Analysis of Cerium Ion Transport in Anode Side Catalyst Layer for Improving Polymer Electrolyte Membrane Durability of Polymer Electrolyte Fuel Cells

Abstract

This study deals with Polymer Electrolyte Fuel Cells (PEFC), which are used in automobiles and household fuel cells. Currently, there are two main challenges for the practical application of PEFCs: durability and cost reduction. The target value for durability is required to be 40,000 hours or more. However, the current durability is about 10,000 hours. Therefore, there is an urgent need to investigate the causes of deterioration and to take countermeasures. One of the causes of degradation is chemical degradation of the polymer electrolyte membrane. When the byproduct hydrogen peroxide encounters impurities such as iron ions, hydroxyl radicals are generated, which attack and degrade the polymer electrolyte membrane. To prevent this degradation, a substance that inactivates hydroxy radicals (radical scavengers) has been featured. Radical scavengers have been added to the polymer electrolyte membranes to prevent this degradation and are now being used in practical applications. One of the most useful radical scavengers is Ce ion. However, it has been observed experimentally that Ce ions in the PEM migrate to the cathode catalyst layer during cell operation. As a result, the Ce ions in the electrolyte membrane are reduced and the electrolyte membrane is degraded by radicals. Conversely, ceria added to the anode microporous layer ionizes and diffuses into the PEM. Because the ceria is ionized, it provides the Ce ions for the electrolyte membrane. However, how Ceion is transported in the catalyst layer and under what conditions it is transported have not been clarified. If Ce ion is not transported sufficiently from the microporous layer to the PEM, it becomes less effective as a radical scavenger and degradation progress. In addition, excessive Ce ion transport also interferes with proton transport in the PEM and reduces the output capacity of the PEFC. However, the cost of running experiments for over 40,000 hours is quite expensive, and the phenomena occurring inside the nanostructured electrolyte membrane are difficult to analyze experimentally. Therefore, simulation analysis is useful to understand the Ce ion transport mechanism in the catalyst layer for controlling Ce ion transport toward improvements of the durability of the electrolyte membrane while maintaining fuel cell performance.In this study, molecular dynamics (MD) simulations have been performed for analysis of structure of Ce ions within ionomers and dynamic properties of Ce ions in catalyst layer. Catalyst layer consists of carbon, polymer chains, Ce ions and solvent molecules, which are water molecules and hydronium ions. A Nafion chain, which has the chemical structure with equivalent weight =1114 was employed. The modified version of the DREIDING force field reported by Mabuchi et al. was used in this study. We have adopted the BOND potential of the COMPASS force field, which is based on the Anharmonic aSPC/Fw Water Model for water molecules and does not consider the Grotthuss mechanism to analyze the effect of cerium ions due to water convection. Water content λ, which indicates the ratio of solvent molecules to sulfonic groups in Nafion, was set at 5, 9 and 13. Twenty polymer chains, 140 hydronium ions and 20 Ce ions were placed in the simulation box with the two-dimensional periodic boundary conditions in the in-plane directions to ensure charge neutrality. The concentration of Ce ions in this case corresponds to 1.1 wt % of the Nafion mass in the membrane. This is a typical concentration for Ce-Nafion composite membranes. We applied to include ionic dipole interaction. For the other molecules, the general 12-6 LJ-type was used, and the arithmetic mixing rule was applied to calculate interaction between them and cerium ions, water oxygen and oxygen of hydronium. The annealing process was applied in order to equilibrate the system. After annealing, the production run was 10 ns using NVE ensemble. The time step was set at 1 fs, and the sampling interval was 5,000 steps. The temperature of the system was set at 323 K, 363 K and 403 K. We will determine where the Ce ions are transported in the catalyst layer by examining the density distribution and diffusion coefficient of Ce ions while changing the initial position of Ce ions. We will also report transport factors by analyzing the structural properties around Ce ions in the presentation. Figure 1