2024-03-28T08:54:08Zhttps://www.tdx.cat/oai/requestoai:www.tdx.cat:10803/1319442017-08-31T19:51:01Zcom_10803_1col_10803_46
nam a 5i 4500
Electrònica
Electrónica
Electronics
Física de l'estat sòlid
Física del estado sólido
Solid state physics
Piles de combustible
Pilas de combustible
Fuel cells
Ceràmiques electròniques
Cerámica electrónica
Electronic ceramics
Font d'alimentació
Fuente de alimentación
Power supply
Microelectromechanical systems (MEMS)
Sistemas microelectromecánicos
Sistemes microelectromecànics
Integration of thin film based micro solid oxide fuel cells in silicon technology
[Barcelona] :
Universitat de Barcelona,
2014
Accés lliure
http://hdl.handle.net/10803/131944
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Garbayo Senosiain, Iñigo,
autor
1 recurs en línia (225 pàgines)
Tesi realitzada a l'Institut de Microelectrònica de Barcelona (IMB-CNM, CISC)
Tesi
Doctorat
Universitat de Barcelona. Departament d'Electrònica
2013
Universitat de Barcelona. Departament d'Electrònica
Tesis i dissertacions electròniques
Tarancón Rubio, Albert,
supervisor acadèmic
Sabaté Vizcarra, Neus,
supervisor acadèmic
TDX
In the last decades, there has been a huge proliferation of portable devices. Among them, consumer electronics such as mobile phones, music players, e-books, etc. are greatly extended. In order to provide these devices with the required autonomy, a power supply system has to be integrated within the device packaging. This impels the search of integrated power sources that could satisfy the requirements of high power density, long operation lifetime and low cost. Up to now, batteries have been commonly used as power supply for these devices. However, as functionalities increase, the need high off-grid power supply and storage exponentially increases. Just entering on the 4th generation (4G) era on consumer electronics devices, some studies suggest that the already optimized batteries are probably reaching their energy density limit and no longer can be considered for reliably powering high-performance devices.
Therefore, in the last years, many research groups around the world have focused their attention on the development of efficient alternatives to batteries, as power supply for the new high-performance portable devices working on the low power regime (1−20W). Due to their long lifetime, high power density and integrability, probably the most promising alternative is the development of micro fuel cells. Among them, micro solid oxide fuel cells (micro SOFC) present the highest values of specific energy densities (by unit mass and/or volume), mainly due to their higher operating temperature and subsequent capability of operate directly on hydrocarbon fuels. The most extended design for micro SOFC devices is based on the fabrication of accessible freeKstanding membranes of the functional layers, i.e. a thin electrolyte covered by an anode and a cathode one at each side (electrodes), supported on silicon-based microfabricated platforms. The use of silicon as supporting material has been found to be very convenient as it is the principal material used in microfabrication technology and therefore there exist a wide and well-known series of techniques already developed for its micromachining. This allows the fabrication of functional membranes, while ensuring robustness on the system.
This thesis encompasses the design, fabrication and characterization of thin film-based micro solid oxide fuel cells integrated in silicon. The development of micro SOFC was carried out in three different ways; (i.) presenting new designing strategies for the optimization of the free-standing membranes, (ii.) fabricating thermo-mechanically stable thin film electrolytes and (iii.) suggesting and implementing new more reliable thin film electrode materials.
On one side, two different membrane designs are micro fabricated using silicon micro machining technology. First, the fabrication of a basic square design was firstly addressed, where the main concerns were placed on the adaptation of the fabrication flow to the Clean Room capabilities at IMB-CNM (CSIC). Then, an innovative large-area membrane was designed and fabricated. This second design was based on the use of doped silicon slab grids as robust support for the larger freeKstanding areas, allowing the fabrication of x30 larger membranes than previous basic designs.
Yttria-stabilized zirconia (YSZ), the state-of-the-art electrolyte material in bulk SOFC, was used for the fabrication of thin film free-standing electrolytic membranes. Dense, fully crystalline and homogeneous films were obtained, as required for the fabrication of effective electrolytes, thus avoiding shortcuts between electrodes and/or gas leakages. An exhaustive study on the thermoKmechanical stability of the electrolytic membranes was performed, paying special attention to the evolution of the stress with fabrication conditions. Finally, target values of resistance associated to the electrolyte (Area Specific
Resistance, ASR= 0.15 Ωcm(2)) were obtained at temperatures as low as 400℃ for 250 nm-thick YSZ membranes, thus presenting them as suitable electrolyte for micro SOFC operating in the intermediate range of temperatures (IT range, 400 − 800℃).
Several materials were tested as thin film electrodes for their use in micro SOFC. First, although widely used by other authors in previous reports of micro SOFC systems, thin film metallic electrodes (porous Pt) were found to be thermally instable under micro SOFC operating temperatures. This impelled the search for alternative materials as either cathode or anode. For the cathode side, porous La(0.6)Sr(0.4)CoO(3-δ) (LSC) thin films were fabricated and implemented in real micro SOFC configurations, i.e. free-standing membranes. Sufficient conductivity for their use as cathode films was measured, and no degradation was observed in the whole operating range. The thermo-mechanical stability of LSC/YSZ/LSC membranes was ensured up to 700℃. Target values of ASR required for SOFC cathode/electrolyte bi-layers (0.30Ω cm(2)) were achieved in the IT range (700℃). For the anode side, porous Pt-Ce(0.8)Gd(0.2)O(1.9-δ) (Pt-CGO) thin film cermets were fabricated. Porous CGO films below 1m thick had to be fabricated due to delamination problems. Percolation of Pt into the porous ceramic network was ensured by thermal treatment and observed by SEM. Anode electrochemical performance was tested on Pt-CGO/YSZ/CGO-Pt symmetrical membranes. Target values for the anode/electrolyte biKlayer were reached again at temperatures of ca. 700℃.
In addition, the fabrication of thermally stable metal-based current collectors was also addressed. A non-conventional lithographic step, i.e. nanosphere lithography was used in order to define a patterned grid on both sides of the functional membranes. Dense Pt grids were fabricated thermo-mechanically stable, and their durability was ensured during real micro SOFC operating conditions.
Finally, a fully ceramic-based micro SOFC was presented here for the first time. The three functional components of the fuel cell, i.e. cathode, electrolyte and anode, were fabricated by using the previously developed thin films. Thus, LSC/YSZ/CGO-Pt free-standing membranes were fabricated, and finally Pt current collectors were implemented on both sides. Thermo-mechanical stability of the micro SOFC membrane was proved till 750℃, extending the up-to-now reported operating temperatures of micro SOFC and therefore allowing the use of ceramic electrodes. A maximum power density of 100 mW/cm(2) was measured at 750℃ under pure H2 as fuel and synthetic air as oxidant. These results represented the first report on a second generation of more reliable micro SOFC systems, based on ceramics instead of thermally instable metal-based electrodes.
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