Effect of hydrothermal temperature on structural, optical and electrochemical properties of α-MnO2 nanostructures for supercapacitor application

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

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.

Investigation of the Defect and Intensity-Dependent Optical Limiting Performance of MnO2 Nanoparticle-Filled Polyvinylpyrrolidone Composite Nanofibers

To enhance the optical limiting behavior triggered by nonlinear absorption (NA), wide-band gap MnO2 nanoparticles were incorporated into polyvinylpyrrolidone (PVP) polymer nanofibers at various concentrations. SEM images of the composite nanofibers showed that MnO2 nanoparticles are well entrapped in the nanofibers. With an increase in MnO2 nanofiller concentration, a widened optical band gap energy and an increased Urbach energy were observed. As the concentration of MnO2 nanofiller in PVP increased, the NA behavior became more pronounced but weakened with higher input intensity. This behavior was attributed to the filling of the localized defect states by one photon absorption (OPA). The NA mechanisms of the composite nanofibers were examined, considering their band gap energies and localized defect states. Although all of the composite nanofibers had OPA, sequential/simultaneous two photon absorption (TPA), and excited state absorption mechanisms, the higher concentration of the MnO2 nanofiller led to stronger NA behavior due to its more defective structure. The highest optical limiting behavior was observed for composite nanofibers with the highest concentration of MnO2 nanofiller. The results obtained show that these composite nanofibers with a high linear transmittance and an extended band gap energy can be used in optoelectronic applications that can operate in a wide spectral range. Furthermore, their robust NA behavior, coupled with their promising optical limiting characteristics, positions them as strong contenders for effective optical limiting applications.