bluuebunny/crossref_metadata_2025 huggingface dataset:
Low Temperature Densification and Electrical Property of a Carbonate-Added Proton Conducting Ceramic
Authors: Li Xue , Xu Nansheng , Zhange Lingling , Huang Kevin
Published: April 2011

Proton conducting yttrium-doped barium zirconate (BZY) is one of the most studied materials for SOFC applications due to its high bulk proton conductivity, good chemical stability under CO2 and mechanical robustness. However, it requires a very high temperature (1700-2100ºC) and long sintering time (>24 h), even with nano-sized powers, to sinter into a dense ceramic. In this paper, we demonstrate a unique approach to make a dense BZY at much lower temperatures. The method involves prefabrication of a porous BZY matrix by solid-state reaction from micro-sized powders of BaCO3, ZrO2 and Y2O3 and densification by impregnating the porous BZY matrix with a carbonate eutectic mixture (e. g., 62 mol% Li2CO3 and 38 mol% K2CO3) at 600oC. Thus fabricated solid-oxide and carbonate composite is also expected to be a mixed proton and carbonate-ion conductor (MPCC). The initial results show that there are no chemical reactions between BZY and carbonate. The microstructure of the composite MPCC is dense with the carbonate phase filling out the pores in the BZY matrix. The measured effective ionic conductivity reaches 0.33 and 0.38 S/cm at 600ºC in wet air and in wet H2, respectively. The performance of a MPCC-based fuel cell is, however, severely limited by the electrode (Ag) resistance, which is expected to be improved with better electrode materials in future study.
Apatite Coatings Deposited by DC Magnetron Sputtering
Authors: Briois Pascal , Mazataud C , Fourcade Sébastien , Mauvy Fabrice , Grenier Jean-Claude , Billard Alain
Published: April 2011

In this study, La-Si metallic coatings were synthesized by Direct Current Magnetron Sputtering of lanthanum and silicon targets in pure argon atmosphere. After the deposition stage, the ceramic apatite-structure coatings were obtained by thermal oxidation in air. The structural and chemical features of these films have been assessed by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The electrical properties were determined by complex impedance spectroscopy in planar configuration. The films with a La/Si atomic ratio close to the apatite composition La9.33(SiO4)6O2 deposited on different substrates were initially amorphous. After thermal oxidation at 1173 K in air, the coating crystallised under the expected apatite structure. SEM observation revealed that the film compactness and thickness increased after thermal oxidation. The electrical measurements carried out under air as a function of temperature (1200 to 800 K) show only one contribution for the apatite layer on the Nyquist diagram. The electrical properties were controlled by the Arrhenius law with an activation energy (Ea = 0.7 eV) slightly lower than that of the literature data. The ionic conductivity is around 10-2 S.cm-1 at 1175K.
Multilayered SOFC Anode Structure with Electroless Ni-YSZ for Enhancement of Cell Performance
Authors: Mukhopadhyay Madhumita , Mukhopadhyay Jayanta , Das Sharma Abhijit , Basu Rajendra N.
Published: April 2011

High performance SOFC is fabricated using multilayer anode support with graded porosity. The graded anode fulfills the requirement of proper fuel oxidation at anode, effective electrochemical reaction at anode/electrolyte interface, thermal compatibility among cell components and long term stability. In such multilayer anode, conventional cermet (40 vol % Ni) helps in proper fuel diffusion and catalytic oxidation. Novel electroless anode (28-32 vol % Ni) is employed adjacent to YSZ electrolyte for effective electrochemical activity and thermal compatibility. Because of the presence of unique core (YSZ)-shell (Ni) microstructure, electroless anode is found to extend the anodic electrochemical reaction from electrolyte/anode interface to the bulk. Significant performance enhancement with current density of ~3.3 Acm-2 is observed for single cells fabricated with multilayer anode at 800degC, 0.7V. The multilayer anode is found to reduce the cell degradation from ~11 % (conventional anode) to ~1.6 % /1000 h during long term performance (~2000 h).
Influence of Tertiary Phases Incorporated into Ni-Based Cermets by Solution Precursor Plasma Spraying (SPPS) on Anode Stability
Authors: Lay Elisa , Metcalfe Craig , Kesler Olivera
Published: April 2011

The Solution Precursor Plasma Spray (SPPS) process was successfully used to deposit cermet coatings which exhibit fine microstructures with high surface area. MgO addition in Ni-YSZ and Ni-SDC cermets results in the solid solution (Ni,Mg)O formation, and nickel particles are finer than without magnesia. In contrast, small tin additions do not influence the microstructure. Influences of MgO and Sn on the chemical stability of cermets in anodic operating conditions are also discussed. It was found that MgO addition helps to prevent carbon deposition, whereas tin does not influence its rate.
3D Imaging of Nickel Oxidation States using Full Field X-ray Absorption Near Edge Structure Nanotomography
Authors: Nelson George , Harris William , Izzo John , Grew Kyle N. , Chiu Wilson K. , Chu Yong , Yi Jaemock , Andrews Joy , Liu Yijin , Pianetta Piero
Published: April 2011

Reduction-oxidation (redox) cycling of the nickel electrocatalyst phase in the solid oxide fuel cell (SOFC) anode can lead to performance degradation and cell failure. A greater understanding of nickel redox mechanisms at the microstructural level is vital to future SOFC development. Transmission x-ray microscopy (TXM) provides several key techniques for exploring oxidation states within SOFC electrode microstructure. Specifically, x-ray nanotomography and x-ray absorption near edge structure (XANES) spectroscopy have been applied to study samples of varying nickel (Ni) and nickel oxide (NiO) compositions. The imaged samples are treated as mock SOFC anodes containing distinct regions of the materials in question. XANES spectra presented for the individual materials provide a basis for the further processing and analysis of mixed samples. Images of composite samples obtained are segmented, and the distinct nickel and nickel oxide phases are uniquely identified using full field XANES spectroscopy. Applications to SOFC analysis are discussed.
Characterization and Carbon Tolerance of New Au - Mo - Ni/GDC Cermet Powders for use as Anode Materials in Methane Fuelled SOFCs
Authors: Niakolas Dimitrios K. , Athanasiou Michael , Neophytides Stylianos G. , Bebelis Symeon
Published: April 2011

The present study concerns the characterization and carbon tolerance behavior of novel Au - Mo - Ni/GDC cermet powders, which have the potential to be used as anodes in CH4 fuelled SOFCs. Specifically, commercially available NiO/GDC cermet anode was modified with Au and/or Mo, via the deposition-precipitation and/or deposition-coprecipitation methods, resulting in new binary and ternary composites with different structure and carbon tolerance, regarding the reaction of catalytic methane dissociation. The most interesting observations concern the reduced form of the prepared powders, where a bimetallic Ni - Au alloy was formed in the Au - modified sample and a bimetallic Ni - Mo alloy in the Mo - modified cermet. In the ternary composite materials there seems to be formed a trimetallic Ni - Au - Mo alloy which, due to a synergistic action, exhibits an enhanced carbon tolerance compared to that of Au - Ni/GDC and Mo - Ni/GDC powders.
Study of Ga Doped LSCM as an Anode for SOFC
Authors: Ghosh Abhijit , Azad Abul , Irvine John T.
Published: April 2011

A phase pure La0.75Sr0.25Cr0.4Ga0.1Mn0.5O3 (LSCGM) was synthesized by the solid state reaction method. LSCGM crystallized with rhombohedral symmetry in the space group R-3c. AC impedance measurements using symmetrical cell arrangement showed that the area specific resistance (ASR) was 0.69 W cm2 at 850°C in wet 5%H2. A three-electrode arrangement with thick electrolyte was used to study the anodic characterization in isolation at various temperatures in wet hydrogen (97 vol% H2 and 3vol% H2O) and methane (97 vol% CH4 and 3 vol% H2O) over a period of 100 hr. For H2, the open circuit voltage (OCV) was found to decrease with temperature, whereas a different trend was observed in CH4. The value of anodic polarization (RP) at ~868OC was found to be 0.65 Ωcm2 and 2.0 Ωcm2 in wet hydrogen and methane respectively. At 900OC, the corresponding values were 0.45 and 1.65 Ωcm2 respectively. The microstructural study revealed a good LSCMG - YSZ interface even after 100 hr of operation.
Preliminary Studies of the Ba-Doped La/Sr Chromo-Manganite Series as New SOFC Anode Materials
Authors: Lay Elisa , Gauthier Gilles , Dessemond Laurent
Published: April 2011

Three perovskites with the general formula La0.75BaxSr0.25-xCr0.5Mn0.5O3 (x = 0, 0.10 and 0.25) have been synthesized. For all materials, the symmetry is rhombohedral (space group R-3c). A p-type semiconducting behaviour was evidenced. In comparison to La0.75Sr0.25Cr0.5Mn0.5O3, the total conductivity of barium containing compounds is higher in argon and it is nearly independent of barium content in humidified hydrogen above 700C. Steady state and impedance spectroscopy measurements were performed on dense pin-shaped electrodes (x = 0 and 0.25) in H2-3% H2O between 800 and 900C. For x = 0.25, anodic performances are interesting for an applicability as a SOFC anode. Origins of individual contributions to the electrode reaction mechanism are discussed.
Correlation between Microstructure and Electrochemical Characteristics of Ni-YSZ Anode Subjected to Redox Cycles
Authors: Muroyama Hiroki , Sumi Hirofumi , Kishida Ryo , Kim Jin-Young , Matsui Toshiaki , Eguchi Koichi
Published: April 2011

Reoxidation of nickel catalyst in anode generally leads to deterioration of cell performance in solid oxide fuel cells (SOFCs). In this study, we aimed to clarify the correlation between microstructural and electrochemical characteristics during reduction-oxidation (redox) cycles for Ni-YSZ anode. Cell performance was deteriorated after the redox cycle because of the increase in polarization resistance of the anode. From the focused ion beam-scanning electron microscopy (FIB-SEM) analysis of Ni-YSZ anode, the continuous increase in surface area of nickel phase was observed after the each redox cycles, which indicated that the nickel particles became finer and more complicated shape. The length of triple phase boundary (TPB) in the Ni-YSZ anodes decreased continuously with the redox cycles. These results indicated that the increase in polarization resistance was caused by the decrease in the TPB length during the redox cycles for the Ni-YSZ anode.
Evaluation of Sn-Modified Ni/YSZ SOFC Anodes for the Direct Utilization of Methane
Authors: Singh Anand , Hill Josephine M.
Published: April 2011

Carbon formation on conventional Ni/YSZ anodes is a major problem when SOFC are operated with hydrocarbons. Carbon formation reduces the operational stability and life of SOFC. Electrolyte-supported SOFC with Sn-modified Ni/YSZ anodes (target Sn loading of 1 wt% with respect to Ni) have been fabricated and evaluated for the direct utilization of CH4 at 1073 K. Addition of Sn enhances the shrinkage of Ni/YSZ anodes during the sintering stage of fabrication process, leading to more densely packed microstructures and better electrochemical performances. Sn was not detected in the anode after the sintering stage, however. The high sintering temperature (1723 K) and the low thickness (40 - 100 µm) of the anodes used in this study appear to have enabled the removal of Sn species from the anode, which resulted in Sn not being present in the anodes at levels sufficient to improve the carbon tolerance of the Ni/YSZ anodes.
In-Situ Measurement of SOFC Anode Surface Processes
Authors: Brightman Edward J. , Maher Robert , Ivey Douglas G. , Offer Greg , Brandon N.P.
Published: April 2011

One of the remaining challenges facing state-of-the-art nickel-based SOFC anodes is their susceptibility to degradation induced by redox cycling and chemical poisoning, most notably by carbon and sulphur. An enhanced scientific understanding of chemical interactions on the surface of SOFC electrodes is therefore critical to the development of robust nickel-based anodes. This paper draws together work on advanced electrochemical studies, thin-foil samples, and Raman spectroscopy studies of anode materials, together with results from a new customized SOFC test rig using combined in situ Raman and electrochemical measurements. The in situ test rig opens up a whole range of new research possibilities and these will be discussed.
Impedance Studies on Solid Oxide Fuel Cells with Yttrium-Substituted SrTiO3 Ceramic Anodes
Authors: Ma Qianli , Tietz Frank , Leonide André , Ivers-Tiffée Ellen
Published: April 2011

Yttrium-substituted SrTiO3 is considered as a very promising anode material for solid oxide fuel cells. Impedance spectra of cells based on Sr0.895Y0.07TiO3 (SYT) ceramic anode substrates under different operating conditions have been studied in this work. The comparison of ohmic and polarization resistance between the cells based on SYT and conventional Ni-YSZ cermet anodes is described in detail. The different electrode polarization resistances for the SYT-based cells are separated from experimental impedance data by a complex nonlinear least-squares approximation combined with distribution function of relaxation times analysis. The anode kinetics plays a dominant role in the overall polarization losses which is related to four phases involved in the anode reaction and the extended surface diffusion and spill-over processes. According to the obtained data, the potential of performance increase for the SYT-based cells is evaluated.
Understanding Performance Losses at Ni-Based Anodes Due to Sulphur Exposure
Authors: Birss Viola I. , Deleebeeck Lisa , Paulson Scott , Smith Tyler
Published: April 2011

While Ni-YSZ (yttria-stabilized zirconia) cermets have excellent properties for use as solid oxide fuel cell (SOFC) anodes, they are susceptible to performance loss in the presence of sulfur (S) in the fuel stream. In this work, we have electrochemically determined the extent of poisoning and recovery of Ni-YSZ electrolyte-supported half-cells in 5 ppm H2S (in H2) between 600 and 850°C, comparing these results with S adsorption/removal data obtained using TGA. It is shown by both approaches that the coverage of Ni by adsorbed S species (Sads) increases as the temperature decreases, as expected. However, the extent of Sads coverage, determined from performance loss, is significantly less than predicted from the mass gain during S adsorption. Furthermore, anodes exposed to H2S at 600°C show an unusual irreversible S poisoning effect when polarized at 250 mV, not seen in TGA experiments.
Mg and Fe Modified Ni/GDC Cermets as Sulfur Tolerant Anodes of Solid Oxide Fuel Cells
Authors: Zhang Lan , Jiang San Ping
Published: April 2011

Sulfur-tolerance of the Mg and Fe modified/doped Ni/ Gd2O3-CeO2 9Ni/GDC) anodes is studied at 800oC in H2S-H2. The polarization and impedance behavior of Ni/GDC cermet anodes indicate that Mg and Fe doping in the Ni phase of the cermet significantly improves the sulfur tolerance and performance stability of Ni/GDC anodes. SEM examination indicates that the microstructure change of the Ni phase of the Mg and particular Fe modified Ni/GDC is significantly smaller in comparison to that of pure Ni/GDC cermet anodes. The enhancement in the sulfur tolerance of Ni95Mg5/GDC and Ni95Fe5/GDC anodes is most likely due to the suppressed microstructure change of the Ni phases due to the presence of Mg and Fe dopants in the Ni/GDC cermets.
Effect of Redox Cycling on Mechanical Properties of Ni-YSZ Cermets for SOFC Anodes
Authors: Sukino Shinji , Watanabe Satoshi , Sato Kazuhisa , Iguchi Fumitada , Yugami Hiroo , Kawada Tatsuya , Mizusaki Junichiro , Hashida Toshiyuki
Published: April 2011

Effects of redox cycling on the mechanical properties of Ni-YSZ cermets were investigated by using an in-situ small punch (SP) testing method. The Ni contents in the Ni-YSZ cermets tested in this study were 0, 15, 30 and 50 vol. %. Electrical conductivity measurements suggested that the Ni particles formed a network-like structure at the Ni contents greater than 20 vol. %. Experimental results obtained from the in-situ SP tests at 800åC demonstrated that ductile-like deformation was induced under reducing environment (at an oxygen partial pressure of 10-18atm) for the Ni contents of 30 and 50vol. %. The ductile-like behavior was attributable to the formation of Ni network. The first complete re-oxidation of Ni-YSZ cermets then caused critical damage and resulted in drastic strength reduction, probably due to Ni agglomeration. This study revealed the first evidence for the presence of potential plastic deformation capacity in Ni-YSZ cermets under reducing environments.
A Study of the Rheological Properties of NiO/ScSZ Screen-Printing Inks and Their Application to SOFC Anodes
Authors: Somalu Mahendra Rao , Brandon N.P. , Yufit Vladimir
Published: April 2011

A series of rheological tests including steady state, oscillatory amplitude stress sweep and creep-recovery tests were performed using inks having 0-5 wt% binder. The steady state test revealed that the thixotropic behaviour of inks increased with increasing binder content. Oscillation test exhibited improved network strength in the inks having higher binder content. Moreover, creep-recovery test demonstrated the percentage of ink recovery of 64 - 68% for inks having 3-5 wt% binder, indicating that inks with 3 wt% binder were optimum in this study. Also, improved electrical and electrochemical properties were shown by anodes fabricated using inks with 3 wt% binder than that of 1 and 2 wt% binder due to increased network strength as confirmed by the oscillation and creep-recovery tests. In conclusion, from the point of both ink rheology and performance, ink with 3 wt% binder was determined to be optimum in this study.
Gas Transport and Internal Reforming Chemistry in SOFC Anode Supports and Structures
Authors: Richards Amy E. , Sullivan Neal , Kee Robert J. , McNeeley Michael , Babiniec Sean
Published: April 2011

Efforts to address deposit formation in hydrocarbon-fueled solid oxide fuel cells have motivated development of advanced anode materials and architectures. The Separated Anode Experiment has been developed to decouple anode internal-reforming processes from electrochemical processes, providing a unique method of evaluating novel anodes and supports. Here, we present experimental and numerical results and compare performance of a Ni-YSZ anode support to a porous metallic support. Additionally, a computational model is developed and utilized as a tool in understanding gas transport and internal reforming processes, and for optimization of anode morphological design.
Performance of Sm0.95Ce0.05Fe1-xNixO3-δPerovskite as Anode Materials under Methane Fuel for Low Temperature Solid Oxide Fuel Cells (LT -SOFC)
Authors: Bukhari Syed M. , Giorgi Javier B.
Published: April 2011

Sm0.95Ce0.05Fe1-xNixO3-δ (x=0-0.05) perovskite materials were investigated for their candidacy as anodes for Low temperature Solid Oxide Fuel Cell (LT-SOFC). Electrolyte supported button cells were made and tested with hydrogen and dry methane fuels. A three electrode geometry was used and electrochemical impedance measurements were carried out revealing that Ni doping does improve the performance of the resulting anodes as indicated by a decrease in charge transfer resistance values. The value of charge transfer resistance is lowest for x=0.03 (1.65 Ωcm2 at 600oC) and only light coking was observed.
Thermal Imaging of Solid Oxide Fuel Cell Anode Degradation with Dry and Wet Ethanol Fuel Flows
Authors: Pomfret Michael B. , Steinhurst Daniel A. , Owrutsky Jeffrey C.
Published: April 2011

Near-infrared thermal imaging has been used to study the effects of humidifying ethanol fuel flows on the surface temperature of SOFC anodes. Temperatures were monitored for SOFCs operating with dry and wet ethanol fuel flows at 800 ºC. The results indicate a dramatic decrease in the amount of carbon that is formed when ethanol flows are humidified. Humidifying the fuel stream reduces both the cooling observed under ethanol fuel flow conditions and the heating observed under post-fuel flow electrochemical oxidation conditions. The spatial variation of the temperature is also reduced. Thermal imaging is capable of detecting fatigue in functioning cells because temperature variations observed in situ provide early signs of SOFC failure, therefore thermal imaging is a viable system diagnostic.
Catalytic Activities and Electrochemical Properties of Y and Fe Co-Doped SrTiO3-Based Composite Anodes for Solid Oxide Fuel Cells (SOFCs)
Authors: Yoon Song Seol , Kim Young Han , Hwang Haejin , Ji Mi Jung , Choi Byung Hyun
Published: April 2011

In this study, Fine Y and Fe co-doped SrTiO3 powers were synthesized via solid state reaction and pechini method. Perovskite-based anodes were proposed to enhance the anode performance. Doping with Y for Sr allows the Y0.08Sr0.92FexTi1-xO3-δ perovskite to be stable at 800°C in a reducing atmosphere. Sintering the oxde anode at 1200°C is found to be optimum to obtain not only good interfacial adhesion, but also a fine grain structure. The Y0.08Sr0.92Fe0.25Ti0.75O3-δ anode, which is sintered at 1200°C, showed the lowest polarization resistance. Electrochemical properties and catalytic activity for CH4 oxidation of the composite anode were investigated by gas chromatography and ac-impedance spectroscopy.
Influence of Anode Thickness on Cell Performance in Internal Reforming Operation of SOFCs
Authors: Lee Yi-Hsuan , Sumi Hirofumi , Muroyama Hiroki , Matsui Toshiaki , Eguchi Koichi
Published: April 2011

The internal reforming operation is one of the advantages for solid oxide fuel cells. In this study, we aimed to elucidate the influence of anode thickness in the electrolyte-supported cells on the performance stability and deterioration behavior with a supply of humidified methane fuel. The increase in the anode thickness enhanced the open circuit voltage, indicating the enhancement of methane conversion in the thick electrode layer. The cells with the 20- and 30-μm thick anodes attained stable performance at 0.2 A/cm2. In contrast, for the anode in 10-μm thickness, the terminal voltage gradually decreased and suddenly oscillated. This behavior corresponded to the gradual increase in ohmic resistance and the drastic change in the anodic overpotential. From SEM observation and Raman analysis, the deposited amorphous carbon obstructed the pores in this anode. Thus, it was concluded the performance deterioration was induced by the deposition of low-conductive carbon species and the concomitant inhibition of gas diffusion.
Synthesis and Characteristics of Nano-Ceria Supported Bimetallic Catalysts For S-Tolerant SOFCs
Authors: Bozeman Joe , Marruffo Alexis , Barney Ian , Jackson Allen , Mukhopadhyay Sharmila , Huang Hong
Published: April 2011

Cu-Pt bimetal and Pt-skin structured Cu-Pt catalysts supported on nanocrystalline CeO2 are synthesized via the low-cost sol-gel approach followed by impregnation processing. Crystal structure, particle size, bulk and surface compositions of the catalyst are systematically characterized. The average particle size of the catalytic composites is 63 nm. The ceria supporter has a fluorite structure, while Cu and Pt contents, not detected by XRD, may exist in amorphous phases. The Pt-skin structured nanocomposite is analyzed based on the surface elemental and bulk compositional analyses. No Cu is detected on the surface based on XPS analyses, however, EDX bulk compositional analyses confirm the co-existence of Cu and Pt. These two kinds of catalytic composites may have potentials to serve as sulfur-resistant anode in solid oxide fuel cells.
Reverse Cell Bias for the Prevention of Ni Oxidation during Air Exposure
Authors: Young Jason L. , Vedharathinam Vedasri , Birss Viola I.
Published: April 2011

The primary aim of this work was to prevent Ni oxidation by reverse biasing of the cell during air exposure at SOFC operating temperatures. As the area specific polarization resistance (Rp, inversely proportional to the H2 oxidation activity) of electrolyte-supported Ni-YSZ anodes normally decreases after a single redox cycle, the approach used here was to determine if reverse polarization during air exposure can prevent this performance enhancement. The use of a constant cathodic potential did not appear to prevent Ni oxidation, although it is likely that an insufficiently negative potential was applied. However, anode performance enhancement was prevented by the application of a constant applied current density during air exposure. However, these results are not fully conclusive, as the extent of Ni oxidation could not be determined analytically in situ. Notably, observations, such as an increase in the observed cathodic overpotential during negative biasing and the decrease in Rp after air exposure (with applied cathodic currents) still need to be understood.
Sulfur Poisoning of SOFCs:Dependence on Operational Parameters
Authors: Yoshizumi Tomoo , Uryu Chie , Oshima Toshihiro , Shiratori Yusuke , Ito Kohei , Sasaki Kazunari
Published: April 2011

Sulfur species are major impurities in practical hydrocarbon-based-fuels, chemisorption of which on Ni anode catalysts results in possible performance degradation of SOFC systems. In the present study, degradation of SOFC performance caused by sulfur in pre-reformed CH4 has been evaluated under various operational conditions such as current density, fuel utilization, pre-reforming ratio, steam-to-carbon ratio, and operational temperature.
Elementary Kinetic Numerical Simulation of Electrochemical CO Oxidation on Ni/YSZ Pattern Anodes
Authors: Yurkiv Vitaliy , Utz Annika , Weber André , Ivers-Tiffée Ellen , Volpp Hans-Robert , Bessler Wolfgang G.
Published: April 2011

Results of a numerical simulation analysis of recent experimental data obtained in a comprehensive study of electrochemical CO oxidation on well-defined Ni/YSZ patterned model anodes [Utz et al. J. Power Sources, doi:10.1016/j.powsour. 20.10.056 (2010)] are presented. A computational model representing the coupled behavior of heterogeneous chemistry and electrochemistry in terms of elementary reactions was developed, which allows for a quantitative description of the complete experimental data set, which covers a wide range of CO/CO2 gas compositions (4.0•102 Pa ≤ pCO ≤ 5.1•104 Pa and 9.5•102 Pa ≤ pCO2 ≤ 9.2•104 Pa) and operating temperatures (973 K ≤ T ≤ 1073 K). In the framework of the presented model a direct mechanistic interpretation of the experimentally observed electrochemical characteristics is obtained.
Optimization of Solid Oxide Fuel Cell Ni-CGO Anode Porosity
Authors: Tamm Kadi , Kivi Indrek , Anderson Erik , Möller Priit , Nurk Gunnar , Lust Enn
Published: April 2011

Microstructure of supporting Ni-CGO anodes for intermediate temperature solid oxide fuel cells (SOFC) has been systematically optimized by changing the amount of a pore former in the anode raw material. In the second series of experiment the influence of pore former particle size distribution on the anode microstructure has been studied. All single cells prepared have been analyzed electrochemically as well as by different structural analysis methods. For the identification of characteristic frequencies and corresponding parameters of cathodic and anodic processes the single cell impedance spectra were recorded and compared analytically as a function of applied potential, partial pressure of hydrogen and partial pressure of oxygen. The data obtained have been analyzed in detail using non-linear regression analysis fitting and differential derivate of impedance modulus versus frequency dependence methods.
Fabrication of Direct Oxidation Solid Oxide Fuel Cell Anodes Using a Novel Atmospheric Plasma Spraying Technique
Authors: Cuglietta Mark , Kesler Olivera
Published: April 2011

Composite coatings of CuO and samaria-doped ceria (SDC) have been fabricated by atmospheric plasma spraying using a combination of suspension and dry-powder plasma spray techniques. The coatings prepared using this novel hybrid technique demonstrate a finer microstructure than similar coatings prepared using conventional dry-powder methods. The coatings consist of fully-melted and partially-melted phases of both CuO and SDC. During deposition, a large portion of the CuO was reduced to Cu2O. The influence of processing conditions on deposition efficiency and porosity were investigated, with a plasma power of 77 kW leading to a maximum in coating porosity. Improvement of the dispersion of the CuO suspension by adjusting the pH led to a finer network of CuO particles in the fabricated coating.
Analysis of Microscopic Anode Structure Effects on an Anode-Supported SOFC Including Knudsen Diffusion
Authors: Andersson Martin , Lu Xinrong , Yuan Jinliang , Sundén Bengt
Published: April 2011

In this study a two dimensional CFD (COMSOL Multiphysics) is employed to study the effect of anode microscopic structures on the transport phenomena and reactions for an anode-supported solid oxide fuel cell (SOFC). FCs can be considered as energy devices, involving multiple processes, such as (electro-) chemical reactions, heat exchange, gas- and ionic transport. All these complex processes are strongly integrated, needing modeling as an important tool to understand the couplings between mass-, heat-, momentum transport and chemical reactions. For the porous material, the Knudsen diffusion is taken into account in this study. The chemical- and electrochemical reaction rates depend on temperature, material structure, catalytic activity, degradation and partial pressure among others. It is found that the anode thickness and also the anode pore size need to be optimized to achieve high cell efficiency, when the Knudsen diffusion effects are included.
Performances of Metal Particle-Dispersed Ceria Hydrogen Electrodes in Reversible SOFCs
Authors: Uchida Hiroyuki , Nishida Ryuta , Tatsuzawa Msahito , Nishino Hanako , Kakinuma Katsuyoshi , Watanabe Masahiro
Published: April 2011

We have synthesized nanometer-sized Ni100−XCoX (X = 0 to 50) alloy catalysts supported on hollow particles of samaria-doped ceria [SDC, (CeO2)0.8(SmO1.5)0.2] by spraying a mixed solution of the corresponding metal nitrates and acetates in an atmospheric pressure plasma. The performances of Ni80Co20/SDC catalyst in both an SOFC and a steam electrolysis (p[H2O] = 0.6 atm) were found to be the highest among alloys and pure Ni (X = 0) supported on the SDC at 800 to 900oC.
Experimental Study of the Ohmic Resistance between the Interconnect and the Ni-CGO Cermet
Authors: Magnière Corentin , Di Iorio Stéphane , Morel Bertrand
Published: April 2011

In this experimental study, ohmic resistance between Ni-CGO/3YSZ/Ni-CGO symmetrical cells and cylindrical Crofer 22APU pins have been studied at 820°C by Electrochemical Impedance Spectroscopy (EIS). Measurements have been carried out at open-circuit voltage in 2% H2/Ar, with the addition of a Ni woven mesh of 100 meshes/cm2 and by varying the mechanical load on the pins. A rapid increase of this ohmic resistance, including cell ohmic resistance and contact resistance, from 0.50 to 0.54 ohm.cm2 during 100 hours of ageing highlights the presence of an evolution which has to be explained. It has been shown that this degradation is rather independent of the possible Crofer 22APU pins corrosion and of the applied mechanical load, but depends on the oxygen partial pressure during ageing. A modification of the microstructure of Ni-CGO electrode or a change of the 3YSZ electrolyte conductivity could explain the present degradation of the ohmic resistance.
High Performance LSM-ESB Cathode on ESB Electrolyte for Low to Intermediate Temperature Solid Oxide Fuel Cells
Authors: Lee Kang Taek , Jung Doh Won , Yoon Hee Sung , Camaratta Matthew , Sexson Nicolas , Wachsman Eric D.
Published: April 2011

Using a highly conductive (Er2O3)0.20(Bi2O3)0.80 (ESB) phase, the performance of conventional La0.80Sr0.20MnO3-δ (LSM) cathodes was dramatically improved. The ESB phase was utilized not only as the ion-conducting phase in the LSM-ESB composite cathode, but also as an electrolyte coupled to LSM-ESB cathode. The electrode area specific resistance (ASR) measured from a symmetric cell consisting LSM-ESB electrodes on an ESB electrolyte was only 0.08 Ω-cm2 at 700oC, which is ~60 % lower than that of LSM-ESB on GDC electrolytes (0.19 Ω-cm2). This significantly small ASR exemplifies the synergic effect of the ESB phase both in the cathode bulk and at the electrolyte/electrode interface. The maximum power density (MPD) of the anode-supported SOFC with a LSM-ESB cathode on an ESB/GDC bilayered electrolyte reached 1013 mW/cm2 at 650 oC. The measured MPDs at IT are to date the highest reported ones for SOFCs using LSM-bismuth oxide composite cathodes.
Quantification of Microstructural Change at the Interface between (La,Sr)MnO3+δ Cathode and YSZ Electrolyte Upon Discharge Operation
Authors: Matsui Toshiaki , Mikami Yuichi , Muroyama Hiroki , Eguchi Koichi
Published: April 2011

The microstructural change at the interface between LSM cathode and YSZ electrolyte upon discharge was quantitatively evaluated by FIB-SEM technique to elucidate the activation process of cathode. The performance of the cell was enhanced soon after the current passage up to ca. 5 h at 1000°C due to the reduction in cathodic impedance. In response to this behavior, the LSM/YSZ interfacial structure changed depending on the time duration of discharge; the roughness of YSZ surface both in and out of contact with LSM increased. Furthermore, the closed pores were formed at the well-adhered LSM/YSZ interface. The TPB-length of the as-prepared sample was evaluated to be 1.26 μm/μm2, whereas after the discharge at 200 mA/cm2 for 300 min the total TPB-length was increased to 1.72 μm/μm2. In the latter case, however, the active TPB-length was 1.35 μm/μm2. Then the contribution rate of TPB-length to the performance enhancement of cathode was discussed.
Oxygen Non Stoichiometry in Nanocrystalline La0.5Sr0.55CoO3-x Thin Films
Authors: Wang Shuangyan , Cho Sungmee , Wang Haiyan , Jacobson Allan
Published: April 2011

Nano-crystalline La0.5Sr0.5CoO3-x (LSCO) thin films with a thickness of ~400 nm were deposited on a polycrystalline CGO substrate using pulsed laser deposition. AC impedance measurements were performed on symmetric cells between 400-600 oC in 0.01 ≤ pO2 ≤ 1 atm. The oxygen non-stoichiometry in LSCO thin films was obtained by utilizing a hypothetical cell where oxygen transport was limited by the surface exchange reaction. It was found that the oxygen non-stoichiometry was about one order of magnitude lower than that observed for bulk LSCO. At 600 oC, in air, the area specific resistance (ASR) of the LSCO thin film is about 0.06 ohm cm2.
Microstructure of Sol-Gel Derived Nanoscaled La0.6Sr0.4CoO3-δ Cathodes for Intermediate-Temperature SOFCs
Authors: Dieterle Levin , Bockstaller Pascal , Gerthsen Dagmar , Hayd Jan , Ivers-Tiffée Ellen , Guntow Uwe , Kübel Christian
Published: April 2011

Nanocrystalline La0.6Sr0.4CoO3-δ thin films with a nominal Sr-content of x=0.4 were deposited on Ce0.9Gd0.1O1.95 electrolyte substrates by low-temperature sol-gel processing. Microstructure, element distribution and electrochemical properties of the LSC thin films were studied after thermal treatment, which included calcination and extended annealing at 700 °C or 800 °C. Transmission electron microscopy combined with Debye electron diffraction and electron tomography was applied for the investigation of the grain-size distribution and porosity. The local chemical composition and element distribution was analyzed by energy-dispersive X-ray spectrometry on the nanoscale. Area specific polarization resistances as low as 0.023 Ωcm2 at 600 °C were measured applying electrochemical impedance spectroscopy. These low resistance values are facilitated by a substantial increase of the inner surface area of the porous thin-film cathode and amplified by an enhanced surface exchange, resulting from precipitation of Co3O4.
Performance of Metal-Supported Composite and Single-Phase Cathodes Based on LSCF and SSC
Authors: Harris Jeffrey , Kesler Olivera
Published: April 2011

Cathodes based on La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Sm0.5Sr0.5CoO3-δ (SSC) were fabricated by axial-injection plasma spraying using dry powder feedstock. Composite cathodes were produced with up to 40 wt. % Ce0.80Sm0.20O2-δ (SDC) in the feedstock. The processing parameters were varied to produce cathodes with different mechanical properties and microstructures. The best-performing LSCF-SDC cathode had a polarization resistance (Rp) of 0.101 Ωcm² at 750°C, and the best-performing SSC cathode had a polarization resistance of 0.0056 Ωcm².
Low Temperature Preparation of LaNi1-xFexO3 as New Cathode Material for SOFC - Advantage of Liquid Phase Mixing Method -
Authors: Niwa Eiki , Uematsu Chie , Miyashita Erina , Ohzeki Takeshi , Hashimoto Takuya
Published: April 2011

Preparation of LaNi1-xFexO3, which is one of the candidate materials of solid oxide fuel cell cathode with high electrical conductivity and resistance against Cr poisoning, was examined with liquid phase mixing method and solid state reaction method. Single phase LaNi1-xFexO3 has successfully been prepared irrespective of composition by low temperature sintering as 750 oC with liquid phase mixing method, whereas large amount of raw materials have remained with solid state reaction method by sintering at the same temperature. It can be ascribed to more homogenous cation distribution in raw powder material prior to sintering with liquid phase mixing method. It has also been revealed that LaNi1-xFexO3 with x lower than 0.3 is thermodynamically unstable in air above 1000 oC. LaNi0.6Fe0.4O3 showed superior property as cathode material with high electrical conductivity, thermodynamic stability and appropriate sintering property.
Towards a Fundamental Understanding of the Oxygen Reduction Mechanism
Authors: Wachsman Eric D. , Armstrong Eric N.
Published: April 2011

The kinetics of oxygen reduction are thermally activated, therefore as solid oxide fuel cell (SOFC) operating temperature is lowered, the rates of chemical reactions drop exponentially, and activation polarization at the cathode dominates SOFC performance. Hence, cathode polarization is widely regarded to be the key issue hindering reduced temperature SOFC development. In order to understand the various mechanistic contributions to cathode polarization and apply this knowledge to development of lower-polarization/lower-temperature SOFC cathodes, we have embarked on a multi-faceted, multi-disciplinary approach to deconvolute the various contributions to SOFC cathode polarization. This approach includes FIB/SEM to quantify the cathode microstructure and heterogeneous catalysis techniques and O-isotope exchange to evaluate the O2 reduction mechanism at the gas-solid reaction interface. These results are then combined (and contrasted) with more conventional electrochemical polarization techniques to try and elucidate each of the mechanisms as a function of composition and microstructure. The progress to date is summarized.
Performance Analysis and Development Strategies for Solid Oxide Fuel Cells
Authors: Ivers-Tiffée Ellen , Hayd Jan , Klotz Dino , Leonide André , Han Feng , Weber André
Published: April 2011

Solid oxide fuel cells are of great interest for a diverse range of applications. Within the past 10 years, an increase in power density by one order of magnitude, a lowering of the operating temperature by 200 K, and degradation rates lowered by a factor of 10 have been achieved on the cell and stack level. However, there is still room for further enhancement of the overall performance by suitably tailoring the cell components on a micro- and nanostructural level. The efficiency of the electrochemically active single cell is characterized by the linear ohmic losses within the electrolyte and by nonlinear polarization losses at the electrode-electrolyte interfaces. Both depend on material composition and operation conditions (temperature and time, fuel utilisation and gas composition). The efficiencies of ASCs will be discussed for various material combinations in the temperature range of technological interest (between 550 °C and 850 °C).
Surface Cation Segregation and its Effect on the Oxygen Reduction Reaction on Mixed Conducting Electrodes Investigated by ToF-SIMS and ICP-OES
Authors: Kubicek Markus , Limbeck Andreas , Frömling Till , Hutter Herbert , Fleig Jürgen
Published: April 2011

La0.6Sr0.4CoO3-δ (LSC) thin film electrodes on yttria stabilized zirconia (YSZ) were investigated by impedance spectroscopy, inductively coupled plasma optical emission spectrometry (ICP-OES) and time of flight secondary ion mass spectrometry (ToF-SIMS). Effects caused by different film deposition temperatures, thermal annealing and chemical etching were studied. Correlations between changes in electrode polarization resistance of oxygen reduction and surface composition were found. The surface cation composition was analyzed by ToF-SIMS and ICP-OES measurements of in situ etched LSC films allowing depth resolved compositional analysis with nominally sub nm resolution. High resolution scanning electron microscopy images illustrate changes in surface morphology.
Viable AC Two-Probe Impedance Spectroscopy Based on Spatially-Limited Contact Probe for SOFC Cathode
Authors: Lee Jong-Ho , Ji Ho-I , Kim Hae-Ryoung , Son Ji-Won , Hwang Jin-Ha
Published: April 2011

We reports on a unique characterization method for the electrode reactions of SOFCs via impedance spectroscopy combined with a spatially-constricted contact based on a robust electrolyte which is expected to give more reliable measurement set-up. Various electrode materials were chosen to validate our method and the corresponding impedance phenomena were analyzed to give an insight for optimization of the SOFC electrode. The ramifications of "limited-contact" impedance spectroscopy are discussed for an exhaustive understanding of electrode-related polarizations in SOFCs
Multi-Scale Assessment of Cr Contamination Levels in SOFC Cathode Environment
Authors: Schuler J. Andreas , Schuler Albert J. , Wuillemin Zacharie , Hessler-Wyser Aïcha , Ludwig Christian , Van Herle Jan
Published: April 2011

This study aims to quantify in a holistic approach chromium (Cr) contamination in solid oxide fuel cell (SOFC) stack testing using adapted tools: i) a hot gas sampling method to analyze volatile Cr species in the air flux at the cathode inlet location; ii) a rapid quantification method for Cr as condensed matter in cathode material of post-test samples. The hot air sampling method reveals itself as a reproducible and time-resolved quantification technique for Cr trace amounts in a gas flow; this technique is seen as a promising evaluation tool for Cr contamination issues in SOFC systems. The quantification method reveals severe Cr-poisoning in a cell. The combined findings indicate that Cr contamination generated by system components located upstream the cell must be suppressed by hindering the access of Cr pollutants to the cathode compartment.
Degradation of Solid Oxide Fuel Cell Performance by Cr-Poisoning
Authors: Kornely Michael , Neumann A. , Menzler N.H. , Weber André , Ivers-Tiffée Ellen
Published: April 2011

Performance and stability of solid oxide fuel cells (SOFC) have been continuously improved over the past 10 years. But whenever the individual cells are connected by a metallic interconnector (MIC), the stack performance undergoes a pronounced performance and degradation. Possible cause, inter alia, is the effect of Cr-evaporation from the MIC and, thus, Cr-poisoning of the cathode. In this work we examined the difference in cell performance and short-term stability during a galvanostatic test for 280h by using an inert or a flowfield made of Crofer22APU (chromia forming alloy). A detailed analysis of the impedance spectra by the distribution of relaxation times (DRT) and a subsequent Complex Nonlinear Least Squares (CLNS) fit enabled the separation of anodic and cathodic polarization resistance. During the test, the cathode polarization resistance increased about +144 % in the presence of Crofer22APU. In contrast, the polarization resistance decreased by 3.2% in the absence of Crofer22APU.
In-situ Investigation of the Chromium Induced Degradation of the Oxygen Exchange Kinetics of the IT-SOFC Cathode Material La0.6Sr0.4CoO3-δ
Authors: Bucher Edith , Yang Min , Sitte Werner
Published: April 2011

The long-term stability of the oxygen exchange properties of the IT-SOFC cathode material La0.6Sr0.4CoO3-d vs. chromium poisoning is investigated at 600°C by in-situ measurements of the chemical surface exchange coefficient kchem and the chemical diffusion coefficient Dchem of oxygen by applying the dc-conductivity relaxation method. The as-prepared sample shows high values of kchem=3x10-4 cm s-1 and Dchem=1x10-6 cm2s-1, which are only negligibly affected by the treatment in a dry atmosphere for 1000 h without and with the presence of a Cr-source, respectively. However, humidification of the carrier gas results in a strong decrease in both kinetic parameters. Post-test surface analysis of the degraded samples by X-ray photoelectron spectroscopy identifies Cr- and Si-poisoning as well as significant Sr-enrichment within 150 nm depth as the origin of the degradation in a humid atmosphere.
Is Chromium Poisoning of LSM Cathodes Avoidable
Authors: De Haart L.G.J. , Neumann A. , Menzler N.H. , Vinke I.C.
Published: April 2011

The degradation of anode supported cells with LSM cathodes exposed to various chromium sources was analyzed and post-test analyses were conducted to observe the cathode microstructure and to determine the chromium content. Three different degradation phases could be identified each corresponding to a different process. The analyses showed that severe chromium poisoning can be alleviated but not generally avoided.
Stability and Performance of LSCF-Infiltrated SOFC Cathodes: Effect of Nano-Particle Coarsening
Authors: Shah Megna , Hughes Gareth , Voorhees Peter W. , Barnett Scott A.
Published: April 2011

La0.6Sr0.4Fe0.8Co0.2O3-δ - Gd doped Ceria (LSCF-GDC) symmetric cathode cells were electrochemically tested at 600°C, after aging at various times and temperatures ranging from 650-850°C. The observed increases in polarization resistance were fit using a combined surface resistance and power-law coarsening kinetics model. Best fits were obtained for a t0.25 power law applicable to coarsening of isolated particles on a surface, with a surface diffusion activation energy range of 2.52 to 2.86eV. The model predicts that the resistance would increase from 0.2 to 0.28-0.35 Ωcm2 (by 39-74%) over 40,000 h at 600°C, due to coarsening. Additionally, evolution of the infiltrated LSCF particles was seen in SEM images of the same location before and after aging treatment. La1-xSrxMnO3-δ (LSM) coarsening was also observed for infiltrated LSM- yttria-stabilized-zirconia (YSZ) cathodes, with a corresponding increase in polarization resistance.
Microstructural Effects on the Oxygen Exchange Kinetics of La0.7Sr0.3MnO3 Thin Films
Authors: Yan Lu , Kavaipatti Balasubramaniam , Chang Kee-Chul , You Hoydoo , Salvador Paul
Published: April 2011

We investigated the relative contribution of the native surface and of the grain boundaries intersecting the surface to the chemical surface exchange of La0.7Sr0.3MnO3 (LSM) thin films. The chemical surface exchange coefficients, kchem, were determined using electrical conductivity relaxation (ECR). Two kchem values were obtained, indicating two surface exchange processes occurred in parallel. These two kchem values had different activation energies (Ea), which were interpreted as belonging to (1) the native surface response of individual grains/variants and (2) the variant / grain boundaries of the textured films. The first process has an activation energy of Ea,1 ≈ 1.57 eV, while Ea,2 ≈ 0.87 eV. The kchem,2 values are almost 3 orders of magnitude higher than the kchem,1 values at low temperatures. At higher temperatures, the data can be fit with one kchem that has an intermediate value of Ea, indicative of a transition region.