Cobalt selenide nanorods used as a high efficient counter electrode for dye-sensitized solar cells

Electrochimica Acta 168 (2015) 69–75

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Cobalt selenide nanorods used as a high efficient counter electrode for dye-sensitized solar cells Jia Dong, Jihuai Wu * , Jinbiao Jia, Shaoyun Wu, Pei Zhou, Yongguang Tu, Zhang Lan Eng Res. Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou, 362021, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 December 2014 Received in revised form 12 March 2015 Accepted 31 March 2015 Available online 2 April 2015

Cobalt selenide (CoSe2) nanorods are prepared by hydrothermal method and used as an efficient Pt-free counter electrode (CE) for dye-sensitized solar cells (DSSCs). Field emission scanning electron microscopy observes that CoSe2 mostly exhibits a nanorod morphology, which facilitates change carrier transfer from their surface to redox electrolyte. Cyclic voltammogram measurement indicates that CoSe2 electrode has larger current density than Pt electrode. Electrochemical impedance spectroscopy shows that the CoSe2 electrode with optimal condition has low series resistance of 8.034 V
cm2 and has low charge-transfer resistance of 0.097 Vcm2. Under simulated solar light irradiation with intensity of 100 mWcm2 (AM 1.5), the DSSC based on the CoSe2 CE achieves a power conversion efficiency of 8.38 %, which is higher than the solar cell based on Pt CE (7.83%). ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Dye-sensitized solar cell counter electrode cobalt selenide

1. Introduction Dye-sensitized solar cells (DSSCs) have attracted considerable interest since it was reported by O'Regan and Gratzel in 1991 [1]. As a new kind of solar cell, DSSC displays many virtues such as low cost, easy preparation, good photovoltaic performance and environmental benignity compared with traditional photovoltaic devices [2]. A typical DSSC is consist of a TiO2 nanocrystalline film sensitized with dye molecules, a redox electrolyte containing the iodide/triiodide (I/I3) redox couple and a counter electrode (CE) whose main role is to collect the electrons from the external circuit and catalyze the reduction of I3 to I at the CE/electrolyte interface [3]. Usually, the counter electrodes of DSSCs are fabricated by loading platinum (Pt) on fluorine-doped tin oxide (FTO) glass. The conductive glass loaded by Pt has high conductivity, outstanding electrocatalytic activity and good chemical stability, allowing the charge on the electrode/electrolyte interface to move quickly and efficiently. However, the highly effective Pt CE of the DSSC is produced by a high-temperature hydrolysis or sputtering process. Besides, as a noble metal, Pt is expensive. So it is important to develop alternative counter electrodes with low cost, high conductivity

* Corresponding author. Tel.: +86 595 22693899; fax.: +86 595 22692229 E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.electacta.2015.03.226 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

and electrocatalytic activity for the resuction of triiodide. For this reason, much effort has been made to exploit competent substitutes for Pt CEs. The substitutes reported so far include carbon materials [4–7], conducting polymers [8–12] and inorganic compounds such as nitride [13–15], carbide [16,17], etc. Metal chalcogenide is a prospective CE material in DSSCs owing to their high conductivity and excellent catalytic activity for the reduction of I3 [3]. Among them, the metal sulfides are the most researched CE materials [3,18–24]. Recently, Hsu et al. synthesized CoS nanoparticles by CTAB-assisted preparation of a metal organic framework, ZIF-67, and subsequent oxidation and sulfide conversion to CoS [18]. The generated CoS nanoparticles was used as CE for DSSCs, leading to an improved efficiency of 8.1%. Metal oxides also are used as CE materials in DSSCs [25,26], using WO2 as a CE, the DSSC obtained power conversion efficiency of 7.25% [25], which can match the performance of the DSSC based on a Pt CE. Metal selenides are fewer reported as CE materials [27–29], although they possess good electrochemical properties. Recently, Sun et al. prepared single-crystal cobalt selenide (CoSe2) nanorods by one-step hydrothermal reaction, using the CoSe2 as CE, the DSSC exhibited a power conversion efficiency of 10.20% versus 8.17% for the Pt CE [27]. Here, we present another mild hydrothermal reduction method for preparing cobalt selenide (CoSe2) nanorods. The DSSC based on as-prepared CoSe2 CE produces a power conversion efficiency of

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8.38% versus 7.83% for the DSSC based on Pt CE under simulated solar light irradiation with intensity of 100 mWcm2 (AM 1.5).

160  C, and 180  C, and the corresponding products are designated as CoSe2-140, CoSe2-160, and CoSe2-180, respectively.

2. Experimental

2.3. Fabrication of DSSCs

2.1. Materials

To reduce the recombination of the electrons on the conductive glass with the holes, a underlayer of TiO2 compact film was prepared by spin-casting the toluene solution of TiO2 QDs [31] on the conductive glass once and subsequently sintered at 450  C for 30 min in air. Then a layer of TiO2 nanocrystal with size about 10–20 nm anode film was also prepared according to the reference [31]. After sintered at 450  C for 30 min, the above TiO2 anode was immersed in TiCl4 aqueous solution (0.05 M) at 70  C for 30 min to form a scattering layer, and then sintered again on the same condition. The resultant TiO2 photoanodes were soaked in an ethanol solution of N719 dye (2.5
104 moll1) for 24 h to obtain dye-sensitized TiO2 photoanode. The dye-adsorbed TiO2 photoanode and the CoSe2 counter electrode or Pt counter electrode was clipped together. One drop of liquid electrolyte was injected into the interspace between the two electrodes, and a dye-sensitized solar cell thus was fabricated. The liquid electrolyte contained 0.10 M tetramethyl ammonium iodide, 0.1 M tetraethyl ammonium iodide, 0.1 M tetrabutyl ammonium iodide, 0.1 M sodium iodide, 0.1 M potassium iodide, 0.1 M lithium iodide, 0.05 M iodine and 0.50 M 4-tert-butyl-pyridine in acetonitrile.

Unless noted otherwise, all chemicals were of AR grade quality and were used as received. Sodium hydroxide, oleic acid, absolute ethyl alcohol, hydrazine hydrate (80 wt%), cyclonexane, tetra-nbutyl titanate, toluene, nitric acid, glacial acetic acid, poly (ethylene glycol) (molecule weight, 20000), Triton X-100, acetonitrile, tetramethyl ammonium iodide, tetraethyl ammonium iodide, tetrabutyl ammonium iodide, potassium iodide, iodine, were purchased from Sinopharm Chemical Reagent Co., Ltd China. Selenium powder, cobalt chloride hexahydrate, titanium tetrachloride sodium iodide, 4-tert-butyl-pyridine, lithium iodide, and lithium perchlorate were purchased from Aladding. Conducting glass plates (FTO glass, Fluorine doped tin oxide over-layer, sheet resistance 15 Vcm1) were purchased from Nippon Glass Co. JP. Sensitizing dye N719 {cis-[(dcbH2)2 Ru (SCN)2]2, 2(n-C4H9)4N+(dcbH2 = 2, 20 -bipyridine-4, 40 -dicarboxylic acid)} was purchased from Dye sol. 2.2. Synthesis of cobalt selenide nanorods CEs The CoSe2 nanorods were synthesized via a one-step hydrothermal reaction similar to the method by Wang et al. [30]. Briefly, a mixture containing 1.0 g sodium hydroxide, 0.2 g selenium powder, 0.3 g cobalt chloride hexahydrate, 8 ml oleic acid, 32 ml absolute ethyl alcohol, 10 ml hydrazine hydrate (80 wt%) and 14 ml of deionized water was added to a 100 ml autoclave and reacted at a certain temperature for 12 h. After cooled down to room temperature, the black precipitate at the bottom of the autoclave was collected, centrifuged and washed with absolute ethyl alcohol for several cycles. Then, the products were re-dispersed in hexamethylene with concentration of 0.0625 gml1 to form cobalt selenide nanorods ink. Cobalt selenide counter electrodes (CEs) were fabricated by directly drop-casting the ink on the cleaned FTO glass and dry at room temperature. According to the reference [30] the hydrothermal reaction temperature was chosen at 140  C,

2.4. Measurements The morphologies of CoSe2 samples were observed by a field emission scanning electron microscopy (FESEM) (SU8000, HITACHI) and a field emission transmission electron microscopy (FETEM) (Tecnai F30). The crystal structures were analyzed by X-ray diffraction (XRD, Cu Ka radiation, SmartLab 3 kW, Rigaku, Japan). Photovoltaic tests were carried out by measuring the Current-Voltage (J-V) characteristic curves on a Keithley 2400 source meter under the illumination of AM1.5G simulated solar light coming from an AAA solar simulator (Newport-94043A) equipped with a Xe lamp (450 W) and an AM1.5G filter. The light intensity was adjusted with a reference Si solar cell (Oriel-91150). The cyclic voltammetry (CV) curves were obtained in acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4, using

Fig. 1. (a) XRD patterns of CoSe2-140, CoSe2-160, CoSe2-180 powders and standard CoSe2, (b) Energy-dispersive X-ray spectrum of CoSe2-160.

J. Dong et al. / Electrochimica Acta 168 (2015) 69–75

Fig. 2. FESEM images of (a) CoSe2-140, (b) CoSe2-160, (c) CoSe2-180; FETEM images of (d) CoSe2-140, (e) CoSe2-160, (f) CoSe2-180.

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CHI660E setup with a three-electrode system at a scan rate of 50 mVs1. The three-electrode system comprises an Ag/AgCl/KCl (3 M) reference electrode, a CE of platinum sheet, and a working electrode of FTO glass supported CoSe2 nanorods. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency range of 100 mHz100 kHz at an AC amplitude of 5 mV (peak to peak) by a Zennium electrochemical workstation (IM6). The impedances were recorded 4 points per decade in lower frequency and 10 points when the frequency above 66 Hz per decade, and the impedance spectra were analyzed with the Zview software. Tafel polarization curves were recorded on the same Workstation by assembling symmetric cell. 3. Results and discussion 3.1. Phase and compositions Fig. 1a depicts the XRD patterns of CoSe2 powder samples synthesized at 140  C, 160  C and 180  C. All the XRD peak positions (degrees) can be well indexed to orthorhombic CoSe2 (PDF#530449). Besides, the chemical compositions of the prepared cobalt selenide nanorods were determined by EDX, the Se/Co atomic number ratios for CoSe2-140, CoSe2-160, and CoSe2-180 are 1.00/2.15, 1.00/2.00, and 1.00/1.88, respectively. The measured atomic ratios are all close to the stoichiometry of CoSe2. Fig. 1b shows the EDX spectra of CoSe2-160. 3.2. Morphology observation The field emission scanning electron microscopic (FESEM) images of the films of CoSe2-140, CoSe2-160 and CoSe2-180 are shown in Fig. 2a  2c, respectively. It can be clearly seen that the obtained CoSe2 mostly exhibits nanorod morphology. FESEM images also reveal that with the increase of hydrothermal reaction temperature the number of nanorods decreases. The CoSe2 nanorod structures were further investigated by field emission transition electron microscopy (Fig. 2d  2f). As shown in FETEM images, under 140  C hydrothermal reaction, the diameter and the length of CoSe2-140 nanorods are approximately 30 nm and 300 nm, respectively; under 160  C, the diameter and the length are 80 nm and 330 nm, respectively. In 180  C, the diameter and the length are 100 nm and 600 nm, respectively. That is to say, the diameter and length of CoSe2 nanorods increase with the increase

Fig. 3. Photocurrent-voltage curves of the DSSCs based on CoSe2-140, CoSe2-160, CoSe2-180, and Pt CEs under one sun illumination (AM 1.5G).

Table 1 Photovoltaic data and electrochemical parameters of DSSCs with different CEs. CEs

VOC (V)

JSC (mAcm2)

FF

PCE (%)

CoSe2-140

0.750  0.005

16.65  0.04

0.644  0.002

8.04  0.03

CoSe2-160 CoSe2-180 Pt

0.743  0.004 0.750  0.005 0.743  0.004

17.04  0.03 15.44  0.05 16.88  0.05

0.662  0.002 0.639  0.003 0.624  0.002

8.38  0.04 7.40  0.03 7.83  0.03

of hydrothermal reaction temperature. In DSSCs, the morphologies of catalytic materials have an important influence on the catalytic reactions due to the catalytic reactions occur on the surface of the counter electrodes. In this experiment, the nanorod architecture facilitates the transfer of charge carriers from their surface to redox electrolyte. 3.3. Photovoltaic performances Fig. 3 compares the photovoltaic performance of the DSSCs based on different CEs including Pt and various CoSe2 nanorod CEs. The detailed photovoltaic performance parameters including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF) and power conversion efficiency (PCE) from the J-V curves are summarized in Table 1. The DSSC with CoSe2-160CE achieves a remarkable PCE of 8.38%, JSC of 17.04 mAcm2, VOC of 0.743 V and FF of 0.662, whereas the DSSC with sputtered Pt CE gives a PCE of 7.83%, JSC of 16.88 mAcm2, VOC of 0.743 V and FF of 0.624. For DSSCs with CoSe2 nanorod CE, PCE follows the order of CoSe2-160 > CoSe2-140 > CoSe2-180, JSC and FF also follow this order. It can be clearly seen from Table 1 that the improvement in the photovoltaic performance of the DSSCs based on CoSe2 CEs compared with the DSSC based on Pt CE is mainly due to the increase in FF. The distinction in FF is related to the resistance inside a DSSC. 3.4. Cyclic voltammetry Cyclic voltammetry (CV) was used to investigate electrocatalytic activity of CEs toward to iodide/triiodide (I/I3) redox couple. Fig. 4 compares the CV curves of I/I3 redox couple on CoSe2 nanorod CEs and Pt CE, respectively. In regard to CE function, two parameters for evaluating catalytic activity are the peak current density of Red1 (|Jred1|) and peak-to-peak potential

Fig. 4. Cyclic voltammograms for CoSe2-140, CoSe2-160, CoSe2-180, and Pt CEs.

J. Dong et al. / Electrochimica Acta 168 (2015) 69–75

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Table 2 Extracted data from CV and EIS curves. CEs

Jred1 (mAcm2)

Epp (mV)

Jox1/|Jred1|

RS (Vcm2)

RCT (Vcm2)

CoSe2-140

4.221 0.004

558.0  0.2

0.810  0.002

8.783  0.002

0.132  0.002

CoSe2-160 CoSe2-180 Pt

4.459  0.004 3.072  0.003 0.776  0.002

478.0  0.2 522.5  0.3 396.5  0.2

0.915  0.002 0.903  0.002 1.363  0.003

8.034  0.002 15.170  0.004 12.860  0.003

0.097  0.002 0.932  0.002 1.923  0.004

separation (Epp) for the pair peaks. The larger |Jred1| value means the fast catalytic reaction speed, and the smaller Epp value means the smaller overpotential for the catalytic reaction. As summarized in Table 2, the |Jred1| values increases in the order of CoSe2-160 > CoSe2-140 > CoSe2-180 > Pt, suggesting the catalytic reaction speed of CoSe2 CEs are all superior to Pt CE. However, the Epp of CoSe2 nanorod CEs are all bigger than Pt CE, which decrease the catalytic activity. Moreover, the ratio of Jox1/|Jred1| is a parameter to evaluate the reversibility of the redox reaction toward I/I3. The obtained Jox1/|Jred1| values from CoSe2 CEs are all closer to 1.0 than Pt, manifesting a more reversible redox reaction for I3 $ I. From the discussion above, CoSe2 nanorods have excellent electrocatalytic activity for reduction of I3. 3.5. Electrochemical impedance spectroscopy To further understand the improved performance of the DSSCs with different CEs, electrochemical impedance spectroscopy (EIS) measurements were carried out. EIS is widely used to estimate the catalytic activity of an electrocatalyst through investigating the charge transfer process. Fig. 5 shows the Nyquist plots obtained from symmetric dummy cell, relative series resistance (RS) and charge transfer resistance (RCT) are listed in Table 2. In a DSSC, RS is related to the collection of electrons from the external circuit and RCT value was directly related to the number of catalytic sites [32]. As shown in Table 1, CoSe2-140CE and CoSe2-160CE display lower RS than Pt, indicating a higher electron-conducting ability. Besides, the RCT of CoSe2 CE is all smaller than that of Pt CE, manifesting a high electrocatalytic activity for I3 reduction which could enhance the FF of DSSCs [33]. The RS and RCT values of CoSe2 CEs follow the order of CoSe2-160 > CoSe2-140 > CoSe2-180, which are in a good agreement with JSC and FF values obtained from J-V curves.

3.6. Tafel curves Fig. 6 shows the Tafel polarization curves measured on the symmetrical cells used in EIS experiments. In the Tafel curve, the intersection of the tangents to the cathodic branch at zero potential ordinate is exchange current density (J0), which is associated with RCT by the equation [22]: J0 = RT/nFRCT. In the equation, R is the gal constant, T is the absolute temperature, n is the number of electrons contributing to the charge transfer at the interface, and F is Faraday's constant. As shown in Fig. 6, it is apparent that the J0 follows an order of CoSe2-160 > CoSe2-140 > CoSe2-180 > Pt, generating an RCT order of CoSe2-160 < CoSe2-140 < CoSe2-180 < Pt, which is in accordance with RCT values derived from EIS. In addition, the limiting diffusion current density (Jlim) could also be obtained from the Tafel polarization curve. Jlim is determined by the diffusion of the redox species between the two identical CEs, it can be expressed by the equation [34]: Jlim = 2nFCDn/l, where l is the distance between the electrodes in a dummy cell, n is the number of electrons involved in the reduction of I3, and C is the I3 concentration. Fig. 6 shows that the Jlim follows an order of CoSe2140 > CoSe2-180 > CoSe2-160 > Pt, which means the diffusion impedance of electron in electrolyte follows the order of CoSe2140 < CoSe2-180 < CoSe2-160 < Pt. 3.7. Long-term stability of the catalytic activity The electrochemical stability of the CoSe2-160 film CE was examined by consecutive cyclic voltammetry, using 60 cycles of scan. Fig. 7a shows the cyclic voltammograms, the correlations between the peak current densities and the number of scans were summarized in Fig. 7b. It can be seen that there is only a slight change between the 1st and 60th scan, indicating that the CoSe2

Fig. 5. (a) Nyquist plots of CoSe2-140, CoSe2-160, CoSe2-180, and Pt CEs; (b) the partial enlargement of Fig. 5 (a)

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Fig. 6. (a) Tafel polarization curves of the dummy cells with CoSe2-140, CoSe2-160, CoSe2-180 and Pt CEs; (b) the partial enlargement of Fig. 6 (a).

Fig. 7. (a) Cyclic voltammograms of CoSe2-160 after subjected to 60 cycles. (b) The peak current density stability as a function of cycle.

film is a reliable material which can replace Pt as an efficient CE of a DSSC. 4. Conclusions Results show that the CoSe2 obtained by 160  C hydrothermal reaction has better electrocatalytic activity than Pt electrode, and has low series resistance and charge transfer resistance. Under simulated solar light irradiation with intensity of 100 mWcm2 (AM 1.5), the DSSC based on the optimal CoSe2 CE achieves a power conversion efficiency of 8.38 %, and this efficiency is superior to that of the DSSC assembled with Pt CE whose power conversion efficiency is 7.83%. The results presented in this work suggest that cobalt selenide is a promising low-cost and high-performance counter electrode material for application in DSSCs. Acknowledgement The authors gratefully acknowledge the financial supporting by the National Natural Science Foundation of China (Nos. 51472094, U1205112, 21301060, and 61306077).

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