低共熔溶剂中直接电解还原V_(2)O_(5)制备VO_(2)的研究

Direct Electrolysis of V_(2)O_(5) to VO_(2) in Choline Chloride-Ethylene Glycol Deep Eutectic Solvent

利用氯化胆碱-乙二醇低共熔溶剂(ChCl-EG DES)无毒、价廉、性质稳定且合成工艺简单的特性,以其为电解质,模压成型的多孔五氧化二钒(V_(2)O_(5))块体为阴极,石墨片为阳极,在搅拌速率300 r·min^(-1),温度80℃,槽电压2.8 V条件下直接电解还原V_(2)O_(5)块体制得二氧化钒(VO_(2))。采用循环伏安曲线测试方法探讨了直接还原V_(2)O_(5)的电化学行为,通过X射线衍射(XRD)与扫描电镜(SEM)检测分别研究了槽电压对产物物相和形貌的影响规律,并探讨了V_(2)O_(5)直接电解还原的过程。循环伏安分析结果表明,80℃下在ChCl-EG DES中填入铂微孔电极的V_(2)O_(5)粉末能够被还原为VO_(2);在2.4~2.8 V范围内,随着槽电压的不断升高,阴极反应速率明显加快,V_(2)O_(5)块体被逐渐还原为VO_(2);电解还原过程发生在电解质、集流体以及钒氧化物这三者的接触点处,还原反应由块体表层不断向其内部推进。该方法具有工艺流程简单、反应温度低、对设备要求低以及环境友好的优点,为低温电化学制备VO_(2)粉体提供了一条新途径。

Vanadium as a strategic metal was widely used in many fields of modern industry.Vanadium pentoxide(V_(2)O_(5))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 V_(2)O_(5) 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(VO_(2))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 VO_(2) powders at low temperature.The electrochemical behavior of direct reduction of V_(2)O_(5) 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 V_(2)O_(5) powders for the preparation of porous V_(2)O_(5) 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 V_(2)O_(5) 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 V_(2)O_(5) 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 V_(2)O_(5) 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 V_(2)O_(5) pellet in ChCl-EG DES was discussed based on the ex⁃perimental phenomena.The results of cyclic voltammetry analysis showed that V_(2)O_(5) powders filled with Pt microporous electrode could be reduced to VO_(2) 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/V_(2)O_(5)/electrolyte threephase reaction point expanding with the pellet surfaces.Then a new formed VO_(2)/V_(2)O_(5)/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 V_(2)O_(5).When the cell voltage rose to 2.6 V,the phase of VO_(2) 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 VO_(2).Scanning electron microscope(SEM)images of the raw material and products showed that V_(2)O_(5) 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 V_(2)O_(5) to VO_(2).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 VO_(2)/V_(2)O_(5)/electrolyte reaction point in the pellet interior.Besides,the large electric contact resistance between porous VO_(2) and V_(2)O_(5) 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 V_(2)O_(5) to porous VO_(2) was proposed.This method provided a new way for the electrochemical preparation of VO_(2) from V_(2)O_(5) at low temperature.