Cathode materials for solid oxide fuel cells: a review

Understanding Structure-Activity Relationships in Sr1- xY xCoO3-δ through in Situ Neutron Diffraction and Electrochemical Measurements

In this work, we report a systematic study on temperature-dependent local structural evolution, oxygen stoichiometry, and electrochemical properties of an oxygen-deficient perovskite Sr_0.7Y_0.3CoO_3-δ (SYC30) for oxygen electrocatalysis. The obtained results are then closely compared with its analogue Sr_0.9Y_0.1CoO_3-δ (SYC10) of different crystal structures to establish structure-activity relationships. The comparison shows that both SYC30 and SYC10 consist of alternate layers of oxygen-deficient Co1-polyhedra and oxygen-saturated Co2-octahedra with Co1-polyhedra being responsible for V_o^•• migration. It is also found that the distribution and concentration of oxygen vacancies within the Co1-layer are, respectively, less symmetrical and lower in SYC30 than those in SYC10, making the former unfavorable for oxygen transport. A molecular orbital energy analysis reveals that the energy gap between Fermi level and O 2p level in the active Co1-polyhedra is larger in SYC30 than that in SYC10, further suggesting that SYC10 is a better oxide-ion conductor and thus a better electrocatalyst for oxygen reduction reaction, which is unambiguously confirmed by the subsequent electrochemical measurements.

In Situ Impedance Analysis of Oxygen Exchange on Growing La0.6Sr0.4CoO3-δ Thin Films

The further development of solid oxide fuel and electrolysis cells (SOFC/SOEC) strongly relies on research activities dealing with electrode materials. Recent studies showed that under operating conditions many perovskite-type oxide electrodes are prone to changes of their surface composition, leading to severe changes of their electrochemical performance. This results in a large scatter of data in literature and complicates comparison of materials. Moreover, little information is available on the potentially excellent properties of surfaces immediately after preparation, that is, before any degradation by exposure to other gas compositions or temperature changes. Here, we introduce in situ impedance spectroscopy during pulsed laser deposition (IPLD) as a new method for electrochemical analysis of mixed ionic and electronic conducting (MIEC) thin films during growth. First, this approach can truly reveal the properties of as-prepared MIEC electrode materials, since it avoids any alterations of their surface between preparation and investigation. Second, the measurements during growth give information on the thickness dependence of film properties. This technique is applied to La_0.6Sr_0.4CoO_3-δ (LSC), one of the most promising SOFC/SOEC oxygen electrode material. From the earliest stages of LSC film deposition on yttria-stabilized zirconia (YSZ) to a fully grown thin film of 100 nm thickness, data are gained on the oxygen exchange kinetics and the defect chemistry of LSC. A remarkable reproducibility is found in repeated film growth experiments, not only for the bulk related chemical capacitance but also for the surface related polarization resistance (±10%). Polarization resistances of as-prepared LSC films are extraordinarily low (2.0 Ω cm² in 40 μbar O₂ at 600 °C). LSC films on YSZ and on La_0.95Sr_0.05Ga_0.95Mg_0.05O_3-δ (LSGM) single crystals exhibit significantly different electrochemical properties, possibly associated with the tensile strain of LSC on LSGM.

Nanointegrated, High-Performing Cobalt-Free Bismuth-Based Composite Cathode for Low-Temperature Solid Oxide Fuel Cells

Cost-effective cathodes that actively catalyze the oxygen reduction reaction (ORR) are one of the major challenges for the technological advancement of low-temperature solid oxide fuel cells (LT-SOFCs). In particular, cobalt has been an essential element in electrocatalysts for efficiently catalyzing the ORR; nevertheless, the cost, safety, and stability issues of cobalt in cathode materials remain a severe drawback for SOFC development. Here, we demonstrated that by appropriate nanoengineering, we can overcome the inherent electrocatalytic advantages of cobalt-based cathodes to achieve comparable performance with a cobalt-free electrocatalyst on a bismuth-based fast oxygen ion-conducting scaffold that simultaneously enhances the performance and stability of LT-SOFCs. Consequently, the peak power density of the SOFCs reaches 1.2 W/cm² at 600 °C, highest performance of a cobalt-free-based cathode that has been ever reported. In addition, by the surface-protecting effect of covered nanoelectrocatalysts, the evaporation of highly volatile bismuth is greatly suppressed, resulting in an 80% improvement in performance durability, the best among all reported bismuth-based fuel cells.

High-Performance Solid Oxide Fuel Cell with an Electrochemically Surface-Tailored Oxygen Electrode

State-of-the-art cathodes for solid oxide fuel cells (SOFCs), such as (La,Sr)MnO₃ -(Y₂ O₃ )_0.08 (ZrO₂ )_0.92 (LSM-YSZ), suffer from sluggish oxygen reduction reaction (ORR) kinetics at reduced temperatures, leading to a significant decline in their performance. Herein, we report a tailored SOFC cathode with high ORR activity at intermediate temperatures using a simple but effective approach based on "electrochemical" surface modification. The proposed process involves chemically assisted electrodeposition (CAED) of a metal hydroxide (LaCo(OH)_x ) on LSM-YSZ surfaces followed by in situ thermal conversion of LaCo(OH)_x to perovskite-type LaCoO₃ (LCO) nanoparticles during the SOFC startup. This method facilitates easy loading of the LCO nanoparticles with a precisely controlled morphology without the need for repeated deposition/annealing processes. An anode-supported SOFC with the LCO-tailored LSM-YSZ electrode exhibits a remarkably increased power density, approximately 180 % at 700 °C, compared with an SOFC with the pristine electrode as well as excellent long-term stability, which are attributed to the beneficial role of the CAED-derived LCO nanoparticles in enlarging the active areas for ORR and promoting oxygen adsorption/diffusion. This work demonstrates that controlled surface tailoring of the cathode by CAED could be an effective approach for improving the performance of SOFCs at reduced temperatures.

An insight into the effects of transition metals on the thermal expansion of complex perovskite compounds: an experimental and density functional theory investigation

ABO₃ perovskite-type LaCoO₃, LaCo_0.5Fe_0.5O₃, LaCo_0.5Ni_0.5O₃, and LaCo_0.5Mn_0.5O₃ complex oxides were synthesized using the sol-gel method. The microstructure and the thermal expansion behavior of the oxides were investigated. Insights into phonon dispersion and the effects of doped transition metals on thermal expansion were obtained by first-principles calculations based on density functional theory. Thermal expansion coefficients (TECs) at finite temperatures were obtained by quasi harmonic approximation. B-site doping was found to reduce the size of the crystalline grains and the TECs of perovskite compounds. The average TECs of LaCoO₃, LaCo_0.5Fe_0.5O₃, LaCo_0.5Ni_0.5O₃, and LaCo_0.5Mn_0.5O₃ were 25 × 10^-6 K^-1, 11 × 10^-6 K^-1, 12.5 × 10^-6 K^-1, and 5 × 10^-6 K^-1, respectively, in the 0-1000 K temperature range. In addition, the TECs of LaCo_0.5Fe_0.5O₃ and LaCo_0.5Ni_0.5O₃ matched well with those of some common electrolytes.

How To Get Mechanistic Information from Partial Pressure-Dependent Current-Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes

The oxygen incorporation and evolution reaction on mixed conducting electrodes of solid oxide fuel or electrolysis cells involves gas molecules as well as ionic and electronic point defects in the electrode. The defect concentrations depend on the gas phase and can be modified by the overpotential. These interrelationships make a mechanistic analysis of partial pressure-dependent current-voltage experiments challenging. In this contribution it is described how to exploit this complex situation to unravel the kinetic roles of surface adsorbates and electrode point defects. Essential is a counterbalancing of oxygen partial pressure and dc electrode polarization such that the point defect concentrations in the electrode remain constant despite varying the oxygen partial pressure. It is exemplarily shown for La_0.6Sr_0.4FeO_3-δ (LSF) thin film electrodes on yttria-stabilized zirconia how mechanistically relevant reaction orders can be obtained from current-voltage curves, measured in a three-electrode setup. This analysis strongly suggests electron holes as the limiting defect species for the oxygen evolution on LSF and reveals the dependence of the oxygen incorporation rate on the oxygen vacancy concentration. A virtual independence of the reaction rate from the oxygen partial pressure was empirically found for moderate oxygen pressures. This effect, however, arises from a counterbalancing of defect and adsorbate concentration changes.

Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides

In recent years, the lowering of the operation temperature of solid oxide fuel cells (SOFCs) has attracted much attention owing to the trade-off between the best performance and the life span of SOFCs. For this challenge, new active sites on the Ni surfaces in a Nickel-Yttria-Stabilized Zirconia (Ni-YSZ) cermet anode of SOFCs have been created by deposition of trace amounts of platinum oxide (PtO_x ) followed by an activation step of the anode at 1073 K in a hydrogen flow. The internal resistance (IR) free value (185 mA cm^-2 at 0.8 V) observed for the single cell with an anode sputtered with a trace amount of PtO_x (Pt content in anode: from 9 to 91 ppm) at 973 K is conspicuously higher than that of a similar single cell with a nonsputtered cermet anode (85 mA cm^-2 ) at 0.8 V and 1073 K. Transmission electron microscopy microanalysis shows that the defect structure is formed on a partially oxidized Ni surface by active Pt species. Also, surface atomistic simulation on NiO (111) predicts the formation of Frenkel defect clusters with Pt cations, which partially cover the Ni surface. The formation of Frenkel defect clusters on the partially oxidized Ni surface (i.e., creation of new active sites for formation of water molecules) promotes the anode reaction, resulting in improvements in the anode performance of SOFC single cells at 973 K. Design of the aforementioned new active sites on Ni through sputtering of trace amounts of PtO_x provides a great opportunity for "radical innovation" in the design of intermediate-temperature SOFCs.

Development of a hybrid molecular beam epitaxy deposition system for in situ surface x-ray studies

A portable metalorganic gas delivery system designed and constructed to interface with an existing molecular beam epitaxy chamber at beamline 33-ID-E of the Advanced Photon Source is described. This system offers the ability to perform in situ X-ray measurements of complex oxide growth via hybrid molecular beam epitaxy. The performance of the hybrid molecular beam epitaxy system while delivering metalorganic source materials is described. The high-energy X-ray scattering capabilities of the hybrid molecular beam epitaxy system are demonstrated both on oxide films grown solely from the metalorganic source and ABO₃ oxide perovskites containing elements from both the metalorganic source and a traditional effusion cell.

Layer-by-Layer Epitaxial Growth of Defect-Engineered Strontium Cobaltites

Control over structure and composition of (ABO₃) perovskite oxides offers exciting opportunities since these materials possess unique, tunable properties. Perovskite oxides with cobalt B-site cations are particularly promising, as the range of the cation's stable oxidation states leads to many possible structural frameworks. Here, we report growth of strontium cobalt oxide thin films by molecular beam epitaxy, and conditions necessary to stabilize different defect concentration phases. In situ X-ray scattering is used to monitor structural evolution during growth, while in situ X-ray absorption near-edge spectroscopy is used to probe oxidation state and measure changes to oxygen vacancy concentration as a function of film thickness. Experimental results are compared to kinetically limited thermodynamic predictions, in particular, solute trapping, with semiquantitative agreement. Agreement between observations of dependence of cobaltite phase on oxidation activity and deposition rate, and predictions indicates that a combined experimental/theoretical approach is key to understanding phase behavior in the strontium cobalt oxide system.

A facile synthesis strategy to couple porous nanocubes of CeO2 with Ag nanoparticles: an excellent catalyst with enhanced reactivity for the ‘click reaction’ and carboxylation of terminal alkynes

CeO₂–Ag nanocomposite was synthesized by modifying the surface of CeO₂ with DMP and its catalytic activity was shown for click reaction and carboxylation of terminal alkynes.

A new structure family of oxide-ion conductors Ca0.8Y2.4Sn0.8O6 discovered by a combined technique of the bond-valence method and experiments

Mg₃TeO₆-type Ca_0.8Y_2.4Sn_0.8O₆ was found as a new structure family of oxide-ion conductors by BVE calculations followed by experiments.

Unveiling the effects of A-site substitutions on the oxygen ion migration in A2−xA′xNiO4+δ by first principles calculations

First principles calculations unveil the effects of A-site substitutions on the interstitial oxygen formation and migration energy in A_2−xA′_xNiO_4+δ.

The Chemical Evolution of the La0.6Sr0.4CoO3-δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO₃ (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O₂ reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.

Gopinath C, Bala T. Deposition of Au nanoparticles inside porous CeO2 nanocubes using Langmuir–Blodgett technique

AuCl₄-ions are reduced to Au⁰ inside the pores of CeO₂ nanocubes by oleylamine, which has the dual role of capping and reducing agent.

Structure Peculiarities of Micro- and Nanocrystalline Perovskite Ferrites La1-x Sm x FeO3

Micro- and nanocrystalline lanthanum-samarium ferrites La_1-x Sm _x FeO₃ with orthorhombic perovskite structure were obtained by using both solid state reactions (x = 0.2, 0.4, 0.6 and 0.8) and sol-gel synthesis (x = 0.5) techniques. Obtained structural parameters of both series of La_1-x Sm _x FeO₃ are in excellent agreement with the "pure" LaFeO₃ and SmFeO₃ compounds, thus proving formation of continuous solid solution in the LaFeO₃-SmFeO₃ system. Peculiarity of La_1-x Sm _x FeO₃ solid solution is divergence behaviour of unit cell dimensions with increasing x: systematic decrease of the a and c lattice parameters is accompanied with increasing b parameter. Such behaviour of the unit cell dimensions in La_1-x Sm _x FeO₃ series led to crossover of the a and c perovskite lattice parameters and formation of dimensionally tetragonal structure near x = 0.04. Linear decrease of the unit cell volume of La_1-x Sm _x FeO₃ with decreasing x according with the Vegard's rule indicate absence of short-range ordering of R-cations in the LaFeO₃-SmFeO₃ system.

Fabrication of Lanthanum Strontium Cobalt Ferrite-Gadolinium-Doped Ceria Composite Cathodes Using a Low-Price Inkjet Printer

In this work, we have successfully fabricated lanthanum strontium cobalt ferrite (LSCF)-gadolinium-doped ceria (GDC) composite cathodes by inkjet printing and demonstrated their functioning in solid oxide fuel cells (SOFCs). The cathodes are printed using a low-cost HP inkjet printer, and the LSCF and GDC source inks are synthesized with fluidic properties optimum for inkjet printing. The composition and microstructure of the LSCF and GDC layers are successfully controlled by controlling the color level in the printed images and the number of printing cycles, respectively. Anode-support type SOFCs with optimized LSCF-GDC composite cathodes synthesized by our inkjet printing method have achieved a power output of over 570 mW cm^-2 at 650 °C, which is comparable to the performance of a commercial SOFC stack. Electrochemical impedance analysis is carried out to establish a relationship between the cell performance and the compositional and structural characteristics of the printed LSCF-GDC composite cathodes.

A Combined Variable-Temperature Neutron Diffraction and Thermogravimetric Analysis Study on a Promising Oxygen Electrode, SrCo0.9Nb0.1O3-δ, for Reversible Solid Oxide Fuel Cells

The present study investigates the temperature-structure-stoichiometry relationship of a promising oxygen electrode SrCo_0.9Nb_0.1O_3-δ over a temperature (T) range from room temperature (RT) to 900 °C. The techniques employed are variable-temperature neutron diffraction (VTND) and thermogravimetric analysis (TGA). At T < 75 °C, VTND reveals a tetragonal (P4/mmm) structure with a G-type magnetic ordering. Above 75 °C, the nucleus structure remains the same, while the magnetic ordering disappears. A phase transition from tetragonal (P4/mmm) to cubic (Pm3̅m) is observed at 412 °C, where the two Co sites and three O sites in the P4/mmm phase converge to one equivalent site, respectively. The phase transition temperature coincides with the peak temperature of oxygen uptake obtained by TGA. It is also observed that the Nb dopant has no preferred Co site to occupy. The oxygen vacancies are mostly located at the O3 site surrounding the Co2 site in the P4/mmm structure. The intermediate-spin state of Co³⁺ at the Co2 site is responsible for the observed distortions of CoO₆ octahedra, i.e., elongation of Co2O₆ octahedra and shortening of Co1O₆ octahedra along the c-axis, which is a phenomenon known as Jahn-Teller distortion. At high temperatures, large thermal displacement factor for O^2- is observed with high concentration of oxygen vacancies, providing a structural environment favorable to high O^2- conductivity in Nb-doped SrCoO₃-based oxygen electrode materials.

Nanoscale Probing of Elastic-Electronic Response to Vacancy Motion in NiO Nanocrystals

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes via first-order reversal curve I-V measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools.

Chromium Poisoning Effects on Surface Exchange Kinetics of La0.6Sr0.4Co0.2Fe0.8O3-δ

The presence of Cr has already been reported in literature to cause severe degradation to La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF). However, fundamental understanding of Cr effects on the surface exchange kinetics is still lacking. For the first time, in situ gas phase isotopic oxygen exchange was utilized to quantitatively determine Cr effect on oxygen exchange kinetics of LSCF powder as a function of temperature and water vapor. Our investigations revealed that the formation of secondary phases such as SrCrO₄, Cr₂O₃, Cr-Co-Fe-O, and La-Co-Fe-O can affect both the oxygen dissociation step and overall surface exchange. Specifically, Cr-containing secondary phases on the surface not only decrease the active sites for surface reactions but also alter the nearby stoichiometry of the LSCF matrix, thereby limiting surface oxygen transport. In addition, water molecules actively participate in the surface reactions and can further block the active sites.

O vacancy formation in (Pr/Gd)BaCo2O5.5 and the role of antisite defects

In search for materials for intermediate temperature solid oxide fuel cells, (Pr/Gd)BaCo₂O_5.5 is investigated by first principles calculations. Antisite defects are considered as they may modify the electronic and O diffusion properties but are rarely studied in double perovskite oxides. Octahedrally coordinated Co atoms are shown to realize intermediate and high spin states for PrBaCo₂O_5.5 and GdBaCo₂O_5.5, respectively, while pyramidally coordinated Co atoms always have high spin. It turns out that O vacancy formation is significantly easier in PrBaCo₂O_5.5 than in GdBaCo₂O_5.5, the difference in formation energy being hardly modified by antisite defects. While pyramidally coordinated Co atoms are not affected, we show that the presence of antisite defects causes parts of the octahedrally coordinated Co atoms to switch from intermediate to high spin.

Controlling Oxygen Mobility in Ruddlesden-Popper Oxides

Discovering new energy materials is a key step toward satisfying the needs for next-generation energy conversion and storage devices. Among the various types of oxides, Ruddlesden–Popper (RP) oxides (A₂BO₄) are promising candidates for electrochemical energy devices, such as solid oxide fuel cells, owing to their attractive physicochemical properties, including the anisotropic nature of oxygen migration and controllable stoichiometry from oxygen excess to oxygen deficiency. Thus, understanding and controlling the kinetics of oxygen transport are essential for designing optimized materials to use in electrochemical energy devices. In this review, we first discuss the basic mechanisms of oxygen migration in RP oxides depending on oxygen nonstoichiometry. We then focus on the effect of changes in the defect concentration, crystallographic orientation, and strain on the oxygen migration in RP oxides. We also briefly review their thermal and chemical stability. Finally, we conclude with a perspective on potential research directions for future investigation to facilitate controlling oxygen ion migration in RP oxides.

Chemically Driven Enhancement of Oxygen Reduction Electrocatalysis in Supported Perovskite Oxides

Perovskite oxides have the capacity to efficiently catalyze the oxygen reduction reaction (ORR), which is of fundamental importance for electrochemical energy conversion. While the perovskite catalysts have been generally utilized with a support, the role of the supports, regarded as inert toward the ORR, has been emphasized mostly in terms of the thermal stability of the catalyst system and as an ancillary transport channel for oxygen ions during the ORR. We demonstrate a novel approach to improving the catalytic activity of perovskite oxides for solid oxide fuel cells by controlling the oxygen-ion conducting oxide supports. Catalytic activities of (La_0.8Sr_0.2)_0.95MnO₃ perovskite thin-film placed on different oxide supports are characterized by electrochemical impedance spectroscopy and X-ray absorption spectroscopy. These analyses confirm that the strong atomic orbital interactions between the support and the perovskite catalyst enhance the surface exchange kinetics by ∼2.4 times, in turn, improving the overall ORR activity.

Thermal Behaviour of Sm0.5 R 0.5FeO3 (R = Pr, Nd) Probed by High-Resolution X-ray Synchrotron Powder Diffraction

Mixed ferrites Sm0.5Pr0.5FeO3 and Sm0.5Nd0.5FeO3 with orthorhombic perovskite structure isotypic with GdFeO3 were synthesized by solid-state reaction technique in air at 1473 K. Structural parameters obtained at room temperature prove a formation of continuous solid solutions in the SmFeO3-PrFeO3 and SmFeO3-NdFeO3 pseudo-binary systems. Sm0.5Pr0.5FeO3 and Sm0.5Nd0.5FeO3 show strongly anisotropic nonlinear thermal expansion: thermal expansion in the b direction is twice lower than in the a and c directions. The average linear thermal expansion coefficients of Sm0.5Pr0.5FeO3 and Sm0.5Nd0.5FeO3 in the temperature range of 298-1173 K are in the limits of (9.0-11.1) × 10(-6) K(-1), which is close to the values reported for the parent RFeO3 compounds. Subtle anomalies in the lattice expansion of Sm0.5Pr0.5FeO3 and Sm0.5Nd0.5FeO3 detected at 650-750 K reflect magnetoelastic coupling at the magnetic ordering temperature T N.

Polarization-Induced Interface and Sr Segregation of in Situ Assembled La0.6Sr0.4Co0.2Fe0.8O3-δ Electrodes on Y2O3-ZrO2 Electrolyte of Solid Oxide Fuel Cells

Application of cobaltite-based electrodes such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) on Y₂O₃-ZrO₂ (YSZ) electrolyte in solid oxide fuel cells (SOFCs) generally requires the use of a doped ceria barrier layer to prevent the interaction between LSCF and YSZ during sintering at high temperatures. In this paper, we report for the first time an in situ assembly approach to directly incorporate LSCF cathode to YSZ electrolyte without the use of a doped ceria barrier layer and without presintering at high temperatures. A Ni-YSZ anode-supported YSZ electrolyte cell with an in situ assembled LSCF electrode exhibits a peak power density of 1.72 W cm^-2 at 750 °C. However, the cell performance degrades significantly at 500 mAcm^-2 and 750 °C. The results indicate that cathodic polarization not only induces the formation of the interface but also accelerates the Sr segregation. The segregated Sr migrates to the LSCF electrode/YSZ electrolyte surface and forms an SrO layer. Using a Sr-free LaCoO_3-δ composite cathode overcomes the Sr segregation problem, showing an excellent peak power density of 1.2 Wcm^-2 and good stability at 750 °C for over 100 h. The present study shows that cobaltite-based perovskites can be directly used on YSZ-based electrolyte via the in situ assembly providing an effective means to advance the application of highly active mixed ionic/electronic conducting cathodes to YSZ electrolyte-based SOFCs.

A computational study of doped olivine structured Cd2GeO4: local defect trapping of interstitial oxide ions

Computational modelling techniques have been employed to investigate defects and ionic conductivity in Cd₂GeO₄. We show due to highly unfavourable intrinsic defect formation energies the ionic conducting ability of pristine Cd₂GeO₄ is extremely limited. The modelling results suggest trivalent doping on the Cd site as a viable means of promoting the formation of the oxygen interstitial defects. However, the defect cluster calculations for the first time explicitly suggest a strong association of the oxide defects to the dopant cations and tetrahedral units. Defect clustering is a complicated phenomenon and therefore not trivial to assess. In this study the trapping energies are explicitly quantified. The trends are further confirmed by molecular dynamic simulations. Despite this, the calculated diffusion coefficients do suggest an enhanced oxide ion mobility in the doped system compared to the pristine Cd₂GeO₄.

Phases and Interfaces from Real Space Atomically Resolved Data: Physics-Based Deep Data Image Analysis

Advances in electron and scanning probe microscopies have led to a wealth of atomically resolved structural and electronic data, often with ∼1-10 pm precision. However, knowledge generation from such data requires the development of a physics-based robust framework to link the observed structures to macroscopic chemical and physical descriptors, including single phase regions, order parameter fields, interfaces, and structural and topological defects. Here, we develop an approach based on a synergy of sliding window Fourier transform to capture the local analog of traditional structure factors combined with blind linear unmixing of the resultant 4D data set. This deep data analysis is ideally matched to the underlying physics of the problem and allows reconstruction of the a priori unknown structure factors of individual components and their spatial localization. We demonstrate the principles of this approach using a synthetic data set and further apply it for extracting chemical and physically relevant information from electron and scanning tunneling microscopy data. This method promises to dramatically speed up crystallographic analysis in atomically resolved data, paving the road toward automatic local structure-property determinations in crystalline and quasi-ordered systems, as well as systems with competing structural and electronic order parameters.

Operation of Thin-Film Electrolyte Metal-Supported Solid Oxide Fuel Cells in Lightweight and Stationary Stacks: Material and Microstructural Aspects

In this study we report on the development and operational data of a metal-supported solid oxide fuel cell with a thin film electrolyte under varying conditions. The metal-ceramic structure was developed for a mobile auxiliary power unit and offers power densities of 1 W/cm² at 800 °C, as well as robustness under mechanical, thermal and chemical stresses. A dense and thin yttria-doped zirconia layer was applied to a nanoporous nickel/zirconia anode using a scalable adapted gas-flow sputter process, which allowed the homogeneous coating of areas up to 100 cm². The cell performance is presented for single cells and for stack operation, both in lightweight and stationary stack designs. The results from short-term operation indicate that this cell technology may be a very suitable alternative for mobile applications.

New Rhenium-Doped SrCo1-xRexO3-δ Perovskites Performing as Cathodes in Solid Oxide Fuel Cells

In the aim to stabilize novel three-dimensional perovskite oxides based upon SrCoO_3−δ, we have designed and prepared SrCo_1−xRe_xO_3−δ phases (x = 0.05 and 0.10), successfully avoiding the competitive hexagonal 2H polytypes. Their performance as cathode materials in intermediate-temperature solid oxide fuel cells (IT-SOFC) has been investigated. The characterization of these oxides included X-ray (XRD) and in situ temperature-dependent neutron powder diffraction (NPD) experiments for x = 0.10. At room temperature, SrCo_1−xRe_xO_3−δ perovskites are defined in the P4/mmm space group, which corresponds to a subtle tetragonal perovskite superstructure with unit-cell parameters a = b ≈ a_o, c = 2a_o (a_o = 3.861 and 3.868 Å, for x = 0.05 and 0.10, respectively). The crystal structure evolves above 380 °C to a simple cubic perovskite unit cell, as observed from in-situ NPD data. The electrical conductivity gave maximum values of 43.5 S·cm⁻¹ and 51.6 S·cm⁻¹ for x = 0.05 and x = 0.10, respectively, at 850 °C. The area specific resistance (ASR) polarization resistance determined in symmetrical cells is as low as 0.087 Ω·cm² and 0.065 Ω·cm² for x = 0.05 and x = 0.10, respectively, at 850 °C. In single test cells these materials generated a maximum power of around 0.6 W/cm² at 850 °C with pure H₂ as a fuel, in an electrolyte-supported configuration with La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) as the electrolyte. Therefore, we propose the SrCo_1−xRe_xO_3−δ (x = 0.10 and 0.05) perovskite oxides as promising candidates for cathodes in IT-SOFC.

The Sulphur Poisoning Behaviour of Gadolinia Doped Ceria Model Systems in Reducing Atmospheres

An array of analytical methods including surface area determination by gas adsorption using the Brunauer, Emmett, Teller (BET) method, combustion analysis, XRD, ToF-SIMS, TEM and impedance spectroscopy has been used to investigate the interaction of gadolinia doped ceria (GDC) with hydrogen sulphide containing reducing atmospheres. It is shown that sulphur is incorporated into the GDC bulk and might lead to phase changes. Additionally, high concentrations of silicon are found on the surface of model composite microelectrodes. Based on these data, a model is proposed to explain the multi-facetted electrochemical degradation behaviour encountered during long term electrochemical measurements. While electrochemical bulk properties of GDC stay largely unaffected, the surface polarisation resistance is dramatically changed, due to silicon segregation and reaction with adsorbed sulphur.