A Molecular Dynamics Study of Transport Properties in Degraded Nafion Membranes

Abstract

Degradation of the polymer membrane is known as one of the important factors to determine the lifetime of polymer electrolyte fuel cells (PEFCs). The membrane electrode assembly of a PEFC is made from various components, e.g. the catalyst layers, the gas diffusion layers, the micro-porous layers and the polymer electrolyte membrane. Generally, the lifetime of a PEFC is specified by the degradation of a polymer electrolyte membrane. Degradation of the membrane can be classified into three categories: thermal, mechanical, and chemical. For Nafion membranes which are often used in PEFCs, the chemical degradation is more significant because Nafion membranes have excellent mechanical and thermal stability. Since the chemical degradation, which is mainly due to the attack of radicals formed during the operation, is inevitable, the knowledge to estimate the performance of the degraded membrane is required. Proton conductivity is one of the typical indicators for chemical degradation of the polymer electrolyte membranes. The proton conductivity in a degraded Nafion membrane becomes lower than the one in the pristine Nafion membrane. The transport properties in the membranes strongly influenced by the molecular morphology of the multicomponent system which includes polymers, water/hydronium molecules, and several fragments generated by the degradation. Although the degraded Nafion membranes had been investigated experimentally and numerically, evaluation of the relationship between transport properties and degradation levels, taking into account several fragments generated by the degradation, had not been considered. In addition, it is difficult to control the degradation experimentally because the degradation occurs in molecular scale. In the present study, we have carried out molecular dynamics simulations of a degraded Nafion membrane system. The effect of a degradation mechanism on transport properties, which is identified by what kind of radical react on, is important because the combination of fragments is considered to determine the water structure. However the degradation mechanism is still under discussion. Here, a proposed mechanism by Ghassemzadeh et al.1 have been adopted to create a degraded Nafion and generated fragments. In their mechanism, the attack of the hydroxyl radical, which is one of the most reactive species in a degraded Nafion membrane, causes unzipping of side chain. Consequently, HF, CO2, CF3, and HOC2F4SO3 molecules are generated in the reaction. A modified DREIDING force field2-4 have been employed for degraded Nafion molecules and fragments. Water/ hydronium molecule interacts with anharmonic potential5 to describe Grothus mechanism with aTS-EVB model6. For side chain molecules of a degraded Nafion monomer and fragment molecules, the equilibrium bond lengths, the equilibrium angles and the partial charges have been determined by DFT calculations. In the simulation systems, there were four ten-unit degraded Nafion molecules, whose degradation level is defined by changing the number of broken side chains, equimolar generated fragments, and water/hydronium molecules with varying water contents. We have conducted simulations in various conditions, such as different water contents and degradation levels. Diffusion coefficient of each molecule have been used as their transport properties. To evaluate molecular structures, radial distribution functions have been calculated in various degradation levels. We have evaluated the relationship between diffusion coefficient and degradation levels.