Understanding the mechanism of hydrogen transport in imidazolyl polymers doped Nafion membranes via molecular dynamics simulations: Case of PVMZ/Nafion

Decreasing gas penetration has noticeable effects on the durability and performance of Nafion based proton exchange membranes (PEMs). Experimental studies showed that adding imidazolyl groups into Nafion could reduce the hydrogen permeation in the membrane, while causing the substantial increase in power density. Nonetheless, the microscopic mechanism of doping on the reduction of hydrogen permeation is remain unclear. In this work, molecular dynamics simulation was performed to understand the transport behaviors of hydrogen in polyvinylimidazole (PVMZ)/Nafion composite membranes at the molecular level. The simulation results indicate that there are different degrees of effects about the polymer phase and aqueous domain on the diffusion kinetics of hydrogen in Nafion and PVMZ/Nafion at low and high hydration levels. With the increase of water content, the hydrogen molecules will migrate from the polymer phase to the polymer/water interface in Nafion and PVMZ/Nafion membranes. At low water content, the more free volume and the more pores in Nafion membrane are conducive to the diffusion of hydrogen than that in PVMZ/Nafion membrane to a greater extent. For the aqueous domain, the smaller and the rougher surface of water clusters in PVMZ/Nafion membranes have a certain limiting effect on the diffusion of hydrogen molecules than that in Nafion membranes at high water content. Consequently, at low water contents (lambda <= 9), the self-diffusion coefficient of hydrogen (DH2) increases slowly from 0.55 to 1.01 ( x 10-5 cm2/s) in Nafion membrane and from 0.26 to 0.53 ( x 10-5 cm2/s) in PVMZ/ Nafion membrane. In contrast, at high water contents (lambda >= 9), the DH2 increases rapidly from 1.01 to 2.35 ( x 10-5 cm2/s) in Nafion membranes and from 0.53 to 1.74 ( x 10-5 cm2/s) in PVMZ/Nafion membranes. The current work can provide a fundamental understanding for experimentally designing new type Nafion based PEMs for hydrogen fuel cells.