Structure Formation and Characterization of PEMFC Catalyst Layers Blended with Multi-Walled Carbon Nanotubes

Mass transport properties of catalyst layers (CLs) for proton exchange membrane fuel cells (PEMFCs) are strongly depends on its structure. Structure control, therefore, is important to achieve higher cell performance. However, the CLs have fabricated by trial and error approach. Understanding mechanism and key parameters of the structure determination is necessary. In this study, we especially focused on pore structure in the CLs.

The pore structure can depend on aggregate structure of carbon black (CB) which is used as a catalyst support. To clarify effects of the material structure on the pore structure of the CLs, different kinds of sub-micron structured carbon, which are carbon black and multi-walled carbon nanotubes (MWCNTs), were blended in catalyst ink. The carbon black is same type as the catalyst support. MWCNTs have completely different structure with the carbon black in sub-micron scale. Therefore, two kinds of MWCNTs were blended with the platinum-supported carbon in the catalyst ink and the effects on the structure and performance were investigated. One of the MWCNTs is 20-50 nm (CNT2050), which is almost the same as the particle diameter of the carbon black (ca. 30 nm), and the other is 40-70 nm in diameter (CNT4070), which is larger than the particle diameter. The CLs were fabricated by doctor blading and a decal transfer method. Both centrifugal dispersion and ultrasonic treatment were performed to prepare homogeneous catalyst ink. Same platinum supported carbon (TEC10E50E, TKK) was used as a catalyst. Non-platinum supported carbon materials were blended in the catalyst ink at 50 wt.% of all carbon materials including platinum-supported carbon. All of fabricated CLs contain the same amount of platinum, 0.08 mg/cm².

Porosities of the fabricated CLs are 59% (CB), 51% (CNT2050) and 42% (CNT4070), respectively. The porosity of the CL blended with CB is almost the same as a previous report[1]. On the other hand, the catalyst layers blended with MWCNTs showed lower porosity. To evaluate micro-scale structure, microscopic observation was performed as shown in Figure 1 and micro-cracks were counted from obtained images. Area ratio of the micro-cracks to all active area is 0.48% (CB), 0.14% (CNT2050) and 0.0037% (CNT4070), respectively. The CLs blended with the MWCNTs showed fewer micro-cracks than the conventional catalyst layer. Figure 2 shows SEM images of the CLs blended with CB and CNT4070. Upper images are in-plane and lower images are through-plane. The cross-sectional structure was obtained by cross-section polishing method which uses broad argon ion beam[2]. Figure 1 (b) is in-plane of the CL blended with CNT4070 and shows large and deep pore which is not shown in the CL blended with CB, Figure 1 (a). On the other hand, such large pores can’t be observed in through-plane direction, Figure 1 (d). It is showed that the CL blended with CNT4070 has heterogeneous pore structure. Porous structure near catalyst can be compact, although the other area contains large pores.

Effects of this characteristic pore structure on the cell performance were evaluated. Polarization curves of the CLs supplying air at different relative humidity (RH) are shown in Figure 3. More drastic voltage drop at high current density is indicated in CNT4070 at high RH condition, although both CLs show almost the same performance at low RH condition. These results suggest that structure of pores which are located near catalyst affect property of flooding.

Acknowledgment

This work was supported by JSPS KAKENHI (25889053). The authors are grateful to Nano-Micro Fabrication Consortium in Kawasaki City for performing the cross-sectional analysis of the CLs.

Reference

[1] T. Suzuki, S. Tsushima and S. Hirai, J. Power Sources, 233, 269 (2013).

[2] T. Suzuki, S. Tsushima and S. Hirai, Int. J. Hydrogen Energy, 36, 12361 (2011).