bluuebunny/crossref_metadata_2025 huggingface dataset:
A Critical Review of Models of the H2/H2O/Ni/SZ Electrode Kinetics
Authors: Mogensen Mogens , Ho̸gh Jens , Hansen Karin V. , Jacobsen Torben
Published: May 2007

Various models of the H2/H2O/Ni/SZ (SZ = stabilized zirconia) electrode kinetics have been presented in the literature in order to explain the reported experimental data. However, there has been a strong tendency of using a limited set of data to verify a given model, disregarding other data sets, which do not fit the model. We have inspected some models in the literature, and problems (e.g. no quantitative model has explained the large variation in reported values of apparent activation energy of the electrode kinetics) as well as strengths of the models are discussed. We point out experimental findings that a useful model must be able to explain such as difference in sensitivity to poisoning by H2S due to differences in the detailed composition of the SZ and large change in apparent activation energy by change in cermet preparation. Finally, we will point out some elements, which seem important for any realistic and useful mathematical model of the H2/H2O/Ni/SZ electrode.
SOFC Anode Performance Enhancement through Precipitation of Nanoscale Catalysts
Authors: Madsen Brian D. , Kobsiriphat Worawarit , Wang Yingmin , Marks Laurence D. , Barnett Scott
Published: May 2007

This paper describes a new method for improving the performance of oxide anodes by precipitating metal nano- clusters on oxide surfaces during the initial stages of SOFC operation, without additional processing steps. Transmission electron microscope and x-ray photoelectron spectroscopy observations showed that nano-clusters of Ni or Ru metal precipitated onto lanthanum chromite (La0.8Sr0.2Cr1-yXyO3-d, X = Ni, Ru) surfaces, respectively, after exposure to hydrogen at 800{degree sign}C. While Ni nano-clusters coarsened over ~300 h at 800{degree sign} C, Ru nano-cluster size was stable at <= 5 nm. SOFC tests were done with the doped lanthanum chromite anodes on LSGM electrolyte-supported cells. Ru and Ni nano-cluster nucleation improved cell performance and reduced anode polarization resistance during the first 50-100 h of cell operation.
Towards Understanding the Impedance Response of Ni/YSZ Anodes
Authors: Sonn Volker , Leonide André , Ivers-Tiffee Ellen
Published: May 2007

A comprehensive impedance study on the hydrogen oxidation in Ni/8YSZ Cermet anodes has been realized. A major problem in this respect concerns the physical interpretation of empirical 'equivalent circuits' used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: I) at first by the deconvolution of a distribution function of relaxation times (DRT) four different processes and their characteristic relaxation times have been identified. Two parts at frequencies <1 kHz represent a gas-conversion process or respectively a gas diffusion, whereas two parts at higher frequencies (2-30 kHz) are associated with the electro-oxidation of hydrogen at active sites. II) Subsequently the last mentioned processes were fitted to a transmission line model describing the electronic and ionic transport properties of the Ni/8YSZ Cermet. This approach resulted in the determination of 1) the total ionic conductivity of the Ni/8YSZ Cermet and 2) the spatial extension of the electrochemical active zone into the electrode
Microstructural Changes of Ni/YSZ Cermet under Repeated Redox Reaction in Environmental Scanning Electron Microscope (ESEM)
Authors: Nakagawa Yoichi , Yashiro Keiji , Sato Kazuhisa , Kawada Tatsuya , Mizusaki Junichiro
Published: May 2007

Ni/8YSZ pellet was exposed to the high temperature (1073 K) redox cycle repeatedly and the micro-structural changes were observed with environmental scanning electron microscope (ESEM). It was revealed that the conditions for observing with ESEM, low gas pressure and electron beam, do not affect the mechanism of micro-structural changes, therefore it can be said that the observation of micro-structural changes under the pseudo SOFC operating condition with ESEM is reliable. However the remarkable agglomeration of Ni particles was observed only on the surface, not inside the sample.
Reaction Compounds in the Vicinity of Ni-SDC/LSGM Interface and Performance of SOFC Single Cells with Different Anode Thickness
Authors: Kawahara Koichi , Suda Seiichi , Suzuki Msaya , Kawano Mitsunobu , Yoshida Hiroyuki , Inagaki Toru
Published: May 2007

Ni-SDC/LSGM/LSC single cells with different anode thickness were fabricated. The cell performance depended strongly on the anode thickness; the performance monotonically increased with increasing the anode thickness up to 100μm. SEM observations revealed that the fine particles were distributed in the vicinity of anode/electrolyte interface with 15-20μm thickness. Although fine particles distributed in the vicinity of the interface, the cells with thicker anode did not show significant performance drop. However, cells with thinner anode than 15μm showed critical low performance. The fine particles, therefore, would have a great effect on the cell performance. The fine particles were revealed to be oxides consisting mainly of Mg by TEM/EDX analysis. Since the fine particles were oxides even after the cell performance test, it was suggested that the much lower electronic conductivity of fine particles determined the lower performance of cells with thinner anode.
Anode Polarization in Liquid Tin Anode Solid Oxide Fuel Cell
Authors: Tao Thomas , Slaney Mike , Bateman Linda , Bentley Jeff
Published: May 2007

The Liquid Tin Anode Solid Oxide Fuel Cell (LTA-SOFC) is a modified version of SOFC that allows the direct conversion of carbonaceous fuels. The LTA-SOFC uses conventional SOFC electrolytes and cathodes but its anode is liquid tin allowing direct oxidization of fuel without reforming or other fuel processing. A porous ceramic separator was introduced in CellTech Power's Gen 2 and Gen 3 LTA-SOFC designs. The separator holds the liquid tin anode in place while allowing free exchange of fuel molecules and products. This paper describes anode polarization of LTA-SOFC and a capillary model for porous tin anode separator
Preparation of Standard Gas and Precise Analysis of Reformate Gas
Authors: Tanaka Yohei , Kato Ken , Negishi Akira , Kato Tohru , Nozaki Ken
Published: May 2007

Reformate gas was analyzed by micro-gas chromatograph (mGC), Fourier Transform Infrared spectroscopy (FTIR), and quadrupole mass spectrometry (QMS). By using gravimetrically calibrated mass flow controllers, preparation of stable and quantifiable steam was achieved by catalytic combustion of hydrogen, which can produce standard reformates with high precision by addition of CO2, CO, or CH4. It was demonstrated that gas analysis of reformates can be done with high precision ({plus minus}0.35 mol.%) by mGC and FTIR.
Mechanical Properties of Anode-Supported Micro-Tubular Solid Oxide Fuel Cells
Authors: Mohammadi Alidad , Pusz Jakub , Smirnova Alevtina L. , Sammes Nigel M.
Published: May 2007

Mechanical properties of the pellets imitating an anode- supported micro-tubular Solid Oxide Fuel Cell (SOFC) using a Nickel Oxide-Gadolinia Doped Ceria (NiO-GDC) anode, GDC electrolyte and (La,Sr)(Co,Fe)O3-based cathode have been tested. Two GDC powders from different manufacturers, Anan Kasei and DKKK, were selected for investigation and compared to traditional Yttria Stabilized Zirconia (YSZ) electrolyte. The mechanical properties, such as Young's modulus, hardness, and biaxial flexure strength of the electrolyte layer and NiO- GDC anode were evaluated and correlated with the SEM morphological characterization.
Coke Formation in Hydrocarbons-Containing Fuel Gas and Effects on SOFC Degradation Phenomena
Authors: Timmermann Henrik , Sawady Walter , Campbell Dennis , Weber Andre , Reimert Rainer , Ivers-Tiffee Ellen
Published: May 2007

The influence of hydrocarbons containing fuel gases on cell performance and coke formation is investigated in this work, wherein acetylene is applied as model compound for the coke formation. The dependence of coke yield on acetylene amount (volume fraction of 0.05% to 0.50%) and gas composition is analyzed. Further parameters, which are varied, are temperature (650 {degree sign}C to 850 {degree sign}C) and operation time. It could be shown, that the mass of coke, that is formed on the cell, depends strongly on acetylene volume fraction and temperature. As a consequence acetylene has a severe influence on the electrical cell performance and acetylene concentration and temperature can be correlated to degradation effects.
Al-Doped (La,Sr)TiO3+δ Anodes for the Direct Oxidation of Methane in SOFCs
Authors: Miller David , Irvine John T.
Published: May 2007

Perovskite (La,Sr)TiO3+δ compounds have been identified as suitable materials for solid oxide fuel cell (SOFC) anodes, due to their high conductivity and stability in reducing conditions. Additionally, when using hydrocarbon fuels, these materials have been proven resistant to carbon deposition. Previous research has shown that the fuel cell performance can be significantly improved by replacing the titanium on the perovskite B-site with other elements. (La,Sr)TiO3+δ compounds, doped with aluminium, were synthesised by solid state reaction. Structural analysis, by XRD and TEM, revealed the structure of these compounds varies with the aluminium content and the amount of oxygen excess (δ). The electrical properties of these materials were analysed using A.C. impedance and four point D.C. conductivity measurements. The conductivity of the compounds was shown to be dependent on the structure. The use of these materials as anodes, in preliminary fuel cell tests has revealed that some compositions have catalytic activity for the direct oxidation of methane. Open cell potentials as high as 1.14V were obtained at 950{degree sign}C.
High Tolerant to Carbon Deposition Ni-based Electrodes under Internal Steam Reforming Conditions
Authors: Gavrielatos Ilias , Neophytides Stylianos
Published: May 2007

Carbon tolerant Ni/YSZ anode modified with 1% (atomic ratio) Au with respect to Ni was tested under methane rich steam reforming conditions. The electrode proved to be high tolerant to C deposits, even at methane to water ratios as high as 3, at temperatures ranging between 700{degree sign}C-900{degree sign}C. The NiAu anode was prepared by combustion synthesis at 600{degree sign}C and sintered under air at 850{degree sign}C. The SEM images have shown that the Au modified Ni/YSZ electrode was nanostructured with particle size ranging between 50-100nm.
Understanding Nickel Oxidation and Reduction Processes in SOFC Systems
Authors: Young Jason L. , Vedahara Vedasri , Kung Sherman , Xia Shen , Birss Viola
Published: May 2007

In Solid Oxide Fuel Cells, Ni-YSZ cermets have many advantages, but degradation can be severe if the anode is exposed to an oxidizing environment. It has been found, in this study and others, that the primary cause of degradation is due to a >60% volume change from Ni to Ni oxide. It is shown here that the Ni is reduced and oxidized in a shorter time period for each redox cycle after the first full cycle. For reduction, this is likely due to the development of a larger surface area, as the cellular NiO surface structure is observed. Also, controlled oxidation of the anode leads to reduced series and reaction resistance values and the Van der Pauw technique indicates that the conductivity of the Ni-YSZ electrode improves after the first redox cycle. Thus, as long as there is enough space for the Ni to expand as it is oxidized, there are actually benefits to cell performance from redox cycling.
Comparative Analysis of Thermal and Redox Cycling for Microtubular SOFCs
Authors: Kendall Kevin , Dikwal Chinnan M. , Bujalski Waldemar
Published: May 2007

One of the key problems of SOFCs is their poor cycling performance which can arise due to thermal expansion or redox stresses. In some instances, one redox cycle is sufficient to destroy a planar Ni/YSZ anode supported cell. Microtubular SOFCs appear to be more resistant to cycling because the thermal and redox gradients are smaller (1). The purpose of this paper is to compare the cycling performance of microtubular SOFCs under both thermal and redox conditions. The objective was to cycle in temperature from ambient to the operating condition several times at high ramping rates up to 8000ºC/min to measure the degradation rate per cycle. Then this degradation was compared with that measured under redox cycling conditions. The relative expansion of the cells was also determined by dilatometry. The conclusion was that redox cycling was more damaging than thermal cycling because much larger expansion and degradation took place.
Microstructure and Effective Thickness of Cermet Anode for SOFC
Authors: Fukunaga Hiroshi , Ohno Tomohito , Arai Chikao , Takatsuka Toru , Yamada Koichi
Published: May 2007

The effective thickness or the active thickness of Ni-SDC cermet SOFC anode was investigated for two types of anode with different microstructure. One was prepared by using large particle, and the other by small particle. The performance of each cells were measured for various anode thickness. The performance of the anode increased by increasing the anode thickness when prepared by large particles. The dependence of the anode thickness was as large as 120μm. On the other hand, the anode prepared by small particles showed little dependence on anode thickness. This large effective thickness of the electrode was achieved probably due to the high sintering temperature. Although this structure may not be typical, it indicates an alternative method for the improvement of the anodic performance of SOFC.
Microstructural and Electrochemical Study of Charge Transport and Reaction Mechanisms in Ni/YSZ Anode
Authors: Bredikhin Ivan , Sinitsyn Vitaly , Aronin Alexander , Kuritsyna Irina , Bredikhin Sergey
Published: May 2007

The microstructure and chemical changes in the Ni-YSZ heterojunctions were studied by scanning, transmission, and high-resolution electron microscopy and X-ray diffraction. It was shown that new nano-size Ni grains are formed at Ni-YSZ interface at the cell operation. The following two stage reaction mechanism was proposed for hydrogen oxidation on the nano-size Ni grains: 1. NiO + Н2 Þ Н2О + Ni; 2. O2-(YSZ) + Ni Þ NiO + 2e. This mechanism implies that the SOFC will operate during some time even after switching off the hydrogen flow. We have observed and investigated current-voltage characteristics of SOFC after cutting off the hydrogen pass through anode chamber. Electrochemical properties of SOFC were investigated as a function of Ni volume content for two different geometry of anode current collector. Good correlations of experimentally measured SOFC characteristics with results of numeric calculations were found.
Effect of Graded Porous Structure on Ni-YSZ Anode Performance
Authors: Jono Kaori , Suda Seiichi , Hattori Masatoshi
Published: May 2007

SOFC anode requires long three phase boundaries (TPBs) and appropriate gas diffusion passes for the fast transport of both fuel and exhaust gases, but the area where gas diffusion passes are especially required would be different from the area suitable for electrochemical reaction in the anode. Then gradient porous structures of SOFC anodes have been proposed to fulfill the anodic functions in adequate anodic areas. Ni- YSZ anodes with various gradient pore-distributions were then prepared using pore-formers by screen-printing method. The relationship between maximum power densities and the pore size and porosity distributions was investigated to identify an effective gradient porous structure that can improve anode performance. The gradient porous structure improved anodic polarization. Pore structure contains two factors of pore size and porosity but the gradient in pore size was more effective in improving anode performance.
Electrochemical and Microstructural Characterization of the Solid Oxide Fuel Cell Anode Prepared by Co-precipitation
Authors: Mosch Sindy , Trofimenko Nikolai , Kusnezoff Mihail , Betz Thomas , Kellner Marco
Published: May 2007

Nickel oxide and yttria stabilised zirconia (NiO/YSZ) cermets have been prepared using a chemical co-precipitation of hydroxides. The influence of different factors on the microstructure, mechanical and electrochemical properties of the anode were investigated. The electrochemical tests were carried out on dense 8YSZ and 10Sc1CeSZ tapes (50x50x0.150 mm, Kerafol GmbH) with symmetrically screen-printed cathodes and anodes with a lateral dimension of 40x40 mm. The cells were sintered in co-firing. The anodes were characterized by impedance spectroscopy at open circuit potential and under current load at temperatures of 850-950{degree sign}C in hydrogen/steam atmosphere (H2:H2O=1:1). The electrode polarization resistance (Rp) of the spectra was identified and the influences of operating temperature (850{degree sign}C, 900{degree sign}C and 950{degree sign}C), current load and pH2O on impedance spectra of anode were discussed. The long-term stability of the cell with an anode prepared by co-precipitation was carried out over 1000 hours under current density of 430-450 mA/cm2@0.7V at temperature 850{degree sign}C.
Influence of Composite Structure on Electrochemical and Mechanical Properties of Thick Ni-YSZ Anode
Authors: Sato Kazuyoshi , Okamoto Go , Abe Hiroya , Naito Makio
Published: May 2007

Appropriate mechanical milling of two kinds of metal oxide powders often induces strong bonding between powder particles, resulting in their composite powder (1). In this study, the mechanically derived NiO-YSZ composite powder was used as the composite precursor for fabricating the anode supported SOFC in a ceramic processing route. The composite precursor finally provided the anode with homogeneous porous structure consisting of precisely mixed Ni (NiO) and YSZ networks, which showed good electrochemical and mechanical properties. Contrarily, the anode made from a conventional powder mixture of NiO and YSZ powders had inhomogeneous porous structure containing the large Ni (NiO) clusters, resulting in less electrochemical property. In addition, the large clusters governed the mechanical strength of the anode.
Electrochemical Behaviors of Mixed Conducting Oxide Anodes for SOFC
Authors: Nakamura Takashi , Kobayashi Tsuneyuki , Yashiro Keiji , Sato Kazuhisa , Kawada Tatsuya , Mizusaki Junichiro
Published: May 2007

Studies were made on electrochemical behaviors of LaCrO3 based, SrTiO3 based, and CeO2 based oxide anodes. Steady state polarization and AC impedance measurements were carried out in H2-H2O-Ar gas-mixtures. Electrode microstructure was observed by SEM before and after electrochemical measurements. The CeO2 based anodes showed higher performance than the LaCrO3 based and the SrTiO3 based oxide anodes. Morphological changes during electrochemical measurements were observed in niobia-doped ceria anodes. Gadolinia-doped ceria anodes showed the highest electrode performance and high redox tolerance. The electrode performance did not show clear relation with the electrode microstructure.
Synthesis and Study of Ti-O Based Materials for SOFC Anode Application
Authors: Kozhukharov Vladimir , Tsvetkova Yana V. , Bebelis Simeon , Kournoutis Vassilis
Published: May 2007

The aim of this work was to synthesize and characterize Ti-O based materials for use as anodes in SOFCs. The compositions 50 mol% SrTiO3 + 50 mol% (La2O3+TiO2), 50 mol% SrTiO3 + 50 mol% 3CeO2.La2O3, 50 mol% SrTiO3 + 50 mol% CeO2, 50 mol% (La2O3+TiO2) + 50 mol% 3CeO2.La2O3 and 50 mol% (La2O3+TiO2) + 50 mol% CeO2 were studied. The samples were prepared using the solid state reaction method followed by sintering at 1300 {degree sign}C for 3 hours. The characterization of all compositions included XRD, FTIR and SEM analysis. From the X-ray diffraction data, SrTiO3, La2Ti2O7 and CeO2 were identified as the basic phases. The SEM analysis showed that all compositions had a microstructure suitable for SOFC anodes. Conductivity measurements in pellets were also performed to obtain information about the electrochemical behavior of the materials. All tested compositions can be considered as potentially promising for use as alternative anode materials.
A Novel Three Electrode Design for Electrochemical Investigations of an Intermediate Temperature SOFC
Authors: Offer Gregory , Brett Daniel J. , Brandon Nigel P.
Published: May 2007

A novel 3-electrode intermediate temperature solid oxide fuel cell (IT-SOFC) design is reported. Ni/CGO:CGO:LSCF/CGO 3- electrode pellet cells were investigated at 600oC with a ring- disc cathode operating in air. Polarisation curves and AC impedance spectra at high and low suction rates of air and at high current densities are shown to demonstrate the effective operation of the reference electrode; comparison is made with operation in 2-electrode mode where the counter and reference are shorted. It has been shown that incorrect placement of the reference electrode can lead to significant errors in the measurement of the working electrode, but that these can be compensated for using impedance spectroscopy.
Effect of Ethene Impurity on Performance of Solid Oxide Fuel Cell
Authors: Aarva Anja , Noponen Matti , Kiviaho Jari
Published: May 2007

A pre-reforming unit is needed if liquid hydrocarbon fuels, such as gasoline or diesel, are used in solid oxide fuel cell (SOFC) systems. Although pre-reforming removes most of the higher hydrocarbons, there may still be some fractions left in the reformate flow. Ethene was chosen to be the subject of this work because it has been analyzed to be the most significant impurity in diesel reformate. In this work, an anode supported unit cell was used to examine the behavior and performance of a SOFC when ethene was added to hydrogen feed. The ethene concentration in the feed was increased stepwise from 0 ppm to 1000 ppm and the current was kept constant for 100 hours at each concentration level. Changes in the cell performance were monitored by impedance spectroscopy and polarization measurements. It was detected that ethene does not accelerate cell degradation.
Fuel Impurity Tolerance of Solid Oxide Fuel Cells
Authors: Sasaki Kazunari , Adachi S. , Haga K. , Uchikawa M. , Yamamoto J. , Iyoshi A. , Chou J.-T. , Shiratori Y. , Itoh K.
Published: May 2007

While SOFCs are the most flexible fuel cells with respect to fuel selection, fuel impurities in practical SOFC fuels can cause degradation of electrodes and thus electrochemical performance. The understanding of fuel impurity poisoning and the increase in tolerance are therefore of technological relevance for SOFC commercialization. Along with thermochemical calculations on the stability of typical fuel impurities under SOFC operational conditions, fuel impurity poisoning of SOFCs is analyzed by using various impurity- containing fuel gases with respect to operational temperature, type and concentration of impurities, fuel composition, and steam-to-carbon ratio. We have revealed that poisoning depends on operational conditions and types of fuels and impurities. The poisoning effect becomes significant for internal- reforming SOFCs operated with humidified methane-based fuels. Poisoning mechanisms are discussed, based on cell voltage and durability data.
Recent Advances on the Development of NiCu Alloy Catalysts for IT-SOFCs
Authors: La Rosa Daniela , Lo Faro Massimiliano , Monforte Giuseppe , Antonucci Vincenzo , Aricò Antonino Salvatore , Sin Agusti
Published: May 2007

A SOFC anodic cermet composed by Ce0.9Gd0.1O1.95 (CGO) and Ni0.49Cu0.51 alloy has been investigated and optimized for the CH4 electro-oxidation in IT-SOFCs. An appropriate synthesis has been developed to obtain optimal electrochemical properties. The anode is characterized by high surface area and suitable catalytic activity towards oxidation of dry methane. Significant improvements on the performances have been obtained by tailoring bulk and surface composition of the alloy. Polarizations curves have been recorded in the temperature range from 700{degree sign}C to 800{degree sign}C. Impedance spectroscopy measurements have been carried out at different cell potentials. The life- time behaviour in dry methane, and the electrochemical stability have been investigated at 750 {degree sign}C. A variation of the electrochemical properties during continuous operation has been observed and interpreted in terms of anode morphology modifications. No significant carbon deposition has been observed after prolonged operation at 750{degree sign}C in the presence of dry methane and CGO electrolyte.
Electrochemical Activity of Ni1-XCoX-Based Cermet Anode for Methane Oxidation
Authors: Weerapakkaroon Wichapun , Ogasa Kazuhito , Sato Kazunori
Published: May 2007

Partial substitution of Co atoms for Ni atoms in the Ni-based cermet anode resulted in a decrease in the anodic overvoltage and interfacial resistance between the anode and electrolyte. This effect has been confirmed for yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) electrolytes. The Ni1-XCoX-ScSZ (-YSZ) cermet anode showed an increase in the cell performance with increasing amount of Co content up to approximately 0.5. The decrease in the anodic overvoltage and the interfacial resistance contributes to an enhancement of the cell performance. The results of electrochemical measurements and microstructural characterization revealed that the enhancement of the direct oxidation of methane is caused by the modification of microstructure due to a solid- solutioning effect in the starting NiO-CoO binary system.
Microstructure and Electrochemical Properties of Ni1-xMgxO-ScSZ Cermet Anodes for Biogas Fueled SOFCs
Authors: Shiratori Yusuke , Sasaki Kazunari
Published: May 2007

To enhance catalytic activity for internal reforming of hydrocarbon-fuels, catalyst based on Ni1-xMgxO was prepared. Small particles of Ni with ca. 50-100 nm in size were formed by the reduction of the mixed oxide, whereas Ni particles formed from NiO easily agglomerated resulting in smaller catalyst area in the cermet anodes. Ni1-xMgxO-ScSZ anode was fabricated and applied to biogas-fueled SOFC. In using Ni0.9Mg0.1O-ScSZ, cell voltages about 850 and 950 mV for humidified and dry biogases, respectively, were achieved at 900 oC in 50 h operation under the constant current density of 200 mA cm-2.
Anode Performance of Ni/(CeO2)1-x(LnO1.5)x (Ln: Lanthanoids) in SOFCs Using Hydrocarbon Fuels
Authors: Asamoto Makiko , Miyake Shinji , Saito Akihiro , Yamaura Hiroyuki , Yahiro Hidenori , Itagaki Yoshiteru , Sadaoka Yoshihiko
Published: May 2007

The anode performances of Ni/(CeO2)1-x(LnO1.5)x (Ln: La, Pr, Nd, Sm, Gd, and Yb) were investigated in direct-methane solid oxide fuel cells. Among the anodes tested in the present study, Ni/CeO2-Sm2O3 and Ni/CeO2-Yb2O3 composite anodes showed the relatively low anodic overpotential. The anodic overpotential of Ni(x wt%)/(CeO2)1-y(SmO1.5)y depended strongly on Ni content, while it less depended on Sm content. The catalytic activity of methane oxidation was measured for Ni/LnDC at 973 K and Ni/CeO2 and Ni/CeO2-Yb2O3 samples showed the high catalytic activity. This result suggests that the activity for the direct oxidation of methane by O2 is not main factor controlling the anodic overpotential in direct methane solid oxide fuel cell.
Steam Reforming of Ethanol over Ni on High Surface Area Ceria Support: Influence of Redox Properties on the Catalyst Stability and Product Selectivities
Authors: Laosiripojana Navadol , Assabumrungrat Suttichai , Charojrochkul Sumittra
Published: May 2007

The steam reforming of ethanol over Ni on high surface area CeO2 support, synthesized by a surfactant-assisted approach, (Ni/CeO2 (HSA)) were studied under solid oxide fuel cell (SOFC) operating conditions. The catalyst provides significantly higher reforming reactivity and excellent resistance toward carbon deposition compared to Ni/Al2O3 and Ni on conventional ceria (Ni/CeO2 (LSA)) under the same conditions. At the temperature above 800{degree sign}C, the main products from the reforming processes over Ni/CeO2 (HSA) were H2, CO, and CO2 with small amount of CH4 depending on the inlet ethanol/steam ratio, whereas high hydrocarbon compounds i.e., C2H4 and C2H6 were also observed from the reforming of ethanol over Ni/CeO2 (LSA) and Ni/Al2O3 in the range of conditions studied (700-1000{degree sign}C). The excellent ethanol reforming performances of Ni/CeO2 (HSA) in terms of stability, reactivity, and product selectivities are due to the high redox property of CeO2 (HSA). During the ethanol reforming process, in addition to the reactions on Ni surface, the gas- solid reactions between the gaseous components presented in the system (C2H5OH, C2H6, C2H4, CH4, CO2, CO, H2O, and H2) and the lattice oxygen (Ox) on ceria surface also take place. Among these redox reactions, the reactions of adsorbed surface hydrocarbons with the lattice oxygen (Ox) can eliminate the formation of high hydrocarbons (C2H6 and C2H4), which easily decompose and form carbon species on Ni surface.
Effect of Steam on Direct Oxidation of Methane over Pd-Ni Electrocatalyst Supported on Lanthanum Chromite Anode
Authors: Yamanaka Ichiro , Ito Tateo , Nabae Yuta , Hatano Masaharu
Published: May 2007

Effects of the ratio of Pd and Ni on the La0.8Sr0.2CrO3 (LSCr) and Ce0.8Sm0.2O1.9 (SDC) composite anode were clarified on the oxidation of dry methane during SOFC reactions using La0.83Sr0.17Ga0.8Mg0.2O3 (LSGM) electrolyte. The highest current density, faster oxidation of methane, was obtained at the Pd and Ni ratio of 75:25(mol). Amounts of carbon deposition were quite small for all Pd-Ni/oxides- composite anodes under the open and closed circuit conditions. Effects of steam were studied on the oxidation of methane over various Pd- Ni/composite-anodes. Higher current densities were obtained above the Pd and Ni ratio of 75:25 at P(H2O) from 3 to 10 kPa. These data proposed the Pd(75)-Ni(25) supported the LSCr-SDC-composite anode is suitable for the oxidation.
SOFC-MGT Hybrid System Using Natural Gas Mixed with Biomass Gasified Fuel
Authors: Sucipta Made , Kimijima Shinji , Suzuki Kenjiro
Published: May 2007

Much attention is now paid to renewable energy resources such as biomass fuel related to the worldwide effort to suppress the global warming. In this article, study is made to see how the performance of SOFC-MGT hybrid system is affected if natural gas, normal fuel of the hybrid system, is mixed or replaced by biomass gasified fuel (bio-fuel). It is found that the efficiencies of the SOFC module and of the hybrid system noticeably decrease when natural gas is completely replaced by bio-fuel but that SOFC-MGT hybrid system can still be operated at a reasonable condition. To explain these results, discussion is given also to the change of the spatial distributions of temperature and concentration of some chemical species inside the internal reforming type SOFC module.
Electrical Interaction Model of Planar SOFC Stack Fed with Natural Gas
Authors: Bultel Yann , Gautier L , Zahid Mohsine , Stevens Philippe , Klein Jean-Marie
Published: May 2007

Solid oxide fuel cells (SOFC) are energy conversion devices that produce electricity and heat directly from a fuel such as natural gas. Little attention has been paid to the electrical behaviour of a SOFC stack. This paper presents an electrical interaction model of a planar anode supported intermediate temperature SOFC stack with direct internal reforming. The SOFC stack model is built up of multiple single repeat units stacked on top of each other and takes into account, amongst other parameters, contact resistances and gas flow. This assembly is sandwiched between two end plates. A combined model with mass, charge and heat balances has been developed with a special attention to the description of electrical behavior. Such an approach can provide a picture of the two dimensional distribution of potential, current density, and temperature. Simulations are performed to analyse the impact of changes in material conductivities, electrical configuration and operation conditions.
Experiences with a CFD Based Two Stage SOFC Stack Modeling Concept and Its Application
Authors: Froning Dieter , Blum Ludger , Gubner Andreas , de Haart L. G. , Spiller Martin , Stolten Detlef
Published: May 2007

The parallel development of kW-range SOFC stacks and their systems at the Forschungszentrum Jülich requires distinguished modeling capabilities by validated SOFC models targeting the three-dimensional stack structure itself as well as the entire system. For the full three-dimensional (3D) geometric resolution of a stack, Computational Fluid Dynamics (CFD) models are used whereas a one-dimensional (1D) behavior model based on ordinary differential equations (ODE) covers the system modeling requirements. The CFD model is using advanced data structures to reduce the computing time significantly. However, Computing times in the range of seconds or below required by system modeling are featured only by the 1D model. Both SOFC models are based on the same physical equations for electrochemical conversion and internal methane reforming. The application of these models is shown in practice, focusing on choosing the right model for an application, and how both models were validated against each other and against experiments.
Investigation of Locally Resolved SOFC Characteristics along the Flow Path
Authors: Metzger Patrik , Friedrich K. A. , Schiller Günter , Müller-Steinhagen Hans
Published: May 2007

Solid oxide fuel cells (SOFCs) are characterized locally in a segmented cell arrangement. The tests were performed in counter flow operation for hydrogen concentrations from 2 % to 100 % to identify concentration limitations and to optimize fuel utilization. Cell characterisations were obtained for different cell concepts by voltage/current density U(i) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilization. Especially at high fuel utilizations approaching 100% a dramatic dependence of power density along the flow path occurs which is associated with increasing degradation. The observation of inhomogeneous temperature distribution emphasizes the importance of a thermal management adapted to the cell operation characteristics, particularly when the stack itself only has a low thermal mass.
Experimental and Numerical Studies on Flow Uniformity in Interconnects and its Influence on a Single Planar Solid Oxide Fuel Cell
Authors: Huang Chia-Ming , Shy Shenqyang (Steven) , Lee C. H.
Published: May 2007

This paper studies experimentally and numerically flow uniformity in various interconnects and its influence to temperature distributions and cell performance of a planar solid oxide fuel cell (SOFC). A transparent hydraulic platform is first established to evaluate the degree of flow uniformity in many different modules of interconnects using the same rib- channels for both the cathode and the anode or using Ni-mesh without rib-channels only for the anode side. This novel setup allows direct imaging and measurements of flow uniformity in both rib-channels and Ni-mesh, so that these experimental data can be used to validate numerical flow data. A three- dimensional numerical model using CFD-RC package with sub- models is then used to simulate various transport phenomena and electrochemical reactions of a single cell with the consideration of different degrees of flow uniformity in interconnects. It is found that previous modules using either single-inlet/single-outlet or double-inlets/single-outlet designs cannot provide uniform velocity distributions in these rib-channels, which can result in serious non-uniform temperature gradients in the cell. We propose a new design, using small guide vanes equally-spaced around the feeder of the double-inlet module on the cathode side and using Ni-mesh on the anode side, which features a highest degree of flow uniformity and least non-uniform temperature gradients in the cell among all different modules studied.
Numerical Simulation of Electrochemical and Transport Processes in Solid Oxide Fuel Cells
Authors: Ho Thinh X. , Kosinski Pawel , Hoffmann Alex C. , Wærnhus Ivar , Vik Arild
Published: May 2007

This paper presents a comprehensive model for a unit solid oxide fuel cell (SOFC). It covers all parts of the cell including fluid and porous solid phases. The electrochemical and transport processes involved in an SOFC can thereby be taken into account. The commercial computational fluid dynamic package Star-CD along with built-in user-defined subroutines is adapted for an anode-supported unit cell. Some results such as gas concentration, temperature distribution and current density are presented at the end of the paper.
Parameter Effects on Transport Phenomena in Conjunction with Internal Reforming Reactions in Intermediate Temperature SOFCs
Authors: Yuan Jinliang , Wang Wei Guo
Published: May 2007

Various transport phenomena in conjunction with chemical reactions are strongly affected by solid oxide fuel cells (SOFCs) configurations and properties of involved porous anode. The considered composite duct is relevant for an intermediate temperature (600-800oC) SOFC and consists of a porous anode layer for the internal reforming reactions of methane, the fuel gas flow duct and solid plate. In this paper, a fully three-dimensional calculation method is developed to simulate and analyze internal reforming reactions, with purpose to reveal the importance of design and operating parameters outlined as three characteristic ratios. The SOFC conditions such as mass balances associated with the internal reforming reactions and gas permeation to/from the porous anode are applied in the analysis. The results show that the characteristic ratios have significant effects on the transport phenomena and overall reforming reaction performance.
Simulation and Modeling Study of Anode-Supported Planar SOFC Operating with Synthesis Gas
Authors: Suwanwarangkul Rapeepong , Wattana Kulpat , Charojrochkul Sumittra , Croiset Eric , Douglas Peter L. , Entchev Evgeniy
Published: May 2007

A two-dimensional model of an anode-supported planar SOFC operating with synthesis gas was developed based on co-flow and counter-flow configurations. Mass, energy and charge transport equations coupled with electrochemical reactions (H2, CO oxidation and O2 reduction) as well as chemical (water- gas shift) reaction were considered in the model. The performance comparison obtained from the model reveals that the cell performance for counter-flow configuration is higher than that of co-flow configuration. Unfortunately, due to the high temperature gradient at the fuel inlet region, the counter-flow configuration may not be the best mode of operation.
Modeling and Analysis of a Micro-Tubular Solid Oxide Fuel Cell (SOFC) Operating at Intermediate Temperatures
Authors: Serincan Mustafa Fazil , Smirnova Alevtina L. , Sammes Nigel M.
Published: May 2007

A two-dimensional (2D), axial symmetric, steady state, non- isothermal computational model incorporating radiation heat transfer, flow and transport of species in porous gas diffusion electrodes has been developed to study the performance of a micro tubular SOFC. Anode, cathode, electrolyte and flow channels are modeled as separate domains to have an accurate characterization of the system. Co- and counter-flow of anode and cathode reactants are discussed to address their effects on the performance of the SOFC. The effects of the operating conditions on polarization curve of the cell are outlined. Some rules for furnace design to improve the overall system efficiency are deduced. The temperature gradients inside the cell are compared for different scales of the system. It is shown that for smaller cells the temperature gradients are reasonably minor than that for those with bigger dimensions. Simulation results are used to verify the model with the experimental data provided in the literature for the cells with the same electrode and electrolyte material and the same geometry. Polarization curve for the model compares reasonably well with the experimental data.
Finite Element Analysis of Thermal Stress Distribution in Planar SOFC
Authors: Lin Chih-Kuang , Chen Tsung-Ting , Chen An-Shin , Chyou Yau-Pin , Chiang Lieh-Kwang
Published: May 2007

A 3-D finite element analysis model of a 3-cell stack based on a prototype planar SOFC stack design was constructed to perform thermal stress analyses. The thermal stress distributions at shutdown and steady-state stages as well as their dependence on the initial stress-free temperature and operation cycles were evaluated. Modeling results indicated the glass-ceramic sealant was the most critical part in terms of structural integrity, as shear fracture of such a component at operation temperature was predicted. Localized plastic deformation was predicted for the metallic interconnect and frame at both shutdown and steady-state conditions. An increase in the initial stress-free temperature would significantly increase the thermal stresses in all of the components at both shutdown and steady-state stages. A notable increase of thermal stress with increasing cycle number in the positive electrode-electrolyte-negative electrode (PEN) plate at steady state as well as in the glass- ceramic sealant at shutdown stage was predicted.
Oriented-Control Lumped Model of a SOFC Stack: Thermal and Electrochemical Response to External Perturbations
Authors: Baratella Andrea , Corradetti Alessandro , Desideri Umberto , Lisbona Pilar , Lunghi Piero
Published: May 2007

Solid oxide fuel cells (SOFC) are considered a promising technology to be adopted as future possible solution for decentralised combined heat and power (CHP). To operate a SOFC stack in an effective way, the control of some system operation parameters such as anode and cathode feeds, temperature or power output is mandatory. A deep knowledge of transient behaviour is a key requirement to get fuel cell plants ready for an industrial development. In this paper, a 1 kWe SOFC stack is modelled and a possible control strategy is proposed to provide rapid system response to perturbations. The model has been implemented using the commercial software package MATLAB/Simulink. As a result of an external perturbation of steady state conditions, the fuel cell stack control system has to regulate temperature, partial pressure and utilization of flows to avoid degradation of the stack performances.
Non-reacting Flow Distributions under Various SOFC Stack Configurations
Authors: Wattana Wassachol , Charoensuk Jarruwat , Phoocharoen Niwat , Charojrochkul Sumittra
Published: May 2007

A 3-dimensional, Computational Fluid Dynamics (CFD) model for fuel cell stack simulations has been developed using STAR- CD with effect of buoyancy force. A flow distribution in a planar fuel cell stack with straight gas channels is considered with variation in gas feeding directions. An electrochemical interaction is neglected. Eight flow configurations comprising of three parameters were investigated, i.e.; i) vertical and horizontal orientations of the cell stack, ii) U-shape and Z-shape flow patterns and iii) upward and downward feeding directions. Of these configurations the velocity distributions across the stack were compared. The better flow distribution was observed as the air with U- shape flow pattern and upward feeding. For fuel side, the flow with U-shape with downward yields a relatively better flow distribution. Moreover, the results indicate that the effect of feeding direction is small in the stack with Z-shape flow.
3-D Simulations of Charge and Mass Distribution in Tubular SOEC
Authors: Deseure Jonathan , Klein Jean-Marie , Bultel Yann , Dessemond Laurent
Published: May 2007

Electrochemical systems that could be employed with reversible functions has been developed: electric production in a solid oxide fuel cell (SOFC) mode and hydrogen production in a solid oxide electrolysis cell (SOEC). This paper proposes a model solution using the CFD-Ace software package to simulate the behaviour of a tubular SOEC. Modelling is based on solving conservation equations of mass, momentum, energy, species and electric current by using a finite volume approach on 3D grids of arbitrary topology. The electrochemical behaviour of porous electrode is described using Butler-Volmer equations at triple phase boundaries. The 3D layouts of the current densities, electronic and ionic potentials allow the analysis of the respective ohmic drops. Simulations indicate that the principal damaging risks at the electrolyte/anode interface originate from high flux densities of oxygen and from the possible re-oxidation of cermet-based cathode.
Modeling of SOFC Anodes Based on the Stochastic Reconstruction Scheme
Authors: Suzue Yoshinori , Shikazono Naoki , Kasagi Nobuhide
Published: May 2007

A novel modeling scheme of SOFC anodes based on the stochastic structure reconstruction technique and the Lattice Boltzmann method (LBM) is proposed for performance assessment and optimization of anode microstructures. Cross-sectional microscopy images are processed to obtain phase-distinguished images (i.e., Ni, YSZ and pore), and a three-dimensional model microstructure is reconstructed from the two-point correlation functions of the microscopy image. Then, the diffusivity, the electrical conductivity and the overpotential of the reconstructed anode are obtained through the LBM simulation. The electronic conductivity is predicted slightly smaller than the experimental result. However, the optimal sintering temperature is predicted to be around 1350 ºC, which is in good agreement with the literature data. Thus the proposed method predicts the anode performance for a given microstructure, and is considered as a promising tool for designing SOFC anodes.
Development of Three-Dimensional Porous Structure Simulator for Optimizing Microstructure of SOFC Anode
Authors: Koyama Michihisa , Tsuboi Hideyuki , Hatakeyama Nozomu , Endou Akira , Takaba Hiromitsu , Kubo Momoji , Del Carpio Carlos A. , Miyamoto Akira
Published: May 2007

Microstructure of anode strongly influences the overpotential characteristics and the best microstructure depends on the anode materials and the operation conditions. Because the experimental optimization of microstructure requires a number of trials and errors, the development of a simulation approach that supports optimizing the anode microstructure is of great importance. In this study, we present a new simulation method, three-dimensional porous structure simulator for optimizing the microstructure of porous anode. The developed method supports the quantitative evaluations of microstructure, cross-sectional areas, triple-phase boundary, and so on. We applied the developed method to Ni-YSZ cermet anode for SOFC. Electrical properties and overpotential characteristics were calculated for cermet anodes with different microstructure. Quantitative estimation of overpotential for Ni-YSZ cermet anode with different microstructure was realized.
3D-Modelling and Performance Evaluation of Mixed Conducting (MIEC) Cathodes
Authors: Rüger Bernd , Weber Andre , Ivers-Tiffee Ellen
Published: May 2007

Mixed electronic/ionic conducting (MIEC) cathode materials are qualified for the application in intermediate-temperature SOFCs. The area specific resistance of MIEC cathodes is likewise determined by chemical composition and microstructure. A three-dimensional finite element method model, which is capable to analyze and predict the performance of MIEC cathodes, was developed. Representations for the actual cathode microstructure are automatically generated according to characteristic measures like cathode thickness, composition, porosity and average particle size. The model calculates the spatial distributions of the oxygen activity, the diffusive oxygen flux in the pores as well as the ionic current density inside the MIEC-cathode and inside the electrolyte material. Based on these values the area specific resistance of the cathode and the penetration depth of the ionic current can be evaluated. This paper will analyze the effect of surface-exchange kO and bulk-diffusion coefficient DO as well as the influence of microstructure properties on the cathode performance.