[email protected] CoreShell Nanomesh Electrodes for

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Flexible Electronics

[email protected] Core–Shell Nanomesh Electrodes for Transparent Flexible Supercapacitors Tengfei Qiu, Bin Luo, Michael Giersig, Eser Metin Akinoglu, Long Hao, Xiangjun Wang, Lin Shi, Meihua Jin, and Linjie Zhi* Future electronic devices, such as wearable displays, laptops, smart phones and e-papers are expected to be thin, light, all transparent and flexible.[1–5] Therefore, transparent flexible supercapacitors (TFSCs) are attracting increasingly attention as energy storage devices in next generation electronics. Recently, while efforts have been dedicated to fabricate successfully TFSCs with various assembly structures,[6,7] feasible fabrication of high-performance TFSCs with reasonable areal/gravimetric capacity, charge/discharge ability and cycling stability is still a big challenge. Transparent flexible electrodes (TFEs) play a key role in the development of high-performance TFSCs. Generally, the widely selected electrode materials for supercapacitors are metal oxides, carbon materials and conducting polymers.[8–10] Among the metal oxides, manganese dioxide has been extensively studied due to its superior capacitor performance enabled by its large specific capacitance, low cost, and environmental-friendly nature.[11,12] However, the low intrinsic electric conductivity[13,14] as well as the low transparency[15] of MnO2 has limited its application as a TFE material in high-performance supercapacitors. The key issue in the fabrication of TFEs is that most of the active materials and current collectors are opaque. A commonly selected approach for the fabrication of TFEs is to combine transparent current collector, for instance indium tin oxide (ITO), with a very thin layer of active material, of which the thickness is much less than its optical absorption length.[16,17] However, the brittle nature of ITO as well as the finite availability of indium hinder its practical application in industry.[18] Thus, several ITO-free carbon based TFSCs have been demonstrated with carbon nanotubes or graphene as the TFEs.[19–21] Nevertheless, in these cases, one still needs to restrictedly control the amount of active electrode materials T. F. Qiu, Dr. B. Luo, L. Hao, X. J. Wang, L. Shi, Dr. M. H. Jin, Prof. L. J. Zhi National Center for Nanoscience and Technology Zhongguancun, Beiyitiao No.11 Beijing 100190, P. R. China E-mail: [email protected] Prof. M. Giersig, E. M. Akinoglu Department of Physics Freie University Berlin Arnimallee 14, 14195 Berlin, Germany DOI: 10.1002/smll.201401250 small 2014, DOI: 10.1002/smll.201401250

to keep the electrode to be thin and transparent,[22] leading to a limited value of areal capacitance of the TFSCs. Another promising alternative approach is to fabricate a nano- or micro-structured electrode by combining patterned active materials with a patterned current collector. The transparency of the electrode can be realized through a patterned structure, a conductive network, that covers only a small fraction of the whole surface area.[21] Using this approach, high capacitance materials can be reasonably utilized as TFEs to enhance the performance of TFSCs. Actually, comb-like pattern structures have been successfully used as electrodes in the fabrication of in-plan micro-supercapacitors,[6,22] although these patterned structures have no functions as TFEs for particularly conventional sandwich-like supercapacitors. In addition, metallic grids have been selected to successfully fabricate transparent batteries, in which the grids serve exactly as a TFE.[21] Nevertheless, TFSCs based on the sandwich fabrication of TFEs that integrate a high specific capacitance and ultra fast charge/discharge ability have not yet been achieved. To this end, a sophisticated architecture efficiently combining a metallic nanomesh and a high capacitance material at the nanoscale is expected to form a TFE with enhanced performance in transparent flexible energy storage devices. Among many well-known nanofabrication methods, nanosphere lithography[23] is a popular technique due to its facility, low cost and high throughput in the fabrication of large-area highly-ordered patterns.[24] Metallic nanomesh electrodes prepared by nanosphere lithography combine sufficient transparency, high conductivity and unique structuredetermined properties, and provide promising potential in photoelectronic applications.[25–27] In this work, we have developed for the first time a novel [email protected] core–shell nanomesh structure on a flexible polymeric substrate through nanosphere lithography combined with electro-deposition processing. The direct growth of MnO2 on a well-conformed Au nanomesh, which acts as a TFE with high transparency, not only introduces metal oxides successfully into a TFE but realizes an efficient contact between the current collector and the active materials needed for a fast charge transport during charging and discharging processes. As a result, a high areal capacitance of 4.72 mF cm−2 and ultrahigh rate capability up to 50 V s−1 have been achieved on such hybrid films. Moreover, the core– shell nanomesh structure can be employed as an electrode to

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Figure 1. a) Schematic illustration for the fabrication of the [email protected] nanomesh-based TFSC. i) Au nanomesh is prepared using a monolayer of PS particles as a template. ii) [email protected] nanomesh is fabricated by electrodepositing a layer of MnO2 onto the Au nanomesh. iii) A gel-electrolyte film is sandwiched between two [email protected] nanomesh electrodes. b) Schematic diagram of ion diffusion pathways and charge transport channels of the [email protected] nanomesh electrode.

fabricate transparent flexible supercapacitors, which exhibit excellent flexibility and reasonable cycling stability. Noticeably, the strategy developed here provides opportunities as well for continuous fabrication of large number of TFSCs which can be connected in series or in parallel, to improve the output potential and/or current. The overall synthetic procedure of an [email protected] nanomesh-based TFSC is illustrated in Figure 1. First, the Au nanomesh is produced on a polyethylene terephthalate (PET) substrate by nanosphere lithography using a monolayer of hexagonally close-packed polystyrene (PS) particles as a template (Figure S1,S2, Supporting Information). The transmittance and sheet resistance of the film can be well controlled by tuning the original particle size, reactive ion etching (RIE) parameters, and the Au film thickness. In this work, the average applied PS particle size is 700 nm, and the prepared Au nanomesh film with a thickness of 50 nm has a sheet resistance of 13–18 Ω sq−1. MnO2 is then conformably electrodeposited onto the patterned Au nanomesh, during which the mass loading of the MnO2 can be easily tuned by adjusting the deposition time. Finally, the [email protected] nanomesh-based TFSC is assembled by sandwiching a LiClO4/ PVA gel-electrolyte between two [email protected] nanomesh films on the PET substrates. Detailed synthetic procedures are provided in the experimental section. Scanning electron microscopy (SEM) characterizations are used to determine the morphology change of the Au

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nanomesh before and after electrodeposition (Figure 2a,b). It is clear that a uniform and continuous nanomesh of abundant petal-shaped MnO2 nanosheets is formed along the embedded Au core nanomesh, preserving the original shape of the Au nanomesh (Figure S3, Supporting Information). The morphology, structure and composition of the [email protected] MnO2 nanomesh are further investigated by means of transmission electron microscopy (TEM) and electron dispersive X-ray spectroscopy (EDX). As shown in Figure 2c, the [email protected] MnO2 core–shell structure is verified by TEM and EDX spectroscopy of the nanomesh fragments scraped off the substrate. The surface of the Au nanomesh is tightly coated with loose and soft MnO2 nanosheets. The high resolution TEM (HRTEM) images show that the MnO2 nanosheets grown on the Au core mesh exhibit thicknesses of 2–3 nm. Such small thicknesses enable the nanosheets to be mechanically flexible. This is confirmed by the experimental test for folding the [email protected] nanomesh film. No obvious change of its sheet resistance has been observed even at a bending radius of 1 mm.The lattice fringes observed in the HRTEM image exhibit a lattice spacing around 2.4 Å, which is consistent with the interlayer spacing of (006) planes of birnessite-type MnO2.[28] For aqueous electrolytes, the hydrophilicity of the electrodes can significantly improve the performance of thin film supercapacitors as an easy access of the ions to the electrode/ electrolyte interface is allowed. Therefore, the hydrophilicity

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small 2014, DOI: 10.1002/smll.201401250

[email protected] Core–Shell Nanomesh Electrodes for Transparent Flexible Supercapacitors

Figure 2. a) SEM image of the Au nanomesh. b) SEM, c,d) TEM, and e) Au, Mn, O elemental mapping images of [email protected]δ-MnO2 core–shell nanomesh peeled from PET substrate. f) Photographs of water droplet shape on 1) PET substrate, 2) Au mesh/PET, 3) [email protected]–200s/PET, and 4) [email protected]–400s/PET surfaces. Contact angle: 86.5°, 104.5°, 32.0°, and 28.9°, correspondingly.

of [email protected] core–shell nanomesh films on PET substrates is further investigated by means of water contact angle measurement as shown in Figure 2f. Two [email protected] nanomesh films with different MnO2 mass loadings, named [email protected]– 200s and [email protected]–400s, are prepared with different electrodeposition times. A PET substrate and an Au nanomesh film on PET substrate are used as reference samples. The water contact angles on the [email protected]–200s/PET and [email protected] MnO2–400s/PET surfaces are 32.0° and 24.9° respectively and decrease with increasing mass loading of MnO2, indicating that both surfaces are hydrophilic. By comparison, the larger contact angle of 86.5° on the PET substrate surface shows that the PET substrate is less strongly hydrophilic; and the largest contact angle of 104.5° on the Au nanomesh/PET surface reveals that the Au nanomesh/PET surface is hydrophobic. Thus, obviously, the high hydrophilicity of [email protected] core–shell nanomesh is mainly due to the hydrophilic nature of MnO2 nanosheets. The transparency of the [email protected] nanomesh structure is also characterized by the UV–vis spectroscopy (Figure 3a). [email protected]–200s, and [email protected]–400s have a transmittance of 60% and 48% (λ = 550) respectively, while the Au nanomesh has a transmittance of 71%. The [email protected] nanomesh is transparent in the whole visible spectrum and exhibits similar transmittance spectra to the Au nanomesh, although the increasing mass loading of MnO2 slightly decreases the transparency of the film. To demonstrate the high electrochemical performance of the [email protected] nanomesh as an electrochemical electrode, cyclic voltammetry (CV) measurements are performed using a three-electrode set-up in 1 m Na2SO4 solution (Figure S4,

Figure 3. a) Transmittance and b) CV curves of the bare Au nanomesh, [email protected]–200s and [email protected]–400s films. Scan rate, 500 mV s−1. c) CV curves of [email protected]–400s at different scan rates. d) The areal capacitance of Au nanomesh, [email protected]–200s and [email protected]–400s as a function of the current density. small 2014, DOI: 10.1002/smll.201401250

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Supporting Information). As shown in Figure 3b, the CV curves for both [email protected]–200s and [email protected]–400s exhibit symmetrical rectangular shapes at a scan rate of 500 mV s−1, indicating ideal capacitive behavior. The areal current density values of the CV curve for the bare Au nanomesh is much lower than those of [email protected]–200s and [email protected]–400s, suggesting that the capacitance of the [email protected] nanomesh electrode is mainly contributed by the incorporated MnO2 nanosheets rather than the Au nanomesh. The CV curves for [email protected]–400s are investigated at various scan rates from 1 to 50 V s−1 (Figure 3c). At a high scan rate of 20 V s−1, the CV curve still presents a nearly symmetrical rectangular shape. The excellent CV shape at such a high scan rate, which is unusual for other MnO2-included materials, exhibits a rapid current response to the voltage change, indicative of the fast diffusion of the electrolyte in the films. This can attribute to the unique structure of the [email protected] composite film. The combination of ultra thin hydrophilic MnO2 nanosheets with a fully accessible surface area, and the strong coupling between MnO2 and the highly conductive Au nanomesh lead to this high rate capability. The [email protected] nanomesh electrode is also analyzed using galvanostatic charge/discharge measurements. Figure 3d shows the areal capacitances of the bare Au nanomesh, [email protected]–200s and [email protected]–400s at different areal current densities. The areal capacitance is calculated from discharging slope after the IR drop. [email protected]–400s preserves 64% of its areal capacitance (from 4.72 to 3.01 mF cm−2) as the current density increases from 5 to 80 µA cm−2. The maximum areal capacitance of 4.72 mF cm−2 obtained for [email protected]–400s is higher than those of many previously reported TFEs, such as, graphene,[29] carbon nanotube (CNT)/Ag nanowires,[30] nano-engineered carbon,[31] and reduced graphene oxide/Cellulose nanofibers hybrid.[20] In marked contrast, the maximum areal capacitance of the bare Au nanomesh is 5.7 µF cm−2, which is as small as one thousandth that of [email protected]–400s. Thus the contribution of Au nanomesh to the capacitance of the film can be ignored. The mass loadings of MnO2 are calculated to be 4.4 µg cm−2 and 9.0 µg cm−2 corresponding to different depositing time, 200s and 400s (S2). Owing to much less mass loading of MnO2, [email protected]–200s has a smaller areal capacitance of 1.9 mF cm−2 at 5 µA cm−2 compared with [email protected]–400s. At low current densities, the areal capacitance increases with the increase of MnO2 mass, which means that almost all the MnO2 contributes to the energy storage. This is due to the strong contact between MnO2 and Au grid allowing a small resistance and nanometer-scale diffusion pathways. Remarkably, the calculated gravimetric capacitance of [email protected]–400s reaches 524 F g−1 at a current density of 0.56 A g−1. This value is also higher than many previously reported values for currently available TFEs such as reduced graphene oxide,[32,33] indium tin oxide (ITO)/polyethylene naphathalate (PEN),[34] polyaniline/CNT.[35] The excellent areal and gravimetric capacitances of [email protected] nanomesh electrodes further confirm the advantage of its unique structure by fully utilizing the active material. To further explore the advantages of this novel design for real applications, TFSCs based on the [email protected] nanomesh

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films on PET substrates are tested. Figure 4a shows a symmetrical TFSC sealed inside a transparent flexible Plydimethylsiloxane (PDMS) film. The device is fully transparent through which the repeated characters “NCNST” beneath it can be seen clearly. The full device with packaging exhibits a transmittance of 36% (λ = 550 nm) as shown in Figure S5. The nearly rectangular shapes and symmetry of the CV scans indicate its typical pseudocapacitive nature (Figure 4b). No obvious change is observed in the CV curves at scan rates of 10 mV s−1 when the device is bent with a bending angle of 90°, revealing good flexibility of our device. The cycling performance of the device at flat and bending state is also tested. As shown in Figure 4c, the discharge capacitance of the flat device decreases slightly from 795 µF cm−2 to 680 µF cm−2 after 500 cycles at 5 µA cm−2 with a retention of 85.5%. The cycling test is performed on the device at the bending state (bending radius = 5 mm) for another 500 times after one day resting. There is actually a small increase in capacitance when the supercapacitor is bent, which may be caused by the resting process. The bent device retains approximately 85% of its initial capacitance (from 730 µF cm−2 to 620 µF cm−2) after 500 cycles, exhibiting similar cycling stability with the flat device. The coulombic efficiency is also shown in Figure 4d. The device shows a stable coulombic efficiency of greater than 95% over 1000 cycles. The cycle stability may be ascribed to the strong coupling between the Au nanomesh and MnO2 nanosheets and the soft and neutral gel electrolyte partially protects the MnO2 layer from mechanical expansion during the ion insertion/removal process and the dissolution of Mn2+ during the charge/discharge cycling which may cause a capacitance decrease. The galvanostatic charging/ discharging curves of the device for the first five cycles and at different current densities are shown in the Figure 4f and Supporting Information Figure S6. The linear voltage versus time profiles, the symmetrical charge/discharge characteristics, and a quick I–V response represent good capacitive characteristics for our supercapacitor. Finally, to illustrate the feasibility of the FTCFs to integrate the storage element as close as possible to the electronic circuit, two supercapacitors were integrated together, as shown in Figure 4e. The CV curve of such a two in-series supercapacitor group exhibits a rectangular shape within a selected potential range from 0 to 2 V. The charge/discharge curve (Figure 4f) indicates that the charge potential could be up to 2 V, which is two times that of a single supercapacitor. In summary, we have proposed and realized a novel approach to pattern the TFEs at the nanoscale for the fabrication of TFSCs. A new core–shell nanomesh structure where MnO2 is in situ grown on the surface of Au nanomesh has been designed and fabricated through the combination of nanosphere lithography method and an electrodeposition process. The efficient contact between the Au current collector and the MnO2 nanosheets provides significantly enhanced electrochemical properties including high areal/ gravimetric capacitance and excellent rate capability for TFSCs. These findings provide useful insights in the design and fabrication of unique 2D core–shell nanomesh electrodes

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small 2014, DOI: 10.1002/smll.201401250

[email protected] Core–Shell Nanomesh Electrodes for Transparent Flexible Supercapacitors

Figure 4. a) Optical images of a TFSC based on [email protected] electrodes sealed in a PDMS film, flat and bent. The repeated characters “NCNST” can be seen clearly through the TFSC device. b) CV curves of the device at different scan rates. c) CV curves of the device at flat and bending state. d) Cycling stability and coulombic efficiency of the device at flat and bending state. e) CV curve of two supercapacitors in series at the scan rate of 500 mV s−1. f) The galvanostatic charging/discharging curves of a single device and 2 devices connected in series at 5 µA cm−2.

for potential high-efficiency energy storage application in future transparent flexible electronics.

Experimental Section Preparation of [email protected] Nanomesh Films: The Au nanomesh film was obtained using a monolayer of close-packed periodic hexagonal polystyrene (PS) particles as a template. First, the PS particles were self-assembled on the water surface and deposited onto a PET substrate and the substrate was dried in air. Next, oxygen reactive-ion etching (RIE) was applied at 60 W in flowing 30 sccm O2 for 70 s to reduce the size of PS particles. Subsequently, an Au thin film with a thickness of 50 nm was evaporated onto the substrate. A thin layer of Cr (5 nm) was used as adhension layer. The next step was to peel off the PS particles on top of the PET substrates with an adhesive tape. Two different [email protected] nanomesh films, named [email protected]–200s and [email protected]–400s, were prepared by coating the Au nanomesh with different MnO2 mass loadings. In detail, the Au nanomesh films were immersed into the manganese perchlorate plating solution containing 2 mm small 2014, DOI: 10.1002/smll.201401250

Mn(ClO4)2, 50 mm LiClO4. MnO2 was electrodeposited under +0.9 V verse Ag/AgCl, KCl (sat’d) with different depositing times, 200 s and 400 s. Afterwards, the electrode was rinsed with Millipore water and dried in an oven at 60 °C. Preparation of the Transparent Flexible Supercapacitor Device: The LiClO4/PVA gel electrolyte was prepared by mixing PVA powder with water (1 g of PVA/10 mL of H2O) and LiClO4 (0.34 g). The mixture was then heated to around 80 °C under vigorous stirring. After cooling down and evaporating about 48 h under ambient conditions, the electrolyte solidified to form an adhesive gel membrane. A transparent flexible supercapacitor was finally produced by placing the gel electrolyte membrane between two [email protected]– 200s electrodes. Material Characterization: The morphology, microstructure and composition of the samples were investigated by FE-SEM (Hitachi S4800) and FE-TEM (FEI Tecnai G2 20 ST) measurements. The water contact angles were measured on a Krüss contact angle goniometer (DSA100).The sheet resistances of the films were analyzed on a four-probe electro-resistance analyzer, and their transmittance curves were detected on a Perkin-Elmer UV–vis–NIR spectrometer.

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Electrochemical Measurement: The electrochemical properties of [email protected] nanomesh electrodes were analyzed using a three-electrode setup to measure the cyclic voltammetry (CV) and galvanostatic charge-discharge (CD) behavior in 1 m Na2SO4 aqueous solution, where [email protected] nanomesh film was used as the working electrode, Ag/AgCl as the reference electrode, and a piece of Pt as the counter electrode. Capacitance Calculations: The areal and gravimetric capacitances of the electrode were calculated from the galvanostatic charge/discharge curve according to the following equations: C areal = i /[ −( ΔV/Δt )Ae ]

(1)

C gravimetric = i /[ −( ΔV/Δt)me ]

(2)

where Careal and Cgravimetric refer to the area capacitance and gravimetric capacitance of the electrode, respectively. i is the applied current, –ΔV/Δt is the slope of the discharge curves after the IR drop. Ae is the area of the electrode including the area of the [email protected] MnO2 nanomesh and the interspaces within the mesh. me is the mass of MnO2. The mass of MnO2 is calculated from the charge pass for the electrochemical deposition according to Faraday’s laws of electrolysis,[36,37] on the assumption that Faraday current efficiency for deposition is 100%. In a typical process, the electrochemical deposition was carried out at 0.9 V for 200 s and 400 s to give a totally passed charge of 9.8 mC and 20.1 mC correspondingly at the Au nanomesh electrode with 1 cm2. The mass loadings of MnO2 are calculated to be 4.4 µg cm−2 and 9.0 µg cm−2 corresponding to different depositing time, 200s and 400s. The areal capacitance of the TFSC device is calculated from the galvanostatic charge/discharge curve according to the equation: C d,areal = i/[ −( ΔV/Δt )Ad ]

(3)

where Cd,areal refer to the areal capacitance of the TFSC device. i is the applied current, –ΔV/Δt is the discharging slope after the IR drop, Ad is the overlap area of the electrodes.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors acknowledge the support from the Ministry of Science and Technology of China (No.2012CB933403), the National Natural Science Foundation of China (Grant No. 21173057, 21273054), the Beijing Municipal Science and Technology Commission (Z121100006812003), and the Chinese Academy of Sciences.

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Received: May 7, 2014 Published online:

small 2014, DOI: 10.1002/smll.201401250