Novel Ultrathin Nanoflake Assembled Porous MnO 2 /Carbon Strip Microspheres for Superior Pseudocapacitors Guiling Wang, †,§ Yuwen Liu, †,§ Guangjie Shao,* ,†,‡,§ Lingxue Kong, ‡ and Weimin Gao* ,‡ † State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China ‡ Institute for Frontier Materials, Deakin University, Geelong, Victoria 3220, Australia § College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China * S Supporting Information ABSTRACT: A novel hierarchical MnO 2 /carbon strip (MnO 2 / C) microsphere is synthesized via galvanostatic charge- discharge of a MnO@C matrix precursor where the carbon is from a low-cost citric acid. This hierarchical structure is composed of manganese oxides nanoflakes and inlaid carbon strips. The ultrathin nanoflakes assemble to form porous microspheres with a rippled surface superstructure. Due to its improved conductivity and remarkable increased phase contact area, this novel structure exhibits an excellent electrochemical performance with a specific capacitance of 485.6 F g -1 at a current density of 0.5 A g -1 and an area capacitance as high as 4.23 F cm -2 at a mass loading of 8.7 mg cm -2 . It also shows an excellent cycling stability with 88.9% capacity retention after 1000 cycles. It is speculated that the present low-cost novel hierarchical porous microspheres can serve as a promising electrode material for pseudocapacitors. KEYWORDS: Carbon sources, Manganese oxides, Pseudocapacitor, Galvanostatic charge-discharge, Porous ■ INTRODUCTION Manganese oxides are considered to be a promising material for pseudocapacitors because of their high theoretical specific capacitance of up to 1370 F g -1 as the oxidation state of Mn ion changes from 4+ to 3+ over a potential window of 0.8 V, 1 their low cost imparted by the natural abundance, and their environmentally friendliness. The practical capacitive behavior of MnO 2 materials is, however, far from the theoretical value due to their poor electrical conductivity (approximately 10 -5 to 10 -6 S cm -1 ) and mechanical instability. 2-4 To overcome this shortage, many efforts have been made, to introduce conductive materials, such as carbonaceous materials, 5-9 metals, 10-13 conducting polymers, 14-16 and metal oxides. 6,10,17 Among of these conductive materials, carbon is generally considered to be one of the most promising candidates for their high electrical conductivity, large surface area, and good mechanical stability. It has been demonstrated that these carbon-based materials can lead to an increase in specific capacitance. For example, Mn 3 O 4 /amorphous-carbon nanoparticles synthesized via a green chemistry route, exhibited a capacitance of up to 522 F g -1 at a specific current density of 1 A g -1 . 18 CNT sheet/MnO x composites were synthesized by electrodepositing MnO x on CNT multisheets. When the thickness of MnO x is ∼70 nm, the specific capacitance of the composites has a high value of 1250 Fg -1 . After the thickness increased to 800 nm, the capacitance is only 190 F g -1 . 19 Note that the most notable enhancement in the specific capacitance has been obtained from the samples having a manganese oxide content of no more than 70% and at an active-material loading density of less than 1 mg cm -2 . 20-23 Both the low manganese oxide content and the low active materials loading will significantly affect the overall energy density per area of the electrodes. In recent years, many researchers have worked on the technique of increasing the utilization of active materials and loading of manganese oxide. 24-27 Lei et al. 28 prepared a supercapacitor electrode composed of graphenes and MnO 2 -coated carbon naontubes between the graphenes. A maximum specific capacitance of 193 Fg -1 was achieved and a capacitance retention of 70% after 1300 cycles was observed at a mass loading of 4 mg cm -2 . To enhance the electrochemically active surface area of MnO 2 , growing ultrathin MnO 2 nanofibers on graphitic hollow carbon spheres was conducted by refluxing the carbon spheres in a KMnO 4 aqueous solution, yielding a sample with an MnO 2 content of 64%, which produced a specific capacitance of 190 F g -1 at a mass loading of 5.0 mg cm -2 . 29 A three-dimensional graphene network loaded with MnO 2 was prepared by electrodeposition, which yielded an area capacitance of 1.42 F cm -2 at an MnO 2 mass loading of 9.8 mg cm -2 . 30 It has been Received: June 20, 2014 Revised: July 28, 2014 Published: August 12, 2014 Research Article pubs.acs.org/journal/ascecg © 2014 American Chemical Society 2191 dx.doi.org/10.1021/sc500379a | ACS Sustainable Chem. Eng. 2014, 2, 2191-2197