Lanthanum Strontium Cobaltite Research Articles

Heater-assisted intense pulsed light irradiation for lanthanum strontium cobaltite thin film electrode fabrication

This study describes the bottom-heater-assisted flash light irradiation method for fabrication of lanthanum strontium cobaltite (La0.6Sr0.4CoO3−δ, LSCO) thin films. Unlike the conventional sintering process, flash light irradiation proceeds instantly under ambient conditions. Since the flash light irradiation process involves the photothermal effect, heat energy supplied by a bottom heater can compensate for decreased irradiation energy. The substrate temperature was varied from room temperature to 200 °C and 300 °C, and the radiation energy requirement for obtaining an optimized LSCO film was decreased with increasing temperature. In addition, the instantaneous temperature difference due to highly intense energy between the top surface and the substrate can be relieved. With the innovative sintering system, an electrolyte-supported solid oxide fuel cell could be fabricated by the flash light irradiation method, and electrochemical characterization of the fuel cells was conducted.

80,000 current on/off cycles in a one year long steam electrolysis test with a solid oxide cell

The current ON/OFF switching of a solid oxide electrolysis cell is treated as elementary step for power variation in a steam electrolyser system. If the cell voltage in the ON mode is adjusted to the thermal neutral voltage, heat generation remains zero in both modes, which largely facilitates the thermal management. To verify whether the cells withstand the switching, an electrolysis durability test with an electrolyte supported solid oxide cell was performed during one year at about 850 °C. The cell consisted of a 3YSZ electrolyte, CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. The test included two operation blocks with each 40,000 cycles of 2 min duration and a current density of −0.7 Acm−2 in the ON mode (−0.07 Acm−2 in OFF mode), as well as steady-state ON periods with 5800 h duration. Voltage degradation was 5 mV/1000 h (0.4%/1000 h) and the increase in the area specific resistance 7 mΩcm2/1000 h, without notable dependence on current cycling. Impedance spectroscopic results were in agreement with the only small switching transients seen in the cell voltage; moreover, they confirmed a dominating ohmic degradation together with minor contributions from gas conversion and reaction, respectively. No electrode delamination was detectable after scheduled test completion.

Segregation Resistant Nanocrystalline/Amorphous (La, Sr) CoO3-(La,Sr)2CoO4 Composite Cathodes for IT-SOFCs

There is a considerable interest in lowering the operating temperature of solid oxide fuel cells. In this respect, (La,Sr)CoO3-(La,Sr)2CoO4 dual phase oxides have attracted much attention as cathode materials due to their enhanced oxygen reduction reaction kinetics. The main drawback in lanthanum strontium cobaltite cathodes is Sr-segregation at operating temperatures which causes a sudden degradation in performance. In the current study, this segregation is verified by a specially designed experiment where a (La0.8Sr0.2)CoO3 - (La0.5Sr0.5)2CoO4 bilayer is deposited and annealed for an extended period of time. Thin film cathodes are then deposited via co-sputtering of (La0.8Sr0.2)CoO3 and (La0.5Sr0.5)2CoO4 yielding non-crystalline structures with acceptable area specific resistance values at temperatures as low as 575°C. The stability of these cathodes is investigated over an extensive range of compositions (La0.8Sr0.2)CoO3: (La0.5Sr0.5)2CoO4 = 0.10:0.90 - 0.90:0.10. Prolonged annealing of cathodes at temperature of the same initial area specific resistance shows an exceptionally stable cathode performance as measured by electrochemical impedance spectroscopy responses. It is therefore concluded that co-sputtered (La0.8Sr0.2)CoO3 - (La0.5Sr0.5)2CoO4 dual phase cathodes with their amorphous/nanocrystalline structures, especially at mid-compositions, provide an extremely stable microstructure with a strong resistance to Sr segregation.

Hydrothermal Synthesis, Characterization, and Sintering Behavior of Core-Shell Particles: A Principle Study on Lanthanum Strontium Cobaltite Coated with Nanosized Gadolinium Doped Ceria

In this work, nanostructured (La0.6Sr0.4)0.99CoO3 (LSC)-Ce0.8Gd0.2O1.9 (CGO) core-shell particles were prepared by precipitating CGO nanoparticles on the surface of LSC particles under hydrothermal conditions. The as-prepared core-shell particles were sintered by spark plasma sintering (SPS) and conventional sintering, and the microstructure evolution and densification behavior were studied. Dense microstructures were reached using both sintering methods at relatively low temperatures. In the case of SPS, the core-shell architecture was partially maintained and nano-structured CGO grains were formed, while conventional sintering led to the formation of larger CGO grains. This work covers a detailed characterization of (a) the individual LSC-CGO core-shell particles and (b) the composites after densification.

Heater-Assisted Intense Pulsed Light Irradiation for Lanthanum Strontium Cobaltite Thin Film Electrode Fabrication

The wet chemical processes show many benefits over vacuum processes on account of their simplicity, versatility, and low cost. The films prepared via this method are advantageous for industrial applications. But inevitably, these processes require post-treatment for sintering and conventional method like furnace is still costly in terms of time and energy. Intense pulsed light irradiation is an innovative approach to proceed the post heat-treatment rather than using furnace which spend much time for process. By employing Xenon arc lamp that be used to cover the spectral range from about 400nm up to 950nm (1), white flash light can be irradiated and sinter the specimen in a moment. In this study, we employed metal organic decomposition method for lanthanum strontium cobaltite (LSCO) film coating and introduce two steps of irradiation process for effective sintering (2); pre-heating step to get rid of remnant organics and main-heating step for further sintering process. Pre-heating is long and weak step that gradually increases the temperature while main-heating is short and strong step that instantly increases. But in case of main-heating, there are some potential risks with process. During main-step process, substrates could be exposed to high flash light energy and could eventually be shattered by thermal shock. For this reason, we had tried to relieve the shock by reducing the input power of intense pulsed light irradiation. Considering that photo-thermal effect is the one of the factor of white flash light sintering process, the shortage of energy for sintering process was filled by employing heat energy supplied by bottom heater. Varying the bottom temperature to RT, 200oC and 300oC, the input energy required for optimized LSCO film was obviously decreased with temperature. Diffraction pattern obtained by XRD also confirms the effect of bottom heat by indicating perovskite peak only for 200oC and 300oC, while flash light irradiated sample at room temperature shows nothing on account of energy shortage. In addition, unlike irradiated sample at room temperature, FE-SEM images of heat-assisted intense pulsed light LSCO show porous surface morphology, which is suitable for cathode of solid oxide fuel cells (SOFCs). Here, electrolyte supported solid oxide fuel cell was fabricated by heat-assisted intense pulsed light irradiation method. Thermal shock to electrolyte was slacken off, and porous cathode could be obtained with this method. Electrolyte substrate could hardly not withstand high irradiation energy under room temperature. Fabricated solid oxide fuel cell shows reasonable performance under low temperature regime. The significance of this work is the feasibility of heater-assisted intense pulsed light irradiation method. These result shows the possibility to expand the applicable targets which are deposited on thermal shock vulnerable substrate. References H.-J. Hwang, W.-H. Chung and H.-S. Kim, Nanotechnology, 23, 485205 (2012). E.-B. Jeon, S.-J. Joo, H. Ahn and H.-S. Kim, Thin Solid Films, 603, 382 (2016).

A Comparative Study of Durability of Solid Oxide Electrolysis Cells Tested for Co-Electrolysis under Galvanostatic and Potentiostatic Conditions

State-of-the-art SOECs consisting of a nickel-yttria stabilized zirconia (Ni-YSZ) fuel electrode, YSZ electrolyte and lanthanum strontium cobaltite ferrite-gadolinium doped ceria (LSCF-GDC) composite oxygen electrode were tested under co-electrolysis (H2O+CO2) conditions. The aim in this study was to compare the SOEC durability under co-electrolysis conditions between galvanostatic and potentiostatic modes. Specifically, the cells were operated at 0.75 A/cm2 (galvanostatic) and at 1.2 V (potentiostatic) at 750°C for over 1000 hours. In both modes, a larger degradation was observed initially for the first 200 hours of testing, followed by a more stable performance over longer operating times. Interestingly, there was a difference in trends of serial and polarization resistances’ evolution. In galvanostatic mode of operation, both increased while for potentiostatic mode only the polarization resistance increased over time. The difference of the degradation was attributed to the overpotentials being experienced by the cells in the respective modes. Trends of the area specific resistance (ASR) and detailed electrochemical analysis of the performance of the cell under durability conditions for both modes indicated that the degradation was due to both the fuel electrode and the oxygen electrode, with an additional contribution from fuel electrode in galvanostatic testing. Microstructural analysis also confirmed the degradation of the active fuel electrode.

Predicting Oxygen Vacancy Polaron Size from D-Orbital Splitting in ABO3 (A=La, Sr; B=Mn, Fe, Co) Perovskites

Mixed Ionic Electronic Conducting (MIEC) transition metal oxide (ABO3) perovskites are utilized for catalytic,1 energy conversion,2,3 and other applications. The oxygen ionic conductivity of these MIECs depends on their oxygen vacancy concentration and ionic mobility. In turn, the oxygen ion concentration and mobility both depend on the charge of the B-site atom,4 which itself depends on factors such as the A site La/Sr ratio, the crystal structure, etc. Previous calculations demonstrated that the Fe atom charge state distribution observed in strontium ferrite (SrFeO3-δ) is caused by differences in the d-orbital splitting of octahedral (Oh) versus square pyramidal (SP) Fe-O coordination (Figure 1).5 This d-orbital splitting ensures that the two electrons left behind when a charge-neutral oxygen vacancy forms in cubic SrFeO3-δ are transferred to the second nearest Fe neighbors,5 resulting in a large oxygen vacancy polaron size that produces strong oxygen vacancy interactions at high oxygen nonstoichiometry. As a result, the oxygen vacancy formation energy increases with oxygen nonstoichiometry in cubic SrFeO3-δ.5 Previous calculations also demonstrated that in orthorhombic lanthanum ferrite (LaFeO3-δ) the formation of a neutral oxygen vacancy produces a polaron that is localized to the oxygen-vacancy-adjacent Fe atoms. Hence, in orthorhombic LaFeO3-δ the oxygen vacancies do not interact with increasing oxygen nonstoichiometry.6 The objective of the present work was to examine whether d-orbital splitting could also explain the oxygen vacancy behavior of other MIEC perovskites. Hence, the impact of d-orbital splitting on oxygen vacancy polaron size and formation energies in lanthanum strontium manganite and lanthanum strontium cobaltite were modeled for the first time. Here, the charge redistribution caused by oxygen vacancy formation was studied for six different compositions: LaFeO3-δ, SrFeO3-δ, LaMnO3-δ, SrMnO3-δ, LaCoO3-δ, and SrCoO3-δ. The GGA+U method with PAW potentials implemented in VASP was utilized for all the structural energy calculations. First, the Hubbard-U parameter was calibrated to describe the charge on the B-site atom (U = 3 was selected for Fe, as done previously).6 The oxygen vacancy formation energy as a function of oxygen non-stoichiometry (δ) was then calculated at 0 K in vacuum by varying the size of the supercell, with oxygen vacancy interactions being enabled by the periodic boundary conditions. The calculated polaron size for neutral-oxygen-vacancy–containing ABO3 structures are summarized in Table 1.In LaMnO3-δ, Mn3+ has an [Ar] 3d4 electronic configuration in Oh coordination. After the formation of a neutral oxygen vacancy, the two Mn atoms adjacent to the oxygen vacancy go from Oh to SP coordination and the excess electrons left behind by the removed oxygen are pushed to the a1g level of the second nearest neighboring Oh-Mn (instead of remaining localized to the b1g level of the SP-Fe). This long range charge transfer results in large polaron size. This behavior is in contrast with that of LaFeO3-δ. However, LaMnO3-δ and SrFeO3-δ exhibit comparable polaron sizes since Fe4+ has an [Ar] 3d4 electronic- configuration. In SrMnO3-δ, Mn4+ has a [Ar] 3d3 electronic configuration in Oh coordination. Therefore, the formation of a neutral oxygen vacancy localizes the electrons to the oxygen vacancy adjacent Mn atoms, producing a small polaron, as explained by the d-orgbial splitting in Figure 1. This d-orbital splitting analysis indicates that the polaron size in LSM > LSF > LSC. Additional work aimed at calculating the oxygen vacancy formation energies of other ABO3 compositions are in progress.

23,000 h steam electrolysis with an electrolyte supported solid oxide cell

An electrolyte supported solid oxide cell of 45 cm2 area was operated in the steam-electrolysis mode during more than 23,000 h before scheduled shutdown, of which 20,000 h with a current density of j = −0.9 A cm−2. The cell consisted of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers between electrolyte and electrodes, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. Voltage degradation in the operation period with j = −0.9 A cm−2 was 7.4 mV/1000 h (0.57%/1000 h) and the increase in the area specific resistance 8 mΩ cm2/1000 h. The final cell voltage was 1.33 V (at 851 °C cell temperature). After dismantling, the cell showed no mechanical damage at electrolyte and H2/H2O electrode; a small fraction of the oxygen electrode was delaminated. Impedance spectroscopy applied at the steady state DC current density confirmed a degradation dominated by an increasing ohmic term, mainly due to ionic conductivity decay in the electrolyte. In addition, a small non-ohmic and at least partly reversible O2 electrode contribution to degradation was identified, affected by a pollution from the (compressor) purge air.

Superior La1 − xSrxCoO3 − δ ceramic electrode fabrication by MOCSD for low-temperature SOFC application

Metal-organic chemical-solution-deposited lanthanum strontium cobaltite (LSC, La1−xSrxCoO3−δ) nanoporous thin films were fabricated for use as cathodes for low-temperature solid oxide fuel cells (LT-SOFCs). Electrochemical performance and durability were evaluated under continuous operation by comparing with typical Pt-cathode SOFCs. In the initial stage of operation, SOFCs with Pt cathodes showed higher output power density for its superior catalytic activity. However, the overall performance of SOFCs with Pt cathodes gradually degraded due to thermal instability, while the performance with LSC cathodes remained stable for 10h of continuous operation. In addition, by taking advantage of wet chemical solution methods, the amount of dopant was systematically varied to investigate its effect on fuel cell performance (La1−xSrxCoO3−δ, x=0.4, 0.5, 0.6). The fuel cell sample with the dopant ratio x=0.5 showed the highest peak power density independent of the operating temperature. This result implies that LSC cathodes deposited by chemical solution deposition show better operational durability than Pt cathodes and provided potentiality of replacing precious metal cathode for low-temperature SOFCs.

Rapid fabrication of chemical-solution-deposited La0.6Sr0.4CoO3−δ thin films via flashlight sintering

This study demonstrates the sintering effectiveness of an intense pulsed light (IPL) sintering method for the fabrication of lanthanum strontium cobaltite (La0.6Sr0.4CoO3−δ, LSCO) thin films via xenon flashlight irradiation. LSCO thin film was coated on silicon (100) wafer via metal organic chemical solution deposition (MOCSD) methods and the flashlight irradiation process preceded instantly under ambient room temperature conditions. The properties of flashlight irradiated films were compared with thermally sintered ones as reference target. Electrical property, surface morphology, and crystallinity were observed to evaluate effectiveness of the flashlight sintering method by employing four-point probe apparatus, FE-SEM, and XRD, respectively. Considering that flashlight sintering process is procedurally accomplished in milliseconds under ambient room temperatures, this method may be able to circumvent several issues from the conventional thermal sintering process in realizing a convenient and available sintering process. The significance of this work lies in the demonstration of xenon flashlight sintering methods of perovskite oxides that are fabricated via wet chemical solution methods.

Correlation between Structural, Chemical, and Electrochemical Properties of La0.6Sr0.4CoO3–dNanopowders for Application in Intermediate Temperature Solid Oxide Fuel Cells

Lanthanum–strontium cobaltites are excellent mixed ionic and electronic conductors and exhibit good activity for the oxygen reduction reaction (ORR) at temperatures higher than 500 °C. This led to these materials being proposed as cathodes for solid-oxide fuel cell devices, either for the standard high temperature (SOFC, 900–1000 °C) or the intermediate temperature (IT-SOFC, 500–700 °C) range. Although it was determined that crystal structure and oxygen vacancies play a central role in these properties, little is known about cobalt speciation at SOFC or IT-SOFC working temperatures. In this study the cobalt speciation and local order of La0.6Sr0.4CoO3–d (LSC) is analyzed in detail for the first time in nano- and microstructured powders by means of in situ experiments using X-ray absorption spectroscopy (XAS) in the temperature range of 20–700 °C. The electrochemical performance of LSC-based cathodes in symmetrical cells under the same conditions is also studied using electrochemical impedance spectroscopy (EIS). Cobalt atoms can be found in any of the three possible oxidation states. Their relative amount depends on temperature and crystallite size. In crystallites of a few tens of nanometers, the ease with which Co atoms change their oxidation state is enhanced. This effect probably facilitates oxide transport in LSC-based cathodes.

In SITU Transmission Electron Microscopy on Operating Electrochemical CELLS

Solid oxide cells (SOC) have the potential of playing a significant role in the future efficient energy system scenario. In order to become widely commercially available, an improved performance and durability of the cells has to be achieved [1]. Conventional scanning and transmission SEM and TEM have been often used for ex-situ post mortem characterization of SOFCs and SOECs [2,3]. However, in order to get fundamental insight of the microstructural development of SOFC/SOEC during operation conditions in situ studies are necessary [4]. In this work, we use advanced chip-based TEM holders to analyse SOFC/SOEC samples during exposure of reactive gas flows, elevated temperatures and electrical biasing in combination. This allows monitoring the nanostructure development under temperature and electrode polarisation conditions similar to operation conditions. Specifically, an in situ analysis of a symmetric SOFC/SOEC cell is analysed inside the TEM under different reaction conditions. In order to perform in situ experiments while drawing a current through the sample, we used a homemade TEM chip [5,6] and an 80-300kV Titan ETEM (FEI Company) equipped with an image corrector and a differential pumping system. A symmetric cell was prepared by depositing three thin films on a strontium titanate (STO) single crystal substrate by pulsed laser deposition (PLD). Lanthanum strontium cobaltite La0.6Sr0.4CoO3- δ (LSC) was chosen as electrode and yttria stabilized zirconia ZrO2: 8% mol Y2O3 (YSZ) as electrolyte. High resolution TEM analysis on PLD samples after the deposition did not reveal any second phase formation at the interface between YSZ and LSC. An in situ experiment was firstly conducted in vacuum at temperature between 25 oC and 900 oC. Secondly, it was repeated in presence of oxygen with an oxygen partial pressure of about 2 mbar and a maximum temperature of 750 oC. Subsequently, the symmetric cell will be exposed to oxygen at 600 oC and 1 V overpotential within the ETEM. In order to do that, a symmetric cell has been placed on a chip with the use of a focus ion beam (FIB) microscope, see figure 1. STEM-EDS with a JEOL 3000F TEM, STEM-EELS and Energy Filter (EF) TEM with a 1MV JEOL ETEM [7] were used for ex situ post mortem analysis. Finally, a bulk symmetric cell, coming from the same batch as the in situ treated TEM samples, was tested in a furnace with similar environmental conditions. This comparison is vital for distinguishing possible surface diffusion effects caused by having a thin lamella for in situ TEM analysis. Electrochemical properties were also investigated by electrochemical impedance spectroscopy (EIS). Figure 2 compared results from two different in situ TEM experiments where for the images (a) and (b) the sample was kept in vacuum and exposed to different temperatures, and for the images (c) and (d) the sample was exposed to different temperatures and an oxygen partial pressure of 2 mbar. The comparison of the two experiments revealed a faster grain growth for the sample exposed to both oxygen and high temperature. For the sample analysed in vacuum, ulterior analysis showed an agglomeration of cobalt at the interface between STN and LSC, figure 3. [1] A. Atkinson et al., Nature Materials 2004, 3, 17-27. [2] R. Knibbe et al., Journal of The Electrochemical Society 2010, 157, B1209-B1217. [3] N. Imanishi et al., Solid State Ionics 2006, 177, 2165-2173. [4] M. L. Traulsen et al., ECS Transactions 2015, 66, 3-20. [5] C. Kallesøe et al., Nano Letters 2012, 12, 2965-2970. [6] S. B. Alam et al., Nano Letters 2015, 15, 6535-6541. [7] N. Tanaka et al., Microscopy 2013, 62, 205-215. Figure 1

Need for In Operando Characterization of Electrochemical Interface Features

It has proven particularly difficult to determine the electrode reaction mechanisms in high temperature solid oxide cells (SOCs) that convert gases. The literature is full of contradictory statements and apparently contradictory findings. Often the same type of electrochemical kinetics that apply to low temperature aqueous systems are assumed valid for SOCs, but in our opinion this has not been fruitful as they do not describe the experimental findings properly. Classical room temperature wet electrochemistry has experienced a huge progress in understanding of the electrode reaction mechanisms during the recent 2 decades. This progress has to a large extent been based on combination of electrochemical characterization and in situ and in operando and in situ surface analysis techniques, which so far have been less developed for high temperature electrochemistry above 300 °C. In spite that such techniques have only recently started becoming available for SOC electrochemistry, they are strongly needed. The high temperature solid-solid and solid-gas interfaces tend to change a lot over time due to segregation of electrolyte and electrode constituents and unavoidable trace impurities on a level of few ppm. Furthermore, a porous electrode for solid-gas reactions has three phase boundaries (TPBs), where the electrolyte, the electrode and the gaseous reactants meet. The current density will be concentrated around the TPB. Also, the TPB seems particularly prone to collect trace impurities and minority components, probably because the TPB zone has many sites with higher free energy relative to the rest of the electrode and electrolyte surface. An example of the segregation is the enrichment of yttria to the yttria stabilized zirconia (YSZ – the common SOFC electrolyte) surface, which takes place during a few hours at operation temperature. Furthermore, most often a silica rich layer will form on top of the yttria enriched layer. These “interphase” (not interface) layers may grow and change over time and with changes in temperature and other test conditions. Such segregation seems to be equally pronounced for surfaces and interfaces of the popular perovskite structured metal oxide electrodes such as lanthanum strontium manganites or cobaltites on which a several nanometer thick skin of strontium rich oxide forms already during cell preparation and it is believed that this is changing significantly during electrode operation. However, our knowledge about the driving forces for and the kinetics of the formation of the interphases is very superficial. Thus, there is a strong need for in operando techniques that can characterize and monitor the development of the mentioned features as function of time and changing experimental conditions with respect to electrical, structural and chemical properties at the nano-scale. Going through the various known techniques, it becomes clear that there are not sufficient in operando techniques available to make a comprehensive electrode characterization, and therefore in situ techniques are usually employed, in which at least one of the operation conditions are fulfilled, e.g. temperature but not atmosphere is matching relevant operation conditions. Finally, our analysis of already published results points out the advantage of combining several different techniques such as electrochemical impedance spectroscopy with in operando scanning probe microscopy and surface sensitive chemical analysis methods. Examples of results will be presented.

Long-term Steam Electrolysis with Electrolyte-Supported Solid Oxide Cells

Steam electrolysis over 11000h11The test was still running at the time of submission of the revised work, exceeding 14 000 h operation time. with an electrolyte-supported solid oxide cell is discussed. The cell of 45cm2 area consists of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel steam/hydrogen electrode. After initial 2500h operation with lower current-density magnitude, the current density was set to j=-0.9Acm−2 and the steam conversion rate to 51%. This led to a cell voltage of 1.185V at 847°C cell temperature. Average voltage degradation was 7.3mV/1000h (<0.6%/1000h), the increase in the area specific resistance was 8mΩcm2/1000h, sufficiently low for application in practical electrolysers. The electrical-to-chemical energy-conversion efficiency was ηel>100% throughout the test (with an external heat source for evaporation). Impedance spectroscopic measurements revealed a degradation almost entirely due to increasing ohmic resistance. The rate of resistance increase was initially faster (up to 40mΩcm2/1000h) and stabilised after several 1000h operation. After 9000h a small (non-ohmic) electrode degradation became detectable (<2mV/1000h), superimposed to ohmic degradation. The small electrode degradation is understood as indication for largely reversible (electrolysis cell/fuel cell) behaviour.

Fabrication and co-sintering of thin tubular IT-SOFC with Cu2O–GDC cermet supporting anode and Li2O-doped GDC electrolyte

The present work describes the fabrication of thin tubular Cu2O-based anode supported solid oxide fuel cells (SOFCs) with dense and crack free 10mol% gadolinia-doped ceria (GDC10) electrolyte by single step co-sintering. Dilatometric analysis was performed to analyze the effects of the Cu2O:GDC10 ratio and of different Li2O doping amounts on sintering behavior. The densification of the electrolyte, deposited on the green anodes by dip coating, was also analyzed. Issues related to insufficient densification and cracking of the electrolyte upon sintering and cooling were discussed with respect to anode sintering behavior and thermal expansion coefficient (TEC) mismatch. Good quality thin tubular cells composed of 35vol% Cu2O–(1mol% Li2O-doped GDC10) anode, 1mol% Li2O-doped GDC10 electrolyte and lanthanum strontium cobaltite ferrite (LSCF)–GDC10 cathode were fabricated by single-step co-sintering at 1020°C.

Evaluation of porous platinum, nickel, and lanthanum strontium cobaltite as electrode materials for low-temperature solid oxide fuel cells

Porous Pt, Ni, and lanthanum strontium cobaltite (LSC) are evaluated as electrode materials for solid oxide fuel cells at the low temperature range under 500 °C. Porous metal electrodes 150 nm thick are prepared by sputtering. Porous LSC was deposited to a typical thickness of 1.5 μm by pulsed laser deposition as the cathode. In terms of fuel cell performance, we confirm that Pt is the best material for both the cathode and the anode under 400 °C, but LSC outperforms Pt as a cathode at temperatures over 450 °C in our configurations. Porous Ni anode is identified as being less effective than the porous Pt. It is determined that these results are closely related to the differences in electrode performance and to morphological changes during fuel cell operation.

Stability of manganese-oxide-modified lanthanum strontium cobaltite in the presence of chromia

In order to restrain the decomposition and conductivity degradation of perovskite-type conductive material in the presence of chromia, manganese oxide modification of lanthanum strontium cobaltite has been studied. La0.7Sr0.3CoO3−δ (LSC) and MnO2-modified LSC coatings are applied onto Ni–Cr alloy and exposed to long-term oxidation text to examine their chemical stability. In a LSC coating, chromium species migrating from the Ni–Cr alloy could induce the decomposition of LSC and produce SrCrO4 and Co–Cr spinel oxides. In contrast, in the MnO2-modified LSC, Sr is stable and the low-conductivity phase SrCrO4 phase is rarely seen even the coated alloy has gone through 1000 h of oxidation tests at 800 °C. It highlights that MnO2 modification could greatly improve the stability of LSC under Cr-rich conditions. The study of solid state reactions reveals that the influence of MnO2 is mainly due to the reaction between MnO2 and LSC, instead of the direct reaction between MnO2 and chromium oxides.

In situ examination of oxygen non-stoichiometry in La0.80Sr0.20CoO3−δ thin films at intermediate and low temperatures by x-ray diffraction

Structural evolution of epitaxial La0.80Sr0.20CoO3−δ thin films under chemical and voltage stimuli was examined in situ using X-ray diffraction. The changes in lattice parameter (chemical expansivity) were used to quantify oxygen reduction reaction processes and vacancy concentration changes in lanthanum strontium cobaltite. At 550 °C, the observed lattice parameter reduction at an applied bias of −0.6 V was equivalent to that from the reducing condition of a 2% carbon monoxide atmosphere with an oxygen non-stoichiometry δ of 0.24. At lower temperatures (200 °C), the application of bias reduced the sample much more effectively than a carbon monoxide atmosphere and induced an oxygen non-stoichiometry δ of 0.47. Despite these large changes in oxygen concentration, the epitaxial thin film was completely re-oxidized and no signs of crystallinity loss or film amorphization were observed. This work demonstrates that the effects of oxygen evolution and reduction can be examined with applied bias at low temperatures, extending the ability to probe these processes with in-situ analytical techniques.

Invited Presentation: Probing Local Ionic Dynamics in Functional Oxides: From Nanometer to Atomic Scales

Vacancy-mediated electrochemical reactions in oxides underpin multiple applications ranging from electroresistive non-volatile memory and neuromorphic logic devices memories, to chemical sensors and electrochemical gas pumps, to energy conversion systems such as fuel cells. Understanding the functionality in these systems requires probing reversible (oxygen reduction/evolution reaction) and irreversible (cathode degradation and activation, formation of conductive filaments) electrochemical processes. In this talk, I summarize recent advances in probing and controlling these transformations locally on nanometer level using scanning probe microscopy. The localized tip concentrates the electric field in the nanometer scale volume of material, inducing local transition. Measured simultaneously electromechanical response (piezoresponse) or current (conductive AFM) provides the information on the bias-induced changes in material. Here, I illustrate how these methods can be extended to study local electrochemical transformations, including vacancy dynamics in oxides such as titanates, La x Sr1-x CoO3, and Y x Zr1-x O2. The formation of electromechanical hysteresis loops and their bias-, temperature- and environment dependences provide insight into local electrochemical mechanisms and can be directly linked to the defect chemistry. In materials such as lanthanum-strontium cobaltite, mapping both reversible vacancy motion and vacancy ordering and static deformation is possible, and can be corroborated by post mortemSTEM/EELS studies. In ceria, a broad gamut of electrochemical behaviors is observed as a function of temperature and humidity. The possible strategies for elucidation of origins of ionic motion at the electroactive interfaces in oxides using high-resolution electron microscopy and combined ex-situ and in-situ STEM-SPM studies are discussed. In the second part of the talk, probing electrochemical phenomena on in-situ grown surfaces with atomic resolution is illustrated.Research supported (SVK) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and partially performed at the Center for Nanophase Materials Sciences, a DOE-BES user facility.

Correlating surface cation composition and thin film microstructure with the electrochemical performance of lanthanum strontium cobaltite (LSC) electrodes

Surface composition of La0.6Sr0.4CoO3−δ electrodes investigated by means of a novel on-line in situ etching procedure with ICP-OES analysis.