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By LZH | 11 August 2023 | 0 Comments

While hydrogen storage applications of vanadium hydride

What is vanadium hydride?
To prepare the vanadium hydride, VCl4 was treated with phenyl lithium in dibutyl ether, and the mixture was filtered to give a black solution, which was subsequently stirred at 100 °C for 48 hours. The ensuing suspension was then filtered to give a black residue, which was dried in vacuo at 100 °C for four hours to afford a black air moisture-sensitive solid (V(IV)-100). As illustrated in the reaction mechanism, we propose that during heat treatment, the vanadium aryl precursor polymerizes via a bimolecular C-H activation process with the loss of benzene to form an organometallic polymer with bridging phenyl groups. An alternate decomposition route could involve multiple bond homolysis reactions or form two biphenyl and vanadium metal equivalents. However, the IR evidence discussed below confirms the presence of hydrocarbon in the polymer, supporting our assignment as an organometallic polymer rather than vanadium metal. The material V(IV)-100 was then treated with hydrogen at 100 bar for 48 hours at 25 °C to give V(IV)-25C-H2. In this step, we propose replacing phenyl ligands with bridging hydrides via the well-documented hydrogenolysis reaction to form a polymeric vanadium(IV) hydride material.

While hydrogen storage applications of vanadium hydride
While hydrogen storage applications of homoleptic vanadium (III) alkyl hydrides have previously been explored by our group,9c the tris(bis(trimethylsilyl) methyl)vanadium(III) precursor required to prepare the material is prohibitively expensive and difficult to synthesize. The original literature procedure for the synthesis of (tris(bis(trimethylsilyl) methyl)vanadium(III)) reported a recrystallized yield of only 14%.17 Both vanadium trichloride, which is insoluble in all solvents it does not react with, and bis(trimethylsilyl) chloromethane are costly reagents to purchase. The freshly-synthesized 1% Na Li dispersion required to synthesize the necessary alkyl Li reagent is difficult and hazardous to prepare. With interest in reducing the cost of materials synthesis to explore electrochemical applications of vanadium hydrides on a bulk scale, it would be beneficial to explore possible synthesis routes for closely analogous vanadium hydrides starting from inexpensive and ether-soluble VCl4 based on the method of Wilkinson18 used for the synthesis of (Me3SiCH2)4V. Furthermore, based on the 18 electron rule, phase pure VH4 could bind up to 4 H2 molecules by the Kubas interaction giving a theoretical maximum gravimetric hydrogen storage value of 14.6 wt% and improving performance over previous vanadium hydrides prepared by our group. Thus, for the reasons cited above, in this paper, we report the synthesis of a multivalent vanadium hydride aryl gel from convenient and inexpensive precursors and explore its properties in electrochemical and hydrogen storage applications.

Preparation of vanadium hydride
By analogy to Wilkinson's synthesis of tetrakis (trimethylsilyl methyl) vanadium from VCl4 and the alkyl lithium,13 phenyl lithium (50 mmol, 25 mL of a 2.0 M solution in dibutyl ether) was stirred at room temperature. VCl4 (2.03 mL, 12.5 mmol) was added dropwise via a syringe. The reaction mixture turned dark brown, increased temperature, and bubbled vigorously. The reaction continued stirring for 15 minutes until it had stopped bubbling and cooled back to room temperature. The mixture was filtered to give a dark brown residue and brown filtrate. Because of the well-documented thermal instability of homoleptic alkyl complexes of vanadium,18, the putative tetraphenyl vanadium (IV) complex was not isolated but used immediately in the next synthesis protocol described below. Working electrodes of V(IV)-25C-H2 and VO2 consisted of 80% active material, 10% superconducting carbon black, and 10% polyvinylidene fluoride (PVDF) by weight. The electrode paste was made by grinding the active material with carbon black to ensure a good mixture of the powders. The powders were then stirred with PVDF and n-methyl-2-pyrrolidinone (NMP) solvent until a homogenous paste was formed. The paste was spread onto the copper foil (current collector). The electrode was dried by heating to 80 °C to bake off any excess NMP solvent before increasing the temperature to 120 °C overnight to allow the electrode material to bind with the current collector. Once completed, the electrodes were punched using a Hohsen electrode punch to produce uniform-sized discs of 15 mm. The same procedure was followed for the respective counter and lithium metal foil reference electrode. The electrolyte used in this investigation is 1.0 M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) solutions at a ratio of 1:1 by volume. Electrochemical cells were assembled in a glove box, and the cell potential window used for these tests was between 1-3.2V vs. Li/Li+. The cyclic voltammetry measurements were carried out at scan rates of 0.5, 1, 2, and 5 VMS-1, and galvanostatic charge-discharge data was recorded at a current density of 1 mA cm-2. All measurements were carried out at room temperature using a Hohsen HS flat cell. Electrochemical AC impedance spectra were obtained using the same Li battery cell setup by applying a sine wave with an amplitude of 0.5 mV over the frequency range from 100 KHz to 0.01 Hz.

Price of Vanadium hydride
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