Study On The Synthesis And Electrochemical Performance Of Nickel-based 2D Nanosheets Electrode Materials

Supercapacitors,as known a new-type electrochemical capacitors,have attract much attention due to their advantages such as high power density,superior rate capability,rapid charging and discharging,long cycle life and low maintenance cost.Although the energy density of most commercially available supercapacitors is much higher than conventional dielectric capacitors,it is still significantly lower than that of batteries and fuel cells.Thus,tremendous research effort has been performed aiming at increasing the energy density of supercapacitors without sacrificing their high power capability.In general,supercapacitors can be divided into two types according to the energy storage mechanism: electrical double layer capacitors（EDLCs）and pseudocapacitors.In pseudocapacitors,fast and reversible redox reactions at the surface or near-surface of electrode materials are carried out to enable energy storage,leading to their higher capacitance than EDLCs storing charges by electrical double layer at the interface of electrode/electrolyte.Nickel-based hydroxides are an important category of electrode materials for application in supercapacitors having advantages of high theory capacitance and low cost.Their main disadvantages are their poor electric conductivity and low stability of charge and discharge,limiting their application as electrode materials.To solve such problems for nickel-based hydroxides,we synthesized nickel hydroxide electrode materials with 2D morphology and study their electrochemical performance in this dissertation as shown in the following.（1）Surface-modified hexagonal β-Ni（OH）2 nanosheets: glycerol modified β-Ni（OH）2 nanosheets with regular hexagonal shape were prepared by a hydrothermal method with morpholine as precipitant and nickel chloride as nickel source.It is considered that the glycerol with high hydrophilicity and the rigid shape of β-Ni（OH）2 nanosheets facilitate the formation of ordered pores inside electrode after electrochemical activation.As a result,the obtained electrode material exhibits a high specific capacitance of 1917 F g^-1 at current density of 1.6 A g^-1 in 3 mol L-1 KOH solution.At high charge and discharge current density of 31.3 A g^-1,this material still remains a high specific capacitance of 1289 F g^-1.（2）Soluble α-Ni（OH）₂ monolayer nanosheets: α-Ni（OH）₂ monolayer nanosheets were successfully synthesized in ethanol solution at 80 °C using NaOH as precipitant.The characterization results indicate that it is difficult for the formation of interlayer hydrogen bond based on water molecules due to the water-deficient environment in absolute ethanol,resulting in the formation of α-Ni（OH）₂ monolayer nanosheets.Because of the atomic thickness of monolayers,α-Ni（OH）₂ monolayer nanosheets exhibited excellent electrochemical performance.The maximum specific capacitance of α-Ni（OH）₂ reached 3074 F g^-1 at scan rate of 5 mV s-1 with a potential window of 0–0.6 V in 3 M aqueous solution.It had a high specific capacitance of 2459 F g^-1 at 2 A g^-1,and still kept a high capacitance of 1182 F g^-1 at high current density of 40 A g^-1.（3）Co-Ni LDHs monolayer nanosheets: Co-Ni LDHs monolayer nanosheets were produced by grinding the solid mixture of nickel nitrate hexahydrate,cobalt nitrate hexahydrate in morpholine and the following aging at room temperature for 10 h.When the molar ratio of Co²⁺ to Ni²⁺ is 0.2,the obtained Co-Ni LDHs monolayer presented the highest electrochemical performance.Its maximum specific capacitance reached 2416 F g^-1 at current density of 1.64 A g^-1 within potential window of 0.6 V in 3 M KOH aqueous solution,and still kept a high capacitance of 1316 F g^-1 at high current density of 35.1 A g^-1.（4）Sandwich-type composite of α-Ni（OH）₂ monolayer/reduced graphene oxide: The sandwich-type structure was built through the electrostatic attraction between the positively charged α-Ni（OH）₂ monolayers and the negatively charged graphene oxide（GO）single layers.The alternately intercalated RGO in composite greatly reduced the internal resistance originating from the α-Ni（OH）₂ component.Consequently,the composite presented high electrochemical performance for the supercapacitor application.