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Tsai C.-L.,Julich Research Center | Tsai C.-L.,Julich Aachen Research Alliance JARA Energy | Roddatis V.,University of Gottingen | Chandran C.V.,Leibniz University of Hanover | And 10 more authors.
ACS Applied Materials and Interfaces | Year: 2016

Al-contaminated Ta-substituted Li7La3Zr2O12 (LLZ:Ta), synthesized via solid-state reaction, and Al-free Ta-substituted Li7La3Zr2O12, fabricated by hot-press sintering (HP-LLZ:Ta), have relative densities of 92.7% and 99.0%, respectively. Impedance spectra show the total conductivity of LLZ:Ta to be 0.71 mS cm-1 at 30 °C and that of HP-LLZ:Ta to be 1.18 mS cm-1. The lower total conductivity for LLZ:Ta than HP-LLZ:Ta was attributed to the higher grain boundary resistance and lower relative density of LLZ:Ta, as confirmed by their microstructures. Constant direct current measurements of HP-LLZ:Ta with a current density of 0.5 mA cm-2 suggest that the short circuit formation was neither due to the low relative density of the samples nor the reduction of Li-Al glassy phase at grain boundaries. TEM, EELS, and MAS NMR were used to prove that the short circuit was from Li dendrite formation inside HP-LLZ:Ta, which took place along the grain boundaries. The Li dendrite formation was found to be mostly due to the inhomogeneous contact between LLZ solid electrolyte and Li electrodes. By flatting the surface of the LLZ:Ta pellets and using thin layers of Au buffer to improve the contact between LLZ:Ta and Li electrodes, the interface resistance could be dramatically reduced, which results in short-circuit-free cells when running a current density of 0.5 mA cm-2 through the pellets. Temperature-dependent stepped current density galvanostatic cyclings were also carried out to determine the critical current densities for the short circuit formation. The short circuit that still occurred at higher current density is due to the inhomogeneous dissolution and deposition of metallic Li at the interfaces of Li electrodes and LLZ solid electrolyte when cycling the cell at large current densities. © 2016 American Chemical Society.

Udomsilp D.,Julich Research Center | Udomsilp D.,Julich Aachen Research Alliance JARA Energy | Roehrens D.,Julich Research Center | Roehrens D.,Julich Aachen Research Alliance JARA Energy | And 6 more authors.
ECS Transactions | Year: 2015

DC conductivity experiments were carried out in order to characterize the area specific resistances (ASR) of various multilayer-structures. They represent the contacting interface between cathode and interconnector of state-of-the-art planar anode-supported SOFCs. The investigation focused on quantifying the influence of various chromium evaporation protection layer materials (MnOx, MnCo1.9Fe0.1O4 (MCF)), perovskitic cathode contact layers (LCC10, LCC12, LSCF), operational parameters during stack joining and the effect of pre-annealing of multi-layer samples on the overall ASR of the model system. The results demonstrate the influence of different material combinations as well as the duration of heat treatment during the joining process on the cell resistance, whereas we have not observed an obvious effect of pre-annealing. © The Electrochemical Society.

Lobe S.,Julich Research Center | Lobe S.,Julich Aachen Research Alliance JARA Energy | Dellen C.,Julich Research Center | Dellen C.,Julich Aachen Research Alliance JARA Energy | And 13 more authors.
Journal of Power Sources | Year: 2016

Thin film batteries based on solid electrolytes having a garnet-structure like Li7La3Zr2O12 (LLZ) are considered as one option for safer batteries with increased power density. In this work we show the deposition of Ta- and Al-substituted LLZ thin films on stainless steel substrates by r.f. magnetron sputtering. The thin films were characterized by XRD, SEM and time-of-flight-secondary ion mass spectrometry (ToF-SIMS) to determine crystal structure, morphology and element distribution. The substrate temperature was identified to be one important parameter for the formation of cubic garnet-structured LLZ thin films. LLZ formation starts at around 650 °C. Single phase cubic thin films were obtained at substrate temperatures of 700 °C and higher. At these temperatures an interlayer is formed. Combination of SEM, ToF-SIMS and XRD indicated that this layer consists of γ-LiAlO2. The combined total ionic conductivity of the γ-LiAlO2 interlayer and the LLZ thin film (perpendicular to the plane) was determined to be 2.0 × 10-9 S cm-1 for the sample deposited at 700 °C. In-plane measurements showed a room temperature conductivity of 1.2 × 10-4 S cm-1 with an activation energy of 0.47 eV for the LLZ thin film. © 2016 Elsevier B.V.

Berger C.M.,Julich Research Center | Berger C.M.,Julich Aachen Research Alliance JARA Energy | Hospach A.,Julich Research Center | Hospach A.,Siemens AG | And 6 more authors.
ECS Transactions | Year: 2015

In a rechargeable oxide battery (ROB) a solid oxide cell (SOC) is combined with an integrated iron oxide base storage for oxygen ions. The cell is operated at 800°C alternately as fuel cell and as electrolyser and the storage material regulates the oxygen partial pressure at the fuel electrode in a range of approximately 10-21-10-18 bar. Repeated charging (electrolysis) and discharging (fuel cell mode) can lead to a degradation of the storage material (particle coarsening, layer formation). In this study the influence of additions of Al2O3, CeO2, Mn3O4, Cr2O3, TiO2, SiO2, and MgO to the Fe2O3 base on these detrimental effects is analysed. Hence, compacted samples are repeatedly oxidised and reduced in a laboratory furnace, where the conditions present in the ROB are simulated. Using XRD and laser microscopy it was found that among the tested oxides only MgO and Al2O3 could mitigate the degradation phenomena to some extent. © The Electrochemical Society.

Deibert W.,Julich Research Center | Deibert W.,Julich Aachen Research Alliance JARA Energy | Ivanova M.E.,Julich Research Center | Ivanova M.E.,Julich Aachen Research Alliance JARA Energy | And 6 more authors.
Journal of Membrane Science | Year: 2015

La5.4WO12-δ (LaWO) is a promising membrane candidate for a variety of H2-related applications due to its appreciable levels of mixed proton-electron conduction and its stability in moist reducing atmospheres at elevated temperatures. Governed by Wagner theory, the H2 permeation performance of a membrane can be enhanced by reducing its thickness. Therefore, the present work deals with preparing LaWO supported membranes with reduced thickness and optimised microstructure. Combining a dense membrane with a porous supporting layer is associated with mismatched sintering rates, which ultimately lead to bending effects. Therefore, the sintering behaviour of both the dense membrane and the porous substrate must be carefully adjusted to each other. For this purpose, single and co-fired membrane and substrate layers were produced by tape casting. Sintering experiments were carried out with an optical dilatometer. The shrinkage and microstructural evolution of the layers were evaluated in terms of the anisotropic shrinkage forces and the membrane rigidness counteracting the substrate shrinkage. The results were used to develop asymmetric LaWO membranes with optimal microstructure. High membrane density was combined with a substrate porosity of ∼30% and minimised bending (40 μm). The LaWO membrane-substrate assembly displayed a He leakage of 10-5 hPa dm3 cm-2 s-1, which is a value that satisfies further practical demands. © 2015 The Authors. Published by Elsevier B.V.

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