Miniature Fuel Cells for Portable Power Applications (Non-Refereed)

Li Jiang Electrical Engineering Department

Tuskegee University Tuskegee, AL 36088 USA

ljiang@tuskegee.edu

Naga S. Korivi Department of Electrical& Computer Engineering

Louisiana State University Baton Rouge, LA 70803 USA

nkoriv1@lsu.edu

Abstract

This paper reports the development of miniaturized fuel cells for the generation of electric power from methanol. The developed fuel cells are made of flexible polymer materials and are capable of generating a peak power output density of 0.2 mW/cm2 from a single device. The output power can be scaled up by employing an array of the developed fuel cells or by connecting them in series or parallel configurations. Due to the small dimensions of the developed device, it can be used for power supply applications for portable devices and appliances. Further, the fabrication of these fuel cells by polymer materials allows some degree of physical flexibility, allowing for stacking of multiple fuel cells in a small volume. This makes the developed devices suitable for powering wearable electronics. Fuel cells are electrochemical devices that convert chemical energy from materials like hydrogen, methanol, and ethanol into electrical energy. Unlike batteries, fuel cells do not require recharging and can generate electric power as long as a fuel supply exists. The recent advances in portable electronic devices such as notebook computers, cellular phones, personal digital assistants (PDAs), and other microdevices, especially microelectromechanical systems (MEMS) have stimulated a need for high-energy, small volume power devices. The supply of power to such devices has been a challenge, as current battery technology does not provide the energy densities needed to sustain power for extended periods. Miniature fuel cell are being seen as an answer to this problem, as such fuel cells would combine the high power density resulting from high energy of liquid hydrocarbon fuels with a high energy-to-weight ratio. These properties make fuel cells particularly well suited for transportation and portable electronic power applications. The developed miniature fuel cell consisted of an anode and cathode separated from each other by a proton exchange membrane (PEM). The PEM is a solid polymer electrolyte which separates the anode and cathode chemical half-reactions, but allows the selective passage of protons through it. The operation of the fuel cell is based on the catalytic oxidation of methanol at an anode and the reduction of oxygen at a cathode. Dilute methanol undergoes oxidation at a platinum (Pt) anode to generate protons, electrons and carbon dioxide. The electrons are

conducted by the anode to the external circuit. To complete the overall reaction, the protons travel through the PEM and reach a Pt cathode where they reduce oxygen to generate water. The electrons and protons produced in the overall reaction contribute to the electric power generated by the device. The PEM was fabricated by spin-coating commercially available Nafion® solution on a plastic substrate. The liquid PEM layer is then cured by heating at 70 oC for one hour in a convection oven. Due to the non-stick nature of the plastic substrate used, the cured PEM layer could be easily debonded from the underlying substrate. The free-standing PEM layer was deposited with 30 nm thick Pt layer on both sides by means of radio-frequency plasma sputtering. An anodic chamber for the fuel was constructed adjacent to the anode. It consisted of groove structure made by a moldable polymer, polydimethylsiloxane (PDMS) for maximizing fuel distribution over the anode surface. The cathodic chamber was constructed adjacent to the cathode and incorporated a special ‘air-breathing’ structure made of PDMS which allowed atmospheric oxygen to reach cathode. Both chambers and PEM are bonded together thermally. The outer packaging was made by thermally bonding layers of flexible polyethyleneterephthalate (PET), commercially available overhead projection sheets. The methanol fuel was injected into the anode chamber by means of a syringe and the fuel cell was operated to generate power. Due to the all-polymer nature of the fuel cell, multiple devices can be fabricated in a small volume and output power can be improved. Further miniaturization is possible making these devices very attractive for portable applications.

1-4244-1029-0/07/$25.00 ©2007 IEEE. 343