Materials design for perovskite SOFC cathodes

Perovskite Oxide Materials for Solar Thermochemical Hydrogen Production from Water Splitting through Chemical Looping

Solar-driven thermochemical hydrogen (STCH) production represents a sustainable approach for converting solar energy into hydrogen (H2) as a clean fuel. This technology serves as a crucial feedstock for synthetic fuel production, aligning with the principles of sustainable energy. The efficiency of the conversion process relies on the meticulous tuning of the properties of active materials, mostly commonly perovskite and fluorite oxides. This Review conducts a comprehensive review encompassing experimental, computational, and thermodynamic and kinetic property studies, primarily assessing the utilization of perovskite oxides in two-step thermochemical reactions and identifying essential attributes for future research endeavors. Furthermore, this Review delves into the application of machine learning (ML) and density functional theory (DFT) for predicting and classifying the thermochemical properties of perovskite materials. Through the integration of experimental investigations, computational modeling, and ML methodologies, this Review aspires to expedite the screening and optimization of perovskite oxides, thus enhancing the efficiency of STCH processes. The overarching objective is to propel the advancement and practical integration of STCH systems, contributing significantly to the realization of a sustainable and carbon-neutral energy landscape.

Visualization of the structural transformation of NiO/YSZ/BZY nanocomposite particles using in situ gas environmental transmission electron microscopy

This study focused on investigating the dynamic structural transformations of spherical NiO/YSZ/BZY triple-phase nanocomposite particles, commonly employed for cermet anodes, during the hydrogen reduction reaction. We utilized both spherical aberration (Cs) corrected transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation modes under a controlled gaseous environment. The environmental gas pressure was set to 1 atm (760 Torr), mirroring real-world conditions. To elucidate pre- and post-hydrogen reduction compositional alterations, we conducted elemental mapping using energy-dispersive X-ray spectroscopy (EDS). Our findings indicated that NiO nanoparticles underwent reduction to Ni particles upon heat treatments in an environment containing H2 gas. Significantly, this reduction of NiO led to the migration of Ni along the external surface of each composite particle, ultimately resulting in the agglomeration at the interparticle spaces among the three NiO/YSZ/BZY nanocomposite particles.

Research Progress on Photocatalytic CO2 Reduction Based on Perovskite Oxides

Photocatalytic CO2 reduction to valuable fuels is a promising way to alleviate anthropogenic CO2 emissions and energy crises. Perovskite oxides have attracted widespread attention as photocatalysts for CO2 reduction by virtue of their high catalytic activity, compositional flexibility, bandgap adjustability, and good stability. In this review, the basic theory of photocatalysis and the mechanism of CO2 reduction over perovskite oxide are first introduced. Then, perovskite oxides' structures, properties, and preparations are presented. In detail, the research progress on perovskite oxides for photocatalytic CO2 reduction is discussed from five aspects: as a photocatalyst in its own right, metal cation doping at A and B sites of perovskite oxides, anion doping at O sites of perovskite oxides and oxygen vacancies, loading cocatalyst on perovskite oxides, and constructing heterojunction with other semiconductors. Finally, the development prospects of perovskite oxides for photocatalytic CO2 reduction are put forward. This article should serve as a useful guide for creating perovskite oxide-based photocatalysts that are more effective and reasonable.

Development of La1.7Ca0.3Ni1-yCuyO4+δ Materials for Oxygen Permeation Membranes and Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

The La_1.7Ca_0.3Ni_1-yCu_yO_4+δ (y = 0.0-0.4) nickelates, synthesized via a solid-state reaction method, are investigated as prospective materials for oxygen permeation membranes and IT-SOFC cathodes. The obtained oxides are single-phase and possess a tetragonal structure (I4/mmm sp. gr.). The unit cell parameter c and the cell volume increase with Cu-substitution. The interstitial oxygen content and total conductivity decrease with Cu-substitution. The low concentration of mobile interstitial oxygen ions results in a limited oxygen permeability of Cu-substituted La_1.7Ca_0.3NiO_4+δ ceramic membranes. However, increasing the Cu content over y = 0.2 induces two beneficial effects: enhancement of the electrochemical activity of the La_1.7Ca_0.3Ni_1-yCu_yO_4+δ (y = 0.0; 0.2; 0.4) electrodes and decreasing the sintering temperature from 1200 °C to 900 °C. Enhanced electrode activity is due to better sintering properties of the developed materials ensuring excellent adhesion and facilitating the charge transfer at the electrode/electrolyte interface and, probably, faster oxygen exchange in Cu-rich materials. The polarization resistance of the La_1.7Ca_0.3Ni_1.6Cu_0.4O_4+δ electrode on the Ce_0.8Sm_0.2O_1.9 electrolyte is as low as 0.15 Ω cm² and 1.95 Ω cm² at 850 °C and 700 °C in air, respectively. The results of the present work demonstrate that the developed La_1.7Ca_0.3Ni_0.6Cu_0.4O_4+δ-based electrode can be considered as a potential cathode for intermediate-temperature solid oxide fuel cells.

CeFeO3-CeO2-Fe2O3 Systems: Synthesis by Solution Combustion Method and Catalytic Performance in CO2 Hydrogenation

Rare-earth orthoferrites have found wide application in thermocatalytic reduction-oxidation processes. Much less attention has been paid, however, to the production of CeFeO₃, as well as to the study of its physicochemical and catalytic properties, in particular, in the promising process of CO₂ utilization by hydrogenation to CO and hydrocarbons. This study presents the results of a study on the synthesis of CeFeO₃ by solution combustion synthesis (SCS) using various fuels, fuel-to-oxidizer ratios, and additives. The SCS products were characterized by XRD, FTIR, N₂-physisorption, SEM, DTA-TGA, and H₂-TPR. It has been established that glycine provides the best yield of CeFeO₃, while the addition of NH₄NO₃ promotes an increase in the amount of CeFeO₃ by 7-12 wt%. In addition, the synthesis of CeFeO₃ with the participation of NH₄NO₃ makes it possible to surpass the activity of the CeO₂-Fe₂O₃ system at low temperatures (300-400 °C), as well as to increase selectivity to hydrocarbons. The observed effects are due to the increased gas evolution and ejection of reactive FeO_x nanoparticles on the surface of crystallites, and an increase in the surface defects. CeFeO₃ obtained in this study allows for achieving higher CO₂ conversion compared to LaFeO₃ at 600 °C.

High-Entropy Perovskites Pr1-xSrx(Cr,Mn,Fe,Co,Ni)O3-δ (x = 0-0.5): Synthesis and Oxygen Permeation Properties

High-entropy perovskite oxides have already been studied in various fields owing to their high-entropy-induced properties. Partial substitution of an element by a lower valence element usually improves the oxygen permeability of perovskite oxides, but high substitution amounts may lead to structural instability. In this work, pure high-entropy perovskites Pr1-xSrx(Cr,Mn,Fe,Co,Ni)O3-δ with high amounts Sr up to x = 0.5 were synthesized via a sol-gel method. Several characterization methods prove that the solubility of Sr increases with higher temperatures of the heating treatment. The ceramic with x = 0.5 shows a transition from semi-conductive to metallic behavior when the temperature reaches 873K. Its oxygen flux is comparable to the low-entropy counterpart La0.6Sr0.4Co0.5Fe0.5O3-δ. A stable run of ca. 46.2 h was documented for oxygen permeation under an air/CO2 gradient.

Electrokinetic Insights into the Triple Ionic and Electronic Conductivity of a Novel Nanocomposite Functional Material for Protonic Ceramic Fuel Cells

Triple ionic and electronic conductivity (TIEC) in cathode materials for protonic ceramic fuel cells (PCFCs) is a desirable feature that enhances the spatial expansion of active reaction sites for electrochemical oxygen reduction reaction. The realization of optimal TIEC in single-phase materials, however, is challenging. A facile route that facilitates the optimization of TIEC in PCFC cathodes is the strategic development of multiphase cathode materials. In this study, a cubic-rhombohedral TIEC nanocomposite material with the composition Ba(CeCo)_0.4 (FeZr)_0.1 O_{3- δ} (BCCFZ) is designed via self-assembly engineering. The material consists of a mixed ionic and electronic conducting phase, BaCo_{1-( x + y + z )} Ce_x Fe_y Zr_z O_{3- δ} (M-BCCFZ), and a dominant proton-conducting phase, BaCe_{1-( x + y + z )} Co_x Zr_y Fe_z O_{3- δ} (H-BCCZF). The dominant cerium-rich H-BCCFZ phase enhances the material's oxygen vacancy concentration and the proton defects formation and transport with a low enthalpy of protonation of -30 ± 9 kJ mol^-1 . The area-specific resistance of the BCCFZ symmetrical cell is 0.089 Ω cm² at 650 °C in 2.5% H₂ O-air. The peak power density of the anode-supported single cell based on BCCFZ cathode reaches 1054 mW cm^-2 at 650 °C with good operation stability spanning over 500 h at 550 °C. These promote BCCFZ as a befitting cathode material geared toward PCFC commercialization.

Further adventures of the perovskite family

The perovskites are an intensely studied class of materials, with a breadth of possible compositions made even wider by the possibility of incorporating molecular ions. Here the context is discussed of a newly reported metal-free perovskite with the H3O+ ion on the B site.

An Unbalanced Battle in Excellence: Revealing Effect of Ni/Co Occupancy on Water Splitting and Oxygen Reduction Reactions in Triple-Conducting Oxides for Protonic Ceramic Electrochemical Cells

Porous electrodes that conduct electrons, protons, and oxygen ions with dramatically expanded catalytic active sites can replace conventional electrodes with sluggish kinetics in protonic ceramic electrochemical cells. In this work, a strategy is utilized to promote triple conduction by facilitating proton conduction in praseodymium cobaltite perovskite through engineering non-equivalent B-site Ni/Co occupancy. Surface infrared spectroscopy is used to study the dehydration behavior, which proves the existence of protons in the perovskite lattice. The proton mobility and proton stability are investigated by hydrogen/deuterium (H/D) isotope exchange and temperature-programmed desorption. It is observed that the increased nickel replacement on the B-site has a positive impact on proton defect stability, catalytic activity, and electrochemical performance. This doping strategy is demonstrated to be a promising pathway to increase catalytic activity toward the oxygen reduction and water splitting reactions. The chosen PrNi_0.7 Co_0.3 O_{3- δ} oxygen electrode demonstrates excellent full-cell performance with high electrolysis current density of -1.48 A cm^-2 at 1.3 V and a peak fuel-cell power density of 0.95 W cm^-2 at 600 °C and also enables lower-temperature operations down to 350 °C, and superior long-term durability.

Structural and Magnetic Properties of BaFeO2.667 Synthesized by Oxidizing BaFeO2.5 Obtained via Nebulized Spray Pyrolysis

A vacancy-ordered perovskite-type compound Ba₃Fe₃O₈ (BaFeO_2.667) was prepared by oxidizing BaFeO_2.5 (P2₁/c) with the latter compound obtained by a spray pyrolysis technique. The structure of Ba₃Fe₃O₈ was found to be isotypic to Ba₃Fe₃O₇F (P2₁/m) and can be written as Ba₃Fe³⁺₂Fe⁴⁺₁O₈. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site, and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen were investigated over a broad temperature range from room temperature to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibited a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO_2.5 at elevated temperatures.

Structural and Catalytic Characterization of La0.6Sr0.4MnO3 Nanofibers for Application in Direct Methane Intermediate Temperature Solid Oxide Fuel Cell Anodes

In the present work, structural and catalytic characterization was performed on La0.6Sr0.4MnO3 (LSM) nanofibers. The LSM nanofibers were obtained using the electrospinning technique. For comparison, LSM powders with identical composition were characterized as well. The LSM powders were prepared through a self-combustion citrate-based procedure. SEM, EDX, XRD, and BET investigations were carried out on both LSM nanofibers and powders, pointing out the different structural features. The LSM nanofibers showed a higher surface area than the LSM powders and a lower presence of strontium oxide on the surface. Results of the H2-Temperature Programmed Reduction (TPR) tests showed evidence of a higher reactivity of the nanofibers compared to the powders. The catalytic characterization was performed utilizing a methane oxidation activity test, revealing a better catalytic performance of the LSM nanofibers: at 800 °C. The methane conversion achieved with the LSM nanofibers was 73%, which compared well with the 50% obtained with powders at 900 °C.

Hf Deposition Stabilizes the Surface Chemistry of Perovskite Manganite Oxide

Stable composition and catalytic activity of surfaces are among the key requirements for materials employed in energy storage and conversion devices, such as solid oxide fuel cells (SOFCs). Perovskite oxides that serve as cathode in SOFCs suffer from segregation of the aliovalent substitutional cations and the formation of an inert, non-conductive phase at the surface. Here, we demonstrate that the surface of the state-of-the-art SOFC cathode material La_0.8Sr_0.2MnO₃ (LSM) is stabilized against the segregation of Sr at high temperature by submonolayer coverages of Hf. The Hf is vapor-deposited onto the LSM thin film surface by e-beam evaporation. Using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we analyze the surface composition of LSM thin films. Half the LSM surface was kept as-prepared, and half was Hf-modified, for a direct comparison of untreated and Hf-treated regions on the same sample. The formation of a binary SrO_x surface species is quantified as descriptor for surface degradation. The onset of Sr segregation is observed at 450 °C on the bare LSM, followed by a substantial advance at 550 °C. Hf-treated regions of the same LSM surface exhibit significantly less Sr surface segregation at 450-550 °C. We interpret this stabilization imparted by Hf to arise from the suppression of the electrostatic attraction of Sr²⁺ cations to surface oxygen vacancies. Doping the surface layer with Hf, that has a higher affinity to oxygen, reduces this attraction by decreasing the surface oxygen vacancy concentration. In doing so, the use of physical vapor deposition highlights the direct role of the metal species in this system and excludes artifacts that could be introduced via chemical routes. The present work demonstrates this stabilizing effect of Hf on the surface of LSM, broadening the relevance of our prior findings on surface metal doping of other perovskite oxides.

Structural, electric and dielectric properties of perovskite based nanoparticles for energy applications

A nanocomposite electrode, obtained by combining two high performance perovskite materials, such as lanthanum strontium cobalt ferrite, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and gadolinium doped ceria, Ce0.85Gd0.15O1.5 (GDC), were investigated as a promising cathode for moderate temperature solid oxide fuel cells (SOFCs). The synthesized material has good conductivity and catalytic performance. The purpose of this synthesis was to prepare a stable and highly performing nanocomposite cathode material. In this research work, LSCF and GDC were separately synthesized by co-precipitation and solid-state reaction method to gain a homogeneous perovskite phase. Varying concentrations of LSCF–GDC composite with GDC (10 wt.%, 20 wt.% and 30 wt.%) were synthesized followed by calcination at 600 °C to remove water content and to achieve an adequate porous structure for oxygen absorption and desorption. These fabricated LSCF, GDC, and the nanocomposite specimens were characterized for microstructure, particle size etc. via. X-ray diffraction method (XRD), scanning electron microscope (SEM) and the laser particle size analyzer. This procedural approach helps to expand new methods for generating bi-functional duel nano-sized perovskites with great performance and stability which can be utilized for advancement of renewable energy sectors especially for rechargeable fuel batteries.

Structural, electric and dielectric properties of perovskite based nanoparticles for energy applications

A nanocomposite electrode, obtained by combining two high performance perovskite materials, such as lanthanum strontium cobalt ferrite, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and gadolinium doped ceria, Ce0.85Gd0.15O1.5 (GDC), were investigated as a promising cathode for moderate temperature solid oxide fuel cells (SOFCs). The synthesized material has good conductivity and catalytic performance. The purpose of this synthesis was to prepare a stable and highly performing nanocomposite cathode material. In this research work, LSCF and GDC were separately synthesized by co-precipitation and solid-state reaction method to gain a homogeneous perovskite phase. Varying concentrations of LSCF–GDC composite with GDC (10 wt.%, 20 wt.% and 30 wt.%) were synthesized followed by calcination at 600 °C to remove water content and to achieve an adequate porous structure for oxygen absorption and desorption. These fabricated LSCF, GDC, and the nanocomposite specimens were characterized for microstructure, particle size etc. via. X-ray diffraction method (XRD), scanning electron microscope (SEM) and the laser particle size analyzer. This procedural approach helps to expand new methods for generating bi-functional duel nano-sized perovskites with great performance and stability which can be utilized for advancement of renewable energy sectors especially for rechargeable fuel batteries.

Rapid Fabrication of Chemical Solution-Deposited Lanthanum Nickelate Thin Films via Intense Pulsed-Light Process

In this paper, we demonstrate the practicality and feasibility of the flash light-sintering method to fabricate the ceramic material perovskite structure for lanthanum nickel oxide (LaNiO3; LNO) thin films using flash light irradiation equipment. LNO thin films are deposited on an Si wafer and Al2O3 substrate via the chemical solution deposition (CSD) method and sintered by a thermal and flash light-irradiation process with a bottom heater. The properties of flash light-sintered LNO thin films are compared with those of thermally sintered films. The surface morphology, crystal development, and electric conductivity of the LNO thin films are measured by field-emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and a four-point probe, respectively. Flash light sintering was accomplished in milliseconds. Through the comparison of thermal sintering and flash light-sintering results, it was confirmed that perovskite LNO thin films deposited by the CSD method can be fabricated by flash light sintering. We show that the flash light sintering method can solve several inherent issues of the conventional thermal sintering method.

Computational study of the mixed B-site perovskite SmBxCo1-xO3-d (B = Mn, Fe, Ni, Cu) for next generation solid oxide fuel cell cathodes

SmCoO3 is a promising perovskite material for the next generation of intermediate temperature solid oxide fuel cells (SOFC), but its potential application is directly linked to, and dependent on, the presence of dopant ions. Doping on the Co-site is suggested to improve the catalytic and electronic properties of this cathode material. Fe, Mn, Ni, and Cu have been proposed as possible dopants and experimental studies have investigated and confirmed the potential of these materials. Here we present a systematic DFT+U study focused on the changes in electronic, magnetic, and physical properties with B-site doping of SmCoO3 to allow cathode optimization. It is shown that doping generally leads to distortion in the system, thereby inducing different electron occupations of the Co d-orbitals, altering the electronic and magnetic structure. From these calculations, the 0 K electronic conductivity (σe) was obtained, with SmMnxCo1-xO3 having the highest σe, and SmFexCo1-xO3 the lowest σe, in agreement with experiment. We have also investigated the impact of dopant species and concentration on the oxygen vacancy formation energy (Ef), which is related to the ionic conductivity (σO). We found that the Ef values are lowered only when SmCoO3 is doped with Cu or Ni. Finally, thermal expansion coefficients were calculated, with Mn-doping showing the largest decrease at low x and at x = 0.75. Combining these results, it is clear that Mn-doping in the range x = 0.125-0.25 would imbue SmCoO3 with the most favorable properties for IT-SOFC cathode applications.

Towards Solar Methanol: Past, Present, and Future

This work aims to provide an overview of producing value-added products affordably and sustainably from greenhouse gases (GHGs). Methanol (MeOH) is one such product, and is one of the most widely used chemicals, employed as a feedstock for ≈30% of industrial chemicals. The starting materials are analogous to those feeding natural processes: water, CO2, and light. Innovative technologies from this effort have global significance, as they allow GHG recycling, while providing society with a renewable carbon feedstock. Light, in the form of solar energy, assists the production process in some capacity. Various solar strategies of continually increasing technology readiness levels are compared to the commercial MeOH process, which uses a syngas feed derived from natural gas. These strategies include several key technologies, including solar-thermochemical, photochemical, and photovoltaic-electrochemical. Other solar-assisted technologies that are not yet commercial-ready are also discussed. The commercial-ready technologies are compared using a technoeconomic analysis, and the scalability of solar reactors is also discussed in the context of light-incorporating catalyst architectures and designs. Finally, how MeOH compares against other prospective products is briefly discussed, as well as the viability of the most promising solar MeOH strategy in an international context.

Anion-Based Pseudocapacitance of the Perovskite Library La1- xSr xBO3-δ (B = Fe, Mn, Co)

We have synthesized a library of perovskite oxides with the composition La_{1- x}Sr _xBO_3-δ ( x = 0-1; B = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH. We find that greater oxygen vacancy content (δ) upon systematic incorporation of Sr²⁺ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. La_0.2Sr_0.8MnO_2.7 exhibited the highest specific capacitance of 492 F g^-1 at 5 mV s^-1 relative to the Fe and Co oxides. In addition, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral and alkaline aqueous electrolytes. We demonstrate that the asymmetric pseudocapacitor cell voltage can be increased by widening the difference between the B-site transition metal redox potentials in each electrode resulting in a maximum voltage window of 2.0 V in 1 M KOH. Among the three pairs of asymmetric pseudocapacitors constructed from SrCoO_2.7, La_0.2Sr_0.8MnO_2.7, and brownmillerite (BM)-Sr₂Fe₂O₅, the BM-Sr₂Fe₂O₅//SrCoO_2.7 combination performed the best with a high energy density of 31 Wh kg^-1 at 450 W kg^-1 and power density of 10 000 W kg^-1 at 28 Wh kg^-1.

Insights into the Surface Reactivity of Cermet and Perovskite Electrodes in Oxidizing, Reducing, and Humid Environments

Understanding the surface chemistry of electrode materials under gas environments is important in order to control their performance during electrochemical and catalytic applications. This work compares the surface reactivity of Ni/YSZ and La_0.75Sr_0.25Cr_0.9Fe_0.1O₃, which are commonly used types of electrodes in solid oxide electrochemical devices. In situ synchrotron-based near-ambient pressure photoemission and absorption spectroscopy experiments, assisted by theoretical spectral simulations and combined with microscopy and electrochemical measurements, are used to monitor the effect of the gas atmosphere on the chemical state, the morphology, and the electrical conductivity of the electrodes. It is shown that the surface of both electrode types readjusts fast to the reactive gas atmosphere and their surface composition is notably modified. In the case of Ni/YSZ, this is followed by evident changes in the oxidation state of nickel, while for La_0.75Sr_0.25Cr_0.9Fe_0.1O₃, a fine adjustment of the Cr valence and strong Sr segregation is observed. An important difference between the two electrodes is their capacity to maintain adsorbed hydroxyl groups on their surface, which is expected to be critical for the electrocatalytic properties of the materials. The insight gained from the surface analysis may serve as a paradigm for understanding the effect of the gas environment on the electrochemical performance and the electrical conductivity of the electrodes.

Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

Oxygen reduction is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in fuel cells and metal-air batteries. In this review, the prospect of perovskite oxides for the oxygen reduction reaction (ORR) in alkaline media is reviewed with respect to fundamental insight into activity and possible mechanisms. For gaining these insights, special emphasis is placed on highly crystalline perovskite films that have only recently become available for electrochemical interrogation. The prospects for applications are evaluated based on recent progress in the synthesis of perovskite nanoparticles. The review concludes with the current understanding of oxygen reduction on perovskite oxides and a perspective on opportunities for future fundamental and applied research.

Charge attachment induced transport - bulk and grain boundary diffusion of potassium in PrMnO3

The transport of potassium through praseodymium-manganese oxide (PrMnO₃; PMO) has been investigated by means of the charge attachment induced transport (CAIT) technique. To this end, potassium ions have been attached to the front side of a 250 nm thick sample of PMO. The majority of the potassium ions become neutralized at the surface of the PMO, while some of the potassium ions diffuse through. Ex situ analysis of the sample by time-of-flight secondary ion mass spectrometry (ToF-SIMS) reveals pronounced concentration profiles of the potassium, which is indicative of diffusion. Two diffusion coefficients have been obtained, namely, the bulk diffusion coefficient and the diffusion coefficient associated with the grain boundaries. The latter conclusion is supported by transmission electron microscopy of thin lamella cut out from the sample, which reveals twin grain boundaries reaching throughout the entire sample as well as model calculations.

A niobium and tantalum co-doped perovskite cathode for solid oxide fuel cells operating below 500 °C

The slow activity of cathode materials is one of the most significant barriers to realizing the operation of solid oxide fuel cells below 500 °C. Here we report a niobium and tantalum co-substituted perovskite SrCo_0.8Nb_0.1Ta_0.1O_3−δ as a cathode, which exhibits high electroactivity. This cathode has an area-specific polarization resistance as low as ∼0.16 and ∼0.68 Ω cm² in a symmetrical cell and peak power densities of 1.2 and 0.7 W cm⁻² in a Gd_0.1Ce_0.9O_1.95-based anode-supported fuel cell at 500 and 450 °C, respectively. The high performance is attributed to an optimal balance of oxygen vacancies, ionic mobility and surface electron transfer as promoted by the synergistic effects of the niobium and tantalum. This work also points to an effective strategy in the design of cathodes for low-temperature solid oxide fuel cells.

Sluggish activity of cathode materials impedes operation of solid oxide fuel cells at low temperatures. Here, the authors report a niobium and tantalum co-doped perovskite cathode exhibiting high electroactivity below 500 °C, and argue that the dopants improve the cathode performance synergistically.

Oxygen electrode reactions of doped BiFeO3 materials for low and elevated temperature fuel cell applications

Bi_0.6Ca_0.4FeO₃ demonstrates potential as an oxygen electrode material (for oxygen evolution and reduction reactions) for operation at room and elevated temperatures.

(Bi,Sr) (Fe1-x,Mx)O3-δ (M = Co, Ni and Mn) Cathode Materials with Mixed Electro-Ionic Conductivity

(Bi,Sr)FeO_3−δ (BSF) cathode materials doped with either Co, Ni or Mn are synthesized by an ethylene diamine tetra-acetic acid (EDTA)-citrate complexing method, and the effects of the doping level on the mixed electronic-ionic conductivity at various temperatures are studied up to 800 °C. The phase purity and solid solution limit are investigated by X-ray diffraction (XRD). The ionic conductivity is measured by the four-probe direct current (DC) method, the valence state of Fe and Mn by X-ray photoelectron spectroscopy (XPS), and the oxygen non-stoichiometry by differential thermo-gravimetric analysis (TGA). The doped ferrites show interesting electronic conductivity dependent on the testing temperature, implying two conductive mechanisms, either controlled by double exchange at lower temperatures or small polaron (electron-oxygen vacancy) conduction at temperatures greater than 400 °C. The results of Co-doped BSF (S50C20) show the best mixed conductivity among the ferrites, and this is used to assemble cells. The cell with a S50C20 cathode in the region of 600–800 °C is improved by 15% in maximum power density greater than the cell with La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) due to the balanced contribution from oxygen ions, vacancies and electrons.

First principles study of confinement effects for oxygen vacancies in BaZrO₃ (001) ultra-thin films

In this contribution, we study possible confinement effects on the atomic and electronic structure, and phonon properties of neutral (V(O)(x) and fully charged (V(O)oxygen vacancies in BaZrO3 (001) ultra-thin films. First principles phonon calculations were performed as a function of film thickness (from 3 to 7 atomic planes) with two complementary DFT methods. The calculations reveal that for both types of vacancies the confinement effect is very short-range; for films containing 5 planes or more, the oxygen vacancy properties are predicted to be similar to those observed in the bulk material.