Application of Electroosmotic Pumps to Low Temperature Fuel Cells.

Proton exchange membrane fuel cells (PEMFCs) are attractive energy conversion devices due to their high efficiency and environmentally benign operation. PEMFCs are being developed for numerous applications ranging from transportation to portable electronics. PEMFCs encompass a general class of fuel cell architectures that employ a solid polymer, typically NafionRTM, as the electrolyte. PEMFCs can be further classified based on the fuel utilized. Hydrogen is the most popular fuel but PEMFCs also operate on various alcohols including ethanol and methanol. Direct methanol fuel cells (DMFCs) in particular oxidize aqueous methanol solutions at the cell anode to yield electrical energy, waste heat, and carbon dioxide. Liquid methanol has roughly half the volumetric energy density of gasoline, making DMFCs attractive for applications with size and weight constraints. This study explores unique aspects of using electroosmotic (EO) pumping structures to remove water from PEMFC cathodes and to supply methanol and water mixtures to DMFCs. Electroosmotic flow is the bulk motion of an electrolyte caused by coulombic interaction of external electric fields and the charges of an electric double layer (EDL). Porous glass EO pumps offer large surface-to-volume ratio and relatively high zeta potential, &zgr;, defined as the potential drop across the diffuse charges of the EDL. EO pumps have no moving parts and can produce high flow rate per package volume. These properties make EO pumps highly suitable for fuel cell applications. Recent experimental and numerical investigations on PEMFCs emphasize water management as a critical factor in the design of robust, high efficiency systems. Although various water management strategies have been proposed, water is still typically removed by pumping air into cathode channels at flow rates significantly higher than required by fuel cell stoichiometry. Such methods are thermodynamically unfavorable and constrain cathode flow channel design. We have developed proton exchange membrane fuel cells (PEMFCs) with integrated planar electroosmotic (EO) pumping structures which actively remove liquid water from cathode flow channels. EO pumps can relieve cathode design barriers and facilitate efficient water management in fuel cells. We demonstrate and quantify the efficacy of EO water pumping using controlled experiments in a single channel cathode flow structure. Our results show that, under certain operating conditions, removing water from the cathode using integrated EO pumping structures improves fuel cell performance and stability. The application of EO pumps for liquid water removal from PEMFC cathodes extends their operational range and reduces air flow rates. With respect to fuel delivery for DMFCs we discuss several EO pump figures of merit pertinent to portable fuel cell applications including flow rate per power, thermodynamic efficiency, and the ratio of EO pump power to fuel cell power. Additionally, we discuss the complex coupling of DMFC and methanl fuel pumps caused by two phase flow in the DMFC anode. Gaseous CO2 in the anode, a byproduct of methanol oxidation, increases the pressure load by nearly one order of magnitude versus single phase flow. We present key considerations for methanol delivery through controlled parametric studies of the two phase pressure drop at the DMFC anode. Finally, we develop a semi-empirical model which predicts trends in DMFC performance and allows for design and analysis of optimal operating conditions. In particular we illuminate the delicate balance between increasing inlet methanol concentration (for reduced EO pump power) and mitigating methanol crossover (favoring...