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Hunt, Adrian

Publications and source records attributed to Hunt, Adrian.

86 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Morphology and chemical behavior of model CsO x /Cu 2 O/Cu(111) nanocatalysts for methanol synthesis: Reaction with CO 2 and H 2

Cs is a promoter of Cu-based catalysts for the synthesis of alcohols from CO 2 hydrogenation. Scanning tunneling microscopy and ambient-pressure x-ray photoelectron spectroscopy were used to study the morphology and chemical properties of surfaces generated by the deposition of cesium on Cu 2 O/Cu(111) and Cu(111) substrates. CsO x nanostructures were formed after Cs metal was deposited on Cu 2 O/Cu(111) at 300 K. The formed CsO x protrude over the surface of copper oxide by 2–4 Å, with the dimension at the base of the nanostructures being in the range of 1–3 nm. Heating to elevated temperature induced significant changes in the size and dispersion of the CsO x nanostructures, and there was a clear reconstruction of the copper oxide substrate, which then exhibited long range order with a hexagonally packed structure. The as-deposited and annealed surfaces of CsO x /Cu 2 O/Cu(111) were more reactive toward CO 2 than plain Cu 2 O/Cu(111) or clean Cu(111). Yet, none of them were stable in the presence of H 2 , which fully reduced the copper oxide at 400–450 K. In CsO x /Cu(111), the CsO x nanoclusters were dispersed all over the metallic copper in no particular order. The CsO x species had an average width of 2 nm and ~1 Å height. The CsO x /Cu(111) systems exhibited the highest activity for the binding and dissociation of CO 2 , indicating that the CsO x -copper interface plays a key role in alcohol synthesis.

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

Gradient Li-rich oxide cathode particles immunized against oxygen release by a molten salt treatment

Lithium-rich transition metal oxide (Li 1+X M 1–X O 2 ) cathodes have high energy density above 900 Wh kg –1 due to hybrid anion- and cation-redox (HACR) contributions, but critical issues such as oxygen release and voltage decay during cycling have prevented their application for years. Here we show that a molten molybdate-assisted LiO extraction at 700 °C creates lattice-coherent but depth (r)-dependent Li 1+X(r) M 1–X(r) O 2 particles with a Li-rich (X ≈ 0.2) interior, a Li-poor (X ≈ –0.05) surface and a continuous gradient in between. The gradient Li-rich single crystals eliminate the oxygen release to the electrolyte and, importantly, still allow stable oxygen redox contributions within. Here, both the metal valence states and the crystal structure are well maintained during cycling. The gradient HACR cathode displays a specific density of 843 Wh kg –1 after 200 cycles at 0.2C and 808 Wh kg –1 after 100 cycles at 1C, with very little oxygen release and consumption of electrolyte. This high-temperature immunization treatment can be generalized to leach other elements to avoid unexpected surface reactions in batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ultrafine CoO nanoparticles as an efficient cocatalyst for enhanced photocatalytic hydrogen evolution

In order to further enhance the performance of photocatalysts, cocatalysts are used to accelerate the photocatalytic reactions. Herein, ultrafine cobalt oxide (CoO) nanoparticles are synthesized through a novel bottom-up strategy and explored as an efficient non-noble cocatalyst to dramatically promote the photocatalytic hydrogen evolution rate of CdS nanorods. CdS/CoO heterostructures, consisting of highly dispersed 3–5 nm CoO nanoparticles anchored on the CdS nanorods, can provide a high photocatalytic hydrogen evolution rate of 6.45 mmol g -1 h -1 (~36 times higher than that of bare CdS nanorods) in the visible-light region (>420 nm). Combined X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy analyses suggest Co–S bond formation between CoO and CdS, which guarantees efficient migration and separation of photogenerated charge carriers. This work provides a new avenue for adopting CoO as an effective cocatalyst for enhanced photocatalytic hydrogen production in the visible-light region.

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

Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au-Supported Fe 2 O 3 Electrocatalysts

Transition metal oxides have gained attention as promising oxygen evolution reaction (OER) electrocatalysts in alkaline electrolytes, but heterogeneities in typical catalyst samples often obscure key structure–property relationships that are essential for developing higher performance materials. Here, we have combined ultrahigh vacuum surface science techniques, electrochemical measurements, and density functional theory (DFT) to quantify structure-dependent OER activity in a series of well-defined electrocatalysts. We describe a direct correlation between the population of Fe edge-site atoms and the OER activity of ultrathin Fe 2 O 3 nanostructures (~0.5 nm apparent height) grown on Au(111) substrates. Hydroxylated Fe atoms residing at edge-sites along the catalyst/support interface were spectroscopically identified as key reaction centers, and these Fe edge-site atoms were estimated to produce OER turnover frequencies approximately 150 times higher than that of Fe atoms on the catalyst surface at an applied potential of 1.8 V vs the reversible hydrogen electrode. Impressively, ultrathin Fe 2 O 3 /Au nanostructures with a high density of catalytically active Fe edge-site atoms outperformed an ultrathin IrOx/Au catalyst at moderate overpotentials. DFT calculations revealed more favorable OER at edge sites along the Fe 2 O 3 /Au interface, with lower predicted overpotentials due to beneficial modification of intermediate binding. Our results demonstrate how a combination of surface science, electrochemistry, and computational modeling can be used to identify key structure–property relationships in a well-defined electrocatalytic system.

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

Synthesis and Characterization of a Molecularly Designed High-Performance Organodisulfide as Cathode Material for Lithium Batteries

An innovative organodisulfide compound, 2,3,4,6,8,9,10,12–Octathia biscyclopenta[b,c]–5,11–anthraquinone–1,7–dithione (TPQD), has been successfully designed, synthesized, and characterized as a cathode material for lithium batteries. A benzoquinone is introduced to coordinate with dithiolane through 1,4–dithianes. The molecular structure, electrochemical performances, and the lithiation/delithiation mechanism of the TPQD cathode have been systematically investigated. TPQD can deliver an initial capacity of 251.7 mAh g -1 at a rate of C/10, which corresponds to the transfer of 4.7 electrons per formula. Highly reversible capacities and stable cyclic performances can be achieved at rates from C/10 to 5 C. Very interestingly, TPQD can retain a capacity of 120 mAh g -1 after 200 cycles at the 5 C rate, which is quite impressive for organodisulfide compounds. X–ray absorption spectroscopy measurements and density functional theory calculation results suggest that such a high capacity is contributed by both O redox of the quinone group and the cleavage and recombination of the disulfide bond. Moreover, the extended π–conjugation structure of the material, introduced by benzoquinone and dithiane, is beneficial for improving the high rate capability and cyclic stability. Furthermore this study illustrates an innovative approach in designing new organodisulfide compounds with improved cyclability and rate capability as cathode materials for high performance lithium batteries.

25 ENERGY STORAGE↗
DOE OSTI2018

Deconvolution of octahedral Pt 3 Ni nanoparticle growth pathway from in situ characterizations

Understanding the growth pathway of faceted alloy nanoparticles at the atomic level is crucial to morphology control and property tuning. Yet, it remains a challenge due to complexity of the growth process and technical limits of modern characterization tools. We report a combinational use of multiple cutting-edge in situ techniques to study the growth process of octahedral Pt3Ni nanoparticles, which reveal the particle growth and facet formation mechanisms. Our studies confirm the formation of octahedral Pt 3 Ni initiates from Pt nuclei generation, which is followed by continuous Pt reduction that simultaneously catalyzes Ni reduction, resulting in mixed alloy formation with moderate elemental segregation. Furthermore, carbon monoxide molecules serve as a facet formation modulator and induce Ni segregation to the surface, which inhibits the (111) facet growth and causes the particle shape to evolve from a spherical cluster to an octahedron as the (001) facet continues to grow.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Subtle and reversible interactions of ambient pressure H 2 with Pt/Cu(111) single-atom alloy surfaces

Pt-Cu single-atom alloys (SAA) have been identified as promising hydrogenation catalysts with an ability to activate H 2 better than the individual metals alone. We studied the interaction between H 2 and Pt/Cu(111) SAA model surface using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). A binding energy shift in the Pt 4f spectrum indicates the presence of adsorbed H at 350 K, occurring because of HeH bond activation. No evidence was found of either surface segregation, subsurface diffusion of Pt atoms, or any effects on the thermal stability of the surface, likely due to the subtle interaction between Pt and H. This shift is reversible upon evacuation of H 2 , which is a sign of a weak interaction. In addition, we studied the effect of H 2 and CO coadsorption and found that while the surface is unaffected by poisoning by a small amount of CO at 350 K, CO can displace adsorbed H on almost half of the surface Pt sites at 300 K. The susceptibility to CO poisoning increases after the surface was heated to 450 K due to changes in the structure of the subsurface layer that enhance the binding of CO to the surface Pt sites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Strongly correlated perovskite lithium ion shuttles

Solid-state ion shuttles are of broad interest in electrochemical devices, nonvolatile memory, neuromorphic computing, and biomimicry utilizing synthetic membranes. Traditional design approaches are primarily based on substitutional doping of dissimilar valent cations in a solid lattice, which has inherent limits on dopant concentration and thereby ionic conductivity. Here, we demonstrate perovskite nickelates as Li-ion shuttles with simultaneous suppression of electronic transport via Mott transition. Electrochemically lithiated SmNiO 3 (Li-SNO) contains a large amount of mobile Li + located in interstitial sites of the perovskite approaching one dopant ion per unit cell. A significant lattice expansion associated with interstitial doping allows for fast Li + conduction with reduced activation energy. We further present a generalization of this approach with results on other rare-earth perovskite nickelates as well as dopants such as Na + . The results highlight the potential of quantum materials and emergent physics in design of ion conductors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structure, Chemistry, and Charge Transfer Resistance of the Interface between Li 7 La 3 Zr 2 O 12 Electrolyte and LiCoO 2 Cathode

All-solid-state batteries promise significant safety and energy density advantages over liquid-electrolyte batteries. The interface between the cathode and the solid electrolyte is an important contributor to charge transfer resistance. Strong bonding of solid oxide electrolytes and cathodes requires sintering at elevated temperatures. Knowledge of the temperature dependence of the composition and charge transfer properties of this interface is important for determining the ideal sintering conditions. To understand the interfacial decomposition processes and their onset temperatures, model systems of LiCoO 2 (LCO) thin films deposited on cubic Al-doped Li 7 La 3 Zr 2 O 12 (LLZO) pellets were studied as a function of temperature using interface-sensitive techniques. X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and energy-dispersive X-ray spectroscopy data indicated significant cation interdiffusion and structural changes starting at temperatures as low as 300 °C. La 2 Zr 2 O 7 and Li 2 CO 3 were identified as decomposition products after annealing at 500 °C by synchrotron X-ray diffraction. X-ray absorption spectroscopy results indicate the presence of also LaCoO 3 in addition to La 2 Zr 2 O 7 and Li 2 CO 3 . On the basis of electrochemical impedance spectroscopy and depth profiling of the Li distribution upon potentiostatic hold experiments on symmetric LCO|LLZO|LCO cells, the interfaces exhibited significantly increased impedance, up to 8 times that of the as-deposited samples after annealing at 500 °C. Here, our results indicate that lower-temperature processing conditions, shorter annealing time scales, and CO 2 -free environments are desirable for obtaining ceramic cathode|electrolyte interfaces that enable fast Li transfer and high capacity.

25 ENERGY STORAGE↗
DOE OSTI2018

Enhancement in Oxygen Reduction Reaction Activity of Nitrogen-Doped Carbon Nanostructures in Acidic Media through Chloride-Ion Exposure

Nitrogen-doped carbon nanostructures (CNx) are promising cathode materials as catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. Incorporation of chlorine into CN x catalysts using a facile methodology can lead to a significant improvement in the ORR activity in acidic media, as confirmed by electrochemical half-cell measurements. The chlorine-containing CN x catalyst (CN x -Cl) is synthesized by soaking CN x powder in 0.3 M HCl. The analysis of near-edge X-ray absorption fine structure spectra collected in the C K-edge region and Fourier-transform infrared spectra confirm the formation of C-Cl bonds in CN x -Cl. X-ray photoelectron spectroscopy (XPS) results reveal the presence of three distinct chlorine species in the CN x -Cl sample: (i) organic chlorine (C-Cl), (ii) anionic chloride in the positively charged environment of a pyridinium ring (N + Cl - ), and (iii) physisorbed ionic chloride. Results from temperature-programmed desorption studies under inert atmosphere corroborate the conclusions from XPS depth profiling analysis. The improvement in ORR activity after exposure of the CN x catalyst to chloride anions can be attributed to the creation of C-Cl functionalities as additional active sites. The difference in the electronegativity of C and Cl atoms results in a net positive charge on adjacent carbon sites, leading to the side-on adsorption of oxygen molecules and breakage of the O-O bond during ORR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Enhanced Stability of Pt-Cu Single-Atom Alloy Catalysts: In Situ Characterization of the Pt/Cu(111) Surface in an Ambient Pressure of CO

The interaction between a catalyst and reactants often induce changes in the surface structure and composition of the catalyst, which, in turn, affect its reactivity. Therefore, it is important to study such changes using in situ techniques under well-controlled conditions. We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to study the surface stability of a Pt/Cu(111) single atom alloy (SAA) in an ambient pressure of CO. By directly probing the Pt atoms, we found that CO causes a slight surface segregation of Pt atoms at room temperature. In addition, while the Pt/Cu(111) surface demonstrates poor thermal stability in UHV, where surface Pt starts to diffuse to the subsurface layer above 400 K, the presence of adsorbed CO enhances the thermal stability of surface Pt atoms. Furthermore, we also found that temperatures above 450 K cause a restructuring of the subsurface layer, which consequently strengthens the CO binding to the surface Pt sites, likely due to the presence of neighboring subsurface Pt atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Hydrogenation of CO 2 on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal–Oxide Interface in Methanol Synthesis

The results of kinetic tests and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the important role played by a ZnO–copper interface in the generation of CO and the synthesis of methanol from CO 2 hydrogenation. The deposition of nanoparticles of ZnO on Cu(100) and Cu(111), θ oxi < 0.3 monolayer, produces highly active catalysts. The catalytic activity of these systems increases in the sequence: Cu(111) < Cu(100) < ZnO/Cu(111) < ZnO/Cu(100). The structure of the copper substrate influences the catalytic performance of a ZnO–copper interface. Furthermore, size and metal–oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO–copper interface favors the binding and conversion of CO 2 into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500–600 K) used for the CO 2 hydrogenation reaction. However, reaction with CO 2 oxidized the zinc, enhancing its stability over the copper substrates.

36 MATERIALS SCIENCE↗