Search

© 2018 American Chemical Society. A high-entropy alloy (HEA) of HfNbTiVZr was synthesized using an arc furnace followed by ball milling. The hydrogen absorption mechanism was studied by in situ X-ray diffraction at different temperatures and by in situ and ex situ neutron diffraction experiments. The body centered cubic (BCC) metal phase undergoes a phase transformation to a body centered tetragonal (BCT) hydride phase with hydrogen occupying both tetrahedral and octahedral interstitial sites in the structure. Hydrogen cycling of the alloy at 500 °C is stable. The large lattice strain in the HEA seems favorable for absorption in both octahedral and tetrahedral sites. HEAs therefore have potential as hydrogen storage materials because of favorable absorption in all interstitial sites within the structure.

© 2016 the Owner Societies. Rare earth (RE) metal borohydrides are receiving immense consideration as possible hydrogen storage materials and solid-state Li-ion conductors. In this study, halide free Er(BH4)3 and Pr(BH4)3 have been successfully synthesized for the first time by the combination of mechanochemical milling and/or wet chemistry. Rietveld refinement of Er(BH4)3 confirmed the formation of two different Er(BH4)3 polymorphs: a-Er(BH4)3 with space group Pa3, a = 10.76796(5) Å, and ß-Er(BH4)3 in Pm3m with a = 5.4664(1) Å. A variety of Pr(BH4)3 phases were found after extraction with diethyl ether: a-Pr(BH4)3 in Pa3 with a = 11.2465(1) Å, ß-Pr(BH4)3 in Pm3m with a = 5.716(2) Å and LiPr(BH4)3Cl in I43m, a = 11.5468(3) Å. Almost phase pure a-Pr(BH4)3 in Pa3 with a = 11.2473(2) Å was also synthesized. The thermal decomposition of Er(BH4)3 and Pr(BH4)3 proceeded without the formation of crystalline products. Rehydrogenation, as such, was not successful. However, addition of LiH promoted the rehydrogenation of RE hydride phases and LiBH4 from the decomposed RE(BH4)3 samples.

A new class of quaternary complex transition metal hydrides (Na2Mg2TH8 (T = Fe, Ru)) have been synthesized and their structures determined by combined synchrotron radiation X-ray and powder neutron diffraction. The compounds can be considered as a link between ionic and complex hydrides in terms of incorporating independently coordinated ionic and covalent hydrogen. These novel isostructural complex transition metal hydrides crystallize in the orthorhombic space group Pbam, where the octahedral complex hydride anion is surrounded by a cubic array of four Mg2+ and four Na+ cations, forming distinct two-dimensional layers. An intriguing feature of these materials is the distorted octahedral coordination of the isolated H− anions by four Na+ and two Mg2+ cations, which form layers between the transition metal containing layers. The vibrational modes of the H− anions and complex hydride anion are independently observed for the first time in a quaternary complex transition metal hydride system by Raman and IR spectroscopy.

The twice hydrogen (H) cycled planetary milled (PM) and cryo milled (CM) NaAlH4 + xTMCl3 (transition metal (TM) = Ti, V, Fe) systems (x > 0.1) have been studied by high resolution synchrotron X-ray diffraction, and high resolution transmission electron microscopy (TEM). Intense primary amorphous (a-) Al1−xTMx halos are evident in diffraction data of PM samples for V and Fe, and in CM samples for Ti, V, and Fe. Weaker primary amorphous Al1−xTix halos are evident in PM samples for Ti. The Ti poor a-Al1−xTix phase observed for NaAlH4 + xTiCl3 (x > 0.1) ranges in composition from a-Al86.5Ti13.5 → a-Al92Ti8. High resolution TEM studies of the Al1−xVx phases in the H cycled PM NaAlH4 + 0.1VCl3 system demonstrates that a nanoscopic composite morphology can exist between face centred cubic (fcc) crystalline (c-) Al1−xVx and a-Al1−xVx phases, with the c-Al1−xVx/a-Al1−xVx composite embedded on the NaAlH4 surface. The amorphous Al1−xVx reaches ca. 28 at.% V.

The twice hydrogen (H) cycled planetary milled (PM) NaAlH4 + xTMCln (transition metal (TM) = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Pd, Pt; 2 < n < 4) and cryo milled (CM) NaAlH4 + xTMCln (TM = Ti, V, Cr, Fe, Ni; 2 < n < 3) systems (x < 0.1) have been studied by high resolution synchrotron X-ray diffraction. Face centred cubic (fcc) A1 crystalline (c-) Al1-xTMx (x < 0.25) solid solutions are evident in PM samples for Sc, Ti, V, Cr, Mn, Zr and Pd while PM samples of Cu, Ni, Pd and Pt display mostly ordered and numerous crystalline Al1-xTMx phases. Very broad reflections in the 2 Å d-spacing region for Cr, Mn, Co, and Pd are identified as partially ordered body centred cubic (bcc) A2 and B2 Al1-xTMx type structures. The amorphous (a-) Al1-xVx phase observed in the H cycled PM NaAlH4 + xVCl3 system ranges in composition from a-Al90V10 (x = 0.02) up to a-Al72V28 (x = 0.1). Across the TM series, the Al1-xTMx particle size ranges as Sc-V 4-25 nm, Cr, Mn, Co <5 nm, Fe 5–15 nm, Ni, Cu <50 nm. A highly ‘compressed’ NaAlH4 phase is observed in the H cycled PM NaAlH4 + 0.1ScCl3 system, with unit cell dimensions of a = 4.9995(2) Å and c = 11.2893(1) Å, compared to the average dimensions of ‘normal’ NaAlH4 across the TM and rare earth (RE) series with a = 5.0228(1) Å and c = 11.3516(1) Å.

The NaAlH4 + xTiCl3 (x < 0.1) system has been studied by a combination of X-ray synchrotron and neutron diffraction, and isotopic H2/D2 scrambling after the completion of the milling process, and the first thermal release of hydrogen (H). An in situ X-ray synchrotron diffraction study of the isochronal release of hydrogen from planetary milled (PM) NaAlH4 + 0.1TiCl3 shows that crystalline (c-) Al1-xTix phases do not form until almost all H is released from the sample, demonstrating that the surface embedded nanoscopic crystalline Al/amorphous (a-) Al50Ti50 composite facilitates the release of H during the very first thermal desorption. Planetary milled (PM) NaAlH4 + xTiCl3 is observed to disproportionate at room temperature, with no NaAlH4 remaining after ca. 200 days. A complete lack of ambient hydrogen release from PM NaAlH4 + 0.1Al (80 nm) measured over 200 days suggests that the nanoscopic a-Al50Ti50 phase is entirely responsible for the hydrogen release during thermal desorption of milled NaAlH4 + xTiCl3. Isotopic H/D exchange has been observed by combined neutron and X-ray synchrotron diffraction on a PM NaAlD4 + 0.04TiCl3 sample, after exposing the milled sample to 20 bar H2 at 50 °C for ca. 6 days. Under these pressure/temperature (P/T) conditions, disproportionation of NaAlD4 is avoided, and ca. 32% of D atoms are exchanged with H atoms. Asymmetrically broadened reflections in the synchrotron data show peak splitting into two unit cell types, one expanded with H, the other remaining close to pure D based unit cell dimensions.The 2-phase model when fitted to the neutron data demonstrates that ca. 56% of D atoms in ca. 58% of all unit cells are exchanged with H, yielding a NaAl(H0.56D0.44)4 composition for the expanded unit cells. HD scrambling (1 bar mixture of H2 and D2 at 23 °C) performed on desorbed H empty PM NaAlH4 + 0.1TiCl3 shows classic H2 + D2 ↔ 2HD equilibrium mixing, demonstrating that nanoscopic Ti containing Al1-xTix surface embedded phases perf

This study elucidates the role of transition metal (TM) additives in enhancing hydrogen (H) reversibility and hydrogenation kinetics for the NaAlH4 system. The isothermal hydrogen absorption kinetics of the planetary milled (PM) NaAlH4 + xTMCln (TM row 1 = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu; row 2 = Zn, Y, Zr, Nb, Mo, Ru, Rh Pd; row 3 = Pt; 2 < n < 5) and cryo-milled (CM) NaAlH4 + xTMCln (TM row 1 = Ti, V, Cr, Fe, Co, Ni, Cu; 2 < n < 3) systems have been measured at 140 °C and 150 bar system pressure. The variation in hydrogenation kinetics across the TM series for NaAlH4 + xTMCln is strongly dependent on the TM species and additive level, milling technique, and the type, structure, and morphological arrangement of nanoscopic Al1−xTMx phases that are embedded on the NaAlH4 surface. In the most interesting case, the surface-embedded Al1−xTix phases in the TiCl3-enhanced NaAlH4 system perform a dual catalytic function, where the outer Al1−xTix surface performs dissociation/recombination of molecular H2 and the inner Al1−xTix surface allows the distortion of a minor number of Al−H bonds from AlH4 − tetrahedra in the vicinity of the subsurface Al1−xTix/NaAlH4 interface. The density of Ti atoms in the subsurface interface (which is Al:Ti composition- and H cycling temperature-dependent) shows the strongest effects on hydrogenation kinetics.

The NaAlH4 + xTiCl3 (x < 0.1) system has been studied by X-ray synchrotron diffraction and transmission electron microscopy (TEM) after the completion of the milling process, for both planetary (PM) and cryo milled (CM) samples. Comparison of the NaAlH4 mosaic size (coherence length) determined from the X-ray synchrotron diffraction lineshape, and measurement of the external powder grain dimensions of ca. 250–300 particles by TEM, reveals that after the completion of the milling process, (1 1 0) {1 1 1} edge dislocated 2–20 nm Al crystallites are dispersed in a Ti rich amorphous (a-)Al1−xTix (0.3 < x < 0.5) matrix. This nano Al/Al50Ti50 composite is embedded on the surface of single crystalline NaAlH4 powder grains. The NaAlH4 single crystal powder grains are moderately defected with uncorrelated defects, induced from the milling process.