(PEFC&E 2nd Place Best Poster Winner) Molecular Analysis of Cerium Ion Transport Properties in Polymer Electrolyte Membrane

Abstract

In order to improve the durability of Polymer electrolyte fuel cell (PEFC), to clarify the cause of deterioration and take measures is necessary. Chemical degradation of polymer electrolyte membrane (PEM) is considered as one of the causes of degradation of PEFC. Hydroxyl radical (OH ∙) is generated by decomposition of hydrogen peroxide generated by oxygen reduction reaction (ORR) during PEFC operation. This OH ∙ has high reactivity, and OH ∙ would degrade the polymer membrane. Based on this degradation mechanism, a method of adding a substance that inactivates OH ∙ before reacting with the polymer (radical scavenger) has been put into practical use. One of the most useful radical scavengers is cerium ions. Cerium ions in the PEM have been reported to migrate during operation and the distribution becomes non-uniform, and degradation occurs at points where there are only a few cerium ions in the membrane. Understanding the cerium ion transport mechanism in PEM is important for controlling cerium ion migration. Because investigating the cerium ion transport in operating PEFC is limited in experiments, nanoscale analysis by numerical simulation is useful to understand the properties of cerium ion transport. In this study, we analyze the distribution and transport properties of cerium ions in the membrane by modeling the PEM containing cerium ions using molecular dynamics (MD) simulations. Diffusion due to the concentration gradient, migration due to the ionomer potential gradient, and convective ion transport due to water crossover are considered as the transport factors of the cerium ions in the membrane. In the present study, the effect of water contents and water flux on the convection ion transport is analyzed. Simulation system is composed of polymer chains, cerium ions, and solvent molecules (water molecules and hydronium ions). Nafion® model was used as polymer and the equivalent weight (EW) was set at 1146. The potential based on the DREIDING force field was adopted for the molecular interaction of Nafion®, and particle mesh Ewald method was used to calculate Coulomb force.The cutoff distance of the LJ potential and Coulomb potential was set at r c=12 Å. The positive charge of the hydronium ion and the cerium ion was balanced with the negative charge of the sulfonic acid group in the system. The number of water molecules was determined by the water content λ which represents the number of water molecules per sulfonate group. The water content was set at λ = 4, 8, 12 and 16, which correspond to the water content in the bulk Nafion® membrane at approximately RH = 30%, 70%, 85% and 100%, respectively. Cerium ion concentration is equivalent to 1.2 wt% cerium ion relative to the polymer mass in the membrane, which is the typical concentration for Ce-Nafion composite membranes. Three-dimensional periodic boundary condition was applied to simulation box. The annealing process was applied in order to equilibrate the system. After annealing, the production run was performed for 15 ns using the NVT ensemble. The temperature in the system was 350 K. The time step was set at 1 fs, and the sampling interval was 10,000 steps. The simulation system is shown in Fig. 1. To evaluate the convection ion transport, an external force was applied to water molecules along the uniaxial direction to obtain flux of water. The carbon atom at the base of the side chain in Nafion® was fixed to prevent the flowing of Nafion®. The convection ion transport coefficient, K convection, is defined as the ratio of the mass flux of cerium ions and the mass flux of water molecules. The mass flux can be obtained by multiplying the concentration of X and the mean velocity of X. In addition, structural analysis will be performed to identify the factors that are dominant in cerium ion transportation. Figure 1