Lithographically Patterned Gold/Manganese Dioxide Core/Shell Nanowires for High Capacity, High Rate, and High Cyclability Hybrid Electrical Energy Storage

We describe the fabrication of arrays of nanowires on glass in which a gold core nanowire is encapsulated within a hemicylindrical shell of manganese dioxide. Arrays of linear gold (Au) nanowires are first prepared on glass using the lithographically patterned nanowire electrodeposition (LPNE) method. These Au nanowires have a rectangular cross-section with a width and height of approximate to 200 and 40 nm, respectively, and lengths in the 1 mm to 1 cm range. Au nanowires are then used to deposit MnO2 by potentiostatic electrooxidation from Mn2+ solution, forming a conformal, hemicylindrical shell with a controllable diameter ranging from 50 to 300 nm surrounding each Au nanowire. This MnO2 shell is delta-phase and mesoporous, as revealed by X-ray diffraction and Raman spectroscopy. Transmission electron microscopy (TEM) analysis reveals that the MnO2 shell is mesoporous (mp-MnO2), consisting of a network of approximate to 2 nm fibrils. The specific capacitance, C-sp, of arrays of gold:mp-MnO2 nanowires is measured using cyclic voltammetry. For a mp-MnO2 shell thickness of 68 +/- 3 nm, core:shell nanowires produce a C-sp of 1020 +/- 100 F/g at 5 mV/s and 450 +/- 70 F/g at 100 mV/s. The cycle stability of this C-sp, however, is extremely limited in aqueous electrolyte, decaying by >90% in 100 scans, but after oven drying and immersion in dry 1.0 M LiClO4, acetonitrile, dramatically improved cycle stability is achieved characterized by the absence of C-sp, fade for 1000 cycles at 100 mV/s. Core:shell nanowires exhibit true hybrid energy storage, as revealed by deconvolution of C-sp into insertion and noninsertion components.