Investigation of the transversal water profile in nafion membranes in polymer electrolyte fuel cells

The in situ resistance of Nafion membranes with different thickness was measured in one-dimensional fuel cells as a function of current density. Except for the thin Nafion 112 membrane, an increase of the ionic resistance with current density tin the range 0 to 1 A/cm(2)) was found. The thicker the membrane, the stronger the increase in the same current density interval. The resistance distribution across the thickness of membranes was determined by using membranes composed from several thin sheets with interlying thin gold wires as potential probes. It was found that the increase of the resistance is always confined to the membrane sheet contacting the anode electrode. These measurements, combined with the results from experiments with membranes of different water content, lead to the conclusion that the resistance increase at the anode side is due to the insufficient compensation of the electro-osmotic drag by the back transport of water to the anode. Based on a solution diffusion mechanism of the water motion in the membrane, the experimental results may be explained by a mechanism whereby the electro-osmotic drag coefficient is independent of the local membrane hydration and the water diffusion coefficient D-H2O, is a strong function of the local membrane water content. The experimental data would, qualitatively, also be in line with a model proposing back transport of water to the anode by convection of water in the submicropores of the membrane. (C) 2001 The Electrochemical Society. All rights reserved.