Oxide Ion Diffusion Research Articles

Effect of Cobalt-Doped Electrolyte on the Electrochemical Performance of LSCFO/CGO Interfaces

Solid oxide fuel cells (SOFCs) are efficient and environmental friendly devices which directly convert hydrogen and fosil fuels into electric energy. The operation temperature of these devices is mainly determined by the nature of its electrolyte. Conventional SOFCs are composed of yttria based electrolytes requiring operation temperatures around 1000ºC, whereas ceria based electrolytes have allowed reducing the temperature down to the 500-700ºC range. Nevertheless, these materials require sinterization temperatures in the 1300-1600ºC range to achieve complete densification, which produces very large grain sizes and poor mechanical properties. In the last years, several research works have demonstrated that the addition of transition metals oxides is effective in improving the densification and thus, lowering the sinterization temperatures of ceria based electrolytes. Such research works, generally focalize on how the addition of these sintering aids affect the properties (density, sintering temperature, conductivity) of the electrolyte. Nonetheless, it is well established that SOFC performance depends not only on the characteristics and performance of its individual components (i.e. cathode, electrolyte and anode) but also on their interfaces. Hence, a comprehensive study on how the addition of electrolyte sintering aids affects the cell performance is still missing. In this work, we have studied La0.4Sr0.6Co0.8Fe0.2O3-d/Ce0.8Gd0.2O2-d/La0.4Sr0.6Co0.8Fe0.2O3-d symmetrical cells containing Ce0.8Gd0.2O2-d electrolytes synthesized by a molten salt route [1] with and without Co oxide doping. La0.4Sr0.6Co0.8Fe0.2O3-d nanostructured cathodes with high electrochemical performance [2,3] were deposited by spin coating techniques on both sides of the electrolyte pellets. The assemblies were studied by Electrochemical Impedance Spectroscopy (EIS), X-Ray Diffraction and Transmission and Scanning Electron Microscopy. EIS results indicate that both electrolyte and cathode impedance response are affected by the addition of the sintering aid to the electrolyte, despite using cathodes with similar microstructure according to SEM observations. A possible explanation for such behavior was founded during TEM characterization which reveals the formation of columnar structures at the cathode/electrolyte interface of the cell with cobalt-doped electrolyte. These columnar structures would negatively affect the cell performance hindering the diffusion of oxide ions from the cathode to the electrolyte. [1] Mendoza-Mendoza E., Padmasree K., Montemayor S. and Fuentes A., J Mater Sci 47 (2012) 6076-6085. [2] Baqué L, Caneiro A, Moreno MS, Serquis A., Electrochem Commun 10 (2008) 1905-1908. [3] Soldati A., Baqué L., Troiani H., Cotaro C., Schreiber A., Caneiro A, Serquis A. Int J Hyd Energy 36 ( 2011) 9180-9188.

Effect of B-Cation Doping on Oxygen Vacancy Formation and Migration in LaBO3: A Density Functional Theory Study

LaBO₃ (B = Cr, Mn, Fe, Co, and Ni) perovskites, the most common perovskite-type mixed ionic-electronic conductors (MIECs), are promising candidates for intermediate-temperature solid oxide fuel cell (IT-SOFC) cathodes. The catalytic activity on MIECbased cathodes is closely related to the bulk ionic conductivity. Doping B-site cations with other metals may be one way to enhance the ionic conductivity, which would also be sensitively influenced by the chemical composition of the dopants. Here, using density functional theory (DFT) calculations, we quantitatively assess the activation energies of bulk oxide ion diffusion in LaBO₃ perovskites with a wide range of combinations of B-site cations by calculating the oxygen vacancy formation and migration energies. Our results show that bulk oxide ion diffusion dominantly depends on oxygen vacancy formation energy rather than on the migration energy. As a result, we suggest that the late transition metal-based perovskites have relatively low oxygen vacancy formation energies, and thereby exhibit low activation energy barriers. Our results will provide useful insight into the design of new cathode materials with better performance.

Atomistic modeling study of surface effect on oxide ion diffusion in yttria-stabilized zirconia

This paper reports the results of a molecular dynamics (MD) study of oxide ion diffusion in cubic yttria-stabilized zirconia (YSZ) using our recently developed interatomic potential [A.M. Iskandarov et al., J. Phys.: Condens. Matter 27, 015005 (2015)], which is based on the Tangney–Scandolo dipole model. We demonstrate that our potential can reproduce oxide ion conductivity in bulk YSZ with yttria concentrations varying from 2 to 14 mol%. We confirm the significant effect of free surfaces on oxygen diffusion by direct MD calculations at elevated temperatures and nudged elastic band analyses. It was found that oxygen vacancies tend to migrate from the (111) surface and locate in the bulk region, suppressing local oxide ion diffusion at the surface. In contrast, higher oxygen vacancy concentration and activation of oxide ion jumps along the 〈110〉 direction facilitate oxygen diffusion at the (110) surface. The MD results for the (110) surface are supported by density functional theory calculations.

ChemInform Abstract: Invited Review: Some Recent Developments in the Atomic‐Scale Characterization of Structural and Transport Properties of Ceria‐Based Catalysts and Ionic Conductors

AbstractReview: 107 refs.

Effects of Ba Adsorption Layer on NO Electrochemical Reduction Reaction Mechanism

This study investigated the electrochemical reduction of NO and O2 on Pt symmetric electrode by adding NOx adsorbents for the removal of NO from the exhausts of diesel engine or gasoline engine. O2 is excess when compared with NO. Thus, a large mount of external electric field should be consumed by electrochemical decomposition of O2 which gives rise to a low selectivity towards NO. Electrochemical reduction of NO has confronted with the difficult of improving NO selectivity in NO/O2 mixtures. Two cells were prepared and tested: single Pt electrode, Pt electrode with Ba(NO3)2 impregnation. Linear sweep voltammetry(0V~2V), cyclic voltammetry(CV:0.5V~-0.5V) and electrochemical impedance spectroscope(EIS) measurements were carried out between 350℃ and 550℃. The Pt electrode was covered with fine particles on the impregnated electrode surface by scanning electron microscope(SEM). It could be observed from the polarization curves that the Ba impregnated electrode showed higher electrochemical performance than the single Pt electrode especially at low temperatures and high voltages (1.25V to 2V). It can be obtained from the CV tests that the Ba impregnated electrode was superior to the Pt electrode in 800ppm NO. When scanned from 0.5V to -0.5V, the electrochemical performance of NO with O2 was far better than NO especially at high voltages and was close to the performance of O2. While changed the sweep direction backward, NO showed the best electrochemical performance than NO with O2 and O2 near open circuit voltage. EIS results at open circuit voltage revealed that the Ba impregnated electrode exhibited lower resistance than the single Pt electrode due to the decreased polarization resistance in the low-frequency region that dominated the impedance spectra in all atmospheres. The polarization resistances in 800ppm NO showed the smallest while in 8% O2 showed the largest in both electrodes possibly resulted from the generated NO2 formed by NO oxidation on Pt sites, the storage of NOx in the form of Ba(NO3)2 in active sites or the direct electrolysis of Ba(NO3)2 in external electric field that enhanced the reactivity and selectivity of NO reduction. From the impedance analysis using by equivalent circuit model R(CR)(CR)(CR) consisted of serial resistance Rs(related with the conductivity of the electrolyte) and 3 (CR) elements. Results showed that the activation energy of Rs for the two electrodes were nearly the same. The activation energy of the total polarization resistance Rp­ for the Ba impregnated electrode and Pt electrode were 0.874eV and 1.396eV respectively in 800ppm NO and 8% O2. The results were very close to the results obtained in the pure 8% O2 atmosphere. And the activation energy were 0.92eV and 1.50eV for the Ba impregnated electrode and Pt electrode. This was very close to the results of O2 reduction on Pt electrode conducted by Bauerle[1] with the activation energy between 1.43eV and 1.78eV demonstrated that the dissociative adsorption of O2 can be the controlling reaction step. The activation energy in the low frequency(0.01Hz-2Hz) for the three electrodes were close to the results showed by Bauerle resulted from the adsorption, surface diffusion, transfer of O2 and NOx intermediates near/at the triple phase boundary(TPB) and the dissociative adsorption of O2 [1]. The activation energy for the Ba impregnated electrode and Pt electrode in the high frequency were 0.608eV and 0.646eV respectively. The high frequency arc indicated the diffusion of oxide ions to the electrode/electrolyte interface and charge transfer of oxide ions from electrode/electrolyte interface to the electrolyte[2-3]. The activation energy in the middle frequency may be attributed to the charge transfer reactions in the electrode and the dissociative adsorption of O2. And the activation energy were 0.657eV and 1.06eV for the Ba impregnated electrode and Pt electrode. Results showed that the Ba impregnated electrode exhibited lower activation energy in middle and low frequency region indicating the improvement mechanism. Reference [1]J.E. Bauerle, J. Phys. Chem. Solids, 30, 2657 (1969). [2]M. J. Jørgensen, M. Mogensen, J. Electrochem. Soc., 148, A438 (2001). [3]M. L. Traulsenz, K. K. Hansen, J. Electrochem. Soc., 158, P156 (2011). Figure 1

Soft X-Ray Absorption and Photoemission Study on the Electronic Structure of (La,Sr)CoO3

A perovskite-type oxide (La,Sr)CoO3 (LSC) has been extensively studied as a cathode material of SOFC due to its high performance. In comparison to the other cathode materials, the bulk property of LSC can be characterized by a large oxygen nonstoichiometry, high oxide ion diffusivity, and high electronic (hole) conductivity, while it is not clear which is the factor governing the high catalytic and cathode performance. On the other hand, discussions have recently been made on the correlation between electronic structure and cathode properties. After evaluation of surface exchange coefficient and defect chemical analyses on a series of solid solution SrTi1-x Fe x O3, it was suggested that the overall cathode reaction kinetics is mainly determined by the density of minority electronic carrier in bulk, in other words, the position of the Fermi energy relative to the electronic band structure [1]. It is also proposed that the oxygen reduction reaction (ORR) activity of a series of perovskite oxides, although the reaction in aqueous electrolyte is concerned, is primarily correlated with the electronic state of the B-site transition element, that is, the occupation of e g orbital and extent of covalency against oxygen [2]. The fundamental understanding on the electronic structure of cathode materials is thus required. While there have been many studies on the electronic structure of LSC by means of x-ray spectroscopies and DFT calculations, most of them are concerned with the electronic transport and magnetic property, and sometimes lack determination of chemical situation of the specimens. In this study, the authors report the comprehensive analyses of the electronic structure for LSC by using soft x-ray absorption and photoemission spectroscopy in consideration of chemistry in bulk and the cathode reaction process. Dense specimens of LSC have been prepared via a citrate process, and the surface has been polished finally with 0.1um diamond abrasive. The specimens have been heat-treated under some conditions of temperature and oxygen partial pressure (Po2), and subjected to x-ray spectroscopic measurements. O K-edge and Co L-edge XAS spectra have been collected at BL-11A and -19B of Photon Factory, KEK, Japan. XPS spectra of the constituent core level and valence band have been collected with an x-ray photoemission spectrometer (PHI5000 / ULVAC-PHI). Fig. 1 shows the O K-edge XAS spectra of La0.6Sr0.4CoO3- d (LSC40) collected in electron yield mode. The main absorption feature (C) observed around 536eV is rather insensitive to the heat-treatment condition, and can be assigned mainly to La5d and Sr4d hybridization orbital. On the other hand, the low energy feature (A-B) ranging 526-531eV show a significant change depending on the heat-treatment condition. It is noted that among the T-Po2 conditions examined, the oxygen nonstoichiometry (3-d) changes in the following order: 2.95 under (T, Po2)=(873K, 1atm), 2.89 (1073K, 10-1atm), 2.83 (1073K, 10-3atm(Ar atmosphere)) [3]. The absorption intensity at the lowest energy part (A) drastically decreases upon reduction. From the XPS measurements, it is observed that the binding energies of the constituent core level, except for Co2p, increases upon reduction. The above observations suggest the following picture. With decreasing oxygen content, the electron hole introduced by Sr-doping reduces, leading to the decrease in the unoccupied density of state corresponding to the absorption feature (A). At the same time, the occupied density of states in the valence band increases and the Fermi energy upward shifts. In contrast to the O K-edge XAS spectra, the Co L-edge absorption spectra are overall less sensitive to the heat-treatment condition, apart from slight energy shift and appearance of shoulder structure. It is widely known that L-edge absorption structure of Co and also the other transition elements significantly changes according to the valence and spin states. It is therefore suggested that the charge compensation for extrinsic defects is practically taken by oxide ion rather than by Co; that is, the electron hole may be expressed as O-. This behavior can be ascribed to the large extent of covalency, and has been observed also for Fe-doped BaZrO3[4]. [1] W.C. Jung and H.L. Tuller, Adv. Energy Mater.1 (2011) 1184. [2] J. Suntivich et al., Nature Chem.3 (2011) 546. [3] M. Kuhn et al., J. Solid State Chem.197 (2013) 38. [4] D. Kim et al., Chem. Mater.26 (2014) 927. Figure 1

Formation of NiAl2O4 Layer on High Temperature Oxidation of Ni/Al2O3 Nanocomposites

Al2O3 composites dispersed with nano-Ni particles, Ni/Al2O3 nanocomposites, are candidates for high temperature structural applications because of excellent mechanical properties. Those nanocomposites have a self-healing function for surface cracks via high-temperature oxidation. In high temperature oxidation of Ni/Al2O3 nanocomposites, inward diffusion of oxide ions occurs mainly to form oxidation zone consisting of NiAl2O4 and Al2O3. Outward diffusion of cations also occurs to develop NiAl2O4 layer on the surface of Ni/Al2O3 nanocomposites. Surface cracks of Ni/Al2O3 nanocomposites are filled out by the surface NiAl2O4 developed by high temperature oxidation. Kinetics on growth of an NiAl2O4 layer was discussed in order to understand kinetics of oxidation and self-healing of Ni/Al2O3nanocomposites at high temperatures. Starting powder of Ni/Al2O3 nanocomposites was prepared by drying and reducing Al2O3 aqueous slurry with Ni nitrate. Volume fraction of Ni was ranged from 1 to 10 vol%. The powder was densified by pulsed electric current sintering at 1400°C for 5 min under 40 MPa in vac. Grain sizes of Al2O3 matrix and Ni particles were equal to 1 μm and 300 nm, respectively. The Ni/Al2O3 nanocomposites were oxidized at 1300°C in air. Thickness of the NiAl2O4 layer was measured as well as the oxidized zone of Ni/Al2O3nanocomposites. Growth of surface NiAl2O3 developed on Ni/Al2O3 nanocomposites,, as well as the oxidized zone, followed a parabolic manner. The parabolic rate constant on the growth of NiAl2O4 layer was proportional to the Ni volume fraction until 5 vol%. Reaching to 10 vol% in Ni volume fraction, the value of the parabolic rate constant increased remarkably. The fast growth of NiAl2O4 layer on 10 vol% Ni/Al2O3 nanocomposite would be caused by connection of NiAl2O4 grains in the oxidized zone because Ni diffusion in NiAl2O4 is faster than that in Al2O3 matrix.

(Invited) Determination of Effective Reaction Area in a Mixed-Conducting SOFC Cathode

Solid oxide fuel cell (SOFC) is one of the most promising next-generation energy conversion devices having high energy conversion efficiency. For its full-scale commercialization, oxygen reduction kinetics at the SOFC cathode, which frequently governs the performance of SOFC, should be improved. In order to achieve high performance of the SOFC cathode, it is necessary to optimize its material and structure. When a porous electrode consisting of a mixed ionic-electronic conducting oxides, such as (La,Sr)CoO3 and (La,Sr)(Co,Fe)O3, is used as an SOFC cathode, electrochemical oxygen reduction reaction occurs not only at the electrode/electrolyte interface but also on the surfaces of the oxide particles. In other words, the area where the electrode reaction occurs, i.e.the effective reaction area, expands to a certain distance from the electrode/electrolyte interface. It is believed that the effective reaction area in a porous mixed-conducting oxide electrode is controlled by the resistance of the surface reaction at the oxide surfaces and the resistance of the oxide ion diffusion in the oxide bulk. Based on this idea, many numerical simulations have been carried out to evaluate the effective reaction area in SOFC electrodes. However, as far as we know, no experimental evaluations of the effective reaction area have been done so far. From the backgrounds mentioned above, in this work, we tried direct observation of the effective reaction area in a SOFC cathode by using experimental manners. The (La,Sr)CoO3 was chosen as a mixed conducting oxide. For quantitative evaluation, the patterned thin film electrode, which was a kind of a columnar electrode modeling the porous electrode, was used. The in situ micro X-ray absorption spectroscopy (XAS) and the 18O/16O isotope exchange measurements under polarization were applied to determine the effective reaction area. Electrochemical impedance spectroscopy measurements were also performed with the same patterned thin film electrode, and the results were compared with those obtained by the XAS and the isotope exchange measurements. A patterned La0.6Sr0.4CoO3- d (LSC) dense film was fabricated on a Ce0.9G0.1O1.95(GDC) electrolyte by means of lithographic techniques. The LSC film was prepared by the pulse laser deposition (PLD) and its thickness was approximately 400 nm. The porous platinum electrodes were used as a reference and a counter electrodes. XAS measurements were carried out at the beam line BL37XU, SPring-8, JASRI, Japan. In this beam line, the incident X-ray from the synchrotron can be focused into micro or sub-micro meter size. The beam size used in this work was typically 0.8 x 0.5 mm. By using the focused X-ray beam, XAS measurements with the high special resolution of less than 1 mm became possible. During the cathodic polarization, the oxygen potential is reduced at the effective reaction area. Such an oxygen potential decrease accompanies the partial reduction of oxides, and the amount of the oxygen potential drop varies with the distance from the electrode/electrolyte interface depending on the amount of the reaction current. In this work, the partial reduction of the LSC electrode was evaluated as a function of the distance from the electrode/electrolyte interface by using in situ micro XAS measurements at the Co K-edge, and then the effective reaction area was determined. In 18O/16O isotope exchange measurements, the cathodic bias was applied to the patterned electrode after the pre-annealing at a desired temperature and p(O2) at least for 1 hour, and then 18O2 was introduced for 5 min. The cell was quenched down to room temperature immediately after the bias was terminated. The distributions of 18O and 16O were investigated by the secondary ion mass spectroscopy (SIMS) measurements. All the measurements were performed in the temperature range of 873-1073 K and the oxygen partial pressure p(O2) range of 10-4-1 bar under cathodic polarization of -0.1-0.25 V. At 873 K, both XAS and oxygen isotope exchange measurements suggested that the electrochemical oxygen reduction reaction takes place in the area of approximately 20 μm from the electrode/electrolyte interface under p(O2) of 1 bar and cathodic bias of -0.2 V. The effective reaction area tended to expand with increasing temperature, decreasing p(O2), and increasing cathodic bias. The results of electrochemical impedance spectroscopy measurements will be also given at the presentation.

Dislocations in SrTiO3: easy to reduce but not so fast for oxygen transport.

The effect of dislocations on the chemical, electrical and transport properties in oxide materials is important for electrochemical devices, such as fuel cells and resistive switches, but these effects have remained largely unexplored at the atomic level. In this work, by using large-scale atomistic simulations, we uncover how a ⟨100⟩{011} edge dislocation in SrTiO3, a prototypical perovskite oxide, impacts the local defect chemistry and oxide ion transport. We find that, in the dilute limit, oxygen vacancy formation energy in SrTiO3 is lower at sites close to the dislocation core, by as much as 2 eV compared to that in the bulk. We show that the formation of a space-charge zone based on the redistribution of charged oxygen vacancies can be captured quantitatively at atomistic level by mapping the vacancy formation energies around the dislocation. Oxide-ion diffusion was studied for a low vacancy concentration regime (ppm level) and a high vacancy concentration regime (up to 2.5%). In both cases, no evidence of pipe-diffusion, i.e., significantly enhanced mobility of oxide ions, was found as determined from the calculated migration barriers, contrary to the case in metals. However, in the low vacancy concentration regime, the vacancy accumulation at the dislocation core gives rise to a higher diffusion coefficient, even though the oxide-ion mobility itself is lower than that in the bulk. Our findings have important implications for applications of perovskite oxides for information and energy technologies. The observed lower oxygen vacancy formation energy at the dislocation core provides a quantitative and direct explanation for the electronic conductivity of dislocations in SrTiO3 and related oxides studied for red-ox based resistive switching. Reducibility and electronic transport at dislocations can also be quantitatively engineered into active materials for fuel cells, catalysis, and electronics.

Edge dislocation slows down oxide ion diffusion in doped CeO₂ by segregation of charged defects.

Strained oxide thin films are of interest for accelerating oxide ion conduction in electrochemical devices. Although the effect of elastic strain has been uncovered theoretically, the effect of dislocations on the diffusion kinetics in such strained oxides is yet unclear. Here we investigate a 1/2<110>{100} edge dislocation by performing atomistic simulations in 4-12% doped CeO2 as a model fast ion conductor. At equilibrium, depending on the size of the dopant, trivalent cations and oxygen vacancies are found to simultaneously enrich or deplete either in the compressive or in the tensile strain fields around the dislocation. The associative interactions among the point defects in the enrichment zone and the lack of oxygen vacancies in the depletion zone slow down oxide ion transport. This finding is contrary to the fast diffusion of atoms along the dislocations in metals and should be considered when assessing the effects of strain on oxide ion conductivity.

Bi1−xNbxO1.5+x (x=0.0625, 0.12) fast ion conductors: Structures, stability and oxide ion migration pathways

A combined experimental and computational study of Bi1−xNbxO1.5+x (x=0.0625 and 0.12) has been carried out using laboratory X-ray, neutron and electron diffraction, impedance measurements and ab-initio molecular dynamics. We demonstrate that Bi0.9375Nb0.0625O1.5625, previously reported to adopt a cubic fluorite-type superstructure, can form two different polymorphs depending on the synthetic method: a metastable cubic phase is produced by quenching; while slower cooling yields a stable material with a tetragonal √2×√2×1 superstructure, which undergoes a reversible phase transition into the cubic form at ~680°C on subsequent reheating. Neutron diffraction reveals that the tetragonal superstructure arises mainly from ordering in the oxygen sublattice, with Bi and Nb remaining disordered, although structured diffuse scattering observed in the electron diffraction patterns suggests a degree of short-range ordering. Both materials are oxide ion conductors. On thermal cycling, Bi0.88Nb0.12O1.62 exhibits a decrease in conductivity of approximately an order of magnitude due to partial transformation into the tetragonal phase, but still exhibits conductivity comparable to yttria-stabilised zirconia (YSZ). Ab-initio molecular dynamics simulations performed on Bi0.9375Nb0.0625O1.5625 show that oxide ion diffusion occurs by O2− jumps between edge- and corner-sharing OM4 groups (M=Bi, Nb) via tetrahedral □M4 and octahedral □M6 vacancies.

Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films.

In this study, the contribution of grain boundaries to the oxygen reduction and diffusion kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. (18)O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen diffusion but also fast oxygen exchange kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.

High temperature crystal chemistry of the n = 3 Ruddlesden–Popper phase LaSr3Fe1.5Co1.5O10 − δ

The crystal chemistry of the n=3 Ruddlesden–Popper phase LaSr3Co1.5Fe1.5O10−δ has been studied in the temperature range 20≤T≤900°C by in situ Neutron Powder Diffraction (NPD), and thermogravimetric and linear expansion measurements. The presence of oxygen vacancies at the O(2) and O(4) crystal sites, in the central perovskite layer, along with the variation of the bottleneck space available for oxygen migration with temperature at T>300°C indicates the O(4)–O(4) jumps predominate during oxide ion diffusion. Absolute oxygen content measurements support oxygen excess (>10.0) at temperatures below 300°C, which is unusual for the n=3 R–P phases. The total expansion in the temperature range 25≤T≤900°C, α=αV3=26.5 (1)×10−6K−1, is twice the values reported for the electrolytes. The linear expansion along the c-axis, αc=34.15 (1)×10−6K−1, is mainly absorbed by the perovskite block while the width of the rock salt layers remains nearly constant. Additionally, the oxygen chemical expansivity (βC) value determined for this layered compound, βC=0.670, was found to be approximately three times larger than those reported for the three dimensional perovskite system La1−xSrxCoO3−δ.

Improving Oxygen Transport in Perovskite-Type LaGaO3 Solid Electrolyte through Strain

Lattice strain is a promising possibility to improve materials performance in view of their application in thin-film devices. In particular, defect and transport properties in ionic conductors may be tailored through strain effects, since defect formation energy and migration barriers are correlated to structural parameters which, in turn, are influenced by strain-induced deformations. In this computational study we predicted that oxide-ion diffusion in perovskite-type lanthanum gallate can be improved through application of tensile strain. The structural deformations required to accommodate tensile lattice strain in the perovskite system are shown to result in a preferential localization of the oxygen vacancies in the equatorial plane of the GaO6 octahedra, while oxide-ion diffusion becomes anisotropic.

ReaxFF Reactive Force-Field Modeling of the Triple-Phase Boundary in a Solid Oxide Fuel Cell.

In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H2/Ni/YSZ and C4H10/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC.

One-Step Synthesis of Carbon Supported Nb2O5 Particles for Electrochemical Capacitor Applications by Oxidation of Layered Nb2C

The crystalline network of orthorhombic niobium oxide (T-Nb2O5) offers two-dimensional transport pathways for fast intercalation of lithium ions, leading to its high and rate independent intercalation pseudocapacitance. Unlike many other lithium intercalation metal oxides, T-Nb2O5 can be charged in short periods of time, making it suitable as a supercapacitor electrode material. This is due to the fast solid-state diffusion and no structural change of T-Nb2O5 upon intercalation of Li ions. For practical applications of Nb2O5 and other metal oxides in supercapacitors, thick electrodes with large mass loadings are necessary. This, however, will result in ohmic losses and ion transport limitations due the relatively low electrical conductivity of the oxide and limited ion diffusion through the electrode thickness. To address these limitations, we have successfully prepared the layered Nb2O5/carbon hybrid materials through controlled oxidation of layered Nb2C (also called Nb2C-MXene). As-prepared Nb2O5/C well inherited the two dimensional structure of the precursor with Nb2O5 nanocrystals formed between the MXene layers. A detailed study of oxidation temperatures and durations was conducted to achieve nanostructured of T- Nb2O5 and XRD, SEM and TEM studies show the uniform formation T-phase of Nb2O5 on carbon sheets after oxidation at 850℃. The electrochemical characterization of thick electrodes of synthesized material show very promising performance, with specific capacitance of about 350 F/g in 1M LiClO4 /EC/DMC (1:1 in volume ratio) electrolyte. The performance of developed electrodes is believed to be highly dependent of layered two-dimensional structure of Nb2O5/carbon hybrid materials.Acknowledgements: Authors acknowledge Dr. Mengqiang Zhao for his help with TEM studies. Support from DOE via Sandia National Laboratory is appreciated.

Oxygen concentration-related impedance spectroscopic studies of La2Mo2O9 oxide ion conductors

The frequency dispersion of impedance, relaxation, and modulus formalism under oxygen, air, and nitrogen atmospheres are performed on La2Mo2−x W x O9 (x = 0, 0.3, 0.5, 0.7) phases from 800 °C to room temperature. Thermal analyses on a dilatometer demonstrate the stabilization of the high-temperature cubic phase when the W content is increased. The whole frequency-dependent relaxations from the bulk effect including different oxide ion diffusion to an interfacial contribution are discussed. The oxygen concentration variation (Po2) has a noticeable influence on the interfacial relaxation of La2Mo2O9 (LMO). A more resistive LMO in the low frequency range with nitrogen treatment is observed differing from the previous conclusions. Besides, an ion dynamic study has been performed and the two sets of overlapping Z″ and M″ peaks in the intermediate temperature range indicate an additional oxide ion jump path in W-doped LMO ceramics.

Invited Presentation: Effect of Elastic Strain and Dislocations on Oxide Ion Diffusion and Oxygen Exchange Kinetics on Oxides

How can the oxygen non-stoichiometry, oxygen diffusion and oxygen reduction kinetics be altered in functional oxides by lattice strain? This question has recently attracted wide interest for enabling a new route for attaining fast oxygen exchange and transport kinetics, and obtaining fundamental answers to this question is important for a range of technologies including high performance solid oxide fuel cells, oxide based sensors, separation membranes and memristors. By applying mechanical stress, the inherent energy landscape of the reactions involved on the material can be altered. For example, in theory one can turn an endothermic reaction into an exothermic one, and reduce the energy barriers by applying lattice strain. This is the so-called mechano-chemical coupling, and when applied on electrochemical systems we can introduce the term mechano-electro-chemistry.In this talk, we will discuss recent work that probes quantitatively the mechanisms by which elastic strain impacts surface reactivity and diffusion kinetics in fluorite and perovskite oxides. Computationally, using ab initio and atomistic simulations, we discovered that lattice strain can accelerate the ionic transport in 8%Y2O3-ZrO2 by reducing the ion migration energy barriers. Combining surface chemical characterization on model thin films with ab initio modeling, we demonstrated for the first time that tensile lattice strain favors oxygen exchange kinetics on both the oxygen-deficient (La,Sr)CoO3-d and (La,Sr)MnO3-d and the oxygen-excess Nd2NiO4+d by impacting the energies of oxygen adsorption and dissociation, oxygen vacancy formation, oxygen diffusion, cation segregation and the electron transfer process. The favorable impact of tensile lattice strain on these elementary reactions was validated at the collective level by quantifying the oxygen surface exchange and diffusion kinetics with isotope exchange measurements on (La,Sr)CoO3-d. Lastly, we will discuss the impact of dislocations in ionic crystals, as doped CeO2 and ZrO2 on the distribution of charged defects and mobility of oxygen vacancies. These results together put forth both elastic strain and the dislocations to be important factors that impressively changes the kinetics of charge transport and reactivity in functional oxides.

(Invited) Increased Oxide Ion Diffusivity and Surface Exchange on Pr2NiO4 Base Oxide by Au Dispersion

Mass transport property in nano-size structure controlled materials are attracting much interest because such effects have a possibility to improve the performance of the electrochemical solid state devices such as Solid Oxide Fuel Cells (SOFCs). There are many approaches and ideas for the improvement of oxide ion conductivity in electrolyte by nano size effects. In this study, we demonstrated the 3 dimension (3D) tensile strain in Pr2NiO4, mixed conductor, was successfully introduced by dispersing Au particles in grain. The maximum lattice distortion estimated from X-ray diffraction measurement was 0.23 % at 2 mol% Au dispersion. Au dispersed Pr2NiO4 shows a large positive effect on oxide ion diffusivity and surface exchange coefficient. The doubled oxygen permeation flux was achieved with dispersing 2 mol% Au in PNCG at 873 K, indicating the tensile strain in PNCG lattice positively works for the oxygen permeation property. Since oxygen permeation is controlled by interstitial oxygen concentration and its mobility, further details study on effects of tensile strain on oxygen diffusivity were studied by using 18O tracer diffusion. The depth profiles of x mol% Au/PNCG (x=0, 1, 2, 3) are studied with SIMS analysis, and the estimated oxygen tracer diffusivity (D*) and the surface exchange coefficient (k*) are much increased with dispersing Au, and the enhancement in D value was well agreed with that in oxygen permeation rate. Redox titration measurement suggests that 3D tensile strain increased amount of excess oxygen. Interstitial oxygen in Pr2NiO4 is introduced in Rock salt layer of K2NiF4 structure, and this is origin for the fast oxygen diffusivity and high surface activity for oxygen dissociation. Recently, there is much attention of surface composition in oxide ionic conductors using the low-energy ion scattering (LEIS) technique to understand the surface reaction and degradation. LEIS analysis was also performed on Au dispersed Pr2NiO4 and it was found that surface of Pr2NiO4 was enriched with Pr. However, Pr segregation was prevented by introduction of Au and so tensile strain also positively works for preventing Pr segregation. On the other hand, surface oxygen content is also decreased by Au dispersion and so surface oxygen vacancy seems to be much enriched. Therefore, increased surface exchange coefficient by Au dispersion could be assigned to the high concentration of oxygen vacancy on surface of Pr2NiO4 dispersed with Au. In summary, although Au dispersion was not uniform, effect of tensile strain on oxide ion mobility could be observed in Pr2NiO4 by Au dispersion. Furthermore, tensile strain affected the surface composition, which may enhance the k* values. If the better dispersion state with fine particles could be achieved, there is a possibility to develop the higher oxide ionic conductor with uniform strained material.

Oxygen Transport in Perovskite-Type Materials for SOFCs Cathodes: What Can We Learn from Quantum Mechanics

Effective, rational design of innovative, low-cost materials with high oxygen reduction reaction (ORR) catalytic activity at intermediate temperatures (ITs) requires a deep understanding of the underlying reaction mechanisms of oxygen incorporation and diffusion in solid oxide fuel cell (SOFC) cathodes.We use state-of-the-art first-principles calculations in order to gain atomic scale insights into the relationships between composition, structure, and transport properties of traditional perovskite-type cathode materials ABO3 (A=La, Sr; B=Mn, Fe, Co, Cr) and the new, promising Sr2Fe1.5Mo0.5O6-δ electrode. Specifically, we study the factors that govern bulk oxide ion diffusion in these materials, namely, the oxygen vacancy formation energy and migration barrier, as well as the surface features that allow the dissociation and incorporation of oxygen.Beyond quantitative agreement with measurements, our results provide new insights into the origin of the observed material behavior. The electronic structure of the B-site transition metal, the A-site cation charge and the B-O bond nature are the main features that determine perovskite cathode performance. The subtle interplay of long-range electrostatics and local electronic effects is ultimately responsible for vacancy formation, ion and transport, and electronic conductivity. All these properties and processes can be fine-tuned by appropriate choices of A- and B-site constituent elements and aliovalent substitutions at each site. The ratio and nature of the additives are crucial for obtaining the desired performance.Our findings provide foundational and systematic understanding of perovskite bulk and surface properties and of the emergent ORR performance so that new generations of effective SOFC cathodes can be developed.[1] Muñoz-García, A. B.; Pavone, M.; Carter, E. A. Chem. Mater. 2011, 23, 4525–4536.[2] Pavone, M.; Ritzmann, A. M.; Carter, E. A. Energy Environ. Sci. 2011, 4, 4933–4937.[3] Muñoz-García, A. B.; Bugaris, D. E.; Pavone, M.; Hodges, J. P.; Huq, A.; Chen, F.; zur Loye, H.-C.; Carter, E. A. J. Am. Chem. Soc. 2012, 134, 6826–6833[4] Muñoz-García, A. B.; Pavone, M.; Ritzmann, A. M.; Carter, E. A. Phys. Chem. Chem. Phys 2013, 15, 6250–6259.[5] Ritzmann, A. M.; Muñoz-García, A. B.; Pavone, M.; Keith, J. A.; Carter, E. A. Chem. Mater. 2013, 25, 3011–3019.[6] Ritzmann, A. M.; Muñoz-García, A. B.; Pavone, M.; Keith, J. A.; Carter, E. A. MRS Commun. 2013, 3, 161–166.