General Procedure for 3-Dimensional Nanostructure Analysis of PEFC Electrocatalyst Layers

Abstract

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. Quantitative evaluation of the nanostructure of electrocatalyst layers is therefore essential for 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 the standard electrocatalyst (TEC10E50E, Tanaka-Kikinzoku) and alternative electrocatalysts using oxide support under development (Pt/Nb-SnO2/GCB, Pt/Nb-SnO2/CNT, and Pt/Nb-SnO2/VGCF) [2, 3]. Here, the purpose of this study is to establish a 3-dimensional observation and evaluation procedure for electrocatalysts with various nanostructures. 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 applied thresholding methods to various microstructures. 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. 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 process was applied. Adjusting the acceleration voltage and beam current during the FIB processing can suppress damages due to local heating to the electrocatalyst layers. FIB processing and SEM observation were repeatedly made in 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 thresholding method, and solid parts and pores were distinguished and separated. Superimposing these images, 3-dimensional reconstruction was made. A typical 3-dimensional reconstruction image is shown in Fig. 1. After this whole procedure, the porosity and pore size distribution was quantitatively obtained from the 3-dimentional reconstruction image. Figure 2 shows the pore size distribution of the Pt/C, Pt/Sn(Nb)O2/GCB, and Pt/Sn(Nb)O2/CNT electrocatalyst layers. In order to establish more general thresholding method, an original thresholding method is considered based on the intensity change in the SEM image. As shown in Fig. 3, the pixel intensity changes significantly at the edge of the pore, it is suggested that this general thresholding method can be applied to various microstructures. It is considered that this 3-dimentional nanostructure observation and evaluation method may be applicable not only to the electrocatalyst layers but also to other porous structures such as gas diffusion layer (GDL) and microporous layer (MPL).