Nanostructural Evolution during Catalyst Layer Formation Studied via Cryo-Electron Microscopy

Abstract

Polymer electrolyte fuel cells (PEFCs) have recently received considerable attention as a next-generation power source, since from the practical viewpoints of energy efficiency and environmental friendliness, they compare favorably with present energy sources1. The catalyst layer (CL) of membrane electrode assembly (MEA) in the PEFC consists from the carbon supported platinum (Pt) nanoparticle and ionomer. The CL is fabricated by coating of the catalyst ink containing the platinum-loaded carbon (Pt/C) catalyst, ionomer, and solvents. The electrochemical activity and transport phenomena in a MEA are affected by the nanostructures in the CL2,3. Though many research activities have been reported on a structural formation in the CL4,5, the dynamics of the CL formation process during the solvent evaporation is still unclear. In this study, we have demonstrated that the structural evolution in the catalyst inks and CLs in drying process was elucidated by cryogenic scanning electron microscopy (cryo-SEM) and cryogenic transmission electron microscopy (cryo-TEM). 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. Acknowledgment 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).