Sandwich structured MnO2/carbon nanosheet/MnO2 composite for high-performance supercapacitors

Abstract

In view of the uniform lamellar structure of potassium citrate-derived carbon nanosheets (PC-Cs) and a good dispersion in water, PC-Cs are prepared via high-temperature carbonization method and acted as sacrificial template for reacting with KMnO4 to generate MnO2 nanosheets arrays on both sides of carbon nanosheet. When the mass ratio of PC-Cs/KMnO4 is 1/15, sandwich structured MnO2/PC-Cs/MnO2 composite is obtained. Through the characterization in microstructure, X-ray diffraction spectrum, X-ray photoelectron spectroscopy and thermogravimetric curve, the sandwich composite has a thickness of 320 nm, containing 79.9% MnO2 and 20.1% residual carbon. When used in three-electrode supercapacitor, the optimized composite delivers a high capacitance of 397 F g−1 at 1 A g−1, with capacitance retention of 93.1% after cycling for 5000 cycles. An asymmetric supercapacitor is assembled by MnO2/PC-Cs/MnO2 positive electrode and PC-Cs negative electrode, which presents a capacitance of 81.5 F g−1 at 0.5 A g−1, with no capacitance decay for 5000 cycles, and the energy density is 21.9 W h kg−1 at 143.8 W kg−1. Compared to existing C/MnO2 composites, sandwich composite displays an obvious superiority, due to the high fraction of MnO2 and nanosheets arrays structure. Moreover, residual carbon layer sandwiched between two MnO2 layers reduces the internal resistance, enhances electronic conductivity, further improving the long-term cycling stability.