ORIGINAL Methanol diffusion rates through the anode diffusion layer in direct methanol fuel cells from limiting current measurements Srikanth Arisetty Æ Suresh G. Advani Æ Ajay K. Prasad Received: 28 February 2007 / Accepted: 15 October 2007 / Published online: 6 November 2007 Ó Springer-Verlag 2007 Abstract Metal foams may be used in direct methanol fuel cells to feed reactants to the catalyst layer and to collect current from the resulting electrochemical reaction. Although the mass transfer from the metal foam to the underlying gas diffusion layer (GDL) is diffusion-domi- nated, it is found that at a fixed methanol concentration, the limiting current density increases with increasing methanol flow rates. This unexpected result is attributed to the more efficient removal of product CO 2 from the GDL. A meth- odology is developed to estimate the effective diffusion coefficient of methanol in the anode diffusion layer from limiting current density measurements, and to extract the fraction of GDL volume occupied by CO 2 . List of symbols a aspect ratio of the GDL C(x,y) concentration of methanol at a position (x, y) in the GDL (M) C o concentration of the methanol feed at inlet (M) D diffusion coefficient of methanol in water (m 2 /s) D eff effective diffusion coefficient of methanol in the GDL (m 2 /s) F Faraday constant (C/mol) H thickness of the GDL (m) J limiting current density (A/cm 2 ) K F permeability of the metal foam (m 2 ) K G permeability of the GDL (m 2 ) K tot total mass transfer coefficient to the catalyst layer (m/s) L length of the GDL (m) Q o methanol flow rate (ml/min) U G , U average velocity of methanol through the GDL (m/s) U F average velocity of methanol through the foam (m/s) Greek symbols u dimensionless concentration g dimensionless x-coordinate h dimensionless y-coordinate e pore fraction in the GDL available for methanol flow e GDL original porosity of the GDL e CO 2 volume fraction of the CO 2 in the GDL Non-dimensional groups Da Damkohler number (JL/(6FHUC o )) Pe Peclet number (UH/D) Sh Sherwood number (K tot H/D) 1 Introduction Direct methanol fuel cells (DMFCs) operate on the prin- ciple of the galvanic cell with a polymer membrane as the electrolyte. In addition to the polymer electrolyte mem- brane (PEM), the other components of a DMFC include anode and cathode catalysts, gas diffusion layers (GDL), and flow fields. A schematic drawing of a DMFC with its associated reactions is shown in Fig. 1. Flow fields are used to supply reactants comprised of methanol and air to the anode and cathode of the cell, respectively; the reactants S. Arisetty  S. G. Advani  A. K. Prasad (&) Fuel Cell Research Laboratory, Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA e-mail: prasad@udel.edu 123 Heat Mass Transfer (2008) 44:1199–1206 DOI 10.1007/s00231-007-0355-3