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.

Urchin-like FeOOH hollow microspheres decorated with MnO2 for enhanced supercapacitor performance

“…One promising approach to improve their performance is to combine other metal oxides in a composite. Manganese dioxide (MnO 2 ) with favorable properties and high theoretical specific capacitance (1370 F g −1 ) makes it a promising electrode material in energy storage and conversion devices [26][27][28][29][30]. Furthermore, three-dimensional (3D) transitional metal oxides/ hydroxides nanostructures assembled from low-dimensional building blocks [31][32][33][34] exhibit enhanced properties compared to their bulk counterparts by offering a large specific surface area, high surface/body ratios, the enhanced permeability for the electrolyte ions, and rich electrochemically active sites [35].…”

Fabrication of Metal Molybdate Micro/Nanomaterials for Electrochemical Energy Storage

“…Transition metal compounds with superior electrochemical characteristics have been widely used as supercapacitors electrode materials . MnO 2 is a promising candidate as an electrode material for supercapacitors because of its superior electrochemical performance, such as low cost and natural abundance . However, it's also reported that the performance of nanostructured MnO 2 as the electrode material in supercapacitors is unsatisfactory .…”

“…[146] MnO 2 is a promising candidate as an electrode material for supercapacitors because of its superior electrochemical performance, such as low cost and natural abundance. [147] However, it's also reported that the performance of nanostructured MnO 2 as the electrode material in supercapacitors is unsatisfactory. [148,149] In order to solve this problem, some composites were formed in the hope of enhancing the electrochemical performance.…”

Hierarchically nanostructured transition metal oxides for supercapacitors

“…Meng et al [137] fabricated hierarchically densely packed porous MnO 2 microspheres doped with Fe 3 O 4 NPs through a one-step low-cost ultrasound-assisted method. The SEM and TEM images (Fig.…”