Research On The Fabrication And Supercapacitor Performance Of Vanadium-Based Composite

Vanadium-based transition metal compounds have been regarded as an ideal electrode material for supercapacitors due to their multi-valence states,rich redox reactions,high electrochemical activity,good structural stability as well as excellent conductivity.Especially,combining with other metal elements into V-based bimetallic compound can significantly improve their capacitive performance in water electrolyte.In order to further improve their capacitive performance,on the one hand,the structure and properties of the electrode material can be optimized by controlling the synthesis conditions of the material,on the other hand,the synergistic effect of the two materials can be used to improve their performance.In particular,the graphene has aroused great attention in the field of supercapacitor due to their good electrical conductivity,excellent structural stability,as well as the ultra-high theoretical specific surface area and the feasibility of mass production.Therefore,compounding graphene with vanadium based bimetallic compounds is a good idea to improve their structural stability,conductivity,rate performance,and excellent capacitance.In this paper,two kinds of vanadium based bimetallic compounds（vanadium manganese based bimetallic compound and vanadium nickel based bimetallic compound）were successfully synthesized and successfully modified with graphene to improve their electrochemical properties.Metal vanadate has aroused great attention in terms of their abundant vanadium resources,low cost and high theoretical capacity.Especially,hydrated metal vanadate has aroused great attention in terms of the larger interlayer spacing and stable structure,in which the interlayer crystal water molecules can act as pillars to stabilize the interior structure.In this paper,a novel hydrated manganese vanadate（Mn V2O6·2H2O）/reduced graphene oxide（r GO）hybrid was fabricated with a methanol-assisted solvothermal method.The Mn V₂O₆·2H₂O spheres with a diameter of 200 nm were uniformly encapsulated by graphene sheets.Especially,the presence of methanol can not only change the anisotropic growth of manganese vanadate,but also preserve the interior crystal water.Meanwhile,a“GO-assisted Ostwald ripen-splitting”mechanism is proposed to explain the formation of Mn V₂O₆·2H₂O/r GO.As a result,the Mn V2O6·2H2O/r GO positive electrode manifests the significantly enhanced specific capacity（1976.3 F/g at 2 A/g）and superior cycling stability（87.5%after 10,000 cycles at 50 A/g）in three-electrode system.In addition,as a novel negative electrode,Mn V2O6·2H2O/r GO was also designed and synthesized with excellent capacitive performance（284.6 F/g at 0.5 A/g）as well as remarkable cycling stability（95.8%after 10,000 cycles in 6 M KOH）.The assembled symmetric supercapacitor device presents a superior energy density of 51.5 Wh/kg（at 849.2 W/kg）within a large potential window（1.7 V）and maintains an excellent cycling stability of87.1%after 6,000 cycles at 10 A/g.Vanadium has been comprehensively used to synthesize V doped Ni-based electrodes due to its low cost,abundant resources and the closeness of its atomic radius to that of nickel.In this paper,by virtue of the[Ni（NH₃）6]²⁺complex ions which can retard the supply of Ni²⁺ions and guarantee the direct and uniform coating of metal hydroxides on graphene substrate,a high-performance Ni V-LDH/r GO hybrid layer was designed as precursor for the subsequent sulfuration and selenization process.Other than the alkaline environment,both the reaction time and GO precursor content are also crucial to forming Ni V-LDH/r GO hybrid layer.After ion exchange treatment,the obtained V-doped nickel sulfide/selenide（named as Ni V-S/r GO or Ni V-Se/r GO）with optimized sulfur/selenium source dosage exhibited excellent capacitive performance（1734.2 C/g and 1577.2 C/g at 2 A/g）and superior cycling stability.A novel asymmetric supercapacitor（ASC）composed of two distinctive V-based electrodes Ni V-X/r GO（X=LDH,S and Se）positive electrode and Mn V₂O₆·2H₂O/r GO negative electrode)was fabricated with exhibit excellent energy density within 1.6 V.Impressively,the Ni V-S/r GO//Mn V2O6·2H2O/r GO device presents a higher energy density（82.4 Wh/kg at 800 W/kg）than that of Ni V-Se/r GO//Mn V₂O₆·2H₂O/r GO（60.0Wh/kg at 800 W/kg）.After 10,000 cycles at10 A/g they also maintain a superior cycling stability of 93.7%and 91.5%,respectively.