Micro-solid oxide fuel cells (micro-SOFC) are predicted to be of high energy density and are potential power sources for portable electronic devices. A micro-SOFC system consists of a fuel cell comprising a positive electrode-electrolyte-negative electrode (i.e. PEN) element, a gas-processing unit, and a thermal system where processing is based on micro-electro-mechanical-systems fabrication techniques. A possible system approach is presented. The critical properties of the thin film materials used in the PEN membrane are discussed, and the unsolved subtasks related to micro-SOFC membrane development are pointed out. Such a micro-SOFC system approach seems feasible and offers a promising alternative to stateof-the-art batteries in portable electronics.

The residual stress and buckling patterns of free‐standing 8 mol.% yttria‐stabilized‐zirconia (8YSZ) membranes prepared by pulsed laser deposition and microfabrication techniques on silicon substrates are investigated by wafer curvature, light microscopy, white light interferometry, and nanoindentation. The 300 nm thin 8YSZ membranes (390 μm × 390 μm) deposited at 25 °C are almost flat after free‐etching, whereas deposition at 700 °C yields strongly buckled membranes with a compressive stress of –1,100 ± 150 MPa and an out‐of‐plane‐displacement of 6.5 μm. These latter membranes are mechanically stable during thermal cycling up to 500 °C. Numerical simulations of the buckling shape using the Rayleigh–Ritz‐method and a Young's modulus of 200 GPa are in good agreement with the experimental data. The simulated buckling patterns are used to extract the local stress distribution within the free‐standing membrane which consists of tensile and compressive stress regions that are below the failure stresses. This is important regarding the application in, e.g., microsolid oxide fuel cell membranes which must be thermomechanically stable during microfabrication and device operation.

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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 on 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 screenprinted heaters for start-up. Thermal independence of the device from the externally powered heater is achieved by exothermic reforming reactions 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.% H 2 .An open-circuit voltage of 1.0 V and maximum power density of 47 mW cm -2 at 565 °C is achieved with the on-chip produced hydrogen at the micro-SOFC membranes.

Lanthanum-strontium-cobalt oxide (LSC) thin films fabricated by pulsed laser deposition (PLD) can be used as electrodes for micro-solid oxide fuel cells (micro-SOFCs) in the intermediate temperature range of 450-550 °C. The LSC thin films deposited under a high oxygen chamber pressure are nanocrystalline after thermal treatment at 550 °C and have a nanoporous microstructure. The buckling pattern of the free-standing yttria-stabilised-zirconia (YSZ) membrane influences the survival rate of the free-standing LSC|YSZ|LSC cells. Electrochemical impedance spectroscopy is used to investigate the properties of the LSC/YSZ interface in free-standing quasi-symmetrical LSC|YSZ|LSC membranes. LSC electrodes with a thickness of ~150 nm had an area specific resistance (ASR) of 3.7 Ωcm2 at 550 °C.

Scanning laser vibrometry was used to investigate the mechanical stability of free‐standing micro‐solid oxide fuel cell (micro‐SOFC) membranes. Arrays of square‐shaped 460 nm thin micro‐SOFC membranes were fabricated on silicon substrates using pulsed laser deposition for the yttria‐stabilized zirconia electrolyte and magnetron sputtering for the platinum electrodes. Resonance frequency, displacement and acceleration measurements were carried out using interferometry analysis of the membrane reflection. The resonance frequencies scale with the reciprocal of the membrane length. At the resonance, the 390 × 390 μm2 micro‐SOFC membranes exhibit an out‐of‐plane displacement of ca. 1.2 μm only. All free‐standing micro‐SOFC membranes survive the resonant vibration without rupturing. These results are promising for the failure‐free implementation of micro‐SOFC in portable electronic devices.

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.