Preparation Of Molybdenum/Cobalt-based Ternary Transition Metal Oxides Core-Shell Nanomaterials And Their Application In Supercapacitors

Since the beginning of this century,it has become an urgent task to find sustainable new energy resources to replace the traditional fossil energy.Among these electrochemical energy storage devices,supercapacitors combine fast charge/discharge ability with large capacity,and show much obvious advantages compared with traditional double-layer capacitors and rechargeable batteries:ultra-long lifespan,being stable and reliable,high power density and environment-friendly.In spite of these advantages,the application of supercapacitors is still limited by low energy density,which generally results in lower efficiency that cannot provide enough power to drive the load.The key to the energy density enhancement of supercapacitorsis is to improve the specific capacitance itself.The most common way is to prepare electrode materials with high capacitance,which involves the selection of electrode materials and modification of material structure.In recent years,ternary transition metal oxides and sulfides have attracted extensive attention due to their extremely high theoretical capacity and simple synthesis methods,and the stability of the core-shell nanomaterials constructed from them has been greatly enhanced.In this paper,we first discussed the rational design of electrode materials to construct novel nanostructures and the chemical means to modify the materials.A unique core-shell structure was constructed by taking nickel foam as the substrate,using transition metal oxide nanorods?wires?as the core framework,and covering the surface with a layer of well-distributed nanorods.Then the core materials were treated with high-reducing solution,and an appropriate amount of oxygen vacancy was introduced into the core materials.The electrochemical activity of the modified core materials was significantly improved.The constructed nano core-shell structure by this way can provide abundant active sites and rapid transfer channels,and the obtained electrode materials own excellent electrochemical properties.To test the performance of these electrode materials from a practical view,the optimal positive electrodes have been assembled into asymmetric supercapacitors for further electrochemical measurements.?1?Hierarchical core-shell NiMoO₄@Ni-Co-S nanorods are first successfully grown on nickel foam by a facile two-step method to fabricate a bind-free electrode.The obtained electrode materials take advantage of the excellent nanostructural properties and outstanding electrochemical performance owing to the synergistic effects of both NiMoO₄ and Ni-Co-S.The prepared core-shell electrode yields a high specific capacitance of 2.27 F cm^-2?1892 F g-1?at a current density of 5 mA cm^-2 and retains a 91.7%specific capacitance even after 6000 cycles.In the electrochemical test of the supercapacitor device,the energy density of the device reaches 2.45 mWh cm^-3 at a power density of 0.131 W cm^-3,and still retains a remarkable 80.3%specific capacitance after 3500 cycles.Its energy storage ability for practical application has also been examined by successfully powering a commercial red LED for a few minutes.?2?To obtain electrode materials with exponentially enhanced performance,we first put forward a synergistic strategy to construct a core-shell reduced ZnCo₂O₄@NiMoO₄·H₂O heterostructure.This constructed nanostructure makes the utmost of reduced ZnCo₂O₄ nanowires and NiMoO₄·H₂O nanosheets,and the performance has been recognized.The reduced ZnCo₂O₄ electrode with moderate oxygen vacancies yields a capacitance of 1.55 F cm^-2,which is significantly increased by 84.5%than that of the pristine electrode.The obtained core-shell reduced ZnCo₂O₄@NiMoO₄·H₂O electrode yields a capacitance of 3.53 F cm^-2,which is marvelously increased by 127.7%than that of the reduced ZnCo₂O₄ electrode,the electrode also shows an excellent 95.4%capacitance retention after 5000 cycles and good electrical conductivity.Electrochemical tests have also been performed on the supercapacitor device.The device delivers a high energy density of 2.55 mWh cm^-3 at a power density of 0.033 W cm^-3,and also shows an outstanding 80.0%capacitance retention after 5000 cycles.Finally,a commercial red LED has been successfully powered for more than 15 min to testify the practicality of the devices.In summary,through the unique nanostructure design and the modification of reduced electrode materials containing oxygen vacancies,our work not only confirms that the electrode materials of different components in the core-shell electrode can achieve efficient collaboration,but also provided a new idea for the next generation of high-performance supercapacitors by integrating different materials.