1

Supplementary Information

Realization of High Performance Flexible Wire Supercapacitors Based on 3-Dimensional NiCo2O4/Ni Fibers

Ananthakumar Ramadoss1, Kyeong-Nam Kang2, Hyo-Jin Ahn2, Sun-I Kim2,

Seung-Tak Ryu*1, and Ji-Hyun Jang *2

1Mixed Signal Integrated Circuit Lab, School of Electrical Engineering, Korea Advanced

Institute of Science and Technology, Daejeon 305-701, Republic of Korea

2School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea

…………………………………………………………………………………………

*Corresponding author: Fax: 82-52-217-3008; Tel: 82-52-217-2922

Email: clau@unist.ac.kr (Ji-Hyun Jang) & stryu@kaist.ac.kr (Seung-Tak Ryu)

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2016

2

A. Materials

All the chemicals used in our experiments were of analytical grade and were directly

used without further purification. The nickel (Ni) wire (0.5 mm), nickel chloride (NiCl4),

ammonium chloride (NH4Cl), cobalt nitrate (Co(NO3)2), nickel nitrate (Ni(NO3)2), polyvinyl

alcohol (PVA), titanium(IV) chloride (TiCl4), fluorine doped tin oxide substrate, chloroplatinic

acid (H2PtCl6), isopropyl alcohol, 1-hexyl-2,3-dimethylimidazolium iodide (99.9%, C-tri),

iodine, 4-tert-buthylpyridine, and lithium iodide in acetonitrile were obtained from Sigma

Aldrich, Korea. Potassium hydroxide (KOH) and hydrochloric acid (HCl) were purchased from

Samchun Pure Chemical Co. Ltd. Korea. TiO2 paste purchased from ENB Korea Co. Ltd. Ultra-

pure (De-ionized) water was used for all experiments.

3

B. Experimental section

Preparation of 3D-Ni on Ni-wire

The 3D-Ni on Ni-wire was electrodeposited by a hydrogen bubble template method 1,

2. Briefly, the Ni-wire was first etched in HCl to remove the oxide layer followed by complete

washing with DI water and ethanol, and the Ni-wire was then used as the cathode for

constructing the 3D-Ni. A platinum mesh was used as an anode (current collector), keeping the

distance between the two electrodes at 1 cm. The 3D-Ni was then electrodeposited using

hydrogen bubbles as templates on the nickel wire at a constant current of 2.5 A for 60 s using

a regulated DC power supply LP-3010 with the electrolyte containing 0.1 M NiCl2 and 2 M

NH4Cl at a pH of 3.5. After the deposition, the 3D-Ni was rinsed several times in DI water to

remove the electrolyte solution from inside the pores and then dried at 50 °C for 12 h.

Preparation of 3D-NiCo2O4/Ni

The NiCo2O4 nanostructures on the 3D-Ni/Ni wire were prepared by the

electrodeposition of bimetallic (Ni, Co) hydroxide followed by thermal transformation into

spinel NiCo2O4. The electrodeposition was performed in a standard three electrode system

consisting of 3D-Ni as the working electrode, a platinum mesh as the counter electrode and a

Ag/AgCl as the reference electrode at room temperature. The bimetallic hydroxide was

electrodeposited on the 3D-Ni/Ni-wire in an aqueous solution containing 1:2 molar ratio of

0.02 M nickel nitrate (Ni(NO3)2.6H2O) and 0.04 M cobalt nitrate (Co(NO3)2.6H2O) at a

constant potential of -1 V for 3, 5 and 10 min using a VMP3 biologic electrochemical

workstation. After electrodeposition, the sample was rinsed several times in DI water and

ethanol and dried in air. Finally, the sample was calcined at 300 °C for 2 h at a ramping rate of

1 °C min-1 in a quartz tube furnace to get the NiCo2O4 nanostructure. The as-prepared 3-, 5-,

and 10-min 3D porous flower-like NiCo2O4 nanostructure/3D-Ni samples were named 3D-

4

NCO/Ni-1, 3D-NCO/Ni-2, and 3D-NCO/Ni-3, respectively.

Figure. S1 Digital image of bimetallic hydroxide deposited at 3, 5, and 10 min

electrodeposition time.

Material characterization

Surface morphology and elemental composition of the as-fabricated electrodes were

examined by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800)

equipped with energy dispersive X-ray spectroscopy (EDS). The phase structure and purity of

the samples were investigated by X-ray diffraction (XRD, Rigaku D/Max-2500/PC) and

Raman spectroscopy (WITec). The surface functionalities and composition of the samples were

analyzed by X-ray photoelectron spectroscopy (K-alpha; Escalab 250Xi model, Thermo Fisher,

UK).

5

Assembly of the flexible fibre symmetric supercapacitor device

The PVA/KOH gel electrolyte was prepared as follows. First, 10 g of PVA was

dissolved in 100 ml of DI water at 90 °C under stirring and was kept until the solution became

clear. Then, 5.6 g of KOH was added in to the solution followed by rigorous stirring at 90 °C

until clear gel was formed. Prior to assembling, the fabricated 3D-NCO/Ni electrodes were

immersed into PVA/KOH gel electrolyte for 5 min and then allowed to solidify at room

temperature. The, the two electrodes were closely and parallel assembled onto a PET substrate

with a separation distance of ~1mm and dried at room temperature overnight to remove excess

water in the electrolyte. After the PVA/KOH gel was solidified, the solid state supercapacitor

was sealed with the tape to prevent it absorbing moisture.

Fabrication of dye-sensitized solar cells (DSSCs)

The fabrication of the DSSCs was performed as follows. First, FTO glass was cleaned

by sonicating in ethanol and acetone for 10 min and O2 plasma treatment for 20 min. TiCl4

treatment was conducted by placing the FTO glass in a 40 mM TiCl4 solution for 30 min at 70

oC. 12 μm of TiO2 paste was deposited on the TiCl4 treated FTO glass by the doctor blade

process, and the sample was subsequently sintered at 550 oC for 60 min. Dye was coated on

the prepared TiO2 paste by dipping in 0.5 mM of N-719 solution of tert-buthanol/acetonitrile

(1:1 vol.) for 12 hours. The Pt counter electrode (CE) was prepared by spin-coating (2000 rpm,

2 times) 30mM of H2PtCl6 solution in IPA on drilled FTO glass and sintering at 450 oC for 30

min. The working electrode and CE were assembled using 60 μm of Surlyn, and an electrolyte

was added through a pre-drilled hole. The composition of the electrolyte was 0.5 M 1-hexyl-

2,3-dimethylimidazolium iodide (99.9%, C-tri), 0.02 M iodine, 0.5 M 4-tert-buthylpyridine,

and 0.05 M lithium iodide in acetonitrile.

6

Electrochemical measurements

The electrochemical properties of the as-prepared samples were carried out in a

standard three electrode system, consisting of the fabricated sample as the working electrode,

platinum mesh as the counter electrode and Hg/HgO as the reference electrode in a 2 M KOH

aqueous solution. As-fabricated solid state symmetric fiber supercapacitor was investigated as

a two electrode system using solid electrolyte, where the 3D-NCO/Ni electrode was used as a

positive and negative electrode. The cyclic voltammetry, galvanostatic charge/discharge and

chronoamperometry measurements were performed using VMP3 biologic electrochemical

workstation. Electrochemical impedance spectroscopy (EIS) measurements were carried out in

the frequency range of 0.1 Hz to 100 kHz using VersaSTAT3 (Princeton Applied Research).

The self-charge/discharge measurement was performed using VersaSTAT3 (Princeton Applied

Research). J-V characterization was carried out by a computerized digital multimeter under 1

Sun illumination (1.5 AM) with an Oriel SOL3A solar simulator.

7

C. Structural and compositional analysis

Figure. S2 XRD spectra of the Ni wire, 3D-Ni and 3D-NiCo2O4/Ni.

The crystal structure of the as-prepared samples was examined using XRD. Figure. S2

shows the XRD pattern of Ni-wire, 3D-Ni and 3D-NiCo2O4/Ni. As shown in Figure. S2 the

three strong diffraction peaks observed at 44.3°, 52.0° and 76.3° belongs to the Ni substrate

(JCPDS file no: 04-0850). The other well-defined diffraction peaks (3D-NiCo2O4/Ni) at the

diffraction angles of 37.1°, 59.1° and 64.9° correspond to the (311), (511) and (440) plane

reflections of spinel NiCo2O4 crystalline structure (JCPDS file no: 20-0781). There were no

other peaks observed which indicates the high purity of the as-prepared samples.

8

Figure. S3 (a) Raman spectrum of 3D-NiCo2O4/3D-Ni. (b-d) XPS spectra of Ni 2p, Co 2p and O 1s for the 3D-NiCo2O4/Ni.

To confirm the structure of the as-prepared sample, an investigation was performed by

Raman spectroscopy. The Raman peaks located at 182, 452, 499 and 643 cm-1 in Figure S3a

correspond to the F2g, Eg, F2g and A1g modes of NiCo2O4, respectively 3, 4. The detailed chemical

composition and oxidation state of 3D-NiCo2O4 were studied by XPS measurements (Figure

S3b-S3d). The XPS core-level spectra of the sample were fitted using the Gaussian/Lorentzian

fitting method. As seen in Figure S3b, the Ni 2p peak was split into 2p3/2 (872.0 eV) and 2p½

(855.1 eV) according to the spin orbit coupling and two shakeup satellites (indicated as “Sat.”)5-

7. The Ni 2p spectrum was split into two spin orbit doublets, characteristics of Ni2+ (853.4 and

870.9 eV) and Ni3+ (855.1 and 872.5 eV) bound to oxygen. The Co 2p spectrum (Figure S3c)

exhibited the presence of two clearly resolved spectral lines at 779.2 and 794.4 eV

9

corresponding to the Co 2p3/2 and Co 2p½ spin orbit components and two shakeup Sat. 5, 6. The

Co 2p spectrum was fitted with two spin-orbit doublets. The first doublet at 779.1 and 794.6

eV corresponds to the Co3+ and the second doublet at 780.7 and 796.8 eV is assigned to the

Co2+ state. The high resolution O 1s spectrum could be deconvoluted into four components as

shown in Figure S3d. The peaks located at 529.1 eV (O1) and 530.4 eV (O2) correspond to

metal-oxygen bonds and hydroxyl groups (OH-), respectively 8, 9. The remaining peaks (O3 &

O4) at 531.3 and 532.2 eV can be ascribed to oxygen deficiencies related to defects in the

materials and physi- and chemi-absorbed water molecules 9, 10. These results, which are in good

agreement with previous reports, confirm the successful fabrication of NiCo2O4.

10

D. Elemental analysis

Figure. S4 FE-SEM (a), EDS spectrum (b) and elemental mapping (c-e) images of three-dimensional porous flower-like Ni/ NiCo2O4 nanostructure film.

Energy dispersive X-ray spectroscopy (EDS) analysis was used to investigate the

elemental composition and distribution of each element present in the 3D-Ni/NiCo2O4/Ni (3D-

NCO/Ni-2). Figure S4a, S4b and S4c-S4e shows the FE-SEM image, EDS spectrum and the

corresponding elemental mapping images of 3D-NiCo2O4/Ni. The EDS spectrum (Figure S4b)

exhibits the characteristic peaks of Ni, Co and O elements present in the sample, which

confirms the formation of NiCo2O4. Further, the elemental mapping images (Figure S4c-e) of

3D-NiCo2O4/Ni clearly show the uniform distribution of Ni, CO, and O elements within the

NiCo2O4 structure.

11

E. Morphological analysis

Figure. S5 FE-SEM images of three-dimensional porous flower-like Ni/NiCo2O4 nanostructures, after deposition for 3 min (a-b) and 10 min (c-d).

12

Figure S6. FE-SEM images of 3D- NiCo2O4/Ni-wire under a bending condition of 90°.

13

F. Electrochemical performance of 3D-NCO/Ni-1, 3D-NCO/Ni-2 and 3D-NCO/Ni-3 electrodes

Figure. S7 CV curves of 3D-NCO/Ni-1 (a), 3D-NCO/Ni-2 (b) and 3D-NCO/Ni-3 (c) electrodes at various scan rates. GCD curves of 3D-NCO/Ni-1 (d), 3D-NCO/Ni-2 (e) and 3D-NCO/Ni-3 (f) electrodes at different current.

To further evaluate the high-power characteristics of the 3D-NCO/Ni electrodes, a CV

study was performed at various scan rates. All the CV curves show a similar shape, and the two

pairs of redox peaks occurred during the cathodic and anodic process are visible in each CV,

14

indicating the pseudocapacitive properties of the electrode (Faradaic reaction: Ni and Co). The

shape of the CV curves was not significantly changed but the peak current increased with

increasing scan rate, indicating that the 3D-NCO/Ni electrodes are beneficial to fast faradaic

reaction and high rate response at higher scan rates. Additionally, with the sweep rate increasing

from 5 to 80 mV s-1, the position of the anodic and cathodic peaks slightly shifted towards

positive and negative potentials, respectively. This phenomenon may be due to the high

electron hopping resistance (internal resistance of the electrode) or the mass-transport

limitation of ions from the electrolyte to the electrode surface in order to satisfy the electronic

neutralization at the electrode during the faradaic reaction 11. But the potential shift of the

anodic (ca. 70 mV) and cathodic (ca. 50 mV) peaks were smaller, suggesting that 3D-NiCo2O4

has low polarization, resulting from the high electrical conductivity (low resistance) of the 3D-

Ni backbone and the good contact between the electroactive materials and the 3D-Ni current

collector. This can be further confirmed through electrochemical impedance spectroscopy (EIS)

(Figure S8).

15

G. Calculation and analysis

Three-electrode analysis

The volumetric, gravimetric and areal capacitance of the as-fabricated electrode

materials were calculated from the cyclic voltammetry and galvanostatic charge/discharge

curves using the following Equations 12-16:

Cyclic Voltammetry:

VS

idVC

2 …………….. (1)

Galvanostatic charge/discharge:

)/( dtdv

IC …………….. (2)

v

CC v …………….. (3)

m

CC g …………….. (4)

A

CC a …………….. (5)

Where C is the capacitance (F), Cv is the volumetric capacitance (F cm-3), Cg is the gravimetric

capacitance (F g-1), Ca is the areal capacitance (F cm-2), S is the scan rate (mV s-1), V is the

voltage window (V), I is the discharge current (A), dv/dt is the average slope of the discharge

curve (V s-1), v is the total volume of the electrode material (cm3), m is the mass of the

electroactive material (g), A is the area of the electrode material (cm2), and ∫idV is the integral

area of the CV curve (A).

16

Device analysis

The volumetric (Cv) and gravimetric (Cg) capacitance was calculated from the cyclic

voltammetry and galvanostatic charge/discharge curves using the equation below 12-16:

Cyclic Voltammetry:

VS

idVCD

2

…………….. (6)

Galvanostatic charge/discharge:

)/( dtdv

IC D …………….. (7)

v

CC D

v …………….. (8)

m

CC D

g …………….. (9)

For single electrode capacitance:

)(4 gvs CC …………….. (10)

Where CD is the device capacitance (F), Cs is the single electrode capacitance (F), v is the total

volume of the device (cm3), and m is the total mass of the device (g)

Energy and power density

Volumetric as well as gravimetric energy density (E) and power density (P) of the

device were calculated from the discharge curves as the follows 16-18;

vm

VdtIE …………….. (11)

t

EP …………….. (12)

Where I is the discharge current (A), ∫Vdt is the integral area of the discharge curve (V s), m is

the total mass of the electroactive material in both electrodes (g), v is the total volume of the

17

device (cm3), and t is the discharge time (s).

H. Electrochemical impedance spectroscopy

Figure. S8 (a) Nyquist plots of Ni-wire and 3D-Ni electrodes. (b) Nyquist plots of 3D-NCO/Ni-1, 3D-NCO/Ni-2 and 3D-NCO/Ni-3 electrodes. The inset is the high frequency region of the Nyquist plots.

To further investigate the fundamental behavior of the fabricated electrodes,

electrochemical impedance spectroscopy (EIS) measurements were performed. Figure S8a

and S6b show the Nyquist plots of the Ni-wire, 3D-Ni, 3D-NCO/Ni-1, 3D-NCO/Ni-2 and 3D-

NCO/Ni-3 electrodes. The inset shows the amplified part of the high frequency region. The

Nyquist plots of Ni and 3D-Ni display (Figure S8a) the negligible charge transferred resistance,

which confirms the good electrical conductivity of the current collectors.

The Nyquist plots of the 3D-NCO/Ni-1, 3D-NCO/Ni-2 and 3D-NCO/Ni-3 electrodes

(Figure S8b) are composed of two distinct parts, a semi-circle arc at the high-frequency region

followed by the linear line at the low frequency region. In the high-frequency region, the

intercept at the Z’-axis of the Nyquist plots indicates the ohmic resistance of the electroactive

material, the electrolyte (Rs) and the electroactive material/ current collector interface 19-21. The

ohmic resistance values of the 3D-NCO/Ni-1, 3D-NCO/Ni-2 and 3D-NCO/Ni-3 electrodes

18

were found to be 0.31, 0.30 and 0.31 Ω, respectively. The semicircle arc in the mid-frequency

region indicates the charge transfer resistance at the electrode/electrolyte interface. The charge

transfer resistances of the 3D-NCO/Ni-1, 3D-NCO/Ni-2 and 3D-NCO/Ni-3 electrodes are

relatively small.

The lower ohmic and charge transfer resistance values of the as-fabricated electrodes

confirmed their good electrical conductivity, which allows fast electron transfer between the

current collector and the electroactive sites to assist the Faradaic reactions. The inclined portion

of the curve (about 45°) in the low frequency region is attributed to the frequency dependence

of ion diffusion at the electrolyte/electrode interface (Warburg impedance)19-21. In addition, the

nearly vertical line in the low frequency region indicates the higher capacitive behavior of the

electrodes.

19

I. Gravimetric and areal capacitance

Figure. S9 Gravimetric (a) and areal (b) capacitances of the 3D-NCO/Ni-1, 3D-NCO/Ni-2and 3D-NCO/Ni-3 electrodes as a function of scan rates. Gravimetric (c) and areal (d) capacitance as a function of current.

20

J. Volumetric and gravimetric capacitance of fiber supercapacitor

Figure. S10 (a) Volumetric and gravimetric capacitances of solid state symmetric device as a function of scan rate. (b) Volumetric and gravimetric capacitances at different current.

21

K. J-V characterization of the fabricated DSSC

Figure. S11 (a) Schematic diagram of fabricated DSSC. (b) Current density/voltage curve of the four fabricated DSSCs connected in series under 1 sun irradiation; the inset shows the digital image of the four DSSCs in series.

22

L. Leakage current and self-discharge of the devices.

Figure. S12 Leakage current (a) and self-discharge (b) curve of the three series-wound fiber supercapacitors.

The leakage current of the serially connected fiber supercapacitors was measured by

keeping the device at a constant voltage of 3 V, after being charged at a constant current of 10

mA. As shown in Figure S12a the leakage current falls rapidly from 0.4 mA to 0.1 mA after

520 s and then beyond 10000 s gradually becomes stable at ~70 µA for a long time. The small

leakage current indicates there are few impurities in the electrode and electrolyte materials in

the fabricated devices.

Furthermore, the open circuit voltage of the serially connected supercapacitors was also

measured after being charged by serially connected DSSCs. Figure S12b presents the typical

self-discharge curve of the serially connected supercapacitors as a function of time courses.

From that curve, we can see that the SC devices exhibited an open circuit voltage of ~1.5 V

after 30 min, which could be maintained for 1 h, which indicates excellent energy storage

performance.

23

References:

1. X. H. Xia, J. P. Tu, Y. Q. Zhang, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, The

Journal of Physical Chemistry C, 2011, 115, 22662-22668.

2. H. Jiang, Y. Guo, T. Wang, P.-L. Zhu, S. Yu, Y. Yu, X.-Z. Fu, R. Sun and C.-P. Wong,

RSC Advances, 2015, 5, 12931-12936.

3. Z.-Q. Liu, K. Xiao, Q.-Z. Xu, N. Li, Y.-Z. Su, H.-J. Wang and S. Chen, RSC Advances,

2013, 3, 4372-4380.

4. W. Li, L. Xin, X. Xu, Q. Liu, M. Zhang, S. Ding, M. Zhao and X. Lou, Scientific Reports,

2015, 5, 9277.

5. Y. Wang, K. Cheng, D. Cao, F. Yang, P. Yan, W. Zhang and G. Wang, Fuel Cells, 2015,

15, 298-305.

6. B. Cui, H. Lin, Y.-z. Liu, J.-b. Li, P. Sun, X.-c. Zhao and C.-j. Liu, The Journal of

Physical Chemistry C, 2009, 113, 14083-14087.

7. V. H. Nguyen and J.-J. Shim, Journal of Power Sources, 2015, 273, 110-117.

8. J. G. Kim, D. L. Pugmire, D. Battaglia and M. A. Langell, Applied Surface Science,

2000, 165, 70-84.

9. J. F. Marco, J. R. Gancedo, M. Gracia, J. L. Gautier, E. Ríos and F. J. Berry, Journal of

Solid State Chemistry, 2000, 153, 74-81.

10. V. M. Jiménez, A. Fernández, J. P. Espinós and A. R. González-Elipe, Journal of

Electron Spectroscopy and Related Phenomena, 1995, 71, 61-71.

11. J. W. Lee, T. Ahn, D. Soundararajan, J. M. Ko and J.-D. Kim, Chemical

Communications, 2011, 47, 6305-6307.

12. Y. Song, X. Cai, X. Xu and X.-X. Liu, Journal of Materials Chemistry A, 2015, 3,

14712-14720.

24

13. A. Ramadoss, B. Saravanakumar, S. W. Lee, Y.-S. Kim, S. J. Kim and Z. L. Wang, ACS

Nano, 2015, 9, 4337-4345.

14. A. Ramadoss, B. Saravanakumar and S. J. Kim, Nano Energy, 2015, 15, 587-597.

15. H. Jin, L. Zhou, C. L. Mak, H. Huang, W. M. Tang and H. L. Wa Chan, Journal of

Materials Chemistry A, 2015, 3, 15633-15641.

16. S. T. Senthilkumar, R. K. Selvan, Y. S. Lee and J. S. Melo, Journal of Materials

Chemistry A, 2013, 1, 1086-1095.

17. B. Akinwolemiwa, C. Peng and G. Z. Chen, Journal of The Electrochemical Society,

2015, 162, A5054-A5059.

18. J. Y. Kim and I. J. Chung, Journal of The Electrochemical Society, 2002, 149, A1376-

A1380.

19. J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang and F. Wei, Carbon, 2010, 48, 3825-3833.

20. W. Zhang, H. Lin, Z. Lin, J. Yin, H. Lu, D. Liu and M. Zhao, ChemSusChem, 2015, 8,

2114-2122.

21. M. Liu, W. W. Tjiu, J. Pan, C. Zhang, W. Gao and T. Liu, Nanoscale, 2014, 6, 4233-

4242.