Search

Using hydrogen to store energy in chemical bonds is a key component of the global strategy to achieving a sustainable future and ameliorating climate change. The challenges associated with handling molecular hydrogen can be solved by using liquid hydrogen carriers. In this perspective, we discuss the concept of bicarbonate–formate cycle, where aqueous solutions of formate ions (HCO2−) are used as hydrogen and energy carriers. Such solutions are composed by earth abundant elements and, in contrast to common liquid organic carriers, are non-flammable, and readily convert to the oxide forms (HCO3−) under reaction with water to release hydrogen, or electrons, at moderate temperatures. We discuss thermodynamic aspects of the bicarbonate–formate cycle and show how it offers the opportunity of combining electrochemical and thermochemical operations, as well as of coupling CO2 capture with energy/hydrogen storage. We emphasize the potential role of electrochemistry in the generation of formate and in the release of energy in the form of electricity. At present, more information on both the fundamental and systems level, is needed to identify the feasible scenarios for using formate/bicarbonate salts for hydrogen/energy storage. It is likely, however, that several strategies, including hybrid electrochemical–thermochemical approaches, will suit different applications. It is also clear that more integration between the disciplines of electrochemistry and heterogeneous catalysis is needed to overcome the challenges for advancing the HCO3−–HCO2− system as a feasible green alternative for storing and transporting energy. [ABSTRACT FROM AUTHOR]

NMR spectroscopy was used to measure the rates of the first and second substitution reactions between iodoalkane (R = Me, 1-butyl) and DABCO in methanol, acetonitrile and DMSO. Most of the reactions were recorded at three different temperatures, which permitted calculation of the activation parameters from Eyring and Arrhenius plots. Additionally, the reaction rate and heat of reaction for 1-iodobutane + DABCO in acetonitrile and DMSO were also measured using calorimetry. To help interpret experimental results, ab initio calculations were performed on the reactant, product, and transition state entities to understand structures, reaction enthalpies and activation parameters. Markov chain Monte Carlo statistical sampling was used to determine a distribution of kinetic rates with respect to the uncertainties in measured concentrations and correlations between parameters imposed by a kinetics model. The reactions with 1-iodobutane are found to be slower in all cases compared to reactions under similar conditions for iodomethane. This is due to steric crowding around the reaction centre for the larger butyl group compared to methyl which results in a larger activation energy for the reaction. [ABSTRACT FROM AUTHOR]

Liquid organic hydrogen carriers will likely be a key element of a future hydrogen economy by enabling the storage and transport of large quantities of hydrogen. Ethanol is a liquid organic hydrogen carrier that is readily available from biological resources, which undergoes a reversible reaction to yield hydrogen and ethyl acetate. The objective of the present study is to obtain a better understanding of the thermodynamic and environmental suitability of the ethanol–ethyl acetate cycle for hydrogen storage applications. The analysis covers three aspects: thermodynamics of the chemical reaction, energy balance of the process, and a first‐order assessment of greenhouse gas emissions. Thermodynamics of the reaction are characterized by a standard Gibbs energy of reaction close to zero which allows the reaction to be shifted between hydrogenation and dehydrogenation within a moderate window of temperature and pressure conditions. The energy demand for dehydrogenation is comparatively small, resulting in an overall system efficiency of 88%. A life cycle greenhouse gas analysis over a 20‐year storage system lifetime gives a carbon intensity of 7.0 kg‐CO2eq/kg‐H2 delivered. These results indicate that the ethanol–ethyl acetate system has considerable promise as a hydrogen carrier and should be the subject of further research. [ABSTRACT FROM AUTHOR]

Coordination complexes of Mg(BH4)2 are of interest for energy storage, ranging from hydrogen storage in BH4 to electrochemical storage in Mg based batteries. Understanding the stability of these complexes is crucial since storage materials are expected to undergo multiple charging and discharging cycles. To do so, we examined the thermal stabilities of the 1 : 1 mixtures of Mg(BH4)2 with different glymes by DSC -- TGA, TPD-MS and powder XRD analysis. Despite their structural similarities, these mixtures show diverse phase transitions, speciations and decomposition pathways as a function of linker length. [ABSTRACT FROM AUTHOR]

Research has been conducted on 1-phenyl-2-phenoxyethanol-1-yl radical. The radical's scission kinetics, its beta-scission energetics and the role of intramolecular hydrogen bonding in scission rate control via ground and transition state stabilization have been investigated and discussed.

Measurement of the diffusion coefficients of benzyl-, sec-phenethyl-, and diphenylmethyl alcohol and the corresponding arylcarbonyls (benzaldehyde, acetophenone, and benzophenone) is done by Taylor's dispersion method. The experimental values are compared to published transient grating measurements of the corresponding aryl ketyls.

Metal borohydrides are considered promising materials for hydrogen storage applications due to their high volumetric and gravimetric hydrogen density. Recently, different Lewis bases have been complexed with Mg(BH4)2 in efforts to improve hydrogenation/dehydrogenation properties. Notably, Mg(BH4)2·xTHF adducts involving tetrahydrofuran (THF; C4H8O) have proven to be especially interesting. This work focuses on exploring the physicochemical properties of the THF-rich Mg(BH4)2·3THF adduct using neutron-scattering methods and molecular DFT calculations. Structural analysis, based on neutron diffraction measurements of Mg(11BH4)2·3TDF (D – deuterium), has confirmed a lowering of the symmetry upon cooling, from monoclinic C2/c to P1¯ via a triclinic distortion. Vibrational properties are strongly influenced by the THF environment, showing a splitting in spectral features as a result of changes in the bond lengths, force constants, and lowering of the overall symmetry. Interestingly, the orientational mobilities of the BH4− anions obtained from quasielastic neutron scattering (QENS) are not particularly sensitive to the presence of THF and compare well with the mobilities of BH4− anions in unsolvated Mg(BH4)2. The QENS data point to uniaxial 180° jump reorientations of the BH4− anions around a preferred C2 anion symmetry axis. The THF rings are also found to be orientationally mobile, undergoing 180° reorientational jumps around their C2 molecular symmetry axis with jump frequencies about an order of magnitude lower than those for the BH4− anions. In contrast, no dynamical behavior of the THF rings is observed with QENS for a more THF-deficient 2Mg(BH4)2·THF adduct. This lack of comparable THF mobility may reflect a stronger Mg2+–THF bonding interaction for lower THF/Mg(BH4)2 stoichiometric ratios, which is consistent with DFT calculations showing a decrease in the binding energy with each additional THF ring in the adduct. Based on the combined experimental and computational results, we propose that combining THF and Mg(BH4)2 is beneficial to (i) preventing weakly bound THF from coming free from the Mg2+ cation and reducing the concentration of any unwanted impurity in the hydrogen and (ii) disrupting the stability of the crystalline phase, leading to a lower melting point and enhanced kinetics for any potential hydrogen storage applications. [ABSTRACT FROM AUTHOR]

Boron compounds have a rich history in energy storage applications, ranging from high energy fuels for advanced aircraft to hydrogen storage materials for fuel cell applications. In this review we cover some of the aspects of energy storage materials comprised of electron-poor boron materials combined with electron-rich nitrogen elements with the goal of moderate temperature release of hydrogen. The parent compounds of ammonium borohydride, ammonia borane, and diammoniate of diborane provide approaches for storing high gravimetric and volumetric densities of hydrogen. Here we provide a review with a historical perspective and current developments in the area of solid state B and N containing compounds. This review highlights developments in synthesis of ammonia borane and its derivatives over the last 80 years. Thermodynamics and kinetics of hydrogen release in the solid state are discussed. By changing either substituents on the boron and nitrogen atoms or the physical environment by embedding in mesoporous scaffolds, the thermodynamics can be modified to reduce the exothermicity of hydrogen release and minimize formation of volatile impurities. Several mechanistic studies are reviewed identifying the key distinctions between homopolar and heteropolar H2 release. Strategies for economical and efficient regeneration of the hydrogen storage materials via chemical transformation are critically reviewed. The limited efficiency of these chemical regeneration has limited some of the potential applications. [ABSTRACT FROM AUTHOR]

The effects of high pressure and temperature on the bonding in ammonia borane (AB), NH3BH3 and decaborane (DB), B10H14 and their interactions with molecular hydrogen (H2) were investigated using Raman spectroscopy in a diamond anvil cell. At 0.7 GPa, AB becomes amorphous between 120 and 127 °C, indicating a positive Clapeyron slope. Heated to 140 °C, AB begins to undergo decomposition to polyaminoborane. The amorphous and decomposed AB does not recrystallize back to AB during slow cooling to room temperature or upon application of high pressure up to 3 GPa, underscoring the challenge of rehydrogenation of decomposed AB. The molecular Raman modes broaden in the reacted phase, and the NH3 modes show no pressure dependence. DB was studied at room temperature up to 11 GPa. The observed frequency dependence with pressure (dν/dP) and mode Grüneisen parameters varied for different spectral groups, and a new transition was identified at approximately 3 GPa. In both DB and heated AB, we found that they could store additional H2 with the application of pressure. We estimate that we can store approximately 3 wt % H2 in heated AB at 3 GPa and 1 wt % H2 in DB at 4.5 GPa. [ABSTRACT FROM AUTHOR]

Incoherent inelastic neutron scattering is used to probe the effects of dihydrogen bonding on the vibrational dynamics in the molecular crystal of ammonia borane. The thermal neutron energy loss spectra of 11B enriched ammonia borane isotopomers (11BH3NH3, 11BD3NH3, and 11BH3ND3) are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics. A harmonic vibrational analysis on NH3BH3 clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50–500 cm-1). Some assignment of modes to simple harmonic motion, e.g., NH3 and BH3 torsion in the molecular crystal is possible, and it is confirmed that the lowest modes are dominated by collective motion. We show that the vibrational dynamics as modeled with ab initio molecular dynamics provides a more complete description of anharmonic and collective dynamics in the low frequency region of the inelastic incoherent neutron scattering spectra when compared to the conventional harmonic approach. [ABSTRACT FROM AUTHOR]

Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 88 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However, some of the modes in the orthorhombic phase appeared to merge 10–12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. In addition, electronic structure calculations are used to assist in the interpretation and assignment of the normal modes. [ABSTRACT FROM AUTHOR]

The nature of a nanophase ammonia borane (AB) is examined by using atomic pair distribution function (PDF) analysis of synchrotron X-ray powder diffraction data to follow the structural evolution of AB embedded within MCM-41 at various temperatures. PDF analyses of composite materials composed of excess AB has shown that the excess AB has formed aggregates outside the mesoporous scaffold and that these aggregates have structural properties similar to neat AB.

The application of iridium complex ([[kappa].sup.3]1,3-[(OP[super t][Bu.sub.2]).sub.2][C.sub.6][H.sub.3]Ir[(H).sub.2] as an efficient catalyst for the rapid dehydrogenation of amine boranes (MeAB) and AB/MeAB mixtures is reported. The reaction yields soluble oligomers and copolymers and provides insight into the kinetic and thermodynamics of reaction materials for hydrogen storage.

The spectroscopic studies of the reaction pathways resulting in the thermal decomposition of solid-state ammonia borane (AB) incorporated in carbon cryogels (CC) are reported. The results reveal that the incorporation of AB in CC enhances the reaction kinetics with residual surface-bound oxygen functional groups.

The role of the rhodium-containing catalysts in the dehyrocoupling of amine borane compounds was determined by using X-ray absorption fine structure (XAFS). The catalytic dehydrocoupling produce metallic rhodium in the presence o air and that the rhodium clusters could be transformed into metallic rhodium by air oxidation of the borane ligands.

An in situ study utilizing X-ray absorption fine structure spectroscopy (XAFS) and (super 11)B NMR methods to evaluate the rhodium-catalyzed dehydrogenation of dimethylamine borane DMAB is presented. XAFS provides chemical and structural information of the local environment of the active rhodium species during the reaction, while (super 11)B NMR allows interrogation of the boron-containing species.

A novel ab initio cavity definition that captures the strength and specific interactions of the solute with a water solvent molecule is discussed. Reactivities in solution, especially that involving polarizable species are better understood through free energy relationships rather than enthalpy changes.

New Arrhenius rate expressions are determined for the abstraction of bromine atom from 2-phenethyl bromide and sulfur atom from propylene sulfide by tributylstannyl radical. The results revealed that increase in rate of bromine atom abstraction by tin radical from the thiiranylmethyl bromide provides evidence for synchronous thiirane ring opening providing an enthalpic enhancement of the bromine atom abstraction rate.

Highly active bimetallic PdAu nanoparticles (NPs) supported on commercial nanocarbons have been prepared through a simple L -arginine-mediated synthetic approach. The as-synthesized catalysts have been used for the dehydrogenation of formic acid (FA). Remarkably, the resultant Pd1Au1 NPs immobilized onto MSC-30 carbon achieved a turnover frequency (TOF) value of 8355 h−1 at 60 °C in aqueous FA solution without addition of sodium formate, much higher than the results of heterogeneous catalysts reported thus far. The simple small-molecule-mediated synthetic methodology developed herein opens a new path for the practical and large-scale development of high-performance catalysts for a wide range of applications. [ABSTRACT FROM AUTHOR]

Hydrogen uptake and release in arene–cycloalkane pairs provide an attractive opportunity for on‐board and off‐board hydrogen storage. However, the efficiency of arene–cycloalkane pairs currently is limited by unfavorable thermodynamics for hydrogen release. It is shown here that the thermodynamics can be optimized by replacement of H in the ‐OH group of cyclohexanol and phenol with alkali or alkaline earth metals. The enthalpy change upon dehydrogenation decreases substantially, which correlates with the delocalization of the oxygen electron to the benzene ring in phenoxides. Theoretical calculations reveal that replacement of H with a metal leads to a reduction of the HOMO–LUMO energy gap and elongation of the C−H bond in the α site in cyclohexanolate, which indicates that the cyclohexanol is activated upon metal substitution. The experimental results demonstrate that sodium phenoxide–cyclohexanolate, an air‐ and water‐stable pair, can desorb hydrogen at ca. 413 K and 373 K in the solid form and in an aqueous solution, respectively. Hydrogenation, on the other hand, is accomplished at temperatures as low as 303 K. Optimized storage: The metalation of the phenoxide–cyclohexanolate pair is a successful strategy to optimize the thermodynamic properties of this type of hydrogen storage material (see figure). In experiments the sodiated pair desorbed hydrogen at 413 K (373 K) in the solid state (aqueous solution), whereas hydrogenation could be accomplished at temperatures as low as 202 K. [ABSTRACT FROM AUTHOR]