Relationship between Surface States of Carbon Support and Electrode Reaction Activity in Cathode Catalyst Layer Toward High Power Output of Polymer Electrolyte Fuel Cells by Large-Scale Reactive Molecular Dynamics Simulations
Authors: Nakamura Tetsuya , Asano Yuta , Ootani Yusuke , Ozawa Nobuki , Kubo Momoji 
Published: February 2024

Higher output of polymer electrolyte fuel cells (PEFC) is required for use in many industrial fields such as the transport industry. PEFCs consist of a gas diffusion layer, a catalyst layer (CL), and a polymer electrolyte membrane. The output of PEFCs depends on the electrode reaction activity of the CL consisting of the carbon support, ionomer, water, and Pt nanoparticles (NPs). For high electrode reaction activity, the optimization of the CL structures such as Pt alloy, pore structures of the carbon support, and I/C (Ionomer carbon ratio), are key factors. To optimize the CL structures, computational simulation, which can propose theoretical design principles, is a promising method. Thus, many researchers have investigated the CL structures with high electrode reaction activity by computational simulation to propose design principles. For example, first-principles calculation can optimize highly active Pt catalysts by considering activation barriers, molecular adsorption structures and surface electronic states, which involves the behavior of atoms and molecules. However, this method cannot consider the effects of carbon support, ionomer, and water because of the limitation in computational costs. On the other hand, macro-scale approach using differential equations can discuss the highly active CL structures since this method can consider multiple structures such as carbon support, ionomer distribution, water content, and Pt loading. However, this approach cannot consider the behavior of atoms and molecules. Therefore, large-scale molecular simulations which, can simultaneously consider the behavior of atoms and molecules, and the CL structures are required. Here, reactive molecular dynamics (RMD) simulation is a practical approach to deal with the behavior of atoms and the chemical reactions in the CL. By using RMD method, we successfully constructed large-scale CL structures and established the CL design methodology both including atomic and molecular effects and clarifying the effect of the pore size of carbon support on the electrode reaction activity [1]. In order to design the higher-performance CL structures, we focused on the distribution of water which mediates proton transfer process. Improving the wettability of carbon support by termination of hydroxyl groups to the carbon support surface can alter the water distribution. Therefore, by large-scale RMD simulations, we investigated the effect of the ratio of the hydroxyl groups on the carbon support surface on water distribution to propose design principles for higher-performance cathode CL structures.
A CL structures model consisting of 1 million atoms was constructed by modeling carbon support, Pt NPs, ionomer, and water. We selected the carbon support, where there were six meso pores on an amorphous carbon sphere. The ratios of hydroxyl groups to the carbon surface atoms were set to 10 and 32% to control the water distribution on the carbon surface. In addition, Pt NPs were put on the carbon support outer surface and interior of the pores. The water film whose thickness is 0.7 nm and ionomer are coated on Pt-supported carbon. Hereafter, we call this structure catalyst particle (CP) model (Fig. 1).
To clarify the effect of the ratio of hydroxyl groups on the carbons support surface, the morphology of the water film was compared in the CP models with different hydroxyl ratios. At the exterior of the pores, the water film was peeled off from the carbon support surface regardless of the ratio of hydroxyl groups. At the interior of the pores, when the ratio of hydroxyl groups was 10%, the water film was peeling from the carbon support surface（Fig. 2(a), (b)). On the other hand, when the ratio of hydroxyl groups was 32%, the water film did not peel off from the carbon support surface, and the structure of the water film was maintained (Fig. 2(c), (d)). Next, to clarify the effect of the water film morphology on the proton conduction to the Pt NPs, the water clusters as proton conduction paths were analyzed. Water clusters were defined as a group of water molecules whose inter-molecular distance is less than 3.5Å. The amount of proton conduction paths to the Pt NPs at the exterior of the pores did not change regardless of the ratio of hydroxyl groups. On the other hand, the amount of proton conduction path to the Pt NPs at the interior of the pores was increased by increasing the ratio of hydroxyl groups (Fig. 3). These results indicate that the increasing the ratio of hydroxyl groups on the carbon support surface to improve the proton conduction path is an effective approach for the design of the higher-performance cathode CL.
[1] T. NAKAMURA, M. KUBO et al. J. Comput. Chem. Jpn.,
20,150-154, (2021).

Figure 1