(Digital Presentation) A Coarse-Grained Molecular Dynamics Study on the Aggregation and Adsorption Behavior of Ionomer from Solution Onto Pt/C Substrate
Authors: Guo Yuting , Mabuchi Takuya , Li Gaoyang , Tokumasu Takashi 
Published: November 2022

The catalyst layer (CL) was an important component in polymer electrolyte fuel cells (PEFCs). In the CL, Nafion ionomers coated the Pt particles on the carbon surface for the proton transfer assistance, and therefore the structure optimization of ionomer films is very important for the improvement of catalytic utilization and proton transport performance. The microscopic structure of ionomer covered carbon and Pt particles in the solution was related to the final structural characteristics of the ionomer film determining the performance of CL. In order to elucidate the structural evolution and drying process of the catalyst ink, in the present study, we carried out coarse-grained molecular dynamics (CGMD) simulations to investigate the adsorption behavior of ionomers and morphology evolution of Nafion ionomer from the aqueous solutions to the Pt/C substrate surface under various solution conditions including ionomer concentration and I/C ratio, and substrate conditions including the size and distribution of Pt particles, and Pt/C ratio. We found that the ionomers formed a cylindrical aggregate in the solution and are easily attracted by Pt particles due to their hydrophilic ionic shells. Although ionomer aggregates are easily to adsorb onto Pt particles, it tended to unfold and spread onto the carbon substrate finally, which is resulted from the hydrophobic attraction of the exposed ionomer backbone to the carbon surface and the hydrophilic attraction of ionomer sulfonate groups to the aqueous solution. The ionomer adsorption displayed a strong correlation with the ionomer concentration in the dilute solution. However, the I/C ratio had little effect on ionomer adsorption. This adsorption phenomenon of ionomers in the dilution solution was found to fit the Langmuir adsorption model very well but failed when the concentration of ionomer became higher. A wettability switching behavior of ionomers is observed because the morphology of ionomers changed from the solution to the surface. Two types of the adsorption behavior of ionomer aggregates in the aqueous solution were observed. The first adsorption behavior was the direct adsorption of ionomer aggregate to the carbon substrate without any interaction with Pt particles. For the ionomer aggregate, exposed apolar backbones of ionomers that were close to the carbon could help the ionomer aggregate adsorb on the hydrophobic carbon surface. During the adsorption process, cylindrical-like shape ionomer aggregates gradually spread onto the carbon surface. Finally, the ionomers were entirely distributed on the carbon surface without aggregating together. The second adsorption behavior was indirect adsorption of ionomer aggregate, where the ionomer aggregates firstly adsorbed onto the Pt particles from solution, and finally to the carbon substrate. In the solution, the apolar backbones of ionomer aggregates located at the core of the aggregate were difficult to contact with the substance outside such as carbon surface, while the anionic groups located at the aggregate-solvent interface were easy to contact with the substance outside. When such Nafion aggregates were close to the Pt particles, the anionic shells of the cylindrical aggregates were easily to adsorb onto hydrophilic Pt particles. When the Nafion aggregate adsorbed onto Pt particles, Nafion still presents a cylinder-like aggregate state until it started to approach the carbon surface. When the Nafion aggregate begin to move onto the carbon substrate, it gradually unfolded due to the strong hydrophobic interaction between the ionomer backbone and the carbon surface. During this process, Nafion ionomers gradually relieved their internal hydrophobic associations between their backbones because the unfolded ionomers created more surface contacts and better anchored themselves to the surface. Therefore, when the exposed carbon surface was large enough, increasing the number and sizes of Pt particles could increase the surface area of Pt particles, which favors the ionomer adsorption. When the space between Pt particles was too small to create enough carbon surface contacts anchoring the ionomer backbone, the coverage of the ionomer dropped rapidly. The present study provided insight into ionomer adsorption from solutions to Pt/C surface, which could contribute to the catalyst ink design to improve the structure of catalyst layers.
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. We also thank W. Shirakawa for her assistance with the simulations.