Zinc-Cobalt Oxide/Sulfide Quantum Dots Anchored On Nitrogen-Doped Graphene For Hybrid Supercapacitor

As a new type of energy storage device,supercapacitor is an irreplaceable part of energy storage because of its rapid charge and discharge,high cycle life,and high power density.The design and preparation of electrode as the core of supercapacitor is naturally the focus of research.Among many electrode materials,zinc-cobalt-based oxide/sulfide has attracted widespread attention from researchers due to rich electrochemical activity,high electrical conductivity,low cost and environmental protection.Currently,although the reported zinc-cobalt-based oxygen/sulfide nanomaterials,such as nanowires,nanosheets,nanorods,and nanoflowers,exhibit superior electrochemical performance,they are of hundreds of nanometers in diameter or even larger,which would greatly restrict the utilization of active materials due to the limited penetration depth?²0nm?of the electrolyte ions during charge and discharge.In addition,current nanomaterial electrodes have a relatively low mass loading(¹ mg cm^-2),and in order to achieve commercial applications,the mass loading needs to reach about 10 mg cm^-2.In view of the above problems,in this paper,we take zinc-cobalt oxide/sulfide as the research object,and its nanomaterials are reduced to the size of quantum dots?<10 nm?and composite with graphene.The best electrochemical performance was obtained by optimizing experimental parameters.The main research contents are as follows:?1?We have designed and synthesized a type of ZnCo₂O₄ quantum dots?QDs?decorating on nitrogen-doped graphene nanosheets?denoted as ZnCo₂O₄/NG?via a simple two-step solvothermal process.By comparing the morphologies and electrochemical properties of ZnCo₂O₄/G,ZnCo₂O₄and NG prepared under the same conditions,it was found that quantum dots and NG can prevent each other from agglomerating and overlapping.Due to the synergistic effect of ZnCo₂O₄ quantum dots and NG,ZnCo₂O₄/NG under the optimal concentration parameters shows high specific capacitance(607.7 F g^-11 at 1 A g^-1),outstanding rate performance(84.5%at 30 A g^-1)and superior cycling stability?89.2%after 5000 cycles?.Electrochemical measurements of asymmetric devices assembled with ZnCo₂O₄/NG and activated carbon showed that the energy density reached 28.3 Wh kg^-1.?2?To further improve the conductivity of the material,we prepared the corresponding zinc-cobalt-based sulfide quantum dots.We have prepared Zn-Co-S/NG composites with ultra-small Zn-Co-S quantum dots modified nitrogen-doped graphene via adding sulfur source in the two-step solvothermal process.At the same time,we achieved different size control of Zn-Co-S nanoparticles in composites by changing the ratio of ethanol/deionized water in the solvent and studied the effect of different sizes on the electrochemical performance.The specific capacitance of Zn-Co-S/NG with the smallest size reaches 710.7 F g^-11 at 1 A g^-1.When the current density is increased 100 times,that is,up to 100 A g^-1,the capacitance of the composites still reaches 443.0 F g^-1,and the corresponding capacitance retention rate is 62.3%,which reflects its super high rate performance.In addition,the electrochemical energy storage performance was studied by assembling and testing asymmetrical devices.The hybrid device reached a maximum energy density of 32.0 Wh kg^-1 at a power density of 0.8 kW kg^-1,when the power density increased to 16 kW kg^-1,the energy density remained at20.6 Wh kg^-1,showing good energy storage performance.?3?To achieve self-support and high mass loading,we achieved the co-assembly of Zn-Co-S/NG composites and porous graphene nanosheets through adding sodium ascorbate,resulting in a novel three-dimensional self-supporting structure Zn-Co-S/HGF.We have optimized the pore formation of graphene as a three-dimensional self-supporting framework,solved the inconsistencies of electron transport and ion transport,and realized the commercial high loading requirement of Zn-Co-S/HGF(10 mg cm^-2).The results show that the optimum Zn-Co-S/HGF-2 electrode has a capacitance of 567 F g^-1 at 10 A g^-11 when the loading capacity is 1 mg cm^-2,and exhibits attenuation of only 9%with mass loading of 10 mg cm^-2.Based on the high loading of single-electrode,we also studied the electrochemical performance of hybrid device assembled with Zn-Co-S/HGF and HGF.The area energy density of the Zn-Co-S/HGF-2//HGF-2 hybrid device is as high as 0.5439 mWh cm^-2 when the area power density is 15.75 mW cm^-2,which indicates the possibility of commercial application.