A New Pore Filling Technology Used for the FIB/SEM Analysis of the Pore Morphology of PEFC Catalyst Layer Cross-Section
Authors: Ghosh Sourov , Ohashi Hidenori , Oshiba Yuhei , Tabata Hiroshi , Yamaguchi Takeo 
Published: February 2020

INTRODUCTION

Polymer electrolyte fuel cells (PEFCs) have gained immense attraction as green and sustainable power sources for both stationary and automobile application. Unfortunately, the degradation of platinum catalyst and carbon support due to the electrochemical oxidation is one of the most serious problems hindering widespread application of PEFC in automobiles [1]. It is very important to understand precisely the structure and property of the catalyst layer, a porous electrode containing carbon-supported platinum mixed with polymer electrolyte (ionomer). In fuel cell cathode, water is generated by oxygen reduction reaction and electro-osmotic drag of proton [2]. Porosity is perhaps the most important factor of MEA microstructure that can effectively control the balance between gas permeability and water removal during long-term operation of fuel cells.

In our present effort we have introduced a new analytical technique to determine (a) overall porosity and pore morphology (b) the state of ionomer in carbon particles. Usually techniques such as mercury intrusion porosimetry (MIP) or BET surface area are used to characterize pore structure and total porosity [3]. Major drawback of such techniques is that analysis of results involves random assumptions about pore shape that are not appropriate for the complex, twisted network of capillary porosity present in MEA. The introduction of new analytical methods for the determination of pore morphology is very important in this context. For the purpose of image analysis a 2D SEM image of catalyst layer cross-section was obtained by FIB fabrication. As the catalyst layer pores and carbon are located in a 3D-space and in SEM image one can often locate a bright spot (for carbon) within a pore, buried inside. It becomes difficult to adjust the threshold to make all the pores with same grey level at the same time. To overcome this problem we proposed to fill the pores with resin to impose two distinct grey levels for pores and carbon, for an improved image analysis.

EXPERIMENTAL

We have adopted advanced digital image analysis technique for the determination of pore shape, structure and porosity. We have designed and assembled a new pressure reactor to fill the pores of the catalyst layer. Epoxy resin and a hardener solution were used to fill the pores. Epoxy resin and the hardener were mixed properly in appropriate ratio with metal (Zr, Co, Ni) acetylacetonate at 50º C for 5 minutes. After that the MEA was immersed into the resin mixture in a glass petri-dish. The petri-dish was then placed in an air tight pressure reactor and degassed for 10 min at 0.05 MPa. Then the vacuum was released and the pressure was increased to 3MPa by an AIR cylinder. The condition was maintained for 15 minute at 50º C temperature. After 15 min heating was stopped and the glass petri-dish was taken out after another 15-20 min and kept overnight for hardening.

RESULT AND DISCUSSION

Figure 1 represents an example of thecross-sectional SEM image analysis sequences for resin filled catalyst layer. A thresholding operation was performed to obtain a binary 2D image (fig. 1b) with black regions precisely represents the pores. This binary image was analyzed digitally considering the pore area equivalent to an ellipse (fig. 1c) and different parameters of the pores were harvested. Porosity of multiple SEM frames has been calculated and it was found that the overall porosity of the cathode catalyst layer decreased by 35% after start-stop durability test. It was also found that the abundance of small pores (between 10-50 nm) increased by ~18% and that of large pores (100-150 nm) decreased by ~70% after start-stop durability test. Pores having the diameter of 50-100 nm were also found to be decreased by ~29% after the durability test. It was found that the thickness of the catalyst layer decreased considerably after start-stop durability test, which depict the extensive carbon corrosion in the MEA. Furthermore, image analysis also revealed the existence of ionomer agglomeration in catalyst layer after start-stop durability test. The complete image analysis will be presented in detail.

ACKNOWLEDGEMENT

This work is financially supported by NEDO, Japan.

REFERENCES

[1] J. P. Meyers, R. M. Darling, J. Electrochem. Soc. 153 (2006) A1432.

[2] H. Zhang, H. Ohashi, T. Tamaki, T. Yamaguchi, J Phys. Chem. C 116 (2012) 7650.

[3] Y. Lee, T. K. Kim, Y. S. Choi, Fuel Cells 13 (2013) 173; S. Park, J-. W. Lee, B. N. Popov, J. Power Sources 163 (2006) 357.