Perovskite and Composite Materials for Intermediate Temperatures Solid Oxide Fuel Cells

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.

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C<T<800 °C) in SOFCs in order to extend their life span, thermal compatibility, cost-effectiveness, and ease of fabrication. However, the major challenges in developing cathode materials for low/intermediate temperature SOFCs include structural stability, catalytic activity for oxygen adsorption and reduction, and tolerance against contaminants such as chromium, boron, and sulfur. This research aims to provide an updated review of the perovskite-based state-of-the-art cathode materials LaSrMnO3 (LSM) and LaSrCOFeO3 (LSCF), as well as the recent trending Ruddlesden-Popper phase (RP) and double perovskite-structured materials SOFCs technology. Our review highlights various strategies such as surface modification, codoping, infiltration/impregnation, and composites with fluorite phases to address the challenges related to LSM/LSCF-based electrode materials and improve their electrocatalytic activity. Moreover, this study also offers insight into the electrochemical performance of the double perovskite oxides and Ruddlesden-Popper phase materials as cathodes for SOFCs.

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.

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.

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.

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.

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.

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.

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

Review on core-shell structured cathode for intermediate temperature solid oxide fuel cells

Development of nickel based cermet anode materials in solid oxide fuel cells – Now and future

A kind of nanostructured Ce0.9Sm0.1O1.95 is prepared by a hydrothermal method. It exhibits large Brunauer‐Emmett‐Teller area (&gt;120 m2 g–1), open mesoporous structure and nanocrystalline sheets. This material shows high activity for oxygen reduction. An area specific resistance below 0.6 Ω cm2 at 600 °C is achieved when it is mixed with Ag paste as a composite cathode in a symmetric cell. This is attributed to its large triple phase boundary area and good ionic conducting property. The influences of microstructure, doping, loading effects of silver nanoparticles and the temperature dependence on oxygen reduction kinetics are investigated by the impedance spectroscopy. Although some further optimisations are still required, this special Ce0.9Sm0.1O1.95 material reported here exhibits promising features of applications in the composite electrodes for intermediate temperature solid oxide fuel cells (IT‐SOFCs).

Effects of In substitution in Y1−xInxBaCo3ZnO7+δ (0 ≤ x ≤ 0.5) cathodes for intermediate temperature solid oxide fuel cells

Enhancing the oxygen reduction reaction activity of SOFC cathode via construct a cubic fluorite/perovskite heterostructure

A mixture of lanthanum strontium cobalt ferrite (LSCF) and nickel oxide (NiO) makes for a desirable cathode material for an IT-SOFC due to its excellent oxygen reduction capability. This study investigates the effect of NiO addition into LSCF cathode on its physical and electrochemical properties. To optimise the amount of NiO addition, both electrochemical impedance spectra and bode phase were used to examine various weight ratios of nickel oxide and LSCF cathode. Brunauer-Emmett-Teller (BET) and thermal analyses validated the electrochemical observation that the LSCF:NiO ratio yields sensible oxygen reduction reaction and stoichiometric findings. Initial characterisation, comprising of phase and bonding analyses, indicated that LSCF-NiO was successfully synthesised at 800 °C using an improved modified sol gel technique. The addition of 5% nickel oxide to LSCF results in the lowest area specific resistance (ASR) value overall. The Bode phase implies that the addition of 5% nickel oxide to LSCF reduces the impedance at low frequencies by 64.28 percent, indicating that a greater oxygen reduction process happened at the cathode. After the addition of 5 wt% NiO, a single LSCF-NiO cell may function at temperatures as low as 650 °C and the LSCF cathode power density is increased by 25.35%. The surface morphology of the LSCF-NiO cathode reveals that the average particle size is less than 100 nm, and mapping analysis demonstrated a homogenous NiO distribution over the cathode layer. Consequently, the synthesis of LSCF-NiO at intermediate temperatures (800–600 °C) revealed outstanding chemical compatibility, bonding characteristics, and electrochemical performance.