Ultrathin, Wrinkled, Vertically-aligned Co(OH)2 ... - ACS Publications

Subscriber access provided by AUT Library

Energy, Environmental, and Catalysis Applications

Ultrathin, Wrinkled, Vertically-aligned Co(OH)2 Nanosheets/Ag Nanowires Hybrid Network for Flexible Transparent Supercapacitor with High Performance Hongwei Sheng, Xuetao Zhang, Yonglu Ma, Pengxiang Wang, Jinyuan Zhou, Qing Su, Wei Lan, Erqing Xie, and Chuanfang (John) Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18609 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Ultrathin,

Wrinkled,

Vertically-aligned

Co(OH)2

Nanosheets/Ag

Nanowires Hybrid Network for Flexible Transparent Supercapacitor with High Performance Hongwei Sheng,a Xuetao Zhang,a Yonglu Ma,a Pengxiang Wang,a Jinyuan Zhou,a Qing Su,a Wei Lan,a* Erqing Xie,a Chuanfang (John) Zhangb* a

Key Laboratory of Special Function Materials and Structure Design, Ministry of

Education, School of Physical Science and Technology, Lanzhou University, Lanzhou,730000, People's Republic of China

b

Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), School of

Chemistry, Trinity College Dublin, Dublin 2, Ireland *Corresponding authors E-mail: [email protected] (W. Lan), [email protected] (C. Zhang).

KEYWORDS: Co(OH)2 nanosheets, vertically alignment, AgNWs network, flexible transparent supercapacitor, wearable electronics

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Developing high-performance, flexible, transparent supercapacitors for wearable electronics represents an important challenge as it requires active materials to be sufficiently transparent without compromising energy storage. Here we manipulate the morphology of the active materials and the junctions on the current collector to achieve optimum electronic/ionic transport kinetics. Two-dimensional Co(OH)2 nanosheets with single- or bi-layered were vertically aligned onto a modified AgNWs network using an electrochemical deposition-UV irradiation approach. The metallic AgNWs network endows high transparency while minimizes the contact resistance with the pseudocapacitive Co(OH)2 nanosheets. The Co(OH)2 nanosheets self-assembled into a three-dimensional array, which is beneficial for the fast ion movements. The rational materials design greatly boosts the electrochemical performance of the hybrid network, including an ultrahigh areal capacitance up to 3108 μC cm-2 (5180 μF cm-2) coupled with long cycling life (20 000 cycles). As a prototype device, the symmetric supercapacitor well combines high energy/power density and excellent mechanical flexibility and long-term performance, suggesting a promising application for the next-generation wearable electronics.


Page 2 of 41

Page 3 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION The popularity of wearable, smart electronics has greatly stimulated the rapid development of energy storage devices with the aim to make them mechanical robust and optically transparent.1-4 Supercapacitors can be rapidly charged/discharged in millions of cycles,5 and thus have been received substantial attentions in the integration of flexible transparent electronics. Producing high-performance flexible, transparent supercapacitors (FTSCs) typically requires the electrode material with abundant redox active sites as well as developed electrical percolative network.6 While the former parameter ensures high capacitance (and as a result, high energy output) of the device, the latter guarantees rapid electrons transport and boosts the rate capability of the transparent electrodes.

To date, plenty of attempts have been reported on transparent electrodes and devices. For example, carbon nanotube (CNT),7-8 graphene,9-12 conducting polymers13 and their nanocomposites14-16 have been employed as electrodes for transparent supercapacitors with fairly good capacitances and optical transmittances. More specifically, single-walled CNT and graphene-based transparent supercapacitors demonstrated the areal capacitance (C/A) of 552 μF cm−2 and 5.8 μF cm-2,8, 10 respectively. Transparent supercapacitors using nanoengineered carbon films and single-walled CNT showcased a retention rate of 84% after 10 000 cycles and 94% after 500 cycles,17, 18 respectively. The relative low capacitance and undesirable cycling performance could be attributed to several causes: (i) the remaining catalysts or other residuals contaminate CNTs or graphene, and limit their lifetime; (ii) the



gradual detach tendency of the electrochemically active materials from the transparent current collector inevitably leads to the capacitance decay; and (iii) the complex and lengthy ion transport paths impede the performance of transparent supercapacitors, especially for high charging-discharging rate. In other words, questing a material that is able to proceed multiple redox reactions is much preferable, especially when the material vertically aligns onto a highly transparent, conductive substrate. Such a materials design can maximize the utilization of active materials and promote the electronic/ionic transport kinetics, resulting in transparent supercapacitors with both high energy density and highpower density. However, this has been proven to be quite challenging, as either the approaches are complicated, or the interaction between the active materials and the substrate is weak, which inevitably leads to the gradual material loss during cycling. Twodimensional (2D) Co(OH)2 are known for their large interlayer spacing, high theoretical pseudocapacitance (3460 F/g) and low cost.19-26 Previous reports on Co(OH)2 are mainly focused on the application of bulk supercapacitors,27-32 and few has shown the application of Co(OH)2 in transparent, flexible supercapacitors. This is due to the poor electronic conductivity and the nanosheets restacking. While incorporating the conductive skeleton (such as Ag nanowires (AgNWs)) can partially solve the conductivity problem, however, the irrational combination of Co(OH)2 and AgNWs typically results in significant contact junctions and low utilization of nanosheets, which significantly limit the capacitance and rate performance of transparent supercapacitors.


Page 4 of 41

Page 5 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Herein, we attempt to address all the issues by controlling the Co(OH)2 nanosheets alignment and optimize the nanosheets/AgNWs interfaces. Through a facile electrochemical deposition coupled with UV irradiation, vertical alignment of ultrathin wrinkled Co(OH)2 nanosheets on the modified AgNWs-based network has been achieved (Scheme 1), enabling directional ion transport and facilitating fast electron mobility. Consequently, the flexible Co(OH)2/AgNWs electrodes display an high areal capacitance (C/A, 3108 μC cm-2 (5180 μF cm-2)) and a long cycle life up to 20 000 cycles. Moreover, the prototype transparent, flexible, solid-state supercapacitors demonstrate a resilient nature, high capacitance and energy/power density. The state-of-the-art electrochemical performance implies the great potential of controlling the nanosheets alignments as well as tuning the network junctions in achieving advanced transparent energy storage devices.

2. EXPERIMENTAL SECTION 2.1. Materials

All chemicals were of analytical grade and used without further purification. Zinc acetate (Zn(CH3COO)2, Xilong Chemical Industry Co. Ltd), cobalt nitrate (Co(NO3)2ˑ6H2O, Tianjin Guangfu Technology Development Co. Ltd), lactic acid (CH₃CH(OH)COOH, Chengdu Kelong Chemical Reagent Factory), potassium chloride (KCl, Chengdu Kelong Chemical Reagent Factory), sodium sulfate (Na2SO4, Chengdu Kelong Chemical Reagent Factory), sodium chloride (NaCl, Chengdu Kelong Chemical Reagent Factory), polyvinyl



alcohol (PVA, average MW 31000-50000, 87-89% hydrolyzed, Sigma-Aldrich), sodium polyacrylate (PAAS, J&K Chemicals) and polyethylene terephthalate were used to synthesize the electrodes and for electrochemical characterization. Ag NWs (average length: 20~30 μm, diameter: 30 nm) were purchased from Suzhou Cold Stone Nano Material Tech Co., China. To improve the surface hydrophilicity, the PET substrates were ultrasonically cleaned for 10 min, in ethanol, deionized water, and then concentrated in a plasma cleaner (HARRICK, PDC-32G-2) using high power under air for 10 min.

2.2. Electrode Preparation

2.2.1 Preparation of the modified AgNWs network Preparation of the ZnO precursor: In a typical procedure, 1g of Zn(CH3COO)2.2H2O was dissolved in 22.5 ml of absolute ethanol. First, the solution was heated by sealing it in a water bath at 60 °C for 1 h. Then, the diluted lactic acid with deionized water (1:2) was added until the solution became colorless and transparent. Before using, the solution was stored at room temperature for 2 days.

Preparation of the AgNWs network: The AgNWs suspension (3 mg/ml) was deposited on PET substrates via spin-coating at 400 rpm, followed by 3500 rpm. After the spincoating procedure was repeated four times, the network film was subsequently dried at 90 °C for 10 min. The ZnO precursor solution was coated the AgNWs network to act as a


Page 6 of 41

Page 7 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


protective layer. Then, the AgNWs network was dried at 65 °C for 10 min and further annealed at 150 °C in Ar gas for 20 min.

2.2.2 Preparation of the Co(OH)2/Ag NWs electrodes

Co(OH)2 nanosheets were electrochemically deposited on the AgNWs network using a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a three-electrode system. During the electrochemical deposition process, the AgNWs network (1 cm × 1.5 cm) was used as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum (Pt) plate as the counter electrode. The electrochemical deposition process was conducted in a 100 mL aqueous electrolyte consisting of 0.01 M Co(NO3)2.6H2O, using static potential of -1.0 V (vs. SCE) and deposition time of 5 min.

After deposition, the obtained electrodes were carefully cleaned with deionized water and ethanol several times, and subsequently dried in air at 60 °C. The Co(OH)2 nanosheets/AgNWs hybrid network electrodes were placed in a petri dish containing deionized water and then exposed to UV light for one hour to improve the stability of Co(OH)2/AgNWs electrodes.

2.3. Fabrication of the Supercapacitors



Two grams of KCl were dissolved in deionized water of 20 ml, and then 2 g of PVA power (molecular weight of about 100 000, hydrolyzed, from Sigma-Aldrich) and 1 ml of PAAS were added in turn under vigorous stirring in a water bath at 85 °C. After the solution had become colorless and transparent, it was cooled to room temperature to get the PVA/PAAS/KCl gel electrolyte. The Co(OH)2/AgNWs electrode was immersed in the gel electrolyte for 1 minute to serve as ionic electrolyte and separator. It was then stored in a fume hood for 10 minutes. After the excess water was vaporized, a pair of electrodes were pressed together to assemble into a symmetric supercapacitor.

2.4. Materials Characterization

The optical transmittances for the single electrode and the assembled supercapacitor were measured by UV-vis spectrophotometer (U-3900H, HITACHI). The sheet resistance was measured by four-point probe (4Probes Tech, RTS-9). Atomic force microscopy (AFM) images were acquired using an MFP-3D system (ASYLUM RESEARCH). The AFM samples were prepared by dropping the ultra-sounded dispersion of the delaminated nanosheets onto a cleaned silicon wafer. The morphology and structures were conducted on field emission scanning electron microscopy (FE-SEM, TESCAN MIRA3 XMU), transmission electron microscopy (TEM) and high resolution TEM (HRTEM, FEI F30). The crystal structures were obtained by X-ray diffraction (XRD, Philips, X'pert pro, Cu Kα, 0.15406 nm) and Raman spectroscopy (JY-HR800 micro-Raman, using a 532 nm wavelength YAG laser with a laser spot diameter of ∼ 600 nm and power attenuator of D


Page 8 of 41

Page 9 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3). The chemical composition was investigated using X-ray photoelectron spectrometer (XPS, PHI-5702, Mg KR X-ray, 1253.6 eV).

2.5. Electrochemical Measurements

The electrochemical tests were performed using a three-electrode system in 2 M KCl aqueous electrolyte on a CHI 660E electrochemical workstation. The synthesized electrode, Pt and Ag/AgCl electrodes were used as working, counter and reference electrodes, respectively.

3. RESULTS AND DISCUSSION

Shown in Scheme 1 is the synthesis of the Co(OH)2 nanosheets on the AgNWs network. During the electrodeposition process of ultrathin wrinkled Co(OH)2 nanosheets, nitrate ions (NO32-) can be reduced to ammonium ions (NH4+) or nitrite ions (NO2-) on the surface of the modified AgNWs network, coupled with the generation of hydroxide ions (OH-). With the increasing of the OH– concentration, cobalt ions (Co2+) in the electrolyte reacts with OH–, resulting in nucleation and growth of Co(OH)2 on the surface of AgNWs network. The morphology evolution of the vertically-aligned Co(OH)2 nanosheets can be attributed to the dominance of the (003)-oriented growth during the electrodeposition process.33 The growth mechanism of electrodeposited Co(OH)2 nanosheets can be described as follows: 34



NO32- + H2O + 2e- → NO2- + 2OH-

NO32- + 7H2O + 8e- → NH4+ + 10OH-

Co2+ + 2OH- → Co(OH)2

Page 10 of 41

(1)

(2)

(3)

Figure 1 shows the morphology and structural information of flexible transparent Co(OH)2/AgNWs electrode. A uniform ZnO protective layer (thickness ~ 5 nm)-coated AgNWs are found on the PET substrate (Figure 1a and inset). The sheet resistance of the pure Ag nanowires network is 20 Ω/□, the sheet resistance is reduced to 12 Ω/□ after coating ZnO layer. Previously we found that by coating a thin layer of ZnO, the junction resistance of AgNWs can be substantially reduced by heat-welding while the antioxidant stability and the adhesion to PET substrate can be dramatically improved.35 This modified AgNWs network serves as transparent current collector for the accommodation of Co(OH)2 nanosheets. SEM images of Co(OH)2/AgNWs electrodes with different magnifications (Figure 1b,c,d), it can be observed that vertically aligned Co(OH)2 nanosheets are uniformly distributed on the AgNWs network. Due to the advantages of in-situ growth for electrodeposition method, the highly conductive AgNWs network current collector and the positive synergistic effect between the positive and negative electrodes, the as-synthesized Co(OH)2 nanosheets are vertically grown and firmly adhered to the AgNWs network, which greatly reduces the contact resistance between Co(OH)2 nanosheets and the AgNWs, while suppresses the detach of active materials from the conductive skeleton. The


Page 11 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interconnected Co(OH)2 nanosheets woven into a 3D porous array on the AgNWs network. The mean pore diameter of the interconnected Co(OH)2 nanosheets is 0.37±0.1 μm (Figure 1e), which can facilitate light transmission and improve electrode transparency. Furthermore, these pore structures can also increase the surface area of active material Co(OH)2 nanosheets and provide shortened ion transport path.

The electrochemically deposited Co(OH)2 nanosheets show the nanosheet structure with many wrinkles (marked with blue squares in the TEM images (Figure 1f, g)). The high transparency under the electron beam indicates their ultrathin characteristic. The element mapping in Figure 1h proves that Co and O elements uniformly distribute in the wrinkled nanosheet structure, in good agreement with the EDS spectrum (see Figure S1a). The HRTEM of the Co(OH)2 nanosheets (Figure 1i) reveals the lattice fringes with a spacing distance of 0.42 nm, corresponding to the (105) and (102) plane of α-Co(OH)2 phase (JCPDS 46-0605). The SEAD pattern (inset of Figure 1i) indicates that the nanosheets are poly-crystallized. As shown in AFM image (Figure 1j), the height variations of Co(OH)2 nanosheets are in the range of 1.3~2.6 nm (Figure 1k), which further confirms that the prepared Co(OH)2 nanosheets are wrinkled and single- or bi-layered.

The XRD pattern (Figure 2a) and Raman spectrum (Figure 2b) indicate the formation of α-Co(OH)2 phase (JCPDS 46-0605).36-39 Three Raman characteristic peaks center at 454 cm-1, 519 cm-1, and 1041 cm-1, corresponding to the Co-O (Ag) symmetric stretching mode, O-Co-O bending mode and the OH deformation mode, respectively. It was reported that α-



Co(OH)2 is composed of brucite-like structural layers made of hexagonal close-packed of hydroxyl ions with Co2+ ions occupying the octahedral sites every two interlayers, and charge balancing anions (e.g., NO32-, CO32-, SO42-, Cl-, etc.) in the interlayers.36 Thus αCo(OH)2 has a large interlayer spacing (usually >7 Å, depending on intercalated anions).36 The full X-ray photoelectron spectroscopy (XPS) survey of the electrode can be found in Figure S1b, indicative of the simultaneous presence of Co and O elements. The deconvoluted spectrum of Co 2p showcases two peaks at 780.9 and 796.7 eV corresponding to the Co 2p3/2 and Co 2p1/2 spin orbit peaks of Co(OH)2, respectively (Figure 2c). The difference (ΔEb =15.8 eV) between the two peaks also confirms the existence of Co(OH)2 phase.40 In addition, four appropriate peaks in the O 1s spectrum (Figure 2d), with the main peak at 531.2 eV which is assigned to be the O from the OH.39 In short, all the characterizations confirm the successful synthesis of Co(OH)2 nanosheets with single- or bi-layered on the AgNWs network.

The optical transmittance, flexibility and electrochemical properties of the assynthesized Co(OH)2/AgNWs electrode were studied, as shown in Figure 3. The electrode transmittance, is varied by altering the electrodeposition time of Co(OH)2 nanosheets onto the AgNWs network (Figure S2a). The transmittance (at 550 nm) of the AgNWs network decreases as Co(OH)2 deposition time increase (Figure 3a). The pure AgNWs film show a high transparency of ~ 90 % at 550 nm and a sheet resistance ~12 Ω/□. In addition, the mass of Co(OH)2 nanosheets on the electrode increases over electrodeposition time.


Page 12 of 41

Page 13 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Generally, during the electrochemical deposition process, it can be found that with the increase of deposition time, Co(OH)2 nanosheets become larger and thicker,41 leading to the disappearance of porous structure between nanosheets. But the redox reactions occur on (or near) electrode surfaces of the Co(OH)2 nanosheets, thus thicker and bigger Co(OH)2 nanosheets might destroy electrical percolative network and specific surface area of the Co(OH)2/AgNWs electrode. Therefore, the over-deposition of the Co(OH)2 nanosheets degrades the overall electrochemical property of Co(OH)2/AgNWs electrodes. Figure S3a shows the Cyclic Voltammetry (CV) curves of Co(OH)2/AgNWs electrode under different voltage windows from 0 to 1 V. We chose a suitable potential window of 0.6 V, owing to CV shape has been well maintained and a little electrochemical polarization that can be surveyed. To obtain the optimum deposition time, the CV curves (at 10 mV s−1) of Co(OH)2/AgNWs electrodes for different deposition time are measured (Figure 3b). As the deposition time increases, the C/A of the Co(OH)2/AgNWs electrodes initially increases but decreases after reaching a peak value at the deposition time of 5 min, as shown in Figure 3c. The Co(OH)2/AgNWs electrode showcased excellent flexibility, best seen by the unchanged CV curves after bending for different angles (Figure S2d). Noted that poor electrochemical stability and conductivity are common problems for transition metal hydroxides, such as Co(OH)2.33 It is reported that UV light irradiation densifies the oxide film via photochemical activation at room temperature.42-43 When the as-synthesized film is exposed to UV irradiation from the mercury light (Cnlight, 25W) with a wavelength of 185 nm (10 %) and 254 nm (90 %), high-energy UV photons can activate metal (M) and



oxygen (O) atoms to facilitate the formation of M–O–M network, forming highperformance and stable films. Based on our experimental data, the Nyquist plots (Figure S2e) of the Co(OH)2/AgNWs electrode before and after irradiating UV light, showing the lower equivalent series resistance (Rs, ~ 12 Ω) and a nearly ideal capacitive behavior. The much reduced Rs of Co(OH)2/AgNWs electrode (by ~ 35 %) indicates that the UV light radiation improves the ionic conductivity. More importantly, the UV irradiation also leads to the improvement of the cycling stability for the Co(OH)2/AgNWs electrodes (Figure 3d). Considering the above experimental results, the ideal deposition time is 5 min for the Co(OH)2/AgNWs electrode coupled with UV irradiation (1 h), thus the optimal electrode is used in the following electrochemical investigations.

The detailed electrochemical properties of flexible transparent Co(OH)2/AgNWs electrodes were studied. To estimate the contribution of the current collector (the AgNWs network) on the total C/A of the electrode, CV curves of the Co(OH)2/AgNWs electrode (Figure 3f) and the AgNWs network (Figure S3e) were measured at various scan rates. And then the influence of the AgNWs network as the current collector at different scan rates on the C/A of the electrode materials is estimated respectively. It is found that the percentage of C/A contribution to total C/A of the AgNWs network is less than 5 %, as shown in the Figure S3b. Compared with the Co(OH)2/AgNWs hybrid electrode, the CV curve of AgNWs network at the low scan rate of 10 mV s−1 approaches a line, showing a very low current. These results indicate C/A mainly originates from the pseudo-capacitance of the


Page 14 of 41

Page 15 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrochemically active Co(OH)2 nanosheets in the Co(OH)2/AgNWs electrodes instead of the double-layer capacitance from AgNWs network. The CV curves of the Co(OH)2/AgNWs electrode at different scan rates are presented in Figure 3f. As the scan rate increases, CV shapes are well retained even at the scan rate up to 100 mV s-1, suggesting rapidly capacitive behavior and outstanding power handling performances. The galvanostatic charge/discharge (GCD) curves (Figure 3g) also conform to these points. The corresponding C/A of the Co(OH)2/AgNWs electrode were calculated from CVs and GCD curves, as shown in Figure 3h. The acquired C/A values using two approaches are comparable, reaching the high C/A of 3108 μC cm−2 (5180 μF cm−2) at 10 mV s-1 and benefiting by the unique electrode structure (Co(OH)2 nanosheets/AgNWs) design, which enables the high surface area and fast ion insertion/desertion rate. The resultant C/A of the Co(OH)2/AgNWs electrodes is superior than previously reported flexible transparent supercapacitor electrodes such as Ag–Au core–shell nanowires,14 graphene,10 carbon nanotubes.8

The electrochemical stability of the single electrode was confirmed by GCD cycling measurements, as depicted in Figure 3i. A C/A retention ratio of 112 % is obtained after 20 000 cycles, illustrating the excellent electrochemical stability of the Co(OH)2/AgNWs electrode. During the first 8000 cycles, the C/A increases gradually, which can be attributed to the electro-activation process of the active Co(OH)2 materials. The Co(OH)2/AgNWs electrode can also retain 118 % and 112 % of the initial C/A after 10 000 and 20 000 cycles,



respectively. This outstanding cycling performance could be attributed to several causes: (i) photochemical activation with UV irradiation can condense the Co(OH)2/AgNWs electrode film, alleviating the exfoliation of the active materials to a certain degree. (ii) the octahedral Co2+ ions are partially substituted by unbonded Zn2+ ions in the ZnO protective layer surface during the GCD cycling process, which can be confirmed by EDS mapping analysis (Figure S3c, d). Such a perfect replacement is mainly attributed to the similar ionic radii of Zn2+ (74.0 pm) and Co2+ (74.5 pm) ions.33 (iii) Synergistic effect of Co(OH)2 nanosheets and AgNWs network, as well as the cross-linked Co(OH)2 nanosheets and the ZnO layer can provide a double protective effect on the AgNWs network. These causes can maximize the charge storage capacity (the capacitance). In addition, the Coulombic efficiency is close to 100 %, implying the excellent charging and discharging behavior of Co(OH)2/AgNWs film electrodes.

In order to demonstrate the electrochemical performance of energy storage devices based on the Co(OH)2/AgNWs electrodes in transparent electronics, the all solid-state FTSCs are constructed into a sandwich structure by assembling the prepared Co(OH)2/AgNWs electrodes as both the positive and negative electrode, with a PVA/KCl gel electrolyte (both the ionic electrolyte and separator). The detailed process can be referred to in Figure S4. The letters under the supercapacitor are clearly visible (Figure 4a). The optical transmittances of the AgNWs network current collector, Co(OH)2/AgNWs electrode, and the assembled supercapacitor were further measured, and the results are shown in Figure


Page 16 of 41

Page 17 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4b. At 550 nm, the single electrode and the assembled supercapacitor exhibit high transmittance of 75 % and 54 %, respectively, which is mainly ascribed to the 3D porous electrode structure. In addition, the overall optical transmittance could be further improved by decreasing the distribution density of the AgNWs network and exploring active materials with higher transparency and theoretical capacitance. The CV curves of the transparent supercapacitors are barely affected after bending various angles (Figure 4c) and many times with 60o bending angle (Figure 4d), confirming that the structural integrity of the transparent supercapacitor is not destroyed after bending. For example, after bending more than 100 times for the designed supercapacitor, the calculated capacitance retention of the devices fluctuates between 100 and 110 %, as shown in Figure S4, which benefits from the excellent mechanical flexibility of the Co(OH)2/AgNWs hybrid network electrodes. Figure 4e shows the CV characterization of the fabricated supercapacitor device under different scan rates ranging from 10 to 20 000 mV s-1. The almost quasi-rectangular shaped CV curves is well retained even at the scan rate reaching 20 000 mV s-1, delivering the rapidly and reversible redox reactions in the supercapacitor device. Identically, this point is also verified by the nearly symmetric sloping GCD curves (Figure 4f) at various current densities from 0.1 to 0.4 mA cm-2. The C/A of the symmetric solid-state transparent supercapacitor device were calculated from CVs and GCD profiles, showcasing 540 μC cm-2 (900 μF cm-2) at 10 mV s-1, as shown in Figure 4g. The C/A (60 μC cm-2) at the ultrahigh voltage scan rate of 20 000 mV s-1 can be also achieved, this capacitance value is comparable to the conventional graphene-based supercapacitors. As demonstrated in Fig.



4h, the EIS results of the all-solid-state devices were acquired, with a magnification of the high frequency range in the inset. The intercept of Nyquist plot on real axis (Rs) is about 9.33 Ω, revealing a good conductivity and low internal resistance of the fabricated device. The slope of EIS curve in low frequency region reflects the good ion diffusion in the gel electrolyte. The electrochemical stability (Figure 4i) of the all solid-state FTSCs was also measured. 91 % of the initial C/A is retained after 10 000 cycles.

To enhance the energy density of power sources for wearable electronics in application, the simplest and most feasible approach is to utilize a series and parallel combination to form supercapacitors arrays (inset of Figure 5c). The corresponding schematic of the FTSC arrays on PET substrate is shown in Figure 5a. The C/A comparison for various transparent supercapacitors is shown in Figure 5b, the C/A (900 μF cm-2) of the Co(OH)2/AgNWsbased transparent supercapacitor is substantially better than the other transparent supercapacitors.6, 9-12, 16 These conventional transparent supercapacitors get excellent C/A (or transmittance) at the cost of transmittance (or C/A), both the high C/A and optical transparency are obtained simultaneously in the Co(OH)2/AgNWs-based FTSC. Energy and power densities of the Co(OH)2/AgNWs based FTSC are calculated and compared with the reported transparent supercapacitors, as shown in Figure 5c. Both power density (28.8 μW cm-2) and energy density (0.04 μWh cm-2) are better than other transparent supercapacitors, which are based on Ti3C2Tx films,44 PEDOT:PSS,13, 45 graphene quantumdot,9 and multilayer reduced graphene oxide thin films.46 As shown in Figure 5d, the


Page 18 of 41

Page 19 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


capacitance value of two supercapacitors in the parallel configuration is almost twice that of the single cell. Moreover, if the two identical supercapacitors are connected in series, the potential window can reach 1.2 V or even larger. Figure S6 shows the CV characterization of the single supercapacitor device, two supercapacitors in parallel and in series at various scan rates ranging from 10 to 20 000 mV s-1. It can be seen that the shape of all CV curves is basically similar, and no other new redox peaks occur without any deformation. Overall, the Co(OH)2/AgNWs-based FTSCs exhibits the excellent electrochemical performance, high transparency and superior flexibility, readily applicable in ‘‘see-through’’ or ‘‘invisible’’ electronics to act as the power sources.



Page 20 of 41

4. CONCLUSIONS In this work, we synthesized ultrathin wrinkled Co(OH)2 nanosheets with single- or bilayered vertically grown on the modified AgNWs network on PET substrates using electrochemical

deposition-UV

irradiation.

The

obtained

flexible

transparent

Co(OH)2/AgNWs electrodes possess the high C/A (3108 μC cm-2 (5180 μF cm-2)) and long cycling stability (117 % after 20 000 times). More importantly, the all-solid-state FTSCs based on Co(OH)2/AgNWs electrodes exhibit excellent electrochemical properties. The FTSC can reach the energy density of 0.04 μWh cm-2 at the high-power density of 28.8 μW cm-2, and great mechanical flexibility without any performance degradation. These results may open new possibilities for wearable and portable FTSC devices, especially if transition metal hydroxides are used as the active materials.

Supporting Information The supplementary data related to this article can be found at ACS Publications website. The computational formulae of the areal capacitance, power/energy density, EDX analysis and XPS survey spectrum of the Co(OH)2/AgNWs electrode, the high resolution XPS spectra of Ag 3d and Zn 2p, the transmittance spectra of Co(OH)2/AgNWs electrode at different deposition time, the photographs of the AgNWs network, CV curves of the Co(OH)2/AgNWs electrode after bending different angles, cycling stability and EIS curves of the Co(OH)2/AgNWs electrode before and after UV irradiation, CV curves of the


Page 21 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co(OH)2/AgNWs electrode under different potential windows, the bar diagram of the capacitive contribution of AgNWs network, CV curves, rate performance and cycling stability of the AgNWs network, Schematic fabrication process the FTSC, CV curves of the FTSCs in one cell, two series and two parallel configurations, CV curves of the comparison of the areal capacitance, transmittance and power/energy density with other reported transparent supercapacitors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W. Lan), [email protected] (C. Zhang) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS



This work was supported by National Natural Science Foundation of China (61874166, U1832149), Natural Science Foundation of Gansu province (18JR3RA292), the Fundamental Research Funds for the Central Universities (lzujbky-2017-k21).


Page 22 of 41

Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Nishide, H.; Oyaizu, K. Materials Science - Toward Flexible Batteries. Science 2008, 319 (5864), 737-738, DOI: 10.1126/science.1151831.

(2) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4 (5), 366-377, DOI: 10.1038/nmat1368.

(3) Zhang, C.; Nicolosi, V. Graphene and MXene-Based Transparent Conductive Electrodes and Supercapacitors. Energy Storage Materials 2019, 16, 102-125, DOI: 10.1016/j.ensm.2018.05.003.

(4) An, T. C.; Cheng, W. L. Recent Progress in Stretchable Supercapacitors. J. Mater. Chem. A 2018, 6 (32), 15478-15494, DOI: 10.1039/c8ta03988g.

(5) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7 (11), 845-854, DOI: 10.1038/nmat2297.

(6) King, P. J.; Higgins, T. M.; De, S.; Nicoloso, N.; Coleman, J. N. Percolation Effects in Supercapacitors with Thin, Transparent Carbon Nanotube Electrodes. ACS Nano 2012, 6 (2), 1732-1741, DOI: 10.1021/nn204734t.



Page 24 of 41

(7) Chen, T.; Peng, H.; Durstock, M.; Dai, L. High-Performance Transparent and Stretchable All-Solid Supercapacitors Based on Highly Aligned Carbon Nanotube Sheets. Sci. Rep. 2014, 4, DOI: 10.1038/srep03612.

(8) Kanninen, P.; Nguyen Dang, L.; Le Hoang, S.; Anoshkin, I. V.; Tsapenko, A.; Seppala, J.; Nasibulin, A. G.; Kallio, T. Transparent and Flexible High-Performance Supercapacitors Based on Single-Walled Carbon Nanotube Films. Nanotechnol. 2016, 27 (23), DOI: 10.1088/0957-4484/27/23/235403.

(9) Lee, K.; Lee, H.; Shin, Y.; Yoon, Y.; Kim, D.; Lee, H. Highly Transparent and Flexible Supercapacitors Using Graphene-Graphene Quantum Dots Chelate. Nano Energy 2016, 26, 746-754, DOI: 10.1016/j.nanoen.2016.06.030.

(10) Chen, T.; Xue, Y.; Roy, A. K.; Dai, L. Transparent and Stretchable High-Performance Supercapacitors Based on Wrinkled Graphene Electrodes. ACS Nano 2014, 8 (1), 10391046, DOI: 10.1021/nn405939w.

(11) Fan, X.; Chen, T.; Dai, L. Graphene Networks for High-Performance Flexible and Transparent

Supercapacitors.

RSC

Adv.

2014,

4

(70),

36996-37002,

DOI:

10.1039/c4ra05076b.

(12) Gao, Y.; Zhou, Y. S.; Xiong, W.; Jiang, L. J.; Mahjouri-samani, M.; Thirugnanam, P.; Huang, X.; Wang, M. M.; Jiang, L.; Lu, Y. F. Transparent, Flexible, and Solid-State


Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supercapacitors Based on Graphene Electrodes. Apl Materials 2013, 1 (1), DOI: 10.1063/1.4808242.

(13) Higgins, T. M.; Coleman, J. N. Avoiding Resistance Limitations in High-Performance Transparent Supercapacitor Electrodes Based on Large-Area, High-Conductivity PEDOT:PSS Films. ACS Appl. Mater. Interfaces 2015, 7 (30), 16495-16506, DOI: 10.1021/acsami.5b03882.

(14) Lee, H.; Hong, S.; Lee, J.; Suh, Y. D.; Kwon, J.; Moon, H.; Kim, H.; Yeo, J.; Ko, S. H. Highly Stretchable and Transparent Supercapacitor by Ag-Au Core-Shell Nanowire Network with High Electrochemical Stability. ACS Appl. Mater. Interfaces 2016, 8 (24), 15449-15458, DOI: 10.1021/acsami.6b04364.

(15) Liu, X. Y.; Gao, Y. Q.; Yang, G. W. A Flexible, Transparent and Super-Long-Life Supercapacitor Based on Ultrafine Co3O4 Nanocrystal Electrodes. Nanoscale 2016, 8 (7), 4227-35, DOI: 10.1039/c5nr09145d.

(16) Qiu, T. F.; Luo, B.; Giersig, M.; Akinoglu, E. M.; Hao, L.; Wang, X. J.; Shi, L.; Jin, M. H.; Zhi, L. J. Au@MnO2 Core-Shell Nanomesh Electrodes for Transparent Flexible Supercapacitors. Small 2014, 10 (20), 4136-4141, DOI: 10.1002/smll.201401250.



Page 26 of 41

(17) Jung, H. Y.; Karimi, M. B.; Hahm, M. G.; Ajayan, P. M.; Jung, Y. J. Transparent, Flexible Supercapacitors from Nano-Engineered Carbon Films. Sci. Rep. 2012, 2, DOI: 10.1038/srep00773.

(18) Yuksel, R.; Sarioba, Z.; Cirpan, A.; Hiralal, P.; Unalan, H. E. Transparent and Flexible Supercapacitors with Single Walled Carbon Nanotube Thin Film Electrodes. ACS Appl. Mater. Interfaces 2014, 6 (17), 15434-15439, DOI: 10.1021/am504021u.

(19) Cao, L.; Xu, F.; Liang, Y. Y.; Li, H. L. Preparation of the Novel Nanocomposite Co(OH)2/Ultra-Stable Y Zeolite and Its Application as A Supercapacitor with High Energy Density. Adv. Mater. 2004, 16 (20), 1853-+, DOI: 10.1002/adma.200400183.

(20) Gupta, V.; Kusahara, T.; Toyama, H.; Gupta, S.; Miura, N. Potentiostatically Deposited Nanostructured Alpha-Co(OH)2: A High Performance Electrode Material for Redox-Capacitors.

Electrochem.

Commun.

2007,

9

(9),

2315-2319,

DOI:

10.1016/j.elecom.2007.06.041.

(21) Chen, H.; Hu, L.; Chen, M.; Yan, Y.; Wu, L. Nickel-Cobalt Layered Double Hydroxide Nanosheets for High-Performance Supercapacitor Electrode Materials. Adv. Funct. Mater. 2014, 24 (7), 934-942, DOI: 10.1002/adfm.201301747.


Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Ji, J.; Zhang, L. L.; Ji, H.; Li, Y.; Zhao, X.; Bai, X.; Fan, X.; Zhang, F.; Ruoff, R. S. Nanoporous Ni(OH)2 Thin Film on 3D Ultrathin-Graphite Foam for Asymmetric Supercapacitor. ACS Nano 2013, 7 (7), 6237-6243, DOI: 10.1021/nn4021955.

(23) Li, H. B.; Yu, M. H.; Wang, F. X.; Liu, P.; Liang, Y.; Xiao, J.; Wang, C. X.; Tong, Y. X.; Yang, G. W. Amorphous Nickel Hydroxide Nanospheres with Ultrahigh Capacitance and Energy Density as Electrochemical Pseudocapacitor Materials. Nat. Commun. 2013, 4, DOI: 10.1038/ncomms2932.

(24) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132 (21), 7472-7477, DOI: 10.1021/ja102267j.

(25) Zhao, C. M.; Wang, X.; Wang, S. M.; Wang, Y. Y.; Zhao, Y. X.; Zheng, W. T. Synthesis

of

Co(OH)2/Graphene/Ni

foam

Nano-Electrodes

with

Excellent

Pseudocapacitive Behavior and High Cycling Stability for Supercapacitors. Int. J. Hydrogen Energy 2012, 37 (16), 11846-11852, DOI: 10.1016/j.ijhydene.2012.05.138.

(26) Ramesh, T. N.; Rajamathi, M.; Kamath, P. V. Ammonia Induced Precipitation of Cobalt Hydroxide: Observation of Turbostratic Disorder. Solid State Sci. 2003, 5 (5), 751756, DOI: 10.1016/S1293-2558(03)00086-4.



(27) Wang, R. T.; Yan, X. B.; Lang, J. W.; Zheng, Z. M.; Zhang, P. A Hybrid Supercapacitor Based on Flower-Like Co(OH)2 and Urchin-Like VN Electrode Materials. J. Mater. Chem. A 2014, 2 (32), 12724-12732, DOI: 10.1039/c4ta01296h.

(28) Jagadale, A. D.; Kumbhar, V. S.; Dhawale, D. S.; Lokhande, C. D. Performance Evaluation of Symmetric Supercapacitor Based on Cobalt Hydroxide [Co(OH)2] Thin Film Electrodes. Electrochim. Acta 2013, 98, 32-38, DOI: 10.1016/j.electacta.2013.02.094.

(29) He, X.; Li, R.; Liu, J.; Liu, Q.; Chen, R.; Song, D.; Wang, J. Hierarchical FeCo2O4@NiCo Layered Double Hydroxide Core/Shell Nanowires for High Performance Flexible All-Solid-State Asymmetric Supercapacitors. Chem. Eng. J. 2018, 334, 15731583, DOI: 10.1016/j.cej.2017.11.089.

(30) He, X.; Liu, Q.; Liu, J.; Li, R.; Zhang, H.; Chen, R.; Wang, J. High-performance AllSolid-State Asymmetrical Supercapacitors Based on Petal-Like NiCo2S4/Polyaniline Nanosheets. Chem. Eng. J. 2017, 325, 134-143, DOI: 10.1016/j.cej.2017.05.043.

(31) Zhao, Y.; He, X.; Chen, R.; Liu, Q.; Liu, J.; Song, D.; Zhang, H.; Dong, H.; Li, R.; Zhang, M.; Wang, J. Hierarchical NiCo2S4@CoMoO4 Core-Shell Heterostructures Nanowire Arrays as Advanced Electrodes for Flexible All-Solid-State Asymmetric Supercapacitors. Appl. Surf. Sci. 2018, 453, 73-82, DOI: 10.1016/j.apsusc.2018.04.159.


Page 28 of 41

Page 29 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Zhao, Y.; He, X.; Chen, R.; Liu, Q.; Liu, J.; Yu, J.; Li, J.; Zhang, H.; Dong, H.; Zhang, M.; Wang, J. A Flexible All-Solid-State Asymmetric Supercapacitors Based on Hierarchical Carbon Cloth@CoMoO4NiCo Layered Double Hydroxide Core-Shell Heterostructures. Chem. Eng. J. 2018, 352, 29-38, DOI: 10.1016/j.cej.2018.06.181.

(33) Tang, J.; Liu, D.; Zheng, Y.; Li, X.; Wang, X.; He, D. Effect of Zn-Substitution on Cycling Performance of Alpha-Co(OH)2 Nanosheet Electrode for Supercapacitors. J. Mater. Chem. A 2014, 2 (8), 2585-2591, DOI: 10.1039/c3ta14042c.

(34) Therese, G. H. A.; Kamath, P. V. Electrochemical Synthesis of Metal Oxides and Hydroxides. Chem. Mater. 2000, 12 (5), 1195-1204, DOI: 10.1021/cm990447a.

(35) Lan, W.; Yang, Z.; Zhang, Y.; Wei, Y.; Wang, P.; Abas, A.; Tang, G.; Zhang, X.; Wang, J.; Xie, E. Novel Transparent High-Performance AgNWs/ZnO Electrodes Prepared on Unconventional Substrates with 3D Structured Surfaces. Appl. Surf. Sci. 2018, 433, 821828, DOI: 10.1016/j.apsusc.2017.10.054.

(36) Liu, Z. P.; Ma, R. Z.; Osada, M.; Takada, K.; Sasaki, T. Selective and Controlled Synthesis of Alpha- and Beta-Cobalt Hydroxides in Highly Developed Hexagonal Platelets. J. Am. Chem. Soc. 2005, 127 (40), 13869-13874, DOI: 10.1021/ja0523338.

(37) Wang, M.; Ma, J. P.; Chen, C.; Zheng, X.; Du, Z. T.; Xu, J. Preparation of SelfAssembled Cobalt Hydroxide Nanoflowers and The Catalytic Decomposition of



Cyclohexyl Hydroperoxide. J. Mater. Chem. 2011, 21 (34), 12609-12612, DOI: 10.1039/c1jm12162f.

(38) Pan, G. X.; Xia, X.; Cao, F.; Tang, P. S.; Chen, H. F. Porous Co(OH)2/Ni Composite Nanoflake Array for High Performance Supercapacitors. Electrochim. Acta 2012, 63, 335340, DOI: 10.1016/j.electacta.2011.12.117.

(39) Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L. Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs. J. Phys. Chem. C 2010, 114 (1), 111-119, DOI: 10.1021/jp908548f.

(40) Xue, T.; Wang, X.; Lee, J.-M. Dual-Template Synthesis of Co(OH)2 with Mesoporous Nanowire Structure and Its Application in Supercapacitor. J. Power Sources 2012, 201, 382-386, DOI: 10.1016/j.jpowsour.2011.10.138.

(41) Liu, Y.-H.; Xu, J.-L.; Gao, X.; Sun, Y.-L.; Lv, J.-J.; Shen, S.; Chen, L.-S.; Wang, S.D. Freestanding Transparent Metallic Network Based Ultrathin, Foldable and Designable Supercapacitors. Energy & Environmental Science 2017, 10 (12), 2534-2543, DOI: 10.1039/c7ee02390a.

(42) Liu, X.; Wang, J.; Yang, G. Amorphous Nickel Oxide and Crystalline Manganese Oxide Nanocomposite Electrode for Transparent and Flexible Supercapacitor. Chem. Eng. J. 2018, 347, 101-110, DOI: 10.1016/j.cej.2018.04.070.


Page 30 of 41

Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Kim, Y.-H.; Heo, J.-S.; Kim, T.-H.; Park, S.; Yoon, M.-H.; Kim, J.; Oh, M. S.; Yi, G.R.; Noh, Y.-Y.; Park, S. K. Flexible Metal-Oxide Devices Made by Room-Temperature Photochemical Activation of Sol-Gel Films. Nature 2012, 489 (7414), 128-U191, DOI: 10.1038/nature11434.

(44) Zhang, C.; Anasori, B.; Seral-Ascaso, A.; Park, S.-H.; McEvoy, N.; Shmeliov, A.; Duesberg, G. S.; Coleman, J. N.; Gogotsi, Y.; Nicolosi, V. Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance. Adv. Mater. 2017, 29 (36), DOI: 10.1002/adma.201702678.

(45) Zhang, C. F.; Higgins, T. M.; Park, S. H.; O'Brien, S. E.; Long, D. H.; Coleman, J.; Nicolosi, V. Highly Flexible and Transparent Solid-State Supercapacitors Based on RuO2/PEDOT:PSS Conductive Ultrathin Films. Nano Energy 2016, 28, 495-505, DOI: 10.1016/j.nanoen.2016.08.052.

(46) Yoo, J. J.; Balakrishnan, K.; Huang, J. S.; Meunier, V.; Sumpter, B. G.; Srivastava, A.; Conway, M.; Reddy, A. L. M.; Yu, J.; Vajtai, R.; Ajayan, P. M. Ultrathin Planar Graphene Supercapacitors. Nano Lett. 2011, 11 (4), 1423-1427, DOI: 10.1021/nl200225J.



Figure Captions:

Graphical Abstract Vertically-aligned Co(OH)2 Nanosheets grown on Ag nanowires network for flexible transparent supercapacitor.

Scheme 1. Schematic of the fabrication procedure for the FTSC based on Co(OH)2/AgNWs electrodes on PET substrate.

Figure 1. Morphological and structural characterizations of the Co(OH)2/AgNWs electrodes. (a) SEM and HRTEM images of the AgNWs network. (b, c, d) SEM images of Co(OH)2/AgNWs electrodes with different magnifications. (e) Pore diameter histogram for the Co(OH)2 nanosheets. (f, g, h) TEM (f), element mapping (g) and HRTEM (h) images of the delaminated Co(OH)2 nanosheets, the inset (i) shows the corresponding selected area electron diffraction (SAED) pattern. (j) AFM image of the delaminated Co(OH)2 nanosheets. (k) Height profiles of the different lines marked in (j).

Figure 2. Spectroscopy analyses of the as-synthesized Co(OH)2/AgNWs electrodes and AgNWs network. (a) XRD pattern and (b) Raman spectrum of the as-synthesized Co(OH)2 nanosheets. High resolution XPS spectra of (c) Co 2p, (d) O 1s, respectively.

Figure 3. Electrochemical characterization of Co(OH)2/AgNWs electrodes. (a) Optical transmittance and mass change of the Co(OH)2/AgNWs electrode as the functions of deposition time. (b) CV curves of the Co(OH)2/AgNWs electrode after different deposition


Page 32 of 41

Page 33 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


time. (c) The relationships of optical transmittance and C/A versus Co(OH)2 deposition time. (d) The schematic of the UV irradiation in DI water. (e) CV curves (at 10 mV s-1) of AgNWs network and Co(OH)2/AgNWs electrode. (f, g) CV curves (f) at different scan rates and GCD curves (g) at different current densities for the Co(OH)2/AgNWs electrode. (h) Rate capability of the Co(OH)2/AgNWs electrodes obtained from the CV and GCD curves. (i) Electrochemical stability and Coulombic efficiency versus cycling number during 20 000 cycles (the inset is the typical GCD curves upon cycling).

Figure 4. Transparency, bending property and electrochemical characterization of the assembled solid-state FTSC. (a) Photographs of a flexible, transparent Co(OH)2/AgNWs electrode and a symmetric solid-state supercapacitor, showing a high transmittance. (b) Transmittance spectra of AgNWs network, Co(OH)2/AgNWs electrode and all solid-state FTSC. (c) CV curves of the FTSC after different bending angles at the scan rate of 1 V s-1. (d) CV curves of the FTSC after different bending times at the scan rate of 10 V s-1, the inset is photograph of the repeated bending test. (e) CV curves of the FTSC at different scan rates ranging from 0.01 to 0.50 V s-1, (f) GCD curves of the FTSC at different current densities ranging from 0.1 to 0.4 mA cm-2. (g) The measured C/A obtained from GCD and CV curves. (h) Nyquist plot of Co(OH)2/AgNWs electrode. (i) Electrochemical cycling stability of the FTSC. Inset shows the typical GCD curves during cycling.

Figure 5. Electrochemical characterization of the FTSC arrays. (a) Schematic of the FTSC arrays fabricated on PET substrates. (b) The measured C/A compared with other reported



FTSCs. Detailed values are presented in Table S1. (c) Ragone plots of symmetric transparent supercapacitors, and comparison with other FTSCs. Detailed information is given in Table S2. (d) CV curves of the FTSCs in one cell, two series and two parallel configurations, the inset is the fabricated supercapacitor arrays.


Page 34 of 41

Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic of the fabrication procedure for the FTSC based on Co(OH)2/AgNWs electrodes on PET substrate. 170x107mm (300 x 300 DPI)



Figure 1. Morphological and structural characterizations of the Co(OH)2/AgNWs electrodes. (a) SEM and HRTEM images of the AgNWs network. (b, c, d) SEM images of Co(OH)2/AgNWs electrodes with different magnifications. (e) Pore diameter histogram for the Co(OH)2 nanosheets. (f, g, h) TEM (f), element mapping (g) and HRTEM (h) images of the delaminated Co(OH)2 nanosheets, the inset (i) shows the corresponding selected area electron diffraction (SAED) pattern. (j) AFM image of the delaminated Co(OH)2 nanosheets. (k) Height profiles of the different lines marked in (j). 170x133mm (300 x 300 DPI)


Page 36 of 41

Page 37 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Spectroscopy analyses of the as-synthesized Co(OH)2/AgNWs electrodes and AgNWs network. (a) XRD pattern and (b) Raman spectrum of the as-synthesized Co(OH)2 nanosheets. High resolution XPS spectra of (c) Co 2p, (d) O 1s, respectively. 170x145mm (300 x 300 DPI)



Figure 3. Electrochemical characterization of Co(OH)2/AgNWs electrodes. (a) Optical transmittance and mass change of the Co(OH)2/AgNWs electrode as the functions of deposition time. (b) CV curves of the Co(OH)2/AgNWs electrode after different deposition time. (c) The relationships of optical transmittance and C/A versus Co(OH)2 deposition time. (d) The schematic of the UV irradiation in DI water. (e) CV curves (at 10 mV s-1) of AgNWs network and Co(OH)2/AgNWs electrode. (f, g) CV curves (f) at different scan rates and GCD curves (g) at different current densities for the Co(OH)2/AgNWs electrode. (h) Rate capability of the Co(OH)2/AgNWs electrodes obtained from the CV and GCD curves. (i) Electrochemical stability and Coulombic efficiency versus cycling number during 20 000 cycles (the inset is the typical GCD curves upon cycling). 169x131mm (300 x 300 DPI)


Page 38 of 41

Page 39 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Transparency, bending property and electrochemical characterization of the assembled solid-state FTSC. (a) Photographs of a flexible, transparent Co(OH)2/AgNWs electrode and a symmetric solid-state supercapacitor, showing a high transmittance. (b) Transmittance spectra of AgNWs network, Co(OH)2/AgNWs electrode and all solid-state FTSC. (c) CV curves of the FTSC after different bending angles at the scan rate of 1 V s-1. (d) CV curves of the FTSC after different bending times at the scan rate of 10 V s-1, the inset is photograph of the repeated bending test. (e) CV curves of the FTSC at different scan rates ranging from 0.01 to 0.50 V s-1, (f) GCD curves of the FTSC at different current densities ranging from 0.1 to 0.4 mA cm-2. (g) The measured C/A obtained from GCD and CV curves. (h) Nyquist plot of Co(OH)2/AgNWs electrode. (i) Electrochemical cycling stability of the FTSC. Inset shows the typical GCD curves during cycling. 170x145mm (300 x 300 DPI)



Figure 5. Electrochemical characterization of the FTSC arrays. (a) Schematic of the FTSC arrays fabricated on PET substrates. (b) The measured C/A compared with other reported FTSCs. Detailed values are presented in Table S1. (c) Ragone plots of symmetric transparent supercapacitors, and comparison with other FTSCs. Detailed information is given in Table S2. (d) CV curves of the FTSCs in one cell, two series and two parallel configurations, the inset is the fabricated supercapacitor arrays. 170x142mm (300 x 300 DPI)


Page 40 of 41

Page 41 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical Abstract Vertically-aligned Co(OH)2 Nanosheets grown on Ag nanowires network for flexible transparent supercapacitor. 170x69mm (300 x 300 DPI)


Ultrathin, Wrinkled, Vertically-aligned Co(OH)2 ... - ACS Publications

Recommend Documents