Nanostructured thin films of La0. 6Sr 0 Nanostructured thin films of La0.6Sr 0.4CoO3-δ via spray pyrolysis for micro-SOFC application Cahit Benel, Azad J. Darbandi, Horst Hahn Michel Prestat, René Tölke, Anna Evans 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. 4/23/2017 | 1-6 June, Nano 2008, Rio de Janeiro

Fundamentals of SOFC Cathode: ½ O2 + 2 e-  O2- Anode: H2 + O2-  H2O + 2 e- Total: H2 + ½ O2  H2O Fuel 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. 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Motivation Losses in SOFC To reduce the losses: Making the whole cell as thin as possible Optimizing of electrode materials and their properties Even 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. National Energy Technology Laboratory Fuel cell handbook. 7th ed. Morgantown, WV: U.S. Department of Energy; 2004. 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Micro-solid oxide fuel cell - State of art Collaboration with ETH Zurich Electrolyte µPEMFC 80 °C pure H2 Cathode µSOFC 350-550 °C hydrocarbons Si Evans, A. et. Al. Journal of Power Sources 194 (2009) 119-129 µDMFC Li-ion batteries Anode In 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. Goal: Nanoparticulate thin film cathode with thickness between 200 nm and 500 nm Ni-MH batteries 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Synthesis of LSC Salt-assisted Spray Pyrolysis La0.6Sr0.4CoO3-δ (LSC) Furnace T Vacuum pump O 2 MFC Filter Control valve Ultrasonic nebulizer p Carrier Gas Water based precursor 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. 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel 4/23/2017 | 1-6 June, Nano 2008, Rio de Janeiro | 5

Results Salt-assisted Spray Pyrolysis As synthesized SEM 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. No reaction between NaCl and LSC phase 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Removal of Salt As synthesized After washing To 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. As synthesized nanopowder washed by DI water to remove NaCl. 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

XRD Before washing After washing Crystallite size ≈ 7 nm As 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. Crystallite size ≈ 7 nm 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Electrochemical Characterization LSC 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) LSC LSC-GDC (10-40 wt %) nanocomposite GDC YSZ GDC LSC Symmetrical cells under OCV 1MHz-0.1Hz 450-650 °C with 50 °C increments PO2=0.01-1 atm The 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 550°C 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. 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Electrochemical Characterization And 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. 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Electrochemical Characterization Dependence of ASR on temperture & GDC concentration * Karageorgakis et. al., Journal of Power Sources 195 (2010) 8152-8161 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Summary Nanocrystalline 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 values 0.78 Ω cm2 (250 nm thickness, @ 600 °C) Next step To check the performance of the LSC functional thin films on free standing electrolytes 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Acknowledgments Financial support: Center for Functional Nanostructures (CFN) Equipment support: Elektrochemie Verbund-Süd 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Thank you for your attention 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel

Free standing electrolyte - State of art Collaboration with ETH Zurich 1. Silicon nitrade deposition 2. Photoresist by spin coating 3. Exposure & Development 4. Plasma etching of silicon nitride 5. Deposition of electrolyte (PLD) 6. KOH wet etching of Si 7. Plasma etching of silicon nitride 26.03.2012 | Gemeinschaftslabors Nanomaterialien | Cahit Benel