Molecular Dynamic Simulations of Proton and Water Transport Mechanism in a Nafion Pore

Herein, nonequilibrium molecular dynamics simulations are conducted to study the hydraulic permeation and vehicle transport of protons in proton exchange membranes (PEMs) with different structures. The results indicate that water molecules and hydronium ions are transported in porous PEMs in the form of clusters. Water molecules are more concentrated in the bulk areas of pores, whereas hydronium ions had a high-density profile near the pore surface areas since sulfonate ions of the side chain exhibit a stronger adsorption effect on hydronium ions. Due to the confined pore size, the slip flow of water molecules and hydronium ions occurs near the pore surfaces and it becomes more significant as side chain separation or pore size increases. As pore size decreases, the reduction of movement region and the deep attractive potential near the pore wall lower the specific enthalpy due to enhanced molecule-wall collisions. In addition, the transport diffusivity coefficients of positively charged hydronium ions are smaller than that of water molecules due to the stronger Coulomb interactions and hydrogen bonding with negatively charged side chains. Transport diffusivity coefficients of water molecules and hydronium ions increase as pore size and side chain separation increase.