S. Moghaddam, E. Pengwang, K. Lin, R.I. Masel, M.A. Shannon
fuel cell, power source, hydrogen generator, microfluidic valve
We report, for the first time, development of a fully integrated millimeter-scale (331 mm3) fuel cell-based power generator with a completely passive control system. Fabrication of this unique power source was enabled through development of a novel self-regulating micro-hydrogen generator . The hydrogen generator stops generating hydrogen automatically when hydrogen is not consumed. The first generation of this device delivered an energy density of 254 W-hr/L. Subsequent generations of this device can potentially reach 1000 W-hr/L. While fabrication of small-scale membrane electrode assembly (MEA) and fuel reservoirs are widely reported in literature, fabrication of a viable millimeter-scale power source has remained a significant challenge. The difficulty is primarily due to the development of a fuel delivery/control system (so-called auxiliary systems) within a fraction of the device volume. Micro-fuel cell architectures suggested in literature (e.g. in ) are scaled downed versions of large-scale systems with numerous auxiliary components. These components can be much larger than the membrane electrode assembly (MEA), greatly reducing the overall device energy density. Additionally, auxiliary components normally require numerous microfabrication steps, have integration difficulties, and require external electronics all of which result in high production cost and complexity of operation. In addition, the auxiliary systems must only consume a fraction of power generated by the device. All these measures are necessary to achieve an energy density above that of the state of the art batteries (e.g. Li-ion). The no-power, self-regulating hydrogen generator consists of hydride and water chambers, with a membrane separating them (cf. Figure 1). Water flows towards the hydride chamber, but stops within the membrane holes due to capillary forces. Water vapor is then diffuses into the hydride chamber resulting in generation of hydrogen (metal hydrides such as LiH, LiAlH4, and CaH2 react with water vapor to produce H2 gas ). When hydrogen use is lower than the generation, gas pressure builds up inside the hydride chamber resulting in membrane deflection and closure of the water gate at a significantly lower pressure than needed to break into the meniscus within the holes. When the H2 use is faster than the generation, the reverse happens, opening the membrane to allow vapor to diffuse to the hydride. The membrane separating the water and hydride chambers is made of polyimide. A 1.3 mm in diameter Cr/Au coated area at the center of the membrane (cf. Figure 2) prevents water diffusion when the membrane is in closed mode. The volume of this control mechanism is 50 nL (about 0.5% of the device volume). The membrane close/open performance was verified (cf. Figure 3) in an apparatus built for this purpose. The hydride chamber was fabricated using KOH etching process. The bottom wall of the hydride chamber was made porous to allow hydrogen to flow out. The membrane electrode assembly (MEA) was fabricated on a SOI wafer through filling 100100 µm2 openings in the device layer with Nafion® and then applying Pt-based catalyst ink on both sides of the membrane. The fuel chamber was loaded with AlLiH4 and then bonded to the water chamber and subsequently to the MEA. Figure 4 show successful operation of the device between on and off states. The I-V characteristic curve of the device, presented in Figure 5, shows typical activation and ohmic polarization losses and that the device current is limited (at operating voltage of less than 0.6 V) by the hydrogen generation rate.