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Solar Energy 83 (2009) 845–849

www.elsevier.com/locate/solener

High-performance and low platinum loading Pt/Carbon black

counter electrode for dye-sensitized solar cells
Pinjiang Li a,b, Jihuai Wu a,*, Jianming Lin a, Miaoliang Huang a,
Yunfang Huang a, Qinghua Li a
a
Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou, Fujian 362021, China
b
Institute of Surface Micro and NanoMaterials, Xuchang University, Xuchang 461000, China

Received 11 June 2008; received in revised form 29 October 2008; accepted 29 November 2008
Available online 25 December 2008

Abstract

Pt/Carbon black counter electrode for dye-sensitized solar cells (DSSCs) was prepared by reducing H2PtCl6 with NaBH4 in carbon
black. The Pt/Carbon black electrode had a high electrocatalytic activity for iodide/triiodide redox reaction. Using the Pt/Carbon black
counter electrode, DSSC achieved 6.72% energy conversion efficiency under one sun illumination. Pt/Carbon black electrode shows the
same energy conversion efficiency and lower cost compared with Pt electrode, which makes it available in DSSCs practical applications.
Ó 2008 Published by Elsevier Ltd.

Keywords: Dye-sensitized solar cell; Counter electrode; Platinum; Carbon; Electrochemistry

1. Introduction platinum film. The roles of the counter electrode are to

collect electrons from external circuit and reduce I3 to
Since the prototype of a dye-sensitized TiO2 nanocrys- I in electrolyte. To keep a low overvoltage and lessen
talline solar cell (DSSC) was reported in 1991 by O’Regan energy losses, the counter electrode should have low resis-
and Gratzel, O’Regan and Gratzel, 1991 it has aroused tance and high electrocatalytic activity for iodide/triiodide
intensive scientific and technological interest and have redox reaction Macyk et al., 2007; Papageorgiou et al.,
evolved a potential alternative to traditional photovoltaic 1996; Papageorgiou et al., 1997; Fang et al., 2004.
devices over the past decade due to its low cost and simple Platinized counter electrode has low resistance and high
preparation procedure. O’Regan and Gratzel, 1991; Nazee- electrocatalytic activity for iodide (I)/triiodide (I3) redox
ruddin et al., 1993; Gratzel, 2004; Wu et al., 2007a; Wu couple in DSSCs. But platinum is one of the costly precious
et al., 2007b. metals. Hence, other materials are attempted to reduce pro-
Generally, a DSSC consists of three main components: duction cost of DSSCs. Some carbonaceous materials such
a dye-covered nanocrystalline TiO2 layer on a transparent as carbon nanotubes, activated carbon, and graphite have
conductive glass substrate, an electrolyte contained been employed as catalysts for counter electrodes to
iodide/triiodide redox couple, and a platinized conductive replace the Pt electrode Suzuki et al., 2003; Kay and
glass substrate as a counter electrode. Counter electrode, Gratzel, 1996; Lindstrom et al., 2001; Imoto et al., 2003.
as one important component in DSSCs, is usually con- However, the conversion efficiency of these cells was rela-
structed with a conducting glass substrate coated with tively low owing to their poor catalytic activity for I3
reduction.
*
Corresponding author. Tel.: +86 595 22693899; fax: +86 595
Carbon materials are inexpensive and show low resis-
22693999. tance and electrocatalytic activity for the reduction of tri-
E-mail address: jhwu@hqu.edu.cn (J. Wu). iodide Wroblowa and Saunders, 1973; Kinoshita, 1987;

0038-092X/$ - see front matter Ó 2008 Published by Elsevier Ltd.

doi:10.1016/j.solener.2008.11.012
846 P. Li et al. / Solar Energy 83 (2009) 845–849

Kamau, 1988. The catalytic active sites of carbon mate- 2.3. Preparation of TiO2 paste
rials usually locate on the edges of carbon crystal sheet.
Consequently the carbon black with low crystalline and a 0.05 mol of acetic acid was mixed with 0.05 mol of tita-
great amount of edges may be more active than that the nium iso-propoxide under stirring at room temperature.
carbon materials with highly orientated such as graphite The mixed solution was rapidly poured into 120 ml distilled
or carbon nanotubes Murakami et al., 2006. In this water with vigorous stirring for 30 min and a white precip-
paper, a Pt/Carbon black counter electrode for DSSC itate was formed. Then, acetic acid (12 ml) and nitric acid
was prepared. The properties of the counter electrode, solution (65 wt%, 1.2 ml) were added. Then the system
including morphology, electrocatalytic activity and the was peptized at 80 °C for 12 h and was autoclaved at
effect of platinum loading on the performance of DSSC 200 °C for 12 h to form a white suspension with some pre-
were investigated. cipitate. The resultant suspension was concentrated to 1/4
of its original volume, PEG-20000 (10 wt% TiO2) and a
2. Experimental few drops of emulsification regent of Triton X-100 was
added to the resultant colloidal solution with stirring. Then
2.1. Materials the colloidal solution was concentrated to form a TiO2
paste of suitable concentration.
Titanium (IV) iso-propoxide and 4-tert-butylpyrldine
(TBP) were purchased from Fluka and used as received. 2.4. Fabrication of DSSC
Chloroplatinic acid (H2PtCl6), carbon black powder, ace-
tonitrile, tetrapropylammonium iodide, potassium iodinate A DSSC (active area of 0.25 cm2) was assembled accord-
and iodine were all purchased from Shanghai Chemical ing to the following procedure. Conducting glass sheet
Agent Ltd. China, and used without further purification. (FTO) was washed with ethanol and immersed in 50 mM
Organometallic dye cis-bis(isothiocyanato)bis(2,20 -bipyri- TiCl4 aqueous solution for 12 h in order to make a good
dyl-4,40 -dicarboxylato) ruthenium (II) [RuL2(NCS)2] was contact between the TiO2 layer and conducting glass sub-
obtained from Solaronix SA (Switzerland), the other strate. A TiO2 electrode (TiO2 film thickness about 6 lm)
reagents came from Shanghai Chemical Agent Ltd. China. was obtained by spreading the TiO2 paste on the conduct-
Conductive glass substrate (FTO glass, Fluorine doped tin ing glass substrate using a ‘‘doctor blade method” and then
oxide over-layer, sheet resistance 8 X cm2 from Hartford sintered at 450 °C for 30 min in air. After cooling to 80 °C,
Glass Co., USA), was used as a substrate for precipitating the TiO2 electrode was dye-sensitized with an organometal-
TiO2 porous film, and was cut into 2  1.5 cm2 sheets. lic dye ([RuL2(NCS)2], 0.5 mM) absolute ethanol solution
for 24 h at room temperature. Afterwards, the dye-sensi-
2.2. Preparation of counter electrode tized TiO2 electrode was rinsed with absolute ethanol and
dried in moisture-free air.
A mixture of carbon black and platinum was prepared A liquid electrolyte was prepared by blending 0.6 M tetra-
by reducing H2PtCl6 with a reducing agent (NaBH4) in car- propylammonium iodide, 0.1 M I2, 0.1 M KI, and 0.5 M
bon black. 300 mg of carbon black powder and 5 ml of iso- TBP in acetonitrille solution. A dye-sensitized solar cell
propanol were added to twice distilled water in a beaker was assembled by dropping a drop of the liquid electrolyte
and ultrasonically stirred for 15 min. H2PtCl6 solution on the dye-sensitized TiO2 porous film electrode. The Pt/Car-
(7.72 mM) with a predetermined Pt/carbon black weight bon black counter electrode was laid over. The two electrodes
ratio was added and ultrasonically stirred for 30 min, fol- were clipped together and a cyanoacrylate adhesive was used
lowed by adding 0.05 mol/L of NaBH4 solution (NaBH4 as sealant to prevent the electrolyte solution from leaking.
was excess to reduce H2PtCl6 fully) and the ultrasonically
stirring for 2 h. After filtering and washing for 3 times, 2.5. Measurements
the deposit substance was sintered at 250 °C for 1 h in
the air. Thus, a mixture powder of Pt and carbon black The microstructure of sample was observed with a JEM-
was obtained. 2000EX transmission electron microscope (JEOL, Japan).
The mixture powder of Pt and carbon black was dis- The crystal structure of samples was investigated by X-
persed in a mixed solution with 2 ml distilled water and ray powder diffraction (XRD) on an X-ray diffractometer
2 ml ethanol. Then, the 30 mg of hydroxyethyl cellulose (D8 ADVANCE, Germany) with Cu Ka radiation. Cyclic
as an adhesive was dissolved in this dispersion to form voltammetry (CV) was carried out in a three electrode one
a paste contained Pt and carbon black. The Pt/Carbon compartment cell with a self-made Pt/Carbon working
black counter electrode was prepared by coating the electrode, Pt foil counter electrode and an Ag/AgCl refer-
paste on FTO conductive glass sheet using doctor-blad- ence electrode dipped in an acetonitrile solution of
ing technique. Then the electrode was sintered at 10 mM LiI, 1 mM I2, and 0.1 M LiClO4. CV performed
180 °C for 1 h and dried in vacuum oven at 120 °C using CHI660B electrochemical measurement system
overnight. (sweep condition: 100 mV s1).
 P. Li et al. / Solar Energy 83 (2009) 845–849 847

The photovoltaic test of dye-sensitized TiO2 nanocrys- tive pair is assigned to the redox reaction Eq. (1) and the
talline solar cells was carried out by measuring the J–V positive pair is assigned to redox reaction Eq. (2) Imoto
characteristic curves under irradiation of white light from et al., 2003; Popov and Geske, 1958; Huang et al., 2007.
a 100 W xenon arc lamp (XQ-500 W, Shanghai Photoelec-
I 
3 þ 2e ¼ 3I

ð1Þ
tricity Device Company, China) under ambient atmo-
sphere. The incident light intensity and the active cell 3I2 þ 2e ¼ 2I
3 ð2Þ
area was 100 mW cm2 and 0.25 cm2, respectively.
Fig. 2 also shows a higher current density of the redox
peak for the Pt/C electrode and carbon black electrode
3. Results and discussion than for the Pt electrode. It could be explained that the car-
bon black usually has large specific surface area, which
3.1. Morphology characterization of the Pt/Carbon black increases effective active area of catalysis, so that the cur-
Counter electrode rent density is improved. The high current density also sug-
gests that the reaction rate was fast, in other word, the
Fig. 1a shows the XRD pattern of the mixture of Pt and charge-transfer resistance (RCT) for the I3/I redox reac-
carbon black (Pt 1.5 wt%). The peak at about 26.6° corre- tion was low on the carbon black electrode Imoto et al.,
sponds to graphite (002) in carbon black. Graphite in car- 2003.
bon black is available for good electrical conductivity for
the counter electrode. Several peaks at 39.4°, 46.0°, 67.4°, 3.3. Photovoltaic performance of DSSCs based on Pt/
and 81.1° were also observed in the XRD pattern, which Carbon black counter electrode
correspond to (111), (200), (220), and (311) of the face-cen-
tered cubic (fcc) lattice of platinum, respectively Schmid, Table 1 summarizes the photoelectric performance of
1992; Teranishi et al., 1999. The widening of the band- the DSSCs using Pt/Carbon black counter electrodes with
widths is due to their small particle size. Fig. 1b is a different platinum loadings. When a carbon black electrode
TEM image of the mixture of Pt and carbon black (Pt without platinum is used as counter electrode, the open-cir-
1.5 wt%). It can be observed that the mean size of platinum cuit photovoltage (Voc) of the DSSC is 724 mV, the short-
particles is 20–30 nm, and platinum nanoparticles were suc- circuit photocurrent density (Jsc) is 8.53 mA/cm2, and the
cessfully supported and homogeneously dispersed on the overall energy conversion efficiency (g) is only 3.76%. Such
carbon black. low conversion efficiency is due to the lower catalytic activ-
ity of the carbon black for I/I3 redox couple. On the
3.2. Cyclic voltammograms for the Pt/Carbon counter other hand, when platinum loading is 1.5 wt%, the Voc
electrode and Isc of the DSSC increase to 753 mV and 14.46 mA/
cm2, and the overall energy conversion efficiency (g) is up
Fig. 2 compares cyclic voltammograms in I2/I system to 6.72%. It indicates that the catalytic activity of the coun-
for Pt plate electrode, carbon black electrode and Pt/Car- ter electrode is improved when the platinum is filled in the
bon electrode at a scan rate of 100 mV s1. The oxida- carbon black. Furthermore, we observe no apparent differ-
tion/reduction peaks were observed in all cases. It ence in Voc, Isc, and g of the DSSCs based on Pt/Carbon
indicates that three materials all have electrocatalytic activ- black Counter electrodes with the platinum loading
ity for I2/I system. There were two pairs of redox waves between 1.5 and 4.0 wt% (shown as Table 1). Therefore,
for the Pt electrode and Pt/C electrode. The relative nega- the 1.5 wt% platinum loading on the carbon black is suffi-

a b

39.4
Intensity / a.u.

26.6

46.0

67.4
81.1

10 20 30 40 50 60 70 80 90
o
2θ /

Fig. 1. (a) X-ray diffraction pattern (b) TEM image for Pt and carbon black mixture (Pt 1.5 wt%).
848 P. Li et al. / Solar Energy 83 (2009) 845–849

0.6 0.6

0.4 0.4
Current density (mA/cm-2)

0.2 0.2

0.0 0.0

-0.2 -0.2

-0.4 -0.4

-0.6 -0.6
(a) Pt electrode (b) C electrode (c) Pt/C electrode
-0.8 -0.8

0.5 0.0 -0.5 -1.0 0.5 0.0 -0.5 -1.0 0.5 0.0 -0.5 -1.0
Voltage (V) Voltage (V) Voltage (V)

Fig. 2. Cyclic voltammograms for Pt plate electrode (a) carbon black electrode (b) and Pt/Carbon black electrode (c) in 10 mM LiI, 1.0 mM I2 acetonitrile
solution containing 0.1 M LiClO4 as the supporting electrolyte. [I]/[I2] = 10/1.

Table 1 ter electrodes exhibited no significant difference in the over-

Photoelectric parameters of dye-sensitized solar cells based on Pt/Carbon all energy conversion efficiencies (g), which are 6.72% and
black counter electrodes with different platinum loadings.
6.63%.
Platinum loading (%) Jsc (mA/cm2) Voc (mV) FF g (%)
0 8.53 724 0.601 3.76 4. Conclusions
1.0 13.59 737 0.614 6.17
1.5 14.46 753 0.616 6.72
In summary, Pt/Carbon black counter electrode for dye-
2.0 14.57 748 0.612 6.67
4.0 14.73 742 0.611 6.68 sensitized solar cells (DSSCs) was prepared by reducing
100 (Pt electrode) 14.86 723 0.617 6.63 H2PtCl6 with NaBH4 in carbon black. Pt/Carbon black
electrode showed high electrocatalytic activity for iodide/
triiodide redox reaction. Using Pt/Carbon black (platinum
1.5 wt%) as counter electrode, DSSC achieved 6.72% over-
16
a all energy conversion efficiency under one sun illumination
14 b (AM1.5, Pin of 100 mW cm2). Pt/Carbon black electrode
shows the same energy conversion efficiency and lower cost
Current density (mA/cm2)

12
compared with Pt electrode, which make it available in
10 DSSCs practical applications.
8
Acknowledgements
6
The authors thank for jointly supporting by the Na-
4
tional Natural Science Foundation of China (No.
2 50572030, 50842027), the Nano Functional Materials Spe-
cial Program of Fujian Province (No. 2005HZ01-4), the
0 Key Project of Chinese Ministry of Education.(No.
0.0 0.2 0.4 0.6 0.8
206074) and Specialized Research Fund for the Doctoral
Voltage (V)
Program of Chinese Higher Education (No.
Fig. 3. Photocurrent-voltage curves of DSSCs with (a) platinized counter 20060385001).
electrode and (b) Pt/Carbon black counter electrode. Under one sun
illumination (AM1.5, Pin of 100 mW cm2). References

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