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

Abstract

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 ≈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 δ-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 ≈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.

MnOx/carbon nanotube/reduced graphene oxide nanohybrids as high-performance supercapacitor electrodes

“…27,28 This capacitance value also approaches the highest values (900-1170 F g − 1 ) ever reported for hybrid structures, which were obtained either from the very low massloading (o10 μg cm − 2 ) of active materials or with a sophisticated current collector (such as nanoporous gold thin films). 26,28,29 In fact, this value (1070 F g − 1 ) is close to the theoretical capacitance of MnO x (C s~1 240 F g − 1 ) by assuming that all Mn atoms are involved in the redox reactions.…”

“…This relatively poor cycling stability is most likely due to the dissolution of Mn atoms into the electrolyte and/or the structural instability caused by the adsorption of cations in the redox reactions. 29 A closer investigation of the electrode after cycling also found that a few flake-shaped nanosheets emerge after cycling (see Supplementary Figure S16), indicating that the structure may be affected by the 'dissolution-redeposition' process. 31 The mechanism of this process assumes that Mn atoms at the surface are first dissolved in the electrolyte during reduction at low potentials; the dissolved Mn are then re-oxidized into insoluble MnO 2 and deposit on the surface.…”

Sodium Charge Storage in Thin Films of MnO2Derived by Electrochemical Oxidation of MnO Atomic Layer Deposition Films

“…Partial film dissolution in aqueous electrolyte prevents the thin MnO 2 films with thicknesses < 40 nm from approaching the highest reported capacitances of >1,000 F/g at CV sweep rates of 5 mV/s. 4,22 This film dissolution occurs during electrochemical oxidation and CV of the MnO 2 samples in aqueous electrolyte and is not accounted for in the calculation of specific mass capacitance. The local atomic structure, morphology, or surface termination of our films may also impact their specific capacitance.…”

“…20 One of the most notable observations for MnO 2 is an exceptionally high specific capacitance of >1,000 F/g when MnO 2 is blended with carbon powder at low mass loadings 4 or deposited as a thin film. 22 * Electrochemical Society Active Member. z E-mail: Steven.George@colorado.edu Typical specific capacitances reported for larger mass loadings or "bulk" MnO 2 in aqueous electrolyte are ∼200 F/g.…”

Extending Electrochemical Quartz Crystal Microbalance Techniques to Macroscale Electrodes: Insights on Pseudocapacitance Mechanisms in MnOx-Coated Carbon Nanofoams

“…12,14,15,32,34 More recent EQCM reports examined MnOx pseudocapacitance phenomena and showed that cation insertion and ejection during reduction and oxidation, respectively, is the primary charge-compensation mechanism. 16,32,35 Traditional quartz crystal microbalance (QCM) experiments rely on the assumption that films are acoustically thin, thus allowing for gravimetric interpretation of the results using the Sauerbrey equation.…”