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Li, Jun

Publications and source records attributed to Li, Jun.

At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Constraining CO coverage on copper promotes high-efficiency ethylene electroproduction

The availability of inexpensive industrial CO gas streams motivates efficient electrocatalytic upgrading of CO to higher-value feedstocks such as ethylene. However, the electrosynthesis of ethylene by the CO reduction reaction (CORR) has suffered from low selectivity and energy efficiency. Here we find that the recent strategy of increasing performance through use of highly alkaline electrolyte—which is very effective in CO 2 RR—fails in CORR and drives the reaction to acetate. We then observe that ethylene selectivity increases when we constrain (decrease) CO availability. Using density functional theory, we show how CO coverage on copper influences the reaction pathways of ethylene versus oxygenate: lower CO coverage stabilizes the ethylene-relevant intermediates whereas higher CO coverage favours oxygenate formation. We then control local CO availability experimentally by tuning the CO concentration and reaction rate; we achieve ethylene Faradaic efficiencies of 72% and a partial current density of >800 mA cm -2 . The overall system provides a half-cell energy efficiency of 44% for ethylene production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Enhanced Ferromagnetism from Organic–Cerium Oxide Hybrid Ultrathin Nanosheets

Room-temperature ferromagnetism in two-dimensional (2D) oxide materials is an intriguing phenomenon for spintronic applications. Here, we report significantly enhanced room-temperature ferromagnetism observed from ultrathin cerium oxide nanosheets hybridized with organic surfactant molecules. The hybrid nanosheets were synthesized by ionic layer epitaxy over a large area at the water–air interface. The nanosheets exhibited a saturation magnetization of 0.149 emu/g as their thickness reduced to 0.67 nm. This value was 5 times higher than that for CeO 2 thin films and more than 20 times higher than that for CeO 2 nanoparticles. The magnetization was attributed to the high concentration (15.5%) of oxygen vacancies stabilized by surfactant hybridization as well as electron transfer between organic and oxide layers. Lastly, this work brings an effective strategy of introducing strong ferromagnetism to functional oxide materials, which leads to a promising route toward exploring new physical properties in 2D hybrid nanomaterials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr 2 Ge 2 Te 6

Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. In this work, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr 2 Ge 2 Te 6 . Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. In conclusion, our observation of topologically nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Dopant-tuned stabilization of intermediates promotes electrosynthesis of valuable C3 products

The upgrading of CO 2 /CO feedstocks to higher-value chemicals via energy-efficient electrochemical processes enables carbon utilization and renewable energy storage. Substantial progress has been made to improve performance at the cathodic side; whereas less progress has been made on improving anodic electro-oxidation reactions to generate value. Here we report the efficient electroproduction of value-added multi-carbon dimethyl carbonate (DMC) from CO and methanol via oxidative carbonylation. We find that, compared to pure palladium controls, boron-doped palladium (Pd-B) tunes the binding strength of intermediates along this reaction pathway and favors DMC formation. We implement this doping strategy and report the selective electrosynthesis of DMC experimentally. We achieve a DMC Faradaic efficiency of 83 ± 5%, fully a 3x increase in performance compared to the corresponding pure Pd electrocatalyst.

42 ENGINEERING↗
DOE OSTI2019

Quadruple bonding between iron and boron in the BFe(CO)3- complex

While main group elements have four valence orbitals accessible for bonding, quadruple bonding to main group elements is extremely rare. Here we report that main group element boron is able to form quadruple bonding interactions with iron in the BFe(CO) 3 - anion complex, which has been revealed by quantum chemical investigation and identified by mass-selected infrared photodissociation spectroscopy in the gas phase. The complex is characterized to have a B-Fe(CO) 3 - structure of C 3v symmetry and features a B-Fe bond distance that is much shorter than that expected for a triple bond. Various chemical bonding analyses indicate that the complex involves unprecedented BΞFe quadruple bonding interactions. Besides the common one electron-sharing σ bond and two Fe→B dative π bonds, there is an additional weak B→Fe dative σ bonding interaction. This finding of the new quadruple bonding indicates that there might exist a wide range of boron-metal complexes that contain such high multiplicity of chemical bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Discharge Permit-1845 Quarterly Status Report October 2019

Trichloroethene (TCE) and nitrate have been identified as constituents of concern in groundwater at the Sandia National Laboratories, New Mexico (SNL/NM) Technical Area (TA)-V Groundwater (TAVG) Area of Concern (AOC) based on detections above the U.S. Environmental Protection Agency (EPA) maximum contaminant levels in samples collected from monitoring wells. The maximum contaminant levels and the State of New Mexico drinking water standards, as specified in 20.6.2.3103 New Mexico Administrative Code (NMAC) for TCE and nitrate (as nitrogen) are 5 micrograms per liter (p,g/L) and 10 milligrams per liter (mg/L), respectively.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Facile Synthesis of Fe/N/S‐Doped Carbon Tubes as High‐Performance Cathode and Anode for Microbial Fuel Cells

Abstract As a renewable energy technology, microbial fuel cell (MFC) has been attracting increasing attention in recent decades. However, practical applications of MFCs has been hampered by the unsatisfactory electrode performance, in particular, at the cathode. Herein, Fe/N/S‐doped carbon hollow tubes were prepared by a facile two‐stage procedure involving hydrothermal treatment and pyrolysis at controlled temperatures. Electrochemical studies showed that the obtained samples exhibited an apparent electrocatalytic activity towards oxygen reduction reaction in both alkaline and acidic media, a performance comparable to that of commercial Pt/C, and the sample prepared at 800 °C stood out as the best among the series with a half‐wave potential of +0.81 V vs. RHE and an electron transfer number of 3.98 at +0.6 V vs. RHE. The Fe/N/S‐doped carbon tubes also exhibited a remarkable performance as an MFC anode by facilitating bacterial growth and electron transfer between the biofilm and electrode. In fact, an MFC based on the carbon tubes as both cathode and anode showed a markedly higher performance (maximum power density 479 W m −3 ) than the control MFC based on a graphene aerogel anode and Pt/C cathode (359 W m −3 ). These results suggest that Fe/N/S‐doped carbon composites can be used for the fabrication of high‐efficiency MFC electrodes.

Yang, Wei↗
DOE OSTI2019

Thickness-dependent magnetic order in CrI 3 single crystals

Two-dimensional (2D) materials with intrinsic ferromagnetism offers unique opportunity to engineer new functionalities in nano-spintronics. One such material is CrI 3 , showing long-range magnetic order in monolayer with the Curie temperature (T c ) of 45 K. In this work, we study detailed evolution of magnetic transition and magnetic critical properties in response to systematic reduction in crystal thickness down to 50 nm. Bulk T c of 61 K is gradually suppressed to 57 K, however, the satellite transition at T*=45 K is observed layer-independent at fixed magnetic field of 1 kOe. The origin of T* is proposed to be a crossover from pinning to depinning of magnetic domain walls. The reduction of thickness facilitates a field-driven metamagnetic transition around 20 kOe with out-of-plane field, in contrast to the continuous changes with in-plane field. The critical analysis around Tc elucidates the mean-field type interactions in microscale-thick CrI 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Anisotropic magnetocaloric effect in Fe3−xGeTe2

Abstract We present a comprehensive study on anisotropic magnetocaloric porperties of the van der Waals weak-itinerant ferromagnet Fe 3− x GeTe 2 that features gate-tunable room-temperature ferromagnetism in few-layer device. Intrinsic magnetocrystalline anisotropy is observed to be temperature-dependent and most likely favors the long-range magnetic order in thin Fe 3− x GeTe 2 crsytal. The magnetic entropy change Δ S M also reveals an anisotropic characteristic between H // ab and H // c , which could be well scaled into a universal curve. The peak value $$-\Delta {S}_{M}^{{\max }}$$ − Δ S M max of 1.20 J kg −1 K −1 and the corresponding adiabatic temperature change Δ T ad of 0.66 K are deduced from heat capacity with out-of-plane field change of 5 T. By fitting of the field-dependent parameters of $$-\Delta {S}_{M}^{{\max }}$$ − Δ S M max and the relative cooling power RCP, it gives $$-{\rm{\triangle }}{S}_{M}^{{\max }}$$ − ∆ S M max ∝ H n with n = 0.603(6) and RCP ∝ H m with m = 1.20(1) when H // c . Given the high and tunable T c , Fe 3− x GeTe 2 crystals are of interest for fabricating the heterostructure-based spintronics device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Understanding Heterolytic H2 Cleavage and Water-Assisted Hydrogen Spillover on Fe3O4(001)-Supported Single Palladium Atoms

The high specific activity, novel properties, and cost effectiveness of single-atom catalysts (SAC) hold great promise and allow for new chemistry in numerous reactions. Here, we focus on fundamental understanding of the critical step in hydrogenation reactions, H2 activation, on a model SAC catalyst comprised of single Pd atoms on reconstructed Fe3O4(001). Using a combination of scanning tunneling microscopy and density functional theory we demonstrate that H2 dissociates heterolytically, ultimately yielding hydroxyls on, and excess charge in, the Fe3O4 support. The hydrogenation can be carried out to high coverages and ultimately leads to lifting of the Fe3O4 reconstruction and Pd reduction and destabilization. Water plays an important role in reduction of the hydrogen diffusion barrier, thereby facilitating redistribution of hydroxyls away from Pd. Our study demonstrates a distinct H2 activation mechanism on single Pd atoms and shows the importance of a reducible support that can facilitate charge transport away from the active site.

Doudin, Nassar M.↗
DOE OSTI2019

Formation of graphene atop a Si adlayer on the C-face of SiC

The structure of the SiC(000$\overline{1}$) surface, the C-face of the {0001} SiC surfaces, is studied as a function of temperature and of pressure in a gaseous environment of disilane (Si 2 H 6 ). Various surface reconstructions are observed, both with and without the presence of an overlying graphene layer (which spontaneously forms at sufficiently high temperatures). Based on cross-sectional scanning transmission electron microscopy measurements, the interface structure that forms in the presence of the graphene is found to contain 1.4–1.7 monolayers (ML) of Si, a somewhat counter-intuitive result since, when the graphene forms, the system is actually under C-rich conditions. Using ab initio thermodynamics, it is demonstrated that there exists a class of Si-rich surfaces containing about 1.3 ML of Si that are stable on the surface (even under C-rich conditions) at temperatures above ~400 K. The structures that thus form consist of Si adatoms atop a Si adlayer on the C-face of SiC, with or without the presence of overlying graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

2018 Annual Site Environmental Report Sandia National Laboratories, Tonopah Test Range, Nevada and Kaua'i Test Facility, Hawai'i

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the United States Department of Energy (DOE) National Nuclear Security Administration. The National Nuclear Security Administration’s Sandia Field Office administers the contract and oversees contractor operations at Sandia National Laboratories, Tonopah Test Range (SNL/TTR) in Nevada and Sandia National Laboratories, Kaua‘i Test Facility (SNL/KTF) in Hawai‘i. Activities at SNL/TTR are conducted in support of DOE weapons programs and have operated at the site since 1957. SNL/KTF has operated as a rocket preparation launching and tracking facility since 1962. DOE and its management and operating contractor for Sandia are committed to safeguarding the environment reassessing sustainability practices and ensuring the validity and accuracy of the monitoring data presented in this Annual Site Environmental Report. This report summarizes the environmental protection, restoration, monitoring programs in place at SNL/TTR and SNL/KTF during calendar year 2018. Environmental topics include air quality, ecology, environmental restoration, oil storage, site sustainability, terrestrial surveillance, waste management, water quality, and implementation of the National Environmental Policy Act. This report is prepared in accordance with and as required by DOE O 231.1B, Admin Change 1, Environment, Safety, and Health Reporting, and has been approved for public distribution.

54 ENVIRONMENTAL SCIENCES↗