Coarse-Grained Molecular Dynamics Simulation on the Impact of the Primary Pores on the Distribution and Morphology of Ionomers within Catalyst Layers during the Drying Process
Authors: Wang Sirui , Guo Yuting , Liu Ruoyu , Huang Sheng-Feng , Tokumasu Takashi 
Published: December 2024

The catalyst layer (CL) plays a crucial role in polymer electrolyte fuel cells (PEFCs) as it facilitates proton transport by coating the carbon surface with platinum (Pt) particles with the assistance of Nafion ionomers. Given the existence of complex pore structures of supported carbon and their potential impact on the distribution and morphological evolution of the ionomers in the CL of PEFCs, this area is essential while remaining underexplored. Herein, we employed the coarse-grained molecular dynamics (CGMD) simulation method to investigate the impact of the primary pores on the distribution and morphological evolution of ionomers from the Nafion ionomers solution to the Pt/C substrate surface during the drying process of the catalyst layer at the nanoscale. We aim to provide a comprehensive understanding of the behavior of ionomers in this process.

To simulate the porous structure of the supported carbon, we employed carbon nanotubes (CNT) structure to represent the primary pores in the MD simulation. By varying the parameters, including the Nafion ionomers to carbon (I/C) ratio, the number and diameter of Pt particles, and the diameter of the CNT, the MD simulation was performed under different conditions. The MD analysis results demonstrated that the structure of the CNT had a significant impact on the distribution and morphology changes of Nafion ionomers during the drying process of the catalyst ink. In the early stage, a small fraction of Nafion ionomers were observed to adsorb directly onto the carbon substrate surface without any interaction with the Pt particles. Meanwhile, the majority of the Nafion ionomers were found to form cylindrical aggregates in the solution, which is attributed to the mutual attraction between the ionomers. During the drying process, the cylindrical Nafion ionomer aggregates were initially attracted to the surface of Pt particles on the carbon substrate due to their hydrophilic sulfonate group shells, undergoing a transformative unfolding upon contacting the carbon substrate. This shift to a lamellar structure is propelled by the hydrophobic attraction of the ionomer backbone and the hydrophilic affinity of the sulfonate groups. Moreover, it is noteworthy that Pt particles with a larger diameter enhance the adsorption of ionomer aggregates by offering a larger surface area during the drying process. Consequently, this leads to the formation of larger bare areas on the carbon substrate surface, even when using a solution with the same I/C ratio. As the drying process further progresses, the Nafion ionomers fully unfold, forming a layered structure on the hydrophobic carbon substrate surface, which finally results in a well-structured ionomer distribution. As the drying process nears completion, ionomer aggregates adhere to the surface of the carbon substrate and form ionomer layers. Specifically, as shown in Fig. 1-c, an adequate I/C ratio will result in the ionomers having a uniform and complete laminar distribution on the carbon substrate along with the edge of CNT. Besides, larger Pt particles resulted in a larger bare area of the carbon substrate (Fig. 1-a). Moreover, the thickness of the ionomer above the primary pore is significantly smaller than that on the surface of the carbon substrate (Fig. 1). These results highlight the significance of taking into account the porous structure when investigating ionomer behavior. The existence of CNT impacts the distribution and morphology of Nafion ionomers during the drying process of the catalyst layer.

This study demonstrates the significant influence of the porous structure of primary pores on the distribution and morphological changes of Nafion ionomers in the catalyst layer. These findings offer valuable insights for the development of more effective catalyst inks and optimization of catalyst layer structures. The results also provide a theoretical foundation for enhancing the catalytic efficiency and proton transport performance of PEFCs.

Acknowledgements

The New Energy and Industrial Technology Development Organization (NEDO) of Japan supported this work under Grant number JPNP20003. The simulations were performed on the Supercomputer system 'AFI-NITY' at the Advanced Fluid Information Research Center, Institute of Fluid Science, Tohoku University. We want to express our sincere gratitude to W. Shirakawa for her invaluable assistance.

Figure 1