Molecular Simulation of Proton Conductivity in Nafion Membrane Contaminated with Ferrous Ion

Abstract

Polymer electrolyte fuel cell (PEFC) is strongly expected as a next generation power supply system due to its high efficiency, high power density, and purity of exhaust gas. However, there are some problems left such as infrastructure of hydrogen gas, durability of cell, and production cost. Performance of PEFC depends on proton conductivity in polymer electrolyte membrane (PEM). Proton conductivity is evaluated by experiment and numerical analysis by many researchers. The parts of separator used for holding PEM are typically made of graphite materials because they require mechanical strength, corrosion resistance, and electric conductivity. However, using graphite material is the cause of cost increase. Alternatively, using metal materials such as stainless alloy for the separator can reduce the cost although this causes the metal ion liquation, which leads to decrease power density of PEFC [1]. To deal with this problem, more information about the behavior of metal ion and its influence on other molecules in PEM is required. However, it is difficult to analyze the effect of metal ion directly in experiment because the proton conductivity depends on the nano-scale PEM and water structure. Therefore, we evaluate changes of proton transport property and nano-structure of hydrated Nafion membrane that is caused by ferrous ion contamination by using molecular dynamics simulation. In this analysis, the aSPC/Fw model was employed for water and hydronium ion [2], the DREIDING force field was employed for Nafion of EW about 1144 [3], and the model of ferrous ion which was based on DREIDING force field and newly modified was employed. The parameters of ferrous ion are shown in table 1. This model was validated by comparing the structural property between ferrous ions and water molecules. The results of distance between ferrous ion and oxygen of water molecule and coordination number that is calculated from RDF and those obtained by experiment [4] are shown in table 2. The 2-state Empirical Valance Bond (EVB) method was used for the description of the proton transfer through the Grotthuss mechanism [5]. It was assumed that all protons exist in the form of hydronium ion. For manipulating the EVB function, the charge of sulfonate groups, hydronium ions, and ferrous ions are weakened by 0.4175 times. For electric balance of the system, the number of hydronium ion was controlled depending on the number of ferrous ion. Amount of ferrous ion contamination was defined as “exchange ratio” (ER) which was the ratio of total ionic value between ferrous ions to all cations in the system. The calculated results of diffusion coefficient of hydronium ions are shown in Fig. 1. The trend that the diffusion coefficient increases with increasing hydration level is in good agreement with experimental result [6]. Characteristic changes of diffusion coefficient of hydronium ions are observed at λ=3. In this case, the diffusion coefficients of hydronium ions do not change in the region of ER≦25%, increase largely at ER=50%, and decrease lower than the region of ER≦25% in the region of ER≧70%. The result of radial distribution functions (RDF) for sulfonate groups in each ER in the case of λ=3 is shown in Fig. 2. In the case of ER=0%, there is no clear structure. Then, in the region ER≦25%, two peaks appear in two places in r=4.9Å and r=7.1Å with increasing ER. Those results suggests that the structural change of PEM occurs by ferrous ion contamination. The result of cluster analysis of water molecules is shown in Fig. 3. The number of water clusters does not change in the region of ER≦25%, decreases at ER=50%, and increases largely than the region of ER≦25% in the region of ER≧70%. This trend is strongly correlated with the result of diffusion coefficient of hydronium ions in Fig. 1. Those results suggest that the ferrous ion contamination causes connecting water clusters and enhancing proton transport in the case of ER=50%. On the other hand, in the case of the region of ER≧70%, ferrous ion contamination causes disintegrating of water clusters and inhibiting proton transport.