Enhanced electrochemical property of La0.6Sr0.4Co0.8Fe0.2O3 as cathode for solid oxide fuel cell by efficient in situ polarization-exsolution treatment

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.

A Superior Catalytic Air Electrode with Temperature-Induced Exsolution toward Protonic Ceramic Cells

Protonic ceramic cells merit extensive exploration, attributed to their innate capabilities for potent and environmentally benign energy conversion. In this work, a temperature-induced exsolution methodology to synthesize SrCo0.5Nb0.5O3-δ (SCN) nanoparticles (NPs) with notably elevated activity on the surface of PrSrCo1.8Nb0.2O6-δ (PSCN) is proposed, directly addressing the extant challenge of restrained catalytic activity prevalent in air electrode materials. In situ assessments reveal that SCN NPs commence exsolution from the matrix at temperatures surpassing 900 °C during straightforward calcination processes and maintain stability throughout annealing. Notably, the resultant SCN-PSCN interface facilitates vapor adsorption and protonation processes, which are poised to enhance surface reaction kinetics pertaining to the proton-involved oxygen reduction and evolution reaction (P-ORR and P-OER). A fuel-electrode-supported protonic ceramic cell leveraging SCN-PSCN as the air electrode manifests compelling performance, attaining a peak power density of 1.30 W·cm-2 in the fuel cell modality and a current density of 1.91 A·cm-2 at 1.3 V in the electrolysis mode, recorded at 650 °C. Furthermore, density functional theory calculations validate that the introduction of SCN NPs onto the PSCN surface conspicuously accelerates electrode reaction rates correlated with P-ORR and P-OER, by significantly mitigating energy barriers associated with surface oxygen and vapor dissociation.

Exsolved catalyst particles as a plaything of atmosphere and electrochemistry

A new type of catalyst preparation yields its active sites not by infiltration but exsolution of reducible transition metals of its own host lattice. These exsolution catalysts offer a high dispersion of catalytically active particles, slow agglomeration, and the possibility of reactivation after poisoning due to redox cycling. The formation of exsolved particles by partial decomposition of the host lattice can be driven by applying a sufficiently reducing atmosphere, elevated temperatures but also by a cathodic bias voltage (provided the host perovskite is an electrode on an oxide ion conducting electrolyte). In addition, such an electrochemical polarisation can change the oxidation state and thus the catalytic activity of exsolved particles. In this work, we investigate the electrochemical switching between an active and an inactive state of iron particles exsolved from thin film mixed conducting model electrodes, namely La_0.6Sr_0.4FeO_3-δ (LSF) and Nd_0.6Ca_0.4FeO_3-δ (NCF), in humid hydrogen atmospheres. We show that the transition between two activity states exhibits a hysteresis-like behaviour in the electrochemical I-V characteristics. Ambient pressure XPS measurements proofed that this hysteresis is linked to the oxidation and reduction of iron particles. Furthermore, it is demonstrated that the surface kinetics of the host material itself has only a negligible impact on the particle exsolution, and that the main impact factors are the surrounding atmosphere as well as the applied electrochemical overpotential. In particular, we suggest a 'kinetic competition' between gas atmosphere and oxygen chemical potential in the mixed conducting electrode and discuss possible ways of how this process takes place.

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

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

Effectively Promoting Activity and Stability of a MnCo2O4-Based Cathode by In Situ Constructed Heterointerfaces for Solid Oxide Fuel Cells

The development of multiphase composite electrocatalysts plays a key role in achieving the efficient and durable operation of intermediate-temperature solid oxide fuel cells (IT-SOFCs). Herein, a self-assembled nanocomposite is developed as the oxygen reduction reaction (ORR) catalyst for IT-SOFCs through a coprecipitation method. The nanocomposite is composed of a doped (Mn_0.6Mg_0.4)_0.8Sc_0.2Co₂O₄ (MMSCO) spinel oxide (84 wt %), an orthorhombic perovskite phase (11.3 wt %, the spontaneous combination of PrO₂ additives and spinel), and a minor Sc₂O₃ phase (4.7 wt %). The surface of the (Mn_0.6Mg_0.4)_0.8Sc_0.2Co₂O₄ phase is activated by the self-assembled nanocoating with many heterogeneous interfaces. Thence, the ORR kinetics is obviously accelerated and an area-specific resistance (ASR) of ∼0.11 Ω cm² is obtained at 750 °C. Moreover, a single cell with the cathode shows a peak power density (PPD) of 1144.1 mW cm^-2 at 750 °C, much higher than that of the cell with the MnCo₂O₄ cathode (456.2 mW cm^-2). An enhanced stability of ∼120 h (0.8 A cm^-2, 750 °C) is also achieved, related to the reduced thermal expansion coefficient (13.9 × 10^-6 K^-1). The improvement in ORR kinetics and stability can be attributed to the refinement of grains, the formation of heterointerfaces, and the enhancement of mechanical compatibility.

Trends and Prospects of Bimetallic Exsolution

Supported bimetallic nanoparticles used for various chemical transformations appear to be more appealing than their monometallic counterparts, because of their unique properties mainly originating from the synergistic effects between the two different metals. Exsolution, a relatively new preparation method for supported nanoparticles, has earned increasing attention for bimetallic systems in the past decade, not only due to the high stability of the resulting nanoparticles but also for the potential to control key particle properties (size, composition, structure, morphology, etc.). In this review, we summarize the trends and advances on exsolution of bimetallic systems and provide prospects for future studies in this field.

Progress of Exsolved Metal Nanoparticles on Oxides as High Performance (Electro)Catalysts for the Conversion of Small Molecules

Utilizing electricity and heat from renewable energy to convert small molecules into value-added chemicals through electro/thermal catalytic processes has enormous socioeconomic and environmental benefits. However, the lack of catalysts with high activity, good long-term stability, and low cost strongly inhibits the practical implementation of these processes. Oxides with exsolved metal nanoparticles have recently been emerging as promising catalysts with outstanding activity and stability for the conversion of small molecules, which provides new possibilities for application of the processes. In this review, it starts with an introduction on the mechanism of exsolution, discussing representative exsolution materials, the impacts of intrinsic material properties and external environmental conditions on the exsolution behavior, and the driving forces for exsolution. The performances of exsolution materials in various reactions, such as alkane reforming reaction, carbon monoxide oxidation, carbon dioxide utilization, high temperature steam electrolysis, and low temperature electrocatalysis, are then summarized. Finally, the challenges and future perspectives for the development of exsolution materials as high-performance catalysts are discussed.

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

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

La0.6Sr0.4Co0.2Fe0.8O3-δ/CeO2 Heterostructured Composite Nanofibers as a Highly Active and Robust Cathode Catalyst for Solid Oxide Fuel Cells

The lack of highly active and robust catalysts for the oxygen reduction reaction (ORR) at the intermediate temperatures significantly hinders the commercialization of solid oxide fuel cells (SOFCs). Here, we report a novel heterostructured composite nanofiber cathode composed of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and CeO₂ nanoparticles, synthesized by using a coaxial electrospinning technique, which exhibits remarkably enhanced ORR activity and durability as compared to single LSCF powder and nanofibers. This cathode achieves a polarization resistance of 0.031 Ω cm² at 700 °C, approximately 1/5 of that for the LSCF powder cathode (0.158 Ω cm²). Such enhancement can be attributed to the continuous paths provided by nanofibers for efficient mass/charge transport and the interdiffusion of La and Ce at the heterointerface which leads to more oxygen vacancy formation. Furthermore, the anode-supported cell with the LSCF/CeO₂ composite cathode shows excellent stability (0.4 V for ∼200 h at 600 °C) because of suppression of Sr segregation in LSCF by introducing CeO₂ and the structure of heterogeneous nanofibers. These results indicate that the microstructure design of this heterostructured composite nanofiber for LSCF/CeO₂ is extremely effective for enhancing ORR activity and stability. This finding may provide a new strategy for the microstructure design of highly active and robust ORR catalysts in SOFCs.