Observation of TiH₅ and TiH₇ in Bulk-Phase TiH₃ Gels for Kubas-Type Hydrogen Storage

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 solidstate 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 TiH 5 and TiH 7 species, in some ways analogous to hypervalent MH 5 and MH 7 (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.

High‐Pressure Raman and Calorimetry Studies of Vanadium(III) Alkyl Hydrides for Kubas‐Type Hydrogen Storage

“…This leads to ag ravimetric performance of 5.4 wt %a t1 20 bar and 298 Kf or V(III)-150 H 2 , which corresponds to av olumetric capacity of 75.35 kgH 2 m À3 based on the measured density of 1.37 gcm À3 .T here is as light degree of hysteresisb etween the adsorption and desorption isotherms above 100 bar,a nd between 107 and 120 bar there is no significant increase or decreasei ns lope, demonstrating that the material had reached saturation and could not adsorb any further hydrogen. Significantly,t his is the first time that saturation of am aterialh as been observed at room temperature for the transition-metal alkyl hydrides and hydrazides previously reported by our group, [8,9,19] further confirming the accuracy of our measurements by ruling out virtual adsorption caused by leaks or inaccurate volumec alibrations. Moreover, the maximum measured performance by this materiali sh igher than that observedf or our chromium(III) alkyl hydride at 120 bar (4 wt %).…”

“…The signals at 4105 cm À1 and higher arise from physisorbed H 2 and the Q-branch of H 2 ,a sp reviously observed in the spectrum of physisorbed H 2 on MOF-5. [20] There are three signals present in the spectrum of V(III)-150 H 2 at 2770, 2929, and3 834 cm À1 that do not appear in the blank and can be assigned to Kubas-bound H 2 .T hese are in as imilar position to the signals for Kubas-bound H 2 in previous work on titanium [8] and chromium alkyl hydrides, [9] the latter of which appear at 2789, 2922, and 3188 cm À1 ,a nd were confirmed as HÀHb ands by comparison to the spectrum of the Cr alkyl hydride under high-pressure D 2 .T he three signals in V(III)-150 H 2 , thus, correspond to different Kubas binding sites, with the signal at 3884cm À1 possibly arising from binding of H 2 to aV IV center,a sK ubas binding to higher oxidation state metals is expected to lead to Ramanb ands at higherf requency,o wing to weaker back bonding.…”

“…The precipitate was dried in vacuo at 100 8Cf or 4hto give ab lack powder[ V(III)-100].T he precipitate was then treated with H 2 gas at 85 bar for 4h at 100 8Ct og ive V(III)-100H 2 .F urther hydrogenation was then carriedo ut at 85 bar and 150 8Cf or 4h to give V(III)-150 H 2 .A ss hown in Figure S1, all three materials exhibit CÀHs tretches in the region of 2800-2965 cm À1 .T he intensity of the CÀHs tretches decrease after progressiveh ydrogenation, most likely owing to replacement of the hydrocarbon ligandsb yahydride. In the Ti analogue, synthesized by hydrogenation of tris[bis(trimethylsilyl)]methyl titanium(III);a n MÀHs tretchi so bserved at 1612 cm À1 [8] and, typically,t ransition metal-hydride stretches come in the 1900 AE 300 cm À1 region, but can often be weak in intensity. [13] In all three samples, there is ab road band centered at 1633 cm À1 ,w hich could be assigned to an MÀHs ignal;h owever,i naprevious study of matrix isolated Fe hydrides, FeÀHb ands in this region were obscured by water OÀHb ands in KBr at 1647 cm À1 .…”

“…In this regard, even for materials with low H 2 adsorption enthalpies (in the range of 4-10 kJ mol), adsorption is an exothermic process, and so the heat released on fueling needs to be dealt with by ah eat management system. [6] In recent work, we have reported Fe, [7] Ti, [8] and Cr alkyl hydride gels [9] for H 2 storage that used the Kubas interaction, at ype of weak chemisorption to transition-metal complexes with2 0-40 kJ mol À1 binding enthalpies. [10] By using the US DOE gravimetric storage-system target of 5.5 wt % [11] as ab enchmark, the Cr III material was the best performing of these three hydrides, reaching5 .07 wt %a t1 60 bar and 298 Kw ith aH 2 adsorptione nthalpy of + 0.37 kJ mol À1 H 2 .T his surprisingly neutral enthalpy of hydrogen binding is optimal for practical applications and suggestst hat minimal loss in performance would occur on incorporation into as imple system utilizing at ypeI tank with no requirement for ah eat management system.…”

Recent progress on Kubas-type hydrogen-storage nanomaterials: from theories to experiments

“…These results show the realization of materials that can serve as H 2 -storage materials at room temperature and ambient pressure. The H 2 capacity of TiH 5 and TiH 7 gels at room temperature was found to be 6 wt% [79]. In addition, TM-ethylene complexes were suggested as potential Kubas-type H 2 storage materials and were found to have a capacity of 12 wt.% at 13 Pa [80].…”