Synthesis Of Mn, Co And Ni Based Chalcogenides And Their Electrochemical Properties

During the past decades, numerous efforts have been devoted to exploit newenergy storage devices with high energy and high power density that can be used inelectrical vehicles. Supercapacitors are one class of energy storage systems that haveattracted tremendous attention due to their superior advantages including highpower/energy density, excellent cycling stability and fast charge/discharge capability.On the basis of energy storage mechanism, there are two types of supercapacitors,namely electrical double-layer capacitors and pseudocapacitors. Recent researchefforts have been made by exploiting novel electrode materials for supercapacitorswith both high energy density and power density. Generally, metal oxides and carbonmaterials are the two major electrode materials of supercapacitor. Among variousmetal oxides, ruthenium dioxide (RuO2) has been widely considered as the optimumelectrode material for supercapacitor. However, its high cost and highly-toxic natureseverely restrict its practical application on a large scale. Therefore, the developmentof low-cost metal oxides as alternative options is highly desirable. The productssynthesized in this paper, CoS2，MnO2，Ni(OH)2，NiS2and NiCo2O4are allalternatives of RuO2for the application of capacitor materials.It is noted that electrode materials play a key role in the development of highperformance supercapacitors in terms of the morphology, size, porosity and so forth.The performances of pseudocapacitive materials are mainly determined by the electrochemical activity and kinetic feature of the electrodes. It is well known that thepseudocapacitance is mainly produced by fast faradaic reactions associated withdouble injection of ions and electrons. To improve the supercapacitor performance, itis crucial to enhance the kinetics of ion and electron transport inside the electrodesand at the electrode/electrolyte interface. An effective way is to fabricate films withnanoporous-structures, which provide a relatively short diffusion pathway for ions aswell as large active surface, leading to high charge/discharge capacities and highutilization of active materials. Spherelike CoS2nanocrystals (NCs) with poroussurfaces were prepared through a modified molten-salt synthesis (MSS) method usingonly Co(NO3)2·6H2O and thiourea as the starting materials. Thiourea acts as a flux,reactant and structure-directing agent in the synthesis of CoS2NCs, which leads tomilder reaction conditions, pure product and porous surface on CoS2NCs. Based onthe effects of reaction time and the ratio of the starting materials on the morphologyand the capacitive properties of the CoS2NCs, a three-step synthesis mechanism ofCoS2nanocrystals was proposed. The specific capacitance CoS2NCs supercapacitorreached913F g-1with a specific surface area of29.30m2g-1. These resultsdemonstrate that CoS2NCs can be produced in large scale by solid-phase synthesisthrough a simple pathway. The excellent electrochemical performances of CoS2NCsmake it a promising electrode material for supercapacitiors.The large specific capacitance of electrochemical supercapacitors is a result oftwo mechanisms that occur at or near the electrode/electrolyte interfaces in thecapacitors. The first is double-layer capacitance, which is a non-Faradic process, andthe second is charge-transfer-reaction pseudocapacitance, which is a Faradic process.These two mechanisms can work separately or together, depending on the activeelectrode materials used in the supercapacitors. Manganese dioxide (MnO2) has beenexplored as a promising electrode material for supercapacitors owing to its hightheoretical capacitance, abundant source, environmental benignity and low toxicity.To improve the electric conductivity of MnO2, researchers have developed compositesin which the MnO2is combined with highly conductive materials such as carbon in various forms or conducting polymers, for applications in both supercapacitors andbatteries. A new MnO2@NiS2/Ni(OH)2heterosructure materials are synthesized witha two-step hydrothermal method, and the morphology is easily developed. Theexistence of1D MnO2nanosticks not only provide a substrate for the junction, butalso facilitate the charge transmission. And the grafting of NiS2/Ni(OH)2sheets ontothe surface of MnO2enlarged the SSA of this material. By this means, SSA of theelectrode materials is improved and capacitor performance is optimized, as high as1010F g-1at a current density of1A g-1. The material also shows a good rate capacityand long-term cycling performance.Among various electrode materials, nickel cobaltite (NiCo2O4) is a verypromising electrode material since it ofers many intriguing advantages of low-cost,abundant resources and environmentally friendly. More significantly, it is reportedthat NiCo2O4possesses a much better electronic conductivity, at least two orders ofmagnitude higher, and higher electrochemical activity than nickel oxides and cobaltoxides. These attractive features are of huge benefit for the development of high-ratesupercapacitors. We have synthesized a novel hybrid composed of MnO2nanowiresand NiCo2O4nanoplates by a simple hydrothermal method without involving anysurfactants or surface modifications. For the MnO2@NiCo2O4prepared in a relativelylong time, the surface of MnO2nanowires is fully covered by the NiCo2O4nanoplatesthat could improve the charge transfer properties, hinder the dissolution of Mn speciesand allow the ion transport to the backbone. Therefor both MnO2nanowires andNiCo2O4nanoplates of the MnO2@NiCo2O4display enhanced electroactivity forcharge storage. Owing to the unique structure, the specific capacitance of theMnO2@NiCo2O4is better than MnO2nanowires. In conclusion, this well-designedhybrid exhibits excellent energy storage properties, which provides an alternative wayof constructing high-performance surpercapacitors without using carbon-orpolymer-based conductive materials. In a word, both the modified molten salt and hydrothermal methods, employed inthis paper, are all green and low-energy approaches for the synthesis of highperformance supercapacitor materials with various structures and components.Moreover, materials involved in this paper (Mn, Co, Ni) are all low-cost, abundant,and environmental friendly. In order to improve the property of electrochemistry, weoptimize the morphologies of electrode materials via the simply adjusting the reactionconditions. Finally, the as-synthesized heterostructured materials would perform apotential application in the fields of electrochemistry.