Symmetric cells with porous La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) electrodes on Gd 0.1 Ce 0.9 O 1.95 (GDC) electrolytes were aged at 800 • C for 800 hours in ambient air. Electrochemical impedance spectroscopy (EIS) measurements performed periodically at 700 • C showed a continuous increase of the polarization resistance from 0.15 to 0.34 · cm 2 . Three-dimensional (3D) tomographic analysis using focused ion beam-scanning electron microscopy (FIB-SEM) showed negligible changes due to the ageing, suggesting that the observed resistance increase was not caused by electrode morphological evolution. However, an increased amount, by a factor of 3, of a water-soluble Sr rich surface phase on the aged LSCF electrode was detected by an etching procedure coupled with inductively coupled plasma-optical emission spectrometry (ICP-OES). The electrochemical analysis in combination with the microstructural parameters determined by FIB-SEM was used to examine the effect of Sr segregation on the rate of oxygen surface exchange, based on the Adler-Lane-Steele (ALS) model. 1-6However, a number of MIEC materials including LSCF exhibit Sr surface segregation, which has been proposed to hinder the oxygen surface exchange process. [7][8][9][10][11] This is believed to be an important issue for the stability of advanced SOFCs utilizing MIEC cathodes. Still, most of the Sr segregation observations have been made on thin film samples, where it is straightforward to measure surface composition using techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS).12-15 Recently Rupp et al. 16 reported a novel technique utilizing chemical etching with on-line inductively coupled plasma optical emission spectrometry (ICP-OES) detection to successfully quantify Sr-rich surface phases on dense La 0.6 Sr 0.4 CoO 3-δ (LSC) thin films. There is only one report of a measurement of a practical porous LSCF electrode that showed Sr segregation, measured via XPS, along with electrochemical performance degradation. 11The distinction between thin film and bulk electrode samples is potentially important. Thin films are often characterized by columnar growth with very high grain boundary densities, they may exhibit significant stress, and the thermal history is usually very different from that of porous electrodes (preparation mostly below 800• C). Although the measured properties of well-prepared polycrystalline thin-film electrodes normally agree reasonably well with values from bulk materials, 17 in some cases such as epitaxial thin film electrodes, properties may deviate significantly.18 Also, SOFC stacks with LSCF cathodes have been shown to provide reasonably stable long-term performance, 19 an observation that appears to be at odds with the Sr segregation and related degradation observed for thin-film LSCF.In this work, we investigated the degradation mechanisms, especially Sr surface segregation, of porous LSCF cathodes. LSCF symmetric-electrode cells with Gd 0.1 Ce 0.9 ...

Quasi-reversible oxygen exchange/point defect relaxation in an amorphous In-Ga-Zn-O thin film was monitored by in situ electrical property measurements (conductivity, Seebeck coefficient) at 200 °C subjected to abrupt changes in oxygen partial pressure (pO2). By subtracting the long-term background decay from the conductivity curves, time-independent conductivity values were obtained at each pO2. From these values, a log-log “Brouwer” plot of conductivity vs. pO2 of approximately −1/2 was obtained, which may indicate co-elimination (filling) of neutral and charged oxygen vacancies. This work demonstrates that Brouwer analysis can be applied to the study of defect structure in amorphous oxide thin films.

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The work functions of various amorphous and crystalline transparent conducting oxides (TCOs) were measured using Kelvin probe. The films, made by pulsed laser deposition, exhibited varying work functions dependent on the composition and deposition parameters. Tin oxide showed the largest work functions of the oxides measured, while zinc oxide showed the lowest. Binary and ternary combinations of the basis TCOs showed intermediate work functions dependent on the endpoint components. Amorphous TCOs, important in OPV and other technological applications, exhibited similar work functions to their crystalline counterparts. UV/ozone treatment of TCOs temporarily increased the work function, consistent with proposed defect mechanisms associated with near-surface changes in carrier content and Fermi level. Finally, a method for facile adjustment of the work function of commercial TCOs by atomic layer deposition (ALD) capping layers was presented, illustrated by the growth of zinc oxide layers on commercial crystalline ITO films.

Nanocrystalline gadolinia‐doped ceria (GDC) specimens with grain sizes ranging from 10 to 100 nm were studied by AC‐impedance spectroscopy over the temperature range of ∼150°–∼300°C, and were analyzed by the nanograin composite model (n‐GCM), which is capable of extracting local properties (grain‐core conductivity, grain‐boundary conductivity, grain‐boundary dielectric constant) and also grain‐boundary width. The grain‐core dielectric constant, a necessary input parameter for the n‐GCM procedure, was measured separately on a microcrystalline GDC specimen sintered from identical powders. In spite of modest increases in grain‐boundary conductivity at the nanoscale, the total conductivity exhibited a monotonic decrease with decreasing grain size. This behavior was attributed to the large increase in the number of grain‐boundary barriers at the nanoscale, which overwhelms the slight increase in grain‐boundary conductivity. An unusual “up‐and‐down” behavior was observed in grain‐boundary conductivity versus grain size, which was accounted for by a similar trend in the preexponential factor versus grain size. Effective grain‐boundary widths, also determined by the n‐GCM, exhibited a similar “up‐and‐down” behavior, which probably reflects the differences in thermal history from specimen‐to‐specimen.

Effective grain core or single crystal dielectric constants of four solid oxide electrolytes—yttria‐stabilized zirconia (YSZ with 8 and 7 mol% yttria), tetragonal zirconia polycrystals (TZP with 3 mol% yttria), strontium‐ and magnesium‐doped lanthanum gallate (LSGM), and gadolinia‐doped ceria (GDC, with 20 mol% gadolinia)—were studied as a function of temperature by AC‐impedance spectroscopy and equivalent circuit fitting. Unlike their undoped counterparts, the acceptor‐doped oxides consistently exhibited an upturn in effective grain core/single crystal dielectric constant in the vicinity of 350–500 K. This temperature‐dependent behavior was attributed to the onset of thermally activated dipolar relaxation—an explanation supported by the appearance of loss tangent peaks at higher temperatures. Differences between “effective” dielectric constants and frequency‐dependent complex dielectric constants for the grain core/single crystal frequency range, and applications of the effective dielectric constant data are discussed.

In recent years, mixed ionic-electronic conducting (MIEC) materials such as (La,Sr)(Fe,Co)O3-δ(LSCF) have been studied and developed as solid oxide fuel cell (SOFC) cathodes, due to their activity for the oxygen reduction reaction at intermediate temperatures (< 800°C) [1-4]. On the other hand, a number of such MIEC materials including LSCF exhibit Sr surface segregation, which has been proposed to hinder the oxygen exchange reaction at the surface [5-8].  In this work, we present the study of long-term degradation mechanisms of porous La0.6Sr0.4Co0.2Fe0.8O3 (LSCF6428) cathodes under thermal annealing. LSCF6428 symmetric electrode cells with Gd0.1Ce0.9O1.95 (GDC) electrolytes were maintained at an elevated SOFC operating temperature of 800°C for ~ 800 hours in ambient air, without current/polarization. As illustrated in Fig. 1, electrochemical impedance spectroscopy (EIS) measurements taken periodically at 700°C showed a polarization resistance increase of ~ 120%, from 0.15 to 0.33 Ω∙cm2. The electrode morphological changes and Sr surface segregation were examined using a combination of three-dimensional (3D) tomography via focused ion beam-scanning electron microscopy (FIB-SEM) and surface composition measurements using XPS and a chemical etching procedure with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection [9]. The 3D imaging showed that there was no coarsening or sintering of the LSCF6428 electrode microstructure that could affect electrochemical performance during annealing. However, the ICP-OES analysis found an increased amount of water-soluble Sr on the surface of annealed samples, from 1.3 nmol Sr/cm2 to 3.9 nmol Sr/cm2, when normalized to the LSCF6428 particle surface measured from 3D image data. Assuming that the measured Sr phase is SrO, the water-soluble Sr amount on freshly prepared cells would correspond to 1.04±0.22 atomic layers and agree well with reports on Sr-doped perovskite-type model thin films suggesting a SrO termination [9]. The Adler-Lane-Steel (ALS) model was then applied, again making use of 3D image data, to examine the effect of Sr surface segregation on the oxygen surface exchange process. Fig. 1. EIS results showing polarization resistance measured for a single LSCF6428 cathode during 800°C anneal. Temperature was temporarily reduced to 700°C during EIS measurements and back to 800°C afterwards. References [1] S. B. Adler, Chem. Rev., 104, 4791 (2004). [2] L. W. Tai, M. M. Nasrallah, H. U. Anderson, D. M. Sparlin, S. R. Sehlin, Solid State Ionics, 76, 259 (1995). [3] L. W. Tai, M. M. Nasrallah, H. U. Anderson, D. M. Sparlin, S. R. Sehlin, Solid State Ionics, 76, 273 (1995). [4] A. Esquirol, N. P. Brandon, J. A. Kilner, M. Mogensen, J. Electrochem. Soc., 151, A1847 (2004). [5] Z. Pan, Q. Liu, L. Zhang, X. Zhang, S. H. Chan, J. Electrochem. Soc., 162, F1316 (2015). [6] L. Zhao, J. Drennan, C. Kong, S. Amarasinghe, S. P. Jiang, J. Mater. Chem. A, 2, 11114 (2014). [7] Y. Liu, K. Chen, L. Zhao, B. Chi, J. Pu, S. P. Jiang, Int. J. Hydrogen Energy, 39, 15868 (2014). [8] S. P. Simner, M. D. Anderson, M. H. Engelhard, J. W. Stevenson, Electrochem. Solid State Lett,, 9, A478 (2006). [9] G. M. Rupp, A. Limbeck, M. Kubicek, A. Penn, M. Stӧger-Pollach, G. Fredbacher, J. Fleig, J. Mater. Chem. A, 2, 7099 (2014). Figure 1