| With the increasingly serious energy problem,researchers pay more attention to energy storage devices.As a new type of energy storage device,supercapacitors have high power density and excellent long cycle stability,but they are often troubled by low energy density in practical applications.In accordance with the classification of energy storage method,there are pseudo capacitor,double-layer capacitor and hybrid supercapacitor.Hybrid capacitor has the advantages of both pseudo capacitor and double-layer capacitor,which has become a focal point in the domain of supercapacitor.Transition metals are one of the most common electrode materials.Nickel-cobalt based compounds are widely used because of their suitable cost and relatively high theoretical capacity.The oxides and hydroxides of transition metals tend to have poor conductivity,and choosing selenides of transition metals can avoid this defect.Although there has been a great deal of research on Ni-Co selenides as super-electric electrode materials,the energy density of devices is still not ideal,so we choose two methods to improve the electrochemical performance of Ni-Co selenides and make them better applied in practice.(1)Starting from the improvement of micro-morphology of electrode materials:Firstly,we added silica microspheres as templates,and synthesized a new porous tremella-like nickel-cobalt selenide by a simple two-step hydrothermal method.The flake and porous structure of this new morphology provide a large number of electrochemical active sites and improve the reaction kinetics.Tests also prove that it greatly improves the electrochemical specific surface area of the material,and the conductivity of the selenized nickel-cobalt based material is greatly improved compared with its hydroxide.The single electrode of the new tremella-like nickel-cobalt selenide can reach 636.2 C g-1 at 1 A g-1,which shows high single electrode capacity,and also shows good rate performance(20A g-1,66.1%)and conductivity.All electrochemical properties far exceed those of nickel-cobalt selenide without template.Finally,for exploring its practical application ability we use the synthesized materials and activated carbon as positive and negative electrodes.The energy density of the assembled hybrid supercapacitor is as high as 53.7 Wh kg-1when the power density is 822.5 W kg-1,and after 10000 cycles,it also keeps the capacity of 86.1%,showing excellent cycle stability.Our assembled hybrid capacitor can successfully light up 15 red bulbs in series for more than 30 minutes,which shows strong practical application ability and great development potential.(2)From the point of view of element doping:Sheet-like nickel-cobalt selenide was grown on foamed nickel by simple two-step hydrothermal method,and then phosphatized at low temperature in a tubular furnace,thus obtaining the final product doped with phosphorus element.Scanning electron microscope observation shows that the microstructure of the material has not changed significantly during the multi-step reaction,and a series of characterization can confirm the doping of phosphorus and the successful synthesis of the nickel cobalt selenide.At 1 A g-1,the specific capacity of phosphorus-doped nickel-cobalt selenide single electrode can reach 679 C g-1,and it shows good rate performance(10 A g-1,51.5%)and excellent conductivity.Compared with nickel-cobalt selenide and nickel-cobalt phosphide,the electrochemical performance of nickel-cobalt selenide doped with phosphorus obviously improved,which is due to the synergistic effect between phosphorus and other elements.At last,we assembled the hybrid supercapacitor with the final product and activated carbon as positive and negative electrodes respectively.During 825 W kg-1,the energy density is 61.2 Wh kg-1,and the retention rate is still 80.5%after10000 cycles at 3 A g-1,showing excellent cycle stability,successfully demonstrating its good practical application ability and unlimited possibilities in the future research and development of supercapacitors. |
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