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Electrochemistry Communications 9 (2007) 2315–2319

Potentiostatically deposited nanostructured a-Co(OH)2:A high performance electrode material for redox-capacitors

Vinay Gupta a,b,*, Teruki Kusahara a, Hiroshi Toyama a, Shubhra Gupta a, Norio Miura a

a Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japanb Japan Science and Technology Agency, Kawaguchi-shi, Saitama 332-0012, Japan

Received 5 June 2007; received in revised form 22 June 2007; accepted 27 June 2007Available online 4 July 2007

Abstract

A high specific capacitance was obtained for a-Co(OH)2 potentiostatically deposited onto a stainless-steel electrode in 0.1 MCo(NO3)2 electrolyte at �1.0 V vs. Ag/AgCl. The structure and surface morphology of the obtained a-Co(OH)2 were studied by usingX-ray diffraction analysis and scanning electron microscopy. A network of nanolayered a-Co(OH)2 sheets was obtained; the averagethickness of individual a-Co(OH)2 sheets was 10 nm, and the thickness of the deposit was several micrometers. The capacitive charac-teristics of the a-Co(OH)2 electrodes were investigated by means of cyclic voltammetry and constant current charge–discharge cycling in1 M KOH electrolyte. A specific capacitance of 860 F g�1 was obtained for a 0.8 mg cm�2 a-Co(OH)2 deposit. The specific capacitancedid not decrease significantly for the active mass loading range of 0.1–0.8 mg cm�2 due its layered structure, which allowed easy pene-tration of electrolyte and effective utilization of electrode material even at a higher mass. This opens up the possibility of using such mate-rials in supercapacitor applications.� 2007 Elsevier B.V. All rights reserved.

Keywords: Cobalt hydroxide; Potentiostatic deposition; Capacitance; Supercapacitor

1. Introduction

The development of high performance power sourcessuch as supercapacitors has become an urgent requirementin recent years due to the unprecedented growth of mobiletechnology. Supercapacitors have the unique ability to pro-vide higher power and longer cyclic life than batteries fornumerous applications such as power sources in next-gen-eration electrical vehicles, as well as mobile electronicdevices [1–3]. Therefore, a lot of research work has beendone in the past few years to improve the performance ofsupercapacitors [4–11]. In a supercapacitor, energy isderived from the stored charge in the active electrode mate-rial. This charge is stored either in the form of an electrical

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.06.041

* Corresponding author. Address: Art, Science and Technology Centerfor Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan. Tel./fax: +81 92 583 7886.

E-mail address: vinay@astec.kyushu-u.ac.jp (V. Gupta).

double layer capacitance or in a form associatedwith a Faradaic process involving redox reactions (redoxcapacitance).

Electrical double layer capacitors (EDLCs) have lowspecific capacitances, because their charge storage is mainlylimited by the available active surface area. In this regard,activated carbons are known to have very high surfaceareas (up to �2500 m2 g�1), but low specific capacitances(up to �280 F g�1) in aqueous electrolytes [8]. Redox-capacitors show higher capacitances than EDLCs due toredox reactions involving the exchange of protons, withless emphasis on surface area, which results in higher utili-zation of electrode material. The conducting polymers andmetal oxides come under this category. However, conduct-ing polymer-based materials also have some disadvantages,which include (i) lower density as compared to metal oxi-des, which can result in a lower volumetric capacitanceand (ii) rapid decreases in specific capacitance at high ratesof charge and discharge, due to destabilization of the

α-Co(OH)2

010 20 30 40 50 60 70 80

Inte

nsit

y / a

.u.

(001)

(002)

(100)

(110)

2θ /° (CuKα)

Fig. 1. X-ray diffraction patterns of cobalt hydroxide depositedpotentiostatically.

2316 V. Gupta et al. / Electrochemistry Communications 9 (2007) 2315–2319

polymer backbone, so that stability may be an issue formany repeated redox processes [13,14].

Therefore, metal oxides (e.g. oxides of Mn, Fe, Co, Ni,Cr, In, Sn, Mo, V etc.) have long been considered as themost promising materials for supercapacitors [12–21].Moreover, metal oxides have higher densities than carbonsor polymers. This is advantageous in terms of volumetricspecific capacitance. Initially, RuO2 generated great inter-est due to its high specific capacitance [13,14], but it has lesspossibility to be commercialized in most applications dueto its high cost. Therefore, the development of alternativelow cost, high performance electrode materials has beenone of the most active areas of electrochemistry duringthe last few years.

Among several possible candidate metal oxides, cobalthydroxide materials are of great interest due to their lay-ered structure with large interlayer spacing [22,23]. A fewrecent studies of the capacitive properties of cobalt hydrox-ide showed specific capacitances in the range of 200–341 F g�1 [24–26]. However, these studies were performedon cobalt hydroxide prepared by precipitation methods.So far, there is no report on the capacitive characteristicsof cobalt hydroxide prepared by electrochemical methods.Here, for the first time, we report the capacitive character-istics of a-Co(OH)2 synthesized by potentiostatic deposi-tion from Co(NO3)2 electrolyte. In this case, a-Co(OH)2

nano-sheets of 10 nm thickness were obtained. We demon-strate the high specific capacitance nature of a-Co(OH)2.Due to the low cost of the starting materials and the highspecific capacitance, a-Co(OH)2 can be a promising mate-rial for supercapacitor applications.

2. Experimental

The chemicals (Co(NO3)2Æ6H2O and 1 M KOH) werepurchased from Wako Pure Chemical Industries, Ltd.Research grade stainless-steel foil (SS, grade 304, 0.1 mmthick) was obtained from Nilaco Corp. The area of theSS for deposition was 48 cm2 (8 cm · 6 cm). The SS waspolished with emery paper to a rough finish, washed freeof abrasive particles and then air-dried. An electrochemicalcell was assembled in a three-electrode configuration inwhich the counter electrode was platinum (Pt), the refer-ence electrode was Ag/AgCl and the working electrodewas SS. An electrolyte solution of 0.1 M Co(NO3)2Æ6H2Owas used for the potentiostatic deposition of Co(OH)2 ontoSS. The potentiostatic deposition of cobalt oxide was car-ried out at a potential of �1.0 V vs. Ag/AgCl.

The deposited electrodes were washed in distilled waterand stirred by using a magnetic paddle in a beaker for sev-eral hours and then dried in an oven at 40 �C overnight.The weight of the deposit was measured by means of amicro-balance (Sartorius, BP211D). All electrochemicaldepositions and capacitive measurements were performedby means of a potentiostat (AUTOLAB�, Eco-Chemie,PGSTAT 30). KOH (1.0 M) electrolyte was used in all elec-trochemical characterizations. The microstructure and the

thickness of the electrode materials were evaluated bymeans of a field emission scanning electron microscope(FE-SEM, JEOL, JSM-6340F). The X-ray diffraction pat-terns were obtained by means of a X-ray diffractrometer(XRD, RIGAKU, R1NT2100) with CuKa radiation(k = 1.5406 A) operating at 40 kV, 20 mA.

3. Results and discussion

Fig. 1 shows the XRD pattern of the deposit. The pat-tern in Fig. 1 consists of four peaks appearing at 7.75,3.81. 2.68 and 1.55 A. The first two peaks are related tod-spacings that are multiples of each other due to multiplereflections from the basal plane and are indexed as (001)and (002). The third and fourth peaks have similar ‘‘saw-tooth’’ line shapes, with a sharp rise at the low-angle sideand pronounced asymmetry on the high-angle side. Thissuggests that they belong to a group of planes differentfrom (00‘) These peaks are indexed as (100) and (110).The XRD pattern corresponds to the well-known a formof layered a-Co(OH)2 [18]. Previously reported XRD pat-terns of a-Co(OH)2 prepared by precipitation methods[24–26], show stronger (00‘) reflections as compared toother reflections, which is not the case in Fig. 1. This isdue to the nano-size of the a-Co(OH)2 sheets along thebasal planes, which results in a loss of intensity of the(00 ‘) reflections, as discussed in the next paragraph.Fig. 2 shows the IR spectra of the obtained deposit. Theabsorption band at �750 cm�1 can be assigned to thevCo–O stretching vibrations and those in the 1400–1500 cm�1 range to the bending vibrations of adsorbedwater molecules. The large absorption band centered at�3050 cm�1 can be assigned to the stretching vibrationof hydrogen-bonded hydroxyl groups in a-Co(OH)2 andadsorbed water molecules, and the sharp vibration at3646 cm�1 to non-hydrogen bonded hydroxyl groups ina-Co(OH)2.

Fig. 3 shows the SEM image of the deposited a-Co(OH)2. The microstructure of the deposit is shown inFig. 3a. It is apparent that a network of a-Co(OH)2 sheets

0

20

40

60

80

100

500 1500 2500 3500Wavenumber / cm-1

% T

ran

smitt

ance

H bonded OH

H free OH

Co-O

Absorbed water

Fig. 2. IR spectra of cobalt hydroxide deposited potentiostatically.

Fig. 3. (a) SEM images of potentiostatically deposited cobalt hydroxide,(b) top view and (c) cross-sectional view.

V. Gupta et al. / Electrochemistry Communications 9 (2007) 2315–2319 2317

is arranged in a packed structure. The average thickness ofa sheet is 10 nm, as shown in Fig. 3b. The cross-section ofthe a-Co(OH)2 deposit is shown in Fig. 3c. It is evident thatthe entire deposit of the a-Co(OH)2 nanostructure is highlymicro-porous.

The capacitive behavior of the material can be estimatedfrom cyclic voltammetry (CV) as well as charge–discharge(CD) cycling. The specific capacitance (SC) based on CVand CD cycling can be calculated as follows:

SC ðCVÞ ðF g�1Þ ¼ QðcÞ= potentialrange� m ðgÞ½ � ð1ÞSC ðCDÞ ðF g�1Þ ¼ iðAÞ � DtðsÞ�=½DEðVÞ � m ðgÞ½ �: ð2Þ

Here, Q is anodic charge, i is the discharge current in am-peres, Dt is the discharge time in seconds corresponding tothe voltage difference (DE) in volts, and m is the electrode(active material) mass in grams. Fig. 4 shows the cyclic vol-tammetric (CV) curves of the deposited a-Co(OH)2 (0.09mg cm�2) at 2 mV s�1 and 10 mV s�1in 1 M KOH electro-lyte solution. The CV consists of a pair of strong redoxpeaks. This indicates that the capacitance characteristicsare mainly governed by Faradaic reactions and not by pureelectric double layer capacitance. These redox peaks are themain contributor to the capacitance of a-Co(OH)2.However, the reported redox peaks were much weakerfor a-Co(OH)2 prepared by precipitation methods and con-sequently showed a low specific capacitance (SC). Due tothe nano-size of the a-Co(OH)2 sheets, effective redox uti-lization of the deposit could be possible in the present caseand a specific capacitance of 860 F g�1was obtained fromthe CV curves. The electrochemical reaction correspondingto the redox peaks can be expressed as follows:

CoðOHÞ2 þOH� () CoOOHþH2Oþ e� ð3Þand the reaction beyond the redox peaks at higher potentialcan be expressed as follows:

CoOOH þOH� () CoO2 þH2Oþ e� ð4ÞFig. 5a shows the charge–discharge characteristics of thedeposited a-Co(OH)2 (0.09 mg cm�2) in the 0–0.4 Vpotential range at various specific currents. The charge–discharge curves appear like mirror images. The single-electrode specific capacitance of 881 F g�1 was obtained

at 1 A g�1. The variation of the specific capacitance withthe specific current is shown in Fig. 5b. Even at the highspecific current of 10 A g�1, the SC value was 772 F g�1.This demonstrates the stable reversible characteristics ofthe deposited a-Co(OH)2 at high currents. This implies thatthe a-Co(OH)2 synthesized here can be of significant

-30

-15

0

15

30

-0.1 0.0 0.1 0.2 0.3 0.4 0.5Potential / V vs. Ag/AgCl

Cur

rent

/ A

g-1

10 mV s-1

2 mV s-1

Fig. 4. CV curves of potentiostatically deposited cobalt hydroxide(0.09 mg cm�2).

0.0

0.1

0.2

0.3

0.4

0 200 400 600Time / s

Vol

tage

/ V

1 A/g2 A/g3 A/g5 A/g

10 A/g

600

650

700

750

800

850

900

0 3 6 9 12Current / A g-1

Sp

ecifi

c ca

pac

itan

ce /

F g

-1

Fig. 5. (a) Charge–discharge cycling curves and (b) dependence of specificcapacitance on current for potentiostatically deposited cobalt hydroxide(0.09 mg cm�2).

0

200

400

600

800

1000

0 0.2 0.4 0.6 0.8Loading mass / mg cm-2

Sp

ecifi

c ca

pac

itan

ce /

F g

-1

α -Co(OH)2

1

ig. 6. Dependence of specific capacitance on the loading mass forotentiostatically deposited cobalt hydroxide.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Z' / ohm

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Z' / ohm

Z"

/ ohm

Z"

/ ohm

0.1 V0.2 V0.3 V0.4 V

Charge

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.1 V0.2 V0.3 V0.4 V

Discharge

Fig. 7. Nyquist plots for the cobalt hydroxide electrode in 1 M KOHsolution at different potentials during (a) charging and (b) discharging.

2318 V. Gupta et al. / Electrochemistry Communications 9 (2007) 2315–2319

importance for practical application. Moreover, the depos-ited a-Co(OH)2 shows very stable SC values even for largedeposited mass, as shown in Fig. 6. In comparison, the SCvalue of RuO2 was reported to exhibit a drastic decrease athigh deposited mass [27]. Another important aspect of asupercapacitor electrode is its resistance. Fig. 7 shows elec-trochemical impedance spectra in the form of Nyquist plots

Fp

for a cobalt hydroxide electrode at various potentials dur-ing charging (Fig. 7a) and discharging (Fig. 7b), where Z 0

and Z00 are the real and imaginary parts of the impedance,respectively. As can be seen in Fig. 7, the plots obtained at0.1 V, 0.2 V, 0.3 V and 0.4 V are composed of a semi-circleat high frequencies, which is related to Faradaic reactions.

0

200

400

600

800

1000

0 500 1000 1500 2000Cycle number

Spec

ific

cap

acit

ance

/ F

g-1

6% 3%

Fig. 8. Cycle life data of potentiostatically deposited cobalt hydroxide(0.09 mg cm�2).

V. Gupta et al. / Electrochemistry Communications 9 (2007) 2315–2319 2319

The slope close to 45� along the imaginary axis (Z00) at lowfrequencies is due to a Warburg impedance (a limiting dif-fusion process), and is not useful for charge storage. FromFig. 7, the observed cobalt hydroxide electrode resistancewas close to 0.05 X and was almost the same in the poten-tial range of 0.1–0.4 V. This shows that the cobalt hydrox-ide electrode is of a highly conducting nature.

Fig. 8 demonstrates the stability characteristics of thedeposited a-Co(OH)2 as a function of cycle number. Adecrease of nearly 6% during the first 1000 cycles (due tostabilization of the structure) and nearly 3% in thesubsequent 1000 cycles was observed, which indicates goodstability characteristics, without significant loss of thecharge–discharge characteristics.

In conclusion, nano-sheets of a-Co(OH)2 were electro-chemically synthesized on SS. The deposits showed veryhigh initial specific capacitance (881 F g�1) and nano-sizemicrostructure of the a-Co(OH)2 sheets, which results ineffective utilization of the electrode material. Moreover,the interlayer spacing of the a-Co(OH)2 was comparableto the size of the hydrated ions (6–7.6 A) intercalatingbetween the highly flat a-Co(OH)2 sheets. It is evident thatthe microstructure of the a-Co(OH)2 in the present case isdifferent from previously reported a-Co(OH)2 samples [21–23]. Thus, the microstructure can play vital a role inimproving the specific capacitance, which in turn dependson the method of preparation. The capacitive characteris-tics of the deposited a-Co(OH)2 presented here can beapplied at large scale to produce inexpensive, high perfor-mance supercapacitors. Further modification of this nano-structure is currently under investigation.

Acknowledgements

The present work was supported by the Japan Scienceand Technology (JST) Agency through the Core Researchfor Evolutionary Science and Technology (CREST) Pro-gram under the Project ‘‘Development of advanced nano-structured materials for energy conversion and storage.’’

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