Preparation of Spherical δ-MnO₂ Nanoflowers by One-Step Coprecipitation Method as Electrode Material for...

Spherical δ-MnO₂ nanoflower materials were synthesized via a facile one-step coprecipitation method through adjusting the molar ratio of KMnO₄ to MnSO₄. 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 KMnO₄ to MnSO₄, the spherical δ-MnO₂ nanoflowers composed of nanosheets with the highest specific surface area (228.0 m² g^-1) were obtained as electrode materials. In the conventional three-electrode system using 1 M Na₂SO₄ as an electrolyte, the specific capacitance of the spherical δ-MnO₂ 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-MnO₂ 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//MnO₂-3.3-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.