Symposium Organizers
Rob Walker Montana State University
Nigel Brandon Imperial College London
Jeff Owrutsky Naval Research Laboratory
Koichi Eguchi Kyoto University
C1: In Situ Studies of SOFCs I
Session Chairs
Monday PM, November 28, 2011
Republic A (Sheraton)
10:00 AM - C1.1
Time-Resolved Correlations between the Structure and Electrical Behavior of La0.6Sr0.4Co0.2Fe0.8O3-δ Epitaxial Thin Films.
Paul Fuoss 1 , Brian Ingram 2 , Edith Perret 1 , Kee-Chul Chang 1 , Tim Fister 2 , Peter Baldo 1 , Jeffrey Eastman 1 , Paul Salvador 3
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Materials Science Department, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractWe have used in-situ grazing incidence x-ray scattering and spectroscopy to correlate the electrochemical behavior and atomic structure of La
xSr
1-xCo
yFe
1-yO
3-δ (LSCF). LSCF is an important cathode material for solid oxide fuel cells (SOFCs) because of its mixed ionic and electronic conduction behavior, and its activity for reducing oxygen under an electrochemical potential. Our focus is on identifying and understanding the structural and chemical changes that occur at elevated temperature during exposure of films to electrochemical potentials at various oxygen partial pressures. Using model half-cells of an approximately 20 nm thick epitaxial LSCF film of composition x=0.6 and y=0.2, grown on a 0.5 mm thick Y
2O
3-stabilized ZrO
2 (YSZ) (001) substrate with an intermediate 20 nm thickness layer of GdO
2-doped CeO
2 (GDC), we observe surprisingly large changes in the LSCF lattice parameter normal to the film surface when small DC potentials (≤ ±3 V) are applied between surfaces of the stack (the GDC and YSZ lattice parameters are unchanged). This occurs despite the expected small voltage drop across the LSCF layer (due to the similar conductivity of all three layers in the heterostructure). From the potential dependence, the behavior appears to arise from a change in either the density or configuration of oxygen vacancies in the LSCF film. We will compare spatially resolved lattice parameter measurements with predictions of the electrical potential derived from simulations.
PHF and JAE are supported by the U. S. Department of Energy (DOE), Basic Energy Sciences, Materials Sciences and Engineering Division. Work by PHF, BJI, EP, and PAS is partially supported by the DOE Solid State Energy Conversion Alliance (SECA). Use of the APS was supported by the DOE, Office of Science, Office of Basic Energy Sciences (BES), under Contract DE-AC02-06CH11357 between UChicago Argonne LLC and the Department of Energy.
10:15 AM - C1.2
In Situ X-Ray Photoelectron Spectroscopy Studies of La0.6Sr0.4CoO3-δ Perovskites Cathodes Close to SOFC Operation Conditions.
JongHoon Joo 1 , Rotraut Merkle 1 , Joachim Maier 1 , Markus Kubicek 2 , Judith Januschewsky 2 , Jürgen Fleig 2 , Andreas Oestereich 3 , Zoltan Hlavathy 3 , Michael Haevecker 3 , Axel Knop-Gericke 3 , Robert Schloegl 3
1 , Max-Planck-Institute for Solid State Research, Stuttgart Germany, 2 , Vienna University of Technology, Vienna Austria, 3 , Fritz-Haber-Institut der MPG, Berlin Germany
Show Abstract Mixed conducting perovskites are commonly used as cathode materials in solid oxide fuel cells (SOFC) because of their high catalytic activity for the oxygen reduction reaction. However, the details of the oxygen reduction reaction are still not clearly understood. The information about the type of adsorbed oxygen species and their concentration is important for a mechanistic understanding of the oxygen incorporation into these cathode materials. X-ray photoelectron spectroscopy (XPS) has been widely used for the analysis of adsorbed species and surface structure. However, the conventional XPS experiments have the severe drawback to operate at room temperature and with the sample under ultrahigh vacuum (UHV) conditions, which is far from the relevant conditions of SOFC operation. The disadvantages of conventional XPS can be overcome to a large extent with a “high pressure” XPS setup installed at the BESSY II synchrotron. It allows sample depth profiling over 2 nm without sputtering by variation of the excitation energy, and most importantly measurements under a residual gas pressure in the mbar range. In this study, we report an investigation of oxygen species and surface composition of dense La0.6Sr0.4CoO3-δ (LSC) films deposited by pulsed laser deposition at different conditions which lead to significantly different electrochemical activity. The nature of adsorbed O or OH species and the surface cation composition, as well as their change upon exposure to humidity and DC current are investigated. Depending on the actual reaction mechanism, in-situ application of a DC-current is expected to significantly increase these adsorbate concentrations. A cathodic current (i.e. oxygen incorporation from gas into the film) leads to a decrease of OH peaks, while anodic current slightly increases OH peaks. The relaxation process after switching off the cathodic current takes longer than expected in case of oxygen diffusion only. The perovskite surface exhibited a complex behavior with respect to the effect of preparation conditions on actual surface composition and the result of water exposure. Electrochemical impedance spectroscopy in dry and humidified O2 also indicates hydroxides at the surface may be involved in the oxygen incorporation reaction at SOFC operation condition.
10:30 AM - **C1.3
In Situ XPS Measurements of Charge-Transfer Overpotentials on Operating Solid Oxide Cells.
Bryan Eichhorn 1 , Chunjuan Zhang 1 , Michael Grass 2 , Yi Yu 1 , Karen Gaskell 1 , Funda Aksoy 2 , Nailia Jabeen 2 , Young Pyo Hong 2 , Gregory Jackson 1 , Zahid Hussain 2 , Zhi Liu 2
1 , University of Maryland, College Park, Maryland, United States, 2 , Advanced Light Source, LBL, Berkelely, California, United States
Show AbstractAn electrochemical double layer (EDL) at interfaces is always one of the key processes that govern cell performances for all electrochemical devices, such as supercapacitors, batteries and fuel cells. Capacitances associated with built-up charges at the double layer can be indirectly measured via electrochemical impedance spectroscopy (EIS), which requires proper identification of capacitance associated with individual processes. Theorists also predict capacitances and overpotentials associated with the EDL at interfaces. However, there has been no direct detection of potential drops associated with the EDL at any interface. We present here the first direct experimental detection of local overpotentials associated with electrochemical double layers built up at gas-solid interfaces by using in situ ambient pressure X-ray Photoelectron Spectroscopy (XPS). The single-chamber polycrystalline YSZ cell consists of one Pt counter electrode and two ceria working electrodes with Au current collectors. Dense thin ceria films were sputtered on top of Au films with elongated Au pads exposed for connections. Underneath the Au films, 30nm thick alumina films were sputtered to block the ionic transport from YSZ to Au. Only one ceria edge has direct contact with YSZ electrolyte to define current flow region. The outer ceria edge is ~500μm away from the nearest Pt electrode edge. All cells were pre-annealed at 700oC before measurements. In situ ambient pressure XPS and electrochemical characterizations were conducted at 700-750oC with 0.5 Torr 1:1 H2 / H2O mixtures.In our previous studies, we have demonstrated the direct measure of local potentials and electrochemical active regions for ceria electrodes in operating solid oxide electrolysis cells and solid oxide fuel cells. We used rigid shifts of XPS kinetic energy (KE) spectra to detect the local electric potentials (V) relative to ground based on . In addition, cerium oxidation state changes at different applied biases were studied by Ce3d and Ce4d XPS spectra to reveal the electrochemically active regions on the ceria surface that extend roughly 150 μm away from the current collector. In this study, we probe the local potentials for surface adsorbates and surface solid species, which show pronounced differences under different applied bias. This finding provides evidence for the existence of electrochemical double layers at gas-solid interfaces. The results provide fundamental mechanistic insights about the surface reaction kinetics, charged species transports, as well as activities for oxidation and electrolysis on operating solid oxide electrochemical cells.
11:30 AM - C1.4
In Situ Study of Surface Chemistry and Crystallography of Highly Active Perovskite Cathodes near SOFC’s Operation Conditions.
Ethan Crumlin 1 , Eva Mutoro 1 , Zhi Liu 2 , Michael Grass 2 , Michael Biegalski 3 , Hans Christen 3 , Hendrik Bluhm 2 , Yang Shao-Horn 1
1 Electrochemical Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Center of Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThere is a large pressure and temperature gap between the ultra-high vacuum and room temperature conditions under which solid oxide fuel cell (SOFC) cathode materials are characterized typically, and SOFC operating conditions at high temperature (~500 – 1000 °C) and ambient pressure (~1 atm). Understanding how the physical and chemical properties of SOFC cathode materials change under operating conditions can provide insights into the mechanism of oxygen reduction reaction (ORR) and lead to material development strategies to improve cathode performance. Our recent work has shown that (001) epitaxial films of La0.8Sr0.2CoO3-δ (LSC)1 can exhibit enhanced surface oxygen exchange rates relative to LSC bulk up to 1 order of magnitude. In this work, we investigate the chemical and structural properties of bulk and (001) epitaxial films at near SOFC operating conditions to provide insights into the physical origin responsible for the observed enhancement.
In situ XRD was conducted to show that epitaxial LSC films were stable from 30 – 550°C at p(O2) of 1 atm, from which the film structural parameters were compared with those of bulk LSC.2 Using near ambient X-ray photoelectron spectroscopy (APXPS) at Lawrence Berkley National Laboratories Advanced Light Source (ALS) synchrotron facility,3 the chemical states and relative atomic concentrations for La, Sr, Co, O from LSC bulk pellet and epitaxial film samples were examined. In this presentation, we will discuss how the cations and oxygen species change as a function of temperature (30 – 520°C) and pressure (1×10-9 – 1×10-3 atm), from which a hypothesis is proposed for enhanced ORR activity on epitaxial LSC1 relative to bulk LSC.
[1] la O', G. J.; Sung-Jin, A.; Ethan, C.; Yuki, O.; Michael, D. B.; Hans, M. C.; Yang, S.-H. Angew. Chem. Int. Ed. 2010, 49, 3. [2] Mastin, J.; Einarsrud, M. A.; Grande, T. Chem. Mater. 2006, 18, 6047. [3] Zhang, C. J.; Grass, M. E.; McDaniel, A. H.; DeCaluwe, S. C.; El Gabaly, F.; Liu, Z.; McCarty, K. F.; Farrow, R. L.; Linne, M. A.; Hussain, Z.; Jackson, G. S.; Bluhm, H.; Eichhorn, B. W. Nature Mater. 2010, 9, 944.
11:45 AM - C1.5
In situ-Scanning Photoelectron Microscopy (SPEM) and µ-Spot XPS Study of Electrocatalytically Highly Active Lanthanum Strontium Cobaltite (LSC) Epitaxial Thin Films for SOFCs at Elevated Temperatures and Applied Biases.
Eva Mutoro 1 , Ethan Crumlin 2 , Hendrik Poepke 1 , Matteo Amati 3 , Majid Kazemian Abyaneh 3 , Michael Biegalski 4 , Hans Christen 4 , Donovan Leonard 4 , Albina Borisevich 4 , Bjoern Luerssen 2 , Marcus Rohnke 2 , Luca Gregoratti 3 , Juergen Janek 2 , Yang Shao-Horn 1
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , Justus-Liebig-University, Giessen Germany, 3 , Synchrotron ELETTRA, Trieste Italy, 4 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe design of high efficiency SOFC materials is limited by the lack of reliable information on elementary processes occurring under operating conditions. Developing in situ-characterization tools that can reveal the physical and chemical properties of electrode surfaces represents a key challenge to obtain fundamental understanding and insights into electrode reaction mechanisms and design of highly active electrode materials. As the surface composition of perovskite cathodes significantly impacts their oxygen reduction reaction (ORR) activity,[1,2] X-ray photoelectron spectroscopy (XPS) is a powerful tool for in situ-diagnostics. In this study, we utilize synchrotron-based scanning photoelectron microscopy (SPEM) and µ-spot XPS at high temperature and applied biases (ESCAmicroscopy beamline,[3] ELETTRA, Italy) to investigate epitaxial thin film lanthanum strontium cobaltite (LSC) cathodes with intrinsically high ORR activity.[1,2,4] Compositionally and structurally well-defined La0.8Sr0.2CoO3–δ (LSC) films have been prepared by pulsed laser deposition (PLD) on (001)-oriented yttria-stabilized zirconia (YSZ) single crystals with a gadolinium-doped ceria (GDC) interlayer and a porous Pt counter electrode.[4] In addition, we introduced surface decorations, a Sr-modification and LSC/(La0.5Sr0.5)2CoO4±δ hetero-structures, improving the oxygen surface exchange coefficient (kq) by 1 and 1-3 orders of magnitude, respectively. Ex situ-sample characterization includes time-of-flight secondary ion mass spectrometry (Tof-SIMS: compositional depth profiling), scanning electron and atomic force microscopy (SEM/AFM: surface morphology/roughness, film thickness), high resolution X-ray diffraction (XRD: phase purity, texture, strain), high resolution scanning transmission electron microscopy (STEM: atomic structure), and electrochemical impedance spectroscopy (EIS: surface exchange coefficient). Experimental challenges and results of the in situ-SPEM and µ-spot XPS investigations will be discussed with regard to general trends and temperature or bias induced cation rearrangements. Our results show how an in situ-approach leads to a better understanding of basic electrode processes, as needed to develop new strategies to improve SOFC cathode performance.References:[1] E. Mutoro, E.J. Crumlin, M.D. Biegalski, H.M. Christen, Y. Shao-Horn, Energy Environ. Sci. 2011, in press (doi:10.1039/C1EE01245B), [2] E.J. Crumlin, E. Mutoro, S.-J.Ahn, G.J. la O’, D.N. Leonard, A.Borisevich, M.D. Biegalski, H.M. Christen, Y. Shao-Horn, J. Phys. Chem. Lett., 2010, 1, 3149, [3] B. Luerssen, E. Mutoro, H. Fischer, S. Günther, R. Imbihl, J. Janek, Angew. Chem. Int. Ed. 45, 2006, 1473, [4] G.J. la O’, S.J. Ahn, E. Crumlin, Y. Orikasa, M.D. Biegalski, H.M. Christen, Y. Shao-Horn, Angew. Chem. Inter. Ed., 2010, 49, 5344-5347.
12:00 PM - C1.6
In Situ Characterization of δ-Bi2O3 Stabilized by Epitaxial Growth on Single Crystal Oxide Substrates.
Danielle Proffit 1 2 , Matthew Highland 1 , Guo-Ren Bai 1 , Stephan Hruszkewycz 1 , Pete Baldo 1 , Seong Keun Kim 1 , Chad Folkman 1 , Paul Fuoss 1 , Dillon Fong 1 , Thomas Mason 2 , Jeffrey Eastman 1
1 Materials Science Division, Argonne National Laboratory, Lemont, Illinois, United States, 2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractOxide ion conductors are critical components in many important energy conversion devices, including solid oxide fuel cells (SOFCs), oxygen separation membranes, and oxidation catalysts. In the search for the best electrolyte for these applications, the high temperature cubic phase of Bi2O3, δ-Bi2O3, has stood out as a unique oxide material. Although it has the largest ionic conductivity of any oxide, reaching 1 S/cm2 at 730°C, the use of this material has been limited by its small range of thermal stability (725-825°C). Previous work has shown that the delta phase can be stabilized to room temperature using dopants, but dopants are known to induce oxygen ordering and degrade the conductivity in this material. Using a different approach, we stabilized the delta phase via synthesis on (001) perovskite single crystal surfaces (SrTiO3 and DyScO3) by metal organic chemical vapor deposition, resulting in the growth of δ-Bi2O3 nanoislands. Synchrotron x-ray scattering observations at controlled temperatures and oxygen partial pressures reveal that the δ-Bi2O3 nanostructures are coherently strained to the substrates at room temperature, but have an unexpected superstructure that may arise due to ordering of the vacant oxygen sites. Annealing the nanostructures at 600°C causes gradual conversion of the (001) oriented delta phase to an unidentified strain-relaxed phase. To improve the continuity of the deposited layer, we have recently employed RF magnetron sputtering from a Bi2O3 target in an oxygen environment. Preliminary results indicate that sputtered islands grow epitaxially on (0001) α-Al2O3 with a different ordered structure than we observed in Bi2O3 grown on SrTiO3 or DyScO3. The potential for achieving high ionic conductivities at lower temperatures in δ-Bi2O3 films and for elucidating the origin of superionic conductivity in oxide materials in general will also be discussed.Use of the Advanced Photon Source was supported by the U. S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), under Contract DE-AC02-06CH11357 between UChicago Argonne LLC and the Department of Energy. T.O.M. acknowledges support of the DOE under Contract No. DE FG02-05ER-46255. Work by D.L.P. was supported in part by an award from the DOE Office of Science Graduate Fellowship Program. Work at Argonne was supported by the DOE, Basic Energy Sciences, Materials Sciences and Engineering Division.
12:15 PM - C1.7
In Situ Microstructural Investigations of SOFC Electrodes.
Paul Shearing 1 , Rob Bradley 3 , Dan Brett 1 , Jeff Gelb 4 , Arno Merkle 4 , Philip Withers 3 , Nigel Brandon 2 , Farid Tariq 1
1 , Imperial College, London United Kingdom, 3 , Univeristy of Manchester, Manchester United Kingdom, 4 , Xradia Inc, Pleasanton, California, United States, 2 , Univeristy College London, London United Kingdom
Show AbstractIn recent years, the development of high-resolution tomography has provided a platform to explore SOFC electrode microstructures in three-dimensions and with unprecedented detail. Electrodes for SOFC are characteristically complex, porous composite materials, which are expected to fulfill a range of criteria, namely: diffusion, catalysis, electrical and ionic conductivity and mechanical integrity. Historically, the relationship between microstructure and performance had been poorly understood – this has been compounded by microstructural evolution processes, which are known to cause change in the nano-scale architecture of the electrode during processing and long-term operation.Tomography techniques spanning a spectrum of spatial and temporal resolutions are now widely available to the materials engineer. At length scales of relevance for SOFC electrode characterisation, X-ray techniques are uniquely flexible, allowing the implementation of a variety of thermal and gas environments. Furthermore their non-destructive nature enables direct quantification of microstructural evolution processes not possible using destructive 3D imaging tools.These tomography techniques can be effectively utilized in tandem with relevant in-situ spectroscopy tools, including XANES, Raman and high temperature crystal microbalance sensors to explore the relationship between electrode microstructure and macroscopic performance.Here we present the development and implementation of a micro-furnace in-situ of a nano computed tomography system and its application to explore microstructural characterisation under pseudo operating and processing environments. We present developments in the design of a bespoke symmetrical cell geometry which will facilitate combined microstructural and electrochemical measurements in-situ of an X-ray microscope.
12:30 PM - C1.8
Hard X-Ray Fluorescence Measurements of Heteroepitaxial Solid Oxide Fuel Cell Cathode Material.
Jacob Davis 1 , Lincoln Miara 1 , Laxmikant Saraf 3 , Tiffany Kaspar 3 , Srikanth Gopalan 1 2 , Uday Pal 1 2 , Joseph Woicik 4 , Soumendra Basu 1 2 , Karl Ludwig 1 5
1 Division of Materials Science and Engineering, Boston University, Brookline, Massachusetts, United States, 3 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Department of Mechanical Engineering, Boston University, Boston, Massachusetts, United States, 4 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 5 Department of Physics, Boston University, Boston, Massachusetts, United States
Show AbstractUsing crystals grown at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL), synchrotron x-ray studies of surface structure were conducted at beamline X23A2 of the National Synchrotron Light Source at Brookhaven National Laboratory. EXAFS, XANES and TXRF experiments were performed on LSM. Experiments on LSCF thin films are planned for the near future and results will be presented. The LSM-20 samples studied were grown on YSZ and NGO (neodymium gallate). The films on YSZ have a fiber texture. LSM-20 on NGO is heteroepitaxial. The LSCF films were grown on LAO and YSZ with a GDC barrier layer.Fluorescence yield X-ray Absorption Fine Structure (EXAFS) was done for the as-deposited condition, at high temperature (800°C), and at room temperature after the anneal. Comparison of the bulk signal of the as-deposited films and the samples after annealing shows no significant changes.X-ray Absorption Near Edge Spectroscopy (XANES) was also used to examine films grown on both NGO and LAO. There are essentially no changes in the pre-edge behavior for Bulk LSM on either substrate. When examining the surface XANES spectra for these samples, some interesting things become apparent. The as-deposited surface and bulk signals are very similar. However, the main absorption peak broadens in the surface signal upon heating. This broadening remains after cooling to room temperature and is irreversible.Total Reflection X-ray Fluorescence (TXRF) data was taken as a function of angle. Probing with an incident angle larger than the critical angle θc, x-rays penetrate the entire sample and bulk properties are measured. Carefully controlling the incident beam, the sample is scanned through the critical angle. At low angle the beam is totally reflected, and only the topmost nanometers of the film fluoresce.Fluorescence signals from strontium, lanthanum and manganese were monitored. The signal was corrected for background and normalized to the bulk. Plotting the normalized ratio of strontium to the total A site (Sr/(Sr+La)) fluorescence and the ratio of the A site to B site ((Sr+La)/Mn) fluorescence the evolution of surface concentrations are examined at different temperatures. For LSM, the surface A site ratio remains very similar to the bulk ratio, and there is no change upon annealing. As-deposited LSM films grown on both substrates show a higher manganese concentration at the surface than in the bulk of the film. This manganese surface enrichment is enhanced when the heated to 800°C, and remains when cooled to room temperature.
C2: In Situ Studies of SOFCs II
Session Chairs
Monday PM, November 28, 2011
Republic A (Sheraton)
2:30 PM - **C2.1
In Situ Raman Characterisation of SOFC Anodes.
Robert Maher 1 , Gregory Offer 2 , Nigel Brandon 2 , Lesley Cohen 1
1 Physics Department, Imperial College London, London United Kingdom, 2 Department of Earth Science and Engineering, , Imperial College London, London United Kingdom
Show AbstractSolid oxide fuel cells (SOFCs)(1) have many advantages when compared to other fuel cell technologies, particularly for distributed stationary applications. As a consequence they are becoming ever more economically competitive with incumbent energy solutions. One of the primary advantages of the SOFC is the efficient generation of electrical and thermal energy using carbonaceous fuels such as natural gas without the need for expensive catalysts through high temperature operation. However, as with all technologies, improvements in durability, efficiency and cost is enabled through improved understanding of the material interactions which occur during operation. For example, carbon and sulphur poisoning of the anodes are two problems associated with SOFC operation on such fuels which can lead to decreased efficiencies and lifetimes which ultimately increase cost. The effects of these and other detrimental effects must be minimised in order to allow SOFCs to become fully competitive and a full scientific understanding of the processes involved is critical for this. Details of the chemical processes which lead to the formation of these deposits are limited by current indirect monitoring of fuel cell performance and ex-situ characterisation techniques such as SEM. In-situ characterization tools which provide the detailed chemical information necessary to fully address such issues are required in order to drive our understanding further. Raman spectroscopy is a non-invasive and non-destructive optical characterization tool which is ideally suited to the study of chemical processes occurring within operational SOFCs. The molecularly specific signals can be used to extract information on reaction kinetics, temperature distributions and material oxidation state in real time(2). Raman is particularly sensitive to carbon and can provide a great deal of information regarding the characteristics of carbon deposits. In this paper I will discuss the in-situ Raman investigation of SOFC anodes under operation using carbonaceous fuels enabled by a miniaturized SOFC with optical windows.(1) S. Singhal, K. Kendall, High-temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications, 1st ed.; (2) R. C. Maher, L. F. Cohen, P. Lohsoontorn, D. J. L. Brett, and N. P. Brandon, J. Phys. Chem. A., 2008, 112, 1497-1501.
3:00 PM - C2.2
In Situ Optical Study of Pr Doped Ceria: Nonstoichiometry and Kinetics.
Jae Jin Kim 1 , Sean Bishop 1 2 , Nicholas Thompson 1 , Harry Tuller 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 International Institute for Carbon Neutral Energy Research, Kyushu University, Nishi-ku, Fukuoka, Japan
Show AbstractMany oxide materials used in solid oxide fuel cells (SOFC) experience significant changes in oxygen stoichiometry during operation at high temperatures and under reducing/oxidizing conditions. These deviations from stoichiometry often result in major changes in electrical, diffusive, and mechanical properties, which, in turn, can negatively impact device performance. The ability to diagnose a material’s behavior under operating conditions is therefore of importance. Praseodymium doped cerium oxide (PrxCe1−xO2-δ: PCO) is unusual for a ceria based oxide given that it exhibits mixed ionic and electronic conducting (MIEC) characteristics at elevated pO2 (e.g. air) due to partial reduction of Pr to the trivalent state. In the oxidized state, Pr4+ empty energy states within the CeO2 band gap result in a red coloration of the sample, which becomes transparent upon reduction (Pr3+). The change in absorption spectra thus provides an in-situ, non contact method for monitoring the transient kinetics as well as equilibrium Pr oxidation state and in turn nonstoichiometry. For this work, PCO thin films were fabricated by pulsed laser deposition on optically polished substrates. Optical absorption spectra of the films were measured as a function of temperature, pO2, and time, allowing for investigation of nonstoichiometry/defect chemistry and the surface exchange coefficient for different Pr doping concentrations. In addition, the effect of precious metals on the surface exchange rate (e.g. gold) were also examined. These results are compared with data obtained independently from electrical conductivity, thermogravimetric, and dilatometric measurements.
3:15 PM - C2.3
High Water Pressure - High Temperature Autoclave for in situ Raman Study of Fuel Cell/Electrolyser Materials.
Aneta Slodczyk 1 , Oumaya Zaafrani 1 , Philippe Colomban 1
1 , LADIR CNRS UPMC, Paris France
Show AbstractAccording to the recent hydrogen and methanol economy the proton conducting materials appear very interesting as an electrolytic membrane and/or an electrode component of fuel cells, water steam electrolysers and CO2/syngas converters [1,2]. Prior to the long life time requirement their structural and mechanical behaviors as the function of operating conditions: high temperature and high water vapour pressure have to be well determined. Since direct, in situ measurements are extremely difficult when a high temperature is combined with a high water pressure, most of necessary characteristics is actually performed ex situ far from working conditions and/or post mortem. In order to guaranty the reliability of such ex situ experiments they should be validated by the comparison with the in situ ones. Raman scattering is an optical technique very efficient to detect both long and short range order structural modifications that can operate through optical windows using very long working distance optics. Consequently, we designed the autoclave working till 600°C and 50 bars of H2O pressure equipped with the sapphire window. It should be stressed that final device conception required many tests and improvements in order to eliminate the water condensation on inner surface of sapphire (cold) window as well as to avoid the heating of Raman optics. The actual design makes our autoclave compatible with the Raman spectrometers excited with a variety of laser wavelengths. The best results are obtained using an instrument offering high brightness and high detection. Various proton conducting materials have been successfully analysed [3,4]. The technical and scientific difficulties encountered during these studies are discussed. 1. G. Olah, Angew. Chem. Int. Ed. 2005 44 26362. Ph. Colomban Ed. Proton Conductors, Cambridge University Press, Cambridge (1992) 3. A. Slodczyk et al. J. Raman Spectrosc. 40 (2009) 513-5214. Ph. Colomban et al. J. Phys. Soc. Jpn. 79 Suppl. A (2010) 1-6
3:30 PM - **C2.4
Thermal Imaging of Humidified Alcohol-Based Fuels in Solid Oxide Fuel Cells.
Michael Pomfret 1 , Daniel Steinhurst 2 , Jeffrey Owrutsky 1
1 Chemistry Division, U.S. Naval Research Laboratory, Washington, District of Columbia, United States, 2 , Nova Research, Inc., Alexandria, Virginia, United States
Show AbstractNear-infrared (NIR) imaging has been developed as an effective in situ, real-time technique for monitoring surface temperature changes of solid oxide fuel cell (SOFC) anodes in order to characterize and understand fuel/catalyst interactions that are relevant to cell performance and degradation. The method yields spatially-resolved results of SOFC anode surface temperature variations during operation, which depend on fuel cracking, oxidation, and catalyst poisoning reactions. Current studies focus on optimizing materials and conditions for improved performance and durability by using carbon-based fuels with steam reforming in Ni/YSZ SOFCs. Specifically, ethanol stands out as a promising carbon-based fuel due to its availability from a variety of sources, existing distribution infrastructure, and potential for direct utilization in SOFCs. In this work, thermal imaging studies have been carried out for both dry and wet ethanol to characterize the effects of humidification. Steam affects reforming reactions in the gas phase and on the anode, especially regarding the propensity for carbon deposition and how it precedes anode degradation and failure. The studies are carried out for a range of temperatures and cell voltages to understand how the conditions affect the pertinent gas and anode reactions. Thermal imaging results demonstrate that humidification renders anodes less susceptible to failure from ethanol operation. The data show that operation with dry ethanol leads to sharp reductions in the anode surface temperature prior to anode deterioration in the 700-800 °C temperature range. Thermal gradients are observed in large, localized regions where carbon deposits form and where the anode ultimately degrades. The thermal imaging capability allows for real-time, in situ observation of the preliminary stages of anode structural failure. When the fuel stream is humidified (to a water:ethanol molar ratio of 1.125) and allowed to reform internally at 800 °C, the surface temperature changes are smaller than with dry ethanol, suggesting that deposit formation is greatly reduced by humidifying the ethanol feed. However, at temperatures between 700 and 750 °C, humidified ethanol flows form carbon deposits. These effects indicate that internal steam reforming does not occur at a sufficient rate at these temperatures. NIR imaging provides a method to identify thermal gradients on the anode and show that these regions are most susceptible to structural failure. Results from anode-supported cells operated with wet ethanol are compared with those from other fuels that form carbon deposits. Our imaging results show that internal steam reforming can be effective when the operating temperature is 800 °C or higher and that lower operating temperatures require additional reforming to avoid cell degradation.
4:30 PM - **C2.5
Probing SOFC Electrode Surfaces Using Raman Spectroscopy and Scanning Probe Microscopy.
Xiaxi Li 1 , Kevin Blinn 1 , Yingcui Fang 1 3 , Hyungmin Park 1 4 , Mahmoud Mahmoud 2 , Bottomley Lawrence 2 , Soojin Park 4 , Mostafa El-Sayed 2 , Meilin Liu 1
1 School of Materials Science and Engineering, Georgia Tech, Atlanta, Georgia, United States, 3 , Hefei University of Technology, Hefei China, 4 Interdisciplinary School of Green Energy, UNIST, Ulsan Korea (the Republic of), 2 School of Chemistry, Georgia Tech, Atlanta, Georgia, United States
Show AbstractProbing and mapping surface species (e.g, reaction intermediates) and recipient phases on electrode surfaces alongside electrochemical measurements are vital to gaining a fundamental understanding of the mechanisms of electrode reactions, which is imperative to achieving effective optimization of electrode performance or rational design of better electrode materials or structures. In this presentation, we will highlight our recent progress in application of surface enhanced Raman spectroscopy (SERS) and scanning probe microscopy (SPM) to SOFC electrode surfaces for probing carbon and sulfur-containing species relevant to coking and sulfur poisoning. In particular, various approaches have been used to create nano-structured Au and Ag particles on electrode surfaces and the enhancement factors are systematically studied under different conditions.
5:00 PM - C2.6
In Situ Observation of Reduction Zone in Ni-Loaded Pr-Doped Ceria Due to Hydrogen Spillover.
Vaneet Sharma 1 , Peter Crozier 1 , Renu Sharma 1 2 , James Adams 1
1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States, 2 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractRare-earth doped ceria (CeO2) is a potential candidate material for intermediate temperature (500 °C to 700 °C) solid-oxide-fuel-cell (IT-SOFC) anodes and electrolytes. Pr-doped ceria (PDC) is particularly promising, as it possesses an oxygen ion conductivity that is 2 to 3 order of magnitudes higher than that of pure ceria in this temperature range. SOFC anodes are usually produced by combining PDC with Ni metal. The Ni provides a conducting path for the transport of electrons generated at the triple-phase-boundaries (TPBs) to the outer circuit of the SOFC. The mixed-ionic and electronic conductivity (MIEC) exhibited by PDC can also be beneficial for improving the anode performance. This is a result of the Ni surface promoting the low-temperature dissociation of fuel molecules under the gaseous reducing conditions. The atomic hydrogen liberated during dissociation is adsorbed at the Ni surface and can then be transported to PDC via hydrogen spillover. Interactions between the gas phase, the metal and the oxide are of importance for understanding the nanoscale functionality of the SOFC anode. We have investigated the interfacial interactions in Ni loaded PDC nanoparticles under a hydrogen fuel gas atmosphere in situ using an environmental transmission electron microscope (ETEM). High-resolution images and electron energy-loss spectroscopy (EELS) obtained in this way allowed us to correlate the local chemical changes occurring in the PDC nanoparticles with hydrogen spillover from Ni. We find that the hydrogen spillover results in the formation of a reduction zone at the Ni/PDC interface at 420 °C. The spatial extent of the reduction zone was explained in terms of surface diffusion coefficients.
5:15 PM - C2.7
In Situ Nanostructural Characterization of Perovskite Cathodes.
Xueyan Du 1 , Zhiping Luo 2 , Jingbo Liu 1
1 Chemistry, Texas A&M University-Kingsville, Kingsville, Texas, United States, 2 Chemistry, Texas A&M University, College Station,, Texas, United States
Show AbstractSolid oxide fuel cells (SOFC) are drawing significant attention due to their advantages of high energy conversion efficiency, flexible fuel supplies and zero/near-zero emission of air pollutants. A critical challenge hindering SOFC technology is the requirement of the high operating temperatures. Therefore, development of cathode and electrolyte materials with long life-spans and high conductivities, allowing operation at intermediate temperature is becomes an important topic. This study presents the performance and nanostructural characterization of solid oxide fuel cells (SOFCs) half cell devices; operating at intermediate temperatures. The cathode is composed of lanthanum strontium cobalt iron oxide (SrxLa1-xCoyFe1-yO3, LSCF) and samarium doped ceria (SmxCe1-xO2) as electrolyte. The homogeneous and ultrafine cathodic nanoparticles with high specific surface area were prepared via a colloidal method followed by solid state chemistry. The SOFCs were then constructed by integrating nanoparticles into macro-devices. Nanocharacterization was implemented using high temperature (HT) transmission electron microscopy (TEM) and high temperature X-ray powder diffraction (XRD). TEM morphological analyses depict that the diameter of cathode materials is in the range of 20-50 nm, which results in larger surface area and allows the rapid gas diffusion and instantaneous chemisorption of O2. TEM morphology showed that highly crystalline and mono-dispersive LSCF nanoparticles with spherical shapes. The high temperature (HT) TEM was also used to effectively provide phase identification and transition measurement, suggesting that the phase transition was detected at 700 Celsius. As an informative complement to high temperature (HT) XRD was also employed to study the dynamic processes. In this study, the phase transition, crystallite growth and thermal expansion were specifically investigated. The parallel beam geometry was used to eliminate the systematic errors caused from the sample thermal expansion, surface displacement and deformation. The LSCF cathodes show good catalytic performances at the temperature range from 400 to 700 Celsius with the increments of 50 Celsius from the electrochemical impedance spectroscopic and cyclic voltammetry analyses, respectively. The mechanism study indicated that the mass transfer effect can be negligible due to the independence of the sweep rate and gas flow rates. It was also found that a small amount of platinum addition via post-synthesis impregnation into the cathode doubled the LSCF activity. In tandem with our previous study, the migration/deposition of Pt particles to the triple phase boundary can potentially catalyze the ORR. It is also interestingly found that Pt nanoparticles display the self-regeneration capability when DC polarization was applied at various cell operating temperatures.
5:30 PM - C2.8
In Situ Stress Analyses of Constrained Sintered Gadolinium-Doped Ceria Thick Films Containing Sintering Aids.
Jason Nicholas 1 , Brian Sheldon 2 , Sunil Mandowara 3
1 Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, United States, 2 School of Engineering, Brown University, Providence, Rhode Island, United States, 3 , Intel Corporation, Hillsboro, Oregon, United States
Show AbstractConstrained ceramic thick films (such as those utilized in multilayer capacitors, solid oxide fuel cells, piezoelectric actuators, micro-electronic packages, thermal barrier coatings, thick film integrated circuits, microfluidic devices, and sensors) are notoriously difficult to densify. This difficulty results from an in-plane tensile stress that develops as the film attempts to reduce its solid-vapor interfacial area while attached to an invariant substrate. Recently, we performed the first in situ constrained sintering stress analysis of Ce0.9Gd0.1O1.95 (CGO) solid oxide fuel cell electrolyte thick films [1]. Compared to the predictions of traditional sintering theory, these thick films displayed anomalously large (~160 MPa at 550°C), early-stage tensile stresses. These stresses were consistent with those expected from stage-zero inter-particle cohesive forces, and this has led us to hypothesize that grain-boundary extension via elastic bond stretching is the primary mechanism by which CGO particles eliminate their solid-vapor interfacial area during the early stages of constrained sintering. (While stage-zero cohesive forces have been observed between free-sintered particles [2], and have been identified as the primary stress generating mechanism during thin film island growth [3], their effect on constrained sintering has been underappreciated in the past).
The present work describes the in situ stress evolution of constrained Li0.03Ce0.873Gd0.097O1.9065 (Li-CGO), and Co0.03Ce0.873Gd0.097O1.9365 (Co-CGO) thick films. Even though lithium and cobalt reduce the 1400°C sintering temperature of free-sintered CGO to ~800°C and ~1000°C respectively [4], the sub-550°C stress evolution (including the anomalously-large, ~160 MPa tensile stress at 550°C) of constrained Li-CGO and Co-CGO films are identical to that of pure CGO. This suggests that similar early-stage cohesive forces/mechanisms are active for all three compositions. Upon further sintering to 1100°C, Li dramatically reduces the constrained film stresses and raises the sintered film relative density to 85 ± 5% (compared to 59 ± 5% for identically processed pure CGO films), while Co slightly reduces the constrained film stresses and raises the sintered film relative density to 64 ± 5%. These results, which are the first to quantify the effect of sintering aids on the stress evolution of densifying constrained films, demonstrate that in situ analyses can be used as a tool to better understand constrained film densification.
References:
[1] B.W. Sheldon, J.D. Nicholas, S. Mandowara, J. Am. Cer. Soc., 94 (2010) 209-216.
[2] K.E. Easterling, A.R. Tholen, Acta Metallurgica, 20 (1972) 1001.
[3] B.W. Sheldon, A. Rajamani, A. Bhandari, E. Chason, S.K. Hong, R. Beresford, J. Appl. Phys., 98 (2005) 043509[043501]-043509[043509].
[4] J.D. Nicholas, L.C. DeJonghe, Solid State Ionics, 178 (2007) 1187-1194.
5:45 PM - C2.9
High Temperature “Spectro-chrono-amperometry”: Quantitative Correlations between in Situ Raman Spectroscopy and Electrochemical Performance in Solid Oxide Fuel Cells.
John Kirtley 1 , David Halat 1 , Bryan Eigenbrodt 2 , Robert Walker 1
1 (1)Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, United States, 2 , Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, Ohio, United States
Show AbstractDue to their high operating temperatures and electrocatalytic electrodes, SOFCs are susceptible to carbon formation (or “coking”) when operating with hydrocarbon fuels. Carbon deposits can block catalytically active sites leading to losses in performance efficiency and eventual device failure. Quantifying the effects of carbon formation on SOFC behavior has been difficult due to challenges associated with making species-specific, in situ measurements in operating devices. Studies described in this talk used in situ vibrational Raman spectroscopy to monitor the kinetics of graphite growth and potential-driven graphite disappearance in SOFCs operating at 720°C with methane (CH4). After exposing Ni-YSZ cermet anodes held at open circuit voltage (OCV) to CH4, SOFCs were operated under galvanostatic conditions. At OCV vibrational Raman spectroscopy measured the kinetic growth of graphite as evidenced by a single vibrational feature at 1560 cm-1 assigned to highly ordered graphite. (The spectra showed little or no evidence of a feature at 1350 cm-1 that is often assigned to disordered graphite.) Graphite growth followed first order kinetics. After exposing the anode to a pre-determined amount of CH4, the fuel flow was replaced with Ar and the cell was polarized to provide constant current. Cell voltage remained constant as the vibrational intensity of the graphite feature diminished. Experiments showed a steep jump in the cell voltage needed to maintain constant current when the graphite signal in Raman spectra disappeared. Continued operation led to a slowly rising voltage that coincided with observable growth in a vibrational feature at 1100 cm-1 assigned to nickel oxide. By measuring the total current required to reach the “equivalence point” where the spectroscopic graphite signal disappears and the cell voltage rises steeply, we can quantify the total amount of graphite deposited on the SOFC anode.
C3: Poster Session
Session Chairs
Tuesday AM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - C3.1
Optimization of Micro-SOFC Performance through the Control of the Architecture Using a Low Cost Process.
Guillaume Muller 1 3 , Gianguido Baldinozzi 2 , Rose-Noelle Vanier 4 , Armelle Ringuede 3 , Christel Laberty-Robert 1 , Clement Sanchez 1
1 LCMCP, University of Pierre et Marie Curie, Paris France, 3 LECIME, UMR 7575 CNRS, Chimie ParisTech, Paris France, 2 Materiaux fonctionnels pour l’energie, CEA-CNRS-Ecole Centrale Paris, Paris France, 4 UCCS, UMR 8181, University of Sciences and Technologies of Lille, Paris France
Show AbstractMicro-SOFCs (μSOFCs) have been attracting much attention because of reducing operating temperature, their potential as long lasting, rapid re-charging, lightweight portable power sources. They have been demonstrated to exhibit good power density at temperatures as low as 600°C. To be cost competitive and relevant for widespread use, it is crucial to explore the synthesis of thin film via soft-chemistry less expensive than physical ones (PLD, ALD...). Combining dip-coating and sol-gel processes, nanometer scale 10%gadolinium doped ceria (GDC, electrolyte), LSCF (cathode) and 10%GDC-nickel based composite nanoarchitectures films were deposited on a Si wafer and on porous Al2O3 substrates in one-step thermal processing. For the electrodes, the 3-D network is constituted of non-agglomerates nanoparticles of Gd-doped ceria and NiO or LSCF. In this arrangement, particles in the nanoscale are kept because of the presence of secondary phases (NiO, LSCF) and pores. The different interfaces in these hierarchical-porous composites tune the particles size as well as the surface energy. The study of the microstrain through XRD analyses shows that the domain of stability of the studied microstructure is large (200°C) for NiO/GDC cermet. Accordingly, the effect of the microstructure on the mixed ionic conductivities in this range of temperature is low, because their microstructure is stable. These composites can be used as model for understanding the impact of the size of the particle on the transport of ions and electrons. Furthermore, the reduction of the NiO/GDC electrodes were studied through various techniques: XRD, ac-impedance. The microstructures were stable up to temperatures of 600°C. The electrochemical performances I-V curves of these different thin layers were measured in a single gas atmosphere setup. The electrochemical results will be discussed as function of the cathode and anode composition and the microstructure (the temperature and the reducing treatments). Finally, these different hierarchical-porous thin films are favorable for efficient composite electrode and electrolyte for micro-SOFC application.1. A. Evans et al., Journal of Power Sources, 2009, 194, 119–129.2. J. Hierso, O. Sel, A. Ringuede, C. Laberty-Robert, L Bianchi, D. Grosso, and C. Sanchez, Chem. Mater. 2009, 21, 2184–21923. G. Muller, G. Baldinozzi, C. Laberty-Robert, C. Sanchez, Phys. Rev. Lett., 2011, summited
9:00 PM - C3.2
Development of In-situ Observation Technology for Evaluation of SOFC Anode Materials by Environmental Transmission Electron Microscope.
Toshiaki Yamaguchi 1 , Hirofumi Sumi 1 , Koichi Hamamoto 1 , Toshio Suzuki 1 , Yoshinobu Fujishiro 1 , Tomoharu Tokunaga 2 , Katsuhiro Sasaki 2
1 , National Institute of Advanced Industrial Science and Technology (AIST), Nagoya Japan, 2 , Nagoya University, Nagoya Japan
Show AbstractA solid oxide fuel cell (SOFC) is known as one of highly-effective power generating systems, and recently commercialization has just started for application to 1kW-level home power supply. In addition, development to realize power systems for automobile and portable generator is also conducted energetically around the world.One of advantages of the SOFC is that it can use various hydrocarbon gases as a fuel, in addition to hydrogen; however carbon deposition problem at the SOFC anode during the operation has been observed mainly after the electrochemical measurements, using a scanning electron microscope (SEM) and Raman scattering spectroscopy etc., and yet the mechanism of the carbon deposition is still not well-understood. Thus, the development of novel observation technology is necessary to evaluate the phenomena at the SOFC anode directly during the hydrocarbon-supplied operationIn this study, we evaluate a micro-tubular SOFC electrochemically under humidified hydrogen and methane fuels at around 500 - 700C. Then, the anode of the cell is observed by using SEM to refer microstructural changes. As for the in-situ observation method using environmental transmission electron microscope (E-TEM), a piece of the anode layer of the cell is set on a special sample holder of the E-TEM, then the sample is heated-up to a predetermined temperature by a heater element at the sample holder with a flow of some gases.
9:00 PM - C3.3
Atomistic Modeling of Cation and Anion Migration in Yttria-Stabilized Zirconia.
Shotaro Hara 1 , Hiroaki Kimura 1 , Satoshi Izumi 1 , Shinsuke Sakai 1
1 , The University of Tokyo, Toyko Japan
Show AbstractLong-term degeneration processes in solid oxide fuel cell (SOFC), such as a dopant segregation, a creep deformation and the formation of a secondary phase at cathode/electrolyte interface are mainly governed by the cation diffusion. However, the cation diffusion is a considerably slow process compared with the oxygen diffusion, such that a molecular dynamics (MD) is not generally suitable to predict the rate for the cation migration due to its limited time scale. To overcome this issue, we investigate the vacancy-mediated cation migration in YSZ by utilizing an adaptive bond-boost method for safely accelerating atomistic simulations. Using this scheme, we directly access the finite-temperature dynamical process of Y and Zr migration over timescale comparable to laboratory experiments. Our simulations successfully provide an activation enthalpy of 4.8 eV for Y migration, which is in good agreement with an experimental result. Furthermore, temperature-dependent activation free energy is computed in the typical experimental temperature range around 1500K. We have estimated the activation entropy of about 8 kB, indicating that an entropic term of activation energy is not negligible for the accurate prediction of the migration rate. The effect of the dopant concentration on migration rate is also discussed.
9:00 PM - C3.4
Phase-Field Modeling of Three-Phase SOFC Electrode Microstructures.
Qun Li 1 2 , Liangjun Li 1 2 , Longqing Chen 1 2
1 , US DOE-National Energy Technology Laboratory, Morgantown, West Virginia, United States, 2 Material Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractThe electrode microstructures of solid oxide fuel cells (SOFC) are known to have a profound effect on the cell performances. On the other hand, the phase field method is ideally suited for modeling transport phenomena and complex microstructural evolution. In this work, a phase field model for describing three-phase electrode microstructure (i.e., electrode-phase, electrolyte-phase and pore-phase) in SOFC is proposed by the diffuse-interface theory. Conserved composition and non-conserved grain orientation order parameters are simultaneously used to describe the coupled phase coarsening and grain growth in three-phase electrode. The microstructural evolution simulated by the phase field approach demonstrates the significant dependence of morphological microstructure and the output statistic material features on the prescribed kinetic parameters and the three-phase volume fractions. The triple phase boundary (TPB) fraction with the evolution of microstructure are also presented, which is found to have a major degradation in the early evolution. The coupling of phase-field equations for evolving the microstructures and the transport equations through an inhomogeneous microstructure will be discussed. This work can contribute to the microstructural optimization of SOFC performances and to the understanding of long-term baseline degradation.
9:00 PM - C3.5
In Situ Synchrotron X-Ray Studies of the Surface Structure and Composition of La0.6Sr0.4Co0.2Fe0.8O3-δ Thin Films.
Edith Perret 1 , Tim Fister 2 , Hua Zhou 2 , Jeffrey Eastman 1 , Peter Baldo 1 , Brian Ingram 2 , Paul Fuoss 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractLa0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskites are promising mixed-conducting cathodes and oxygen separation materials for SOFCs due to their thermal stability and catalytic activity. Previous studies have shown that oxygen reduction at the cathode is a rate-limiting step in the performance of SOFCs. The determination of structural and compositional surface changes induced by high temperature conditions of working SOFCs is therefore important for the development of high performance cathode materials. Synchrotron x-ray diffraction is an effective tool to study cathode mechanisms and their relation to material properties and microstructures. In the present work, an epitaxial LSCF film (~5 nm) grown on TiO2-terminated Nb-doped SrTiO3 (001) was studied by synchrotron surface x-ray diffraction (SXRD) and x-ray reflectivity at room temperature, 500 °C and 750 °C in a He/O2 mixture with a pO2 of 150 Torr. Specular x-ray reflectivity data was analyzed using Coherent Bragg Rod Analysis (COBRA) and difference map (DM) algorithms. We find that strontium segregates to the surface at all temperatures. A (3x3) surface reconstruction is observed at 500 °C and 750 °C. The interplay between the changes in surface composition and structure of LSCF and its implication on oxygen reduction in SOFCs will be discussed.JAE and PHF are supported by the U. S. Department of Energy (DOE), Basic Energy Sciences, Materials Sciences and Engineering Division. Work by PHF, BJI and EP is partially supported by the DOE Solid State Energy Conversion Alliance (SECA). Use of the APS was supported by the DOE, Office of Science, Office of Basic Energy Sciences (BES), under Contract DE-AC02-06CH11357 between UChicago Argonne LLC and the Department of Energy."
9:00 PM - C3.6
Development of a Technique for In Situ High Temperature TEM Observation of Catalyst in Moisturized Air Atmosphere.
Toshie Yaguchi 1 , Takashi Kanemura 2 , Takahiro Shimizu 3 , Daichi Imamura 3 , Akira Watabe 1 , Takeo Kamino 1
1 , Hitachi High-Technologies Corp., Hitachinaka Japan, 2 , Hitachi High-Tech Manufacturing & Service Corp., Hitachinaka Japan, 3 , Japan Automobile Research Institute, Tsukuba Japan
Show AbstractTo clarify the influence of moisture to the structural changes of heated nano-materials, in-situ high temperature transmission electron microscopy (TEM) has been carried out using a conventional analytical TEM combined with a gas injection-specimen heating holder [1]. The instrument used for the experiment is a Hitachi H-9500 high resolution 300kV analytical TEM equipped with an AMT high resolution CCD camera system [2]. A newly developed moisturized air supply unit connected to a specimen heating holder which is inserted to a specimen stage of the TEM. The unit consists of a humidifier, a duct hose and a thermo hygrometer. In this method, the air from the humidifier is introduced into a specimen chamber of the microscope via gas injection nozzle of the specimen heating holder. Flow rate of the air to be introduced to the specimen chamber is controlled by a needle valve of a gas injection unit of the specimen heating holder. The moisture and the temperature of the air from the humidifier is measured in the vicinity of the needle valve of the specimen heating holder continuously during the in-situ observation, and the measured values are stored in a data logger automatically. A platinum catalyst dispersed on carbon black (Pt/CB) catalyst was used as the specimen. The specimen was wetted by ethanol and mounted on a heating element of the specimen heating holder directly using a small painting brush. Air of high moisture content of 94 %RH was directly injected to the heated Pt/CB catalyst and the morphological changes of the catalyst were observed at high magnification dynamically. TEM images were observed in the air atmosphere of 94%RH at 0.2Pa. Morphological changes of both Pt particles and CB supports at 300 oC are clearly demonstrated. In this condition, contrary to our expectation, active movement of the Pt particles on the CB supports followed by the growth of the Pt particles was observed. To investigate the influence of the humidity to the behavior of the Pt particles, in-situ observation using a dry air was also carried out and we obtained useful information for discussing the catalyst degradation mechanism. AcknowledgementsThis work was partly supported by Grant-in-Aid for Scientific Research (C) 23510136 from the Japan Society for the Promotion of Science (JSPS).References[1] T.Kamino, et al.,Journal of Electron Microscopy 54(6) (2005) 497-503.[2] T.Yaguchi et al.,Proc. Microsc. Microanal. 16 (Suppl .2) (2010) 302-303.
Symposium Organizers
Rob Walker Montana State University
Nigel Brandon Imperial College London
Jeff Owrutsky Naval Research Laboratory
Koichi Eguchi Kyoto University
C4: SOFC Cathodes
Session Chairs
Tuesday AM, November 29, 2011
Republic A (Sheraton)
9:30 AM - C4.1
Thermomechanical Behavior of SOFC Materials.
Haijoon Lee Lee 1 , Elizabeth Kupp 1 , Gary Messing 1
1 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractMany energy systems such as SOFCs and TBCs are processed from, and composed of, layered ceramic components supported on a rigid substrate. These systems are processed at and/or operate at a range of thermal cycles and transients. The reliability and costs of the energy system depends on the ability to predict what conditions lead to failure-causing stresses that arise during operation and processing. Materials can range from completely elastic (E) at room temperature to viscous (η) (i.e., exhibiting time and stress dependent deformation) at high temperature. The intermediate viscoelastic behavior is typical of polycrystalline ceramics and depends on the material, porosity and grain size. By measuring the viscoelastic properties (E, η) of the individual components as a function of temperature we can model the stress states in the energy system during processing and operation. With this knowledge we can understand how to adjust materials, processing and operation conditions to avoid failure. We have developed testing techniques, such as cyclic loading dilatometry (CLD) and creep beam bending (CBB), to measure the thermomechanical properties of individual ceramic components as a function of relative density and microstructure (i.e. porosity and grain size) up to typical processing and operation temperatures. SOFC cathode materials such as La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), [50 wt%(La0.6Sr0.4Co0.2Fe0.8O3)-50 wt%(Ce0.9Gd0.1O2) (LSCF-GDC), La0.8Sr0.2MnO3 (LSM), 50 wt%(La0.8Sr0.2MnO3)-50wt%YSZ (LSM-YSZ), and 8 mol% yttria doped ZrO2 (YSZ)] were analyzed during this study. Samples were prepared by tape casting, which allows preparation of monolithic parts as well as laminar composites that mimic SOFC architectures. Material sintering behaviors (e.g., shrinkage, warpage, densification) and thermomechanical responses (e.g., viscosity, Poisson’s ratio) were determined using CLD and CBB on both individual materials and laminar composites. These behaviors were observed at processing and operation conditions.A thorough understanding of the viscoelastic behavior of these systems over the range of processing and operational temperatures will enable prediction of the mechanical and microstructural stability of SOFCs and the accompanying cell efficiency and lifetime.
9:45 AM - C4.2
Oxygen Bubble Nucleation in Solid Oxide Electrolysis Cells.
O. Comets 1 , Peter Voorhees 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractRunning a Solid Oxide Electrolysis Cell (SOEC) above a critical current can degrade the cell very quickly. Oxygen bubble formation has been observed in the electrolyte near the oxygen electrode, and seems to be a source of this degradation. Applying a load on a SOEC shifts the oxygen potential away from its equilibrium value, driving oxygen ions from the hydrogen to the oxygen electrode. This, along with the applied current in the presence of a finite –however small– electronic conductivity in the electrolyte, is responsible for high oxygen potential under the oxygen electrode. The resulting oxygen pressure in the electrolyte can be large, in certain cases much higher than 1 atm. Furthermore, these high pressures inside the bubble introduce significant stress in the oxide. We address the problem of homogeneous nucleation of bubbles containing oxygen in the electrolyte under conditions of constant current flow. We determine the radius of the critical nucleus and the reversible work for the formation of the critical nucleus. We include the effects of the stress generated by both the oxygen pressure and the surface stress of the oxide-vapor surface, as well as the effects of the applied current density and surface energy. The implications of these results for homogeneous nucleation of bubbles in the oxide of a SOEC will be discussed.
10:00 AM - **C4.3
Towards a Fundamental Understanding of the Oxygen Reduction Mechanism.
Eric Wachsman 1
1 , University of Maryland, College Park, Maryland, United States
Show AbstractThe 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 (in terms of tortuosity and porosity for gas diffusion, solid-phase surface area for gas adsorption/surface diffusion, and triple phase boundaries for the charge transfer reaction) and heterogeneous catalysis techniques (temperature programmed desorption and reaction) and O-isotope exchange to evaluate the O2 reduction mechanism at the gas-solid reaction interface. These results are then combined (and contrasted) with the more conventional electrochemical polarization techniques (impedance spectroscopy and I-V behavior) to try and elucidate each of the mechanisms as a function of composition and microstructure. The progress to date is summarized.
11:30 AM - **C4.5
Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration.
Raymond Gorte 1 , Rainer Kungas 1 , John Vohs 1
1 Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractA detailed understanding of the cathode-electrolyte interface is very important for designing better and more durable fuel cells. Because the structure of electrodes prepared by infiltration of a conductive oxide into a porous scaffold of the electrolyte is relatively simple and easy to control, at least compared to conventional electrodes, we have conducted an extensive investigation into how electrode performance is affected by composition, structure, and pretreatment conditions. The effects of scaffold structure and ionic conductivity, of the conducting perovskite structure and conductivity, and of catalyst addition were all studied and compared to predictions of a mathematical model of the electrodes.
12:00 PM - C4.6
Electronic Structure and Oxygen Reduction Activity at the Heterointerfaces of (La,Sr)CoO3/(La,Sr)2CoO4 Multilayers.
Zhuhua Cai 1 , Yener Kuru 1 2 , Harry Tuller 2 , Bilge Yildiz 1
1 Department of Nuclear Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractStrontium-doped lanthanum cobaltite and related materials are promising cathode candidates for intermediate temperature solid oxide fuel cells.[1] Recently, the hetero-interfaces between the perovskite (La,Sr)CoO3 (LSC113) and the Ruddlesden-Popper (La,Sr)2CoO4 (LSC214) phases have shown highly enhanced oxygen surface exchange kinetics.[2,3,4] Although these recent results indicate the interfacial regions are likely responsible for the enhanced ORR kinetics, the origin of this enhancement at the LSC113/214 interface region is still unknown. Possible hypotheses for the enhanced oxygen exchange rates at/near this hetero-interface include lattice strain influencing oxygen defect formation,[5] dopant cation (Sr) enrichment, and interface electronic band structure changes favoring oxygen reduction. In this work, our aim is to probe the local oxygen reduction activity on the basis of the local electronic structure of this interface, by integrating the use of advanced in situ scanning probe techniques with focused ion beam milling to reach the interfaces in a systematic and controlled manner. For this purpose, high quality multilayers of LSC113 and LSC214 with 20 nm modulation length are formed by pulsed laser deposition on single crystal SrTiO3 (001) substrates, with a resulting crystallographic relationship of (00l)LSC214 //(00l)pcLSC113//(00l) STO (pc: pseudocubic). The buried interfaces are then exposed to the ambient by glancing angle focused ion-beam milling[6] in a geometry feasible for scanning probe measurements. The electronic structure of the exposed interfaces at the surface is then interrogated by in situ scanning tunneling microscopy and spectroscopy (STM, STS) from ambient to high temperature conditions in oxygen. The differences between the “true topography” probed by AFM and the “electronic topography” probed by STM/STS scans across the multilayers directly show that the electronic structure of the hetero-interfaces are modified compared to the bulk properties of the layers. Correlations of the local electronic structure near the LSC113/214 interface with the enhanced ORR kinetics are discussed.1.Adler, S.B. Chem. Rev. 2004, 104.2.Sase, M. et al., Solid State Ionics 2008, 178. 3.Yashiro, K. et al., J. Electrochem. Solid State Lett. 2009, 12.4.Crumlin, E.J. et al, J. Phys. Chem. Lett. 2010, 1.5.Kushima, A. et al., Phys. Rev. B 2010, 82.6.Kuru, Y., et al. submitted 2011.
12:15 PM - C4.7
Potential Dependent Cation Segregation and B-Site Surface Oxidation State in Model Thin Film Perovskite Cathodes.
Kee-Chul Chang 1 , Brian Ingram 2 , Lu Yan 3 , Paul Salvador 3 , Hoydoo You 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractWe used pulsed laser deposited films of La0.6Sr0.4Fe0.8Co0.2O3 (LSCF) and La0.8Sr0.2MnO3 (LSM) on single crystal yttria-stabilized zirconia (YSZ) substrates as model cathodes to better understand the surface processes under solid oxide fuel cell operating conditions. The thin film on YSZ system was run in a half cell configuration between 700°C and 800°C under +1 to -1V of applied potential. We used a combination of X-ray florescence and spectroscopy under total reflection to achieve surface sensitivity in our measurements. Ex situ characterization of samples under ~72 hours of -1V of applied cathodic potential at 700°C showed B-site (Mn in LSM and Co in LSCF) surface segregation with a change in surface B-site oxidation state compared with the bulk. We will further discuss in situ LSCF experiments where we were able to observe the segregation/desegregation of the Co compared with Fe and the accompanying Co oxidation state changes under cathodic/anodic potential.
12:30 PM - C4.8
Spatially Resolved Mapping of Oxygen Reduction/Evolution Reaction on Solid-Oxide Fuel Cell Cathodes with Sub-10 nm Resolution.
Amit Kumar 1 , Donovan Leonard 1 , Stephen Jesse 1 , Mike Biegalski 1 , Hans Christen 1 , Eva Mutoro 2 , Ethan Crumlin 2 , Yang Shao-Horn 2 , Albina Borisevich 1 , Sergei Kalinin 1
1 CNMS, Oak ridge National laboratory, Oak ridge, Tennessee, United States, 2 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Tennessee, United States
Show AbstractSpatial localization of the oxygen reduction/evolution reactions (ORR/OER) on the lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied on the sub-10 nanometer level. Comparison between ESM and structural imaging by scanning transmission electron microscopy (STEM) suggest that the regions of enhanced electrochemical activity are associated with small-angle grain boundaries that act as diffusion pathways for oxygen vacancies. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single structural defect.Acknowledgement :This research was conducted in part at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy.
12:45 PM - C4.9
Static and Dynamic Observations of Bias Induced Effects in Thin Film SOFC Cathode Materials.
Donovan Leonard 1 , Amit Kumar 1 , Stephen Jesse 1 , Sergei Kalinin 1 , Eva Mutoro 2 , Ethan Crumlin 2 , Yang Shao-Horn 2 , Michael Biegalski 1 , Hans Christen 1 , Stephen Pennycook 1 , Albina Borisevich 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMulti-length scale, in situ characterization of thin film SOFC cathodes is necessary to understand structural and chemical parameters responsible for increased ORR activity of Lanthanum Strontium Cobaltite (LSC) cathode heterostructures [1,2]. Observations of structure-property relationships, under conditions approaching operating parameters of SOFC devices, at the resolution from nanometer all the way down to the single defect level, are needed for insight into mechanisms responsible for improved power generation efficiency.Here, we use Scanning Transmission Electron Microscopy (STEM) high angle annular dark field (HAADF) imaging to observe local oxygen stoichiometry and vacancy ordering with atomic resolution. We further explore the effect of local voltage stress in Electrochemical Strain Microscopy on cobaltite causing vacancy injection, ordering and amorphization. For that we use (a) ex situ approach of site specific focused ion beam (FIB) micro-sampling of locally-cycled regions and (b) in situ STM/STEM, where the bias is applied to a sample inside an electron microscope column.ESM studies have revealed both reversible and irreversible effects resulting from bias cycling. Reversible effects included chemical expansion of nanoscale regions of the cathode surface, while irreversible effects included surface roughening. From ex situ STEM data, surface roughening could be attributed to bias-induced amorphization. Changes in oxygen vacancy ordering and elemental distribution in bias-cycled samples were also examined by STEM and electron energy loss spectroscopy (EELS). In situ STM/STEM biasing of LSC samples demonstrated that bias cycling, as opposed to a specific static bias value, is responsible for surface roughening and amorphous layer formation similar to what was induced by ESM. EELS response of the in situ biased samples will also be discussed.Research sponsored by the DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Research supported in part by ORNL’s Shared Research Equipment (SHaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.[1] G. Jose la O’, et al., Angew. Chem. Int. Ed., 49, (2010).[2] E.J. Crumlin, et al., J. Phys. Chem. Lett., 1, 3149 (2010).[3] N. Balke et al., Nature Nanotech., 5, 749 (2010).
C5/B8: Joint Session: SOFC Materials Characterization I
Session Chairs
Juergen Fleig
Harry Tuller
Tuesday PM, November 29, 2011
Constitution B (Sheraton)
2:30 PM - **C5.1/B8.1
FIB/SEM Reconstruction and 3D Simulations of SOFC Electrodes.
Ellen Ivers-Tiffee 1 3 , Jochen Joos 1 , Thomas Carraro 2 , Moses Ender 1
1 Institut für Werkstoffe der Elektrotechnik (IWE), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany, 3 DFG Center for Functional Nanostructures (CFN), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany, 2 Institut für Angewandte Mathematik (IAM), Universität Heidelberg, Heidelberg Germany
Show AbstractThe microstructures of high performance La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Ni/8YSZ electrodes are reconstructed using a dual-beam focused ion beam / scanning electron microscopy (FIB/SEM) system. This method has already proven potential for the detailed analysis of microstructures [1-2]. The corrected reconstruction data are the basis for the calculation of the microstructure parameters (i) volume-specific surface area of the individual phases (ii) volume/porosity fractions, (iii) tortuosity and in case of composite electrodes (iv) triple phase boundary density. These parameters constitute the basis to calculate electrode performance via simplified microstructure models and the accurate determination of these parameters will be shown. However, it would be desirable to fit the reconstructed microstructure directly into a more accurate 3D-model. Therefore a 3D finite element method model was developed, which enables a direct use of the corrected 3D FIB/SEM data. The model is able to calculate (among other things) the spatial distribution of oxygen concentration and the area specific resistance of the electrode. First results of this model, including a parameter sensitivity analysis, are presented.[1] J. R. Wilson, W. Kobsiriphat, R. Mendoza, H. Y. Chen, J. M. Hiller, D. J. Miller, K. Thornton, P. W. Voorhees, S. B. Adler, and S. A. Barnett, nature materials, 5 (7), p. 541 (2006).[2] J. Joos, T. Carraro, A. Weber, and E. Ivers-Tiffée, J. Power Sources, 196, (17), p. 7302 (2010).
3:00 PM - C5.2/B8.2
Imaging Analysis of Aggregation Behavior in Reduced NiO-YSZ.
Laxmikant Saraf 1 , David King 1 , Chongmin Wang 1 , A. Lea 1 , Zihua Zhu 1 , James Strohm 1 , Donald Baer 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractExploring overlooked properties of traditional materials used for solid oxide fuel cell (SOFC) is as important as the development of new materials to improve its ion transport at the intermediate temperature range. NiO-YSZ has been extensively studied as a SOFC anode due to high catalytic activity and affordability NiO to steam reform hydrocarbons for hydrogen generation in anode supported and direct hydrocarbon fuel feed SOFCs. In this study, we explore high resolution imaging analysis of reduced NiO-YSZ to establish its direct correlation with energy dispersive X-ray spectroscopy (EDS) chemical mapping. The correlation among NiKα maps with scanning transmission electron microscopy (STEM) indicate Ni/NiO aggregation behavior in extensively reduced NiO-YSZ. Widely known reduction reaction of NiO into Ni and stability of YSZ in reducing conditions can also be visualized by correlating ZrKα, NiKα and OKα maps. The nickel mobility on the widely varying bulk and intra-particle length scales is discussed in the context of reducing conditions. The aggregation behavior of Ni/NiO in NiO-YSZ is compared with growth and aggregation of pure fluorite structure materials frequently used in SOFCs to discuss the impact of reducing conditions and temperature on grain growth and transport properties.
3:15 PM - **C5.3/B8.3
Microstructural and Electrochemical Studies of Solid Oxide Fuel Cell Durability.
Scott Barnett 1
1 Materials Science, Northwestern Univ, Evanston, Illinois, United States
Show AbstractThis talk will describe accelerated testing of solid oxide fuel cell (SOFC) durability combining 2D and 3D imaging with detailed electrochemical characterization. The aim of accelerated testing is to predict performance over ~40,000 h lifetimes, but meaningful extrapolation of the data requires accurate mechanistic models of degradation mechanisms; quantitative imaging is an aid to developing such models. A few examples of degradation studies will be discussed. First, degradation of cathodes consisting of Gd-doped Ceria infiltrated with (La,Sr)(Fe,Co)O3 is described, and a model based on coarsening of LSCF nano-particles is used to explain the data. Second, 3D FIB-SEM studies are presented showing structural and electrochemical changes in Ni-YSZ anode active layers upon extended annealing, including a surprising effect where pores become increasingly isolated. Third, structural and electrochemical changes in LSM-YSZ electrodes during cyclic operation, between fuel cell cathode mode and electrolyzer anode mode, will be discussed.
3:45 PM - C5.4/B8.4
Correlating Nanostructure and Ion Conductivity in Gadolinium and Praseodymium Doped and Co-Doped Cerias for Solid Oxide Fuel Cell Electrolytes.
William Bowman 1 2 , Albert Talin 2 , Renu Sharma 2 , Vaneet Sharma 1 , Peter Crozier 1
1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States, 2 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractRare-earth doped ceria (CeO2) is a potential candidate material for intermediate temperature (500 °C to 700 °C) solid-oxide-fuel-cell (IT-SOFC) electrolytes. Ceria doped with aliovalent dopants has been found to possess high oxygen-ion conductivity in this temperature range. Understanding the relationship between nanostructure and ionic conductivity is essential to provide fundamental knowledge of the atomic-level migration mechanisms of oxygen ions in these electrolyte materials to allow for their optimization. Here we investigate the variation in ion conductivity with dopant concentration for a series of Pr-doped, Gd-doped and Pr/Gd-co-doped materials synthesized using a spray-drying technique. Ionic conductivity measurements are made using impedance spectroscopy and will be presented for these samples as well as pure ceria specimens. Grain size distribution and composition is expected to affect the results and we present data on these parameters for our samples, determined using scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX). Similarly, the segregation of dopant atoms to grain boundaries is thought to have a significant effect on oxide ion transport through doped ceria. We discuss the correlations between the nanostructure compositional heterogeneity – measured using high spatial resolution electron energy-loss spectroscopy in a scanning transmission electron microscope – and ionic conductivity. We compare our results with those generated by Dholabhai et al [1,2], who have used density functional theory (DFT) and a kinetic-lattice Monte Carlo (KLMC) model to investigate the variation in ion conductivity with dopant concentrations for single crystal binary and tertiary oxides.References:[1] P. P. Dholabhai, J. B. Adams, P. Crozier, R. Sharma, "Oxygen vacancy migration in ceria and Pr-doped ceria: A DFT+U study," The Journal of Chemical Physics, vol. 132, Mar. 2010. [2]P. P. Dholabhai, J. B. Adams, P. Crozier, R. Sharma, "A density functional study of defect migration in gadolinium doped ceria," Physical Chemistry Chemical Physics, no. 12, May 2010; pp. 7904-7910.
4:30 PM - **C5.5/B8.5
Compositional Studies of SOFC Cathode Surfaces by Low Energy Ion Scattering (LEIS).
John Kilner 1 2 , Monica Burriel 1
1 Materials, Imperial College,London, London United Kingdom, 2 I2CNER, Kyushu University, Kyushu Japan
Show AbstractOne of the major obstacles to the optimization of electrodes for high temperature electrochemical devices, such as the cathode for the SOFC or the anode for the SOEC, is the current rudimentary understanding of the oxygen surface exchange process. Despite decades of isotopic, electrochemical and theoretical studies we still have a rather poor understanding of the way that the oxygen in the gas phase interacts with the surface of an oxide. Part of the problem is that we do not understand how the outer surface composition and surface structure relate to the bulk composition, and other factors such as the presence of impurities, under the operating conditions relevant to the SOFC (or SOEC). It has thus become imperative that we investigate the surface composition of these materials so that models of the exchange process can be based upon realistic terminations of the bulk. Low Energy Ion Scattering (LEIS) is a method for the determination of the composition of the outermost atomic layers of materials with high precision and is thus extremely useful for the study of SOFC materials. In this contribution the principles of LEIS will be described with particular attention to their application to the conventional cathode materials such as the perovskites LSM and LSCF, and newer materials such as the Ruddlesden-Popper and double perovskite materials.
5:00 PM - C5.6/B8.6
Neutron Scattering Studies of Fluorite Structured SOFC Electrolytes.
Stefan Norberg 1 2 , Stephen Hull 2 , Sten Eriksson 1 , Dario Marrocchelli 3 , Paul Madden 4
1 Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg Sweden, 2 The ISIS Facility, Rutherford Appleton Laboratory, Chilton United Kingdom, 3 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractFuel cell technology currently attracts significant attention since these devices show considerable potential for efficient power generation in stationary, portable and transport applications. Solid oxide fuel cells (SOFCs) show much promise, given that they can use a variety of fuels such as natural gas, biogas, hydrogen, gasified coal and hydrocarbons. This variation in useable fuels, from renewable energy resources to traditional carbon based fuels, can additionally be utilised with high fuel efficiency, up to 85% for combined heat and power systems, by the direct electrochemical conversion of these fuels into electricity and heat.A core component within a solid oxide fuel cell (SOFC) is the impermeable solid electrolyte, which consists of a polycrystalline oxide ceramic operating at temperatures between 1073 K and 1273 K. The current SOFC electrolyte of choice is yttria stabilized zirconia (YSZ, Zr1-xYxO2-x/2) which for, x > 0.16, adopts the cubic fluorite phase. The oxygen anion conductivity, σ, increases with increasing x, until a maximum value (σ ~ 10-2 Ω-1 cm-1 at 1000 K) is reached at an yttria concentration close to the lower stability limit of the cubic fluorite phase. It is possible to increase the anion conductivity in YSZ by substituting Y3+ by Sc3+ and scandium stabilized zirconia (SSZ) electrolytes show good potential for use in SOFCs operating at temperatures below ~1000 K. However, from the crystal structure point-of-view, SSZ is rather more complex than its YSZ counterpart and the stability field of the highly conducting cubic fluorite structured phase is more limited in composition and temperature. An alternative to the doped zirconias is doped ceria, e.g. Ce1-xMxO2-x/2 with M = Y3+, Gd3+, etc., which also adopts the fluorite structure, but with a larger lattice constant compared to YSZ, displays higher ionic conductivities and offers the possibility of operating SOFCs at temperatures between 873 K and 1073 K.The authors have used neutron total scattering data to probe the local cation and anion ordering in numerous fluorite structured electrolytes, e.g. yttria-doped ceria, yttria- and scandia-doped zirconia, and reduced ceria, in order to access the roles of the cation-anion, cation-vacancy and vacancy-vacancy interactions determining the oxide conductivity in these systems. The results obtained by reverse Monte Carlo (RMC) analysis of the total scattering data have been correlated with impedance spectroscopy and molecular dynamics (MD) studies. The combination of RMC and MD has allowed us to carefully map out the distribution of vacancies in relation to the host and dopant cations, vacancy clustering and oxygen anions in each of these systems which makes it possible to determine the structural factors inhibiting oxygen ion conductivity. These results and their implications will be presented.
5:15 PM - C5.7/B8.7
Local Measurements of Oxygen Vacancy Content and Homogeneity in SOFC Cathode Materials from High-Resolution STEM Images.
Albina Borisevich 1 , Donovan Leonard 1 , Young-Min Kim 1 , Michael Biegalski 2 , Hans Christen 2 , Stephen Pennycook 1 , Sergei Kalinin 2
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractSpatial distribution and dynamics of oxygen vacancies in the material are key parameters determining functionality of solid oxide fuel cell (SOFC) cathodes in the oxygen reduction/evolution reactions. Global chemical expansivity measurements have demonstrated that lattice parameters are very sensitive to the overall oxygen content. However, to evaluate contributions of defects and interfaces to the oxygen transport properties, we need to examine oxygen distribution at the atomic scale. Here, we describe an approach to local oxygen concentration mapping using unit-cell-by-unit-cell lattice parameter mapping by aberration-corrected Scanning Transmission Electron Microscopy (STEM). We examine lantanum/strontium cobaltite (LSCO), which was shown to exhibit good ORR activity in thin film form [1]. Several compositions of LSCO were grown by Pulsed Laser Deposition on Yttria-stabilized Zirconia (YSZ), La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) and NdGaO3 (NGO) substrates. The examined films often exhibit oxygen vacancy ordering, either partial (on YSZ) or uniform (on NGO and LSAT). By mapping lattice parameters of the modulated structures, we are able to quantitatively determine local oxygen content, from the parent stoichiometric perovskite structure (La,Sr)CoO3 to highly oxygen-deficient (La,Sr)CoO2.5 brownmillerite. Lattice parameter maps and Electron Energy Loss (EELS) spectra recorded in the vicinity of grain boundaries, interfaces, and dislocations show local changes in oxygen content and electronic states. Principal Component Analysis (PCA) based statistical methods are developed for automatic determination of characteristic sizes of ordered domains and classification of the types or ordering. These studies provide insight into the phase evolution and defect chemistries in cobaltite-based SOFC cathodes under operational conditions.* This research is sponsored by the Materials Sciences and Engineering Division (AYB, DNL, YMK, SJP) and Scientific User Facilities Division (DNL, MDB, HMC, SVK), Office of BES of the U.S. DOE, and by appointment (YMK) to the ORNL Postoctoral Research Program administered jointly by ORNL and ORISE.[1] G. Jose la O’, et al., Angew. Chem. Int. Ed., 49, (2010).
5:30 PM - C5.8/B8.8
Spatially Resolving Surface Reduced Phases in SOFC Ion Conducting Materials.
Robert Walker 1 , John Kirtley 1 , David Halat 1 , Bryan Eigenbrodt 2
1 Chemistry and Biochemistry, Montana State University, Bozeman, Montana, United States, 2 Chemistry and Biochemistry, University of Maryland, College Park, Maryland, United States
Show AbstractVibrational Raman scattering, X-ray absorption spectroscopy (XAS), and electrochemical impedance spectroscopy were used to characterize the effects of ambient oxidizing and reducing atmospheres on the structural and electronic properties of high temperature ion conducting materials commonly used in SOFCs. These studies were performed in functioning devices at temperatures up to 715C. Raman scattering experiments showed that yttria stabilized zirconia (YSZ) formed a reduced phase that remained stable even when cooled to room temperature and exposed to air. This reduced phase extended approximately 5-10 nm into the bulk. Furthermore, experiments carried out under potential control show clearly that polarizing the SOFC can deplete oxide concentration even further, and this electrochemically depleted oxide region extends hundreds of microns away from the nominal electrode/electrolyte three phase boundary. Complementary ex-situ XAS data show that this surface reduced phase arises primarily from partial reduction of zirconium and not yttrium, and that the observed increase in surface conductivity is the result of excess electron density, not more facile oxide ion transport. In contrast, ceria shows behavior similar to YSZ at high temperatures, namely exposure to a reducing atmosphere leads to a surface reduction, but such a phase is not stable and can not be isolated under room temperature conditions. This observation is attributed to the mixed valence character of the cerium ion as well as ceria's smaller activation energy for oxide diffusion relative to YSZ.
Symposium Organizers
Rob Walker Montana State University
Nigel Brandon Imperial College London
Jeff Owrutsky Naval Research Laboratory
Koichi Eguchi Kyoto University
C6/B10: Joint Session: SOFC Materials Characterization II
Session Chairs
Wednesday AM, November 30, 2011
Constitution B (Sheraton)
9:30 AM - C6.1/B10.1
Measuring Oxygen Reduction/Evolution Reactions in Fuel Cells on the Nanoscale.
Amit Kumar 1 , Francesco Ciucci 2 , Anna Morozovska 3 , Sergei Kalinin 1 , Stephen Jesse 1
1 CNMS, Oak ridge National laboratory, Oak ridge, Tennessee, United States, 2 Interdisciplinary Center for Scientific computing, University of Heidelberg, Heidelberg Germany, 3 Institute of semiconductor physics, National academy of science of Ukraine, Kiev Ukraine
Show AbstractSolid oxide fuel cells (SOFC) based electrochemical energy conversion systems are one of integral components of current and future energy technologies. The energy conversion in these systems is underpinned by ion and vacancy diffusion, electronic transport and solid-gas and solid-liquid reactions at surfaces and triple phase junctions. One of the critical steps in the SOFC and Li-air battery operation leading to large overpotentials and charge-discharge hysteresis is the kinetics of the oxygen oxidation reaction (ORR). It is important thus to explore the mechanisms behind this enhancement which remain elusive, largely due to the lack of experimental techniques capable of probing ORR on the nanoscale. Oxygen vacancies play a significant role in determining the functionality of electro-resistive devices, non-volatile memories based on resistive switching and solid oxide fuel cells. Traditionally, the study of the role of oxygen vacancies in these processes is limited by high activation temperature and macroscopic measurement techniques. Here, we demonstrate spatially resolved local probing of the thermodynamics and kinetics involving the generation and diffusion of oxygen vacancies by utilizing chemical expansivity of these oxides upon application of concentrated electric fields. Using Band excitation Electrochemical strain microscopy (ESM), a strongly confined electric field at tip is used to drive the oxygen vacancies in these oxide materials and the resulting localized electrochemical strain is detected. A high frequency periodic bias is applied on the oxide material and the PFM tip acts as a probe of the local displacement arising due to migration of oxygen vacancies. Local strain hysteresis loops driven by vacancy diffusion have slow dynamics and thus open up. Mapping the loop opening as a function of the final bias allows establishment of the onset and kinetics of the diffusion process. Signal relaxation measurements enable us to locally characterize the diffusion dynamics of the vacancies. In mixed ionic-electronic oxide systems, we also utilize current-voltage measurements to probe the electronic transport . Correlated mapping of the local oxygen vacancy movement and diffusivity has been achieved with a resolution of ~ 30 nm. The mapping of vacancies is shown on purely ionic oxides (Yttrium stabilized zirconia and Samarium doped Ceria). Systematic mapping of ORR/OER activity on bare and Pt-functionalized yttria-stabilized zirconia (YSZ) surfaces is demonstrated. This approach allows directly visualization of ORR\OER activation process at the triple-phase boundary. Acknowledgement : This material is based upon work supported by research division of Materials Science and Engineering, Office of basic energy sciences, DOE.
9:45 AM - C6.2/B10.2
Highly Anisotropic Oxygen Adsorption and Incorporation Kinetics on (La,Sr)2CoO4+δ Surfaces and Hetero-Interfaces.
Jeong Woo Han 1 , Bilge Yildiz 1
1 Laboratory for Electrochemical Interfaces, Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractA2BO4 Ruddlesden-Popper family of materials is being considered as interesting cathode candidates for intermediate temperature solid oxide fuel cells (SOFCs) due to the fast oxygen transport in them. Furthermore, recent reports have demonstrated distinctly enhanced oxygen reduction rate (ORR) at the hetero-interface of (La,Sr)CoO3 and (La,Sr)2CoO4+δ, but the mechanism for such enhancement is not yet known. To unravel the governing mechanism for the ORR activity enhancement at this interface, we first investigate the oxygen surface reactions on LaCoO3 (LCO113) and La2CoO4+δ (LCO214) as reference systems, and then probe the same interactions at the hetero-interface of LCO113/LCO214. We recently reported the static and kinetic energetics of the oxygen incorporation and transport on strained LCO113 surfaces, and the interstitialcy diffusion of oxygen in bulk LCO214. Complementary to those results, here we assess the O2 adsorption on and incorporation into the LCO214 surfaces. We perform first principles-based calculations in the density functional theory (DFT) formalism, to provide an atomic scale view of the adsorption and incorporation mechanisms and kinetics. We identified the reaction sites and energies for the O2 adsorption, its dissociation on LCO214 surface and incorporation into the subsurface. Atop La cation in the vicinity of the interstitial path between the LaO planes on LCO214(100) is a significantly stronger adsorption site than on the CoO2 layer terminated on LCO214(001) (that is equivalent to the LCO113(001) surface). Upon adsorption, O2 dissociates into the interstitialcy path, with an energy barrier (0.77 eV) similar to that of dissociation on the LCO113(001) surface and of oxygen migration barrier in the bulk LCO214 and LCO113. Furthermore, lattice strain at the LCO214/LCO113 heterointerface (on LCO214(100)) strengthens the adsorption, and at the same time provides larger space for oxygen incorporation. These two processes balance, and thus, no effective improvement is found for the dissociation and incorporation with increasing tensile strain. Based on our results, the strongly favored adsorption on the (100) plane of LCO214 plays a major role on the recently reported enhancement of the ORR near the LCO214/LCO113 interface, which has the LCO214(100) surface exposed to oxygen.
10:00 AM - C6.3/B10.3
First Principles Kinetic Monte Carlo Simulations of the Interaction of Oxygen with LaMnO3 Surfaces.
Ghanshyam Pilania 1 , R. Ramprasad 1
1 Materials Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractPerovskite type lanthanum based transition metal oxides are frequently explored as the cathode material for solid oxide fuel cells (SOFCs). Their utility in SOFCs derives from their ability to catalyze the oxygen reduction reaction (ORR), as well as their low cost, high-temperature stability, and adequate thermal expansion properties. Apart from their use in SOFCs these materials are also promising for many other technologically important applications such as photocatalysis and for removal of NOx gases in diesel engine auto-exhausts. The underlying processes in all the above mentioned applications involve interaction of the perovskite surface with the gas phase oxygen. Therefore, acquiring a fundamental understanding of the nature of the interaction between oxygen and the perovskite surfaces is crucial to explain the role of oxygen in such technological processes.Experimental techniques such as temperature programmed desorption and reaction spectroscopies are employed to provide useful insights into the binding energetics of adsorbates or reactants at the solid surfaces. However, the obtained experimental data contains only a limited amount of information on the spatial arrangement of the surface species at a given temperature and pressure. More importantly, such techniques cannot directly distinguish between the various possible elementary or help in the identification of the rate limiting steps involved. Here we have employed first-principles based kinetic Monte Carlo (kMC) simulations to investigate the relative stability of the clean as well as molecular and atomic oxygen covered LaMnO3 surfaces over a vast range of temperatures and oxygen partial pressures. For definiteness, the (001) MnO2-terminated surface was considered. The energetics as well as the activation energies of various surface reactions were computed using density functional theory. The elementary surface reactions included adsorption, desorption, surface dissociation, and the surface diffusion of molecular and atomic oxygen. The computed energetics and barriers were then used in large-scale kMC simulations to predict the surface oxygen content and configuration at various combinations of temperature and pressure, thereby yielding a surface phase diagram. Owing to the state-of-the-art theory, algorithms and computations employed, these results are believed to represent the real situation with high fidelity. On the basis of our surface phase diagram results, we are able to identify the conditions most favored for the dissociative adsorption of molecular oxygen. These are (temperature and pressure) conditions corresponding to partial atomic oxygen coverage. In other words, the “phase boundaries” as predicted by our kMC simulations are identified to be the catalytically active regions for ORR. Our methodology will be applied to study doped oxides (e.g., LSM), and may effectively be used to design materials with enhanced activities at predetermined temperatures and pressures.
10:15 AM - C6.4/B10.4
Structural Characterization of Epitaxial Fluorite-Type Solid Electrolyte Thin Films.
Weida Shen 1 , Jun Jiang 1 , Joshua Hertz 1
1 Department of Mechanical Engineering, University of Delaware, Newark, Delaware, United States
Show AbstractThin film electrolytes are being heavily investigated within both fundamental studies as well as for practical use within thin film and bulk devices. The structural and crystallographic properties of thin films are often highly dependent upon the substrate and growth conditions. As these properties are suspected to consequently affect the electrochemical properties of the films, determination of the processing-property relationships are critical to understanding and, ultimately, improving device performance. Complicating this understanding, in part, is the lack of highly suitable substrates with a close lattice match to the typical fluorite-type solid electrolytes, ceria and zirconia. Here, we present microstructural and crystallographic characterization of films deposited via sputtering onto heteroepitaxial substrates. Particular attention is paid to the presence of lattice strains and interfacial regions with a high concentration of planar defects, as these are supposed causes for the recent reports of high apparent conductivity in these films.
10:30 AM - C6.5/B10.5
Measurement of the Debye Screening Length in an Oxide Ion Conductor.
Marisa Frechero 1 2 , Mirko Rocci 1 , Rainer Schmidt 1 , Mario Diaz-Guillen 1 , Oscar Dura 1 , Alberto Rivera-Calzada 1 , Jacobo Santamaria 1 , Carlos Leon 1
1 Fisica Aplicada III, Universidad Complutense Madrid, Madrid Spain, 2 , On leave from Dpto Quimica- Universidad Nacional del Sur, Bahia Blanca Argentina
Show AbstractBroadband dielectric spectroscopy experiments in a bicrystal of the oxide ion conductor 9 mol % yttria stabilized zirconia (YSZ) with a symmetrical -12°/12° [110] tilt grain boundary have allowed us to measure and characterize ion transport across a single grain boundary. We have obtained important microscopic parameters that determine ion transport properties at the nanoscale such as the built-in potential at the interfacial plane, the space charge layer thickness, and the Debye screening length. The values are found to be in remarkable agreement with the predictions of simple theoretical space charge models, in contrast to previous estimates from measurements on polycrystalline YSZ ceramic samples of similar composition.
11:15 AM - **C6.6/B10.6
Geometrically Asymmetric Electrodes for Probing Electrochemical Reaction Kinetics.
Sossina Haile 1 , Yong Hao 1 , Kenji Sasaki 1 , Mary Louie 1 , WooChul Jung 1
1 , Caltech, Pasadena, California, United States
Show AbstractElectrochemical reactions can exhibit considerable asymmetry, with the polarization behavior of oxidation at a given electrode differing substantially from that of reduction. The reference-less, microcontact electrode geometry, in which the electrode overpotentials are geometrically constrained to the working electrode (by limiting its area) is experimentally convenient, particularly for fuel cell studies, because the results do not rely on accurate placement of a reference electrode nor must oxidant and reductant gases be sealed off from one another. Here, the conditions under which the critical assumption of this geometry applies – that the overpotential at the large-area counter electrode can be ignored – are numerically assessed. Several experimental examples of insight gained from this method are presented.
11:45 AM - C6.7/B10.7
Unreliability of the Simultaneous Determination of kchem and Dchem through Conductivity Relaxation.
Rosemary Cox-Galhotra 1 , Steven McIntosh 1 2
1 Chemical Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractElectrical conductivity relaxation is utilized to determine the oxygen surface exchange coefficient, kchem, and bulk oxygen diffusion coefficient, Dchem, of mixed ionic and electronic conducting oxides for solid oxide fuel cell cathodes. In this technique, the conductivity of a dense sample is recorded during an instantaneous step change in gas-phase pO2. Bar samples of material are typically utilized, and kchem and Dchem are extracted from each data set. In this study we examine the reliability of this simultaneous fit procedure for bar samples of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF 6428). We suggest that discrepancies in the literature reported values of these parameters are due to the existence of multiple solutions to the data fit, which render this technique unreliable for bulk samples. To obtain accurate values of kchem and Dchem, the sample thickness must be varied significantly such that analysis occurs completely in the kchem regime (thin film) or Dchem regime (thick sample) and the number of adjustable parameters collapses to one.LSCF 6428 was synthesized using a modified Pechini procedure. The powder was calcined, pressed into pellets and sintered at 1523K for 12 h. Bar samples 1.2 cm x 0.5 cm x ~0.07 cm were cut from these dense (>95% theoretical), polished pellets and four-point Au conductivity contacts attached. Relaxation measurements were performed in six logarithmically spaced pO2 steps from 100% to 3.3% O2. Full details available elsewhere [1].The values of kchem and Dchem for LSCF 6428 determined in our study were within one to two orders of magnitude greater than those previously reported [2,3]. Direct comparison of our electrical conductivity data with published values [3] suggested that our sample was nominally identical. Furthermore, measurements on samples prepared identically yielded results within 0.1 orders of magnitude. The samples do not appear to be problematic. We then turned our attention to the fitting procedure and investigated the influence of the number of roots determined on the resulting values of kchem and Dchem. We found a strong variation in especially kchem with the number of roots, but no significant change in the quality of the fit. From this we conclude that the simultaneous determination of both parameters from a single data set is flawed due to the presence of multiple, equally correct solutions of kchem and Dchem. As mentioned previously, using thin film samples for conductivity relaxation allows Dchem to be eliminated from the analysis, resulting in a one-parameter fit of kchem. We will demonstrate this method for dense, polycrystalline thin film samples of the layered perovskite PrBaCo2O5+δ (PBCO), deposited onto SrTiO3 substrates using spray pyrolysis. References[1] R. Cox-Galhotra and S. McIntosh, Solid State Ionics,181 (2010) 1429.[2] H.J.M. Bouwmeester, et al., J. Solid State Electrochem., 8 (2004) 599.[3] J.A. Lane, et al., Solid State Ionics, 121 (1999) 201.
12:00 PM - C6.8/B10.8
Detailed Electrochemical Analysis of Nanoscaled La0.6Sr0.4CoO3-δ as Intermediate Temperature SOFC Cathode.
Jan Hayd 1 3 , Levin Dieterle 2 3 , Dagmar Gerthsen 2 3 , Andre Weber 1 , Ellen Ivers-Tiffee 1 3
1 Institut für Werkstoffe der Elektrotechnik (IWE), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany, 3 DFG Center for Functional Nanostructures (CFN), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany, 2 Laboratorium für Elektronenmikroskopie (LEM), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany
Show AbstractLow-temperature operation (400 to 600 °C) of solid oxide fuel cells has generated new concepts for materials choice, interfacial design and electrode microstructures. Nanoscaled and nanoporous La0.6Sr0.4CoO3-δ thin film cathodes derived from metal-organic precursors (metal organic deposition, MOD) with a film thickness of 200 nm and an average grain and pore size in the sub 100 nm regime were investigated with the focus on the reaction kinetics by a systematic temperature (T = 400 – 600°C) and oxygen atmosphere (pO2 = 0.01 – 0.5 atm) variation. Electrochemical impedance spectroscopy was applied, followed by a detailed data analysis including calculating the distribution function of relaxation times (DRT) and CNLS fit. Five processes were identified. Four processes are temperature activated, whereas one process at low frequencies exhibits a slight deactivation with temperature. Furthermore, the three low and mid frequency processes are oxygen partial pressure dependent, whereas the two high frequency processes remain unaffected by pO2 variations. We report a record performance for nanoscaled LSC cathodes of 9 mΩxcm2 at 600 °C, 75 mΩxcm2 at 500 °C and 1.95 mΩxcm2 at 400 °C, measured in synthetic air. These results were, in the most part, facilitated by a substantial increase of the inner surface area of the porous thin film cathode, but also nanoparticulate Co3O4 might be the cause of further enhanced oxygen exchange kinetics.
12:15 PM - C6.9/B8.9
Investigation the Rate Determining Steps of Oxygen Reduction on LSCF Cathode Using Nonlinear EIS.
Ning Xu 1 , Jason Riley 1 , John Kilner 1 , Cortney Kreller 2
1 Materials, Imperial College London, London United Kingdom, 2 Chemical Engineering, University of Washington, Seattle, Seattle, Washington, United States
Show AbstractLanthanum cobaltite substituted with strontium and iron, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), has attracted much attention as a promising candidate for intermediate temperature solid oxide fuel cells (IT-SOFC) in the past decade, [1] [2] because of its high mixed ionic-electronic conductivity and good thermal stability. As a consequence of the high ionic-electronic conductivity, oxygen reduction can take place competitively along the surface of the LSCF grains and/or through the bulk of the electrode material. [3] A systematic understanding of the rate limiting processes under different operation conditions has yet to be achieved.Compared to the traditional electrochemical impedance spectroscopy (EIS), nonlinear EIS (NLEIS) has been demonstrated to be more powerful in separating entangled mechanisms and distinguishing theoretical models with subtle differences. Wilson et al. [4] [5] performed NLEIS analysis on strontium-doped lanthanum cobaltite (LSC) and revealed that O2 reduction on this cathode material is limited by dissociative adsorption.In this work, NLEIS measurements are adopted as an extension to the polarization dependent EIS measurements which were performed on a symmetric cell with gadolinium-doped ceria (CGO) as the electrolyte. LSCF ink made from ball-milling commercial powder with ink vehicle was screen-printed onto both sides of a dense CGO pellet (sintered at 1200 oC for 5h) to form a symmetric cell. Cross sectional SEM was used to characterize the microstructure of the electrode-electrolyte interface. Cross-sectional SEM was used to characterize the microstructure of the electrode-electrolyte interface. Obvious distortions were found between the EIS results under different excitations when the cell is operated in the low temperature regime (<500 oC), while no distortions were observed at high temperature (>700 oC); this might imply a switch of the dominant process. The dominant process of low operation temperature is therefore more nonlinear. NLEIS measurements were then carried out at the same temperature points, the results of which showed very interesting features in harmonic responses. For example, the appearance of appreciable even harmonic responses on the symmetric cell suggests an asymmetric anodic/cathodic mechanism. Moreover, harmonic responses were also detected at high temperature with similar amplitudes to those at low temperature in NLEIS; whereas in the polarization dependent EIS results, the nonlinearity at high temperature was completely masked by the increased fundamental response.[1] A. Esquirol, N. Brandon, J. Kilner, and M. Mogensen, J. Electrochemical Society, 151 (2004) 1847;[2] J. Lane, S. Benson, D. Waller and J. Kilner, Solid State Ionics 121 (1999) 201;[3]M. Prestat, J. Koenig and L. Gauckler, J. Electroceram 18 (2007) 87;[4] J. Wilson, M. Sase, T. Kawada and S. Adler, Electrochemical and Solid-State Letters 10 (2007): 86;[5] J. Wilson, D. Schwartz and S. Adler, Electro. Acta, 51 (2006), 1389.
12:30 PM - C6.10/B10.10
A New La0.5Sr0.5Mn0.5Co0.5O3-δ Anode for Low-temperature Solid Oxide Fuel Cells.
Ainara Aguadero 1 2 , Domingo Perez-Coll 3 , Jose Antonio Alonso 1 , Stephen Skinner 2 , John Kilner 2
1 , ICMM-CSIC, Madrid Spain, 2 Materials, Imperial College London, London United Kingdom, 3 , ICV-CSIC, Madrid Spain
Show AbstractIn this work we have developed a new oxygen-defective La0.5Sr0.5Mn0.5Co0.5O3-δ perovskite as an anode for low temperature solid oxide fuel cells (SOFC) (< 600 C). The reduction of the oxygen-stoichiometric La0.5Sr0.5Mn0.5Co0.5O3-δ perovskite oxide at 500 C for 6h leads to a hypostoichiometric phase with oxygen hypostoichiometry of δ = 0.6(1) oxygen atoms per formula unit. The neutron power diffraction study reveals a slight deviation from the cubic symmetry that can be structurally defined in the Pbnm space group, with a random distribution of the metals over the A and B positions of the perovskite and oxygen vacancies. The quantity of oxygen refined in the structure is O2.52(2) with high values of isotropic thermal factors (up to 4.5(4) Å2 for O1 and 6.3 (1) Å2 for O2) in the 150 to 600 C temperature range indicating significant mobility of oxide ions in this structure; on the other hand, this material has shown an extraordinary reaction rate and selectivity for the conversion of alkylaromatics, for instance p-xylene in TPA, with respect to the standard homogeneous catalyst [1]. The thermal expansion coefficient has been observed to be around 14x10-6 K-1 while the polarization resistance values with ceria-based electrolytes in H2(10%) vary from 0.02 to 2 Ωcm2 in the 600 to 350 C temperature range, suggesting this material as a promising low-temperature SOFC anode.[1] A. Aguadero, H. Falcón, J. M. Campos-Martín, J. L. García-Fierro, J. A. Alonso Angewandte chemie international edition. Accepted DOI : 10.1002/anie.201007941
C7: SOFC Anodes and Fuels
Session Chairs
Wednesday PM, November 30, 2011
Republic A (Sheraton)
2:30 PM - **C7.1
Separated Anode Experiment to Measure Gas Transport and Methane Reforming within Solid-Oxide Fuel Cell Anodes.
Robert Kee 1 , Amy Richards 1 , Huayang Zhu 1 , Neal Sullivan 1
1 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractSolid-oxide fuel cell (SOFC) performance depends upon multi-component gas transport, thermal catalytic chemistry, and electrochemical charge transfer within porous composite (e.g., cermet) anode structures. This paper describes an experiment that is designed specifically to measure gas transport and catalytic chemistry within the anodes. An anode-support structure without electrolyte or cathode layers is sandwiched within ceramic manifolds into which two rectangular gas flow channels are machined. The anode support is typically on the order of a millimeter thick. The gas channels have hydraulic diameters and lengths of approximately one millimeter and three centimeters, respectively. The upper channel typically carries a fuel mixture and the lower channel carries gases that represent products of charge-transfer chemistry (e.g., H2O and CO2), which are also hydrocarbon-reforming agents. The assembly is operated in a furnace at typical SOFC operating temperatures around 800 °C. Gases enter the channels at flow rates ranging between 50 and 200 standard cubic centimeters per minute. Because the anode support is porous, gases are free to cross-diffuse between channels and participate in heterogeneous reforming reactions. The exhaust composition of each channel is measured by gas chromatography. Physically based computational models, which model plug flow in the channels and reactive porous-media transport within the anode, are used to assist quantitative experimental interpretation. Anode support structures are initially evaluated using inert gases in both channels, providing information about gas transport in the absence of catalytic reforming chemistry. Using computational models that represent multicomponent porous-media transport with a Dusty-Gas Model, important physical parameters that are difficult to measure directly (e.g., tortuosity) can be derived from these experiments. With the porous-media parameters established, the experiments are used to evaluate catalytic reforming performance. In this case, the models incorporate detailed multistep chemical kinetics mechanisms. The presentation will discuss the experiments themselves as well as example results from alternative anode structures and materials.
3:00 PM - C7.2
Microstructural Change in Anode during Operation of Solid Oxide Fuel Cells.
Koichi Eguchi 1 , Hiroki Muroyama 1 , Toshiaki Matsui 1
1 Graduate School of Engineering, Kyoto University, Kyoto Japan
Show AbstractSolid oxide fuel cells (SOFCs) are one of the attractive power generation systems. Presently, this system is in an advanced stage of development, and the durability and reliability need to be improved before the commercialization. The cermet of nickel and yttria-stabilized zirconia (YSZ) is widely used as an anode in SOFCs. Because the fuel cells are operated at high fuel utilizations, the downstream part of the cermet anode is subjected to the severe conditions due to the depletion of fuel and high steam concentration. The anode is exposed to the highly-humidified fuel even at the upstream part. In such conditions, the oxidization and agglomeration of nickel proceed during a long-term operation, leading to performance deterioration. Recently the direct measurement of electrode in a three dimensional (3D) space by using the focused ion beam–scanning electron microscopy (FIB–SEM) has attracted much attention as a powerful technique for microstructural analysis. In this study, then, the microstructural change in cell components with an elapsed time was quantified by the FIB–SEM technique to elucidate the degradation of anode.The performance stability of electrolyte-supported cell (Ni–YSZ | YSZ | LSM) was examined at 1000°C by feeding humidified hydrogen fuel. The degradation behavior was significantly dependent on the fuel humidity and cermet composition. In the case of Ni–YSZ with a volume ratio of 50 to 50, peculiar phenomena were observed. When the fuel of 30% H2O–70% H2 was supplied to the anode at the terminal voltage of 0.7 V, the current density decreased gradually soon after the discharge up to 69 h, followed by a sudden drop in current density. After the subsequent open-circuit holding, the performance was partially recovered in discharge operation. In the case of 40% H2O–60% H2, irreversible performance deterioration was observed accompanied with a drastic decrease in volume-specific TPB-length.This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan (Development of System and Elemental Technology on Solid Oxide Fuel Cell Project).
3:15 PM - C7.3
Direct Methane Fueled Thin Film Solid Oxide Fuel Cells with Ruthenium Anodes.
Yuto Takagi 1 2 , Suhare Adam 1 , Bo-Kuai Lai 1 , Kian Kerman 1 , Shriram Ramanathan 1
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Core Device Development Group, SONY Corporation, Kanagawa Japan
Show AbstractSolid-oxide fuel cells (SOFCs) can efficiently convert chemical energy into electricity from a wide variety of fuels including hydrogen, hydrocarbons and bio-fuels. Among various hydrocarbons direct supply of methane to SOFCs has been investigated, as methane is a major component of natural gas and also highly contained in bio-gas. In this study, thin film micro-solid oxide fuel cells (μSOFCs) utilizing nanoporous ruthenium (Ru) anodes were fabricated and investigated for direct methane operation. Thin film of 8 mol% yttria-stabilized zirconia was fabricated as free-standing electrolytes, with porous platinum and porous Ru deposited as cathode and anode electrodes, respectively. μSOFCs were tested with methane as the fuel and air as the oxidant, exhibiting an open circuit voltage of 0.71 V and a peak power density of 450 mW/cm2 at 500 °C, without visually detectable carbon deposition. Anode electrodes were investigated by SEM for structural change and Auger spectroscopy for carbon deposition. It was found that morphology evolution in anode electrodes were strongly dependent on the fuels (namely, methane or hydrogen) used, and possible mechanisms leading to the observations are discussed. Auger spectroscopy revealed minimal amount of carbon increase after methane operation. Further investigations in stabilizing electrode nanostructures will be presented.
3:30 PM - **C7.4
Carbon Formation on the Functional and Conduction Layers of an SOFC Anode.
Josephine Hill 1
1 Chem & Pet Engineering, University of Calgary, Calgary, Alberta, Canada
Show AbstractSolid-oxide fuel cells (SOFC) have tremendous potential as power generators but are complex devices. In our research over the past few years, we have sought to better understand these devices such that they can be operated effectively with hydrocarbon and alcohol fuels rather than just hydrogen. Specifically, we have focused on the anode compartment in which electrochemical oxidation, reforming, cracking, and/or other reactions occur, and through which reactants, products and electrons are transported. Carbon formation in the anode compartment is a potential problem when operating the SOFC on fuels other than hydrogen. Although some carbon formation may be beneficial as the electrical conductivity of the anode is improved, significant carbon formation can result in blocking of the pores, and destruction of the anode, especially if the anode contains nickel. The exact amount of carbon that is significant depends on the fuel, anode composition and current density. The amount of carbon formation is significantly reduced at the triple phase boundary (i.e. in the functional layer), and this carbon is more reactive during regeneration with either oxygen or hydrogen. Carbon formation can be reduced by using an anode with distinct functional and conduction layers. This bi-layer structure can be optimized in terms of electrochemical activity in the functional layer and in terms of conductivity and/or reforming activity in the conduction layer. Results obtained with various anode bi-layer structures will be presented.
4:30 PM - C7.5
Optimization of Syngas Generation for Micro-Solid Oxide Fuel Cell Application Using Rhodium Doped Ceria Zirconia Nanoparticles.
Alejandro Santis 1 , Majid Nabavi 1 , Dimos Poulikakos 1
1 Department of Mechanical and Process Engineering, ETH Zurich, Zurich Switzerland
Show AbstractThis paper aims at the investigation and optimization of n-butane to syngas conversion for intermediate temperature, portable micro-scale solid oxide fuel cell (SOFC) power plant applications. The main goal of this study is the efficient partial oxidation of n-butane at temperatures between 450°C and 550 °C.The catalytic reaction is carried out in the presence of 2 wt% Rh/CexZr1-xO2 nanoparticles in small size disk-shaped reactors at an intermediate temperature range between 400 °C and 550 °C. The nanoparticles were prepared in a one-step process by flame spray synthesis. The capability of flame-made Rh/CexZr1-xO2 catalyzing the partial oxidation reaction of n-butane is investigated for different nanoparticle sizes and Ce:Zr (substrate) ratios (x=0.3, 0.5, 0.7) synthesized using different precursor solution and oxygen dispersion ratios. Different fuel to oxygen (C/O) ratios are also investigated in order to optimize the catalytic reaction in terms of syngas selectivity and hydrogen yield. In the experiments, controlled flows of synthetic air (80 vol% N2, 20 vol% O2) and n-butane were mixed at specified ratios at room temperature before entering the reactor. All measurements were carried out under steady flow conditions and repeated at least twice. An online automated gas chromatograph was used to measure the sample effluent composition.The results show that syngas generation performance depends on nanoparticle size, Ce:Zr ratios and the chosen C/O ratio. It is demonstrated that C/O ratios lower than stochiometric values (C/O < 1.0) deliver high hydrogen yield and syngas selectivities at all investigated temperatures. At residence times as low as 65 ms and a C/O ratio of 0.8, a carbon monoxide selectivity of 52% and hydrogen yield of 56% are enhanced to 56% and 61%, respectively, by using a substrate ratio of 0.3:0.7 instead of 0.5:0.5. A nanoparticle size decrease from 11 nm to less than 5 nm also shows higher n-butane to syngas conversion.A further enhancement of the partial oxidation reaction suggests a combination of optimized parameters and variation of catalytic reactor size for investigations involving a thermally self-sufficient reaction.This study provides us with insight into the optimized C/O-ratio and catalyst that could result in acceptable hydrogen and carbon monoxide yields within the desired low operating temperature range for an optimized portable SOFC micro power plant.Keywords: Micro solid oxide fuel cell, Butane reforming, self-sustained micro-reactor, catalytic nanoparticles.
4:45 PM - C7.6
Phase Transformation Related Conductivity Degradation of NiO Doped YSZ: The In Situ Micro-Raman Analysis.
Haruo Kishimoto 1 , Keiji Yashiro 2 , Taro Shimonosono 1 , Manuel Brito 1 , Katsuhiko Yamaji 1 , Teruhisa Horita 1 , Harumi Yokokawa 1 , Junichiro Mizusaki 2
1 , National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan, 2 , Tohoku University, Sendai, Miyagi, Japan
Show AbstractCubic stabilized zirconia, such as YSZ, is widely used as electrolyte material in SOFCs because of its high oxide ion conductivity, high oxide ion transport number and high chemical, thermal and mechanical stabilities. Nevertheless, several authors have reported conductivity degradation in cubic stabilized zirconia: 1. After long time annealing at a high temperature (1273 K) and a few thousand hours, phase transformation from cubic into tetragonal phase is observed [1]. 2. Conductivity degradation of NiO doped YSZ occurs after treatment under reducing condition at a high temperature within a dozens of hours [2].Needless to say that conductivity degradation of the electrolyte material leads to decrease of electrical efficiency of SOFC system. In this study, conductivity degradation behaviour of NiO doped YSZ is examined focusing on the phase transformation behaviour of YSZ. The conductivity of 1mol% NiO doped 8mol% Y2O3 stabilized zirconia (1Ni-8YSZ) was measured at open circuit voltage (OCV) under SOFC condition, in which wet hydrogen and dry air were fed to the anode and cathode side, respectively. In-situ Raman spectroscopy measurement was carried out at the anode side surface while monitoring conductivity. The conductivity of 1Ni-8YSZ monotonically decreased within several ten hours after hydrogen was fed to the anode side. Correspondingly phase transformation of 1Ni-8YSZ also gradually proceeded from cubic to tetragonal phase under SOFC conditions. [1] Ex, K. Nomura et al., Solid State Ionics, 132 (2000) 235, B. Butz et al., Solid State Ionics, 177 (2006) 3275[2] Ex, W.G. Coors et al., Solid State Ionics, 180 (2009) 246, A. Lefarth et al., ECS Trans., 35 (2011) 1581-1586
5:00 PM - C7.7
Observation of Electrochemical Phenomena in Solid Oxide Fuel Cells under Operating Conditions Using In Situ Scan Probe Techniques.
Stephen Nonnenmann 1 , Dawn Bonnell 1
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractMicro SOFCs (mu-SOFCs) utilize a reduction in the electrolyte thickness to effectively decrease the ionic diffusion path length, thus greatly reducing the operating temperature (~350C), yielding promise as a low power source alternative for portable electronics [1]. This reduction in operating temperature allows for more accessible conditions to experimentally observe electrochemical phenomena at the electrode/electrolyte interface. Conductive atomic force microscopy techniques (cAFM) have previously been implemented to spatially resolve phenomena, aqueous domains, and proton conduction within Nafion proton exchange membranes [2,3]. More recently, Vels Hansen et al. [4] demonstrated the ability of a controlled atmosphere high temperature SPM (CAHT-SPM) to resolve conductive regimes within a stabilized zirconia symmetrical cell up to temperatures of 650C. Here, we designed and developed a miniature reaction chamber for use with a standard commercial atomic force microscope, isolating fuel cell conditions (temperature, gas environment) from the surrounding ambient environment. High resolution spatial mapping of the conductive regimes at the electrode/electrolyte interface of a cross-sectional slab of a mu-SOFC using cAFM will be presented. Design challenges will also be discussed.References:[1]A. Evans, A. Bieberle-Hütter, J. L.M. Rupp, and L. J. Gauckler, J. Power Sources 194 (2009) 119.[2]R. O’Hayre, M. Lee, F.B. Prinz, J. Appl. Phys. 95 (2004) 8382[3]D. A. Bussian, J. R. O’Dea, H. Metiu, and S.K. Buratto, Nano Letters 7 (2007) 227.[4]K. Vels Hansen, T. Jacobsen, A.-M. Nørgaard, N. Ohmer, and M. Mogensen, Electrochem. Solid St 12 (2009) B144.
5:15 PM - C7.8
In Situ Strain Measurements and Its Influence on Ionic Conductivity in Hetero-Nanostructured Electrolytes of Solid Oxide Fuel Cells.
Hoda Amani Hamedani 1 , Hamid Garmestani 1 , Klaus Dahmen 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractDevelopment of high performance low-temperature solid oxide fuel cells (SOFCs) requires design of new electrolyte architectures with enhanced ionic transport properties. Toward this goal, the present work investigates the ion transport phenomena at the nanoscale in nanostructured electrolytes. A two-phase system of yttria-stabilized zirconia (Y2O3)x(ZrO2)1–x (YSZ) and Sr-doped TiO2 (STO) is used as an oxygen deficient compound for development of novel well-ordered one-dimensional composite nanostructures with enhanced charge transfer characteristics at the interface. Using high-resolution x-ray diffraction (XRD), in-situ measurement of lattice strain at elevated temperatures is performed to assess the dependence of space charge and strain fields at the YSZ/STO interface on strontium and yttrium dopant content. The interfacial oxygen ionic conductivity of the multilayer systems with different dopant content is determined using electrochemical impedance spectroscopic (EIS) measurements. Our findings elucidate the potential role of lattice strain up to a critical doping content in oxygen-vacancy formation and enhanced oxygen exchange at the YSZ/STO interface.
5:30 PM - C7.9
In Situ Analysis on Carbon Infiltration into Ni-YSZ Cermet Anode of Solid Oxide Fuel Cell.
Yongmin Kim 1 , Chang Won Yoon 1 , Suk Woo Nam 1
1 Center for Fuel Cell Research, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractMany people intend that SOFC possesses capability to directly utilize carbon-containing fuels such as hydrocarbons and carbon monoxide for power generation. However, carbon deposition and consequent carbon infiltration induced by carbon-containing fuels easily deactivate Ni-YSZ cermets commonly used for SOFC anode and finally lead to catastrophic fracture of Ni composites by the expansion of carbon filament. Carbon infiltration alters electrical and mechanical properties of Ni-YSZ cermets. In this work, electrical conductivity measurement, TGA analysis and volumetric expansion measurement by CCD camera were used to monitor change of electrical and mechanical properties in Ni-YSZ cermets in the midst of carbon infiltration. Electrical conductivity of Ni-YSZ specimens decreased when Ni-YSZ specimens were exposed to feeding gases where carbon formation by carbon-containing fuels was thermodynamically favorable, which means that the carbon activity > 1. The decrease in electrical conductivity is likely to be resulted from the breakage of the electron conducting network composed of Ni particles by carbon infiltration and formation of Ni carbide and filamentous carbon. Moreover, the decrease rate of electrical conductivity was correlated to the carbon activity determined by mole fraction of carbon-containing fuels and addition of steam to the feeding gas and actual amount of carbon formation confirmed by the conventional TGA analysis. Therefore, electrical conductivity measurement can be utilized as a facile means to directly investigate carbon infiltration in the Ni-based anode. Additionally, the expansion of the filamentous carbon inside of Ni-YSZ specimens leaded to overall volumetric expansion of the specimens and evolution of change in volume was recorded when carbon infiltration occurred. The propensity of volumetric expansion seems to be closely related to catastrophic fracture of Ni-YSZ cermets as a result of carbon infiltration and the understanding in volume expansion behavior is expected to enhance the accuracy of the aforementioned electrical conductivity measurement technique which might be compromised by change in dimension of specimens.
5:45 PM - C7.10
Silver Composites for Highly Stable Cathode Current Collectors for Long Term SOFC Operations.
Ayhan Sarikaya 1 , Vladimir Petrovsky 1 , Fatih Dogan 1
1 Materials Science and Engineering, Missouri University of Science and Technology, Rolla, Missouri, United States
Show AbstractLong term stability has been a crucial issue for the future applications of the solid oxide fuel cells (SOFCs). Current collectors for the cathodes have been among the most vulnerable components of the SOFCs due to their operation in oxidizing atmospheres at relatively high temperatures. Ag and Ag based LSM (lanthanum-strontium manganite) composites have been evaluated in order to develop low cost current collectors with high stability and compatibility with other components. Effect of the LSM additions on the microstructural development of the composites was investigated by SEM and image analysis techniques before and after the long term electrochemical measurements on symmetrical cells based on YSZ (Y-stabilized ZrO2) electrolytes and screen printed LSM-YSZ cathodes. Gas diffusion limitation problem on the cathode – current collector interface by densification of pure Ag upon time was eliminated by stabilizing the porous microstructure with LSM additions up to 25 vol% LSM. Conductivity of the Ag-LSM composite current collector remained nearly unchanged and no degradation has been observed after 600h of measurements in air at 800oC. Recent results on the relationship between the composite microstructure, electrochemical performance and long term stability of the cathode current collectors will be discussed.
Symposium Organizers
Rob Walker Montana State University
Nigel Brandon Imperial College London
Jeff Owrutsky Naval Research Laboratory
Koichi Eguchi Kyoto University
C8: SOFC In Situ Materials Characterization
Session Chairs
Thursday AM, December 01, 2011
Republic A (Sheraton)
10:00 AM - **C8.1
In Situ Neutron Diffraction Studies of Solid Oxide Fuel Cell Materials.
Steven McIntosh 1
1 Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractA significant barrier to progress in the field of high temperature solid oxide electrochemistry is a lack of experimental techniques that can probe the chemical and crystallographic properties of materials under working conditions. Solid oxide cells (SOCs) typically operate at >700oC at atmospheric pressure, in both oxidizing and reducing environments. Most analytical techniques cannot probe materials at these high temperatures in these harsh gas environments. Neutron diffraction is one technique that can achieve this goal due to the large penetration depth of neutrons; however in-situ neutron diffraction remains a niche experiment with no central user facilities available with this capability. Most current neutron measurements are performed under vacuum at room temperature or below.As a structural probe neutrons are especially suitable for the crystallographic study of these solid oxide materials using traditional Rietveld method along with Pair Distribution Function (PDF) analysis to study more local disorder effects. Many problems related to octahedral tilting in perovskites, phase transition, order-disorder phenomenon, presence of anionic vacancies, and visualization of diffusion paths in ionic conductors can be addressed by neutron scattering experiments.In this presentation I will discuss results for a recently developed in-situ neutron cell for the POWGEN beamline at the Spallation Neutron Source. I will present studies of the high temperature structure and oxygen stoichiometry of the SOFC anode materials Sr2MgMO6-δ (SMMO) and La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM). The SMMO structure shows distinct differences in crystal symmetry when compared to the previously reported room temperature structure, while LSCM serves as an example of our ability to map structure and properties over a wide range of conditions. Both samples show distinct anisotropy in the oxygen thermal ellipsoids, indicating preferred pathways for oxygen migration at high temperature.
10:30 AM - **C8.2
Transport Studies in Mixed Proton, Oxygen Ion and Electron (Hole) Conductors: Fuel Cells, Electroyzers, and Driven Fuel Cells.
Anil Virkar 1 , Niladri Dasgupta 1 2 , Greg Tao 2
1 Materials Science & Engineering, University of Utah, Salt Lake City, Utah, United States, 2 , Materials and Systems Research, Inc., Salt Lake City, Utah, United States
Show AbstractThis talk is on experimental and theoretical studies on high temperature fuel cells and electrolyzers. Analysis of transport through mixed proton, oxygen ion and electron (hole) conducting membranes is examined in terms of Onsager equations. The predominant transport is by protons and oxygen ions with electronic conductivity being small. The only permeation of H2, O2, and H2O is assumed to occur via a coupled transport of H+, O2-, and e (and/or h). Transport is analyzed for three cases: (1) Fuel cell under normal operating conditions, (2) A driven fuel cell, and (3) Electrolyzer. Transport equations are presented in the Onsager format. All coefficients are given in terms of cell parameters and the operating conditions. Spatial variations of chemical potentials of all neutral species through the membrane are determined as a function of operating conditions. Implications of the analysis performed and the results obtained will be discussed. A possible experimental procedure for the measurement of transport coefficients in the various modes of operation will be described. Experimental results will be presented on cells made of predominantly oxygen ion conducting materials operated in fuel cell and electrolyzer modes.
11:30 AM - **C8.3
Numerical Simulation of SOFC Electrode Polarization Using Three-Dimensional Microstructure Reconstructed by FIB-SEM.
Naoki Shikazono 1 , Nobuhide Kasagi 2
1 Institute of Industrial Science, University of Tokyo, Tokyo Japan, 2 Department of Mechanical Engineering, University of Tokyo, Tokyo Japan
Show AbstractA three-dimensional numerical simulation can provide information which cannot be obtained from experiments and be a powerful tool for investigating reaction phenomena at SOFC electrodes. In the present study, a dual-beam focused ion beam-scanning electron microscope is used to reconstruct three dimensional microstructures of the electrode, and electrode polarization is predicted by a lattice Boltzmann method. Ni-YSZ anode as well as mixed ionic and electronic conducting cathode (LSCF) are investigated. Local three-dimensional distributions of electrochemical potential and current densities inside the electrode microstructure are obtained. Large non-uniformities of potential and current streamlines are presented for the Ni-YSZ anode, while those are drastically improved in the LSCF cathode. Present method will be an effective tool for investigating local potential fields which affect local reactions, diffusions and physical properties of the SOFC electrodes.
12:00 PM - C8.4
In Situ Nanoscale Measurements of the Redox Behavior of Ceria Based Materials for SOFC Application.
Vaneet Sharma 2 , Peter Crozier 2 , Renu Sharma 1 2 , James Adams 2
2 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States, 1 Center for nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractCeria based materials have been recognized as potential candidates for solid-oxide-fuel-cell (SOFC) components. Anode performance is enhanced because ceria exhibits mixed ionic and electronic conduction (MIEC), due to its ability to switch between Ce4+ and Ce3+ oxidation states. Oxygen ion conductivity is improved in the intermediate temperature range (500 °C to 700 °C) when ceria is doped with aliovalent oxides like PrO2, Gd2O3, Sm2O3, Y2O3, ZrO2. In Pr-doped ceria (PDC), the total conductivity (ionic as well as electronic) has been found to be almost 2 to 3 orders of magnitude higher than that for pure ceria: this has been attributed to the presence of high concentrations of oxygen vacancies created during the low-temperature (≈ 200 °C) reduction of Pr4+ to Pr3+. In addition to its conductivity, the redox behavior of the PDC anode component in an SOFC is critical for the efficient conversion of chemical energy (from oxidation of hydrogen or hydrocarbons) to electrical energy. We have found that the nanoscale chemical heterogeneity inherent in PDC synthesized by spray drying method can affect SOFC anode performance.[1] We investigate this in detail, using an environmental scanning/transmission electron microscope (ESTEM) to follow the redox behavior of individual PDC nanoparticles.[2,3] We correlated the nanoscale redox behavior of individual particles with their chemical composition, structure (defect density) and morphology (size) using temperature-resolved low- and high-resolution imaging and electron energy-loss spectroscopy (EELS). The considerable overlap of the Ce and Pr signals in the energy-loss spectra can make analysis problematic, so we developed a new quantitative EELS method to resolve this problem, which we present. Finally, we also investigated the effect of chemical composition on the macroscopic redox behavior of PDC by thermo-gravimetric analysis (TGA) and compared this with the observed nanoscale behavior.References:[1]. V. Sharma et al., Chem. Phys. Lett. 495, (2010) 280-286.[2]. R. Sharma etal,, Microsc. & Microanal. (2003) 912 CD[3]. R. Sharma et al., Phil. Mag. 84, (2004) 2731-2747
12:15 PM - C8.5
Correlation of Surface Electronic Structure to Oxygen Reduction Activity on SrTi1-xFexO3 Model Cathode System.
Yan Chen 1 , WooChul Jung 2 , JaeJin Kim 2 , Zhuhua Cai 1 , Harry Tuller 2 , Bilge Yildiz 1
1 Laboratory for Electrochemical Interfaces,Department of Nuclear Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractKnowledge pertaining to the strength of oxygen adsorption, barriers to oxygen dissociation and incorporation into sub-surface regions and their relation to the surface electronic structure are essential for attaining highly active solid oxide fuel cell cathodes. While the d-band structure is a well established descriptor of ORR activity on transition metals, such a simple activity descriptor for perovskite oxide (ABO3) SOFC cathodes does not yet exist. The aim of this study is to indentify key characteristics of the cathode surface electronic structure that fundamentally correlate with ORR activity; such predictive knowledge could accelerate the design of new cathodes materials. Towards this objective, the SrTi1-xFexO3 (STF) solid solution is chosen as a model material system given its mixed conductivity coupled with the capability of controlling the magnitude of conductivity over orders of magnitude through Fe substitution. We probe the surface electronic structure for various levels of Fe content in STF (Sr(Ti0.95Fe0.05)O3 (STF5), Sr(Ti0.65Fe0.35)O3 (STF35) and SrFeO3(SFO)) and correlate the surface electronic structure differences to their ORR activities. The STF films were grown by pulsed layer deposition (PLD) on single crystal yttria-stabilized zirconia (YSZ) substrates. High resolution probing of the surface topographic and electronic structure is achieved via in situ Scanning Tunneling Microscopy and Spectroscopy (STM, STS) at high temperature and in O2 environment. Angle revolved X-ray Photoelectron Spectroscopy (XPS) within the same chamber is used to obtain the chemical composition with depth sensitivity from bulk to surface. The surface chemistry of the STF films was found to exhibit Sr enrichment, increasing with higher Fe content. The surface electronic structure as a function of Fe content evolves contrary to the dependence of bulk band gap on Fe content in this material; at 345 oC and oxygen pressure of 10^-3 mbar, STS shows that there is no gap present for the STF5 surface, and the band gap is 2.5eV to 3.6eV for STF35 and SFO, respectively, increasing with Fe content. The variation in the surface band gap is found to be driven by the energy position of the top of the valance band (VB), with a lower VB top for SFO, while the bottom of the conduction band (CB) is the same for STF35 and SFO. Furthermore, upon the partial removal of the Sr-segregated layer from the SFO surface by etching, the VB top position shifts upwards while the CB bottom position stays unchanged. These surface chemistry and electronic structure results suggest that: 1) the larger amount of SrO-related phase segregation on the surface may inhibit electron transfer to the adsorbed oxygen, and 2) the magnitude of the band gap and the positions of the bands relative to the vacuum level may play an important role in determining the ease of electron transfer to oxygen in ORR on SOFC cathodes.
12:30 PM - C8.6
Dynamic 3D FEM Model for Mixed Conducting SOFC Cathodes.
Andreas Haeffelin 1 , Jochen Joos 1 , Moses Ender 1 , Andre Weber 1 , Ellen Ivers-Tiffee 1 2
1 Institut für Werkstoffe der Elektrotechnik (IWE), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany, 2 DFG Center for Functional Nanostructures (CFN), Karlsruher Institut für Technologie (KIT), Karlsruhe Germany
Show AbstractElectrochemical impedance spectroscopy (EIS) is an established method for the analysis of complex electrochemical systems such as solid oxide fuel cells (SOFC) enabling a deconvolution of individual electrochemical processes at the electrodes [1]. In case of mixed ionic/electronic conducting (MIEC-) cathodes, a Gerisher type impedance is commonly applied to describe the coupling of oxygen surface exchange and oxygen bulk diffusion in the mixed conductor [2,3]. In these models nonlinear effect are not considered and the microstructure of the electrode is described by homogenized parameters as porosity and tortuosity. Therefore these models fail for signal amplitudes exceeding the linear regime as well as for inhomogeneous microstructures as graded electrode structures. We present a dynamic finite element method (FEM) model implemented in the commercial software package COMSOL Multiphysics. The model is based on our formerly published stationary model [4,5], which was extended to perform simulations in the time domain. The impedance spectra of a MIEC-cathode is calculated from a simulation of the current response due to a sinusoidal voltage stimulus at different frequencies or an appropriate voltage step.The model was validated by performing low-level signal simulations on a homogeneous microstructure and comparing them with the results calculated according to the homogenized models [2,3].By disabling the loss contribution of individual processes in the model as i) gas diffusion in the pores, ii) oxygen exchange between the gas phase and the mixed conductor, iii) oxygen ion diffusion in the mixed conductor iv) charge transfer between the MIEC cathode/electrolyte interface the impact of these processes on the impedance spectra will be presented for various MIEC cathodes differing in microstructure. Further studies will take the nonlinear behavior of the system into account. Due to the high computing power demands of the model, a rapid method for impedance modeling via potential step will be tested. [1] A. Leonide, V. Sonn, A. Weber and E. Ivers-Tiffée, J. Electrochem. Soc., 155 (1), p. B36 (2008).[2] S. B. Adler, J. A. Lane and B. C. H. Steele, J. Electrochem. Soc., 143 (11), p. 3554 (1996).[3] S. B. Adler, Solid State Ionics, 111 (1-2), p. 125 (1998).[4] B. Rüger, A. Weber and E. Ivers-Tiffée, ECS Trans., 7, p. 2065 (2007).[5] J. Joos, T. Carraro, A. Weber and E. Ivers-Tiffée, J. Power Sources, 196 (17), p. 7302 (2010).
12:45 PM - C8.7
Synthesis and Evaluation of Solid Oxide Fuel Cells Having Meso-Structured Ni-YSZ and LSM-YSZ Composite Electrode.
Jungdeok Park 1 , Jing Zou 1 , Nigel Sammes 1 , Jong-shik Chung 1
1 Department of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractThe in-situ techniques using catalyst impregnation methods provide the desired nano-structure of LSM(La0.8Sr0.2MnO3)-YSZ(yttria stabilized zirconia) composite cathodes. Among these techniques, wet chemistry methods such as cellulose-GNP or precipitation give composite cathodes with a large surface area. And its polarization resistance also is a smaller than that of solid state methods using physical mixing. But cathodes derived by wet chemistry methods are weak for the high temperature sintering. The meso-structured LSM-YSZ composite cathodes are prepared by silica hard template to overcome these limitations. This cathode having nano contacts between LSM and YSZ shows the superior performances than the reference cathode derived simple mixing. The synthesis conditions are investigated for electrochemical performances and structural analysis. The LSM-YSZ composite cathodes were prepared by 3-times repetition for LSM precursors impregnation to meso-porous YSZ backbone derived by KIT-6 silica template. This cathode shows the Rp value of 0.3 Ωcm2 at 800oC which is the half value of reference cathode derived by simple physical mixing. And its maximum powder density is 855 mWcm-2 at 800oC.Acknowledgements. This work was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (20093020030030) grant funded by the Korea Government Ministry of Knowledge Economy.