Nanostructured thin films of La 0.6 Sr 0.4 CoO 3-δ via spray pyrolysis for micro-SOFC application...

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26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Ben el Nanostructured thin films of La 0.6 Sr 0.4 CoO 3-δ via spray pyrolysis for micro- SOFC application Cahit Benel, Azad J. Darbandi, Horst Hahn Michel Prestat, René Tölke, Anna Evans

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  • Nanostructured thin films of La0.6Sr 0.4CoO3- via spray pyrolysis for micro-SOFC application

    Cahit Benel, Azad J. Darbandi, Horst HahnMichel Prestat, Ren Tlke, Anna Evans

  • Fundamentals of SOFC

    Anode: H2 + O2- H2O + 2 e-

    Cathode: O2 + 2 e- O2- Total: H2 + O2 H2O

  • MotivationLosses in SOFCTo reduce the losses: Making the whole cell as thin as possible Optimizing of electrode materials and their properties National Energy Technology Laboratory Fuel cell handbook. 7th ed. Morgantown, WV: U.S. Department of Energy; 2004.

  • Micro-solid oxide fuel cell - State of artCollaboration with ETH ZurichEvans, A. et. Al. Journal of Power Sources 194 (2009) 119-129PEMFC80 Cpure H2DMFCLi-ion batteriesNi-MH batteriesSOFC350-550 ChydrocarbonsCathodeAnodeElectrolyteSiGoal:Nanoparticulate thin film cathode with thickness between 200 nm and 500 nm

  • Synthesis of LSCSalt-assisted Spray PyrolysisLa0.6Sr0.4CoO3- (LSC)

  • ResultsSalt-assisted Spray PyrolysisAs synthesizedNo reaction between NaCl and LSC phase

  • Removal of SaltAs synthesizedAfter washingAs synthesized nanopowder washed by DI water to remove NaCl.

  • XRDBefore washingAfter washingCrystallite size 7 nm

  • Electrochemical Characterization

    Symmetrical cells under OCV1MHz-0.1Hz450-650 C with 50 C incrementsPO2=0.01-1 atm Yttria stabilized zirconia (YSZ) substrates

    Ce0.8Gd0.2O1.9 (GDC) buffer layer via spin coating (950 C for 2 h)

    LSC functional layers via spin coating (550 C for 1 h)LSCLSC-GDC (10-40 wt %) nanocomposite

    LSCGDCYSZLSCGDC

  • Electrochemical Characterization

  • Electrochemical CharacterizationDependence of ASR on temperture & GDC concentration* Karageorgakis et. al., Journal of Power Sources 195 (2010) 8152-8161

  • SummaryNanocrystalline single phase LSC via SASP

    Nanoparticulate thin films of LSC and LSC-GDC (10-40%) with thicknesses between 200 and 500 nm by single step spin coating

    LSC-GDC (30%) nanocomposite films showed the lowest ASR values0.78 cm2 (250 nm thickness, @ 600 C)

    Next stepTo check the performance of the LSC functional thin films on free standing electrolytes

  • Financial support: Center for Functional Nanostructures (CFN)

    Equipment support:

    Acknowledgments Elektrochemie Verbund-Sd

  • Thank you for your attention

  • Free standing electrolyte - State of artCollaboration with ETH Zurich1. Silicon nitrade deposition2. Photoresist by spin coating3. Exposure & Development4. Plasma etching of silicon nitride 5. Deposition of electrolyte (PLD)6. KOH wet etching of Si7. Plasma etching of silicon nitride

    *| 1-6 June, Nano 2008, Rio de Janeiro| *Thank you for kind introduction.This work is a collaboration between us and nonmetallic inorganic materials group in ETH Zurich. So I will talk about nanostructured thin films of LSCO for micro solid oxide fuel cell application.

    | 1-6 June, Nano 2008, Rio de JaneiroFuel cells are electrochemical devices. They convert chemical energy of fuel gas into electrical energy. The schematic shows the basic structure of a solid oxide fuel cell. It consists of a solid electrolyte layer in contact with an anode and a cathode.So how does it work?At cathode side we have oxygen gas, and at anode side we have fuel gas, which is Hydrogen here. At cathode layer, oxygen molecules are reduced to oxygen ions according to this reaction. Then oxygen ions difuse through the solid electrolyte towards anode side and react with the fuel gas according to this reaction. During this process electrons are carried by oxygen ions from the cathode layer to the anode. So if those two layers are connected externally, an electric current is obtained continuously from this electrochemical process. This is the electrical energy. | 1-6 June, Nano 2008, Rio de JaneiroEven though fuel cells are highly efficient, the actual cell performance is far from the ideal case. Here a typical voltage-current plot is presented. Under ideal conditions, one would expect constant voltage as indicated here. However, there are some losses within the cell. At low current densities, activation-related losses are dominant. These losses come from the activation energy of the electrochemical reactions at the electrodes. As the current increases, ohmic losses start to dominate. These losses are because to the ionic resistance in the electrolyte and electrodes, and electronic resistance in electrodes and other components of fuel cell. So to improve the cell performance, one can make the whole cell as thin as possible. In this way the diffusion path length for oxygen ions would be decreased. Also cell performance can be improved by optimization of electrode materials and their properties. | 1-6 June, Nano 2008, Rio de JaneiroIn this sense, micro solid oxide fuel cell membranes have been developed by nonmetallic inorganic materials group in ETH Zurich. A micro solid oxide fuel cell consists of a thin free standing electrolyte and thin film electrode layers. Compared to conventional SOFC systems, the whole cell is very thin. And this make it possible to reduce the operation temperature even below 600 C. It is predicted that micro SOFC systems have higher specific energy and energy density compared to other power supply sources.Therefore it is expected that they can be integrated with portable electronic devices with power requirements upto 20 W, such as mobile phones and laptops.And our part in this collaboration to prepare thin film cathodes on this free standing electrolyte layer.

    | 1-6 June, Nano 2008, Rio de Janeiro*| 1-6 June, Nano 2008, Rio de Janeiro| *For the cathode this material system was chosen and for synthesis Salt-assisted spray pyrolysis method was used. The synthesis setup consists of a nebulizing chamber, a pyrolysis zone, and a powder collector. The precursor solution is water based and it consists of the nitrate salts of La, Sr, and Co in stoichiometric ratios. In addition, NaCl is dissolved in precursor solution. Then the precursor is continiously nebulized and delivered into the pyrolysis zone by the oxygen flow. The nanoparticles are formed in the pyrolysis zone and then collected here in the collector.

    | 1-6 June, Nano 2008, Rio de JaneiroSEM images show the typical morphology of the samples.The synthesis method depends on the distribution of NaCl on nanoparticle surfaces and this prevents the nanoparticles from agglomerating and sintering. Also X-ray diffraction experiments confirm that there is no reaction between NaCl and LSC phase.| 1-6 June, Nano 2008, Rio de JaneiroTo remove the NaCl from the system, as synthesized powder is washed by distilled water several times. If the SEM images before and after washing are compared, we can see that the morphology of the powder is improved. Washed powder consists of well seperated nanoparticles.| 1-6 June, Nano 2008, Rio de JaneiroAs I mentioned before, no reaction between NaCl and LSC was observed. XRD experiments on washed powder confirm the complete removal of NaCl and the formation of desired single LSC phase. The average crystallite size is calculated as around 7 nm by Rietveld refiment.| 1-6 June, Nano 2008, Rio de JaneiroThe performance of the thin film cathodes was evaluated using Electrochemical impedance spectroscopy. So this is how we prepared the symmetrical samples. Yttria stabilized zirconia substrates were used as electrolyte. To avoid the chemical reaction between YSZ substrate and LSC, a gadolinum doped ceria layer was deposited on both sides of YSZ substrate. Water based stabilized dispersions of LSC and LSC-GDC were prepared and LSC functional layers were spin coated on both sides of the cell. Finally, the samples were sintered at 550C for 1 hour to achieve better adhesion.

    Here we see two cross section of the functional layers. This one is about 250 nm and the other one is around 500 nm.

    The samples were characterized under open circuit conditions in the range from 1MHz to 0.1Hz. The measurements were performed at temperature between 450 C and 650 C with increments of 50 C. And oxygen partial pressures between 0.01 atm and 1 atm were used for each measurement.

    | 1-6 June, Nano 2008, Rio de JaneiroAnd this is just a representative impedance spectra from the sample with thickness of 500 nm and 30 wt % of GDC. It was measured at 600 C under different partial pressures of oxygen. By fitting these data at each temperature and concentration with equivalent curcuits, area specific resistance values are calculated.

    | 1-6 June, Nano 2008, Rio de Janeiro| 1-6 June, Nano 2008, Rio de Janeiro| 1-6 June, Nano 2008, Rio de Janeiro| 1-6 June, Nano 2008, Rio de Janeiro