Pt-free and efficient counter electrode with nanostructured CoNi2S4 for dye-sensitized solar cells

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#### OPEN

CONFERENCE
PROCEEDINGS

ISFM2014
...............

SUBJECT AREAS:

SOLAR CELLS

NANOSCALE MATERIALS

Received
2 October 2014

Accepted
16 December 2014

Published
23 March 2015

Correspondence and
requests for materials

should be addressed to

[LL. (lli@suda.edu.cn;](mailto:lli@suda.edu.cn)

[liang.li0216@gmail.](mailto:liang.li0216@gmail.com)

[com)](mailto:liang.li0216@gmail.com)

## Pt-free and efficient counter electrode with nanostructured CoNi2S4 for dye-sensitized solar cells

Zhiwei Shi, Kaimo Deng & Liang Li

College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology,
Soochow University, Suzhou 215006, P. R. China.

The counter electrode has a great influence on the performance of the dye-sensitized solar cells (DSSCs). The
research and development of Pt-free counter electrode is becoming one of the hot areas in the field of DSSCs.
Herein, we successfully synthesized a ternary metal sulfide (CoNi2S4) nanostructure on FTO substrate by
hydrothermal method and investigated its application as counter electrode. The as-synthesized sample
could exhibit better electrocatalystic property than that of Pt, and corresponding DSSCs have comparable
conversion efficiency with typical Pt catalyzed cells. The easy synthesis, low cost and excellent
electrocatalytic property may help the CoNi2S4 nanostructure stand out as an alternative counter electrode
in DSSCs.

ye sensitized solar cells have attracted extensive attentions in the past two decades as one of the most
promising third generation solar cells, due to the excellent properties compared with silicon based solar
cells such as low cost, relatively high efficiency, flexibility and easy fabrication 1–3 . In general, a DSSC
# D
consists of an n-type semiconductor with a large band-gap (also named photoanode), dye molecules as sensitizer,
a redox electrolyte and a counter electrode 4–5 . By using newly synthesized ruthenium dyes and nanosized TiO2
particles, Gratzel opened up a brand new era for the field of DSSCs and achieved an unprecedented efficiency over
12% 6 . However, this value is still lower than that of crystalline silicon based solar cells. The contemporary research
and development of DSSCs mainly focus on the enhancement of efficiency and cost reduction. Great efforts have
been devoted to developing efficient dye, employing photoanodes with different materials, structures and morphology and trying out other new kind of electrolyte. These developments have been reviewed in the recent
articles 7 . On the other hand, the cost reduction can be generally realized by using cost-efficient materials and
adopting low-temperature synthesis and processing 8–9 . The platinum sputtered conducting glass usually serves as
the counter electrode in a typical DSSC. It is known that Pt is extremely expensive and its reserve is quite limited.
Meanwhile, the fabrication of Pt counter electrode needs heat treatment and it is unfavorable for the development
of flexible plastic substrate. So there is an urgent need to explore alternative Pt-free materials with a cheaper price
as the counter electrode in the large scale production of DSSCs.

As one of the most important part in DSSCs, the counter electrode acts as a catalyst by reducing the redox
species after the electron injection so that the dye molecules can be regenerated. To fulfill this function, counter
electrode materials should possess superior catalytic activity and chemical stability against the corrosive electrolyte in the DSSCs. As a traditional choice of counter electrode materials in DSSCs, Pt satisfies both the requirements thanks to its outstanding properties 10 . In fact, quite a few alternative materials, such as carbon materials 11–13

and conducting polymers 14–15, have been used and investigated to replace Pt. In recent years, transition metal
sulfides, such as NiS, CoS, WS and MoS2, have also found their application in DSSCs as counter electrodes 16–18 .
For example, Ho et. al. 16 explored the performance of DSSCs using CoS as the counter electrode and the
maximum conversion efficiency was nearly the same as that of the reference cell with a sputtered Pt counter
electrode. The excellent performance of metal sulfide is largely related to the excellent electrocatalytic ability. In
particular, transition metal sulfides have a variety of advantages over other counterparts such as easy synthesis,
low cost and chemical stability, all of which render them attractive candidates as counter electrodes. However, it is
noted that few ternary transition metal sulfides have been explored as counter electrodes in DSSCs 19,20 . Ternary
transition metal sulfides usually have a high electronic conductivity and they are also expected to have rich redox
reactions due to the existence of nickel and cobalt ions 21 . In this report, for the first time, we investigate the
synthesis of CoNi2S4 nanoparticle film and its application as counter electrode in DSSCs.

Figure 1 | XRD patterns of bare FTO substrate and samples after the
synthesis of 90 min and 300 min.

Results
Fig. 1 shows the XRD patterns of the FTO substrates after hydrothermal reaction of 90 min and 300 min. It is noted that all the XRD
patterns are dominated by the diffraction peaks from FTO substrates.
Only one (311) peak at 31.4u is indexed to the phase (JCPDS, card no:
43-1477) of CoNi2S4 and no other impurity phase can be found. The
XRD pattern of the sample underwent hydrothermal reaction of
20 min exhibits no obvious difference compared with that of the
bare FTO substrate, indicating a small amount of products are
formed on the substrate. To confirm the existence of CoNi2S4, we
performed X-ray photoelectron spectroscopy (XPS) experiments
and the results are presented in Fig. 2. The XPS survey (Fig. 2a)

and high-resolution spectra of Ni 2p (Fig. 2b), Co 2p (Fig. 2c) and
S 2p (Fig. 2d) match well with the reported data of CoNi2S422–24. The

Ni2p spectrum have two spin–orbit doublets characteristic of Ni 21

and Ni 31, and two shake-up satellites (identified as ‘‘Sat’’). Similarly,
the Co 2p spectrum can also be considered by two spin–orbit doublets and two shake-up satellites. The S 2p spectrum shows two main
peaks and one shake-up satellite. The peak at 162.4 eV is typical of
metal-sulphur bonds, while the peak centered at 161.3 eV is attributed to sulphur ion in low coordination at the surface of samples.

The scanning electron microscopic (SEM) images of the FTO
substrates with different reaction time are shown in Fig. 3. It can
be seen that CoNi2S4 particles with an average diameter of 20 nm
grow on the surface of the FTO substrate after the synthesis reaction
of 20 min, leaving most surface areas of the crystalline grains uncovered (Fig. 3a, 3b). Due to the small amount of particles, the sample
(denoted as NCS-20) still remains transparent as can also be found in
the inset of Fig. 3a. The low loading of CoNi2S4 particles is responsible for the difficulty in detecting the X-ray diffraction peaks from the
CoNi2S4 phase and taking the cross-section SEM image. After
90 min of the synthesis reaction, a continuous network with porous
CoNi2S4 structure is formed on the substrate (Fig. 3c). The porous
structure with a high surface area is beneficial for the catalytic reaction and the fast diffusion of I 2 /I32 ions. The cross-section SEM

image of the sample (NCS-90) shown in Fig. 4a indicates that the
thin layer with a thickness of 500 nm is deposited evenly on the
surface of the FTO substrate. For the sample with synthesis reaction
of 300 min (NCS-300), the crystalline grains of the products become
larger while the pore volume diminishes (Fig. 3d). Meanwhile, the
increased reaction time results in more CoNi2S4 deposition on the
substrate and an 800 nm thick film is obtained (Fig. 4b). To confirm
the stoichiometry of these films, we mapped the elemental distribution of Ni, Co and S on the surface of samples, as shown in
Fig. 5. Fig. 5a indicates the distribution of all three elements on the

Figure 2 | (a) XPS survey spectra, (b) Ni 2p, (c) Co 2p, (d) S 2p XPS spectra of the products deposited on the surface of FTO substrate after 90 min
reaction

Figure 3 | SEM images the CoNi2S4 nanostructure synthesized under different reaction time: (a, b) 20 min, (c) 90 min and (d) 300 min. Inset shows the
corresponding digital photos.

surface of scanned area, and the distribution of individual Ni, Co and
S are also shown in Fig. 5b–5d, respectively. From these images, we
can see that these elements are distributed uniformly. In addition, the
relative ratio of elements is 8%516%530%, which is very close to a
standard value (15254) of CoNi2S4.

To investigate the application of the CoNi2S4 as counter electrode,
we studied the electrocatalytic properties of the counter electrode
with the as-synthesized three types of CoNi2S4 nanostructures as
well as the Pt film. The cyclic voltammetry (CV) curves of different
counter electrodes under I 2 /I32 electrochemical system using a

three-electrode method are presented in Fig. 6. It can be found that
all the CV curves, except that for the NCS-20, exhibit two pairs of
redox peaks. The one at more negative potentials corresponds to the
reaction 25 :

density is, the better the electrocatalytic ability will be 26 . Meanwhile, another two conclusions can be made. First, the CV curve of
NCS-20 is quite different from the other curves and there is no redox
peaks at more negative potentials, indicating the poor electrocatalytic
properties of the sample toward I32 reduction27. This behavior is

properties of the sample toward I32 reduction27. This behavior is

most likely related to the small amount of CoNi2S4 deposition.
Second, the existence of the two pair redox peaks for the NSC-90
and NSC-300 sample shows that both of them have the electrocatalytic ability for reducing the triidide ions. Their cathodic peak current
densities are higher than those of Pt, and the NSC-300 has the highest
value among all the counter electrodes. This implies that they have
better electrocatalytic ability toward I 2 /I32 redox couple than that of

better electrocatalytic ability toward I 2 /I32 redox couple than that of

Pt.

To understand the charge transfer process at the interface between
the electrolyte and the counter electrode, we measured the electrochemical impedance spectra (EIS) by using the symmetric sandwich-type configuration in order to eliminate the influence of the
photoanode. The Nyquist plots for different counter electrodes are
shown in Fig. 7. In each curve, there are two well-defined semicircles.
The first semicircle at the high frequency region is related to the
impedance of charge transfer process occurring at the counter electrode and electrolyte interface, while the other at lower frequency
range can be assigned to the Nernst diffusion impedance within the
electrolyte. Since a thin Surlyn film (60 mm) is used in the measurement, the Nernst diffusion impedance is insignificant and we will not
discuss it in detail Among all the counter electrodes the NCS 20

{z2e{<3I{ ð1Þ

and the other at more positive potentials relates to the reaction:

3I2z2e { <2I3{ ð2Þ

Since the counter electrode is the place where the reduction reaction
of I32 in DSSCs is carried out, we will mainly focus on the oxidation

of I32 in DSSCs is carried out, we will mainly focus on the oxidation

and reduction of I 2 /I32 at the more negative potentials. It is well

accepted that the electrocatalytic ability of a counter electrode for
I32 reduction in DSSCs can be reflected by the cathodic peak current

I32 reduction in DSSCs can be reflected by the cathodic peak current

at more negative potential The higher the cathodic peak current

Figure 4 | Cross-section SEM image of CoNi2S4 nanostructure (a) NCS-90 and (b) NCS-300.

counter electrode has the largest semicircle in the Nyquist plot, suggesting that there is a large charge-transfer resistance (Rct) 17, which is
in accordance with the poor performance of reducing I32 ions in the

in accordance with the poor performance of reducing I32 ions in the

CV measurement. The Rct value of the NCS-90 is lower than that of
the NCS-300. The Pt counter electrode has the lowest Rct value.
Besides the charge transfer impedance, the sheet resistance of the
counter electrode (RS) is another important parameter affecting the
performance of counter electrode and it can also be obtained from
EIS. We found that the NCS-90 has the lowest sheet resistance and
the other counter electrodes have similar values. This demonstrates
that CoNi2S4 has a prominent conductivity. Due to the increased
thickness of the deposited film, the sheet resistance of the NCS-300
is larger than that of the NCS-90. Based on the above results, the
NCS-90 has the best catalytic capability and the lowest Rct, which is
due to its proper film thickness. If the thickness is very small (NCS20), the low amounts of nanoparticles cannot provide enough catalysis sites for chemical reaction, and if the thickness very large
(NCS-300), the I 2 /I32 solution cannot penetrate pores located in

(NCS-300), the I 2 /I32 solution cannot penetrate pores located in

films and thus the catalytic effect is also decreased.

The photocurrent density versus photovoltage (J-V) curves of the
DSSCs based on different counter electrodes are depicted in Fig. 8.
The photovoltaic parameters including the short circuit current density (Jsc), open circuit voltage (Voc), FF and energy conversion efficiency (g) corresponding to the DSSCs are summarized in the inset.
It can be found that DSSCs based on NSC-20 counter electrode
shows a rather low FF as a result of its poor electrocatalytic property.
As for the NSC-90 counter electrode, the Jsc and FF value of the
DSSCs increases obviously, leading to enhanced conversion efficiency from 2.49% to 4.61%. Both the excellent electrocatalytic property and the low sheet resistance contribute to the enhanced
conversion efficiency. However, the efficiency of NSC-300 counter
electrode-based DSSCs decreases a lot, chiefly arising from the
decreased Jsc. This can be blamed on the increased charge transfer
resistance at the counter electrode as confirmed by the EIS analysis.
In short, the NSC-90 counter electrode exhibits the best performance
among the three different NSC nanostructures. Its conversion efficiency is comparable to that of the Pt counter electrode-based DSSCs.
In spite of a less satisfactory electrocatalytic property of the Pt
counter electrode, DSSCs using Pt as catalyst still possess the highest
conversion efficiency among all the DSSCs in our study. It should be
pointed out that the conversion efficiency of DSSCs in our study is
less than that of typical DSSCs. This is primarily because the TiO2
photoanode of the DSSCs is about 8 mm in thick and lacks the scattering layer and our concern is mainly focused on the counter electrode giving less attention on the conversion efficiency

Discussions
Due to the high conductivity and excellent electrocatalytic property,
Pt is widely used in DSSCs as counter electrode. The record conversion efficiency of DSSCs is also obtained by employing Pt as the
catalyst. However, its large-scale application has to face two serious
questions. First, Pt is expensive in price and scarce in reserve. Second,
the Pt counter electrode is usually fabricated by thermal decomposition of HPtCl4 or sputtering technology. Both the high temperature
heat-treatment and the sputtering process are not beneficial to
reduce the cost. So it is quite necessary to use an alternative catalyst
with comparable electrocalalytic ability and cheaper price. Our study
shows that ternary sulfide nanostructures could possess a better electrocatalytic ability than Pt. DSSCs assembled with the ternary sulfide
CoNi2S4 nanostructures (NiCoS-90) as counter electrode has a similar conversion efficiency compared with DSSCs using Pt counter
electrode. It is particularly important that the metal sulfide counter
electrodes are synthesized by a conventional hydrothermal method
at a low temperature of 130uC and no further calcination is required
to complete the reaction. In addition, the raw materials in the synthesis are inexpensive and have a rich storage in the earth. These
distinct characteristics would make it possible for the application of

Figure 5 | Elemental mapping results of CoNi2S4 nanostructure film
(NCS-90). (a) The whole element distribution on the surface and the inset
is relative quantity, and (b–d) individual Co, Ni and S element
distribution

Figure 6 | CV curves of different counter electrodes.

CoNi2S4 nanostructures as counter electrode but not at the expense
of conversion efficiency.

In summary, we have synthesized different CoNi2S4 nanostructures on FTO substrates with a conventional hydrothermal
method at temperature as low as 130uC and investigate their application as counter electrode in DSSCs. CV test shows that the CoNi2S4
nanostructures could exhibit better electrocatalytic property than Pt.
The DSSCs with CoNi2S4 nanostructures as counter electrode shows
comparable conversion efficiency compared with Pt catalyzed
DSSCs. The excellent electrocatalytic properties, simple synthesis
and low cost of the CoNi2S4 nanostructures may make the large-scale
production of DSSCs become a more achievable goal.

Methods
Synthesis CoNi2S4 nanoparticles film. The CoNi2S4 nanoparticles films were
synthesized via a one-step hydrothermal reaction. Briefly, 2 mmol Cobaltous acetate
and 4 mmol Nickel acetate were dissolved in 30 ml deionized water, followed by the
addition of 0.1 g Polyvinylpyrrolidone, 1 mmol NH4F, and 2 mmol Urea. Then the
mixture was transferred to a capped bottle with a FTO growth substrate facing down
in the precursor at 130uC for 20 min, 90 min, 300 min respectively. After reaction,
the samples were rinsed with deionized water and dried in vacuum oven at 60uC. It
should be noted that we just obtained the nanosheet-like films when we modulated
the relative amounts of NH4F (2–10 mmol) and Urea (3–15 mmol), as shown in Fig.
S1.

Fabrication of DSSC devices. A 20 nm-sized TiO2 (P25, Degussa, Germany) layer
(8 mm) was printed on FTO glass. When the obtained films sintered at 500uC for
30 min were cooled to 90uC, they were immersed in an ethanol solution containing
0.5 mM of N719 dye (cisbis(isothiocyanato) bis (2,29-bipyridyl-4,49-dicarboxylic
acid) ruthenium(II)) (Solaronix) for 24 h, followed by rinsing in ethanol to remove

Figure 7 | EIS spectra of different counter electrodes

Figure 8 | Photocurrent density versus photovoltage curves of DSSCs
based on different counter electrodes.

dye absorbed physically and drying in air. The Pt counter electrode was prepared
by spin-coating 1 mM of chloroplatinic acid (H2PtCl6.6H2O, Aldrich, 99.9%) in 2propanol (Aldrich, 99.7%) onto a FTO substrate and then heating at 350uC for
30 min. The as-prepared Pt film is composed of discontinuous nanoparticles with
a diameter smaller than 20 nm (Fig. S2). The as-prepared CoNi2S4 nanoparticles
films were used directly as counter electrodes. The dye-coated photoanodes were
sealed against Pt or CoNi2S4 counter electrodes with hot melt plastic spacers
(Solaronix, 60 mm thick). The electrolyte (0.1 M LiI, 0.03 M I2, 0.5 M
tetrabutylammonium iodide and 0.5 M 4-tert-butylpyridine in acetonitrile) was
introduced into the gap between two electrodes by a syringe. The active area of
DSSCs was 0.25 cm 2 .

Materials characterization. The morphology of our samples was characterized by
field-emission scanning electron microscope (FESEM, Hitachi SU8010) with an
elemental mapping apparatus. The phase of the products was checked by the X-ray
diffractometer (XRD, D/MAX-III-B-40KV, Cu Ka radiation; l5 0.15418 nm). The
composition was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB
250Xi, Thermo Scientific).

Device measurements. The photovoltaic test (J-V) were performed using a Keithley
2400 source meter under simulated AM 1.5G illumination (100 mW cm 22 ) provided
by a solar light simulator (Newport, 94043A). Cyclic voltammetry (CV) and the
electrochemical impedance spectroscopy (EIS) were recorded on an Autolab
electrochemical workstation (PGSTAT 302N). CV was carried out in a threeelectrode system with different counter electrodes as working electrodes, a Pt foil as
counter electrode, and a Ag/Ag 1 electrode as reference electrode at a scan rate of
50 mV s 21 . The electrodes were immersed into an anhydrous acetonitrile solution
consisting of 0.1 M LiClO4, 10 mM LiI, and 1 mM I2. EIS was carried out with a
symmetric cell assembled with two identical counter electrodes at open-circuit

voltage bias under dark condition at room temperature. The measured frequency
ranged from 10 mHz to 1 MHz with an ac amplitude of 10 mV.

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Acknowledgments
We acknowledge the support from the National Natural Science Foundation (51422206,
51372159, 11304217), 1000 Youth Talents Plan, Jiangsu Shuangchuang Plan, a Project
Supported by Jiangsu Science and Technology Committee for Distinguished Young
Scholars (BK20140009) and Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD).

Author contributions

L.L. designed the experiment and analyzed results. Z.S. performed synthesis experiment
and characterization. K.D. analyzed data and wrote the paper.

Additional information
[Supplementary information accompanies this paper at http://www.nature.com/](http://www.nature.com/scientificreports)
[scientificreports](http://www.nature.com/scientificreports)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Shi, Z., Deng, K. & Li, L. Pt-free and efficient counter electrode with
nanostructured CoNi2S4 for dye-sensitized solar cells. Sci. Rep. 5, 9317; DOI:10.1038/
srep09317 (2015).

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