What is vanadium hydride? A vanadium hydride gel was prepared by thermal treatment of a solution of tetraphenyl vanadium (IV) at 100 °C, followed by hydrogenation of the material to exploit the low mass of the hydrogen ligand for improved gravimetric energy storage. Characterization of the material shows that it is amorphous and consists predominantly of vanadium hydride species in oxidation states between III and IV. The material reversibly stores 5.8 wt% hydrogens at 130 bar and 25 °C and is also significantly more convenient, safer, and less expensive to synthesize than previous vanadium alkyl hydrides reported previously by our group. As anticipated from the valency of the vanadium centers and the low mass of the hydride ligands, the material possesses a promising redox peak capacity of 131 mAhg -1 at a discharge rate of 1.0 mAcm-2; the capacity decreases with repeated cycling until it reaches 47 mAhg -1 after 50 cycles, comparable to bulk phase VO2. While these numbers are approaching practical performance for onboard systems at an affordable cost, this material does not yet present enough recyclability to be utilized as a useful secondary lithium battery material, however improvements in synthesis and nanostructuring could further improve hydrogen storage performance while also leading to a more effective battery cathode material with improved recyclability performance.
Hydrogen Storage Properties of vanadium hydride Hydrogen adsorption-desorption recorded at 25 °C measured between 2-3 mmol total H2 adsorption per isotherm at 130 bar for consistency with increasing pressure until it reaches 120 bar where the isotherm reaches saturation, as demonstrated by a plateau in both the adsorption and desorption isotherms. However, there is some hysteresis and a slight decrease in capacity after saturation. Since saturation of hydrogen uptake is observed, we can be confident in the accuracy of the measurements, as any error introduced from inaccurate volume calibrations or a leak would lead to virtual adsorption in the isotherm, meaning that saturation would never be reached. Furthermore, saturation with hysteresis was observed at 120 bar in our previously studied V(III)-hydride materi repeated ten times with a five-minute vacuum step at 25 °C between each cycle. The capacity at 130 bar for each cycle is shown in Figure 8. The average performance over the ten cycles was 5.77 wt%, and the capacity did not decrease from repeated hydrogen adsorption/desorption cycles. In previous work, the enthalpy of hydrogen adsorption for a V(III) alkyl hydride gel was determined directly by isothermal calorimetry as +0.52 kJ mol-1 H2 over the pressure range of the experiment between 0 and 130 bar.9c We also reported the total enthalpy of hydrogen adsorption for the Cr(III) alkyl hydride (+0.37 kJ mol-1 H2).9b In both systems, the directly measured enthalpy of hydrogen binding by the Kubas interaction is not in the expected 20-40 kJ mol-1 range exhibited by classical Kubas complexes, despite confirming Kubas binding by high-pressure Raman measurements. This supports the mechanism put forward by our group and supported by computations that the hydrogen binding is thermodynamically neutral because the enthalpy of hydrogen binding is offset by an endothermic process of pressure-induced deformation of the material springing open new binding sites with increasing pressure. 31 With Henry's adsorption and an expected entropy of H2 binding of -100 JK-1, reversible adsorption can occur with increasing pressure.11b While the enthalpy of H2 adsorption has not yet been directly measured for V(IV)-25C-H2, because of the similarities in performance and XPS, we expect it to be in a similar range to the values reported for the Cr(III) hydride and V(III) hydride materials.
While the stoichiometry of vanadium hydride For a hypothetical material based on pure phase VH4, up to 4 H2 could bind according to the eighteen electron rule, or 5 H2 in the case of VH3. This gives a theoretical maximum performance of 12.7 wt% for VH4 and 15.7 wt% for VH3, calculated from the final mass at saturation of VH12 and VH13, respectively. Although the measured 5.8 wt% capacity for V(IV)-25C-H2 is lower than this theoretical maximum, this is not surprising considering that there is still hydrocarbon present in the material and that the material is a mixture of different vanadium oxidation states, some of which may not absorb H2. Although both V(III) and V(IV) are confirmed by XPS and expected to bind H2, based on pure VH4, 1 H2 per V gives an expected gravimetric capacity of 3.6 wt% and 2 H2 per V would amount to 7.3 wt%. Therefore based on the 5.8 wt% plateaux, it is reasonable to assume that some V centers bind 1 H2 and some bind 2 H2, similar to what we have determined for previous Ti,9a Cr,9b, and V9c materials. Because the 2.2 m2 /g surface area of V(IV)-25CH2 is very low relative to AX-21, which has a surface area of 2800 m2 /g and much lower adsorption than V(IV)-25C-H2, 19 the hydrogen adsorption of 5.8 wt% can not be due to physisorption but most likely from the Kubas interaction of H2, as directly observed previously by high-pressure Raman spectroscopy for H2 adsorption by TiH3 9a, CrH3, 9b, and VH3. 9c However more physical studies are required on the adsorption mechanism as the nitrogen adsorption determined surface areas may not reflect the surface area available to the much smaller H2 molecule, and there is also the possibility of H2 shuttling through the material by a "bind and rotate" mechanism. Vanadium oxamide materials studied previously by our group also possess low surface areas in the range of 9-30 m2 /g and store hydrogen via the Kubas interaction, as confirmed by Raman measurements. Still, these materials have capacities of only 0.87 wt% at 85 bar and 25 °C, likely due to the bulky oxamide linking ligands which could hinder H2 coordination while adding excess weight, thus cutting into gravimetric performance.
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