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

How reduction temperature influences the structure of perovskite-oxide catalysts during the dry reforming of methane

Dry reforming of methane is a promising reaction to convert CO2 and combat climate change. However, the reaction is still not feasible in large-scale industrial applications. The thermodynamic need for high temperatures and the potential of carbon deposition leads to high requirements for potential catalyst materials. As shown in previous publications, the Ni-doped perovskite-oxide Nd0.6Ca0.4Fe0.97Ni0.03O3 is a potential candidate as it can exsolve highly active Ni nanoparticles on its surface. This study focused on controlling the particle size by varying the reduction temperature. We found the optimal temperature that allows the Ni nanoparticles to exsolve while not yet enabling the formation of deactivating CaCO3. Furthermore, the exsolution process and the behaviour of the phases during the dry reforming of methane were investigated using in situ XRD measurements at the DESY beamline P02.1 at PETRA III in Hamburg. They revealed that the formed deactivated phases would, at high temperatures, form a brownmillerite phase, thus hinting at a potential self-healing mechanism of these materials.

Atomic-Scale Behavior of Perovskite-Supported Ir-Pd-Ru Nanoparticles under Redox Atmospheres

An advanced materials solution utilizing the concept of "smart catalysts" could be a game changer for today's automotive emission control technology, enabling the efficient use of precious metals via their two-way switching between metallic nanoparticle forms and ionic states in the host perovskite lattice as a result of the cyclical oxidizing/reducing atmospheres. However, direct evidence for such processes remains scarce; therefore, the underlying mechanism has been an unsettled debate. Here, we use advanced scanning transmission electron microscopy to reveal the atomic-scale behaviors for a LaFe0.95Pd0.05O3-supported Ir-Pd-Ru nanocatalyst under fluctuating redox conditions, thereby proving the reversible dissolution/exsolution for Ir and Ru but with a limited occurrence for Pd. Despite such selective dissolution during oxidation, all three elements remain cooperatively alloyed in the subsequent reduction, which is a key factor in preserving the catalytic activity of the ternary nanoalloy while displaying its self-regenerating functionality and control of particle agglomeration.

Dielectric Barrier Plasma Discharge Exsolution of Nanoparticles at Room Temperature and Atmospheric Pressure

Exsolution of metal nanoparticles (NPs) on perovskite oxides has been demonstrated as a reliable strategy for producing catalyst-support systems. Conventional exsolution requires high temperatures for long periods of time, limiting the selection of support materials. Plasma direct exsolution is reported at room temperature and atmospheric pressure of Ni NPs from a model A-site deficient perovskite oxide (La0.43Ca0.37Ni0.06Ti0.94O2.955). Plasma exsolution is carried out within minutes (up to 15 min) using a dielectric barrier discharge configuration both with He-only gas as well as with He/H2 gas mixtures, yielding small NPs (<30 nm diameter). To prove the practical utility of exsolved NPs, various experiments aimed at assessing their catalytic performance for methanation from synthesis gas, CO, and CH4 oxidation are carried out. Low-temperature and atmospheric pressure plasma exsolution are successfully demonstrated and suggest that this approach could contribute to the practical deployment of exsolution-based stable catalyst systems.

Squeezing Out Nanoparticles from Perovskites: Controlling Exsolution with Pressure

Nanoparticle exsolution has emerged as a versatile method to functionalize oxides with robust metallic nanoparticles for catalytic and energy applications. By modifying certain external parameters during thermal reduction (temperature, time, reducing gas), some morphological and/or compositional properties of the exsolved nanoparticles can be tuned. Here, it is shown how the application of high pressure (<100 bar H2) enables the control of the exsolution of ternary FeCoNi alloyed nanoparticles from a double perovskite. H2 pressure affects the lattice expansion and the nanoparticle characteristics (size, population, and composition). The composition of the alloyed nanoparticles could be controlled, showing a reversal of the expected thermodynamic trend at 10 and 50 bar, where Fe becomes the main component instead of Ni. In addition, pressure drastically lowers the exsolution temperature to 300 °C, resulting in unprecedented highly-dispersed and small-sized nanoparticles with a similar composition to those obtained at 600 °C and 10 bar. The mechanisms behind the effects of pressure on exsolution are discussed, involving kinetic, surface thermodynamics, and lattice-strain factors. A volcano-like trend of the exsolution extent suggests that competing pressure-dependent mechanisms govern the process. Pressure emerges as a new design tool for metallic nanoparticle exsolution enabling novel nanocatalysts and surface-functionalized materials.

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.

Electro-tuned catalysts: voltage-controlled activity selection of bimetallic exsolution particles

In this work, we show how the activity states of bimetallic Ni0-Fe0 catalysts exsolved from Nd0.6Ca0.4Fe0.97Ni0.03O3-δ (NCFNi) can be influenced electrochemically. The NCFNi parent oxide was employed in the form of thin film mixed conducting model electrodes, which were operated in a humid hydrogen atmosphere. By precisely controlling the oxygen chemical potential in the parent oxide electrode via applying an electrochemical polarisation, we managed to selectively exsolve Ni nanoparticles from the perovskite lattice and study their catalytic activity switching characteristics. To be able to track the surface chemical changes during the switching process, electrochemical polarisation experiments were combined with near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) measurements. This in situ analytical approach allowed relating the difference we observed in the switching behaviour of Ni particles here and of Fe particles in a recent study, to a different kinetic interplay between electrochemical driving force and atmosphere. We propose that slow oxygen transport through nickel oxide, located at the particle/perovskite interface, is mainly responsible for the observed difference to iron exsolutions, which becomes especially evident for larger nickel particles. In addition, in the case of bimetallic exsolutions and with applied bias voltage as a control parameter, we are able to reversibly switch between three different activity states, namely bimetallic Ni0-Fe0 (medium activity), pure Ni0 (high activity), and the inactive oxides.

Challenges and Advancements in the Electrochemical Utilization of Ammonia Using Solid Oxide Fuel Cells

Solid oxide fuel cells utilized with NH3 (NH3-SOFCs) have great potential to be environmentally friendly devices with high efficiency and energy density. The advancement of this technology is hindered by the sluggish kinetics of chemical or electrochemical processes occurring on anodes/catalysts. Extensive efforts have been devoted to developing efficient and durable anode/catalysts in recent decades. Although modifications to the structure, composition, and morphology of anodes or catalysts are effective, the mechanistic understandings of performance improvements or degradations remain incompletely understood. This review informatively commences by summarizing existing reports on the progress of NH3-SOFCs. It subsequently outlines the influence of factors on the performance of NH3-SOFCs. The degradation mechanisms of the cells/systems are also reviewed. Lastly, the persistent challenges in designing highly efficient electrodes/catalysts for low-temperature NH3-SOFCs, and future perspectives derived from SOFCs are discussed. Notably, durability, thermal cycling stability, and power density are identified as crucial indicators for enhancing low-temperature (550 °C or below) NH3-SOFCs. This review aims to offer an updated overview of how catalysts/electrodes affect electrochemical activity and durability, offering critical insights for improving performance and mechanistic understanding, as well as establishing the scientific foundation for the design of electrodes for NH3-SOFCs.

New trends in nanoparticle exsolution

Many relevant high-temperature chemical processes require the use of oxide-supported metallic nanocatalysts. The harsh conditions under which these processes operate can trigger catalyst degradation via nanoparticle sintering, carbon depositions or poisoning, among others. This primarily affects metallic nanoparticles created via deposition methods with low metal-support interaction. In this respect, nanoparticle exsolution has emerged as a promising method for fabricating oxide-supported nanocatalysts with high interaction between the metal and the oxide support. This is due to the mechanism involved in nanoparticle exsolution, which is based on the migration of metal cations in the oxide support to its surface, where they nucleate and grow as metallic nanoparticles partially embedded in the oxide. This anchorage confers high robustness against sintering or coking-related problems. For these reasons, exsolution has attracted great interest in the last few years. Multiple works have been devoted to proving the high catalytic stability of exsolved metallic nanoparticles in several applications for high-temperature energy storage and conversion. Additionally, considerable attention has been directed towards understanding the underlying mechanism of metallic nanoparticle exsolution. However, this growing field has not been limited to these types of studies and recent discoveries at the forefront of materials design have opened new research avenues. In this work, we define six new trends in nanoparticle exsolution, taking a tour through the most important advances that have been recently reported.

Advancements and prospects of perovskite-based fuel electrodes in solid oxide cells for CO2 electrolysis to CO

Carbon dioxide (CO2) electrolysis to carbon monoxide (CO) is a very promising strategy for economically converting CO2, with high-temperature solid oxide electrolysis cells (SOECs) being regarded as the most suitable technology due to their high electrode reaction kinetics and nearly 100% faradaic efficiency, while their practical application is highly dependent on the performance of their fuel electrode (cathode), which significantly determines the cell activity, selectivity, and durability. In this review, we provide a timely overview of the recent progress in the understanding and development of fuel electrodes, predominantly based on perovskite oxides, for CO2 electrochemical reduction to CO (CO2RR) in SOECs. Initially, the current understanding of the reaction mechanisms over the perovskite electrocatalyst for CO synthesis from CO2 electrolysis in SOECs is provided. Subsequently, the recent experimental advances in fuel electrodes are summarized, with importance placed on perovskite oxides and their modification, including bulk doping with multiple elements to introduce high entropy effects, various methods for realizing surface nanoparticles or even single atom catalyst modification, and nanocompositing. Additionally, the recent progress in numerical modeling-assisted fast screening of perovskite electrocatalysts for high-temperature CO2RR is summarized, and the advanced characterization techniques for an in-depth understanding of the related fundamentals for the CO2RR over perovskite oxides are also reviewed. The recent pro-industrial application trials of the CO2RR in SOECs are also briefly discussed. Finally, the future prospects and challenges of SOEC cathodes for the CO2RR are suggested.

In Situ Self-Assembled Active and Stable Ir@MnOx/La0.7Sr0.3Cr0.9Ir0.1O3-δ Interfaces for CO2 Electrolysis

Solid oxide electrolysis cells are prospective approaches for CO2 utilization but face significant challenges due to the sluggish reaction kinetics and poor stability of the fuel electrodes. Herein, we strategically addressed the long-standing trade-off phenomenon between enhanced exsolution and improved structural stability via topotactic ion exchange. The surface dynamic reconstruction of the MnOx/La0.7Sr0.3Cr0.9Ir0.1O3-δ (LSCIr) catalyst was visualized at the atomic scale. Compared with the Ir@LSCIr interface, the in situ self-assembled Ir@MnOx/LSCIr interface exhibited greater CO2 activation and easily removable carbonate intermediates, thus reached a 42 % improvement in CO2 electrolysis performance at 1.6 V. Furthermore, an improved CO2 electrolysis stability was achieved due to the uniformly wrapped MnOx shell of the Ir@MnOx/LSCIr cathode. Our approach enables a detailed understanding of the dynamic microstructure evolution at active interfaces and provides a roadmap for the rational design and evaluation of efficient metal/oxide catalysts for CO2 electrolysis.

Electrocatalysis in Solid Oxide Fuel Cells and Electrolyzers

Interest in energy-to-X and X-to-energy (where X represents green hydrogen, carbon-based fuels, or ammonia) technologies has expanded the field of electrochemical conversion and storage. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for these processes. Their unmatched conversion efficiencies result from favorable thermodynamics and kinetics at elevated operating temperatures (400-900 °C). These solid-state electrochemical systems exhibit flexibility in reversible operation between fuel cell and electrolysis modes and can efficiently utilize a variety of fuels. However, electrocatalytic materials at SOC electrodes remain nonoptimal for facilitating reversible operation and fuel flexibility. In this Review, we explore the diverse range of electrocatalytic materials utilized in oxygen-ion-conducting SOCs (O-SOCs) and proton-conducting SOCs (H-SOCs). We examine their electrochemical activity as a function of composition and structure across different electrochemical reactions to highlight characteristics that lead to optimal catalytic performance. Catalyst deactivation mechanisms under different operating conditions are discussed to assess the bottlenecks in performance. We conclude by providing guidelines for evaluating the electrochemical performance of electrode catalysts in SOCs and for designing effective catalysts to achieve flexibility in fuel usage and mode of operation.

Ammonolysis-Driven Exsolution of Ru Nanoparticle Embedded in Conductive Metal Nitride Matrix to Boost Electrocatalyst Activity

Exsolution is an effective method for synthesizing robust nanostructured metal-based functional materials. However, no studies have investigated the exsolution of metal nanoparticles into metal nitride substrates. In this study, a versatile nitridation-driven exsolution method is developed for embedding catalytically active metal nanoparticles in conductive metal nitride substrates via the ammonolysis of multimetallic oxides. Using this approach, Ti1-xRuxO2 nanowires are phase-transformed into holey TiN nanotubes embedded with exsolved Ru nanoparticles. These Ru-exsolved holey TiN nanotubes exhibit outstanding electrocatalytic activity for the hydrogen evolution reaction with excellent durability, which is significantly higher than that of Ru-deposited TiN nanotubes. The enhanced stability of the Ru-exsolved TiN nanotubes can be attributed to the Ru nanoparticles embedded in the robust metal nitride matrix and the formation of interfacial Ti3+─N─Ru4+ bonds. Density functional theory calculations reveal that the exsolved Ru nanoparticles have a lower d-band center position and optimized hydrogen affinity than deposited Ru nanoparticles, indicating the superior electrocatalyst performance of the former. In situ Raman spectroscopic analysis reveals that the electron transfer from TiN to Ru nanoparticles is enhanced during the electrocatalytic process. The proposed approach opens a new avenue for stabilizing diverse metal nanostructures in many conductive matrices like metal phosphides and chalcogenides.

Maximizing Photoelectrochemical Performance in Metal-Oxide Hybrid Composites via Amorphous Exsolution-A New Exsolution Mechanism for Heterogeneous Catalysis

Exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle-perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond's low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

Local Structures of Ex-Solved Nanoparticles Identified by Machine-Learned Potentials

In this study, we identify the local structures of ex-solved nanoparticles using machine-learned potentials (MLPs). We develop a method for training machine-learned potentials by sampling local structures of heterointerface configurations as a training set with its efficacy tested on the Ni/MgO system, illustrating that the error in interface energy is only 0.004 eV/Å2. Using the developed scheme, we train an MLP for the Ni/La0.5Ca0.5TiO3 ex-solution system and identify the local structures for both exo- and endo-type particles. The established model aligns well with the experimental observations, accurately predicting a nucleation size of 0.45 nm. Lastly, the density functional theory calculations on the established atomistic model verify that the kinetic barrier for the dry reforming of methane are substantially reduced by 0.49 eV on the ex-solved catalysts compared to that on the impregnated catalysts. Our findings offer insights into the local structures, growth mechanisms, and underlying origin of the catalytic properties of ex-solved nanoparticles.

Comparative Study of Exsolved and Impregnated Ni Nanoparticles Supported on Nanoporous Perovskites for Low-Temperature CO Oxidation

This study investigated the redox exsolution of Ni nanoparticles from a nanoporous La0.52Sr0.28Ti0.94Ni0.06O3 perovskite. The characteristics of exsolved Ni nanoparticles including their size, population, and surface concentration were deeply analyzed by environmental scanning electron microscopy (ESEM), transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR). Ni exsolution was triggered in hydrogen as early as 400 °C, with the highest catalytic activity for low-temperature CO oxidation achieved after a reduction step at 500 °C, despite only a 10% fraction of Ni exsolved. The activity and stability of exsolved nanoparticles were compared with their impregnated counterparts on a perovskite material with a similar chemical composition (La0.65Sr0.35TiO3) and a comparable specific surface area and Ni loading. After an aging step at 800 °C, the catalytic activity of exsolved Ni nanoparticles at 300 °C was found to be 10 times higher than that of impregnated ones, emphasizing the thermal stability of Ni nanoparticles prepared by redox exsolution.

Fe-Ni-based alloys as highly active and low-cost oxygen evolution reaction catalyst in alkaline media

NiFe-based oxo-hydroxides are highly active for the oxygen evolution reaction but require complex synthesis and are poorly durable when deposited on foreign supports. Herein we demonstrate that easily processable, Earth-abundant and cheap Fe-Ni alloys spontaneously develop a highly active NiFe oxo-hydroxide surface, exsolved upon electrochemical activation. While the manufacturing process and the initial surface state of the alloys do not impact the oxygen evolution reaction performance, the growth/composition of the NiFe oxo-hydroxide surface layer depends on the alloying elements and initial atomic Fe/Ni ratio, hence driving oxygen evolution reaction activity. Whatever the initial Fe/Ni ratio of the Fe-Ni alloy (varying between 0.004 and 7.4), the best oxygen evolution reaction performance (beyond that of commercial IrO2) and durability was obtained for a surface Fe/Ni ratio between 0.2 and 0.4 and includes numerous active sites (high NiIII/NiII capacitive response) and high efficiency (high Fe/Ni ratio). This knowledge paves the way to active and durable Fe-Ni alloy oxygen-evolving electrodes for alkaline water electrolysers.

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.

Nanotechnologies in ceramic electrochemical cells

Although they are emerging technologies for achieving high-efficiency and green and eco-friendly energy conversion, ceramic electrochemical cells (CECs), i.e. solid oxide electrolysis cells (SOECs) and fuel cells (SOFCs), are still fundamentally limited by their inferior catalytic activities at low temperature, poor thermo-mechanical stability, high material cost, etc. The materials used in electrolytes and electrodes, which are the most important components in CECs, are highly associated with the cell performances. Therefore, rational design of electrolytes and electrodes with excellent catalytic activities and high stabilities at relatively low cost is a meaningful and valuable approach for the development of CECs. Nanotechnology is a powerful tool for improving the material performances in CECs owing to the favourable effects induced by the nanocrystallization of electrolytes and electrodes. Herein, a relatively comprehensive review on the nanotechnologies implemented in CECs is conducted. The working principles of CECs and the corresponding challenges were first presented, followed by the comprehensive insights into the working mechanisms of nanocrystalline materials in CECs. Then, systematic summarization and analyses of the commonly used nano-engineering strategies in the fabrication of CEC materials, including physical and chemical methods, were provided. In addition, the frontiers in the research of advanced electrolyte and electrode materials were discussed with a special emphasis on the modified electrochemical properties derived from nanotechnologies. Finally, the bottlenecks and the promising breakthroughs in nanotechnologies were highlighted in the direction of providing useful references for rational design of nanomaterials for CECs.

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.

Co-Exsolution of Ni-Based Alloy Catalysts for the Valorization of Carbon Dioxide and Methane

ConspectusThe reversible coexsolution mechanism of perovskite oxides is emerging as an alternative method for synthesizing alloy catalyst nanoparticles. Co-exsolution is a partial decomposition process where multiple B cations diffuse from the bulk of a solid precursor and nucleate on the surface. The unique properties of exsolved alloy catalysts, including improved dispersion, thermal stability, and compositional malleability, make them particularly useful for converting CO2 into chemical commodities and fuels. However, the coexsolution of alloys is still in development, and fundamental insights into the alloying mechanism, formation of nanoparticles, and defect chemistry are needed.This Account examines the solid-state chemistry of perovskite oxide precursors and reaction parameters that can be altered to control the assembly or exsolution of Ni-based alloys. The characteristics of bulk perovskite oxide precursors heavily influence the exsolved alloy catalyst nanoparticle assembly, growth, and composition. Inherent defects, such as oxygen vacancies and grain boundaries, primarily facilitate the transport of catalytic B-cation dopants from the bulk to the surface. An example of how bulk defects can affect the properties of Ni-based alloy catalysts is demonstrated through the formation of NiFe from La(Fe, Ni)O3. The A/B cation ratio plays a significant role in determining the size and composition of NiFe nanoparticles, which directly impacts their catalytic performance. Using in situ X-ray absorption spectroscopy (in situ XAS), the dynamic behavior of exsolved NiFe nanoparticles can be observed in different reaction environments (oxidation, reduction, and dry reforming of methane) by tracking the oxidation state and local environment of the Ni K-edge and Fe K-edge using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), respectively. Time-resolved experiments with in situ XAS showed that NiFe nanoparticle growth starts at ∼280 °C and transforms from predominantly Ni to NiFe at higher reduction times and temperatures.The challenges of exsolution of higher-order Ni-based alloys, such as 3(NiFeCo), 4(NiCoCuPd), and 5(NiFeCoCuPd) element nanoparticles, to improve the catalyst properties are discussed. The size, concentration, and reducibility of the dopant cation can alter the exsolution kinetics, alloy nanoparticle growth dynamics, and catalyst performance. The size and composition of exsolved Ni-based alloys affect the effectiveness of catalysts in the dry reforming of methane. Large NiFeCo nanoparticles separated from Pd and Cu can lead to catalyst deactivation, but using a complex alloy with smaller NiFeCoPdCu nanoparticles results in a stable performance. The use of in situ XANES reveals how the dry reforming of methane reaction conditions can induce changes in the NiFe with the rapid redissolution of Fe back into the lattice.The dynamicity of the exsolved Ni-based alloy nanoparticles and implications for their regeneration after aging or exposure to waste gas contaminants are discussed. Finally, we summarize the Account and provide promising future directions.

Quantifying Electrochemical Driving Force for Exsolution in Perovskite Oxides by Designing Graded Oxygen Chemical Potential

Metal nanoparticles exsolved and anchored at the parent perovskite oxide surfaces can greatly enhance the activity and antisintering stability for high-temperature (electro-) chemical catalytic reactions. While exsolution of nanoparticles triggered by using conventional high-temperature thermal reduction suffers from slow kinetics, using an electrochemical driving force can promote the exsolution rate. However, a quantitative correlation between the applied electrochemical driving force and the spatial density of exsolved nanoparticles remains unknown. In this work, we use a specially designed electrochemical device to induce a spatially graded voltage in a La_0.43Ca_0.37Ti_0.94Ni_0.06O_3-δ electrode, in order to systematically investigate the effect of electrochemical switching on exsolution. With increasing driving force, which leads to decreasing oxygen chemical potential, the density of nanoparticles was observed to increase dramatically, while the average particle size remained roughly constant. We further identified oxygen vacancy pairs or clusters as the preferential nucleation sites for exsolution. Our work provided a high-throughput platform for the systematic study of exsolution of perovskite oxides targeted for fuel electrode materials with improved electrocatalytic performance and stability.

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.

Use of A-Site Metal Exsolution from a Hydrated Perovskite Titanate for Combined Steam and CO2 Reforming of Methane

Metal segregation from a perovskite oxide (ABO₃) usually referring to "redox metal exsolution" has recently been used for in situ preparation of a well-designed catalyst where metal nanoparticles are homogeneously and strongly embedded on perovskite scaffolds upon reduction. The exsolution concept of B-site transition metal ions has grown, but several issues such as segregation of A-site alkaline-earth metal ions (altering electronic structures of the perovskite surface, causing deformation of perovskite structures, or creating undesirable products via side reactions) and carbon formations on metal nanoparticles should be addressed for stable catalysts in greenhouse gas (CO₂ or CH₄) conversion. Here, we suggest a new approach to designing metal-perovskite composite catalysts via A-site metal segregation from a hydrated perovskite titanate. In situ formation of A-site-deficient hydrated CaTiO₃ accompanied with Ni exsolution solids leads to ∼78 and 65% of CH₄ and CO₂ conversion, respectively, suppressing carbon formations and alkaline-earth metal segregations in combined steam and carbon dioxide reforming of methane at 700 °C. It would help to design active and stable metal-perovskite catalysts for energy and environmental 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.

Advancing Electrode Properties through Functionalization for Solid Oxide Cells Application: A Review

Hydrogen energy has emerged as the only renewable which is capable of sustaining the prevalent energy crisis in conjunction with other intermittent sources. In this connection, solid oxide cell (SOC) is the most sustainable solid-state devices capable of recycling and reproducing green hydrogen fuel. It is operable in reversible modes viz, fuel cell (FC) and electrolysis cell (EC). SOC is capable of engaging multiple fuels thereby promoting carbon neutral planet. The all-solid design further augments the optimization of cost, efficiency, durability and endurance at higher temperature. Electrodes are therefore, an important component which is responsible for electrocatalytic processing of fuel and oxidant for higher recyclability of cell/stack. The present review article embarks a detailed overview on the past and present status of electrode composition, heterointerface engineering applicable for SOC's. Recent trends in electrode engineering and the possibilities for advancement in SOC is also reviewed with respect to both experimental and computational aspects.

Exsolution of Ru Nanoparticles on BaCe0.9 Y0.1 O3-δ Modifying Geometry and Electronic Structure of Ru for Ammonia Synthesis Reaction Under Mild Conditions

Green ammonia is an efficient, carbon-free energy carrier and storage medium. The ammonia synthesis using green hydrogen requires an active catalyst that operates under mild conditions. The catalytic activity can be promoted by controlling the geometry and electronic structure of the active species. An exsolution process is implemented to improve catalytic activity by modulating the geometry and electronic structure of Ru. Ru nanoparticles exsolved on a BaCe_0.9 Y_0.1 O_3-δ support exhibit uniform size distribution, 5.03 ± 0.91 nm, and exhibited one of the highest activities, 387.31 mmol_NH3  g_Ru ^-1  h^-1 (0.1 MPa and 450 °C). The role of the exsolution and BaCe_0.9 Y_0.1 O_3-δ support is studied by comparing the catalyst with control samples and in-depth characterizations. The optimal nanoparticle size is maintained during the reaction, as the Ru nanoparticles prepared by exsolution are well-anchored to the support with in-plane epitaxy. The electronic structure of Ru is modified by unexpected in situ Ba promoter accumulation around the base of the Ru nanoparticles.

Nanoparticle Exsolution from Nanoporous Perovskites for Highly Active and Stable Catalysts

Nanoporosity is clearly beneficial for the performance of heterogeneous catalysts. Although exsolution is a modern method to design innovative catalysts, thus far it is predominantly studied for sintered matrices. A quantitative description of the exsolution of Ni nanoparticles from nanoporous perovskite oxides and their effective application in the biogas dry reforming is here presented. The exsolution process is studied between 500 and 900 °C in nanoporous and sintered La0.52 Sr0.28 Ti0.94 Ni0.06 O3±δ . Using temperature-programmed reduction (TPR) and X-ray absorption spectroscopy (XAS), it is shown that the faster and larger oxygen release in the nanoporous material is responsible for twice as high Ni reduction than in the sintered system. For the nanoporous material, the nanoparticle formation mechanism, studied by in situ TEM and small-angle X-ray scattering (SAXS), follows the classical nucleation theory, while on sintered systems also small endogenous nanoparticles form despite the low Ni concentration. Biogas dry reforming tests demonstrate that nanoporous exsolved catalysts are up to 18 times more active than sintered ones with 90% of CO2 conversion at 800 °C. Time-on-stream tests exhibit superior long-term stability (only 3% activity loss in 8 h) and full regenerability (over three cycles) of the nanoporous exsolved materials in comparison to a commercial Ni/Al2 O3 catalyst.

Fast Surface Oxygen Release Kinetics Accelerate Nanoparticle Exsolution in Perovskite Oxides

Exsolution is a recent advancement for fabricating oxide-supported metal nanoparticle catalysts via phase precipitation out of a host oxide. A fundamental understanding and control of the exsolution kinetics are needed to engineer exsolved nanoparticles to obtain higher catalytic activity toward clean energy and fuel conversion. Since oxygen release via oxygen vacancy formation in the host oxide is behind oxide reduction and metal exsolution, we hypothesize that the kinetics of metal exsolution should depend on the kinetics of oxygen release, in addition to the kinetics of metal cation diffusion. Here, we probe the surface exsolution kinetics both experimentally and theoretically using thin-film perovskite SrTi_0.65Fe_0.35O₃ (STF) as a model system. We quantitatively demonstrated that in this system the surface oxygen release governs the metal nanoparticle exsolution kinetics. As a result, by increasing the oxygen release rate in STF, either by reducing the sample thickness or by increasing the surface reactivity, one can effectively accelerate the Fe⁰ exsolution kinetics. Fast oxygen release kinetics in STF not only shortened the prereduction time prior to the exsolution onset, but also increased the total quantity of exsolved Fe⁰ over time, which agrees well with the predictions from our analytical kinetic modeling. The consistency between the results obtained from in situ experiments and analytical modeling provides a predictive capability for tailoring exsolution, and highlights the importance of engineering host oxide surface oxygen release kinetics in designing exsolved nanocatalysts.

Electrochemical Activation Applied to Perovskite Titanate Fibers to Yield Supported Alloy Nanoparticles for Electrocatalytic Application

Active bi-metallic nanoparticles are of key importance in catalysis and renewable energy. Here, the in situ formation of bi-metallic nanoparticles is investigated by exsolution on 200 nm diameter perovskite fibers. The B-site co-doped perovskite fibers display a high degree of exsolution, decorated with NiCo or Ni₃ Fe bi-metallic nanoparticles with average diameter about 29 and 35 nm, respectively. The perovskite fibers are utilized as cathode materials in pure CO₂ electrolysis cells due to their redox stability in the CO/CO₂ atmosphere. After in situ electrochemical switching, the nanoparticles exsolved from the perovskite fiber demonstrate an enhanced performance in pure CO₂ electrolysis. At 900 °C, the current density of solid oxide electrolysis cell (SOEC) with 200 µm YSZ electrolyte supported NiFe doped perovskite fiber anode reaches 0.75 Acm^-2 at 1.6 V superior to the NiCo doped perovskite fiber anode (about 1.5 times) in pure CO₂ . According to DFT calculations (PBE-D3 level) the superior CO₂ conversion on NiFe compared to NiCo bi-metallic species is related to an enhanced driving force for C-O cleavage under formation of CO chemisorbed on the nanoparticle and a reduced binding energy of CO required to release this product.

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.

Perovskite-Socketed Sub-3 nm Copper for Enhanced CO2 Electroreduction to C2

In situ socketing metal nanoparticles onto perovskite oxides has shown great potential in heterogeneous catalysis, but its employment in boosting ambient CO₂ electroreduction (CER) is unexplored. Here, a CER catalyst of perovskite-socketed sub-3 nm Cu equipped with strong metal-support interactions (SMSIs) is constructed to promote efficient and stable CO₂ -to-C₂₊ conversion. For such a catalyst, plentiful sub-3 nm ellipsoid Cu particles are homogeneously and epitaxially anchored on the perovskite backbones, with concomitant creation of significant SMSIs. These SMSIs are able to not only modulate electronic structure of active Cu and facilitate adsorption/activation of key intermediates, but also to strengthen perovskite-Cu adhesion and intensify resistance to structural degradation. Beneficial from these advantageous merits, when evaluated in CER, it performs comparably to or better than most reported Cu-based heteronanostructures. Relative to a physical-mixture counterpart, it features marked improvements (up to 6.2 folds) in activity and selectivity for C₂₊ , together with greatly boosted stability (>80 h). This work gives a new avenue to rationally design more advanced Cu-based heteronanostructures for CER.

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.

In Situ Control of the Eluted Ni Nanoparticles from Highly Doped Perovskite for Effective Methane Dry Reforming

To design metal nanoparticles (NPs) on a perovskite surface, the exsolution method has been extensively used for efficient catalytic reactions. However, there are still the challenges of finding a combination and optimization for the NPs' control. Thus, we report in situ control of the exsolved Ni NPs from perovskite to apply as a catalyst for dry reforming of methane (DRM). The La_0.8Ce_0.1Ti_0.6Ni_0.4O₃ (LCTN) is designed by Ce doping to incorporate high amounts of Ni in the perovskite lattice and also facilitate the exsolution phenomenon. By control of the eluted Ni NPs through exsolution, the morphological properties of exsolved Ni NPs are observed to have a size range of 10~49 nm, while the reduction temperatures are changed. At the same time, the chemical structure of the eluted Ni NPs is also changed by an increased reduction temperature to a highly metallic Ni phase with an increased oxygen vacancy at the perovskite oxide surface. The optimized composite nanomaterial displays outstanding catalytic performance of 85.5% CH₄ conversion to produce H₂ with a value of 15.5 × 10¹¹ mol/s·gcat at 60.2% CO conversion, which shows the importance of the control of the exsolution mechanism for catalytic applications.

An exsolution constructed FeNi/NiFe2O4 composite: preferential breaking of octahedral metal-oxygen bonds in a spinel oxide

Exsolution is an ingenious strategy for the in situ construction of metal- or alloy-decorated oxides and, due to its promising energy related catalysis applications, has advanced from use in perovskites to use in spinels. Despite its great importance for designing target composites, the ability to identify whether active metal ions at octahedral or tetrahedral sites will preferentially exsolve in a spinel remains unexplored. Here, an inverse spinel NiFe2O4 (NFO) was employed as a prototype and FeNi/NFO composites were successfully constructed via exsolution. The preferential breaking of octahedral metal-oxygen bonds in the spinel oxide was directly observed using Mössbauer and X-ray absorption spectroscopy. This was further verified from the negative segregation energies calculated based on density-functional theory. One exsolved FeNi/NFO composite exhibits enhanced electrochemical activity with an overpotential of 283 mV at 10 mA cm-2 and a long stability time for the oxygen evolution reaction. This work offers a unique insight into spinel exsolution based on the preferential breaking of chemical bonds and may be an effective guide for the design of new composite catalysts where the desired metal ions are deliberately introduced to octahedral and/or tetrahedral sites.

Precise Modulation of Triple-Phase Boundaries towards a Highly Functional Exsolved Catalyst for Dry Reforming of Methane under a Dilution-Free System

Dry reforming of methane (DRM) has been emerging as a viable solution to achieving carbon neutrality enhanced by the Paris Agreement as it converts the greenhouse gases of CO2 and CH4 into industrially useful syngas. However, there have been limited studies on the DRM catalyst under mild operating conditions with a high dilution gas ratio due to their deactivation from carbon coking and metal sintering. Herein, we apply the triple-phase boundary (TPB) concept to DRM catalyst via exsolution phenomenon that can secure elongated TPB by controlling the Fe-doping ratio in perovskite oxide. Remarkably, the exsolved catalyst with prolongated TPB shows exceptional CO2 and CH4 conversion rates of 95.9 % and 91.6 %, respectively, stable for 1000 hours under a dilution-free system. DFT calculations confirm that the Lewis acid of support and Lewis base of metal at the TPB promote the adsorption of reactants, resulting in lowering the overall CO2 dissociation and CH4 dehydrogenation energy.

Boosting the stability of perovskites with exsolved nanoparticles by B-site supplement mechanism

Perovskites with exsolved nanoparticles (P-eNs) have immense potentials for carbon dioxide (CO₂) reduction in solid oxide electrolysis cell. Despite the recent achievements in promoting the B-site cation exsolution for enhanced catalytic activities, the unsatisfactory stability of P-eNs at high voltages greatly impedes their practical applications and this issue has not been elucidated. In this study, we reveal that the formation of B-site vacancies in perovskite scaffold is the major contributor to the degradation of P-eNs; we then address this issue by fine-regulating the B-site supplement of the reduced Sr₂Fe_1.3Ni_0.2Mo_0.5O_6-δ using foreign Fe sources, achieving a robust perovskite scaffold and prolonged stability performance. Furthermore, the degradation mechanism from the perspective of structure stability of perovskite has also been proposed to understand the origins of performance deterioration. The B-site supplement endows P-eNs with the capability to become appealing electrocatalysts for CO₂ reduction and more broadly, for other energy storage and conversion systems.

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.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Who Does the Job? How Copper Can Replace Noble Metals in Sustainable Catalysis by the Formation of Copper–Mixed Oxide Interfaces

Following the need for an innovative catalyst and material design in catalysis, we provide a comparative approach using pure and Pd-doped LaCu_xMn_1–xO₃ (x = 0.3 and 0.5) perovskite catalysts to elucidate the beneficial role of the Cu/perovskite and the promoting effect of Cu_yPd_x/perovskite interfaces developing in situ under model NO + CO reaction conditions. The observed bifunctional synergism in terms of activity and N₂ selectivity is essentially attributed to an oxygen-deficient perovskite interface, which provides efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles. The latter promotes the decomposition of the intermediate N₂O at low temperatures, enhancing the selectivity toward N₂. We show that the intelligent Cu/perovskite interfacial design is the prerequisite to effectively replace noble metals by catalytically equally potent metal–mixed-oxide interfaces. We have provided the proof of principle for the NO + CO test reaction but anticipate the extension to a universal concept applicable to similar materials and reactions.

The Perfect Imperfections in Electrocatalysts

Modern day electrochemical devices find applications in a wide range of industrial sectors, from consumer electronics, renewable energy management to pollution control by electric vehicles and reduction of greenhouse gas. There has been a surge of diverse electrochemical systems which are to be scaled up from the lab-scale to industry sectors. To achieve the targets, the electrocatalysts are continuously upgraded to meet the required device efficiency at a low cost, increased lifetime and performance. An atomic scale understanding is however important for meeting the objectives. Transitioning from the bulk to the nanoscale regime of the electrocatalysts, the existence of defects and interfaces is almost inevitable, significantly impacting (augmenting) the material properties and the catalytic performance. The intrinsic defects alter the electronic structure of the nanostructured catalysts, thereby boosting the performance of metal-ion batteries, metal-air batteries, supercapacitors, fuel cells, water electrolyzers etc. This account presents our findings on the methods to introduce measured imperfections in the nanomaterials and the impact of these atomic-scale irregularities on the activity for three major reactions, oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Grain boundary (GB) modulation of the (ABO₃ )_n type perovskite oxide by noble metal doping is a propitious route to enhance the OER/ORR bifunctionality for zinc-air battery (ZAB). The perovskite oxides can be tuned by calcination at different temperatures to alter the oxygen vacancy, GB fraction and overall reactivity. The oxygen defects, unsaturated coordination environment and GBs can turn a relatively less active nanostructure into an efficient redox active catalyst by imbibing plenty of electrochemically active sites. Obviously, the crystalline GB interface is a prerequisite for effective electron flow, which is also applicable for the crystalline surface oxide shell on metal alloy core of the nanoparticles (NPs). The oxygen vacancy of two-dimensional (2D) perovskite oxide can be made reversible by the A-site termination of the nanosheets, facilitating the reversible entry and exit of a secondary phase during the redox processes. In several instances, the secondary phases have been observed to introduce the right proportion of structural defects and orbital occupancies for adsorption and desorption of reaction intermediates. Also, heterogeneous interfaces can be created by wrapping the perovskite oxide with negatively charged surface by layered double hydroxide (LDH) can promote the OER process. In another approach, ion intercalation at the 2D heterointerfaces steers the interlayer spacing that can influence the mass diffusion. Similar to anion vacancy, controlled formation of the cation vacancies can be achieved by exsolving the B-site cations of perovskite oxides to surface anchored catalytically active metal/alloy NPs. In case of the alloy electrocatalysts, incomplete solid solution by two or more mutually immiscible metals results in heterogeneous alloys having differently exposed facets with complementary functionalities. From the future perspective, new categories of defect structures including the 2D empty spaces or voids leading to undercoordinated sites, the multiple interfaces in heterogeneous alloys, antisite defects between anions and cations, and the defect induced inverse charge transfer should bring new dimensionalities to this riveting area of research.

Evolution of surface and sub-surface morphology and chemical state of exsolved Ni nanoparticles

Nanoparticle formation by dopant exsolution (migration) from bulk host lattices is a promising approach to generate highly stable nanoparticles with tunable size, shape, and distribution. We investigated Ni dopant migration from strontium titanate (STO) lattices, forming metallic Ni nanoparticles at STO surfaces. Ex situ scanning probe measurements confirmed the presence of nanoparticles at the H₂ treated surface. In situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) revealed reduction from Ni²⁺ to Ni⁰ as Ni dopants migrated to the surface during heating treatments in H₂. During Ni migration and reduction, the Sr and Ti chemical states were mostly unchanged, indicating the selective reduction of Ni during treatment. At the same time, we used in situ ambient pressure grazing incidence X-ray scattering (GIXS) to monitor the particle morphology. As Ni migrated to the surface, it nucleated and grew into compressed spheroidal nanoparticles partially embedded in the STO perovskite surface. These findings provide a detailed picture of the evolution of the nanoparticle surface and subsurface chemical state and morphology as the nanoparticles grow beyond the initial nucleation and growth stages.

The 10th Anniversary of Nanomaterials-Recent Advances in Environmental Nanoscience and Nanotechnology

As a result of the rapid growth of nanoscience and nanotechnology, including advanced methods of fabrication and characterization of nanostructured materials, great progress has been made in many fields of science, not least in environmental catalysis, energy production and sustainability [...].

A Robust Approach to In Situ Exsolve Highly Dispersed and Stable Electrocatalysts

Catalysts made of in situ exsolved metal nanoparticles often demonstrate promising activity and high stability in many applications. However, the traditional approach is limited by perovskites as prevailing precursor and requires high temperature typically above 900 K. Here, with the guidance of theoretical calculation, an unprecedented and substantially facile technique is demonstrated for Cu nanoparticles exsolved from interstitially Cu cations doped nickel-based hydroxide, which is accomplished swiftly at room temperature and results in metal nanoparticles with a quasi-uniform size of 4 nm, delivering an exceptional CO faradaic efficiency of 95.6% for the electrochemical reduction of CO₂ with a notable durability. This design principle is further proven to be generally applicable to other metals and foregrounded for guiding the development of advanced catalytic materials.