Freestanding transparent metallic network based ultrathin, foldable and designable supercapacitors

Yan-Hua Liu a, Jian-Long Xu *b, Xu Gao b, Yi-Lin Sun c, Jing-Jing Lv b, Su Shen a, Lin-Sen Chen *a and Sui-Dong Wang *b
aCollege of Physics, Optoelectronics and Energy, Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province & Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou, Jiangsu 215006, P. R. China. E-mail: lschen@suda.edu.cn
bInstitute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China. E-mail: xujianlong@suda.edu.cn; wangsd@suda.edu.cn
cInstitute of Microelectronics, Tsinghua National Laboratory for Information Science and Technology (TNList), Tsinghua University, Beijing 100084, P. R. China

Received 20th August 2017 , Accepted 29th September 2017

First published on 29th September 2017


Fully integrated ultrathin, transparent and foldable energy storage devices are essential for the development of smart wearable electronics, yet typical supercapacitor electrodes are substrate-supported which limits their thickness, transparency and mechanical properties. Employing freestanding transparent electrodes with no substrate support could bring ultrathin, foldable and designable supercapacitors closer to reality. Herein, we report a freestanding, ultrathin (<5 μm), highly conductive (3 × 104 S cm−1), highly transparent (>84% transmittance) and foldable metallic network electrode, loaded with MnO2 by electrochemical deposition, as a supercapacitor electrode. The freestanding metallic network electrode is fabricated via a simple and low-cost laser direct-writing micro-patterning technique followed by a selective electrodeposition process, where the metallic network patterns, network periods, metal thickness and also the electrode film patterns can be designed for different applications. The obtained freestanding MnO2@Ni network electrode delivers an outstanding areal capacitance of 80.7 mF cm−2 and long-term performance stability (96.3% after 10[thin space (1/6-em)]000 cycles). Moreover, the symmetric solid-state supercapacitors employing the freestanding MnO2@Ni network electrode not only show high areal capacitance as well as high optical transparency (>80% transmittance), but also can be tailored, attached, folded, rolled up, and crumpled into any object or various shapes with only slight performance degradation. The advent of such freestanding transparent metallic network electrodes may open up a new avenue for realizing fully integrated ultrathin, foldable and designable supercapacitors towards self-powered wearable electronics.



Broader context

With the growing demand for smart wearable electronic devices, it is essential to develop fully integrated ultrathin, transparent and foldable energy storage systems. In order to bring this to reality, employing freestanding transparent electrodes without a substrate support is required. This article reports the design and fabrication of a freestanding, ultrathin (<5 μm), highly conductive (3 × 104 S cm−1), highly transparent (>84%) and foldable metallic network electrode, loaded with MnO2 by electrochemical deposition, as a supercapacitor electrode. In our process, a facile laser direct-writing micro-patterning followed by selective electrodeposition method beneficial for designable electrodes is adopted. The highly uniform and interconnected Ni network electrode film provides a platform for constructing a monolithic MnO2@Ni electrode film, rendering a superior electron and ion percolation network. The resultant symmetric solid-state supercapacitor device not only shows good electrochemical performance as well as high transparency, but also can be attached, folded and even crumpled with almost no performance degradation. Furthermore, this technique is fully compatible with industrial manufacturing, and offers a paradigm for developing portable and wearable energy storage devices and systems.

Introduction

Transparent flexible energy storage devices are critical components in power-integrated wearable electronic systems, including shape-conformable sensors, self-powered rolled-up displays, and foldable capacitive touch screens.1–5 In particular, compared with other energy storage devices, supercapacitors are an ideal option owing to their rapid charge/discharge, high power density and long cycling life.6,7 Significantly, the supercapacitor electrode must be simultaneously transparent, bendable, foldable and even twistable to adapt the devices for complex shaped, rolled-up or foldable applications.8,9 However, current industrial indium tin oxide (ITO) based transparent conductive electrodes (TCEs) have a serious weakness in realizing fully flexible electronics due to their inherent brittleness under repeated bending cycles.10 In recent years, the development of metallic TCEs has been considered as an effective approach for achieving high-performance flexible supercapacitor electrodes. Metallic TCEs possess high conductivity, high thermal and air stability, easy tuning of electrical and optical properties, low fabrication cost and excellent scalability to large-area applications,11–13 and are superior to other TCEs based on conducting polymers,14 3D carbon,15 graphene16 or other organic–inorganic hybrid materials.17 To date, the reported metallic TCEs are mostly substrate-supported, commonly employing polyethylene terephthalate (PET), polyethylene (PE), cyclic olefin copolymer (COC), etc. as the flexible substrates.18–20 However, the substrate-supported nature of metallic TCEs limits their applications due to the low glass-transition temperature (<150 °C), low optical transmittance, large thickness (usually 50–200 μm) and poor foldable capability of typical flexible substrates.21,22

Undoubtedly, if freestanding paper-like metallic TCEs with high conductivity, excellent transparency and flexibility are successfully developed, the substrate-supported nature induced limitations of conventional metallic TCEs could be overcome. Freestanding paper-like metallic electrode films can be ultrathin, highly conductive and super flexible, opening up a world of application opportunities for power-integrated foldable and optoelectronic systems. Flexible freestanding metallic electrode films based on metal nanowires/nanofibers have been developed by vacuum filtration and electrospinning methods recently.23–25 For example, Wang et al. fabricated a freestanding Pt-nanofiber-based TCE film with a sheet resistance of 16 Ω sq−1 and an optical transparency of 80% by an electrospinning and controlled annealing process.26 However, the inter-nanowire/nanofiber junction resistance between nanowires/nanofibers often leads to poor conductivity, and thus an additional process is always required either by thermal treatment, selective welding, mechanical pressing or other chemical modifications.23–26 Moreover, this fabrication process is complicated and time-consuming, and is also difficult to scale up to large areas due to the nonuniform electrical conductivity distribution because of the random alignment of metal nanowires. In contrast with metal-nanowire-based freestanding metallic TCEs, a patterned metallic network can eliminate the inherent percolation between nanowires, large junction resistance and random nanowire distribution, benefitting from its designable nature and patterning.27 Nevertheless, patterned metal network based freestanding metallic TCEs have never been reported.

Herein, we achieve a freestanding ultrathin metallic network TCE film via a simple metal selective electrodeposition method combined with laser-direct writing patterning techniques. The solution processability of the electrodeposition process allows for scaling up to large areas and high performance without adopting complicated vapour filter and high-temperature annealing processes. The as-fabricated freestanding ultrathin Ni network TCE film possesses not only extraordinary optoelectronic performance (electrical conductivity ∼3 × 104 S cm−1 and optical transparency >84%), but also excellent flexibility such that it can be repeatedly bent, folded and even crumpled into a ball shape with only slight performance degradation. Significantly, the freestanding, highly uniform and interconnected Ni network electrode film provides a platform for constructing a monolithic MnO2@Ni network film, which shows exceptional areal capacitance, rate capability and long-term cycling stability. Furthermore, the solid-state supercapacitors based on the MnO2@Ni network electrodes are ultrathin (∼20 μm), highly transparent (∼80%) and super flexible enough to be attached, folded and even crumpled to any object or shape with only slight performance degradation. This can be attributed to the Ni network electrode being “freestanding, ultrathin, highly conductive, optically transparent and super-flexible” in nature and the excellent electrochemical properties of MnO2. Overall, this strategy offers a method for the simple and controlled fabrication of ultrathin, foldable and designable supercapacitors for transparent wearable and portable electronic systems.

Experimental

Fabrication of a freestanding Ni network electrode

Fig. 1 graphically illustrates the fabrication process of the freestanding transparent MnO2@Ni network electrode. In a typical process, a photoresist was firstly spin-coated onto a pre-cleaned ITO glass substrate (Step I) and then micro-patterns were generated by using laser direct writing techniques and development, exposing the ITO glass surface in micro-groove patterns (Step II). In the following step, Ni metal was selectively deposited to fill the pre-defined micro-grooves and gradually formed a uniform metallic network via a selective electrodeposition process (Step III). The typical Ni network thickness was about 1.5–5 μm and the whole electrodeposition process takes only several minutes. Next, the Ni network could be easily peeled off from the ITO glass substrate (Step IV), producing a freestanding Ni network electrode, and there was no need to remove the photoresist.
image file: c7ee02390a-f1.tif
Fig. 1 Graphical illustration of the fabrication process of the freestanding MnO2@Ni network supercapacitor electrode.

Electrodeposition of MnO2 nanoflakes

The MnO2 nanoflakes were electrochemically deposited onto the freestanding Ni network electrode by using a three-electrode configuration, consisting of the freestanding Ni network as the working electrode, platinum wire as the counter electrode and Ag/AgCl as the reference electrode at room temperature. The electrolyte solution contained 20 mM MnSO4 and 100 mM Na2SO4 and MnO2 was grown by applying a constant potential of 0.92 V versus Ag/AgCl for various periods of time. Then, the obtained freestanding MnO2@Ni network electrode film was thoroughly washed with deionized water to remove the residual electrolyte and dried in ambient air.

Fabrication of all-solid-state supercapacitors

The symmetric all-solid-state supercapacitor devices were assembled by using freestanding MnO2@Ni network electrode films both as the positive and negative electrode, and PVA/LiCl gel as the electrolyte. The PVA/LiCl gel electrolyte was prepared as follows: 5 g of LiCl and 12 g of PVA powder were dissolved in 120 mL of deionized water under constant stirring at 90 °C until formation of a clean gel. Then, the MnO2@Ni network electrode painted with silver paste was soaked in the PVA/LiCl gel electrolyte for 5 min and then allowed to solidify. Finally, the device was prepared successfully by assembling two identical electrodes together and leaving it in air for two days until the electrolyte was wholly solidified.

Material characterization and electrochemical measurements

The collimated transmittance spectra in the visible range (400–800 nm) were recorded using a UV-vis-spectrophotometer (UV-2550, SHIMADZU). Field-emission scanning electron microscopy (SEM) (FESEM: JEOL, JSM-5400, USA) was used to study the microstructures of the freestanding MnO2@Ni network electrode. The chemical composition of the electrochemically deposited MnO2 nanoflakes was measured by an Axis Ultra DLD X-ray photoelectron spectroscopy (XPS) instrument. All electrochemical measurements were performed using a CHI 660B electrochemical workstation (Shanghai CH Instrument Company, China). For material electrochemical performance analyses, the tests were initially performed in a standard three-electrode system with a 1 M Na2SO4 electrolyte at room temperature. Here, the freestanding MnO2@Ni network electrode, a platinum wire, and an Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. Moreover, the electrochemical behaviours of all-solid-state supercapacitor devices were examined in a two-electrode system.

Results and discussion

Our strategy for fabricating the freestanding Ni network electrode involves four steps (Fig. 1): photoresist spin-coating, laser direct-writing patterning, Ni metal electrodepositing, and finally freestanding Ni network peel-off. This process is simple, effective and low-cost. Firstly, the micro-grooves derived from the pre-defined topological patterns and generated from the laser direct-writing techniques offer high flexibility to tailor the arrangements, period, aspect ratio and thickness for the production of a freestanding Ni network electrode for different applications; then, the uniform metallic network is deposited by an electrodeposition process and the whole process only takes several minutes, eliminating the standard vacuum-based metal film deposition techniques such as thermal evaporation, electron beam evaporation, magnetron sputtering, etc. Moreover, the patterned micro-grooves on the ITO glass substrate as a mold for the metallic network can be utilized repeatedly, which significantly reduces the template cost and improves the electrode production efficiency.

The prepared freestanding Ni network electrode is as thin as 1.5–5 μm and can be freely tailored to arbitrary shapes for adapting to different applications. Excellent flexibility of the freestanding ultrathin Ni network is revealed in deformation tests: it can light up an LED array as shown in Fig. 2a; then when it is rolled onto a pencil, or even crumpled into a ball and released, the freestanding Ni network electrode basically retains its original appearance and still can light up the LED array, with only a few new creases observed (Fig. 2b–d). No structural cracks are found in the released Ni network electrode and its transparency is also not affected, revealing its excellent mechanical flexibility. Importantly, the transparent “gossamer-thin” Ni network electrode can be conformably attached to arbitrary complex shapes such as a Rubik's cube, plant leaf, coloured pencil, etc., as depicted in Fig. 2e–g. Meanwhile, the clearly visible butterfly picture placed beneath the electrode illustrates its high optical transmittance, also confirmed in Fig. 2h. The freestanding Ni network electrode exhibits a high optical transparency with 80–85% transmittance in the visible range (especially 84.39% at 550 nm). Apart from the electro-optical properties, the mechanical flexibility and stability of the freestanding flexible Ni network electrodes are also crucial for practical applications.28,29 The electrical conductivity properties before and after repetitive folding processes were characterized, and the sheet resistance variation versus cycles of repeated folding properties are plotted in Fig. S1 (ESI). Even after 2000 cycles of repeated folding, the sheet resistance of the freestanding Ni network is almost unchanged, showing only ∼0.7% variation (ΔR/Rinitial, ΔR and Rinitial represent the sheet resistance variation and initial sheet resistance, respectively), indicating the excellent mechanical flexibility and electrical stability properties of our freestanding Ni electrodes. A representative SEM image of the freestanding Ni network electrode is presented in Fig. 3a. The freestanding Ni strings form a highly uniform and interconnected network with a period of 200 μm, which can be ascribed to the spatially uniform network pattern derived from the laser direct writing techniques. The interconnected Ni string microstructure can be further confirmed by the high magnification SEM image (Fig. 3b), and this interconnected structure avoids the large junction resistance which is a common bottleneck of metal nanowire based TCEs. Therefore, the freestanding Ni network electrode is “ultrathin and high optically transparent” and its unique uniform interconnected network structure makes it a promising candidate for various energy applications such as supercapacitors, lithium-ion batteries, P-doped metal compounds for the hydrogen evolution reaction (HER), etc.30–32


image file: c7ee02390a-f2.tif
Fig. 2 Freestanding ultrathin transparent and foldable Ni network electrode film: (a) digital photographs of the freestanding ultrathin transparent Ni network electrode, which can light up an LED indicator array. Then (b) after being rolled up onto a pencil, (c) after being crumpled, and (d) after being released to the flat state, the freestanding Ni network electrode remains highly conductive and can still light up the LED array. (e–g) Digital photographs of the freestanding Ni network electrode when attached to (e) a Rubik's cube, (f) a plant leaf, and (g) a colored pencil, indicating its transparent “gossamer-thin” nature. (h) Transmittance spectra (400–800 nm) of the freestanding Ni network electrode before and after MnO2 electrochemical deposition.

image file: c7ee02390a-f3.tif
Fig. 3 (a) SEM image of the freestanding Ni network electrode. Scale bar: 400 μm. High-magnification SEM images of the convex Ni network (b) before and (c) after MnO2 nanoflake electrochemical deposition. Scale bar: 4 μm. Inset of (c) shows the high resolution SEM image of the MnO2 nanoflakes deposited on Ni strings. Scale bar: 200 nm. EDS mapping (area distribution of Ni, Mn and O elements) of the freestanding Ni network electrode (d) before and (e) after MnO2 nanoflake electrochemical deposition, demonstrating the uniform distribution of MnO2 on the Ni strings. Scale bar: 1 μm.

For the preparation of a supercapacitor electrode, a facile electrochemical deposition method was adopted to directly grow MnO2 with different reaction times on the freestanding Ni network. It is noteworthy that a very low optical transmittance loss of 0.48% is observed in the freestanding MnO2@Ni network electrode compared to the bare Ni network electrode without MnO2, as shown in Fig. 2h. MnO2 is uniformly electrochemically deposited on the Ni string surface including the convex areas, as can be seen in Fig. 3c. High-magnification SEM and energy dispersive spectroscopy (EDS) were performed to investigate the microstructure and the element distribution of MnO2 electrodeposited on the Ni strings. The MnO2 nanoflakes exhibit a porous structure and are estimated to be 100–150 nm in length and 10–30 nm in width (inset of Fig. 3c). This porous structure with such nanosizes results in large specific surface area, which is beneficial for its excellent electrochemical performances. Obviously, the EDS mapping images (Fig. 3d and e) reveal that each element (Mn, O, Ni) has a uniform distribution throughout the whole Ni string area, including the convex surfaces. It can be attributed to the well-distributed freestanding metallic network, which facilitates more efficient ion transport and transfer over the entire Ni network surface and thus boosts the electrochemical performance. The chemical composition of the MnO2 nanoflakes electrodeposited on the Ni network is further measured by XPS. The Mn 2p XPS spectrum (Fig. S2, ESI) shows that the binding energies of Mn 2p3/2 and Mn 2p1/2 along with a spin-energy separation of 11.8 eV well agree with those reported for MnO2, demonstrating a 4+ oxidation state for Mn.33,34 Therefore, above all SEM, EDS and XPS results prove the presence of the MnO2 nanoflakes and their uniform distribution throughout the freestanding Ni network, along with its high optical transparency and high electrical conductivity, making it suitable for flexible transparent supercapacitor electrode applications.

The electrochemical properties of the unique freestanding MnO2@Ni network electrode were firstly evaluated by cyclic voltammetry (CV) characterization with a standard three-electrode configuration containing 1 M Na2SO4 electrolyte at room temperature. The relationship between the MnO2 electrochemical deposition time and the electrochemical properties is examined. The transmittance spectra and CV curves of the freestanding MnO2@Ni network electrode at a scan rate of 10 mV s−1 with different MnO2 deposition times ranging from 2 min to 10 min are illustrated in Fig. S3 (ESI). With the increase of MnO2 deposition time, the optical transmittance steadily decreases, while the areal capacitance initially increases and then reduces after reaching a peak value, as shown in Fig. 4a. It is noteworthy that the optical transmittance decrease is only 1.4%, whereas, a large areal capacitance variation can be observed, achieving the largest capacitance when the deposition time is 6 min. For the electrodeposition process, it can be found that MnO2 nanoflakes become obviously bigger and thicker with an increase in MnO2 deposition time. However, the oxidation–reduction reaction process occurs on the surface layer of the MnO2 nanoflakes, and thicker and bigger MnO2 nanoflakes might decrease the electrical conductivity and specific surface area of the MnO2@Ni electrode.35,36 Therefore, further increase in the deposition time will decrease the areal capacitance and the overall electrochemical performance. Moreover, the dependence of optical transmittance and electrochemical properties on the Ni network period is also investigated (Fig. S4, ESI). With the Ni network period ranging from 200 to 25 μm, the optical transmittance decreases from 83.91% to 35.98%, while the specific areal capacitance increases from 80.7 to 131.5 mF cm−2. Thus, the specific capacitance could be improved by increasing the MnO2 deposition time or decreasing the network period. However, larger MnO2 mass loadings or a smaller Ni network period will decrease the electrode optical transparency. It is a trade-off between transparency and electrochemical properties, however, both high optical transparency and excellent electrochemical properties should be simultaneously achieved for transparent supercapacitor electrode applications. Based on the above experimental results, the optimized deposition time is 6 min for the freestanding MnO2@Ni network electrode with a 200 μm network period, and the optimized electrode is used in the following electrochemical study.


image file: c7ee02390a-f4.tif
Fig. 4 Electrochemical performance of the freestanding ultrathin transparent MnO2@Ni network electrode: (a) optical transmittance and areal capacitance versus MnO2 electrochemical deposition time relationships, (b) CV curves of the freestanding MnO2@Ni network electrode at the deposition time of 6 min in a 1 M Na2SO4 electrolyte, (c) variation of the areal capacitance versus scan rate obtained from the CV curves, (d) galvanostatic charge/discharge curves of the freestanding MnO2@Ni network electrode at the deposition time of 6 min at various current densities ranging from 0.25 to 5 mA cm−2, (e) imaginary impedance Z′′ versus real impedance Z′ (Nyquist plot) in the frequency range from 0.1 to 100 kHz. Inset shows the magnified part for the low real impedance part, (f) areal capacitance retention versus cycling time during 10[thin space (1/6-em)]000 cycles at a scan rate of 0.1 V s−1.

Fig. 4b shows the CV curves of the freestanding MnO2@Ni network electrode in a potential window of 0–0.8 V versus an Ag/AgCl reference electrode with the scan rate ranging from 5 to 100 mV s−1. The almost quasi-rectangular shaped CV curves demonstrate the excellent charge storage characteristics and fast response of the freestanding MnO2@Ni network electrode, and such a shape is well retained even at a scan rate up to 100 mV s−1. Note that the freestanding Ni network electrode without active MnO2 exhibits negligible capacitance (Fig. S5, ESI), indicating that Ni does not contribute to the quite large electric current response of the freestanding MnO2@Ni network electrode. At a low scan rate of 5 mV s−1, the areal capacitance (CA) extracted from Fig. 4b is calculated to be 80.7 mF cm−2, decreasing to 47.5 mF cm−2 with the scan rate increasing to 100 mV s−1, as shown in Fig. 4c, which indicates the good rate performance of the electrode. The electrochemical properties shown above are superior to most reported flexible transparent supercapacitor electrodes such as Ag–Au core–shell nanowires,37 PEDOT:PSS,38 graphene,39 carbon nanotubes,40etc., which can be ascribed to high optical transparency and excellent conductivity of the freestanding Ni network and also high electrochemical properties of the electrodeposited MnO2 nanoflakes. Moreover, except for the CV measurements, the galvanostatic charge–discharge (GCD) curves at different current densities ranging from 0.25 to 5.0 mA cm−2 exhibit nearly symmetrical features without obvious IR drop and also small deviation from the ideal triangle shape (Fig. 4d). The results demonstrate excellent electrochemical capacitive characteristics and reversible Faradaic reactions between Na+ ions in the electrolyte and the electrodeposited MnO2 nanoflakes.41,42 Impressively, as shown above, the freestanding MnO2@Ni network electrode shows high optical transmittance (83.91% at 550 nm) and large areal capacitance (80.7 mF cm−2 at a scan rate of 5 mV s−1), making it a promising candidate as a flexible transparent supercapacitor electrode. Moreover, besides MnO2, our method could be extended to other supercapacitor active electrode materials such as metal oxides, polymers, carbon-based materials, etc. when the electrolyte solution is neutral or alkaline because the metal Ni will be dissolved in acidic solution. Other supercapacitor electrode materials such as Ni(OH)2, Co(OH)2, MoS2, etc. have been successfully electrochemically deposited onto the freestanding Ni network electrode. For example, the Co(OH)2@Ni network electrode also exhibits excellent electrochemical capacitive characteristics and reversible Faradic reactions between K+ ions in the KOH electrolyte and the deposited Co(OH)2 nanosheets (CV and GCD curves are shown in Fig. S6, ESI). Detailed experimental fabrication and characterization methods can be seen in the ESI. The excellent electrochemical performance shown in Fig. S6 (ESI) proves that our method can be extended to other supercapacitor active electrode materials besides MnO2 in this paper.

In addition, the frequency responses of the electrode film in the frequency range from 0.01 to 100 kHz are also examined by electrochemical impedance spectroscopy (EIS) measurements as presented in Fig. 4e. The observed straight line in the Nyquist curve reveals an ideal capacitive behaviour of the electrode, and the equivalent series resistance (Rs), reflecting the conductivity of the freestanding MnO2@Ni network electrode, can be estimated from the x-intercept of the plots (inset of Fig. 4e). The extracted Rs value is as low as 4.3 Ω, which proves the high electrical conductivity of the electrode. Furthermore, the cycling stability is a critical parameter for practical supercapacitor applications and thus long cycling tests are performed to explore the electrochemical stability of the electrode. After 10[thin space (1/6-em)]000 cycles of repetitive cycling, compared with the original electrode, 96.3% of the original capacitance and thus high electrochemical cycling stability can be maintained, as demonstrated in Fig. 4f. This could be ascribed to the unique hierarchical micro/nano structure of the freestanding MnO2@Ni network electrode. The highly electrically conductive Ni network electrode endows the MnO2 nanoflakes with high electrical conductivity. Simultaneously, the electrodeposited MnO2 nanoflakes on the Ni network electrode with a porous structure and thus a large specific surface can provide more redox active sites for facilitating the quick migration of electrolyte ions into the composites, and the small sizes of the MnO2 nanoflakes can also shorten electron transfer paths, which both promote the electron transport efficiency between the Ni network electrode and the MnO2 nanoflakes.43,44

In order to demonstrate the superior performance of our freestanding MnO2@Ni network electrode for electrochemical energy storage device applications in flexible electronics, ultrathin flexible transparent all-solid-state supercapacitor devices were constructed by assembling the freestanding MnO2@Ni network electrode as both the positive and negative electrode, with a PVA/LiCl gel electrolyte (both the ionic electrolyte and separator). The fabricated supercapacitor device achieves a high optical transparency with ∼80% transmittance in the visible range (especially 80.82% at 550 nm), and only a 3–4% transmittance decrease compared with the electrode, as presented in Fig. 5a. This is much higher than that of most other reported transparent flexible supercapacitors, making the present device promising for future smart invisible electronics applications.37–40,45 The overall high optical transmittance could be attributed to the freestanding ultrathin nature without substrate loss, metallic network structure and high transmittance of the electrolyte. It could be further improved by decreasing the Ni string width, enlarging the metallic network period or exploring electrochemical materials with higher transparency. Fig. 5b and c show the rate-dependent CV characterization of the solid-state supercapacitor device at various scan rates ranging from 10 mV s−1 to 20 V s−1. The device shows well-shaped CV curves, even for a scan rate as high as 20 V s−1, indicating the good energy capacitive performance and excellent rate capability. For a low voltage scan rate of 10 mV s−1, the CA of the solid-state supercapacitor device is calculated to be 10.6 mF cm−2, decreasing to 3.0 mF cm−2 when the scan rate increases to 100 mV s−1, as shown in Fig. 5d. Moreover, CA decreases to 0.7 mF cm−2 with the scan rate further increasing to 20 V s−1. The electrochemical, optical and mechanical properties of our fabricated freestanding MnO2@Ni network electrode based flexible transparent supercapacitor and other reported flexible transparent supercapacitors are summarized in Table S1 (ESI). As can be seen from Table S1 (ESI), the overall performances of our fabricated supercapacitor device including thickness, transparency, areal capacitance, cycling retention, etc. are much superior to other reported flexible transparent supercapacitor devices utilizing electrodes such as graphene paper, graphene quantum dots/graphene, Ag–Au core–shell nanowires, RuO2/PEDOT:PSS, PEDOT:PSS/Ag grid electrodes, etc.37,39,46–48 Though it exhibits such high capacitance as described above, these flexible transparent supercapacitor devices based on the freestanding MnO2@Ni network electrode still store very low energy, which may be not suitable for practical applications. A very simple approach is to utilize a series and parallel combination of the supercapacitor devices in order to achieve the required specific voltage and capacitance according to applications.49 As shown in Fig. 5e, the tandem configuration of two supercapacitors in parallel exhibits a capacitance value that is almost twice that of a single cell. Moreover, two identical cells were also put in series, giving rise to an operation voltage window extended up to 1.6 V or even larger, as compared to that of a single cell with an operation voltage window of 0.8 V. In addition, the electrochemical stability of the solid-state supercapacitor device was tested by repeated CV cycling tests as shown in Fig. 5f. During the initial stages, it is demonstrated that the capacitance increases steadily until the 4000th cycle, implying the surface activation of the MnO2@Ni active material. In general, this steady capacitance increase during the initial stages can be attributed to the electro-activation process of the MnO2 active materials along with the CV cycling, which is often observed in both metal oxides and carbon-based materials.50–52 Subsequently, the capacitance value stabilized without any capacitance loss and even slightly increased as the cycling number further increases, demonstrating its excellent electrochemical cycling stability.


image file: c7ee02390a-f5.tif
Fig. 5 (a) Transmittance spectra of the freestanding Ni network, freestanding MnO2@Ni network electrode and solid-state supercapacitor device, (b and c) CV curves of the solid-state supercapacitor device utilizing the freestanding MnO2@Ni network electrode as the electrode at various scan rates ranging from 0.01 to 20 V s−1, (d) variation of the areal capacitance versus scan rate obtained from the CV curves, (e) CV curves of the supercapacitor device in one cell, two series and two parallel combination configurations, (f) capacitance retention versus cycling number properties during 10[thin space (1/6-em)]000 cycles at a scan rate of 20 V s−1, (g) photographs of the ultrathin and foldable solid-state supercapacitor device under different conditions: flat, 2-fold and 4-fold, all three are attached to a colored pencil, indicating its transparent and foldable properties, and (h) CV curves of the solid-state supercapacitor device in flat, 2-fold, 4-fold, crumpled and after release conditions.

As expected, the solid-state supercapacitor device utilizing the freestanding MnO2@Ni network electrode exhibits exceptional flexible abilities to withstand various deformations, promising for power sources of rolled-up or foldable electronics. Due to its ultrathin thickness and superior flexibility, it can be attached to any shaped subjects such as a ball, pencil, etc.Fig. 5g shows photographs of the solid-state supercapacitor device attached to a coloured pencil and a “No Smoking” sign under flat, two-fold and four-fold conditions, indicating its high transparency, superior flexibility and shape-conformable properties. To further verify its high flexibility properties, more severe bending conditions were introduced where the solid-state device was crumpled into a ball shape and its electrochemical behaviors were also measured (Fig. S7, ESI). The CV curves of the device under flat conditions (without bending), shaped configuration (two-fold, four-fold and crumple) and then back to the original state almost remain the same, as shown in Fig. 5h, which benefits from the excellent flexibility of the freestanding MnO2@Ni network electrode and its ultrathin nature. Combined with the high optical transparency of the device (80.82% at 550 nm), the solid-state device utilizing the freestanding MnO2@Ni network electrode clearly exhibits high transparency, good electrochemical behaviors, superior flexibility and ease of transfer or attachment to any-shaped subjects, readily applicable in “see-through” or “invisible” electronics such as electronic skin.

Conclusions

Exploring high-performance freestanding electrode films is critical for achieving ultrathin, transparent and foldable supercapacitor devices. In this work, we demonstrate a novel, simple and highly controlled approach to make a freestanding ultrathin metallic network electrode by a laser direct-writing micro-patterning technique combined with a selective electrodeposition method. The obtained freestanding ultrathin Ni network TCE processes both extraordinary optoelectronic performance (optical transparency >84% and electrical conductivity ∼3 × 104 S cm−1) and excellent flexibility such that it can be repeatedly bent, folded and even crumpled into a ball shape with only slight performance degradation. Significantly, after the loading of MnO2 as the active material, the freestanding MnO2@Ni network electrode shows outstanding rate performance, excellent cycle stability and high capacitance. More importantly, the ultrathin (∼20 μm), highly transparent (∼80.82%) and ultra-flexible solid-state supercapacitors based on the freestanding MnO2@Ni network electrode not only exhibit excellent electrochemical properties but also can be tailored, attached, folded, rolled up, and crumpled into any objects or various shapes with only slight performance degradation. Therefore, the controlled, solution-processable and potentially scalable production of the freestanding, ultrathin and super-flexible Ni network electrode presented here offers a new paradigm for developing transparent wearable and portable electronic devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from the Natural Science Foundation of China (No. 61405133, 61705152, 91323303, 61575133, 61675143 and 11661131002) and the Natural Science Foundation of Jiangsu Province (No. BK20140348 and BK20160328). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology, and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

Electronic supplementary information (ESI) available: Normalized variations in sheet resistance of the freestanding Ni network electrode as a function of the repetitive folding cycling times, XPS spectra of electrochemically deposited MnO2 on the freestanding Ni network, transmittance and electrochemical properties of the freestanding MnO2@Ni network electrode with different electrochemical deposition times, transmittance and electrochemical properties of the freestanding MnO2@Ni network electrode with different Ni network periods, CV curves of the freestanding Ni network before and after MnO2 electrochemical deposition, experimental procedures and electrochemical behaviors of the freestanding Co(OH)2@Ni network electrode film including CV and GCD curves, and images of the ultrathin, transparent and foldable solid-state supercapacitor devices under different conditions: flat, 2-fold, 4-fold, crumpling and then back to the original flat case. See DOI: 10.1039/c7ee02390a

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