Preparation Of Layered Ni(OH) 2 /expanded Graphite Composite By Pulse Electrodeposition And Its Electrochemical Properties

For supercapacitors,the energy storage characteristics are mainly determined by the properties of electrode materials.Expanded Graphite,as a new type of carbon material,has the advantages of large specific surface area and excellent electrical conductivity.It has been regarded as an ideal electrode material for electric double layer capacitors,but it is easily restack to graphite.Ni(OH)2 as a pseudocapacitive electrode material,has a high theoretical specific capacitance,but its electrical conductivity is poor and its particles are easily aggregated during charge-discharge process.Ni(OH)2/graphite composite has the advantages of both graphite and Ni(OH)2 and can overcome their shortcomings,and thus attracted intensive and extensive attentions.In this thesis,cost-effective expanded graphite(EG)will be used as a support,and pulsed electrochemical deposition method will be adopted to synthesize a layered Ni(OH)2/EG nanocomposites.Firstly,Ni/EG was prepared by pulsed electrochemical deposition,then followed by an in-situ electrochemical activation process,thus Ni/EG composites was transformed to Ni(OH)2/EG nanocomposites.Preparation parameters of Ni(OH)2/EG nanocompsosites were investigated by single-factor experiments,such as slurry preparation,pulse electrodeposition,and electrochemical activation.Finally,the Ni(OH)2/EG composites prepared under the optimal conditions was used as a positive electrode and commercial activated carbon(AC)as a negative electrode,an assembled unit supercapacitor was tested.The crystal structures of EG and the Ni(OH)2/EG composites were characterized by X-ray diffraction and selected-area electron diffraction.The Ni(OH)2 contentin the Ni(OH)2/EG composites was analyzed by thermogravimetric analysis.The morphology was observed by scanning electron microscopy and transmission electron microscopy.The electrochemical performance was tested by cyclic voltammetry(CV),galvanostatic discharge-charge(GCD),and electrochemical impedance spectroscopy(EIS).(1)Investigation of the slurry preparation conditions is the first step to study the Ni(OH)2/EG composite.Particle size,slurry composition,binder type,and conductive agent were considered.CV results demonstrated that particle size of EG was 80 mesh,Nafion was used as the binder,slurry was mixed with Ni(OH)2/EG:CNTs:Nafion mass ratio of 85:10:5,and the ultrasonic duration last 6 hours,the obtained Ni(OH)2/EG composites exhibited the best electrochemical performances.(2)During the electrodeposition process,deposition potential,deposition time,plusc working time,nickel ion concentration,and deposition temperature were investigated.According to the results,when the deposition potential was-3 V,0.05 M nickel solution was used as the electrolyte,the deposition time was 15 min,the working pulse time(Ton = 3 ms),the off time(Toff= 1 ms),and the deposition temperature was set at 30 ?,the electrochemical performance of Ni(OH)2/EG was the best.(3)In the process of electrochemical activation,KOH concentration,CV scan rate,and potential window were investigated.When 6 mol L-1 KOH was used as the electrolyte,the CV scan rate was 80 mV s-1,the potential window was 0.15-0.55 V,the results show that the composites has the highest specific capacitance of 947 F g-1 at CV scan rate of 2 mV s-1,and the capacitance retention was 90%after running charge-discharge 1000 times at current density of 10 A g-1.(4)Assembly of unitcapacitors.Firstly a symmetrical AC//AC capacitor with a mass loading of 7 mg cm-2 was assembled,when operating at voltage window of 1.2 V,achieved a specific capacitance of 32.7 F g-1,and a power density of 610 W Kg-1 and energy density of 6.3 Wh kg-1.Then an asymmetric AC//Ni(OH)2/EG supercapacitor with total mass loading of 6.5 mg cm-2 was assembled,with a working voltage window of 1.4 V,the specific capacitance was 70 F g-1,and the energy density was 31.1 Wh kg-1 at a power density of 520 W kg-1.In summary,Ni(OH)2/EG composite,prepared by electrochemical deposition using EG as raw material,has advantages of high specific capacitance,good rate capability and long cycle life.The asymmetric AC//Ni(OH)2/EG capacitor also shows good rate capability,long cycle life,high power density and energy density.The preparation technique of Ni(OH)2/EG composites is facile,low-cost,and easily scaled-up.Therefore,Ni(OH)2/EG will have potential and promising applicationsin supercapacitors.