Metal Exsolution to Enhance the Catalytic Activity of Electrodes in Solid Oxide Fuel Cells

A High-Performance and Durable Direct-Ammonia Symmetrical Solid Oxide Fuel Cell with Nano La0.6Sr0.4Fe0.7Ni0.2Mo0.1O3-δ-Decorated Doped Ceria Electrode

Solid oxide fuel cells (SOFCs) offer a significant advantage over other fuel cells in terms of flexibility in the choice of fuel. Ammonia stands out as an excellent fuel choice for SOFCs due to its easy transportation and storage, carbon-free nature and mature synthesis technology. For direct-ammonia SOFCs (DA-SOFCs), the development of anode catalysts that have efficient catalytic activity for both NH3 decomposition and H2 oxidation reactions is of great significance. Herein, we develop a Mo-doped La0.6Sr0.4Fe0.8Ni0.2O3-δ (La0.6Sr0.4Fe0.7Ni0.2Mo0.1O3-δ, LSFNM) material, and explore its potential as a symmetrical electrode for DA-SOFCs. After reduction, the main cubic perovskite phase of LSFNM remained unchanged, but some FeNi3 alloy nanoparticles and a small amount of SrLaFeO4 oxide phase were generated. Such reduced LSFNM exhibits excellent catalytic activity for ammonia decomposition due to the presence of FeNi3 alloy nanoparticles, ensuring that it can be used as an anode for DA-SOFCs. In addition, LSFNM shows high oxygen reduction reactivity, indicating that it can also be a cathode for DA-SOFCs. Consequently, a direct-ammonia symmetrical SOFC (DA-SSOFC) with the LSFNM-infiltrated doped ceria (LSFNM-SDCi) electrode delivers a superior peak power density (PPD) of 487 mW cm-2 at 800 °C when NH3 fuel is utilised. More importantly, because Mo doping greatly enhances the reduction stability of the material, the DA-SSOFC with the LSFN-MSDCi electrode exhibits strong operational stability without significant degradation for over 400 h at 700 °C.

Ru-Ce0.7Zr0.3O2-δ as an Anode Catalyst for the Internal Reforming of Dimethyl Ether in Solid Oxide Fuel Cells

The development of direct dimethyl ether (DME) solid oxide fuel cells (SOFCs) has several drawbacks, due to the low catalytic activity and carbon deposition of conventional Ni-zirconia-based anodes. In the present study, the insertion of 2.0 wt.% Ru-Ce0.7Zr0.3O2-δ (ruthenium-zirconium-doped ceria, Ru-CZO) as an anode catalyst layer (ACL) is proposed to be a promising solution. For this purpose, the CZO powder was prepared by the sol-gel synthesis method, and subsequently, nanoparticles of Ru (1.0-2.0 wt.%) were synthesized by the impregnation method and calcination. The catalyst powder was characterized by BET-specific surface area, X-ray diffraction (XRD), field emission scanning electron microscopy with an energy-dispersive spectroscopy detector (FESEM-EDS), and transmission electron microscopy (TEM) techniques. Afterward, the catalytic activity of Ru-CZO catalyst was studied using DME partial oxidation. Finally, button anode-supported SOFCs with Ru-CZO ACL were prepared, depositing Ru-CZO onto the anode support and using an annealing process. The effect of ACL on the electrochemical performance of cells was investigated under a DME and air mixture at 750 °C. The results showed a high dispersion of Ru in the CZO solid solution, which provided a complete DME conversion and high yields of H2 and CO at 750 °C. As a result, 2.0 wt.% Ru-CZO ACL enhanced the cell performance by more than 20% at 750 °C. The post-test analysis of cells with ACL proved a remarkable resistance of Ru-CZO ACL to carbon deposition compared to the reference cell, evidencing the potential application of Ru-CZO as a catalyst as well as an ACL for direct DME SOFCs.

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.

Enhancing the Electrochemical Performance in Symmetrical Solid Oxide Cells through Nanoengineered Redox-Stable Electrodes with Exsolved Nanoparticles

Symmetrical solid oxide cells (SSOCs) have recently gained significant attention for their potential in energy conversion due to their simplified cell configuration, cost-effectiveness, and excellent reversibility. However, previous research efforts have mainly focused on improving the electrode performance of perovskite-type electrodes through different doping strategies, neglecting microstructural optimization. This work presents novel approaches for the nanostructural tailoring of (La0.8Sr0.2)0.95Fe1-xTixO3-δ (LSFTx, x = 0.2 and 0.4) electrodes using a single-step spray-pyrolysis deposition process. By incorporating these electrodes into a Ce0.9Gd0.1O1.95 (CGO) porous backbone or employing a nanocomposite architecture with nanoscale particle size, we achieved significant improvements in the polarization resistance (Rp) compared with traditional screen-printed electrodes. To further boost the fuel oxidation performance, a Ni-doping strategy, coupled with meticulous microstructural optimization, was implemented. The exsolution of Ni nanoparticles under reducing conditions resulted in remarkable Rp values as low as 0.34 and 0.11 Ω cm2 in air and wet H2 at 700 °C, respectively. Moreover, an electrolyte-supported cell with symmetrical electrodes demonstrated a stable maximum power density of 617 mW cm-2 at 800 °C. These findings highlight the importance of combining electrode composition optimization with advanced morphology control in the design of highly efficient and durable SSOCs.

Microwave-Driven Exsolution of Ni Nanoparticles in A-Site Deficient Perovskites

Exsolution has emerged as a promising method for generating metallic nanoparticles, whose robustness and stability outperform those of more conventional deposition methods, such as impregnation. In general, exsolution involves the migration of transition metal cations, typically perovskites, under reducing conditions, leading to the nucleation of well-anchored metallic nanoparticles on the oxide surface with particular properties. There is growing interest in exploring alternative methods for exsolution that do not rely on high-temperature reduction via hydrogen. For example, utilizing electrochemical potentials or plasma technologies has shown promising results in terms of faster exsolution, leading to better dispersion of nanoparticles under milder conditions. To avoid limitations in scaling up exhibited by electrochemical cells and plasma-generation devices, we proposed a method based on pulsed microwave (MW) radiation to drive the exsolution of metallic nanoparticles. Here, we demonstrate the H2-free MW-driven exsolution of Ni nanoparticles from lanthanum strontium titanates, characterizing the mechanism that provides control over nanoparticle size and dispersion and enhanced catalytic activity and stability for CO2 hydrogenation. The presented method will enable the production of metallic nanoparticles with a high potential for scalability, requiring short exposure times and low temperatures.

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.

A Recent Review of Electrospun Porous Carbon Nanofiber Mats for Energy Storage and Generation Applications

Electrospun porous carbon nanofiber mats have excellent properties, such as a large surface area, tunable porosity, and excellent electrical conductivity, and have attracted great attention in energy storage and power generation applications. Moreover, due to their exceptional properties, they can be used in dye-sensitized solar cells (DSSCs), membrane electrodes for fuel cells, catalytic applications such as oxygen reduction reactions (ORRs), hydrogen evolution reactions (HERs), and oxygen evolution reactions (OERs), and sensing applications such as biosensors, electrochemical sensors, and chemical sensors, providing a comprehensive insight into energy storage development and applications. This study focuses on the role of electrospun porous carbon nanofiber mats in improving energy storage and generation and contributes to a better understanding of the fabrication process of electrospun porous carbon nanofiber mats. In addition, a comprehensive review of various alternative preparation methods covering a wide range from natural polymers to synthetic carbon-rich materials is provided, along with insights into the current literature.

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.

Enhanced Electrolysis of CO2 with Metal–Oxide Interfaces in Perovskite Cathode in Solid Oxide Electrolysis Cell

The application of solid oxide electrolysis cell in CO2 electroreduction is a hot research topic at present, but the development of low−cost catalysts with high catalytic activity has always been a challenge for this work. Herein, we use NiCu alloy nanoparticles to modify the perovskite LSCM electrode to build a metal–oxide active interface to obtain high catalytic performance. At 850 °C, 4.66 mL min−1 cm−2 CO productivity and 97.7% Faraday current efficiency were obtained. In addition, the current remained stable during the 100 h long−term test, indicating that the active interface has the dual effect of improving catalytic performance and maintaining cell durability.

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

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

Nanoscale interface engineering for solid oxide fuel cells using atomic layer deposition

Atomic layer deposition (ALD), which is already actively used in the semiconductor industry, has been in the spotlight in various energy fields, such as batteries and fuel cells, given its unique ability to enable the nanoscale deposition of diverse materials with a variety of compositions onto complex 3D structures. In particular, with regard to ceramic fuel cells, ALD has attracted attention because it facilitates the manufacturing of thin and dense electrolytes. Furthermore, recently, electrode surfaces and electrode/electrolyte interface modification are arising as new research strategies to fabricate robust fuel cells. In this mini-review, we present a brief overview of ALD and recent studies that utilize ALD in ceramic fuel cells, such as manufacturing thin film electrolytes, stabilizing electrodes, functionalizing electrodes, and modifying the chemistry of electrode surfaces. We also propose research directions to expand the utility and functionality of the ALD techniques.