Analysis of the Influence of Cerium Ions on Mass Transport Properties in Polymer Electrolyte Membrane by Molecular Dynamics Simulation
Authors: Ishikawa Kyohei , Mabuchi Takuya , Tokumasu Takashi 
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) are highly expected as a promising technology for stationary power generation, mobile electronics, and hybrid engines in automobiles. However, durability is one of the critical problems for practical use. Chemical decomposition of polymer electrolyte membrane (PEM) by hydroxyl radicals generated from hydrogen peroxide generated during PEFCs operation has been reported as one of the causes of PEFCs degradation, and as a countermeasure, a method of adding cerium ions into the PEM has been studied and has been applied to practical use. The distribution of cerium ions in PEM and the influence of cerium ions on mass transport properties in the membrane has not been confirmed, and it is difficult to analyze the phenomenon that occurs inside PEM with nanostructure by experiment, so analysis by simulation is required. As an example, we describe the influence on proton transport properties. Proton transport properties are important for the improvement of efficiency of PEFCs, and it is greatly influenced by the structure in PEM. In the PEM, it is known that solvent molecules gather around hydrophilic groups in the polymer to form a structure called water cluster. Inside this water cluster, proton conduction by the fast spreading grotthuss mechanism become dominant. Therefore, it is very important to evaluate proton transport properties in relation to the structure of water cluster.

In this study, molecular dynamics (MD) simulations have been performed for analysis of structure and proton transport properties in PEM containing cerium ions. PEM consists of polymer chains, cerium ions and solvent molecules, which are 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 3, 6, 9, 12. Four polymer chains and 40 hydronium ions were placed in the simulation box to ensure charge neutrality, reducing 3 hydronium ions per cerium ion. The number of cerium was varied to be from 0% to 20% of the number of sulfonic groups of Nafion. The annealing process was applied in order to equilibrate the system. Sampling time was 24 ns. The temperature in the system was 350 K. We have obtained diffusion coefficient of proton as transport property analysis and radial distribution function (RDF), coordination number, number of cluster, and cluster size as structural property analysis.

At low water content, diffusion coefficient of protons increases by adding cerium ions. It was also found that at low water content, cerium ions were present around sulfonic groups and the number of clusters increased and the cluster size decreased as the number of cerium ions increased. From the above, at low water content, cerium ions attract the surrounding water molecules and clusters are connected to make better diffusion path of protons. At high water content, diffusion coefficient of protons decreases by adding cerium ions. Also, the distribution of cerium ions spreads not only around sulfonic groups but also within clusters. Cluster size is large compared to that at low water content and it is not affected by in the number of cerium ions. From the above, at high water content, clusters are grown to a sufficient size with or without cerium ions. And, due to the presence of highly charged cerium ions in the cluster, it repulses with protons having the same positive charge, thereby reducing the diffusion of protons.

In addition, after long-term operation of PEFC, it is reported that the distribution of cerium ions in PEM concentrate on the cathode side and PEM degrades. Therefore, as the future work, we investigate the dominant factor of cerium ion transport in PEM.