Characterization of Agglomerated Particles in a PEFC Electrode Slurry by Soft X-Ray Radiography

Abstract

Polymer electrolyte fuel cells (PEFCs) have been commercialized for various applications, such as automobiles and household power sources, because of their low operating temperature and high power density. A PEFC comprises a polymer electrolyte membrane as the electrolyte and electrodes consisting of a catalyst layer and a gas diffusion layer. The catalyst layer plays a role as an electrochemical reaction field with a porous structure and is generally prepared by mixing platinum-supported carbon and ionomer in a solvent to make a suspension (electrode slurry), which is then coated on a substrate and dried. Mixed particles aggregate in the electrode slurry, which affects the porous structure of the catalyst layer. In order to improve the cell performance of PEFCs by controlling the structure of catalyst layers, it is necessary to understand and characterize the electrode slurry [1-2].In this study, we carried out a temporal visualization of the electrode slurry by a soft X-ray radiography system (Mars-Tohken X-ray Inspection, TUX-9000D) to clarify the agglomeration/dispersion state and migration phenomena of particles in the electrode slurry. PEFC electrode slurry is a black opaque suspension and its structure is hardly probed by visible light. In order to observe the electrode slurry by soft X-ray radiography, we used a house-made PTFE container. The container was placed on the sample stage of the soft X-ray imaging system for observation. In order to investigate the effect of interaction between particles, four types of electrode slurries with different non-volatile ratios (7.0, 0.70, 0.40, and 0.11%) were prepared for observation. The ionomer-to-carbon ratio of all electrode slurries was kept at 1.0. The dispersions were prepared by a planetary centrifugal mixer for 30 minutes (2000 rpm for the revolution and 800 rpm for the rotation) just before soft X-ray visualization.Figure 1 shows a soft X-ray radiograph of electrode slurry (non-volatile ratio: 7.0%). The image captured a spatial variation of X-ray absorption in the electrode slurry. High X-ray absorption regions correspond to high density in the slurry, attributed to agglomerates of platinum-supported carbon particles and ionomers. Their diameters were about 10 – 40 μm, and less variation was found in the non-volatile ratio of the electrode slurry from 0.11 to 7.0%. However, the number of agglomerated particles varied depending on the non-volatile ratio of the electrode slurry. The less the non-volatile ratio, the less the number of agglomerated particles observed, suggesting that migrations in the electrode slurry were much affected by the non-volatile component in the slurry. We performed a time-series visualization of the electrode slurry placed in the X-ray radiography system for several hours. Sedimentation behaviors of the agglomerated particles were observed in all electrode slurries. The captured images were processed to quantitatively characterize sedimentation rates in the slurry. It was found that the less the non-volatile ratio, the more the sedimentation rate of the agglomerated particles.AcknowledgmentsThis work was supported by JSPS KAKENHI Grant Number 18K13702 and 21H04540.[1] T. Suzuki et al., ECS Trans., 86-13 (2018), 193.[2] S. Khandavalli et al., ACS Appl. Mater. Interfaces, 10 (2018), 43610. Figure 1