High-performance supercapacitors based on MnO₂ tube-in-tube arrays

Abstract

The MnO 2 tube-in-tube arrays supported on carbon fiber cloth (MnO 2 TTAs/CFC) were designed and synthesized. As a robust integrated 3D electrode with high utilization rate and fast ion transport, the MnO 2 TTAs/CFC exhibits high areal specific capacitance (Csp) of 322 mF·cm -2 (~1007 F/g for MnO 2 at 0.125 A/g) and superior cycling stability (96.4% retention of the initial Csp after 2000 cycles) at a high scan rate of 100 mV/s. The assembled flexible all-solid-state symmetric supercapacitors (SSCs) based on MnO 2 TTAs/CFC electrodes show a high volumetric energy density of 0.073 mW·h/cm 3 at the power density of 25 W/kg and high cycling stability (96.4% retention of the initial Csp after 2000 cycles).This study demonstrates that the 3D MnO 2 TTAs/CFC electrodes hold great potential for the flexible energy storage devices.Here the designed MnO 2 TTAs/CFC significantly improve the mass loading of active material, meanwhile the void space in MnO 2 TTAs/CFC will avoid effectively "dead volume" and thus will improve the ion transport and the utilization rate of electrode material. Because of unique structures, the designed MnO 2 TTAs/CFC electrodes exhibit high C sp (322 mF/cm 2 , 1007 A/g) and outstanding cycling stability (95.1% retention of the initial C sp after 2000 cycles at a high scan rate of 100 mV/s). The flexible all-solid-state symmetric supercapacitors (SSCs) are assembled basing on the MnO 2 TTAs/CFC electrodes and they exibit high volumetric energy density of 0.073 mWh/cm 3 (11.3 Wh/kg) at the power density of 25.0 W/kg and high cycling stability (96.4% retention of the initial C sp after 2000 cycles). Experimental SectionAll chemical reagents used in this study were analytical (AR) grade. Electrodeposition was carried out in a simple three-electrode cell via HDV-7C transistor potentiostatic apparatus, and the graphite electrode (spectral grade, 1.8 cm 2 ), carbon fiber cloth (CFC, 0.7×0.5 cm 2 ) and Ag/AgCl electrode were utilized as counter electrode, working electrode and reference electrode, respectively. Before electrodeposition, the CFC was successively rinsed with ethanol and distilled water in ultrasonic bath for 5 min, and repeatedly cleaned three times. Firstly, ZnO MRAs were electrodeposited on the CFC in 0.01 M Zn(NO 3 ) 2 +0.05 M NH 4 NO 3 solution at 0.4 mA/cm 2 at 70 °C for 90 min (ZnO MRAs/CFC). Secondly, MnO 2 was electrodeposited on ZnO MRAs in solution of 0.02 M Mn(CH 3 COO) 2 +0.20 M Na 2 SO 4 at 0.45 V (vs. Ag/ AgCl) for 1 min and ZnO@MnO 2 MRAs/CFC were obtained. Thirdly, ZnO layer was further electrodeposited on the ZnO@MnO 2 MRAs for 15 min to form ZnO@MnO 2 @ZnO MRAs/CFC. Fourthly, MnO 2 layer were further deposited on the ZnO@MnO 2 @ZnO MRAs to form multilayered ZnO@MnO 2 @ZnO@MnO 2MRAs. Finally, the fabricated ZnO@MnO 2 @ZnO@MnO 2 MRAs were immersed in 2.5 wt% aqueous ammonia for 3 h to remove ZnO completely and the MnO 2 TTAs/CFC were fabricated. In addition, for the comparative study, the MnO 2 STAs/CFC electrodes were fabricated via the similar synthesis method by etching ...

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