Increasing μDMFC efficiency by passive CO₂bubble removal and discontinuous operation

Abstract

A new concept that enables fully passive CO 2 gas bubble removal in micro direct methanol fuel cells (µDMFCs) is presented. The original concept behind the presented degassing structure (flowfield) is based on microchannels with a T-shaped cross section. These channels have defined tapering angles over their cross section (α) and along their axis (β). The tapered channel design creates an intrinsic transport mechanism that removes the gas bubbles from the electrodes by capillary forces only. Computational fluid dynamic (CFD) simulations have been used to determine applicable opening angles of α = 5 • and β = 1.5 •. The experimental verification was done by using a transparent flowfield to show the passive bubble removal as well as with a fully operational µDMFC. During the operation, the fuel cell delivered an output of up to 8 mW cm −2 without the need for external pumping in short-term measurements. During the long-term measurements, discontinuous pumping showed the highest fuel cell efficiency compared to the continuously pumped fuel supply.

An active micro-direct methanol fuel cell with self-circulation of fuel and built-in removal of CO2 bubbles

“…Existing lateral venting approaches to remove gas bubbles from microfluidic devices [4,5] are prone to leakage when the medium is methanol, especially under pressure fluctuation during operation. This problem may be addressed by employing a thin polymer membrane to allow gas diffusion [6] or designing microchannel of specific geometries to temporarily alleviate the bubble clogging [7,8]. However, producing a sufficiently high removal rate of the continuously generated gas bubbles, for a fuel cell under high loads, presents serious technical challenges.…”

Introduction to Direct Alcohol Fuel Cells

“…Litterst et al [65] developed a μDMFC using poly(methyl methacrylate) (PMMA) and graphite-filled polypropylene as substrates. The authors used an original T-shaped channel design, with defined tapering angles over their cross section and along their axes, which facilitates the removal of CO 2 bubbles.…”

Capillary driven movement of gas bubbles in tapered structures

“…Opening angles up to 7°are sufficient and useful. In Litterst et al 2006, for example, opening angles of 2a = 5°where successfully applied to lDMFCs.…”

“…The described effect was already applied for the passive removal of gas bubbles appearing in the anode flow field of a lDMFC by Litterst et al (2006) and Paust et al (2009a, b). In this article, the physical effects that control the gas bubble's movement in such tapered channels are studied, in particular for a channel width that is about ten times the bubble diameter.…”