Preparation of Spherical δ-MnO 2 Nanoflowers by One-Step Coprecipitation Method as Electrode Material for Supercapacitor.

Spherical δ-MnO 2 nanoflower materials were synthesized via a facile one-step coprecipitation method through adjusting the molar ratio of KMnO 4 to MnSO 4 . The influence of the molar ratio of the reactants on the crystal structure, morphology, and electrochemical performances was investigated. At a molar ratio of 3.3 for KMnO 4 to MnSO 4 , the spherical δ-MnO 2 nanoflowers composed of nanosheets with the highest specific surface area (228.0 m 2 g -1 ) were obtained as electrode materials. In the conventional three-electrode system using 1 M Na 2 SO 4 as an electrolyte, the specific capacitance of the spherical δ-MnO 2 nanoflowers reached 172.3 F g -1 at a current density of 1 A g -1 . Moreover, even after 5000 cycles at a current density of 5 A g -1 , the GCD curves remained essentially unchanged, and the specific capacitance still retained 86.50% of the maximum value. The kinetics of the electrode reaction were preliminarily studied through the linear potential sweep technique to observe diffusion-controlled contribution toward total capacitance. For the spherical δ-MnO 2 nanoflower electrode material, diffusion-controlled contribution accounted for 65.1% at low scan rates and still remained significant at high scan rates (100 mV s -1 ), indicating excellent utilization efficiency of the bulk phase. The as-fabricated asymmetric supercapacitor HFC-7//MnO 2 -3.3-ASC presented a prominent specific energy of 16.5 Wh kg -1 at the specific power of 450 W kg -1 . Even when the specific power reached 9.0 kW kg -1 , the energy density still retained 9.5 Wh kg -1 .