Preperation And Electrochemical Propreties Of Graphene/Ni（OH）₂ Electrode Materials For Supercapacitor

With the energy and environment problem becoming more and more serious in the modern modern society, long cycle life, high power output and environmental low-toxic performance make supercapacitor widespread use in many fields. However, lower energy density limits further development of supercapacitor. Transition metal hydroxide, especially nickelous hydroxide has high theoretical specific capacitance and energy density, but poor electrical conductivity. What’s more, serious inflation happers during charging and discharging process, thus bringing safe problem to the supercapacitor. Graphene is a two dimensional plane like carbon material with excellent electrical conductivity and stable chemical performance. So that we can bring together these two materials to synthesis a composite electrode material aiming at obtaining ideal capacitive propertiesIn this thesis, we first of all study the synthesis methods of Ni(OH)2 and (active)graphene/Ni(OH)2 materials. This is a low cost, easy operation and environment friendly method that can be introduced to industrial production. Then we research the capacitive properties of the as-prepared electrode materials which deliver stable long cycle life and high power density and energy density. The concrete conclusion are as follow:(1) Ni(OH)2 electrode material was synthesized by chemical precipitation method using KOH, NaOH and ammonia solution as the precipitator, respectively. At a scan rate of 1 mV/s, these three materials Ni-K, Ni-Na and Ni-N delivered a specific capacitance of 1672F/g,1381F/g and 950F/g. When the current density is lA/g, their specific capacitance are 1144F/g,935F/g and 503F/g. It can be concluded that Ni-K perform better capacitive property. In another word, the KOH precipitator can make the Ni(OH)2 a better performance. After a 5000 long cycle test, the specific capacitance of Ni-K decrease from 917F/g to 543F/g, retaining 59.2% of the initial capacitance(2) Graphene material was prepared by the modified Hummers method. Then the graphene/Ni(OH)2 composites with various ratios (1:1,1:3,1:5,1:10) were synthesized. It can be seen from the result that with the adding of graphene material, the specific capacitance of the composite electrode materials rise a lot. At a scan rate of 1mV/s, the GN5 composite obtained a specific capacitance of 24070F/g and performed 2200F/g and 1266F/g at a current density of lA/g to 40A/g. This is better than the pure Ni(OH)2 of 1144F/g and 603F/g. During a 5000 long cycle test, a specific capacitance of 1510F/g and 1257F/g was obtained in GN5 material with the capacitance retain of 83.2%.The GN5 material exhibited a specific energy density of 53.2 Wh/kg at a power density of 12.8 kW/kg,even 92.4Wh/kg at the power density of of 251W/kg. The excellent capacitive property and energy performance made GN5 the best component in the four composite electrode materials. Besides, we have also researched the electrochemical performance of the prepared electrode materials by symmetrical & asymmetrical supercapacitor and lithium battery test. After a 5000 cycles the specific capacitance of GNS symmetrical supercapacitor decreased from 123F/g to 117F/g, showing a capacitance retention of 95.1%, it’s largest specific energy density and specific power density are 18.75 Wh/kg and 6.74kW/kg, separately. For the GN5//GNS asymmetrical supercapacitor, the largest specific energy density and specific power density are 62.8 Wh/kg and5.54kW/kg, separately, owing to the increasing of working voltage to 1.5V. It can be concluded that the GN5 material show perfect capability performance of the GNS/Ni(OH)2 materials.(3)An active idea was introduced to synthesis active graphene aiming at acquiring large specific superficial area, finally to improve the long cycle life of Ni(OH)2 material. As the current density rise from 1 A/g to 40A/g,the specific capacitance of AGN10 material decrease from 2450F/g to 1398F/g, higher than that of pure Ni(OH)2.After a 5000 long cycle test at 10A/g, AGN10 material showed only 6.5% capacitance loss, from 1603F/g to 1499F/g, exhibiting a more stable cycle property than that of all graphene/Ni(OH)2 material. The AGN10 also presented considerable power density of 15.1kW/kg and the highest energy density of 103Wh/kg in the four active graphene/Ni(OH)2 materials. All the conclusion implied that the 1:10 component ratio of active graphene/Ni(OH)2 can get a perfect cycle performance and will turn into the next generation of supercapacitor electrode material.