Dual-Doping Strategy for Lowering the Thermal Expansion Coefficient and Promoting the Catalytic Activity in Perovskite Cobaltate Air Electrodes for Solid Oxide Cells

Lowering the thermal expansion coefficient (TEC) and promoting the catalytic activity of cobalt-based perovskite air electrodes is crucial for efficient solid oxide cells (SOCs) devices. However, the co-achievement of both merits has usually been largely compromised in most scenarios. Herein, a dual-doping strategy to manipulate the properties of perovskite cobaltate electrocatalyst is reported in which a high valence element of Ta5+ is incorporated into B-site to significantly suppress the dynamic reduction of Co4+ species and reduces the TEC value from PrBaCo2O5+δ (PBC, 17.8 × 10⁻6 K-1) to PrBaCo1.96Ta0.04O5+δ (PBCT, 12.5 × 10⁻6 K-1) and suppresses the oxygen loss in SOCs operation condition, revealing the improved structural stability. Meanwhile, the Ca2+ is doped into A-site of Ta-incorporated candidate, further decreasing the covalency of Co─O bonds and facilitating the formation of oxygen vacancies, benefiting the oxygen exchange kinetics and leading to a low polarization resistance of 0.026 Ω cm2 (800 °C) in as-prepared PrBa0.8Ca0.2Co1.96Ta0.04O5+δ (PBCCT) electrode. The cell with PBCTT demonstrates remarkable robustness during a 50 h thermal cycling test (25 cycles). Moreover, it delivers a high current density of 1.44 A cm⁻2 (1.6 V, 800 °C), as well as attractive durability over 100 h for pure CO2 electrolysis.

Recent Advances in Barium Cobaltite-Based Perovskite Oxides as Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are considered as advanced energy conversion technologies due to the high efficiency, fuel flexibility, and all-solid structure. Nevertheless, their widespread applications are strongly hindered by the high operational temperatures, limited material selection choices, inferior long-term stability, and relatively high costs. Therefore, reducing operational temperatures of SOFCs to intermediate-temperature (IT, 500-800 °C) range can remarkably promote the practical applications by enabling the use of low-cost materials and enhancing the cell stability. Nevertheless, the conventional cathodes for high-temperature SOFCs display inferior electrocatalytic activity for oxygen reduction reaction (ORR) at reduced temperatures. Barium cobaltite (BaCoO3-δ)-based perovskite oxides are regarded as promising cathodes for IT-SOFCs because of the high free lattice volume and large oxygen vacancy content. However, BaCoO3-δ-based perovskite oxides suffer from poor structural stability, inferior thermal compatibility, and insufficient ionic conductivity. Herein, an in-time review about the recent advances in BaCoO3-δ-based cathodes for IT-SOFCs is presented by emphasizing the material design strategies including functional/selectively doping, deficiency control, and (nano)composite construction to enhance the ORR activity/durability and thermal compatibility. Finally, the currently existed challenges and future research trends are presented. This review will provide valuable insights for the development of BaCoO3-δ-based electrocatalysts for various energy conversion/storage technologies.

Suppressing Structure Delamination for Enhanced Electrochemical Performance of Solid Oxide Cells

Reversible solid oxide cells (rSOCs) have significant potential as efficient energy conversion and storage systems. Nevertheless, the practical application of their conventional air electrodes, such as La0.8Sr0.2MnO3-δ (LSM), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), and PrBa0.8Ca0.2Co2O5+δ (PBCC), remains unsatisfactory due to interface delamination during prolonged electrochemical operation. Using micro-focusing X-ray absorption spectroscopy (µ-XAS), a decrease (increase) in the co-valence state from the electrode surface to the electrode/electrolyte interface is observed, leading to the above delamination. Utilizing the one-pot method to incorporate an oxygen-vacancy-enriched CeO2 electrode into these air electrodes, the uniform distribution of the Co valence state is observed, alleviating the structural delamination. PBCC-CeO2 electrodes exhibited a degradation rate of 0.095 mV h-1 at 650 °C during a nearly 500-h test as compared with 0.907 mV h-1 observed during the 135-h test for PBCC. Additionally, a remarkable increase in electrolysis current density from 636 to 934 mA cm-2 under 1.3 V and a maximum power density from 912 to 989 mW cm-2 upon incorporating CeO2 into PBCC is also observed. BSCF-CeO2 and LSM-CeO2 also show enhanced electrochemical performance and prolonged stability as compared to BSCF and LSM. This work offers a strategy to mitigate the structural delamination of conventional electrodes to boost the performance of rSOCs.

Oxygen Migration Pathways in Layered LnBaCo2O6-δ (Ln = La - Y) Perovskites

Layered LnBaCo2O6-δ perovskites are important mixed ionic-electronic conductors, exhibiting outstanding catalytic properties for the oxygen evolution/reduction reaction. These phases exhibit considerable structural complexity, in particular, near room temperature, where a number of oxygen vacancy ordered superstructures are found. This study uses bond valence site energy calculations to demonstrate the key underlying structural features that favor facile ionic migration. BVSE calculations show that the 1D vacancy ordering for Ln = Sm-Tb could be beneficial at low temperatures as new pathways with reduced barriers emerge. By contrast, the 2D vacancy ordering for Ln = Dy and Y is not beneficial for ionic transport with the basic layered parent material having lower migration barriers. Overall, the key criterion for low migration barriers is an expanded ab plane, supported by Ba, coupled to a small Ln size. Hence, Ln = Y should be the best composition, but this is stymied by the low temperature 2D vacancy ordering and moderate temperature stability. The evolution of the oxygen cycling capability of these materials is also reported.

Cross-Validation of the Remarkably High Surface Oxygen Exchange Kinetics of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ: A Combined Thin-Film Mass Relaxation and Bulk Electrical Conductivity Relaxation Study

In this work, we measure the oxygen kinetic properties of double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), a material widely used as the air electrode in solid oxide electrochemical cells, by mass relaxation (MR) and electrical conductivity relaxation (ECR) experiments. MR studies are carried out using thin films deposited on a gallium phosphate piezocrystal microbalance, and ECR studies are performed using a bulk bar sample with 97% theoretical density. Measurements are performed at 600 °C over the temperature oxygen partial pressure range from 10-4 to 0.21 atm. Despite the differences in experimental formats and surface microstructural features, the ks values extracted from the two methods are found to be in good agreement with one another. The rate constant is found to increase with oxygen partial pressure with a power law dependence, rising from 1.0 × 10-6 cm/s at 3.2 × 10-4 atm to 1.2 × 10-4 cm/s at 0.24 atm, as averaged over the oxidation and reduction directions. The rates in the oxidation direction are observed to be slightly higher than those in the reduction direction for a given pair of pO2 values, suggesting that the final pO2 value controls the overall relaxation behavior. The power law exponent describing the dependence of ks on pO2 is found to be 0.74 ± 0.01. The ECR study of the bulk sample reveals that even with a diffusion length of 1.8 mm, the relaxation process is largely free of diffusion limitations, indicating that PBSCF has the high bulk transport properties required for a double-phase boundary oxidation/reduction pathway.

An Efficient High-Entropy Perovskite-Type Air Electrode for Reversible Oxygen Reduction and Water Splitting in Protonic Ceramic Cells

Reversible protonic ceramic electrochemical cells (R-PCECs) are emerging as ideal devices for highly efficient energy conversion (generating electricity) and storage (producing H₂ ) at intermediate temperatures (400-700 °C). However, their commercialization is largely hindered by the development of highly efficient air electrodes for oxygen reduction and water-splitting reactions. Here, the findings in the design of a highly active and durable air electrode are reported: high-entropy Pr_0.2 Ba_0.2 Sr_0.2 La_0.2 Ca_0.2 CoO_{3- δ} (HE-PBSLCC), which exhibits impressive activity and stability for oxygen reduction and water-splitting reactions, as confirmed by electrochemical characterizations and structural analysis. When used as an air electrode of R-PCEC, the HE-PBSLCC achieves encouraging performances in dual modes of fuel cells (FCs) and electrolysis cells (ECs) at 650 °C, demonstrating a maximum power density of 1.51 W cm^-2 in FC mode, and a current density of -2.68 A cm^-2 at 1.3 V in EC mode. Furthermore, the cells display good operational durabilities in FC and EC modes for over 270 and 500 h, respectively, and promising cycling durability for 70 h with reasonable Faradaic efficiencies. This study offers an effective strategy for the design of active and durable air electrodes for efficient oxygen reduction and water splitting.

Nonstoichiometry, Defect Chemistry and Oxygen Transport in Fe-Doped Layered Double Perovskite Cobaltite PrBaCo2-xFexO6-δ (x = 0-0.6) Membrane Materials

Mixed conducting cobaltites PrBaCo_2-xFe_xO_6-δ (x = 0-0.6) with a double perovskite structure are promising materials for ceramic semi-permeable membranes for oxygen separation and purification due to their fast oxygen exchange and diffusion capability. Here, we report the results of the detailed study of an interplay between the defect chemistry, oxygen nonstoichiometry and oxygen transport in these materials as a function of iron doping. We show that doping leads to a systematic variation of both the thermodynamics of defect formation reactions and oxygen transport properties. Thus, iron doping can be used to optimize the performance of mixed conducting oxygen-permeable double perovskite membrane materials.

Kinetic Atomic-Scale Reproducibility of the Oxygen Reduction Reaction Process and a Newly Suggested Strong Correlation Descriptor: A Case Study of BaCo0.75Fe0.25O3-δ

The sluggish oxygen reduction reaction (ORR) is a central issue for energy conversion technologies, particularly in the cathodes of solid oxide fuel cells. The recognition of atomic-level kinetics of the ORR is the key solution. Herein, we take BaCo_0.75Fe_0.25O₃ (BCF) perovskite cathode as a model to investigate the ambiguous ORR mechanism by density function theory and ab initio molecular dynamics. The oxygen dissociation process was found as the rate-determining step, and the performance of BCF series perovskite could be well-characterized by the dissociation barrier energy. Further electronic structure analysis demonstrated that the Pr (Nd)-O_dis bond accepted electrons during the oxygen dissociation process, resulting in reduction of the barrier energy. Finally, strong correlations between rare earth 4f electrons and B-site transition metal 3d electrons were found to be another underlying descriptor to determine the electrochemical activity. We expected that the method could be universally applied to design or screen other high-performance perovskite cathodes.

Promoted Performance of Layered Perovskite PrBaFe2O5+δ Cathode for Protonic Ceramic Fuel Cells by Zn Doping

Proton-conducting solid–oxide fuel cell (H-SOFC) is an alternative promising low-temperature electrochemical cell for renewable energy, but the performance is insufficient because of the low activity of cathode materials at low temperatures. A layered perovskite oxide PrBaFe1.9Zn0.1O5+δ (PBFZ) was synthesized and investigated as a promising cathode material for low-temperature H-SOFC. Here, the partial substitution of Fe by Zn further enhances the electrical conductivity and thermal compatibility of PrBaFe2O5+δ (PBF). The PBFZ exhibits improved conductivity in the air at intermediate temperatures and good chemical compatibility with electrolytes. The oxygen vacancy formed at the PBFZ lattice due to Zn doping enhances proton defects, resulting in an improved performance by extending the catalytic sites to the whole cathode area. A single cell with a Ni-BZCY anode, PBFZ cathode, and BaZr0.7Ce0.2Y0.1O3-δ (BZCY) electrolyte membrane was successfully fabricated and tested at 550–700 °C. The maximum power density and Rp were enhanced to 513 mW·cm−2 and 0.3 Ω·cm2 at 700 °C, respectively, due to Zn doping.

Dual nature of high-temperature electronic transport in layered perovskite-like cobaltites: exhaustive consideration of experimental features observed

Complex oxides with the general formula Pr_1-xY_xBaCo_2-yNi_yO_6-δ (x = 0, 0.1, y = 0, 0.2) were successfully synthesized via combustion of organo-metallic precursors. The high-temperature dc conductivity of the obtained sintered materials was studied in a wide range of oxygen partial pressures and temperatures by means of the 4-probe method. The resulting dependencies were juxtaposed with the previously published data on oxygen non-stoichiometry for the oxides considered. The comprehensive analysis of these datasets in attempts to explain the observed trends has shown the large inadequacy of currently existing conduction models. Consequently, a new model approach was developed to account for the numerous experimental and theoretical peculiarities being characteristic for the cobaltites with a layered double perovskite structure. One of the key propositions made postulates mixed nature of the band structure for materials studied with spin states of Co ions acting as a spatial descriptor of a particular type of conductivity: semiconducting or a metallic one. The abovementioned hypothesis was validated by magnetic, thermodynamic and structural arguments obtained both theoretically and experimentally. The models suggested were shown to be adequate in describing large arrays of data collected. Additionally, the reasons behind doping and temperature/pressure influences on the respective conductivity changes in Pr_1-xY_xBaCo_2-yNi_yO_6-δ were uncovered. Transport and thermodynamic parameters determined were used to evaluate transference numbers and mobilities of different charge carriers which revealed the dominating role of metallic conductivity under oxidative conditions and the superiority of semiconducting charge transport in reducing environments. The obtained conclusions were further supported by utilizing the derived model equations for successful description of conductivity/non-stoichiometry data for other layered cobaltites. Also, interesting correlations between cation composition, thermodynamic and transport properties were found. Finally, general review of the formulated approach was made and further research directions were proposed.

Progress and prospects of reversible solid oxide fuel cell materials

Reversible solid oxide fuel cell (RSOFC) is an energy device that flexibly interchanges between electrical and chemical energy according to people's life and production needs. The development of cell materials affects the stability and cost of the cell, but also restricts its market-oriented development. After decades of research by scientists, a lot of achievements and progress have been made on RSOFC materials. According to the composition and requirements of each component of RSOFC, this article summarizes the research progress based on materials and discusses the merits and demerits of current cell materials in electrochemical performance. According to the efficiency of different materials in solid oxide fuel cell (SOFC mode) and solid oxide electrolyzer (SOEC mode), the challenges encountered by RSOFC in the operation are evaluated, and the future development of RSOFC materials is boldly prospected.

Boosting the surface oxygen activity for high performance Iron-based perovskite oxide

Surface oxygen activities always play an important role in various heterogeneous reaction processes. In this study, the surface oxygen activity of studied perovskite oxides is greatly enhanced after the composition and morphology are tuned. It is worth noting that the surface oxygen activity is enhanced correspondingly, accompanied by higher surface area, better reducibility, and superior low-temperature reactivity of studied catalysts. The sample introduced with nickel atom and nanorods structure possesses higher surface oxygen activity and vacancies with superior performance including T₁₀ at 221 °C and T₉₀ at 243 °C, nearly 90 °C elevations. Double perovskite oxides, especially with nanorods structure are verified to be composed of more surface active oxygen, which could be related to low-temperature redox ability and superior oxygen vacancies. Based on the DFT calculation, introducing nickel element is confirmed to be able to efficiently boost the generation of oxygen vacancies and adsorption of oxygen molecular, in accord with the analysis of characterization. To sum up, the strategy of introducing the nickel atom and nanorods structure could effectively tune the surface oxygen activity and generate more oxygen vacancies, which would be beneficial to the catalytic performance of toluene catalytic oxidation correspondingly.

Pr- and Sm-Substituted Layered Perovskite Oxide Systems for IT-SOFC Cathodes

In this study, the phase synthesis and electrochemical properties of A/A//A///B2O5+d (A/: Lanthanide, A//: Ba, and A//: Sr) layered perovskites in which Pr and Sm were substituted at the A/-site were investigated for cathode materials of Intermediate Temperature-Operating Solid Oxide Fuel cells (IT-SOFC). In the PrxSm1−xBa0.5Sr0.5Co2O5+d (x = 0.1–0.9) systems, tetragonal (x < 0.4) and orthorhombic (x ≥ 0.5) crystalline structures were confirmed according to the substitution amount of Pr, which has a relatively large ionic radius, and Sm, which has a small ionic radius. All of the layered perovskite oxide systems utilized in this study presented typical metallic conductivity behavior, with decreasing electrical conductivity as temperature increased. In addition, Pr0.5Sm0.5Ba0.5Sr0.5Co2O5+d (PSBSCO55), showing a tetragonal crystalline structure, had the lowest conductivity values. However, the Area-Specific Resistance (ASR) of PSBSCO55 was found to be 0.10 Ω cm2 at 700 °C, which is lower than those of the other compositions.

High-Throughput Computational Screening of Cubic Perovskites for Solid Oxide Fuel Cell Cathodes

It is a present-day challenge to design and develop oxygen-permeable solid oxide fuel cell (SOFC) electrode and electrolyte materials that operate at low temperatures. Herein, by performing high-throughput density functional theory calculations, oxygen vacancy formation energy, E_vac, data for a pool of all-inorganic ABO₃ and A^I_0.5A^II_0.5BO₃ cubic perovskites is generated. Using E_vac data of perovskites, the area-specific resistance (ASR) data, which is related to both oxygen reduction reaction activity and selective oxygen ion conductivity of materials, is calculated. Screening a total of 270 chemical compositions, 31 perovskites are identified as candidates with properties that are between those of state-of-the-art SOFC cathode and oxygen permeation components. In addition, an intuitive approach to estimate E_vac and ASR data of complex perovskites by using solely the easy-to-access data of simple perovskites is shown, which is expected to boost future explorations in the perovskite material search space for genuinely diverse energy applications.

Enhanced Electrochemical Properties of Non-stoichiometric Layered Perovskites, Sm1-xBaCo2O5+d, for IT-SOFC Cathodes

In this study, electrochemical properties of layered perovskites having non-stoichiometric compositions (Sm_1−xBaCo₂O_5+d, x = 0, 0. 01, 0.02, 0.03, 0.04, 0.05, 0.10, and 0.15) were analyzed for the direct application of cathode materials for Intermediate Temperature-operating Solid Oxide Fuel Cells (IT-SOFC). From the Sm_1−xBaCo₂O_5+d oxide systems calcined at 1,100°C for 8 h, single phase (SmBaCo₂O_5+d, SBCO_1) was maintained only in the case of the x = 0 composition. In the compositions of x = 0.05–0.10, BaCoO_2.6 was mixed with the pattern of SBCO. In addition, in the composition of x = 0.15, it was confirmed that BaCoO_2.6 and CoO phases coexisted with SBCO. In the compositions of Sm_1−xBaCo₂O_5+d, the overall Area Specific Resistance (ASR) values decreased as the removal amount of Sm increased from x = 0–0.10; then, the values increased for compositions from x = 0.15. For example, the ASRs of SBCO_1, Sm_0.95BaCo₂O_5+d (SBCO_0.95), Sm_0.90BaCo₂O_5+d (SBCO_0.90), and Sm_0.85BaCo₂O_5+d (SBCO_0.85) measured at 600°C were 0.301, 0.147, 0.119, and 0.179 Ω cm², respectively. In particular, SBCO_0.90 was found to have an excellent ASR property of about 0.035 Ω cm² at 700°C. Typical properties of the metal–insulator transition (MIT) electrical conductivity were shown in all measured compositions. The temperature at which MIT occurred increased as the non-stoichiometric composition increased.

The Brønsted-Evans-Polanyi relationship in oxygen exchange of fuel cell cathode material SrCo0.9Ta0.1O3-δ with the gas phase

Perovskite related oxides ABO3-δ exhibiting mixed ionic-electronic conductivity (MIEC) possess large deviations from the oxygen stoichiometry. When providing excellent application potential, this feature also makes it very difficult to study the reaction mechanism between such oxides and molecular oxygen, also known as the oxygen reduction reaction. The complexity of the theoretical interpretation of kinetic experiments originates from the significant dependence of the kinetic and equilibrium properties of MIEC oxides on δ. It is proposed to consider such grossly nonstoichiometric oxides having different oxygen nonstoichiometry as chemical homologues participating in the oxygen exchange reaction and forming a series continuous in δ. The continuous homologous series approach is considered using the example of SrCo0.9Ta0.1O3-δ, an SOFC cathode material. The equilibrium and kinetic properties of the oxide were studied by new methods of oxygen partial pressure relaxation and oxygen release. Linear free-energy relationships have been discovered in the homologous series: thermodynamic and kinetic enthalpy-entropy compensations, as well as the Brønsted-Evans-Polanyi relation. A relationship has been established between the change in the observed LFERs and the morphotropic phase transition in the oxide.

Rare earth elements based oxide ion conductors

Rare-earth-elements-based oxide ion conductors with various structures and their structure-property relationships were systematically presented and summarized, which can provide new insight and guidance for the development of new oxide ion conductors.

Investigation of Structural and Thermal Evolution in Novel Layered Perovskite NdSrMn2O5+δ via Neutron Powder Diffraction and Thermogravimetric Analysis

Neutron diffraction is one of the best methods for structural analysis of a complex, layered perovskite material with low symmetry by accurately detecting the oxygen positions through octahedral tilting. In this research, the crystal structure of NdSrMn2O5+δ was identified through X-ray diffraction (XRD) and neutron powder diffraction (NPD) at room temperature (RT), which indicated the formation of a layered structure in orthorhombic symmetry in the Pmmm (no. 47) space group. Rietveld refinement of the neutron diffraction data has confirmed the orthorhombic symmetry with unit cell parameters (a = 3.8367 (1) Å, b = 3.8643 (2) Å, and c = 7.7126 (1) Å), atomic positions, and oxygen occupancy. Thermogravimetric analysis revealed the total weight loss of about 0.10% for 20–950°C temperature, which occurred mainly to create oxygen vacancies at high temperatures. Rietveld analyses concurred with the XRD and neutron data allowing correlation of occupancy factors of the oxygen sites.

Probing One-Dimensional Oxygen Vacancy Channels Driven by Cation-Anion Double Ordering in Perovskites

Visualizing the oxygen vacancy distributions is highly desirable for understanding the atomistic oxygen diffusion mechanisms in perovskites. In particular, the direct observation of the one-dimensional oxygen vacancy channels has not yet been achieved in perovskites with dual ion (i.e., cation and anion) ordering. Here, we perform atomic-resolution imaging of the one-dimensional oxygen vacancy channels and their structural dynamics in a NdBaCo₂O_5.5 double perovskite oxide. An in situ heating transmission electron microscopy investigation reveals the disordering of oxygen vacancy channels by local rearrangement of oxygen vacancies at the specific temperature. A density functional theory calculation suggests that the possible pathway of oxygen vacancy migration is a multistep route via Co-O and Nd-O_v (oxygen vacancy) sites. These findings could provide robust guidance for understanding the static and dynamic behaviors of oxygen vacancies in perovskite oxides.

High-Temperature Structural and Electrical Properties of BaLnCo2O6 Positrodes

The application of double perovskite cobaltites BaLnCo₂O_6−δ (Ln = lanthanide element) in electrochemical devices for energy conversion requires control of their properties at operating conditions. This work presents a study of a series of BaLnCo₂O_6−δ (Ln = La, Pr, Nd) with a focus on the evolution of structural and electrical properties with temperature. Symmetry, oxygen non-stoichiometry, and cobalt valence state have been examined by means of Synchrotron Radiation Powder X-ray Diffraction (SR-PXD), thermogravimetry (TG), and X-ray Absorption Spectroscopy (XAS). The results indicate that all three compositions maintain mainly orthorhombic structure from RT to 1000 °C. Chemical expansion from Co reduction and formation of oxygen vacancies is observed and characterized above 350 °C. Following XAS experiments, the high spin of Co was ascertained in the whole range of temperatures for BLC, BPC, and BNC.

Electrode Materials for Supercapacitors: A Review of Recent Advances

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

Self-Assembled Ruddlesden-Popper/Perovskite Hybrid with Lattice-Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

The oxygen evolution reaction (OER) is pivotal in multiple gas-involved energy conversion technologies, such as water splitting, rechargeable metal-air batteries, and CO₂ /N₂ electrolysis. Emerging anion-redox chemistry provides exciting opportunities for boosting catalytic activity, and thus mastering lattice-oxygen activation of metal oxides and identifying the origins are crucial for the development of advanced catalysts. Here, a strategy to activate surface lattice-oxygen sites for OER catalysis via constructing a Ruddlesden-Popper/perovskite hybrid, which is prepared by a facile one-pot self-assembly method, is developed. As a proof-of-concept, the unique hybrid catalyst (RP/P-LSCF) consists of a dominated Ruddlesden-Popper phase LaSr₃ Co_1.5 Fe_1.5 O_10-δ (RP-LSCF) and second perovskite phase La_0.25 Sr_0.75 Co_0.5 Fe_0.5 O_3-δ (P-LSCF), displaying exceptional OER activity. The RP/P-LSCF achieves 10 mA cm^-2 at a low overpotential of only 324 mV in 0.1 m KOH, surpassing the benchmark RuO₂ and various state-of-the-art metal oxides ever reported for OER, while showing significantly higher activity and stability than single RP-LSCF oxide. The high catalytic performance for RP/P-LSCF is attributed to the strong metal-oxygen covalency and high oxygen-ion diffusion rate resulting from the phase mixture, which likely triggers the surface lattice-oxygen activation to participate in OER. The success of Ruddlesden-Popper/perovskite hybrid construction creates a new direction to design advanced catalysts for various energy applications.

Direct evidence of boosted oxygen evolution over perovskite by enhanced lattice oxygen participation

The development of oxygen evolution reaction (OER) electrocatalysts remains a major challenge that requires significant advances in both mechanistic understanding and material design. Recent studies show that oxygen from the perovskite oxide lattice could participate in the OER via a lattice oxygen-mediated mechanism, providing possibilities for the development of alternative electrocatalysts that could overcome the scaling relations-induced limitations found in conventional catalysts utilizing the adsorbate evolution mechanism. Here we distinguish the extent to which the participation of lattice oxygen can contribute to the OER through the rational design of a model system of silicon-incorporated strontium cobaltite perovskite electrocatalysts with similar surface transition metal properties yet different oxygen diffusion rates. The as-derived silicon-incorporated perovskite exhibits a 12.8-fold increase in oxygen diffusivity, which matches well with the 10-fold improvement of intrinsic OER activity, suggesting that the observed activity increase is dominantly a result of the enhanced lattice oxygen participation.

Self-sustainable protonic ceramic electrochemical cells using a triple conducting electrode for hydrogen and power production

The protonic ceramic electrochemical cell (PCEC) is an emerging and attractive technology that converts energy between power and hydrogen using solid oxide proton conductors at intermediate temperatures. To achieve efficient electrochemical hydrogen and power production with stable operation, highly robust and durable electrodes are urgently desired to facilitate water oxidation and oxygen reduction reactions, which are the critical steps for both electrolysis and fuel cell operation, especially at reduced temperatures. In this study, a triple conducting oxide of PrNi_0.5Co_0.5O_3-δ perovskite is developed as an oxygen electrode, presenting superior electrochemical performance at 400~600 °C. More importantly, the self-sustainable and reversible operation is successfully demonstrated by converting the generated hydrogen in electrolysis mode to electricity without any hydrogen addition. The excellent electrocatalytic activity is attributed to the considerable proton conduction, as confirmed by hydrogen permeation experiment, remarkable hydration behavior and computations.

While producing renewable fuel is crucial for a sustainable energy economy, there is still a need for active and durable materials capable of efficient fuel generation and utilization. Here, authors demonstrate a triple-conductive oxide as an oxygen electrode for H₂ or electricity production.

Self-Diffusion in Perovskite and Perovskite Related Oxides: Insights from Modelling

Perovskite and perovskite related oxides are important materials with applications ranging from solid oxide fuel cells, electronics, batteries and high temperature superconductors. The investigation of physical properties at the atomic scale such as self-diffusion is important to further improve and/or miniaturize electronic or energy related devices. In the present review we examine the oxygen self-diffusion and defect processes in perovskite and perovskite related oxides. This contribution is not meant to be an exhaustive review of the literature but rather aims to highlight the important mechanisms and ways to tune self-diffusion in this important class of energy materials.

A Cobalt-Free Multi-Phase Nanocomposite as Near-Ideal Cathode of Intermediate-Temperature Solid Oxide Fuel Cells Developed by Smart Self-Assembly

An ideal solid oxide fuel cell (SOFC) cathode should meet multiple requirements, i.e., high activity for oxygen reduction reaction (ORR), good conductivity, favorable stability, and sound thermo-mechanical/chemical compatibility with electrolyte, while it is very challenging to achieve all these requirements based on a single-phase material. Herein, a cost-effective multi-phase nanocomposite, facilely synthesized through smart self-assembly at high temperature, is developed as a near-ideal cathode of intermediate-temperature SOFCs, showing high ORR activity (an area-specific resistance of ≈0.028 Ω cm² and a power output of 1208 mW cm^-2 at 650 °C), affordable conductivity (21.5 S cm^-1 at 650 °C), favorable stability (560 h operation in single cell), excellent chemical compatibility with Sm_0.2 Ce_0.8 O_1.9 electrolyte, and reduced thermal expansion coefficient (≈16.8 × 10^-6 K^-1 ). Such a nanocomposite (Sr_0.9 Ce_0.1 Fe_0.8 Ni_0.2 O_{3- δ} ) is composed of a single perovskite main phase (77.2 wt%), a Ruddlesden-Popper (RP) second phase (13.3 wt%), and surface-decorated NiO (5.8 wt%) and CeO₂ (3.7 wt%) minor phases. The RP phase promotes the oxygen bulk diffusion while NiO and CeO₂ nanoparticles facilitate the oxygen surface process and O^2- migration from the surface to the main phase, respectively. The strong interaction between four phases in nanodomain creates a synergistic effect, leading to the superior ORR activity.

Identification of oxygen diffusion mechanisms in Nd1-xAExBaInO4-x/2 (AE = Ca, Sr, Ba) compounds through molecular dynamics

Molecular dynamics simulations have been widely adopted to study oxygen ion diffusion mechanisms in materials for application in solid oxide fuel cells. Indeed, understanding the fundamental aspects of oxygen diffusion is important to develop new materials for this application. In this work, Nd1-xAExBaInO4-x/2 (AE = Ca, Sr, Ba) compounds have been studied by MD simulations focusing on oxygen diffusion mechanisms. Two general clustering methods were used, namely a convex hull classification method and a DBSCAN machine learning algorithm, to identify oxygen ion diffusion pathways. Here, relevant details are provided for an efficient use of these two approaches during MD analysis of ion conductors. The calculations show that Ca is the most favorable dopant for substituting Nd in NdBaInO4, while Ba is the least desired. Indeed, the substitution of Nd by Ca hardly changes the pristine lattice parameters of NdBaInO4 and leads to the highest oxygen diffusion coefficient compared to other dopants. The oxygen vacancies induced by doping mainly locate on two specific oxygen sites over four oxygen sites available. Concerning the diffusion process, jumps involving these two sites play the main role and are associated with smaller migration enthalpies. For the main diffusion path, ions migrate along the b (2 routes) and c (4 routes) directions. Some other oxygen sites can be considered as barriers for the diffusion process inducing a strong anisotropy in the diffusion process. Additionally, the residence time analysis of oxygen ions confirms that ions at different sites have different motion abilities. As a whole, the approach presented here can be extrapolated to other ion conductors for gaining detailed information about the diffusion process.

Porous and highly conducting cathode material PrBaCo2O6-δ: bulk and surface studies of synthesis anomalies

The paradigm that chemical synthesis reduces the sintering temperature as compared to solid state synthesis seems to be violated in the case of the PrBaCo2O6-δ double perovskite. The sintering temperatures for pure phase samples synthesized through the solid state route (P-SSR) and the auto-combustion route (P-ACR) were found to be 1050 and 1150 °C, respectively. The porous microstructure of P-SSR is suitable for SOFC cathode materials while that of P-ACR is pore free. High-resolution transmission electron microscopy, Raman and scanning tunneling microscopy studies reveal that there is crystal growth on a smooth surface with a preferred orientation. Our results show that this anomalous synthesis behaviour is due to anisotropic surface nucleation growth. Thermodynamically, the higher decomposition temperature in the chemical route is due to stronger electron-phonon coupling and the higher value of change in entropy. The variation in the Co-O-Co bond angle reveals Jahn-Teller vibrational anisotropy in the-b plane leading to the anisotropic synthesis behaviour. This anisotropy is the reason for the violation of the paradigm.

PrBaCo2O6−δ-Ce0.8Sm0.2O1.9 Composite Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells: Stability and Cation Interdiffusion

The single-phase oxide PrBaCo2O6−δ and composites (100 − y)PrBaCo2O6−δ-yCe0.8Sm0.2O1.9 (y = 10–30 wt.%) were investigated as cathode materials for intermediate-temperature solid oxide fuel cells. The chemical compatibility, cation interdiffusion, thermal expansion and dc conductivity were studied. As a result, strong interdiffusion of Pr and Sm was found between PrBaCo2O6−δ and Ce0.8Sm0.2O1.9. This leads to only insignificantly decreasing thermal expansion coefficient of composite with increasing fraction of Ce0.8Sm0.2O1.9 and, thus, mixing PrBaCo2O6−δ with Ce0.8Sm0.2O1.9 does not improve thermal expansion behavior of the cathode material. Moreover, formation of poorly-conducting BaCeO3, caused by chemical interaction between the double perovskite and doped ceria, was shown to lead to pronounced drop in the electrical conductivity of the composite cathode material with increasing Ce0.8Sm0.2O1.9 content.

Electric field control of resistive switching and magnetization in epitaxial LaBaCo2O5+δ thin films

The low operating temperature and volatile characteristics of the magnetization change are the main obstacles for the practical applications of spintronic and magnetic memories. In this work, both the resistive switching and magnetization switching are realized in Pt/LaBaCo2O5+δ (LBCO)/Nb-doped SrTiO3 (Nb-STO) devices at room temperature through an electric field. Unlike the traditional approach of an external stress inducing a volatile magnetization change, the magnetization in the Pt/LBCO/Nb-STO device is modulated by an electrical field, along with the resistive switching. The resistive and magnetization switching can be attributed to the variation of the depletion layer width at the LBCO/Nb-STO interface via oxygen vacancy migration and the increase/decrease of the Co-O-Co bond length, respectively. The present device with the synchronous manipulation of both resistance and magnetization at room temperature can be applied in nonvolatile resistive memories and novel magnetic multifunctional devices.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Scandium Doping Effect on a Layered Perovskite Cathode for Low-Temperature Solid Oxide Fuel Cells (LT-SOFCs)

Layered perovskite oxides are considered as promising cathode materials for the solid oxide fuel cell (SOFC) due to their high electronic/ionic conductivity and fast oxygen kinetics at low temperature. Many researchers have focused on further improving the electrochemical performance of the layered perovskite material by doping various metal ions into the B-site. Herein, we report that Sc3+ doping into the layered perovskite material, PrBaCo2O5+δ (PBCO), shows a positive effect of increasing electrochemical performances. We confirmed that Sc3+ doping could provide a favorable crystalline structure of layered perovskite for oxygen ion transfer in the lattice with improved Goldschmidt tolerance factor and specific free volume. Consequently, the Sc3+ doped PBCO exhibits a maximum power density of 0.73 W cm−2 at 500 °C, 1.3 times higher than that of PBCO. These results indicate that Sc3+ doping could effectively improve the electrochemical properties of the layered perovskite material, PBCO.