| Supercapacitors,one important category of energy storage devices that bridges the gap between dielectric capacitors and batteries with some unique properties such as great power density,long cycle life,and easy maintenance,have attracted active researches in the development of advanced electrode materials.Thereinto,nickle cobalt bimetallic compounds,as typical pseudocapacitive materials,have emerged as the promising candidates for pseudocapacitive supercapacitor electrode owing to its exceptional features of rich redox reactions,ultrahigh theoretical capacitance,abundant resources and environmental friendliness.Nevertheless,its inherent poor electronic conductivity and brittleness would still deteriorate the rate capability and cycling performance of supercapacitor electrodes.As a fascinating carbon material with known virtues of large specific surface area,well-defined porous structure,robust strength,and fast mass and charge transport kinetics,3D graphene network has been considered as a promising substrate for constructing hybrid supercapacitor electrode.The introduction of 3D graphene can not only buffer the volume change of nickle cobalt bimetallic compound during the charge-discharge process,but can provide considerable electric double layer capacitance,further improving the capacitive performance,rate capacity and cycling stability of the electrode.Herein,nickle cobalt bimetallic compound and graphene are chosen as electroactive materials and nickle foam is selected as current collector due to its high conductivity and porous structure.NiCo2O4/graphene and KNixCo1-xPO4/graphene composite electrodes were fabricated by hydrothermal process and electrodeposit process,and their structure,constitute,morphology,specific surface area,pore size distribution and electrochemical properties are studied systematically.Furthermore,asymmetrical supercapacitor devices were also assembled to study the practical application of those electrodes.Detailed researches are as follows:1.Hydrothermally prepared graphene hydrogel?GF?was utilized as an ideal conductive support,in which the porous GH can spontaneously wrap the skeleton of Ni foam and fill up the voids in Ni foam,improving the specific surface of GF.Then a facile one-step electrodeposition method was used to deposit NiCo2O4 nanoflakes on graphene sheets to form a hierarchical structure without further annealing process.The obtained NiCo20O4/graphene composite was denoted as NGF.?1?NGF composites fabricated under different electrodeposit temperature??,25? and 50??were denoted as NGF-0,NGF-25 and NGF-50,respectively.NGF-25 with hierarchical porous structure exhibits a high specific surface area of 206 m2/g,with the pore size ranging from mesopores to macropores.And NGF-25 shows the best electrochemical properties originating from the unique hierarchical structure.The specific capacitance of NGF-25 can reach to 3840 F/cm2 at 2 mA/cm2 and remain 71.6%at 50 mA/cm2.?2?The assembled asymmetric supercapacitor using NGF-25 as positive electrode and GF as negative electrode exhibits a maximum energy density and power density of 65 Wh/kg and 18.9 kW/kg,respectively,and a striking cycling stability with 92%capacitance retention after 5000 charge-discharge cycles at 2 mA/cm2.2.KNixCo1-xPO4·H2O/graphene composite with different Ni/Co ratio were fabricated through a simple one-step hydrothermal process,which are denoted as KNiP04 H2O/graphene?KNP/G?,KNi0.33Co0.6-7PO4·H2O/graphene?KNCP-0.5/G?,KNi0.67Co0.33PO4·H2O/graphene?KNCP-2/G?and KCoP04 H2O/graphene?KCP/G?,respectively.?1?KNiXCo1-xPO4·H2O nanosheets can disperse on the graphene sheets uniformly.Especially,KNCP-2 nanosheets interconnecting with each other to form flower-like structure anchor on graphene sheets,which facilitating the electron transfer and mass transport.The KNCP-2/G with hierarchical porous structure exhibits a high specific capacitance of 3240 mF/cm2 at 2 mA/cm2 and 72.1%retention at 50 mA/cm2,revealing good rate capability.?2?The assembled asymmetric supercapacitor using KNCP-2/G as positive electrode and active carbon as negative electrode exhibits a maximum energy density and power density of 34.3 Wh/kg and 11.3 kW/kg,respectively,and an exceptional cycling stability can be obtained as well. |
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