- [Show abstract] [Hide abstract] ABSTRACT: A vanadium aryl hydride gel was prepared by thermal decomposition and subsequent hydrogenation of tetraphenyl vanadium and evaluated for electrochemical and hydrogen storage performance. Characterization by IR, XRD, XPS, nitrogen adsorption, and TGA suggests that the material consists predomi-nantly of a mixture of vanadium centers in oxidation states of II-IV bound together by bridging hydride and phenyl groups. Elec-trochemical properties were explored to probe the reversible oxi-dation state behavior and possible applications to Li batteries, with the hypothesis that the low mass of the hydride ligand may lead to superior gravimetric performance relative to heavier vana-dium oxides and phosphates. The material shows reversible redox activity and has a promising peak capacity of 131 mAhg-1, at a discharge rate of 1 mAcm-2, comparable to bulk VO2 samples also tested in this study. After repeated charge discharge cycling for 50 cycles, the material retained 36% of its capacity. The material also shows improved hydrogen storage performance relative to previ-ously synthesized VH3-based gels, reaching a reversible gravimet-ric storage capacity of 5.8 wt% at 130 bar and 25 °C. Based on the measured density, this corresponds to a volumetric capacity of 79.77 kgH2 m-3, demonstrating that the 2017 US DOE system goals of 5.5 wt% and 40 kg H2 m-3 may be achievable upon con-tainment in a Type 1 tank and coupling to a fuel cell.
- [Show abstract] [Hide abstract] ABSTRACT: Reversible hydrogen storage under ambient conditions has been identified as a major bottleneck in enabling a future hydrogen economy. Herein we report an amorphous vanadium(III) alkyl hydride gel that binds hydrogen via the Kubas interaction. The material possesses a gravimetric adsorption capacity of 5.42 wt% H2 at 120 bar and 298 K reversibly at saturation with no loss of capacity after 10 cycles. This corresponds to a volumetric capacity of 75.4 kgH2 m-3. Raman experiments at 100 bar confirmed that Kubas binding is involved in the adsorption mechanism.The material possesses an enthalpy of H2 adsorption of +0.52 kJ mol-1 H2 as measured directly by calorimetry, which is practical for use in a vehicle without a complex heat management system.
- [Show abstract] [Hide abstract] ABSTRACT: This paper describes the synthesis and electrochemical properties of mesoporous titanium oxysulfides prepared through the chemical treatment of pristine mesoporous titanium oxide under various synthesis conditions. The materials were doped with sulfur by using hexamethyldisilathiane (HMDST), a strategy that was developed to improve the conductivity of the material, whilst also retaining the porosity and thermal stability. Varying amounts of HMDST and different synthesis temperatures were tested to optimize the surface area and electrochemical performance. Lower temperatures generally yielded materials with superior properties and, even though the conductivity was improved by using higher loading levels of HMDST, it also led to a drop in initial capacity at the highest synthesis temperature of 200°C (137-41mAhg-1). The best performing material was, thus, synthesized by using the highest level of HMDST (3.5mL) at lower heating temperatures (100-150°C). This set of conditions maximizes the combination of surface area, initial capacity, conductivity, and capacity retention, the latter of which was notably superior to that of the pristine material (81 vs. 35%), emphasizing the overall success of this doping strategy in improving the electrochemical properties of these otherwise insulating materials.
- [Show abstract] [Hide abstract] ABSTRACT: Proton conductivity and thermal durability studies were performed on a series of mesoporous Nb₂O₅ composites with naphthalene sulfonate formaldehyde resin polymerized within the pores. The proximity of the sulfonate groups of the polymer to the walls of the oxide mesostructure was deliberately tailored to ensure superior dehydration resistance crucial to proton conductivity. Initially characterized by nitrogen adsorption, XRD, TGA and STEM, subsequent study using impedance spectroscopy over a temperature range of 20-150 °C established their proton conductivity performance. The most promising sample displayed a conductivity of 21.77 mS/cm at 80 °C surpassing the literature value for Nafion 117 (8 mS/cm) as measured in our labs using the same set up. Subsequent thermal durability tests demonstrated that this composite maintains superior conductivity to Nafion 117 at 80 °C for the length of the study (24 h). These observations were rationalized by in depth solid-state NMR studies.
- [Show abstract] [Hide abstract] ABSTRACT: This paper describes the synthesis and characterisation of high surface area mesoporous Ti and Ta oxides with polypyrrole nanowires in the pores. The incorporation of polymer was used to improve the electron conductivity into the channels inside these high surface area (400-1000 m2/g) materials in order to exploit surface redox sites for possible pseudocapacitive Li storage. Synthesis was achieved using catalyst-free UV initiated polymerisation of vapour-loaded pyrrole monomer. The best materials showed improved conductivity for both the Ti and Ta oxides as well as improved Li capacity (190 mAh/g) relative to the pristine material (128 mAh/g) and superior capacity retention (49% as compared to 22%) for the Ti composites. The retention in surface area was also 87% compared to 49% reported previously for analogous materials synthesized by catalyst-initiated methods, which only yielded Li capacities of 170 mAh/g, further highlighting the superiority of this new photochemical approach.
- [Show abstract] [Hide abstract] ABSTRACT: In this paper we present amorphous chromium(III) hydride gels that show promise as reversible room temperature hydrogen storage materials with potential for exploitation in mobile applications. The material uses hydride ligands as a light weight structural feature to link chromium(III) metal centres together which act as binding sites for further dihydrogen molecules via the Kubas interaction, the mode of hydrogen binding confirmed by high pressure Raman spectroscopy. The best material possesses a reversible gravimetric storage of 5.08 wt% at 160 bar and 25 °C while the volumetric density of 78 kgH2m3 compares favourably to the DOE ultimate system goal of 70 Kg/m3.1 The enthalpy of hydrogen adsorption is + 0.37 kJ mol-1 H2 as measured directly at 40 °C using an isothermal calorimeter coupled directly to a Sieverts gas sorption apparatus. These data support a mechanism confirmed by computations in which the deformation enthalpy required to open up binding sites is almost exactly equal and opposite to the enthalpy of hydrogen binding to the Kubas sites, and suggests that this material can be used in on-board applications without a heat management system.
- [Show abstract] [Hide abstract] ABSTRACT: This paper describes the synthesis and characterisation of high-surface-area mesoporous titanium oxides with polypyrrole nanowires within the pores, and the subsequent variation of synthesis parameters such as polymer-loading level and pore size to improve performance. These modifications are employed to improve the electron conductivity of the amorphous host and exploit the high internal surface areas of over 800 m2g−1 for potential use as a lithium battery cathode material, once fully optimised, with fast charge-transfer kinetics expected from the proximity of the vast majority of the redox sites at, or near, the surface of the inner pore walls. A full structural characterisation, in addition to electrochemical assessments, of the composite materials is presented and compared to the pristine mesoporous titanium oxide hosts. The best synthesis conditions were achieved with 5 % polymer loading and the largest pore sized host materials. Excessive polymer loading and smaller pore sizes lead to decreased performance, possibly due to inhibition of Li+ transport. The C18 templated TiO2 composite produced the best capacity retention at 58 % retention, and the C12 composite produced the highest initial capacity of 170 mAh g−1 by using a current density of 1 mAcm−2.
- [Show abstract] [Hide abstract] ABSTRACT: Proton conductivity in a series of mesoporous niobium and tantalum metal oxide (mX2O5) composites of naphthalene sulfonic acid formaldehyde resin (NSF) that are resistant to moisture loss at temperatures greater than 50 °C is reported. The investigation focuses on the effect to proton conductivity by changing pore size and metal in the mesostructure of the mX2O5 system and thus, a series of mX2O5-NSF composites were synthesized with C6, C12, and C18 templates. These were characterized by XRD, thermogravimetric analysis, nitrogen adsorption, and scanning TEM and then studied using impedance spectroscopy to establish proton conductivity values at various temperatures ranging from 25 to 150 °C. The most promising sample displayed a conductivity of 21.96 mS cm−1 at 100 °C, surpassing the literature value for Nafion 117 (ca. 8 mS cm−1). 1H and 13C solid state NMR studies the mX2O5-NSF composites demonstrate that the oligomeric nature of the NSF is preserved while in contact with the mX2O5 surface, thus facilitating conductivity.
- [Show abstract] [Hide abstract] ABSTRACT: This paper describes the synthesis and characterisation of amine-templated mesoporous titanium oxide with polythiophene nanowires in the pores. These materials were designed to improve the electron conductivity of the ca. 1000 m(2)/g mesostructure in order to exploit the redox sites within the pores. An in depth characterisation of the synthesised composites including an electrochemical assessment of these materials is presented, and compared to the pristine mesoporous titanium oxide. The results demonstrate successful synthesis of conducting nanowires within the pores of the mesoporous titanium oxides by utilising vapour diffusion and subsequent in situ polymerisation, whilst retaining the mesostructure of the titanium oxide host. The mesoporous titanium oxide produced a peak capacity of 301 mAh/g at current densities of 0.2 mA cm(-2). The polythiophene nanowires improve the conductivity of the material with a slight drop in capacity.
- [Show abstract] [Hide abstract] ABSTRACT: In this report we attempt to synthesize materials resistant to dehydration by exploiting the interaction of sulfonate groups with the hydrophilic surfaces of the inner pore walls of mesoporous titanium oxides to form channels for proton conduction. Thus, six mesoporous titanium oxide composites of naphthalene sulfonate formaldehyde (NSF) were synthesised, fully characterised and formed into pellets for potentiostatic impedance measurements. The most promising sample, a NSF composite of mesoporous TiO2 (mTiO(2)), displays a proton conductivity of 1.837 mS cm(-1) at 100 degrees C surpassing that of a pellet of Nafion 117 constructed as a reference under the same conditions (1.143 mS cm(-1)). This material also has greater conductivity than pure hydrated NSF (0.122 mS cm(-1)), confirming a synergistic interaction between the NSF and the oxide mesostructure in the proton conductivity mechanism. Both H-1 and C-13 solid state NMR studies of the NSF material and the mTiO(2)-NSF composites demonstrate that the oligomeric nature of the NSF is preserved while in contact with the mTiO(2) surface, thus facilitating conductivity.
- [Show abstract] [Hide abstract] ABSTRACT: An amorphous Fe(II) hydride material approximating FeH2 in composition (FeH2−xRx(Et2O)y where R = mesityl) has been isolated as a bulk powder in the solid state. This was accomplished under moderate reaction conditions by the reaction of bis(mesityl) iron(II) in toluene and hydrogen gas at 100 bar and 298 K to give a 1:5 mixed phase amorphous material of Fe(0) and the iron (II) hydride. This represents an important advance because FeH2 has never been synthesised in bulk form. The material shows ferromagnetic behaviour with a magnetic susceptibility of 1.25 Bohr magnetons per formula unit at 10 K.
- [Show abstract] [Hide abstract] ABSTRACT: The lack of an efficient hydrogen storage material has so far hindered the implementation of hydrogen as an energy vector, that is, a substance that allows the transfer through space and time of a certain quantity of energy from its original source. This work presents porous Ti(III) hydride gels as a promising new hydrogen storage material, exploiting the first example of a solid-state homoleptic metal hydride that binds further H-2 ligands using the Kubas interaction. These materials use bridging hydride ligands as an ultralightweight structural feature to support a microporous network of Ti binding sites for molecular H-2 chemisorption. High-pressure Raman spectroscopy confirmed the first evidence of TiH5 and TiH7 species, in some ways analogous to hypervalent MH5 and MH7 (M = Si, Ge, Sn) species. The material with the highest capacity has an excess reversible storage of 3.49 wt % at 140 bar and 298 K without saturation, corresponding to a volumetric density of 44.3 kg/m(3), comparable to the DOE 2017 volumetric system goal of 40 kgH(2)/m(3). However, extrapolations show that the phase-pure material is capable of binding at least 6 wt % hydrogen reversibly at room temperature.
- [Show abstract] [Hide abstract] ABSTRACT: Our established method of modeling transition metal based H2 storage materials is extended to include the desirable and achievable targets of hydrazine linked Cu(I), Cu(II) and Ni(II). Two coordinate Cu(I) H2 binding site representations bind two H2 molecules through the reversible Kubas interaction with a theoretical maximum storage capacity of 4.27%wt.
- [Show abstract] [Hide abstract] ABSTRACT: Models of two linked M(III) and M(II) (M = Ti, V, Cr) binding sites in hydrazine-linked hydrogen storage materials have been studied quantum chemically using density functional theory. The results compare favorably with previous experimental and computational results. Strong evidence is observed that the H2 molecules bind to the metal in a Kubas manner. As seen previously in monometallic analogues,(1, 2) altering the transition metal across the first row of the periodic table reduces the number of H2 molecules that can be bound, and replacing a hydrazide ligand with a hydride increases the M-H2 interaction energy. Evidence is presented for metal–metal interactions, which can influence the H2 binding enthalpy and may help to explain the observed metallic properties and rising H2 binding enthalpies with coverage of the experimental materials. An alternate explanation for the rising enthalpies is also proposed, involving a pressure-induced deformation of the structure with concomitant twisting of the bonds into conformations that allow more optimal binding of an H2 ligand.
- [Show abstract] [Hide abstract] ABSTRACT: The Cr(II) binding sites of an experimentally realised hydrazine linked hydrogen storage material have been studied computationally using density functional theory. Both the experimentally determined rise in H(2) binding enthalpy upon alteration of the ancillary ligand from bis[(trimethylsilyl)methyl] to hydride, and the number of H(2) molecules per Cr centre, are reproduced reasonably well. Comparison with analogous Ti(II), V(II) and Mn(II) systems suggests that future experiments should focus on the earliest 3d metals, and also suggests that 5 and 7 wt% H(2) storage may be possible for V(II) and Ti(II) respectively. Alteration of the metal does not have a large effect on the M-H(2) interaction energy, while alteration of the ancillary ligand bound to the metal centre, from bis[(trimethylsilyl)methyl] or hydride to two hydride ligands, THF and only hydrazine based ligands, indicates that ancillary ligands that are poor π-acceptors give stronger M-H(2) interactions. Good evidence is found that the M-H(2) interaction is Kubas type. Orbitals showing σ-donation from H(2) to the metal and π-back-donation from the metal to the dihydrogen are identified, and atoms-in-molecules analysis indicates that the electron density at the bond critical points of the bound H(2) is similar to that of classical Kubas systems. The Kubas interaction is dominated by σ-donation from the H(2) to the metal for Cr(II), but is more balanced between σ-donation and π-back-donation for the Ti(II) and V(II) analogues. This difference in behaviour is traced to a lowering in energy of the metal 3d orbitals across the transition series.
- [Show abstract] [Hide abstract] ABSTRACT: Manganese(II) hydrazide gels designed for Kubas-type hydrogen storage were synthesized from the reaction between bis(trimethylsilylmethyl) manganese and anhydrous hydrazine. The synthetic materials were characterized by X-ray powder diffraction, nitrogen adsorption, X-ray photoelectron spectroscopy, infrared spectroscopy, and elemental analysis. Hydrogen storage measurements were conducted on materials with hydrazine:Mn ratios of 0.5:1, 1:1. 1.5:1, and 2:1. The best results were obtained with the 1:1 material, which demonstrated as high as 1.01 wt % and 32 kg/m(3) at 77 K and 85 bar and 1.06 wt % and 24.2 kg/m(3) at 298 K at 85 bar. Measurements up to 143 bar at 298 K led to an adsorption value of 1.65 wt % and 37.7 kg/m(3) without saturation. The isosteric enthalpy of this material rose with surface coverage from 1.3 to 39 kJ/mol. Excess void space was removed from the materials by compressing at 500 Psi and the resulting compressed gel pellets possess an absolute volumetric adsorption of 18.97 kg/m(3) at 298 K and 85 bar. The mechanism of hydrogen storage was probed with electron paramagnetic resonance (EPR), which showed signal enhancements when sample was held under 1 bar hydrogen, both at room temperature and at 77 K. This was interpreted as a perturbation due to hydrogen binding to the Mn center.
- [Show abstract] [Hide abstract] ABSTRACT: Direct experimental evidence for dihydrogen-Ti binding on a silica-supported Ti organometallic complex using detailed sorption and inelastic neutron scattering (INS) measurements is presented. The results show that the 300 K hydrogen sorption saturates at 3 mgg -1, or approximately 25% of the 77 K capacity. The signatures of molecularly adsorbed hydrogen, namely the low-energy rotational peaks and recoil background, are found to be entirely absent. The surface oxygen atoms that do not contribute to the attachment of the organometallic are passivated by hydrogen atoms, which are also fully relaxed. INS investigations of the original Kubas complexes show that original Kubas complexes possess relatively open faces for dihydrogen bonding, such that the quasi-2D rotations of the coordinated dihydrogen are a reasonable description of the local environment.
- [Show abstract] [Hide abstract] ABSTRACT: We developed mesoporous and nanostructured TiO2 anodes with very high specific surface, up to more than 500 m2 g−1, that show good promise for lithium battery applications. Modifying the surfactant to form a carbon coating inside the pores at low temperature shows better promises than normal carbonization processes, since the high temperature needed is too high and results in the crystal formation and the loss of porosity.
- [Show abstract] [Hide abstract] ABSTRACT: Molecular models of the M-H(2) binding sites of experimentally characterised amorphous vanadium hydrazide gels are studied computationally using gradient corrected density functional theory, to probe the coordination number of the vanadium in the material and the nature of the interaction between the metal and the H(2) molecules. The H(2) is found to bind to the vanadium through the Kubas interaction, and the first quantum theory of atoms-in-molecules analysis of this type of interaction is reported. Strong correlation is observed between the electron density at the H-H bond critical point and the M-H(2) interaction energy. Four coordinate models give the best reproduction of the experimental data, suggesting that the experimental sites are four coordinate. The V-H(2) interaction is shown to be greater when the non-hydrazine based ligand, THF, of the experimental system is altered to a poorer π-acceptor ligand. Upon altering the metal to Ti or Cr the M-H(2) interaction energy changes little but the number of H(2) which may be bound decreases from four (Ti) to two (Cr). It is proposed that changing the metal from V to Ti may increase the hydrogen storage capacity of the experimental system. A 9.9 wt% maximum storage capacity at the ideal binding enthalpy for room temperature performance is predicted when the Ti metal is combined with a coordination sphere containing 2 hydride ligands.
University of South Wales
Понтиприте, Wales, United Kingdom
- Sustainable Environment Research Centre
University of Windsor
Windsor, Ontario, Canada
- Department of Chemistry and Biochemistry
University of SussexBrighton, England, United Kingdom
Massachusetts Institute of Technology
Cambridge, Massachusetts, United States
- Department of Chemical Engineering