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

Ni-Doped SFM Double-Perovskite Electrocatalyst for High-Performance Symmetrical Direct-Ammonia-Fed Solid Oxide Fuel Cells

Ammonia has emerged as a promising fuel for solid oxide fuel cells (SOFCs) owing to its high energy density, high hydrogen content, and carbon-free nature. Herein, the electrocatalytic potential of a novel Ni-doped SFM double-perovskite (Sr1.9Fe0.4Ni0.1Mo0.5O6-δ) is studied, for the first time, as an alternative anode material for symmetrical direct-ammonia SOFCs. Scanning and transmission electron microscopy characterization has revealed the exsolution of Ni-Fe nanoparticles (NPs) from the parent Sr2Fe1.5Mo0.5O6 under anode conditions, and X-ray diffraction has identified the FeNi3 phase after exposure to ammonia at 800 °C. The active-exsolved NPs contribute to achieving a maximal ammonia conversion rate of 97.9% within the cell's operating temperatures (550-800 °C). Utilizing 3D-printed symmetrical cells with SFNM-GDC electrodes, the study demonstrates comparable polarization resistances and peak power densities of 430 and 416 mW cm-2 for H2 and NH3 fuels, respectively, with long-term stability and a negligible voltage loss of 0.48% per 100 h during ammonia-fed extended galvanostatic operation. Finally, the ammonia consumption mechanism is elucidated as a multistep process involving ammonia decomposition, followed by hydrogen oxidation. This study provides a promising avenue for improving the performance and stability of ammonia-based SOFCs for potential applications in clean energy conversion technologies.

In situ investigation of ruthenium doped lanthanum nickel titanium double perovskite and its exsolution behaviour

Exsolution, an innovative method for fabricating perovskite-based oxides decorated with metal nanoparticles, has garnered significant interest in the fields of catalyst fabrication and electrochemical devices. Although dopant exsolution from single perovskite structures has been extensively studied, the exsolution behaviour of double perovskite structures remains insufficiently understood. In this study, we synthesized B-site double perovskite Ru-doped lanthanum nickel titanates with a 7.5 at% A-site deficiency, and systematically investigated the exsolution process that formed nickel metal nanoparticles on the material surface, across a broad reduction temperature range of 350-1000 °C. Both Ex situ and in situ characterization revealed that small, uniform Ni nanoparticles exsolved at low temperatures, whereas the exsolution of ruthenium required higher reduction temperatures beyond 1000 °C. Within the reduction temperature range of 350-500 °C, a notable finding is the reconstruction of exsolved nanoparticles, implying that Ni particles exist in a thermodynamically metastable state. Electrochemical impedance spectroscopy (EIS) showed a decreased area specific resistance (ASR) during the progress of exsolution. The increase in current density of a full solid oxide cell (SOC) in electrolysis mode and the doubling of peak power density in fuel cell mode attributed to the exsolution of Ni nanoparticles highlight the potential application of metal exsolution in electrode materials for SOCs.

Accelerating metal nanoparticle exsolution by exploiting tolerance factor of perovskite stannate

The octahedral symmetry in ionic crystals can play a critical role in atomic nucleation and migration during solid-solid phase transformation. Similarly, octahedron distortion, which is characterized by Goldschmidt tolerance factor, strongly influences the exsolution kinetics in the perovskite lattice framework during high-temperature annealing. However, a fundamental study on manipulating the exsolution process by octahedron distortion is still lacking. In this study, we accelerate Ni metal exsolution on the surface of perovskite stannates by increasing the [BO6] octahedron distortion in the lattices. Decreasing the A-site ionic radius (rBa2+ = 161 pm → rSr2+ = 144 pm → rCa2+ = 134 pm) increased the density of exsolved Ni nanoparticles by up to 640% (i.e., 47 particles μm-2 of Ba(Sn, Ni)O3 → 304 particles μm-2 of Ca(Sn, Ni)O3) after the identical exsolution process. Based on the theoretical calculation and experimental characterization, the decrease in crystal symmetry by octahedral distortion promoted the Ni exsolution owing to the boosted Ni migration by weakening the bond strength and generating domain boundaries. The findings highlight the importance of octahedral distortion to control atomic migration through the perovskite lattice framework and provide a strategy to tailor the density of uniformly populated nanoparticles in nanocomposite oxides for multifunctional material design.

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.

Enhancing the CO2 Adsorption of the Cobalt-Free Layered Perovskite Cathode for Solid-Oxide Electrolysis Cells Gains Excellent Stability under High Voltages via Oxygen-Defect Adjustment

Solid-oxide electrolysis cells are a clean energy conversion device with the ability to directly electrolyze the conversion of CO2 to CO efficiently. However, their practical applications are limited due to insufficient CO2 adsorption performance of the cathode materials. To overcome this issue, the A-site cation deficiency strategy has been applied in a layered perovskite PrBaFe1.6Ni0.4O6-δ (PBFN) cathode for direct CO2 electrolysis. The introduction of 5% deficiency at the Pr/Ba site leads to a significant increase in the concentration of oxygen vacancies (nonstoichiometric number δ of oxygen vacancies increased from 0.093 to 0.132), which greatly accelerates the CO2 adsorption performance as well as the O2- transport capacity toward the CO2 reduction reaction (CO2RR). CO2 temperature-programmed desorption indicates that A-site cation-deficient (PrBa)0.95Fe1.6Ni0.4O6-δ (PB95FN) shows a larger desorption peak area and a higher desorption temperature. PB95FN also exhibits a greater presence of carbonate in Fourier transform infrared (FT-IR) spectroscopy. The electrical conductivity relaxation test shows that the introduction of the 5% A-site deficiency effectively improves the surface oxygen exchange and diffusion kinetics of PB95FN. The current density of the electrolysis cell with the (PrBa)0.95Fe1.6Ni0.4O6-δ (PB95FN) cathode reaches 0.876 A·cm-2 under 1.5 V at 800 °C, which is 41% higher than that of PB100FN. Moreover, the PB95FN cathode demonstrates excellent long-term stability over 100 h and better short-term stability than PB100FN under high voltages, which can be ascribed to the enhanced CO2 adsorption performance. The PB95FN cathode maintains a porous structure and tightly binds to the electrolyte after stability testing. This study highlights the potential of regulating oxygen defects in layered perovskite PrBaFe1.6Ni0.4O6-δ cathode materials via incorporation of cation deficiency toward high-temperature CO2 electrolysis.

Revealing the detrimental CO2 reduction effect of La0.6Sr0.4FeO3-δ-derived heterostructure in solid oxide electrolysis cells

Solid oxide electrolysis cells hold unique Faraday efficiency and favored thermodynamic/kinetics for CO2 reduction to CO. Perovskite oxide-based composite materials are promising alternatives to Ni-based cermet electrodes in SOECs. However, contrary results of the electrocatalytic activity over single-phase perovskite oxide exist and the rationale of the negative effect is not well revealed. In this work, two-phase perovskite materials with various complementary properties and unique interfaces are self-assembled, which was realized by "subtractive" defect-driven phase separation. The obtained heterostructure electrodes showed reduced performance over that of single-phase materials although the cyclic stability was improved. The main reasons for the performance degradation are the decrease of electrical conductivity, oxygen vacancy concentration while increasing the average valence state of B-site Fe cations, and electrode surface Sr aggregation. This work highlights the self-assembly method and insight into the rational design and synthesis of active electrodes/catalysts for CO2 conversion in solid oxide cells.

Smart Dual-Exsolved Self-Assembled Anode Enables Efficient and Robust Methane-Fueled Solid Oxide Fuel Cells

Perovskite oxides have emerged as alternative anode materials for hydrocarbon-fueled solid oxide fuel cells (SOFCs). Nevertheless, the sluggish kinetics for hydrocarbon conversion hinder their commercial applications. Herein, a novel dual-exsolved self-assembled anode for CH4 -fueled SOFCs is developed. The designed Ru@Ru-Sr2 Fe1.5 Mo0.5 O6-δ (SFM)/Ru-Gd0.1 Ce0.9 O2-δ (GDC) anode exhibits a unique hierarchical structure of nano-heterointerfaces exsolved on submicron skeletons. As a result, the Ru@Ru-SFM/Ru-GDC anode-based single cell achieves high peak power densities of 1.03 and 0.63 W cm-2 at 800 °C under humidified H2 and CH4 , surpassing most reported perovskite-based anodes. Moreover, this anode demonstrates negligible degradation over 200 h in humidified CH4 , indicating high resistance to carbon deposition. Density functional theory calculations reveal that the created metal-oxide heterointerfaces of Ru@Ru-SFM and Ru@Ru-GDC have higher intrinsic activities for CH4 conversion compared to pristine SFM. These findings highlight a viable design of the dual-exsolved self-assembled anode for efficient and robust hydrocarbon-fueled SOFCs.

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.

Enhancing Oxygen Evolution Reaction Performance in Prussian Blue Analogues: Triple-Play of Metal Exsolution, Hollow Interiors, and Anionic Regulation

Prussian blue analogs (PBAs) are promising catalysts for green hydrogen production. However, the rational design of high-performing PBAs is challenging, which requires an in-depth understanding of the catalytic mechanism. Here FeMn@CoNi core-shell PBAs are employed as precursors, together with Se powders, in low-temperature pyrolysis in an argon atmosphere. This synthesis method enables the partial dissociation of inner FeMn PBAs that results in hollow interiors, Ni nanoparticles (NPs) exsolution to the surface, and Se incorporation onto the PBA shell. The resulting material presents ultralow oxygen evolution reaction (OER) overpotential (184 mV at 10 mA cm-2 ) and low Tafel slope (43.4 mV dec-1 ), outperforming leading-edge PBA-based electrocatalysts. The mechanism responsible for such a high OER activity is revealed, assisted by density functional theory (DFT) calculations and the surface examination before and after the OER process. The exsolved Ni NPs are found to help turn the PBAs into Se-doped core-shell metal oxyhydroxides during the OER, in which the heterojunction with Ni and the Se incorporation are combined to improve the OER kinetics. This work shows that efficient OER catalysts could be developed by using a novel synthesis method backed up by a sound understanding and control of the catalytic pathway.

Atomistic Insights into Medium-Entropy Perovskites for Efficient and Robust CO2 Electrolysis

Solid oxide electrolysis cells (SOECs) show great promise in converting CO2 to valuable products. However, their practicality for the CO2 reduction reaction (CO2RR) is restricted by sluggish kinetics and limited durability. Herein, we propose a novel medium-entropy perovskite, Sr2(Fe1.0Ti0.25Cr0.25Mn0.25Mo0.25)O6-δ (SFTCMM), as a potential electrode material for symmetrical SOEC toward CO2RR. Experimental and theoretical results unveil that the configuration entropy of SFTCMM perovskites contributes to the strengthened metal 3d-O 2p hybridization and the reduced O 2p bond center. This variation of electronic structure benefits oxygen vacancy creation and diffusion as well as CO2 adsorption and activation and ultimately accelerates CO2RR and oxygen electrocatalysis kinetics. Notably, the SFTCMM-based symmetrical SOEC delivers an excellent current density of 1.50 A cm-2 at 800 °C and 1.5 V, surpassing the prototype Sr2Fe1.5Mo0.5O6-δ (SFM, 1.04 A cm-2) and most of the state-of-the-art electrodes for symmetrical SOECs. Moreover, the SFTCMM-based symmetrical SOEC demonstrates stable CO2RR operation for 160 h.

Enhancing Electrochemical CO2 Reduction on Perovskite Oxide for Solid Oxide Electrolysis Cells through In Situ A-Site Deficiencies and Surface Carbonate Deposition Induced by Lithium Cation Doping and Exsolution

Solid oxide electrolysis cells (SOECs) hold enormous potential for efficient conversion of CO2 to CO at low cost and high reaction kinetics. The identification of active cathodes is highly desirable to promote the SOEC's performance. This study explores a lithium-doped perovskite La0.6- x Lix Sr0.4 Co0.7 Mn0.3 O3-δ (x = 0, 0.025 0.05, and 0.10) material with in situ generated A-site deficiency and surface carbonate as SOEC cathodes  for CO2 reduction. The experimental results indicate that the SOEC with the La0.55 Li0.05 Sr0.4 Co0.7 Mn0.3 O3-δ cathode exhibits a current density of 0.991 A cm-2 at 1.5 V/800 °C, which is an improvement of ≈30% over the pristine sample. Furthermore, SOECs based on the proposed cathode demonstrate excellent stability over 300 h for pure CO2 electrolysis. The addition of lithium with high basicity, low valance, and small radius, coupled with A-site deficiency, promotes the formation of oxygen vacancy and modifies the electronic structure of active sites, thus enhancing CO2 adsorption, dissociation process, and CO desorption steps as corroborated by the experimental analysis and the density functional theory calculation. It is further confirmed that Li-ion migration to the cathode surface forms carbonate and consequently provides the perovskite cathode with an impressive anti-carbon deposition capability, as well as electrolysis activity.

Artificial intelligence-aided preparation of perovskite SrFexZr1-xO3-δ catalysts for ozonation degradation of organic pollutant concentrated water after membrane treatment

Membrane technology has been widely used to treat wastewater from a variety of industries, but it also results in a large amount of concentrated wastewater containing organic pollutants after membrane treatment, which is challenging to decompose. Here in this work, a series of perovskite SrFe_xZr_1-xO_3-δ catalysts were prepared via a modified co-precipitation method and evaluated for catalytic ozone oxidative degradation of m-cresol. An artificial neural intelligence networks (ANN) model was employed to train the experimental data to optimize the preparation parameters of catalysts, with SrFe_0.13Zr_0.87O_3-δ being the optimal catalysts. The resultant catalysts before and after reduction were then thoroughly characterized and tested for m-cresol degradation. It was found that the co-doping of Fe and Zr at the B-site and the improvement of oxygen vacancies and oxygen active species by reduction dramatically increased TOC removal rates up to 5 times compared with ozone alone, with the conversion rate of m-cresol reaching 100%. We also proposed a possible mechanism for m-cresol degradation via investigating the intermediates using GC-MS, and confirmed the good versatility of the reduced SrFe_0.13Zr_0.87O_3-δ catalyst to remove other common organic pollutants in concentrated wastewater. This work demonstrates new prospects for the use of perovskite materials in wastewater treatment.

Ni/NiO Exsolved Perovskite La0.2Sr0.7Ti0.9Ni0.1O3-δ for Semiconductor-Ionic Fuel Cells: Roles of Electrocatalytic Activity and Physical Junctions

A semiconductor-ionic fuel cell (SIFC) is recognized as a promising technology and an alternative approach to reduce the operating temperature of solid oxide fuel cells. The development of alternative semiconductors substituting easily reduced transition metal oxide is a great challenge as high activity and durability should be satisfied simultaneously. In this study, the B-site Ni-doped La_0.2Sr_0.7Ti_0.9Ni_0.1O_3-δ (LSTN) perovskite is synthesized and used as a potential semiconductor for SIFC. The in situ exsolution and A-site deficiency strategy enable the homogeneous decoration of Ni/NiO nanoparticles as reactive sites to improve the electrode reaction kinetics. It also supports the formation of basic ingredient of the Schottky junction to improve the charge separation efficiency. Furthermore, additional symmetric Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) electrocatalytic electrode layers significantly enhance the electrode reaction activity and cells' charge separation efficiency, as confirmed by the superior open circuit voltage of 1.13 V (close to Nernst's theoretical value) and peak power density of 650 mW cm^-2 at 550 °C, where the latter is one order of magnitude higher than NCAL electrode-free SIFC. Additionally, a bulk heterojunction effect is proposed to illustrate the electron-blocking and ion-promoting processes of the semiconductor-ionic composite electrolyte in SIFCs, based on the energy band values of the applied materials. Overall, we found that the energy conversion efficiency of novel SIFC can be remarkably improved through in situ exsolution and intentional introduction of the catalytic functionality.

Ti/Ni co-doped perovskite cathode with excellent catalytic activity and CO2 chemisorption ability via nanocatalysts exsolution for solid oxide electrolysis cell

Carbon dioxide (CO₂) gas is the main cause of global warming and has a significant effect on both climate change and human health. In this study, Ni/Ti co-doped Sr_1.95Fe_1.2Ni_0.1Ti_0.2Mo_0.5O_6-δ (SFNTM) double perovskite oxides were prepared and used as solid oxide electrolysis cell (SOEC) cathode materials for effective CO₂ reduction. Ti-doping enhances the structural stability of the cathode material and increases the oxygen vacancy concentration. After treatment in 10% H₂/Ar at 800°C, Ni nanoparticles were exsolved in situ on the SFNTM surface (Ni@SFNTM), thereby improving its chemisorption and activation capacity for CO₂. Modified by the Ti-doping and the in situ exsolved Ni nanoparticles, the single cell with Ni@SFNMT cathode exhibits improved catalytic activity for CO₂ reduction, exhibiting a current density of 2.54 A cm⁻² at 1.8 V and 800°C. Furthermore, the single cell shows excellent stability after 100 h at 1.4 V, indicating that Ni/Ti co-doping is an effective strategy for designing novel cathode material with high electrochemical performance for SOEC.