High efficient PANI/Pt nanofiber counter electrode used in dye-sensitized solar cell page 1
High efficient PANI/Pt nanofiber counter electrode used in dye-sensitized solar cell page 2
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High efficient PANI/Pt nanofiber counter electrode used in dye-sensitized solar cell

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  • 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 ORegan 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 polyanilineplatinum 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 isdeposited 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

    nanofibers 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 substrates 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 peakscorresponding to the C, N, Sn and O elements, while the PANI/Pt

    sample (ESI,{ Fig. S2b) shows a new peak corresponding to Ptbeside 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 PANIcharacteristic 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 CH 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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  • 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) Photocurrentvoltage 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.

    This journal is The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 40624064 | 4