A spontaneous and passive waste-management device (PWMD) for a micro direct methanol fuel cell

This paper introduces a passive waste-management device (PWMD) for a micro direct methanol fuel cell to exhale CO₂ gas and to gather and transport water and methanol residue during operation. It passively employs condensation, temperature gradient, surface tension gradient and droplet coalescence to accumulate liquid, separate liquid and gas, and transport droplets without the need of external power. CO₂ gas can be breathed out through hydrophobic micro holes with the assistance of buoyancy force while water/methanol vapor is condensing into droplets, coalescing with smaller droplets and is transported toward a cooler and more hydrophilic waste tank through wettability gradient. The wettability gradient is prepared by diffusion-controlled silanization with a gradient from 117° to 28° and is radial toward the outer boundary, which can drive droplets down to 1 νl with a speed of up to 20 mm s^-1. With the assistance of coalescence along with the wettability gradient, the condensed water droplets can reach a double speed of 40 mm s^-1. The maximum water removal rate of the PWMD can approach 6.134 νl s^-1 cm^-2, which is at least one order of magnitude higher than the demand of a standard νDMFC with power generation ability of 100 mW cm^-2. © 2007 IOP Publishing Ltd.