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Lithographically patterned gold/manganese dioxide core/shell nanowires for high capacity, high rate, and high cyclability hybrid electrical energy storage
Wenbo Yan, Jung Yun Kim, Wendong Xing, Keith Donavan, Talin Ayvazian, and Reginald M. Penner
Chem. Mater., Just Accepted Manuscript ? DOI: 10.1021/cm3011474 ? Publication Date (Web): 24 May 2012 Downloaded from http://pubs.acs.org on May 29, 2012

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Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright ? American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Lithographically patterned gold/manganese dioxide core/shell nanowires for high capacity, high rate, and high cyclability hybrid electrical energy storage
Wenbo Yana, Jung Yun Kimb, Wendong Xinga, Keith C. Donavana, Talin Ayvazianb, Reginald M. Pennera,b* a Department of Chemistry, University of California, Irvine, CA 92607-2025, bDepartment of Chemical Engineering and Materials Science, University of California, Irvine, CA 92607-2025
Corresponding Author: Reginald M. Penner, email: rmpenner@uci.edu.

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 crosssection with a width and height of ≈200 nm 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 nm 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 MnO2 shell is mesoporous (mp-MnO2), consisting of a network of ≈2 nm fibrils. The specific capacitance, Csp, 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 Csp of 1020 ± 100 F/g at 5 mV/s and 450 ± 70 F/g at 100 mV/s. The cycle stability of this Csp, 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 Csp fade for 1000 cycles at 100 mV/s. Core:shell nanowires exhibit true hybrid energy storage, as revealed by deconvolution of Csp into an insertion pseudocapacitance and a noninsertion pseudocapacitance. Keywords: electrodeposition | microfabrication | acetonitrile | cyclic voltammetry | birnessite

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Introduction Manganese dioxide, MnO2, was first proposed as a lithium ion insertion electrode by Altung and Jacobsen in 19811. Later in 1999, Lee and Goodenough2 proposed MnO2 as a cheaper alternative to ruthenium dioxide in supercapacitors. Like the ruthenium centers in RuO2, Mn ions in MnO2 can access multiple redox states (III and IV) and this imparts the ability of MnO2 to store energy faradaically. The faradic capacity of MnO2 has two components: A noninsertion capacity derived from redox charge storage by Mn centers located at the wetted surface of the material according to: MnO2(surface) + X+ + e- ? MnOOX(surface) (1)

where X is H+, Li+, or another cation. These surface Mn centers can be charge-compensated by ions present in the electrolyte without the requirement for these ions to intercalate into the material. Mn centers in the bulk of MnO2 can also contribute to faradaic charge storage because many polytypes of this material (e.g., α, β, γ, δ, and λ)3 contain 1D (α, β, γ, λ), 2D (δ), or 3D (λ) channels through which charge compensating cations such as H+ and Li+ can move: MnO2 (s) + X+ (aq.) + e- ? MnOOX (s) Augmenting the faradaic capacity of MnO2 is a non-faradaic or double-layer capacity that is enabled by the moderate electrical conductivity of this material which for bulk MnO2 is in the range from 0.006 S/cm to 300 S/cm4. Thus, MnO2 is capable of functioning as a hybrid energy storage material that has the capability to store charge using both faradiac and non-faradaic mechanisms. The theoretical faradaic specific capacity of an MnO2 half-cell based upon reactions (1) and (2) is 1118 F/g, assuming charge/discharge across a 1.0 V window. Efforts to achieve this specific capacity, Csp, have provided motivation for exploring nano-MnO2 as battery and supercapacitor electrode material5-9. Pseudo-capacitive and non-faradaic energy storage at the wetted surfaces of nano-MnO2 enhances both the gravimetric capacity and the rate capabilities of this material as the diffusion lengths for cations are reduced. However a survey of Csp values for nano-MnO2 half-cells10 reveals that the vast majority remain in the 200-600 F/g range. Some ? (2)

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exceptions to this generalization are worth noting: Pang et al.11 reported Csp values of 698 F/g for dip-coated sol-gel-derived films of MnO2. Toupin et al.12 measured Csp values as high as 1380 F/g for dendritic nanoparticles of MnO2 prepared by electrodeposition. Lang et al.13 prepared composites of nanoporous gold with nanocrystalline MnO2 demonstrating enhanced electronic conductivity and Csp values of 1145 F/g. However the preponderance of lower Csp values implies that electrochemically inaccessible Mn centers persist even in nano-MnO2 where the critical dimension is typically in the 10-50 nm range. The tremendous surface areas of nanomaterials is a double-edged sword. The progress of kinetically-controlled surface reactions that lead to decomposition is also scaled by the surface areas of these materials. For MnO2, a major source of cycle instability is the solubility of Mn2+ which is obtained by over-reducing the oxide in water 14: MnO2 (s) + 4H+ (aq.)+ 2e- ? Mn2+ (aq.) + 2H2O (aq.) Confining the negative limit of the material to 0.0V vs. Ag/AgCl provides some protection against this mechanism of dissolution,15 but with few exceptions the cycle stability of nanoMnO2 cathodes in aqueous solutions continues to be problematic. Several innovative strategies have been proposed to enhance the stability of MnO2 in aqueous electrolytes: Long and Rolison16 electropolymerized ultrathin poly(phenylenediamine) layers over highly dispersed MnO2 but the influence of these layers on cycle stability was not reported in that study. Brousse et al.17 showed that asymmetic capacitor systems involving a MnO2 positive electrode showed dramatically improved cycle stability when oxygen was excluded from aqueous K2SO4 electrolyte. In that work, 195,000 cycles were obtained with just 12% Csp fade for some cells, but Csp was relatively low at 21 (± 2) F/g17. Recently, we described the use of lithographically patterned nanowire electrodeposition (LPNE)18-20 to prepare mesoporous δ-MnO2 (henceforth: mp-MnO2) nanowires.10 In that work, arrays of 20 nm × 400 nm mp-MnO2 nanowires produced a specific capacitance, Csp ≈ 900 F/g at 5 mV/s and ≈ 400 F/g at 100 mV/s.10 These Csp values compare favorably with the highest seen previously for this material10 but the cycle stability of this Csp was not reported in that paper. A second issue involves the gold current collector. The mp-MnO2 nanowires were not removed from the LPNE template for electrochemical characterization in that work. This means that (3)

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electrical contact for each nanowire, along its entire length, was provided for by a 5 ?m wide gold film on which each MnO2 nanowire was electrodeposited. This scheme completely eliminates ohmic drops along the axis of the mp-MnO2 nanowires, enabling the characterization of their electrochemical properties, but it would be completely impractical in any type of real battery electrode.10 In this paper, we scale down the dimensions of the current collector to those of the mpMnO2 nanowire itself. This is accomplished by creating a gold nanowire first, using the LPNE method, and then synthesizing a hemicylindrical shell of mp-MnO2 on top of the gold nanowire to produce a gold core:mp-MnO2 shell nanowire that is millimeters in total length. The properties of arrays of these Au core:mp-MnO2 shell nanowires for storing energy are evaluated. These hybrid nanostructures show both an ultra-high specific capacity of 1020 ± 100 F/g at 5 mV/s and 450 ± 70 F/g at 100 mV/s (somewhat higher than we achieved previously10) coupled with absolute cycle stability of this capacity to 1000 cycles. This unique combination of properties is imparted by two innovation features of these nanowires: 1) The MnO2 shell is mesoporous with a critical dimension of just 2 nm. This morphology “unlocks” the Mn centers that are electrochemically isolated in nano-MnO2 having a larger critical dimension, and, 2) Core:shell nanowires synthesized in aqueous solutions can be stabilized by transfer into dry acetonitrile electrolytes. In acetonitrile, the dissolution of MnO2 during cycling – the most common mode of instability for this material - is completely eliminated. These nanowires establish the feasibility of creating durable, ultra-high capacity, battery/supercapacitor electrodes based upon this coreshell nanowire paradigm. Experimental Section Preparation of Au/MnO2 Core:Shell Nanowires. The LPNE process18-20 depicted in Fig.1A was used to fabricate Au/MnO2 core:shell nanowires. A 40 nm thick of nickel ?lm was thermally evaporated on top of precleaned 1”×1” squares of soda lime glass. Then a positive photoresist (PR, Shipley, S1808) layer was deposited by spin-coating, followed by soft-baking at 90°C for 30 min, and a contact mask was used to pattern this PR layer with a 365 nm UV light source combined with a shutter and photolithographic alignment ?xture (Newport, 83210i-line, 1.80s). After developing this exposed PR region for 15 s (Shipley, MF-319) and rinsing with Millipore water (Mill-Q, ρ > 18 MΩ · cm), the exposed nickel was removed by etching in 0.80 M nitric

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acid for 7 min to produce a horizontal trench with a typical width of 300 nm. The whole device with the pattern created was dipped into commercial Au plating solution (Clean Earth Solutions?), then Au was potentiostatically electrodeposited using a Princeton Applied Research 2263 potentiostat at -0.9 V verse Hg/HgCl2, KCl (sat’d) (SCE) by using a one compartment three-electrode electrochemical cell with Pt foil as counter electrode. After completing the electrodeposition, rinsing the device with acetone and Millipore water will create a device that only has patterned Ni pads with Au NWs growing on their edges. Then half of the device was immersed into nitric acid, making sure that Ni pads are perpendicular to the etching solution surface, for 6 min to remove the Ni. After etching, the device was rinsed with Millipore water and air-dried. The second etching with nitric acid resulted in parallel gold nanowires are free standing on one end but still in good contact with nickel on the other. The Au NWs with Ni was used as a electrical contact while the Au NWs without Ni were immersed into manganese perchlorate plating solution, containing 2mM Mn(ClO4)2, 50 mM LiClO4.18 MnO2 was electrodeposited under +0.6V verse Hg/Hg2SO4, K2SO4 (sat’d) (MSE).21 When the deposition was completed, the glass slide was removed from the plating solution, rinsed with Millipore water and dried in the oven at 90°C for 30 min prior to electrochemical assessment. This process created a hybrid nanowire consisting of a Au nanowire as the core and shell composed of mpMnO2. Ultrapure, ultradry acetonitrile required for cycle life testing was prepared by processing reagent grade acetonitrile with a Jorg Meyer Phoenix SDS column purification system. LiClO4 was battery grade from Sigma-Aldrich, with a purity of 99.99%. Electrochemical Characterization. All electrochemical measurements were performed by a one-compartment three-electrode cell using a Princeton Applied research 2263 potentiostat. The speci?c capacitance determination was conducted in 1.0M LiClO4 (battery grade, dry, 99.99%) in dry acetonitrile. Prior to each measurement, the acetonitrile is pre-saturated with N2 gas. A Pt foil was used as counter electrode with a saturated mercurous sulfate reference electrode (MSE) for the capacitance measurement. All these potentials are quoted with respect to this reference (EMSE = +0.640V verse normal hydrogen electrode). QCM (Quartz crystal microbalance). In order to characterize the coloumbic efficiency of MnO2 electrodeposition, QCM was used. The QCM measurements were performed with a Stanford Research Systems (SRS) QCM200 Quartz Crystal Microbalance Digital Controller, in conjunction with a QCM25 5MHz Crystal Oscillator equipped with an Au-coated 5MHz quartz

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crystal (area = 1.37 cm2). The sensitivity factor (Cf) of the immersed quartz/gold electrode was calibrated by galvanostatically electrodepositing silver from aqueous 1.0 mM Ag2SO4, 0.1M H2SO4. A second calibration, and a second Cf value appropriate for the emersed electrode, was obtained after drying the Ag ?lm under vacuum for 2 hours at room temperature. QCM analysis measures the frequency change, ?f, that is correlated with the mass loading, m, onto the quartz/Au electrode via the Sauerbrey equation22:

Δf = ?C f ? Δm

(4)

For the immersed gold QCM crystal, the value for Cf obtained from the silver calibration in the silver plating solution was 55.9(±0.7) Hz cm2 ug?1; for the dried silver deposit it was 49(±1) Hz cm2 ug?1. MnO2 was directly electrodeposited onto the gold surface at room temperature and the frequency changes were recorded in-situ. After 10s deposition, the probe was pulled out from the plating solution, removed from the controller, rinsed with Millipore water and air-dried, then vacuum dried for 2 hours at room temperature. After drying, the probe was mounted to the QCM controller and the frequency was recorded. Each electrodepostion was repeated in triplicate (Fig.1). Mean mass versus mean charge data for three replicate samples (Fig.1) produced slopes of 3.23(±0.1) ?g/mC for the in-situ measurement and 0.78 ?g/mC for the dried ?lm. This latter value was used to determine MnO2 shell masses from the deposition charge as required for the calculation of Csp. Structural Characterization. Micrographs of MnO2/Au NWs are acquired by a Philips XL30 FEG SEM (Field Emission Gun Scanning Electron Microscope) operated at 10 keV equipped with an energy dispersive X-ray detector (Ultradry silicon drift X-ray detector). All samples except the sample for EDX mapping were sputtered with a thin layer of Au/Pd before imaging to prevent charging. Transmission electron microscopy (TEM) images and selective area electron diffraction (SAED) patterns were obtained using a Philips CM 20 TEM operating at 200 keV. The Au and MnO2 core:shell NWs were synthesized on a Si grid with 20 nm thick Si3N4 windows (Ted Pella, Inc). Atomic force microscopy (AFM) images and amplitude traces were acquired using an Asylum Research, MFP-3DTM AFM equipped with Olympus, AC160TS tips in a laboratory air ambient. The thickness of MnO2 shells were measured by AFM based on 40 nm Au NW arrays

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(Fig.3). In order to obtain the affect of 1.0M LiClO4/CH3CN on the MnO2/Au NWs, the thickness of MnO2 shell was measured before and after 1000 cyclic voltammograms by AFM (Fig.5). Grazing-incidence X-ray diffraction (GIXRD) patterns were obtained using a Rigaku Ultima III high resolution X-ray diffractometer employed the parallel beam optics with a ?xed incident angle of 0.3 . The X-ray generator was operated at 40 kV and 44 mA with Cu Kα
°

irradiation. The JADE 7.0 (Materials Data, Inc.) X-ray pattern data processing software was used to analyze acquired patterns and estimate the respective grain diameter size. Raman spectra were collected using a Renishaw inVia Raman Microscope equipped with the EasyConfocal optical system (spatial resolution: <1 ?m) and red laser (wavelengths: 785 nm). WiRE 3 software was used to acquire the data and images. Determination of Specific Capacitance (Csp). Csp of MnO2 NWs was calculated from cyclic voltammograms (CV) acquired in 1.0 M LiClO4/CH3CN using Eq. (5):
Q ΔE ? m

Csp =

(5)

where Q is the total charge integrated from the CV, associated with scanning from -0.40V to +0.50V, ?E is 0.9V, and m is the dry mass of the MnO2 obtained from QCM-calibrated coulometry. Determination of the “b” value. The scan rate (ν)-dependent current, i ?, is described by the generic equation: i ?= aνb ? ?23-?‐25 ?where a is a constant that depends upon the electrode area, the concentration of Mn3+/4+ redox centers, and the diffusion coef?cient for charge transport in the MnO2. Two limiting values exist for b: b = 0.5 applies for voltammetric processes that are controlled by diffusion to a planar surface, where the diffusion may occur either in the solution or within the MnO2. On the other hand, b = 1.0 applies to surface-limited processes including double-layer charging and the pseudo-capacitive charge transfer at surface Mn3+/4+ centers. We observe both of these limits in the data of Fig 5b. At E = -0.05V, b = 0.79, closer to 1.0, indicating that processes 1 and 2 dominate the current. At E = 0.50V b = 0.55 indicating that the diffusion of ions within the MnO2 dominates the current. Deconvolution of Specific Capacitance. The Csp can be deconvoluted into two components

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corresponding to the two values of b already mentioned above: b = 1.0 corresponding to the case i (ν ) ∝ ν, encompassing both double-layer charging and the pseudo-capacitance of Mn centers located at wetted MnO2 surfaces, and b = 0.5, corresponding to the case i (ν ) ∝ ν1/2 which accounts for the current derived from diffusion-limited cation intercalation/deintercalation. The total current measured in a voltammetric scan at each potential, i (ν), is given by the sum of these two components: ?23-?‐25

i(v ) = k1v + k 2v1/ 2

(6)

where k1 and k2 are scan rate independent constants. By plotting the quotient (i (ν)/ν1/2) versus ν1/2 the values of k1 and k2 are obtained from the slope and intercept, respectively: ?23-?‐25 ?

i(v ) = k1v1/ 2 + k 2 1/ 2 v

(7)

Porosity Measurement. The porosity of the MnO2 is obtained if the volume of these core:shell NWs are known, using:

porosity = 1 ?

ρ exp t ρ theor

(8)

where ρexpt is the ratio of mass to volume, and ρtheor is 3.40 g/cm3 as reported for the birnessite MnO2 26. To calculate the exact volume of MnO2 shells, a hemicylindrical cross section of the core:shell NWs is assumed. Mp-MnO2 was electrodeposited onto linear Au nanowires (40 nm in thickness) spaced by 5 ?m inside a 1mm × 1mm area created by photolithography, simplifying the counting of individual core:shell NWs with an optical microscope. The length and width of the core:shell NWs was measured by SEM. The deposition of MnO2 was carried out for durations of 5s, 10s, and 20s, during which the deposition charge was recorded in order to estimate the mass of the MnO2 using the QCM calibration already discussed above.

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Results and Dicussion. Au core:mp-MnO2 shell nanowires were prepared using LPNE (Fig 2a). Briefly, a linear array of gold nanowires on glass was first prepared as previously described.18 These nanowires had a rectangular cross section with a height of 40 nm, a width of 150-250 nm, a length of 0.1-1 cm, and an interwire spacing of 5 ?m. Then a δ-MnO2 shell was electrodeposited on top of these gold nanowires by oxidation from a Mn2+ solution according to: Mn2+(s) + (2+n)H2O (l) + xLi+(aq.) ? Lix(Mn3+xMn4+1-x)O2 . nH2O(s) + 4H+(aq.) + (2-x)eusing the procedure of Popov et al.21 who reported x = 0.35 and n ≈ 1.0 using a potentiostatic growth potential of +0.60 V vs. saturated mercurous sulfate (MSE). In that work, δ-phase MnO2 was obtained. Recently10, we demonstrated that reaction (9) could be used to prepare nanowires of mesoporous δ-MnO2 by applying a series of +0.60 V × 0.5s growth pulses spaced by 20s at open circuit. The resulting δ-MnO2 is composed of a network of 2-5 nm fibrils and is characterized by a porosity of ≈50%.10 In this case, the use of a train of +0.60 growth pulses drives dendritic, diffusion-controlled growth of MnO2, creating this unique mesoporous morphology. Here, δ-MnO2 having a similar mesoporous morphology was obtained by potentiostatic electrodeposition at +0.60V vs. MSE, without pulsing (Fig 2b). We attribute the rapid MnO2 deposition rate produced by efficient hemicylindrical diffusion of Mn2+ to the gold nanowires. The thickness of the δ-MnO2 shell, controlled using the deposition time, can be directly observed and its thickness measured using atomic force microscopy (AFM) (Fig 3). The δ-MnO2 shell thickness was adjustable over the range from 50 to 250 nm (Fig 2c). Scanning electron microscope images of these nanowire arrays (Fig 2d,e) reveals that the electrodeposited δ-MnO2 forms a uniform, conformal shell on the gold nanowires. Elemental analysis using electron dispersive x-ray spectroscopy (EDX) shows that manganese is concentrated within this shell (Fig 4a,b), confirming the formation of a MnOx layer. Grazing incidence x-ray diffraction (GIXRD) reveals two strong reflections that can be indexed to δ-phase MnO2 (JCPDS, 43-1456). Scherrer analysis of the (001) reflection yields a mean grain diameter of ?5 nm for both films and nanowire samples. This re?ection derives from (9)

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the d = 7.1 ? periodicity of the O-Mn-O prismatic layers arrayed along the c-axis of δ-MnO2. This structural assignment is further corroborated by Raman spectroscopic analysis of these structures (Fig 4d). Raman spectra for film and core:shell nanowires are both dominated by three prominent peaks at 505, 572, and 652 cm-1 that are characteristic of δ-MnO227. The peak at 652 cm?1 has been assigned to the symmetric stretching vibration (Mn-O) of the MnO6 groups, and the peak at 572 cm?1 is assigned to the (Mn-O) stretching in the basal plane of the MnO6 sheet.27 A weaker transition at 505 cm?1 is not prominent in most of the typical δ-phase MnO2. The mesoporous nature of the δ-MnO2 is apparent in the transmission electron micrograph (TEM) of a single core:shell nanowire (Fig 4e). This shell has a high porosity ranging from 45 – 81%, and its internal structure consists of a network of 1-4 nm fibrils. From high magnification TEM images (Fig 4f, inset), the diameter of these fibrils can be directly measured and a histogram of these diameters (Fig 4f) shows that these structures are narrowly distributed in diameter, with a mean diameter of 2.2 (± 0.7) nm corresponding to just three times the prominent c-axis periodicity of 0.71 nm. This critical dimension is amongst the smallest achieved for MnO2 nanomaterials5,28,29. The properties of these Au core:mp-MnO2 shell nanowires for storing charge is of interest. In prior work10, we have carried out the electrochemical characterization of mesoporous MnO2 in aqueous LiClO4 electrolytes, achieving a high Csp of 923 (± 24) F/g at 5 mV/s and 484 (± 15) F/g at 100 mV/s, but as already indicated, we did not evaluate the cycle stability of these materials. Here, voltammetry in aqueous 1.0 M LiClO4 yielded an even higher Csp in excess of 1200 F/g at 5 mV/s and 398 (± 32) F/g at 100 mV/s (data not shown), however this capacity faded rapidly on subsequent cycles (Fig 5d) as a consequence of dissolution of the mesoporous δ-MnO2 shell. Our approach for mitigating this stability issue involved the transfer of core:shell nanowires from aqueous LiClO4 to LiClO4 in acetonitrile. This was accomplished by first water rinsing these nanowires, air drying them at 90 oC for at least 30 minutes, and finally transferring to dry, N2 sparged, 1.0 M LiClO4, acetonitrile for voltammetric characterization. The cyclic voltammetry for a typical array of 141 (± 17) nm (Fig 3a) and 68 (± 3) nm (Fig 3b) Au core:mpMnO2 shell nanowires in acetonitrile (Fig 5a) closely resembles that seen in aqueous electrolytes except that the Csp is reduced by 25% at all scan rates. At 100 mV/s, for example, the mean Csp for identical nanowire arrays is reduced from 398 (± 32) F/g in 1.0 M LiClO4 (aq) to 299 (± 41) F/g in 1.0 M LiClO4 (CH3CN). At 5 mV/s, these nanowires exhibit Csp values even larger than ? 10 ?

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we reported earlier10, in the range from 1020 (± 100) F/g (68 nm shell) to 260 (± 50) F/g (175 nm shell). More importantly, the retained Csp shows no fade for 1000 cycles (Fig 4d) and the thickness of the mp-MnO2 shell, measured using atomic force microscopy, increased slightly after cycling as compared to its initial state (Fig 6). The sensitivity of these nanowires to water is further emphasized by observing the cycle stability in wet acetonitrile (0.004% water), where capacity fade is much improved relative to water, but a monotonic loss occurs culminating in a 50% loss at 600 cycles. Salts of divalent cations like Mn2+ have a far lower solubility in acetonitrile than in water. It follows that eliminating water from the electrolyte prevents reaction (3) from occurring. The measured Csp for arrays of Au core:mp-MnO2 shell nanowires can be deconvoluted into an insertion capacity, rate-limited by cation diffusion in the MnO2, and a non-insertion capacity that is not diffusion controlled, as previously described by Conway24 and Dunn25. As already indicated above, the non-insertion capacity - denoted as ‘∝ν’ in Fig 5 - has two components: a faradaic pseudo-capacitance derived from reaction (1) and the double-layer capacity of the material. The voltammetric current derived from these two processes increases in direct proportion to the potential scan rate, ν30. The diffusion-controlled insertion capacity on the other hand shows a ν dependence30. The scan-rate independent non-insertion capacity was
1/2

nearly constant across a wide range of shell thicknesses, ranging from 88 F/g to 68 F/g for shell thicknesses of 74 nm and 188 nm, respectively. The insertion capacity, in contrast, was strongly dependent on the shell thickness, decreasing from 1010 F/g for a shell thickness of 74 nm to 260 F/g for a 188 nm shell thickness at the scan rate of 5 mV/s. The higher capacity of thinner MnO2 shells is interesting and not explained by the data presented here. In particular, this effect does not appear to be rooted in the critical dimension of the MnO2 material as this does not change appreciably for shells of varying thickness, based upon our TEM analyses. Our hypothesis is that this effect is a consequence of the ohmic drop within the mp-MnO2 which can be expected to increase with shell thickness. Conclusions. δ-phase MnO2 nanowires containing an integral gold nanowire current collector have been prepared using a variant of the LPNE process. These Au core:mp-MnO2 shell nanowires are certain to have an extremely low volumetric capacity because they are patterned at low areal ? 11 ?

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density on glass so their practical utility is limited, but they provide clear proof that concentric metal oxide:metal hybrid structures have the potential to function efficiently as battery and supercapacitor electrodes that are characterized by ultrahigh Csp coupled with excellent cycle stability. Specifically, these hybrid nanostructures show both an ultra-high specific capacity of 1020 ± 100 F/g at 5 mV/s and 450 ± 70 F/g at 100 mV/s coupled with absolute cycle stability of this capacity to 1000 cycles. This performance is made possible by shrinking the critical dimensions of the MnO2 to 2 nm, and by imparting stability to the mp-MnO2 shell by transferring it into an ultra-dry acetonitrile electrolyte. In principle, acetonitrile should make possible considerably higher charge storage capacities, enabled by expanding the potential limits and we hope to explore that possibility in future work. We are also interested in learning to what extent the thickness of the mp-MnO2 shell can be increased while the desirable rate properties intrinsic to the mp-MnO2 are retained. Acknowledgement. This material is based upon work supported as part of the Nanostructures for Electrical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. Valuable discussions with Professor Phil Collins, Dr. Israel Perez, and Brad Corso are gratefully acknowledged. All electron microscopy was carried out in the Laboratory for Electron and X-ray Instrumentation (LEXI) at University of California, Irvine. We also thank Aaron Halpern for producing a version of Figure 1a.

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References. (1) Altung, S.; Jacobsen, T. Electrochim Acta 1981, 26, 1447. (2) Lee, H. Y.; Goodenough, J. B. J Solid State Chem 1999, 144, 220. (3) Devaraj, S.; Munichandraiah, N. J Phys Chem C 2008, 112, 4406. (4) Preisler, E. J Appl Electrochem 1976, 6, 311. (5) Rolison, D. R.; Long, R. W.; Lytle, J. C.; Fischer, A. E.; Rhodes, C. P.; McEvoy, T. M.; Bourga, M. E.; Lubers, A. M. Chem Soc Rev 2009, 38, 226. (6) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S. Chem Rev 2004, 104, 4463. (7) Whittingham, M. S. Chem Rev 2004, 104, 4271. (8) Arthur, T. S.; Bates, D. J.; Cirigliano, N.; Johnson, D. C.; Malati, P.; Mosby, J. M.; Perre, E.; Rawls, M. T.; Prieto, A. L.; Dunn, B. Mrs Bull 2011, 36, 523. (9) Long, J. W.; Belanger, D.; Brousse, T.; Sugimoto, W.; Sassin, M. B.; Crosnier, O. Mrs Bull 2011, 36, 513. (10) Yan, W. B.; Ayvazian, T.; Kim, J.; Liu, Y.; Donavan, K. C.; Xing, W. D.; Yang, Y. G.; Hemminger, J. C.; Penner, R. M. Acs Nano 2011, 5, 8275. (11) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J Electrochem Soc 2000, 147, 444. (12) Toupin, M.; Brousse, T.; Belanger, D. Chem Mater 2004, 16, 3184. (13) Lang, X. Y.; Hirata, A.; Fujita, T.; Chen, M. W. Nat Nanotechnol 2011, 6, 232. (14) Mulvaney, P.; Cooper, R.; Grieser, F.; Meisel, D. J Phys Chem-Us 1990, 94, 8339. (15) Ataherian, F.; Lee, K. T.; Wu, N. L. Electrochim Acta 2010, 55, 7429. (16) Long, J. W.; Rhodes, C. P.; Young, A. L.; Rolison, D. R. Nano Lett 2003, 3, 1155. (17) Brousse, T.; Taberna, P. L.; Crosnier, O.; Dugas, R.; Guillemet, P.; Scudeller, Y.; Zhou, Y.; Favier, F.; Belanger, D.; Simon, P. J Power Sources 2007, 173, 633. (18) Xiang, C. X.; Kung, S. C.; Taggart, D. K.; Yang, F.; Thompson, M. A.; Guell, A. G.; Yang, Y. A.; Penner, R. M. Acs Nano 2008, 2, 1939. (19) Xiang, C. X.; Yang, Y. G.; Penner, R. M. Chem Commun 2009, 859. (20) Menke, E. J.; Thompson, M. A.; Xiang, C.; Yang, L. C.; Penner, R. M. Nat Mater 2006, 5, 914. (21) Nakayama, M.; Kanaya, T.; Lee, J. W.; Popov, B. N. J Power Sources 2008, 179, 361. (22) Sauerbrey, G. Z Phys 1959, 155, 206. (23) Brezesinski, K.; Wang, J.; Haetge, J.; Reitz, C.; Steinmueller, S. O.; Tolbert, S. H.; Smarsly, B. M.; Dunn, B.; Brezesinski, T. J Am Chem Soc 2010, 132, 6982. (24) Liu, T. C.; Pell, W. G.; Conway, B. E.; Roberson, S. L. J Electrochem Soc 1998, 145, 1882. (25) Wang, J.; Polleux, J.; Lim, J.; Dunn, B. J Phys Chem C 2007, 111, 14925. (26) Post, J. E.; Veblen, D. R. Am Mineral 1990, 75, 477. (27) Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P. Solid State Ionics 2003, 159, 345. (28) Long, J. W.; Rolison, D. R. Acc Chem Res 2007, 40, 854. (29) Wei, W. F.; Cui, X. W.; Chen, W. X.; Ivey, D. G. Chem Soc Rev 2011, 40, 1697. (30) Bard, A. J.; Faulkner, L. R. Electrochemical methods : fundamentals and applications; 2nd ed.; Wiley: New York, 2001.

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Figure 1. Determination of x (Eq. 8) for MnO2 electrodeposition using quartz crystal microbalance (QCM) gravimetry. Mean mass versus mean charge for three replicate depositions showing the slope obtained for the in-situ mass measurement (blue) and that of the vacuum dried ?lm (red). Assuming all of the mass change derives from the removal of super?cial water and that n = 1, this slope corresponds to x = 0.58 (Eq. 8). The wet state of the ?lm has ?19 super?cial water molecules per Mn in this case. Also shown (dashed line) is the predicted slope for the case of x = 0 and n = 0 in Eq. 9.

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Figure 2. Synthesis of Au core:mp-MnO2 shell nanowires. a) Five-step LPNE process flow: Step 1: photolithographic patterning of a photoresist layer on a 40 nm thick nickel film (gray) on glass (blue), Step 2: etching of nickel film for 7 minutes using nitric acid. This process forms a horizontal trench at the perimeter of the patterned region (inset). Step 3: Electrodeposition of gold nanowires within this trench from an aqueous commercial gold plating solution. Step 4: Removal of photoresist and nickel. Step 5: Electrodeposition of δ-MnO2 on the gold nanowire array. b) Cyclic voltammogram of Mn2+ plating solution (2mM Mn(ClO4)2, 50 mM LiClO4) showing the potential used for potentiostatic δ-MnO2 growth, c) Thickness of the hemicylindrical δ-MnO2 layer as measured using atomic force microscopy. d,e) Scanning electron micrographs of a linear array of Au core:mp-MnO2 shell nanowires prepared at 5 ?m pitch on glass.

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Figure 3. Atomic force micrographs of MnO2/Au core:shell NWs. a) A linear array of MnO2/Au core:shell NWs on glass (image size: 80 ?m × 80 ?m). This image shows two uncoated gold nanowires and 13 Au:MnO2 core:shell nanowires. b) A single Au NW, with a height of 30 nm, prepared by LPNE on glass (image size 5 ?m× 5 ?m) (c) A single Au:MnO2 core:shell nanowire with a MnO2 shell thickness of 120 nm.

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Figure 4. Characterization of Au core:mp-MnO2 shell nanowires. a,b) Scanning electron micrograph (a) and Mn elemental map (b) for a Au core:mp-MnO2 shell nanowire showing the co-localization of the Mn signal within the shell of the nanowire. Spurious contrast in (a) is caused by the fact that the sample could not be overcoated with metal to eliminate charging in this experiment. c) Grazing incidence x-ray diffraction (GIXRD) of a δ-MnO2 film on gold (blue trace) and an array of thousands of Au core:mpMnO2 shell nanowires (red trace). d) Raman spectra of a δ-MnO2 film on gold (blue trace) and a single Au core:mp-MnO2 shell nanowire (red trace). e) Transmission electron micrograph (TEM) of a Au core:mpMnO2 shell nanowire. f) Histogram of δ-MnO2 fibril diameters obtained from TEM images such as that shown in the inset. The mean fibril diameter is 2.2 ± 0.7 nm.

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Figure 5. Electrochemical characterization of gold:δ-MnO2 core:shell nanowires. a,b) Cyclic voltammograms (100 mV/s) for arrays of core:shell nanowires on glass (shell thickness =141 ± 17 nm), and for the same number of bare gold nanowires (40 nm (h) × 200 nm (w)). Three CVs in 1.0 M LiClO 4, acetonitrile are shown: Gold nanowires only (blue trace), and scan 1 (green) and scan 1000 (red) for an array of gold:δ-MnO2 core:shell nanowires. b) Scan rate dependence of the CV for an array of core:shell nanowires. The value of b is calculated at two potentials: +0.5V and -0.05V as indicated (see SI). c) Csp versus scan rate for core-shell nanowires with the indicated shell thicknesses. d) Csp as a function of the number of charge-discharge cycles (from -0.4 V to +0.5V vs. MSE). Shown are data for core-shell nanowires in aqueous 1.0 M LiClO4 (black trace), in 1.0 M LiClO4 prepared with as received Fisher acetonitrile (99.9% purity) containing 0.004% of water, and 1.0 M LiClO4 prepared with ultradry acetonitrile from a Glasscontour Solvent SystemTM.

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Figure 6. AFM images of a single Au:MnO2 nanowire before (a) and after (b) voltammetric cycling in dry acetonitrile for 1000 cycles at 100 mV/s as described by. The mean height of this nanowire increased from 120 nm to 140 nm.

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Figure 7. Deconvolution of two capacity components: Insertion capacity (∝ν1/2, yellow) and noninsertion capacity (∝ν, blue). (a,b) CVs at 100 mV/s (a) and 5 mV/s (b) for an array of ≈800 Au core:mp-MnO2 shell nanowires having a 74 nm shell thickness showing the two capacity components after deconvolution. (c,d) Bar graphs of the two Csp components versus ν for core:shell nanowires having two different shell thicknesses: 74 nm (c) and 188 nm (d).

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