Numerical Analysis of Proton Conductivity in Hydrated Nafion Membrane Contaminated with Iron Ion

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. The parts of separator used for holding polymer electrolyte membrane (PEM) are typically made of carbon materials because they require mechanical strength, corrosion resistance, and electric conductivity. However, using carbon material is the cause of cost increase. Using metal materials such as stainless alloy for separator can reduce the cost. However, by using metal material metal ion liquate out, which leads to accelerate chemical degradation of PEM and depress power density [1]. For this problem, more information about the behavior of metal ion and influence to other molecules in PEM is required. It is difficult to measure it directly in experiment because proton conductivity depends on nano-scale Nafion and water structure. Therefore, we use molecular dynamics method to evaluate it.

In this analysis, Flexible three-centered water model [2] is employed for water and hydronium ion, and DREIDING force field [3] was employed for Nafion of EW about 930 and ferric ion. It was assumed that all protons exist in the form of hydronium ion. For electric balance of the system, number of hydronium ion was controlled depending on the number of ferric ion. Amount of ferric ion contamination is defined as “exchange ratio” (ER) which equals number of total ionic value of contamination ion divided by number of sulfo group.

The calculated result of radial distribution functions (RDF) for sulfo groups in each ER is shown in Fig. 1. In the case of “without ferric ion”, the distribution has 1st peak at 4.8 Å and it becomes lower as the distance is away from the peak. In the other case of “contaminated with ferric ion”, the 1st peak become higher and the 2nd peak emerges with increasing contaminated ions. Those results suggest that the structure of Nafion changes with increasing contaminated ions. Another result of RDFs of sulfo group, hydronium ion, and water molecule for contaminated ferric ion in case of ER of 41.7% are shown in Fig. 2. Strong peak is observed at 3.0 Å for water and 4.2 Å for sulfo group. Otherwise, hydronium ion has lower and further peak (6.5 Å) than water molecules. This result suggests that hydronium ion keeps away from ferric ion because of their electric force. Because of this repulsive effect, proton conductivity is decreased in contaminated with ferric ion case.

Comparing between Fig.1 and Fig.2, distance of the emerged 2nd peak at 7.8 Å in Fig.1 is almost twice as  the distance of the peak for ferric ion and sulfo group shown in Fig.2. This result suggests that contaminated ferric ion make bridge structures between sulfo groups with each other. Fig.3 shows a snapshot of the simulation system around ferric ion. This result suggests that the macroscopic mechanical property of PEM such as Young’s constant is changed due to the change of the nanostructure in the PEM.

References

[1] A. Pozio, R.F. Silva, M. De Francesco, L.Giorgi Electrochim Acta 48 (2003) 1543-1549

[2] M. Levitt, M. Hirshberg, R. Sharon, K.E. Laidig, V. Daggett, J. Phis. Chem. B, 101 (1997) 5051-5061

[3] S.L. Mayo, B.D. Olafson, W.A. Goddard ΙΙΙ, J. Phis. Chem. (1990), 94, 8897-8909