John A. Kilner, Imperial College London
Juergen Janek, Institut Justus-Liebig-Universit
Bilge Yildiz, Massachusetts Institute of Technology
Tatsumi Ishihara, Kyushu University
I3: Strain Effects I - Dedicated to Arthur S. Nowick
Monday PM, November 26, 2012
Hynes, Level 3, Room 310
2:30 AM - *I3.01
Metastable Thin Films for Energy Applications: On Structural Lattice Anomalies and Electrical Transport
Jennifer L.M. Rupp 1 2 3 Bilge Yildiz 2 Harry Tuller 3
1ETH Zurich Zurich Switzerland2Massachusetts Institute of Technology Cambridge USA3Massachusetts Institute of Technology Cambridge USAShow Abstract
Ionically conducting thin films are of important for integration into miniaturized Si-based devices in the areas of energy processing. For single phase solid solutions such as doped-ceria electrolytes in fuel cells, the lattice constant is generally linearly dependent on the ion radius of the dopant with a classic “order parameter” describing cation-cation and oxygen vacancy long range order at these high dopant levels. Characteristics such as oxygen vacancy formation, their concentration and ion mobility rely on anion-cation spacing and coordination, and the nature of the dopant in these solid solution systems. State-of-the-art solid solutions are processed through sintering at temperatures far higher than those where transport properties and structural characteristics are investigated; stable structures and constant diffusion lengths persist. Despite the growing interest unequivocal elucidation of near and long range structural order changes and their implication for ionic diffusivities and mobilities have been unclear for films processed at low temperature far from classical sintering conditions. This work presents a case study experiment in which Ce0.8M0.2O1.9-x (M=Sc3+, Lu3+, Gd3+ and La3+) thin films are compared to sintered pellets with respect to their structural, chemical and electrical properties. In general, an increase in dopant radii resulted in a systematic lattice expansion for the films similar to that of the pellets. However, exceptionally large decreases in lattice constant and (micro-)strain were observed for the fully crystalline films with increasing temperature anneals, contrary to that observed for the pellets. Raman spectroscopy on the films confirms the compaction of the O-Ce-O bonds upon annealing. There is also a dependency of strain on solute-host radii mismatch. Scandia, prone to clustering to oxygen vacancies, reveals the largest microstrain and lattice strain changes. Gd-doped films exhibit a comparable activation energy (Ea) to bulk pellets of 0.71 eV for T>450°C. However, for T<450°C, the Ea decreases from 1.1 to 0.7 eV in the films annealed at 1000 oC compared to those annealed at 600 oC with frozen-in structural states. Interestingly, this coincides with the strain decrease from 0.3% to almost zero, lattice constant compression by -1.5%, and a shift in Raman. These results point to a decrease in the space available for migration and potentially a reduction in defect concentration upon high-temperature annealing of the films, recovering them from their metastable nature. This is thin film processing and doping dependent. This is especially relevant for T<450°C and Gd doped ceria films as electrolyte used in micro-fuel cells. Fundamentally, one may also remark that the observed structural changes are larger for a given dopant in a ceria film then to the effect of doping with different cations in bulk pellets.
3:00 AM - *I3.02
Atomistic Simulations of Oxygen Vacancy Migration in Strained CeO2 Electrolytes
Roger A De Souza 1 Judith Hinterberg 1 Tobias Zacherle 1
1RWTH Aachen University Aachen GermanyShow Abstract
There is increasing interest in enhancing the ionic conductivity of fluorite-based oxides, such as zirconia and ceria, through nanostructuring. Experimental investigations, however, give conflicting reports, with the nanoscale effect varying from slight decreases, through modest increases, to a “colossal” enhancement. In addition, the reasons for these nanoscale effects are seldom clear. Two effects are often claimed, without quantitative treatment, to be responsible: altered point-defect distributions in interfacial space-charge layers or modified migration barriers in the vicinity of the interface due to lattice strain. In this work the effect of lattice strain on oxygen vacancy migration in the fluorite-structured oxide CeO2 is investigated at the atomistic level. Static lattice simulation techniques, employing density functional theory (DFT) calculations, are used to determine the migration energies for oxygen vacancies, ΔEmig, in ceria lattices subjected to isotropic, uniaxial or biaxial strain. Analysis of the data yields the activation volumes, ΔVmig, and activation enthalpies, ΔHmig. Enhancement of the oxygen-ion conductivity of an oxide heterostructure through space-charge effects is also discussed.
3:30 AM - I3.03
Determination of the Role of Strain on Oxygen Ion Conductivity in Epitaxial Zirconia and Ceria Multilayers
Weida Shen 1 Jun Jiang 2 Joshua L. Hertz 1 2
1University of Delaware Newark USA2University of Delaware Newark USAShow Abstract
Solid oxide fuel cells (SOFCs) normally need to operate at >800 °C due in part to low oxygen ion conductivity of the traditional yttria stablized zirconia (YSZ) electrolyte. Finding ways to improve the low temperature oxygen ion conduction of the electrolyte, thereby decreasing the operating temperature, will play a decisive role in utilization of SOFCs in stationary or vehicular power applications. Recently, nanostructured multilayers that consist of two oxygen ion conductors (doped zirconia and ceria) or one oxygen ion conductor and one insulating oxide (e.g.,YSZ/Y2O3) have been reported to exhibit higher oxygen ion conductivity by orders of magnitude compared to that of either of their constituent layer materials. Particular interest is focused on the presence of lattice strains and interfacial regions with a high concentration of planar defects, as these are supposed to be the causes for the higher oxygen ion conductivity in these multilayers. These results remain controversial and difficult to repeat, especially in heteroepitaxial growth in which a large lattice mismatch exists at the interface. Here we present the structural and electrical properties of multilayers composed of ceria-zirconia solid solutions deposited via sputtering onto single crystal Al2O3 substrates. A unique thin film deposition route has been used, allowing the fabrication of multilayer films with composition controlled at the nanometer level. The strain induced at the interface between two layers was adjusted by altering the Ce/Zr ratios. The aim of this study is to quantify the influence of strain on oxygen ion vacancy mobility and concentration.
3:45 AM - I3.04
Effect of Lattice Expansion on the Ionic Conductivity in Gd-doped Ceria
Pratik Dholabhai 1 2 James B Adams 1 Peter A Crozier 1 Renu Sharma 1 3
1Arizona State University Tempe USA2Brookhaven National Laboratory Upton USA3National Institute of Standards and Technology Gaithersburg USAShow Abstract
One of the most important goals in Solid Oxide Fuel Cell (SOFC) research is to decrease the operating temperature. To realize this objective, it is imperative to identify electrolyte materials that show enhanced conductivity in the intermediate temperature range (773 K to 1073 K). Previously, we developed a Kinetic Lattice Monte Carlo (KLMC) model that takes into account repulsive interactions between vacancies to investigate oxygen diffusion in doped ceria and to compute ionic conductivity as a function of temperature and dopant concentration. One effect not included in this KLMC model was the effect of dopants on lattice parameter expansion that is experimentally observed. The effect of lattice parameter expansion for 20 % mole fraction of dopant content in Gd-doped ceria (GDC) was calculated using density functional theory (DFT+U) for an expanded 96-atom lattice, and resulted in slightly lower activation energies for oxygen vacancy migration. Inclusion of these revised energies in the KLMC model showed roughly an order of magnitude increase in ionic conductivity for GDC in the intermediate temperature range. Reasons for this remarkable increase in ionic conductivity as a function of lattice parameter expansion in GDC will be discussed. The large effect of lattice parameter expansion on ionic conductivity suggests that the addition of other dopants and co-dopants that expand the lattice further may be beneficial, and this effect may be an important guide in searching for new dopants for ceria. Overall, the ionic conductivities computed using the revised KLMC model are in reasonable agreement with experiment.
I4: Electrolytes I
Monday PM, November 26, 2012
Hynes, Level 3, Room 310
4:30 AM - *I4.01
From Perovskite to Apatite: Atomic-scale Insights into Defects, Transport and Surfaces of SOFC Materials
Saiful Islam 1
1University of Bath Bath United KingdomShow Abstract
It is clear that fundamental materials research is a crucial component in the discovery and characterisation of ionic and mixed conducting materials for solid oxide fuel cells (SOFCs). In this context, advanced computational techniques are now well-established tools for probing the properties of energy materials on the atomic- and nano-scale. This presentation will highlight recent studies on oxide-ion, proton and mixed conductors  focusing on perovskite-type materials (such as doped BaZrO3 and La2CoO4) and novel structures containing tetrahedral units (such as Si/Ge-based apatites). Key properties investigated include ion transport mechanisms, defect-dopant nano-cluster formation and surface structures. Our studies are closely correlated with related experimental work on these materials (e.g. diffraction, conductivity, solid-state NMR).  C. Tealdi et al., J. Mater. Chem., 22, 8969 (2012); P. Panchmatia et al., Angewandte Chemie, 50, 9328 (2011); L. Malavasi et al., Chem. Soc. Rev. 39, 4370 (2010).
5:00 AM - I4.02
Interstital Containing Complex Oxides as Potential IT-SOFC Electrolytes
Stephen Skinner 1 Ryan Bayliss 1 Ruth Sayers 2 Miguel Laguna-Bercero 3
1Imperial College London London United Kingdom2University of Liverpool Liverpool United Kingdom3Universidad de Zaragoza Zaragoza SpainShow Abstract
Energy generation and storage is an increasingly important area of materials development, with several competing technologies vying for commercialisation, each of which present technological challenges. One leading technology is the solid oxide fuel cell which typically is based on a ceramic oxide electrolyte with fast oxide ion conduction. Currently there are only a limited number of electrolyte materials available to developers due to the stingent conditions imposed on the materials (stable in a wide pO2 range, fast ion conduction, no reactivity with other components etc) and hence there is a need for new electrolyte materials. Electrolytes are typically based on a simple cubic structure type with oxygen vacancies introduced through substituion with aliovalent cations. In this work we report on an alternative strategy: that of introducing structurally complex materials with excess oxygen content leading to fast ion conduction. Here we report on the preparation of a substituted LaNbO4 based oxide in which additional oxygen content is accommodated through the adoption of a superstructure leading to interstital ion conducting pathways. The clearly oxygen excess materials have then been processed as dense ceramics and formed into single fuel cells with conventional anode and cathode layers, demonstrating no reactivity under operating conditions and generating a reasonable power ouput of > 100mW cm2 at 900oC with an OCV of 1V on a thick electrolyte. These are promising data that offer considerable scope for optimisation.
5:15 AM - I4.03
Al-doping Effects on the Oxygen Conduction in Apatite-type Lanthanum Silicate
Ting Liao 2 Taizo Sasaki 1 Ziqi Sun 3
1National Institute for Materials Science Tsukuba Japan2University of Queensland Brisbane Australia3University of Wollongong Wollongong AustraliaShow Abstract
The apatite-type lanthanum silicate is an ionic conductor showing the oxygen migration, and is regarded as one of the candidates of the electrolyte for the mid-temperature solid oxide fuel cell. We have recently indicated theoretically that the interstitialcy diffusion is the main conduction mechanism of La9.33(SiO4)6O2, and that it is strongly perturbed by the intrinsic lanthanum vacancy. From this theory, it will be expected that Al-doped silicate, La10(Si(2/3)Al(1/3)O4)6O2, would exhibit a lower activation energy by the elimination of the lanthanum vacancy. However, experiments have shown that the activation energy is not affected much by the doping. Here we propose the model on the effects of the Al doping to the oxygen conduction on the basis of the density-functional calculations. Presented are the calculation results of the stable configuration, the formation enthalpies, and the diffusion path of the oxygen related defects. The result has shown that the interstitial oxygen ion in the Al-doped apatite resides at the site forming the pyramid structure with the AlO4 cluster. This is contrast to that in the starting material, La9.33(SiO4)6O2, the split interstitial. It is predicted by the calculated formation enthalpies that this configuration stabilizes the oxygen interstitial to the oxygen vacancy even at rather oxygen poor condition. Moreover, similar to La9.33(SiO4)6O2, the oxygen interstitial in the double negatively-charged state will exhibits a lower activation energy in the migration than the positively charged vacancy. In spite of the preference of the oxygen interstitial, due to the significant change of the stable structure and the diffusion path, the substitution by Al will not lower oxygen a migration barrier of the oxygen interstitial. This research was partly supported by Grant-in-Aid for Scientific Research (C) (No. 22560663) from Japan Society for the Promotion of Science.
5:30 AM - I4.04
Novel Oxide-ion Conduction Mechanism via Tetrahedral Moieties in Apatite-type Fast Ionic Conductors
Katsuyuki Matsunaga 1 2 Kouta Imaizumi 1 Kazuaki Toyoura 1
1Nagoya University Nagoya Japan2Japan Fine Ceramic Center Nagoya JapanShow Abstract
Rear-earth silicate/germanate materials having an apatite-type crystal structure are known to exhibit high oxide-ion conductivities, and are expected as electrolytes for solid oxide fuel cells. However, the oxide-ion conductivity has to be further improved for the practical application, and thus the atomic-level ionic conduction mechanism is of scientific and industrial importance and should be clarified in more detail. In this study, first-principles calculations were performed to investigate the oxide-ion conduction mechanism in lanthanum silicate and germanate and discuss a physical origin of the fast oxide-ion conduction. A first-principles plane-wave based PAW method was used for the present calculations and minimum energy diffusion pathways and their potential barriers for oxide-ion conduction in different crystallographic directions were analyzed. In the case of lanthanum silicate, it was found that the energetically most stable interstitial O5 site is located close to the O4 column along the c axis. During the diffusion process, the interstitialcy mechanism with O4 is likely to occur for O5 diffusion along the c axis. In contrast, during O5 diffusion normal to the c axis, the interstitial O5 ion can favorably interact with SiO4 groups and form SiO5-like units as intermediate low-energy structure. After the SiO5 formation, an oxide ion in the SiO5 unit was readily released into the interstitial region in the crystal lattice. This result shows a novel oxide-ion diffusion mechanism that SiO4 groups can act as relay points of interstitial oxide ions to diffuse in lanthanum silicate. Such a relay mechanism was also confirmed for oxide-ion conduction in lanthanum germanate, which should be a key to explain the large oxide-ion conductivity observed experimentally.
5:45 AM - I4.05
Pulsed Laser Deposition of Gadolinium-doped Barium Zirconate Thin Films for Applications in Intermediate Temperature Solid Oxide Fuel Cells
Alex W. Skinner 1 Eric H. Remington 1 Alessandra Zenatti 2 Daniel Z. de Florio 2 Renato P Camata 1 2
1University of Alabama at Birmingham Birmingham USA2Universidade Federal do ABC Santo Andre BrazilShow Abstract
A major goal of ongoing research in Solid Oxide Fuel Cells (SOFCs) is the reduction of the operating temperature from the high temperature range (800-1000°C) to intermediate temperatures (500-750°C). Such a reduction could solve numerous reliability problems in SOFC systems and reduce the overall cost of cell stacks. This goal can be achieved by the development of new electrolyte materials with high ionic conductivity and/or by the reduction of the thickness of the electrolyte. In this work, we explore the use of pulsed laser deposition (PLD) in the synthesis of thin-film electrolytes based on gadolinium-doped barium zirconate (BZG). BZG has recently been predicted to have high protonic conductivity on par with or perhaps even superior to that of yttrium-doped barium zirconate, which currently holds the record for the highest measured thin-film protonic conductivity. BZG deposition was carried out using ablation targets prepared from commercially available barium zirconate (BaZrO3) and gadolinium oxide (Gd2O3) powders. The masses of the constituent powders were adjusted in suitable mixtures to yield targets containing 5, 10, and 15 mol. % of Gd in BaZrO3. Targets of pure BaZrO3 were also prepared for comparison. Targets where pressed at 2800 psi and annealed at 1200°C for 12 hours in air. By ablating these targets in a controlled environment, PLD is achieved on various substrates including silicon, platinum, and MgO. PLD was performed using a KrF excimer laser (248 nm) with energy density between 1 and 2 J/cm2 and pulse repetition rate of 30 Hz. Substrates were maintained at temperatures ranging from 500°C to 700°C. Deposition took place in O2 atmosphere at various pressures in the 40-200 mTorr range in a vacuum system with base pressure below 5.0×10minus;7 Torr. Based on deposition rate calibration data, PLD was carried out for sufficient time to achieve film thicknesses in the 1-2 mu;m range. X-ray diffraction (XRD) measurements on as-deposited films show reflections consistent with polycrystalline BaZrO3 in the cubic structure when films are deposited at temperatures above 650°C and O2 pressure of 200 mTorr. Scanning electron microscopy (SEM) studies on these crystalline films reveal smooth, high-quality surfaces with average grain size of approximately 200 nm. As-deposited films synthesized at O2 pressures in the 40-150 mTorr tended to be amorphous, as indicated by XRD measurements and by the absence of grain structure in the SEM analysis. All films crystallized upon post-deposition annealing at 850°C in O2-rich environment. Energy dispersive X-ray measurements indicate the successful incorporation of Gd in the films in concentrations that scale with the original Gd content in the ablation targets. Targets prepared with 5, 10, and 15 mol. % of Gd yielded films with Gd concentration of approximately 0.5, 1.0, and 1.9 at. %, respectively. Results of electrochemical impedance spectroscopy measurements on obtained films will be discussed.
I1:Cathode Surfaces I
Monday AM, November 26, 2012
Hynes, Level 3, Room 310
9:30 AM - *I1.01
Multilayer and Thin Film Mixed Conducting Oxides: Surface and Interface Effects
Harry L Tuller 1 Bilge Yildiz 2 Yan Chen 2 Di Chen 1 Sean Bishop 1 3
1MIT Cambridge USA2MIT Cambridge USA3Kyushu University Fukuoka JapanShow Abstract
The typical electrode in solid oxide fuel cells is complex both compositionally and microstructurally leading to difficulties in achieving reproducible performance and isolating the key rate limiting steps controlling performance. As a result, there has been great interest, in recent years, in studying dense thin films offering simple geometries and microstructures and good compositional reproducibility. Furthermore, thin films lend themselves well to high resolution analysis by scanning probe techniques including scanning tunneling microscopy (STM) capable of interrogating the surface electronic structure of the films. In this presentation, we begin by reviewing recent progress made in correlating the oxygen exchange kinetics exhibited by Sr(Ti,Fe)O3-δ mixed conductors with their electronic structure and the impact that segregated surface Sr species has on these kinetics. We next examine means for measuring oxygen nonstoichiometry in (Ce,Pr)O2-δ thin films and correlating that with their cathodic performance. We also demonstrate that these films exhibit significant chemical expansion under similar operating conditions. We finish by reporting recent studies on (La,Sr)CoO3-δ/(La,Sr)2CoO4+δ multilayers by STM which serve to explain the exceptionally high surface exchange rates exhibited by this materials couple.
10:00 AM - I1.02
Assessing First Principles Based Descriptors for Predicting Oxygen Reduction and Oxygen Revolution Reaction Activities of Transition Metal Perovskites
Yueh-Lin Lee 1 Yang Shao-Horn 1 Dane Morgan 2
1Massachusetts Institute of Technology Cambridge USA2University of Wisconsin-Madison Madison USAShow Abstract
Transition metal (TM= Cr, Mn, Fe, Co, and Ni) perovskites are current, and potentially future cost-effective catalysts for electrochemical energy conversion devices such as fuel cells, electrolytic water splitting devices, metal air batteries, etc. The materials have low cost as well as good ability to catalyze the oxygen reduction reaction (ORR) and oxygen evolution reactions (OER). While an ab initio Density Functional Theory (DFT) based descriptor approach has been very valuable in understanding and optimizing catalytic activity in ORR/OER metal catalysts, prediction of ORR/OER activity for TM perovskites using first-principles based descriptors is less well developed. Such descriptors are potentially more challenging to establish due to intrinsic errors of DFT for strongly correlated electronic systems and the significant complexity of TM perovskite surfaces. For example, while basic DFT methods predict clear trends in d-electron filling and oxygen binding energetics, it is not established that these trends correlate as expected with relevant experimental data on oxygen surface coverage, thermogravimetry, and catalytic activity. In this talk, a DFT based database, including both DFT-GGA and GGA+U approaches, are used to assess various electronic structure and binding energy based descriptors that have been proposed to effectively correlate with experimentally measured ORR/OER activities of TM perovskites. We assess these descriptors against measured low temperature ORR/OER and high temperature ORR activity data, and highlight issues in their application.
10:15 AM - I1.03
Sources of Chemical Heterogeneities on the Surface of Thin Film Cathodes
Wonyoung Lee 1 Jeong Woo Han 1 Zhuhua Cai 1 Bilge Yildiz 1
1Massachusetts Institute of Technology Cambridge USAShow Abstract
The slow rate of oxygen reduction reaction (ORR) at the cathode is one of the main barriers for implementation of high-performance solid oxide fuel cells (SOFCs) at intermediate temperatures (500-700 °C). The structural, chemical and electron transfer properties of the cathode surface are of importance to improve the ORR reactivity. However, the dynamic nature of surface structure and chemistry, driven by the harsh conditions of high temperatures and oxygen partial pressure in SOFCs, has resulted in the difficulty to probe and fundamentally understand the origin of the activation and deactivation of ORR kinetics on the surfaces. Cation segregation and phase separation on the surface of perovskite oxides has been commonly observed, but the exact mechanism for the segregation and its impact on the surface activity remains controversial. In this work, we systematically assessed the mechanisms of the cation segregation and phase separation on perovskite thin films by consideration of the elastic and electrostatic interactions. The effects of the elastic energy on the cation segregation were investigated with two control parameters, A-site stoichiometry and the lattice mismatch between the dopant (Ca, Sr, Ba) and the host cation (La) in doped lanthanum manganite as model system. The effects of the electrostatic energy were investigated by controlling the oxygen chemical potential during annealing, which influences the distribution of charged oxygen defects in the films. The surface cation stoichiometry, bonding states, and the surface structure were investigated at the film surface. The higher chemical stability upon annealing was observed on A-site deficient Sr-doped lanthanum manganite (LSM), and on the lanthanum manganite films with smaller size mismatch between the dopant and the host cation. The relatively higher stability is considered to be due to the excess space available to dopant in the A-site sub-lattice in the bulk. The oxygen vacancy distribution near the film surface affected the cation segregation because of the electrostatic interactions between the dopants and the oxide ion vacancies. Observed difference in extent of surface segregation and phase separation with the control of the elastic and electrostatic interactions suggests the different electronic and electrochemical properties on the surface. Density functional theory (DFT) calculations are performed to develop a physical model for the mechanisms of the cation segregation on perovskite surfaces. Electronic and electrochemical investigations of the film surface will be discussed to elucidate the impact of cation segregation and phase separation on the ORR kinetics at the cathode surface.
10:30 AM - I1.04
Dynamic Response of La0.6Sr0.4Co0.2Fe0.8O3-delta; Cathode under Applied Electrochemical Potential
Edith Perret 1 E. Mitchell Hopper 1 Jeffrey A. Eastman 1 Peter M. Baldo 1 Kee-Chul Chang 1 Brian J. Ingram 2 Hoydoo You 1 Paul H. Fuoss 1
1Argonne National Laboratory Argonne USA2Argonne National Laboratory Argonne USAShow Abstract
La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskites are promising mixed-conducting cathodes for solid oxide fuel cells (SOFCs) due to their catalytic activity. Previous studies have shown that the oxygen reduction process at the cathode surface is a rate-limiting step in the performance of SOFCs . Using in-situ synchrotron x-ray diffraction, the oxygen exchange reaction mechanism under applied electrochemical potential was studied using pseudo half-cells. The cells consisted of an epitaxial LSCF thin film (60 nm) grown on an yttria stabilized zirconia (YSZ) substrate with a gadolinium doped ceria (GDC) interlayer. The dependence of oxygen vacancy concentration on electrochemical potential was spatially resolved by measuring the c-lattice parameter with a 50 mu;m x-ray beam as a function of applied cathodic and anodic overpotential at different oxygen partial pressures (1.5, 15 and 150 Torr) in a temperature range from 350°C-600°C. Typically, the out-of-plane c-lattice parameter of LSCF increases with increasing oxygen vacancy concentration and temperature. We measured both the absolute change as well as the time dependence of the change in lattice parameters upon application of an electric potential with a time resolution of a tenth of a second. The transition from an initial state to steady state upon potential application is described in terms of a characteristic time constant tau;. Activation energies were extracted from the slopes of lines fitted to Arrhenius plots for both the lattice parameter and electrical current data. The time constants, surface exchange rates and activation energies of the current and of lattice parameter shifts are compared to determine average versus local (from the x-ray measurements) behavior. Time dependent correlations between oxygen vacancy formations, oxygen transfer rates and oxygen ion diffusion processes are revealed. 1. S. B. Adler, Chemical Reviews, 104, 4791 (2004).
10:45 AM - I1.05
Oxygen Exchange Kinetics on Solid Oxide Fuel Cell Cathode Materials - Mechanistic Interpretation and the Importance of Defects
Rotraut Merkle 1 Lei Wang 1 2 Anja Wedig 1 Yuri A. Mastrikov 1 3 Eugene A. Kotomin 1 Joachim Maier 1
1MPI for Solid State Research Stuttgart Germany2MIT Cambridge USA3University of Latvia Riga LatviaShow Abstract
The mixed conducting perovskites investigated as SOFC cathode materials cover a variety of compositions ranging from (La,Sr)MnO3-d (LSM) to (La,Sr)(Co,Fe)O3-d (LSCF) and (Ba,Sr)(Co,Fe)O3-d (BSCF). Measurements on pore-free samples (18O exchange for dense ceramics, e.g. ; impedance spectroscopy for dense PLD films on YSZ substrates, e.g. [2,3]) allows us to obtain effective rate constants k for the oxygen surface exchange reaction (unaffected by morphology changes as it would be the case for porous films). For the mentioned perovskites, the k values span a range of 5 orders of magnitude at 750°C. The analysis of possible correlations of k with (bulk) materials properties emphasizes the importance of ionic conductivity - i.e. high oxygen vacancy concentration as well as vacancy mobility - as a key factor for the surface oxygen exchange rate . This interpretation is corroborated by ab initio calculations for (La,Sr)MnO3-d , which indicate that the approach of an oxygen vacancy to oxygen intermediates adsorbed on the surface is the rate-determining step for a number of perovskites. Measurements of surface rate constant as well as oxygen diffusivity for BSCF perovskite films indicate that for this family of materials k has a linear correlation with the ionic conductivity . Having identified the importance of a high oxygen vacancy concentration as well as mobility for fast oxygen exchange, materials such as (Bi,Sr)(Co,Fe)O3-d are currently explored .  R.A. De Souza, J.A. Kilner, Solid State Ionics 126 (1999) 153  F. S. Baumann, J. Fleig, G. Cristiani, B. Stuhlhofer, H.-U. Habermeier, J. Maier, J. Electrochem. Soc. 154 (2007) B931  L. Wang, R. Merkle, J. Maier, J. Electrochem. Soc. 157 (2010) B1802  L. Wang, R. Merkle, Y.A. Mastrikov, E.A. Kotomin, J. Maier, J. Mat. Res. (2012) doi: 10.1557/jmr.2012.186  Y.A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin, J. Maier, J. Phys. Chem. C 114 (2010) 3017  A. Wedig, R. Merkle, B. Stuhlhofer, H.-U. Habermeier, J. Maier, E. Heifets, Phys. Chem. Chem. Phys. 13 (2011) 16530
I2: Cathodes I
Monday AM, November 26, 2012
Hynes, Level 3, Room 310
11:30 AM - *I2.01
Investigation of LSM Composite Cathode Stability during Operation and Its Effect on Long Term Durability of SOFCs
Hsiang-Jen Wang 2 Zhengliang Xing 1 Zhien Liu 1 Richard Goettler 1 Mark De Guire 2 Arthur Heuer 2
1Rolls-Royce Fuel Cell Systems (US) Inc. North Canton USA2Case Western Reserve University Cleveland USAShow Abstract
Small scale cells (including button cells, and one to five cells printed on an inert substrate) with segmented-in-series cell design were fabricated using a LSM composite as cathode, a Ni cermet as anode, and ScSZ as the electrolyte. These cells were tested under simulated fuel cell system operating conditions with moisture on the cathode side and reformate fuel (through the reversed water shift reaction of H2 and CO2) on the anode side. After 2000 to 8000 hours of testing, the LSM composite cathodes, especially the cathode/electrolyte interface, were characterized using transmission electron microscope (TEM) and electron energy-loss spectroscopy (EELS) for second phase formation, microstructural evolution, phase stability, and the presence of impurities. The detailed analysis revealed changes in the LSM and ionic phase during long-term operation under fuel cell system conditions. AC impedance was measured before and after durability tests of small-scale cells to understand electrochemical process changes associated with cathode chemistry and microstructural changes, and to deconvolute contributions to area specific resistance (ASR) from different components. The effect of cathode stability during fuel cell operation on long-term degradation will be discussed. A mitigation approach to improve LSM cathode stability during fuel cell operation was investigated. Small-scale cells with a new cathode formulations were tested under the same conditions. Cathode long-term durability and lower degradation rate were demonstrated. Post-test analysis confirmed that the modified LSM composite shows better phase stability after long term fuel cell operation.
12:00 PM - I2.02
Degradation and Stability of Complex Perovskites for Energy Applications
Maija Kuklja 1 David Fuks 2 Onise Sharia 1 Yuri A. Mastrikov 1 3 Eugene Kotomin 3 4
1University of Maryland College Park USA2Ben Gurion University of the Negev Beer Sheva Israel3University of Latvia Riga Latvia4Max Planck Institute for Solid State Research Stuttgart GermanyShow Abstract
Complex perovskite materials with the ABO3 lattice structure promise to improve efficiency of energy conversion devices due to a high concentration of oxygen vacancies and enhanced oxygen reactions. (Bax/LaxSr1minus;x)(Co1minus;yFeyO3minus;δ) (BSCF/LCSF) perovskites are considered among perspective materials for cathodes in SOFC and oxygen permeation membranes because a good oxygen exchange performance, mixed ionic and electronic conductivity, and low oxygen vacancy diffusion activation barrier. However, understanding of the interplay between the chemical composition, structural order/disorder, and crystalline stability in most perovskites is extraordinarily complex and essentially unexplored. Our first principles DFT and thermodynamics calculations of an ideal BSCF crystal, the crystal containing defects, a set of solid solutions, and phase diagrams suggest that while the hexagonal phase of BSCF is favored over the cubic phase for small oxygen deficiency, at low temperatures, the material decomposes into a mixture of cubic and hexagonal phases, which are likely to form grain boundaries and surface interfaces. This instability is considered a disadvantage for fast oxygen transport chemistry and impedes the applicability of BSCF-based SOFC. We discuss possible mechanisms of defect-induced (in)stability in the context of available experiments, explain the observed SOFC performance reduction, and provide insights on enhancing energy conversion in devices. This work is supported partly by GIF (project 1-1025-5-10/2009) and NSF.
12:15 PM - I2.03
Quantitative Microstructure Analysis and Electrochemical Activity of La0.6Sr0.4CoO3-delta; Electrodes Deposited by Spray Pyrolysis
Omar M. Pecho 1 2 Lorenz Holzer 1 Zhen Yang 3 Thomas Hocker 1 Robert J. Flatt 2 Julia Martynczuk 3 Ludwig J. Gauckler 3 Michel Prestat 3
1Zurich University of Applied Sciences (ZHAW) Winterthur Switzerland2ETH Zurich Zurich Switzerland3ETH Zurich Zurich SwitzerlandShow Abstract
Mixed ionic-electronic conducting La0.6Sr0.4CoO3-δ (LSC) is one of the state-of-the-art cathode materials for thin film and miniaturized SOFC  operated at intermediate temperatures. The electrochemical kinetics is believed to be limited by oxygen incorporation at the perovskite/air interface. Hence, for porous electrodes, increasing the number of sites for oxygen exchange either by making the electrode thicker or by producing nanosized LSC grains can improve the cathode performance. In this work, nanoporous LSC cathodes are deposited by spray pyrolysis onto gadolinium-doped ceria (GDC) electrolyte substrates. The as-deposited films are dense an amorphous. Different porous microstructures are produced by changing the heat-treatment, 600°C and 800°C for 4 hours in air in this study. XRD and DSC analyses reveal that the layers are crystalline after heat treatment. The electrode microstructure is analyzed using focus-ion beam nanotomography, continuous phase size distribution (c-PSD) and mercury intrusion porosimetry PSD (MIP-PSD). The latter methods, based on the analysis of 2D-micrographs and 3D-reconstructions allow for the determination of the phase sizes, volume fractions, surface areas and percolation levels of the various phases present in the annealed films. The area specific resistance (ASR) of symmetrical LSC/GDC/LSC cell is measured in air between 400 and 600°C by impedance spectroscopy. The 800°C annealed LSC layers, with an average grain size (r50) of ca. 30 nm show higher electrochemical performance than those annealed at 600°C (r50 = 12 nm), ca. 0.12 Omega;cm2 and 0.35 Omega;cm2, respectively. This is attributed to an improved percolation level of pores (ca. 99% vs. 60%) making more LSC surface available for oxygen exchange. The principle of c-PSD and MIP-PSD will be described. More results on the relationships between microstructural features (such as percolation factor, constrictivity, and tortuosity) and oxygen reduction activity will be reported.  A. Evans, S. Karalicacute;, J. Martynczuk, M. Prestat, R. Tölke, Z. Yáng, and L.J. Gauckler, ECS Transactions, 45 (2012) 333.  B. Muench and L. Holzer, Journal of the American Ceramic Society, 91 (2008) 4059.
12:30 PM - I2.04
Local Studies of Oxygen Vacancy Content, Vacancy Ordering, and Defect Configurations in SOFC Cathode Materials with High-resolution STEM
Albina Y Borisevich 1 Young-Min Kim 1 Anna N Morozovska 2 Donovan N Leonard 1 Michael D Biegalski 1 Eugene A Eliseev 2 Sergei V Kalinin 1
1Oak Ridge National Laboratory Oak Ridge USA2National Academy of Science of Ukraine Kiev UkraineShow Abstract
Lanthanum strontium cobaltites LaxSr1-xCoO3-δ (LSCO) are promising materials for solid oxide fuel cells, electrochemical sensors, and other energy storage and conversion technologies, with properties exhibiting a complex interplay of magnetic, ionic, and electronic phenomena. Spatial distribution and mobility of oxygen vacancies (influenced by vacancy ordering) are key to determining their functionality. Here, we determine local vacancy concentration using lattice parameter mapping by aberration-corrected Scanning Transmission Electron Microscopy (STEM), extending the concept of chemical expansivity to sub-unit cell scale. We use the collected data to develop a theoretical description of the vacancy ordering behavior. For the study we examine several compositions of LSCO grown by Pulsed Laser Deposition on Yttria-stabilized Zirconia (YSZ), La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) and NdGaO3 (NGO) substrates. The studied films show oxygen vacancy ordering, where alternating cobalt oxide planes are oxygen-depleted or stoichiometric. Vacancy ordering is not uniform across the samples and has multiple topological defects. Intriguingly, types and behaviour of these defects show similarities to classical ferroic systems including ferroelectrics and ferroelastics, such as “domain walls” separating vacancy ordered regions with different ordering direction, anti-phase boundaries with oxygen sublattice displaced by a fraction of the ordered unit cell (cation sublattice stays intact), and relaxor-like microstructures with small domains of ordered phase incorporated into disordered matrix. We can therefore treat vacancy ordering as a ferroic transition and develop a Ginzburg-Landau-Devonshire (GLD) description for this phenomenon. Using atomically resolved data on defect configurations in LSCO, we demonstrate that the gradient and interfacial terms for GLD description can be quantitatively determined. These results suggest that a predictive theory can be developed for vacancy ordering transitions, facilitating optimization of material performance. * This research is sponsored by the Materials Sciences and Engineering Division (YMK, SJP, SVK, DNL, AYB), Office of BES of the U.S. DOE, and by appointment (YMK) to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE. Research partially conducted at Center for Nanophase Materials Sciences (MDB), with is supported at Oak Ridge National Laboratory by Office of Basic Energy Sciences of the US DOE.
12:45 PM - I2.05
A High-temperature Moessbauer Study of Iron-doped Ruddlesden-Popper Phases Lan+1NinO3n+1
Klaus Dieter Becker 1 Piotr Gaczynski 1 Salvatore Cusenza 1 Tobias Klande 2 Armin Feldhoff 2
1Technische Universitaet Braunschweig Braunschweig Germany2Leibniz-University Hannover Hannover GermanyShow Abstract
In the past years, the layered rare earth nickelate Ruddlesden-Popper (R-P) type phases, Lnn+1NinO3n+1, have attracted attention mostly as potential alternate cathode materials for solid-oxide fuel cells (SOFC) as well as for oxygen separation membranes or nature gas separation. The lanthanum nickelate R-P structure is generated by stacking up n perovskite-type layers (LaNiO3) separated by rock salt layers (LaO) along the c-axis. The n = 1 member of this series, La2NiO4+x, adopts the K2NiF4 structure and consists of alternating perovskite and rock salt layers, whereas the n = infin; member corresponds to the three dimensional perovskite LaNiO3. La2NiO4+x can accommodate an extraordinarily large oxygen excess (0 < x le; 0.3), see e.g. Refs. [1,2]. In contrast the R-P compounds with n = 2 and 3 are mostly oxygen deficient. Herein we present a Mössbauer investigation of the Lan+1Nin-y57FeyO3n+1 series for n = 1, 2, and 3 with y = 0.02, 0.05, 0.1, and 0.9 in the temperature range from room temperature up to 1000°C and at oxygen activities, aO2, ranging between logaO2 = 0 and logaO2 = minus;4. The temperature dependent measurements provide information on the evolution of local structure around the nuclear probes. Especially in the case of the oxygen excess material La2Ni1-yFeyO4+x, the spectra show a clear dependence on oxygen activity and it is concluded that quadrupolar interactions increase with decreasing stoichiometry parameter x. Possible origins of these uncommon results and their connection with the oxygen disorder of the material are discussed. References  S. J. Skinner, Solid State Sci. 5 (2003) 419  A. Aguadero, J.A. Alonso, M.J. Martinez-Lope, M.T. Fernandez-Diaz, M.J. Escudero, L. Daza, J. Mater. Chem 16 (2006) 3402