Electro-osmotic drag coefficient of Nafion membrane with low water Content for Proton exchange membrane fuel cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) are recognized as promising clean power sources in automotive applications. However, there are still confusions and contradictions about the electro-osmotic drag (EOD) coefficient of Nafion membrane with low water content. In this work, the molecular dynamics (MD) simulation is utilized to predict the EOD coefficient in dehydrated Nafion membranes with low water content λ=1, 1.5 and 2 at 350 K for fuel cells under low relative humidity. Micro-structurally, it is found that when the dehydrated Nafion membrane gradually becomes more hydrated, or λ increases from 1 to 2, more water molecules are bound to Nafion side chains and larger atom clouds are formed. In order to predict the EOD coefficient, mass flows of hydronium ions and water molecules are analyzed. The hydronium ions flow much faster than water molecules in the direction of electric field. Consequently, based on the mass flow values, for the Nafion membrane with low water content λ=1, 1.5, and 2, values of the EOD coefficients are 0.098, 0.149 and 0.255, respectively. It is further discovered that the EOD coefficient for Nafion membrane with low water content exactly agree with the linear model nd=2.5λ/22.