Ž . Journal of Power Sources 83 1999 204–216 www.elsevier.comrlocaterjpowsour The impact of mass transport and methanol crossover on the direct methanol fuel cell K. Scott a, ) , W.M. Taama a , P. Argyropoulos a , K. Sundmacher b a Chemical and Process Engineering Department, UniÕersity of Newcastle upon Tyne, Newcastle Upon Tyne, NE1 7RU, UK b Max-Planck-Institut fur Dynamik Komplexer Technischer Systeme, D-39120 Magdeburg, Germany Received 16 March 1999; received in revised form 28 April 1999; accepted 22 June 1999 Abstract The performance of a liquid feed direct methanol fuel cell based on a Nafion w solid polymer electrolyte membrane is reported. The cell utilises a porous Pt–Ru-carbon supported catalyst anode. The effect of cell temperature, air cathode pressure, methanol fuel flow rate Ž 2 . and methanol concentration on the power performance of a small-scale 9 cm area cell is described. Data reported is analysed in terms of semi-empirical models for the effect of methanol crossover by diffusion on cathode potential and thus cell voltage. Mass transfer characteristics of the anode reaction are interpreted in terms of the influence of carbon dioxide gas evolution and methanol diffusion in the carbon cloth diffusion layer. Preliminary evaluation of reaction orders and anode polarisation agree with a previous suggested mechanism for methanol oxidation involving a rate limiting step of surface reaction between adsorbed CO and OH species. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Direct methanol fuel cell; Mass transport; Methanol crossover 1. Introduction Ž . The direct methanol fuel cell DMFC , based on a solid Ž . polymer electrolyte SPE in the form of a proton conduct- ing membrane, has the attraction of no liquid acidic or alkaline electrolyte and uses methanol, either as vapour or liquid. The structure of the DMFC is a composite of two porous electrocatalytic electrodes on either side of a solid Ž . polymer electrolyte SPE membrane. The direct methanol Ž . fuel cell DMFC is a promising power source for a range of applications including transportation and portable power Ž . sources. The thermodynamic reversible potential 298 K for the overall cell reaction in the DMFC is 1.214 V, which is comparable to 1.23 V for the hydrogen fuel. A current advantage of the hydrogen cell is that hydrogen oxidation at the anode is very fast and consequently the performance of the hydrogen cell is better than that of methanol cell. For methanol the oxidation kinetics are inherently slower as a result of intermediates formed dur- ) Corresponding author. Tel.: q44-191-222-8771; fax: q44-191-222- 5292; E-mail: k.scott@ncl.ac.uk wx ing methanol oxidation 1 . Oxidation of intermediates to carbon dioxide requires the adsorption of oxygen contain- Ž . ing species e.g., OH, H O . Adsorption of these species 2 does not occur substantially until potentials well above the wx reversible potential of the anode 2 . In fuel cells, platinum alone is not a sufficiently active methanol oxidation elec- trocatalyst and the promotion of methanol oxidation has been actively studied. Currently significant results have been achieved with the use of binary catalysts, notably Pt–Ru. With these catalysts the second metal forms a surface oxide in the potential range for methanol oxidation wx 3. Recent developments in electrode fabrication techniques and better cell designs have brought dramatic improve- ments in cell performance in small-scale DMFCs operating on vaporised fuel. Typically, power densities higher than 0.18 W cm y2 are achievable, and power densities higher y2 wx than 0.3 W cm have been reported 4 . To date an essential condition for the high performance of a DMFC is the use of low concentrations of methanol. At concentra- tions higher than approximately 2 mol dm y3 , the cell voltage declines significantly due to permeation of Ž w . methanol through the SPE Nafion membrane, i.e., wx methanol crossover 4 . This permeation results in a mixed 0378-7753r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0378-7753 99 00303-1