Understanding the phase dependent energy storage performance of MnO2 nanostructures

Abstract

We demonstrate charge storage mechanisms of four kinds of MnO2 polymorphs (α, β, γ, and δ) both in micro- and nanodimensions successfully synthesized by hydrothermal and microwave irradiation techniques. We observed that layered δ-MnO2, comprised of self-assembled nanoflakes oriented in different directions, shows a significantly improved capacitive behavior. The maximum achieved specific capacitance is 518 F/g at a current density of 3 A/g in a 3M KOH electrolyte solution, exhibiting a large capacity retention of 83.95% over 2000 consecutive charge/discharge cycles at a current density of 10 A/g. State of the art Density Functional Theory (DFT) simulations have also been performed to support experimental data. The quantum capacitance presented from DFT simulations predicts that the δ phase exhibits highest quantum capacitance, whereas it is lowest for the β phase supporting the experimental trend. Also, the structural features of wide tunnel size (∼7 Å) for the δ phase facilitates favorable insertion of cations, whereas narrow tunnel size (∼1.89 Å) for the β phase restricts the diffusion of charge particles yielding poor capacitance performance.