燃料電池の膨張触媒層の3次元構造のクライオSTEMトモグラフィー
Authors: Shimanuki Junichi , Arao Masazumi , Katayama Shota , Imai Hideto , Ito Yoshiko , Nishino Yuri , Miyazawa Atsuo 
Published: December 2024

In order to improve the power generation properties of fuel cells, it is important to understand and control the fine pore structure of the fuel cell catalyst layer, which affects the flow of gas and water. It is inferred that the ionomer having a hydrophilic group is in a swollen state when a fuel cell is operating because of the high temperature and high humidity state of the catalyst layer.

Therefore, in order to understand the flow of gas and water, information is required not only on the pore structure due to Pt/C aggregates but also on the distribution state of the swollen ionomer. A three-dimensional observation method is needed to obtain the three-dimensional structural information necessary for detailed analysis of the correlation between performance evaluation results, such as electrical characteristics, and electrode structures.

Detailed investigation of the fine pore structure of a fuel cell catalyst layer requires electron microscopy techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) due to the scale involved. However, since a typical electron microscope is a vacuum device, materials containing moisture are observed in a dry state. Furthermore, data are obtained as images, which are basically two-dimensional.

Cryo-electron microscopy is a method that allows samples containing water and particles in liquid to be observed while being kept in a wet state without drying them, as the sample is quickly frozen and observed in that condition with an electron microscope. Cryo-electron microscopy has been established mainly in biological fields, and there are many issues to be solved in adapting it to investigations of materials. For example, in biological fields, objects to be observed are basically composed of light elements, while heavy elements are included in the materials field. In addition, the solvents are not only water but also mixed ones of organic solvents, and the freezing conditions and tendency of electron beam irradiation damage during observation are also different, requiring a different approach from that used in biological fields.

Electron tomography is a three-dimensional observation method using TEM, in which information on the three-dimensional structure of a sample can be obtained by continuously tilting it and performing Radon-inverse Radon conversion from the obtained transmission image. Since many images are acquired during continuous tilt imaging, it is important to control the electron beam dose to the sample for samples that are susceptible to electron beam irradiation damage.

To address these issues, a low-dose scanning transmission electron microscopy (STEM) mode creates a parallel electron beam without converging the beam, which suppresses electron beam irradiation damage and emphasizes scattering contrast, though it has the disadvantage of reducing resolution. It is thus expected that this method will also be effective in distinguishing between ionomers and carbon. Since the catalyst layer contains heavy elements such as precious metals, STEM observation is considered to be even more optimal as it has higher transparency than TEM.

In this research, we used cryo-STEM tomography to analyse differences in the pore structure in the swollen ionomer of catalyst layers of samples with different electrical properties, and investigated the correlation between the pore structure and those properties.

Three types of catalyst layers samples were fabricated using commercial catalyst powder (TEC10V30E, TKK) and Nafion as the ionomer with different ionomer-to-carbon ratios (I/C:0.1, 0.5, 1.0) for the analyses. Cell performance was investigated on MEAs with the catalyst layers for the cathodes, respectively. The performance at 80°C and 80% was lower for the samples with lower I/C. The performance of the I/C=0.1 sample was especially low, which was primarily attributed to the low catalyst utilization and high proton resistance. In order to understand these phenomena, we investigated in detail the three-dimensional distribution of the ionomer in the porous structure composed of Pt/C aggregates and compared the results found for the samples.

This work is partly supported by NEDO FC-Platform project.