| Supercapacitors have attracted much attention due to their fast charging/discharging rate,long cycle life,green and high reliability.Owing to the Faraday redox reaction,transition metal-based supercapacitors can store more energy than carbon-based electric double-layer capacitors.Amoung them,transition metal oxide/hydroxide-derived materials,attributed to their inherent high electrical conductivity,are considered to be one of the most outstanding electrode materials for supercapacitors.So far,these materials with various nanostructures are synthesized.However,their actual capacitance and energy density are still relatively low.Therefore,how to effectively control their structure and morphology to improve their electrochemical performance needs to be further explored.That is of great significance to meet the requirements for high specific capacitance,high energy density and high rate capability of supercapacitors.This dissertation has carried out systematic research on the development of high-performance transition metal-based electrode materials,and the following meaningful results are achieved:（1）To solve the poor conductivity of transition metal-based compounds,a one-pot method is developed to simultaneously construct nickel-cobalt-based compound with conductive carbon-based material.Firstly,a low-temperature calcination method is used to simultaneously prepare an oxygen-rich Ni Co2O4/nitrogen-deficient g-C3N4 composite.The employed g-C3N4 not only acts as a substrate to tightly anchor the Ni Co2O4,but also induces the formation of oxygen-vacancy-rich Ni Co2O4.Meanwhile,the newly formed Ni Co2O4 species unexpectedly promote the denitriding process of g-C3N4 and catalyze the formation of graphite-like sp2 carbon structure.The simultaneously synthesis method can increase the rivet action and enhance the stability between carbon materials and Ni Co2O4 nanoparticles.Due to above advantages,the prepared composites can exhibit a high specific capacitance of 277.5 m Ah g-1 at 2 A g-1 and an excellent rate performance of 78.66%even the current density increasing 10-fold.In addition,by introducing boric acid,a one-step method is used to simultaneously prepare amorphous Ni/Co borate embedded into nitrogen-doped carbon/carbon nitride framework.Moreover,the obtained sample exhibits a much higher capacitance than similar borate.A series of subsequent characterizations discover that the Ni/Co borate undergoes an unexpected phase and structure reconstruction process during the test process to form the actual active material Co（OH）2/Ni（OH）2 with ultra-thin nanosheets.At the same time,the phase and structure transformation process mechanism of Ni/Co borate during the test is revealed.（2）To solve the poor ion diffusion ability caused by inappropriate microstructure of transition metal-based compounds,an anion induction-assisted electrochemical activation strategy is developed to construct high performance transition metal-based compounds with nanosheet structure.Firstly,thiourea is introduced and a simple calcination method is used to prepare a pomegranate-like Ni/Co-S nanoparticles coated with N,S co-doped carbon,which is then transformed into Ni/Co oxyhydroxide nanoflowers decorated with carbon fragments by electrochemical activation strategy.Moreover,the nanoflower is composed of a large number of sheet-like structures,which can fully expose the active sites.The flaky structure combined with the dotted carbon fragments and in-situ generated oxygen vacancies can ensure the Ni/Co oxyhydroxide nanoflowers with excellent electrical conductivity and ion transport capability.Thus,the obtained sample exhibits an excellent specific capacitance of 237 m Ah g-1,even the mass loading as high as 7 mg cm-2.In addition,the highly loaded Ni Co2O4 on nickel foam is prepared by molten salt method,then anion phosphorus is introduced into Ni Co2O4 to obtain phosphorus functionalized Ni Co2O4 by phosphating treatment.Subsequently,the above phosphorus functionalized Ni Co2O4 is transformed into oxygen vacany-rich Ni/Co oxyhydroxide porous nanosheets array by in-situ electrochemical activation strategy.Owing to the unique structure and surface properties,Ni/Co oxyhydroxide nanosheets can release an areal capacitance of 1.51 m Ah cm-2 at 1 m A cm-2,which is much higher than the original Ni Co2O4.Finally,an electrochemical activation strategy is used to treat the pre-prepared Fe3C/Fe nanoparticles to synthesize Fe3O4 nanosheets.When used as the negative electrode of supercapacitors,it can exhibit excellent specific capacitance as high as 353 m A h g-1 at 2 A g-1,much higher than iron-based materials.（3）To solve the low utilization of active materials when the mass loading of transition metal-based compound is high,a simple molten salt method is developed to build a hierarchical structure on nickel foam.Firstly,only using nitrate as the precursor,a simple calcination method is used to prepare a self-supporting Ni Co2O4 material,which is composed with 2D sheet layer and 3D hollow nanosphere core-shell structure,easily achieving a high mass loading of 30 mg cm-2.Due to the sufficient active sites and buffer space provided by the hierarchical structure,the self-supporting electrode material exhibits a remarkable areal capacitance(1.29 m Ah cm-2,4 m A cm-2),and excellent rate performance(80%,from 4 m A cm-2 to 50 m A cm-2)and high cycle life after 5000 cycles（113.7%）.Likewise,using only nitrate as the precursor,the Ni/Co oxyhydroxide with hierarchical hollow nanorod arrays is prepared by the low-temperature molten salt method assisted by the electrochemical activation method.Owing to the enough exposed active sites ensured by fully stripped lamellar structure and the buffer space provided by the hollow structure,the nanorods array exhibits an ultra-high areal capacitance of 2.82 m Ah cm-2 and specific capacitance of 353 m Ah g-1,as well as a high rate performance of 62%(from 1 m A cm-2 to 50 m A cm-2)and 84%capacitance retention after 5000 continuous charge and discharge cycles. |
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