Direct Methanol Fuel Cells (DMFCs) are promising candidates for portable power sources because of their high energy density, lightweight, compactness, simplicity as well as easy and fast recharging [1-3]. Initially, DMFC stacks have been developed for auxiliary power units (APUs) purpose or for electrotraction using a classical stacked cells configuration. Recently, various stack configurations have been proposed for portable applications[4-7]. Moreover, to increase efficiency, it is necessary to reduce power losses from auxiliaries (i.e. pump and fan used to feed methanol and air into the stack). Hence passive-feed DMFCs have been proposed. Under this configuration, DMFCs operate without any external devices for methanol feed and air blower. Oxygen can diffuse into the cathode from the ambient due to an air-breathing action of the cell (partial pressure gradient), whereas methanol can reach the catalytic layer from a reservoir through capillary action.
In the present paper, two different designs of monopolar DMFC mini-stacks (3 cells) operating at room temperature are presented. Performances were investigated by steady-state galvanostatic polarizations and chrono-potentiometric measurements as well as optimisation of operation parameters, such as methanol concentration and catalyst loading. From this analysis, it is derived that 4 mg cm-2 Pt loading in the presence of unsupported catalysts appears to be the best compromise using a methanol concentration ranging from 2M up to 20 vol.%.

Acknowledgments
The authors acknowledge support from “Regione Piemonte” (Italy) through the Micro Cell project (Delibera della Giunta Regionale n° 25-14654 del 31/01/05).