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Carbon Black Composite as Counter Electrode for Dye-Sensitized Solar Cells
Accepted Manuscript Title: Facile Synthesis of NiCo2 O4 /Carbon Black Composite as Counter Electrode for Dye-Sensitized Solar Cells Authors: Yanan Wang, Nianqing Fu, Pin Ma, Yanyan Fang, Lumei Peng, Xiaowen Zhou, Yuan Lin PII: DOI: Reference:
S0169-4332(17)31369-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.057 APSUSC 35992
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-3-2017 27-4-2017 7-5-2017
Please cite this article as: Yanan Wang, Nianqing Fu, Pin Ma, Yanyan Fang, Lumei Peng, Xiaowen Zhou, Yuan Lin, Facile Synthesis of NiCo2O4/Carbon Black Composite as Counter Electrode for Dye-Sensitized Solar Cells, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile Synthesis of NiCo2O4/Carbon Black Composite as Counter Electrode for Dye-Sensitized Solar Cells
Yanan Wanga,b, Nianqing Fuc, Pin Maa,b, Yanyan Fanga, Lumei Penga, Xiaowen Zhoua, Yuan Lina,b,* a
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, CAS
Research/Education Center for Excellence in Molecular Science, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China b
c
University of Chinese Academy of Sciences, Beijing 100049, China
School of Material Science and Engineering, South China University of Technology, Guangzhou
510640, China
Highlight The NiCo2O4/C composite is successfully prepared by a facile solution route. The NiCo2O4/C is applied as counter electrode for the reduction of triiodide in the dye-sensitized solar cells. The NiCo2O4/C composite possesses a better electronic conductivity and a higher electrochemical activity than NiO/C and Co3O4/C. The NiCo2O4/C CE shows the photovoltaic performance of 6.27% and the fill factor of 0.60. Due to the synergistic effect between C and NiCo2O4, the NiCo2O4/C CE possesses both high efficiency and stability. Abstract NiCo2O4/carbon black (NiCo2O4/C) composite is successfully synthesized by a facile solution route, and used as counter electrode (CE) for dye-sensitized solar cells (DSCs). The DSCs based on NiCo2O4/C composite CE achieves a power conversion efficiency of 6.27%, which is much higher than that of NiO/C (5.07%), Co3O4/C *Corresponding
author. Tel.: +86 10 8261 5031; fax: +86 10 8261 7315 E-mail address: [email protected] (Y. Lin)
(4.82%) or pristine C (4.34%). Also, the fill factors of DSCs devices with the NiCo2O4/C CE are better than that of other CEs. Compared to pristine C, the NiCo2O4/C composite has a marked improvement on electrocatalytic performance for the reduction of triiodide. Due to the synergistic effect between C and NiCo2O4, the NiCo2O4/C CE possesses both high efficiency and stability. Key words
Counter electrode NiCo2O4/carbon black composite Dye-sensitized
solar cells 1. Introduction Since the first prototype from O’Regan and Grätzel, dye sensitized solar cells (DSCs) have attracted growing interests and are generally regarded as a promising solution to global energy and environmental problems [1,2]. Typically, a DSC consists of a dye-sensitized porous semi-conductor photoanode, an electrolyte containing an iodide/triiodide (I-/I3-) redox couple, and a counter electrode [3]. The counter electrode (CE) plays a crucial role in catalysing the reduction reaction of I3- to I- to regenerate the sensitizer [4,5]. So far, the Pt electrode is still the preferred counter electrode in DSCs owing to its outstanding electrocatalytic activity and superior conductivity. However, Pt CE is expensive and suffers from a long-term stability problem under highly corrosive I-/I3- redox couple. As alternatives to Pt CE, conductive
polymers
[6-8],
carbon-based
materials
[9-12],
and
metallic/semiconductive compounds [13-15] have been reported. Among these materials, carbon is a promising material to supplant Pt owing to their advantages of low-cost, environmental friendliness and large surface. However, the
carbon materials generally have low intrinsic electrocatalytic activity [16]. Meanwhile, transition metal compounds possess the poor electron transport efficiency, which finally restricted their catalytic activity for the reduction of triiodide [17]. Thus, hybrid materials based on carbon materials and metallic compounds can be a reasonable alternative to satisfy inevitable requirements of a low cost and high electric conductivity. Recently, composites of transition metal compound and carbon material were constructed and used as CEs for DSCs, such as Fe3O4/carbon black [17], Co3O4/mesoporous carbons [18], RuO2/grapheme [19], RGO/MWCNTs/NiO [20]. The power conversion efficiency of DSCs based on these hybrid composites CEs is much higher than that of devices with pure transition metal compounds CEs or pure carbon materials CEs and is comparable to that of Pt CE based DSCs. Little attention has been paid to the use of multiple transition metal compounds, especially the multiple oxide compounds. Some multiple transition metal compounds have shown good performance as CE in DSCs, such as Cu2ZnSnS4, Cu2ZnSnSe4, CoFeO4 and NiCo2S4 [21-23]. These results indicate that multiple transition metal compounds may provide excellent performance in DSCs. The electronic structures of transition metal compounds are similar to that of Pt or Pt-like materials. In addition, multiple transition metal compounds are low-cost and environmentally-friendly. Among them, the binary transition metal oxide NiCo2O4 exhibits excellent electrochemical activity due to its high electronic conductivity. Particularly, it was demonstrated that NiCo2O4 material possesses a richer redox chemistry, a much better electronic conductivity, and higher electrochemical activity than nickel oxides or
cobalt oxides [24]. Further modification results in more extensive applications of the fabricated NiCo2O4 catalysts, such as effective negative electrode of sodium-ion or lithium-ion battery [25], methanol oxidation [26] and supercapacitor [27]. Exploring more applications of NiCo2O4 catalyst (e.g., as electrocatalyst for DSCs) is highly desired. Its application in DSCs, however, received less attention. To date, to the best of our knowledge, there are few works [28,29] to report NiCo2O4 and its composites as electrocatalysts for the reduction of triiodide to iodide in DSCs. Employing NiCo2O4 as CE for DSCs was firstly reported by Li and coworkers [28]. However, the power conversion efficiency of DSCs based on NiCo2O4 can only achieve of a number of 0.78%. The result may because the highly corrosive and interacting with CEs of organic liquid electrolyte. Ma and coworkers [29] reported that NiCo2O4/RGO was used as CE for DSCs, exhibiting a power conversion efficiency of 6.17%. However，the NiCo2O4/RGO was synthesized by a two-step hydrothermal approach. Conventional hydrothermal [29,30] and solvothermal [31,32] were used for the preparation of Ni and Co based bimetal oxide electrocatalysts with different morphologies. However, most of them involve in use of toxic chemicals such as NH4F, high temperatures and other non-eco-friendly solvents. In this study, a low cost, environmental-benign solution process has been developed to synthesize highly electrocatalytic NiCo2O4/C composites. Scanning electron microscopy reveals that NiCo2O4 nanoparticles are grown on the surface of C. The composite is coated on the conducting fluorine-doped tin oxide (FTO) conductive glass to construct NiCo2O4/C CE used in DSCs. The NiCo2O4/C CE is demonstrated higher catalytic activity and
stability for the reduction of triiodide to iodide, yielding a power conversion efficiency of 6.27%, which is 44% higher than that of the C CE (4.34%) based DSCs. This good performance could be attribute to synergistic effect between C and NiCo2O4 nanoparticals. 2. Experimental Section 2.1. Preparation of NiCo2O4/C composite 120 mg carbon black (C) powder was first mixed with 50 mL of ethylene glycol dissolved with 2.5 mmol of nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) and 5 mmol of cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O). The mixture was sonicated for 1 h to achieve a homogeneous dispersion. The solution was then heated up to 160 °C using an oil bath and the reaction was lasted for 1 h. Then the product was collected by centrifugation and washed with deionized water and ethanol for several times after the solution was cooled down to room temperature naturally. The obtained powder was annealed at 300 °C for 2 h in air with a heating rate of 1 °C min-1. NiO/C and Co3O4/C composites were obtained with a similar synthetic process of NiCo2O4/C, but only containing 2.5 mmol of nickel acetate tetrahydrate or 5 mmol of cobalt acetate tetrahydrate and 120 mg C. All prepared products were preserved for further characterization and use. 2.2. Fabrication of CEs 0.5 g NiCo2O4/C composite powder was grinded with 2 mL n-butanol and 100 μL binder solution (10 mg mL-1 polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone). This obtained paste was coated onto FTO glass by a doctor-blade
technique and then heated at 70 oC for 1 h under vacuum to prepare the NiCo2O4/C CE. Pure carbon black CE, NiO/C CE and Co3O4/C CE, which were employed as reference, were prepared via the same method as that for the NiCo2O4/C CE. The thickness of films was fixed by tapes. For comparison, the Pt-FTO electrode was fabricated by thermal decomposition of H2PtCl6 (5 mM in isopropanol) on FTO glass at 390 oC for 30 min. 2.3. Fabrication of DSCs and measurement The
TiO2
colloidal
solution was
prepared
by hydrolysis
of
titanium
tetrabutyltitanate according to our previous work [33]. TiO2 electrodes were prepared by depositing a TiO2 colloidal paste onto FTO substrates using the doctor-blade method and then sintered at 450 °C for 30 min. The thickness of the TiO2 film was controlled to be about 10 μm. The as-prepared TiO2 electrodes were immersed into the ethanol solution of N3 dye (0.5 mM) for 12 h at room temperature. The dye-sensitized TiO2 electrodes were then rinsed by ethanol and dried in air. A series of DSCs were fabricated with traditional sandwich-type configuration by using a dye-anchored TiO2 photoelectrode, counter electrode and an electrolyte containing 0.5 M LiI, 0.3 M 1-butyl-3-methylimidazolium iodide, 0.05 M I2 and 0.5 M 4-tert-butyl pyridine in 3-methoxypropionitrile. 2.4. Characterization The morphologies of the composite and pristine C were characterized using a Hitachi S4800 scanning electron microscopy (SEM) after sputtering the samples with platinum for 120 s. Energy dispersive X-ray measurements were conducted using the
EDAX system attached to the same microscope. X-ray diffraction (XRD) patterns were recorded on a Rikaku X-ray diffractometer (Cu Kα radiation). High-resolution transmission electron microscope (HRTEM) images were recorded with a Hitachi Corp., JEM-2011 electron microscope. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG Scientific Escalab 220i-XL spectrometer with standard Al Kα radiation. The photocurrent density–voltage (J-V) curves of the DSCs were measured employing a computer-programmed Keithley 2611 Source Meter under illumination (AM 1.5, 100 mW cm-2) by an oriel Newport solar simulator (model:91160-1000). The light intensity of 100 mW cm-2 was calibrated by a NIST-certified monocrystalline Si solar cell. The active area was 0.2 cm2 fixed by a light shading mask. In this work, the photovoltaic parameters were obtained from the reverse scan. Electrochemical impedance spectroscopy (EIS) and the stability test were carried out on DSCs under 100 mW cm-2 illumination at open-circuit voltage situation employing Solartron 1255B frequency response analyzer and Solartron SI 1287 electrochemical interface system. The EIS data was fitted using ZView software. Cyclic voltammograms (CVs) were performed on electrochemical workstation (CHI630C) at a scan rate of 50 mV s-1. The electrolyte was an acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4. In the three-electrode system, the NiCo2O4/C, NiO/C, Co3O4/C, pristine C and thermal-deposited Pt were as working electrode, a Pt wire as counter electrode, and a SCE as reference electrode. 3. Results and discussion 3.1. Morphologies of pristine C and NiCo2O4/C composite
The morphologies of the pristine C and synthesized NiCo2O4/C were investigated by SEM measurement. Fig.1a, b and Fig.1c, d show the SEM images of the pristine C and the NiCo2O4/C composite with different magnifications, respectively. Simply, sphere-like morphologies can be seen from the Fig.1c, 1d. Magnified SEM (Fig.1d) clearly shows a rough surface, suggesting the successful decoration of the carbon black with bimetal oxide nanoparticles. Fig.1e shows elemental mapping results for the distribution of C, Ni, Co and O elements based on the NiCo2O4/C composite material. The content of Ni and Co is 3.42wt% and 7.09wt% respectively, close to the stoichiometric molar ratio of 1:2 in the NiCo2O4. Also, it is evident that the distribution of the C, Ni, Co and O are quite uniform throughout the whole hybrid materials. These results further proved that NiCo2O4 nanoparticles were attached on the carbon black. This rough surface of NiCo2O4/C composite is believed to benefit reduction of triiodide. Fig.1f and 1g present cross-sectional SEM images of pristine C CE and NiCo2O4/C CE. Apparently, the thickness of films was about 4 μm. 3.2. X-ray diffraction and TEM characterization Fig.2a shows the XRD patterns of all investigated composite materials. As is shown in the Fig.2a, the diffraction peak at 25.8° for three investigated samples can be assigned to the C. For the nickel oxide/C, all new diffraction peaks can be indexed as nanocrystalline NiO (JCPDS no.47-1049), while all new diffraction peaks of cobalt oxide/C can be assign to Co3O4 (JCPDS no.42-1467). When the reaction precursors containing cobalt and nickel sources are mixed together with C in this work, all new diffraction peaks at 18.9°, 31.1°, 36.7°, 44.6°, 59.1°, 65.0° can be indexed to the (111),
(220), (311), (400), (511) and (400) reflections of the spinel NiCo2O4 crystalline structure (JCPDS no.20-0781), respectively. Fig.2b and 2c show low and high magnification TEM images of NiCo2O4/C. Obviously, NiCo2O4 nanoparticles disperse uniformly on the surface of C. From the high-magnified TEM image (Fig.2c), the particle sizes of the NiCo2O4 nanoparticles are mostly in the range of 6-10 nm. In addition, it can be observed that these NiCo2O4 nanoparticles are grown on the C surface with certain space in between. This feature could increase the exposed sites and thus contribute to the enhanced catalytic performance [34]. To further characterize the structures, high resolution TEM analyses are performed and the results are shown in Fig.2d. The image of several NiCo2O4 nanoparticles shows the high crystallinity with clear lattice fringes. The inset in Fig.2d shows the enlarged view of the well-defined lattice fringes, which can be readily indexed to (311) crystal planes of the NiCo2O4. 3.3. XPS analysis In order to understand the chemical composition and the oxidation states of the metals in NiCo2O4/C composite, the NiCo2O4/C composites were subjected to XPS analyses and the results are shown in Fig.3. The survey spectrum indicates the presence of elements Ni, Co, O, and C as well as absence of any impurities. The results of the present investigation are consistent with those of the literature [25-27]. By using Gaussian fitting, the Co2p spectrum (Fig.3b) is best fitted with two spin-orbit doublets that are corresponding to Co2+ and Co3+ and two shake-up satellites (identified as “Sat.”). Similarly, the Ni2p spectrum (Fig.3c) is best fitted considering
two spin-orbit doublets characteristic of Ni2+ and Ni3+and two shake-up satellites. Thus the results indicate that the NiCo2O4/C composite contains Ni2+/Ni3+ and Co2+/Co3+. In its structure, the solid state redox couples Co3+/Co2+and Ni3+/Ni2+ are present, which provide a notable catalytic activity [35]. Further the O1s spectrum (Fig.3d) reveals the presence of metal oxygen bonds by exhibiting a peak at 529.8 eV. The peak at 532.6 and 534 eV are ascribed to high number of defect sites with a low oxygen coordination and hydroxyl species of surface-adsorbed water molecules, respectively [36]. 3.4. Photovoltaic performance As electrocatalyst, the NiCo2O4/C composite is used as CE material for DSCs. For comparison, thermal-decomposed Pt CE, NiO/C and Co3O4/C were also prepared. The photocurrent density–voltage (J-V) characteristic curves are shown in Fig. 4. The detailed photovoltaic parameters, such as the open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill factor (FF), and the power conversion efficiency (η), are summarized in Table 1. The distributions of photovoltaic parameters are also summarized in Table 1. As shown in Fig. 4, the DSCs made with NiCo2O4/C exhibit a Jsc of 16.08 mA cm-2, a Voc of 650 mV, and a FF of 0.60, resulting in η of 6.27% (Table 1). Obviously, the conversion efficiency of DSCs with NiCo2O4/C CE is higher than that of the cell with NiO/C (η=5.07%), Co3O4/C (η=4.82%) and pristine C (η=4.34%). This performance enhancement may attribute to the improvement of electrocatalytic activity and conductivity of the NiCo2O4/C composite. However, lower Voc and FF were observed for the DSCs based on the
NiCo2O4/C in comparison with that of Pt CE. It should be noted that the used NiCo2O4/C composite here was fabricated with nickel and cobalt precursor molar ratio of 1:2. In this work, we also prepared NixCoyO4/C composite with different molar ratios of nickel and cobalt precursors (1:1 and 1:3) as CEs for DSCs. The results (Fig.4 and Table 1) indicate that the DSCs made of NiCo2O4/C with nickel and cobalt precursor molar ratio of 1:2 give the best photovoltaic performance among all investigated NixCoyO4/C CEs, implying that excess nickel or cobalt precursor in synthetic process could be unfavourable for the formation of NiCo2O4/C with high solar cell performance. The high performance device based NiCo2O4/C composite could be ascribed to the superior electrocatalytic activity of nanocrystalline NiCo2O4 in composite, which has been confirmed by a series of electrochemical characterizations. 3.5. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is an effective and widely used tool to reveal the charge transfer process and further evaluate the catalytic activity of a catalyst. The EIS of DSCs were measured under the illumination of simulated AM 1.5 G solar light (100 mW cm-2), and the corresponding Nyquist plots are presented in Fig.5. Two semicircles are observed on each Nyquist plot. One in high-frequency region corresponds to the charge-transfer process at the electrolyte/CE interface (Rct), which characterizes the electrocatalytic ability of CE for the reduction of triiodide. The other in low-frequency region is assigned to the charge-transport in the TiO2 film and charge-transfer resistance at TiO2 electrode/electrolyte interface (RR) [37]. The Rct
is determined by the diameter of the semicircle presented in the high-frequency region and smaller value means higher electrocatalytic activity of the investigated CE [38]. The high frequency intercept on the real axis represents the Ohmic series resistance (Rs), which usually decides the adhesion property of materials on FTO conductive substrate [8]. The equivalent circuit of this type cells is described in the inset of Fig.5a. The fitted values of Rs, Rct and RR are summarized in table 2. Among all the investigated CEs, thermal-deposited Pt CE based DSCs exhibits the smallest Rs value, and NiCo2O4/C, Co3O4/C, NiO/C and C based devices show close Rs, in agreement with the fact that these CEs were fabricated under the same condition. The value of Rct for devices based on NiCo2O4/C, NiO/C, Co3O4/C, and C CE is 2.2 Ω, 2.6 Ω, 2.8 Ω and 5.7 Ω, respectively. Compared to Co3O4/C and NiO/C CEs, the obtained smaller Rct value (2.2 Ω ) from NiCo2O4/C CE is very close to the Rct value (1.8 Ω) obtained by Pt CE (Table 2), indicating good electrocatalytic activity of NiCo2O4/C CE. This result also demonstrates that NiCo2O4/C CE can catalyze the reduction of I3as effectively as the Pt CE does. This is important for improving the performance of NiCo2O4/C CE based DSCs. The outstanding electrocatalytic activity of NiCo2O4/C CE can be attributed to the hybrid structure, which may provide effective electrocatalytic active sites. Different CEs with different catalytic activity would result in the variation of reduction rate for triiodide. So while the current flows through the different devices, RR value would differ due to the different concentration of I2 on the surface of TiO2. The value of RR for devices with NiCo2O4/C CE and C CE is 29.3 Ω and 55.0 Ω, respectively. After introduction of NiCo2O4 into C, RR
decreases. The total internal resistance for a DSC device can be expressed as Rtotal=Rs+Rct+RR
(1)
The total internal resistance is inversely related to the photovoltaic performance [39]. As shown in Table 2, the DSCs device with Pt CE shows the lowest value of Rtotal among the five kinds of DSCs, which lead to the highest power conversion efficiency (7.38%). The devices with the pristine C CE have the largest Rtotal (88.7 Ω), consistent with its lowest efficiency (4.34%). The incorporation of NiCo2O4 into the C leads to a significant decrease of Rtotal of the device to 58.3Ω. The decrease of Rtotal helps to improve FF and the power conversion efficiency of DSCs [39]. As a result, the DSC with NiCo2O4/C composite CE presents a power conversion efficiency of 6.27%, which is comparable to that of the device with thermal-deposited Pt CE (7.38%). Introducing the NiCo2O4 to the carbon black could enhance the catalytic performance for the triiodide reduction, and thus the photovoltaic performance of device based on NiCo2O4/C CE. 3.6. Cyclic voltammetry measurement Catalytic activities for the composites toward triiodide reduction process were further evaluated by cyclic voltammetry (CV) using a three-electrode system [40]. The corresponding CV curves are shown in Fig.6. Obviously, all measured CEs exhibit two pairs of redox peaks. The relative positive pair is due to the redox reaction of I3-/I2, and the relative negative one belongs to the reaction of I-/I3-. The triiodide ions have to be effectively reduced on the surface of CE during the operation of DSCs. Hence, the negative redox pair is the research focus of the CV analysis [38]. As
shown in Fig.6, the separation of cathodic and anodic peaks potential (ΔEp) for Pt CE is 0.68 V. The ΔEp for pristine C CE is 1.10 V, whereas, the ΔEp for NiCo2O4/C CE is 0.90 V. ΔEp commences to decrease after incorporating NiCo2O4 into carbon black, indicating that triiodide ions are reduced more readily on the composite electrode than on the pristine C electrode [8,13]. In addition, the current reduction peak for the different electrodes increases in the order of C< Co3O4/C < NiO/C < NiCo2O4/C, which indicates that the reduction reaction is faster on the composite electrodes than on the pristine C. It could be ascribed to the difference components and surface morphologies between the composites and the pristine C. In comparison with the pristine C, the NiCo2O4/C composite can provide more reactive sites for triiodide reduction because of its electrocatalytic activity [34]. Furthermore, synergistic powers of C and NiCo2O4, such as increased electrocatalytic activity, electron transport and surface area, enhance the DSCs performance with NiCo2O4/C composite CE. 3.7 Stability test Additionally, the photocurrent density-time curves were measured under persistent illumination over 1200 s to study the stability of the DSCs. Aiming at a better comparison, all the photocurrent density curves of DSCs were normalized. As shown in Fig.7, the current densities of DSCs base on Pt and NiCo2O4/C CEs decrease firstly and then tend to stable. The decrease of the current density could be attributed to an evaporation of the electrolyte, resulting from that all the DSCs were measured under an open system without any sealing. It is observed that the current densities can remain about 88% after illumination for 1200s s for DSCs devices with NiCo2O4/C
and Pt CE, indicating that these DSCs have good stability. The C CE based DSCs reveals poor stability with the current density retention of about 70%. These results demonstrate that the NiCo2O4/C composite CE is more stable for catalyzing triiodide than the pure C CE. 4. Conclusion In summary, NiCo2O4/carbon black (NiCo2O4/C) composite is successfully synthesized by economical and environmental friendly method and is used as counter electrode material for DSCs. The obtained NiCo2O4/C composite demonstrates a superior catalytic activity toward triiodide reduction. The DSCs with NiCo2O4/C composite based CE presents a power conversion efficiency of 6.27%, which is remarkably superior to that of NiO/C CE (5.07%), Co3O4/C CE (4.82%) or pristine C CE (4.34%). The FF of the DSCs with NiCo2O4/C CE is 0.60, which is higher than that of NiO/C CE (0.51), Co3O4/C (0.49) or pristine C CE (0.48). Furthermore, the NiCo2O4/C composite is prepared by a facile solution method, so it is a promising material to be used as a plastic counter electrode for flexible dye-sensitized solar cells. Acknowledgments This work was supported by National Natural Science Foundation of China (51303186, 51673204), National Materials Genome Project (2016YFB0700600).
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Figure Captions: Fig.1. Low and high magnification SEM images of pristine C (a, b) and NiCo2O4/C composite (c, d). Mapping image of NiCo2O4/C composite (e). Cross-sectional SEM images of pristine C CE (f) film and NiCo2O4/C composite CE film (g). Fig.2. XRD patterns of NiCo2O4/C, NiO/C and Co3O4/C (a). Low and high magnification TEM images of NiCo2O4/C composite (b, c). High- resolution TEM image of NiCo2O4/C composite (d). Fig.3. XPS spectrum of NiCo2O4/C: survey spectrum (a), Co2p (b), Ni2p (c), O1s (d). Fig.4. Current density-voltage (J-V) curves of the cells under simulated 1.5 AM illuminations with an active area of 0.20 cm-2. Fig.5 Nyquist plots the DSCs fabricated with Pt, NiCo2O4/C, Co3O4/C, NiO/C composite and pristine C electrodes. The inset (a) is equivalent circuit of DSCs and the inset (b) is the high frequency part of Nyquist plots. Fig.6. Cyclic voltammograms for C, NiO/C, Co3O4/C, NiCo2O4/C and Pt electrode at a scan rate of 50 mV s-1 in acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 with platinum wire and a SCE electrode used as counter electrode and reference electrode, respectively. Fig.7 Photocurrent density as a function of time under persistent illumination (AM 1.5 simulated sun light) for 1200s.
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Tables Table1 Photovoltaic parameters of DSCs assembled with various CEs Counter electrodes
Jsc (mA cm-2)
Voc (mV)
FF
η (%)
NiCo2O4/C
16.08 14.61 16.85
650 650 630
0.60 0.63 0.58
6.27 5.98 6.16
NiO/C
16.30 17.10 16.45
610 610 590
0.51 0.48 0.55
5.07 5.00 5.33
Co3O4/C
16.12 16.47 15.98
610 630 655
0.49 0.45 0.44
4.82 4.67 4.60
Ni:Co=1:1
15.97 16.31 15.98
605 575 605
0.54 0.53 0.56
5.22 4.97 5.41
Ni:Co=1:3
15.86 16.27 16.50
605 595 595
0.55 0.54 0.52
5.28 5.22 5.10
Pt C
17.48 15.06
665 600
0.63 0.48
7.38 4.34
Note: parallel three samples for DSCs measurements under each experimental condition. All
underlined data were consistent with the Fig.4.
Table 2 Nyquist fitting parameters of Pt, NiCo2O4/C, Co3O4/C, NiO/C composite and C counter electrodes Counter electrodes
Rs (Ω)
Rct (Ω)
RR (Ω)
Rtotal (Ω)
Pt NiCo2O4/C NiO/C Co3O4/C C
22.4 26.8 28 29 28
1.8 2.2 2.6 2.8 5.7
25.3 29.3 38.4 50.0 55.0
49.5 58.3 69.0 81.8 88.7
S0169-4332(17)31369-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.057 APSUSC 35992
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-3-2017 27-4-2017 7-5-2017
Please cite this article as: Yanan Wang, Nianqing Fu, Pin Ma, Yanyan Fang, Lumei Peng, Xiaowen Zhou, Yuan Lin, Facile Synthesis of NiCo2O4/Carbon Black Composite as Counter Electrode for Dye-Sensitized Solar Cells, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile Synthesis of NiCo2O4/Carbon Black Composite as Counter Electrode for Dye-Sensitized Solar Cells
Yanan Wanga,b, Nianqing Fuc, Pin Maa,b, Yanyan Fanga, Lumei Penga, Xiaowen Zhoua, Yuan Lina,b,* a
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, CAS
Research/Education Center for Excellence in Molecular Science, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China b
c
University of Chinese Academy of Sciences, Beijing 100049, China
School of Material Science and Engineering, South China University of Technology, Guangzhou
510640, China
Highlight The NiCo2O4/C composite is successfully prepared by a facile solution route. The NiCo2O4/C is applied as counter electrode for the reduction of triiodide in the dye-sensitized solar cells. The NiCo2O4/C composite possesses a better electronic conductivity and a higher electrochemical activity than NiO/C and Co3O4/C. The NiCo2O4/C CE shows the photovoltaic performance of 6.27% and the fill factor of 0.60. Due to the synergistic effect between C and NiCo2O4, the NiCo2O4/C CE possesses both high efficiency and stability. Abstract NiCo2O4/carbon black (NiCo2O4/C) composite is successfully synthesized by a facile solution route, and used as counter electrode (CE) for dye-sensitized solar cells (DSCs). The DSCs based on NiCo2O4/C composite CE achieves a power conversion efficiency of 6.27%, which is much higher than that of NiO/C (5.07%), Co3O4/C *Corresponding
author. Tel.: +86 10 8261 5031; fax: +86 10 8261 7315 E-mail address: [email protected] (Y. Lin)
(4.82%) or pristine C (4.34%). Also, the fill factors of DSCs devices with the NiCo2O4/C CE are better than that of other CEs. Compared to pristine C, the NiCo2O4/C composite has a marked improvement on electrocatalytic performance for the reduction of triiodide. Due to the synergistic effect between C and NiCo2O4, the NiCo2O4/C CE possesses both high efficiency and stability. Key words
Counter electrode NiCo2O4/carbon black composite Dye-sensitized
solar cells 1. Introduction Since the first prototype from O’Regan and Grätzel, dye sensitized solar cells (DSCs) have attracted growing interests and are generally regarded as a promising solution to global energy and environmental problems [1,2]. Typically, a DSC consists of a dye-sensitized porous semi-conductor photoanode, an electrolyte containing an iodide/triiodide (I-/I3-) redox couple, and a counter electrode [3]. The counter electrode (CE) plays a crucial role in catalysing the reduction reaction of I3- to I- to regenerate the sensitizer [4,5]. So far, the Pt electrode is still the preferred counter electrode in DSCs owing to its outstanding electrocatalytic activity and superior conductivity. However, Pt CE is expensive and suffers from a long-term stability problem under highly corrosive I-/I3- redox couple. As alternatives to Pt CE, conductive
polymers
[6-8],
carbon-based
materials
[9-12],
and
metallic/semiconductive compounds [13-15] have been reported. Among these materials, carbon is a promising material to supplant Pt owing to their advantages of low-cost, environmental friendliness and large surface. However, the
carbon materials generally have low intrinsic electrocatalytic activity [16]. Meanwhile, transition metal compounds possess the poor electron transport efficiency, which finally restricted their catalytic activity for the reduction of triiodide [17]. Thus, hybrid materials based on carbon materials and metallic compounds can be a reasonable alternative to satisfy inevitable requirements of a low cost and high electric conductivity. Recently, composites of transition metal compound and carbon material were constructed and used as CEs for DSCs, such as Fe3O4/carbon black [17], Co3O4/mesoporous carbons [18], RuO2/grapheme [19], RGO/MWCNTs/NiO [20]. The power conversion efficiency of DSCs based on these hybrid composites CEs is much higher than that of devices with pure transition metal compounds CEs or pure carbon materials CEs and is comparable to that of Pt CE based DSCs. Little attention has been paid to the use of multiple transition metal compounds, especially the multiple oxide compounds. Some multiple transition metal compounds have shown good performance as CE in DSCs, such as Cu2ZnSnS4, Cu2ZnSnSe4, CoFeO4 and NiCo2S4 [21-23]. These results indicate that multiple transition metal compounds may provide excellent performance in DSCs. The electronic structures of transition metal compounds are similar to that of Pt or Pt-like materials. In addition, multiple transition metal compounds are low-cost and environmentally-friendly. Among them, the binary transition metal oxide NiCo2O4 exhibits excellent electrochemical activity due to its high electronic conductivity. Particularly, it was demonstrated that NiCo2O4 material possesses a richer redox chemistry, a much better electronic conductivity, and higher electrochemical activity than nickel oxides or
cobalt oxides [24]. Further modification results in more extensive applications of the fabricated NiCo2O4 catalysts, such as effective negative electrode of sodium-ion or lithium-ion battery [25], methanol oxidation [26] and supercapacitor [27]. Exploring more applications of NiCo2O4 catalyst (e.g., as electrocatalyst for DSCs) is highly desired. Its application in DSCs, however, received less attention. To date, to the best of our knowledge, there are few works [28,29] to report NiCo2O4 and its composites as electrocatalysts for the reduction of triiodide to iodide in DSCs. Employing NiCo2O4 as CE for DSCs was firstly reported by Li and coworkers [28]. However, the power conversion efficiency of DSCs based on NiCo2O4 can only achieve of a number of 0.78%. The result may because the highly corrosive and interacting with CEs of organic liquid electrolyte. Ma and coworkers [29] reported that NiCo2O4/RGO was used as CE for DSCs, exhibiting a power conversion efficiency of 6.17%. However，the NiCo2O4/RGO was synthesized by a two-step hydrothermal approach. Conventional hydrothermal [29,30] and solvothermal [31,32] were used for the preparation of Ni and Co based bimetal oxide electrocatalysts with different morphologies. However, most of them involve in use of toxic chemicals such as NH4F, high temperatures and other non-eco-friendly solvents. In this study, a low cost, environmental-benign solution process has been developed to synthesize highly electrocatalytic NiCo2O4/C composites. Scanning electron microscopy reveals that NiCo2O4 nanoparticles are grown on the surface of C. The composite is coated on the conducting fluorine-doped tin oxide (FTO) conductive glass to construct NiCo2O4/C CE used in DSCs. The NiCo2O4/C CE is demonstrated higher catalytic activity and
stability for the reduction of triiodide to iodide, yielding a power conversion efficiency of 6.27%, which is 44% higher than that of the C CE (4.34%) based DSCs. This good performance could be attribute to synergistic effect between C and NiCo2O4 nanoparticals. 2. Experimental Section 2.1. Preparation of NiCo2O4/C composite 120 mg carbon black (C) powder was first mixed with 50 mL of ethylene glycol dissolved with 2.5 mmol of nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) and 5 mmol of cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O). The mixture was sonicated for 1 h to achieve a homogeneous dispersion. The solution was then heated up to 160 °C using an oil bath and the reaction was lasted for 1 h. Then the product was collected by centrifugation and washed with deionized water and ethanol for several times after the solution was cooled down to room temperature naturally. The obtained powder was annealed at 300 °C for 2 h in air with a heating rate of 1 °C min-1. NiO/C and Co3O4/C composites were obtained with a similar synthetic process of NiCo2O4/C, but only containing 2.5 mmol of nickel acetate tetrahydrate or 5 mmol of cobalt acetate tetrahydrate and 120 mg C. All prepared products were preserved for further characterization and use. 2.2. Fabrication of CEs 0.5 g NiCo2O4/C composite powder was grinded with 2 mL n-butanol and 100 μL binder solution (10 mg mL-1 polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone). This obtained paste was coated onto FTO glass by a doctor-blade
technique and then heated at 70 oC for 1 h under vacuum to prepare the NiCo2O4/C CE. Pure carbon black CE, NiO/C CE and Co3O4/C CE, which were employed as reference, were prepared via the same method as that for the NiCo2O4/C CE. The thickness of films was fixed by tapes. For comparison, the Pt-FTO electrode was fabricated by thermal decomposition of H2PtCl6 (5 mM in isopropanol) on FTO glass at 390 oC for 30 min. 2.3. Fabrication of DSCs and measurement The
TiO2
colloidal
solution was
prepared
by hydrolysis
of
titanium
tetrabutyltitanate according to our previous work [33]. TiO2 electrodes were prepared by depositing a TiO2 colloidal paste onto FTO substrates using the doctor-blade method and then sintered at 450 °C for 30 min. The thickness of the TiO2 film was controlled to be about 10 μm. The as-prepared TiO2 electrodes were immersed into the ethanol solution of N3 dye (0.5 mM) for 12 h at room temperature. The dye-sensitized TiO2 electrodes were then rinsed by ethanol and dried in air. A series of DSCs were fabricated with traditional sandwich-type configuration by using a dye-anchored TiO2 photoelectrode, counter electrode and an electrolyte containing 0.5 M LiI, 0.3 M 1-butyl-3-methylimidazolium iodide, 0.05 M I2 and 0.5 M 4-tert-butyl pyridine in 3-methoxypropionitrile. 2.4. Characterization The morphologies of the composite and pristine C were characterized using a Hitachi S4800 scanning electron microscopy (SEM) after sputtering the samples with platinum for 120 s. Energy dispersive X-ray measurements were conducted using the
EDAX system attached to the same microscope. X-ray diffraction (XRD) patterns were recorded on a Rikaku X-ray diffractometer (Cu Kα radiation). High-resolution transmission electron microscope (HRTEM) images were recorded with a Hitachi Corp., JEM-2011 electron microscope. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG Scientific Escalab 220i-XL spectrometer with standard Al Kα radiation. The photocurrent density–voltage (J-V) curves of the DSCs were measured employing a computer-programmed Keithley 2611 Source Meter under illumination (AM 1.5, 100 mW cm-2) by an oriel Newport solar simulator (model:91160-1000). The light intensity of 100 mW cm-2 was calibrated by a NIST-certified monocrystalline Si solar cell. The active area was 0.2 cm2 fixed by a light shading mask. In this work, the photovoltaic parameters were obtained from the reverse scan. Electrochemical impedance spectroscopy (EIS) and the stability test were carried out on DSCs under 100 mW cm-2 illumination at open-circuit voltage situation employing Solartron 1255B frequency response analyzer and Solartron SI 1287 electrochemical interface system. The EIS data was fitted using ZView software. Cyclic voltammograms (CVs) were performed on electrochemical workstation (CHI630C) at a scan rate of 50 mV s-1. The electrolyte was an acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4. In the three-electrode system, the NiCo2O4/C, NiO/C, Co3O4/C, pristine C and thermal-deposited Pt were as working electrode, a Pt wire as counter electrode, and a SCE as reference electrode. 3. Results and discussion 3.1. Morphologies of pristine C and NiCo2O4/C composite
The morphologies of the pristine C and synthesized NiCo2O4/C were investigated by SEM measurement. Fig.1a, b and Fig.1c, d show the SEM images of the pristine C and the NiCo2O4/C composite with different magnifications, respectively. Simply, sphere-like morphologies can be seen from the Fig.1c, 1d. Magnified SEM (Fig.1d) clearly shows a rough surface, suggesting the successful decoration of the carbon black with bimetal oxide nanoparticles. Fig.1e shows elemental mapping results for the distribution of C, Ni, Co and O elements based on the NiCo2O4/C composite material. The content of Ni and Co is 3.42wt% and 7.09wt% respectively, close to the stoichiometric molar ratio of 1:2 in the NiCo2O4. Also, it is evident that the distribution of the C, Ni, Co and O are quite uniform throughout the whole hybrid materials. These results further proved that NiCo2O4 nanoparticles were attached on the carbon black. This rough surface of NiCo2O4/C composite is believed to benefit reduction of triiodide. Fig.1f and 1g present cross-sectional SEM images of pristine C CE and NiCo2O4/C CE. Apparently, the thickness of films was about 4 μm. 3.2. X-ray diffraction and TEM characterization Fig.2a shows the XRD patterns of all investigated composite materials. As is shown in the Fig.2a, the diffraction peak at 25.8° for three investigated samples can be assigned to the C. For the nickel oxide/C, all new diffraction peaks can be indexed as nanocrystalline NiO (JCPDS no.47-1049), while all new diffraction peaks of cobalt oxide/C can be assign to Co3O4 (JCPDS no.42-1467). When the reaction precursors containing cobalt and nickel sources are mixed together with C in this work, all new diffraction peaks at 18.9°, 31.1°, 36.7°, 44.6°, 59.1°, 65.0° can be indexed to the (111),
(220), (311), (400), (511) and (400) reflections of the spinel NiCo2O4 crystalline structure (JCPDS no.20-0781), respectively. Fig.2b and 2c show low and high magnification TEM images of NiCo2O4/C. Obviously, NiCo2O4 nanoparticles disperse uniformly on the surface of C. From the high-magnified TEM image (Fig.2c), the particle sizes of the NiCo2O4 nanoparticles are mostly in the range of 6-10 nm. In addition, it can be observed that these NiCo2O4 nanoparticles are grown on the C surface with certain space in between. This feature could increase the exposed sites and thus contribute to the enhanced catalytic performance [34]. To further characterize the structures, high resolution TEM analyses are performed and the results are shown in Fig.2d. The image of several NiCo2O4 nanoparticles shows the high crystallinity with clear lattice fringes. The inset in Fig.2d shows the enlarged view of the well-defined lattice fringes, which can be readily indexed to (311) crystal planes of the NiCo2O4. 3.3. XPS analysis In order to understand the chemical composition and the oxidation states of the metals in NiCo2O4/C composite, the NiCo2O4/C composites were subjected to XPS analyses and the results are shown in Fig.3. The survey spectrum indicates the presence of elements Ni, Co, O, and C as well as absence of any impurities. The results of the present investigation are consistent with those of the literature [25-27]. By using Gaussian fitting, the Co2p spectrum (Fig.3b) is best fitted with two spin-orbit doublets that are corresponding to Co2+ and Co3+ and two shake-up satellites (identified as “Sat.”). Similarly, the Ni2p spectrum (Fig.3c) is best fitted considering
two spin-orbit doublets characteristic of Ni2+ and Ni3+and two shake-up satellites. Thus the results indicate that the NiCo2O4/C composite contains Ni2+/Ni3+ and Co2+/Co3+. In its structure, the solid state redox couples Co3+/Co2+and Ni3+/Ni2+ are present, which provide a notable catalytic activity [35]. Further the O1s spectrum (Fig.3d) reveals the presence of metal oxygen bonds by exhibiting a peak at 529.8 eV. The peak at 532.6 and 534 eV are ascribed to high number of defect sites with a low oxygen coordination and hydroxyl species of surface-adsorbed water molecules, respectively [36]. 3.4. Photovoltaic performance As electrocatalyst, the NiCo2O4/C composite is used as CE material for DSCs. For comparison, thermal-decomposed Pt CE, NiO/C and Co3O4/C were also prepared. The photocurrent density–voltage (J-V) characteristic curves are shown in Fig. 4. The detailed photovoltaic parameters, such as the open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill factor (FF), and the power conversion efficiency (η), are summarized in Table 1. The distributions of photovoltaic parameters are also summarized in Table 1. As shown in Fig. 4, the DSCs made with NiCo2O4/C exhibit a Jsc of 16.08 mA cm-2, a Voc of 650 mV, and a FF of 0.60, resulting in η of 6.27% (Table 1). Obviously, the conversion efficiency of DSCs with NiCo2O4/C CE is higher than that of the cell with NiO/C (η=5.07%), Co3O4/C (η=4.82%) and pristine C (η=4.34%). This performance enhancement may attribute to the improvement of electrocatalytic activity and conductivity of the NiCo2O4/C composite. However, lower Voc and FF were observed for the DSCs based on the
NiCo2O4/C in comparison with that of Pt CE. It should be noted that the used NiCo2O4/C composite here was fabricated with nickel and cobalt precursor molar ratio of 1:2. In this work, we also prepared NixCoyO4/C composite with different molar ratios of nickel and cobalt precursors (1:1 and 1:3) as CEs for DSCs. The results (Fig.4 and Table 1) indicate that the DSCs made of NiCo2O4/C with nickel and cobalt precursor molar ratio of 1:2 give the best photovoltaic performance among all investigated NixCoyO4/C CEs, implying that excess nickel or cobalt precursor in synthetic process could be unfavourable for the formation of NiCo2O4/C with high solar cell performance. The high performance device based NiCo2O4/C composite could be ascribed to the superior electrocatalytic activity of nanocrystalline NiCo2O4 in composite, which has been confirmed by a series of electrochemical characterizations. 3.5. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is an effective and widely used tool to reveal the charge transfer process and further evaluate the catalytic activity of a catalyst. The EIS of DSCs were measured under the illumination of simulated AM 1.5 G solar light (100 mW cm-2), and the corresponding Nyquist plots are presented in Fig.5. Two semicircles are observed on each Nyquist plot. One in high-frequency region corresponds to the charge-transfer process at the electrolyte/CE interface (Rct), which characterizes the electrocatalytic ability of CE for the reduction of triiodide. The other in low-frequency region is assigned to the charge-transport in the TiO2 film and charge-transfer resistance at TiO2 electrode/electrolyte interface (RR) [37]. The Rct
is determined by the diameter of the semicircle presented in the high-frequency region and smaller value means higher electrocatalytic activity of the investigated CE [38]. The high frequency intercept on the real axis represents the Ohmic series resistance (Rs), which usually decides the adhesion property of materials on FTO conductive substrate [8]. The equivalent circuit of this type cells is described in the inset of Fig.5a. The fitted values of Rs, Rct and RR are summarized in table 2. Among all the investigated CEs, thermal-deposited Pt CE based DSCs exhibits the smallest Rs value, and NiCo2O4/C, Co3O4/C, NiO/C and C based devices show close Rs, in agreement with the fact that these CEs were fabricated under the same condition. The value of Rct for devices based on NiCo2O4/C, NiO/C, Co3O4/C, and C CE is 2.2 Ω, 2.6 Ω, 2.8 Ω and 5.7 Ω, respectively. Compared to Co3O4/C and NiO/C CEs, the obtained smaller Rct value (2.2 Ω ) from NiCo2O4/C CE is very close to the Rct value (1.8 Ω) obtained by Pt CE (Table 2), indicating good electrocatalytic activity of NiCo2O4/C CE. This result also demonstrates that NiCo2O4/C CE can catalyze the reduction of I3as effectively as the Pt CE does. This is important for improving the performance of NiCo2O4/C CE based DSCs. The outstanding electrocatalytic activity of NiCo2O4/C CE can be attributed to the hybrid structure, which may provide effective electrocatalytic active sites. Different CEs with different catalytic activity would result in the variation of reduction rate for triiodide. So while the current flows through the different devices, RR value would differ due to the different concentration of I2 on the surface of TiO2. The value of RR for devices with NiCo2O4/C CE and C CE is 29.3 Ω and 55.0 Ω, respectively. After introduction of NiCo2O4 into C, RR
decreases. The total internal resistance for a DSC device can be expressed as Rtotal=Rs+Rct+RR
(1)
The total internal resistance is inversely related to the photovoltaic performance [39]. As shown in Table 2, the DSCs device with Pt CE shows the lowest value of Rtotal among the five kinds of DSCs, which lead to the highest power conversion efficiency (7.38%). The devices with the pristine C CE have the largest Rtotal (88.7 Ω), consistent with its lowest efficiency (4.34%). The incorporation of NiCo2O4 into the C leads to a significant decrease of Rtotal of the device to 58.3Ω. The decrease of Rtotal helps to improve FF and the power conversion efficiency of DSCs [39]. As a result, the DSC with NiCo2O4/C composite CE presents a power conversion efficiency of 6.27%, which is comparable to that of the device with thermal-deposited Pt CE (7.38%). Introducing the NiCo2O4 to the carbon black could enhance the catalytic performance for the triiodide reduction, and thus the photovoltaic performance of device based on NiCo2O4/C CE. 3.6. Cyclic voltammetry measurement Catalytic activities for the composites toward triiodide reduction process were further evaluated by cyclic voltammetry (CV) using a three-electrode system [40]. The corresponding CV curves are shown in Fig.6. Obviously, all measured CEs exhibit two pairs of redox peaks. The relative positive pair is due to the redox reaction of I3-/I2, and the relative negative one belongs to the reaction of I-/I3-. The triiodide ions have to be effectively reduced on the surface of CE during the operation of DSCs. Hence, the negative redox pair is the research focus of the CV analysis [38]. As
shown in Fig.6, the separation of cathodic and anodic peaks potential (ΔEp) for Pt CE is 0.68 V. The ΔEp for pristine C CE is 1.10 V, whereas, the ΔEp for NiCo2O4/C CE is 0.90 V. ΔEp commences to decrease after incorporating NiCo2O4 into carbon black, indicating that triiodide ions are reduced more readily on the composite electrode than on the pristine C electrode [8,13]. In addition, the current reduction peak for the different electrodes increases in the order of C< Co3O4/C < NiO/C < NiCo2O4/C, which indicates that the reduction reaction is faster on the composite electrodes than on the pristine C. It could be ascribed to the difference components and surface morphologies between the composites and the pristine C. In comparison with the pristine C, the NiCo2O4/C composite can provide more reactive sites for triiodide reduction because of its electrocatalytic activity [34]. Furthermore, synergistic powers of C and NiCo2O4, such as increased electrocatalytic activity, electron transport and surface area, enhance the DSCs performance with NiCo2O4/C composite CE. 3.7 Stability test Additionally, the photocurrent density-time curves were measured under persistent illumination over 1200 s to study the stability of the DSCs. Aiming at a better comparison, all the photocurrent density curves of DSCs were normalized. As shown in Fig.7, the current densities of DSCs base on Pt and NiCo2O4/C CEs decrease firstly and then tend to stable. The decrease of the current density could be attributed to an evaporation of the electrolyte, resulting from that all the DSCs were measured under an open system without any sealing. It is observed that the current densities can remain about 88% after illumination for 1200s s for DSCs devices with NiCo2O4/C
and Pt CE, indicating that these DSCs have good stability. The C CE based DSCs reveals poor stability with the current density retention of about 70%. These results demonstrate that the NiCo2O4/C composite CE is more stable for catalyzing triiodide than the pure C CE. 4. Conclusion In summary, NiCo2O4/carbon black (NiCo2O4/C) composite is successfully synthesized by economical and environmental friendly method and is used as counter electrode material for DSCs. The obtained NiCo2O4/C composite demonstrates a superior catalytic activity toward triiodide reduction. The DSCs with NiCo2O4/C composite based CE presents a power conversion efficiency of 6.27%, which is remarkably superior to that of NiO/C CE (5.07%), Co3O4/C CE (4.82%) or pristine C CE (4.34%). The FF of the DSCs with NiCo2O4/C CE is 0.60, which is higher than that of NiO/C CE (0.51), Co3O4/C (0.49) or pristine C CE (0.48). Furthermore, the NiCo2O4/C composite is prepared by a facile solution method, so it is a promising material to be used as a plastic counter electrode for flexible dye-sensitized solar cells. Acknowledgments This work was supported by National Natural Science Foundation of China (51303186, 51673204), National Materials Genome Project (2016YFB0700600).
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Figure Captions: Fig.1. Low and high magnification SEM images of pristine C (a, b) and NiCo2O4/C composite (c, d). Mapping image of NiCo2O4/C composite (e). Cross-sectional SEM images of pristine C CE (f) film and NiCo2O4/C composite CE film (g). Fig.2. XRD patterns of NiCo2O4/C, NiO/C and Co3O4/C (a). Low and high magnification TEM images of NiCo2O4/C composite (b, c). High- resolution TEM image of NiCo2O4/C composite (d). Fig.3. XPS spectrum of NiCo2O4/C: survey spectrum (a), Co2p (b), Ni2p (c), O1s (d). Fig.4. Current density-voltage (J-V) curves of the cells under simulated 1.5 AM illuminations with an active area of 0.20 cm-2. Fig.5 Nyquist plots the DSCs fabricated with Pt, NiCo2O4/C, Co3O4/C, NiO/C composite and pristine C electrodes. The inset (a) is equivalent circuit of DSCs and the inset (b) is the high frequency part of Nyquist plots. Fig.6. Cyclic voltammograms for C, NiO/C, Co3O4/C, NiCo2O4/C and Pt electrode at a scan rate of 50 mV s-1 in acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 with platinum wire and a SCE electrode used as counter electrode and reference electrode, respectively. Fig.7 Photocurrent density as a function of time under persistent illumination (AM 1.5 simulated sun light) for 1200s.
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Tables Table1 Photovoltaic parameters of DSCs assembled with various CEs Counter electrodes
Jsc (mA cm-2)
Voc (mV)
FF
η (%)
NiCo2O4/C
16.08 14.61 16.85
650 650 630
0.60 0.63 0.58
6.27 5.98 6.16
NiO/C
16.30 17.10 16.45
610 610 590
0.51 0.48 0.55
5.07 5.00 5.33
Co3O4/C
16.12 16.47 15.98
610 630 655
0.49 0.45 0.44
4.82 4.67 4.60
Ni:Co=1:1
15.97 16.31 15.98
605 575 605
0.54 0.53 0.56
5.22 4.97 5.41
Ni:Co=1:3
15.86 16.27 16.50
605 595 595
0.55 0.54 0.52
5.28 5.22 5.10
Pt C
17.48 15.06
665 600
0.63 0.48
7.38 4.34
Note: parallel three samples for DSCs measurements under each experimental condition. All
underlined data were consistent with the Fig.4.
Table 2 Nyquist fitting parameters of Pt, NiCo2O4/C, Co3O4/C, NiO/C composite and C counter electrodes Counter electrodes
Rs (Ω)
Rct (Ω)
RR (Ω)
Rtotal (Ω)
Pt NiCo2O4/C NiO/C Co3O4/C C
22.4 26.8 28 29 28
1.8 2.2 2.6 2.8 5.7
25.3 29.3 38.4 50.0 55.0
49.5 58.3 69.0 81.8 88.7