| Owing to the increasing energy demand together with the deteriorating environment and decreasing fossil fuels resources,the development of highly efficient energy conversion and storage devices is one of the key challenges of both fundamental and applied research in energy technology.Supercapacitors,which act as a bridge between high specific energy batteries and the high specific power electrolytic capacitors,have attracted increasing interest because of their high specific capacitance,fast recharge capability,and cycle efficiency.In general,supercapacitors can be divided into two types based on the energy storage mechanism and the electrode materials:pseudocapacitors and electrical double-layer capacitors(EDLCs).In general,the EDLCs with carbonaceous materials can offer higher power density(10.0 kW kg-1)and excellent cycling ability(10 000 cycles).However,the pseudocapacitors based on faradaic materials can deliver much higher specific capacitance and specific energy(100 Wh kg-1).In order to simultaneously take advantage of EDLCs' and pseudocapacitors' energy storage mechanisms,the hybrid materials combining conducting components and electroactive materials are the natural target.The main research contents and results are as follows:1.The hybrid nanostructure of NiCo2O4@rGO with the controllable composition was synthesized by depositing graphene oxide(GO)on the NiCo2O4 scaffold combined with chemical reduction.By combining NiCo2O4 mesoporous nanosheets with reduced graphene oxide(rGO),the hybrid electrode shows significant improvement in electrical conductivity,as well as more electrochemically active sites and short transport path lengths for both electrons and ions.Moreover,resulting from the Schottky electric field at the interface of NiCo2O4 and rGO,NiCo2O4 exhibits superior electron collection efficiency,thus contributing to the redox reactions on/near the surface of NiCo2O4.Benefiting from the rational structural features and excellent electrical conductivity,the designed NiCo2O4@rGO hybrid electrode demonstrates high specific capacitance(3.6 F cm-2 at a current density of 5 mA cm-2),good rate capability and superior cycling stability(90%of the initial capacitance can be retained after 2 000 charge/discharge cycles).2.An asymmetric supercapacitor offers opportunities to effectively utilize the full potential of the different potential windows of the two electrodes for a higher operating voltage,resulting in an enhanced specific capacitance and significantly improved energy without sacrificing the power delivery and cycle life.To achieve high energy and power densities,we have synthesized an all-solid-state asymmetric supercapacitor with a wider voltage range using Fe-doped Co3O4 and three-dimensional reduced graphene oxide(3DrGO)as the positive and negative electrodes,respectively.In contrast to undoped Co3O4,the increased density of states and modified charge spatial separation endow the Fe-doped Co3O4 electrode with greatly improved electrochemical capacitive performance,including high specific capacitance(1997 F g-1 and 1757 F g-1 at current densities of 1 and 20 A g-1,respectively),excellent rate capability,and superior cycling stability.Remarkably,the optimized all-solid-state asymmetric supercapacitor can be cycled reversibly in a wide range of 0-1.8 V,thus delivering a high energy density(270.3 W h kg-1),high power density(9.0 kW kg-1 at 224.2 W h kg-1),and excellent cycling stability(91.8%capacitance retention after 10 000 charge-discharge cycles at a constant current density of 10 A g-1).3.We design and synthesize a novel hybrid nanostructure composed of interconnected Ni3S2 mesoporous nanosheets and 3D reduced graphene oxide(3DrGO)supported by Ni foam through an electrodeposited method combined with a high-temperature postanealing processes.The good electrochemical capacitive activity of Ni3S2 and superior conductivity of 3DrGO synergistically lead to the remarkable electrochemical performances when utilized as electrodes for supercapacitors.In addition,an all-solid-state symmetric supercapacitor device is prepared based on the Ni3S2/3DrGO as electrodes and a filter paper in-between as separator.The fabricated supercapacitor device possesses remarkable electrochemical capacitor performance with specific energy of 59 Wh kg-1 at a specific power of 0.525 kW kg-1 and specific power of 3.675 kW kg-1 at a specific energy of 46 Wh kg-1.Remarkably,92%of the capacitance can be retained after 30 000 cycles at 10 A g-1.4.Exploring efficient all-solid-state flexible supercapacitors is particularly attractive to face the rapid growing demand of powers for flexible and wearable energy storage devices.Herein,we report a novel strategy to prepare high-performance all-solid-state flexible asymmetric supercapacitors characterized by nanostructured Ni3S2 nanorods as positive electrode and three-dimensional reduced graphene oxide(3DrGO)as negative electrode.Due to the tunable morphological structures and novel electronic properties of heazlewoodite phase Ni3S2 and interconnected porous 3DrGO,the synthesized electrode materials exhibit high specific capacitances,excellent rate performance and cycling stability.Furthermore,the constructed asymmetric supercapacitor can able to be cycled reversibly in the voltage range of 0-1.3 V,but still delivers high energy density(184.5 W h kg-1),high power density(19.5 kW kg-1 at 137.1 W h Kg-1),and excellent cycling stability(with 90.4%specific capacitance retained even after 5 000 cycles).Moreover,the device exhibits good flexibility without performance degradation. |
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