Materials for hydrogen-based energy storage - past, recent progress and future outlook

Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered; porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage

LiCe(BH 4) 3Cl, a new lithium-ion conductor and hydrogen storage material with isolated tetranuclear anionic clusters

Mechanochemical synthesis using CeCl 3-MBH 4 (M = Li, Na or K) mixtures are investigated and produced a new compound, LiCe(BH 4) 3Cl, which crystallizes in a cubic space group I4̄3m, a = 11.7204(2) Å. The structure contains isolated tetranuclear anionic clusters [Ce 4Cl 4(BH 4) 12] 4- with a distorted cubane Ce 4Cl 4 core, charge-balanced by Li + cations. Each Ce atom coordinates three chloride ions and three borohydride groups via the Ε 3- BH 3 faces, thus completing the coordination environment to an octahedron. Combination of synchrotron radiation powder X-ray diffraction (SR-PXD), powder neutron diffraction and density functional theory (DFT) optimization show that Li cations are disordered, occupying 2/3 of the 12d Wyckoff site. DFT calculation indicates that LiCe(BH 4) 3Cl is stabilized by higher entropy rather than lower enthalpy, in accord with the disorder in Li positions. The structural model also agrees well with the very high lithium ion conductivity measured for LiCe(BH 4) 3Cl of 1 à - 10 -4 Scm -1 at T = 20 °C. In situ SR-PXD reveals that the decomposition products consist of LiCl, CeB 6 and CeH 2. The Sieverts measurements show that 4.7 wt % H 2 is released during heating to 500 °C. After rehydrogenation at 400 °C and p(H 2) = 100 bar for 24 h an amount of 1.8 wt % H 2 is released in the second dehydrogenation. The 11B MAS NMR spectra of the central and satellite transitions for LiCe(B(D/H) 4) 3Cl reveal highly asymmetric manifolds of spinning sidebands from a single 11B site, reflecting dipolar couplings of the 11B nuclear spin with the paramagnetic electron spin of the Ce 3+ ions. © 2012 American Chemical Society

Structural Evolution during Lithium- and Magnesium-Ion Intercalation in Vanadium Oxide Nanotube Electrodes for Battery Applications

Multiwalled vanadium oxide nanotubes are an intriguing class of materials due to their complex and functional structure. They have especially gained attention as an electrode material for rechargeable ion batteries exhibiting Li-ion storage capacities up to 250 mAh/g. The pristine nanotube materials and their electrochemical properties have previously been investigated extensively; however little knowledge exists on the structural transformations induced by ion storage in vanadium oxide nanotube electrodes. In this work, the changes in the atomic-scale and nanoscale structure during lithium- and magnesium-ion storage in two types of vanadium oxide nanotubes are investigated by operando powder X-ray diffraction and total X-ray scattering. Linear expansion and contraction of the VO$_x$ layers with vanadium reduction and oxidation are observed, while the interlayer spacing is found to be drastically dependent on the nature of the templating molecules or ions residing in the interlayer space of the pristine tubes. The study demonstrates how certain conditions will lead to destruction of the multilayer structure of the tubes, while leaving the individual VO$_x$ layers intact. Pair distribution function analysis reveals that vanadium reduction induces a change in the vanadium coordination geometry in the VO$_x$ layers from square pyramidal to octahedral

Novel Alkali Earth Borohydride Sr(BH₄)₂ and Borohydride-Chloride Sr(BH₄)Cl

Two novel alkali earth borohydrides, Sr(BH4)2 and Sr(BH4)Cl, have been synthesized and investigated by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and Raman spectroscopy. Strontium borohydride, Sr(BH4)2, was synthesized via a metathesis reaction between LiBH4 and SrCl2 by two complementary methods, i.e., solvent-mediated and mechanochemical synthesis, while Sr(BH4)Cl was obtained from mechanochemical synthesis, i.e., ball milling. Sr(BH4)2 crystallizes in the orthorhombic crystal system, a = 6.97833(9) Å, b = 8.39651(11) Å, and c = 7.55931(10) Å (V = 442.927(10) Å3) at RT with space group symmetry Pbcn. The compound crystallizes in α-PbO2 structure type and is built from half-occupied brucite-like layers of slightly distorted [Sr(BH4)6] octahedra stacked in the a-axis direction. Strontium borohydride chloride, Sr(BH4)Cl, is a stoichiometric, ordered compound, which also crystallizes in the orthorhombic crystal system, a = 10.8873(8) Å, b = 4.6035(3) Å, and c = 7.4398(6) Å (V = 372.91(3) Å3) at RT, with space group symmetry Pnma and structure type Sr(OH)2. Sr(BH4)Cl dissociates into Sr(BH4)2 and SrCl2 at 170 °C, while Sr(BH4)2 is found to decompose in multiple steps between 270 and 465 °C with formation of several decomposition products, e.g., SrB6. Furthermore, partly characterized new compounds are also reported here, e.g., a solvate of Sr(BH4)2 and two Li–Sr–BH4 compounds

Potassium zinc borohydrides containing triangular [Zn(BH4)3]-- and tetrahedral [Zn(BH4)xCl4-x]2-- anions

Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH4)3 and K2Zn(BH4)xCl4-x form in ball-milled KBH4:ZnCl2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K3Zn(BH4)xCl5-x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH4)3 contains an anionic complex [Zn(BH4)3]− with D3 (32) symmetry, located inside a rhombohedron K8. KZn(BH4)3 contains 8.1 wt % of hydrogen and decomposes at 385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn2(BH4)5 (M = Li, Na) and NaZn(BH4)3. The decomposition temperature is much lower than for KBH4. Monoclinic K2Zn(BH4)xCl4-x contains a tetrahedral complex anion [Zn(BH4)xCl4-x]2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K2ZnCl4 (structure type K2SO4). K2Zn(BH4)xCl4-x releases BH4 starting from 395 K, forming Zn and KBH4. As the reaction proceeds and x decreases, the monoclinic distortion of K2Zn(BH4)xCl4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K2ZnCl4. Tetragonal K3Zn(BH4)xCl5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs3CoCl5 structure type possibly with the space group P42/ncm. K3Zn(BH4)xCl5-x decomposes nearly at the same temperature as KZn(BH4)3, i.e., at 400 K, with the formation of K2Zn(BH4)xCl4-x and KBH4, indicating that the compound is an adduct of the two latter compounds