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18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on HoNiGe by Materials Project

HoNiGe crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Ho is bonded in a 12-coordinate geometry to six equivalent Ni and six equivalent Ge atoms. There are a spread of Ho–Ni bond distances ranging from 2.92–3.15 Å. There are a spread of Ho–Ge bond distances ranging from 2.94–3.07 Å. Ni is bonded in a 10-coordinate geometry to six equivalent Ho and four equivalent Ge atoms. There are a spread of Ni–Ge bond distances ranging from 2.45–2.71 Å. Ge is bonded in a 10-coordinate geometry to six equivalent Ho and four equivalent Ni atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on HoNiGe by Materials Project

HoNiGe crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Ho is bonded in a 12-coordinate geometry to six equivalent Ni and six equivalent Ge atoms. There are a spread of Ho–Ni bond distances ranging from 2.95–3.21 Å. There are a spread of Ho–Ge bond distances ranging from 2.97–3.16 Å. Ni is bonded in a 10-coordinate geometry to six equivalent Ho, one Ni, and three equivalent Ge atoms. The Ni–Ni bond length is 2.68 Å. There are two shorter (2.40 Å) and one longer (2.64 Å) Ni–Ge bond lengths. Ge is bonded in a 10-coordinate geometry to six equivalent Ho, three equivalent Ni, and one Ge atom. The Ge–Ge bond length is 2.72 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Operando carbon corrosion measurements in fuel cells using boron-doped carbon supports

Carbonaceous materials are the most common catalyst supports in proton exchange membrane fuel cell (PEMFCs), yet their corrosion is one of the limiting factors in achieving high durability. Herein, we doped carbon supports with boron (B) to increase the corrosion-resistance of the support. Two types of B-doped carbons were synthesized and studied as platinum support materials. Further, they varied in their morphologies, surface areas, and the types of boron species. The durability of Pt/B-doped carbon catalysts was investigated using the US-DOE catalysts’ supports accelerated stress test (AST) and a mass-spectrometer connected to the fuel cell effluent stream to quantify the mass of corroded carbon support in operando. The addition of boron to the carbon increased the stability of Pt catalysts in long-term usage of PEMFC. After 4000 AST cycles, more than 50% of initial current density was preserved for the boron-containing systems, while less than 30% of it remained with Vulcan carbon (Pt/V). Also, the Pt/B-doped carbon samples demonstrated better electrochemical active surface area (ECSA) stability when compared to Pt/V. Carbon loss measurements showed that B-doped carbons have higher resistance to electrochemical corrosion than unmodified carbon. Specifically, the substitutional boron-doped carbon demonstrated an extremely high stability and low corrosion rate.

25 ENERGY STORAGE↗
DOE OSTI2022

Mixed-Metal Nickel–Iron Oxide Aerogels for Oxygen Evolution Reaction

Alkaline electrolyte membrane electrolyzers are a promising technology to efficiently produce clean hydrogen without the use of critical raw materials. At the heart of these electrolyzers are the electrocatalysts, which facilitate the cathodic and anodic reactions, with the latter oxygen evolution reaction (OER) being the most sluggish. In recent years, aerogels have become a very well-studied class of materials due to their unique properties, including very high surface area. Until now, aerogels have not been used to catalyze the OER by themselves but were mainly considered catalyst supports. Here, mixed-metal nickel–iron oxide aerogels were synthesized with a modified epoxide route synthesis and tested as OER catalysts. Depending on the Ni/Fe ratio, they show very high catalytic activity and low overpotential to reach 10 mA cm –2 (at η = 380 mV). This activity is beyond that of the existing state-of-the-art platinum group metal-free OER catalysts.

PGM free catalysts↗
DOE OSTI2023

Biomimetic Fe–Cu Porphyrrole Aerogel Electrocatalyst for Oxygen Reduction Reaction

The development of bioinspired catalysts for oxygen reduction reaction is one of the most prominent pathways in the search for active materials to replace Pt-based catalysts in fuel cells. Herein, we report innovative bioinspired catalysts using a directed synthetic pathway to create adjacent Cu and Fe sites. This catalyst is composed of a covalent 3D framework in an aerogel form. Aerogels are high surface area and porous hierarchical structures that can allow the formation of ultrahigh active site density and optimized mass transport of reactants and products to and from the catalytic sites. The aerogel-based catalyst exhibits high performance in a half-cell in 0.1 M KOH, with an onset potential of 0.94 V vs RHE and half-wave potential of E 1/2 = 0.80 V vs RHE, high selectivity toward the four-electron reduction of oxygen to hydroxide anions, and high durability. These results are well-translated to the anion exchange membrane fuel cell (AEMFC), reaching an open circuit potential of 0.97 V and iR-corrected peak power density of 0.51 W cm –2 . Based on density functional theory calculations, the improved activity relative to the Fe-porphyrin and Cu-corrole is ascribed to the effect of the extended carbon network and the proximity of the metal sites.

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

Modular Iron–Bipyridine-Based Conjugated Aerogels as Catalysts for Oxygen Reduction Reaction

Aerogels have a large surface area and a porous structure, which make them an attractive catalyst support materials for fuel cells. Modifying the aerogels’ building blocks and introducing catalytic sites into their backbones allow them to function as both catalysts and supports, thereby increasing the density and distribution of catalyst active sites. In this work, we studied conjugated aerogels containing iron–bipyridine catalytic sites for the oxygen reduction reaction. To demonstrate the variation in physical and electrochemical properties of these aerogels, a series of aerogels were synthesized by a Glaser coupling reaction. Iron salt was added to the three-dimensional gel to produce iron–bipyridine complexes and obtain atomically dispersed catalytic sites. The electrocatalytic activity and electrical conductivity of the aerogels were increased after their heat treatment to yield Fe-doped carbon aerogels. The control of metal loadings was accomplished by changing the bipyridine content during the aerogel synthesis, resulting in 0.9–4.1 wt % Fe. In conclusion, the results show a correlation between Fe loading, aerogel structure, and catalytic activity, achieving high performance for the oxygen reduction reaction in both acidic and alkaline media.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Studying the Durability of Molybdenum Carbide Aerogel as an Electrocatalyst Support for Proton Exchange Membrane Fuel Cells

Carbon-supported platinum nanoparticles (Pt/C) are currently the state-of-the-art catalyst in proton exchange membrane fuel cells (PEMFCs). Unfortunately, the carbon support lacks the ability to stabilize the metal catalyst, as platinum tends to dissolve and agglomerate, significantly compromising the durability. Herein, we synthesized a ceramic material, molybdenum carbide aerogel (MCAG), and utilized it as a Pt support for PEMFCs, as an alternative for conventional carbon supports. N 2 adsorption and XRD analysis showed that the MCAG possesses a combination of high porosity and a well-defined ceramic crystalline structure. The Pt/MCAG system was studied for its electrocatalytic activity toward ORR in a half-cell and demonstrated satisfactory reaction kinetics and electrochemical active surface area, comparable to the commercial Pt/C. For durability examination, an accelerated stress test (AST) in a single cell was conducted with the Pt/MCAG catalyst at the cathode, following the U.S. DOE electrocatalyst AST protocol. Intermediate polarization curves and cyclic voltammograms were recorded over the course of the AST to monitor the aging process. The results indicated that the MCAG support exhibited superior durability in long-term fuel cell operation compared to a carbon support. The Pt/MCAG system demonstrated stable behavior during the progressive stages of the AST, retaining more than 50% of its initial performance by the end of the test, whereas carbon-supported Pt preserved only 30%.

25 ENERGY STORAGE↗
DOE OSTI2022

Proteolytic processing induces a conformational switch required for antibacterial toxin delivery

Many Gram-negative bacteria use CdiA effector proteins to inhibit the growth of neighboring competitors. CdiA transfers its toxic CdiA-CT region into the periplasm of target cells, where it is released through proteolytic cleavage. The N-terminal cytoplasm-entry domain of the CdiA-CT then mediates translocation across the inner membrane to deliver the C-terminal toxin domain into the cytosol. Here, we show that proteolysis not only liberates the CdiA-CT for delivery, but is also required to activate the entry domain for membrane translocation. Translocation function depends on precise cleavage after a conserved VENN peptide sequence, and the processed ΔVENN entry domain exhibits distinct biophysical and thermodynamic properties. By contrast, imprecisely processed CdiA-CT fragments do not undergo this transition and fail to translocate to the cytoplasm. These findings suggest that CdiA-CT processing induces a critical structural switch that converts the entry domain into a membrane-translocation competent conformation.

59 BASIC BIOLOGICAL SCIENCES↗
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 OSTI2020

Magnetic field induced structural changes in magnetite observed by resonant x-ray diffraction and Mössbauer spectroscopy

When a magnetic field is applied to a single crystal of magnetite at 124 K > T > 50 K , the monoclinic c M axis, which is the easy magnetization axis, switches to one of the $\langle$100$\rangle$ cubic directions, nearest to the direction of the magnetic field, and the phenomenon known as an axis switching (AS) occurs. Here, a global symmetry probe, resonant x-ray scattering, and a local probe, Mössbauer spectroscopy, are used to better understand the mechanism of axis switching. The behavior of three subsystems ordered below the Verwey transition temperature T V , i.e., lattice distortion, an orbital, and charge orderings, was observed via resonant x-ray scattering as a function of an external magnetic field. This was preceded by calculation of selected peak intensities using the fdmnes code. The Mössbauer spectroscopy studies confirmed that the magnetic field triggers electronic rearrangements and atomic displacements. The structure observed after the process of axis switching is very similar to the one obtained after cooling below T V with the magnetic field applied along one of the initial $\langle$100$\rangle$ cubic directions and distinct from the cooling in the absence of a magnetic field. From all the experimental observations of the phenomenon done so far, it is clear that AS starts from the fluctuations between octahedral iron orbitals that ultimately lead to the Verwey transition, but also to the higher-temperature trimeron dynamics. Therefore, further observation of the axis switching may be a key point to the understanding of a majority of strongly correlated electronic behavior in magnetite as well as in other transition metal oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Trimeron-phonon coupling in magnetite

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe 3 O 4 ) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic Cc structure revealing the strong coupling between the charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise due to fluctuation effects, which are not included in the calculation method. Despite this shortcoming, we argue that the main spectral features can be understood assuming that the strong trimeron-phonon coupling is extended above the Verwey transition, with lattice dynamics influenced by the short-range order instead of the average cubic structure. Our results indicate the validity of trimerons (and trimeron-phonon coupling) to explain the physics of magnetite much beyond their original formulation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Pit Disassembly and Processing/Oxidation Capacity (PDP) for Disposition of Surplus Plutonium at Los Alamos (FY2021 Workshop 1) [Slides]

This workshop will establish NNSA/LANL consensus on the planning basis for: updating the 2017 Life Cycle Cost Estimate (LCCE) pre-conceptual estimate by identifying the specific scope elements that will be included; identifying the specific scope elements that will be included in the Line Item project; and identifying the preliminary agenda items for the next PDP workshop.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗