Water and methanol transport through polymer electrolytes in elevated temperature fuel cells.

摘要

Elevated temperatures will minimize the effects of catalyst poisoning and enhance the catalyst activity in methanol/air proton exchange membrane (PEM) fuel cells. In order to develop an elevated temperature PEM fuel cell, a stable polymer electrolyte membrane with high ionic conductivity at the operating temperature is required. Such a membrane should also possess desirable properties including negligible electronic conductivity, an ionic transference number of the reacting ion (proton) which tends to one, and low methanol permeability to increase the fuel efficiency.; This study addresses the sorption and transport properties of water and methanol vapor in polymer electrolytes, that are required for rational design of elevated temperature fuel cells. Specifically, water and methanol vapor sorption, solubility, diffusivity, permeability coefficients, electro-osmotic drag coefficient (representing the number of water molecules dragged with each migrating proton) as well as the transference number were measured as a function of water (or methanol) activity at temperatures up to 200{dollar}spcirc{dollar}C. Comparisons and distinctions among the PBI (polybenzimidazole), the Nafion{dollar}spcircler{dollar}/H{dollar}rmsb3POsb4,{dollar} and conventional Nafion{dollar}spcircler{dollar}117 polymer electrolytes are provided.; Based on these properties, a transport model applicable to elevated temperature direct methanol/air PEM fuel cell is derived and solved numerically using a flexible polyhedron search. The model focuses on water management, fuel utilization and ohmic loss of the membrane-electrodes assembly. The model results can be used to maintain an optimal chemical species distribution in the PEM fuel cell, provide sufficient proton conductivity in the membrane, and avoid cathode flooding and anode dehydration, thus assuring long term PEM fuel cell performance.