Combining Microscopy and Synchrotron X-Ray Diagnostics to Study Catalyst Degradation in PEFCs

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Polymer electrolyte fuel cells (PEFCs) suffer from performance degradation during operation due to transformations in the catalyst (Pt or Pt/Co particles) and their carbon support. However, nano-structural diagnostic methods are not readily applicable in the humid, high-temperature, and gas environments of PEFC operation. This hinders elucidation of the mechanisms behind catalyst property evolution (size, shape, lattice parameters), which directly affects the electrocatalytic surface area (ECSA), a critical factor in catalyst performance.1 Here, we employ a combined methodology to study the changes in catalyst particle properties using identical location scanning transmission electron microscopy (IL-STEM)2 and Operando synchrotron X-ray measurements, particularly small angle X-ray scattering technique (SAXS)3. While IL-STEM offers high-resolution imaging for estimating local particle size, shape, and lattice parameters, SAXS provides statistically reliable data on the overall particle size distribution.4 These complementary techniques, with their inherent strengths, allow for a comprehensive understanding of the mechanisms involved during particle transformation.Electrochemical cycling of the catalysts was performed using a square wave potential pattern cycling between 0.6 and 1.0 VRHE at 6 seconds/cycle. A gold STEM grid was mounted on a specially designed working electrode for electrochemical cycling, which is performed ex-situ. STEM images, of the same areas, before and after electrochemical cycling were recorded using JEOL NEOARM operated at an accelerating voltage of 200 kV. SAXS experiments were conducted at BL40B2 beamline of SPring-8. An electrochemical cell (PECC-2, Zahner, Germany) was modified to increase X-ray transmittance. The thickness of its electrolyte layer was minimized to 1.5 mm, the upstream window was a 7.5 µm thick Kapton film. Its downstream window, a 50 µm thick graphite window onto which a catalyst film was deposited, acted as the working electrode. A PILATUS 2M detector was used to record scattering data with exposure time of 100 seconds.Scanning transmission electron microscopy revealed a comparable mean particle radius before electrochemical cycling (1.60 ± 0.44 nm) to that obtained by small-angle X-ray scattering (1.62 ± 0.32 nm), Table 1. Furthermore, STEM analysis of the same locations before (607 particles) and after (494 particles) cycling showed a similar trend in particle size growth as observed in operando SAXS measurements. However, the estimated sizes after cycling differed slightly, with SAXS data indicating a larger mean radius (2.08 ± 0.53 nm) compared to STEM (1.88 ± 0.49 nm). This discrepancy is attributed to the smaller probed volume by STEM, which was also limited to the same locations, potentially excluding other growing particles from analysis compared to the bulk characterization offered by SAXS.These results highlight the potential of combining microscopy with synchrotron X-ray SAXS for studying catalyst growth mechanisms with greater accuracy. In addition, by incorporating complementary synchrotron X-ray techniques like wide-angle X-ray scattering (WAXS) and X-ray fluorescence (XRF) during analysis, a more comprehensive understanding of catalyst degradation can be achieved. This combined approach can pave the way for the development of more informed catalyst engineering strategies.Table 1 summarising the mean particle sizes obtained from both methods. BEFORE (nm)AFTER (nm)Peak radiusMean radiusFWHMPeak radiusMean radiusFWHMSTEM1.38 ± 0.011.60 ± 0.440.86 ± 0.031.66± 0.021.88 ± 0.490.95 ± 0.04SAXS1.47 ± 0.011.62 ± 0.320.60 ± 0.021.84 ± 0.022.08 ± 0.531.04 ± 0.04 References Lyth, S. M. & Mufundirwa, A. Electrocatalysts in polymer electrolyte membrane fuel cells. Heterog. Catal. Adv. Des. Charact. Appl. 2, 571–592 (2021).Yu, H. et al. Tracking Degradation in Individual Catalyst Nanoparticles Under Fuel Cell-Relevant Cycling Conditions by Identical-Location STEM. Microsc. Microanal. 28, 2614–2617 (2022).C. Smith, M., A. Gilbert, J., R. Mawdsley, J., Seifert, S. & J. Myers, D. In Situ Small-Angle X-ray Scattering Observation of Pt Catalyst Particle Growth During Potential Cycling. J. Am. Chem. Soc. 130, 8112–8113 (2008).Mufundirwa, A. et al. Contrast variation method applied to structural evaluation of catalysts by X - ray small - angle scattering. Sci. Rep. 1–9 (2024) doi:10.1038/s41598-024-52671-7.