Molecular Dynamics Simulations of Cerium Ion Transport Phenomena in Polymer Electrolyte Membranes of Polymer Electrolyte Fuel Cells
Authors: Suzuki Hiroto , Mabuchi Takuya , Tokumasu Takashi 
Published: November 2022

As energy demand increases, global warming progresses, and energy resources are scarce in the future, expectations for fuel cells, which generate electricity through a chemical reaction between hydrogen and oxygen, are rising. There are various types of fuel cells, which are classified according to the electrolyte. This study deals with polymer electrolyte fuel cells (PEFCs), 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 h or more. However, the current durability is about 10,000 h, so an investigation into the causes of deterioration and countermeasures are urgently needed. One of the causes of degradation is the chemical deterioration of polymeric membranes. When the by-product hydrogen peroxide comes into contact with impurities such as iron ions, hydroxyl radicals (・OH) are generated, which attack and decompose the polymer membrane. In order to suppress this degradation, research have been conducted to add a substance that renders hydroxyl radicals inactive (radical scavenger) to polymer membranes, and this has been put to practical use. One of the most useful radical scavengers is cerium ions. However, it has been reported that cerium ions migrate in the electrolyte membrane, resulting in non-uniform distribution in the proton exchange membranes (PEMs), and in places where the concentration is low, and degradation progresses. The addition of too much radical scavenger hinders proton transport in the electrolyte membrane, reducing the output capacity of the PEFC. Therefore, understanding the cerium ions transport mechanism in the PEM is important to control cerium ions migration and improve the durability of the electrolyte membrane to maintain its performance as a fuel cell. However, the cost of operating experiments over 40,000 h is enormous, and it is difficult to analyze the phenomena occurring inside nanostructured electrolyte membranes by experiment, so simulation analysis is required.

In this study, molecular dynamics simulations have been performed for analysis of structure and cerium ions transport properties in PEM, consisting of polymer chains, cerium ions, water molecules, and hydronium ions. In all simulations, Nafion chain, which has the chemical structure with EW=1114, has been employed. Water content λ, which indicates the ratio of solvent molecules to sulfonic groups in Nafion, was changed to 4, 9, 14.

Fig. 1 shows the self-diffusion coefficient of cerium ions at a cerium ions content of 1.1 wt% (20 pcs added) as a function of temperature for each water content, indicating that the coefficient increases with increasing temperature for all water contents. In addition, the increase in the self-diffusion coefficient of cerium ions with temperature is small at low water content (λ = 4), and the rate of increase in the self-diffusion coefficient of cerium ions with temperature becomes larger as the water content increases. From the radial distribution functions (RDFs) of cerium ions around the sulfone group, cerium ions are distributed at about 4.15 Å around the sulfone group at low water content, while at high water content they are distributed at about 6.0 Å. From the RDF of water molecules around the sulfone group, water molecules are located at about 4 Å around the sulfone group. The results indicate that cerium ions coordinate directly to the sulfone groups at low water content because fewer water molecules collect around the sulfone groups. Conversely, at high water content, water molecules cover the sulfone groups, and cerium ions coordinate outside the sulfone groups, which lead to the increase of the diffusion coefficient.

Acknowledgments

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan, Grant number JPNP20003. It was performed on the Supercomputer system “AFI-NITY” at the Advanced Fluid Information Research Center, Institute of Fluid Science, Tohoku University.

Figure 1