The catalyst ink was prepared by blending a Pt/C catalyst, ionomer (Nafion®) and solvents (water/1-propranol (NPA) =1:1 in volume ratio). The solid content of the catalyst ink was 10 wt.%, and the weight ratio of ionomer to carbon ratio was 1.0. The catalyst ink was frozen using a high-pressure freezer (EM ICE, Leica Microsystems, Vienna, Austria). The block surface of the frozen catalyst ink for the cryo-SEM observation was then trimmed with a cryo ultramicrotome (EM UC7/FC7T, Leica Microsystems, Vienna, Austria). The CLs were prepared by following procedure. The catalyst ink was coated on Cu substrates (3 mm diameter) placed on a PTFE film using a film applicator. The Cu substrates coated catalyst ink were picked up in 60 and 180 sec after the coating, and soaked into liquid ethane for freezing (CL60 and CL180, respectively). The dried CL was prepared by drying over an hour at room temperature (CLdry). The frozen catalyst ink and catalyst layers on the Cu substrates were observed with a scanning electron microscope (JSM-6701F, JEOL, Tokyo, Japan) incorporated with a liquid nitrogen cooled-stage (EM VCT100, Leica Microsystems, Vienna, Austria).
The cryo-SEM image of the catalyst ink is shown in Fig. 1(a). The brighter particle and darker region correspond to the Pt catalyst nanoparticles and the frozen solvents, respectively. The Pt/C particles formed the several hundred nanometers to micrometer size agglomerates in the catalyst ink. The cryo-SEM images of the surface structure on the CL60 and CL180 are shown in Fig. 1(b) and (c), respectively. The bright particles with several-hundred nanometers correspond to the frost on the sample surface. In CL60, the shape of sample surface of the CL was flat and the agglomerates of Pt/C particles observed in the catalyst ink could not be seen on the surface. In CL180, the several ten micrometers agglomerates of the Pt/C particles could be seen in the CL. In contrast, the solvents and ionomers exhibited the network structure, which connected between the Pt/C agglomerates. In CLdry, the network structure does not exist any longer as shown in Fig. 1(d). In the CL formation process, the size of agglomerated of the Pt/C particles grew to larger agglomerates induced by solvent evaporation. The structure of the CL was changed the liquid–like to the solid via the network structure. The submicron structural evolution in the CL formation process during the solvent evaporation is demonstrated by the cryo-SEM observations. The further nanostructural investigations of the catalyst ink and the CL using cryo-TEM are now in progress.
This presentation is based on results obtained from the PEMFC Research and Development Program for “Highly‐Coupled Analysis of Phenomena in MEA and its Constituents and Evaluation of Cell Performance” commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
1. Mehta, V. and Cooper, J. S. Journal of Power Sources 114, 32–53 (2003).
2. Holdcroft, S. Fuel Cell Catalyst Layers: A Polymer Science Perspective. Chem. Mater. 26, 381–393 (2014).
3. Takeshi Terao, Gen Inoue, Motoaki Kawase, Norio Kubo, Makoto Yamaguchi, Kouji Yokoyama, Tomomi Tokunaga, Kazuhiko Shinohara, Yuka Hara, and Toru Hara. Journal of Power Sources 347, 108–113 (2017).
4. Fan Xu, HangYu Zhang, Jan Ilavsky, Lia Stanciu, Derek Ho, Matthew J Justice, Horia I Petrache, and Jian Xie. Langmuir 26, 19199–19208 (2010).
5. Takahashi, S., Mashio, T., Horibe, N., Akizuki, K. and Ohma, A. ChemElectroChem 2, 1560–1567 (2015).