Microstructural Construction of Nickel and Cobalt Based Hydroxides and the Research on Their Electrochemical Properties

Author:Bai Xue

Supervisor:jing xiao yan


Degree Year:2018





The consumption of fossil fuels and environmental pollution increase serious threat to the sustainable development of humankind.In response to these challenges,a large amounts of research have been contributed to the development and utilization of renewable energy sources such as solar energy and wind energy.Taking into account the intermittent issue of these energy sources,it is necessary to employ energy storage systems to transfer and store energy by a stable and controllable method.Supercapacitors are widely used as high power output energy storage devices due to their excellent characteristices of high power density,fast charge and discharge rate,and excellent cycle stability.However,in comparison with battery system,its low energy density has lead to short duration that limites the development and application.In order to solve this problem,nickel and cobalt based hydroxides were selected as research materials in this dissertation,a variety of different methods(electrostatic self-assembly,template sacrificel method,hydrothermal method,and dip-coating method)were employed to construct composite materials as high conductive materials and pseudocapacitive materials.The electrochemical performance of fabricated electrodes were investigated,and finally,the electrodes were assembled by carbon material forming asymmetric supercapacitors to achieve improvement of energy density and power density simultaneously.Ni-Al LDHs was the main object in the first part of dissertation.In order to resolve the difficulty of stacking among layers,separation of layers by an exfoliation method in two-dimensional scale was proposed.Ni-Al LDHs nanosheet colloid with positive charge was obtained via exfoliation method,however,the poor electrical conductivity of this type of material was not solved.A negatively charged GO dispersion was prepared by the modified Hummers method.The NAL/RGO composites with chiffon like structure were successfully prepared by electrostatic self-assembly and subsequent alkali reduction.The maximum energy and power densities of assembled NAL/RGO//AC were 46.4 Wh kg-1 and 6080.2 W kg-1,respectively.After 2000 cycles,the retention of specific capacitance was 92.5%.In order to further explore the morphology structure and optimization of performance,the Ni-Co LDHs and Co-Co LDHs hollow structures were respectively prepared via template sacrifice method.Considering the high conductivity of graphene,it can effectively compensate the poor conductivity of two materials.The composite materials consisting of Ni-Co LDHs,Co-Co LDHs and graphene were prepared by in-situ deposition and chemical etching.When the amount of graphene was 15 mg,two composites displayed excellent electrochemical performance.The specific capacitance of Ni-Co LDHs/G15 reached 1265.2F g–1 at current density of 1 A g-1.The maximum specific capacitance of Co-Co LDHs/G15was 1205.1 F g–1.At same time,two electrode materials all exhibited excellent cycling stability.After 2000 cycles,the capacitance retention of Ni-Co LDHs/G15 and Co-Co LDHs/G15 were 92.9%and 96.5%,respectively.In order to solve the influence of conductive adhesive on electrochemical performance of material during preparation process of the electrodes,a three-dimensional graphene foam nickel skeleton(3D RGO NF)was prepared by a simple dip-coating method.Followed by a hydrothermal method,Ni-Co DH nanowires were loaded on the skeleton.According to the results of SEM and TEM measurements,Ni-Co DH exhibited interconnected nanowire morphology.The nanowires were approximately 5μm in length and approximately 30 nm in width.The maximum specific capacitance of electrode material was 1041.1 F g-1 at current density of 2.5 mA cm-2.When the current density increased 20 times,the specific capacitance can still reach 627.4 F g-1,with 60.2%capacitance retention.However,the active material of this electrode was easy to falling off during cycling test.According to an improved synthesis method,a 3D RGO skeleton was successfully prepared by addition of a small amount of PTFE to the GO dispersion and then etching nickel foam away by acid.The doposition of ternary NiCoAl LDHs nanosheets on to 3D RGO was performed via a facile hydrothermal reaction.In order to match the asymmetric capacitors with high performance,an ordered mesoporous carbon was successfully synthesized using sucrose as the carbon source with SBA-15 as the template.NiCoAl LDHs/3D RGO exhibited high specific capacitance of1421.1 F g-1 and excellent rate capability(when the current density increased 15 times,capacitance retention still remained 67.8%).The hydrophobicity of the chemically modified carbon was improved with a typical property of double-layer capacitor.The electrode material achieved a maximum specific capacitance of 289.0 F g-1 at a current density of 1 A g-1.NiCoAl LDHs/3D RGO//MMC all-solid-state asymmetric capacitor exhibited a high energy density of 57.1 Wh kg-1 and a power density of 8650.0 W kg-1,as well as an excellent mechanical stability.Nickel,cobalt-based hydroxide can not only be used as an active material alone but also be combined with other pseudocapacitance materials to form a hierarchical structure,which could increase the active area and improve the electrochemical performance.The Co3O4@Ni(OH)2 core-shell electrode material with nanosheet array structure was synthesized via a two-step hydrothermal method.According to the results of SEM and TEM tests,the average size of core-shell material was 2.5μm with 250 nm in thickness,and the length of Ni(OH)2 shell was about 100 nm.The maximum specific capacitance of Co3O4@Ni(OH)2material was 1308.0 F g-1 in 2 mol L-1 KOH solution as electrolyte,as well as excellent rate performance(when current density increased 10 times,the capacitance retention was 46%).The assembled device of Co3O4@Ni(OH)2//AC all solid state supercapacitor achieved a maximum energy density of 40.0 Wh kg-1.