|Nanotube array architecture, triple-layered structure, and high conductivity in electrodes provide ion and electron “superhighways”. Credit: ACS, Li et al. Click to enlarge.|
Researchers at Sun Yat-sen University in Guangzhou, China designed and synthesized novel MnO2/Mn/MnO2 sandwich-like nanotube arrays for supercapacitors. In testing reported in the ACS journal Nano Letters, the hybrid MnO2/Mn/MnO2 sandwich-like nanotube arrays exhibited an excellent rate capability with a high specific energy of 45 Wh/kg and specific power of 23 kW/kg and excellent long-term cycling stability (less 5% loss of the maximum specific capacitance after 3,000 cycles).
The high specific capacitance and charge−discharge rates offered by the sandwich-like nanotube arrays make them promising candidates for supercapacitor electrodes, combining high-energy densities with high levels of power delivery, the researchers suggest.
Supercapacitors—also called ultracapacitors or electrochemical capacitors (ECs)—offer high power density, fast charging−discharging rate, and excellent cycle stability. Various materials, including carbon materials, transition-metal oxides, conducting polymers, and hybrid composites have been widely studied as electrodes for these devices, the team notes.
There has been extensive interest in developing the inexpensive transition-metal oxide electrodes, such as MnO2, Co3O4, NiO, VOx, and TiO2 for supercapacitors. MnO2 is an attractive electrode material owing to its high theoretical specific capacitance, low cost, natural abundance, and environmental friendliness. However, poor electrical conductivity (10−5∼10−6 S/cm) remains a major challenge and limits rate capability for high power performance.
Recent work has explored possible solutions such as hybrid composite nanostructures in which thin MnO2 layers were loaded on highly conductive materials such as metal, conducting polymer, carbon nanotube, or graphene for enhanced performance.
The enhanced performance was also obtained by coating MnO2 onto SnO2 nanowires, ZnO nanorods, and Zn2SnO4 nanowires. In all of the above cases, MnO2 is of relatively low weight fraction and usually has excellent rate and cycling performance; however, the energy and power densities of electrodes are sacrificed.
To realize the practical applications for high-performance ECs that needs large capacitance and high energy storage, here we design and synthesize novel MnO2/Mn/MnO2 sandwich-structured nanotube arrays (SNTAs) with high MnO2 weight fraction...The aligned SNTAs represent a new prime example of materials with a well-defined pore structure.—Li et al.
The team outlined four primary reasons for taking this approach:
The middle crystalline metal Mn layer in the sandwich provides electron “superhighways”—highly conductive cores—for charge storage and delivery, which overcomes the key weakness (the limited electric conductivity) of MnO2.
The MnO2/Mn/MnO2 SNTAs relax the transport of ions because of the hollow nanostructures. In addition, the double thin layers of MnO2 in SNTAs would enable fast, reversible Faradaic reactions and provide short ion diffusion paths.
The SNTAs with double MnO2 shells would obviously enhance the utilization rate of MnO2 material because of anisotropic morphology, large specific surface area, and hollow nanostructures.
The SNTAs directly growing on conductive substrate have an excellent electrical contact with current collectors, and this would let each MnO2/Mn/MnO2 nanotube effectively participate in electrochemical reactions with almost no “dead” volume.
The maximum specific capacitances of 937 F/g at a scan rate of 5 mV/s by cyclic voltammetry (CV) and 955 F/g at a current density of 1.5 A/g by chronopotentiometry were achieved for the MnO2/Mn/MnO2 sandwich-like nanotube arrays in solution of 1.0 M Na2SO4.
Qi Li, Zi-Long Wang, Gao-Ren Li, Rui Guo, Liang-Xin Ding, and Ye-Xiang Tong (2012) Design and Synthesis of MnO2/Mn/MnO2 Sandwich-Structured Nanotube Arrays with High Supercapacitive Performance for Electrochemical Energy Storage. Nano Letters doi: 10.1021/nl301748m