Numerical Analysis of Nanostructure Around Ferrous Ion in Hydrated Nafion Membrane

Abstract

Polymer electrolyte fuel cell (PEFC) is strongly expected as a next generation power source because of 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 strongly on proton transport property in polymer electrolyte membrane (PEM). Proton transport property of PEM has been evaluated by experiments and numerical simulations by many researchers. The parts of separator used for holding PEM and gas flow channel are typically made of graphite materials because they require mechanical strength, corrosion resistance, and electric conductivity. However, using graphite material causes increasing the cost. 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 in power density of PEFC [1]. To deal with this problem, more information about the behavior of metal ion and its influence on proton transport in PEM is required. However, it is difficult to analyze the effect of metal ion directly in experiment because the proton transport property depends on the nano-scale structure of PEM and water molecules. In our previous work [2], characteristic changes in diffusion coefficients of proton were observed with ferrous ion contamination by using molecular dynamics simulations. Ferrous ion contamination has two totally different effects on proton transport. One is the connecting water clusters and enhancing proton transport at ER = 50%. The other is disintegrating water clusters and inhibiting proton transport at ER >= 70%. However, the accuracy of those results was insufficient because the model of ferrous ion was treated as the static charged particle with coulomb interaction. In regard to this problem, the polarizable force field model is effective to describe ferrous ion complex [3]. In this study, we analyze the structural property around the ferrous ion with more accurate force field model of ferrous ion considering with induced dipole interaction. Polarizability is considered on the ferrous ion atom and the oxygen atom in water molecule. The model of water molecule is developed based on aSPC/Fw model [4]. We investigated hydrated Nafion membrane in the case of water content λ=3, 6, 9, 12. We evaluated the radial distribution function between ferrous ion and water molecule, hydronium ion, or sulfonate groups. The cluster analysis of water region will be carried out. Finally we discussed the correlation between the concentration of ferrous ion and nanoscale structural changes of Nafion membrane, and estimated the change of proton transport property.