Direct Electrolysis of V2O5 to VO2 in Choline Chloride-Ethylene Glycol Deep Eutectic Solvent

Vanadium as a strategic metal was widely used in many fields of modern industry. Vanadium pentoxide（V2O5）had a large proportion of reserves in vanadium resources. Its extraction and reduction were related to the development and application of vanadium resources. Therefore，the reduction of V2O5 to low-priced vanadium oxides could help to broaden the application of vanadium. In addi⁃ tion，based on the sudden changes of the electrical，magnetic and optical properties brought by the phase transition of vanadium ox⁃ ide，it had a good application prospect in the fields of optical disk dielectric materials，optoelectronic switches，thermistors，tempera⁃ ture sensors and so on. As a typical thermotropic phase change compound，the preparation of vanadium dioxide（VO2）was a hot re⁃ search topic. Deep eutectic solvents（DESs）had the characteristics of simple synthesis，biodegradability，wide electrochemical win⁃ dow，cheap and easy to obtain，which made it possible for the electrochemical preparation of VO2 powders at low temperature. The electrochemical behavior of direct reduction of V2O5 on platinum microporous electrode was investigated by cyclic voltammetry（CV）test with a three-electrode system on an electrochemical workstation. Before the electrolysis experiment，ammonium bicarbonate （NH4HCO3）was served as pore-making agent to add into V2O5 powders for the preparation of porous V2O5 pellets by mold pressing. The pellets were heated at 80 ℃ for 4 h in vacuum drying oven to remove NH4HCO3，resulting in lots of porosity in the pellet. Then V2O5 pellet was wrapped with stainless steel mesh and then attached to a titanium current collector. Small scale potentiostatic electrolysis ex⁃ periment with a two-electrode arrangement was carried out in 50 cm3 cell and the choline chloride-ethylene glycol（ChCl-EG）DES was used as electrolyte. On the direct reduction process，the porous V2O5 pellet（1.0 cm in diameter and 1.0 g weight）and a graphite sheet （20 mm×15 mm）were used as cathode and anode，respectively. The cell was purged with Ar flow and the constant voltages of 2.4~2.8 V were controlled by a direct current power supply for 20 h. The temperature was maintained at 80 ℃ and the stirring speed was 300 r·min-1 by a magnetic paddle. The electrolytic reduction process was analyzed in combination with the apparent changes of V2O5 blocks after different electrolytic time at 2.8 V cell voltage. The reduction process had been analyzed by the current-time curves during the constant voltage electrolysis，and the reaction phenomena at different voltages of 2.4~2.8 V were compared. The surface and cross section of the products were observed and the morphologies of the products were analyzed by scanning electron microscope. The phase of the prod⁃ ucts was detected by X-ray diffractometer（XRD）. The reaction process of V2O5 pellet in ChCl-EG DES was discussed based on the ex⁃ perimental phenomena. The results of cyclic voltammetry analysis showed that V2O5 powders filled with Pt microporous electrode could be reduced to VO2 in the electrochemical window of ChCl-EG DES at 80 ℃. The current-time curve of the reduction process firstly in⁃ creased and then stabilized over the constant voltage electrolysis. This could be attributed to the current collector/V2O5/electrolyte three-phase reaction point expanding with the pellet surfaces. Then a new formed VO2/V2O5/electrolyte reaction point expanded into the pellet interior. In the range of 2.4~2.8 V，the cathodic reaction rate increased obviously with the increase of cell voltage. The color of the pel⁃ let surface changed from yellow to blue and black，and its surface gradually became porous. XRD results showed that when the cell voltage was 2.4 V，and the phase was mainly still V2O5. When the cell voltage rose to 2.6 V，the phase of VO2 was generated after elec⁃ trolysis. When the cell voltage increased to 2.8 V，the diffraction peak of the pellet after electrolysis corresponded to the phase of VO2. Scanning electron microscope（SEM）images of the raw material and products showed that V2O5 pellet after the direct electrolysis be⁃ came loose and porous as increased cell voltage. The particle size of the products was decreased. These results indicated that the rais⁃ ing of cell voltage can effectively accelerate the electrochemical reduction rate of solid V2O5 to VO2. At the same time，with the prolon⁃ gation of reaction time，the direct reduction of oxides was slower than the initial stage because of the slow expanding rate of the VO2/ V2O5/electrolyte reaction point in the pellet interior. Besides，the large electric contact resistance between porous VO2 and V2O5 also had significant influence on the current. With the increase of cell voltage，the driving force of the reaction increased，and the reduction rate could be accelerated during the direct electrolysis of the pellet interior，which reflected on the raising of current. In addition，a schematic diagram of direct electrolytic reduction process for solid V2O5 to porous VO2 was proposed. This method provided a new way for the electrochemical preparation of VO2 from V2O5 at low temperature. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.