Journal of Power Sources 127 (2004) 187–196 Influence of diffusion layer properties on low temperature DMFC A. Oedegaard a,∗ , C. Hebling b , A. Schmitz b , S. Møller-Holst c , R. Tunold a a Norwegian University of Science and Technology, NO-7491 Trondheim, Norway b Fraunhofer Institute for Solar Energy Systems ISE, D-79110 Freiburg, Germany c SINTEF Materials Technology, NO-7465 Trondheim, Norway Abstract The effect of the diffusion layer on the performance and mass transport in a direct methanol fuel cell at ambient conditions is reported. Car- bon cloths with variable Teflon contents and pore sizes, carbon paper and a metal wire cloth were investigated. Membrane-electrode-assemblies (MEAs) for direct methanol fuel cells (DMFCs) prepared after an in-house receipt are used, giving reproducible results after a pre-treatment involving polarisation with hydrogen and air. Long-term effects and methanol crossover were also briefly investigated. Adding Teflon to the diffusion layer leads to better gas transport, as gas and liquid transport takes place in different paths. Thus, the fuel cell power output is more stable. The same effect was seen with increasing pore size. Carbon paper is found not suitable as a diffusion layer for low temperature DMFC. The metal wire cloth yielded best performance giving 15.8 mW cm -2 . © 2003 Elsevier B.V. All rights reserved. Keywords: DMFC; Diffusion layer; MEAs; Ambient conditions 1. Introduction The direct methanol fuel cell is a promising power con- verter for a wide range of applications from small sensors and portable electronics up to stationary power plants. Low temperature operation and the use of liquid fuel make DMFC systems very simple, without the need for humid- ification and thermal management systems, fuel vaporiser or reformer. The use of liquid methanol as fuel provides high energy density and fast and convenient refuelling. Hence, the DMFC may become a viable substitute to bat- teries especially for portable electronics. Cell performance vary significantly with operating conditions. State-of-the-art power densities are in the order of 15 mW cm -2 for single cell passive systems [1] and up to 50 mW cm -2 for small stacks operating at higher temperatures [2]. However, there are some challenges associated with DM- FCs as power converters. Compared to the hydrogen fed proton exchange membrane fuel cell (PEMFC), the anode oxidation kinetics of methanol are by far slower. Differ- ent reaction mechanisms have been proposed [3,4], and although no consensus about the correct reaction path has been reached, it is commonly recognised that intermediates ∗ Corresponding author. Present address: Fraunhofer Institute for Solar Energy Systems, Freiburg 79110, Germany. Tel.: +49-761-4588-5213. E-mail address: anders@ise.fhg.de (A. Oedegaard). such as carbon monoxide are adsorbed to the active catalyst sites, thus blocking further methanol oxidation. Binary cata- lysts such as PtRu are used to improve the reaction kinetics. With these catalysts the second metal forms a surface oxide or hydroxide in the potential range of methanol oxidation [5]. Another problem is related to the methanol permeation through the polymer membrane, commonly referred to as methanol crossover. Methanol has many similar physical properties as water, and since proton transport through the membrane is facilitated by water, this implies that methanol molecules also follow this path. Crossover increases with temperature and methanol concentration [6]. The effect of crossover is two-fold; the reduction in cell current as well as in cell voltage. Even at low methanol concentrations and ambient conditions, the cell voltage declines significantly due to a mixed potential at the cathode. There, methanol is directly oxidised with a corresponding reduction of oxygen. Thus, lowering the cell voltage resulting in a loss of power and methanol. Carbon monoxide produced during operation is adsorbed on the platinum catalyst surface, thus block- ing further reactions. For all applications, and especially for portable applications, the use of high concentration methanol solution is favourable, as it gives the highest energy density. This increases the crossover, however. The third major difficulty regarding DMFCs is related to the carbon dioxide evolution at the anode. According to the 0378-7753/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2003.09.015