Research On Capacitance Performance Of Green And Low Cost Ni(OH)₂/Expanded Graphite Composite

A supercapacitor as a new type of energy storage device,owing to its high power density and long cycle life,has been extensively investigated.However,its low energy density and high price have limited its further applications.Electrode material is one of the most important components in the supercapacitor which almost determines its performance and price.Nickel hydroxide?Ni?OH?₂?becomes one of the research hotspots for the electrode materials in recent years,due to its high theoretical specific capacitance and low toxicity.However,its poor electronic conductivity?10-17 S/cm?hinders the electron transfer and reduces the efficiency of the redox reactions during charge and discharge processes,thus leads to low power density and poor rate capability.To solve this issue,an effective way is to introduce highly conductive carbon materials.In this thesis,a green and efficient approach will be adopted to develop the electrode materials with low cost and excellent electrochemical performance.Firstly,cost-effectively commercial expanded graphite?EG?is chosen as a raw material,and the Ni?OH?₂/EG composite with high specific capacitance will be prepared by a green and non-toxic electrochemical method.However,the larger Ni?OH?₂ particle size in the composite leads to poor rate performance and cycling stability.Compared to unitary metallic hydroxides,metallic double hydroxides usally can provide more redox sites,enhance the electronic conductivity,and have synergistic effects.In order to improve the rate performance and cycle stability of Ni?OH?₂/EG composites,cobalt?Co?will be introduced to obtain smaller particle size,then more surface active sites and lower resistance can be achieved.So the nickel-cobalt double hydroxide/EG?Ni-Co DHs/EG?composites with superior excellent electrochemical performance will be synthesized.Finally,mixed acid?concentrated sulfuric acid and nitric acid?and calcination will be respectively used to pretreat EG,and the effects of surface properties of carbon material on the performance of Ni-Co DHs electrode will be investigated.Finally,an aqueous electrolyte-based asymmetric supercapacitor?ASC?will be assembled,in which the obtained composite materials is used as a positive electrode and a commercial activated carbon?AC?is used as a negative electrode,6 M KOH solution and nonwoven fabrics are used as the electrolyte and diaphragm,respectively.The main results are listed in the following.?1?The optimal conditions for preparation of Ni?OH?₂/EG composites are as follows:The deposition potential was set at-1.1 V versus SCE electrode,and the electrolyte solution was 0.1 M Ni?NO3?2·6H2O in a 1:10?v:v?H20/DMF mixed solvent.The mass ratio of CNTs was 10%and binder was Nafion solution.The Ni?OH?₂/EG composite exhibits a layered structure composed of thin nanosheets by SEM.The inserted Ni?OH?₂ nanoparticles lead to expanded interlayer spacing and create some pore structures,which is beneficial to the access of electrolyte ions.It delivers a satisfactory initial specific capacitance of 1719.5 F/g at the discharge current density of 1 A/g,and the capacitance only decreases to 1181.3 F/g even at 10 A/g,showing a remarkable rate capability of 68.7%.?2?The specific capacitance of AC/Ni?OH?₂/EG ASC is 67 F/g at the current density of 0.5 A/g.When the current density increases to 10 A/g,the specific capacitance decreases to 43.5 F/g,showing a remarkable rate capability of 64.9%.The AC//Ni?OH?₂/EG ASC can deliver a prominent energy density of 69.34 Wh/kg at power density of 1.05 kW/kg,and long cycling lifespan of 79%initial capacitance after 1000 cycles at 5 A/g.?3?When Ni²⁺ and Co²⁺ molar ratio is 2:1,the obtained Ni-Co DHs/EG composite demonstrates the best electrochemical performances.The specific capacitance is 1246 F/g at 0.5 A/g;when the current density increases to 10 A/g,the specific capacitance decreases to 1143.8 F/g,showing a superior excellent rate capability of 91.8%.The capacitance retention of Ni-Co DHs/EG composite is 81%after charge-discharge 1000 cycles at 10 A/g.The cycle stability is much higher than Ni?OH?₂/EG composite due to the introduction of Co.?4?EG was modified by the mixed acid of H2SO4 and HNO3 with a volume ratio of 3:1 and high temperature calcination at 900 ? for 3 h,the modified EG was denoted as a-c-EG.Electrochemical tests show that the cyclic stability of Ni-Co DHs/a-c-EG composite is greatly improved.This is due to the fact that the oxygen-containing functional groups are generated on EG surface during the mixed acid treatment and then turns into defects when calcined at the elevated temperature,and Ni-Co DHs particles are preferentially nucleated at the defects.Moreover,The specific capacitance of AC//Ni-Co DHs/a-c-EG is 82.2 F/g at 0.5 A/g,while the capacitance still retains 61.2%at 10 A/g.Besides,the capacitance retention is 96.8%after charge-discharge 1000 times at 5 A/g.When the power density is 1.35 kW/kg,the energy density can reach 98.6 Wh/kg.Even at a higher power density of 27.03 kW/kg,the energy density can still be maintained at 60.36 Wh/kg.In summary,the Ni?OH?₂/EG composite prepared in this thesis shows excellent initial specific capacitance.The carbon material EG is cost-effective and the entire preparation process is green,non-toxic and simple.Introduction of Co can improve the rate performance and cycle stability of nickel hydroxides.Carbon materials with defects on the surface coupled with Ni-Co DHs,this composite indicates excellent specific capacitance,superior rate performance,and improved cycle stability,it can provide very good prospect for supercapacitor applications.