Preparation Of Nickel Hydroxide-based Composites And Their Application In Supercapacitors

Supercapacitors（SCs）as a novel type of green energy storage device,have attracted more attentions due to its excellent charge-discharge rates,high power densities and high safety,which will have promising applications in electric vehicles and renewable energy systems.However,the relatively low energy density has limited its wide commerical application.As a vital component of SCs,electrode materials are manily responsible for the electrochemical performance of SCs including the energy/power density and lifespan.Nickel hydroxide（Ni（OH）₂）has been widely investigated as a battery-type supercapacitor material because of its high theoretical capacitance(2082 F g^-1),good stability,easy synthesis and low cost.However,the irreversible phase charge during the cyclic charge-discharge process and the relatively low conductivity lead to a visible decrement of the electrochemical performance of Ni（OH）₂,especially at high scan rates or current densities.Herein,in order to overcome weaks of Ni（OH）₂,this work will modify the Ni（OH）₂-based composite materials by adopting several stratgies.Meanwhile,the electrochemical performances of the Ni（OH）₂-based electrode in three-electrode system and hybrid supercapacitors（HSC）system were also evaluated.The main research contents and conclusions of this work are as follows.（1）The La-decorated Ni（OH）₂ nanosheets（La-Ni（OH）₂）were fabricated via a one-pot microwave hydrothermal method.The electrochemical behavior of the La-Ni（OH）₂ electrode material could be modulated by expediently adjusting the amount of La precursor in the preparation process.When the amount of La precursor was 0.3 mmol,the La-Ni（OH）₂-0.3electrode had the best electrochemical property with a specific capacity of 209.8 m A h g^-1 at1 A g^-1,which was 2.1 times higher than that of Ni（OH）₂ electrode.The assembled La-Ni（OH）₂-0.3//active carbon（AC）capacitor provides the highest energy density(47.5 W h k g^-1)at the power density of 375.0 W kg^-1 and good capacity retention（83.3%of intial specific capacity）after 5000 charge/discharge cycles.The prominent electrochemical performance of La-Ni（OH）₂-0.3 is mainly attributed to its high surface area,large amount of mesopores and enhanced conductivity induced by the introduction of La.（2）Ni（OH）₂ nanosheets（as the shell component）were grown in situ on ZIF-67-derived CoS nanoparticles（as the core component）deposited on carbon cloth（CC）to form the CoS@Ni（OH）₂/CC core-shell heterostructure.The CoS@Ni（OH）₂/CC electrode possesses a superior specific capacity of 561.6 m A h g^–1 at a current density of 1 A g^–1,which is superior to the pure Ni（OH）₂/CC electrode(199.4 m A h g^–1).The assembled CoS@Ni（OH）₂/CC//AC capacitor provides an energy density of up to 90.8 W h kg^–1 at a power density of 800 W Kg^–1,higher than most of the reported Ni（OH）₂-related devices,and possesses a capacity retention of 71.7%over 5000 charge-discharge cycles.The outstanding electrochemical performance of CoS@Ni（OH）₂/CC is chiefly due to the mediation of the Co²⁺/Co³⁺redox cycle for the rapid conversion of Ni²⁺into Ni³⁺,which greatly boosts the charge-transfer efficiency in the electrochemical reactions.In addition,the constructed core-shell heterostructure promotes the exposure of the electrochemical active area of the main active substance and reduces the charge transfer resistance,which leads to an enhanced utilization of the electrodes.（3）Metal element doping and construction of core-shell heterostructures have been effective ways to enhance the electrochemical performance of Ni（OH）₂ electrode in the two aforementioned chapters.In this chapter of work,Mn-doped Ni（OH）₂ nanosheets with oxygen vacancies were in-situ grown on the Co₃O₄ nanorods deposited on CC to form the Co₃O₄@Mn-Ni（OH）₂/CC core-shell heterostructures.When the current density is at 1 A g^–1,The optimized Co₃O₄@Mn-Ni（OH）₂/CC electrode has an outstanding specific capacity of 313.4 m A h g^–1,which is 6.4 times higher than that of the unmodified Ni（OH）₂/CC electrode(48.9 m A h g^–1).The assembled Co₃O₄@Mn-Ni（OH）₂/CC//AC capacitor displays a marvelous energy density of 65.5 W h kg^–1 at a power density of 800 W kg^–1 and the capacity retention was up to 93.0%even after 10000 charging-discharging cycles,far better than that of Co₃O₄@Ni（OH）₂/CC//AC capacitor（36.5%）and Mn-Ni（OH）₂/CC//AC capacitor（53.9%）.The excellent electrochemical properties of Co₃O₄@Mn-Ni（OH）₂/CC is mainly due to the Mn-doped inhibits the irreversible phase transition of Ni（OH）₂ during the charging and discharging process and enhances the recycling stability of the material.Moreover,the existence of oxygen vacancies facilitates the redox reactions occurring on the electrode material.Meanwhile,the core-shell heterostructure promotes the exposure of active sites and reduces charge transfer resistance,which in turn improves the utilization of the electrode.