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

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.

Investigation of structural, morphological and magnetic properties of nanostructured strontium hexaferrite through co-precipitation technique: Impacts of annealing temperature and Fe/Sr ratio

M-type strontium hexaferrite (SrM) were successfully synthesized from Sr²⁺ and Fe³⁺ precursor salt through co-precipitation technique. Different higher sintering temperatures (800-1000 °C) were used to get the desired SrM with variation of Fe³⁺/Sr²⁺ mole ratio as well. The characterization of SrM and its properties were investigated using modern instrumental techniques viz. X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer, Scanning Electron Microscopy, Vibrating Sample Magnetometer, UV-Visible NIR Spectrometer, Impedance Analyzer and Thermal Conductivity Meter. The phase of the synthesized SrM were confirmed by comparing the XRD patterns with the standard ICDD data and Reitvelt Refinement for the SrM having Fe³⁺/Sr²⁺ ratio 10 and SrM with distinct annealing temperature were performed. The structural parameters, particle size (75 nm-318 nm) and shape of the as prepared samples were changed with calcination temperature as well as mole ratio. The saturation magnetization (73.77-24.27 emu/g), coercivity (3732.28-642.10 Oe) and remanant magnetization (39.15-8.86 emu/g) were varied with calcination temperature and composition. The dielectric properties, optical properties and thermophysical properties were measured for the SrM keeping Fe³⁺/Sr²⁺ ratio 10 calcined at 1000 °C. The synthesized SrM can be applied in magnetic recording media and as photocatalyst due to its low coercivity (2764.48 Oe), high saturation magnetization (73.77 emu/g) and low band gap energy (Eg-2.04 eV) respectively.

Exsolution constructed FeNi/NiFe2O4 composite: preferentially breaking of octahedral metal-oxygen bonds in spinel oxide

Exsolution is an ingenious strategy for the in situ construction of metal- or alloy-decorated oxides and, due to its promising energy related catalysis applications, has advanced from use in perovskites to use in spinels. Despite its great importance for designing target composites, the ability to identify whether active metal ions at octahedral or tetrahedral sites will preferentially exsolve in a spinel remains unexplored. Here, an inverse spinel NiFe₂O₄ (NFO) was employed as a prototype and FeNi/NFO composites were successfully constructed via exsolution. The preferential breaking of octahedral metal–oxygen bonds in the spinel oxide was directly observed using Mössbauer and X-ray absorption spectroscopy. This was further verified from the negative segregation energies calculated based on density-functional theory. One exsolved FeNi/NFO composite exhibits enhanced electrochemical activity with an overpotential of 283 mV at 10 mA cm⁻² and a long stability time for the oxygen evolution reaction. This work offers a unique insight into spinel exsolution based on the preferential breaking of chemical bonds and may be an effective guide for the design of new composite catalysts where the desired metal ions are deliberately introduced to octahedral and/or tetrahedral sites.

The preferential breaking of octahedral metal–oxygen bonds is exploited to construct an exsolved FeNi/NFO composite for an efficient oxygen evolution reaction.

Undoped Sr2MMoO6 Double Perovskite Molybdates (M = Ni, Mg, Fe) as Promising Anode Materials for Solid Oxide Fuel Cells

The chemical design of new functional materials for solid oxide fuel cells (SOFCs) is of great interest as a means for overcoming the disadvantages of traditional materials. Redox stability, carbon deposition and sulfur poisoning of the anodes are positioned as the main processes that result in the degradation of SOFC performance. In this regard, double perovskite molybdates are possible alternatives to conventional Ni-based cermets. The present review provides the fundamental properties of four members: Sr₂NiMoO_6-δ, Sr₂MgMoO_6-δ, Sr₂FeMoO_6-δ and Sr₂Fe_1.5Mo_0.5O_6-δ. These properties vary greatly depending on the type and concentration of the 3d-element occupying the B-position of A₂BB’O₆. The main emphasis is devoted to: (i) the synthesis features of undoped double molybdates, (ii) their electrical conductivity and thermal behaviors in both oxidizing and reducing atmospheres, as well as (iii) their chemical compatibility with respect to other functional SOFC materials and components of gas atmospheres. The information provided can serve as the basis for the design of efficient fuel electrodes prepared from complex oxides with layered structures.

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.

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.

Platinum-Decorated Ceria Enhances CO2 Electroreduction in Solid Oxide Electrolysis Cells

CO₂ electroreduction by solid oxide electrolysis cells (SOECs) can not only attenuate the greenhouse effect, but also convert surplus electrical energy into chemical energy. The adsorption and activation of CO₂ on the cathode play an important role in the SOEC performance. La_0.6 Sr_0.4 Co_0.2 Fe_0.8 O_3-δ -Ce_0.8 Sm_0.2 O_2-δ (LSCF-SDC; SDC=samarium-doped ceria) is a promising SOEC cathode. However, its electrocatalytic activity still needs to be improved. In this study, Pt/SDC interfaces are constructed by decorating Pt nanoparticles onto the SDC surface. Electrochemical measurements indicate that the polarization resistance of the SOEC is decreased from 0.308 to 0.120 Ω cm² , and the current density is improved from 0.913 to 1.420 A cm^-2 at 1.6 V and 800 °C. Physicochemical characterizations suggest that construction of the Pt/SDC interfaces increases the oxygen vacancy concentration on the cathode and boosts CO₂ adsorption and dissociation, which leads to enhanced CO₂ electroreduction performance in SOECs.

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.

Molten Salt Synthesis of High-Performance, Nanostructured La0.6Sr0.4FeO3-δ Oxygen Electrode of a Reversible Solid Oxide Cell

Nanoscale perovskite oxides with enhanced electrocatalytic activities have been widely used as oxygen electrodes of reversible solid oxide cells (RSOC). Here, La_0.6Sr_0.4FeO_3−δ (LSF) nanoscale powder is synthesized via a novel molten salt method using chlorides as the reaction medium and fired at 850 °C for 5 h after removing the additives. A direct assembly method is employed to fabricate the LSF electrode without a pre-sintering process at high temperature. The microstructure characterization ensures that the direct assembly process will not damage the porosity of LSF. When operating as a solid oxide fuel cell (SOFC), the LSF cell exhibits a peak power density of 1.36, 1.07 and 0.7 W/cm² at 800, 750 and 700 °C, respectively, while in solid oxide electrolysis cell (SOEC) mode, the electrolysis current density reaches 1.52, 0.98 and 0.53 A/cm² under an electrolysis voltage of 1.3 V, respectively. Thus, it indicates that the molten salt routine is a promising method for the synthesis of highly active perovskite LSF powders for directly assembled oxygen electrodes of RSOC.

The Conductivity and Dielectric Properties of Neobium Substituted Sr-Hexaferrites

The Nb3+ ion substituted Sr hexaferrites (SrNb_xFe_12−xO19 (x = 0.00–0.08) hexaferrites (HFs)) were fabricated via a citrate-assisted sol-gel approach. X-ray powder diffractometer analysis affirmed the pureness of all products. The crystallite sizes of the products which were estimated from Scherrer equation were in the 36–40 nm range. The chemical component of the samples was proved by Energy-dispersive X-ray spectroscopy (EDX) and Elemental mapping. The hexagonal morphology of all products was confirmed by Field Emission Scanning Electron Microscopy (FE-SEM). The electrical conduction mechanisms and dielectric properties of a variety of Nb3+ions-substituted SrNb_xFe_12−xO19 HFs were investigated by a complex impedance system. Dielectric parameters such as conductivity, dielectric constant, dielectric loss, dielectric tangent loss and complex modulus, were studied at temperatures up to 120 °C in a frequency range varying from 1.0 Hz to 3.0 MHz for several Nb ratios. The frequency dependence of the conductivity was found to comply with the power law with diverse exponents at all frequencies studied here. Subsequently, incremental tendencies in dc conductivity with temperature indicate that the substituted Sr-HFs leads to a semiconductor-semimetal like behavior. This could be attributable to a feature of conduction mechanism which is based on the tunneling processes. Additionally, the dielectric dispersion pattern was also explained by Maxwell–Wagner polarization in accordance with the Koop’s phenomenological theory.

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.