FIB-SEM 3-Dimensional Nanostructure Observation of PEFC Electrocatalyst Layers
Authors: Manabe Hiromichi , Nakazato Yoshiki , Okumura Makito , Matsuda Junko , Noda Zhiyun , Hayashi Akari , Sasaki Kazunari 
Published: February 2020

Introduction

The electrocatalyst layer of polymer electrolyte fuel cells (PEFCs) has a complicated porous nanostructure. Since the structure allows gas, proton, and electron transport, cell performance depends strongly on their 3-dimensional nanostructure. Quantitatively evaluation of the nanostructure of electrocatalyst layers is therefore essential in improving PEFC electrochemical performance.[1] Observation and quantification procedure using Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) have been applied to electrocatalyst layers using a standard material (electrocatalyst: TEC10E50E, Tanaka-Kikinzoku). Here, the purpose of this study is to establish a 3-dimensional observation and evaluation procedure for electrocatalysts with various structures. Such a methods is applied to alternative electrocatalysts using oxide support under development (Pt/Nb-SnO2/GCB, Pt/Nb-SnO2/CNT, and Pt/Nb-SnO2/VGCF).[2],[3]

Experimental

For each single cell using the standard electrocatalyst, Pt/Nb-SnO2/GCB, Pt/Nb-SnO2/CNT, and Pt/Nb-SnO2/VGCF for the cathode electrocatalyst layers, FIB processing conditions and various image processing procedures were examined. In particular, since it is difficult to distinguish between solid and pore in SEM images due to complicated porous microstructure, we evaluated and optimized 7 different methods for local image thresholding. The porosity of various electrocatalyst layers was examined by FIB-SEM, and was compared with that derived from the thickness of the electrocatalyst layers with known materials compositions to specify the optimal local thresholding method. Furthermore, pore size distribution of the electrocatalyst layers was quantified. Suitable local thresholding method was proposed for electrocatalyst layers with various 3D porous nanostructures.

Result and discussion

For each single cell using the standard Pt/C electrocatalyst, Pt/Nb-SnO2/GCB, Pt/Nb-SnO2/CNT, and Pt/Nb-SnO2/VGCF, FIB-SEM micrographs are shown in Fig.1. Adjusting the acceleration voltage and beam current during the FIB processing can suppress damages due to local heating to the electrocatalyst layer. FIB processing and SEM observation were repeatedly performed every 20 nm of the cross sectioning of the electrocatalyst layers. After that, image processing was carried out for the observed SEM images by the automatic local thresholding method, and solid parts and pores were distinguished and separated. Superimposing these images, 3-dimensional reconstruction was made. Figure 2 shows the SEM images of the standard Pt/C electrocaltalyst layers after image processing by various local thresholding methods[1], indicating that porosity and pore size distribution depend strongly on the thresholding methods. After this whole procedure, the correlation between the pore size distribution and the image threshold method was discussed.

References

[1] M. Okumura et al., J. Electrochem. Soc.,
164(9), F928 (2017).

[2] A. Masao et al., Electrochem. Sorid-state Lett.,
12(9), B119 (2009)

[3] F. Takasaki et al., J. Electrochem. Soc.,
158(10), B1270 (2017)

Figure 1