Thermodynamically neutral Kubas-type hydrogen storage using amorphous Cr(iii) alkyl hydride gels

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 kgH2 m(-3) compares favourably to the DOE ultimate system goal of 70 kg m(-3). 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.

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.…”

“…Typically,H 2 binding enthalpies are measured for physisorption materials by using the isosteric method. [2] However,a pplying this technique to transition-metal hydrazide and hydride materials studied by our group, which absorb H 2 through the Kubas interaction, [8][9]19] always gives enthalpies that rise with surfacec overage, contrary to what is normally observed for physisorption materials. This rising trend was also observed in recents tudies by Andalibiand co-workers [21] on Kubas H 2 storage in organometallic coppers pecies supported on carbon.…”

“…Moreover, the maximum measured performance by this materiali sh igher than that observedf or our chromium(III) alkyl hydride at 120 bar (4 wt %). [9] To rationalize the storagev alues in the V system,w emustc onsider that the adsorption of H 2 through the Kubas interaction by VH 3 to give VH 5 would equate to ag ravimetric capacity of 3.6 wt %. Further adsorption to give VH 7 corresponds to ac apacity of 7wt%.O wing to excess mass presenti nt he materials from residual hydrocarbon, lower weightp ercentage values than the theoretical maximum capacity calculated by using the 18-electron rule are also expected.…”

“…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.…”