The cathode catalyst layer (CL) is one of the principal components for proton exchange membrane fuel cells. CL is typically prepared by drying a catalyst ink consisting of Pt-C (platinum on a carbon power support), ionomer, and solvents (water/alcohol mixtures). Under good solvent conditions for the polymer backbone, where solvents dissolve the backbone chains, ionomers exhibit polyelectrolyte-type behavior, forming loose ionic aggregates. The interaction between ionomer and solvents controls the ionomer conformations in the solutions, which determines the final microstructure of ionomer in CLs. The characterization of ionomer structures has been extensively studied using experimental measurements such as small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS).^1-2 They observed that hydrophobic backbones of Nafion form the cylindrical-like core of self-assembled ionomer structures while ionic groups are located at the periphery of this aggregates. Theoretically, while most of studies have primarily focused on the solid membrane state with water volume fraction significantly below 50%, ionomer and carbon aggregations in solutions are also an important phenomenon to understand the self-assembly behaviors of ionomers as key structure-forming elements in CL. In this study, ionomer and carbon aggregations in a mixture of 1-propanol (NPA) and water have been investigated using coarse-grained molecular dynamics simulations. To perform simulations of larger systems and longer time spans compared with atomistic simulations, the reduced spatial resolution of the bead representation based on the relative free energy difference was used as a coarse-grained model. The dependence of NPA content on the ionomer structures was studied by systematically changing the NPA content in the system. The self-assembly behavior of ionomers into cylindrical bundle-like aggregates was observed for all NPA content solutions (Figure 1). Ionomer aggregates showed different behavior of formation at different NPA contents. The radius of an ionomer bundle was found to become smaller as NPA content increases, which is in good agreement with the trend seen in experimental measurements.³ The effects of carbon particles on the ionomer aggregation for various ionomer concentrations will also be discussed in the presentation.

References

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2. Jiang, S.; Xia, K.-Q.; Xu, G., Effect of Additives on Self-Assembling Behavior of Nafion in Aqueous Media. Macromolecules 2001, 34, 7783-7788.
3. Yamaguchi, M.; Matsunaga, T.; Amemiya, K.; Ohira, A.; Hasegawa, N.; Shinohara, K.; Ando, M.; Yoshida, T., Dispersion of Rod-Like Particles of Nafion in Salt-Free Water/1-Propanol and Water/Ethanol Solutions. J. Phys. Chem. B 2014, 118, 14922-14928.