Flexible Array of Microsupercapacitor for Additive Energy Storage

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Functional Inorganic Materials and Devices

Flexible Array of Micro-supercapacitor for Additive Energy Storage Performance over a Large Area Buddha Deka Boruah, Arnab Maji, and Abha Misra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02660 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

Flexible Array of Micro-supercapacitor for Additive Energy Storage Performance over a Large Area Buddha Deka Boruaha, Arnab Majia and Abha Misraa* a

Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, Karnataka,

India 560012 *Corresponding Author: Abha Misra *Email: [email protected]

1

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

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Abstract On-chip micro-supercapacitor (MSC) pattern is obtained by layer-by-layer spray deposition of both manganese dioxide (MnO2) nanoparticles coated carbon nanotubes (MnO2-CNTs) and MnO2 nanosheets decorated reduced graphene oxide (MnO2-rGO) on mechanically robust, flexible polyethylene terephthalate. Layer-by-layer patterning of MSC electrode offers rapid inplane diffusion of electrolyte ion in electrode with respect to the layered electrode and hence increase in the synergistic response under energy storage process. Ultra-high capacitance and energy density of 7.43 mF/cm2 (32300 mF/cm3) and 0.66 μWh/cm2 (2870 μWh/cm3), respectively are obtained. A robust electrochemical response was measured under multiple bending of the solid-state flexible MSC as well as under repetitive cycles (~5000).

KEYWORDS: synergistic effect, layer-by-layer pattern, flexible, in-plane on-chip microsupercapacitor, high-performance.

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INTRODUCTION Recently, development of portable energy conversion and storage devices have extensively drawn research interest in the modern society.1-2 Portable devices offer many additional advantages such as ultra-thin, super-light, wearable, mechanical flexibility, etc.3-7 Among them, microscale flexible energy storage devices have drawn great research interest to offer compatibility with the modern microelectronic systems,8,9 where currently microbatteries are employed.10-12 However, microbatteries provide low power density and slow rapid chargedischarge capability that limits the overall cycling life. Microsupercapacitors (MSCs) are introduced to deliver the ultra-high power density, rapid charge and discharge rates along with long cycle life.13-15 On the other hand, the energy density of MSCs is lower than that of microbatteries.16 Therefore, the improvement in energy density of on-chip flexible MSCs is being considered as a primary research challenge. Hybrid nanomaterials are preferable for supercapacitor (SC) electrodes to introduce the synergistic

effects

of

both

electrical

double-layer

capacitance

(EDLC)

and

pseudocapacitance.16,17-20 Among them, carbon based nanomaterials, e.g., carbon nanotubes (CNTs), reduced graphene oxide (rGO), activated carbon, carbon foam, etc are widely used for the EDLC response, whereas manganese dioxide (MnO2) for pseudocapacitance in SC. However, during the fabrication of MSCs, aggregation and stacking of MnO2-rGO or MnO2-CNTs hybrid nanomaterials greatly minimizes the electroactive surface area thus reduces the electrochemical performance. Therefore, an alternative approach is highly desirable for the excellent performance of flexible MSC composed of MnO2-carbon hybrid nanomaterials for the advance microelectronic systems. Herein, layer-by-layer patterning of electrodes based on MnO2 nanoparticles (NPs) decorated CNTs (MnO2-CNTs) and MnO2 nanosheets (NSs) decorated rGO 3



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(MnO2-rGO) nanocomposites is presented for the first time. Array of flexible MSC with efficient energy storage performance is obtained on cost-effective flexible polyethylene terephthalate (PET) substrate. This method introduces higher electroactive surface area of the electrodes, shorter diffusion length of ions and synergistic contribution of electrode material for the efficient electrochemical performance than previously reported MSCs. RESULTS AND DISCUSSION The microstructural investigation of the as-prepared samples was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), where Figure S1a,b depicts the SEM images of MnO2-CNTs at low and high magnifications, which shows a uniform coating of MnO2 NPs on CNTs. Similarly, Figure S1c,d shows the SEM images of as-grown MnO2 NSs on rGO sheets. TEM image of MnO2-CNTs (Figure 1a) implies that the average size of MnO2 NPs are 10 to 30 nm. Figure 1b deicts the high-resolution TEM (HRTEM) image of MnO2, where the measured interplanar spacing is 0.69 nm corresponding to the (110) plane.21 Moreover, Figure 1c,d shows the TEM images of as-grown MnO2 NSs on the rGO at low and high magnifications. The calculated interplanar spacing is measured to be of 0.25 nm that corresponds to (200) plane of MnO2 (Figure 1e).17 Elemental analysis (energy dispersive spectroscopy) of the MnO2-CNTs (Figure S2) and MnO2-rGO (Figure S3) confirms the impurity free as-synthesized samples. In the Raman spectra (Figure 1f), the G band appears at around 1580 cm-1 which is the characteristic of sp2 hybridization of C-C bonds and the D band (1352 cm1

) corresponds to the degree of disorderness in carbon lattice. The appeared high intensity D band

in rGO as compared to CNTs (Figure 1f) is mainly originated due to the existence of residual oxygen based groups.20,22 In addition, the peak originated due to MnO2 is related to the Mn–O stretching vibration.20,23 4


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a

b

c MnO2

0.69 nm (110)

20 nm

5 nm

d

e

200 nm

rGO

f 0.25 nm (200)

50 nm

5 nm

g

h

j

k

i

l

Figure 1. (a) TEM image of MnO2 coated CNT and (b) HRTEM image of MnO2. TEM images of as-grown MnO2 NSs on rGO at (c) low and (d) high magnifications. (e) HRTEM image of MnO2 NS. (f) Raman spectra of MnO2-CNTs and MnO2-rGO. High resolution XPS spectra of MnO2-rGO: (g) Mn 2p spectrum, (h) O1s spectrum and (i) C1s spectrum for rGO. (j) Mn 2p 5



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spectrum, (k) O1s spectrum and (l) C1s spectrum are the high-resolution XPS spectra of MnO2CNTs. Further X-ray photoelectron spectroscopy (XPS) is used to analysis for the samples as shown in Figure S4,5. Figure 1g shows high resolution Mn 2p spectrum of the MnO2-rGO sample, where two distinct characteristic peaks are obserbed at around 642 eV and 653.7 eV related to Mn 2p3/2 and Mn 2p1/2. The energy difference of 11.7 eV between the peaks of Mn 2p3/2 and Mn 2p1/2 demonstrates +4 valance of Mn.24 Figure 1h depicts O 1s spectrum of MnO2rGO, where the characteristic peaks are observed at around 529.7 eV and 531.2 eV, which are related to the Mn–O–Mn and Mn–O–H bonds.24,25 Three other distinct characteristic peaks at around 284.61 eV (C-C/C=C bonds), 286.20 (C–OH bond) and 288.22 eV (C–OOH/C=O bonds) are appeared in C 1s spectrum of the MnO2-rGO (Figure 1i).26 Moreover, Figure 1j shows the Mn 2p spectrum of MnO2-CNTs, where two distinct characteristic peaks corresponding to Mn 2p3/2 and Mn 2p1/2 with 12.2 eV energy difference are also observed. The characteristic peak of Mn–O–Mn bond is observed at around 530 eV in O 1s spectrum of the MnO2-CNTs (Figure 1k), whereas peaks around 284.8 eV (C-C/C=C bonds) and 286.7 eV (C–OH bond) are observed in the C 1s spectrum as depicted in Figure 1l. X-ray diffraction (XRD) patterns of MnO2-rGO and MnO2-CNTs samles are shown in Figure 2a. The originated diffraction peaks at around 2θ = 29.8o, 38.3o and 44.5o corresponding to the diffraction planes of (100), (101) and (111), respectively of the MnO2-rGO demonstrate βMnO2 crystal structure of as-grown MnO2 NS on rGO sheets.17 Also, a broad peak at around 2θ = 24o is attributed to the presence of rGO in MnO2-rGO. The peaks corresponding to MnO2CNTs are observed at around 2θ = 31.5o (310), 36.2o (400), 38.2o (211) and 41.5o (301), 6


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respectively along with the diffraction peak at 2θ = 26.2 o (002) originated due to CNTs (Figure 2a). The XRD pattern of the MnO2-CNTs suggests that as-coated MnO2 NPs have α-MnO2 crystal structure.27

a

b

c

d

Au sputtering

Materials deposition

Final MSC

e

f

Figure 2. (a) XRD patterns of MnO2-rGO and MnO2-CNTs. (b) Schematic illustration of different layer in MSC and (c) fabrication steps involved in the MSC and inset shows the digital photograph of an MSC. (d) Digital photographs of MSC array on the flexible A4 PET sheet. (e) CV and (f) areal capacitance plots of MnO2-CNTs MSC, MnO2-rGO MSC, RMSC and LMSC at 50 mV/s. 7



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Spray deposition of MnO2-rGO and MnO2-CNTs materials was carred out to fabricate layered MSC (LMSC) as shown in schematic (Figure 2b). Figure 2c depicts the schematic of the fabrication steps involved in the LMSC. First, stainless steel (SS) mask was fixed on the PET substrate and then Cr/Au (25/200 nm) electrodes were deposited on the respective SS mask patterned followed by sputtering technique and then MnO2-rGO and MnO2-CNTs were spray deposited for large area deposition on flexible substrate. Inset in Figure 2c depicts digital photograph of a single as-fabricated LMSC. Figure 2d shows the large array of the LMSCs on flexible, standard A4 PET paper. The MnO2-rGO MSC, MnO2-CNTs MSC and randomly mixed of MnO2-rGO and MnO2-CNTs MSC (RMSC) were also fabricated via spray deposition and the electrochemical responses were evaluated in PVA/H3PO4 electrolyte at room temperature. The advantages associated with the PVA/H3PO4 electrolyte are as follows: (i) large dispersion ability of H3PO4 into PVA matrix;28 (ii) smaller ionic radius of H+ than Na+, K+, OH-, Cl-, etc ions; (iii) excellent ionic conductivity.28 Figure 2e depicts cyclic voltammetry (CV) plots of the MSCs at a fixed scan rate of 50 mV/s over the working voltage of 0.8 V. It is clearly noticed from the Figure 2e that the CV plots of all the MSCs are rectangular in nature without any redox peak due to MnO2 which is mainly attributed to the charge-discharge (CD) entire cycle of the MSCs at a pseudoconstant rate.29-31 An increase in the area of CV curve of LMSC as compared to RMSC, MnO2-CNTs MSC and MnO2-rGO MSC reveals the enhanced capacitive performance. Figure 2f depicts the comparison of specific areal capacitance of the MSCs, where 312% (5.89 mF/cm2), 173.4% (3.91 mF/cm2) and 79.4% (2.57 mF/cm2) areal capacitance enhancement was observed in LMSC, RMSC and MnO2-rGO MSC, respectively as compared to MnO2-CNTs MSC (1.43 mF/cm2). The cross-sectional views of the LMSC (Figure S6a-c) and RMSC (Figure S7a-c) shows more porosity in the LMSC thus provides faster diffusion for electrolyte ions by 8


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introducing more electrochemical surface area, low charge transfer resistance, good cation intercalation for enhanced capacitive response of the LMSC. Therefore, the layer-by-layer deposition of MnO2-rGO and MnO2-CNTs of LMSC electrode introduces rapid in-plane diffusion of electrolyte ions with respect to the layered electrode materials that increase the overall energy storage capacity. Moreover, in the LMSC synergistic role is played by its components, for example, rGO provides the EDLC response, CNTs offer electrical conductivity of the electrode along with the EDLC response and MnO2 induces pseudocapacitive response during the electrolytic process. Also, it has been observed that layer-by-layer patterning of electrode structures offer much better electrical conductivity than those of randomly mixed composites for excellent electrochemical performance.32-34 The increase in capacitance of RMSC as compared to alone MnO2-CNTs MSC as well as MnO2-rGO MSC is mainly due to the above mentioned synergistic effects. Also, the observed higher energy storage performance in the MnO2-rGO MSC as compared to the MnO2-CNTs MSC could be due to the higher specific surface area of MnO2-rGO then MnO2-CNTs, where the measured specific areas using single point BET (Brunauer–Emmett–Teller, SMART SORB 93) measurement are 80.7 m2/g and 46.9 m2/g for the MnO2-rGO and MnO2-CNTs samples, respectively. Therefore, the capacitance of an electrode not only depends on the material electrode composition for synergistic effects, but the microstructure plays a significant role. Figure 2f compares the performance of MnO2-rGO MSC, MnO2-CNTs MSC and RMSC with the LMSC, which suggests that layer-by-layer deposition of electrode materials offers improved capacitance than randomly mixed counterparts.

9



a

0o

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b

20o

50o

c

f

e

d

g This work [13] [35] [42] [15] [47]

[45]

[36] [37] [43] [48]

[46]

[44]

Figure 3. (a) Digital photographs of the LMSC at different bent angles (0 – 70o) and (b) CV responses of the LMSC. CV profiles of LMSC with respect to (c) scan rate and (d) working voltage. (e) Areal and (f) volumetric capacitance plots of the LMSC at different scan rates. (g) Areal capacitance vs volumetric capacitance plot of the LMSC with the previously reported MSCs. 10


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The capacitive response of the flexible LMSC was further evaluated by CV measurements at bending angles from 0 to 70o as depicted in Figure 3a and Figure 3b shows the respective CV responses. At all bending angles, as-fabricated LMSC displays stable electrochemical performance and hence, suitable for flexible MSC applications. Figure 3c depicts the CV profiles of the LMSC at different scan rates and the rectangular shapes of CV at all the scan rates from 5 mV/s to 100 mV/s confirms stable energy storage performance of the LMSC. Study of CVs at different applied voltages (0.4 – 0.8 V) (Figure 3d) also demonstrates that the LMSC remains stable at different voltages. Figure 3e,f shows the specific areal and volumetric capacitances of the LMSC. It is observed that both at lower as well as higher scan rates, LMSC offers excellent capacitances of 7.43 mF/cm2 (32.3 F/cm3) at 5 mV/s and 5.5 mF/cm2 (23.8 F/cm3) at 100 mV/s, respectively, which are higher than reported highperformance MSCs, e.g., methane plasma reduced rGO (0.0807 mF/cm2 and 17.9 F/cm3 at 10 mV/s),35 laser reducted rGO (0.51 mF/cm2 and 3.1 F/cm3 at 20 mV/s),36 exfoliated graphene and PEDOT:PSS (1.08 mF/cm2 and 5.4 F/cm3 at 10 mV/s),13 activated carbon (2.1 mF/cm2 at 2.7 F/cm3 at 1 mV/s),37 cobalt hydroxide/rGO (2.7 mF/cm2 and 54 F/cm3 at 0.040 mA/cm2),38 MWCNT/Mn3O4 (8.9 F/cm3 at 0.1 A/cm3),39 rGO/PANI (17.4 F/cm3 at 20 mV/s),40 RuOx (12.6 mF/cm2 at .38 mA/cm2)41 and so on as shown in Figure 3g. The CV and areal capacitance plots with the scan rates of the MnO2-CNTs MSC (Figure S8a,b), MnO2-rGO MSC (Figure S9a,b) and RMSC (Figure S10a,b) are provided in the supporting information. The calculated capacitance retentions at 100 mV/s as compared to 5 mV/s of the MSCs are 73.7% for LMSC, 47.95% for RMSC, 50% for MnO2-rGO MSC as well as 52.12% for MnO2-CNTs MSC, respectively. The observed higher capacitance retention of the LMSC as compared to RMSC, MnO 2-rGO MSC and MnO2-CNTs MSC is due to the rapid in-plane diffusion of electrolyte ions in layer-by-layer 11



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electrode structure. Likewise, the increase in the capacitance retention in MnO2-CNTs MSC as compared to MnO2-rGO MSC could attributed to the shorter diffusion length of electrolyte ions as well as higher electrical conductivity of CNTs. Due to the physical mixing of MnO 2-rGO and MnO2-CNTs in RMSC electrode offers relatively lower capacitance retention of 47.95% at 100 mV/s as compared to 5 mV/s.

a

c

b

d

Figure 4. (a) CD profiles of the LMSC. (b) and (c) are the areal capacitance / coulombic efficiency plots of the LMSC with respect to the current density (0.01 – 0.10 mA/cm2) and voltage (0.4 – 0.8 V). (d) Capacitance retention plot for 5000 CD cycles of the LMSC.

12


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Moreover, the CD measurements were evaluated to further investigate the energy storage response of the LMSC. Figure 4a shows the typical CD profiles of the MSC at different current densities ranging from 0.01 to 0.10 mA/cm2 over the working voltage of 0.8 V. Symmetric CD plots as well as negligible voltage (IR) drop of the LMSC at all the current densities denote efficient redox activity and hence superior electrochemical performance.9,49 Also, the as-fabricated LMSC offers the stable capacitance even at different voltages varying from 0.4 to 0.8 V (Figure S11). Figure 4b depicts the areal capacitance (obtained from CD plots) plot with the current density of the LMSC, where excellent coulombic efficiency (  t d t c 100% ; where t c and t d are the charge time and discharge time) of nearly 98.2% (average value) demonstrates better charge and discharge reversibility of the LMSC.50 The plot of the areal capacitance and coulombic efficiency of the LMSC at different voltages (0.4 – 0.8 V) are shown in Figure 4c. The capacitance stability of the LMSC, cyclic CD measurement was performed for 5000 CD cycles. Figure 4d shows the capacitance retention of LMSC, where 98.9% capacitance is retained after 5000 CD cycles, which is higher than previously reported MSCs.9,49,51 Electrochemical impedance spectroscopy (EIS) was performed for the MSCs, where Nyquist plots of the RMSC and LMSC recorded in the frequency range of 0.01 Hz to 10 kHz at the ac perturbation of 5 mV (Figure 5a). In the low frequency range, the Nyquist plot shows straight line variation, that suggests the EDLC response close to the ideal capacitor of the RMSC and LMSC. The charge transfer resistance of the RMSC and LMSC are negligible and hence efficient ionic transfer between the electrodes and the electrolyte, where the steep Nyquist plots of the RMSC and LMSC indicates the short ion diffusion pathway for faster ion diffusion in 13



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electrodes.15,35 The calculated equivalent series resistances (ESRs) are 20.26 Ω/cm2 and 18.46 Ω/cm2 for the RMSC and LMSC, where the decrease in the ESR of LMSC as compared to RMSC is mainly attributed to the better electrical conductivity of layer-by-layer patterning of electrode structure than those of randomly mixed composites electrode structure.32-34

a

b

c

d This work [56] [15] [42]

[36]

[55]

Figure 5. (a) Nyquist plots of the RMSC and LMSC. (b) Bode plot of the LMSC. (c) Areal energy density and volumetric energy density plots and (d) Ragone plot of the LMSC. Figure S12,13 shows the Nyquist plots of the MnO2-rGO MSC and MnO2-CNTs MSC, where the measured ESRs were found to be 14.8 Ω/cm2, 10.2 Ω/cm2, respectively. Likewise, the decrease in the ESRs of MnO2-rGO MSC and MnO2-CNTs MSC as compared to RMSC as well as LMSC are mainly originated due to decrease in the intrinsic resistance of the pristine materials 14


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(MnO2-CNT or MnO2-rGO) as compared to the physically mixed of MnO2-CNT and MnO2rGO, because physically integrated composite material offers relatively higher capacitance than the pristine counterpart. Moreover, the lower ESR of MnO2-CNT MSC is mainly attributed to the higher electrical conductivity of CNTs as compared to rGO in MnO2-rGO MSC. Figure 5b shows the Bode plot of the LMSC, where the obtained high characteristic frequency is 0.98 Hz and the respective time constant is ~1 s at -45o, where, the higher characteristic frequency demonstrates ac line filtering capability of the LMSC.15 Energy density is considered as a key parameter that defines the LMSC performance. Figure 5c shows the areal energy density and volumetric energy plots of the LMSC. The obtained areal / volumetric energy densities at the lower scan rate (5 mV/s) and higher scan rate (100 mV/s) are 0.66 μWh/cm2 / 2870 μWh/cm3 and 0.49 μWh/cm2 / 2116.6 μWh/cm3, respectively. Likewise the obtained values are larger than the reported MSCs, e.g., 680 μWh/cm3 for rGO-CNT MSC,45 0.38 μWh/cm2 for 3D graphene MSC,52 430 μWh/cm3 for laser-written rGO MSC,36 0.074 μWh/cm2 for graphene quantum dots MSC,42 1800 μWh/cm3 for LbLMWNT/Mn3O4 MSC,53 2400 μWh/cm3 for LIG-FeOOH//LIG-MnO2 MSC,9 1.51 mWh/cm3 for rGO/PANI MSC,54 1 mWh/cm3 for pyrolysis photoresist MSC,47 etc. As-fabricated LMSC exhibits superior volumetric energy and power densities than reported MSCs (Figure 5d). Moreover, the calculated volumetric energy / power densities of the LMSC are 1468.6 μWh/cm3 / 17.4 mW/cm3 at 0.01 mA/cm2 and 1188.4 μWh/cm3 / 174 mW/cm3 at 0.1 mA/cm2, respectively. Therefore, the results of as-fabricated LMSC reveal high-performance, robust and stable on-chip energy storage device.

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CONCLUSION In summary, this work presents a simple, cost-effective LMSC, assembled using layerby-layer coating of hybrid MnO2-CNTs and MnO2-rGO through alternate spray deposition for excellent electrochemical performance. LMSC exhibited significantly higher electrochemical performance than RMSC due to the efficient synergistic response as well as in-plane diffusion of ions in the layered electrode. As-fabricated LMSC offers excellent areal capacitance (7.43 mF/cm2) and volumetric capacitance of 32.3 F/cm3 at 5 mV/s along with outstanding energy density of 0.66 μWh/cm2 (2870 μWh/cm3). Only, 1.1% capacitance degradation was recorded over 5000 CD cycles of the MSC. Moreover, LMSC displayed high flexibility along with stable capacitive response. Hence, as-fabricated LMSC is necessary for flexible, energy efficient inplane on-chip energy storage device applications in modern microelectronic. EXPERIENTAL SECTION Material synthesis. Modified Hummers method was followed during the synthesis of graphene oxide (GO) and rGO.57 The syhthesis of CNTs was performed as follows: first, 1g ferrocene was dissolved into 50 ml toluene. Then, a piece of clean silicon wafer (SiO2 (300 nm)/Si) of size 2 × 2 cm2

was loaded into the CVD furnace. Thereafter, the temperature of the furnace was

increased to 825 oC in argon environment (800 sccm flow rate) and the ferrocene solution was then fed into the furnace in presence of argon gas. Finally, the furnace was cooled down under the presence of argon gas.58 For the growth of MnO2 NPs coated CNTs, 60 mg CNTs was first dispersed into 60 ml (30 ml deionized (DI) water + 30 ml ethylene glycol) solution with the help of ultra-sonication process (5 h). Next, 1g of potassium permanganate was dissolved into CNTs solution followed 16


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by stirring for another 2 h. Afterwards, the solution was transferred into borosil screw cap (100 ml) bottle and maintained at 180 oC for another 8 h.27 Finally, the sample was collected followed by vacuum filtration process and cleaned with ethanol and DI water in sequence. Likewise, the synthesis of MnO2-rGO sample was as follows: 50 mg rGO was added into 50 ml DI water and dispersed properly with ultra-sonication process for 5 h. Then, 1 mmol sodium persulfate and 1 mmol manganese (II) acetate tetrahydrate were added into the rGO solution and maintained at 120 °C for another 12 h. Fabrication of MSC. Initially, PET substrates were cleaned with isopropyl alcohol, acetone, DI water which were dried with nitrogen (N2) gas. Thereafter, interdigitated electrodes pattern SS masks were fixed on PET substrates with the help of adhesive tape. Afterward, Cr/Au (25/200 nm) electrodes were deposited on the respective patterns via sputtering deposition technique. Finally, the electroactive materials were spray deposited on the top the electrode at 180 oC using N2 gas. During the fabrication of LMSC, MnO2-CNTs and MnO2-rGO were spray deposited alternatively for four layers, whereas same amount of MnO2-CNTs and MnO2-rGO were mixed for RMSC. Likewise, for the fabrication of MnO2-CNTs MSC and MnO2-rGO MSC, same amount of total mass (around 2.3 g) of electroactive materials were spray deposited under identical condition. The sizes of each electrode finger of MSCs are 1000 µm (width), 13300 µm (length) and 700 µm (gap between electrode fingers). The total active areas of MSCs are 1.5 × 1.3 cm2 with ~ 2 µm electrode thickness. Figure S14 depicts the schematic of MSC which showing the size of the MSC. Electrochemical Measurements. During the preparation of PVA/H3PO4 gel electrolyte, 3g PVA was added into 30 ml DI water then maintained at 95 °C for 1 h under stirring. Thereafter, 6 ml 17



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of H3PO4 was added and maintained at the same condition for another 1 h. Finally, as-prarared PVA/H3PO4 electrolyte was drop-casted on MSCs active areas and then kept inside the laminar airflow chamber for 12 h to evaporate the excess water. Then the electrochemical performances of MSCs were studied by CV, CD and EIS using the electrochemical workstation of model number CHI660E. CV as well CD measurements were carried out at different scan rates and working voltages. Calculation. The following formulas were used to calculate the MSCs parameters as follows:5962

Vf

Areal capacitance (based on CV profile): C S 

1 IdV S (V f  Vi ) Vi

Areal capacitance (based on CD profile) ; C S 

I t d S (V f  Vi ) Vf

1 IdV Volumetric capacitance (based on CV profile): CV  V (V f  Vi ) Vi

Volumetric capacitance (based on CD profile): CV 

It d V (V f  Vi )

(V f  Vi ) 2 1 Areal energy density: ES   CS  2 3600 (V f  Vi ) 2 1 Volumetric energy density: EV   CV  2 3600

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Areal power density: PS 

ES  3600 ; and t d

Volumetric power density: PV 

EV  3600 t d

where, S , V ,  , I , Vi and V f are the total MSC area, total MSC volume, scan rate, discharge current, initial voltage limit and final voltage limit. ASSOCIATED CONTENT Supporting Information. SEM images of MnO2-rGO and MnO2-CNTs samples; EDS and XPS spectra of the samples; Cross-sectional SEM images of LMSC and RMSC at low and high magnifications; CV plots of MnO2-CNTs MSC, MnO2-rGO MSC and RMSC; areal capacitance plots MnO2-CNTs MSC, MnO2-rGO MSC and RMSC; CD profileds LMSC at different working voltage; Nyquist plots of MnO2-rGO MSC and MnO2-CNTs MSC; Schematic depicts the size of the MSC. ACKNOWLEDGEMENTS AM would like to thank the Department of Science and Technology (DST) for funding the project under the fast track grant (Grant no. DST-1682). REFERENCES 1. Cheng, T.; Zhang, Y.; Lai, W. Y.; Huang, W. Stretchable Thin‐film Electrodes for Flexible Electronics with High Deformability and Stretchability. Adv. Mater. 2015, 27, 3349-3376.

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Energy-Storage

Performance.

ACS

Appl.

Energy

Mater.

2018,

DOI:

10.1021/acsaem.7b00358.

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Table of Contant (TOC):

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Flexible Array of Microsupercapacitor for Additive Energy Storage

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