The exsolution of transition-metal nanoparticles from non-stoichiometric perovskites presents a powerful strategy for creating high-activity, regenerable fuel cell electrodes. This presentation highlights two recent advances that exploit this approach across distinct electrochemical systems.

First, we examine the A-site-deficient perovskite La0.4​Sr0.4​Sc0.9​Ni0.1​O3−δ​. Guided by thermodynamic assessment and density functional theory (DFT), this material was designed for the favorable segregation of nickel. Upon hydrogen reduction, it yields uniformly distributed Ni nanoparticles that lower the area-specific resistance to an exceptional 0.055 Ω cm² at 800 °C in humid H2​, validating models of heterogeneous nucleation and growth.

Second, the mixed-conducting perovskite BaCo0.4​Fe0.4​Zr0.1​Y0.1​O3−δ​ is presented as a bifunctional electrode for a symmetric protonic ceramic fuel cell. DFT reveals that its performance is driven by the reversible exsolution and dissolution of Co-Fe nanoparticles, a process governed by defect chemistry and orbital interactions. The resulting self-recovering electrodes deliver a peak power density of approximately 350 mW cm⁻² with H2​ and demonstrate remarkable fuel flexibility and extended lifetime with methanol or methane.

By synthesizing mechanistic insights from these complementary systems, this talk establishes clear design principles for engineering advanced electrodes. We outline how tailoring cation chemistry, vacancy concentration, and redox protocols can precisely control nanoparticle nucleation, size, and regenerability, charting a strategic path toward efficient, durable, and commercially viable fuel cells.

Mono-, bi- and tri-metallic nanoparticle exsolution from perovskite oxides has become a widespread strategy to boost the performance of solid oxide cell (SOC) electrodes by enhancing the electrocatalytic activity at the solid/gas interface [¹]. A particular interesting group are redox stable perovskites, designed to exsolve nanoparticles and able to perform as anode or cathode in symmetric cells at intermediate temperatures (500 – 700 °C) with high efficiencies^{[1, 2]}. These perovskites are usually highly functionalized oxides with complex stoichiometries.

While high resolution scanning and transmission electron microscopies and electron spectroscopies are able to characterize the NP decoration over a wide length scale, or access quite accurately nanoparticle composition after exsolution, in-situ and operando synchrotron-based methods such as ambient pressure photoelectron and absorption fine structure spectroscopies (AP-XPS/NEXAFS) offer the required surface sensitivity, chemical and elemental specificity needed to distinguish the role of each element during exsolution near the solid/gas interface. Synchrotron-based in-situ X-ray diffraction on these materials offers a complementary structural perspective of the transformations undergone by the electrode materials during thermal exsolution.

In this contribution, we showcase this combination of in-situ/operando synchrotron-based methods with electron microscopies and spectroscopies applied to Sr(Ti,Fe)O_3-d perovskites ^[3-5] and Sr₂FeMoO_6-d double perovskites, doped with Ni and Co^[6]. In a next step, Ni-doped Sr(Ti,Fe)O_3-d perovskites were used to fabricate model cells for a device-driven optimization applying polarization and other electrochemical techniques such as voltammetry, chronoamperometry and electrochemical impedance spectroscopy while monitoring the electrode evolution with synchrotron-based techniques. Polarization can modify the surface chemistry of the working electrode, offering opportunities for tuning NP composition or regenerating the working electrode material by reversing exsolution or investigate SOCs in fuel or electrolysis (SOFC/SOEC) operation modes.

Solid Oxide Cells (SOC) offer a promising route for CO₂ conversion and energy storage in a net-zero future, where carbon-neutral fuels are urgently needed to mitigate climate change. High-temperature CO₂ electrolysis is particularly attractive as it allows for carbon-neutral production of synthetic fuels, but cathode deactivation and coking remain major challenges.

Exsolution is widely regarded as a beneficial phenomenon in heterogeneous (electro)catalysis, often linked to enhanced activity and coking resistance. However, in the context of high-temperature CO₂ electrolysis, we reveal that exsolved metallic iron nanoparticles in fact lead to a decrease in cell performance. Using well-defined thin film electrodes based on three ferrite perovskites—La_0.6Ca_0.4FeO_3‒δ, Nd_0.6Ca_0.4FeO_3‒δ, and Pr_0.6Ca_0.4FeO_3‒δ —we combine in-situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) with electrochemical impedance spectroscopy to track changes in surface chemistry and performance under CO₂ electrolysis conditions simultaneously. Our measurement strategy makes use of precisely controlling the chemical oxygen potential in the model electrodes via the applied voltage. With this approach it was possible to explore the kinetics of the same electrode first without any exsolutions, then decorated with exsolved metallic iron particles, and finally again with re-oxidised exsolutions.

Our results demonstrate a clear correlation between Fe exsolution and a decrease in CO₂ splitting activity. Notably, this behavior contrasts with the beneficial effects of exsolution observed in H₂O electrolysis. Our findings challenge the general assumption that metal exsolution is universally advantageous and emphasize the need for mechanism-specific catalyst design.

The catalytic activity of exsolved particles is strongly linked to their size and population. However, which parameters affect particle size as well as how this size in turn affects other properties of the exsolved particles is often only poorly understood.

In this contribution, we explore the interplay between the size of exsolved particles and their re-dissolution behavior using a novel approach that allows for the control of the sample’s oxygen activity under Ultra-High Vacuum (UHV) conditions. Utilizing a Solid Oxide Cell (SOC) like design, we can precisely control the oxygen activity in the working electrode by applying a bias relative to an oxygen ion buffering counter electrode, while excluding any effects of gas phase adsorbates. Applying this Electrochemical oXygen Activity ConTrol (EXACT), SrTi_0.3Fe_0.7O_3-δ (STF) was repeatedly reduced and oxidized at 600 °C while the exsolution behavior was monitored via X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). Simultaneously, oxygen stoichiometry changes were tracked by coulometry to correlate surface processes with changes of the bulk material.

With this method, we have achieved the in-situ observation of the formation and growth of Fe particles upon reduction and the complex Fe particle re-oxidation behavior. We find that repeated oxidation and reduction (redox-cycling) enhances particle agglomeration and that the resulting change in particle size alters the re-dissolution behavior. While re-dissolution is observable for small particles, larger particles get oxidized without re-incorporation into the perovskite lattice at the investigated temperature. Overall, we can show that the exact voltage program for exsolution formation can strongly influence the properties of the exsolved particles.

Along with their prolonged lifetime and regenerability, what distinguishes the exsolution catalysts from conventional ones is the homogeneous distribution and tailorable size of the metallic particles. These features can be precisely controlled by tuning intrinsic parameters (such as perovskite defect chemistry and dopant concentration) as well as extrinsic parameters (such as time, temperature, and oxygen partial pressure) during the exsolution process.

Electron microscopy has proven to be a powerful tool for monitoring the morphological evolution of exsolved particles [1]. However, complementary techniques are required to obtain volumetric information on changes in nanoparticle size, population, and distribution. For this purpose, small-angle X-ray scattering (SAXS) offers a valuable opportunity [2,3]. Taking advantage of the electron density contrast between the metallic particles and the host oxide, SAXS provides a unique and direct method to monitor the evolution of both surface (i.e., exogenous) and internal (i.e., endogenous) particles.

In this contribution, I will present the growth behavior of exogeneous and endogeneous nickel nanoparticles exsolved from nanoporous and sintered matrices, as studied via SAXS. Particular focus will be placed on the effects of exsolution temperature and nickel dopant concentration on nanoparticle morphology evolution. The development of in situ SAXS experiments for studying nickel nanoalloys will also be discussed.

In alkaline water electrolysis (AWE), the oxygen evolution reaction (OER) is a key limiting electrochemical reaction because of its complex reaction pathway through a four-electron transfer process, and high overpotential. This results in sluggish kinetics and increased energy consumption, due to hindering efficiency and scalability of hydrogen production, thus posing stringent catalyst requirements. One approach to overcome this is through rational catalyst design of such as perovskite oxides and the exsolved nanoparticle systems. Perovskite oxides (ABO₃) have emerged as highly versatile materials in this context, offering tuneable structures, compositional flexibility, and mixed ionic–electronic conductivity. These properties allow for the optimisation of catalytic performance, particularly for the OER and hydrogen evolution reaction, positioning perovskites as promising, scalable catalysts for efficient AWE systems. Exsolution refers to the thermally driven formation of catalytically active metal nanoparticles, which emerge from the perovskite lattice and become anchored to the surface under reducing conditions, thereby enhancing catalytic performance through increased surface reactivity and stability.

This study examines the perovskite oxide Sr₀.₉₅Ti₀.₃Fe₀.₆Cu₀.₁O₃ (STFCu), a composition deliberately engineered to incorporate earth-abundant, non-precious elements while aiming to achieve a synergistic balance between structural robustness and enhanced electrocatalytic performance. The low cost, abundance, and compatibility of copper make it suitable as both a lattice dopant and electrode component.

The present work focuses on optimising the sol-gel synthesis of STFCu perovskite by controlling calcination time and temperature to achieve a pure phase perovskite. Phase purity and crystal structure are assessed using X-ray diffraction (XRD), with the overarching goal of comprehensively evaluating their structural, catalytic, engineering, and economic viability. A central objective involves the investigation of nanoparticle exsolution behaviour, examined using scanning electron microscopy (SEM). To assess the functional implications of this phenomenon, electrochemical characterisation (including cyclic voltammetry, linear sweep voltammetry, and Tafel slope) were conducted, revealing that surface–exsolved nanoparticles play a pivotal role in promoting reaction kinetics and electron transfer during the OER. The findings contribute to a more nuanced understanding of structure–activity relationships in copper–doped perovskites, and offer a foundation for the rational design of advanced OER electrocatalysts that combine high activity, long-term durability, and economic feasibility.

Perovskite oxides have already demonstrated their exceptional capabilities in energy conversion applications. [1-2] In particular, the exsolution of metallic nanoparticles by employing the perovskite as a conductive matrix has broadened even more the potential of those materials as potential catalysts for large-scale operations. [3,4]

Although, these catalysts exhibit high catalytic performance, they typically utilize noble metals such as Pt, Ir, Pd etc., as catalytic active sites. This reliance on noble metals usage increases manufacturing costs and limits their industrial applicability.

Here, we present a novel synthetic perspective on devising high-performance exsolution catalysts without the presence of noble metals. Specifically, the surface engineering of the exsolution nanoparticles can provide a plethora of new catalytic active species such as phosphides, sulfides etc., that can enhance the catalytic performance and simultaneously provide low cost and high chemical stability.

In this work, we investigated the impact of the surface modification of exsolved Co nanoparticles as potential electrocatalysts for hydrogen (HER) and oxygen (OER) evolution reactions. The results suggest, that the surface engineering of the metal-based electrocatalysts is crucial enhance the electrocatalytic activity further. In particular, the modified catalyst exhibited a remarkable performance towards electrocatalytic water splitting, yielding overpotentials (η₁₀) of 280 mV and 390 mV for HER and OER, respectively. Additionally, the mass activity of the modified catalysts has increased by ⁓ x10 factor in regards to the unmodified materials. Also, the stability of catalysts was investigated and the results indicate that they exhibit exceptional stability at 10 mA cm^-2 for at least 3 days. All the catalysts were characterized by various physicochemical and electrochemical techniques unveiling the structural, morphological, and electrochemical properties. These findings demonstrate that the rational synthetic engineering of the surface of the exsolved nanoparticles can provide noble-free catalysts that exhibit high electrocatalytic performance and low cost that have the potential to replace the usage of noble metals.

Coating Strategies for Integrating Exsolved Nanoparticles onto Metallic Foam

Keywords: Metallic foams, exsolution, powder metallurgy, coating processes, alkaline water electrolysis (AEL)

Porous metallic foams are gaining attention as high-performance catalyst supports in energy applications, offering up to 1000 times the surface area of plain substrates along with excellent electrical conductivity, low pressure drop, and superior thermal and chemical stability. These properties make them ideal platforms for heterogeneous catalysis and electrochemical energy conversion, particularly in alkaline water electrolysis (AEL).

To enhance functionality, ceramic coatings based on perovskites containing exsolvable dopants are applied to the foam surface. Upon thermal treatment in reducing atmosphere, these powders form metallic nanoparticles on the surface of the parent perovskite, that enhance chemical activity and increase the effective surface area of the electrode. Key challenges include achieving strong adhesion, uniform distribution across the foam structure, and a stable interface between coating and substrate.

The quality and strength of the perovskite-to-substrate connection is analyzed using scanning electron microscopy (SEM), with additional insights gained from electrochemical measurements. By bridging material functionality with scalable electrode design, tailored coatings facilitate the industrial implementation of exsolution-based surface technologies in sustainable energy systems.

Catalysts enable important chemical transformations for industrial applications, as well as for emerging technologies for the decarbonization of our societies, such as water electrolysis and the production of green hydrogen. Very often, catalysts experience structural and compositional changes during operating conditions, most notably the oxygen evolution reaction (OER), that hinder their rational design. For example, perovskite-based catalysts form an amorphous surface layer even in contact with electrolyte solutions, as well as during operating conditions.[1] The initial catalyst form is termed as “precatalyst” and currently, we have no control over the final structure of the precatalyst, which is simply termed as the “catalyst”. Controlling and monitoring the structural and compositional changes between precatalysts and the catalysts will contribute to a predictive and truly rational catalysts design. Several works addressing this issue have emerged and they look into the surface amorphization of catalysts during the oxygen evolution reaction (OER),[2, 3] but there are no studies trying to stabilize the precatalyst, ultimatelybeing the catalyst as well. This is an important aspect as such precatalysts/catalysts will provide us with insights on the catalytic activity of the crystalline phase compared to the amorphous one. This knowledge gap in perovskite-based materials is being discussed in this contribution. We have discovered that A-site deficiency in perovskite-based oxides has a dual role, first to induce exsolution, but also to stabilize the surface of the perovskite and retain its crystalline phase under harsh OER conditions in alkaline environment.[4] We consistently observe that A-site deficient perovskites perform better than their stoichiometric analogues, while exsolution boosts further the catalytic activity towards the OER. We generalize this strategy for suppression of the surface amorphization with relevant studies in other perovskite oxides catalysts for the OER.

The development of superior exsolved materials for energy applications necessitates a comprehensive understanding of their atomic-scale structure and the structural modifications that occur both during formation and under operational conditions. A core challenge in characterising exsolved materials is how to extract detailed atomic-resolution information under real-world conditions. This study reports the monitoring of the full nucleation and growth mechanism of exsolved single-metal NPs by in situ thermal scanning transmission electron microscopy (STEM) investigations at the atomic scale. This high-resolution in situ microscopy study allowed to observe atomic diffusion, nucleation sites, the evolution of the host crystal structure, and the role of evolving host defects in the early stages of nucleation, allowing to correlate the defect formation in the host oxide to the nanoparticle formation at the surface during exsolution looking at the process holistically: both atom to nanoparticles nucleation, and perovskite structural evolution at the same scale. Next, to tackle the limited understanding of exsolved materials' exceptional catalytic properties, a combined approach adopting complementary operando characterisation techniques was developed to study the chemical, structural, and microstructural features that determine the catalytic behaviour mechanisms in exsolved fluorite and spinel materials applied to the catalytic process of CO₂ hydrogenation to methanol. By carrying out a complementary in situ-operando study on the behaviour of this category of catalytic materials, a NP-support dynamic synergy was revealed for the exsolved materials, which elucidated their improved performance. By near ambient pressure (NAP)-XPS the evolution of the materials surface composition and reaction intermediate species during catalytic processing was studied, also monitored, together with the identification and role of active sites, by in situ/operando FTIR studies. Through operando STEM, this complementary set of information was coupled with the morphology evolution of the exsolved catalysts, revealing a dynamic faceting and restructuring of the exsolved particles, as well as a reversible surface speciation during reaction, providing insights into the catalytic mechanism of the exsolved materials studied. Using such a combined approach allowed to identify the key factors responsible for the enhanced catalytic activity of the developed exsolved materials while determining the structure-activity-selectivity relationship under dynamic conditions.

Perovskite oxides offer a sustainable platform for catalyzing the oxygen evolution reaction (OER) in alkaline water electrolysis through the use of earth-abundant and cost-effective alternatives to platinum group metals (PGM). While iridium and ruthenium-based anodes demonstrate the highest activity towards the OER, cobalt oxides offer an opportunity to achieve similar performance with fewer economic limitations. This can be further improved by employing exsolution to maximise the surface area of cobalt active sites with enhanced stability, contributing to additional environmental benefits in extended catalyst lifespan.

However, cobalt mining raises several social implications with human right violations including child and forced labor as well as health concerns due to its classification as a heavy metal and 2A carcinogen. Currently, one of the most reported perovskite-based catalyst due to its high performance is BSCF (Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d) with 80at% Co-loading. Here we show that reducing doping in cobalt-doped perovskites, when coupled with exsolution, can improve nanoparticle morphology and population, enhancing OER activity. Preliminary findings had shown that our materials with just 10at% cobalt, Sr_0.95Ti_0.3Fe_0.7-xCo_xO_3-d (x = 0.10, STFC10) matched the electrocatalytic activity of BSCF in alkaline media. Furthermore, we demonstrate that the exsolution of STFC5 (x = 0.05) with 5at% Co-loading can achieve a higher density of smaller nanoparticles than in STFC10 (x = 0.10).

These results indicate that cobalt loading can be significantly lowered and OER performance enhanced by using the STF perovskite family. We employed a range of physiochemical characterization techniques including XRD for confirmation of phase purity and information on the perovskite crystal structure; SEM for insight into surface morphology and nanoparticle arrangement; and XPS for the elemental composition and oxidation states of the exsolved components. Furthermore, electrochemical performance was evaluated using linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 1M KOH.

The growing demand for energy conversion and storage technologies, particularly reversible Symmetrical Solid Oxide Cells (S-SOCs), has been driven by the need for efficient, low-emission energy solutions in stationary power generation and systems that integrate intermittent renewable energy sources. In S-SOCs, both the cathode and anode are composed of the same material, placing stringent requirements on electrode materials. These materials must exhibit mixed ionic-electronic conductivity, redox stability, and high electrocatalytic activity [1].

In this work, we present catalytically active, redox-stable nanofibrous perovskites decorated with in situ exsolved nanocatalysts as novel, high-performance electrode materials for S-SOCs. Perovskite compounds with the general formula Ln_0.9(Ba, Sr)_0.9(Fe, Mn)_1.8(Co, Ni)_0.2O_6−δ (where Ln = selected lanthanides) were synthesized via the electrospinning technique, which promotes enhanced gas diffusion and improved electrochemical performance of the electrode layers (see Figure 1a, b). Structural and microstructural characterization, performed using X-ray diffraction, transmission electron microscopy equipped with an energy-dispersive detector and Raman spectroscopy, enabled us to elucidate the influence of morphology and Fe-Co-Ni doping on the reversible in situ exsolution/dissolution process of nanoparticles. A self-assembled S-SOC incorporating the most promising electrode materials demonstrated a power density exceeding 950 mW cm⁻² when fuelled with wet hydrogen at 850 °C. Excellent long-term stability confirmed that the combined use of multiple material optimization strategies, including in situ exsolution and the electrospinning technique, effectively meets the stringent requirements for high-performance electrodes in S-SOCs. The stable performance achieved in this study significantly surpasses previously reported power densities for symmetrical cells employing Mn-based electrodes [2–5].

Exsolution has gained attention as a versatile electrode functionalization method for solid oxide electrochemical cells ¹. One of the key advantages of exsolution is the possibility to exsolve alloys or multielement nanoparticles, which can unlock unprecedented electrocatalytic functionalities. However, controlling exsolution of several elements aiming at equimolar metallic concentrations in the nanoparticles entails several mechanistic hurdles, governed by the kinetics of cation reduction. In order to understand in deeper detail the governing principles of multielemental nanoparticle exsolution, Sr₂Fe_1.2Co_0.1Ni_0.1Cu_0.1Mo_0.5O_6-δ, perovskite is selected as a model.

In this work, we have evaluated the impact of temperature and time on (1) the size and the population of the nanoparticles and (2) the composition of the exsolved nanoparticles. Short exsolution times (2-6 h) enabled the formation of nanoparticles mainly composed of Cu-Ni metals, while longer times (24 h) generate Janus ² type nanoparticles at similar treatment temperatures. Janus nanoparticles have the special feature of having two distinguished compositional zones, one mainly composed by Cu while the other side has Co, Ni and Fe. Lower exsolution temperatures (700 ºC) give nanoparticles composed mainly of Cu and Ni, while higher temperatures (900 ºC), for the same time and atmosphere, lead to nanoparticles composed of the four metals but mainly of Fe. These results could be a great step when tuning the nanoparticle composition since Fe has been reported to have the most energy-demanding exsolution metal among the studied ³. Other relevant tests on this material help to comprehend how oxidation steps, wet exsolution, or electrode exsolution work, with encouraging results such as morphological changes and optimized nanoparticle population when in situ electrode exsolution is performed.

Finally, the studied material has been characterized as electrode in solid-oxide fuel cells in a symmetrical configuration, where the polarization resistance was 2.94 Ω cm² after 24 h of exsolution, characterising their electrochemical behaviour in both cathodic and anodic operation. When tested in non-symmetrical fuel cells, as anode, polarization resistances as low as 0.85 Ω · cm² were reached  700 ºC.

This work shows the high tunability that could be achieved in multielemental nanoparticle exsolution, mainly in terms of composition but also in terms of size, shape and exsolution conditions. These results highlight the influence of processing parameters in the composition of the multielemental nanoparticles, providing guidelines for compositional fine tuning, with high influence in electrocatalytic activity and selectivity.

Exsolution of transition-metal cations from perovskite-oxide hosts has emerged as an outstanding route for producing oxide-supported metal nanoparticles: Various transition-metal cations can be incorporated into the host lattice under oxidising conditions at sintering temperatures and exsolved as metallic nanoclusters after a reducing treatment at much lower temperatures. Despite extensive investigations over the past decade, there is no consistent, comprehensive and fundamental description of why exsolution occurs. Furthermore, it is unclear how the exsolving cations can be sufficiently mobile within a perovskite lattice at temperatures well below those used for sintering.

In this study we used hybrid Density-Functional-Theory (DFT) calculations to examine these two central issues: why exsolution occurs and how it occurs. From our results we proposed a single model that explains diverse experimental observations; why transition-metal cations (but not host cations) exsolve from perovskite lattices upon reduction; why different transition-metal cations exsolve under different conditions; why the metal nanoparticles are embedded at the surface; why the oxide’s surface orientation affect behaviour; why exsolution occurs surprisingly rapidly at relatively low temperatures; and why the re-incorporation of exsolved species involves far longer times and much higher temperatures. Our model’s foundation is that the transition-metal cations are completely reduced to metal atoms within the perovskite lattice as the Fermi level is shifted upwards within the bandgap. This understanding of the exsolution phenomenon provides the basis for a facilitated optimisation of current exsolution systems and for the accelerated development of new exsolution systems.

Bonkowski, A., Wolf, M.J., Wu, J., Parker, S.C., Klein, A. and De Souza, R.A.,  A single model for the thermodynamics and kinetics of metal exsolution from perovskite oxides. J. Am. Chem. Soc. 2024, 146, 23012-23021.

While most focus on entropy designed single-phase materials, we are exploring the potential to design and utilize secondary phases in high-entropy oxides (HEOs) to introduce tunable and reversible composite properties [1. Vahidi (2024) Adv. Func. Mater.]. I will discuss our recent works developing high-entropy-oxide-derived nanocomposite electroceramic thin films using a flexible approach – “exsolution self-assembly” (ESA) – that combines traditional immiscibility driven self-assembly concepts with defect-chemistry-governed exsolution phenomena into [2. Guo (2024) Matter; 3. Guo (2024) Appl. Phys. Lett]. During physical vapor deposition, ESA yields HEO-based nanocomposite thin films with intricate multi-element nanostructures and precisely tailorable surfaces. In an ongoing study, we’ve used this approach for high-throughput electrocatalyst library synthesis coupled with CO₂ reduction reaction activity screening by electrochemical cell microscopy [4. Xuan (In revision)]. Importantly, by utilizing a suite of multiscale characterization down to the atomic-scale, these works provide a set of design guidelines for novel self-assembled nanocomposite oxide thin films incorporating entropy-designed materials, which offer a vast compositional landscape to explore and develop.

Metal exsolution reactions yield a high density of finely dispersed nanoparticles at the surface of functional oxides, enabling the synthesis of efficient electrocatalysts. The exsolution behaviour of reducible metals from host oxides and the nanoparticle self-assembly is closely linked to the oxides’ defect structure. Consequently, defect engineering has emerged as a strategy to control the properties of exsolution catalysts, with a primary focus on modifying point defect concentrations in exsolution-active host oxides.

We explore dislocation engineering to tune nucleation sites for metal nanoparticles formed under the reducing reaction conditions. For this purpose, we developed a novel approach to induce laterally confined regions of increased dislocation densities into oxide thin films. This method is based on mechanical deformation of single-crystal substrates followed by the deposition of epitaxial thin films using pulsed laser deposition. Based on this methodology, exsolution-active Ni-doped strontium titanate model systems with defined areas of pre-engineered dislocations are synthesized, which enable the investigation of the role of dislocations in metal exsolution reactions on the atomic scale.

We use environmental scanning transmission electron microscopy with simultaneous bulk-sensitive and surface-sensitive image detection to study the formation of dislocation-associated metal nanoparticles. The in-situ analysis reveals a clear correlation between the presence of pre-engineered bulk dislocations in the exsolution-active oxide and the formation of surface nanoparticles. Two major reasons for the dislocation-associated nanoparticle formation are identified. First, the accumulation of exsolution-active acceptors along dislocations, driven by electrostatic interactions and lattice strain. Second, lattice distortions that are expected to decrease the energy barrier for nanoparticle nucleation during metal exsolution reactions.

As society becomes increasingly aligned with achieving net-zero emissions, significant efforts have been dedicated to exploring renewable alternatives to fossil fuels to sustainably meet the globes increasing chemical and energy demands. Oxidation of glucose to formic acid presents promise as a route to utilise biomass as a renewable feedstock to address these demands [1]. As a raw feedstock formic acid is utilised in food, textile and pharmaceutical production [1], and had a market value of USD 2.4 billion in 2024. In regards to addressing energy demands, it could be regarded as a liquid hydrogen carrier, with a greater volumetric density than liquid hydrogen, 53.4 g H₂/L [2]. However, current methods for glucose conversion to formic acid utilise high pressure, high temperature and glucose:catalyst ratios of ca. 2:1 [3-5].

Here we show that a series of perovskites containing exsolved Ru or Ni are active for glucose oxidation to formic acid under more sustainable reaction conditions. We found that complex oxides, Sr(Mo,M)O_4γ-δ where M = Ru or Ni, in a pure perovskite, pure scheelite and perovskite/scheelite mixture all produced formic acid, with the pure perovskite phase demonstrating the highest activity - a sustained formic acid yield of ca. 20 % over the 90-minute duration. This value is lower than that of alternative promising Mo-MnO_x catalysts [5] (ca. 65 %). However, our values relate to lowered temperatures (100 vs. 160 ^oC), lowered pressures (ambient compressed air flow vs. 30 MPa O₂) and increased glucose:catalyst ratios (10:1 vs. 2:1). Our results demonstrate the promise of perovskite materials to aid in the production of formic acid utilising green feedstocks and sustainable reaction conditions to address global chemical and energy needs. We anticipate these results to be the foundations for development of more tailored perovskite materials to facilitate green formic acid production with sustainable reaction conditions.

Ammonia (NH₃) plays a crucial role in global fertilizer production and is a promising hydrogen carrier, yet its conventional production via the Haber-Bosch process is highly energy-intensive and CO₂-emitting. Direct electrochemical ammonia synthesis using ceramic proton-conducting cells (PCCs) poses a sustainable alternative for this established synthesis route. For the application of PCCs for ammonia synthesis, novel catalysts for the nitrogen reduction, and ammonia formation reaction are necessary. Here, we investigated the formation kinetics and morphology of FeRu nanoparticle catalysts formed by metal exsolution from the perovskite ceramic BaZr₀.₅₋ₓCe₀.₂Fe₀.₂RuₓY₀.₁O₃₋_δ. Metal exsolution is an easy and direct method to synthesize metal nanoparticles using a direct thermal reduction route to selected cations from an oxide host. We utilized ex and in situ transmission electron microscopy techniques to understand the alloy chemistry, formation kinetics and morphology of bimetallic FeRu nanoparticles to optimize their properties for NH₃ synthesis. Furthermore, we tested the performance of the catalyst in a bench-scale thermal reactor for NH₃ synthesis and made first steps to integrate the exsolution-active ceramic electrode into state of the art PCC half cells for future evaluation of its performance in a proof-of-concept configuration for ammonia synthesis. 

Over the last decade, exsolution technique has emerged as a relevant alternative to deposition methods for generation of nanoparticle-based robust catalysts. This exsolution process leads to stable and active nanoparticles anchored to oxide supports, which in turn grants more efficient and durable catalysts for driving high-interest reactions, such as hydrogen production through ammonia cracking. Nevertheless, some challenges remain unsolved for expanding exsolution to certain materials, such as cerium oxide (CeO₂)-based systems due to the limited solubility of transition metals.

In this work, CeO₂ lattice modifications -partial substitution of Ce with Gd- enabled an adequate introduction of highly-active metals, namely Ru and Rh. Latter exsolution led to highly-dispersed Ru, Rh and -unprecedent- RuRh alloyed nanoparticles formation (ca. 3 nm). These functionalized materials were employed as catalysts for ammonia decomposition process, exhibiting outstanding performance and long-term stability, even outperforming Ru-impregnated materials, especially Ru-exsolved@Ce_0.8Gd_0.2O_2-δ. These exceptional results were achieved despite the notably low metal loading (~0.7 wt.% Ru) and surface areas. Catalytic performance of the exsolved materials was tested under different temperatures and space velocities, leading to efficient hydrogen production along 260 hours, at 600 and 400 ºC (~110 h and ~150 h respectively), with no evidence of degradation affecting the exsolved nanoparticles, nor the support. Lastly, a comparative with other state-of-the-art catalysts evidenced the outstanding potential of exsolved CeO₂-based catalysts, which allows the reduction of metal loading requirements, leading to more efficient catalysts.