manganese-oxide hybridized nanostructures as electrodes for creation of fiber-shaped high-energy-density supercapacitors

Journal Pre-proof Thicker carbon-nanotube/manganese-oxide hybridized nanostructures as electrodes for creation of fiber-shaped high-energy-density supercapacitors Wei Gong, Bunshi Fugetsu, Zhipeng Wang, Takayuki Ueki, Ichiro Sakata, Hironori Ogata, Fei Han, Mingda Li, Lei Su, Xueji Zhang, Mauricio Terrones, Morinobu Endo PII:

S0008-6223(19)30801-2

DOI:

https://doi.org/10.1016/j.carbon.2019.08.004

Reference:

CARBON 14485

To appear in:

Carbon

Received Date: 19 June 2019 Revised Date:

10 July 2019

Accepted Date: 1 August 2019

Please cite this article as: W. Gong, B. Fugetsu, Z. Wang, T. Ueki, I. Sakata, H. Ogata, F. Han, M. Li, L. Su, X. Zhang, M. Terrones, M. Endo, Thicker carbon-nanotube/manganese-oxide hybridized nanostructures as electrodes for creation of fiber-shaped high-energy-density supercapacitors, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.08.004. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Thicker carbon-nanotube/manganese-oxide hybridized nanostructures as electrodes for creation of fiber-shaped supercapacitors of high energy densities *, Zhipeng Wang a, Takayuki Ueki b, Ichiro Sakata

a, b

Hironori Ogata c, Fei Han d, Mingda Li d, Lei Su e, Xueji Zhang e, Mauricio Terrones

f, g

Wei Gong

a,

*, Bunshi Fugetsu

b,

, ,

Morinobu Endo f

The CNT/MnO2 hybrid nanostructures are deposited on carbon fibers and the resultant CNT/MnO2@CF electrodes displaying specific volumetric capacitances of 527 F cm-3. An assembling symmetric flexible cell with the CNT/MnO2@CF coaxial electrodes gives a volumetric energy density of 8.14 mWh cm-3 which is high enough for powering a certain flexible and portable electronic system.

1

Thicker carbon-nanotube/manganese-oxide hybridized nanostructures as electrodes for creation of fiber-shaped high-energy-density supercapacitors Wei Gong a, *, Bunshi Fugetsu b, *, Zhipeng Wang a, Takayuki Ueki b, Ichiro Sakata a, b, Hironori Ogata c, Fei Han d, Mingda Li d, Lei Su e, Xueji Zhang e, Mauricio Terrones f, g, Morinobu Endo f

a

School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan

b

Institute for Future Initiatives, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan

c

Graduate School of Science and Engineering, Hosei University, Koganei, Tokyo 184-8584,

Japan d

Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77

Massachusetts Avenue, Cambridge, MA 02139, USA e

Research Centre for Bioengineering and Sensing Technology, University of Science and

Technology Beijing, Haidian District, Beijing 100083, China f

Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano

380-8553, Japan g

Department of Physics, Department of Chemistry, Department of Materials Science and

Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA

* Corresponding author. E-mail addresses: [email protected] (W. Gong), [email protected] (B. Fugetsu).

1

Abstract This work demonstrates a high-energy-density and flexible supercapacitor as a potential energy source for smart electronics devices. Cathode and anode are fiber-shaped electrodes with manganese oxide (MnO2) being electrochemically inserted into densely interconnected carbon nanotube (CNT) networks as active domains, while carbon fibers (CF) serve as current collectors. The CNT/MnO2 hybrids are built up as a co-axial shell with an optimized thickness of 1.44 µm surrounding CF. Specific volumetric capacitance is found as high as 527 F cm-3 when a 1.0 M Na2SO4 aqueous solution is used as electrolyte; when a solid electrolyte (polyvinyl alcohol and lithium chloride, PVA/LiCl) is used, the specific volumetric capacitance is found as high as 492 F cm-3. These values, to the best of our knowledge, are the highest values of the specific volumetric capacitance among all the MnO2-based fiber-shaped electrodes reported in previous literature. An all-solid-state (PVA/LiCl) symmetric fiber-shaped supercapacitor cell is assembled and a volumetric energy density of 8.14 mWh cm-3 which is high enough for driving a portable LED device, is obtained. Our fiber-shaped supercapacitor cell is safe, flexible, and capable of powering smart electronic devices.

2

1. Introduction Power source for devices has long been the bottleneck encountered in flexible electronics commercialization. It must be bendable, wearable, lightweight, and at the same time, both energy density and power density must be high enough to meet immediate- and long-term operations. Fiber-shaped supercapacitors (FSC) have attracted extensive attention; their wearable nature, lightweight and high flexibility make FSC highly desirable for powering the flexible electronics devices [1-4]. Similar to conventional cylindrical- and planar-shaped supercapacitors, FSC store energies via either electrical double layer (EDL) principle or Faradaic electrochemical interactions [1,5-9]. Active materials in electrodes are the key elements which determine the performance of FSC. Carbon-based materials are the most popular active materials in EDL-FSC [4,10-12]. Cations and anions (namely, the electrolytes) form cation-EDL and anion-EDL, nearby anode and cathode, respectively; energies (electrons) are stored entirely via the carbon-based domains. This unique simplicity of energy storage gives distinguishing characteristics of high power density and excellent cycling stability to the carbon-based EDL-FSC. Energy density depends entirely on densities of EDL (cation-EDL and anion-EDL). EDL are ultra-thin layers generated at interfaces of activedomain/electrolyte. High surface area carbons, such as activated carbon [13-15], carbon nanotubes (CNT) [16], carbon onion [14,17], reduced graphene oxide (rGO) [18-20], and graphene [8,21-25], are the typical carbon-based active materials capable of generating highdensity EDL; the ultimate energy of EDL-FSC, however, is still much lower, in comparison with the energy stored by using Faradaic-FSC (pseudo-FSC). Pseudo-FSC stores energies via Faradaic electrochemical interactions. Theoretical (ultimate) energy density is governed entirely by the absolute amount of the active (redox) materials in electrodes; the actual energy density, on the other hand, is tunable, depending on degrees of electrical activation of the redox materials. Manganese oxide (MnO2) is a popular redox material for pseudo-FSC, which stores energies (electrons) via Faradaic electrochemical interactions, as is illustrated in Equation 1 [26-29]. Two key reactions occur in electrical activation of MnO2: i) cations (C+) of electrolytes penetrate into crystal cavities of MnO2; ii) at the same time, electrode delivers electrons in to MnO2 to maintain charge neutrality; this kind of Faradaic electrochemical interaction is also referred to as electrochemical intercalation [30].

3

+

+

⇄

⋯

(1)

Actual energy density lies in the degree of electrochemical activation of the redox materials in electrodes; electrical conductivity of the redox material is the key which determines degrees of the electrochemical activation. MnO2 is electrically insulating in nature; the thickness for electrons possibly penetrate into MnO2 is theoretically predicted to be less than 420 nm [5,26,27]. Several pioneering solutions have been established to compensate for the poor electrical conductivity of MnO2. Typical solutions are i) thinning the size of MnO2 down to nano meters via electrochemical deposition of MnO2 on surfaces of a large-surfacearea yet electrically conductive substance [31-36], ii) modifying the conductivity of MnO2 by doping other conductive elements [37-45], and iii) wrapping of nano-sized MnO2 via electrical-conductive yet ultra-thin sheets, such as purely CNT-based sheet [46,47]. Low mass loadings of MnO2 and low degrees of electrochemical activation of MnO2 are still the major drawbacks remaining to overcome. It is demonstrated in our previous studies that high mass loadings and high degrees of electrochemical activation of MnO2 are achievable via the formation of a new type of CNT/MnO2

hybridized

nanostructures

[30].

Nano-sized

MnO2

structures

are

electrochemically inserted into densely interconnected CNT-networks. CNT-networks can be visualized as nano-sized capillaries and MnO2 domains as nano-sized tissue cells. Electrons are rapidly delivered via the highly electric conductive CNT-networks; nano-sized MnO2 domains are thereby being effectively activated. This self-electric-conductive CNT/MnO2 nano-hybrid can be loaded up to a thickness of 150 µm while rates of transfer of electrons and electrolytes remain excellent. In our previous study, nickel wires with a diameter of 200 µm were used as current collectors; this restricted the flexibility and wearability of the final FSC. This difficulty, as demonstrated experimentally, is finally overcome by using carbon fibers (CF) as current collectors in this study. Moreover, by replacing nickel wires by CF, the specific volumetric capacitance is increased from 177 F cm-3 to 527 F cm-3. To the best of our knowledge, which is the highest values of specific volumetric capacitances among all the MnO2-based fiber-shaped electrodes ever reported in previous literature. It is noted here that the so-called purely CNT-based fibers, especially the fibers consisted of entirely singlewalled CNT, are also highly desirable as current collectors for fabricating FSC [9,48-57].

4

However, the ultra-high costs and the very limited availability have been the major drawbacks encountered in commercialized scale applications of the purely CNT-based fibers.

2. Experimental section 2.1. Preparation of aqueous dispersions containing mono-dispersed CNT As-grown, entangled multi-walled CNT powders (NC7000TM), were purchased from Nanocyl S.A. (Belgium). They were further dispersed into aqueous suspensions via the following steps: i) 30 g of the as-purchased CNT powders were pre-dispersed in to 1000 ml deionized water using a ball mill system, ii) 10 g of sodium cholate (Wako Chemicals) were then introduced and the slurry was then milled for about 50 minutes via a beads mill (Multi Lab DYNO-Mill, 0.6 mm zirconium beads), iii) 5.0 g of PVP (Wako Chemicals) and 5.0 g of hydroxypropyl cellulose (Wako Chemicals) were introduced and the slurry was further milled till the CNT disperse individually. Degrees of dispersion (D90 < 60 nm) were confirmed via size distribution analysis using a dynamic light scattering analyzer (HORIBA Dynamic Light Scattering Particle Size Analyzer LB-550). Cholate functioned as the dispersant while PVP and hydroxypropyl cellulose as stabilizers. 2.2. Preparation of the CNT/MnO2@CF electrodes A piece of triple-bundled carbon fibers (diameter of the individual CF is about 7.24 µm) was dipped into the mono-dispersed CNT suspension for 10 seconds and then rapidly removed. Subsequently, the CNT coated CF was dried using a heat gun. After drying, the fiber electrode was subjected to surfactant/stabilizer removal by washing with abundant ethanol and deionized water and again dried at room temperature. Nanostructured MnO2 domains were anodically electrodeposited into the CNT-networks via cyclic voltammetry. CF being coated with the CNT-networks was used as the working electrode, Pt foil as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The fiber electrode was immersed into an aqueous solution containing 0.1 M Mn(Ac)2 and 0.1 M Na2SO4. Cyclic voltammograms were recorded between 0.4 and 1.0 V (vs. Ag/AgCl) at a certain scan rate (from 2 to 50 mV s-1) for 1 cycle. After electrodepositing of the nanostructured MnO2 domains, the CNT/MnO2@CF electrode was rinsed in deionized water and then dried at room temperature. The two steps were repeated several times until desirable CNT/MnO2@CF electrodes were obtained. 5

2.3. Assembly of the symmetric supercapacitors (cells) The PVA/LiCl gel electrolyte was prepared by heating a mixture of 4 g PVA (Mw 89000 ~ 98000) and 8.5 g LiCl in 40 mL deionized water at 90 oC for 3 h. Two fiber-shaped CNT/MnO2@CF electrodes with the same length were immersed in the PVA/LiCl electrolyte solution at 50 oC for 10 min. The fiber electrodes with electrolyte were placed parallel to prepare the symmetric cells and their position was adjusted under an optical microscope (Olympus, SZX16). One end of each electrode was connected to a 180 µm diameter Cu wire using silver paste for electrochemical performance measurements. 2.4. Characterization The materials were characterized by X-ray diffraction pattern (XRD, SuperLab, Rigaku) using a Cu radiation (Kα, λ = 0.15406 nm), field emission scanning electron microscope (FESEM, Hitachi SU-8020) equipped with an energy-dispersive X-ray spectroscopy (EDX, HORIBA X-Max), scanning electron microscope (SEM, JEOL JSM-6390, Japan) and transmission electron microscope (TEM, JEOL JEM-2100F). The cross-sectioned electrodes were prepared using a cross-section polisher (CP, JEOL SM-09010) and cryo ion slicer (JEOL IB-09060CIS). All electrochemical measurements including CV curves, galvanostatic charge/discharge curves, EIS (that was obtained with a sinusoidal potential excitation of 5 mV in the frequency range from 100 kHz to 0.01 Hz), and cycling performance measurements were carried out using an electrochemical workstation (CHI 760E, CH Instruments). The electrochemical performance of the electrode materials was measured in a three-electrode system with 1.0 M Na2SO4 electrolyte solution. The CNT/MnO2@CF electrode, a platinum foil, and an Ag/AgCl electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. The electrochemical performance of the symmetric supercapacitor cells was evaluated in a two-electrode system. 2.5. Calculation of the electrochemical performances The specific volumetric capacitance (Csp,V) of the single electrode-based pseudo-capacitive material in a three-electrode cell was calculated from galvanostatic charge/discharge curves, according to the following equation:

6

=

.

×∆ ×∆

(2)

where I denotes the constant discharge current; ∆t denotes the time for a full discharge; Vfiber is the volume of the pseudo-capacitive fiber-shaped electrode, and ∆V is the voltage drop on discharge (excluding the Vdrop). The capacitance of the supercapacitor cell (Ccell) in a two-electrode cell was calculated from their galvanostatic charge/discharge curves at different current densities based on the following equation:

= /( !/ ") (3)

where i is the discharging current and dV/dt is the slope of the discharge curve. The cell volumetric capacitances of the FSC (Ccell,V) was calculated according to the following equation:

,

=

/!

(4)

where Vcell refers to the cell volume of the FSC. The volumetric energy density of the cell (Ecell,V) was obtained from the following equation:

&

,

=

,

∆& /(2 × 3600)

(5)

7

where ∆E is the operating voltage window in volts. The volumetric power density of the cell (Pcell,V) was calculated from the galvanostatic curves at different charge/discharge current densities by using the following equation:

+

,

=&

,

× 3600/",-

./01

(6)

where tdischarge is the discharge time.

3. Results and discussion 3.1. Preparation of CNT/MnO2@CF Electrochemical deposition is a highly desirable approach to prepare the fiber-shaped electrodes for establishing the pseudo-FSC [9,18-20,30,48,50,58-65], due to its simplicity, precise controllability, and ease of operation. The process for fabricating CNT/MnO2 hybridized coaxial fiber-shaped electrodes with CF as current collectors (denoted as CNT/MnO2@CF electrode) is shown in Fig. 1a. Two key steps are involved: step (i) establishing the densely interconnected CNT-networks via “dipping and drying” and step (ii) inserting the nanostructured MnO2 domains into the densely interconnected CNT-networks via electrochemical deposition through cyclic voltammetry (CV). Step (i) and step (ii) are conducted repeatedly until a CNT/MnO2 hybridized shell with a desirable thickness is established. Thickness, mass loading, and mass ratio of CNT/MnO2 of the CNT/MnO2 hybridized shell is controllable by controlling the CV scan rate.

8

d

b

Carbon fiber

f

CNTs

MnO2

a

c

e

g

h

Fig. 1. Preparation of the CNT/MnO2 hybridized fiber-shaped electrode. (a) Schematic illustration of the fabrication of the coaxial fiber-shaped CNT/MnO2 electrode with CF as the current corrector (CNT/MnO2@CF). (b) SEM images of the outer surface and (c) crosssectional SEM images of the fracture end area of the bare CF. (d) SEM images of the outer surface and (e) cross-sectional SEM images of the fracture end area of CNT-networks@CF. (f) SEM images of the outer surface and (g) cross-sectional SEM images of the fracture end area of CNT/MnO2@CF. (h) Photograph of a bent CNT/MnO2@CF electrode with high flexibility.

Table S1 summarizes the detailed data together with the specific capacitance for four typical CNT/MnO2@CF electrodes established at a CV scan rate of 50 mV s-1, 20 mV s-1, 10 mV s-1 and 5 mV s-1, respectively. Data on the diameter of a single monofilament of the CF, namely, the current collector, is also given in Table S1. Slowing CV scan rate, thereby longing the time of electrochemical deposition, gives thicker CNT/MnO2 hybridized shells. For example, the thickness of the CNT/MnO2 hybridized shell was 3.19 µm of the electrode obtained at the CV scan rate of 5 mV s-1; while it was 1.44, 0.80, and 0.67 µm, when the CV scan rate was used at 10, 20, and 50 mV s-1, respectively. Electrode obtained at CV scan rate of 10 mV s-1 gives the best specific capacitance, which is 527 F cm-3. Mass ratios of MnO2/CNT increased from 60.6wt% to 93.9wt%, as the time of electrochemical deposition increased from 50 mV s-1 to 5 mV s-1. Scanning electron microscopy (SEM) images of the pristine carbon fibers (Fig. 1b and c), densely interconnected CNT-networks formed over the CF’s surfaces (Fig. 1d and e) and the nano-sized MnO2 being electrochemically inserted into

9

the CNT-networks (Fig. 1f and g) are shown in Fig. 1b-g, respectively. The resultant CNT/MnO2@CF electrodes are highly flexible and bendable (Fig. 1h). 3.2. Characterization of CNT/MnO2 hybridized nanostructures Structures of the CNT/MnO2 hybridized shell are characterized by using SEM and energy dispersive spectroscopy (EDS). Fig. 2a shows an SEM image of a low-resolution view of a typical CNT/MnO2@CF electrode; coaxial architecture with a uniformed thickness of the CNT/MnO2 hybridized shell is observed. Fig. 2b and c show SEM images of the outmost layer which is the surfaces of the CNT/MnO2@CF electrode obtained by electrochemically inserting MnO2 into the 5th layer of CNT-networks. MnO2 are seen as flake-shaped and porous nanostructures; this is a typical architecture of the electrochemical deposited MnO2 [60,62,66]. Fig. 2d shows an SEM image of a cross-section of the CNT/MnO2@CF electrode; the thickness of the CNT/MnO2 hybridized shell is around 1.44 µm. The first layer of the densely interconnected CNT-networks attached firmly on the surfaces of carbon fibers (i.e., the current collectors); thickness of the CNT-networks is approximately 800 nm (Fig. S1a). MnO2 are inserted into the densely interconnected CNT-networks via CV electrochemical deposition (Fig. S1). Formation of a thin MnO2 layer with CNT-networks as scaffolders via electrochemical deposition is a well-established methodology for fabricating electrodes for supercapacitors [9,30,67]. Structures of CNT/MnO2 hybrids established in our study, however, is unique and this is confirmed based on the data on EDS elemental mapping for carbon (C), manganese (Mn), oxygen (O), and sodium (Na) elements (Fig. S2) and the linescan EDS data (Fig. S3).

b

a

c

50 μm

500 nm

10

250 nm

d

e

200 nm

5 μm

g

f

500 nm

10 nm

Fig. 2. Microstructures of the CNT/MnO2 hybridized nanostructures. (a) An SEM image of the full view of the CNT/MnO2@CF electrode. (b) The SEM image of MnO2 nanosheets on the surface of the CNT/MnO2 hybridized nanostructures. (c) Cross-sectional SEM image of the fracture end area of MnO2 nanosheets on the CNT/MnO2 hybrid. (d) The cross-sectional SEM image of CNT/MnO2@CF electrode obtained by sectioning with a focused Ga ion beam. (e) The high magnification SEM image of the cross-section of CNT/MnO2 hybrid. (f) A TEM image of the cross-section of CNT/MnO2 hybrid. (g) A high-resolution TEM image of the cross-section of the CNT/MnO2 hybrid.

Thicker CNT/MnO2 hybridized shells, thereby a larger mass loading of MnO2, are obtainable by electrochemically linking several the preliminary CNT/MnO2 hybrid blocks (layers). The preliminary CNT/MnO2 blocks are seamlessly connected, as seen from the highresolution observations. Fig. 2e and f are typical images of a cross-section of the CNT/MnO2 hybridized shell which consisted of 5 preliminary CNT/MnO2 layer, observed by using SEM and transmission electron microscopy (TEM), respectively. The densely interconnected CNTnetworks are the nano-meshed networks. MnO2 domains are also nano-sized particles and being firmly inserted (stabilized) within the nano-meshed CNT-networks. In other words, CNT-networks can be visualized as nano-sized capillaries and MnO2 domains as nano-sized 11

tissue cells. Electrons are effectively delivered via the electrically conductive CNT-networks and as a result, effective electrical activation of the nano-sized MnO2 domains is obtained. It is noted here that CNT-networks, in this study, are established without the use of any kind of binders; the monodispersed CNTs are self-assembling to form the interconnected CNTnetworks, the interconnected CNT-networks are then firmly immobilized via electrochemical deposition of nanosized MnO2 particles throughout the CNT-networks. X-ray diffraction (XRD) patterns of the CF/CNT/MnO2@CF electrodes show very weak diffraction intensity (Fig. S4), due to the tetragonal phase of α-MnO2 (JCPDS: 44-0141) produced via the CV electrodeposition process. The high-resolution TEM (HRTEM) image confirms further that the CF/CNT/MnO2 hybrid consists of nanocrystalline MnO2 and multiwalled CNT (MWCNT) with an average layer spacing of approximately 0.24 and 0.34 nm, corresponding to the (400) plane of MnO2 and the (002) plane of MWCNT, respectively (Fig. 2g). Fig. S5 shows SEM images of full views for some typical electrodes: a 5/5-layered CNT/MnO2@CF prepared at the CV scan rate of 10 mV s-1 and the CF current collector is directly wrapped by the densely interconnected CNT-networks as the 1st layer of the CNTnetworks (Fig. S5a); a 5-layered MnO2 and 4-layered CNT prepared at the same scan rate (10 mV s-1) but the 1st layer of MnO2 is directly electrochemical deposited on the CF current collector (Fig. S5b), cracks appeared to the terminal layer of MnO2; a 5/5-layered CNT/MnO2@CF prepared at a CV scan rate of 20 mV s-1 (Fig. S5c) and a 5/5-layered CNT/MnO2@CF prepared at a CV scan rate 50 mV s-1 (Fig. S5d), respectively. A 5/5layered CNT/MnO2@CF prepared at CV scan rate of 2 mV s-1 is also given in Fig. S5e, the overall thickness of the CNT/MnO2 hybridized shell is about twice the value of the thickness of the CNT/MnO2 shell prepared at CV scan rate of 10 mV s-1 but is still showing excellent mechanical properties, although cracks appeared. Serious cracks are seen on the electrode prepared at 2 mV s-1 without CNT-networks on the terminal surfaces (Fig. S5f). Bridging of cracked MnO2 domains, as is shown in Fig. S5g and h, is also a key feature of the densely interconnected CNT-networks. 3.3. Electrochemical characterization of the CNT/MnO2@CF electrodes

12

b700

50 mV s-1 20 mV s-1 10 mV s-1 5 mV s-1

4000 2000

Volume capacitance (F cm-3)

Current density (mA cm-3)

a 6000

0 -2000 -4000

50 mV s-1 10 mV s-1

600

20 mV s-1 5 mV s-1

500 400 300 200 100 0

0.0

6000

0.4 0.6 Potential (V)

0.8

5L-CNT

4L/5L-CNT/MnO2

5L-MnO2

5L/5L-CNT/MnO2

0

d

500

1000 1500 2000 Current density (mA cm-3)

2500

0.06 5L-CNT

4L/5L-CNT/MnO2

5L-MnO2

5L/5L-CNT/MnO2

4000 0.04 -Z'' (Ω cm3)

-3

Current density (mA cm )

c

0.2

2000 0

0.02

-2000 10 mV s-1

-4000

0.00

0.0

0.2

0.4 Potential (V)

0.6

0.00

0.8

0.03

0.06 Z' (Ω cm3)

0.09

0.12

Fig. 3. Electrochemical performance of CNT/MnO2@CF electrodes in the aqueous electrolyte (1M Na2SO4). (a) CV curves and (b) specific volumetric capacitance of four 5/5layered-CNT/MnO2@CF electrodes prepared a CV scan rate of 5, 10, 20, and 50 mV s-1, respectively. (c) CV curves and (d) Nyquist plots of a 5/5-layered CNT/MnO2@CF, 4/5layerd CNT/MnO2@CF, 5-layered entire MnO2@CF and 5-layered CNT@CF, each prepared at CV scan rate of 10 mV s-1.

Electrochemical performances of CNT/MnO2@CF electrodes are firstly evaluated via a three-electrode cell containing 1.0 M Na2SO4 solution as the electrolyte. Fig. 3a shows CV curves of three optimized 5/5-layered (5-layered CNT-networks and 5-layered MnO2 nanodomains) electrodes, each was prepared at a CV scan rate of 50 mV s-1, 20 mV s-1, and 10 mV s-1, respectively. The CV curve of a 5/5-layered CNT/MnO2@CF electrode but with an excessive thickness (3.19 µm), prepared at a scan rate of 5 mV s-1 for the electrochemical deposition, is also given in Fig. 3, for demonstrating the detrimental effects of the internal resistance of the active materials on the charge/discharge (Quasi-rectangular-shapes) performance of the CNT/MnO2 hybridized electrode. Electrode prepared at CV scan rate of 10 mV s-1 gave the highest capacitance, followed by the electrode prepared at the CV scan rate of 20 mV s-1 and then the electrode prepared at the scan rate of 50 mV s-1. Quasi13

rectangular-shaped CV curves and symmetrically shaped galvanostatic charge/discharge curves are observed, implying the fact that these three optimized electrodes are undergoing with an ideal pseudocapacitive principle [60,67]. On the other hand, the thickness of the CNT/MnO2 hybridized shells was in an order of 5 mV s-1 > 10 mV s-1 > 20 mV s-1 > 50 mV s-1. Same trends are observed when the electrodes are tested via galvanostatic charge/discharge (GCD) experiments (capacitance vs. current density), as shown in Fig. 3b. Electrode prepared at CV scan rate of 10 mV s-1 also shows an excellent performance at high current density: the capacitance remains at 373.3 F cm-3 even at a high current density of 3000 mA cm-3; at a lower current density of 188.5 mA cm-3, the capacitance is 526.9 F cm-3 (Fig. 3b). To the best of our knowledge, this is the highest values of specific volumetric capacitance among all the MnO2-based fiber-shaped electrodes reported in previous literature. The specific capacitance in weight of a 5/5-layered CNT/MnO2@CF prepared at the CV rate of 10 mV s-1 for the electrochemical deposition, is calculated to be 263.5 F g-1 at a current density of 1000 mA g-1. Note here that the volumetric capacitance is a more meaningful parameter for evaluating the energy storage performance of the fiber-shaped supercapacitor than the term of gravimetric capacitance based on the electrode active materials. Table S2 summarizes data on the specific volumetric capacitance of the MnO2-based FSC, acquired from some typical publications in recent years. Core-shell structured electrodes and coreless structured electrodes are selected. The so-called core-shell type of electrodes are the electrodes which consist of macroscopic solid fibers or wires as current-collectors, i.e., the cores; the cores have no contributions for energy storage. The coreless type of electrodes are the electrodes which contain microscopic or nanoscopic fibers and/or particles as the current collectors; this kind of current collectors are capable of storing energies as well. Specific volumetric capacitances are calculated via either the 2-electrode system or the 3-electrode system. Our CNT/MnO2@CF electrode gives 527 F cm-3, which is about 9 times higher than that of the capacitance reported for the MnO2@CF electrode [60] where 1 M Na2SO4 aqueous solution is used as the electrolyte in both cases. When the PVA/LiCl solid electrolyte is used, our electrode shows 492 F cm-3 which is more than 14 times higher than that of the capacitance reported for MnO2@CNT-based-yarn [50]. In fact that the capacitance provided by our CNT/MnO2@CF electrode is comparable even better than the capacitance reported for electrode with RuO2 as the redox active materials. A coreless type of RuO2/rGO/CNT gave 1054 F cm-3 [68]; the sole CNT/MnO2 shell of our CNT/MnO2@CF gave 1079.3 F cm-3. This

14

volumetric capacitance is calculated based on the net volume of the CNT/MnO2 shell (i.e., without considering the volume of the CF core). Fig. 3c shows CV curves of four typical electrodes: 5-layered CNT-networks as the entire active domains, 5-layered MnO2 as the entire active domains, 5-layered-CNT-networks/5layered-MnO2 and the 1st CNT-networks is built on the CF current collector as the active domains, and 5-layered-MnO2/4-layered-CNT-networks but the 1st MnO2 layer is built on the CF current collector as the active domains. CV scan rate for preparation of each electrode was the same, 10 mV s-1. The volume capacitances of each electrode are calculated based on the CV curve areas; gain, the 5/5-layered CNT/MnO2 electrode shows the best capacitance (245 F cm-3), followed by 4/5-layered electrode CNT/MnO2 (188.8 F cm-3), 5-layered entire MnO2 electrode (177 F cm-3), and then the 5-layered entire CNT electrode (23.7 F cm-3). Nyquist plots (Fig. 3d) give detailed insights into the charge-transfer resistance (Rct) involved in the active domains. Again, the best electrode, namely, the 5/5 layered CNT/MnO2@CF electrode, shows an Rct value of 0.006 Ω cm-3, which is 1/10th of the Rct value of the 5layered entire MnO2@CF electrode (Rct 0.062 Ω cm-3). The Rct value for the 4/5-layered CNT/MnO2@CF electrode is found to be 0.021 Ω cm-3 and 0.001 Ω cm-3 for the 5-layered entire CNT electrode, respectively. 3.4. Performances of all-solid-state FSC PVA-LiCl is a safer yet solid electrolyte. All-solid-state supercapacitors (cells) are assembled by using PVA-LiCl as electrolyte; electrodes consisted of 5/5-layered CNT/MnO2 and the 1st CNT-networks are built on the CF current collector are used as both cathode and anode. The overall volume of the model cell, including two electrodes and PVA-LiCl solid electrolytes, is estimated to be ~2.6 × 10-5 cm3; length (L) of the model cell is around 4 cm (Fig. S6). Quasi-rectangular-shaped CV curves (Fig. 4a) and triangular-shaped galvanostatic charge/discharge curves (Fig. 4b) are obtained, demonstrating excellent capacitive performances provided by this model cell. The cell is capable of providing a volumetric capacitance of ~91.6 F cm-3 at a low current density (104.7 mA cm-3) and 73.9 F cm-3 at a high current density (1047.0 mA cm-3; Fig. 4c). Ccell,V is normalized vs. the whole cell volume. Long-termed charge/discharge cycling stabilities are examined; current density is constant at 2617.6 mA cm-3. The model cell shows a capacitance retention ratio better than 95.3% after 7000 cycles of charge/discharge (Fig. 4d); this again demonstrates excellent cycling stabilities of the model cell. The model cell is also bendable. Fig. 4e shows the

15

capacitance retention as a function of bending cycles; the capacitance retention ratio remains better than 95.1% after a 1000 bending cycling test (bending angle is from 0 to 180 degrees). Data on CV curves and capacitance retention ratios of the model cell at other bending degrees are shown in Fig. S7.

a 1200 900 600 300 0 -300

0.2 0.0

-900 0.2

0.4 0.6 Potential (V)

0

0.8

200

400 600 Time (s)

800

1000

d 100

80 5L/5L-CNT/MnO2

60

all-solid-state symmetric FSC

40 20

80

0.8

60

Potential (V)

Capacitance retention (%)

c 100 Cell capacitance (F cm-3)

0.4

-600

0.0

40

0.6 0.4 0.2 0.0

20

6990

7000

Cycle number

0

0 0

200

400 600 800 1000 Current density (mA cm-3)

0

1200

e

f Energy density (Wh cm-3)

100 Capacitance retention (%)

1047 mA cm-3 523.5 mA cm-3 392.6 mA cm-3 261.8 mA cm-3

0.6 Potential (V)

Current density (mA cm-3)

b 0.8

10 mV s-1 7.5 mV s-1 5 mV s-1 2 mV s-1

Bending 180o

80 60 40 20 0 0

200

400 600 Bending cycles

800

10-2

MnO2@GCF

This work CNT/MnO2@CF

(ref.69)

rGO/CNT@ Graphite fiber(ref.70)

10-3

MnO2@CF(ref.60) PANI@CF(ref.71)

rGO/MnO2@CF(ref.72) AuPd/MnO2@Cu wire(ref.58) CNT@CF(ref.74)

Activated CF(ref.73)

10-4 MnO2@CNT Fiber(ref.49)

10-3

1000

1000 2000 3000 4000 5000 6000 7000 Cycle number

10-2 10-1 Power density (W cm-3)

100

Fig. 4. Electrochemical performances of all-solid-state FSC. (a) CV curves of the symmetric supercapacitor cell measured at different scan rates. (b) Galvanostatic charge/discharge curves at various current densities. (c) Volumetric cell capacitance at different current densities. The inserted image shows the schematic of a typical FSC which is constructed by 16

using two CNT/MnO2@CF electrodes. (d) Cycling performance of the model FSC; the inserted figure shows the galvanostatic charge/discharge curves from the 6990th to 7000th cycle between 0 and 0.8 V at 2617.6 mA cm-3. (e) Capacitance retention after 1000 cycles up to 180° bending angle; the inserted photos show non-bent and after bent (180° bending degree) the model cell used in the bending cycle evaluation. (f) Energy and power densities of the same model supercapacitor cell compared with typical published data on the fibershaped supercapacitor cells.

Ragone plots of the model cell are shown in Fig. 4f; data on energy-density vs. powerdensity, namely the Ragone plots of state-of-the-art FSC appeared in recent literature [49,58,60,69-74] are also given in Fig. 4f, for comparison. Our model cell shows a volumetric energy density of 8.14 mWh cm-3 at a lower power density (31.7 mW cm-3) and 6.57 mWh cm-3 at a higher power density (370.7 mW cm-3). Both Ecell,V and Pcell,V are normalized v.s. whole cell volume. Table S3 summarizes data on the cell volumetric capacitance, the cell maximum power density, and the cell energy density at the optimized power density, acquired from typical literature published in recent years. Data normalized via whole cell (cathode, anode, and solid electrolyte) are denoted by a hash mark, data normalized via the volume of cathode and anode (without containing the electrolyte volume) are denoted by a star mark. Information regarding the active materials, electrolyte, and working potentials are also included in Table S3. Our cell gave 91.6 F cm-3 (cell volumetric capacitance), 463.8 mW cm-3 (cell max power density), and 12.72 mWh cm-3 at a power density of 39.63 mW cm-3, respectively. The volumetric capacitance and the energy density of our cell are the highest values among the MnO2-based all-solid-state FSCs. The max power density of our cell, on the other hand, is less favorable; this drawback can be overcome by using single-walled CNT for establishing the CNT-networks of the CNT/MnO2 hybridized shell. Noted here that a commercially available product of multi-walled CNT is used in this study for the formation of the CNT/MnO2 hybridized shells. Ruthenium oxide (RuO2) based cell showed excellent data on all the three functionalities [68], RuO2 is a very unique redox-active substance, it is highly electrically conductive yet highly toxic and very expensive. Three single cells are assembled in series (Fig. 5a and b) for widening the operating voltage; other three model cells are linked in parallel (Fig. 5c and d) for enhancing the working current. The three cells being connected in series exhibit a 2.4 V charge/discharge voltage window (Fig. 5a); discharge time is identical to that of a single cell (Fig. 5b). The parallelly linked three cells, on the other hand, yield tripled currents and tripled discharge times as did by the single cell; the working voltage window is identical to that of the single 17

cell (Fig. 5c and d). An integrated cell which is assembled by parallelly inking of two 3serial-linked cells is shown to be capable of powering a blue light-emitting diode (Fig. 5e), demonstrating the applicability in portable and wearable electronics systems.

a1200

b 3.0 2.5

300 0 -300 Single cell 3 cells connected in series

0.0

1.5 1.0

0.5

1.0 1.5 Potential (V)

2.0

0.0 0

2.5

d 1.0

Single cell 3 cells connected in parallel

3000

30

60

90 120 Time (s)

150

180

Single cell 3 cells connected in parallel

0.8

2000

Potential (V)

Current density (mA cm-3)

2.0

0.5 -600 -900

c

Single cell 3 cells connected in series

600

Potential (V)

Current density (mA cm-3)

900

1000 0

0.6 0.4

-1000

0.2

-2000

0.0 0.0

0.2

0.4 0.6 Potential (V)

0

0.8

100

200

300 400 Time (s)

500

600

e

Fig. 5. Assembly of high-performance FSC and their application. (a) Typical CV curves and (b) galvanostatic charge/discharge curves of one single FSC cell and three FSC cells connected in series. (c) Typical CV curves and (d) galvanostatic charge/discharge curves of one single FSC cell and three FSC cells connected in parallel. (e) Photograph of a lightemitting diode (LED) powered by a typical integrated FSC cell containing 6 single FSC. 18

4. Conclusions We demonstrated a simple yet scalable approach to fabricating high-energy-density fibershaped supercapacitors (FSC) to overcome the bottleneck which has long been encountered in power sources in flexible/portable electronics commercialisations. The formation of selfconductive pseudo-active domains, i.e., the CNT/MnO2 hybrids via electrochemical inserting of nano-sized MnO2 into densely interconnected CNT-networks is the key to achieve this goal. The thickness of the preliminary CNT/MnO2 hybridized unit (CNT/MnO2 hybridized thin layer) is around 300 nm; thicker yet seamless CNT/MnO2 hybridized active domains, thereby a larger mass loading of the redox active substance, is obtainable via an electrochemical layer/layer-building approach. Essential features of CNT-networks involved in the CNT/MnO2 hybridized active domains can be summarized as followings: i) bridging between current collectors (carbon fibers) and MnO2 domains for transfer of electrons, ii) templating shapes and sizes of MnO2 in to nano-sized CNT-networks, iii) linking between the neighbouring preliminary CNT/MnO2 hybrid units into seamless and thicker structures, and iv) enhancing mechanical flexibility and absorbing internal stresses while the electrode is being bended. CNT-networks can be visualized as nano-sized capillaries and MnO2 domains as nano-sized tissue cells. Electrons (energies) are rapidly delivered into MnO2 domains via CNT-networks; MnO2 domains are therefore being effectively activated. CNT-networks are a kind of nano-meshes, this unique meshing structure has prevented MnO2 possibly be growing up in to larger-sized domains during the electrochemical deposition [5,9,31,32,54]. Our optimized CNT/MnO2 hybridized domains with a thickness of 1.44 µm provided a volumetric capacitance of 1079.3 F cm-3, which reached about 1/6 of the theoretical value of the volumetric capacitance of MnO2. Commercially available carbon fibers (CF) were used as the macroscopic current collectors; commercially available multi-walled CNT of CVD products were used as the preliminary elements for building the CNT-networks. Despite using these commonalities of the starting materials, our CNT/MnO2@CF electrodes also the assembled cells provided the highest energy density among the MnO2-based FSC reported in previous literature. All-solid-state cells with PVA/LiCl as the electrolyte can be assembled in parallel and serial manners to widen the working windows of voltages and/or the working currents without diminishing the electrochemical performances of the individual cells. The excellent integrability together with the feasible scalability of our CNT/MnO2@CF-based FSC shows a firm step toward achieving the goal of commercialization of the power sources in the flexible 19

and portable electronics systems. CNT-networks play critical rules on determining the performance of the CNT/MnO2 hybridized electrodes; targeting to commercialization by using high-quality CNT, including the high-quality single-walled CNT, is undertaken, in this research group.

Acknowledgements This work is supported in part by grants from the Project of Saitama Prefectural IndustryAcademia Collaborative Development Project Subsidy. W.G. acknowledges support from Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. W.G. also acknowledges Mr. Shigeru Ohtsuka and Mr. Toshio Ito for their help in the measurement of the SEM and TEM images in the University of Tokyo.

References [1]

D. Yu, Q. Qian, L. Wei, W. Jiang, K. Goh, J. Wei, J. Zhang, Y. Chen, Chem. Soc. Rev. 44 (2015) 647-662.

[2]

L. Dong, C. Xu, Y. Li, Z. H. Huang, F. Kang, Q. H. Yang, X. Zhao, J. Mater. Chem. A 4 (2016) 4659-4685.

[3]

F. Meng, Q. Li, L. Zheng, Energy Storage Mater. 8 (2017) 85-109.

[4]

L. Li, Z. Wu, S. Yuan, X. B. Zhang, Energy Environ. Sci. 7 (2014) 2101-2122.

[5]

G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797-828.

[6]

J. R. Miller, P. Simon, Science 321 (2008) 651-652.

[7]

J. Chmiola, C. Largeot, P. L. Taberna, P. Simon, Y. Gogotsi, Science 328 (2010) 480483.

[8]

D. Yu, K. Goh, H. Wang, L. Wei, W. Jiang, Q. Zhang, L. Dai, Y. Chen, Nat. Nanotechnol. 9 (2014) 555-562.

[9]

C. Choi, J. A. Lee, A Y. Choi, Y. T. Kim, X. Leprό, M. D. Lima, R. H. Baughman, S. J. Kim, Adv. Mater. 26 (2014) 2059-2065.

[10] Y. Gogotsi, P. Simon, Science 334 (2011) 917-918. [11] X. Yang, C. Cheng, Y. Wang, L. Qiu, D. Li, Science 341 (2013) 534-537. [12] Q. Wang, J. Yan, Z. Fan, Energy Environ. Sci. 9 (2016) 729-762. 20

[13] S. Zhai, W. Jiang, L. Wei, H. E. Karahan, Y. Yuan, A. K. Ng, Y. Chen, Mater. Horiz. 2 (2015) 598-605. [14] K. Jost, C. R. Perez, J. K. McDonough, V. Presser, M. Heon, G. Dion, Y. Gogotsi, Energy Environ. Sci. 4 (2011) 5060-5067. [15] K. Jost, D. Stenger, C. R. Perez, J. K. McDonough, K. Lian, Y. Gogotsi, G. Dion, Energy Environ. Sci. 6 (2013) 2698-2705. [16] X. Chen, L. Qiu, J. Ren, G. Guan, H. Lin, Z. Zhang, P. Chen, Y. Wang, H. Peng, Adv. Mater. 25 (2013) 6436-6441. [17] D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P. L. Taberna, P. Simon, Nat. Nanotechnol. 5 (2010) 651-654. [18] Y. Meng, Y. Zhao, C. Hu, H. Cheng, Y. Hu, Z. Zhang, G. Shi, L. Qu, Adv. Mater. 25 (2013) 2326-2331. [19] Y. Li, K. Sheng, W. Yuan, G. Shi, Chem. Commun. 49 (2013) 291-293. [20] L. Liu, Y. Yu, C. Yan, K. Li, Z. Zheng, Nat. Commun. 6 (2015) 7260. [21] J. Carretero-González, E. Castillo-Martínez, M. Dias-Lima, M. Acik, D. M. Rogers, J. Sovich, C. S. Haines, X. Lepró, M. Kozlov, A. Zhakidov, Y. Chabal, R. H. Baughman, Adv. Mater. 24 (2012) 5695-5701. [22] H. Sun, X. You, J. Deng, X. Chen, Z. Yang, J. Ren, H. Peng, Adv. Mater. 26 (2014) 2868-2873. [23] L. Kou, T. Huang, B. Zhang, Y. Han, X. Zhao, K. Gopalsamy, H. Sun, C. Gao, Nat. Commun. 5 (2014) 3754. [24] Y. Ma, P. Li, J. W. Sedloff, X. Zhang, H. Zhang and J. Liu, ACS Nano 9 (2015) 13521359. [25] W. Jiang, S. Zhai, Q. Qian, Y. Yuan, H. E. Karahan, L. Wei, K. Goh, A. K. Ng, J. Wei, Y. Chen, Energy Environ. Sci. 9 (2016) 611-622. [26] W. Wei, X. Cui, W. Chen, D. G. Ivey, Chem. Soc. Rev. 40 (2011) 1697-1721. [27] M. Toupin, T. Brousse, D. Bélanger, Chem. Mater. 16 (2004) 3184-3190. [28] H. Y. Lee, J. B. Goodenough, J. Solid State Chem. 144 (1999) 220-223. [29] H. Y. Lee, V. Manivannan, J. B. Goodenough, Comptes Rendus Chimie 2 (1999) 565577. [30] W. Gong, B. Fugetsu, Z. Wang, I. Sakata, L. Su, X. Zhang, H. Ogata, M. Li, C. Wang, J. Li, J. Ortiz-Medina, M. Terrones, M. Endo, Commun. Chem. 1 (2018) 16. [31] Y.-T. Wu, C.-C. Hu, J. Electrochem. Soc. 151 (2004) A2060-A2066.

21

[32] C. Y. Lee, H. M. Tsai, H. J. Chuang, S. Y. Li, P. Lin, T. Y. Tseng, J. Electrochem. Soc. 152 (2005) A716-A720. [33] Z. Zhang, F. Xiao, L. Qian, J. Xiao, S. Wang, Y. Liu, Adv. Energy Mater. 4 (2014) 1400064-1400072. [34] B. Anothumakkool, S. Kurungot, Chem. Commun. 50 (2014) 7188-7190. [35] Z. Su, C. Yang, B. Xie, Z. Lin, Z. Zhang, J. Liu, B. Li, F. Kang, C. P. Wong, Energy Environ. Sci. 7 (2014) 2652-2659. [36] Z. Qiao, X. Yang, S. Yang, L. Zhang, B. Cao, Chem. Commun. 52 (2016) 7998-8001. [37] K. R. Prasad, N. Miura, Electrochem. Commun. 6 (2004) 1004-1008. [38] Y.-S. Chen, C.-C. Hu, Electrochem. Solid-State Lett. 6 (2003) A210-A213. [39] M. Nakayama, A. Tanaka, S. Konishi, K. Ogura, J. Mater. Res. 19 (2004) 1509-1515. [40] K.-Q. Ding, J. Chin. Chem. Soc. 55 (2008) 543-549. [41] J. Wen, X. Ruan, Z. Zhou, J. Phys. Chem. Solids 70 (2009) 816-820. [42] M. Nakayama, A. Tanaka, Y. Sato, T. Tonosaki, K. Ogura, Langmuir 21 (2005) 59075913. [43] H. Kim, B. N. Popov, J. Electrochem. Soc. 150 (2003) D56-D62. [44] Y. Li, H. Xie, Ionics 16 (2010) 21-25. [45] X. Pang, Z.-Q. Ma, L. Zuo, Acta Phys. -Chim. Sin. 25 (2009) 2433-2437. [46] C. Choi, K. M. Kim, K. J. Kim, X. Leprό, G. M. Spinks, R. H. Baughman, S. J. Kim, Nat. Commun. 7 (2016) 13811. [47] P. Shi, L. Li, L. Hua, Q. Qian, P. Wang, J. Zhou, G. Sun, W. Huang, ACS Nano 11 (2017) 444-452. [48] J. Ren, L. Li, C. Chen, X. Chen, Z. Cai, L. Qiu, Y. Wang, X. Zhu, H. Peng, Adv. Mater. 25 (2013) 1155-1159. [49] P. Xu, B. Wei, Z. Cao, J. Zheng, K. Gong, F. Li, J. Yu, Q. Li, W. Lu, J. H. Byun, B. S. Kim, Y. Yan, T. W. Chou, ACS Nano 9 (2015) 6088-6096. [50] C. Choi, H. J. Sim, G. M. Spinks, X. Leprό, R. H. Baughman, S. J. Kim, Adv. Energy Mater. 6 (2016) 1502119-1502126. [51] X. Cheng, J. Zhang, J. Ren, N. Liu, P. Chen, Y. Zhang, J. Deng, Y. Wang, H. Peng, J. Phys. Chem. C 120 (2016) 9685-9691. [52] J. Yu, W. Lu, J. P. Smith, K. S. Booksh, L. Meng, Y. Huang, Q. Li, J.-H. Byun, Y. Oh, Y. Yan, T.-W. Chou, Adv. Energy Mater. 7 (2017) 1600976-1600983. [53] Q. Zhang, J. Sun, Z. Pan, J. Zhang, J. Zhao, X. Wang, C. Zhang, Y. Yao, W. Lu, Q. Li, Y. Zhang, Z. Zhang, Nano Energy 39 (2017) 219-228. 22

[54] Z. Lu, Y. Chao, Y. Ge, J. Foroughi, Y. Zhao, C. Wang, H. Long, G. G. Wallace, Nanoscale 9 (2017) 5063-5071. [55] M. Li, M. Zu, J. Yu, H. Cheng, Q. Li, Small 13 (2017) 1602994-1603000. [56] Z. Zhou, Q. Zhang, J. Sun, B. He, J. Guo, Q. Li, C. Li, L. Xie, Y. Yao, ACS Nano 12 (2018) 9333-9341. [57] Q. Li, Q. Zhang, J. Sun, C. Liu, J. Guo, B. He, Z. Zhou, P. Man, C. Li, L. Xie, Y. Yao, Adv. Sci. 6 (2019) 1801379-1801388. [58] Z. Yu, J. Thomas, Adv. Mater. 26 (2014) 4279-4285. [59] Y. Huang, H. Hu, Y. Huang, M. Zhu, W. Meng, C. Liu, Z. Pei, C. Hao, Z. Wang, C. Zhi, ACS Nano 9 (2015) 4766-4775. [60] J. Zhang, X. Zhao, Z. Huang, T. Xu, Q. Zhang, Carbon 107 (2016) 844-851. [61] C. Shen, Y. Xie, M. Sanghadasa, Y. Tang, L. Lu, L. Lin, ACS Appl. Mater. Interfaces 9 (2017) 39391-39398. [62] A. Rafique, A. Massa, M. Fontana, S. Bianco, A. Chiodoni, C. F. Pirri, S. Hernández, A. Lamberti, ACS Appl. Mater. Interfaces 9 (2017) 28386-28393. [63] Q. Chen, Y. Meng, C. Hu, Y. Zhao, H. Shao, N. Chen, L. Qu, J. Power Sources 247 (2014) 32-39. [64] C. Choi, S. H. Kim, H. J. Sim, J. A. Lee, A. Y. Choi, Y. T. Kim, X. Leprό, G. M. Spinks, R. H. Baughman, S. J. Kim, Sci. Rep. 5 (2015) 9387. [65] C. Choi, J. H. Kim, H. J. Sim, J. Di, R. H. Baughman, S. J. Kim, Adv. Energy Mater. 7 (2017) 1602021-1602027. [66] N. Yu, H. Yin, W. Zhang, Y. Liu, Z. Tang, M. Q. Zhu, Adv. Energy Mater. 6 (2016) 1501458-1501466. [67] S. L. Chou, J. Z. Wang, S. Y. Chew, H. K. Liu, S. X. Dou, Electrochem. Commun. 10 (2008) 1724-1727. [68] S. Zhai, C. Wang, H. E. Karahan, Y. Wang, X. Chen, X. Sui, Q. Huang, X. Liao, X. Wang, Y. Chen, Small 14 (2018) 1800582-1800594. [69] D. Yu, K. Goh, Q. Zhang, L. Wei, H. Wang, W. Jiang, Y. Chen, Adv. Mater. 26 (2014) 6790-6797. [70] S. Zhai, H. E. Karahan, L. Wei, X. Chen, Z. Zhou, X. Wang, Y. Chen, Energy Storage Mater. 9 (2017) 221-228. [71] H. Jin, L. Zhou, C. L. Mak, H. Huang, W. M. Tang, H. L. W. Chan, Nano Energy 11 (2015) 662-670. [72] Z. Zhang, F. Xiao, S. Wang, J. Mater. Chem. A 3 (2015) 11215-11223. 23

[73] D. Yu, S. Zhai, W. Jiang, K. Goh, L. Wei, X. Chen, R. Jiang, Y. Chen, Adv. Mater. 27 (2015) 4895-4901. [74] V. T. Le, H. Kim, A. Ghosh, J. Kim, J. Chang, Q. A. Vu, D. T. Pham, J. H. Lee, S. W. Kim, Y. H. Lee, ACS Nano 7 (2013) 5940-5947.

24