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Zachman, Michael J.

Publications and source records attributed to Zachman, Michael J..

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

Crowded supported metal atoms on catalytically active supports may compromise intrinsic activity: A case study of dual-site Pt/α-MoC catalysts

Increasing the surface population of supported catalytic sites, assuming these sites are stable, is considered a straightforward approach to improving the overall catalytic performance. Here, we report an exception represented by the Pt/α-MoC catalysts featuring atomically dispersed Pt. The Pt/α-MoC catalysts display very high activity for the reverse water gas shift reaction with near 100% CO selectivity for CO 2 :H 2 ratios from 0.25 to 4 and from 250 to 400 °C. Despite the excellent performance, the intrinsic activity per Pt-centric catalytic center declines as the Pt loading increases from 0.1 to 1.0 wt%. With the dispersed Pt evolving from isolated atoms to fully exposed ensembles, the shrinking inter-Pt-atom space impedes CO 2 activation at the critical Pt-Mo interfaces, where the Pt shall temporarily take the -O intermediates. The Pt, even as atomically dispersed without noticeable sintering, is underutilized in such a crowded state. This caution for high-loading catalyst design is translational to other systems where the direct catalytic roles of the supports are crucial.

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

State-of-the-Art Electron Microscopy For Physical Sciences Research

The ability to provide high-resolution information about a variety of materials has made electron microscopy a valuable tool for the physical sciences research community. In recent years, many advanced electron microscopy techniques have been developed to provide access to new classes of materials or provide new types of information about materials. This Methods Collection focuses on a selection of these state-of-the-art techniques, including those that provide access to information about materials in liquids and gases and those that provide new or enhanced information about more conventional solid materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Liquid Phase Exfoliation of Chemically Prelithiated Bilayered Vanadium Oxide in Aqueous Media for Li-Ion Batteries

Bilayered vanadium oxides are attractive for energy storage due to their high initial specific capacities, which could be stabilized by integrating the bilayers with conductive nanoflakes often produced in a form of aqueous dispersions. Therefore, exfoliation of the bilayered vanadium oxides in water with high yield is desirable. Here, this work introduces the first aqueous exfoliation of chemically prelithiated bilayered vanadium oxide (i.e., δ-Li x V 2 O 5 ·nH 2 O or LVO) followed by vacuum filtration to produce a free-standing film, exhibiting a lamellar stacking of the bilayered vanadium oxide nanoflakes as evidenced by scanning electron microscopy. Due to the hydrated nature of bilayered vanadium oxides, the relationship between interlayer water content and the vacuum drying temperature (105 °C vs 200 °C) was studied using X-ray diffraction, thermogravimetric analysis, and Raman spectroscopy. It was found that vacuum drying the LVO nanoflakes at 200 °C enabled more efficient removal of crystallographic water than drying at 105 °C, and did not induce a phase transformation. Scanning transmission electron microscopy confirmed the layered structure of the samples, which was more well-ordered in the 200 °C case and had no clear boundaries between flakes at the atomic scale. Furthermore, electrochemical testing in nonaqueous Li-ion cells revealed that vacuum drying at 200 °C led to improvements in ion storage capacity and electrochemical stability. Improvements in electrochemical charge storage properties of the electrodes obtained via LVO exfoliation and free-standing film assembly in water dried at 200 °C reveal that conventional battery electrode drying protocols need to be revised as new electrochemically active materials are synthesized, such as hydrated layered oxides with expanded interlayer regions. The remaining capacity fading can be attributed to the structural LVO degradation, dissolution of vanadium oxide in electrolyte, and parasitic effects of the remaining interlayer water molecules. Our results establish an environmentally friendly and safe approach to obtain two-dimensional (2D) bilayered vanadium oxide nanoflakes and create a pathway to constructing novel 2D heterostructures for improved performance in energy storage applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Elucidating the impact of the ionomer equivalent weight on a platinum group metal-free PEMFC cathode via oxygen limiting current

Leveraging the interactions between ionomer and catalyst can increase the performance of proton exchange membrane fuel cells. The impacts of the equivalent weight (EW) of perfluorosulfonic acid–based ionomers on the platinum group metal-free electrode structure and fuel cell performance have not been fully explored. Four membrane electrode assemblies (MEAs) were prepared by using a commercial Fe–N–C catalyst, two perfluorosulfonic acid ionomers with different EWs, that is, Aquivion 720 (A720) and Nafion 1100 (N1100), and two ionomer-to-catalyst (I/C) ratios. The four MEAs were characterized to understand the impact of the ionomer EW and content on the capacitance, proton conductivity, and mass transport on the cathode. The mass transport resistance was measured for the first time using a new oxygen reduction reaction limiting current method enabling to couple the effects of oxygen diffusion with liquid water generation. Low EW ionomer combined with a moderate I/C results in improved performance due to its enhanced proton conductivity. However, when used at high I/C, it can cause severe water flooding at high current density due to the enhanced liquid water uptake, especially at high relative humidity, resulting in lower catalyst utilization and higher mass transport resistance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Microscale spatial distribution and soil organic matter persistence in top and subsoil

The spatial distribution of organic substrates and microscale soil heterogeneity significantly influence organic matter (OM) persistence as constraints on OM accessibility to microorganisms. However, it is unclear how changes in OM spatial heterogeneity driven by factors such as soil depth affect the relative importance of substrate spatial distribution on OM persistence. This work evaluated the decomposition and persistence of 13 C and 15 N labeled water-extractable OM inputs over 50 days as either hotspot (i.e., pelleted in 1–2 mm-size pieces) or distributed (i.e., added as OM < 0.07 μm suspended in water) forms in topsoil (0–0.2 m) and subsoil (0.8–0.9 m) samples of an Andisol. We observed greater persistence of added C in the subsoil with distributed OM inputs relative to hotspot OM, indicated by a 17% reduction in cumulative mineralization of the added C and a 10% higher conversion to mineral-associated OM. A lower substrate availability potentially reduced mineralization due to OM dispersion throughout the soil. NanoSIMS (nanoscale secondary ion mass spectrometry) analysis identified organo-mineral associations on cross-sectioned aggregate interiors in the subsoil. On the other hand, in the topsoil, we did not observe significant differences in the persistence of OM, suggesting that the large amounts of particulate OM already present in the soil outweighed the influence of added OM spatial distribution. Here, we demonstrated under laboratory conditions that the spatial distribution of fresh OM input alone significantly affected the decomposition and persistence of OM inputs in the subsoil. On the other hand, spatial distribution seems to play a lower role in topsoils rich in particulate OM. The divergence in the influence of OM spatial distribution between the top and subsoil is likely driven by differences in soil mineralogy and OM composition.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Ordered CoPt oxygen reduction catalyst with high performance and durability

A high-performance and durable polymer electrolyte membrane fuel cell cathode catalyst composed of an ordered L10–CoPt core and a thin Pt shell was designed and prepared. Under practical fuel cell membrane electrode assembly testing conditions, the cathode catalyst showed an outstanding initial mass activity of 0.6 A/mgPt, which satisfies the U.S. Department of Energy performance target while also meeting the durability target of less than 40% loss in mass activity after 30,000 accelerated stress test voltage cycles. The high structural stability of the L10–CoPt@Pt-shell catalyst was confirmed by postmortem materials characterization, and the origin of this robustness was revealed by density functional theory calculations, where the barriers for diffusion of Co atoms were observed to be significantly increased in the ordered intermetallic core.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

High Velocity, Low‐Voltage Collective In‐Plane Switching in (100) BaTiO 3 Thin Films

Abstract Ferroelectrics are being increasingly called upon for electronic devices in extreme environments. Device performance and energy efficiency is highly correlated to clock frequency, operational voltage, and resistive loss. To increase performance it is common to engineer ferroelectric domain structure with highly‐correlated electrical and elastic coupling that elicit fast and efficient collective switching. Designing domain structures with advantageous properties is difficult because the mechanisms involved in collective switching are poorly understood and difficult to investigate. Collective switching is a hierarchical process where the nano‐ and mesoscale responses control the macroscopic properties. Using chemical solution synthesis, epitaxially nearly‐relaxed (100) BaTiO 3 films are synthesized. Thermal strain induces a strongly‐correlated domain structure with alternating domains of polarization along the [010] and [001] in‐plane axes and 90° domain walls along the [011] or [01] directions. Simultaneous capacitance–voltage measurements and band‐excitation piezoresponse force microscopy revealed strong collective switching behavior. Using a deep convolutional autoencoder, hierarchical switching is automatically tracked and the switching pathway is identified. The collective switching velocities are calculated to be ≈500 cm s −1 at 5 V (7 kV cm −1 ), orders‐of‐magnitude faster than expected. These combinations of properties are promising for high‐speed tunable dielectrics and low‐voltage ferroelectric memories and logic.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Nitrogen-coordinated single atom iron sites (FeN 4 ) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. Still, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface. We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN 4 sites into highly stable pyridinic N-coordinated FeN 4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mg Pt cm -2 ). The encouraging stability improvement represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications.

08 HYDROGEN↗
DOE OSTI2022

Understanding Recoverable vs Unrecoverable Voltage Losses and Long-Term Degradation Mechanisms in Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) have recently shown excellent progress in terms of their performance – e.g., achievable power and current density. However, very few AEMFCs have been demonstrated with the ability to operate for a long duration (>1000 h). In addition, it is unknown whether performance losses observed during operation are reversible, irreversible, or a combination of the two. In this study, a high-performance AEMFC operated continuously at 600 mA/cm 2 for 3600 h (150 days) at 80 °C with H 2 /O 2 reacting gases was demonstrated. Throughout testing, the electrochemical properties of the AEMFC were probed to provide information about performance degradation pathways and their degree of reversibility. It was found that a portion of the performance loss that occurs during AEMFC operation was due to suboptimal reaction conditions and can be recovered. At the end of the experiment, the cell was disassembled, and its structure and composition were evaluated at the nanoscale by aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. The structure and composition of the electrode were compared to cells at the beginning of their operational life. It was found that the primary mechanism for long-term AEMFC performance loss was catalyst agglomeration. During the operational time, there was no evidence of significant polymer degradation, likely due to the high hydration state of the cell. By documenting the long-term changes in high-performing AEMFCs, this work provides important information for the systematic design of cell components and demonstrates the importance of controlling cell operation, which can aid in the commercialization and widespread deployment of low-cost, long-life AEMFCs.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2022

Atomically Dispersed Dual‐Metal Site Catalysts for Enhanced CO 2 Reduction: Mechanistic Insight into Active Site Structures

Abstract Carbon‐supported nitrogen‐coordinated single‐metal site catalysts (i.e., M−N−C, M: Fe, Co, or Ni) are active for the electrochemical CO 2 reduction reaction (CO 2 RR) to CO. Further improving their intrinsic activity and selectivity by tuning their N−M bond structures and coordination is limited. Herein, we expand the coordination environments of M−N−C catalysts by designing dual‐metal active sites. The Ni‐Fe catalyst exhibited the most efficient CO2RR activity and promising stability compared to other combinations. Advanced structural characterization and theoretical prediction suggest that the most active N‐coordinated dual‐metal site configurations are 2N‐bridged (Fe‐Ni)N 6 , in which FeN 4 and NiN 4 moieties are shared with two N atoms. Two metals (i.e., Fe and Ni) in the dual‐metal site likely generate a synergy to enable more optimal *COOH adsorption and *CO desorption than single‐metal sites (FeN 4 or NiN 4 ) with improved intrinsic catalytic activity and selectivity.

Li, Yi↗
DOE OSTI2022

Recreating Fuel Cell Catalyst Degradation in Aqueous Environments for Identical-Location Scanning Transmission Electron Microscopy Studies

The recent surge in interest of proton exchange membrane fuel cells (PEMFCs) for heavy-duty vehicles increases the demand on the durability of oxygen reduction reaction electrocatalysts used in the fuel cell cathode. This prioritizes efforts aimed at understanding and subsequently controlling catalyst degradation. Identical-location scanning transmission electron microscopy (IL-STEM) is a powerful method that enables precise characterization of degradation processes in individual catalyst nanoparticles across various stages of cycling. Recreating the degradation processes that occur in PEMFC membrane electrode assemblies (MEAs) within the aqueous cell used for IL-STEM experiments is vital for generating an accurate understanding of these processes. In this work, we investigate the type and degree of catalyst degradation achieved by cycling in an aqueous cell compared to a PEMFC MEA. While significant degradation is observed in IL-STEM experiments performed on a traditional Pt catalyst using the standard accelerated stress test potential window (0.6-0.95 VRHE), degradation of a PtCo catalyst designed for heavy-duty vehicle use is very limited compared to that observed in MEAs. We therefore explore various experimental parameters such as temperature, acid type, acid concentration, ionomer content, and potential window to identify conditions that reproduce the degradation observed in MEAs. We find that by extending the cycling potential window to 0.4-1.0 V RHE in an electrolyte containing Pt ions, the degraded particle size distribution and alloy composition better match that observed in MEAs. In particular, these conditions increase the relative contribution of Ostwald ripening, which appears to play a more significant role in the degradation of larger alloy particles supported on high-surface-area carbons than coalescence. Results from this work highlight the potential for discrepancies between ex situ aqueous experiments and MEA tests. While different catalysts may require a unique modification to the AST protocol, strategies provided in this work enable future in situ and identical-location experiments that will play an important role in the development of robust catalysts for heavy-duty vehicle applications.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2022

Quantifying the electrochemical active site density of precious metal-free catalysts in situ in fuel cells

We report advances in the development of precious-group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes have produced active catalysts that reduce the performance gap to the incumbent Pt-based materials. However, utilization of state-of-the-art PGM-free catalysts for commercial applications is currently impeded by their relatively low durability. Methods designed to study catalyst degradation in the operation of fuel cells are therefore critical for understanding durability issues and, ultimately, their solutions. Here we report the use of Fourier-transform alternating current voltammetry as an electrochemical method for accurate quantification of the electrochemically active site density of PGM-free cathode catalysts, and to follow their degradation in situ during the operation of polymer electrolyte fuel cells. Using this method, we were able to quantify the electrochemical active site density, which will enable the elucidation of degradation mechanisms of PGM-free ORR catalysts in situ in fuel cells.

25 ENERGY STORAGE↗
DOE OSTI2022

Robust Atomic-Resolution Imaging of Lithium in Battery Materials by Center-of-Mass Scanning Transmission Electron Microscopy

The performance of energy storage materials is often governed by their structure at the atomic scale. Conventional electron microscopy can provide detailed information about materials at these length scales, but direct imaging of light elements such as lithium presents a challenge. While several recent techniques allow lithium columns to be distinguished, these typically either involve complex contrast mechanisms that make image interpretation difficult or require significant expertise to perform. In this work, we demonstrate how center-of-mass scanning transmission electron microscopy (CoM-STEM) provides an enhanced ability for simultaneous imaging of lithium and heavier element columns in lithium ion conductors. Through a combination of experiments and multislice electron scattering calculations, we show that CoM-STEM is straightforward to perform and produces directly interpretable contrast for thin samples, while being more robust to variations in experimental parameters than previously demonstrated techniques. As a result, CoM-STEM is positioned to become a reliable and facile method for directly probing all elements within energy storage materials at the atomic scale.

25 ENERGY STORAGE↗
DOE OSTI2021

Application of Molecular Catalysts for the Oxygen Reduction Reaction in Alkaline Fuel Cells

Here, the development of precious group metal-free (PGM-free) catalysts for the oxygen reduction reaction is considered as the main thrust for the cost reduction of fuel cell technologies and their mass production. Within the PGM-free category, molecular catalysts offer an advantage over other heat-treated PGM-free catalysts owing to their well-defined structure, which enables further design of more active, selective, and durable catalysts. Even though non-heat-treated molecular catalysts with exceptional performance have been reported in the past, they were rarely tested in a fuel cell. Herein, we report on a molecular catalyst under alkaline conditions: fluorinated iron phthalocyanine (FeFPc) supported on cheap and commercially available high-surface area carbon-BP2000 (FeFPc@BP2000). It exhibits the highest activity ever reported for molecular catalysts under alkaline conditions in half-cells and fuel cells.

25 ENERGY STORAGE↗
DOE OSTI2021

Co-precipitation induces changes to iron and carbon chemistry and spatial distribution at the nanometer scale

Association of organic matter (OM) with mineral phases via co-precipitation is expected to be a widespread process in environments with high OM input and frequent mineral dissolution and re-precipitation. In contrast to surface area-limited adsorption processes, co-precipitation may allow for greater carbon (C) accumulation. However, the potential sub-micrometer scale structural and compositional differences that affect the bioavailability of co-precipitated C are largely unknown. In this work, we used a combination of high-resolution analytical electron microscopy and bulk spectroscopy to probe interactions between a mineral phase (ferrihydrite, nominally Fe 2 O 3 •0.5H 2 O) and organic soil-derived water-extractable OM (WEOM). In co-precipitated WEOM-Fe, nanometer-scale scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) revealed increased Fe(II) and less Fe aggregation relative to adsorbed WEOM-Fe. Spatially distinct lower- and higher-energy C regions were detected in both adsorbed and co-precipitated WEOM-Fe. In co-precipitates, lower-energy aromatic and/or substituted aromatic C was spatially associated with reduced Fe(II), but higher-energy oxidized C was enriched at the oxidized Fe(III) interface. Therefore, we show that co-precipitation does not constitute a non-specific physical encapsulation of C that only affects Fe chemistry and spatial distribution, but may cause a bi-directional set of reactions that lead to spatial separation and transformation of both Fe and C forms. In particular, we propose that abiotic redox reactions between Fe and C via substituted aromatic groups (e.g., hydroquinones) play a role in creating distinct co-precipitate composition, with potential implications for its mineralization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗