Silver Nanoparticle-Induced Growth of Nanowire-Covered Porous MnO₂ Spheres with Superior Supercapacitance

We report a facile, low-cost, ultrasound-assisted synthesis of nanowire-covered porous MnO₂ spheres with superior supercapacitance at high charging rates with long-term durability. The use of catalytic silver nanoparticles is crucial to the growth mechanism in the initial stage, and the resulting silver oxides later grow the nanowires in such a way that they always terminate the wires, thus automatically covering the structures and increasing conductivity. The optimal Ag₂O–MnO₂ structures have a specific capacitance of 536.4 F/g at 5 mV/s. At a high scan rate of 100 mV/s, only 200 F/g remain for the reported carbon nanotube/MnO₂ material with an excellent capacitance at low scan rate (1230 F/g, 1 mV/s), while the Ag₂O–MnO₂ reported here still has 417.2 F/g. The material reaches a stable region of 91.3% capacitance retention over 10000 charge/discharge cycles at 5 A/g

Hierarchically Porous MnO₂ Microspheres Doped with Homogeneously Distributed Fe₃O₄ Nanoparticles for Supercapacitors

Hierarchically porous yet densely packed MnO₂ microspheres doped with Fe₃O₄ nanoparticles are synthesized via a one-step and low-cost ultrasound assisted method. The scalable synthesis is based on Fe²⁺ and ultrasound assisted nucleation and growth at a constant temperature in a range of 25–70 °C. Single-crystalline Fe₃O₄ particles of 3–5 nm in diameter are homogeneously distributed throughout the spheres and none are on the surface. A systematic optimization of reaction parameters results in isolated, porous, and uniform Fe₃O₄–MnO₂ composite spheres. The spheres’ average diameter is dependent on the temperature, and thus is controllable in a range of 0.7–1.28 μm. The involved growth mechanism is discussed. The specific capacitance is optimized at an Fe/Mn atomic ratio of <i>r</i> = 0.075 to be 448 F/g at a scan rate of 5 mV/s, which is nearly 1.5 times that of the extremely high reported value for MnO₂ nanostructures (309 F/g). Especially, such a structure allows significantly improved stability at high charging rates. The composite has a capacitance of 367.4 F/g at a high scan rate of 100 mV/s, which is 82% of that at 5 mV/s. Also, it has an excellent cycling performance with a capacitance retention of 76% after 5000 charge/discharge cycles at 5 A/g