A thermally self-sustained micro-power plant with integrated micro-solid oxide fuel cells, micro-reformer and functional micro-fluidic carrier

Low temperature micro-solid oxide fuel cell (micro-SOFC) systems are an attractive alternative power source for small-size portable electronic devices due to their high energy efficiency and density. Here, we report a thermally self-sustainable reformer – micro-SOFC assembly. The device consists of a micro-reformer bonded to a silicon chip containing 30 micro-SOFC membranes and a functional glass carrier with gas channels and screen-printed heaters for start-up. Thermal independence of the device from the externally powered heater is achieved by this exothermic reforming reaction above 470 °C. The reforming reaction and the fuel gas flow rate of the n-butane/air gas mixture controls the operation temperature and gas composition on the micro-SOFC membrane. In the temperature range between 505 °C and 570 °C, the gas composition after the micro-reformer consists of 12 vol% to 28 vol% H2. An open-circuit voltage of 1.0 V and maximum power density of 47 mW/cm2 at 565 °C is achieved with the on-chip produced hydrogen at the micro-SOFC membranes

Micro-solid oxide fuel cells running on reformed hydrocarbon fuels

Micro‐solid oxide fuel cell (micro‐SOFC) systems are predicted to have a high energy density and specific energy and are potential power sources for portable electronic devices. A micro‐SOFC system is under development in the frame of the ONEBAT project [1‐3]. In this presentation, we report on the fabrication and characterization of a sub‐system assembly consisting of a startup heater and a micro‐reformer bonded to a Si chip with electrochemically‐active micro‐SOFC membranes. A functional carrier including fluidic channels for gas feed and integrated heaters was bonded to a microreformer with an overall size of 12.7 mm x 12.7 mm x 1.9 mm [4‐7]. As a catalyst, a foam‐like material made of ceria‐zirconia nanoparticles doped with rhodium was used to fill the 58.5 mm3 reformer cavity. This micro‐reformer allows for high methane and butane conversion of > 90 % with a hydrogen selectivity of > 80 % at 550 °C in the reformer [7, 8]. A silicon chip with 30 free‐standing micro‐SOFC membranes (390 μm x 390 μm) with a thickness of less than 500 nm was bonded to the carrier‐reformer assembly described above. The micro‐SOFC membrane consisted of an yttria‐ stabilized zirconia thin film electrolyte. Both Pt‐based and ceramic‐based electrode materials were tested regarding the thermal stability and carbon poisoning at temperatures below 600 °C. The functional‐carrier mirco‐reformer micro‐SOFC assembly was electrochemically tested with hydrocarbon fuel between 300 °C and 600 °C. The fuel cell performance and the microstructural evolution of the anode are discussed as well

Micro-solid oxide fuel cells as power supply for small portable electronic equipment

Micro-solid oxide fuel cell (SOFC) systems are anticipated for powering small, portable electronic devices, such as laptop, personal digital assistant (PDA), medical and industrial accessories. It is predicted that micro-SOFC systems have a 2-4 higher energy density than Li-ion batteries [1]. However, literature mainly focuses on the fabrication and characterization of thin films and membranes for micro-SOFC systems [2-12]; the entire system approach is not yet studied in detail. We will therefore discuss in this paper the entire approach from the fabrication of thin films and membranes up to the complete system, including fuel processing, thermal management and integration

Stochastic 3D modeling of La0.6Sr0.4CoO3−δ cathodes based on structural segmentation of FIB–SEM images

A stochastic microstructure model is developed in order to describe and simulate the 3D geometry of two-phase microstructures (solid and pore phase), where the solid phase consists of spherical particles being completely connected with each other. Such materials appear e.g. in La0.6Sr0.4CoO3−δ (LSC) cathodes of solid oxide fuel cells, which are produced by screen printing and sintering of a paste consisting of LSC powder manufactured by flame spray synthesis. Thus, as a model type, we consider (fully parameterized) random sphere systems which are based on ideas from stochastic geometry and graph theory. In particular, the midpoints of spheres are modeled by random point processes. In order to assure the complete connectivity of the spheres, a modified version of the relative neighborhood graph is introduced. This graph controls the radii of spheres such that a completely connected sphere system is obtained. The model parameters are exemplarily fitted to three different materials for LSC cathodes, produced with sintering temperatures of 750, 850 and 950°C, respectively. Finally, the goodness of fit is validated by comparing structural characteristics of real and simulated image data

Synthesis and characterization of nanoparticulate La0.6Sr0.4CoO3−δ cathodes for thin-film solid oxide fuel cells

Nanocrystalline La0.6Sr0.4CoO3−δ (LSC) powder with an ultrafine microstructure is synthesized via salt-assisted spray pyrolysis and subsequently stabilized in water-based dispersions. Nanoparticulate cathode thin films of LSC and LSC–GDC (gadolinium doped ceria) nanocomposites (with 10–40 wt% of GDC) are prepared via single step spin coating on yttria stabilized zirconia (YSZ) substrates. In order to prevent the chemical reaction between the cathode and the electrolyte, a thin buffer layer of GDC is deposited using spin coating on the YSZ substrates. The electrochemical performance of the thin film cathodes is measured by impedance spectroscopy on symmetrical cells in the temperature range of 450–650 °C. LSC cathode thin films (250 nm thick) with 30 wt% GDC content exhibit the lowest area specific resistance (ASR) values of 0.32, 0.78 and 2.04 Ω cm2 in ambient air at 650, 600 and 550 °C, respectively

Residual stress and buckling patterns of yttria-stabilised-zirconia thin films for micro-solid oxide fuel cell membranes

Free-standing yttria-stabilised-zirconia (YSZ) thin films can be found in today's miniaturised gas sensors and as electrolytes in micro-solid oxide fuel cell membranes. 8 mol.% YSZ thin films prepared by pulsed laser deposition on silicon substrates are investigated by wafer curvature and nanoindentation. The 300nm thin 8YSZ films deposited at 700°C have a compressive stress of -1100±150 MPa and a Young's modulus of 205±20 GPa at 25 °C. The corresponding free-standing 8YSZ membranes are investigated by light microscopy and white light interferometry. The 8YSZ membranes deposited at 700°C have a buckling shape with a C4z-rotational symmetry and buckling amplitude of 6.5μm at 25°C. Numerical simulations of the buckling patterns using the Rayleigh-Ritz-method are in good agreement with the experimental data. These simulated buckling patterns are used to extract the local stress distribution. This is important regarding the application of YSZ membranes in micro-solid oxide fuel cells which must be thermomechanically-stable during device operation

Miniaturized free-standing SOFC membranes on silicon chips (A0704)

Due to their high specific energy and high energy density, miniaturized low-temperature (350-550°C) solid oxide fuel cells, hereafter abbreviated “micro-SOFC”, are believed to constitute one of the technologies that could help satisfy the continuously increasing electric energy demand for mobile devices such as laptops and camcorders. Using thin film and MEMS technologies, cathode-electrolyte-anode layer assemblies as thin as 1 μm are deposited on silicon substrates that are micromachined to form arrays of free-standing membranes (surface area: 390x390 μm2 at ETH Zurich). Proof of concept was already established by several groups and high power densities of several hundreds of mW/cm2 have been reported at temperatures as low as 350 °C. In Switzerland, the OneBat® consortium consisting of eight research groups is working on the development of the micro-SOFC technology covering various aspects such as membrane fabrication and characterization, reformer catalysis, thermal management and system development. After a brief presentation of the consortium activities as well as the state-of-the-art of the micro-SOFC research worldwide, this contribution will lay emphasis on the core of the micro-SOFC technology, namely the electrochemical cells, and address key-aspects for their further development: fabrication and thermomechanical stability of free-standing membranes, development of cost-effective thin film deposition techniques, and development of thermally stable electrodes