Understanding the phase dependent energy storage performance of MnO2 nanostructures

We demonstrate charge storage mechanisms of four kinds of MnO2 polymorphs (alpha, beta, gamma, and delta) both in micro- and nanodimensions successfully synthesized by hydrothermal and microwave irradiation techniques. We observed that layered delta-MnO2, comprised of self-assembled nanoflakes oriented in different directions, shows a significantly improved capacitive behavior. The maximum achieved specific capacitance is 518F/g at a current density of 3A/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 10A/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 delta phase exhibits highest quantum capacitance, whereas it is lowest for the beta phase supporting the experimental trend. Also, the structural features of wide tunnel size (similar to 7 angstrom) for the delta phase facilitates favorable insertion of cations, whereas narrow tunnel size (similar to 1.89 angstrom) for the beta phase restricts the diffusion of charge particles yielding poor capacitance performance.