Understanding the mechanism of nitrogen transport in the perfluorinated sulfonic-acid hydrated membranes via molecular dynamics simulations

Abstract

In perfluorinated proton exchange membrane fuel cells (PEMFCs), the diffusion of inert nitrogen across the membranes can lead to its accumulation at the anode, which will lead to the failure of the cells. However, such a diffusion mechanism has not been clarified up to now. In this work, molecular dynamics simulations were performed to investigate the effect of water content and equivalent weight (EW) value on the diffusion of nitrogen molecules in perfluorinated sulfonic-acid (PFSA) hydrated membranes. It is found that the nitrogen molecules tend to distribute in the polymer phase and show the restricted regional motion at relatively low water content. However, with the increase of water content and the decrease of the EW value, the distribution and movement of nitrogen molecules show completely different behaviors. Nitrogen molecules are forced to migrate to the polymer/water interface and have a higher probability to diffuse along the water clusters. Meanwhile, the diffusion of nitrogen molecules is promoted and they show a jump-like motion. This study provides a fundamental and comprehensive understanding of nitrogen transport in PFSA hydrated membranes. • The distribution and diffusion dynamics of nitrogen have great differences for PFSA membranes with different water content and main chain length. • Nitrogen distributes in the polymer phase and is restricted in a specific region at low water content. • Nitrogen distributes at the polymer/water interface and diffuses along the polymer/water interface with high water content and low EW value.