Portable electronic devices are already an indispensable part of our daily life; and their increasing number and demand for higher performance is becoming a challenge for the research community. In particular, a major concern is the way to efficiently power these energy-demanding devices, assuring long grid independency with high efficiency, sustainability and cheap production. In this context, technologies beyond Li-ion are receiving increasing attention, among which the development of micro solid oxide fuel cells (μSOFC) stands out. In particular, μSOFC provides a high energy density, high efficiency and opens the possibility to the use of different fuels, such as hydrocarbons. Yet, its high operating temperature has typically hindered its application as miniaturized portable device. Recent advances have however set a completely new range of lower operating temperatures, i.e. 350-450°C, as compared to the typical <900°C needed for classical bulk SOFC systems. In this work, a comprehensive review of the status of the technology is presented. The main achievements, as well as the most important challenges still pending are discussed, regarding (i.) the cell design and microfabrication, and (ii.) the integration of functional electrolyte and electrode materials. To conclude, the different strategies foreseen for a wide deployment of the technology as new portable power source are underlined.
This work presents current achievements on the fabrication and characterization of an all-Si based planar thermoelectric microgenerator. Ordered dense arrays of Vapor-Liquid-Solid (VLS) grown p-type Si nanowires (Si NWs) are integrated in predefined thermally isolated microstructures as nanostructured thermoelectric active material. Optimizations in device processing and architecture that improved both thermal and electrical performances of the microgenerator resulted in a 70 fold increase in power output. Furthermore, the performance of microgenerators with Si NWs is compared to that of microgenerators with micron-sized Si beams as active material. Additionally, a 60 fold improvement in power output is observed by placing a cold-finger on top of the thermally isolated microstructure to demonstrate the effect of a heat exchanger, which is currently being implemented on the microgenerator.
A novel design of a fuel-flexible micro-reactor for hydrogen generation from ethanol and methane is proposed in this
work. The micro-reactor is fully fabricated with mainstream MEMS technology and consists of an array of more than
20000 through-silicon vertically aligned micro-channels per cm<sup>2</sup> of 50 μm in diameter. Due to this unique configuration,
the micro-reformer presents a total surface per projected area of 16 cm<sup>2</sup>/cm<sup>2</sup> and per volume of 320 cm<sup>2</sup>/cm<sup>3</sup>. The active
surface of the micro-reformer, i.e. the walls of the micro-channels, is homogenously coated with a thin film of Rh-
Pd/CeO<sub>2</sub> catalyst. Excellent steam reforming of ethanol and dry reforming of methane are presented with hydrogen
production rates above 3 mL/min·cm<sup>2</sup> and hydrogen selectivity of ca. 50% on a dry basis at operations conditions
suitable for application in micro-solid oxide fuel cells (micro-SOFCs), i.e. 700-800ºC and fuel flows of 0.02 mLL/min for
ethanol and 36 mLG/min for methane (corresponding to a system able to produce one electrical watt).
Silicon nanowires thermoelectric properties are much better than those of silicon bulk. Taking advantage of silicon
microfabrication techniques and compatibilizing the device fabrication with the CVD-VLS silicon nanowire growth, we
present a thermoelectric microgenerator based on silicon nanowire arrays with interdigitated structures which enhance
the power density compared to previous designs presented by the authors. The proposed design features a thermally
isolated silicon platform on the silicon device layer of an SOI silicon wafer. This silicon platform has vertical walls
exposing <111< planes where gold nanoparticles are deposited by galvanic displacement. These gold nanoparticles act as
seeds for the silicon nanowires. The growth takes place in a CVD with silane precursor, and uses the Vapor-Solid-Liquid
synthesis. Once the silicon nanowires are grown, they connect the silicon platform with the silicon bulk. The proposed
thermoelectric generator is unileg, which means that only one type of semiconductor is used, and the second connection
is made through a metal. In addition, to improve the thermal isolation of the silicon platform, multiple trenches of silicon
nanowire arrays are used, up to a maximum of nine. After packaging the device with nanowires, we are able to measure
the Seebeck voltage and the power obtained with different operation modes: harvesting mode, where the bottom device
is heated up, and the silicon platform is cooled down by natural or forced convection, and test mode, where a heater
integrated on the silicon platform is used to produce a thermal gradient.
The present study is devoted to analyze the compatibility of yttria-stabilized zirconia thin films prepared by pulsed laser
deposition technique for developing new silicon-based micro devices for micro solid oxide fuel cells applications. Yttriastabilized
zirconia free-standing membranes with thicknesses from 60 to 240 nm and surface areas between 50x50 μm<sup>2</sup>
and 820x820 μm<sup>2</sup> were fabricated on micromachined Si/SiO2/Si3N4 substrates. Deposition process was optimized for
deposition temperatures from 200ºC to 800ºC. A complete mechanical study comprising thermomechanical stability,
residual stress of the membranes and annealing treatment as well as a preliminary electrical characterization of ionic
conductivity was performed in order to evaluate the best processing parameters for the yttria-stabilized zirconia