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Collateral Advantages of a Gel Electrolyte for MnO Nanowire Capacitors: Higher Voltage; Reduced Volume Mya Le Thai, Shaopeng Qiao, Rajen Kumar Dutta, Gaurav Jha, Alana F Ogata, Girija Thesma Chandran, and Reginald M. Penner ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00172 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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ACS Energy Letters

Collateral Advantages of a Gel Electrolyte for MnO2 Nanowire Capacitors: Higher Voltage; Reduced Volume Mya Le Thai,† Shaopeng Qiao,‡ Rajen K. Dutta,‡ Gaurav Jha,† Alana Ogata,† Girija Thesma Chandran,† and Reginald M. Penner∗,† †Department of Chemistry, University of California, Irvine, CA 92697 ‡Department of Physics and Astronomy, University of California, Irvine, CA 92697

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Abstract:

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Recently we demonstrated that symmetric, all Au@δ-MnO2 core@shell nanowire

capacitors can achieve cycle stability to 100,000 cycles and beyond in a poly(methylmethacrylate) (PMMA) gel electrolyte. Here we examine the limits of the PMMA gel to confer this extraordinary stability, in terms of the accessible maximum voltage, Vmax , and the thickness of the PMMA gel electrolyte layer. Two conclusions are: 1) The PMMA gel permits the Vmax to be increased by 50% from 1.2 V to 1.8 V, allowing the specific energy to be increased 5-6 fold, and, 2) The PMMA gel layer thickness can be reduced from 180 µm (previously) to 2 µm while simultaneously utilizing two layers of nanowires, and patterning nanowires in each layer at 5× higher density. For this nanowire “sandwich” architecture, a net increase in volumetric capacity of 600× up to 500 mF/cm3 can be achieved while retaining cycle stability to 100,000 cycles.

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ACS Energy Letters

In either batteries or capacitors, the ultra-high surface area:volume ratio associated with nanowire-based electrodes increases charge/discharge rates and enhances power. For example, we have demonstrated 1,2 that symmetrical all-nanowire capacitors based upon arrays of Au@δ-MnO2 core@shell nanowires produce a specific capacity, Csp , of 20-40 F/g at high charge/discharge rates of 100 mV/s. For this system, the thinner the nanowires, the higher the accessible Csp . 1,2 The disadvantage of nanowires is their limited cycle stability which is typically a fraction of that available using thin films of the same battery materials. For symmetrical Au@δ-MnO2 nanowire capacitors operating in 1.0 M LiClO4 in propylene carbonate (PC) or acetonitrile, a reversible capacity of 4000-8000 cycles is reproducibly observed. 1,2 Recently, we reported 2 that adding 20 w/w% poly(methylmethacrylate)(PMMA) to the liquid electrolyte (1.0 M LiClO4 in propylene carbonate) extends the cycle lifetime to 100,000 cycles or more. In that study, we exploited a symmetrical, all-nanowire capacitor consisting of two interpenetrating arrays of 375 ultra-long (5 mm) core@shell, Au@δ-MnO2 nanowires 1 lithographically patterned at 20 µm pitch onto a glass surface using the LPNE process. 3–5 A 180 µm thick layer of the PMMA gel electrolyte layer was coated onto the nanowire array in these experiments. When these all-nanowire capacitors were cycled using the PMMA gel electrolyte for 100,000 cycles across 1.2 V at 100 mV/s - corresponding to continuous cycling for more than 5 weeks - the specific capacity actually increased in most trials. 2 In this letter, we explore the answers to two questions related to these earlier results: First, does the PMMA gel electrolyte extend the permissible Vmax ? Vmax in the range from 0.8 V - 1.0 V has typically been used for symmetrical MnO2 capacitors, including those involving nanostructured composites with graphene, carbon nanotubes, etc. 6–13 In our prior work with symmetrical Au@δ-MnO2 nanowire arrays, involving both gel and liquid electrolytes, Vmax = 1.2 V has been used. 1,2 We report here that that the PMMA gel electrolyte enables the use of a Vmax of 1.8 V while retaining both Csp and cycle stability to 100,000. In theory, this 50% increase in Vmax relative to 1.2 V translates into an energy enhancement of 2.25 times, 2 based upon E = (1/2)CVmax . Experimentally, however, we measure a much larger increase

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Figure 1: The Au@MnO2 nanowire “sandwich” capacitor. a) Schematic diagram (plan view) showing a array of Au@δ-MnO2 nanowires in one layer of the “sandwich” capacitor. The PMMA gel layer is omitted, exposing the nanowires. b) Schematic diagram (side view) the Au@δ-MnO2 all nanowire capacitor. A dgel = 1.0 µm gel layer covers the nanowires in each of the two layers. c) Schematic diagram (side view) showing two layer “sandwich” capacitor showing a dgel = 2.0 µm PMMA gel electrolyte layer. d) Photograph of a “sandwich” capacitor containing two Au@δ-MnO2 nanowire layers on glass, as shown schematically in (c). e) Low magnification SEM image of Au@δMnO2 nanowires, spaced laterally by 5 µm. f) Higher magnification image of a single Au@δ-MnO2 nanowire. SEM images for other thicknesses of the δ-MnO2 shell are shown in Figure 3. g) SEM image of a gold nanowire like that at the center of each of the Au@δ-MnO2 nanowires shown in (f).

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ACS Energy Letters

in energy of 5-6 fold because C increases with Vmax across the entire potential window. The second question we address here involves the feasibility of dramatically reducing the thickness of the gel layer, dgel , from the value of 180 µm used in our prior work. 2 This question is answered by designing a “sandwich” capacitor architecture that exploits two, more densely packed nanowire 2D layers and dgel = 2 µm of the PMMA gel electrolyte. This sandwich capacitor represents a “unit cell” of a many-layered, 3D capacitor consisting of tens or hundreds of planar nanowire layers, spaced by PMMA gel electrolyte layers. We demonstrate that even when dgel is reduced from 180 µm (in our prior work) to 2 µm, two layers of nanowires are used instead of one, the nanowire packing density within both layers are increased by a factor of 5, and Vmax is increased by 50% to 1.8 V, the cycle stability to 100,000 cycles is retained. PMMA gel electrolytes, consisting of 10-50 w/w% PMMA in propylene carbonate (PC), and mixtures of PC with ethylene carbonate (EC), together with lithium salts, have been investigated and applied in batteries for more than 25 years. 14–16 Interest in PMMA gel electrolytes was initially sparked by the unusually high ionic conductivities of these gels relative to those of other polymers such as polyvinylidene fluoride, poly(ethylene oxide), and others. 16 The PMMA gel employed in this study (20 w/w% PMMA gel, 1.0 M LiClO4 , PC) has a room temperature conductivity of ≈10−3 S/cm; one fifth of the conductivity of 1.0 M LiClO4 , PC. 14–16 Au@δ-MnO2 Core@Shell Nanowire “Sandwich” Capacitors. Au@δ-MnO2 nanowires can be patterned onto glass surfaces using the process flow depicted in Figure 2. 1,2,17 In this study, we pattern these nanowires at a pitch (center-to-center distance) of 5 µm instead of 25 µm, 1,2,17 increasing the nanowire packing density by a factor of 5. This array of 4000 nanowires are 1.0 cm in total length - increased two-fold from prior work. Finally, two 4000 nanowire layers supported on glass slides are placed face-to-face to construct a nanowire sandwich capacitor. One of these nanowire layers is configured as the (+)-electrode and the other as the (-)-electrode (Figure 1c). These nanowire layers are separated by a PMMA

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Figure 2: Nine-step process flow for the fabrication of a single, high density layer of 4000 Au@MnO2 nanowires patterned at 5 µm pitch on glass, with dimensions as indicated in Figure 1a,b. The processes corresponding to the numbered steps are described in the experimental methods.

gel electrolyte layer that is just 1.6 - 2.0 µm in thickness. As depicted in Figure 1b,c, this thickness, dgel = 2dP R where dP R is the thickness of the Shipley 1808 PR layer applied in Figure 2, Step 7. This PR layer acts like a gasket to fix the total PMMA gel thickness and to provide a tight seal around the perimeter of the capacitor containing the gel and nanowires. The PMMA gel is applied to both nanowire layers by spin-coating, and the sandwich is then formed. Finally, the capacitor is hermetically sealed by applying hot glue to the edges of the glass slides, as seen in the photograph of Figure 1d. The Au@δ-MnO2 nanowires are prepared by using the LPNE process to prepare a linear array of gold nanowires 40 nm × 200 nm, and then electrodepositing δ-MnO2 according to the following half-reaction: 5,18

4+ + − Mn2+ + 2H2 O + xLi+ −→ Lix Mn3+ x Mn1−x O2 + 4H + (2 − x)e

(1)

This deposition process is carried out in an aqueous plating solution of 2 mM Mn(ClO4 )2 , 50 mM LiClO4 . Previously we have shown that “x” is in the range from 0.0 - 0.05 for the as-deposited δ-MnO2 prepared at +0.60 V vs. MSE. 2,17 Scanning electron microscope (SEM) images and atomic force microscopy (AFM) to-

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ACS Energy Letters

Figure 3: SEM images (top) and AFM images and height versus distance amplitude traces for gold and Au@δ-MnO2 core@shell nanowires prepared with three δ-MnO2 shell thicknesses : a,b) Uncoated gold nanowire comprising the core of Au@δ-MnO2 core@shell nanowires. c,d) Au@δMnO2 core@shell nanowire prepared by electrodepositing MnO2 onto the gold nanowire for tdep = 5 s. e,f) tdep = 40 s. g,h) tdep = 300 s. The mean nanowire heights (indicated at bottom) equal the sum of the MnO2 shell thickness and the gold nanowire height (39 ± 3 nm). The performance of a sandwich capacitor containing nanowires prepared with tdep = 40 s (dM nO2 = 302 - 39 = 263 nm) is shown in (i) and (j). i) [inset] Voltammograms at scan rates from 1 mV/s to 500 mV/s across Vmax = 1.2V, and a plot of Csp versus scan rate from these data. j) Plot of Csp at 100 mV/s up to 100,000 cycles showing 10-12 F/g over the 5 week interval during which these data were collected.

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pographs (Figure 3a-h) confirm the dimensions of the gold nanowire core (a,b) and the distinctive morphology of the δ-MnO2 shells prepared using tdep = 5 s (c,d), 40 s (e,f) and 300 s (g,h). The thicknesses of these shells is obtained from the AFM images show here, by subtracting the 39 nm mean height of the gold nanowire from the total mean height of Au@δ-MnO2 nanowires. δ-MnO2 shell thicknesses, dM nO2 = 65 (±16) nm, 263 (±18) nm, and 367 (±26) nm.

Increasing Vmax . The performance of the sandwich capacitor at Vmax = 1.2 V, with respect to Csp and cycle stability, is indistinguishable from that of single layer, interdigitated nanowire capacitors. 2 For example, a sandwich capacitor with dM nO2 = 263 nm (tdep = 40 s) shows a maximum Csp at 1 mV/s of 335 F/g (Figure 3i) which corresponds to a halfcell capacity of four times this value or 1340 F/g - approximately equal to the expected theoretical Faradaic capacity of MnO2 . At more rapid charge/discharge rates of 100 mV/s, Csp is 10-12 F/g, also as previously observed. 2 Whereas the cycle stability in liquid PC electrolyte is normally 4000 - 8000 cycles for single layer capacitor systems, 2 here we obtain cycle stability to at least 100,000 scans with an average coulombic efficency (C.E) of 96 % (Figure 3j). These data demonstrate that the new challenges imposed by the high density sandwich capacitor architecture - particularly the 2 µm PMMA electrolyte thickness - is not detrimental either to Csp or cycle stability. These conclusions apply to Vmax = 1.2 V. As already indicated above, Vmax values for symmetrical MnO2 capacitors are typically lower than this, in the range from 0.80 V to 1.0 V. 6–13 A larger Vmax is desirable because the total energy, E, is proportional to Vmax 2 :

2 E = (1/2)CVmax

(2)

Does the sandwich capacitor architecture and PMMA gel electrolyte allow Vmax to be increased beyond 1.2 V? Cyclic voltammograms for sandwich capacitors with nanowires prepared with dM nO2 = 65 nm (Figure 4a) show that an irreversible process is present at cell 8


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ACS Energy Letters

Figure 4: Charge storage performance for Au@δ-MnO2 nanowire sandwich capacitors. All data here was acquired using the PMMA gel electrolyte. a) Cyclic voltammograms (CVs) at 200 mV/s for nanowires prepared with a MnO2 deposition time, tdep = 5 s (dM nO2 = 65 nm) showing Vmax values of 1.2 V, 1.5 V, and 1.8 V as indicated. b) CVs at 200 mV/s for Vmax = 1.2 V and all three tdep values: 5 s, 40 s, and 300 s. c) b) CVs at 200 mV/s for Vmax = 1.8 V and all three tdep values: 5 s, 40 s, and 300 s. d, e, f) Csp versus scan rate for MnO2 nanowire arrays corresponding to all three tdep values: 5 s, 40 s, and 300 s, corresponding δ-MnO2 shell thicknesses, dM nO2 , are 65, 263 and 367 nm, respectively.)

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potentials above ≈1.5 V. Since this is a symmetrical capacitor, the asymmetry seen in these CVs indicates that (+) and (-) electrodes are not, in fact, identical electrochemically. The irreversible current in the CVs of Figure 4a is necessarily produced by two reactions occurring an an equal rate at both electrodes: An oxidation at the (+) electrode and a reduction at the (-) electrode. At least one of these reactions must be irreversible. This irreversible process is responsible for a depression of the coulombic efficiency (C.E.) to 92-94%, in this case. With other cells we have studied, average C.E. values in the range from 85% to 98% have been observed. Normally, irreversible processes tax the cycle stability of the system because they consume Li+ , causing depletion of Li+ within the cell, or they degrade the host material. Either of these processes will result in irreversible capacity loss. Surprisingly, we demonstrate below that this irreversible electrochemistry does not compromise the cycle stability of these systems. By definition, Csp = Q/(Vmax mM nO2 ), where Q is the charge stored, and mM nO2 is the total mass of MnO2 in the capacitor. For a solid state, parallel plate capacitor, Csp is independent of Vmax but this is not what we observe. Instead, Csp increases by a factor up to 2.0 as Vmax is increased from 1.2 V to 1.8 V. This can be seen most easily in the cyclic voltammograms of Figure 4a: The current envelope, ic (E), at a potential E, is given by: iC (E) = 2C(E)ν where ic (E) is the total anodic plus cathodic charging/discharging current at E, C(E) is the capacitance at E, and ν is the potential scan rate. From Figure 4a, it is apparent value of ic (E), and therefore C(E), approximately doubles as Vmax increases from 1.2 V to 1.8 V. This increased in C(E) not only increases Csp , it also leads to an increase in energy beyond that expected for the increase in Vmax alone in Eq 2. Because of the influence of Vmax on C(E), cell energy at Vmax = 1.8 V is increased by a factor of 5 to 6 compared with 1.2 V, instead of the 2.25–fold increase predicted by Eq. 2 (Table 2). Thinner MnO2 shells on these Au@δ-MnO2 nanowires correlate with improved rate capability (Figure 4d-f), just as previously observed. 1,2,17 At 1 mV/s, both the tdep = 5 s (65 nm) and 40 s (263 nm) samples produce Csp = 300 - 325 F/g which translates into a half-

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ACS Energy Letters

cell Csp value of four times this value, or 1200 F/g - 1300 F/g; approximately equaling the theoretical Faradaic capacity of MnO2 . The thickest MnO2 shell obtained using tdep = 300 s (367 nm), does not achieve this maximum value. At higher scan rates, Csp rapidly declines, but thinner MnO2 shells show better retention of Csp at high scan rates (Figure 4d-f). This behavior has been attributed to the influence of rate-limiting Li+ insertion and solid-state diffusion of Li on charge storage, not only for MnO2 17,18 but also for other transition metal oxides. 19–22 These Csp versus scan rate data also show that while a larger MnO2 shell does yield a higher total capacity (Table 1), Csp moves in the opposite direction, becoming higher with decreasing MnO2 thickness (Figure 4d-f). This trend, also documented previously, 1,2 demonstrates that the specific capacity of the MnO2 shell is degraded as the shell thickness increases. We hypothesize that two factors contribute to this inefficiency: First, the efficiency of energy storage by the MnO2 shell can be reduced by I 2 R losses across its thickness caused by its high electrical resistivity (10−4 Ω cm < ρ < 10−3 Ω cm). 23 Second, Mn centers, buried deep within crystalline MnO2 , may become electrically isolated when the transport of charge-compensating ions to these centers is impeded. These capacity loss mechanisms have not yet been experimentally confirmed.

Influence of Vmax and Sandwich Architecture on Cycle Stability. Hermetically sealed sandwich capacitors were assembled and operated in moist (RH ≈55%) laboratory air. To assess the influence of Vmax on the cycle stability of Au@δ-MnO2 nanowire sandwich capacitors, these devices were first cycled to Vmax = 1.2 V at 200 mV/s for 10,000 cycles. Vmax was then increased to 1.5 V for an additional 10,000 cycles, and finally to 1.8 V for an additional 80,000 cycles before terminating the experiment (Figure 5). Csp versus cycle data for all three dM nO2 values (Figure 5) demonstrates that no capacity fade was observed in these experiments. In fact, a slow and steady increase in Csp was usually seen across 100,000 cycles, as documented in the data for the devices of Fig 5 a and b. The device of Figure 5c showed an increase in Csp to 11 F/g at 27,000 cycles, and the Csp stabilized at this

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Table 1: Energy Storage Metrics for Au@δ-MnO2 Nanowire Arrays in Single Layer 2 and Sandwich Capacitors dM nO2 a (nm)

Vmax b (V)

Ctotal c (µF)

Vol. Csp d (mF/cm3 )

25 " "

143 222 300

1.2 1.2 1.2

0.45 0.90 2.5

0.35 0.62 0.85

5 " " "

65(±16) 263(±18) 263(±18) 367(±26)

1.8 1.2 1.8 1.8

57 38 73 230

130 62 180 510

Device NW pitch Architecture (µm)

single layer

sandwich

Literature Reference

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" " this work " " "

a c

dM nO2 = Thickness of the δ-MnO2 shell as measured by AFM. b Vmax = Maximum applied voltage. Ctotal = Total capacitance = Qtotal /Vmax . d Vol. Csp = Volumetric capacitance = Ctotal /PMMA gel volume, excluding the volume of the glass slides.

value for the remainder of the test. To our knowledge, Vmax values as large as 1.8 V have not been successfully employed in any symmetrical or asymetrical MnO2 -based capacitors (Table 2). 2,24–27 Using our methodology, this Vmax is not only useable, but it is compatible with elevated Csp performance coupled with ultra-long cycle stability, to 100k cycles. This paper begins to answer the question: Is a 3D nanowire capacitor, consisting of stacked, planar 2D nanowire layers separated by thin PMMA gel electrolyte layers, feasible? Could such capacitors produce the ultra-high cycle stability - to 100,000 cycles - previously observed for single layers of nanowires operating in a PMMA gel electrolyte? 2 We have addressed this question by designing a “sandwich” capacitor architecture in which two 4000 nanowire layers are separated by a very thin 2 µm layer of PMMA gel electrolyte. This sandwich capacitor can be thought of as a “unit cell” of a many-layered, 3D capacitor consisting of tens or hundreds of planar nanowire layers, spaced by micron-scale PMMA gel electrolyte layers. For Au@δ-MnO2 core@shell nanowire capacitors, this sandwich capacitor architecture provides two advantages: First, it enables the Vmax to be increased from 1.2 V to 1.8 V, 12


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ACS Energy Letters

Figure 5: Cycle stability for three Au@δ-MnO2 core@shell nanowire capacitors. Csp and the coulombic efficiency (C.E.) are plotted as a function of the number of cycles, as Vmax was progressively increased from 1.2 V (initially), to 1.5 V (at 10,000 cycles), and finally to 1.8 V (at 20,000 cycles). Data, acquired at 200 mV/s, is presented for three devices: a) tdep = 5 s (MnO2 shell thickness = 65 nm), b) tdep = 40 s (MnO2 shell thickness = 263 nm), c) tdep = 300 s (MnO2 shell thickness = 367 nm).

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increasing the specific energy by a factor of 2.25 (theoretically) and 5-6-fold in practice. Second, it allows the volumetric capacity to be increased by a factor of more than 100 - a combined consequence of increasing the nanowire density in each layer, and reducing the thickness of the PMMA gel electrolyte layer from 180 µm to 2 µm. A volumetric capacity of 500 mF/cm3 at 100 mV/s is thereby obtained. This number does not represent a limit. For example, a 2.5x higher packing volumetric density of nanowires is obtained by further reducing the inter-wire pitch to 2 µm from 5 µm. Significantly, the cycle stability of the Csp is observed up to 100,000 cycles for sandwich capacitors operating at Vmax = 1.8 V across a range of MnO2 shell thicknesses. These results demonstrate the feasibility of 3D nanowire capacitors with a cycle stability of 100,000+ cycles – consisting of many, densely packed, planar nanowire layers separated by micron-scale PMMA gel electrolyte layers.

Acknowledgments This work was supported by Nanostructures for Electrical Energy Storage (NEES II), an Energy Frontier Research Center (EFRC) 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, Professor Suzanna Siwy, Eric Wong and Tim Plett are gratefully acknowledged. SEM data were acquired at the LEXI facility (lexi.eng.uci. edu/) at UCI.

Supporting Information for Publication Experimental methods are provided for the following processes and procedures: i) Au@δMnO2 nanowire fabrication, ii) Preparation of the gel electrolyte, iii) sandwich capacitor assembly, iv) procedures for electrochemical characterization, and, v) structural characterization are provided. X-ray powder diffraction data are also provided.

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ACS Energy Letters

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via Self-Limiting Electroless Deposition: Implications for Electrochemical Capacitors. Nano Lett 2007, 7, 281–286. (9) Jiang, H.; Zhao, T.; Ma, J.; Yan, C.; Li, C. Ultrafine Manganese Dioxide Nanowire Network for High-Performance Supercapacitors. Chem Commun 2011, 47, 1264–1266. (10) Xia, H.; Feng, J.; Wang, H.; Lai, M. O.; Lu, L. MnO2 Nanotube and Nanowire Arrays by Electrochemical Deposition for Supercapacitors. J Power Sources 2010, 195, 4410– 4413. (11) Nam, H. S.; Kwon, J. S.; Kim, K. M.; Ko, J. M.; Kim, J. D. Supercapacitive Properties of a Nanowire-Structured MnO2 Electrode in the Gel Electrolyte Containing Silica. Electrochim Acta 2010, 55, 7443–7446. (12) Liu, W.; Lu, C.; Wang, X.; Tay, R. Y.; Tay, K. High-Performance Microsupercapacitors Based on Two-Dimensional Graphene/Manganese Dioxide/Silver Nanowire Ternary Hybrid Film. ACS Nano 2015, 1528–1542. (13) He, Y.; Chen, W.; Li, X.; Zhang, Z.; Fu, J.; Zhao, C.; Xie, E. Freestanding ThreeDimensional Graphene/MnO2 Composite Networks as Ultralight and Flexible Supercapacitor Electrodes. ACS Nano 2012, 7, 174–182. (14) Bohnke, O.; Frand, G.; Rezrazi, M.; Rousselot, C.; Truche, C. Fast Ion Transport in New Lithium Electrolytes Gelled with PMMA. 1. Influence of Polymer Concentration. Solid State Ionics 1993, 66, 97–104. (15) Bohnke, O.; Frand, G.; Rezrazi, M.; Rousselot, C.; Truche, C. Fast Ion Transport in New Lithium Electrolytes Gelled with PMMA. 2. Influence of Lithium Salt Concentration. Solid State Ionics 1993, 66, 105–112. (16) Bohnke, O.; Rousselot, C.; Gillet, P.; Truche, C. Gel Electrolyte for Solid-State Electrochromic Cell. J Electrochem Soc 1992, 139, 1862. 16


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Table 2: Recent Literature Metrics for Asymmetric and Symmetric MnO2 Capacitors +/− Electrode

Electrolytea

Vol. Csp b (mF/cm3 )

Energy Wh/kgc (Vmax )

Cycles At Faded

Aq. LiOH Aq. Na2 SO4 Aq. Na2 SO4 LiCl/PVA gel

n.a. n.a. n.a. 650

n.a. (0.70 V) 6–21 (1.8 V) 7–21 (0.80 V) n.a. (1.8 V)

1.5k 1k 5k 5k

Aq. KOH Aq. KOH

13,000 - 30,000 n.a.

n.a. (1.5 V) 40–105 (1.6 V)

10k 3k

Aq. Na2 SO4 Aq. Na2 SO4 Aq. Na2 SO4

n.a. 80 - 130 n.a.

4–23 (1.80 V) 30–50 (0.80 V) 8–27 (1.70 V)

3k 5k 4k

Aq. Na2 SO4 PVA,LiCl,ethanol PC: PMMA

140 - 175 8,000 - 11,000e n.a.

32–35 (0.80 V) n.a. (1.0 V) 3–17 (0.40 V)

10k 15k 50k

Literature Reference

Assymmetric MnO2 /AC G-MnO2 /G G-MnO2 /G H-TiO2 @MnO2 / H-TiO2 @C CNT−MnO2 /CNT Ni(OH)2 , MnO2 @CNT/C MnO2 nanotubes/AC NCTs/ANPDM/C MnO2 ,CNT/AC

27 6 28 29

30 31

32 33 34

Symmetric MnO2 ,CNT MnO2 @MWCNT R-MnO2 ,CNT textile Au@δ-MnO2 dM nO2 = 65 nm 263 nm 367 nm

35 36 37

this work

PC: PMMA " "

130 180 510

a Abbreviations:

37 (1.8 V) 16 (1.8 V) 5.4 (1.8 V)

>100k " "

AC - activated carbon, ac - activated carbon, ACN - activated carbon nanofibers, C - carbon, G - graphene, MeCN - acetonitrile, MWCNT - multiwalled carbon nanotube, NCTs/ANPDM – N-doped-carbon-tubes/Au-nanoparticles-doped-MnO2 , PC - propylene carbonate, PEDOT - poly(ethylenedioxythiophene), PMMA - poly(methylmethacrylate), PVA poly(vinyl alcohol). b Specific capacity (volumetric), in F/g, measured at 100 mV/s unless otherwise specified. c Gravimetric energy density, calculated based upon the mass of the active material only. d Cycle stability, as defined by the authors. e This C sp is based upon the volume of the fiber, exclusive of the gel electrolyte layer.

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