title: Hierarchically porous MnO2 microspheres doped with homogeneously distributed Fe3O4 nanoparticles for supercapacitors.
abstract: 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 Fe(2+) 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 r = 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.
authors.affiliation: National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and Institute of Materials Engineering, Nanjing University , Nanjing, Jiangsu 210093, People's Republic of China.
authors.forename: Jian
authors.initials: J
authors.lastname: Zhu
authors.forename: Shaochun
authors.initials: S
authors.lastname: Tang
authors.forename: Hao
authors.initials: H
authors.lastname: Xie
authors.forename: Yuming
authors.initials: Y
authors.lastname: Dai
authors.forename: Xiangkang
authors.initials: X
authors.lastname: Meng
keywords: composite materials; hierarchically porous structures; manganese dioxide; supercapacitors