Evaluating Effects of Cerium Concentration and Water Content on Diffusivity of Cerium Ions in PEFC Using Molecular Dynamics Simulations

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Polymer electrolyte fuel cell (PEFC) is highly regarded as a promising technology. High durability is needed to achieve the large scale spread of PEFC. Chemical degradation of polymer electrolyte membrane (PEM) is one of the most critical problems that decreases the longevity and reduce functionality of a PEFC. Hydroxyl radicals and hydroperoxyl radicals are generated by the decomposition of hydrogen peroxide generated during PEFC operation. These radicals have high reactivity and degrade the polymer membrane. A method of adding a substance that quench radicals before reacting with the polymer (radical scavenger) has been put into practical use. One of the most useful radical scavengers is a cerium ion. Cerium ions in the PEM have been reported to migrate and the distribution become non-uniform which causes local degradation. Furthermore, if we add exceeded amount of cerium ions, proton conductivity decreases. There are three factors that can make cerium ions migrate in PEM: concentration gradient, water flux, and electrical potential. These factors are considered to be influenced by cerium concentration, temperature, and water content. Separating these effects in a real system and numerically evaluating the effects of many more variables is difficult. Therefore, nanoscale numerical analysis is a realistic approach to elucidate the transport properties of cerium ions. In this study, molecular dynamics simulations were used to evaluate the transport properties of each factor of cerium ions in polymer electrolyte membranes.Simulation system is composed of Nafion chains, cerium ions, and solvent molecules (water molecules and hydronium ions). The number of water molecules was determined by the water content λ which represents the number of solvent molecules per sulfonate group. The water content was set at λ = 3, 5, 7 that correspond the real conditions. Cerium ion concentrations are set at 5.38, 7.53, and 9.69 mg/cm3, which is the typical concentration mentioned in previous studies [1]. The number of hydronium ions was adjusted according to the concentration of cerium ions so that the positive charge is balanced by the negative charge of the sulfonic acid group of Nafion in the system. Three-dimensional periodic boundary condition was applied to simulation box. The temperature in the system was 323, 363, and 403 K. After equilibration, production run was performed for 50 ns using the NVE ensemble to calculate diffusion coefficient. To analyze the effect of water flux, a force in the x direction to the water molecules was applied. The force magnitude was determined by confirming that there was no change in water molecule polarization and membrane structure. After steady state was confirmed, and a 50 ns sampling run was performed using the NVT ensemble. The time step was set at 1 fs, and the sampling interval was 10 ps.The diffusion coefficients of cerium ions were obtained using mean square displacements. Fig.1(a) shows that the higher temperature, the greater the diffusion coefficient of cerium ions. Fig.1(b) indicates that when λ=3, the diffusivity was higher at higher concentrations, but the rate of increase in diffusivity with increasing water content was higher at lower concentrations, and at λ=7, the diffusion coefficient of cerium ions was larger at lower concentrations. This result indicates diffusivity vary depending on the concentration of cerium ions, which has not been previously considered by experiment. To evaluate water flux influence, the ratio of the molar flux of cerium ions to the molar flux of water molecules (K conv) in the x direction was taken. Molar fluxes were obtained from the mean displacement of the molecules. The fluxes were compared under conditions of cerium ion concentrations at 9.69 mg/cm3, because they cannot be accurately evaluated when the concentrations are low. Fig.1 (c) shows that K conv takes lower value as temperature increases and the lowest value when λ=5 at all temperatures. As Fig.1(d) shows, the reason for the larger K conv at lower temperatures is that the mean displacement of cerium ions is not so different at 323 K~363 K, but the mean displacement of water at 323 K is significantly smaller than at other temperatures. Fig.1 (e) indicates that at water contents of 3 and 5, the mean displacement of water increases dramatically, even though the change in the mean displacement of cerium ions is small. These results can lead to optimization of water and temperature management for control of cerium ion distribution.AcknowledgmentThe New Energy and Industrial Technology Development Organization (NEDO) of Japan supported this work under Grant number JPN20003.Reference[1] A. M. Baker., et al., Cerium Migration during PEM Fuel Cell Accelerated Stress Testing, J. Electrochem. Soc. 163, F1023 Figure 1