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A MEMs based planar micro fuel cell with self breathing cathode side

: Wagner, S.; Hahn, R.; Krumbholz, S.; Reichl, H.

Reichl, H.; Griese, H.; Pötter, H. ; Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration -IZM-, Berlin:
Driving forces for future electronics : Joint International Congress and Exhibition Electronics Goes Green 2004+; September 6 - 8, 2004, Berlin, Germany. Proceedings
Stuttgart: Fraunhofer IRB Verlag, 2004
ISBN: 3-8167-6624-2
ISBN: 978-3-8167-6624-7
Joint International Congress and Exhibition "Electronics goes green 2004+" <2, 2004, Berlin>
Fraunhofer IZM ()

Despite increasingly efficient components and low-power electronics, the energy demands of portable electronic products such as next-generation mobile phones, wearable computers, autonomous sensors and microsystems will rise dramatically in the future due to their growing functionality. As improvements in battery technology have so far been limited to energy density increases of only a few percent per annum, over the past few years many R&D activities have concentrated on alternative forms of portable power supply. One of the most promising candidates are micro fuel cells (FC) based on Polymer Electrolyte Membranes (PEM). Compared to batteries, which are the major contributor of heavy metal pollution in domestic waste, fuel cells may be the environmental friendly alternative. For many applications an integrated power source has to meet the criteria of a flat geometry. Thus, fuel cells with a planar design are advantageous compared to the conventional stack design. Planar fuel cells can be integrated in the housing of electronic devices. In this packaging concept the planar fuel cell can serve as part of the housing itself.
At the Fraunhofer IZM a technology for planar PEM micro fuel cells was developed, which is based on wafer level technologies and the use of metal-polymer foils. Thus an economical production of large numbers ensured as well as space-saving integration electronic devices. Peripheral elements like pumps or fans are not needed due to the self breathing function of the planar fuel cell. The entire fuel cell is less than 200µm thick and has very low-volume consumption. The demonstrated cells have a size of 1 cm² and an active area of 0,54 cm². Serial interconnection of individual cells is attained with gang bonding technologies. A dispensing technique or screen printing is used for integrated planar sealing. A micro structured flowfield on the anode side is used instead of diffusion layers. This implies a direct contact of the ribs of the flowfield with the membrane electrode assembly (MEA). Different designs of the flowfield with dimensions down to 10 µm were analysed. Due to the poor electrical conductivity of the catalyst layer on the MEA, a voltage drop occurs in the MEA in a direction perpendicular to the gas channel. Therefore the channels have to be quite narrow in order to minimise ohmic losses. As the ribs cover the catalyst layer, gas transport under the ribs is limited. These effects are analysed by comparative measurements of varying flowfield structures . An optimisation of the dimensions between the channels and ribs was carried out. The influence of flowfield dimensions on the pressure loss in micro structures was also be investigated. The operation behaviour of the MEMs fuel cells like stability, water management and temperature distribution are examined. The cell prototypes are electrically characterised by V/I-measurements. The measurements were carried out at various temperature and under natural air convection of the cathode. Stable operation of the micro cells was achieved at 80 mW/cm² in long-term tests at a temperature between 10 °C and 60 °C and a relative humidity of 10 and 90 %. In the medium range of temperature and humidity the micro fuel cell reached a power density of 120 mW/cm². Due to the ultra low profile design of the fuel cells these values correspond to a power density by 4000 ... 6000 W/l. For comparison: conventional PEM fuel cell stacks of 100 and 1000 W have power densities between 100 and 200 W/l. Li-Ion batteries have power densities of 500... 2000 W/l. Most other button cells however have a substantially smaller power output. The prototypes supply a stable current of 20 mA (40 mA with optimum conditions) at a voltage of 1.5 V. At this time long term measurements with 2400 hours have been carried out.