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Flower-like nickel cobalt sulfide microspheres modified with nickel sulfide as Pt-free counter electrode for dye-sensitized solar cells
Journal of Power Sources 304 (2016) 266e272
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Flower-like nickel cobalt sulfide microspheres modified with nickel sulfide as Pt-free counter electrode for dye-sensitized solar cells Jinghao Huo, Jihuai Wu*, Min Zheng, Yongguang Tu, Zhang Lan Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The NiCo2S4/NiS microspheres are synthesized by a two-step hydrothermal method. The NiCo2S4/NiS shows flower-like morphology and has a large specific surface area. NiCo2S4/NiS CE exhibits excellent electrocatalytic activity and conductivity. The DSSC with NiCo2S4/NiS CE obtains a PCE of 8.80% than that of DSSC with Pt CE.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 July 2015 Received in revised form 12 November 2015 Accepted 17 November 2015 Available online xxx
The nickel cobalt sulfide/nickel sulfide (NiCo2S4/NiS) microspheres which exhibit flower-like morphologies are synthesized by a two-step hydrothermal method. Then the NiCo2S4/NiS microspheres are deposited on a fluorine doped SnO2 substrate by spin-casting the isopropyl alcohol solution of asprepared microspheres. The cyclic voltammetry, electrochemical impedance spectroscopy and Tafel tests are employed to measure the electrochemical performance of NiCo2S4/NiS counter electrode. The NiCo2S4 and NiS all are used to improve the conductivity and electrocatalytic ability of the films, and the NiS can also increase the specific surface area of microspheres. The dye-sensitized solar cells (DSSCs) with the NiCo2S4/NiS counter electrode exhibite a power conversion efficiency of 8.8%, which is higher than that of DSSC with Pt counter electrode (8.1%) under the light intensity of 100 mW cm2 (AM 1.5 G). © 2015 Published by Elsevier B.V.
Keywords: Dye-sensitized solar cells Counter electrode Sulfide Flower-like microsphere
1. Introduction As energy consumption and environmental issues are becoming more prominent, researchers pay more attention to the dyesensitized solar cells (DSSCs) due to their easy fabrication, low cost and high conversion efficiency of solar energy [1,2]. Generally, a typical DSSC is a sandwich structure including a dye-sensitized
* Corresponding author. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.jpowsour.2015.11.062 0378-7753/© 2015 Published by Elsevier B.V.
semiconductor (TiO2 or ZnO or SnO2 et al.) photoanode on fluorine doped SnO2 (FTO) glass, a I3 =I redox couple contained electrolyte, and a platinum (Pt) counter electrode (CE) [3e5]. Today researchers concentrate on exploring Pt-free catalytic materials to replace Pt because of its exorbitant price and easy corrosion. Inorganic materials like transition-metal sulfides are one of the most important materials for CEs [6]. To date, more and more attention has been paid to metal sulfides especially cobalt sulfide, nickel sulfide and their solid solution, such as CoS, CoS2, Co9S8, Ni9S8, NiS, Ni3S4, NiS2 NiCo2S4 and CoNi2S4 [7e12]. Among these sulfides materials, NiCo2S4 is a ternary metal sulfide and contains
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higher electrochemical characteristics than that of nickel sulfides and cobalt sulfides [13]. As a promising electrode material, NiCo2S4 was used in DSSCs [14,15], supercapacitors [11,16e18] and lithiumion batteries [19]. In 2013, Xiao et al. [15] prepared a NiCo2S4 hollow nanorod arrays CE for quantum dot sensitized solar cells (QDSCs), which attained a higher power conversion efficiency (PCE) of 4.22% than that of QDSCs with Pt CE (3.05%). Shi et al. [20] successfully synthesised of NiCo2S4 nanosheet films from NiCo2O4 nanosheet films and these two nanomaterials were directly used to be CE and photocathode of p-type DSSCs. These two electrode materials made the photocurrent density have an improvement of 2.989 mA cmˉ2 for p-type DSSCs. Banerjee et al. [19] prepared NiCo2S4 nanoneedles by sulphurization of NiCo2O4 to replace Pt as CE material, and NiCo2S4 CE made the DSSCs obtain an impressive PCE of 6.9%, compared with that of DSSCs based on Pt CE (7.7%). Chou et al. [21] firstly synthesized a transparent NiCo2S4 CE by electrophoretic deposition, and the DSSCs with this transparent CE obtain a PCE of 6.14%. In this paper, we fabricated a flower-like NiCo2S4/NiS microspheres by a simple two-step hydrothermal method which is similar to prepared Co9S8 nanotubes [22]. Then the NiCo2S4/NiS hybrid material was coated on FTO glass by spin-casting the isopropyl alcohol solution of as-prepared material. With the combined effect of NiCo2S4 and NiS, the NiCo2S4/NiS composite exhibited the large specific surface area, high conductivity and excellent electrocatalytic activity for the reduction of I3 to I . Based on this NiCo2S4/NiS CE, the PCE of DSSCs can attain a high value of 8.8% and improve about 8.24%, compared to DSSCs with Pt CE (8.1%).
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dried in vacuum at 60 C for 24 h. Followed by a hydrothermal process with Na2S$9H2O, the precursor of (Ni, Co)2(CO3) (OH)2 is transformed into metal sulfides (NiCo2S4/NiS). In short, 400 mg of precursor and 1.2 g of Na2S$9H2O were dispersed in 60 mL deionized water under intense stirring for 10 min, then the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, and heated at 180 C for 8 h. After cooling to room temperature naturally, the underlying precipitation (NiCo2S4/NiS microspheres) was collected and washed with deionized water and anhydrous ethanol three times, respectively. Finally, the NiCo2S4/NiS powder was dried in vacuum at 60 C for 24 h. The Co9S8, NiS and NiCo2S4 samples were prepared with different content of NiCl2$6H2O and CoCl2$6H2O by the same experimental procedures as that of NiCo2S4/NiS (Table 1). 2.3. Preparation of counter electrodes In a simple way, 100 mg of NiCo2S4/NiS microspheres was dissolved in 10 mL isopropyl alcohol solution and treated with ultrasonic oscillation for about 20 min to be an uniformly dispersed solution. Then the NiCo2S4/NiS was deposited on a FTO substrate by spin-casting the as-prepared isopropyl alcohol solution at a rate of 4000 rpm for 20 s. After the FTO substrate was thermally treated at 80 C for 10 min, the second NiCo2S4/NiS layer was spin-casting on the first layer. The same processes were done when preparing the third NiCo2S4/NiS layer. After three NiCo2S4/NiS layers done, the NiCo2S4/NiS CE was fabricated. The same method of NiCo2S4/NiS CE was used to prepare the Co9S8, NiS and NiCo2S4 CEs.
2. Experimental 2.4. Fabrication of DSSCs 2.1. Chemicals and materials The nickel chloride hexahydrate (NiCl2$6H2O), cobalt chloride hexahydrate (CoCl2$6H2O), urea, sodium sulfide nonahydrate (Na2S$9H2O), isopropyl alcohol, acetone, ethanol, tetra-n-butyl titanate, titanium tetrachloride (TiCl4), acetonitrile (ACN), tetraethyl ammonium iodide, tetrabutyl ammonium iodide, tetramethyl ammonium iodide, 4-tert-butyl pyridine (tBP), lithium perchlorate (LiClO4), sodium iodide (NaI), potassium iodide (KI), lithium iodide (LiI), and iodine (I2) were all A. R. grade and purchased from Sinopharm Chemical Reagent Co. Ltd, China. N719 dye (Ru[LL'-(NCS)2], L ¼ 2, 2'-bipyridyl-4, 4'-dicarboxylic acid, L' ¼ 2, 2'bipyridyl-4, 4'-ditetrabutylammonium carboxylate) was acquired from Dye sol. The FTO glass (20 cm 15 cm, 14 U sq1) and a Pt CE (2.5 cm 2 cm, prepared by magnetron sputtering) were purchased from Wuhan Lattice Solar Energy Technology Co. Ltd, China. And the FTO glass was cut into pieces (1.5 cm 1.5 cm) and washed with cleaner, acetone and isopropyl alcohol in turn. Finally, the FTO glass was stored in anhydrous ethanol. 2.2. Synthesis of NiCo2S4/NiS microspheres The flower-like NiCo2S4/NiS microspheres were synthesized by a simple two-step hydrothermal method. The first step, NiCl2$6H2O and CoCl2$6H2O were mixed with urea in deionized water to form a precursor of (Ni, Co)2(CO3) (OH)2. In brief, 7 mmol of NiCl2$6H2O, 3 mmol of CoCl2$6H2O and 10 mmol of urea were added into 60 mL deionized water under intense stirring for 10 min, then the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, heated in an oven at 130 C for 10 h. After the autoclave cooled to room temperature naturally, the underlying precipitation was the precursor and washed with deionized water and anhydrous ethanol three times, respectively. Finally, the precursor powder was
TiO2 photoanodes were prepared by spin-casting a ~160 nm TiO2 under layer and doctor-blading technique to form a 12 mm TiO2 nanocrystalline layer [23,24]. Then the TiO2 photoanodes were immersed into 0.05 M TiCl4 solution at 70 C for 0.5 h. After that, the photoanodes were sintered at 450 C in a muffle furnace for 0.5 h. When the temperature dropped to 80 C, the TiO2 photoanodes were took out and soaked in a 0.3 mM N719 dye ethanol solution in the room temperature for 24 h. Finally, the TiO2 photoanodes were took out from solution and washed with anhydrous ethanol. The size of photoanodes was ~0.12 cm2 (0.3 cm 0.4 cm). The TiO2 photoanode and an as-prepared CE were fabricated a typical sandwich cell, which these two electrodes were adhered together with a cyanoacrylate adhesive and a I3 =I electrolyte was injected into the gap between them. The I3 =I electrolyte was an ACN solution of tetramethyl ammonium iodide, (0.1 M), tetrabutyl ammonium iodide (0.1 M), tetraethyl ammonium iodide (0.1 M), NaI (0.1 M), KI (0.1 M), LiI (0.1 M), I2 (0.05 M), and tBP (0.5 M). 2.5. Characterizations and measurements The crystal structures of Co9S8, NiCo2S4/NiS powders and their precursors were measured by X-ray Diffraction (XRD, Bruker D8,Cu Ka, l ¼ 1.5418 Å) at scanning rate of 5 minˉ1. The composition of NiCo2S4/NiS hybrid was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo). XPS was performed by using Al Ka (hn ¼ 1486.6 eV) with power of 150 W and beam spot of 500 mm. XPS data were calibrated by C1s (284.8 eV) and fitted with XPSPEAK 4.0 software. The surface Morphologies of as-prepared products were performed by a field emission scanning electron microscopy (FESEM, SU8010, Hitachi). Energy dispersive X-ray spectroscopy (EDS) analysis was performed to observe the elemental type of the NiCo2S4/NiS CEs. The surface roughness of NiCo2S4 and NiCo2S4/NiS CEs was conducted with atomic force
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Table 1 The experimental formulas of preparing NiS, NiCo2S4/NiS, NiCo2S4 and Co9S8 materials by a two-step hydrothermal method. NiCl2$6H2O þ CoCl2$6H2O (60 mL H2O)
10 mmol þ 0
7 mmol þ 3 mmol
3.33 mmol þ 6.67 mmol
0 þ 10 mmol
Product (step 1) Product (step 2)
Ni2(CO3) (OH)2 NiS
(Ni, Co)2(CO3) (OH)2 NiCo2S4/NiS
(Ni, Co)2(CO3) (OH)2 NiCo2S4
Co(CO)0.35Cl0.20(OH)1.10$1.74H2O Co9S8
microscopy (AFM, Nanoscope Ⅲ a, Veeco). Surface areas of Co9S8, NiCo2S4 and NiCo2S4/NiS samples were measured with a BrunauerEmmett Teller (BET) sorptometer (ASAP 2020, Micromeritics) using nitrogen adsorption at 77 K. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel tests were used to study the electrochemical properties of CEs by electrochemical working station (IM6, Zahner, Germany). CV tests used a three-electrode cell to study the electrocatalytic ability of CEs for I3 =I and the scanning potential range was from 0.4 V to 1.4 V with a scan rate of 50 mV s1. The three-electrode cell includes as-prepared CEs as work electrode, a Pt sheet as CE and an Ag/AgCl reference electrode. Meanwhile an ACN solution of 10 mM LiI, 1 mM I2, and 100 mM LiClO4 was used to be electrolyte solution. EIS measurement was conducted with a traditional symmetrical cell from 100 kHz to 100 mHz under bias voltage of 0 V and the corresponding amplitude of 5 mV. After the tests, all EIS data were fitted with ZsimpWin software. Tafel curves were recorded from 1 V to 1 V at the scan rate of 10 mV s1. Photocurrent-photovoltage (JeV) tests of DSSCs with asprepared CEs and Pt CE were measured with a solar simulator (PVIV-94043A, Newport, USA) under simulated solar illumination of 100 mW cm2 (AM 1.5 G). A black mask was used on the surface of DSSC to avoid stray light and all the tests were performed in the air and at room temperature.
3. Results and discussion Fig. 1a shows typical XRD patterns of Co9S8 and its precursor. All the diffraction peaks of precursor can be indexed to crystalline Co(CO)0.35Cl0.20(OH)1.10$1.74H2O (JCPDS card file No. 38-547). And all of the reflection peaks in Co9S8 sample pattern at 15.4 , 29.8 , 31.2 , 39.5 , 47.6 , 52.1, 54.7 can be indexed to the (111), (311), (222), (331), (511), (440) and (531) planes of face-centered cubic Co9S8 (JCPDS card file No. 65-1765). When adding 7 mmol NiCl2$6H2O to replace CoCl2$6H2O, the diffraction peaks of precursor ((Ni, Co)2(CO3) (OH)2) can be indexed to Ni2(CO3) (OH)2 (JCPDS card file No. 35-0501) (Fig. 1b). This means the presence of Co ions does not affect the crystal structure of Ni2(CO3) (OH)2, but only has an influence for lattice parameters [16]. With 7 mmol NiCl2$6H2O in experimental procedures, the diffraction peaks of the final product at 16.3 , 26.8 , 31.6 , 38.3 , 50.5 and 55.3 corresponding to the (111), (220), (311), (400), (511) and (440) planes of cubic type NiCo2S4 (JCPDS card file No. 20e0782). There are some other small diffraction peaks at 34.7, 53.5 , 60.9 and 65.7 have the same diffracted angle as the (101), (110), (103) and (201) planes of NiS (JCPDS card file No. 02-1280). It turned out that the final product was a compound of NiCo2S4 and NiS. To further explore the surface elemental composition of NiCo2S4/NiS samples, XPS analysis was conducted and shown in Fig. 2. There are four elements: Ni, Co, S and O were tested in the sample of survey san (Fig. 2a) and the C element is used to be a reference. As shown in Fig. 2b of Ni 2p XPS, the energy difference between Ni 2p3/2 (857.0 eV) and Ni 2p1/2 (874.2 eV) is 17.2 eV, which indicated the existence of both Ni2þ and Ni3þ [25]. Meanwhile the energy difference between Co 2p3/2 (778.5 eV) and Co 2p1/2 (798.1 eV) is over 15 eV and demonstrated the existence of both Co2þ and Co3þ (Fig. 2c) [26e28]. According to Fig. 2d, there are three main peaks of 161.4 eV, 162.6 eV and 169.3 eV in the high
Fig. 1. XRD patterns of as-prepared materials: (a) Co(CO)0.35Cl0.20(OH)1.10$1.74H2O and Co9S8, (b) (Ni, Co)2(CO3) (OH)2 and NiCo2S4/NiS.
resolution scan of S 2p spectrum and these three peaks can be ascribed to the S 2p3/2, S 2p1/2 and the shakeup satellite, respectively. And furthermore, the peak at 161.4 eV of S 2p represents the existence of S2 ion and the component 162.6 eV can be ascribed to the sulphion in low coordination at the surface of hybrid. So on the near surface of the NiCo2S4/NiS hybrid, there are ions of Ni2þ, Ni3þ, Co2þ, Co3þ and S2, which match with literature results for NiCo2S4. In Fig. 2e, the main peak of 532.1 eV at O 1s XPS proves the existence of MO (M ¼ Ni or Co) [29]. Fig. 2f is the EDS spectrum of the NiCo2S4/NiS, which illustrates the element types of C, O, Co, Ni and S elements and the atomic percent of every element. From Fig. 2f can be obtained that the atomic percents of O, Ni, Co and S in NiCo2S4/NiS are 23.20%, 25.62%, 10.42% and 20.29%, respectively. Using XRD and XPS analysis, EDS data suggest that the NiCo2S4 and NiS are main compositions of hybrid and some oxides are formed in the hybrid. Fig. 3 was the FESEM images of the surface morphologies of precursors and as-obtained materials. Fig. 3a is the surface
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Fig. 2. XPS spectra of NiCo2S4/NiS hybrid: (a) Survey scan, (b) high resolution scan of the Ni 2p peak, (c) Co 2p peak, (d) S 2p peak, (e) O 1s peak and the related fittings and (f) EDS of NiCo2S4/NiS CE.
morphology of Co(CO)0.35Cl0.20(OH)1.10$1.74H2O, which shows many nanoneedles, with diameters of ~200 nm, and lengths of several micrometers. In contrast, Fig. 3b shows the surface morphology of Co9S8 with a tubular structure. Fig. 3c gives a FESEM image of precursor ((Ni, Co)2(CO3) (OH)2) of NiCo2S4, which consists of many micron flower-like structures and a mass of nanorods radially grown from these common centre. After a hydrothermal process of precursor and Na2S$9H2O, the morphology of NiCo2S4 has changed a lot from (Ni, Co)2(CO)3(OH)2. In Fig. 3d, it is clear to see that the bunches of flower-like structures are nanotubes instead of nanorods. When the NiCl2$6H2O all replaced CoCl2$6H2O in hydrothermal process, the NiS product was some micron balls with nanosheets and nanowires (Fig. 3e). Fig. 3f shows the surface morphology of NiCo2S4/NiS matericals prepared by 7 mmol of NiCl2$6H2O and 3 mmol of CoCl2$6H2O in the first hydrothermal step. The NiCo2S4/NiS consisted of flower-like structures with the diameters of several micrometers. The inset of Fig. 3f is the highpowered FESEM picture and it is clear to see that the flower-like structures were composed of numerous nanowires and some nanoparticles. To further estimate the surface roughness of NiCo2S4 and NiCo2S4/NiS CEs, the AFM measurement was conducted and AFM images (3 3 mm2) were recorded in Fig. 4. The root mean square (RMS) surface roughness was used to evaluate the surface roughness of CEs and the values of RMS were calculated from AFM images [30]. The values of RMS for NiCo2S4 and NiCo2S4/NiS CEs are 54.6 and 100.5 nm. The higher surface roughness of NiCo2S4/NiS CEs results in a larger active area for the reactions with the electrolyte. The increasing of RMS surface roughness is due to the NiCo2S4 and NiS together to increase the surface area. Fig. 5 shows the N2 adsorptionedesorption isotherms of Co9S8, NiCo2S4 and NiCo2S4/NiS samples for comparison. According to the classification of International Union of Pure and Applied Chemistry
(IUPAC), these isotherms are typical Ⅲ isotherms with H3 hysteresis loops [31]. The BET surface areas of Co9S8, NiCo2S4 and NiCo2S4/NiS are 16.90 m2 gˉ1, 12.00 m2 gˉ1 and 27.79 m2 gˉ1, respectively. This is an evidence to sufficiently justify that the doping of NiS to NiCo2S4 materials can improve the surface area of NiCo2S4/NiS composite materials. Meanwhile, the large surface area can benefit the contact of NiCo2S4/NiS CE and electrolyte thus improving the photoelectric property of DSSCs with NiCo2S4/NiS CE. Fig. 6a shows the CV analysis of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs. The CV curves of these CEs all have similar shapes with two pairs of redox peaks. This indicated that these as-prepared CEs possess remarkably similar catalytic activity with Pt CE. In fact, the left pair of redox peaks in the low potential range due to Eq. (1) and the right one is assigned as Eq. (2). Science the function of CE for a DSSC is to catalyze the reduction of I3 to I , so the left pair of redox peaks has an important impact for the photovoltaic property of DSSCs. The smaller overpotential of peak separation (Epp) between left pair of redox peaks means that the reduction of I3 to I is easy to carry out. The values of Epp for these CEs were recorded in Table 2. The value of Epp for the Pt CE is ~504 mV, which is higher than that of as-prepared CEs and the values of Epp for NiCo2S4/NiS and NiCo2S4 both were ~378 mV. Thus it is clear that all the asprepared CEs especially NiCo2S4/NiS and NiCo2S4 CEs are in favor of the reduction of I3 to I than Pt CE. Meanwhile, the higher current density of left cathodic peak (Jred-left) illustrates that the CE has an excellent electrocatalytic activity for the reduction of I3 =I . From Fig. 6a and Table 2, it is clear to get that the Co9S8 and NiCo2S4 CEs had similar values of Jred-left, and NiS made NiCo2S4/NiS CE have a higher Jred-left (0.774 mA cmˉ2). This maybe that the higher specific surface area and surface roughness of NiCo2S4/NiS benefits the contact between the electrolyte and CE and leads a good capacitive behaviour for higher current. Though the value of Jred-left for NiCo2S4/NiS CE is smaller than that of Pt CE (1.002 mA cmˉ2), the
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Fig. 3. FESEM images of as-prepared materials: (a) Co(CO)0.35Cl0.20(OH)1.10∙1.74H2O, (b) Co9S8, (c) (Ni, Co)2(CO3) (OH)2, (d) NiCo2S4, (e) NiS and (f) NiCo2S4/NiS, respectively.
Fig. 4. Surface roughness images of CEs: (a) NiCo2S4 and (b) NiCo2S4/NiS.
smaller Epp and Jred-left together made NiCo2S4/NiS CE demonstrate good electrocatalytic activity for the reduction of I3 =I .
I3 þ 2e 43I
(1)
3I2 þ 2e 42I3
(2)
EIS and Tafel analyses were employed to further study the electrocatalytic activity and the conductivity of CEs. Fig. 6b shows the Nyquist plots of EIS for NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs and the inset is the equivalent circuit diagram for fitting. There are four parameters in the equivalent circuit diagram: Rs is the series resistance of the sheet resistance of CEs, external wire
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Fig. 5. N2 adsorptionedesorption isotherms of Co9S8, NiCo2S4 and NiCo2S4/NiS.
resistance and the electrolytic resistance, Rct and CPE represent the charge transfer resistance and double layer capacitance at the electrolyte/CE interface, and ZW is the Nernst diffusion impedance of the I3 =I redox couples in the electrolyte. The correlation parameters of Rs and Rct were summarized in Table 2. Usually, smaller Rs and Rct mean that the conductivity and electrocatalytic activity of CEs are excellent and it is beneficial to increase the fill factor (FF) and power conversion efficiency (PCE) of DSSCs. From Table 2 can be obtained that the Pt-free CEs have similar values of Rs and the value of Rs for NiCo2S4/NiS CE is 9.6 U cm2, which is higher than that of Pt CE (7.0 U cm2). However, the Rct of NiCo2S4/NiS CE (2.2 U cm2) was smallest among these Pt-free CEs, this illustrated that the introduction of NiS can significantly improve the electrocatalytic activity of NiCo2S4/NiS CE for the reaction of I3 =I . Meanwhile, the Rct of NiCo2S4/NiS CE is also smaller than that of Pt CE (5.2 U cm2), and this would improve the short-circuit current density (Jsc) and PCE of DSSCs. Tafel polarization curves of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs were recorded in Fig. 6c. In Tafel analysis, exchange current density (J0) and limiting diffusion current density (Jlim) are two crucial parameters to evaluate the electrocatalytic activity of CEs. The Tafel curve of NiCo2S4/NiS CE shows a larger slope than that of other CEs, meaning a higher J0. From Eq. (3), it is easy to learn that the larger J0 made CEs have smaller Rct and excellent electrocatalytic activity and the values of Rct were calculated and listed in Table 2. In spite of the different Rct values between Tafel curves and EIS measurement, their change trends are consistent. In addition, the NiCo2S4/NiS CE has a higher value of Jlim than that of Pt CE and other as-prepared CEs. The relationship of Jlim and diffusion coefficient of I3 (D) is shown in Eq. (4). This demonstrates that I3 is easy to diffuse in electrolyte with NiCo2S4/NiS CE and higher Jlim is an important factor to increase photovoltaic performance of DSSCs.
J0 ¼ RT=nFRct
(3)
Jlim ¼ 2neDCNA =l
(4)
Where R ¼ gas constant, T ¼ absolute temperature, n ¼ electron number involved in the reaction, F ¼ Faraday constant, Rct ¼ charge-transfer resistance, D ¼ diffusion coefficient of I3 , C ¼ concentration of I3 in the electrolyte, l ¼ spacer thickness. Fig. 7 shows the J-V curves of DSSCs with NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs and relevant parameters were collected in Table 3. The DSSC with NiCo2S4/NiS CE exhibited an open-circuit voltage (Voc) of 744 mV, a high Jsc (17.7 mA cmˉ2), a FF of 0.67 and
Fig. 6. (a) CV, (b) EIS and (c) Tafel curves of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs, respectively. And the inset in (b) is the equivalent circuit diagram for fitting EIS data.
a high PCE of 8.8%, which is a significant improvement compared to that of DSSC with Pt CE (8.1%). Furthermore, the PCE of DSSC with NiCo2S4/NiS CE is higher than that of DSSCs with Co9S8 (7.7%) or NiS (6.9%) or NiCo2S4 (8.5%) CE. It is mainly because the Jsc and FF improved with the introduction of NiS for NiCo2S4/NiS CE. And the result of Jsc was agreed with the Rct data of EIS analysis. In addition, the smaller Rs and Rct made the DSSC with NiCo2S4/NiS CE has a higher FF and an excellent PCE. These photovoltaic characteristics
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Table 2 CV, EIS and Tafel data of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs. CE
Epp (mV)
JRed-left (mA cmˉ2)
Rs (U cm2)
a
NiCo2S4/NiS NiCo2S4 Co9S8 NiS Pt
378 378 612 666 504
0.774 0.687 0.688 0.699 1.002
9.6 10.6 12.2 10.3 7.0
2.2 4.0 9.4 14.4 5.2
a b
Rct (U cm2)
b
Rct (U cm2)
3.8 4.5 8.6 11.3 4.8
The values of Rct were obtained from EIS. The values of Rct were obtained from Tafel plots.
Acknowledgment The authors gratefully acknowledge the financial supporting by the National Natural Science Foundation of China (Nos. 91422301, U1205112, 21301060, and 61306077, 61474047), the Key Project of the Chinese Ministry of Education (212206), the Programs for Prominent Young Talents and New Century Excellent Talents in Fujian Province University, and the Promotion Program for Yong and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-YX102). References
Fig. 7. JeV curves of the DSSCs based on NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs under full sunlight illumination (100 mW cm2, AM 1.5 G).
Table 3 The photovoltaic data of the DSSCs based on NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs. CE
Voc (mV)
Jsc (mA cmˉ2)
FF
PCE (%)
NiCo2S4/NiS NiCo2S4 Co9S8 NiS Pt
744 743 741 735 736
17.7 17.4 16.2 14.9 16.5
0.67 0.66 0.64 0.63 0.67
8.8 8.5 7.7 6.9 8.1
demonstrate that the NiCo2S4/NiS hybrid material is a good conductive catalyst for CEs of DSSCs. 4. Conclusions In summary, the flower-like NiCo2S4/NiS microspheres are synthesized by a simple two-step hydrothermal method. The doping of NiS to NiCo2S4 can not only increase the specific surface area of the NiCo2S4/NiS composite, but also increase the surface roughness, conductivity and electrocatalytic activity of NiCo2S4/NiS CE. In addition, the CV, EIS and Tafel analyses indicated that the electrocatalytic activity of NiCo2S4/NiS CE for the reduction of I3 to Iˉ was superior to that of Pt CE. The DSSC with a NiCo2S4/NiS CE shows a perfect PCE of 8.8%, which is higher than that of DSSC with Pt CE (8.1%). Therefore, the as-prepared NiCo2S4/NiS is a kind of low-cost and high efficient hybrid material for counter electrodes of DSSCs.
[1] B. O’regan, M. Gratzel, Nature 353 (1991) 737e740. [2] J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan, G. Luo, Chem. Rev. 115 (2015) 2136e2173. [3] H. Koo, Y. Kim, Y. Lee, W. Lee, K. Kim, N. Park, Adv. Mater. 20 (2008) 195e199. [4] G. Shang, J. Wu, M. Huang, J. Lin, Z. Lan, Y. Huang, L. Fan, J. Phys. Chem. C 116 (2012) 20140e20145. [5] Z. Tang, J. Wu, M. Zheng, J. Huo, Z. Lan, Nano Energy 2 (2013) 622e627. [6] H. Sun, D. Qin, S. Huang, X. Guo, D. Li, Y. Luo, Q. Meng, Energy Environ. Sci. 4 (2011) 2630e2637. [7] J. Huo, J. Wu, M. Zheng, Y. Tu, Z. Lan, J. Power Sources 293 (2015) 570e576. [8] W. Guo, C. Chen, M. Ye, M. Lv, C. Lin, Nanoscale 6 (2014) 3656e3663. [9] J. Jin, X. Zhang, T. He, J. Phys. Chem. C 118 (2014) 24877e24883. [10] Z. Wan, C. Jia, Y. Wang, Nanoscale 7 (2015) 12737e12742. [11] Y. Li, L. Cao, L. Qiao, M. Zhou, Y. Yang, P. Xiao, Y. Zhang, J. Mater. Chem. A 2 (2014) 6540e6548. [12] Z. Shi, K. Deng, L. Li, Sci. Rep. 5 (2015) 09317. [13] W. Chen, C. Xia, H. Alshareef, ACS Nano 8 (2014) 9531e9541. [14] A. Banerjee, K. Upadhyay, S. Bhatnagar, M. Tathavadekar, U. Bansode, S. Agarkar, S. Ogale, RSC Adv. 4 (2014) 8289e8294. [15] J. Xiao, X. Zeng, W. Chen, F. Xiao, S. Wang, Chem. Commun. 49 (2013) 11734e11736. [16] H. Wan, J. Jiang, J. Yu, K. Xu, L. Miao, L. Zhang, H. Chen, Y. Puan, CrystEngComm 15 (2013) 7649e7651. [17] H. Chen, J. Jiang, L. Zhang, H. Wan, T. Qi, D. Xia, Nanoscale 5 (2013) 8879e8883. [18] J. Xiao, L. Wan, S. Yang, F. Xiao, S. Wang, Nano Lett. 14 (2014) 831e838. [19] R. Zou, Z. Zhang, M. Yuen, M. Sun, J. Hu, C. Lee, W. Zhang, NPG Asia Mater. 7 (2015) e195. [20] Z. Shi, H. Lu, Q. Liu, F. Cao, J. Guo, K. Deng, L. Li, Nanoscale Res. Lett. 9 (2014) 1e6. [21] S. Chou, J. Lin, 227th ECS Meet. Abstr. 37 (2015), 2023e2023. [22] H. Wan, X. Ji, J. Jiang, J. Yu, L. Miao, L. Zhang, S. Bie, H. Chen, Y. Ruan, J. Power Sources 243 (2013) 396e402. [23] L. Que, Z. Lan, W. Wu, J. Wu, J. Lin, M. Huang, J. Power Sources 268 (2014) 670e676. [24] G. Yue, J. Wu, Y. Xiao, J. Lin, M. Huang, Z. Lan, L. Fan, Energy 54 (2013) 315e321. [25] J. Pu, F. Cui, S. Chu, T. Wang, E. Sheng, Z. Wang, ACS Sustain. Chem. Eng. 2 (2014) 809e815. [26] L. Fu, Z. Liu, Y. Liu, B. Han, P. Hu, L. Cao, D. Zhu, Adv. Mater. 17 (2005) 217e221. [27] B. Ernst, S. Libs, P. Chaumette, A. Kiennemann, A. Appl, Catal. A 186 (1999) 145e168. [28] J. Xu, P. Gao, T. Zhao, Energy Environ. Sci. 5 (2012) 5333e5339. [29] J. Xiao, L. Wan, S. Yang, F. Xiao, S. Wang, Nano Lett. 14 (2014) 831e838. [30] M. Shehzad, A. Qaiser, A. Javaid, F. Saeed, Synth. Met. 200 (2015) 164e171. [31] R. Tao, J. Wu, H. Xue, X. Song, X. Pan, X. Fang, X. Fang, S. Dai, J. Power Sources 195 (2010) 2989e2995.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Flower-like nickel cobalt sulfide microspheres modified with nickel sulfide as Pt-free counter electrode for dye-sensitized solar cells Jinghao Huo, Jihuai Wu*, Min Zheng, Yongguang Tu, Zhang Lan Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The NiCo2S4/NiS microspheres are synthesized by a two-step hydrothermal method. The NiCo2S4/NiS shows flower-like morphology and has a large specific surface area. NiCo2S4/NiS CE exhibits excellent electrocatalytic activity and conductivity. The DSSC with NiCo2S4/NiS CE obtains a PCE of 8.80% than that of DSSC with Pt CE.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 July 2015 Received in revised form 12 November 2015 Accepted 17 November 2015 Available online xxx
The nickel cobalt sulfide/nickel sulfide (NiCo2S4/NiS) microspheres which exhibit flower-like morphologies are synthesized by a two-step hydrothermal method. Then the NiCo2S4/NiS microspheres are deposited on a fluorine doped SnO2 substrate by spin-casting the isopropyl alcohol solution of asprepared microspheres. The cyclic voltammetry, electrochemical impedance spectroscopy and Tafel tests are employed to measure the electrochemical performance of NiCo2S4/NiS counter electrode. The NiCo2S4 and NiS all are used to improve the conductivity and electrocatalytic ability of the films, and the NiS can also increase the specific surface area of microspheres. The dye-sensitized solar cells (DSSCs) with the NiCo2S4/NiS counter electrode exhibite a power conversion efficiency of 8.8%, which is higher than that of DSSC with Pt counter electrode (8.1%) under the light intensity of 100 mW cm2 (AM 1.5 G). © 2015 Published by Elsevier B.V.
Keywords: Dye-sensitized solar cells Counter electrode Sulfide Flower-like microsphere
1. Introduction As energy consumption and environmental issues are becoming more prominent, researchers pay more attention to the dyesensitized solar cells (DSSCs) due to their easy fabrication, low cost and high conversion efficiency of solar energy [1,2]. Generally, a typical DSSC is a sandwich structure including a dye-sensitized
* Corresponding author. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.jpowsour.2015.11.062 0378-7753/© 2015 Published by Elsevier B.V.
semiconductor (TiO2 or ZnO or SnO2 et al.) photoanode on fluorine doped SnO2 (FTO) glass, a I3 =I redox couple contained electrolyte, and a platinum (Pt) counter electrode (CE) [3e5]. Today researchers concentrate on exploring Pt-free catalytic materials to replace Pt because of its exorbitant price and easy corrosion. Inorganic materials like transition-metal sulfides are one of the most important materials for CEs [6]. To date, more and more attention has been paid to metal sulfides especially cobalt sulfide, nickel sulfide and their solid solution, such as CoS, CoS2, Co9S8, Ni9S8, NiS, Ni3S4, NiS2 NiCo2S4 and CoNi2S4 [7e12]. Among these sulfides materials, NiCo2S4 is a ternary metal sulfide and contains
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higher electrochemical characteristics than that of nickel sulfides and cobalt sulfides [13]. As a promising electrode material, NiCo2S4 was used in DSSCs [14,15], supercapacitors [11,16e18] and lithiumion batteries [19]. In 2013, Xiao et al. [15] prepared a NiCo2S4 hollow nanorod arrays CE for quantum dot sensitized solar cells (QDSCs), which attained a higher power conversion efficiency (PCE) of 4.22% than that of QDSCs with Pt CE (3.05%). Shi et al. [20] successfully synthesised of NiCo2S4 nanosheet films from NiCo2O4 nanosheet films and these two nanomaterials were directly used to be CE and photocathode of p-type DSSCs. These two electrode materials made the photocurrent density have an improvement of 2.989 mA cmˉ2 for p-type DSSCs. Banerjee et al. [19] prepared NiCo2S4 nanoneedles by sulphurization of NiCo2O4 to replace Pt as CE material, and NiCo2S4 CE made the DSSCs obtain an impressive PCE of 6.9%, compared with that of DSSCs based on Pt CE (7.7%). Chou et al. [21] firstly synthesized a transparent NiCo2S4 CE by electrophoretic deposition, and the DSSCs with this transparent CE obtain a PCE of 6.14%. In this paper, we fabricated a flower-like NiCo2S4/NiS microspheres by a simple two-step hydrothermal method which is similar to prepared Co9S8 nanotubes [22]. Then the NiCo2S4/NiS hybrid material was coated on FTO glass by spin-casting the isopropyl alcohol solution of as-prepared material. With the combined effect of NiCo2S4 and NiS, the NiCo2S4/NiS composite exhibited the large specific surface area, high conductivity and excellent electrocatalytic activity for the reduction of I3 to I . Based on this NiCo2S4/NiS CE, the PCE of DSSCs can attain a high value of 8.8% and improve about 8.24%, compared to DSSCs with Pt CE (8.1%).
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dried in vacuum at 60 C for 24 h. Followed by a hydrothermal process with Na2S$9H2O, the precursor of (Ni, Co)2(CO3) (OH)2 is transformed into metal sulfides (NiCo2S4/NiS). In short, 400 mg of precursor and 1.2 g of Na2S$9H2O were dispersed in 60 mL deionized water under intense stirring for 10 min, then the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, and heated at 180 C for 8 h. After cooling to room temperature naturally, the underlying precipitation (NiCo2S4/NiS microspheres) was collected and washed with deionized water and anhydrous ethanol three times, respectively. Finally, the NiCo2S4/NiS powder was dried in vacuum at 60 C for 24 h. The Co9S8, NiS and NiCo2S4 samples were prepared with different content of NiCl2$6H2O and CoCl2$6H2O by the same experimental procedures as that of NiCo2S4/NiS (Table 1). 2.3. Preparation of counter electrodes In a simple way, 100 mg of NiCo2S4/NiS microspheres was dissolved in 10 mL isopropyl alcohol solution and treated with ultrasonic oscillation for about 20 min to be an uniformly dispersed solution. Then the NiCo2S4/NiS was deposited on a FTO substrate by spin-casting the as-prepared isopropyl alcohol solution at a rate of 4000 rpm for 20 s. After the FTO substrate was thermally treated at 80 C for 10 min, the second NiCo2S4/NiS layer was spin-casting on the first layer. The same processes were done when preparing the third NiCo2S4/NiS layer. After three NiCo2S4/NiS layers done, the NiCo2S4/NiS CE was fabricated. The same method of NiCo2S4/NiS CE was used to prepare the Co9S8, NiS and NiCo2S4 CEs.
2. Experimental 2.4. Fabrication of DSSCs 2.1. Chemicals and materials The nickel chloride hexahydrate (NiCl2$6H2O), cobalt chloride hexahydrate (CoCl2$6H2O), urea, sodium sulfide nonahydrate (Na2S$9H2O), isopropyl alcohol, acetone, ethanol, tetra-n-butyl titanate, titanium tetrachloride (TiCl4), acetonitrile (ACN), tetraethyl ammonium iodide, tetrabutyl ammonium iodide, tetramethyl ammonium iodide, 4-tert-butyl pyridine (tBP), lithium perchlorate (LiClO4), sodium iodide (NaI), potassium iodide (KI), lithium iodide (LiI), and iodine (I2) were all A. R. grade and purchased from Sinopharm Chemical Reagent Co. Ltd, China. N719 dye (Ru[LL'-(NCS)2], L ¼ 2, 2'-bipyridyl-4, 4'-dicarboxylic acid, L' ¼ 2, 2'bipyridyl-4, 4'-ditetrabutylammonium carboxylate) was acquired from Dye sol. The FTO glass (20 cm 15 cm, 14 U sq1) and a Pt CE (2.5 cm 2 cm, prepared by magnetron sputtering) were purchased from Wuhan Lattice Solar Energy Technology Co. Ltd, China. And the FTO glass was cut into pieces (1.5 cm 1.5 cm) and washed with cleaner, acetone and isopropyl alcohol in turn. Finally, the FTO glass was stored in anhydrous ethanol. 2.2. Synthesis of NiCo2S4/NiS microspheres The flower-like NiCo2S4/NiS microspheres were synthesized by a simple two-step hydrothermal method. The first step, NiCl2$6H2O and CoCl2$6H2O were mixed with urea in deionized water to form a precursor of (Ni, Co)2(CO3) (OH)2. In brief, 7 mmol of NiCl2$6H2O, 3 mmol of CoCl2$6H2O and 10 mmol of urea were added into 60 mL deionized water under intense stirring for 10 min, then the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, heated in an oven at 130 C for 10 h. After the autoclave cooled to room temperature naturally, the underlying precipitation was the precursor and washed with deionized water and anhydrous ethanol three times, respectively. Finally, the precursor powder was
TiO2 photoanodes were prepared by spin-casting a ~160 nm TiO2 under layer and doctor-blading technique to form a 12 mm TiO2 nanocrystalline layer [23,24]. Then the TiO2 photoanodes were immersed into 0.05 M TiCl4 solution at 70 C for 0.5 h. After that, the photoanodes were sintered at 450 C in a muffle furnace for 0.5 h. When the temperature dropped to 80 C, the TiO2 photoanodes were took out and soaked in a 0.3 mM N719 dye ethanol solution in the room temperature for 24 h. Finally, the TiO2 photoanodes were took out from solution and washed with anhydrous ethanol. The size of photoanodes was ~0.12 cm2 (0.3 cm 0.4 cm). The TiO2 photoanode and an as-prepared CE were fabricated a typical sandwich cell, which these two electrodes were adhered together with a cyanoacrylate adhesive and a I3 =I electrolyte was injected into the gap between them. The I3 =I electrolyte was an ACN solution of tetramethyl ammonium iodide, (0.1 M), tetrabutyl ammonium iodide (0.1 M), tetraethyl ammonium iodide (0.1 M), NaI (0.1 M), KI (0.1 M), LiI (0.1 M), I2 (0.05 M), and tBP (0.5 M). 2.5. Characterizations and measurements The crystal structures of Co9S8, NiCo2S4/NiS powders and their precursors were measured by X-ray Diffraction (XRD, Bruker D8,Cu Ka, l ¼ 1.5418 Å) at scanning rate of 5 minˉ1. The composition of NiCo2S4/NiS hybrid was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo). XPS was performed by using Al Ka (hn ¼ 1486.6 eV) with power of 150 W and beam spot of 500 mm. XPS data were calibrated by C1s (284.8 eV) and fitted with XPSPEAK 4.0 software. The surface Morphologies of as-prepared products were performed by a field emission scanning electron microscopy (FESEM, SU8010, Hitachi). Energy dispersive X-ray spectroscopy (EDS) analysis was performed to observe the elemental type of the NiCo2S4/NiS CEs. The surface roughness of NiCo2S4 and NiCo2S4/NiS CEs was conducted with atomic force
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Table 1 The experimental formulas of preparing NiS, NiCo2S4/NiS, NiCo2S4 and Co9S8 materials by a two-step hydrothermal method. NiCl2$6H2O þ CoCl2$6H2O (60 mL H2O)
10 mmol þ 0
7 mmol þ 3 mmol
3.33 mmol þ 6.67 mmol
0 þ 10 mmol
Product (step 1) Product (step 2)
Ni2(CO3) (OH)2 NiS
(Ni, Co)2(CO3) (OH)2 NiCo2S4/NiS
(Ni, Co)2(CO3) (OH)2 NiCo2S4
Co(CO)0.35Cl0.20(OH)1.10$1.74H2O Co9S8
microscopy (AFM, Nanoscope Ⅲ a, Veeco). Surface areas of Co9S8, NiCo2S4 and NiCo2S4/NiS samples were measured with a BrunauerEmmett Teller (BET) sorptometer (ASAP 2020, Micromeritics) using nitrogen adsorption at 77 K. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel tests were used to study the electrochemical properties of CEs by electrochemical working station (IM6, Zahner, Germany). CV tests used a three-electrode cell to study the electrocatalytic ability of CEs for I3 =I and the scanning potential range was from 0.4 V to 1.4 V with a scan rate of 50 mV s1. The three-electrode cell includes as-prepared CEs as work electrode, a Pt sheet as CE and an Ag/AgCl reference electrode. Meanwhile an ACN solution of 10 mM LiI, 1 mM I2, and 100 mM LiClO4 was used to be electrolyte solution. EIS measurement was conducted with a traditional symmetrical cell from 100 kHz to 100 mHz under bias voltage of 0 V and the corresponding amplitude of 5 mV. After the tests, all EIS data were fitted with ZsimpWin software. Tafel curves were recorded from 1 V to 1 V at the scan rate of 10 mV s1. Photocurrent-photovoltage (JeV) tests of DSSCs with asprepared CEs and Pt CE were measured with a solar simulator (PVIV-94043A, Newport, USA) under simulated solar illumination of 100 mW cm2 (AM 1.5 G). A black mask was used on the surface of DSSC to avoid stray light and all the tests were performed in the air and at room temperature.
3. Results and discussion Fig. 1a shows typical XRD patterns of Co9S8 and its precursor. All the diffraction peaks of precursor can be indexed to crystalline Co(CO)0.35Cl0.20(OH)1.10$1.74H2O (JCPDS card file No. 38-547). And all of the reflection peaks in Co9S8 sample pattern at 15.4 , 29.8 , 31.2 , 39.5 , 47.6 , 52.1, 54.7 can be indexed to the (111), (311), (222), (331), (511), (440) and (531) planes of face-centered cubic Co9S8 (JCPDS card file No. 65-1765). When adding 7 mmol NiCl2$6H2O to replace CoCl2$6H2O, the diffraction peaks of precursor ((Ni, Co)2(CO3) (OH)2) can be indexed to Ni2(CO3) (OH)2 (JCPDS card file No. 35-0501) (Fig. 1b). This means the presence of Co ions does not affect the crystal structure of Ni2(CO3) (OH)2, but only has an influence for lattice parameters [16]. With 7 mmol NiCl2$6H2O in experimental procedures, the diffraction peaks of the final product at 16.3 , 26.8 , 31.6 , 38.3 , 50.5 and 55.3 corresponding to the (111), (220), (311), (400), (511) and (440) planes of cubic type NiCo2S4 (JCPDS card file No. 20e0782). There are some other small diffraction peaks at 34.7, 53.5 , 60.9 and 65.7 have the same diffracted angle as the (101), (110), (103) and (201) planes of NiS (JCPDS card file No. 02-1280). It turned out that the final product was a compound of NiCo2S4 and NiS. To further explore the surface elemental composition of NiCo2S4/NiS samples, XPS analysis was conducted and shown in Fig. 2. There are four elements: Ni, Co, S and O were tested in the sample of survey san (Fig. 2a) and the C element is used to be a reference. As shown in Fig. 2b of Ni 2p XPS, the energy difference between Ni 2p3/2 (857.0 eV) and Ni 2p1/2 (874.2 eV) is 17.2 eV, which indicated the existence of both Ni2þ and Ni3þ [25]. Meanwhile the energy difference between Co 2p3/2 (778.5 eV) and Co 2p1/2 (798.1 eV) is over 15 eV and demonstrated the existence of both Co2þ and Co3þ (Fig. 2c) [26e28]. According to Fig. 2d, there are three main peaks of 161.4 eV, 162.6 eV and 169.3 eV in the high
Fig. 1. XRD patterns of as-prepared materials: (a) Co(CO)0.35Cl0.20(OH)1.10$1.74H2O and Co9S8, (b) (Ni, Co)2(CO3) (OH)2 and NiCo2S4/NiS.
resolution scan of S 2p spectrum and these three peaks can be ascribed to the S 2p3/2, S 2p1/2 and the shakeup satellite, respectively. And furthermore, the peak at 161.4 eV of S 2p represents the existence of S2 ion and the component 162.6 eV can be ascribed to the sulphion in low coordination at the surface of hybrid. So on the near surface of the NiCo2S4/NiS hybrid, there are ions of Ni2þ, Ni3þ, Co2þ, Co3þ and S2, which match with literature results for NiCo2S4. In Fig. 2e, the main peak of 532.1 eV at O 1s XPS proves the existence of MO (M ¼ Ni or Co) [29]. Fig. 2f is the EDS spectrum of the NiCo2S4/NiS, which illustrates the element types of C, O, Co, Ni and S elements and the atomic percent of every element. From Fig. 2f can be obtained that the atomic percents of O, Ni, Co and S in NiCo2S4/NiS are 23.20%, 25.62%, 10.42% and 20.29%, respectively. Using XRD and XPS analysis, EDS data suggest that the NiCo2S4 and NiS are main compositions of hybrid and some oxides are formed in the hybrid. Fig. 3 was the FESEM images of the surface morphologies of precursors and as-obtained materials. Fig. 3a is the surface
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Fig. 2. XPS spectra of NiCo2S4/NiS hybrid: (a) Survey scan, (b) high resolution scan of the Ni 2p peak, (c) Co 2p peak, (d) S 2p peak, (e) O 1s peak and the related fittings and (f) EDS of NiCo2S4/NiS CE.
morphology of Co(CO)0.35Cl0.20(OH)1.10$1.74H2O, which shows many nanoneedles, with diameters of ~200 nm, and lengths of several micrometers. In contrast, Fig. 3b shows the surface morphology of Co9S8 with a tubular structure. Fig. 3c gives a FESEM image of precursor ((Ni, Co)2(CO3) (OH)2) of NiCo2S4, which consists of many micron flower-like structures and a mass of nanorods radially grown from these common centre. After a hydrothermal process of precursor and Na2S$9H2O, the morphology of NiCo2S4 has changed a lot from (Ni, Co)2(CO)3(OH)2. In Fig. 3d, it is clear to see that the bunches of flower-like structures are nanotubes instead of nanorods. When the NiCl2$6H2O all replaced CoCl2$6H2O in hydrothermal process, the NiS product was some micron balls with nanosheets and nanowires (Fig. 3e). Fig. 3f shows the surface morphology of NiCo2S4/NiS matericals prepared by 7 mmol of NiCl2$6H2O and 3 mmol of CoCl2$6H2O in the first hydrothermal step. The NiCo2S4/NiS consisted of flower-like structures with the diameters of several micrometers. The inset of Fig. 3f is the highpowered FESEM picture and it is clear to see that the flower-like structures were composed of numerous nanowires and some nanoparticles. To further estimate the surface roughness of NiCo2S4 and NiCo2S4/NiS CEs, the AFM measurement was conducted and AFM images (3 3 mm2) were recorded in Fig. 4. The root mean square (RMS) surface roughness was used to evaluate the surface roughness of CEs and the values of RMS were calculated from AFM images [30]. The values of RMS for NiCo2S4 and NiCo2S4/NiS CEs are 54.6 and 100.5 nm. The higher surface roughness of NiCo2S4/NiS CEs results in a larger active area for the reactions with the electrolyte. The increasing of RMS surface roughness is due to the NiCo2S4 and NiS together to increase the surface area. Fig. 5 shows the N2 adsorptionedesorption isotherms of Co9S8, NiCo2S4 and NiCo2S4/NiS samples for comparison. According to the classification of International Union of Pure and Applied Chemistry
(IUPAC), these isotherms are typical Ⅲ isotherms with H3 hysteresis loops [31]. The BET surface areas of Co9S8, NiCo2S4 and NiCo2S4/NiS are 16.90 m2 gˉ1, 12.00 m2 gˉ1 and 27.79 m2 gˉ1, respectively. This is an evidence to sufficiently justify that the doping of NiS to NiCo2S4 materials can improve the surface area of NiCo2S4/NiS composite materials. Meanwhile, the large surface area can benefit the contact of NiCo2S4/NiS CE and electrolyte thus improving the photoelectric property of DSSCs with NiCo2S4/NiS CE. Fig. 6a shows the CV analysis of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs. The CV curves of these CEs all have similar shapes with two pairs of redox peaks. This indicated that these as-prepared CEs possess remarkably similar catalytic activity with Pt CE. In fact, the left pair of redox peaks in the low potential range due to Eq. (1) and the right one is assigned as Eq. (2). Science the function of CE for a DSSC is to catalyze the reduction of I3 to I , so the left pair of redox peaks has an important impact for the photovoltaic property of DSSCs. The smaller overpotential of peak separation (Epp) between left pair of redox peaks means that the reduction of I3 to I is easy to carry out. The values of Epp for these CEs were recorded in Table 2. The value of Epp for the Pt CE is ~504 mV, which is higher than that of as-prepared CEs and the values of Epp for NiCo2S4/NiS and NiCo2S4 both were ~378 mV. Thus it is clear that all the asprepared CEs especially NiCo2S4/NiS and NiCo2S4 CEs are in favor of the reduction of I3 to I than Pt CE. Meanwhile, the higher current density of left cathodic peak (Jred-left) illustrates that the CE has an excellent electrocatalytic activity for the reduction of I3 =I . From Fig. 6a and Table 2, it is clear to get that the Co9S8 and NiCo2S4 CEs had similar values of Jred-left, and NiS made NiCo2S4/NiS CE have a higher Jred-left (0.774 mA cmˉ2). This maybe that the higher specific surface area and surface roughness of NiCo2S4/NiS benefits the contact between the electrolyte and CE and leads a good capacitive behaviour for higher current. Though the value of Jred-left for NiCo2S4/NiS CE is smaller than that of Pt CE (1.002 mA cmˉ2), the
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Fig. 3. FESEM images of as-prepared materials: (a) Co(CO)0.35Cl0.20(OH)1.10∙1.74H2O, (b) Co9S8, (c) (Ni, Co)2(CO3) (OH)2, (d) NiCo2S4, (e) NiS and (f) NiCo2S4/NiS, respectively.
Fig. 4. Surface roughness images of CEs: (a) NiCo2S4 and (b) NiCo2S4/NiS.
smaller Epp and Jred-left together made NiCo2S4/NiS CE demonstrate good electrocatalytic activity for the reduction of I3 =I .
I3 þ 2e 43I
(1)
3I2 þ 2e 42I3
(2)
EIS and Tafel analyses were employed to further study the electrocatalytic activity and the conductivity of CEs. Fig. 6b shows the Nyquist plots of EIS for NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs and the inset is the equivalent circuit diagram for fitting. There are four parameters in the equivalent circuit diagram: Rs is the series resistance of the sheet resistance of CEs, external wire
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271
Fig. 5. N2 adsorptionedesorption isotherms of Co9S8, NiCo2S4 and NiCo2S4/NiS.
resistance and the electrolytic resistance, Rct and CPE represent the charge transfer resistance and double layer capacitance at the electrolyte/CE interface, and ZW is the Nernst diffusion impedance of the I3 =I redox couples in the electrolyte. The correlation parameters of Rs and Rct were summarized in Table 2. Usually, smaller Rs and Rct mean that the conductivity and electrocatalytic activity of CEs are excellent and it is beneficial to increase the fill factor (FF) and power conversion efficiency (PCE) of DSSCs. From Table 2 can be obtained that the Pt-free CEs have similar values of Rs and the value of Rs for NiCo2S4/NiS CE is 9.6 U cm2, which is higher than that of Pt CE (7.0 U cm2). However, the Rct of NiCo2S4/NiS CE (2.2 U cm2) was smallest among these Pt-free CEs, this illustrated that the introduction of NiS can significantly improve the electrocatalytic activity of NiCo2S4/NiS CE for the reaction of I3 =I . Meanwhile, the Rct of NiCo2S4/NiS CE is also smaller than that of Pt CE (5.2 U cm2), and this would improve the short-circuit current density (Jsc) and PCE of DSSCs. Tafel polarization curves of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs were recorded in Fig. 6c. In Tafel analysis, exchange current density (J0) and limiting diffusion current density (Jlim) are two crucial parameters to evaluate the electrocatalytic activity of CEs. The Tafel curve of NiCo2S4/NiS CE shows a larger slope than that of other CEs, meaning a higher J0. From Eq. (3), it is easy to learn that the larger J0 made CEs have smaller Rct and excellent electrocatalytic activity and the values of Rct were calculated and listed in Table 2. In spite of the different Rct values between Tafel curves and EIS measurement, their change trends are consistent. In addition, the NiCo2S4/NiS CE has a higher value of Jlim than that of Pt CE and other as-prepared CEs. The relationship of Jlim and diffusion coefficient of I3 (D) is shown in Eq. (4). This demonstrates that I3 is easy to diffuse in electrolyte with NiCo2S4/NiS CE and higher Jlim is an important factor to increase photovoltaic performance of DSSCs.
J0 ¼ RT=nFRct
(3)
Jlim ¼ 2neDCNA =l
(4)
Where R ¼ gas constant, T ¼ absolute temperature, n ¼ electron number involved in the reaction, F ¼ Faraday constant, Rct ¼ charge-transfer resistance, D ¼ diffusion coefficient of I3 , C ¼ concentration of I3 in the electrolyte, l ¼ spacer thickness. Fig. 7 shows the J-V curves of DSSCs with NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs and relevant parameters were collected in Table 3. The DSSC with NiCo2S4/NiS CE exhibited an open-circuit voltage (Voc) of 744 mV, a high Jsc (17.7 mA cmˉ2), a FF of 0.67 and
Fig. 6. (a) CV, (b) EIS and (c) Tafel curves of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs, respectively. And the inset in (b) is the equivalent circuit diagram for fitting EIS data.
a high PCE of 8.8%, which is a significant improvement compared to that of DSSC with Pt CE (8.1%). Furthermore, the PCE of DSSC with NiCo2S4/NiS CE is higher than that of DSSCs with Co9S8 (7.7%) or NiS (6.9%) or NiCo2S4 (8.5%) CE. It is mainly because the Jsc and FF improved with the introduction of NiS for NiCo2S4/NiS CE. And the result of Jsc was agreed with the Rct data of EIS analysis. In addition, the smaller Rs and Rct made the DSSC with NiCo2S4/NiS CE has a higher FF and an excellent PCE. These photovoltaic characteristics
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Table 2 CV, EIS and Tafel data of NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs. CE
Epp (mV)
JRed-left (mA cmˉ2)
Rs (U cm2)
a
NiCo2S4/NiS NiCo2S4 Co9S8 NiS Pt
378 378 612 666 504
0.774 0.687 0.688 0.699 1.002
9.6 10.6 12.2 10.3 7.0
2.2 4.0 9.4 14.4 5.2
a b
Rct (U cm2)
b
Rct (U cm2)
3.8 4.5 8.6 11.3 4.8
The values of Rct were obtained from EIS. The values of Rct were obtained from Tafel plots.
Acknowledgment The authors gratefully acknowledge the financial supporting by the National Natural Science Foundation of China (Nos. 91422301, U1205112, 21301060, and 61306077, 61474047), the Key Project of the Chinese Ministry of Education (212206), the Programs for Prominent Young Talents and New Century Excellent Talents in Fujian Province University, and the Promotion Program for Yong and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-YX102). References
Fig. 7. JeV curves of the DSSCs based on NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs under full sunlight illumination (100 mW cm2, AM 1.5 G).
Table 3 The photovoltaic data of the DSSCs based on NiCo2S4/NiS, NiCo2S4, Co9S8, NiS and Pt CEs. CE
Voc (mV)
Jsc (mA cmˉ2)
FF
PCE (%)
NiCo2S4/NiS NiCo2S4 Co9S8 NiS Pt
744 743 741 735 736
17.7 17.4 16.2 14.9 16.5
0.67 0.66 0.64 0.63 0.67
8.8 8.5 7.7 6.9 8.1
demonstrate that the NiCo2S4/NiS hybrid material is a good conductive catalyst for CEs of DSSCs. 4. Conclusions In summary, the flower-like NiCo2S4/NiS microspheres are synthesized by a simple two-step hydrothermal method. The doping of NiS to NiCo2S4 can not only increase the specific surface area of the NiCo2S4/NiS composite, but also increase the surface roughness, conductivity and electrocatalytic activity of NiCo2S4/NiS CE. In addition, the CV, EIS and Tafel analyses indicated that the electrocatalytic activity of NiCo2S4/NiS CE for the reduction of I3 to Iˉ was superior to that of Pt CE. The DSSC with a NiCo2S4/NiS CE shows a perfect PCE of 8.8%, which is higher than that of DSSC with Pt CE (8.1%). Therefore, the as-prepared NiCo2S4/NiS is a kind of low-cost and high efficient hybrid material for counter electrodes of DSSCs.
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