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ABSTRACT: We have synthesized a new metastable metal hydride with promising hydrogen storage properties. Body centered cubic (bcc) magnesium niobium hydride (Mg(0.75)Nb(0.25))H(2) possesses 4.5 wt% hydrogen gravimetric density, with 4 wt% being reversible. Volumetric hydrogen absorption measurements yield an enthalpy of hydride formation of -53 kJ mol(-1) H(2), which indicates a significant thermodynamic destabilization relative to the baseline -77 kJ mol(-1) H(2) for rutile MgH(2). The hydrogenation cycling kinetics are remarkable. At room temperature and 1 bar hydrogen it takes 30 minutes to absorb a 1.5 μm thick film at sorption cycle 1, and 1 minute at cycle 5. Reversible desorption is achieved in about 60 minutes at 175 °C. Using ab initio calculations we have examined the thermodynamic stability of metallic alloys with hexagonal close packed (hcp) versus bcc crystal structure. Moreover we have analyzed the formation energies of the alloy hydrides that are bcc, rutile or fluorite.
Physical Chemistry Chemical Physics 07/2012; 14(31):10904-9. · 3.57 Impact Factor
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Li Zhang,
Chris M. B. Holt,
Erik J. Luber,
Brian C. Olsen,
Huatao Wang,
Mohsen Danaie,
Xinwei Cui,
XueHai Tan,
Vicki W. Lui,
W. Peter Kalisvaart, David Mitlin
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ABSTRACT: A simple methodology is developed to directly synthesize three-dimensional (3D) electrochemically supercapacitive arrays, consisting of multiwalled carbon nanotubes conformally covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or Inconel electrodes. These nanostructures demonstrate a respectable specific capacitance of 289 F g–1, which is achieved in 1 M KOH electrolyte at a scan rate of 20 mV s–1. The well-connected highly electrically conductive structures exhibit a superb rate capability; at a very high scan rate of 1000 mV s–1 there is less than a 20% drop in the capacitance relative to 20 mV s–1. Such rate capability has never been reported for VN and is highly unusual for any other oxide or nitride. These 3D arrays also display nearly ideal triangular voltage profiles during constant current charge–discharge cycling. Analysis of the post-electrochemically cycled samples indicates negligible changes occurring in the VN nanocrystallite morphology, but a modification in the structure of the oxidized surface. We envision that the direct synthesis approach employed in this study may serve as a “drop-in” platform for large-scale commercial fabrication of a variety of carbon nanotube-supported functional materials that require excellent electrical conductivity to the underlying support.
11/2011;
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ABSTRACT: A hydrothermal process was employed to create a variety of Co3O4 nanostructures. We demonstrate that nominally minor differences in the synthesis temperature (50, 70°, or 90 °C) yield profound variations in the oxide microstructure, with lath-like, necklace-like and net-like morphologies of different scales resulting. This in turn resulted in significant variations in the supercapacitive performance that ranged from mediocre to superb. Specifically, the mesoporous net-like Co3O4 nanostructures that were synthesized at 50 °C exhibited very favorable electrochemical properties: The net-like Co3O4 had a specific capacitance of 1090 F/g at a mass loading of 1.4 mg/cm2. At this high mass loading, such performance has not been previously reported. SEM and TEM analysis of these samples revealed an interconnected array of sub-10 nm crystallites interspersed with a high volume fraction of similar scale pores. The poorer performing microstructures were both coarser and much less porous.
08/2011;
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ABSTRACT: We present a neutron reflectometry study of deuterium absorption in thin films of Al-containing Mg alloys capped with a Ta/Pd, Ni/Pd and Ti/Pd-catalyst bilayer. The measurements were performed at room temperature over the 0–1 bar pressure range under quasi-equilibrium conditions. The modeling of the measurements provided a nanoscale representation of the deuterium profile in the layers at different stages of the absorption process. The absorption mechanism observed was found to involve spillover of atomic deuterium from the catalyst layer to the Mg alloy phase, followed by the deuteration of the Mg alloy. Complete deuteration of the Mg alloy occurs in a pressure range between 100 and 500 mbar, dependent on the type of bilayercatalyst. The use of a Ti/Pd bilayercatalyst yielded the best results in terms of both storage density and kinetic properties.
Journal of Alloys and Compounds 05/2011; 509(18):5466–5471. · 2.29 Impact Factor
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ABSTRACT: The effect of chromium and vanadium alloying on the hydrogenation of a magnesium thin film is studied by neutron reflectometry. Immediate formation of a blocking MgD(2) layer is observed in pure Mg, however in the alloyed film deuteration is rapid and almost completely homogeneous.
Chemical Communications 03/2011; 47(14):4294-6. · 6.17 Impact Factor
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ABSTRACT: We report that surface templated and supported palladium nanoparticles self assemble on ETS-10 type molecular sieve surfaces by simple exchange and activation procedures in the absence of a reductant. This procedure is similar to the one previously reported for silver nanoparticle self assembly on ETS-10. We observed a bimodal distribution with particle sizes ranging from 2-5 and 15-30 nm. This simple, economical method generates high concentrations (approximately 12 wt% of total composite) of uniform, metallic palladium nanoparticles that are multiply twinned and thermally stable making them potentially unique for advanced catalytic and electronic applications.
Journal of Nanoscience and Nanotechnology 03/2011; 11(3):2537-9. · 1.56 Impact Factor
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Philosophical Magazine. 01/2011; 91(25):3393-3405.
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ABSTRACT: Titanium based molecular sieves, such as ETS-10, have the ability to exchange silver ions and subsequently support self assembly of stable silver nanoparticles when heated. We report that a high surface area sodium titanate (resembling ETS-2) displays a similar ability to self template silver nanoparticles on its surface. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show high concentrations of silver nanoparticles on the surface of this sodium titanate, formed by thermal reduction of exchanged silver cations. The nanoparticles range in size from 4 to 12 nm, centered at around 6 nm. In addition to SEM and TEM, XRD and surface area analysis were used to characterize the material. The results indicate that this sodium titanate has a high surface area (>263 m2/g), and high ion exchange capacity for silver (30+ wt%) making it an excellent substrate for the exchange and generation of uniform, high-density silver nanoparticles.
Journal of Nanoscience and Nanotechnology 12/2010; 10(12):8448-51. · 1.56 Impact Factor
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ABSTRACT: We examine hydrogen sorption cycling of 1.5 μm thick magnesium thin films containing a bimetallic chromium titanium catalyst. At 200 °C these nanocomposites absorb 5 wt % hydrogen in several seconds, and desorb in 10–20 minutes. In several compositions, there is negligible hydrogenation kinetics or capacity degradation even at over 100 cycles. Equally importantly, the ternary films require minimal activation, achieving rapid magnesium hydride formation and decomposition from cycle one. Pressure-composition isotherms display well-known enthalpies of MgH2. Transmission electron microscopy analysis supports a hypothesis that such extreme kinetics is due to the presence of a nanodispersed Cr Ti phase in Mg matrix.
Applied Physics Letters 08/2010; 97(8):083106-083106-3. · 3.84 Impact Factor
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ABSTRACT: We have examined comilling with unpurified single-walled carbon nanotubes (SWCNTs) as a method to promote hydrogenation/dehydrogenation cycling kinetic stability in nanocrystalline magnesium hydride (MgH2). The synthesized material was a true nanocomposite consisting of MgH2 covered by highly defective SWCNTs coupled to catalytic metal nanoparticles and mixed with amorphous carbon. The nanocomposite was hydrogen sorption cycled at 300 °C using a volumetric Sievert’s type apparatus. Identically milled pure MgH2 was used as a baseline. The microstructure of both materials was analyzed in detail using cryo-stage transmission electron microscopy (TEM) as well as other techniques. The nanocomposite shows markedly improved kinetic performance, both during initial postmilling desorption and during subsequent cycling. Activation energy analysis demonstrates that any catalytic effect due to the metallic nanoparticles is lost during cycling. Improved cycling performance is instead achieved as a result of the carbon allotropes preventing MgH2 particle agglomeration and sintering. Even after 35 absorption/desorption cycles, the SWCNTs remain covering the MgH2 surfaces. Sorption cycling creates a dramatic difference in the particle size distributions between the nanocomposite system and the baseline, whereas the two were nearly identical at the onset of testing. In a separate experiment performed at more aggressive pressure conditions, the nanocomposite received over 100 sorption cycles with fairly minor kinetic degradation.
01/2010;
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ABSTRACT: We examined hydrogen sorption in 1.5 μm thick Mg–Fe–V films, using the binary alloys as baselines. At 200 °C both Mg–V and Mg–Fe–V absorb in tens of seconds, and desorb in tens of minutes. The ternary alloys show minimal kinetic or capacity degradation even after 105 absorption/desorption cycles. Pressure—composition isotherms yield the well-known enthalpies of α-MgH2 formation (decomposition), agreeing with x-ray diffraction results. The x-ray spectrum also shows a broad hump centered near (011) reflection of CsCl-type Fe–V phase. Our hypothesis is that a densely distributed nanoscale Fe–V acts both as a potent hydrogen dissociation catalyst and a heterogeneous nucleation site.
Applied Physics Letters 01/2010; 96(1):013108-013108-3. · 3.84 Impact Factor
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ABSTRACT: We describe an analytic model for polycrystalline thin film growth based on competition between grains with conic geometries. This model is valid for all film thicknesses and is verified using level set simulations in 2+1 dimensions. We study the effects of nonuniform initial grain distributions on growth statistics. These results provide a possible explanation for discrepancies between experimentally measured and theoretical scaling laws.
Physical Review E 01/2010; 81(1 Pt 1):011601. · 2.26 Impact Factor
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ABSTRACT: This study focused on hydrogen sorption properties of 1.5 μm thick Mg–10 at. % Fe–10 Ti, Mg–15 at. % Fe–15 Ti, and Mg–20 at. % Fe–20 Ti films. We show that the alloys display remarkable sorption behavior: At 200 °C the films are capable of absorbing nearly 5 wt % hydrogen in seconds and desorbing in minutes. Furthermore this sorption behavior is stable over cycling. In the Mg–15 at. % Fe–15 Ti alloy there is no kinetic or capacity degradation even after 100 absorption/desorption cycles. Pressure–composition isotherm data for Mg–10 at. % Fe–10 Ti indicates that the sorption enhancement is due to improved kinetics rather than any altered thermodynamics. We envision these alloys as becoming the material of choice for a variety of sensing and storage applications.
Applied Physics Letters 09/2009; 95(10):103114-103114-3. · 3.84 Impact Factor
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ABSTRACT: We used a nanoscale (5 nm Ta/5 nm Pd) bilayer catalyst to achieve remarkable desorption kinetics for thin films. Full hydrogen desorption occurred at 100 °C with a noticeable desorption even at room temperature. This is a significant improvement relative to the 175 °C needed to fully desorb an identical film with a single Pd layer acting as the catalyst. Neutron reflectometry confirmed that the Ta/Pd bilayer remained intact both after hydrogen absorption and following the hydrogen desorption. We used x-ray diffraction analysis to gather complementary information regarding the crystal structure of the as-synthesized, sorbed and desorbed film.
Applied Physics Letters 06/2009; 94(24):241901-241901-3. · 3.84 Impact Factor
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ABSTRACT: We have examined the effect of single-walled carbon nanotube (SWCNT)-metallic nanoparticle additions on the hydrogen desorption behavior of MgH(2) after high-energy co-milling. The metallic nanoparticles were the catalysts used for the SWCNT growth. The co-milling consisted of high-energy planetary milling in an inert argon environment of the hydride powder mixed with the SWCNTs. Identically milled pure MgH(2) powders were used as a baseline. The composites were tested using a combined differential scanning calorimeter and thermogravimetric analyzer, while the microstructures were examined using a variety of techniques including x-ray diffraction and transmission electron microscopy (TEM). We found that the SWCNT-nanoparticle additions do have an influence on the desorption kinetics. However, the degree to which they are effective depends on the composite's final state. The optimum microstructure for sorption, obtained after 1 h of co-milling, consists of highly defective SWCNTs in intimate contact with metallic nanoparticles and with the hydride. This microstructure is optimum, presumably because of the dense and uniform coverage of the defective SWCNTs on the MgH(2) surface. Prolonged co-milling of 7 h destroys the SWCNT structure and reduces the enhancement. Even after 72 h of co-milling, when the SWCNTs are completely destroyed, the metallic nanoparticles remain dispersed on the hydride surfaces. This indicates that the metallic nanoparticles alone are not responsible for the enhanced sorption and that there is indeed something catalytically unique about a defective SWCNT-metal combination. Cryo-stage TEM analysis of the hydride powders revealed that they are nanocrystalline and in some cases multiply twinned. To our knowledge this is the first study where the structure of milled alpha- MgH(2) has been directly imaged. Since defects are an integral component of hydride-to-metal phase transformations, such analysis sheds new insight regarding the fundamental microstructural origins of the sorption enhancement due to mechanical milling.
Nanotechnology 06/2009; 20(20):204016. · 3.98 Impact Factor
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ABSTRACT: Deuterium absorption in Mg70Al30 thin films coated with a Pd layer and a Ta/Pd bilayer were investigated using in situ neutron reflectometry at room temperature and deuterium pressures up to 1.3 bar. The approach used provides a detailed profile, at the nanoscale, of the deuterium content inside the specific layers that constitute the films. It is found that Mg70Al30 can store up to 5 wt.% under these mild conditions following a two-step mechanism. The latter involves the deuteration of the top and bottom catalyst layers first, followed by the main Mg70Al30 layer. The presence of deuterium throughout the films in the early absorption stages evidences atomic deuterium spillover from the catalyst layers. The addition of a Ta layer between the Pd and Mg70Al30 was found to allow observable absorption at a pressure 10 times lower than on the Ta-free sample, without affecting the storage capacity. Our measurements imply that this improvement in kinetics is due to a lowering of the nucleation barrier for the formation of the hydride phase in the Mg70Al30 layer.
International Journal of Hydrogen Energy.
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ABSTRACT: We studied the catalytic effects of Titanium, Iron and FeTi intermetallic on the desorption kinetics of magnesium hydride. In order to separate the catalytic effects of each element from additional synergistic and alloying effects, Mg–Ti and Mg–Fe mixtures were studied as a baseline for Mg–Fe–Ti elemental and Mg–(FeTi) intermetallic composites. Sub-micron dimensions for MgH2 particles and excellent nanoscale catalyst dispersion was achieved by high-energy ball-milling as confirmed by analytical electron microscopy techniques. The composites containing Fe shows desorption temperature of 170 K lower than as-received MgH2 powder, which makes it suitable to be cycled at relatively low temperature of 523 K. Furthermore, the low cycling temperature prevents the formation of Mg2FeH6. In sorption cycling tests, Mg-10% Ti and Mg-10% (FeTi), after about 5 activation cycles, show fast desorption kinetics initially, but the kinetics also degrade faster than for all other composites, eventually slowing down by a factor of 7 and 4, respectively. The ternary Mg–Fe–Ti composite shows best performance. With the highest BET surface area of 40 m2/g, it also shows much less degradation during cycling. This is attributed to titanium hydride acting as a size control agent preventing agglomeration of particles; while Fe works as a very strong catalyst with uniform and nanoscale dispersion on the surface of MgH2 particles.
International Journal of Hydrogen Energy 36(11):6711-6722. · 4.05 Impact Factor
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ABSTRACT: We have tested the hydrogen storage cycling behavior of bulk centimeter-scale magnesium – titanium and magnesium – stainless steel multilayer composites synthesized by accumulative roll-bonding (ARB). Roll-bonding of either titanium or stainless steel with magnesium allows the reversible hydrogen sorption of the resulting composite at 350 °C. Identically roll-bonded pure magnesium can hardly be absorbed at this temperature. In the composites, the kinetics of the first cycle of absorption (also called “activation”) improves with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Increasing the loading amount of the second phase also accelerates the kinetics. This holds true up to a threshold limit. Microscopy analysis performed on 1–2 wt.% hydrogen absorbed composites demonstrates that MgH2 formed exclusively on various heterogeneous nucleation sites. During activation, MgH2 nucleation occurred at the Mg-hard phase interfaces. During the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of “internal” free surfaces such as cracks.Research highlights► We prepared Mg–Ti & Mg–SS composites by accumulative roll-bonding at room conditions. ► These composites can reversibly absorb hydrogen at 350 °C. ► The hydride forming phase in both cases is MgH2. ► Kinetics of the first absorption cycle is enhanced by addition of both Ti & SS. ► Heterogeneous nucleation of MgH2 during the first cycle occurs close to Ti/SS.
International Journal of Hydrogen Energy 36(4):3022-3036. · 4.05 Impact Factor
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ABSTRACT: In this study we report on the solid-state dewetting of ultrathin Ni films on amorphous SiO2. The dewetting process is monitored in situ using time resolved differential reflectometry (TRDR). The time resolved differential reflectivity signal during dewetting is found to exhibit a rich behavior, which is intimately connected the changes in morphology. Finite-difference time-domain simulation is used to explain the observed reflectivity data, where experimentally acquired atomic force microscope heightmaps are used as simulation inputs. From ex situ atomic force microscope heightmaps, the sequential processes of grain growth, grain boundary grooving, hole growth, and particle coarsening are observed. Grain growth of ultrathin films prior to dewetting is critically important in determining the particle density, which has been largely unexplored in previous dewetting studies. Kinetic analysis of the TRDR data revealed two rate-limiting processes, with activation energies of 0.31±0.04 and 0.59±0.06 eV. We hypothesize that these kinetic pathways correspond to Ni grain growth and surface mass self-diffusion on the Ni(111) planes, respectively.
Phys. Rev. B. 82(8).
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ABSTRACT: We used electrochemical testing and theoretical calculations based on density functional theory (DFT) to examine the oxygen reduction reaction (ORR) activity of platinum electrocatalyst supported on several forms of niobium oxide. Bilayer electrocatalysts were synthesized in the form of 5 nm thick Pt layers (ca. 0.01 mg/cm2), deposited on 5 or 10 nm thick niobium oxide and backed by glassy carbon (GC) electrodes. The NbO and NbO2 supports enhance the specific electrochemical activity of Pt relative to the identically synthesized baseline system of Pt on GC but have no positive effect on the mass activity. The electrochemical stability of the Pt/NbO2 bilayer system was investigated by potential cycling with up to 2500 cyclic voltammetry (CV) cycles. After 2500 cycles, data indicates minimal electrochemical area loss. With the use of DFT calculations, we have evaluated effects of oxygen incorporation on stability, electronic structure, and electrochemical activity of Pt|NbxOy systems. Calculations predict a transfer of electronic charge density from Nb, NbO, and NbO2 to Pt and a reverse case for Nb2O5. However, the experimental ORR activity does not follow the trends predicted by the d-band model. yes yes
