Theoretische Modellierung der elektro-physikalischen Eigenschaften, der Struktur und Funktion von Niedertemperatur-Ionenaustauschmembranen

Abstract

Electrophysical properties of polymer-electrolyte membranes (PEM), used as proton conductors and separators in polymer electrolyte fuel cells (PEFC), were studied and overpotential losses due to coupled transports of water and protons were calculated. The models focus or1 the perfluorinated sulfonic acid ionomers, which hitherto are the material of choice in PEFC. The properties of these PEM are determined by their phase-separated morphology, consisting of water containing pathways for proton and water transport and hydrophobic parts which provide mechanical stability and elasticity. In order to rationalize the water distribution in the porous polymer membrane and its effect an the proton conductivity, a random network model of proton transport was proposed, which takes into account the main features of the water distribution and of the specific swelling behavior. The specific bulk conductivity and capacity were calculated as functions of the water content within the effective medium approach. The obtained proton conductivity shows, in certain cases, a quasi-percolation behavior with a strong increase above a critical water content and a smail residual conductivity below this value (the one for the residual conductivity along pore Walls in the dry membrane) . The calculated geometric capacity possesses a sharp maximum at the percolation threshold. A comparison with experimental conductivity data shows, that the low percolation thresholds, obtained in the model for Nafion-type membranes, can be explained by the existence of a well connected network of pores (of a few nm diameter) in which water is homogeneously distributed already at low water contents. A serious problem for low temperature fuel cells is the partial dehydration of the membrane under working conditions . A model, which takes into account the electroosmotic drag of water molecules from anode to cathode counteracted by a backflow in a hydraulic pressure gradient, was considered . A balance between these fluxes is established in the stationary state, determining the gradient in water content across the membrane. Local values of proton conductivity, hydraulic permeability and electroosmotic coefficient are functions of the local water content . The latter is a function of the local capillary pressure in membrane pores . This function was measured, using a Standard porosimetry metho