Water channel morphology of non-perfluorinated hydrocarbon proton exchange membrane under a low humidifying condition

Abstract

Water channel formation of non-perfluorinated proton exchange membranes (PEMs) under a low humidifying condition is a very important issue, due to weaker phase separation between hydrophilic and hydrophobic moieties than in the case of perfluorinated PEMs such as Nafion. In this study, we performed Molecular dynamics (MD) simulations of hydrated sulfonated polyimide (SPI) models, one of the representative non-perfluorinated PEMs, under different temperature and humidifying conditions by removing water molecules continuously, reflecting experimental conditions of actual low humidifying fuel cell. The water channel morphology of sulfonated polyimide (SPI) models had no apparent temperature dependence. The hydrated SPI models show weak water channel formation even in a fully hydrated condition (lambda = 16.4), consistent with our previous study, and they do not display significant temperature dependence on the water molecule distribution. As the lambda value decreases from 16.4 to 2 (i.e., low humidifying conditions), the water molecules in the hydrated SPI models are evenly reduced. In particular, when the lambda value of the hydrated SPI model decreases from 8.5 to 6, the size of the water clusters is significantly narrowed and the clusters become segregated, and this is also confirmed by an X-ray scattering analysis. As a result, the proton conducting performance of hydrated SPI models shows similar behavior with the change in water channel morphologies, which will be very important to design a novel non-perfluorinated hydrocarbon PEM with high performance for practical fuel cell systems. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.