Study On Preparation Of Ni-based Hydroxide/Graphene Composite Materials And Their Supercapacitive Performance

With rapiding depletion of fossil fuels and growing environmental pollution,environmentally benign electric vehicles are bound to enter a great number of families.Due to their advantages of larger power density, longer cycle life and non-maintenance,supercapacitors have been recognized as ideal auxiliary power sources for electric vehicles(EVs). However, supercapacitors at present can only deliver relatively low energy density,which seriously limits their applications in the feids of EVs. Therefore, synthesis ofelectrode materials with high energy density has become the research focus in the area.Layered Ni-based hydroxides exhibit large specific capacitance and low electricalconductivity, while graphene possesses the merits of large surface area, high electricalconductivity, good chemical tolerance. By the aid of the synergistic effect between thecomponents of the Ni-based hydroxide and graphene, it is feasible to obtain ahigh-performance composite material for supercapacitors.This dissertation concerns investigations on homogeneous growth of Ni-basedhydroxide nanoparticles on graphene sheets in order to achieve composite materials withhigh power density, high energy density and good cycling performance. The main resultswere summarized as follows:(1) Effect of various solvent systems used for hydrothermal synthesis was studied onelectrochemical performance of the obtained Ni(OH)2/graphene composite materials.Using graphene, nickel nitrate, sodium hydroxide as starting materials, three types ofNi(OH)2/graphene composites were synthesized by a hydrothermal method in the solventsof water, water+glycol and glycol, respectively. The results show that the nickel hydroxidein the composite synthesized in the solvent of water is mainly composed of-Ni(OH)2,while the one obtained in the solvents of glycol and water+glycol mainly consists of-Ni(OH)2with a particle size of~10nm. In comparison with the Ni(OH)2/graphenecomposites systhesized in the solvents of water and glycol, the one obtained in the mixedsolvent of water+glycol shows a higher supercapacitive performance. The electrodeexhibites a specific capacitance of1965and756F/g at scan rate of2and50mV/s,respectively. At charge/discharge current densities of1and40A/g, its capacitance was determined to be2020and1002F/g, respectively. The electrode exhibites a specificenergy density of98.23Wh/kg at a power density of1.18kW/kg, still delivering a highenergy density of37.79Wh/kg when increasing power density to11.34kW/kg. However,during2000cycles at a current density of10A/g, the electrode showed a poor cyclingperformance, which really needs to be improved.(2) Influence of the metal ion doping was studied on supercapacitive performance ofthe Ni(OH)2/graphene composite materials. The Ni0.8M0.2(OH)δ(M=Zn, Co, Al, Y)/graphene composites were synthesized by a hydrothermal method in a water+glycol mixedsolvent, using graphene, nickel nitrate, zinc nitrate, cobalt chloride, aluminum nitrate,yttrium nitrate and sodium hydroxide as starting materials. The results show that theNi-based hydroxides in the obtained composites after incorporation of Zn, Co and Al ionsare mainly composed of-Ni(OH)2, and the Ni-based hydroxide in the Y-doped compositemainly comprises-Ni(OH)2. Also, the doped nickel hydroxides in the synthesizedcomposites grow homogeneously on the graphene sheets. Discharge efficiency and cyclestability of the Ni(OH)2/graphene composite materials are improved in varying degrees bythe doping of Zn, Co, Al and Y ions. Particularly, the Ni-Al hydroxide/RGO compositematerial shows the best rate capability and cycle stability. The Ni-Al hydroxide/grapheneelectrode exhibites a specific capacitance of1222and461F/g at scan rate of2and50mV/s, respectively. At charge/discharge current densities of1and40A/g, its capacitancewas measured to be1460and830F/g, respectively. The electrode presents a considerablehigh specific energy density of61.13Wh/kg at a power density of0.73kW/kg, stillpreserving a high energy density of23.08Wh/kg when increasing power density to6.92kW/kg. Furthermore, after2000cycles at a current density of10A/g, it shows11.9%capacitance decay, exhibiting much improved cycling stability.(3) The final chapter in the dissertation involves investigations on further improvingsupercapacitive performance of Ni-Al hydroxide/graphene composite materials by the Coion doping. Ni1-xAl0.2Cox(OH)δ(x=0,0.05,0.1,0.15)/graphene composites weresynthesized by a hydrothermal method in a water+glycol mixed solvent, using graphene,nickel nitrate, cobalt chloride, aluminum nitrate, sodium hydroxide as starting materials.The results show that the Ni-Al-Co hydroxides in the obtained composites present a crystalstructure similar to-Ni(OH)2, and the Ni-Al-Co hydroxide nanoparticles uniformly growon graphene. Supercapacitive performance of the Co-doped Ni-Al hydroxide/graphenecomposite materials is obviously improved. Among the obtained Ni-Al-Cohydroxide/graphene composites, the one with Co ion content x=0.05shows the highest supercapacitive performance. The electrode exhibites a specific capacitance of1274and530F/g at scan rate of2and50mV/s, respectively. At current densities of1and40A/g, itscapacitance was determined to be1303and882F/g, respectively. It delivers a high energydensity of63.69Wh/kg at a power density of0.76kW/kg, retaining an energy density of26.19Wh/kg when increasing power density to7.90kW/kg. Most importantly, its cyclingstability is greatly improved, no capacitance fading being observed during2000cycles at acurrent density of10A/g.