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Molecular Dynamics Study of Adsorption Phenomenon of Aromatic Hydrocarbon Ionomer in Catalytic Layer of Polymer Electrolyte Fuel Cell

In response to the current global energy crisis and environmental problems, fuel cells are an optional solution. Among them, polymer electrolyte fuel cell (PEFC) has high expectations because of its low operating temperature and suitability for applications in households and mobility devices. Since PEFC requires the use of proton exchange membranes, the transmission efficiency and durability of the exchange membrane is a major concern. To improve the proton transport efficiency, which is generally required, a catalytic layer is integrated into the PEFC. The catalyst layer of PEFC generally contains platinum catalyst covered with the ionomer membrane. Proton conductivity increases with ionomer thickness, while oxygen permeability decreases with ionomer thickness. Therefore, in order to improve the power generation efficiency of PEFC, it is important to design the catalyst layer. In addition, the fluorinated ionomer membrane material (such as Nafion) used for PEFC has good proton conductivity and chemical stability, but the synthesis cost is relatively high. As a consequence, new materials such as composite materials and non-fluorine-based materials have received a lot of attention in recent years. In this study, we focus on a new ionomer material of aromatic hydrocarbon (named SPP-BP) and elucidate the adsorption state of the ionomer on the platinum surface on a nanoscale using molecular dynamics (MD) simulations. Then, the structural properties and water diffusion characteristics of the ionomers of new material (SPP-BP) and conventional Nafion were analyzed based on the MD simulation results.
 The degree of polymerization (m, p, s) of the monomers were given as 20, 5, and 23 (molecular weight 7.38 kDa), respectively, based on the reported experimental values of SPP-BP. For the polymer composition, molecular models of a random type (r-type), which shows a randomness of monomers, were used. A molecular model of Nafion was also created as the conventional fluorinated membrane materials for comparison with the SPP-BP. The platinum surface was built with a three layers of platinum atoms in a face-centered cubic lattice structure. Regarding the calculation potentials, the DREIDING forcefield was used for the interactions with the molecules. While for the intermolecular potential, the Spohr potential was used for Pt-H2O and Pt-H3O+ interactions, and the others were applied with the Lennard Jones (LJ). The parameters of those potentials were based on the previous studies.
 In the ionomer membrane, water content is an important indicator that affects the transfer of protons and substances. Based on the water absorption results obtained in the experiment, the water content of SPP-BP was calculated to be between 1.8 and 7.2. Therefore, the water content during the MD simulation was used 3, 5, and 7. The computational size was 55.5 Å × 52.9 Å in the transverse direction of the ionomer membrane (X and Y directions), and the thickness of the membrane adsorbed on the platinum surface was about 50-60 Å. A vacuum region of about 40 Å was set at the top of the system. To calculate the diffusion coefficient and the cluster character of water, multiple simulations with different initial structure were considered. Furthermore, in order to compare to the Nafion, the number of polymers and the computational size was adjusted for containing the same water content.
 Based on the simulation results, the density distribution of water, hydrophilic sulfo groups, and main body of polymer was analyzed. By the analysis of density distribution, the structure of the ionomer at the interface-vacuum, near the Pt surface and inside was studied. In addition, the orientation of the sulfo group of SPP-BP and Nafion near the Pt surface was analyzed. Besides, the water cluster characteristics and water diffusion properties of the ionomers of SPP-BP and Nafion were also investigated.
 Acknowledgements: This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan through the fund for ECCEED'30 Projects (No. JPNP20003). Numerical simulations were performed on the Supercomputer system "AFI-NITY" at the Advanced Fluid Information Research Center, Institute of Fluid Science, Tohoku University.