(444b) Electroosmotic Pumps for Fuel Delivery to Direct Methanol Fuel Cells

Buie, C. R. - Presenter, Stanford University
Kim, D. - Presenter, Stanford University
Litster, S. E. - Presenter, Stanford University
Santiago, J. G. - Presenter, Stanford University

Direct methanol fuel cells (DMFCs) have recently gained popularity as high power density energy conversion devices and potential replacements to conventional batteries. Benefits of DMFCs for portable applications include long run times, capability for refueling as opposed to recharging, and silent operation. Some challenges to widespread commercialization include overall system cost (including pumps, thermal management devices, packaging, etc.), CO2 removal at the anode, and system efficiency. Currently, one approach to reducing system cost and improving reliability is employing passive reactant delivery methods. Passive systems can possess higher reliability than conventional active systems (as they have few moving parts) but suffer from increased anodic losses resulting from CO2 generation. Gaseous CO2 at fuel cell anode can reduce the active area and lower system power output.

Electroosmotic flow is the bulk motion of an electrolyte caused by coulombic interaction of external electric fields with electric double layers (EDLs). Porous glass EO pumps offer large surface-to-volume ratio and relatively high zeta potential, ζ, defined as the potential drop associated with the diffuse charges of the EDL. Electroosmotic (EO) pumps have no moving parts and produce flow rates on the order of several ml/min. This work reports the development of miniature DMFCs that utilize electroosmotic pumps for methanol delivery. We have designed a 2 cm2 DMFC with forced convection at the anode and free convection at the cathode (air breathing). Performance of the DMFC was characterized in the form of polarization curves for a variety of methanol/water solutions, flow rates, and anode flow field designs using syringe pumps. This data was subsequently compared to DMFC polarization for a cell supplied methanol by a porous glass electroosmotic pump. We will discuss gross and net power output of the system with an integrated electroosmotic pump supplying a variety of methanol solutions. Preliminary experiments reveal that electroosmotic pumps can supply 8M methanol to a 2 cm2 DMFC using only 2% of the fuel cell power. While increased methanol concentration can reduce power consumed via EO pumping (by reducing ionic current), increased concentration also results in higher methanol crossover in the DMFC. We are currently exploring this trade-off further and will present an optimization balancing DMFC power output with EO pump parasitic loss.