Reverse Monte Carlo Modeling for Catalyst Nanoparticles in Polymer Electrolyte Fuel Cells By High-Energy X-Ray Diffraction Measurement

Sakurai, Yoshiharu

Polymer electrolyte fuel cells (PEFCs) consist of a gas diffusion layer (GDL) and an electrode on each side, and a polymer electrolyte membrane (MEA) between the electrodes. The main components of the MEA are catalysts, electrolyte membranes and electrodes. In this study, the local atomic structures of catalytic nanoparticles in MEA were investigated by high-energy X-ray diffraction measurement and reverse Monte Carlo (RMC) modeling. The purpose of this study is to show guidelines for improving the performance of the PEFCs that contribute to industry by clarifying the differences in each catalyst structure and linking them with catalyst performance such as catalytic activity and catalyst durability.

In this conference, we report the structural work on the platinum nanoparticles-supported carbon catalyst TEC10V30E (manufactured by TANAKA KIKINZOKU KOGYO K.K.). Although this sample is a commercial product, it is important to investigate its structure because it can be used as a standard sample for evaluating various catalyst samples in the future.

RMC modeling based on the structure factor S(Q) obtained from high-energy X-ray diffraction measurement is suitable to determine the 3-dimensional (3D) atomic structures of nanoparticles. High-energy X-ray diffraction measurements were performed using a two-axis diffractometer at the beamline BL04B2 in SPring-8; the energy was used 61.4 keV corresponding to 0.202 Å in wavelength. RMC simulations were performed to obtain structure information from RMC-generated 3D configuration models using RMC_POT software in the case of non-periodic boundary conditions for experimental structure factor data. Figure 1 shows the experimental structure factor of the Pt nanoparticles in TEC10V30E and the RMC-simulated structure factor. For the Pt nanoparticle studied here, the initial configuration of the RMC simulations is a spherical shape with 3.8 nm in diameter based on the fcc bulk crystal structure. The agreement between the experiment and the RMC-simulated S(Q) is satisfactory. The 3D structure model of the Pt nanoparticle obtained by RMC simulations is shown in Fig. 1. It was found that the deviation from the lattice position was larger on the surface than on the inside in the obtained structure model.

This research approach is expected to accelerate the creation of new catalysts for fuel cells by providing local atomic-scale structural information. It is also important to create the database of atomic arrangement structure data that is the basis of material design and material research.

In the meeting, data recorded from actual catalyst samples will also be presented, including those from the MIRAI fuel cell (manufactured by Toyota Motor Corp.).

This work was performed under the NEDO FC-Platform project.