Effect of temperature and external electric field on proton transport in ordered and amorphous proton exchange membranes: A molecular dynamics study

Abstract

Proton exchange membrane (PEM) is a key component of proton exchange membrane fuel cell, and its proton conductivity directly affects the performance of fuel cell. Imparting order to Nafion molecules is one of potential effective approaches that can significantly improve the proton transport performance of PEM, but the underlying mechanism of this effect as well as the regulatory mechanism is unknown. In order to investigate the regulatory mechanism of temperature, electric field, and water content on the transport mechanism of the PEM and compare the transport properties between ordered and amorphous membranes, this study examined the proton migration rate of ordered and amorphous membranes under different electric field and water contents at 300 K and 350 K with molecular dynamics methods. The simulation results indicate that the ordered PEM exhibits better proton conductivity compared with the amorphous PEM due to its ordered structure with smoothly connected proton transport channels. The proton conductivity of the ordered PEM with a water content of 16 exhibits a remarkably high proton conductivity of 0.153 S/cm at a temperature of 350 K. Additionally, it was found that increasing electric field intensity, temperature, and hydration level can enhance the diffusion of water molecules and hydrated protons and improve the proton conductivity of the PEM. Especially, at low electric field intensity, ordered PEM demonstrates superior proton conductivity compared to amorphous PEM. However, as the electric field strength increases to 0.7 V/nm, the proton diffusion behaviour of amorphous PEM gradually surpasses that of ordered PEM. Moreover, the electric field can enhance the anisotropic diffusion behaviour of PEM. As the electric field increases from 0 V/nm to 0.7 V/nm, the diffusion coefficient parallel to the direction of the electric field increases by three orders of magnitude, while the diffusion coefficient perpendicular to the direction of the electric field increases by only one order of magnitude.