Water and methanol uptakes in nafion membranes and membrane effects on direct methanol cell performance

This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion(R) (E.I. DuPont de Nemours and Company) membranes of different eq uivalent weight (EW). Methanol and water uptakes in 1100 and 1200 EW Nafion membranes were determined by weighing P2O5-dried and methanol solution-equ ilibrated membranes. Both methanol and water uptakes in the 1200 EW membran e were about 70-74% of those in the 1100 EW membrane. The methanol crossove r rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell oper ating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1200 EW membrane w as 52% of that through an 1100 EW membrane. To resolve the cathode and anod e performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathod e can be easily flooded. as shown in a DMFC using 1100 EW membrane. An incr ease in methanol crossover rare decreases the DMFC cathode potential at ope n circuit. At a high cell current density, the DMFC cathode potential can a pproach that of a H-2/air cull. (C) 2000 The Electrochemical Society. S0013 -4651(99)05-093-4. All rights reserved.