Hierarchical Ni-Co-S@Ni-W-O core-shell nanosheet arrays on nickel foam for high-performance asymmetric supercapacitors

Nickel cobalt sulfides （Ni-Co-S） have attracted extensive attention for application in electronic devices owing to their excellent conductivity and high electrochemical capacitance. To facilitate the large-scale practical application of Ni-Co-S, the excellent rate capability and cyclic stability of these compounds must be fully exploited. Thus, hierarchical Ni-Co-S@Ni-W-O （Ni-Co-S-W） core/shell hybrid nanosheet arrays on nickel foam were designed and synthesized herein via a facile three-step hydrothermal method, followed by annealing in a tubular furnace under argon atmosphere. The hybrid structure was directly assembled as a free-standing electrode, which exhibited a high specific capacitance of 1,988 F·g^-1 at 2 A·g^-1 and retained an excellent capacitance of approximately 1,500 F·g^-1 at 30 A·g^-1, which is superior to the performance of the pristine Ni-Co-S nanosheet electrode. The assembled asymmetric supercapacitors achieved high specific capacitance （155 F·g^-1 at 1 A·g^-1）, electrochemical stability, and a high energy density of 55.1 W·h·kg^-1 at a power density of 799.8 W·kg^-1 with the optimized Ni-Co-S-W core/shell nanosheets as the positive electrode, activated carbon as the negative electrode, and 6 M KOH as the electrolyte.

Flexible,stretchable,and transparent InGaN/GaN multiple quantum wells/polyacrylamide hydrogel-based light emitting diodes

Visualization is a direct,efficient,and simple interface method to realize the interaction between human and machine,whereas the flexible display unit,as the major bottleneck,still deeply hinders the advances of wearable and virtual reality devices.To obtain flexible optoelectronic devices,one of the effective methods is to transfer a high-efficient and long-lifetime inorganic optoelectronic film from its rigid epitaxial substrate to a foreign flexible/soft substrate.Additionally,piezo-phototronic effect is a fundamental theory for guiding the design of flexible optoelectronic devices.Herein,we demonstrate a flexible,stretchable,and transparent InGaN/GaN multiple quantum wells(MQWs)/polyacrylamide(PAAM)hydrogel-based light emitting diode coupling with the piezo-phototronic effect.The quantum well energy band and integrated luminous intensity(increased by more than 31.3%)are significantly modulated by external mechanical stimuli in the device.Benefiting from the small Young's modulus of hydrogel and weak Van der Waals force,the composite film can endure an extreme tensile condition of about 21.1%stretching with negligible tensile strains transmitted to the InGaN/GaN MQWs.And the stable photoluminescence characteristics can be observed.Moreover,the hydrogen-bond adsorption and excellent transparency of the hydrogel substrate greatly facilitate the packaging and luminescence of the optoelectronic device.And thus,such a novel integration scheme of inorganic semiconductor materials and organic hydrogel materials would help to guide the robust stretchable optoelectronic devices,and show great potential in emerging wearable devices and virtual reality applications.