Development of high-performance supercapacitor electrodes using novel

ordered mesoporous tungsten oxide materials with high electrical

conductivityw

Songhun Yoon,*aEunae Kang,

bJin Kon Kim,

bChul Wee Lee

aand Jinwoo Lee*

b

Received 1st September 2010, Accepted 15th October 2010

DOI: 10.1039/c0cc03594g

An ordered mesoporous WO3�x material was employed for use

as a supercapacitor electrode. This material exhibited a high

rate capability and an excellent capacitance (366 lF cm�2,

639 F cm�3), which were probably attributed to the large

ordered mesopores, high electrical conductivity, and high

material density.

Nanostructured materials have attracted much attention for

energy related devices.1 Recently, supercapacitors (SCs) have

been investigated as promising high power energy sources for

use in digital communications, hybrid electric vehicles (HEVs),

and EVs.2 It is well-known that pseudocapacitors utilizing the

charges accumulated during a Faradaic reaction exhibit a

higher capacitance than electric double-layer capacitors

(EDLCs). Generally, various transition metal (Ru, Mn, Zn,

W, and Ni) oxides and conducting polymer (polypyrrole and

polyaniline) materials have been employed as pseudocapacitor

electrodes.3–6 Among them, Ru- and Mn-based oxides have

shown a good pseudocapacitive performance.3,4 In particular,

Ru- and Mn-based oxides have maximum specific capacitances

(Csp) of 768 and 243 F g�1, respectively. Considering the

high cost of Ru metal, Mn oxide has been considered as the

more suitable of the two metals for practical use as an

electrode material in supercapacitors. However, the low

electrical conductivity and poor rate performance of Mn oxide

pseudocapacitors have been their major demerits.4 In order to

overcome these intrinsic drawbacks of Mn oxide electrodes,

novel pseudocapacitors based on more conductive metal

oxides and having highly developed pore structures are highly

sought after.

This study reports the preparation and evaluation of a

highly conductive, mesoporous tungsten oxide for use as a

pseudocapacitor electrode.z Recently, we developed a new

ordered mesoporous WO3�x (hereafter m-WO3�x) by employing

KIT-6 mesoporous silica as a hard template.5 m-WO3�x was

synthesized using the reported procedure.5 m-WO3�x showed

high electrical conductivity (1.76 S cm�1) comparable to that

of ordered mesoporous carbon, highly interconnected ordered

pores, and a large surface area (54.3 m2 g�1), which make the

structure ideal for use as an electrode material for super-

capacitors (Fig. S1–S3, ESIw). Fig. 1(a) shows the schematic

representation of the structure of m-WO3�x and its merits

when employed in supercapacitors. The transmission electron

microscopy (TEM) image shows that the m-WO3�x has

uniform and ordered mesopores (Fig. 1(b)). The well-defined

mesoporosity and high electrical conductivity (as listed in

Table 1) also make m-WO3�x highly suitable for use as a

support material in fuel cells.5 For the same reasons, high rate

capability and high capacitance are expected in the m-WO3�xsupercapacitor application.6 The preparation and detailed

structural comparison of m-WO3 and m-WO3�x are presented

in the ESI.w It is noteworthy that the crystal structure of

m-WO3�x (cubic WO3�x phase-JCDPS 461096) is identical to

that of non-porous bulk WO3�x (b-WO3�x), except for peak

broadening caused by nanometre-scale wall thickness in

m-WO3�x. The supercapacitor performance was compared

for three electrodes fabricated from b-WO3�x, m-WO3�x,

and m-WO3. To the best of our knowledge, this is the first

investigation of an ordered, mesoporous metal-oxide material

with high electrical conductivity, for use in supercapacitor

application.

Fig. 2 shows the cyclic voltammograms of the three electrodes,

which were measured from �0.1 to 0.8 V vs. SCE at a scan rate

of 5 mV s�1. The characteristics of the peaks of supercapacitors

were clearly observed for all three electrodes. The large

increase in current observed for the m-WO3�x electrode is

indicative of a high capacitance per unit area (Carea). A

pseudocapacitive anodic peak and a cathodic peak were

observed at 0.1 and 0.0 V vs. SCE, respectively, for the

b-WO3�x electrode, as shown in the inset of Fig. 2. Identical

peak potentials were observed in the anodic scans of the

m-WO3 and m-WO3�x electrodes; however, the cathodic peak

was relevant to unclear additional reduction reaction that was

Fig. 1 (a) Schematic explanation of the improved, high capacitance,

highly conductive and mesoporous WO3�x electrode. (b) TEM image

of mesoporous WO3�x.

a Green Chemical Technology Division, Korea Research Institute ofChemical Technology (KRICT), Daejeon 305-600, Korea.E-mail: yoonshun@krict.re.kr

bDepartment of Chemical Engineering, Pohang University of Scienceand Technology, Kyungbuk 790-784, Korea.E-mail: jinwoo03@postech.ac.kr

w Electronic supplementary information (ESI) available: TEM images(Fig. S1), N2 isotherm and pore size distribution (Fig. S2), XRD, SAXS(Fig. S3), cyclic voltammograms (Fig. S4), galvanostatic charge–discharge profiles (Fig. S5), and ac-impedance spectroscopy (Fig. S6)for the three electrode materials. See DOI: 10.1039/c0cc03594g

This journal is �c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 1021–1023 | 1021

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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probably attributable to the higher surface area than

b-WO3�x.5 On the basis of the scan rate change, it was

observed that the peak position of b-WO3�x remained invariant

and its current was proportional to the scan rate, indicating

that the redox reaction on WO3�x was pseudocapacitive

(Fig. S4, ESIw). Furthermore, increases in the peak current

and changes in the peak positions were observed for the other

two electrodes, which were probably attributed to larger time

constants resulting from the high capacitance of the two

electrodes.

Fig. 3 shows the galvanostatic charge–discharge profiles for

the three electrodes, which were measured at 1 mA cm�2

current. Similar to the trend observed in Fig. 2, the capacity

increased in the order of b-WO3�x o m-WO3 o m-WO3�x. It

is evident that a characteristic pseudocapacitive behaviour was

observed for all three electrodes. The steep increase in the

potential above 0.4 V correlates with the observed decrease in

the current in the cyclic voltammogram of Fig. 2. It should be

noted that the inverse value of the slope in Fig. 3 corresponds

to the capacitance of the supercapacitors.6,7 The measured

specific capacitance (Csp) as listed in Table 1 was estimated by

this method. A high Csp was observed for m-WO3�x(199 F g�1). The capacitance per unit area, Carea, was calculated

from the BET surface area (ABET), where Carea = Csp/ABET

and the values are listed in Table 1. Carea of the m-WO3�xelectrode was calculated as 366 mF cm�2. This value for Carea

of the m-WO3�x electrode is quite high when compared with

Carea of Mn-oxide-based supercapacitors, which is less than

200 mF cm�2. The volumetric capacitance (Cvol) of individual

materials presented in Table 1 was obtained as indicated in

footnote f of this table. Cvol of the m-WO3�x electrode

was relatively high with a value of 639 F cm�3, which is much

larger than the Mn oxide value (o500 F cm�3),4 and there-

fore, a higher volumetric energy density is expected in the

m-WO3�x electrode. From a practical point of view, Cvol is

more significant than Csp because of the smaller spaces

required for charge storage devices in real applications such

as HEV, plug-in HEV, and EV.8 Further, based on practical

considerations, although Ru oxide electrodes show a high

supercapacitor performance, the m-WO3�x material may be

a more suitable alternative for supercapacitor application

because of its high conductivity and low cost.

In order to investigate the rate capability of the three

electrodes, the applied current was varied from 1 to 20 mA

cm�2 as shown in Fig. S5 (ESIw). The change of Csp with

applied current obtained from these results is displayed

in Fig. 4. Note that the electrode loading was as high as

30 mg cm�2 in order to obtain a high energy density. When

compared with the results reported in the literature, it is

evident that the m-WO3�x electrode exhibited a high rate

capability.6 Notably, the rate capability of the m-WO3�xelectrode was comparable to a reported EDLC electrode

which employed MSC-25 microporous carbon, further sub-

stantiating the high rate capability of the m-WO3�x electrode.6

When the low loading of the EDLC electrode (10 mg cm�2)

and its intrinsically fast charging mechanism are taken into

consideration, it can be seen that the pseudocapacitive

m-WO3�x electrode exhibited a truly excellent rate capability,

which was confirmed by electrochemical impedance spectra at

open circuit voltage (Fig. S6(a), ESIw). Furthermore, a similar

capacitance decrease was observed for m-WO3�x and m-WO3

electrodes as shown in Fig. 4. The conducting agent, such as

carbon black, is typically needed for enough electrical percolation

within electrode. Because of the high electrical conductivity of

the m-WO3�x electrode, a small amount of conducting agent

Table 1 The physical parameters and supercapacitor properties of b-WO3�x, m-WO3�x and m-WO3 tungsten oxide electrodes

Sample Vporea/cm3 g�1 ABET

b/m2 g�1 kc/S cm�1 Cspd/F g�1 Carea

e/mF cm�2 Cvolf/F cm�3

b-WO3�x — — 50.8 18.4 — —m-WO3�x 0.18 54.3 1.76 199 366 639m-WO3 0.16 46.0 — 109 236 451

a The pore volume fromN2 sorption experiment. b The surface area calculated by BETmethod. c The conductivity was measured by van der Pauw

4-probe methods. d The specific capacitance measured from Fig. 3. e The capacitance per unit area (Csp/ABET).f The volumetric capacitance from

dividing Csp by calculated density (dcal(WO3�x)= 3.21 g cm�3, dcal(WO3) = 4.14 g cm�3).

Fig. 2 Cyclic voltammograms for b-WO3�x, m-WO3�x and m-WO3

electrodes, measured at 5 mV s�1 scan rate in 2.0 M H2SO4. The inset

is the magnification of b-WO3�x electrode.

Fig. 3 The galvanostatic charge–discharge voltage profiles recorded

for b-WO3�x, m-WO3�x and m-WO3 tungsten oxide electrodes at

1 mA cm�2.

1022 | Chem. Commun., 2011, 47, 1021–1023 This journal is �c The Royal Society of Chemistry 2011

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(5 wt%) was added in order to enhance the mechanical

strength of the electrode. However, the m-WO3 electrode

contained enough carbon black (20%) for electric percolation

within the electrode, which indicated that resistance from

electron transport can be disregarded for both electrodes.

Since the two electrodes have a similar mesopore structure,

the electrolyte transport resistance within the pores was, in all

probability, similar. Hence, it is expected that a similar rate

capability of the two electrodes is reasonable. The cyclability

of m-WO3�x was also investigated for up to 1200 cycles

(Fig. S6(b), ESIw). A stable cycle performance was observed,

which further demonstrates the potential for the use of

m-WO3�x as a supercapacitor electrode material. This result

also shows that WO3�x is quite stable in the presence of an

acidic electrolyte, as we also observed in the fuel cell applica-

tion of m-WO3�x.5

In conclusion, ordered mesoporous WO3�x (m-WO3�x) with

high electrical conductivity was employed as an electrode for

pseudocapacitors. The m-WO3�x shows high volumetric capacity

and rate capability, which are attributed to its high electrical

conductivity and ordered mesoporosity. These features render

conductive mesoporous transition metal oxides highly promising

as electrode materials for supercapacitors.

Notes and references

z The electrode fabrication method was identical to the literatureprocedure except for the electrode loading of 30 mg cm�2.5 Super-capacitor performance of the electrodes was analyzed with a three-electrode configuration in an aqueous 2.0 M H2SO4 electrolyte. A Ptflag and SCE (saturated calomel electrode) were used as the counterand reference electrode, respectively. Cyclic voltammetry and galvano-static charge–discharge cycling were carried out using an Iviumpotentiostat in the potential range of �0.1 to 0.8 V (vs. SCE). Thecalculated density of mesoporous materials (dcal) was obtained fromtrue density (dtrue) and pore volume (Vpore); dcal = 1/(Vpore + dtrue

�1).From dcal, Cvol was estimated; Cvol = Csp � dcal. dtrue of WO3 andWO3�x was 7.61 g cm�3.

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Fig. 4 Rate capabilities of b-WO3�x, m-WO3�x and m-WO3 tungsten

oxide electrodes. The specific capacitance was obtained from the slope

of the anodic branch in Fig. S5 (ESIw).

This journal is �c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 1021–1023 | 1023

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