1

On the electrochemical synthesis and charge storage properties of

WO3/polyaniline hybrid nanostructures

Gergely F. Samu1,2, Kriszta Pencz1, Csaba Janáky1,2,*, Krishnan Rajeshwar3,*

1Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Square 1, H6720,

Szeged, Hungary 2MTA-SZTE „Lendület” Photoelectrochemistry Research Group, Rerrich Square 1, H6720, Szeged, Hungary

3Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019

ABSTRACT

The aim of this study is to tailor the electrochemical synthesis of hybrid materials, consisting

of nanoporous tungsten trioxide (WO3) and polyaniline (PANI) for application as

supercapacitor electrodes. Nanoporous WO3, synthetized by the anodization of tungsten foils,

acted as the active material framework for this assembly. With the variation of the anodization

voltage, host materials with different morphological features were prepared. Subsequently,

electrodeposition of PANI was carried out by potentiodynamic cycling in highly acidic media.

Through alteration of the number of deposition cycles the amount of deposited PANI was

varied. This parameter had a decisive impact on the morphology of the resulting hybrids as

confirmed by SEM images. Cyclic voltammetry and galvanostatic charge discharge

measurements were carried out to characterize the charge storage properties of the synthetized

hybrids. By comparing electrochemical data with structural properties, structure-property

relationships were established, and under optimal condition 350 F g-1 and 200 mF cm-2 were

obtained.

Keywords: polyaniline, WO3, supercapacitor, electrochemical deposition, composite

corresponding authors

C. Janáky: janaky@chem.u-szeged.hu, Rerrich Sq. 1, Szeged, Hungary, H6720; Tel: +36-62-546393

K. Rajeshwar: rajeshwar@uta.edu, 76013 Planetarium Place 1, Arlington, TX, USA

This article is dedicated to Mikhail A. Vorotyntsev on the occasion of his 70th birthday in

recognition of his valuable contribution to the electrochemistry of redox active films.

2

Introduction

New devices for harnessing renewable energy sources are an essential part of future

economics. The overall efficiency of these devices, however, is limited by the intermittency of

the underlying renewable energy sources. To address this issue the development of high

efficiency energy storage devices is critically needed. In this vein, this study focuses on

materials exhibiting pseudocapacitive properties. In this type of electrochemical capacitor

device, both ion adsorption and fast reversible redox processes on the electrode surface

contribute to the overall charge storage capability. Both transition metal oxides and conducting

polymers (CPs) possess such attributes [1, 2].

In terms of electro-optical properties WO3 is one of the most intensively studied

transition metal oxides. As a relatively narrow bandgap semiconductor (Eg=2.6 eV) it is a

promising candidate for visible light harvesting photoanodes [3, 4]; furthermore, it possesses

electrochromic properties which makes it an ideal component of smart windows and displays

[5, 6]. However, unlike other transition metal oxides (e.g., RuO2), the pseudocapacitive nature

of this oxide remains as yet, incompletely exploited. The enhanced electroactivity of WO3 in

acidic media is rooted in the formation of tungsten bronze (HxWO3), supported by H+

uptake/release as charge compensation during the (W6+/W5+) redox process [7].

Research in this area has mainly focused on revealing the relationship between charge

storage capability and the oxide structure as summarized in the first part of Table 1. For

example, a tenfold increase in the specific capacitance was achieved by fabricating nanoporous

structures [8]. This enhancement was attributed to the greater extent of electrolyte penetration

into the nanostructure, thus increasing the active area of the electrode participating in the charge

storage process. Also an increase in the charge storage capability of WO3 was found by

obtaining crystal networks with precise ordering as demonstrated for hexagonal WO3 (h-WO3)

[9].

The methods employed in the preparation of the above mentioned materials are similar

in a sense that all of them require a binding agent to fix the active material on the supporting

electrode surface. This however may increase charge transfer resistance at the phase boundaries

between the different materials. These findings establish several important criteria for the

preparation procedure for the active electrode material: (i) large surface area (ii) controlled

morphology (iii) lack of binding agent. As an example, we note that the above summarized

criteria were ultimately met when h-WO3 nanowire arrays were fabricated directly on 3D

3

carbon cloth electrodes; the resultant hybrid exhibited superior performance when compared to

pure WO3 electrodes [10].

Anodization is a well-known method for preparing transition metal oxides [4, 11] with

effective control over the morphology of the produced nanostructures (pore size [12], layer

thickness [13–15], and structural features [16, 17]). Furthermore, it circumvents the use of

binding agents, as the nanostructured metal oxide is directly grown from the respective metal

foil.

Hybrid materials based on nanostructured transition metal oxides and CPs play a

continuously increasing role in both energy conversion and storage. Their utilization as

electrode materials for solar energy harvesting [19, 20], electrochromic devices and

supercapacitors [1, 20] has received well-deserved scientific attention in recent years. These

composite materials often outperform their pristine counterparts by uniting the beneficial

properties of the constituents.

In the case of supercapacitors, such hybrid formation can effectively address the

relatively poor cycling stability of CPs [2], while harnessing their complementary

pseudocapacitive properties with transition metal oxides. Several preparation procedures are

known to graft CPs into metal oxide nanostructures. Among them electrochemical methods are

often found to be more versatile than its rivals such as physical infiltration, mechanical mixing,

in situ chemical formation etc [21]. The main virtue of the electrochemical preparation strategy

is its intrinsic ability to achieve higher filling ratio in the nanostructured material while retaining

precise control over the morphology and the amount of the electrogenerated CP [21].

Polyaniline (PANI) is often selected as the CP component for such hybrid materials,

because of its remarkable chemical and electrochemical stability, versatile redox behavior,

electrochromic property, and large specific capacitance [22]. The use of WO3/PANI hybrids is

further supported by the similarity in the ion exchange properties of the two materials (both

electron and proton conduction) [23, 24]. As summarized in the second part of Table 1,

fabrication of WO3/PANI hybrids have been carried out using various techniques; however, the

performance of these materials has remained disappointingly low in all cases.

In this study we build on companion efforts [23, 24] and electrochemically graft PANI

on nanoporous WO3 electrodes prepared by anodization. The effect of different polymer

loadings on the charge storage properties of the hybrids was evaluated and the key

morphological aspects were studied. Our goal was to study the establish structure-property

relationships between the morphological attributes and the electrochemical properties of the

hybrid materials.

4

Table 1.

Experimental

Materials

All chemicals used were of analytical grade. Tungsten (W) foils were purchased from Sigma-

Aldrich (99.9% purity, 0.25 mm thickness), while aniline and sulphuric acid were from VWR

International. Aniline monomer was freshly distilled before use. All other chemicals were used

as received. Ultrapure water (Milli-Q, ρ=18.2 MΩ cm) was used for the preparation of all

solutions.

Synthesis of nanoporous WO3 by anodization

Prior to anodization, the commercially purchased W foils were mechanically polished to

mirror finish using silicon carbide sandpaper with successively finer roughness. This

procedure was followed by three subsequent 5 min steps of ultrasonication in 2-

propanol, acetone and finally ultrapure water, to remove any attached organic

contaminants. Anodization was performed in a two electrode cell setup, where the W foil

served as the anode and a Pt foil as the cathode. The W foil was pressed between a set

of O-rings in the electrochemical cell, leaving 0.63 cm2 exposed to the electrolyte, and

the electric contact was located on the backside of the sample. The electrolyte consisted

of 0.15 M NaF in ultrapure water. The anodization was carried out at different voltages

using a programmable DC Power Supply (Voltcraft PSP 1803), while the procedure was

monitored by a Digital Multimeter (Keithley 2000). Both instruments were controlled

by custom-written LabView software. Immediately after anodization was completed, the

sample was carefully washed with ultrapure water and dried in N2 stream. To obtain the

desired crystalline phase of WO3, the anodized samples were annealed for 1 h at 450 °C

with a heating ramp of 10 °C min-1 (Thermo Scientific Heraeus K114 Furnace).

Electrochemical synthesis of WO3/PANI hybrids

For the electrochemical synthesis of the hybrids, a classical three-electrode

electrochemical cell was used, where the nanoporous WO3 functioned as the working

electrode, a Pt foil as a counterelectrode and a Ag/AgCl (3.0 M NaCl with a potential of

+200 mV vs. SHE) reference electrode completed the cell setup. A solution of 0.2 M

aniline in 0.5 M H2SO4 was used as the polymerization medium. Electrosynthesis (using

5

an Autolab PGSTAT10 instrument) of the hybrid materials built on previous findings in

our research groups [23]. Two distinctly different strategies were employed to synthetize

hybrids. Both of them were based on potentiodynamic deposition which was carried out

using a cycling protocol between E=-0.2 – 1.1 V for different cycle numbers (1-10) at a

sweep rate of 100 mV s-1. The key difference resided in the deposition methodology;

namely in the first set of experiments, separate WO3 electrodes were used for different

PANI amounts, while in the second set of experiments, the same WO3 electrode was

used to subsequently deposit gradually increasing amounts of PANI.

Electrochemical and morphological characterization

Cyclic voltammetry (CV) and galvanostatic charge-discharge measurements were carried out

in a monomer free 0.5 M H2SO4 solution with the same setup described for the electrosynthesis.

Before and during measurements the solution was deaerated by purging N2 through the solution.

CV measurements were carried out in a potential window of E=-0.5 – 0.7 V at four different

sweep rates between 10-100 mV s-1 for all samples. Charge-discharge measurements were

carried out with charging (and discharging) currents ranging from 0.2 – 0.5 mA with cut-off

limits at -0.5 V and 0.7 V respectively. Three cycles were performed and the last cycle was

used for quantification purposes. The morphology of the nanoporous WO3 and the hybrids was

studied using a field emission scanning electron microscope (Hitachi S-4700). All experiments

were performed at room temperature (20 ± 2 °C).

Estimation of the deposited PANI amounts

From the recorded I vs. t curves during potentiodynamic deposition the electrosynthesized

polymer mass can be calculated [25] by making the following assumptions: (i) the redox

reactions attributed to the forming polymer are completely reversible, thus the calculated excess

positive (anodic) charge from the CVs can be attributed to the oxidation of monomeric units

and the incorporation of anions into the thin PANI layer (ii) electrochemical oxidation of aniline

monomer is a two electron process (iii) in highly acidic media a highly doped polymer is formed

(xA=0.5) with A = HSO4- as the compensating anion [25]. The following expression was used

to calculate the mass of the deposited PANI:

𝑚𝑃𝐴𝑁𝐼 =((𝑀𝑎𝑛𝑖𝑙𝑖𝑛𝑒 − 2 𝐴𝐻) + 𝑥𝐴 𝑀𝐴) 𝑄𝑝𝑜𝑙

(2 + 𝑥𝐴) 𝐹

6

where Maniline, MA and AH are the molar or atomic masses of the monomeric unit, compensating

anion and hydrogen, respectively. Qpol is the calculated polymerization charge and F is the

Faraday constant.

Direct measurement of the mass of electrodeposited PANI amounts was also performed for

samples with 3 different polymer loadings. It was found that, within the accuracy of our weight

measurements, the measured data paralleled their calculated counterparts, thus we used the

latter ones during further data analysis.

Calculation of the charge storage capacity

As seen in Table 1, there is no generally accepted protocol to measure charge storage capacity.

Charge-discharge measurement, compared to CV, however, is considered as a more reliable

method for this purpose. Thus in this study only capacitance values obtained from charge-

discharge measurements are shown. In this vein, to calculate the capacitance of the samples the

following expression was used:

𝐶 =𝐼 ∆𝑡

∆𝐸

where I (A) is the constant current applied during measurement, Δt (s) is the average of

measured charging and discharging time and ΔE (V) is the potential window of the

measurements (without IR drop correction).

To evaluate the contribution of the varying amount PANI to the overall specific capacitance of

the material, the measured capacitance of the WO3 framework was always subtracted from that

of the hybrid material at a given charging/discharging current. The resulting PANI related

capacitance was than divided by the deposited mass of the polymer calculated by the method

discussed earlier.

𝐶𝑛𝑜𝑟𝑚,𝑃𝐴𝑁𝐼 =(𝐶ℎ𝑦𝑏𝑟𝑖𝑑 − 𝐶𝑊𝑂3

)

𝑚𝑃𝐴𝑁𝐼

Results and Discussion

Preparation of WO3 samples via anodization

7

Electrochemical anodization was employed to fabricate nanoporous WO3 electrodes. To obtain

porous structures (instead of a compact metal oxide layer) it is necessary to carry out the

anodization in a solution of a complexing agent (in our case F-). Although there is wealth of

examples in the literature for this process,[4, 16, 26] its reproducibility falls below that of TiO2;

therefore precise control and monitoring of the synthesis parameters is indeed important.

Figure 1A shows current density vs. time traces recorded during anodization of different W

samples at a constant voltage of 60 V. At the first stage, which is magnified in the inset, the

voltage was gradually increased to 60 V, with a rate of 500 mV s-1. During this regime, the

evolution of two separate peaks can be discerned at well-defined voltages, which are also

present during the anodization of Ti foils [27]. The first peak was assigned to the fast-field aided

ion-transport (presumably F-) through the forming oxide layer, while the second to the increase

in electrode area due to pore formation. Subsequently, the two competing reactions (oxide

formation and dissolution) reached a steady state level, indicated by the near-constant current

density.

Figure 1.

Anodization experiments were performed at different voltages (Fig. 1B) to reveal the

decisive role of this parameter on the morphology of the prepared WO3 samples. Note that this

property is of prime importance from a supercapacitor perspective. With decreasing anodization

voltage a significant decline in the steady-state current density was observed, which is closely

related to the porosity of the prepared WO3 (see also SEM images in Fig. 3). This notion is

further verified by cyclic voltammetric (Fig. 2A) and charge-discharge measurements

performed on the oxide samples (Fig. 2B.) At all charging-discharging currents the decrease of

areal capacitance is observed at decreasing anodization voltages, which can originate from the

difference in the actual surface area of the samples, if we assume that there is no chemical

difference between the samples. Another characteristic of these data is the decreasing areal

capacitance with increasing charging currents (as confirmed by both CV and galvanostatic

charge-discharge measurements; see Fig. 2). This trend is caused by the gradually smaller

contribution of pseudocapacitive behavior to the overall capacitance of the oxide. However for

active materials for supercapacitor applications, a high charge/discharge rate is an important

requirement so in this study the emphasis was given to capacitance values obtained at higher

current densities.

8

Figure 2.

To visualize the morphological variances caused by the different preparation voltage,

SEM images were recorded for the heat treated WO3 samples (Fig. 3). With increasing voltage,

the nanoporous structure loses its initial ordered nature and becomes rather randomly

distributed. Furthermore, this is followed by the increase in the etched out amount of WO3 from

the initial compact oxide layer. These results are consistent with prior literature data on

nanoporous WO3 prepared under similar conditions [28].

Figure 3.

Preparation of WO3/PANI hybrids on distinct anodized samples

Potentiodynamic deposition (together with slow galvanostatic methods for some monomers) is

considered an ideal synthesis technique for obtaining oxide/CP hybrids, where mass transport

can be hindered by diffusion processes; this problem is exacerbated for nanostructured

electrodes [21]. In Fig. 4A, 5 potentiodynamic polymerization cycles are shown for aniline

electropolymerization. With the progress of polymerization, the gradual evolution of PANI-

related redox peaks is seen. By increasing the number of polymerization cycles the

polymerization charge increases (Fig. 4B), which should be proportional to the deposited

amount of polymer. This trend is also reflected in Fig. 4C, which shows the CVs of hybrids

with different compositions recorded at 50 mV s-1 sweep rate.

It is apparent that both the deposited PANI and the underlying nanoporous WO3 retained

its electroactivity in the hybrid material even at higher loadings (Fig. 4D). This may be

attributed to facile H+ transport throughout the hybrid material. Furthermore the preservation

of WO3 related redox peaks in the hybrid may indicate that the underlying nanoporous WO3 is

still accessible to H+ from the solution phase. Alternatively, the deposited PANI may be acting

as a relay layer for the transport of H+ [29], however, from CV results alone, differentiation of

these two different mechanisms is not possible.

For supercapacitor application, however, this complementary behavior is beneficial,

because pseudocapacitive contribution to the overall capacitance can be expected in the whole

potential window. In the case of this hybrid material the positive potential regime is limited by

the irreversible oxidation of PANI to its insulating form (pernigraniline), while in the negative

regime, reduction of water (or H+) is a restricting factor. Thus an optimized working potential

window of 1.2 V (from E = -0.5 to 0.7 V) was selected.

9

Figure 4.

Electrochemical characterization of the prepared hybrids

In Fig. 5, CVs recorded at four different sweep rates are presented for hybrids prepared by 3

cycles (A) and 10 polymerization cycles (B). As the deposition cycle number is increased a

thicker polymer layer is formed which is indicated by the increase in the peak current densities.

However in the second case, diffusion limitation of mass transport may arise. This manifests in

the relationship between the recorded peak current densities and the sweep rate. To uncover

this phenomenon the anodic and cathodic peaks, originating from the leucoemeraldine –

emeraldine transformation of PANI, were scrutinized. In the case of the sample with moderate

PANI content (3 cycles) we found that the peak current densities are linearly proportional to

the sweep rate (inset of Fig. 5A), whereas in the case of high PANI content, a linear

proportionality was found to the square root of the sweep rate (inset of Fig. 5B). This confirms

that in the second case redox transformation is diffusion limited (as opposed to the first case

which is clearly not), which in terms of supercapacitor electrodes means that longer charging

and discharging times are necessary to completely transform the electrode material.

Figure 5.

Morphological characterization of the prepared WO3/PANI hybrids

To investigate the morphological changes induced by the increasing amount of electrodeposited

PANI on the nanoporous WO3 template SEM images were taken (Fig. 6). It is obvious that with

increasing number of polymerization cycles the growth of PANI became more pronounced. In

the first few cycles the deeper pores of the nanoporous WO3 template were filled (1 cycle – B)

and thickening of the outstretched branches of the nanostructure was observed (3 cycles – C).

As the polymerization propagates, further PANI growth occurs predominantly at the top of the

sample (5 cycles – D) until complete coverage is reached (10 cycles – E). It is interesting to

note that the polymerization proceeds in a fashion that conserves the initial morphology of the

underlying nanoporous WO3 template, which is retained even after the voids are completely

filled, resulting in fractal like PANI coverage at prolonged depositions. This open structure can

be especially favorable for supercapacitor applications, since high surface area remains

preserved.

Figure 6.

10

Evaluation of the contribution of PANI to the overall charge capacity

The results of charge-discharge measurements conducted on hybrids with different PANI

loadings, at a charging/discharging current of 0.4 mA, are summarized in Fig. 7. As seen in Fig.

7A, the specific capacitance values (related to PANI) do not show a significant variation for the

hybrid samples with different polymer content. This indicates that under our circumstances, the

PANI content is accessible for all the materials, which can be rooted in the above discussed

nanostructured nature of the hybrids. As for the areal capacity, a continuous and linear increase

can be seen with the increasing PANI content, further supporting our earlier conclusions. The

maximum values of the normalized capacitance (Cspec ≈ 350 F g-1 and Carea ≈ 200 mF cm-2) are

among the best values obtained for this kind of hybrid materials [1][37].

Figure 7.

The stability of the hybrid material fabricated with 3 potentiodynamic cycles was evaluated by

exposing it to strenuous galvanostatic charge-discharge experiments at a charging/discharging

current of 0.5 mA. After 200 cycles the hybrid retained 93.4% of its initial capacitance value,

underlining that hybrid formation is indeed an effective strategy for combating the low cycling

stability of PANI [30].

Figure 8.

Consecutive potentiodynamic synthesis of WO3/PANI hybrids

To dismiss the possible inconsistencies arising from the use of separate WO3 samples for each

measurement, consecutive potentiodynamic deposition was also carried out, using the same

sample throughout the whole preparation and characterization procedure. In this manner, the

effect of increasing PANI content on the capacitive performance was studied by successive

overpolymerization on the previously formed hybrid. Figure 9A shows CVs at a sweep rate of

50 mV s-1 for the hybrids prepared with different deposition cycles. The same gradual increase

in the PANI related electroactivity can be observed as shown for the previous materials in Fig.

4C above. The charge-discharge measurements showed similar behavior: with increasing

polymer content the charging/discharging times increase. The capacitance values obtained from

these experiments are very close to those presented earlier (Cspec ≈ 330 F g-1 and Carea ≈ 170 mF

cm-2).

Figure 9.

11

This consecutive deposition procedure was employed to fabricate hybrids, where the

nanoporous WO3 host materials were anodized at different voltages. As opposed to the trend in

the initial capacitance values (Fig. 2B), the specific capacitance, related to PANI was highest

for the WO3 etched at 40V and lowest for the 60 V sample (Fig. 10). At this point, we presume

that the more ordered structure, together with a larger number of seeds for PANI deposition,

are the primary reasons for this peculiar trend.

Figure 10.

Conclusions

In this study a systematic investigation of nanostructured WO3/PANI hybrid electrodes was

carried out from a supercapacitor application perspective. A two-step electrochemical protocol

was employed to synthesize a series of WO3/PANI assemblies with varying PANI content. The

first step was the carefully controlled anodization of W-foils to form the nanoporous WO3 host

framework. Subsequently, PANI was electrografted on the oxide surface through

potentiodynamic cycling. The most important finding deduced from SEM studies was that the

hybrid retained its nanostructured nature even at very high PANI loadings. This nanostructures

morphology was reflected in the specific capacitance of the hybrids, namely that high values

were obtained even at high PANI amounts (as directly seen from the areal capacitance values).

The best capacitance values (obtained at relatively fast charge-discharge rates) were 350 F g-1

and 200 mF cm-2.

Acknowledgement

CJ thanks the Hungarian Academy of Sciences for financial support through its “Lendület”

excellence program. One of us (KR) thanks Sid Richardson Carbon & Energy Co., Fort Worth,

TX for partial funding support.

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15

Tables

Table 1

Material Preparation

method

Charge - discharge measurement Cyclic voltammetry

Reference Potential

window Current density

Specific

capacitance

Potential

window Sweep rate

Specific

capacitance

WO3 Spin-coating n.a. n.a. n.a. -1.0 - 1.0 V 50 - 1000 mV s-1 240 - 90 F cm-2 [31]

WO3 Sol-gel method -0.2 - 0.8 V 0.008 - 0.16 mA cm-2 n.a. -0.5 - 0.8 V 5 - 100 mV s-1 2 - 2 mF cm-2 [32]

WO3 Chemical bath

deposition -0.2 - 1.0 V 0.02 mA cm-2 2.8 mF cm-2 -0.5 - 0.7 V 50 mV s-1 4 mF cm-2 [33]

W18O49 Solvothermal

synthesis -0.5 - 0.0 V 0.08 - 0.7 mA cm-2 25 - 17.5 mF cm-2 n.a. n.a. n.a. [34]

WO3 Potentiodynamic

deposition -0.5 - 0.7 V 0.5 - 5.0 A g-1 75 - 30 F g-1 n.a. n.a. n.a. [35]

WO3

Microwave

assisted hydrothermal

synthesis

n.a. n.a. n.a. -0.6 - 0.2 V 25 - 200 mV s-1 293 - 200 F g-1 [36]

WO3-x Template

assisted chemical

synthesis

-0.1 - 0.8 V 1 - 20 mA cm-2

18.4 - 10 F g-1

n.a. n.a. n.a. [8] m-WO3-x 199 -144 F g-1

m-WO3 110 - 75 F g-1

h-WO3 Hydrothermal

synthesis -0.5 - 0.0 V 0.5 - 5.0 A g-1 421.8 - 311 F g-1 -0.5 - 0.0 V 2 - 20 mV s-1 417.8 - 201.9 F g-1 [9]

h-WO3 nanowire

array

Hydrothermal

synthesis -0.6 - 0.3 V 0.5 - 8.0 A g-1 680 - 240 F g-1 n.a. n.a. n.a. [10]

PANI Potentiodynamic

deposition -0.5 - 0.7 V 0.5 - 5.0 A g-1 160 - 135 F g-1 n.a. n.a. n.a. [35]

PANI Potentiodynamic

deposition -0.2 - 0.8 V 0.008 - 0.16 mA cm-2 n.a. -0.5 - 0.8 V 5 - 100 mV s-1 75 - 12 mF cm-2 [32]

PANI Chemical

synthesis -0.4 - 0.8 V 0.02 mA cm-2 9.2 mF cm-2 -0.5 - 0.7 V 50 mV s-1 17 mF cm-2 [33]

PANI Galvanostatic

deposition 0.0 - 0.8 V 0.08 - 0.7 mA cm-2 10 - 7.5 mF cm-2 n.a. n.a. n.a. [34]

WO3/PANI

Potentiodynamic

codeposition on

carbon cloth

-0.5 - 0.7 V 0.5 - 5.0 A g-1 168 - 120 F g-1 n.a. n.a. n.a. [35]

WO3/PANI

Chemical

synthesis on

WO3 prepared via chemical

bath deposition

-0.2 - 0.8 V 0.02 mA cm-2 4.1 mF cm-2 -0.5 - 0.7 V 50 mV s-1 10 mF cm-2 [33]

WO3/PANI

Potentiodynamic

deposition on WO3 prepared

via sol-gel

method

-0.2 - 0.8 V 0.008 - 0.16 mA cm-2 12 - 4 mF cm-2 -0.5 - 0.8 V 5 - 100 mV s-1 25 - 9 mF cm-2 [32]

W18O49/PANI

Galvanostatic

electrodeposition

on chemically prepared W18O49

-0.5 - 0.8 V 0.08 - 0.7 mA cm-2 5 mF cm-2 n.a. n.a. n.a. [34]

16

Figures

Figure 1.

Figure 2.

Figure 3.

0.20 0.25 0.30 0.35 0.40 0.45 0.50

20

40

60

80

100

120

140

B 60 V

50 V

40 V

C / m

F c

m-2

I / mA

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-15.0

-10.0

-5.0

0.0

5.0

10.0

j /

mA

cm

-2

E / V

10 mV s-1

25 mV s-1

50 mV s-1

100 mV s-1

A

17

Figure 4.

Figure 5.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

-20

-10

0

10

20

30

A

j / m

A c

m-2

E / V

1. cycle

2. cycle

3. cycle

4. cycle

5. cycle

0 2 4 6 8 10

0

20

40

60

80

100

120

B

Qpol /

mC

Polymerization Cycle Number

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-20

-10

0

10

20

30

C

j / m

A c

m-2

E / V

WO3

1 cycle PANI

3 cycles PANI

5 cycles PANI

10 cycles PANI

40 V 50 V 60 V

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

D

j /

mA

cm

-2

E / V

WO3

PANI

WO3/PANI

12 m

A c

m-2

5 m

A c

m-2

26 m

A c

m-2

18

Figure 6.

Figure 7.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-10

-5

0

5

10

15

A

j / m

A c

m-2

E / V

10 mV s-1

25 mV s-1

50 mV s-1

100 mV s-1

0 20 40 60 80 100-8

-4

0

4

8

12

j /

mA

cm

-2

/ mV s-1

A B

D

C

E

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-25

0

25

50

j /

mA

m-2

E / V

10 mV s-1

25 mV s-1

50 mV s-1

100 mV s-1

B

2 4 6 8 10

-20

0

20

40

j / m

A c

m-2

/ (mV s-1)

0.5

19

Figure 8.

Figure 9.

Figure 10.

0 2 4 6 8 10

0

100

200

300

400AC

norm

, P

AN

I / F

g-1

Deposition cycle number

0 2 4 6 8 10

0

40

80

120

160

200

B

Cn

orm

, P

AN

I / m

F c

m-2

Deposition cycle number

0 5 10 15 20 25 30 35 40

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

A

E / V

t / sec

0 40 80 120 160 200

0

10

20

30

40

B

Cnorm

, P

AN

I / m

F c

m-2

Cycle number

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-15

-10

-5

0

5

10

15

20A

j /

mA

cm

-2

E / V

WO3

1 cycle PANI

3 cycles PANI

5 cycles PANI

7 cycles PANI

10 cycles PANI

0 20 40 60 80 100 120 140 160

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

B

E / V

t / sec

WO3

1 cycle PANI

3 cycles PANI

5 cycles PANI

7 cycles PANI

10 cycles PANI

20

Captions

Table 1. Representative examples of WO3 and PANI based materials and their charge storage

properties.

Figure 1. Anodization curves of distinct W foils registered at 60 V in 0.15 M NaF (A).

Anodization curves of W foils at different voltages in 0.15 M NaF (B). The inset shows the

voltage sweep section with a rate of 500 mV s-1 in both cases.

Figure 2. Cyclic voltammograms recorded in 0.5 M H2SO4 solution at different sweep rates for

WO3 samples anodized at 60 V (A). Panel (B) shows the effect of preparation voltage on the

areal capacitance of WO3 samples (as derived from galvanostatic charge-discharge

measurements).

Figure 3. SEM images of heat treated WO3 samples prepared with different anodization

voltages.

Figure 4. Potentiodynamic deposition of PANI for 5 cycles on nanoporous WO3 (A), the

variation of polymerization charge with increasing cycle number (B), recorded CVs in 0.5 M

H2SO4 solution at 50 mV s-1 sweep rate for samples prepared with different PANI loadings (C),

and comparison of the voltammetric behavior of WO3, PANI, and WO3/PANI samples under

the same condition (D).

Figure 5. Cyclic voltammograms recorded in 0.5 M H2SO4 solution at different sweep rates for

WO3/PANI samples prepared with 3 cycles (A) and 10 cycles of potentiodynamic

polymerization. The insets in Figs. 5A and 5B show the linear dependence of the PANI related

anodic and cathodic peak current densities on the potential sweep rate (A) and square root of

the sweep rate (B) respectively.

Figure 6. SEM images of pristine WO3 (A) and four WO3/PANI hybrids prepared by 1, 3, 5

and 10 potentiodynamic cycles of (B, C, D and E).

0 2 4 6 8 10

250

300

350

400

450

500

60 V

50 V

40 V

C / F

g-1

Deposition cycle number

21

Figure 7. The variation of the PANI associated specific capacitance (at a charging/discharging

current of 0.4 mA) with the number of deposition cycles. The specific capacitance was

normalized with the deposited polymer mass (A) and the geometric area of the electrode

exposed to the solution (B).

Figure 8. Stability evaluation of the hybrid material prepared by 3 potentiodynamic cycles on

a nanoporous WO3 host prepared at 60 V. Specific capacitance values (B) determined from 200

galvanostatic charge-discharge curves (A) were recorded at a charging/discharging current of

0.5 mA.

Figure 9. CVs recorded at 50 mV s-1 sweep rate for WO3/PANI hybrids prepared by

consecutive potentiodynamic deposition, where the summary of the total preparation cycle

numbers is denoted (A) and the related charge-discharge measurements carried out at a 0.4 mA

charging/discharging current.

Figure 10. The variation of the PANI associated specific capacitance (at a charging/discharging

current of 0.4 mA) with the number of deposition cycles for WO3 hosts anodized at different

voltages.