Sodium Doped Strontium Silicates as Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

Among the various oxygen conducting materials, Sr and Mg doped lanthanum gallate (LSGM) is attractive for use as an electrolyte in intermediate temperature Solid Oxide Fuel Cells (SOFC). the LSGM compositions show high oxygen conductivity as well as excellent stability over the range of oxygen partial pressures encountered in SOFC operation. Lowering the operating temperature is expected to provide a more expeditious route to commercial realization of Solid Oxide Fuel Cell (SOFC) based power systems. the intermediate operating temperature (600 to 800°C) allows the use of lower cost materials in both the stack and the balance of plant, and significantly slows down potential deleterious materials interaction, thus extending the useful long-term performance and thermal cycle capability. the primary challenge in long-term stability of LSGM-based fuel cell operation is the chemical reaction between the nickel anode and the LSGM electrolyte. A chemical modification to the anode was found to reduce the reaction. the modified anode composition is also shown to perform equivalent to standard NiO based anode. Single cells using thin (30 micron) electrolyte supported on a thicker anode were found to operate at a power density of greater than 500 mW/cm2 at 700°C. Stack test verification using thick electrolyte supported cells show promising performance. Stack tests using thin electrolytes are planned.

Planar solid oxide fuel cells (SOFCs), which are intended to be used in the intermediate temperature range (600–800 °C) and whose supporting anode contains various amount of Cu in Li‐doped gadolinia‐doped ceria matrix, are produced and characterized in the present work. Intermediate temperature solid oxide fuel cell (IT‐SOFC) performances are investigated by recording polarization and power density at different temperatures. Electrochemical impedance spectroscopy (EIS) measurements are carried out to analyze how internal resistances change as function of temperature and, above all, for variable Cu content. The gadolinia‐doped ceria electrolyte microstructure and the cell integrity after sintering are also shown to depend on the initial CuO content in the anode. The electronic conductivity increases with Cu content within the anodic cermet. When the anode contains more than 25 vol.% CuO, a dense Li‐doped gadolinia‐doped ceria electrolyte can be obtained by sintering at 900 °C. Conversely, CuO load in excess to 50% in the anode is detrimental because of the formation of macroscopic cracks within the electrolyte. Intermediate temperature solid oxide fuel cells produced with an anodic layer containing 50 vol.% CuO show the lowest impedance parameters.

A review of doped lanthanum gallates as electrolytes for intermediate temperature solid oxides fuel cells: From materials processing to electrical and thermo-mechanical properties

Thermoelectric characterization of an intermediate temperature solid oxide fuel cell system directly fed by dry biogas

A new challenge in the field of Solid Oxide Fuel Cells (SOFCs) concerns the reduction of their operating temperature down to 700°C. Apatite ceramics are interesting alternatives for SOFC electrolytes due to their high ionic conductivity at this temperature. The present work reports on the manufacturing and characterization of La9SrSi6O26.5 coatings obtained by atmospheric plasma spraying at two different plasma spray powders. The microstructure and the composition of the as-sprayed and heat-treated coatings were investigated by several techniques including X-Ray Diffraction, Inductively Coupled Plasma - Atomic Emission Spectroscopy as well as Scanning and Transmission Electron Microscopy. The porosity level of the coatings was evaluated by the Archimedean method and image analysis. The studies revealed that the as-sprayed apatite coatings were composed of an amorphous phase, a crystalline apatite phase and chemical heterogeneities due to Si volatilization in the high-temperature plasma. Furthermore, a heat treatment made it possible to obtain denser, fully crystallized apatite coatings and also improved their ionic conductivity.

Proton conducting oxides such as BaZr0.1Ce0.7Y0.1Yb0.1O3 (BZCYYb) are attractive electrolyte materials for intermediate temperature (~400-600oC) solid oxide fuel cells (IT-SOFC) due to their higher ionic conductivity at intermediate temperature range compared to widely-used oxide-ion conducting oxides (e.g. Yttria-stabilized zirconium, YSZ) that are used in conventional high temperature (>700oC) SOFC. Whether proton conductivity also offers additional benefit for the cathode process has not been well-studied yet. In this study, two most promising cathode materials for oxide ion conducting IT-SOFC - Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and SrCo0.8Nb0.1Ta0.1O3−δ (SCNT) as well as their corresponding composites with proton conducting electrolyte (i.e., BSCF-BZCYYb and SCNT-BZCYYb) are evaluated as the cathode for proton conducting IT-SOFC using electrolyte supported cell structured from 750oC to 450oC. Comparisons have been made based on electrochemical impedance spectroscopy (EIS), current-voltage characteristics (IV) tests, as well as microstructure features. The implications from these studies are discussed and the directions for designing new proton conducting IT-SOFC cathodes with further improved performance are pointed out based on the insights gained from these studies.

Operation of SOFCs at intermediate temperatures (500-800 C) requires new combinations of electrolyte and electrode materials that will provide both rapid ion transport across the electrolyte and electrode--electrolyte interfaces and efficient electrocatalysis of the oxygen reduction and fuel oxidation reactions. This project concentrates on materials and issues associated with cathode performance that are known to become limiting factors as the operating temperature is reduced. The specific objectives of the proposed research are to develop cathode materials that meet the electrode performance targets of 1.0 W/cm{sup 2} at 0.7 V in combination with YSZ at 700 C and with GDC, LSGM or bismuth oxide based electrolytes at 600 C. The performance targets imply an area specific resistance of {approx}0.5 {Omega}cm{sup 2} for the total cell. The research strategy is to investigate both established classes of materials and new candidates as cathodes, to determine fundamental performance parameters such as bulk diffusion, surface reactivity and interfacial transfer, and to couple these parameters to performance in single cell tests. In this report, the oxygen exchange kinetics of a P2 composition are described in detail. The oxygen exchange kinetics of the oxygen deficient double perovskite LnBaCo{sub 2}O{sub 5.5+{delta}} (Ln=Pr and Nd) have been determined by electrical conductivity relaxation. The high electronic conductivity and rapid diffusion and surface exchange kinetics of PBCO suggest its application as cathode material in intermediate temperature solid oxide fuel cells.

Influence of K+ substitution on germanium doped strontium silicate (Sr3-3xK3xSi3-3yGe3yO9-δ; 0 ≤ x ≤ 0.20, y=0.1) for application as solid electrolyte

Performance durability is one of the essential requirements for solid oxide fuel cell materials operating in the intermediate temperature range (500–700 °C). The trade‐off between desirable catalytic activity and long‐term stability challenges the development and commercialization of electrode materials. Here an oxygen cathode material, Ba0.5Sr0.5(Co0.7Fe0.3)0.69−xMgxW0.31O3−δ (BSCFW‐xMg), that exhibits excellent electrocatalytic performance through the addition of an optimized amount of Mg to the self‐assembled nanocomposite Ba0.5Sr0.5(Co0.7Fe0.3)0.69W0.31O3−δ (BSCFW) by simple solid‐state reaction is reported. Distinct from the bulk and surface approaches to introduce vacancies and defects in materials design, here the Mg2+ ions concentrate at the single perovskite/double perovskite interface of BSCFW with dislocations and Mg2+‐rich nanolayers, resulting in stressed and compositionally inhomogeneous interface regions. The interfacial chemistry within these nanocomposites provides an additional degree of freedom to enable performance optimization over single phase materials and promotes the durability of alkaline‐earth based fuel cell materials.

AC complex impedance spectroscopy studies were conducted between 600-800 C on symmetrical cells that employed strontium-and-magnesium-doped lanthanum gallate electrolyte, La{sub 0.9}Sr{sub 0.1}Ga{sub 0.8}Mg{sub 0.2}O{sub 3} (LSGM). The objective of the study was to identify the materials system for fabrication and evaluation of intermediate temperature (600-800 C) solid oxide fuel cells (SOFCs). The slurry-coated electrode materials had fine porosity to enhance catalytic activity. Cathode materials investigated include La{sub 1-x}Sr{sub x}MnO{sub 3} (LSM), LSCF (La{sub 1-x}Sr{sub x}Co{sub y}Fe{sub 1-y}O{sub 3}), a two-phase particulate composite consisting of LSM-doped-lanthanum gallate (LSGM), and LSCF-LSGM. The anode materials were Ni-Ce{sub 0.85}Gd{sub 0.15}O{sub 2} (Ni-GDC) and Ni-Ce{sub 0.6}La{sub 0.4}O{sub 2} (Ni-LDC) composites. Experiments conducted with the anode materials investigated the effect of having a barrier layer of GDC or LDC in between the LSGM electrolyte and the Ni-composite anode to prevent adverse reaction of the Ni with lanthanum in LSGM. For proper interpretation of the beneficial effects of the barrier layer, similar measurements were performed without the barrier layer. The ohmic and the polarization resistances of the system were obtained over time as a function of temperature (600-800 C), firing temperature, thickness, and the composition of the electrodes. The study revealed important details pertaining to the ohmic and the polarization resistances of the electrode as they relate to stability and the charge-transfer reactions that occur in such electrode structures.

Solid Oxide Fuel Cells (SOFCs) are a promising technology to produce electric energy from chemical energy. To achieve this, the development of new materials with enhanced properties are still needed. In this work, the synthesis of nanostructured powders to be used as components of an Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) was carried out through different synthesis methods. So, co-precipitation, sol-gel combustion, and hydrothermal synthesis methods were compared in the synthesis of the Gadolinium Doped Cerium G0,2C0,8O1,9 (GDC) used as electrolytic material. Cathodic material (La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) cobaltite type perovskite) was obtained by coprecipitation method. The materials were characterized by Fourier Transformed Infrared Spectroscopy (FTIR), Thermal Gravimetrical Analysis (TGA), Scanning Electron Microscopy (SEM-EDS), X-ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). The synthesized materials showed to be nanostructured. The chemical composition measured by EDS was constant in the entire sample and it has the expected stoichiometry in both materials. On the other hand, X-ray patterns exhibited well-defined peaks characteristic of ordered structures and with a high degree of crystallinity, corresponding with the information reported in the literature. From FTIR results, the characteristic peaks related to the presence of perovskites can be observed in the case of LSCF when it was compared to the results found in the literature. Nanostructured LSCF cathodic material was effectively synthesized by coprecipitation method. In the case of GDC highly crystalline material was obtain by sol-gel combustion method. Based on the results, the employed method allows to obtain homogeneous materials suitable to be used in IT-SOFC.

Solid oxide fuel cells (SOFCs) have drawn remarkable attention in recent years due to their distinctive advantages of high efficiency, fuel variety and low environment pollution. Along the way to designing of new cathode materials for SOFCs, an understanding of the mechanism of oxygen reduction reaction (ORR) plays a key role, especially the interaction between O2 molecule and surface of cathode. The results of the first principles calculations of the atomic and electronic structure of a layered perovskite LaSrCo0.5Feo.5O4-δ used as a cathode material for solid oxide fuel cells (SOFCs) are presented and discussed. The oxygen vacancy formation energies is found to be smaller than LaSrCoO4 and slightly lower than the other perovskite, e.g. (La, Sr)MnO3, which widely used as a cathode because of its excellent catalytic activity. From a detailed analysis of difference electron densities, the presence of oxygen vacancies can induce local charge redistribution. Furthermore, a strong hybridization between Co-Oad and Fe-Oad for molecular adsorption was obtained by analysis of density of states. Our results show how the mixed B-site ions produce the excellent adsorption and conduction performance of LaSrCo0.5Feo.5O4-δ and fully support its potential application in next-generation SOFCs.

Effect of the symmetric cell preparation temperature on the activity of Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as cathode for intermediate temperature Solid Oxide Fuel Cells

Preparation and Characterization of Anodes for Intermediate Temperature Solid Oxide Fuel Cells

The bulk-to-surface Sr-segregation can seriously compromise the stability of oxygen electrocatalysis in Sr-doped perovskite oxides such as La1-xSrxCoO3-δ and La1-xSrxCo1-yFeyO3-δ and limit their practical applications as cathode materials in solid oxide fuel cells. This work aims to acquire fundamental knowledge of Sr-segregation process under practical conditions in solid oxide fuel cells and develop suppression method to ensure the long-term stability through surface modifications. In this work, the pristine and ZrO2 atomic layer deposition (ALD) modified La0.6Sr0.4CoO3- d (LSCo) epitaxial films were used as the model systems to investigate how the temperature, time and surface coating affect the surface concentration of Sr via in situ synchrotron-based ambient pressure XPS. This information was also correlated with their electrochemical performances characterized by electrochemical impedance spectroscopy. We also report a Sr-segregation-free SrCo0.9Ta0.1O3-δ (SCT) multifunctional coating on a benchmark cathode LSCF as a means of enhancing ORR activity and durability against Cr-poison.