Three-dimensional nanostructure analysis of non-stained Nafion in fuel-cell electrode by combined ADF-STEM tomography

Microscopy, 2024, 73(4), 318–328 DOI: https://doi.org/10.1093/jmicro/dfae002 Advance Access Publication Date: 13 January 2024 Article Three-dimensional nanostructure analysis of non-stained Nafion in fuel-cell electrode by combined ADF-STEM tomography Takuji Ube * Nano-scale Characterization Center, JFE Techno-Research Corporation, Kawasaki-Ku, Kawasaki City, Kanagawa, Tokyo 2100855, Japan * To whom correspondence should be addressed. E-mail: The polymer electrolyte fuel cell (PEFC) is one of the strongest candidates for a next-generation power source for vehicles which do not emit CO2 gas as exhaust gas. The key factor in PEFCs is the nano-scaled electrochemical reactions that take place on the catalyst material and an ionomer supported by a carbon support. However, because the nano-scaled morphological features of the key materials in the catalyst compound cannot be observed clearly by transmission electron microscopy, improvement of PEFC performance had been approached by an imaginal schematic diagram based on an electrochemical analysis. In this study, we revealed the nano-scaled morphological features of the PEFC electrode in three dimensions and performed a quantitative analysis of the nanostructure by the newly developed ‘Combined ADFSTEM tomography technique’. This method combines information from plural annular darkfield detectors with different electron collection angles and can emphasize the difference of the electron scattering intensity between the ionomer and carbon in the cross-sectional image of the reconstructed three-dimensional (3D) data. Therefore, this segmentation method utilizing image contrast does not require a high electron beam current like that used in energy dispersive X-ray analysis, and thus is suitable for electron beam damage-sensitive materials. By eliminating the process of manually determining the thresholds for obtaining classified component data from grayscale data, the obtained 3D structures have sufficient accuracy to allow quantitative analysis and specify the nano-scaled structural parameters directly related to power generation characteristics. Key words: PEFC, tomography, 3D, STEM, segmentation, ionomer Introduction The phase-out of fossil fuel combustion vehicles has become a global trend in the 2020s, but in order to prevent further global warming, next-generation power sources that do not emit CO2 exhaust gas will be required [1,2]. The polymer electrolyte fuel cell (PEFC) [3] is a promising power source with the potential to become an alternative to the internal combustion engine (ICE), on the same level as battery electric vehicles (BEVs). The key part of electric generation by a fuel-cell system is the electrochemical reactions that take place on the surface of a catalyst metal surrounded by an ionomer which is supported by carbon black. Because the nanostructure of these PEFC components, and especially the three-phase interface of the components, plays the role of transporting gases (hydrogen, air and water vapor), protons and electrons, the nanostructure of these components significantly affects PEFC performance [4,5]. Furthermore, since low cost is also important for practical application, the developers of PEFC power sources strongly require optimization of the use of Pt [6] and reduction of the amount used [7]. The scanning transmission electron microscope (STEM) is one of the most powerful tools in material science for observation with high spatial resolution in various types of analyses using a finely converged electron beam. However, two extremely difficult problems arise in quantitative observation of these PEFC nanostructures by STEM: One is the poor electron resistivity of ionomers [8], and the second is the very small difference in contrast between the ionomer and the carbon support due to their similar densities and atomic numbers. To overcome this problem, previous reports on (S)TEM analysis of ionomers including PEFC catalysts at the single-nanometer level have examined the staining method [8–11], cryogenic technique [12,13] and freehand segmentation [14–16], but neither observation of a unmodified PEFC catalyst nor an elimination of arbitrariness has been established. Here, we propose an analysis technique for a nanoscaled unmodified PEFC catalyst by combined annular darkfield scanning electron microscope (ADF-STEM) tomography. We also developed an arbitrariness-free segmentation recipe for extracting binary images of materials and demonstrated quantitative evaluation using reconstructed 3D data, especially for the fraction of ionomer versus the carbon support. Methods Sample preparation In this study, a commercially available catalyst powder for PEFCs, TEC10E50E (Tanaka Kikinzoku Kogyo K.K., Japan) was used. The Pt nanoparticles were supported on a Received 23 August 2023; Revised 23 November 2023; Editorial Decision 11 December 2023; Accepted 11 January 2024 © The Author(s) 2024. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Microscopy, 2024, Vol. 73, No. 4 319 Table 1. Electron collection angles of STEM detectors (unit: mrad) Detector Inner angle Outer angle HAADF MAADF LAADF BF 37 14 8 200 35 12 6 Electron microscopy data acquisition The electron microcopy experiments were performed using a Talos F200X (Thermo Fisher Scientific Inc., USA) equipped with a quad windowless energy dispersive X-ray (EDX) detector system (Super-X). The acceleration voltage of the incident electron beam was 200 kV. EDX spectral imaging was performed for 20 μs as the dwell time. More than 300 frames of images were acquired to ensure a high signal-to-noise ratio, and also to suppress damage of the ionomer. Three ADF detectors were inserted on the optical axis of the penetrating electron beam, and their scattered electron collection angles were set as shown in Table 1 by controlling the camera length of a STEM instrument parameter. Combined ADF-STEM tomography was performed using Tomography STEM software (Thermo Fisher Scientific Inc., USA). Series tilt images were taken from −78∘ to +80∘ at 1∘ intervals. The STEM images were acquired using the three ADF detectors and one BF detector with 2 048 × 2 048 px resolution and a pixel size of 0.183 nm. The reconstruction area was cropped to 1 024 × 1 024 px from the aligned image stack, and the finally reconstructed field of view was 187 nm cubed. 3D-reconstruction and segmentation The obtained series tilt image stacks were aligned by two steps of the cross-correlation method and bead tracking method in order to reconstruct the most accurate tomogram possible. These image alignment calculations were performed only for the HAADF image stac (...truncated)