reduced graphene oxide nanocomposite as Pt-free counter electrode with enhanced electrocatalytic performance in dye-sensitized solar cells

Carbon 122 (2017) 381e388

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Facile synthesis of Co0.85Se nanotubes/reduced graphene oxide nanocomposite as Pt-free counter electrode with enhanced electrocatalytic performance in dye-sensitized solar cells Hong Yuan a, Qingze Jiao a, b, Jia Liu a, Xiufeng Liu a, Yongjian Li a, Daxin Shi a, Qin Wu a, Yun Zhao a, Hansheng Li a, * a b

School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China School of Chemical Engineering and Materials Science, Beijing Institute of Technology, Zhuhai, 519085, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 May 2017 Received in revised form 20 June 2017 Accepted 28 June 2017 Available online 29 June 2017

A novel nanocomposite of Co0.85Se nanotubes with reduced graphene oxide (Co0.85Se/RGO) is prepared by a facile hydrothermal approach coupled with in situ selenization, which is applied as counter electrode in dye-sensitized solar cells. Electrochemical analysis shows that the Co0.85Se/RGO composite displays a higher electrocatalytic activity for the reduction of triiodide respect to its single components due to the unique hollow structure of Co0.85Se as well as the synergistic effects of high catalytically active Co0.85Se and conductive RGO. As a consequence, the dye-sensitized solar cell (DSSC) fabricated with the Co0.85Se/RGO electrode presents a high photovoltaic conversion efficiency of 7.81%, exceeding the cell based on a Pt electrode (7.55%). Moreover, a considerable electrochemical stability is also achieved when the material was used as a counter electrode in a I
/I
3 electrolyte. Therefore, the Co0.85Se/RGO exhibits a huge potential as efficient and low-cost Pt-free counter electrode material to replace Pt allowing for large-scale fabrication of DSSCs. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Cobalt selenide Reduced graphene oxide Counter electrode Dye-sensitized solar cells

1. Introduction Over the past decades, dye-sensitized solar cells (DSSCs) have attracted remarkable attention due to their low cost, simple structure, facile fabrication and high efficiency [1e3]. Recently, DSSCs have achieved remarkable photoelectric conversion efficiency (PCE) of 13%; this achievement results from great efforts that researchers have directed toward feasible and practical applications [4]. A standard DSSC generally comprises a dye-sensitized TiO2 nanocrystalline photoanode, an electrolyte containing the redox couple (I
/I
3 ) and a counter electrode (CE). The CE as a key component in DSSCs plays a considerable role in collecting electrons from external circuit and catalyzing the reduction of triiodide [5]. Platinum is widely applied as CEs materials thanks to its high conductivity and excellent electrocatalytic activity. However, its low abundance and high cost as well as instability in I
/I
3 electrolyte strongly limits the large-scale application of DSSC technologies [6,7]. Therefore, great efforts are put to develop alternative Pt-

* Corresponding author. E-mail address: [email protected] (H. Li). http://dx.doi.org/10.1016/j.carbon.2017.06.095 0008-6223/© 2017 Elsevier Ltd. All rights reserved.

free CE materials ensuring low-cost, high-conductivity and an excellent electrocatalytic performance. Promising candidates are carbon materials [8e11], conductive polymers [12e14] and transition metal compounds including chalcogenides [15e18], oxides [5] and nitrides [19]. Within the class of chalcogenides, selenides have attracted considerable attention owing to their distinctive Pt-like electronic configuration and splendid electrochemical performance [20,21]. Recently, illuminating by excellent electrocatalytic activity in fuel cell, cobalt selenide applied as CE in DSSC exhibits the enormous potential to replace the Pt [22,23]. To date, cobalt selenides with a variety of structure and morphology, such as graphene-like Co0.85Se [22], hollow Co0.85Se nanoparticles [24], CoSe2 nanorods [25], petal-like Co0.85Se nanosheets [26], have been reported to efficiently elevate the catalytic activity for the reduction of triiodide. Evidently, an efficient strategy is certainly to develop a rational design of three-dimensional nanoscale hierarchical structures to further improve the electrocatalytic performance of cobalt selenide. As we all know, hierarchical hollow structural nanotube is widely considered as efficient construction for DSSC, because this unique three-dimensional structure not only provides more

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electrocatalytic active sites derived from the inner surface and outer surface of tubes, but also contributes to high porosity for efficient electrolyte diffusion [15,27]. Unfortunately, there are few references which have reported hierarchical Co0.85Se nanotubes as CE applied in DSSC. Furthermore, as one of important semiconducting materials, cobalt selenide possesses a relatively low intrinsic electrical conductivity not allowing for the required fast electron transfer due to unique energy band structure [28]. Graphene, as the ultrathin twodimensional honeycomb carbon materials, has attracted great scientific interests owing to its unique physical and chemical properties [29]. Considering its superior conductivity and large specific surface area, graphene has been proven as an ideal conductive matrix to support extraneous materials, resulting in an advanced electrocatalytic materials for energy application [30e32]. Hence, the possibility of fabricating a hybrid of cobalt selenide and conductive graphene would significantly alleviate or solve above issues, thus improving the conductivity and catalytic performance of cobalt selenide for the reduction of I
3. Herein, we report a facile approach to synthesize highperformance Co0.85Se nanotubes/reduced graphene oxide (Co0.85Se/RGO) used as CE material in DSSC for the first time. On one hand, the exceptional electron-transfer pathway of RGO accelerates the electron transfer from the external circuit to the active sites at the interface between the CE and electrolyte. On the other hand, the abundant mesoporous structure derived from the hollow structure and small grain size of Co0.85Se nanotubes is favorable for maximizing the contact between inner active sites and the electrolyte. When applied as the CE in DSSC, the DSSC with Co0.85Se/ RGO CE exhibited a high photovoltaic conversion efficiency of 7.81% versus 7.55% for Pt CE under the same conditions. Furthermore, the Co0.85Se/RGO composite displayed an excellent electrochemical stability. Therefore, the proposed strategy to combine this unique Co0.85Se nanotubes and RGO offers a great potential for low-cost Ptfree CE material, which can significantly contribute to the largescale fabrication of low-cost DSSCs.

for 1 hour. Subsequently, 280 mL of deionized water at 60  C were added into flask. When cooling to 50  C, 40 mL of H2O2 and 120 mL of hydrochloric acid were added in sequence. Then, the mixture was centrifuged and washed with deionized water. Finally, the dispersed solvent of GO was dialyzed for one week before used. 2.3. Preparation of Co0.85Se nanotubes/RGO composite

2. Experimental

The Co0.85Se nanotubes/RGO nanocomposite was synthesized through a facile two-step method. Firstly, a novel Co(CO3)0.35Cl0.20(OH)1.10 nanorods/RGO precursor was prepared by a simple hydrothermal method referring to our previous work [15]. A typical precursor preparation procedure was as follows. 3 mmol cobalt chloride hexahydrate and 30 mg GO were dissolved in 20 mL distilled water to form uniformly dispersion. Another 20 mL water solution containing 0.67 g PVP was added to the above dispersion under stirring. Then, 6 mmol urea was added into the mixed solution, and then the mixture was transferred into a 50 mL autoclave and reacted at 120  C for 12 h. After reaction, the precursor samples were achieved by centrifugation, washing with distilled water and absolute ethanol several times. Afterwards, Co0.85Se nanotubes/RGO composite was prepared by in situ selenization of NaHSe solution. 0.1 g of the as-obtained precursor was added into 40 mL of 0.05 mM NaHSe solution under stirring for 30 min. Then, the dispersed solution was transferred into hydrothermal reactor and heated at 160  C for 24 h. After cooling down to the room temperature, the black samples were obtained by centrifugation, and washing with distilled water and absolute ethanol for several times. Subsequently, the as-prepared Co0.85Se/RGO dispersed solution was under ultrasonic treatment for 1 h, and sprayed-coating onto FTO conductive glass substrates. Finally, the films were dried in vacuum at 80  C for 24 h. The bare Co0.85Se and RGO were also prepared by the same hydrothermal process except with the absence of GO and cobalt chloride hexahydrate, respectively. It is note that the NaHSe solution was prepared as follows [34]. 0.32 g NaBH4 and 0.32 g Se powder were added into 4 mL Argon saturated deionized water under Ar flow, and then the fresh NaHSe solution was obtained after reacting.

2.1. Materials

2.4. Fabrication of DSSCs

Cobalt chloride hexahydrate, urea, Se powder, polyvinylpyrrolidone (PVP, K30), Sodium borohydride, tert-butyl alcohol, acetonitrile, pristine graphite powder, concentrated sulfuric acid, sodium nitrate, potassium permanganate, H2O2, hydrochloric acid and absolute ethanol were obtained from Sinopharm Chemical Reagent Co. Ltd, (Beijing, China). 4-tert-butylpyridine (TBP) was purchased from J&K scientific Co., Ltd (Beijing, China). 1,2dimethyl-3-propylimidazolium iodide (DMPII), lithium iodide (LiI), lithium perchlorate (LiClO4) and iodine (I2) were obtained from Aladdin Reagent Co. Ltd (Shanghai, China). N719 dye (cis-di(isothiocyanato)-bis-(2,20 -bipyridyl-4,40 -dicarboxylato) ruthenium (II) bis-tetrabutyl ammonium) was purchased from Dyesol (Australia). All chemicals were of AR grade and used without further purification.

A TiO2 photoanodes were prepared with a simple doctor-blade method to coat commercial TiO2 paste on the FTO conductive glass referring to our previous work [5]. The generally process was as follows. FTO conductive glasses were cleaned and pretreated with 50 mM TiCl4 at 70  C for 30 min. Then, a 20 nm-sized TiO2 paste (~6 mm in thickness) and a 400 nm-sized TiO2 paste (~7 mm in thickness) were scraped on the FTO substrate with a doctor-blade method, followed by annealling in a muffle furnace at 450  C and 500  C for 15 min, respectively. After cooling to room temperature, the TiO2 films were immersed in 50 mM TiCl4 aqueous solution for 30 min at 70  C. The TiO2 films were then followed by sintering at 500  C. After cooling to 80e120  C, the as-prepared TiO2 films were soaked in a 0.5 mM of N719 dye in a mixture of acetonitrile and tertbutyl alcohol (1:1 vol ration) for 24 h. Then a dye-sensitized TiO2 photoanodes were obtained. DSSC was assembled with a dye-sensitized TiO2 photoanode, a counter electrode, and an acetonitrile electrolyte to form a sandwich-type cell. The photoanode was separated from the counter electrode by a 30 mm thick Surlyn film. The electrolyte containing 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.06 M LiI, 0.03 M I2 and 0.5 M 4-tert-butyl pyridine in acetonitrile solvent was injected into the gap between the two electrodes. As a control, a Pt CE was prepared by thermally decomposing a drop of 20 mM H2PtCl6 ethanol solvent onto the FTO

2.2. Preparation of graphene oxide GO was synthesized by a modified Hummer's methods [33]. Simply, 2 g of pristine graphite powder, 46 mL of concentrated sulfuric acid and 1.0 g of sodium nitrate were added into 1 L flask and stirred at 5  C for 30 min. Then, 6 g of potassium permanganate were added into the above mixture under intense stirring at 15  C for 2 h, and then the system was maintained at 40  C for 1 h. Afterwards, 92 mL of deionized water were added and stirred at 98  C

H. Yuan et al. / Carbon 122 (2017) 381e388

conductive glass substrate at 400  C for 15 min under ambient atmosphere. 2.5. Characterization X-ray powder diffraction (XRD) analysis of samples was collected by an Ultima IV X-ray diffractometer (Rigaku, Japan) with CuKa radiation. Thermogravimetric analysis (TGA) was carried out on a TG/DTA 6200 (SII Nano Technology Inc., Japan), by heating the samples under air atmosphere from room temperature to 900  C with a heating rate of 10 K min
1. The morphology and microstructure of the samples were characterized by field emission scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). Energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectra (XPS) were carried out to analyze the elemental composition and bonding configurations. Fourier transform infrared spectrometry (FT-IR) analysis was recorded by using Nicolet iS10 Fourier transform infrared spectrometer (Thermo scientific, America). The Raman spectroscopy was carried out through a Renishaw in Via-Reflex Raman Microscope with an argon-ion laser at an excitation wavelength of 532 nm. Electrochemical impedance spectroscopy (EIS) and Tafel polarization on a symmetric dummy cell were measured under the dark condition on the electrochemical working station (CHI 604E). Cyclic voltammetry (CV) measurement was conducted by using a three-electrode electrochemical cell in acetonitrile solution containing 0.1 M LiClO4, 10 mM LiI and 1 mM I2. The Co0.85Se/RGO, bare Co0.85Se, RGO and Pt counter electrodes were used as the working electrode, whereas platinum plate as the counter electrode, and Ag/AgCl electrode as the reference electrode. Photocurrent-photovoltage (J-V) curves of the DSSCs were measured under 100 mW cm
2 with AM 1.5 illuminations (Zolix SS150), and the light intensity was calibrated using a Si solar cell (National Institute of Metrology, China). 3. Results and discussion Fig. 1a illustrates a facile two-step synthetic process for the Co0.85Se/RGO hybrids. Firstly, owing to electrostatic adsorption, the Co2þ precursors are attracted to the surface of GO by the sufficient oxygen-containing groups (-COOH, -OH, -O-, etc.). Then, the Co(CO3)0.35Cl0.20(OH)1.10 nanorods/RGO precursors are uniformly grown and supported on the surface of RGO through a simple hydrothermal method. Finally, the as-prepared precursors are treated by in situ selenization of NaHSe solution, yielding Co0.85Se nanotubes/RGO composite. The structure and morphology of the as-prepared Co(CO3)0.35Cl0.20(OH)1.10 nanorods/RGO precursor were determined by SEM (Fig. 1b and Fig. S1) and XRD (Fig. 1c). It is obvious that the precursor exhibits homogeneous rod-like structure as well as uniform morphology and smooth surface. XRD analysis of nanorods precursor manifests that the diffraction peaks can be indexed to JCPDS card of Co(CO3)0.35Cl0.20(OH)1.10 (PDF#38-0547). Fig. 1d shows the typical XRD pattern of the as-made Co0.85Se/RGO samples prepared through in situ selenization of precursor based on the Kirkendall effect [35]. Obviously, the diffraction peaks could be attributed to hexagonal structure of Co0.85Se, which is in agreement with its standard card (PDF#52-1008). Moreover, the relative broad diffraction peaks reveal the formation of the small sample sizes. The absence of the peak of RGO (002) plane indicates that the restacking of the RGO sheets is restrained by Co0.85Se supported on both sides of RGO sheet. The diffraction peaks of bare Co0.85Se prepared without RGO can partial indexed to CoSe2 (PDF#530449), indicating that the presence of RGO is beneficial to the conversion of Co0.85Se. Based on the thermal-gravimetric analysis

383

(Fig. S2), the loading amount of RGO in the Co0.85Se/RGO composite is about 17.8 wt%. Fig. 2a and b shows the micromorphologies of as-obtained Co0.85Se/RGO samples. Evidently, the Co0.85Se/RGO is selforganized into a hollow nanotube structure. Meanwhile, the surface of Co0.85Se nanotubes is rough, indicating composition of small sizes of nanoparticles. In addition, a relatively high length-diameter ratio can be clearly observed, which is conducive to the longitudinal transfer of electron. The energy dispersive X-ray spectroscopy (EDX) result (Fig. S3) manifests that the element ratio of Co to Se of as-prepared samples is close to 0.85:1. TEM are conducted to give further insight into their microstructure and morphology. The corresponding TEM image (Fig. 2c) reveals a clearly and rough hollow tube structure with folded RGO sheets. The nanotubes are with diameter of about 70 nm and the wall thickness of 10 nm. As a result, the hollow structure and small size in composites offer an effective mesoporous structure, which can facilitate the diffusion of electrolyte and be favorable for the accessibility of inner electrochemical active sites for electrolyte ion (I
3 ). Meanwhile, the presence of RGO can provide effective electron transfer pathways, promoting the rapid electrochemical reactions. Crystal lattice fringes of the (101) planes (0.27 nm) of Co0.85Se are well defined and can be clearly detected from high-resolution TEM investigation (Fig. 2d), evidently indicating the relatively high crystallinity. The diffraction rings of the selected-area electron diffraction (SAED) pattern (inset of Fig. 2c) could be indexed to (101) and (110) crystalline planes of Co0.85Se, which shows its polycrystalline nature. XPS was further conducted to investigate the chemical composition of as-prepared hybrid materials. As shown in Fig. 3a, the survey spectra of Co0.85Se/RGO and Co0.85Se samples clearly reveal the existence of Co and Se elements; besides, the content of O element for Co0.85Se/RGO and RGO samples is inferior to that for GO sample. For the high resolution C 1s spectrum of Co0.85Se/RGO (Fig. 3b), the peaks at 284.5 eV, 285.1 eV, 286.2 eV and 287.8 eV are assigned to the C¼C-C, CeO, C¼O and C¼OeO configurations, respectively [30]. Compared with the C 1s spectrum of pristine GO (Fig. 3e), the peak intensities including C¼O (286.6 eV) and C¼OeO (287.8 eV) species are significantly decreased, indicating an efficient deoxidization and reduction of GO to RGO. The reduction of GO is further demonstrated by FT-IR characterization shown in Fig. S4, where the intensities of absorption peaks at 1050, 1750 and 3300 cm
1 associated with oxygen-containing groups are significantly weakened or entirely removed for the Co0.85Se/RGO sample. The high resolution Co 2p spectrum is shown in Fig. 3c. The peaks at 778.6 eV and 793.6 eV could be assigned to Co 2p3/2 and Co 2p1/2, respectively, and the peaks at about 797.6 eV and 802.5 eV are fitted with satellite peaks of Co 2p. The high resolution of Se 3d spectrum in Fig. 3d exhibits two peaks at 54.6 eV and 59.2 eV, which are ascribed to Se 3d5/2 and Se 3d3/2, respectively. The Raman spectra of as-prepared Co0.85Se/RGO hybrid were also characterized as shown in Fig. 3f. Specially, the spectrum of this hybrid shows two clear broader peaks at 1329 cm
1 and 1590 cm
1, representing to the degree of disorder (D band) and the degree of graphitic (G band) of sp2 bonded carbon in composite, respectively. The intensity ratios of D band to G band for Co0.85Se/RGO are decreased compared with GO, indicating the effective reduction for GO. The peaks at 672 cm
1, 512 cm
1 and 171 cm
1 for Co0.85Se/ RGO are corresponding to the peaks of Co0.85Se, demonstrating the formation of Co0.85Se. It is well known that the thickness of CEs is an important factor to influence the catalytic activity. Hence, the thickness of Co0.85Se/ RGO CEs was optimized by characterized the photoelectric conversion efficiency of DSSCs with CEs with different thickness as shown in Fig. S5 and Table S1. The thicknesses of CE were estimated by spraying different volumes of Co0.85Se/RGO dispersion onto the

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Fig. 1. (a) Formation process of the Co0.85Se nanotubes/RGO composite. (b) SEM image and (c) XRD patterns of the as-prepared Co(CO3)0.35Cl0.20(OH)1.10 nanorods/RGO precursor. (d) XRD patterns of the as-prepared Co0.85Se/RGO, bare Co0.85Se and GO samples. (A colour version of this figure can be viewed online.)

conductive substrate. Fig. 4a presents the photovoltaic performances of DSSCs fabricated with Co0.85Se/RGO CE with the best thickness under full irradiation at 100 mW cm
2. For comparison, the photocurrent-photovoltage (J-V) curves of the DSSCs with RGO, bare Co0.85Se and Pt were also conducted with the same thickness. The corresponding photovoltaic parameters are listed in Table 1. It is obvious that these cells exhibit similar Voc of 0.706 V, except the cell with RGO CE. At the same time, the cell with RGO CE also displays low photovoltaic performance, suggesting inferior intrinsic electrocatalytic capability for the triiodide reduction [36]. Due to the relative high catalytic activity for bare Co0.85Se compared with RGO, the enhanced PCE of 5.34% was obtained. Nevertheless, the PCE of bare Co0.85Se CE is still inferior to that of Pt CE (7.55%), which could be attributed to the relatively low intrinsic conductivity and nonuniform morphology (Fig. S6). Note that the cell with the Co0.85Se/RGO hybrid exhibits a remarkable PCE of 7.81% (Jsc ¼ 16.01 mA cm
2, Voc ¼ 0.706 V, FF ¼ 0.69), which is even higher than those of Pt CE (PCE ¼ 7.55%, Jsc ¼ 15.44 mA cm
2, Voc ¼ 0.707 V, FF ¼ 0.69). The enhanced performance could be attributed to the combination of the catalytically active Co0.85Se nanotube with conductive RGO. On one hand, the hollow structure and small grain size of Co0.85Se nanotubes offer high porosity and sufficient active sites, thus facilitating the electrolyte diffusion and the accessibility of more inner electrocatalytically active sites for redox couple. On the other hand, the high conductive RGO provides faster electron transfer paths and accelerates the transfer of electron collected from external circuit to active sites, thus promoting
electrochemical reaction kinetics for the reduction of I
3 to I at CE and the regeneration of dye at photoanode (Fig. 4b). Therefore, the

synergistic effects of Co0.85Se nanotubes and RGO result in the high conversion efficiency. In order to realize the catalytic activity of different CEs, the CV measurements were carried out as shown in Fig. 5. Typically, two pairs of reduction and oxidation peaks can be clearly observed. The right pair is attributed to oxidation reactions: 3I2 þ 2e
/ 2I
3 , and

the left pair is ascribed to the reduction reaction: I
3 þ 2e / 3I . Generally, the cathodic reduction peak current density represents the catalytic activity of electrode, and the peak separation between the oxidation and reduction peaks (Epp) is correlated with the overpotential and electrochemical reaction rate [37]. A high reduction peak current and low Epp are positive to the electrocatalytic activity for the reduction of I
3 . It can be clearly observed that the reduction peak current densities of different CEs are in the order of Co0.85Se/RGO > Pt > Co0.85Se > RGO, indicating that the electrocatalytic activities are also in the same order. In addition, lower Epp for Co0.85Se/RGO CE reflects lower overpotential and faster electrochemical reaction rate. Therefore, the higher redox peak current density and lower Epp for Co0.85Se/RGO CE are responsible for the higher electrocatalytic capacity for the triiodide reduction, corresponding to the results of photovoltaic performance. The EIS measurements were also employed to elucidate the electrochemical performance based on the symmetrical dummy cells fabricated with two identical electrodes (CE/electrolyte/CE). The corresponding Nyquist plots of Co0.85Se/RGO, bare Co0.85Se, RGO and Pt CEs are shown in Fig. 6a. It is obvious that two semicircles can be observed, in which the left arc at the high frequency area is related to the charge transfer resistance (Rct) and constant

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385

Fig. 2. (a) SEM images, (b) the magnified SEM image, (c) TEM image, (d) high-resolution TEM image of as-prepared Co0.85Se/RGO sample. Inset of (c) is selected-area electron diffraction (SAED) pattern of as-prepared Co0.85Se/RGO samples.

Fig. 3. (a) XPS survey spectra for Co0.85Se/RGO, Co0.85Se, RGO and GO samples. High-resolution spectra of (b) C1 s, (c) Co 2p and (d) Se 3d for Co0.85Se/RGO, (e) high-resolution spectra of C1 s for GO. (f) Raman spectra of Co0.85Se/RGO, bare Co0.85Se and GO. (A colour version of this figure can be viewed online.)

phase element (CPEdl) at the CE/electrolyte interface, representing the electrocatalytic activity of CEs for the reduction of I
3 . The high-

frequency intercept on the real axis is attributed to the series resistance (Rs). The right arc at the low frequency area is ascribed to

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Fig. 4. (a) Photocurrent-photovoltage curves of DSSCs based on Co0.85Se/RGO, bare Co0.85Se, RGO and Pt CEs. (b) Schematic diagram about work process of Co0.85Se nanotubes/RGO CE in DSSC. (A colour version of this figure can be viewed online.)

Table 1 Detailed photovoltaic parameters of DSSCs fabricated with Co0.85Se/RGO, bare Co0.85Se, RGO and Pt CEs. Samples

Voc (V)

Jsc (mA cm
2)

FF

PCE (%)

RGO Co0.85Se Co0.85Se/RGO Pt

0.721 0.706 0.706 0.707

8.86 14.51 16.01 15.44

0.16 0.52 0.69 0.69

1.01 5.34 7.81 7.55

Nernst diffusion impedance (ZN) of redox couple in the electrolyte. The spectra were fitted with Randies equivalent circuit (inset of Fig. 6a) using the Z-view software. The detailed impedance data are shown in Table 2. Due to the enhanced conductivity of RGO matrix, the Co0.85Se/RGO CE shows the lower Rs values compared with bare Co0.85Se. Simultaneously, a smaller Rct of Co0.85Se/RGO CE is also observed, which is lower than that of bare Co0.85Se and RGO CEs, even lower than that of Pt CE, indicative of supreme electrocatalytic activity for the triiodide reduction. The integration of low Rct and small Rs can efficiently lower charge transfer resistance and facilitate the transfer of electron, thus contributing to the improvement of photovoltaic performance for DSSCs. Moreover, the Co0.85Se/RGO

CE exhibits a relatively small ZN value, originating from abundant mesoporous structure in composite. It is well known that the values of the CPEdl can be deduced by the following equation: CPEdl ¼ (CPEdl-T)
1(ju)-(CPEdl
P), where j2 ¼
1, u is the frequency, CPEdl-T and CPEdl-P are the frequency-independent parameters of the CPEdl [38]. CPEdl-T is corresponding to the active surface area, while CPEdl-P is related to the porosity of CE. It can be seen that the value of CPEdl-T for Co0.85Se/RGO CE is larger than that of other CE, which indicates a high active surface area. A decreased value of CPEdl-P of the Co0.85Se/RGO CE demonstrates an increased pore structure, corresponding to low ZN. These results also manifest an increased catalytic activity. The low value of CPEdl-T and high value of CPEdl-P for Pt CE can be attributed to the thin film structure. These results indicate that the Co0.85Se/RGO CE possesses admirable catalytic activity for the reduction of triiodide. Tafel polarization curves as shown in Fig. 6b were recorded by using the same symmetrical cells used in EIS test. It can be seen that the slopes for the cathodic or anodic branches of polarization curves are in the order of Co0.85Se/RGO > Pt > Co0.85Se > RGO. The slope is positively correlated to the exchange current density (J0). Therefore, a relatively large slope for Co0.85Se/RGO CE manifests a higher J0, further indicative of higher catalytic activity. The J0 can also be deduced from the following equation:

J0 ¼

Fig. 5. CV curves of Co0.85Se/RGO, bare Co0.85Se, RGO and Pt CEs at a scan rate of 50 mV s
1. (A colour version of this figure can be viewed online.)

RT nFRct

where R is the gas constant, T is absolute temperature, n is the electron number involved in the redox reaction, F is the Faraday constant. Apparently, J0 is inversely related to Rct, which is consistent to the results of EIS. In addition, the intersection of the cathodic branch with the vertical axis reflects the limiting diffusion current density (Jlim), which is interrelated to the diffusion capability of redox couple in electrolyte. Due to the presence of sufficient mesoporous structure, the Co0.85Se/RGO exhibits a higher Jlim. A higher J0 and Jlim determine that Co0.85Se/RGO is an excellent electrocatalyst for the reduction of triiodide. Fig. 7 presents the CV curves of the Co0.85Se/RGO CE at the different scan rates. It can be clearly seen that the reduction and oxidation peak current gradually aggrandize with increasing the scan rate from 50 mV s
1 to 120 mV s
1. Moreover, a good linear relation between redox peak current density and the square root of the scan rate is observed. The result reveals that the electrochemical reaction at the interface of CE and electrolyte is conducted

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387

Fig. 6. (a) Nyquist plots and (b) Tafel polarization curves of the symmetrical dummy cells based on Co0.85Se/RGO, bare Co0.85Se, RGO and Pt CEs. (A colour version of this figure can be viewed online.)

Table 2 Fitted EIS parameters of the symmetrical dummy cells based on Co0.85Se/RGO, bare Co0.85Se, RGO and Pt CEs. Samples

Rs (U)

Rct (U)

ZN (U)

CPEdl-T (F)

RGO Co0.85Se Co0.85Se/RGO Pt

23.5 27.94 21.9 20.56

92.8 6.46 0.95 3.30

88.32 18.23 8.73 9.29

4.67 2.14 3.17 8.70

   

10
7 10
7 10
5 10
7

CPEdl-P 0.97 1.12 0.79 1.03

(introducing 50 mg GO) is mainly ascribed to the restack of RGO sheets and reduced density of catalytically active Co0.85Se in composite (Fig. S7c). As a consequence, the content of RGO in composite not only has a greatly effect on the morphology and structure of composite, but also is correlated to the electric conductivity of hybrid, which further determines the catalytic performance for the reduction of triiodide and the PCE of DSSC. Considering the needs for the practical application, the elec-

Fig. 7. (a) CV curves of Co0.85Se/RGO CE at different scan rates (from inner to outer: 50, 70, 90 and 120 mV s
1, respectively), (b) the linear relationship between cathodic and anodic peak current densities and the square root of scanning speed in CV curves. (A colour version of this figure can be viewed online.)

by the ionic diffusion, and CE material does not react with electrolyte species. In order to investigate the affection on the content of RGO in composite to the catalytic performance for the reduction of I
3 and PCE of DSSC, the Co0.85Se/RGO containing different RGO content were prepared by introducing different GO dosage in composite as shown in Fig. S7. According to the electrochemical tests as shown in Fig. S8, it is obvious that the aforementioned Co0.85Se/RGO composite with 17.8 wt% loading of RGO prepared by introducing 30 mg GO exhibits the optimal electrocatalytic capability for reducing I
3 accompanying with highest PCE. As for Co0.85Se/RGO composite with low content of RGO (introducing 10 mg GO in composite), it reveals relatively low electrocatalytic activity for the reduction of triiodide with an inferior PCE, which could be attributed to the nonuniform morphology and structure of Co0.85Se (Fig. S7a). Besides, the low electric conductivity proved by relatively low Rs value in Nyquist plots (Fig.S8b) should be also responsible for low performance, because it is difficult to observe the existence of RGO in composite. In contrast, the inferior electrocatalytic performance for the Co0.85Se/RGO hybrid with relatively high dosage of RGO

trochemical stability of Co0.85Se/RGO CE was performed by a continuous CV scanning measurement at a scan rate of 50 mV s
1 as shown in Fig. 8. After the consecutive 20 cycles CV test, the anodic and cathodic peak current densities and peak positions have no obvious variation, suggesting the excellent electrochemical stability in I
/I
3 electrolyte. The superior electrochemical performance and outstanding electrochemical stability demonstrate that the Co0.85Se/RGO CE is a good and potential substitute for Pt in DSSCs. 4. Conclusions In summary, a novel Co0.85Se nanotube/RGO composite was successfully prepared by a facile synthesis routes for DSSC application. In this unique hybrid, conductive RGO offers faster electron transfer paths to inner active sites, and the hollow structure and small size of Co0.85Se nanotubes provide sufficient electrolyte diffusion channel and much accessible inner active sites. Benefited from the synergistic effects, the Co0.85Se/RGO exhibits enhanced electrocatalytic activity for the reduction of triiodide. The DSSC with Co0.85Se/RGO CE presents a high photovoltaic conversion

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Fig. 8. CV curves of Co0.85Se/RGO CE obtained for successive 20 cycles at a scan rate of 50 mV s
1.

efficiency of 7.81%, outperforming the cell with Pt CE. At the same time, the Co0.85Se/RGO CE also shows the remarkable electrochemical stability. In consideration of the excellent photovoltaic performance and outstanding stability, the Co0.85Se/RGO is of a huge potential as efficient and low-cost Pt-free counter electrode material in large-scale fabrication of DSSCs. Simultaneously, our work also provides an effective approach for application in other energy storage and conversion devices. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 21576025). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2017.06.095. References [1] B. O'Regan, M. Gratzel, A low-cost high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737e740. [2] A. Yella, H.W. Lee, H. Nok Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, et al., Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629e634. [3] M. Wu, X. Lin, Y. Wang, L. Wang, W. Guo, D. Qi, et al., Economical Pt-free catalysts for counter electrodes of dye-sensitized solar cells, J. Am. Chem. Soc. 134 (2012) 3419e3428. [4] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B.F.E. Curchod, N. AshariAstani, et al., Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem. 6 (2014) 242e247. [5] H. Yuan, Q. Jiao, S. Zhang, Y. Zhao, Q. Wu, H. Li, In situ chemical vapor deposition growth of carbon nanotubes on hollow CoFe2O4 as an efficient and low cost counter electrode for dye-sensitized solar cells, J. Power Sources 325 (2016) 417e426. [6] X. Cui, J. Xiao, Y. Wu, P. Du, R. Si, H. Yang, et al., A graphene composite material with single cobalt active sites: a highly efficient counter electrode for dyesensitized solar cells, Angew. Chem. Int. Ed. 55 (2016) 6708e6712. [7] M.J. Ju, I.Y. Jeon, J.C. Kim, K. Lim, H.J. Choi, S.M. Jung, et al., Graphene nanoplatelets doped with n at its edges as metal-free cathodes for organic dyesensitized solar cells, Adv. Mater. 26 (2014) 3055e3062. [8] H. Wang, Y.H. Hu, Graphene as a counter electrode material for dye-sensitized solar cells, Energy Environ. Sci. 5 (2012) 8182e8288. [9] V.-D. Dao, L.L. Larina, H. Suh, K. Hong, J.-K. Lee, H.-S. Choi, Optimum strategy for designing a graphene-based counter electrode for dye-sensitized solar cells, Carbon 77 (2014) 980e992. [10] S. Hwang, M. Batmunkh, M.J. Nine, H. Chung, H. Jeong, Dye-sensitized solar cell counter electrodes based on carbon nanotubes, ChemPhysChem 16 (2015)

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