Timothy A. Strobel

Ultrahigh volumetric capacitance and cyclic stability of fluorine and nitrogen co-doped carbon microspheres

Highly porous nanostructures with large surface areas are typically employed for electrical double-layer capacitors to improve gravimetric energy storage capacity; however, high surface area carbon-based electrodes result in poor volumetric capacitance because of the low packing density of porous materials. Here, we demonstrate ultrahigh volumetric capacitance of 521 F cm−3 in aqueous electrolytes for non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen synthesized by low-temperature solvothermal route, rivaling expensive RuO2 or MnO2 pseudo-capacitors. The new electrodes also exhibit excellent cyclic stability without capacitance loss after 10,000 cycles in both acidic and basic electrolytes at a high charge current of 5 A g−1. This work provides a new approach for designing high-performance electrodes with exceptional volumetric capacitance with high mass loadings and charge rates for long-lived electrochemical energy storage systems.

Raman spectroscopic studies of hydrogen clathrate hydrates

Raman spectroscopic measurements of simple hydrogen and tetrahydrofuran+hydrogen sII clathrate hydrates have been performed. Both the roton and vibron bands illuminate interesting quantum dynamics of enclathrated H(2) molecules. The complex vibron region of the Raman spectrum has been interpreted by observing the change in population of these bands with temperature, measuring the absolute H(2) content as a function of pressure, and with D(2) isotopic substitution. Quadruple occupancy of the large sII clathrate cavity shows the highest H(2) vibrational frequency, followed by triple and double occupancies. Singly occupied small cavities display the lowest vibrational frequency. The vibrational frequencies of H(2) within all cavity environments are redshifted from the free gas phase value. At 76 K, the progression from ortho- to para-H(2) occurs over a relatively slow time period (days). The rotational degeneracy of H(2) molecules within the clathrate cavities is lifted, observed directly in splitting of the para-H(2) roton band. Raman spectra from H(2) and D(2) hydrates suggest that the occupancy patterns between the two hydrates are analogous, increasing confidence that D(2) is a suitable substitute for H(2). The measurements suggest that Raman is an effective and convenient method to determine the relative occupancy of hydrogen molecules in different clathrate cavities.

Water cavities of sH clathrate hydrate stabilized by molecular hydrogen.

X-ray diffraction and Raman spectroscopic measurements confirm that molecular hydrogen can be contained within the small water cavities of a binary sH clathrate hydrate using large guest molecules that stabilize the large cavity. The potential increase in hydrogen storage could be more than 40% when compared with binary sII hydrates. This work demonstrates the stabilization of hydrogen in a hydrate structure previously unknown for encapsulating molecular hydrogen, indicating the potential for other inclusion compound materials with even greater hydrogen storage capabilities.

Tetra-n-butylammonium Borohydride Semiclathrate: A Hybrid Material for Hydrogen Storage

In this study, we demonstrate that tetra-n-butylammonium borohydride [(n-C(4)H(9))(4)NBH(4)] can be used to form a hybrid hydrogen storage material. Powder X-ray diffraction measurements verify the formation of tetra-n-butylammonium borohydride semiclathrate, while Raman spectroscopic and direct gas release measurements confirm the storage of molecular hydrogen within the vacant cavities. Subsequent to clathrate decomposition and the release of physically bound H(2), additional hydrogen was produced from the hybrid system via a hydrolysis reaction between the water host molecules and the incorporated BH(4)(-) anions. The additional hydrogen produced from the hydrolysis reaction resulted in a 170% increase in the gravimetric hydrogen storage capacity, or 27% greater storage than fully occupied THF + H(2) hydrate. The decomposition temperature of tetra-n-butylammonium borohydride semiclathrate was measured at 5.7 degrees C, which is higher than that for pure THF hydrate (4.4 degrees C). The present results reveal that the BH(4)(-) anion is capable of stabilizing tetraalkylammonium hydrates.

Raman Studies of Methane−Ethane Hydrate Metastability

The interconversion of methane-ethane hydrate from metastable to stable structures was studied using Raman spectroscopy. sI and sII hydrates were synthesized from methane-ethane gas mixtures of 65% or 93% methane in ethane and water, both with and without the kinetic hydrate inhibitor, poly(N-vinylcaprolactam). The observed faster structural conversion rate in the higher methane concentration atmosphere can be explained in terms of the differences in driving force (difference in chemical potential of water in sI and sII hydrates) and kinetics (mass transfer of gas and water rearrangement). The kinetic hydrate inhibitor increased the conversion rate at 65% methane in ethane (sI is thermodynamically stable) but retards the rate at 93% methane in ethane (sII is thermodynamically stable), implying there is a complex interaction between the polymer, water, and hydrate guests at crystal surfaces.

Chemical−Clathrate Hybrid Hydrogen Storage: Storage in Both Guest and Host

Hydrogen storage from two independent sources of the same material represents a novel approach to the hydrogen storage problem, yielding storage capacities greater than either of the individual constituents. Here we report a novel hydrogen storage scheme in which recoverable hydrogen is stored molecularly within clathrate cavities as well as chemically in the clathrate host material. X-ray diffraction and Raman spectroscopic measurements confirm the formation of beta-hydroquinone (beta-HQ) clathrate with molecular hydrogen. Hydrogen within the beta-HQ clathrate vibrates at considerably lower frequency than hydrogen in the free gaseous phase and rotates nondegenerately with splitting comparable to the rotational constant. Compared with water-based clathrate hydrate phases, the beta-HQ+H2 clathrate shows remarkable stability over a range of p-T conditions. Subsequent to clathrate decomposition, the host HQ was used to directly power a PEM fuel cell. With one H2 molecule per cavity, 0.61 wt % hydrogen may be stored in the beta-HQ clathrate cavities. When this amount is combined with complete dehydrogenation of the host hydroxyl hydrogens, the maximum hydrogen storage capacity increases nearly 300% to 2.43 wt %.

Investigation of exotic stable calcium carbides using theory and experiment

It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions. We find that Ca5C2, Ca2C, Ca3C2, CaC, Ca2C3, and CaC2 have stability fields on the phase diagram. Among these, Ca2C and Ca2C3 are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance are the base-centered monoclinic phase (space group C2/m) of Ca2C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P21/c) of CaC with zigzag C4 groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca5C2 with semimetallic behaviour.It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions using the USPEX code. We find that Ca5C2, Ca2C, Ca3C2, CaC, Ca2C3 and CaC2 have stability fields on the phase diagram. Among these, Ca2C and Ca2C3 are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance is the base-centred monoclinic phase (space group C2/m) of Ca2C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P21/c) of CaC with zigzag C4 groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca5C2 with semi-metallic behaviour.

Synthesis of Mg2C: A Magnesium Methanide

Carbides, which have been intensively studied for more than half a century, still remain a major center of scientific and technological attention. A large number of new promising phases have been predicted to exhibit exceptional structural and electronic properties, as well as high-temperature superconductivity. In particular, magnesium compounds containing Mg C and C C bonds are quite fascinating from both fundamental science and synthesis perspectives. The properties of such compounds are determined by the nature of the chemical bonds present, allowing a variety of different materials to be suggested, such as ionic semiconductors, superhard sp and/or sp carbon networks intercalated with Mg, 9] and novel polymeric carbides. Furthermore, the intrinsic nature of Mg C chemical bonding is of great importance to polar organometallic compounds and to understanding the covalent/ionic nature of carbanions. The ambient-pressure chemistry of the Mg C system was studied quite thoroughly in the past. Magnesium forms an acetylide-type carbide, MgC2, [13] similar to all other alkalineearth metals. Mg also forms Mg2C3, [14] a derivative of propadiene (H2C=C=CH2), which is unique for the alkalineearth metals and is one of only a handful of examples that contain the rare [C=C=C] group. Herein, we present the formation of a third carbide of magnesium, namely Mg2C. This compound is stabilized at pressures above 15 GPa, but is fully recoverable to ambient conditions and contains the very unusual C methanide anion. 15] Both in situ and ex situ X-ray diffraction experiments revealed the formation of magnesium carbide, Mg2C, directly from a stoichiometric mixture of the elements at pressures between 15–30 GPa and temperatures of 1775–2275 K (Figure 1). Samples were recovered in powder form, which have a brown color, and Rietveld analysis indicates that the compound takes on the antifluorite structure (Li2O) in the cubic crystal system with space group Fm3̄m (No. 225) with lattice parameter a = 5.4480(4) . A comparison of local bonding environments (structural coordination) for Mg2C is presented in Figure 1b. Contrary to Mg2C3 and MgC2, Mg2C does not contain covalent C C bonds. According to our structural data, the ambient-pressure Mg C distance in Mg2C (2.36 ) is larger than the minimal Mg C distances in both Mg2C3 (2.21 ) and MgC2 (2.17 ) and smaller than the Mg C distance in Al2MgC2 (2.487), where Mg has octahedral coordination. Carbon within Mg2C is coordinated eightfold by magnesium, whereas carbon coordination within Mg2C3 and MgC2 is much more sophisticated. If the whole carbon anions are considered as structural units, Mg2C3 and Mg2C have the same coordination number 8, but in the first case they form a distorted and elongated dodecahedron, while in the second case the coordination polyhedron is a regular cube. In MgC2 the C2 dumbbell coordination number is 6 (elongated octahedron). Among the Group 2 elements, beryllium forms the only known methanide-type carbide. Be2C, as well as a second known methanide, Al4C3, are quite hard, low-compressibility compounds with a large degree of covalent bonding character (the ionic/covalent nature is described below). The minority phase synthesis of Li4C was reported previously, but minimal yields (0–10%) have precluded definitive characterization. Although never experimentally observed until now, the isostructural magnesium analogue of Be2C, namely Mg2C, was first suggested by Corkill and Cohen about twenty years Figure 1. a) X-ray diffraction data with MoKa radiation (*), Rietveld refinement (c), and difference (at bottom). Tick marks are shown for Mg2C (top) and MgO impurity (bottom). b) Carbon and magnesium coordination in Mg2C. c) NMR spectrum of Mg2 C (99% of isotope purity).

Vibrational dynamics, intermolecular interactions, and compound formation in GeH4–H2 under pressure

Optical microscopy, spectroscopic and x-ray diffraction studies at high-pressure are used to investigate intermolecular interactions in binary mixtures of germane (GeH(4)) + hydrogen (H(2)). The measurements reveal the formation of a new molecular compound, with the approximate stoichiometry GeH(4)(H(2))(2), when the constituents are compressed above 7.5 GPa. Raman and infrared spectroscopic measurements show multiple H(2) vibrons substantially softened from bulk solid hydrogen. With increasing pressure, the frequencies of several Raman and infrared H(2) vibrons decrease, indicating anomalous attractive interaction for closed-shell, nonpolar molecules. Synchrotron powder x-ray diffraction measurements show that the compound has a structure based on face-centered cubic (fcc) with GeH(4) molecules occupying fcc sites and H(2) molecules likely distributed between O(h) and T(d) sites. Above ca. 17 GPa, GeH(4) molecules in the compound become unstable with respect to decomposition products (Ge + H(2)), however, the compound can be preserved metastably to ca. 27 GPa for time-scales of the order of several hours.

Synthesis of β‑Mg 2 C 3 : A Monoclinic High-Pressure Polymorph of Magnesium Sesquicarbide

A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg2C3), the new monoclinic C2/m structure (β-Mg2C3) contains linear C3(4-) chains that are isoelectronic with CO2. Unlike α-Mg2C3, which contains alternating layers of C3(4-) chains oriented in opposite directions, all C3(4-) chains within β-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of β-Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C═C stretching near 1200 cm(-1) and (13)C resonances confirming the presence of the rare allylenide anion.