What is vanadium hydride?
The vanadium hydride materials V(IV)-100 and V(IV)-25C-H2 were tested for room temperature hydrogen storage to compare their activity to V(III) alkyl hydride gels previously synthesized by our group. 9c Hydrogen PCT adsorption-desorption isotherms were thus recorded at 298 K and compared to standard isotherm of AX-21 recorded under the same conditions at 2-3 mmol total adsorption of H2 for accuracy. The initial material V(IV)-100 reached a maximum of 2.1 wt % at 130 bar and 25 °C without saturation, as shown in Figure 7. After solid-state hydrogenation to make V(IV)-25C-H2, the hydrogen storage performance improved to reach a maximum of 5.8 wt% at 130 bar and 25 °C shown in figure 7. The adsorption isotherm increases in a linear fashion.
Proposed mechanism for the synthesis of vanadium(IV) hydride
Infrared spectroscopy was used to gauge hydrocarbon loss on hydrogenation of V(IV)-100 by observing the relative intensity of the C-H stretch in the 2900 – 2960 cm-1 region. The infrared spectra of V(IV)-100 and V(IV)-25C-H2 are shown in the placement of phenyl ligands with hydrides further to completion. While the precise molecular formula of these two materials cannot be determined from the TGA data because of the unknown composition of the final combustion product arising from ambiguous oxidation states and possible vanadium carbide formation, the sum of our data is consistent with the formulation of V(IV)-25CH2 as a vanadium aryl hydride with the general formula VHxy(C6H5)y. X-Ray photoelectron spectroscopy (XPS) was carried out to determine the oxidation states of the vanadium species present in V(IV)-100 and V(IV)-25C-H2, and the data is shown in figures 1 and 2, respectively, with the baseline corrected XPS figures displayed in S7 and S8. In the vanadium 2p/3 region of V(IV)-100, there is a broad emission centered around 516 eV, consistent with an average oxidation state of V(IV). Peak fitting of this emission demonstrates that there are multiple oxidation states of vanadium present in the material. A vanadium (V) species can be simulated at 516.9 eV, while the simulated emission at 515 eV can be assigned to V(V) compared to the XPS of V2O5. 21 The simulation at 515.3 eV can be attributed to a V(III) species as it is close in value to the emission reported for VCl3 at 515 eV. 22 After hydrogenation of V(IV)-100 to give V(IV)-25C-H2, the broad emission moves to 513 eV, as shown in Figures 1 and 2. Peak fitting of this emission demonstrates that there are still multiple oxidations.
States of vanadium present in the material
The material's states of vanadium are present, but an overall reduction has occurred in hydrogenation. Simulation shows that a vanadium (IV) species is now present, as demonstrated by the emission at 516.1 eV, which is close to the emission of V(IV) at 516.3 eV for V2O4. 23 The two minor simulated emissions at 515.3 eV and 514.3 eV are similar in intensity. They can both be attributed to V(III) species as they are close to the emission reported for V2O3 at 515.7,23, suggesting that these are two different V(III) species with slightly different environments. The emission with the largest intensity falls at 513.8 eV and can also be attributed to a V(III) species compared to the 513.9 eV reported for V(OH)3. 24 The small emission at 512.2 eV can be assigned to V(0)25 and demonstrates that excessive reduction leads to the formation of V metal. The oxidation state distribution of V(IV)-25C-H2 is similar to that observed for lower valent vanadium hydride gels synthesized previously by our group from V(III) alkyl precursors.9c Overall, this demonstrates that this important new hydrogen storage material can be readily accessed indirectly by reduction of more conveniently prepared V(IV) aryl species as opposed to directly from more elusive and expensive V(III) alkyl compounds while also highlighting the flexibility of oxidation states in this class of materials. These properties can be exploited for electrochemical energy storage or catalysis.
Electrochemical assessment of vanadium hydride
Because V(IV)-25C-H2 contains various V oxidation states, further characterization by electrochemical methods and possible exploitation as a cathode material is warranted. The electrochemical properties of V(IV)-25C-H2 were thus assessed using various electrochemical methods. The potential window used for these studies was slightly different from what is typical for V(IV) materials as V(IV)-25C-H2 became unstable above 3.2 V. So, we could not reach the maximum 4 V normally employed for measuring other vanadium compounds. 15,26 we first tested a VO2 commercial powder as an external standard to compare the electrochemical behavior.
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