High efficient PANI/Pt nanofiber counter electrode used in dye-sensitizedsolar cell{

Ziying Tang, Jihuai Wu,* Min Zheng, Qunwei Tang, Qin Liu, Jianming Lin and Jiangli Wang

Received 1st February 2012, Accepted 3rd March 2012

DOI: 10.1039/c2ra20180a

A one-dimensional PANI nanofiber supported Pt nanoparticle

film is prepared by a two-step electrochemical deposition method.

The PANI/Pt film possesses high conductivity, surface area and

catalytic activity. A dye-sensitized solar cell based on the PANI/Pt

film achieves a high light-to-electric energy conversion efficiency

of 7.69%.

Since the first prototype of a dye-sensitized solar cell (DSSC) was

reported by O’Regan and Gratzel1 in 1991, it has aroused intensive

interest over the past few decades due to its low cost, simple

preparation procedure and high conversion efficiency over 12%.2

However, determining how to enhance its efficiency and decline costs

is still a crucial issue. The counter electrode, as an important and

expensive component in DSSCs, should have a low resistance and

high electrocatalytic activity for an I2/I32 redox reaction to keep a

low overvoltage and decelerated charge recombination.3 On the

other hand, in order to reduce the cost of DSSCs, conductive

polymers and carbon materials, such as PANIs (polyanilines),4

polypyrrole (PPy),5 carbon nanotubes (CTNs),6 graphene,7 poly(3,4-

ethylenedioxythiophene) (PEDOT)8,9 have been widely attempted.

Among conductive polymers, PANI is one of the most

attractive conducting polymers, due to its easy synthesis, high-

conductivity, good environmental stability and interesting redox

properties.10 The PANI nanofiber has attracted more interest

because of its surface-to-volume ratios and potential applications

in electrochemical devices.11 PANI nanofibers can be facilely

synthesized by either chemical oxidation12 or electrochemical

polymerization13 under mild conditions. Recently, several oxida-

tion polymerization methods to fabricate polyaniline nanofibers

without surfactants or templates have been developed, such as

interfacial polymerizations,14 rapidly mixed reactions,12 dilute

polymerizations,15 and two-step growths etc.16

Here, a polyaniline–platinum hybrid nanofiber (PANI/Pt) film is

directly grown on a conductive glass substrate by a facile

electrochemical deposition method on an electrochemical work-

station. The resultant PANI/Pt film is used as the counter electrode

for a DSSC, based on the PANI/Pt counter electrode, and the DSSC

achieves a high conversion efficiency of 7.69%.

Scheme 1 shows the synthesis of the hybrid PANI/Pt film

electrode, which is prepared by a two-step electrochemical

deposition method (see ESI{). Firstly, a PANI nanofiber film is

deposited onto a conductive glass substrate; secondly, the Pt

nanoparticles are further deposited onto the surface of the PANI

nanofibers, thus the hybrid PANI/Pt film on an indium tin oxide

(ITO) glass substrate is formed.

Fig. 1 gives the morphologies of the Pt, PANI and PANI/Pt

electrodes at different magnifications. Fig. 1a and b are the scanning

electron microscope (SEM) images of the PANI electrode, it can be

seen that the PANI nanofibers possess a large amount of pores and a

one-dimensional structure, which provides a high effective surface

area for the PANI film.17 The SEM images of the PANI/Pt are also

presented in Fig. 1c and d, it can be observed that the PANI/Pt

nanofiber’s one-dimensional structure still remains. Comparing the

morphologies of the pristine PANI and the hybrid PANI/Pt, the

PANI/Pt hybrid has a much rougher surface than the pristine PANI

fiber does, which indicates that the Pt nanoparticles have been

deposited onto the surface of the PANI nanofibers.11 Fig. 1e and f

are the SEM images of the Pt electrode, the Pt particles are ball-like

and separately deposited on the substrate’s surface. From the

magnified SEM image (Fig. 1f), the Pt particles are agglomerated

and exist independently with a diameter of about 200 nm. The

agglomerated structure of Pt is disadvantageous for its electro-

catalytic performance.

The formation of the PANI/Pt hybrid nanofibers was character-

ized by energy dispersive X-ray spectroscopy (EDS). The EDS

spectrum of the PANI sample (ESI,{ Fig. S2a) shows the peaks

corresponding to the C, N, Sn and O elements, while the PANI/Pt

sample (ESI,{ Fig. S2b) shows a new peak corresponding to Pt

beside that of the C, N, Sn and O elements, which indicates the

existence of Pt and confirms the successful synthesis of PANI/Pt

hybrid nanofibers.

To further detect the structures of the PANI nanofibers and the

PANI/Pt hybrid nanofibers, Fourier transform infrared spectroscopy

(FTIR) spectra of the PANI and PANI/Pt nanofibers were measured

and are shown in Fig. S3 (ESI{). For the PANI sample, all the PANI

characteristic absorption peaks are observed in the spectra.

Compared with the Pt sample, the PANI/Pt sample has no obvious

absorption peak changes except that the peaks are slightly red or

blue shifted. For example, the band at 1120 cm21 in the PANI

sample (corresponding to the C–H in-plane deformation, which has

been used by Chiang and MacDiarmid18 as a measure of the extent

Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou,362021, P.R. China. E-mail: jhwu@hqu.edu.cn{ Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra20180a/

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4062 | RSC Adv., 2012, 2, 4062–4064 This journal is � The Royal Society of Chemistry 2012

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of the electron delocalization in PANI) red shifts to 1140 cm21 in the

PANI/Pt sample; this implies that the PANI/Pt film has a higher

degree of protonation and electrical conductivity than the PANI

sample.19 The FTIR spectra further confirm the formation of the

PANI/Pt nanofibers after the second electrodeposition process.

Using an I2/I32 redox as the supporting electrolyte, the cyclic

voltammogram curves of the I2/I32 redox mediator for the PANI, Pt

and PANI/Pt electrodes are shown in Fig. 2a. In DSSCs, electrons

are injected into a photo-oxidized dye from I2 ions in the electrolyte

[eqn (1)], and the I32 ions produced are reduced on the counter

electrode [eqn (2)].20

3I2 2 2e2 = I32 (1)

I32 + 2e2 = 3I2 (2)

The current peak of the positive potential (around 0.5 V) is

assigned to the oxidation reaction (eqn (1)) and the current peak of

the negative potential (about 20.2 V) is assigned to the reduction

reaction (eqn (2)).21,22 In Fig. 2a, the PANI/Pt electrode shows a

much larger current density for the I32 reduction and I2 oxidation

than both of the Pt and PANI electrodes, which means a faster redox

reaction rate and a better electrocatalytic activity for the I2/I32 redox

couple on the PANI/Pt electrode. This is ascribed to the unique 1-D

nanofiber structure, the large active surface area and the increased

catalytic active sites of the PANI/Pt electrode.23 According to the

SEM observation, the PANI/Pt film is interconnected and micro-

porous, thus, this structure is favorable for the electrolyte permeation

and I32 reduction. Moreover, the Pt nanoparticles are uniformly

dispersed on the surfaces of the 1-D PANI nanofibers, which

provides more catalytic active sites and faster electron transportation

channels, logically, leading to an enhanced electrochemical activity of

the PANI/Pt hybrid film electrode.

Electrochemical impedance spectroscopy (EIS) measurements

were carried out to compare the charge transfer and ion transport

characteristics of the different electrodes. In Fig. 2b, the EIS results

show well-defined single semicircles over the high frequency range,

followed by short straight lines in the low-frequency region for the Pt

electrodes. The PANI/Pt electrode has the lowest Rct of 2.51 V cm2,

which is lower than both that of the PANI (4.53 V cm2) and

Fig. 2 (a) Cyclic voltammograms (CVs) of the PANI, Pt and PANI/Pt

electrodes using an acetonitrile solution containing 0.1 M LiClO4, 0.01 M LiI

and 0.001 M I2 as the supporting electrolyte, scan rate = 10 mV s21; (b) EIS

spectra of the cells with two identical electrodes, the PANI, Pt and PANI/Pt

were used as the working electrodes, respectively (Rs is serial resistance, Cdl

is the constant phase element, Rct is the charge-transfer resistance and Zw is

the diffusion impedance). (c) Photocurrent–voltage curves of the DSSCs with

the PANI, Pt and PANI/Pt electrodes.

Scheme 1 The two-step electrodeposition of the PANI/Pt hybrid electrode.

Fig. 1 SEM images of the PANI electrode (a, b), PANI/Pt electrode (c, d)

and Pt electrode (e, f) at different magnifications.

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Pt (4.73 V cm2) electrodes. The lower Rct for the PANI/Pt electrode

implies that the reduction of I32 is more advantageous on the PANI/

Pt electrode than that of the other two electrodes.8 In view of the

excellent electrocatalytic activity, and lower charge transfer resis-

tance, it is expected that the DSSC based on a PANI/Pt counter

electrode can achieve an improved performance.

Fig. 2c shows the photocurrent–voltage curves of the DSSCs with

the PANI, Pt and PANI/Pt electrodes under a simulated solar light

illumination of 100 mW cm22. The photovoltaic parameters of the

DSSCs such as short current density (JSC), open voltage (VOC), fill

factor (FF) and the light-to-electric energy conversion efficiency (g)

are listed in Table 1.

Among the three DSSCs, the DSSC with the PANI counter

electrode shows the smallest JSC, which may be ascribed to the lower

conductivity of the PANI film. While the DSSC with the Pt counter

electrode has the smallest light-to-electric conversion efficiency and

FF, which may be ascribed to the aggregation and discontinuous

distribution of the Pt particles on the substrate. The DSSC with the

PANI/Pt electrode shows the best photovoltaic performance and a

light-to-electric conversion efficiency of 7.69%, which is a great

improvement when compared with the DSSCs with the Pt and PANI

counter electrodes. The higher light-to-electric efficiency for the

DSSC with the PANI/Pt electrode is attributed to the following

reasons: (i) according to the SEM images, a thin layer of Pt

nanoparticles was evenly coated on the PANI nanofibers, which

provides good conductivity and more catalytic active sites for the

reduction of I32 compared with the PANI counter electrode; (ii) the

microporous PANI/Pt nanofibers have higher accessible surface

areas24 compared with the Pt counter electrode, which facilitates the

electrolyte–electrode interfacial contact and contributes to the

enhanced charge collection efficiency; (iii) from the CV and EIS

measurements, the PANI/Pt hybrid counter electrode shows an

enhanced electrochemical activity and lower Rct compared with the

PANI and Pt electrodes. The above reasons are beneficial for the

I32/I2 redox couple regeneration and the electron transportation,

logically, the photovoltaic performance of the DSSC with a PANI/Pt

electrode can be improved.

In summary, a one-dimensional PANI nanofiber supported Pt

nanoparticle film was prepared by a two-step electrochemical

deposition method. The PANI/Pt film possesses high conductivity,

high surface area and high catalytic activity. Using the PANI/Pt film

as a counter electrode, a dye-sensitized solar cell achieves a light-to-

electric energy conversion efficiency of 7.69% under a simulated solar

illumination with an intensity of 100 mW cm22, which is higher than

those with pure PANI or Pt counter electrodes.

Acknowledgements

This work was supported by the National High Technology

Research and Development Program of China (No.

2009AA03Z217) and the National Natural Science Foundation

of China (Nos. 90922028 and 51002053). Dr Bin Xu of the

Institute of Urban Environment, Chinese Academy of Sciences,

is also acknowledged for his assistance in the SEM measure-

ments.

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Table 1 The photovoltaic performance of the DSSCs with PANI, Ptand PANI/Pt counter electrodes

Counterelectrode

Rct / V cm22 JSC / mA cm22 VOC / V FF g / %

PANI 4.53 13.4 0.728 0.676 6.58Pt 4.73 13.8 0.752 0.628 6.52PANI/Pt 2.51 14.3 0.766 0.704 7.69

4064 | RSC Adv., 2012, 2, 4062–4064 This journal is � The Royal Society of Chemistry 2012

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