NEW CATHODE MATERIALS FOR INTERMEDIATE TEMPERATURE SOLID OXIDE FUEL CELLS

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.

Triple perovskite has been recently developed for the intermediate temperature solid oxide fuel cell (IT-SOFC). The performance of Nd1.5Ba1.5CoFeMnO9−δ (NBCFM) cathodes for IT-SOFC is investigated in this work. The interfacial polarization resistance (RP) of NBCFM is 1.1273 Ω cm2~0.1587 Ω cm2 in the range of 700–800 °C, showing good electrochemical performance. The linear thermal expansion coefficient of NBCFM is 17.40 × 10−6 K−1 at 40–800 °C, which is significantly higher than that of the electrolyte. In order to further improve the electrochemical performance and reduce the thermal expansion coefficient (TEC) of NBCFM, Ce0.8Sm0.2O2−δ (SDC) is mixed with NBCFM to prepare an NBCFM-xSDC composite cathode (x = 0, 10, 20, 30, 40 wt.%). The thermal expansion coefficient decreases monotonically from 17.40 × 10−6 K−1 to 15.25 × 10−6 K−1. The surface oxygen exchange coefficient of NBCFM-xSDC at a given temperature increases from 10−4 to 10−3 cm s−1 over the range from 0.01 to 0.09 atm, exhibiting fast surface exchange kinetics. The area specific resistance (ASR) of NBCFM-30%SDC is 0.06575 Ω cm2 at 800 °C, which is only 41% of the ASR value of NBCFM (0.15872 Ω cm2). The outstanding performance indicates the feasibility of NBCFM-30% SDC as an IT-SOFC cathode material. This study provides a promising strategy for designing high-performance composite cathodes for SOFCs based on triple perovskite structures.

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.

The strontium doped lanthanum cuprate with tetragonal perovskite structure has been considered a potential cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). This study presents work on the performance evaluation of using La 0.75 Sr 0.23 CuO 2.5-δ (LSCu) as cathode material for IT-SOFC carried out by the Ni-YSZ anode supported single cell. The cell consists of a 600-μm-thick porous Ni/YSZ anode as cell support, a yttria-stabilized zirconia (YSZ) thin film layer (ca. 8 μm) as electrolyte, and a porous LSCu layer as cathode (ca. 40 pm). For the performance testing, the maximum power density at 800°C was 0.65 W/cm 2 with 150 and 500 seem of hydrogen and air flow, respectively. However, the area-specific resistances (ASRs) of the cell were 0.32, 0.75, and 1.21 Ω cm 2 at 800, 750, and 700°C, respectively. For further improvement, the ASR will be decreased by LSCu compositional modification and microstructure optimization of the LSCu cathode layer.

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.

The microstructure and elemental distribution of the gadolinium-doped ceria (GDC) thin film electrolyte, Ni-GDC cermet anode, and the interface between them were comprehensively characterized by high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX) operated in scanning TEM (STEM) mode. HR-TEM observations show newly appeared microstructure (i.e., superstructures) formations at both GDC and metallic Ni grains at the electrolyte−anode interface. STEM-EDX mapping and line scan analyses illustrate that not only can Ni diffuse into GDC grains as previously reported but also Ce and Gd can diffuse into metallic Ni particles with equal diffusion lengths as that of Ni diffusion. Such mutual diffusion is independent of ionic radii and can result in the valence state change of diffusing ions, verified by electron energy loss spectroscopy investigations. Therefore, the mutual diffusion and related microstructural evolutions are elucidated to be dominating factors that lead to the interfacial layer formation between anode and electrolyte, which is identified to have a considerable influence on the ionic conductivity behavior in intermediate temperature solid oxide fuel cells.

A kind of nanostructured Ce0.9Sm0.1O1.95 is prepared by a hydrothermal method. It exhibits large Brunauer‐Emmett‐Teller area (>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).

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

Ni-free redox reversible perovskite oxide La0.5Sr0.5Fe0.8Cu0.1Ti0.1O3-δ (LSFCT) has been synthesized and in-situ reduced to a functional anode R-LSFCT for intermediate temperature solid oxide fuel cells (ITSOFC). Power generating properties of the cells in various fuels, and the stability of the cell in H2 are investigated by using scandia-stabilized ZrO2 electrolyte and Pr0.5Sr0.5Fe0.8Cu0.2O3-δ cathode. It is found that reasonably high power densities are achieved on the cells with various fuels. The maximum power density (MPD) of the cell with wet H2 as fuel is achieved for values of 690, 531 and 368 mW cm−2 at 800, 750 and 700°C, respectively, and the MPDs of the cells in H2 containing 54 ppm H2S and wet syngas (H2 + 50% CO) fuels are achieved 618 and 512 mW cm−2 at 800°C, respectively. Moreover, the cell in wet H2 demonstrates a quite stable output at 800°C under 0.7 V for 100 h without any decrease. This work also shows that doping Ti stabilizes the perovskite structure of LSFCT in 5% H2-Ar and suppresses the segregation of Sr at surface of LSFCT/R-LSFCT, which may account for the improved cell performance and stability comparing with the undoped R-La0.5Sr0.5Fe0.8Cu0.2O3-δ anode.

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

The materials La2−xSrxNiO4+δ(x=0,0.1) with K2NiF4 structure have been studied in order to explore their potential use as cathodes for intermediate temperature solid oxide fuel cells. The introduction of Sr in the La2NiO4+δ lattice produces an enhancement of the electric conductivity but is accompanied by a decrease of ionic transport species. Neutron powder diffraction data show a transition from orthorhombic to tetragonal symmetry with the incorporation of Sr in the LaO sublattice. Excess oxygen determined by iodometric titration shows that strontium insertion generates a decrease of the interstitial oxygen. In order to avoid the concomitant decrease of the ionic conductivity, the system underwent heat treatments under high oxygen pressure (200bar, 650°C). As a result, the O2− treated La1.9Sr0.1NiO4+δ exhibited an increase of the amount of interstitial oxygens (δ=0.17) but with improved electronic properties. The polarization resistances measured for these materials vary between 8Ωcm2 and 0.1Ωcm2 in air at the temperature range of 700–975°C. This remarkable behavior enables us to propose this material as an alternative cathode for IT-SOFC.

Carbon nanotubes (CNTs)/La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) composite films have been fabricated by electrophoretic codeposition on Ce0.9Gd0.1O1.95 (CGO) substrates. CNTs are used as a sacrificial phase to produce ordered porous LSCF cathodes for intermediate temperature solid oxide fuel cells. The synthesis of LSCF powder by a modified sol–gel route is presented. The possible mechanism of formation of CNT/LSCF composite nanoparticles in suspension is discussed. Moreover the optimal suspension composition and the conditions for achieving successful electrophoretic deposition (EPD) of CNTs/LSCF composite nanoparticles were evaluated. Experimental results showed that the CNTs were homogeneously distributed and mixed with LSCF nanoparticles forming a mesh‐like structure, which resulted in a highly porous LSCF film when the CNTs were burned out during heat treatment in air at 800°C for 2 h. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffraction (XRD) techniques were employed to characterize the microstructure of the precursors and of the composite films.

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

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

Powders of Bi2Ru2O7.3, Pb2Ru2O6.5 and Y2Ru2O7 were prepared and their reactivity with Ce0.9Gd0.1O1.95 (CGO) electrolyte powders examined. Bi2Ru2O7.3 and Pb2Ru2O6.5 reacted with CGO but Y2Ru2O7 appeared stable when heated in contact with CGO powder at 900C for 24 h. Symmetrical electrodes of Y2Ru2O7-x were deposited on CGO ceramic pellets, either by tape-casting or by electrostatic assisted chemical vapor deposition (EACVD) techniques, and cathode resistivities determined by impedance spectroscopy. Undoped Y2Ru2O7 electrodes exhibited very high area specific resistivities (ASR) at 627C (∼4000 Ωcm2), but by doping with SrO the resistivity was reduced almost 100×to 47 Ωcm2. The behavior of the Y2Ru2O7 cathodes was interpreted in terms of available oxygen ion transport data for the Gd2Ti2O7 series, and it was concluded that optimization of pyrochlore ruthenate compositions should be possible to improve further the oxygen reduction behavior of ruthenate cathodes for intermediate temperature solid oxide fuel cells.