Influence of Copper‐based Anode Composition on Intermediate Temperature Solid Oxide Fuel Cells Performance

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.

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.

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.

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.

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.

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

The strontium-lanthanum cobalt-rich perovskites are the most extensively studied mixed conductor SOFC cathode materials[1-3]. Recently, different methods have been developed to improve their microstructure and to make them suitable for use as electrodes in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) [1, 3-5]. It is well known that reducing particle size leads to an increase in surface area, thereby reducing the electrode polarization resistance. However, the elevated temperatures typical of SOFC operation can cause particles coarsening, a dominant contribution to long-term degradation for nanomaterials [6, 7]. Another cause of degradation is Sr-segregation on the surface of these perovskites, which blocks the O-surface exchange sites [8, 9] and decreases the effective surface. In this work we present the study of the cell performance and long-term stability of a nanoscaled La0.4Sr0.6Co0.8Fe0.2O3-δ cathode obtained by the combustion method (LSCF4682) at 800˚C. Details about the synthesis method, the microstructure and the mechanism of oxygen reduction reaction for this cathode has been previously reported in [4]. The unique microstructure achieved by this method, which exhibits a high surface area, good interconnectivity of grains and a high volume fraction of mesopores induces a change of mechanism controlling the O-surface rate. Below 600 °C, the mechanism of reaction is dominated by the O-surface and transport rates, whereas as T increases the mass transfer due to the O2gas diffusion becomes important [4]. SOFC tests were performed on single cells of anode–electrolyte–cathode systems. The samples were constructed from a porous (Sr0.8La0.2TiO3-α) STO-LSGM supported multilayer cell [10]. The anode consists of a Ni infiltrated porous LSGM separated from the cathode side by a thin dense LSGM electrolyte. The cathode was prepared as a bilayer electrode consisting of a thin LSGM porous interlayer and a thicker LSCF4682-LSGM 50% composite. Figure 1a shows post mortemSEM images of the tested cell, and Figures 1 b and c show the cathode and anode microstructures. Figure 2 shows I-V and I-P curves collected between 500 and 650 °C. A maximum power density of 1.23 Wcm-2was achieved at 650 °C suggesting that this is a competitive IT-SOFC assembly. Above 600 °C the gas mass transfer controls the power density at high current density in agreement with the oxygen reduction mechanisms previously proposed [4]. The time-dependent evolution of cathode polarization resistance for this material deposited in symmetrical LSCF/LSGM/LSCF cell was studies by Electrochemical Impedance Spectroscopy (EIS) and compared with that of a commercial La0.6Sr0.4Co0.2Fe0.8O3-δ cathode (Praxair LSCF6428). In order to reduce the time for data acquisition, an accelerated aging test was performed, where ageing would take place at 800 °C, with brief reduction in temperature to 700 °C for EIS collection. Figure 3a shows the initial and 500 h EIS spectra of cells manufactured with either, the commercial LSCF6428 and the LSCF4682 synthetized by us. Despite that the degradation rate of both cathodes are similar, the low LSCF4682 polarization resistance guarantees a competitive IT-SOFC cathode during longer-term operation. Figure 3b indicates that the time evolution of total cathode polarization resistance (Rtot) is due to both contributions by O-surface exchange and O2gas diffusion suggesting a microstructural change with time. Figure Caption Figure 1. a) SEM cross-section images of SOFC full cell b) cathode consisting of LSCF4682 as synthetized powder c) Ni-LSGM infiltrate anode. Figure 2. Voltage and power density versus current density measured between 200 sccm air and 100 sccm humidified H2 as a function of temperature. Figure 3. a) Comparison between EIS spectra taken at 700 °C at the beginning and after 500 h of the 800 ˚C aging test. b) Resistance evolution with time for High (RHF) and Low (RLF) frequency contribution to Total cathode polarization resistance (Rtot) .

Characterization of novel GdBa0.5Sr0.5Co2−xFexO5+δ perovskites for application in IT-SOFC cells

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

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.

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.

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.

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.