Electropolymerization Polyoxometalate (POM) - American Chemical

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Electropolymerization Polyoxometalate (POM)-Doped PEDOT Film Electrodes with Mastoid Microstructure and Its Application in DyeSensitized Solar Cells (DSSCs) Chunchen Yuan, Shuangshuang Guo, Shiming Wang, Lin Liu, Weilin Chen,* and Enbo Wang* Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Ren Min Street No. 5268, Changchun, Jilin, 130024, People’s Republic of China ABSTRACT: A polyoxometalate (POM)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) hybrid film counter electrode was successfully fabricated by electropolymerization in an environmentally friendly aqueous solution. The hybrid film presents a similar surface morphology to lotus leaf, the microstructure of which is a regularly distribution of mastoid shapes. As far as we know, it is the first time that POM-PEDOT hybrid film with mastoid morphology was fabricated. The application of the hybrid film counter electrodes (CEs) in dye-sensitized solar cells (DSSCs) was studied for the first time, and the efficiency of DSSCs based on hybrid film CEs was almost as high as DSSCs with Pt CEs. FTIR results indicated that POM polyanions had already been dispersed into the PEDOT matrix. Cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization test results show that the electrocatalytic ability for I3− reduction of the hybrid film CE is comparable to that of a Pt CE. In addition, the SiW11−PEDOT hybrid film has good electrochemical stability.

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INTRODUCTION Currently, the rising demand and the accelerating consumption of energy threaten the development of society and the economy. More and more efforts have been made, with regard to exploring abundant and renewable energy resources, especially solar energy. Hence, a large number of photovoltaic devices have been fabricated in succession. Among the numerous photovoltaic devices, dye-sensitized solar cells (denoted DSSCs) have aroused great attention, because of their high efficiency and ease of fabrication.1 At present, most of the research on further improving the efficiency of DSSCs has been focused on developing the components of DSSCs, such as semiconductor nanocrystalline TiO2,2−5 counter electrodes (denoted hereafter as CEs),6−8 dye molecules,9−13 and electrolytes.14−17 Fluorine-doped tin oxide (FTO) or tindoped indium oxide (ITO) glass coated with platinum has been widely used as the CEs of DSSCs for their excellent properties, such as high electrocatalytic activity for the conversion of I3− back to I− in the electrolyte and high conductivity. However, more and more queries about the dependence on platinum has been proposed, regarding not only its high cost, which limits its wide application and industrialization, but also the thermal treatment for the synthesis of conventional platinum counter electrodes, which limits the application of plastic substrates in DSSCs. Therefore, finding a new material to replace platinum currently is still an urgent work. In recent years, poly(3,4-ethylenedioxythiophene) (PEDOT) has shown significant promise for the challenge, because of its high conductivity, electrochemical reversibility, good stability at room temperature, and high electrocatalytic activity.18,19 Hence, a large number of researchers have focused on improving the efficiency of PEDOT-based DSSCs.20−22 To our knowledge, all in all, there are two methods of fabricating PEDOT-based counter electrodes: one is coating the as-prepared PEDOT slurry onto the substrate by spin coating (or the doctor blade © 2013 American Chemical Society

method) and the other one is electropolymerizing the monomer EDOT onto conducting substrate directly. The PEDOT used in the former method is generally prepared via chemical oxidative polymerization, in which Fe(III) compounds are commonly used as oxidants. However, trace amounts of Fe(III/II) oxidants existed in the polymer, which would induce low energy conversion efficiency.23,24 Pringle et al. synthesized PEDOT in an ionic liquid and introduced gold chloride as the oxidant for the first time.25 Although the DSSCs based on the counter electrodes that they prepared achieved high performance, the cost of gold chloride is not very economical; therefore, the cost of the solar cell is increased. In addition, PEDOT-PSS is a commercially available form of PEDOT26,27 that exists as a water dispersion and also can be deposited by spin coating. The performance of PEDOT-PSS-based DSSCs can be improved by the addition of various inorganic materials, such as graphene28 and carbon nanotubes (CNTs).29 Moreover, in the same manner as incorporation of graphene and CNTs, many types of nanoparticles (ZnO, NiO, Al2O3, and TiO2)30,31 have been mixed into PEDOT-PSS, which could enhance the film roughness and, therefore, increase the exposed surface area of the counter electrodes. However, the cost of PEDOT-PSS also is not very economical. Different from chemical polymerization, electropolymerization is considerably more economic and efficient. As far as we know, to date, only a few studies on the electrochemical synthesis of the PEDOT counter electrode have been investigated, and most of the PEDOT counter electrodes were synthesized in an organic solvent and ionic liquid,25,32−34 which were not environmentally friendly. Furthermore, studies of the doping of Received: Revised: Accepted: Published: 6694

October 18, 2012 March 18, 2013 April 25, 2013 April 25, 2013 dx.doi.org/10.1021/ie302845z | Ind. Eng. Chem. Res. 2013, 52, 6694−6703

Industrial & Engineering Chemistry Research

Article

Figure 1. Photocurrent−voltage curves for DSSCs with SiW11−PEDOT and PEDOT counter electrodes (CEs) (various sweep cycles (2−40 sweep circles (SCs))).

different anions (TsO−, ClO4−, PSS−) into PEDOT by electropolymerization also have been conducted, but the efficiency of DSSCs based on doped-PEDOT counter electrodes did not satisfy the researchers.35 Hence, it is a good way to introduce a new type of anion into PEDOT by means of electropolymerization, relying on an aqueous solution to improve the efficiency of DSSCs, which also conforms with environmental protection tendencies. Polyoxometalates (denoted as POMs), which are a wellknown class of transition-metal oxide nanoclusters with welldefined structure, reversible multielectron electrochemical reactions, catalytic activity,36−38 and unique photoelectrochemical properties,39 are currently of great interest to researchers in numerous fields. Many polyoxometalates, such as Keggin and Dawson POM anions, are able to accept large numbers of electrons, giving rise to mixed-valence species, which make them greatly attractive in the preparation of modified electrodes and in electrocatalysis.40,41 POMs have been incorporated to conducting polymers to take responsibility for the overall electroactivity ability while the polymeric structure is retained in hybrid materials.42−49 Almeida et al. utilized Keggin-type POM-doped conducting polyanilines as counter electrodes in the DSSCs, although the efficiency of the cells must be increased.50 Recent reports51−55 established that the immobilization of POM redox centers in PEDOT hybrid film showed improved charge propagation dynamics and potential application in the field of electrocatalysis. Consequently, POMs could be able to optimize the electrochemical properties of PEDOT. Herein, we design and fabricate a new type of POM (K8[SiW11O39]·13H2O)-doped PEDOT hybrid film (denoted as SiW11−PEDOT) via electropolymerization in aqueous solution. The hybrid film presents a lotus leaf-like surface with a regularly intermingled small mastoid shape microstructure. To the best of our knowledge, this is the first time that a POM-PEDOT hybrid film with mastoid morphology has been fabricated. Notably, the electrocatalytic activity for I3− reduction of as-prepared SiW11−PEDOT hybrid film is confirmed to be comparable with platinum via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization tests for the first time. Furthermore, the cost of the SiW11−PEDOT counter electrode is lower and the preparation is simpler than that for platinum. Especially, DSSCs based on this new type of hybrid film

electrode exhibit higher conversion efficiency (η), higher short current density (Jsc), and higher open circuit voltage (Voc) than DSSCs with PEDOT-only counter electrodes, which are comparable with the DSSCs based on platinum counter electrodes. To the best of our knowledge, the present paper is the first report on introducing the POM-doped PEDOT hybrid film counter electrode into DSSCs.

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EXPERIMENTAL PROCEDURE 3,4-Dioxyetylenethiophene (EDOT) monomer was kindly donated by Yieldpharma Electronic Chemical Co., LTD (Shenzhen, PRC). N719, electrolyte, and photoanodes were purchased from Heptachroma (Dalian, PRC). 3-Aminopropyltriethoxysilane (denoted APS) and other chemicals were purchased from Aladdin. The potassium salts of the α-Keggin silicotungstates K8[SiW11O39]·13H2O (denoted SiW11) was prepared by previously published procedures.56 All the aqueous solutions were prepared with double-distilled water. All the chemicals were used as received, without further purification. Electrochemical experiments were performed on a Model CS350 electrochemistry station (CH Instruments, Wuhan CorrTest Instrument Corporation, PRC). The performance of the DSSCs were measured under 1 sun illumination (AM = 1.5). The SiW11−PEDOT CE was prepared by electrochemical deposition. Cyclic voltammetry was carried out in threeelectrode system in which a FTO glass was used as working electrode, saturated calomel electrode was used as reference electrode and platinum wire was used as the counter electrode. FTO glass was washed with double-distilled water, isopropyl alcohol, and ethanol in turn before used. Electrolyte A was an aqueous solution of 1 mM SiW11 and 0.02 M EDOT, and the supporting electrolyte was 0.05 M KCl; the pH value was 4.5. PEDOT-only CE was prepared in electrolyte B, which contained EDOT and KCl with the same concentration as electrolyte A. The pH value of electrolyte B was also 4.5. Electrolytes A and B were under ultrasonic treatment for 30 min before use. The voltage range were from −1.0 V to 1.5 V for electrolyte A and from 0.0 V to 1.5 V for electrolyte B. Scan rate was 100 mV/s. SiW11−PEDOT and PEDOT films were grown on FTO after a variation of sweep circles (2, 5, 10, 20, 30, 40, and 50 sweep cycles (denoted hereafter as SCs)). APSSiW11−PEDOT CE was prepared as follows: the cleaned FTO 6695

dx.doi.org/10.1021/ie302845z | Ind. Eng. Chem. Res. 2013, 52, 6694−6703


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Table 1. Photovoltaic Parameters of DSSCs Based on CEs of Different Sweep Cycle Electropolymerization SiW11−PEDOT and PEDOT Film on Fluorine-Doped Tin Oxide (FTO) SiW11−PEDOT

CE number of sweep circles (SCs) 2 5 10 20 30 40

Jsc [mA/cm2] 13.94 17.10 17.10 16.60 16.17 14.57

± ± ± ± ± ±

0.05 0.01 0.01 0.02 0.01 0.06

Voc [mV] 591 640 633 632 625 576

± ± ± ± ± ±

1 1 1 4 2 2

PEDOT η [%]

FF 0.473 0.533 0.499 0.484 0.429 0.394

± ± ± ± ± ±

0.05 0.01 0.01 0.01 0.03 0.02

3.91 5.81 5.42 5.07 4.34 3.31

± ± ± ± ± ±

0.09 0.04 0.04 0.05 0.06 0.03

Jsc[mA/cm2] 12.93 13.35 13.32 12.53 12.06 9.33

± ± ± ± ± ±

0.03 0.01 0.01 0.02 0.05 0.01

Voc [mV] 598 648 629 586 610 588

± ± ± ± ± ±

3 4 4 4 2 3

η [%]

FF 0.531 0.531 0.484 0.447 0.410 0.394

± ± ± ± ± ±

0.04 0.01 0.02 0.03 0.02 0.01

4.10 4.60 4.05 3.29 3.02 2.16

± ± ± ± ± ±

0.05 0.08 0.06 0.07 0.06 0.05

glasses were dipped into 5% (v/v) APS-toluene solution for 8 h, then immersed in HCl (pH 2.0) for 30 min to form the precursor film before use.57,58 SiW11−PEDOT film was grown on APS-FTO by cyclic voltammetry in electrolyte A after 5 sweep cycles. Scan rate was 100 mV/s. To fabricate DSSCs, TiO2 electrode was immersed in N719 dye solution with a concentration of 5 × 10−4 mol/L in dry ethanol for 20 h. The thickness of TiO2 photoanode is 10−12 μm and its active area is 0.36 cm2. The cell was assembled with two electrodes separated by the typical electrolyte (0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile).

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RESULTS AND DISCUSSION Three types of DSSCs were fabricated with SiW11−PEDOT, PEDOT-only, and Pt CEs. SiW11−PEDOT and PEDOT-only CEs were prepared by means of various SCs. The performance of the DSSCs with the aforementioned CEs were measured under irradiation of simulated sunlight (AM = 1.5). Figure 1 shows the photocurrent density−photovoltage (J−V) curves of DSSCs fabricated with SiW11−PEDOT and PEDOT CEs. Open circuit voltage, (Voc), short-circuit current density (Jsc), and fill factor (FF) are three important parameters that are used to calculate the conversion efficiency (η) of solar cells:30 η=

Figure 2. Photocurrent−voltage curves for DSSCs with 5 SC APSSiW11−PEDOT, 5 SC APS-PEDOT, 5 SC SiW11−PEDOT, 5 SC PEDOT, and Pt CEs.

Table 2. Photovoltaic Performance of DSSCs with 5 SC APS-SiW11−PEDOT, 5 SC APS-PEDOT, 5 SC SiW11− PEDOT, 5 SC PEDOT, and Pt CEs

Voc × Jsc × FF Pin

(1)

Increases in Voc, Jsc, and FF would lead to higher efficiency. The resulting values of Jsc, Voc, FF, and η are summarized in Table 1. Comparing the J−V curves and performance parameters of SiW11−PEDOT- and PEDOT CE-based DSSCs, it is obviously that DSSCs with SiW11−PEDOT CEs show notable performance, which are generally superior to that of DSSCs with PEDOT-only CEs. Namely, the efficiency of PEDOT-based DSSCs is significantly increased after the incorporation of SiW11 polyanions. As seen in Table 1 and Figure 1, 5 SCs is the best preparation condition under which the as-prepared SiW11−PEDOT CEs achieved excellent conversion efficiency of η = 5.81%, the Jsc value was increased by 28.4% and the η value was increased by 30.9%, compared to that observed with 5 SC PEDOT-only CE-based DSSCs. We assumed that SiW11 polyanions were dispersed into the polymer matrix electrostatically, with the result of improving the electrocatalytic activity of PEDOT for the conversion of I3− back to I−. Moreover, after introducing the APS precursor film, the performance of 5 SC APS-SiW11−PEDOT CE-based DSSCs is extremely close to that of Pt CE-based DSSCs (see Figure 2 and Table 2); however, the performance of 5 SC APS-PEDOT CE-based DSSCs shows no significant difference from PEDOTonly CE-based DSSCs. We presumed that positively charged

CE

Jsc [mA/cm2]

Voc [mV]

FF

η [%]

APS-SiW11− PEDOT SiW11− PEDOT APSPEDOT PEDOT Pt

16.98 ± 0.01

645 ± 2

0.540 ± 0.01

5.93 ± 0.04

17.10 ± 0.01

640 ± 1

0.533 ± 0.01

5.81 ± 0.04

13.31 ± 0.01

648 ± 3

0.531 ± 0.01

4.58 ± 0.08

13.35 ± 0.01 17.11 ± 0.01

648 ± 4 638 ± 5

0.531 ± 0.01 0.541 ± 0.01

4.60 ± 0.08 5.94 ± 0.06

APS precursor film and negatively charged SiW11 interacted electrostatically with each other; in other words, APS precursor film acted as an adhesion promoter for the subsequently electropolymerized SiW11−PEDOT layer. Therefore, the APS precursor film is supposed to improve the contact between the SiW11−PEDOT film and the substrate. In order to confirm the aforementioned hypothesis, we selected the 5 SC APS-SiW11− PEDOT and APS-PEDOT CEs as samples to carry out a series of studies. A Fourier transform infrared (FTIR) spectra method was used to investigate the interaction between positively oxidized PEDOT and the negatively charged POM polyanions. Figure 3 shows the FTIR spectra of SiW11−PEDOT and SiW11, respectively. SiW11−PEDOT powder was scraped off asprepared electrodes. The assignment of the bands is reported in Table 3. Significant changes were observed for some bands of the SiW11 polyanions in the hybrid film compared to those of 6696



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in FESEM and AFM testing were obtained on indium tin oxide (ITO) glass in order to meet the testing requirement. Figures 4

Figure 4. FESEM images of (a, c) PEDOT-only film and (b, d) SiW11−PEDOT film.

and 5 show the FESEM and AFM images, respectively. As shown in Figure 5, the hybrid film surface is denser than that of the PEDOT-only film. According to FESEM, it is notable that the PEDOT-only film presents a network structure, while the SiW11−PEDOT film displays a lotus leaf-like surface, which shows a microstructure of small mastoid shapes regularly intermingled. The small mastoid shape microstructures are considered to be blocks of SiW11. The root-mean-square roughness (denoted as RMS) was also measured by AFM. The RMS values of SiW11−PEDOT and PEDOT film were 32.54 and 70.34 nm, respectively. It is obviously that the RMS of the film is decreased after doping POM into PEDOT film. The N2 adsorption−desorption method is used to evaluate the surface area of the as-prepared film; the Brunauer−Emmett−Teller (BET) surface areas of the PEDOT-only film and the SiW11−PEDOT film are 11.24 and 0.22 m2/g, respectively. Taking both the RMS and BET surface area into consideration, the PEDOT-only film has a larger electrochemical surface area than the SiW11−PEDOT film. Therefore, the excellent performance of DSSCs with SiW11− PEDOT CEs is expected to profit from the outstanding electrocatalytic activity of SiW11−PEDOT material in reducing I3−. Field-emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FESEM-EDS) is able to analyze the components of the SiW11−PEDOT and PEDOTonly films. The EDS spectra are presented in Figure 6. Elemental tungsten (W) appears in Figure 6a, and the content of elemental silicon (Si) is greater in Figure 6a than in Figure 6b; the obvious differences between Figure 6a and Figure 6b verify the existence of SiW11 polyanions in the hybrid film. In addition, the elements carbon (C), oxygen (O), and sulfur (S) constitute PEDOT. Sodium (Na), aluminum (Al), magnesium (Mg), indium (In), and calcium (Ca) are the components of ITO conducting glass. Elemental silicon exists in both ITO conducting glass and SiW11 polyanions. Thickness is also an important property of CEs, which affects the properties of CEs materials. According to the step profiler test results, the thickness of the PEDOT-only film is 1.6 μm and the SiW11−PEDOT film is 1.3 μm. There is no significant difference between different CE thicknesses. In order to evaluate the electrocatalytic ability of APSSiW11−PEDOT CEs, CV was carried out using two types of electrolytes in a typical three-electrode system. As documented

Figure 3. Fourier transform infrared (FTIR) spectra of SiW11 and SiW11−PEDOT powder.

Table 3. Frequency Values and Assignment of FTIR Bands Observed from SiW11 and SiW11−PEDOT Samples W−O [cm−1] a

sample SiW11 SiW11− PEDOT a

Si−O [cm−1]

corner-shared oxygen

edge-shared oxygen

terminal oxygen

888 917

866 841

794 795

956 979

Tetrahedral oxygen.

the pure SiW11. The characteristic W−Ob (corner-shared) band shifts to lower frequencies of the hybrid film, while the W−Od (terminal) band and the Si−Oa band shift to higher frequencies. The above phenomena indicate not only the existence of SiW11 polyanions but also extremely obvious differences in the environment of SiW11 polyanions between SiW11−PEDOT hybrid material and pure SiW11. It is considered as SiW11 polyanions dispersed in the polymer matrix, where they would be surrounded by positively charged PEDOT, as mentioned in previous work.59 As depicted by Fernandes,60 electropositive PEDOT and SiW11 polyanions interacted with each other by electrostatic interaction during the electropolymerization process. Hence, we deduced that SiW11 polyanions act as a dopant during the formation process of the hybrid film; therefore, it should be able to optimize the electrochemical properties of PEDOT. Atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM) were used to provide detailed information about the surface morphology and the homogeneity of SiW11−PEDOT and PEDOT-only CEs. The CEs used 6697



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Figure 5. AFM of different electrodes, the left ones correspond to PEDOT-only electrode and the right ones correspond to SiW11−PEDOT electrode.

Figure 6. Field-emission scanning electron microscopy, coupled with energy-dispersive spectroscopy (FESEM-EDS), of (a) the SiW11−PEDOT film and (b) the PEDOT-only film.

values of ΔEp are 146, 225, and 132 mV for APS-SiW11− PEDOT, APS-PEDOT, and Pt electrodes, respectively. Moreover, the values of peak current density of APS-SiW11−PEDOT electrode are similar to Pt electrode and higher than APSPEDOT electrode. The results indicate that the electron

in the previous literature,14,15 lower peak separation (ΔEp) and higher peak current density might be associated with higher electron transfer rate. Figure 7a shows the CV results obtained from different electrodes in K3Fe(CN)6 aqueous solution that contains a single-electron transfer medium (Fe2+/Fe3+). The 6698



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Figure 7. Cyclic voltammograms of the APS-SiW11−PEDOT, APS-PEDOT, and Pt electrodes (a) with 6 mM K3Fe(CN)6 aqueous solution and 1 M KNO3 as the supporting electrolyte and (b) with 1 mM I2, 10 mM LiI in 3-methoxypropionitrile, and 0.1 M LiClO4 as the supporting electrolyte. The scan rate is 50 mV/s.

transfer rate of the APS-SiW11−PEDOT electrode is extremely close to that of the Pt electrode, but higher than that of APSPEDOT electrode. Figure 7b shows the CVs of different electrodes in a two-electron transfer medium (I−/I3−) electrolyte, which is able to simulate the inner condition of a real DSSC. Two pairs of redox peaks are obtained, which can be assigned to the following reactions:13 I3− + 2e− ↔ 3I− −

3I 2 + 2e ↔

configuration with two identical APS-SiW11−PEDOT, APSPEDOT, SiW11−PEDOT, PEDOT, and Pt CEs. The frequency range explored was 0.01 Hz to 105 Hz, with the ac amplitude perturbed at 10 mV. The applied bias voltage, between two symmetrical counter electrodes, was set at 0 V. Figure 8 shows

(2)

2I3−

(3)

The values of ΔEp and peak current density are listed in Table 4. It is obviously that the ΔEp values of the APS-SiW11− Table 4. Electrochemical Parameters of Different CEs ΔEp(mV)

CE APS-SiW11− PEDOT APS-PEDOT Pt

Current Density, Jsc (mA/cm2)

left peak

right peak

upper left peak

lower left peak

upper right peak

lower right peak

475

343

1.388

−1.378

1.702

−0.457

598 473

564 256

1.285 1.396

−1.358 −1.326

1.578 1.805

−0.688 −0.464

−

−

Figure 8. Electrochemical impedance spectroscopy (EIS) for different counter electrodes (CEs). The frequency range explored was 0.01 Hz to 105 Hz, with the ac amplitude perturbed at 10 mV. The applied bias voltage, between two symmetrical CEs, was set at 0 V.

I2 /I3−

are PEDOT CEs for both redox reactions of I3 /I and much smaller than that of the APS-PEDOT electrode. Comparing the ΔEp of APS-SiW11−PEDOT CEs and Pt CEs, the ΔEp values of left peak of APS-SiW11−PEDOT CEs and Pt CEs are almost equivalent, while the ΔEp value of right peak of APS-SiW11−PEDOT CEs is slightly larger than the ΔEp value of Pt CEs. Furthermore, the peak current density values of the APS-SiW11−PEDOT electrode are similar to that of the Pt electrode and higher than that of the APS-PEDOT electrode. Distinctly, the electrocatalytic activity for the conversion of I3− back to I− redox reaction of the APS-SiW11−PEDOT electrode is close to the Pt electrode and higher than the APS-PEDOT electrode. According to the resulting values, the incorporation of SiW11 polyanions enhances the electrocatalytic ability of PEDOT. EIS was employed to verify the electrocatalytic activity of APS-SiW11−PEDOT in reducing I3− and to evaluate the contact between the SiW11−PEDOT and the substrate. EIS measurement was taken in a symmetrical dummy cell

the typical Nyquist plot of different CEs. The intercept of the plot and the real axis represents the series resistance (Rs), and the arch on the left of the plot represents the Rct and the double capacitance in the electrode and electrolyte interface. Usually, Rct is related to exchange current density (J0), which is caused by reducing of I3− to I− at the counter electrode. Equation 461 shows their relationship: J0 =

RT nFR ct

(4)

where J0 is the exchange current density, R the gas constant, T the temperature, n the number of electrons in the redox, F the Faraday constant, and Rct the charge transfer resistance. The values of Rct and Rs for different electrodes were obtained by fitting EIS spectra and are summarized in Table 5. The Rs 6699



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to I−, based on the J0 values. In addition, the intersection of the cathodic branch with the Y-axis can be considered as the limiting diffusion current density (Jlim). Jlim is directly proportion to the diffusion coefficient of the I3−, according to eq 5:

Table 5. Electrochemical Parameters of Different CEs counter electrode, CE

Rs [Ω/cm2]

Rct [Ω /cm2]

APS-SiW11−PEDOT APS-PEDOT SiW11−PEDOT PEDOT Pt

20.88 28.13 27.62 27.21 5.45

7.92 14.77 7.67 14.85 6.71

D=

lJlim 2nFC

(5)

where D is the diffusion coefficient of the I3−, l the spacer thickness, and C the I3− concentration. (Jlim and F are as previous defined.) Jlim values of different electrodes are of the same magnitude, while the Jlim values of the APS-SiW11− PEDOT and the Pt electrodes are higher than the APS-PEDOT electrode, illustrating that the diffusion coefficient of the I3− in the electrolyte of the former two ones are higher than the latter one in the symmetrical dummy cell. The excellent diffusion character of the I3− in the APS-SiW11−PEDOT based symmetrical dummy cell electrolyte benefits from the superior electrocatalytic activity of the APS-SiW11−PEDOT CEs. In an APS-SiW11−PEDOT-based symmetrical dummy cell, under the efficient electrocatalytic character of CE, no sooner the I3− have been efficiently reduced to I− nearby the CE than a new mass of I3− diffuses to CE. Thus, a speedy diffusion of the I3− forms, which contributes to high energy conversion efficiency. In conclusion, High values of J0 and Jlim reflect high electrocatalytic activity of the APS-SiW11−PEDOT CE. The Tafel results are generally in agreement with the EIS results. It has been verified by CV, EIS, and Tafel polarization measurement that the electrocatalytic activity of the APSSiW11−PEDOT hybrid film CE can be comparable with Pt as CE material. The excellent electrocatalytic activity gives rise to high photoelectric conversion efficiency. In other words, POMdoped PEDOT hybrid films are expected to be a promising CE material for DSSCs. Stability is also an important property of electrode materials. Figure 10 shows 50 consecutive cyclic voltammograms of the I−/I3− system for the APS-SiW11−PEDOT electrode from −1.0 V to 1.0 V (vs Ag/AgCl). With the increase in cycle times, the CV curves show stable peak current densities, indicating that the APS-SiW11−PEDOT hybrid film has good electrochemical stability.34 Moreover, thermogravimetry (TG) of SiW11 from 0

values of the SiW11−PEDOT electrode and PEDOT electrode are higher than that of the Pt electrode, while the Rs value of the SiW11−PEDOT electrode is reduced after the introduction of the APS precursor film. These phenomena indicate that the APS precursor film is able to improve the bad contact between the SiW11−PEDOT film and the substrate, since the positively charged APS precursor film and the negatively charged SiW11 interacting electrostatically with each other. The Rct value of the APS-SiW11−PEDOT electrode is merely slightly higher than that of the Pt electrode, but is sharply lower than that of the APS-PEDOT electrode. This phenomenon indicates that the new hybrid film show more excellent ability to regenerate I− than the pure APS-PEDOT film, which could make the asprepared hybrid film a promising CE material. The EIS results are coincident with those of CVs. Tafel polarization measurement is a powerful electrochemical characterization method. To further evaluate the electrocatalytic ability of different CEs, Tafel curves were measured in a symmetrical dummy cell similar to those utilized in EIS. In the Tafel zone, a larger slope in the anodic or cathodic branch indicates a higher exchange current density (J0) on the electrode.62 As seen in Figure 9, the cathodic branch of the

Figure 9. Tafel polarization curves of different counter electrodes (CEs).

curves show similar slope for the APS-SiW11−PEDOT and the Pt electrodes, which are higher than APS-PEDOT electrodes, indicating a higher J0 on the surface of APS-SiW11−PEDOT electrode. As we know, J0 is in inverse proportion to Rct, according to eq 3. Considering the EIS results, the J0 value on the APS-SiW11−PEDOT electrode surface is similar to the J0 value on the Pt electrode surface. Namely, the change tendency of J0 obtained from Tafel polarization measurements is generally consistent with the variation of Rct obtained from EIS measurements. We can reach the conclusion that the APSSiW11−PEDOT electrode shows similar electrocatalytic ability to the Pt electrode with regard to catalyzing the reduction of I3−

Figure 10. Fifty (50) consecutive cyclic voltammograms of the I−/I3− system for the APS-SiW11−PEDOT electrode (1 mM I2, 10 mM LiI, 0.1 M LiClO4 in 3-methoxypropionitrile). The scan rate is 50 mV/s. 6700



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°C up to 500 °C was performed under nitrogen at a heating rate of 5 °C min−1. As shown in Figure 11, a weight loss in the

Ph.D station Specialized Research Foundation of Ministry of Education for Universities (No. 20120043120007), the Fundamental Research Funds for the Central Universities (Nos. 11QNJJ014 and 11SSXT141), and the Analysis and Testing Foundation of Northeast Normal University.

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Figure 11. Thermogravimetry (TG) curve of K8[SiW11O39]·13H2O.

temperature range of 50−180 °C corresponded to the loss of water molecules, and no other weight loss is observed, even when heated to 500 °C, suggesting that the chosen POM has good thermal stability, enough to satisfy the temperature requirement of DSSCs.

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CONCLUSION We have successfully electropolymerized a new type of POMdoped PEDOT hybrid film counter electrode (CE) to introduce into DSSCs. POM-doped PEDOT hybrid films show excellent electrocatalytic activity for I3− reduction, which is comparable with Pt; its cost is lower and the preparation is simpler than that for Pt. The POM-doped PEDOT hybrid films overcome the negative effect caused by the shortage of surface area to exhibit low ΔEp and high peak current density, low Rct, and high J0. As supported by measurements results from previously discussed methods, the incorporation of POM enhances the electrocatalytic ability of PEDOT. In addition, the SiW11−PEDOT hybrid film CE has good electrochemical stability. Considering all the advantages of the POM-doped PEDOT hybrid film CE, it could be a promising material to substitute Pt in DSSCs. More importantly, the present work takes the first step for using POMs as dopants in the process of PEDOT replacing Pt to fabricate a high electrocatalytic activity and easy-to-handle CE in DSSCs. Further challenges to be tackled include the enhancement of the POM-doped PEDOT hybrid film surface area in order to improve the performance of POM-PEDOT electrode-based DSSC. Further studies along this line are underway.

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AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected] (W.C.); wangeb889@ nenu.edu.cn and [email protected] (E.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21001022, 21131001, and 21201031), Science and Technology Development Project Foundation of Jilin Province (Nos. 20100169 and 201201072), 6701



Article

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