Preparation And Electrochemical Properties Of Graphene/Metal Sulfide (Oxide) Composite

Transition metal oxides and sulfides have attracted much attention as electrode materials for supercapacitors in view of their chemical and structural reversible pseudocapacitive characteristics, which make them own multiple oxidation states for pseudocapacitance,resulting in higher capacitance in comparison to electric double layer capacitors. However, the intrinsic semicon ducting nature of metal oxides and sulfides show poor electrical conductivity, eventually leading to low utilization of electrode materials and the poor rate capability, thus restricting their development. Graphene has a high conductivity and ultra-large surface area (2600 m2 g-1), therefore it owns excellent capacitive characteristic and cycling stability. However, because of the aggregating nature of graphene due to strong p-p interactions, the large surface area cannot be fully utilized, thus leading to the decrease of capacitance. In this paper, graphene oxide (GO) was employed as a precursor and conductive substrate to prepare transition metal oxides and sulfide nanoparticles/graphene composites with excellent electrochemical performance for supercapacitors. The microstructure and morphology of composites were characterized by XRD, SEM, TEM, Raman and XPS.Meanwhile, the interalationship between the microstructure and the electrochemical performance of the composites was also deeply investigated.First of all, NiS nanoparticles and its composites were synthesized by a facile hydrothermal route. Besides, the reaction temperature and time were also taken account to investigate its influence on the microstructure and electrochemical properties of NiS. At the same time the effect of loading amount of NiS on the microstructure and electrochemical properties of GNS-NiS composite was also investigated; With the loading of NiS gradually increased, more NiS grew on the surface of graphene, which effectively improve the electrical conductivity and specific capacity of the composites. But when NiS content reached a certain value, graphene cannot provide enough surface for the growth of NiS nanosheets, leading to accumulation of NiS, which inhibited the utilization electrochemical active material.GNS-CNTs-NiS ternary composite was also synthesized, in which the NiS nanoparticles uniformly distributed on the surface of GNS and CNTs. More importantly, the composite avoid the aggregation of NiS nanoparticles effectively, improving the utilization of specific surface area, which is advantageous to the electrolyte ion transfer and transportation.Moreover, the conductivity and cycle performance can be further enhanced by the addition of carbon nanotubes due to the conductive bridge constructed by CNTs, further improving the specific capacitance of the hybrid material. A maximum specific capacitance of 2377 F·g-1 at 2 mV·s-1 was obtained, which is 1.5 times higher than pure NiS (1599 F·g-1).Secondly, CoS2 nanoparticles and its composites were prepared using a one-step hydrothermal rout. Subsequently, the microstructure and morphology of composites were characterized and the electrochemical performance of the composites was also tested. The results show that the proper mass loading of CoS2 is beneficial for the growth of CoS2 nanoparticles on the surface of graphene, increasing the ion transport space and accelerating the exchange of the ion, thus leading to a 10% increase of capacitive retention; Carbon nanotubes are beneficial for the electron transfer and electronconductivity, enhancing the utilization of active material.Finally, ultra-small Co3O4 nanoparticles/graphene hybrid material had been synthesized by a facile hydrothermal route and consequent calcination process.The effect of heat treatment temperature on the properties of the GNS-Co3O4 was investigated. The results indicate that the temperature will lead to the great change of crystallinity, influencing the nucleation of Co3O4 on the surface of the graphene. With Co3O4 loading increases gradually, capacitance properties of composites first increased and then decreased and there is an optimal load.Synergistic effect of graphene and Co3O4 can effectively improve the electrochemical performance of the composite material, the GNS-80%Co3O4-300 composite exhibited a high specific capacitance of 461 F·g-1 at 5 mV·s-1, good rate capability, and excellent cycling performance (88% retention of its initial capacitance after 2000 cycles), indicating a promising electrode material for supercapacitors.