Search NASA⌕ Search

SEARCH · Search NASA

Results for “dark-field X-ray microscopy”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Dynamic Tracking of NiFe Smart Catalysts using In Situ X-Ray Absorption Spectroscopy for the Dry Methane Reforming Reaction

The exsolution of nanoparticles from perovskite precursors has been explored as a route to synthesize catalysts with sinter or coke resistance. The characteristics of these exsolved nanoparticles are highly dynamic depending on the redox nature of the environment to which they are subjected. To develop their properties for thermo- and electrocatalytic applications, it is necessary to track the states and behavior of exsolved catalysts with in situ and ex situ characterization. In this study, we conduct in situ X-ray absorption spectroscopy (XAS) along with ex situ scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) and energy-dispersive X-ray spectroscopy analysis of the parent perovskite oxide precursor, LaFe 0.8 Ni 0.2 O 3 , as its structure forms bimetallic NiFe nanoparticles and evolves in oxidative, reductive, and dry methane reforming environments. We develop a theory that NiFe exsolution is a function of the reduction potential where LaFe 0.8 Ni 0.2 O 3 transforms to NiFe alloy supported on LaO x -LaFeO x . The Ni starts to exsolve at 268 °C, while most Fe exsolves at 700 °C. During dry methane reforming conditions, most of the Fe is oxidized by CO 2 during the reaction and re-enters the perovskite as LaFeO 3 , while Ni remains on the surface as nanoparticles in the metallic state. During the oxidative regeneration phase, most of the Fe re-enters the bulk perovskite phase, while Ni is partially regenerated with a small percentage oxidized to large NiO nanoparticles. Finally, this study sheds light on the exsolution and regeneration of bimetallic alloy nanoparticles and the influence of the reaction conditions on their catalyst performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Iridium Incorporation into MnO 2 for an Enhanced Electrocatalytic Oxygen Evolution Reaction

Here, we have investigated the structure and activity of electrocatalysts for the oxygen evolution reaction (OER) that had low loadings of Ir incorporated into the 2D layered MnO 2 , (birnessite, nominally δ-MnO 2 ) and the 3D MnO 2 (pyrolusite, β-MnO 2 ). The Ir-incorporated β-MnO 2 (Ir/β-MnO 2 ) electrocatalysts were prepared for the first time via a thermally induced phase transition of δ-MnO 2 containing 16-22 wt% Ir. This phase transition of δ-MnO 2 to β-MnO 2 was facilitated by the presence of Ir in the structure, as both Ir in IrO 2 and Mn in β-MnO 2 could adopt a thermodynamically favored rutile structure. Extended X-ray absorption fine structure (EXAFS) of Ir/β-MnO 2 showed that the catalyst consisted of Ir substituted into the crystalline β-MnO 2 lattice. 22 wt% Ir/β-MnO 2 (60 μg Ir cm $\ ^{-2}_{geo}$) exhibited an OER overpotential (η) of 337 mV, lower than the η for commercial IrO 2 . This η was constant for 6 h, at 10 mA cm $\ ^{-2}_{geo}$ in 0.5 M H 2 SO 4 . EXAFS, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption near edge structure (XANES) showed that 22 wt% Ir/β-MnO 2 had a strained structure containing ~41% Mn 3+ , an OER active species, along with a modified Ir bond covalency consisting of both Ir-O-Ir and Ir-O-Mn.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Highly Selective Oxygen Reduction to Hydrogen Peroxide on a Carbon-Supported Single-Atom Pd Electrocatalyst

Selective electrochemical two-electron oxygen reduction is a promising route for renewable and on-site H 2 O 2 generation as an alternative to the anthraquinone process. In this study, we report a high-performance nitrogen-coordinated single-atom Pd electrocatalyst, which is derived from Pd-doped zeolitic imidazolate frameworks (ZIFs) through one-step thermolysis. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with X-ray absorption spectroscopy verifies atomically dispersed Pd atoms on nitrogen-doped carbon (Pd-NC). The single-atom Pd-NC catalyst exhibits excellent electrocatalytic performance for two-electron oxygen reduction to H 2 O 2 , which shows ~95% selectivity toward H 2 O 2 and an unprecedented onset potential of ~0.8 V versus revisable hydrogen electrode (RHE) in 0.1 M KOH. Density functional theory (DFT) calculations demonstrate that the Pd-N 4 catalytic sites thermodynamically prefer *–O bond breaking to O–O bond breaking, corresponding to a high selectivity for H 2 O 2 production. This work provides a deep insight into the understanding of the catalytic process and design of high-performance 2e– ORR catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Plasmon assisted synthesis of TiN-supported single-atom nickel catalysts

We report the deposition of single atom nickel catalyst on refractory plasmonic titanium nitride (TiN) nanomaterials supports using the wet synthesis method under visible light irradiation. TiN nanoparticles efficiently absorb visible light to generate photoexcited electrons and holes. Photoexcited electrons reduce nickel precursor to deposit Ni atoms on TiN nanoparticles’ surface. The generated hot holes are scavenged by the methanol. We studied the Ni deposition on TiN nanoparticles by varying light intensity, light exposure time, and metal precursor concentration. These studies confirmed the photodeposition method is driven by hot electrons and helped us to find optimum synthesis conditions for single atoms deposition. We characterized the nanocatalysts using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). We used density functional theory (DFT) calculations to predict favorable deposition sites and aggregation energy of Ni atoms on TiN. Surface defect sites of TiN are most favorable for single nickel atoms depositions. Interestingly, the oxygen sites on native surface oxide layer of TiN also exhibit strong binding with the single Ni atoms. Plasmon enhanced synthesis method can facilitate photodeposition of single atom catalysts on a wide class of metallic supports with plasmonic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Tailoring P2/P3‐Intergrowth in Manganese‐Based Layered Transition Metal Oxide Positive Electrodes via Sodium Content for Na‐Ion Batteries

High-manganese content sodium-ion positive electrodes have received heightened interest as an alternative to contemporary Li-ion chemistries due to their high abundance, low toxicity, and even geographical distribution. However, these materials typically suffer from poor capacity, unstable cycling performance, and sluggish Na + kinetics. Herein, we explore a manganese-based layered transition metal oxide (Na x N 0.25 Mn 0.75 O 2 ) and show by X-ray diffraction (XRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) that careful variation of the sodium content can instigate the formation of a biphasic intergrowth. This intergrown P2/P3 material offered a higher capacity than its monophasic P2 counterpart due to the P3 structure having greater low-voltage Mn 3+/4+ redox. Further, the intergrowth material offers greatly enhanced kinetics and cycling stability when compared to single-phase P3 material, due to the stabilizing nature of the P2 structure, elucidated by galvanostatic intermittent titration technique (GITT) and operando synchrotron X-ray diffraction. These results highlight the beneficial effect that the intergrowth structure has on the electrochemical performance of high-manganese content positive electrode for future sodium-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2025

Unsupported and carbon-supported silver catalysts for oxygen reduction reaction in alkaline media

Quick and easy Ag catalysts preparation via wet chemical synthesis method using only reducing agent (pure-Ag); reducing agent and citric acid as the capping agent (Ag-CA); and carbon support (KetjenBlack 600J), capping agent, and the reducing agent (Ag/C) is demonstrated. The Ag-based electrocatalysts are characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activity of Ag catalysts for O 2 reduction reaction (ORR) in 1 M KOH is evaluated using the rotating (ring)-disc electrode method. SEM and HAADF-STEM results show that the unsupported pure-Ag and Ag-CA catalysts consist mainly of big agglomerates, and Ag/C has the smallest agglomerates and some sub-3 nm Ag nanoparticles. The XPS results reveal that Ag in all the catalysts is in the metallic form (Ag 0 ). Despite consisting of big agglomerates, the Ag-CA catalyst exhibits similar ORR electrocatalytic activity to that of Ag/C. Ag-CA (unsupported) shows the lowest hydrogen peroxide yield. These results are of great importance for the development of Ag-based catalysts that can be prepared in a fast, simple and easily up scalable fashion, for anion exchange membrane fuel cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Pd-Ru pair on Pt surface for promoting hydrogen oxidation and evolution in alkaline media

Hydrogen oxidation reaction in alkaline media is critical for alkaline fuel cells and electrochemical ammonia compressors. The slow hydrogen oxidation reaction in alkaline electrolytes requires large amounts of scarce and expensive platinum catalysts. While transition metal decoration can enhance Pt catalysts’ activity, it often reduces the electrochemical active surface area, limiting the improvement in Pt mass activity. Here, we enhance Pt catalysts’ activity without losing surface-active sites by using a Pd-Ru pair. Utilizing a mildly catalytic thermal pyrolysis approach, Pd-Ru pairs are decorated on Pt, confirmed by extended X-ray absorption fine structure and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory and ab-initio molecular dynamics simulations indicate preferred Pd and Ru dopant adsorption. The Pd-Ru decorated Pt catalyst exhibits a mass-based exchange current density of 1557 ± 85 A g –1 metal for hydrogen oxidation reaction, demonstrating superior performance in an ammonia compressor.

25 ENERGY STORAGE↗
DOE OSTI2025

Oxygen Reduction on Platinum-Nickel and Platinum-Cobalt Alloy Based Catalysts for High Temperature Proton Exchange Membrane Fuel Cells

Catalyst layers, with commercial PtNi/TKK (TECNiE52; platinum 46.5 wt%), PtCo/TKK (TEC36E52; platinum 45.8 wt%) catalysts, and an in-house catalyst PtNi-NC (platinum 38.4 wt%), synthesised using acoustic mixer and a tube furnace, are prepared using three different ionomers: 20 wt% Nafion®, 60 wt% polytetrafluoroethylene (PTFE), and an in-house 5 wt% ionomer (‘Ionomer X’). These inks are bar-coated onto carbon paper gas diffusion layers (GDLs), which are cut into 3 cm × 5 cm pieces. The coated layers are tested for oxygen reduction in a commercial gas diffusion electrode (GDE) test cell, FlexCell® (Gaskatel GmbH, Germany), using 85 wt% phosphoric acid at both room temperature and at 155°C. This study evaluates the polynorbornene (PNB)-based in-house ionomer performance as binder and in-house catalyst oxygen reduction reaction (ORR) activity in comparison to commercial products. The catalyst layers are characterised using X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Among the catalyst layers prepared with PTFE ionomer and tested at 155°C, the in-house catalyst demonstrates the highest ORR activity. Ionomer X proves to be a good candidate to be used as a per- and polyfluoroalkyl-free binder for high-temperature (HT) proton exchange membrane fuel cell (PEMFC) applications.

85 wt% phosphoric acid↗
DOE OSTI2023

Selective oxidation and nickel enrichment hinders the repassivation kinetics of multi-principal element alloy surfaces

Robust and sustained corrosion resistance in multi-principal element alloys (MPEA) requires rapid repassivation, i.e. regrowth of the passive layer once it is damaged or destroyed at the surface. In this study, we show that the repassivation of Al 0.1 CrCoFeNi MPEA in 0.6 M NaCl solution is hindered at pH of ~ 2.4 - 6.8 due to the formation of a Ni-enriched subsurface layer as a result of selective oxidation and dissolution of several principal elements, which can be fully restored at pH of ~ 14 from the oxidation of all principal elements. Specifically, surface characterization via X-ray photoelectron spectroscopy (XPS), high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM), and atom probe tomography (APT), are coupled with density functional theory (DFT) calculations to determine surface composition, oxidation state, and electron work function to uncover the structural origin of the pH-dependent repassivation mechanisms. It was found that selective oxidation of Cr, Co, and Fe in the acidic to neutral solutions altered the surface composition to be significantly enriched in Ni as compared to the bulk. Once the original passive film is destroyed locally by either pitting or tribocorrosion, this altered surface composition exhibited a much poorer repassivation capability due to the increased electron work function and reduced surface reactivity at higher Ni concentration. Finally, these understandings could shed light on the future compositional design of non-equiatomic MPEAs towards sustained repassivation and corrosion resistance over a wide pH range.

36 MATERIALS SCIENCE↗
DOE OSTI2022

PbI 2 Nanocrystal Growth by Atomic Layer Deposition from Pb(tmhd) 2 and HI

Atomic layer deposition (ALD) allows for fine control over the thickness, stoichiometry, and structural defects of materials. ALD provides a suitable route to deposit lead halides, which can further be converted to perovskites for photovoltaics, photoemission, and photodetection, among other applications. Deposition of lead halides by ALD has already begun to be explored; however, the precursors used in published processes are highly hazardous, require expensive fabrication processes, or contain impurities that can jeopardize the optoelectronic properties of metal halide perovskites after conversion. In this work, we deposited lead iodide (PbI 2 ) by a facile ALD process involving only two readily accessible and low-cost precursors. PbI 2 nanocrystals were grown on soda-lime glass (SLG), silicon dioxide support grids, and silicon wafer substrates and provided the groundwork for further investigation into developing lead halide perovskite processes by ALD. Further, the ALD-grown PbI 2 was characterized by annular dark-field scanning transmission electron microscopy (ADF-STEM), atomic force microscopy (AFM), high resolution transmission electron microscopy (HRTEM), X-ray fluorescence (XRF), and X-ray photoemission spectroscopy (XPS), among other methods. This work presents the first step to synthesize lead halide perovskites with atomic control for applications such as interfacial layers in photovoltaics and for deposition in microcavities for lasing.

36 MATERIALS SCIENCE↗
DOE OSTI2022

High-Temperature Pretreatment Effect on Co/SiO 2 Active Sites and Ethane Dehydrogenation

Here, we report the synthesis, optimization, and characterization of Co/SiO 2 for ethane nonoxidative dehydrogenation. Co/SiO 2 is synthesized via strong electrostatic adsorption using the widely available Co(NO 3 ) 2 as the precursor. We demonstrate that high-temperature pretreatment (900 °C) in an inert atmosphere can significantly enhance the initial activity of the Co/SiO 2 catalyst. X-ray absorption near-edge spectroscopy (XANES), temperature-programmed reduction (TPR), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) suggest that highly dispersed Co(II) clusters are more active than Co 0 or CoO x nanoparticles. Fourier transform infrared (FTIR) and isopropanol (IPA) temperature-programmed desorption and density functional theory (DFT) calculations suggest that high-temperature treatment significantly increases the density of active Lewis acid sites, possibly via surface dehydroxylation of the catalyst.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Atom Efficiency of Pd Sites for Methane Combustion: Single Atom Catalysts Versus Nanocatalysts

Methane combustion is an important reaction for energy production and methane removal from the atmosphere. This reaction highly relies on the use of noble metal Pd-based catalysts, which therefore drives the pursuit of catalysts with high atomic dispersion and activity. In this work, Pd/ceria catalysts dominated with Pd single atoms or nanosized Pd clusters (∼1 nm) are prepared and characterized by combining high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and Raman and X-ray absorption spectroscopy (XAS) techniques. By comparing the turnover frequencies (TOF; per every Pd atom) of Pd/ceria single atom catalysts and nanocatalysts, it is found that the atom efficiency of Pd is increased by 10 ∼30 times from single atom catalysts to nanocatalysts. For Pd single atom catalysts, although their activity can be tuned by changing the local structures, the intrinsic activity and number of active sites need to be further improved by engineering the surfaces of supports. For nanosized Pd species, despite the high TOF, the Pd atoms in the bulk structure are not directly participating in the catalytic reaction. Furthermore, this work highlights the importance of increasing the intrinsic activity of individual noble atoms, as well as the homogeneity of their local structures. For Pd/ceria systems reported in this work, our results indicate that from the application point of view, at the current stage, it is not practical to replace Pd nanocatalysts with single atom catalysts for methane combustion.

Pd↗
DOE OSTI2024

Influence of excess silicon on polytype selection during metal-mediated epitaxy of GaN nanowires

For this work, we have examined the origins of polytype selection during metal-mediated molecular-beam epitaxy of GaN nanowires (NWs). High-angle annular dark-field scanning transmission electron microscopy reveals [111]-oriented zinc blende (ZB) NWs and [0001]-oriented wurtzite (WZ) NWs, with SixNy at the interface between individual NWs and the Si (001) substrate. Quantitative energy dispersive x-ray spectroscopy reveals a notably higher Si concentration of 7.0% ± 2.3% in zinc blende (ZB) NWs than 2.3% ± 1.2% in wurtzite (WZ) NWs. Meanwhile, density functional theory calculations show that incorporation of 8 at. % Si on the Ga sublattice inverts the difference in formation energies between WZ and ZB GaN, such that the ZB polytype of GaN is stabilized. This identification of Si and other ZB polytype stabilizers will enable the development of polytype heterostructures in a wide variety of WZ-preferring compounds.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Exfoliation of Cu-Containing Poly(triazine imide): From Three-Dimensional to Two-Dimensional Particle Morphology

Controlling the morphological parameters of extended covalent organic frameworks remains challenging and represents an important yet often elusive metric of consideration. Typically, carbon nitride materials possess local ordering but remain largely amorphous in terms of their long-range order and orientation. This study probes the synthesis of a crystalline carbon nitride, poly(triazine imide) lithium bromide which possesses an atomically-precise extended structure, and demonstrates its exfoliation into a two-dimensional hexagonal sheet-like morphology. Furthermore, a previously unreported carbon nitride material, poly(triazine imide) copper bromide, or PTI-CuBr, was developed through an additional flux-assisted cation-exchange process and is shown to retain its internal Cu cations during solvothermal exfoliation. Characterization by dynamic light scattering and high-angle annular dark-field scanning electron microscopy reveals the morphological changes and captures the high aspect ratio of the thin carbon nitride sheets with <10 nm thickness while maintaining hundreds of nm in width. Additional characterization by energy-dispersive spectroscopy and X-ray photoelectron spectroscopy confirms that the Cu:Br:N molar ratio was maintained within the extended layers throughout the exfoliation process. This top-down synthesis approach differs from typical methods that isolate thin sheets for subsequent metal−cation coordination and illustrates the importance of maintaining oxygen-free conditions to minimize copper clustering. Thus, this new approach is demonstrated to provide a consistent and more homogeneous occupancy of the PTI pore spaces throughout the carbon nitride framework.

Exfoliation↗
DOE OSTI2022

Polytypism in semi-disordered lizardite and amesite by low-dose HAADF-STEM

Serpentine minerals exert important controls on the physical properties of ultramafic rocks and have the potential to influence deformation phenomena in fault zones and to control the release of water in subducted slabs. Sheet serpentine generally, and lizardite and amesite specifically, can adopt alternative crystallographic stacking arrangements called polytypes. Polytypism has been extensively studied in fully ordered crystals, but it remains largely enigmatic in the more common semi-disordered crystals that in long-range analyses such as X-ray diffraction only exhibit random combinations of 0b and ±1/3b interlayer shifts. To date, atomic-resolution imaging to identify locally ordered polytypes has been precluded by the beam-sensitive nature of this hydrous magnesium silicate mineral. Here, in this study, we employed low-dose high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to study the polytypic structure of semi-disordered lizardite and amesite. Because the electron dose was as low as ~6000 e–/Å 2 , it was possible to directly resolve oxygen atomic columns and all the cations with a resolution of ~1 Å and reveal the short-range order. For lizardite, we identified long-period non-standard polytypes, including examples with 3, 4, 8, and 9 layers stemming from the ordering of the octahedral tilt along the a-axis. For amesite, we found short-range ordered polytypes with periodicities of up to 42 Å stemming from the ordering of interlayer shifts along the b-axis. The resolution was sufficient to determine the relative abundance of 6R 2 , 6R 1 , 2H 1 , and 2H 2 polytypes in amesite to be 46.1, 29.6, 7.7, and 1.9%, respectively. This is contrary to the expectation that the most common form of amesite is the 2H 2 polytype, which may be more likely to form macroscopic crystals suitable for conventional X-ray diffraction-based studies. We conclude that HAADF-STEM methods open the way for the characterization of beam-sensitive minerals and to resolve the structural details of less well-ordered (but possibly more abundant) minerals at a unit-cell scale.

58 GEOSCIENCES↗
DOE OSTI2025

Optimal 3D chemical imaging with multimodal electron tomography

Accurate mapping of nanoscale chemistry in three dimensions (3D) has been a longstanding challenge. Modern electron microscopy provides chemical images by electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectrometry (EDX) but requires high fluences that damage specimens. In 3D, the requirements are worse; electron tomography demands many high-fluence chemical maps for reconstruction, creating a tradeoff between resolution, accuracy, and sample survival. Fused multimodal electron tomography (MM-ET) alleviates this requirement by leveraging lower-fluence high-angle annular dark-field (HAADF) images alongside a few chemical maps to dramatically improve chemical resolution. Here, experimental and computational parameter space is systematically explored to determine when MM-ET performs best. Ideal imaging conditions balance sample survival with resolution and chemical specificity; we recommend a tilt range of at least ± 70°, acquiring 40 equally spaced HAADF projections (signal-to-noise > 10), and 7 EELS/EDX maps of each chemistry (signal-to-noise > 4).

36 MATERIALS SCIENCE↗
DOE OSTI2024

Understanding Formation of Irradiation-Induced Defects through 4D-STEM, Electron Tomography, and WBDF-STEM

A major challenge in advancing nuclear materials for next-generation fission and proposed fusion reactors is to comprehensively understand the formation of irradiation-induced defects. Here it is essential to correlate the evolution of irradiation-induced defects and the degradation of mechanical properties, as they collectively dictate the material's lifespan and ensure nuclear safety. Scanning transmission electron microscopy (STEM) based techniques have emerged as indispensable tools for irradiation-induced defect characterization, offering high spatial resolution imaging and chemical analysis, such as electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS). These techniques have been effectively used to obtain an atomic-scale view of the defect structure. Recent advances in electron microscopy, particularly in 4D-STEM, offer detailed insight into microstructural evolution by capturing full 2D diffraction patterns at every pixel position. Using high-speed direct electron detectors, this technology generates a four-dimensional dataset, overcoming the limitations of traditional STEM imaging.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Investigation of Nanoparticle Degradation in Hydrogen Fuel Cell Systems through Automated Electron Microscopy

Proton exchange membrane fuel cells (PEMFC) are promising devices for the deployment of hydrogen-powered heavy-duty vehicles, providing a higher efficiency for similar driving range and fueling time than the existing ones. However, PEMFCs still encounter durability challenges mainly due to catalyst degradation in the cathode. Mitigating these performance losses requires a better understanding of the degradation mechanisms under heavy-duty accelerated stress tests (ASTs) [1]. Scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) are key tools for the analysis of Pt and PtCo nanoparticle size, spatial distribution and composition [2]. Electron tomography is also used to determine the rate and type of degradation of catalyst nanoparticles as a function of their position on the carbon support. In this work, automated data acquisition software, paired with a custom Python code, have been used to study the effect of different accelerated stress tests (ASTs) on nanoparticle coarsening [2]. Figure 1 shows high-angle annular dark-field (HAADF)-STEM images and EDS maps comparing the cathodes of membrane electrode assemblies (MEAs) following an electrocatalyst AST performed under H2/N2 with that of the heavy-duty AST performed under H2/air. We will discuss how AST conditions affect considerably the spatial distribution of the nanoparticles across the electrode between the membrane and microporous layer. Although the median particle size increased more in the MEA aged under the heavy-duty AST, as determined using a high-throughput image analysis, the quantitative EDS measurements demonstrate that the electrocatalyst AST resulted in more Pt and Co dissolution from the cathode, which is another important indicator of electrocatalyst degradation. We will further present the impact of the relative humidity (% RH) on the degradation mechanisms demonstrated using the same approach. Electron tomography has been used to distinguish the Pt nanoparticles residing on the carbon support surface (exterior) from those within the pore structure (interior) in order to determine the relative stability of interior and exterior nanoparticles. As shown in Figure 2, we will compare the Pt catalyst particle size at the beginning of test (BOT) and end of test (EOT), and discuss the importance of automating the electron tomography workflow, i.e. acquisition, reconstruction, and visualization, to increase sampling and determine the standard deviation of these measurement. The outlook for utilizing low-dose cryo-tomography for limiting damage to the catalyst, support, and especially proton-conducting ionomer will also be discussed [3].

Amichi, Lynda↗