Nickel-cobalt Oxide/Sulfide Anchored On Three Dimensional Graphene Composites For Supercapacitors

The electrical energy produced by green and renewable resources such as solar,wind or tidal energy is sufficient to meet the ever-growing energy demand to a certain extent.Supercapacitors,as an essential energy storage device for the development of sustainable energy systems,can provide higher power density,cycle stability and charge-discharge rate than batteries.Among many electrode materials with great potential,transition metal oxides,especially nickel-cobalt-based oxides and sulfides,are favored for their abundant redox active sites,high capacitance and economic and environmental benefits.However,bare they have suffered from poor conductivity and agglomeration,resulting in a significant reduction in the effective contact area with the electrolyte and thereby reducing their electrochemical properties.Graphene with representative 2D honeycomb sheets composed of sp² carbon atoms,has been developed thoroughly in the field of supercapacitors due to its abundant functional groups,high theoretical specific surface area,excellent conductivity and environmental friendly property.However,the actual capacitance of graphene is much lower than its theoretical value due to its poor ionic permeability and surface wettability,insufficient active sites and stack of nanosheets.To date,there are two reliable methods to tackle these issues.One efficacious approach to tailor its surface activity and electrochemical performance is by N doping,which can improve the electronic structure of graphene.Another method is to reduce the stack of graphene sheets by adjusting its morphology,such as the preparation of graphene with wrinkles or 3D architecture.Based on the above discussion,we combined nickel-cobalt-based oxides and sulfides with 3D N-doped graphene framework（NGF）respectively.By designing the structure of the electrodes and devices,we investigated the electrochemical and energy storage properties of the composite materials,making some contributions to the construction of new composite electrodes for supercapacitors.The specific research contents are as follows:（1）NGF was prepared by one-step fast flame reduction method using cost-effective melamine sponges with stable three-dimensional network structure as raw materials.Subsequently,NiCo2S4/NGF with 3D open structure was obtained by compounding NGF as matrix with nanotubular nickel-cobalt binary sulfide（NiCo2S4）.At the same time,bare NiCo2S4 was prepared by the same experimental method without adding NGF.By comparing the morphologies and electrochemical properties of two electrodes,it was found that the addition of NGF can not only effectively prevent the agglomeration of NiCo2S4 nanotubes,but also generate synergistic effect to enhance the electrical performance of the composite materials.The NiCo₂S₄/NGF electrode exhibits specific capacitance as high as 1240 F g^–1 at 1 A g^–1,almost two-fold of its counterpart NiCo2S4.The NiCo2S4/NGF electrode exhibits prominent durability against long-term cycling（80%of its original specific capacitance was kept after 5000cycles）.Moreover,the asymmetric supercapacitor was assembled by applying NiCo2S4/NGF and activated carbon（AC）as positive and negative materials respectively,delivering a decent energy density of 36.8 Wh kg^–1 at 375 W kg^–1.（2）On the basis of the first work,we synthesized NiCo2O4/NGF with hierarchical porous structure by hydrothermal method and subsequent calcination process using NGF as matrix.By comparing the performance of bare NiCo₂O₄,we found that the agglomeration of NiCo2O4 was alleviated after the addition of NGF.The uniform distribution of NiCo2O4 nanosheets exposed many active sites which were originally blocked by agglomeration,and effectively improved its electrochemical performance.As expected,the NiCo₂O₄/NGF possessed specific capacitance of 1198 F g^–1 at 1 A g^–1 and decent durability against long-term cycling（only 3%of its original capacitance was lost over 5000 cycles）.In addition,the NiCo₂O₄/NGF\\AC ASC could deliver an energy density of 32.1 Wh kg^–1 at 188 W kg^–1,and 22 LEDs could be lit by two NiCo2O4/NGF\\AC devices connected in series,demonstrating its practical significance.