Nano "Koosh Balls" of Mesoporous MnO₂ : Improved Supercapacitor Performance through Superior Ion Transport

Manganese dioxide nanomaterials with "Koosh-ball"-like morphology (MnO₂ -KBs) as well as worm-like nanotubes (MnO₂ -NWs) are obtained by employing Tween 20 as the reducing and structure-directing agent, and KMnO₄ as a MnO₂ precursor. Whereas the MnO₂ -KBs are interconnected through tubular extensions, the MnO₂ -NWs are largely disconnected. Both MnO₂ -KBs and MnO₂ -NWs have large BET surface areas (>200 m²  g^-1 ), and are thermally robust up to 300 °C. Electrochemical studies reveal that the highest specific capacitance (C_sp ) obtained for MnO₂ -KBs (272 F g^-1 ) is significantly higher than that of MnO₂ -NWs (129 F g^-1 ). The C_sp values correlate well with the electroactive surface areas of the materials: MnO₂ -KBs have a significantly higher electrolyte-accessible surface area. Electrochemical impedance spectroscopy (EIS) reveals a higher electron-transfer rate at the electrode/electrolyte interface for MnO₂ -KBs than for MnO₂ -NWs. The multiple tubular interconnections between individual MnO₂ -KBs allow improved ion penetration and act as conduits for their propagation, shortening the diffusion distances of the ions from external electrolytes to the interior of the MnO₂ framework. Thus, this work exemplifies the importance of interconnections for enhancing the electrochemical performance of nanomaterials employed for energy storage.