Nanoparticle Surface Modification Enhanced Anode Performance in La1.7Sr0.3Fe0.9Ru0.1O4 for Intermediate Temperature Solid Oxide Fuel Cells

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.

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

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) .

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.

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.

Assessment of finite volume modeling approaches for intermediate temperature Solid Oxide Fuel Cells working with CO-rich syngas fuels

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.

Advanced Engineering Toward High Performance Intermediate Temperature Solid Oxide Fuel Cells

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.

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.

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.

The conventional solid oxide fuel cell electrolyte, Yttria stabilized Zirconia requires high operating temperature (800 - 1000 0C) to achieve sufficient oxide ion conductivity, which increases the cost and exacerbates the instability between the components. Therefore, there is considerable interest in developing alternative electrolytes(s > 10−2 S/ cm) at intermediate temperature (IT, 500 - 700 0C). Hence, here we successfully synthesized IT operatable electrolytes Sr1−xNaxSiO3−δ (x = 0.0, 0.45) by solid state reactions. From the XRD, it is evident that both the samples can be indexed to monoclinic c2/c space group (15). NMR reveals a broad peak at ~ −87 ppm for Sr0.55Na0.45SiO3−δ indicates the presence of amorphous Na2Si2O5 phase. FE-SEM and WDS mapping shows the segregation of amorphous Na2Si2O5 along the grain boundaries. Sr0.55Na0.45SiO3−δ exhibits high conductivity in the measured temperature range, which makes it as electrolyte for IT-SOFCs.

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.

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.

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.