Superior Cyclic Stability and Capacitive Performance of Cation- and Water Molecule-Preintercalated δ-MnO 2 /h-WO 3 Nanostructures as Supercapacitor Electrodes.

The large number of active sites in the layered structure of δ-MnO 2 with considerable interlayer spacing makes it an excellent candidate for ion storage. Unfortunately, the δ-MnO 2 -based electrode has not yet attained the exceptional storage potential that it should demonstrate because of disappointing structural deterioration during periodic charging and discharging. Here, we represent that stable Na ion storage in δ-MnO 2 may be triggered by the preintercalation of K ions and water molecules. Furthermore, the sluggish reaction kinetics and poor electrical conductivity of preintercalated δ-MnO 2 layers are overcome by the incorporation of h-WO 3 in the preintercalated δ-MnO 2 to form novel composite electrodes. The composites contain mixed valence metals, which provide a great number of active sites along with improved redox activity, while maintaining a fast ion transfer efficiency to enhance the pseudocapacitance performance. Based on our research, the composite prepared from preintercalated δ-MnO 2 with 5 wt % h-WO 3 provides a specific capacitance of up to 363.8 F g -1 at a current density of 1.5 A g -1 and an improved energy density (32.3 W h kg -1 ) along with an ∼14% increase in capacity upon cycling up to 5000 cycles. Hence, the interaction between the preintercalated δ-MnO 2 and h-WO 3 nanorods results in satisfactory energy storage performance due to the defect-rich structure, high conductivity, superior stability, and lower charge transfer resistance. This research has the potential to pave the way for a new class of hybrid supercapacitors that could fill the energy gap between chemical batteries and ideal capacitors.