Micro fuel cells ($\mu$-FC) represent promising power sources for portable applications. Today, one of the technological ways to make $\mu$-FC is to have recourse to standard microfabrication techniques used in the fabrication of micro electromechanical systems (MEMS). This paper shows an overview on the applications of MEMS techniques on miniature FC by presenting several solutions developed throughout the world. It also describes the latest developments of a new porous silicon-based miniature fuel cell. Using a silane grafted on an inorganic porous media as the proton-exchange membrane instead of a common ionomer such as Nafion, the fuel cell achieved a maximum power density of 58 mW cm-2 at room temperature with hydrogen as fuel.
Today, the development and miniaturization of portable devices such as cellular phones, PDA or any mobile electronics involve new needs in terms of energy supply and autonomy. In the context of these applications, miniature fuel cells (FC) represent a true alternative to common batteries, e.g. Li-ion batteries, due to their large energy density, their theoretical simplicity (easy and immediate recharge by refilling with the fuel) and their inherent non-polluting aspect.
For the range of powers required (<10 W), recent work [1][2][3] has shown a growing interest for the development of MEMS and CMOS compatible processes for small FC fabrication with silicon, stainless steel foils or polymer technologies. Beyond standard silicon, the use of porous silicon (PS) has also been demonstrated as an attractive material for gas diffusion layer [4], support for catalyst [5][6] or as a reformer [7] in the case of FC. Recent papers [8][9] have also shown the relevance of using PS as the membrane of a FC.
In this paper, we will focus on MEMS-based FC and try to show some of the most recent developments and technologies applied to this kind of miniature FC. We will also present the latest developments concerning a novel silane-grafted porous silicon-based technology for small FC conducted at the FEMTO-ST Institute.
Since the 1980’s, the MEMS technologies have been fully developed at the various requests of microsensors, micro actuators, optical and biomedical systems, and microfluidics. In the future prospects of miniaturization and mass production of small FC, the use of microfabrication techniques and materials appears naturally to be one of the promising solutions to improve the performances of the FC, reducing their fabrication costs and enabling their easier integration with other electronic devices.
In the domain of MEMS-based miniature FC, various solutions have been reported. Since the performances do not seem to be the better criteria to classify FC -actually we can notice in the literature that the power densities of the miniature FC range from a few tenths of µW cm -2 up to several hundreds of mW cm -2 -and as the structures are often the same -two micromachined plates for fuel delivery and current collectors sandwiching a membrane electrodes assembly (MEA) -we will index the different FC by similar materials. Nevertheless we will linger on miniature FC with the best performances. In this paper, we will not focus on the different fuels used (hydrogen for Proton-Exchange-Membrane FC or PEMFC, methanol for Direct Methanol FC or DMFC, ethanol, formic acid) but we will precise for each FC described the fuel supply conditions.
As the base material for MEMS technologies, Si is also the most common material encountered in MEMS-based FC. Its properties and the microfabrication techniques associated to it such as photolithography, wet or dry etching, depositing (sputtering, CVD, thermal oxidation, Tristan Pichonat, Bernard Gauthier-Manuel Recent developments in MEMS-based micro fuel cells etc.) are now well-known and mastered. Another advantage of Si-based FC may also be to facilitate the possible integration of the FC with other electronic devices on the same chip.
Since 2000, Meyers et al (Lucent Technologies) have proposed two alternative designs using Si -a classical bipolar using separate Si wafers for the cathode and the anode and a less effective monolithic design that integrated the two electrodes onto the same Si surface [4,10]. In the bipolar design, both electrodes were constructed from conductive Si wafers. The reactants were distributed through a series of tunnels created by first forming a PS layer and then electropolishing away the Si beneath the porous film. The FC was completed by adding a catalyst film on top of the tunnels and finally by casting a Nafion solution. Two of these membraneelectrode structures were made and then sandwiched together. A power density of 60 mW cm -2 was announced for the bipolar design with H 2 supply.
Unlike Meyer and Maynard, Lee et al (Stanford Univ.) proposed a ‘flip-flop’ µ-FC design where both electrodes are present on the same face [2]. If this design does provide ease of manufacturing by allowing in-plane electrical connectivity, it complicates the gas management. Instead of electrons being routed from front to back, gasses must be routed in crossing patterns, significantly complicating the fabrication process and sealing. Peak power in a four-cell assembly achieved was still 40 mW cm -2 with H 2 as fuel.
Min et al (Tohoky Univ, Japan) reported a variant of this design proposing two structures of µ-PEFC using microfabrication techniques [11], the “alternatively inverted structure” and the “coplanar structure”. These structures use Si substrates with porous SiO 2 layers with Pt-based catalytic electrodes and gas feed holes, glass substrates with micro-gas channels, and a polymer membrane (Flemion S). In spite of a reported enhancement [12], the FC reached poor
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