Preparation and Characterization of Anodes 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.

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

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).

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.

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.

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.

Ceria has higher conductivity than the conventional SOFC electrolyte, yttria-stabilized zirconia (YZS) but has the drawback of measurable electronic conductivity, particularly at high temperatures. The aims of this work research are to synthesize several Gd and Er co-doped ceria materials as solid electrolyte for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) and to obtain the correlation between the average ionic radius of dopants and the value of their conductivity. Three set material have been choosen as target of this research. They are Ce <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x−y</inf> Gd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Er <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2−z</inf> (CeGdEr); Ce <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x−y</inf> Gd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Dy <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2−z</inf> (CeGdDy); and Ce <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x−y</inf> Gd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2−z</inf> (CeGdNd). In this paper, one set material of CeGdEr have been synthesized, they are Ce90Gd5Er5, Ce85Gd5Er10, Ce85Gd10Er5, and Ce80Gd10Er10. Their structures have been investigated using X-Ray Diffraction (XRD) and refined it using Le Bail method. All material have the similar structure that is cubic structure with space group Fm3m, but only a little differences at their lattice parameter. The lattice parameter for Ce90Gd5Er5, Ce85Gd5Er10, Ce85Gd10Er5, and Ce80Gd10Er10 are 5,406(9) Å; 5,434(1) Å; 5,434(1) Å; and 5,430(2) Å, respectively. Moreover, Ce90Gd5rEr5 morphology have been investigated using Scanning Electron Microscope (SEM). The result is Ce90G5Er5 still has pore probably due to the low temperature of sintering. The result of EDS characterization show that there is no impurity in Ce90Gd5Er5 material.

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) .

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.

Low Cr stainless steels could represent a cost-effective alternative to high Cr alloys for use as metallic interconnects at the lower end range of Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Main drawback is represented by their insufficient long-term corrosion resistance, requiring suitable corrosion protection methods to avoid both electrical contact and chrome evaporation related issues. In this work, the stability of a perovskite conversion coating applied onto a commercial 13Cr ferritic stainless steel has been evaluated during prolonged exposure at 700°C in static air atmosphere. The passivation layer was produced with a molten carbonate bath treatment that resulted in the formation of a thin-film passive layer prevalently consisting of a LaFeO3 perovskite oxide. Surface morphology and structure of the perovskite-based passive layer remained almost unchanged during 400 hour oxidation exposure showing thus a promising corrosion protection ability in IT-SOFC oxidizing environments.

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

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

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.