Preparation And Supercapacitor Performance Of The Metal Sulfide Electrode Materials

The ever-worsening energy and global warming issues call for the urgent development of clean and high-performance energy storage devices.As a new class of energy storage devices between conventional capacitors and batteries,supercapacitors have attracted great attention over the past decades due to their prominent characteristics of high power density,long cyclic life,and superior safety.It is well known that the structure and morphology of electrode active materials have a very significant impact on the capacitance performance of energy storage devices.Apart from these widely-used electrode materials,transition-metal sulfides have received increasing attention as electrode materials in view of their high theoretic capacities,abundant availability and low costs.Among them,nickel and cobalt sulfides are favored by researchers because of their low toxicity,large specific capacitance,and simple preparation process.In addition,nickel and cobalt sulfides have various phases,and valence states,which facilitate the reversible redox reaction,and the electronegativity of sulfur elements is lower than oxygen elements,which is conducive to increase the specific capacitance of the active material.Moreover,the better flexible and extended of nickel and cobalt sulfides can effectively reduce the volume effect of the electrode material during rapid charging and discharging,thereby improving the cycle stability of the active material.In this work,by controlling the pH value of the initial solution,severals nickel/cobalt sulfide 3D micro-nano structures were successfully prepared by hydrothermal synthesis method,including hexagonal Ni3S4 flower-like structure,lantern-like CoS structure,and NiCo2S4 amorphous structure.The structure and morphology of the materials were characterized by XRD,SEM,BET,and EDS.The effect of the pH on the morphology variation of the product was discussed.Their capacitance properties were also tested in a three-electrode electrochemical system.The main contents of this paper are as follows:(1)Morphology regulation and supercapacitor performance of nickel sulfide electrode materials.The effect of different pH values on the microstructure of nickel sulfide was investigated by one-step hydrothermal method.The test results show that the pH value is very important to the morphology,structure,and phase of the products.The neutral(pH=7)condition is favorable for the formation of flower-like Ni3S4 micron structure,which has the largest specific surface area(35.8 m2·g-1).Under different collecting fluids(carbon paper,nickel foam)and electrolyte types(KOH or H2SO4),the electrochemical test results show that Carbon paper is superior to nickel foam,and the basic electrolyte is superior to the acid electrolyte.At the current density of 1 A·g-1 and 6 mol·L-1 KOH electrolyte solution,the flower-like Ni3S4 micron-structure obtained at pH=7 has the largest specific capacity(up to 1005 F·g-1),which firstly attributed to the high specific surface area for the flower-like layered structure.Secondly,the thinner surface layer can shorten the diffusion and migration distance of electrolyte ions.(2)Controlled synthesis and electrochemical properties of CoS nanostructures:The morphological and structural transformations of CoS with different pH values(4,7 and 9)were investigated.The result shows that the acidic condition(pH=4)is favor for the formation of CoS lanceolate structures with the high specific surface area(19.2 m2·g-1)and the specific capacity is 791 F·g-1 at a current density of 1 A·g-1 and 6 mol·L-1 KOH electrolyte solution.After 1000 times of continuous charge and discharge test,the specific capacitance retention rate is almost 100%,which indicates that the CoS lanceolate structures have excellent cycle stability.(3)Synthesis and electrochemical properties of nickel-cobalt complex sulfide electrode materials:On the basis of the above studies,the morphology and electrochemical properties of nickel-cobalt composite sulfides at different pH values(4,7 and 9)were further investigated.The results showed that compared with the single sulfide,the alkaline condition(pH=9)is conducive to the acquisition of NiCo2S4 crystals with high specific surface area(47.58 m2·g-1),and the specific capacity is also can be greatly improved to 1403.7 F·g-1 at the current density of 0.5 A·g-1.Compared with single metal sulfide,the specific capacity of the nickel-cobalt composite sulfides is also greatly improved due to the larger specific surface area(47.58 m2·g-1)and the synergistic effect between Ni and Co element.After a constant current charge-discharge cycle about 1000 times,the specific capacity of the material remained at 92%at a high current density of 10 A·g-1,which proved that the cycle stability of the sample was good.The result is also proved that nickel-cobalt composite sulfide(NiCo2S4)with high specific capacitance and good cyclic stability is a good supercapacitors material.