Sulfur-tolerant redox-reversible anode material for direct hydrocarbon solid oxide fuel cells

High-Performance Co-production of Electricity and Light Olefins Enabled by Exsolved NiFe Alloy Nanoparticles from a Double-Perovskite Oxide Anode in Solid Oxide-Ion-Conducting Fuel Cells

Light olefins (LOs) such as ethylene and propylene are critical feedstocks for many vital chemicals that support our economy and daily life. LOs are currently mass produced via steam cracking of hydrocarbons, which is highly energy intensive and carbon polluting. Efficient, low-emission, and LO-selective conversion technologies are highly desirable. Electrochemical oxidative dehydrogenation of alkanes in oxide-ion-conducting solid oxide fuel cell (SOFC) reactors has been reported in recent years as a promising approach to produce LOs with high efficiency and yield while generating electricity. We report here an electrocatalyst that excels in the co-production. The efficient catalyst is NiFe alloy nanoparticles (NPs) exsolved from a Pr- and Ni-doped double perovskite Sr2Fe1.5Mo0.5O6 (Pr0.8Sr1.2Ni0.2Fe1.3Mo0.5O6-δ, PSNFM) matrix during SOFC operation. We show evidence that Ni is first exsolved, which triggers the following Fe-exsolution, forming the NiFe NP alloy. At the same time as the NiFe exsolution, abundant oxygen vacancies are created at the NiFe/PSNFM interface, which promotes the oxygen mobility for oxidative dehydrogenation of propane (ODHP), coking resistance, and power generation. At 750 °C, the SOFC reactor with the PSNFM catalyst reaches a propane conversion of 71.40% and LO yield of 70.91% under a current density of 0.3 A cm-2 without coking. This level of performance is unmatchable by the current thermal catalytic reactors, demonstrating the great potential of electrochemical reactors for direct hydrocarbon conversion into value-added products.

Exsolution of Co-Fe Alloy Nanoparticles on the PrBaFeCoO5+δ Layered Perovskite Monitored by Neutron Powder Diffraction and Catalytic Effect on Dry Reforming of Methane

Reversible exsolution and dissolution of metal nanoparticles (NPs) in complex oxides have been investigated as an efficient strategy to improve the performance and durability of the catalysts for thermal and electrochemical energy conversion. Here, in situ exsolution of Co-Fe alloy NPs from the layered perovskite PrBaFeCoO_5+δ (PBFC) and their dissolution back into the oxide host have been monitored for the first time by in situ neutron powder diffraction and confirmed by X-ray diffraction and electron microscopy. Catalytic tests for dry reforming of methane showed stable operation over ∼100 h at 800 °C with negligible carbon deposition (<0.3 mg/g_cat h). The CO₂ and CH₄ conversions are among the highest achieved by layered double perovskites. The cyclability of the PBFC catalyst and the potential to improve the catalytic activity by adjusting the composition, size, and the NP distribution would pave the way for highly efficient energy conversion applications.

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.

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.

Mutual Conversion of CO-CO2 on a Perovskite Fuel Electrode with Endogenous Alloy Nanoparticles for Reversible Solid Oxide Cells

Reversible solid oxide cells (RSOCs) can efficiently render the mutual conversion between electricity and chemicals, for example, electrolyzing CO₂ to CO under a solid oxide electrolysis cell (SOEC) mode and oxidizing CO to CO₂ under a solid oxide fuel cell (SOFC) mode. Nevertheless, the development of RSOCs is still hindered, owing to the lack of catalytically active and carbon-tolerant fuel electrodes. For improving mutual CO-CO₂ conversion kinetics in RSOCs, here, we demonstrate a high-performing and durable fuel electrode consisting of redox-stable Sr₂(Fe, Mo)₂O_6-δ perovskite oxide and epitaxially endogenous NiFe alloy nanoparticles. The electrochemical impedance spectrum (EIS) and distribution of relaxation time (DRT) analyses reveal that surface/interface oxygen exchange kinetics and the CO/CO₂ activation process are both greatly accelerated. The assembled single cell produces a maximum power density (MPD) of 443 mW cm^-2 at 800 °C under the SOFC mode, with the corresponding CO oxidation rate of 5.524 mL min^-1 cm^-2. On the other hand, a current density of -0.877 A cm^-2 is achieved at 1.46 V under the SOEC mode, equivalent to a CO₂ reduction rate of 6.108 mL min cm^-2. Furthermore, reliable reversible conversion of CO-CO₂ is proven with no performance degradation in 20 cycles under SOEC (1.3 V) and SOFC (0.6 V) modes. Therefore, our work provides an alternative way for designing highly active and durable fuel electrodes for RSOC applications.

A critical review on nano-structured electrodes of solid oxide cells

Renewable energies from solar and wind power are playing an ever increasing role in meeting the tremendous global energy demand with substantially reduced carbon emissions, however, their intermittent nature poses...

Unveiling the key factor for the phase reconstruction and exsolved metallic particle distribution in perovskites

To significantly increase the amount of exsolved particles, the complete phase reconstruction from simple perovskite to Ruddlesden-Popper (R-P) perovskite is greatly desirable. However, a comprehensive understanding of key parameters affecting the phase reconstruction to R-P perovskite is still unexplored. Herein, we propose the Gibbs free energy for oxygen vacancy formation in Pr_0.5(Ba/Sr)_0.5TO_3-δ (T = Mn, Fe, Co, and Ni) as the important factor in determining the type of phase reconstruction. Furthermore, using in-situ temperature & environment-controlled X-ray diffraction measurements, we report the phase diagram and optimum 'x' range required for the complete phase reconstruction to R-P perovskite in Pr_0.5Ba_0.5-xSr_xFeO_3-δ system. Among the Pr_0.5Ba_0.5-xSr_xFeO_3-δ, (Pr_0.5Ba_0.2Sr_0.3)₂FeO_4+δ - Fe metal demonstrates the smallest size of exsolved Fe metal particles when the phase reconstruction occurs under reducing condition. The exsolved nano-Fe metal particles exhibit high particle density and are well-distributed on the perovskite surface, showing great catalytic activity in fuel cell and syngas production.

Activation Strategies of Perovskite-Type Structure for Applications in Oxygen-Related Electrocatalysts

The oxygen-related electrochemical process, including the oxygen evolution reaction and oxygen reduction reaction, is usually a kinetically sluggish reaction and thus dominates the whole efficiency of energy storage and conversion devices. Owing to the dominant role of the oxygen-related electrochemical process in the development of electrochemical energy, an abundance of oxygen-related electrocatalysts is discovered. Among them, perovskite-type materials with flexible crystal and electronic structures have been researched for a long time. However, most perovskite materials still show low intrinsic activity, which highlights the importance of activation strategies for perovskite-type structures to improve their intrinsic activity. In this review, the recent progress of the activation strategies for perovskite-type structures is summarized and their related applications in oxygen-related electrocatalysis reactions, including electrochemistry water splitting, metal-air batteries, and solid oxide fuel cells are discussed. Furthermore, the existing challenges and the future perspectives for the designing of ideal perovskite-type structure catalysts are proposed and discussed.

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.

Unveiling the Interface Structure of the Exsolved Co-Fe Alloy Nanoparticles from Double Perovskite and Its Application in Solid Oxide Fuel Cells

Exsolution of catalytic nanoparticles (NPs) from perovskites has arisen as a flexible method to develop high-performance functional materials with enhanced durability for energy conversion and catalytic synthesis applications. Here, we unravel the interface structure of the in situ exsolved alloy nanoparticles from the double perovskite substrate on the atomic scale. The results show that the Co-Fe alloy NPs exsolved topologically from the {100} facets terminations of the Sr₂FeMo_0.65Co_0.35O_6-δ (SFMC) double perovskite along ⟨100⟩ directions exhibiting the same orientation and identical crystal structure. The lattice planes of these two phases align and insert into each other at the interface, forming a smooth and continuous coherent connection. The presence of moiré patterns at the interface confirms the topological exsolution mechanism. The coherent interface can significantly reduce the interfacial energy and therefore stabilize the exsolved nanoparticles. Therefore, excellent and stable electrochemical performance of the NP-decorated SFMC perovskite is observed as the anode for solid oxide fuel cells. Our contribution promotes a fundamental understanding of the interface structure of the in situ exsolved alloy nanoparticles from perovskite substrate.

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

Exsolution is a novel technology for attaching metal catalyst particles onto ceramic anodes in the solid oxide fuel cells (SOFCs). The exsolved metal particles in the anode exhibit unique properties for reaction and have demonstrated remarkable stabilities under conditions that normally lead to coking. Despite extensive investigations, the underlying principles behind exsolution are still under investigation. In this review, the present status of exsolution materials for SOFC applications is reported, including a description of the fundamental concepts behind metal incorporation in oxide lattices, a listing of proposed mechanisms and thermodynamics of the exsolution process and a discussion on the catalytic properties of the resulting materials. Prospects and opportunities to use materials produced by exsolution for SOFC are discussed.

Progress in the use of electrospun nanofiber electrodes for solid oxide fuel cells: a review

The application of one-dimensional nanofibers in the fabrication of an electrode greatly improves the performance of solid oxide fuel cells (SOFCs) due to its advantages on electron transfer and mass transport. Various mixed ionic-electronic conducting materials with perovskites and Ruddlesden-Popper-type metal oxide structures are successfully electrospun into nanofibers in recent years mostly in solvent solution and some in melt forms, which are used as anode and cathode electrodes for SOFCs. This paper presents a comprehensive review of the structure, electrochemical performance, and development of anode and cathode nanofiber electrodes including processing, structure, and property characterization. The focuses are first on the precursor, applied voltage, and polymer in the material electrospinning process, the performance of the fiber, potential limitation and drawbacks, and factors affecting fiber morphology, and sintering temperature for impurity-free fibers. Information on relevant methodologies for cell fabrication and stability issues, polarization resistances, area specific resistance, conductivity, and power densities are summarized in the paper, and technology limitations, research challenges, and future trends are also discussed. The concluded information benefits improvement of the material properties and optimization of microstructure of the electrodes for SOFCs.

Construction of Multifunctional Nanoarchitectures in One Step on a Composite Fuel Catalyst through In Situ Exsolution of La0.5Sr0.5Fe0.8Ni0.1Nb0.1O3-δ

Multifunctional nanoarchitecture (MNA) on catalysts has attracted great attention because of its capability to improve the performance, durability, and resistance to unwanted side reactions. Such structures, however, are conventionally prepared by deposition methods, which inherently suffer from costly and time-consuming drawbacks. Here, we report a simple one-step process to successfully construct a novel MNA with core-shell nanoparticles anchored at the heterointerface of dual-phase oxide substrates through a phase transition and in situ exsolution of perovskite La_0.5Sr_0.5Fe_0.8Ni_0.1Nb_0.1O_3-δ (LSFNNb0.1) in wet H₂ (3% H₂O) at 800 °C. The core-shell nanoparticles are composed of a Ni-Fe alloy core and a SrLaFeO₄-type layered perovskite oxide shell (RP-Ruddlesden-Popper-layered perovskites), which synergistically improves the electrochemical activity and effectively suppresses aggregation and coarsening of the metallic core. The RP phase also covers the surface of perovskite bulk (SP-single perovskite), forming the heterointerface and preventing further decomposition of the SP phase. The RP/SP heterointerface may improve the kinetics of surface exchange of oxygen species, resulting in the enhancement of performance and durability of the reduced LSFNNb0.1 as an anode for solid oxide fuel cells (SOFCs). A doped zirconia electrolyte-supported single cell with the anode achieves the maximum power density (MPD) of 0.83 W cm^-2 at 800 °C in wet H₂, and the corresponding polarization resistance is as low as 0.15 Ω cm². This work reveals the formation mechanism of the MNA by investigating the evolution of the crystal structure, composition and morphology of LSFNNb0.1, when changing reducing temperature and time in wet H₂ and 5% H₂-Ar. The oxygen vacancies and phase transitions are found to play important roles in the formation of the MNA. The construction of MNAs in one step opens a new opportunity to design and prepare high-performance and stable catalysts for applications in energy conversion and storage.

Redox-Reversible Electrode Material for Direct Hydrocarbon Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) can directly operate on hydrocarbon fuels such as natural gas; however, the widely used nickel-based anodes face grand challenges such as coking, sulfur poisoning, and redox instability. We report a novel double perovskite oxide Sr₂Co_0.4Fe_1.2Mo_0.4O_6-δ (SCFM) that possesses excellent redox reversibility and can be used as both the cathode and the anode. When heat-treated at 900 °C in a reducing environment, double perovskite phase SCFM transforms into a composite of the Ruddlesden-Popper structured oxide Sr₃Co_0.1Fe_1.3Mo_0.6O_7-δ (RP-SCFM) with the Co-Fe alloy nanoparticles homogeneously distributed on the surface of RP-SCFM. At 900 °C in an oxidizing atmosphere, the composite transforms back into the double perovskite phase SCFM. The excellent oxygen reduction reaction catalytic activity and mixed ionic-electronic conductivity make SCFM an excellent cathode material for SOFCs. When SCFM is used as the anode, excellent performance and stability are achieved upon either direct oxidation of methane as a fuel or operation with sulfur-containing fuels. The excellent redox reversibility coupled with outstanding electrical and catalytic properties manifested by SCFM will enable a broad application in energy conversion applications.

In Situ Investigation of Reversible Exsolution/Dissolution of CoFe Alloy Nanoparticles in a Co-Doped Sr2 Fe1.5 Mo0.5 O6- δ Cathode for CO2 Electrolysis

Reversible exsolution and dissolution of metal nanoparticles in perovskite has been investigated as an efficient strategy to improve CO₂ electrolysis performance. However, fundamental understanding with regard to the reversible exsolution and dissolution of metal nanoparticles in perovskite is still scarce. Herein, in situ exsolution and dissolution of CoFe alloy nanoparticles in Co-doped Sr₂ Fe_1.5 Mo_0.5 O_6-δ (SFMC) revealed by in situ X-ray diffraction, scanning transmission electron microscopy, environmental scanning electron microscopy, and density functional theory calculations are reported. Under a reducing atmosphere, facile exsolution of Co promotes reduction of the Fe cation to generate CoFe alloy nanoparticles in SFMC, accompanied by structure transformation from double perovskite to layered perovskite at 800 °C. Under an oxidizing atmosphere, spherical CoFe alloy nanoparticles are first oxidized to flat CoFeO_x nanosheets, and then dissolved into the bulk with structure evolution from layered perovskite back to double perovskite. Electrochemically, CO₂ electrolysis performance can be retrieved during 12 redox cycles due to the regenerative ability of the CoFe alloy nanoparticles. The anchoring of the CoFe alloy nanoparticles in SFMC perovskite via reduction shows enhanced CO₂ electrolysis performance and stability compared with the parent SFMC perovskite.

Gas-Permeable Inorganic Shell Improves the Coking Stability and Electrochemical Reactivity of Pt toward Methane Oxidation

Solid oxide fuel cells produce electricity directly by oxidizing methane, which is the most attractive natural gas fuel, and metal nanocatalysts are a promising means of overcoming the poor catalytic activity of conventional ceramic electrodes. However, the lack of thermal and chemical stability of nanocatalysts is a major bottleneck in the effort to ensure the lifetime of metal-decorated electrodes for methane oxidation. Here, for the first time, this issue is addressed by encapsulating metal nanoparticles with gas-permeable inorganic shells. Pt particles approximately 10 nm in size are dispersed on the surface of a porous La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) electrode via wet infiltration and are then coated with an ultrathin Al₂O₃ layer via atomic layer deposition. The Al₂O₃ overcoat, despite being an insulator, significantly enhances the immunity to carbon coking and provides high activity for the electrochemical oxidation of methane, thereby reducing the reaction impedance of the Pt-decorated electrode by more than 2 orders of magnitude and making the electrode activity of the Pt-decorated sample at 650 °C comparable with those reported at 800 °C for pristine LSCM electrodes. These observations provide a new perspective on strategies to lower the operation temperature, which has long been a challenge related to hydrocarbon-fueled solid oxide fuel cells.

Approaching Durable Single-Layer Fuel Cells: Promotion of Electroactivity and Charge Separation via Nanoalloy Redox Exsolution

Single-layer fuel cells (SLFCs) based on mixed semiconductors and ionic conductors demonstrate  simplified material preparation and fabrication procedure and possess high performance potentially. However, the operational stability and principle of SLFCs have not yet been convinced of either commercialization or fundamental interests. We hereby report on the employment of a perovskite oxide-based phase-structured redox-stable semiconductor prior to determining a possible solution that improves the durability of the SLFC. Feasible working principles are established and an in-depth understanding of the short-circuit-free phenomenon in SLFCs with the mixed ionic and electronic conductors is provided. Additionally, a smart material design and cell structure processing are also proposed. An extended nonstop testing period of up to 2 days confirms the project feasibility and improved durability of the SLFCs, achieved by replacing the unstable lithiated oxide phase with redox-stable perovskite oxide, though the electrochemical performance is sacrificed. The precipitated metal/alloy nanoparticle on perovskite oxide not only improves the electrode reaction kinetics but also facilitates the charge separation and ionic conduction in SLFCs, consequently enhancing the fuel cell performance and electrical efficiency. The results confirmed the potential of stable operation for future practical deployment of SLFCs via appropriate selection of material and cell structure design. It is greatly believed that the physical junction plays a crucial role in overcoming the internal short-circuit issue of SLFCs.

A-Site Ordered Double Perovskite with in Situ Exsolved Core-Shell Nanoparticles as Anode for Solid Oxide Fuel Cells

A highly active anode material for solid oxide fuel cells resistant to carbon deposition is developed. Co-Fe co-doped La_0.5Ba_0.5MnO_3-δ with a cubic-hexagonal heterogeneous stucture is synthesized through the Pechini method. An A-site ordered double perovskite with Co_0.94Fe_0.06 alloy-oxide core-shell nanoparticles on its surface is formed after reduction. The phase transition and the exsolution of the nanoparticles are investigated with X-ray diffraction, thermogravimetric analysis, and high-resolution transmission electron microscope. The exsolved nanoparticles with the layered double-perovskite supporter show a high catalytic activity. A single cell with that anode and a 300 μm thick La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ electrolyte layer exhibits maximum power densities of 1479 and 503 mW cm^-2 at 850 °C with wet hydrogen and wet methane fuels, respectively. Moreover, the single cell fed with wet methane exhibits a stable power output at 850 °C for 200 h, demonstrating a high resistance to carbon deposition of the anode due to the strong anchor of the exsolved nanoparticles on the perovskite parent. The oxide shell also preserves the metal particles from coking.

Cation-swapped homogeneous nanoparticles in perovskite oxides for high power density

Exsolution has been intensively studied in the fields of energy conversion and storage as a method for the preparation of catalytically active and durable metal nanoparticles. Under typical conditions, however, only a limited number of nanoparticles can be exsolved from the host oxides. Herein, we report the preparation of catalytic nanoparticles by selective exsolution through topotactic ion exchange, where deposited Fe guest cations can be exchanged with Co host cations in PrBaMn_1.7Co_0.3O_5+δ. Interestingly, this phenomenon spontaneously yields the host PrBaMn_1.7Fe_0.3O_5+δ, liberating all the Co cations from the host owing to the favorable incorporation energy of Fe into the lattice of the parent host (ΔE_{incorporation} = −0.41 eV) and the cation exchange energy (ΔE_exchange = −0.34 eV). Remarkably, the increase in the number of exsolved nanoparticles leads to their improved catalytic activity as a solid oxide fuel cell electrode and in the dry reforming of methane.

Exsolution is attractive for the preparation of catalytically active metal nanoparticles, but versatility is limited. Here the authors report a technique for selective exsolution through topotactic ion exchange, leading to an electrocatalyst for a solid oxide fuel cell with enhanced performance.

Impact of Strain-Induced Changes in Defect Chemistry on Catalytic Activity of Nd2NiO4+δ Electrodes

It is well known that defect chemistry plays a vital role in determining the electronic structure, ionic conductivity, and catalytic activity of metal oxides, as demonstrated in perovskite-based oxides to achieve desired functionalities. In this work, we explored the possibility of tuning the defect chemistry and hydrogen oxidation reaction (HOR) activity of Nd₂NiO_4+δ model thin films by controlling the lattice strain. Highly textured Nd₂NiO_4+δ thin films with different strain states were prepared on (110)- and (100)-oriented single-crystal yttrium-stabilized zirconium (YSZ) substrates using pulsed laser deposition. Electrochemical impedance spectroscopy results indicated that the NNO(100) film on the YSZ(110) substrate with larger tensile strain in the a- b plane and compressive strain along the c axis exhibited higher HOR activity than the NNO(110) film on the YSZ(100) substrate at 500-600 °C. The enhancement in HOR activity is attributed to the strain-induced difference in the oxygen defect concentration, as confirmed by high-resolution X-ray diffraction analysis. We believe that the correlation among the strain state, defect chemistry, and catalytic properties is helpful for rational design of more efficient electrode materials.

A Highly Efficient and Robust Perovskite Anode with Iron-Palladium Co-exsolutions for Intermediate-Temperature Solid-Oxide Fuel Cells

The low performance and insufficient catalytic activity of perovskite anodes hinder their further application in intermediate-temperature solid-oxide fuel cells (IT-SOFCs). A novel La_0.8 Sr_0.2 Fe_0.9 Nb_0.1 Pd_0.04 O_3-δ (LSFNP) anode material has been developed with Fe-Pd co-exsolutions for IT-SOFCs. Fe⁰ and Pd⁰ metallic nanoparticles are confirmed to exsolve on the surface of the perovskite anode during operation under a hydrogen atmosphere. The introduced Pd exsolutions promote the charge-transfer process slightly and the H₂ -adsorption ability of the La_0.8 Sr_0.2 Fe_0.9 Nb_0.1 O_3-δ (LSFN) parent anode significantly, as metallic Pd is a conductor with excellent catalytic activity and an absorber of hydrogen that can absorb a large amount of H₂ by forming unstable chemical bonds. A single cell with the LSFNP anode exhibits high output performance (maximum power density of 287.6 mW cm^-2 at T=800 °C by using humidified H₂ as the fuel), excellent redox stability, and considerable coking and sulfur tolerances. After the introduction of Pd exsolutions, the increase in the electrochemical performance is more significant under low H₂ concentrations and at low temperatures with a maximum power density ratio of the LSFNP anode cell/LSFN anode cell reaching 18 under 5 % H₂ /argon at T=650 °C. Pd-decorated LSFNP is a high-performance, redox-stable, coking-tolerant, and sulfur-tolerant anode material for IT-SOFCs, making Pd exsolution a reliable nanodecoration strategy to improve the low kinetics of perovskite anodes.

Energetics of Nanoparticle Exsolution from Perovskite Oxides

The presence of active metal nanoparticles on the surface significantly increases the electrochemical performance of ABO₃ perovskite oxide materials. While conventional deposition methods can improve the activity, in situ exsolution produces nanoparticles with far greater stability. The migration of transition metal atoms toward the surface is expected to affect the exsolution process. To study the energetics, we use ab initio computations combined with experiments in a SrTiO₃-based model system. Our calculations show that Ni preferentially segregates toward the (100)-oriented and SrTiO-terminated surfaces, note that this orientation is identical to one reported by the Irvine and Gorte groups. Vacancies in the Sr-site and O-site promote the segregation of Ni, while placing La on the Sr-site has an opposite effect. The corresponding experiments are in agreement with the computational predictions. Fast nanoparticle growth and activity enhancement are found in STO system with Sr vacancies and without La. The approach developed in this Letter could be used to study the mechanism of exsolution in other material systems, and possibly lead to the development of new compositions capable of nanoparticle exsolution with higher activity and stability.

Controlling cation segregation in perovskite-based electrodes for high electro-catalytic activity and durability

Solid oxide cell (SOC) based energy conversion systems have the potential to become the cleanest and most efficient systems for reversible conversion between electricity and chemical fuels due to their high efficiency, low emission, and excellent fuel flexibility. Broad implementation of this technology is however hindered by the lack of high-performance electrode materials. While many perovskite-based materials have shown remarkable promise as electrodes for SOCs, cation enrichment or segregation near the surface or interfaces is often observed, which greatly impacts not only electrode kinetics but also their durability and operational lifespan. Since the chemical and structural variations associated with surface enrichment or segregation are typically confined to the nanoscale, advanced experimental and computational tools are required to probe the detailed composition, structure, and nanostructure of these near-surface regions in real time with high spatial and temporal resolutions. In this review article, an overview of the recent progress made in this area is presented, highlighting the thermodynamic driving forces, kinetics, and various configurations of surface enrichment and segregation in several widely studied perovskite-based material systems. A profound understanding of the correlation between the surface nanostructure and the electro-catalytic activity and stability of the electrodes is then emphasized, which is vital to achieving the rational design of more efficient SOC electrode materials with excellent durability. Furthermore, the methodology and mechanistic understanding of the surface processes are applicable to other materials systems in a wide range of applications, including thermo-chemical photo-assisted splitting of H₂O/CO₂ and metal-air batteries.

Highly efficient electrochemical reforming of CH4/CO2 in a solid oxide electrolyser

Reforming CH₄ into syngas using CO₂ remains a fundamental challenge due to carbon deposition and nanocatalyst instability. We, for the first time, demonstrate highly efficient electrochemical reforming of CH₄/CO₂ to produce syngas in a solid oxide electrolyser with CO₂ electrolysis in the cathode and CH₄ oxidation in the anode. In situ exsolution of an anchored metal/oxide interface on perovskite electrode delivers remarkably enhanced coking resistance and catalyst stability. In situ Fourier transform infrared characterizations combined with first principle calculations disclose the interface activation of CO₂ at a transition state between a CO₂ molecule and a carbonate ion. Carbon removal at the interfaces is highly favorable with electrochemically provided oxygen species, even in the presence of H₂ or H₂O. This novel strategy provides optimal performance with no obvious degradation after 300 hours of high-temperature operation and 10 redox cycles, suggesting a reliable process for conversion of CH₄ into syngas using CO₂.

Niobium Doped Lanthanum Strontium Ferrite as A Redox-Stable and Sulfur-Tolerant Anode for Solid Oxide Fuel Cells

The Nb-doped lanthanum strontium ferrite perovskite oxide La_0.8 Sr_0.2 Fe_0.9 Nb_0.1 O_3-δ (LSFNb) is evaluated as an anode material in a solid oxide fuel cell (SOFC). The effects of Nb partial substitution in the crystal structure, the electrical conductivity, and the valence of Fe ions are studied. LSFNb exhibits good structural stability in a severe reducing atmosphere at 800 °C, suggesting that high-valent Nb can effectively promote the stability of the lattice structure. The concentration of Fe²⁺ increases after Nb doping, as confirmed by X-ray photoelectron spectroscopy. The maximum power density of a thick Sc-stabilized zirconia (ScSZ) electrolyte-supported single cell reached 241.6 mW cm^-2 at 800 °C with H₂ as fuel. The cell exhibited excellent stability for 100 h continuous operation without detectable degeneration. Scanning electron microscopy clearly revealed exsolution on the LSFNb surface after operation. Meanwhile, LSFNb particles agglomerated significantly during long-term stability testing. Impedance spectra suggested that both the LSFNb anode and the (La_0.75 Sr_0.25 )_0.95 MnO_3-δ /ScSZ cathode underwent an activation process during long-term testing, through which the charge transfer ability increased significantly. Meanwhile, low-frequency resistance (R_L ) mainly attributed to the anode (80 %) significantly increased, probably due to the agglomeration of LSFNb particles. The LSFNb anode exhibits excellent anti-sulfuring poisoning ability and redox stability. These results demonstrate that LSFNb is a promising anode material for SOFCs.

A self-assembled dual-phase composite as a precursor of high-performance anodes for intermediate temperature solid oxide fuel cells

A durable high-performance anode composed of Fe–Cu nano-fibers, a Ruddlesden–Popper layered perovskite and a single perovskite is prepared from a dual-phase precursor.