Direct Methanol Fuel Cell: Transport Properties of Polymer Electrolyte Membrane and Cell Performance

Methanol and water absorption in 1100 and 1200 e.w. Nafion ® membranes was determined by weighing P 2 O 5 dried and methanol solution equilibrated membranes. Both methanol and water absorption in the 1200 e.w. membrane is about 70–74 % of that in the 1100 e.w. membrane. The methanol cross-over rate corresponding to that in a direct methanol fuel cell (DMFC) at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions (temperature, humidification and concentration of feed methanol solution). Accounting for the thickness difference between the membrane samples, the methanol cross-over rate through a 1200 e.w. membrane is 52 % of that through a 1100 e.w. membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode (DHE) was used as a reference electrode. Results show that in DMFC operation the cathode could be flooded due to the high water and methanol cross-over rates, especially through the 1100 e.w. membrane at a cell temperature below 80 °C. An increase in methanol cross-over rate as incurred by increasing the concentration of the feed methanol solution, increasing the cell operating temperature or using a membrane more permeable to methanol decreases the cathode potential of the DMFC at open circuit. As the cell current density is increased, the cathode potential of the DMFC can approach the cathode potential of a H 2 /air cell, thanks to the consumption of methanol at the anode and consequent decrease in methanol cross-over rate.