Preparation And Capacitive Properties Of Three-dimensional Holey-reduced Graphene/Ni_xCo_3-xS₄ Composite Materials

Supercapacitor is considered to be an advanced energy storage device with development prospect because of its advantages of high-power density,fast charge discharge and long cycle life.However,the low energy density of the existing supercapacitor can not meet the needs of the actual development.The development of composite electrode materials with excellent electrochemical properties is an effective way to improve the energy density of supercapacitors.In recent years,it has become a research hotspot to synthesize high-performance composite electrode materials by combining graphene-based materials with nickel cobalt bimetallic sulfide with high specific capacity.In this paper,graphene oxide?GO?was prepared by improved Hummer's method,and the electrochemical properties of GO were improved by etched and heteroatom doping.On this basis,Ni_xCo_3-xS₄?x=1 or 2?/nitrogen and sulfur co-doped three-dimensional holey-reduced graphene oxide by solvothermal method and hydrothermal method.The obtained materials are systematically characterized and the capacitive properties are studied,and their application in hybrid supercapacitors is further investigated.The main research contents and results of this paper are as follows:?1?GO was synthysized by improved Hummer's method,and further holey graphene oxide?HHGO?was prepared by hydrothermal method;Nitrogen?N?,sulfur?S?and phosphorus?P?tri-doped holey GO?N,S,P-HHGO?material was synthysized by a hydrothermal approach with ammonium dihydrogen phosphate and L-cysteine as dopants.The electrochemical tests results indicate that the specific capacitance of the N,S,P-HHGO electrode is 295 F g^-1 at 1 A g^-1,the capacity retention rate is71.2%at 20 A g^-1 and the initial capacity retention is 93.5%at 3 A g^-1above 10 000 cycles.The excellent electrochemical behaviors can be attributed to the superior pore-size distribution and the synergistic effect caused by the doping of N,S,P heteroatoms.?2?Co Ni₂S₄/nitrogen and sulfur co-doped three-dimensional holey-reduced graphene oxide?Co Ni₂S₄/N,S-HRGO?composite architectures was prepared by solvothermal and hydrothermal methods.The electrochemical test results show that the specific capacity of the prepared electrode is 233.9 m Ah g^-1 at 1 A g^-1,which is much higher than that of the prepared Co Ni₂S₄ electrode(136.1 m Ah g^-1).In addition,the specific capacity retention rate is still 89.6%after 2000 cycles at 5 A g^-1.The excellent electrochemical performances of Co Ni₂S₄/N,S-HRGO electrode is attributed to the synergistic effect between the two materials,which can effectively promote the degree of electrolyte infiltration on the materials,improve the conduction rate of ions and charges,and alleviate the volume change of electrode materials during the charging and discharging process.?3?Ni Co₂S₄/nitrogen and sulfur co-doped three-dimensional holey-reduced graphene oxide?Ni Co₂S₄/N,S-HRGO?composite architectures were prepared through a two-step hydrothermal approach.The electrochemical test results show that the specific capacity of the Ni Co₂S₄/N,S-HRGO electrode is 184.2 m Ah g^-1 at 1 A g^-1,which is higher than that of Ni Co₂S₄/N,S-RGO(143.1 m Ah g^-1)and Ni Co₂S₄ electrode(96.1 m Ah g^-1).In addition,the capacity retention is 64.9%at 50 A g^-1,and the capacity retention is 91.8%at 10 A g^-1 after 2000 cycles.Furthermore,a hybrid supercapacitor utilizing Ni Co₂S₄/N,S-HRGO electrode as the positive and N,S,P-HHGO electrode as the negative is fabricated,showing a larger energy density of 35.4 Wh kg^-1 at 749.5 W kg^-1 and good cycle stability with 87.5%of capacity retention at 10 A g^-1 after 10000cycles.Moreover,two hybrid supercapacitors linked in series can effectively power a white LED indicator and drive a mini motor,which demonstrates the great potential of the as-assembled hybrid supercapacitor for practical applications.