the area-specific resistance of the electrode layers is found to decrease by 90% from 291 to 29 Ω cm² , suggesting encouraging prospects for these low-temperature rapidly exsolved Ni/Ru alloy nanoparticles in a range of catalytic applications.

Exsolution-Driven Surface Transformation in the Host Oxide

Exsolution synthesizes self-assembled metal nanoparticle catalysts via phase precipitation. An overlooked aspect in this method thus far is how exsolution affects the host oxide surface chemistry and structure. Such information is critical as the oxide itself can also contribute to the overall catalytic activity. Combining X-ray and electron probes, we investigated the surface transformation of thin-film SrTi_0.65Fe_0.35O₃ during Fe⁰ exsolution. We found that exsolution generates a highly Fe-deficient near-surface layer of about 2 nm thick. Moreover, the originally single-crystalline oxide near-surface region became partially polycrystalline after exsolution. Such drastic transformations at the surface of the oxide are important because the exsolution-induced nonstoichiometry and grain boundaries can alter the oxide ion transport and oxygen exchange kinetics and, hence, the catalytic activity toward water splitting or hydrogen oxidation reactions. These findings highlight the need to consider the exsolved oxide surface, in addition to the metal nanoparticles, in designing the exsolved nanocatalysts.

Recent Advances in Perovskite Catalysts for Efficient Overall Water Splitting

Hydrogen is considered a promising clean energy vector with the features of high energy capacity and zero-carbon emission. Water splitting is an environment-friendly and effective route for producing high-purity hydrogen, which contains two important half-cell reactions, namely, the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). At the heart of water splitting is high-performance electrocatalysts that efficiently improve the rate and selectivity of key chemical reactions. Recently, perovskite oxides have emerged as promising candidates for efficient water splitting electrocatalysts owing to their low cost, high electrochemical stability, and compositional and structural flexibility allowing for the achievement of high intrinsic electrocatalytic activity. In this review, we summarize the present research progress in the design, development, and application of perovskite oxides for electrocatalytic water splitting. The emphasis is on the innovative synthesis strategies and a deeper understanding of structure–activity relationships through a combination of systematic characterization and theoretical research. Finally, the main challenges and prospects for the further development of more efficient electrocatalysts based on perovskite oxides are proposed. It is expected to give guidance for the development of novel non-noble metal catalysts in electrochemical water splitting.

A-site deficient La0.52Sr0.28Ti0.94Ni0.06O3by low-pulsed electric current treatment: achieved exsolution of B-site Ni nanoparticles and significant improvement of electrocatalytic properties

Perovskite materials with exsolved nanoparticles have a wide range of applications in energy conversion systems owing to their unique basal plane active sites and excellent catalytic properties. The introduction of A-site deficiency can help the formation of highly mobile oxygen vacancies and remarkably enhance the reducibility of Ni nanoparticles, thus significantly increasing electronic conductivity and catalytic activity simultaneously. Herein, we adopt pulsed electric current (PEC) treatment, a novel approach instead of the long-time high-temperature reduction technique, and for the first time review that the exsolution of minuscule Ni nanoparticles (8-20 nm) could be facilitated on Ni-doped La_0.52Sr_0.28Ti_0.94Ni_0.06O₃(LSTN) anodes with A-site deficiency. Encouragingly, finding that low PEC can successfully lead to nanoparticle exsolution and show a significantly improved oxygen evolution reaction performance of LSTN-PEC (LSTN after PEC treatment) possessing A-site deficiency, the onset potential of LSTN-PEC (500 V) (LSTN after PEC treatment with 500 V-4 Hz-90 s) was advanced by 0.173 V, theR_ctvalue was reduced by 82.38 Ω·cm², and the overpotential was also reduced by 73 mV.

Bulk and surface exsolution produces a variety of Fe-rich and Fe-depleted ellipsoidal nanostructures in La0.6Sr0.4FeO3 thin films

The past several years have seen a resurgence in the popularity of metal exsolution as an approach to synthesize advanced materials proposed for novel catalytic, magnetic, optical, and electrochemical properties. Whereas most studies to-date have focused on surface exsolution (motivated by catalysis), we instead report on the diversity of nanostructures formed in La_0.6Sr_0.4FeO₃ thin films during sub-surface or so-called 'bulk' exsolution, in addition to surface exsolution. Bulk exsolution is a promising approach to tuning the functionality of materials, yet there is little understanding of the nanostructures exsolved within the bulk and how they compare to those exsolved at gas-solid interfaces. This work combines atomic- and nano-scale imaging and spectroscopy techniques applied using a state-of-the-art aberration-corrected scanning transmission electron microscope (STEM). In doing so, we present a detailed atomic-resolution study of a range of Fe-rich and Fe-depleted nanostructures possible via exsolution, along with qualitative and quantitative chemical analysis of the exsolved nanostructures and oxide phases formed throughout the film. Local structural changes in the perovskite matrix, coinciding with nanostructure exsolution, are also characterized with atomic-resolution STEM imaging. Fe exsolution is shown to create local A-site rich domains of Ruddlesden-Popper phase, and some stages of this phase formation have been demonstrated in this work. In particular, phase boundaries are found to be the primary nucleation sites for bulk and surface exsolution, and the exsolved particles observed here tend to be ellipsoidal with shape factor of 1.4. We report a range of nanostructure types (core-shell, bulk core-shell, adjacent, and independent particles), revealing several possible avenues of future exploration aimed to understand the formation mechanism of each exsolution type and to develop their functionality. This work is thus relevant to materials scientists and engineers motivated to understand and utilize exsolution to synthesize materials with predictable nanostructures.

Probing into the In-Situ Exsolution Mechanism of Metal Nanoparticles from Doped Ceria Host

Exsolved nanoparticle catalysts have recently attracted broad research interest as they simultaneously combine the features of catalytic activity and chemical stability in various applications of energy conversion and storage. As the internal mechanism of in-situ exsolution is of prime significance for the optimization of its strategy, comprehensive research focused on the behaviors of in-situ segregation for metal (Mn, Fe, Co, Ni, Cu, Ag, Pt and Au)-substituted CeO2 is reported using first-principles calculations. An interesting link between the behaviors of metal growth from the ceria host and their microelectronic reconfigurations was established to understand the inherent attribute of metal self-regeneration, where a stair-stepping charge difference served as the inner driving force existing along the exsolving pathway, and the weak metal-coordinate associations synergistically facilitate the ceria's in-situ growth. We hope that these new insights provide a microscopic insight into the physics of in-situ exsolution to gain a guideline for the design of nanoparticle socketed catalysts from bottom to top.

Emergence and Future of Exsolved Materials

Supported nanoparticle systems have received increased attention over the last decades because of their potential for high activity levels when applied to chemical conversions, although, because of their nanoscale nature, they tend to exhibit problems with long-term durability. Over the last decade, the discovery of the so-called exsolution concept has addressed many of these challenges and opened many other opportunities to material design by providing a relatively simple, single-step, synthetic pathway to produce supported nanoparticles that combine high stability against agglomeration and poisoning with high activity across multiple areas of application. Here, the trends that define the development of the exsolution concept are reviewed in terms of design, functionality, tunability, and applicability. To support this, the number of studies dedicated to both fundamental and application-related studies, as well as the types of metallic nanoparticles and host or support lattices employed, are examined. Exciting future directions of research are also highlighted.

Nanoparticle Ex-solution for Supported Catalysts: Materials Design, Mechanism and Future Perspectives

Supported metal catalysts represent one of the major milestones in heterogeneous catalysis. Such catalytic systems are feasible for use in a broad range of applications, including renewable energy devices, sensors, automotive emission control systems, and chemical reformers. The lifetimes of these catalytic platforms depend strongly on the stability of the supported nanoparticles. With this regard, nanoparticles synthesized via ex-solution process emphasize exceptional robustness as they are socketed in the host oxide. Ex-solution refers to a phenomenon which yields selective growth of fine and uniformly distributed metal nanocatalysts on oxide supports upon partial reduction. This type of advanced structural engineering is a game-changer in the field of heterogeneous catalysis with numerous studies showing the benefits of ex-solution process. In this review, we highlight the latest research efforts regarding the origin of the ex-solution phenomenon and the mechanism underpinning particle formation. We also propose research directions to expand the utility and functionality of the current ex-solution techniques.

Unveiling the Interface Structure of the Exsolved Co-Fe Alloy Nanoparticles from Double Perovskite and Its Application in Solid Oxide Fuel Cells

Exsolution of catalytic nanoparticles (NPs) from perovskites has arisen as a flexible method to develop high-performance functional materials with enhanced durability for energy conversion and catalytic synthesis applications. Here, we unravel the interface structure of the in situ exsolved alloy nanoparticles from the double perovskite substrate on the atomic scale. The results show that the Co-Fe alloy NPs exsolved topologically from the {100} facets terminations of the Sr₂FeMo_0.65Co_0.35O_6-δ (SFMC) double perovskite along ⟨100⟩ directions exhibiting the same orientation and identical crystal structure. The lattice planes of these two phases align and insert into each other at the interface, forming a smooth and continuous coherent connection. The presence of moiré patterns at the interface confirms the topological exsolution mechanism. The coherent interface can significantly reduce the interfacial energy and therefore stabilize the exsolved nanoparticles. Therefore, excellent and stable electrochemical performance of the NP-decorated SFMC perovskite is observed as the anode for solid oxide fuel cells. Our contribution promotes a fundamental understanding of the interface structure of the in situ exsolved alloy nanoparticles from perovskite substrate.

Exsolution of Catalytically Active Iridium Nanoparticles from Strontium Titanate

The search for new functional materials that combine high stability and efficiency with reasonable cost and ease of synthesis is critical for their use in renewable energy applications. Specifically in catalysis, nanoparticles, with their high surface-to-volume ratio, can overcome the cost implications associated with otherwise having to use large amounts of noble metals. However, commercialized materials, that is, catalytic nanoparticles deposited on oxide supports, often suffer from loss of activity because of coarsening and carbon deposition during operation. Exsolution has proven to be an interesting strategy to overcome such issues. Here, the controlled emergence, or exsolution, of faceted iridium nanoparticles from a doped SrTiO₃ perovskite is reported and their growth preliminary probed by in situ electron microscopy. Upon reduction of SrIr_0.005Ti_0.995O₃, the generated nanoparticles show embedding into the oxide support, therefore preventing agglomeration and subsequent catalyst degradation. The advantages of this approach are the extremely low noble metal amount employed (∼0.5% weight) and the catalytic activity reported during CO oxidation tests, where the performance of the exsolved SrIr_0.005Ti_0.995O₃ is compared to the activity of a commercial catalyst with 1% loading (1% Ir/Al₂O₃). The high activity obtained with such low doping shows the possibility of scaling up this new catalyst, reducing the high cost associated with iridium-based materials.

Recent Advances of First d-Block Metal-Based Perovskite Oxide Electrocatalysts for Alkaline Water Splitting

First d-block metal-based perovskite oxides (FDMPOs) have garnered significant attention in research for their utilization in the water oxidation reaction due to their low cost, earth abundance, and promising activities. Recently, FDMPOs are being applied in electrocatalysis for the hydrogen evolution reaction (HER) and overall water splitting reaction. Numerous promising FDMPO-based water splitting electrocatalysts have been reported, along with new catalytic mechanisms. Therefore, an in-time summary of the current progress of FDMPO-based water splitting electrocatalysts is now considered imperative. However, few reviews have focused on this particular subject thus far. In this contribution, we review the most recent advances (mainly within the years 2014–2020) of FDMPO electrocatalysts for alkaline water splitting, which is widely considered to be the most promising next-generation technology for future large-scale hydrogen production. This review begins with an introduction describing the fundamentals of alkaline water electrolysis and perovskite oxides. We then carefully elaborate on the various design strategies used for the preparation of FDMPO electrocatalysts applied in the alkaline water splitting reaction, including defecting engineering, strain tuning, nanostructuring, and hybridization. Finally, we discuss the current advances of various FDMPO-based water splitting electrocatalysts, including those based on Co, Ni, Fe, Mn, and other first d-block metal-based catalysts. By conveying various methods, developments, perspectives, and challenges, this review will contribute toward the understanding and development of FDMPO electrocatalysts for alkaline water splitting.

Exceptional Tunability over Size and Density of Spontaneously Formed Nanoparticles via Nucleation Dynamics

The ex-solution process, in which metal nanoparticles are grown on a host oxide, can be used to synthesize nanocatalysts with excellent thermal and chemical durability via spontaneous heterogeneous nucleation. However, this technique lacks a means to control the particle size and density because the amounts of ex-solved metal elements vary with the reduction conditions. Here, we devise a strategy to achieve small particle sizes and high particle densities concurrently by controlling the temperature (T), oxygen partial pressure (pO₂) and ramping rate of the temperature. Quantitative analyses of Co particles ex-solved on Sr_0.98Ti_0.95Co_0.05O_3-δ thin films using ex situ SEM and in situ TEM reveal that the increasing T and decreasing the pO₂ lead to smaller particle sizes with higher density levels and vice versa, contrary to common ex-solution examples. We find that nucleation thermodynamics dictates such counterintuitive behaviors of particle characteristics, which are attributed to our specific ex-solution conditions in which particle interactions are minimized and all the Co atoms are ex-solved under highly reducible conditions. We also demonstrated the feasibility of our strategy via CO oxidation with typical powder-based catalysts, suggesting that this method can be extended to various chemical/electrochemical applications.

Metal Nanoparticle Exsolution on a Perovskite Stannate Support with High Electrical Conductivity

In situ exsolution of metal nanoparticles (NPs) is emerging as an alternative technique to deliver thermally stable and evenly dispersed metal NPs, which exhibit excellent adhesion with conducting perovskite oxide supports. Here we provide the first demonstration that Ni metal NPs with high areal density (∼175 μm^-2) and fine size (∼38.65 nm) are exsolved from an A-site-deficient perovskite stannate support (La_0.2Ba_0.7Sn_0.9Ni_0.1O_3-δ (LBSNO)). The NPs are strongly anchored and impart coking resistance, and the Ni-exsolved stannates show exceptionally high electrical conductivity (∼700 S·cm^-1). The excellent conductivity is attributed to conduction between delocalized Sn 5s orbitals along with structural improvement toward ABO₃ stoichiometry in the stannate support. We also reveal that experimental conditions with strong interaction must be optimized to obtain Ni exsolution without degrading the perovskite stannate framework. Our finding suggests a unique process to induce the formation of metal NPs embedded in stannate with excellent electrical properties.

Progress and Opportunities for Exsolution in Electrochemistry

This perspective gives the reader a broad overview of the progress that has been made in understanding the physics of the exsolution process and its exploitation in electrochemical devices in the last five years. On the basis of this progress, the community is encouraged to pursue unreported and under-reported opportunities for the advancement of exsolution in electrochemical applications through new materials discovery.

Modifying the Surface Structure of Perovskite-Based Catalysts by Nanoparticle Exsolution

In heterogeneous catalysis, surfaces decorated with uniformly dispersed, catalytically-active (nano)particles are a key requirement for excellent performance. Beside standard catalyst preparation routines—with limitations in controlling catalyst surface structure (i.e., particle size distribution or dispersion)—we present here a novel time efficient route to precisely tailor catalyst surface morphology and composition of perovskites. Perovskite-type oxides of nominal composition ABO3 with transition metal cations on the B-site can exsolve the B-site transition metal upon controlled reduction. In this exsolution process, the transition metal emerges from the oxide lattice and migrates to the surface where it forms catalytically active nanoparticles. Doping the B-site with reducible and catalytically highly active elements, offers the opportunity of tailoring properties of exsolution catalysts. Here, we present the synthesis of two novel perovskite catalysts Nd0.6Ca0.4FeO3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ with characterisation by (in situ) XRD, SEM/TEM and XPS, supported by theory (DFT+U). Fe nanoparticle formation was observed for Nd0.6Ca0.4FeO3-δ. In comparison, B site cobalt doping leads, already at lower reduction temperatures, to formation of finely dispersed Co nanoparticles on the surface. These novel perovskite-type catalysts are highly promising for applications in chemical energy conversion. First measurements revealed that exsolved Co nanoparticles significantly improve the catalytic activity for CO2 activation via reverse water gas shift reaction.

Enhanced Anode Performance and Coking Resistance by In Situ Exsolved Multiple-Twinned Co-Fe Nanoparticles for Solid Oxide Fuel Cells

The broad and large-scale application of solid oxide fuel cells (SOFCs) technology hinges significantly on the development of highly active and robust electrode materials. Here, Ni-free anode materials decorated with metal nanoparticles are synthesized by in situ reduction of Fe-doping Sr₂CoMo_1-xFe_xO_6-δ (x = 0, 0.05, 0.1) double perovskite oxides under a reducing condition at 850 °C. The exsolved nanoparticles from the Sr₂CoMo_0.95Fe_0.05O_6-δ (SCMF0.05) lattice are Co-Fe alloys with rich multiple-twinned defects, significantly enhancing the catalytic activity of the SCMF0.05 anode toward the oxidation of H₂ and CH₄. The electrolyte-supported single cell with the reuduced SCMF0.05 anode reaches peak power densities as high as 992.9 and 652.3 mW cm^-2 in H₂ and CH₄ at 850 °C, respectively, while maintaining superior stability (∼50 h at 700 °C). The reduced SCMF0.05 anode also demonstrates excellent coking resistance in CH₄, which can be attributed to the increased oxygen vacancies due to Fe doping and the effective catalysis of multiple-twinned Co-Fe alloy nanoparticles for reforming of CH₄ to H₂ and CO. The findings in this work may provide a new insight for the design of highly active and durable anode catalysts in SOFCs.

In Situ Observation of Nanoparticle Exsolution from Perovskite Oxides: From Atomic Scale Mechanistic Insight to Nanostructure Tailoring

Understanding and controlling the formation of nanoparticles at the surface of functional oxide supports is critical for tuning activity and stability for catalytic and energy conversion applications. Here, we use a latest generation environmental transmission electron microscope to follow the exsolution of individual nanoparticles at the surface of perovskite oxides, with ultrahigh spatial and temporal resolution. Qualitative and quantitative analysis of the data reveals the atomic scale processes that underpin the formation of the socketed, strain-inducing interface that confers exsolved particles their exceptional stability and reactivity. This insight also enabled us to discover that the shape of exsolved particles can be controlled by changing the atmosphere in which exsolution is carried out, and additionally, this could also produce intriguing heterostructures consisting of metal-metal oxide coupled nanoparticles. Our results not only provide insight into the in situ formation of nanoparticles but also demonstrate the tailoring of nanostructures and nanointerfaces.

Towards efficient use of noble metals via exsolution exemplified for CO oxidation

Many catalysts and in particular automotive exhaust catalysts usually consist of noble metal nanoparticles dispersed on metal oxide supports. While highly active, such catalysts are expensive and prone to deactivation by sintering and thus alternative methods for their production are being sought to ensure more efficient use of noble metals. Exsolution has been shown to be an approach to produce confined nanoparticles, which in turn are more stable against agglomeration, and, at the same time strained, displaying enhanced activity. While exsolution has been extensively investigated for relatively high metal loadings, it has yet to be explored for dilute loadings which is expected to be more challenging but more suitable for application of noble metals. Here we explore the substitution of Rh into an A-site deficient perovskite titante aiming to investigate the possibility of exsolving from dilute amounts of noble metal substituted perovskites. We show that this is possible and in spite of certain limitations, they still compete well against conventionally prepared samples with higher apparent surface loading when applied for CO oxidation.

Growth Kinetics of Individual Co Particles Ex-solved on SrTi0.75Co0.25O3-δ Polycrystalline Perovskite Thin Films

A precise control of the size, density, and distribution of metal nanoparticles dispersed on functional oxide supports is critical for promoting catalytic activity and stability in renewable energy and catalysis devices. Here, we measure the growth kinetics of individual Co particles ex-solved on SrTi_0.75Co_0.25O_3-δ polycrystalline thin films under a high vacuum, and at various temperatures and grain sizes using in situ transmission electron microscopy. The ex-solution preferentially occurs at grain boundaries and corners which appear essential for controlling particle density and distribution, and enabling low temperature ex-solution. The particle reaches a saturated size after a few minutes, and the size depends on temperature. Quantitative measurements with a kinetic model determine the rate limiting step, vacancy formation enthalpy, ex-solution enthalpy, and activation energy for particle growth. The ex-solved particles are tightly socketed, preventing interactions among them over 800 °C. Furthermore, we obtain the first direct clarification of the active reaction site for CO oxidation-the Co-oxide interface, agreeing well with density functional theory calculations.