Synthesis of various carbon incorporated flower-like

J Solid State Electrochem DOI 10.1007/s10008-016-3407-0

ORIGINAL PAPER

Synthesis of various carbon incorporated flower-like MoS2 microspheres as counter electrode for dye-sensitized solar cells J. Theerthagiri 1 & R.A. Senthil 1 & Prabhakarn Arunachalam 2 & J. Madhavan 1 & M.H. Buraidah 3 & Amutha Santhanam 4 & A.K. Arof 3

Received: 12 May 2016 / Revised: 7 September 2016 / Accepted: 21 September 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract A flower-like molybdenum disulfide microspheres and various carbon materials (acetylene black, vulcan carbon, multi-walled carbon nanotubes, carbon nanofibers, and rice husk ash) incorporated MoS2 microsphere materials were synthesized via convenient single-step hydrothermal method. The obtained MoS2/C materials provide cost-effective and Pt free counter electrodes for DSSCs. Phthaloylchitosan-based polymer electrolyte was used as an electrolyte for DSSCs. The phase formation and purity of the synthesized materials were ascertained by powder X-ray diffractometer. The shape, morphology, and the distribution of the carbon materials in MoS2 microspheres were examined by electron microscope measurements. The electrochemical measurements were revealed that the carbon materials incorporated MoS2 electrode hold low charge transfer resistance at the counter electrode/ electrolyte interface and demonstrate high electrocatalytic activity for the reduction of I3− to I−ions. Among different carbon materials studied, MoS2 doped on CNF offered a positive synergistic effect for the electrocatalytic reduction of I3−. The DSSC fabricated with MoS2/CNF CE and phthaloylchitosanbased polymer electrolyte shown a high power conversion efficiency of 3.17 %, whereas pure MoS2 CE showed only

* J. Madhavan [email protected]

1

Solar Energy Lab, Department of Chemistry, Thiruvalluvar University, Vellore 632 115, India

2

Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

3

Department of Physics, University of Malaya, Centre for Ionics University of Malaya, 50603 Kuala Lumpur, Malaysia

4

National Centre for Nanoscience and Nanotechnology, Guindy campus, University of Madras, Chennai 600025, India

1.04 %. The present study validates the application of carbon incorporated MoS2 as a potential CE in DSSCs. Keywords Molybdenum disulfide . Carbon . Electrocatalytic activity . Counter electrode . Polymer electrolyte . Dye-sensitized solar cell

Introduction In the view of concerns with growing sternness of energy crisis and environmental challenges, dye-sensitized solar cells (DSSCs) have fascinated substantial consideration as a growing applicant for the creation of electricity via solar energy and considered to be eco-friendly processes [1]. These sources of energy offer a cleaner source and it most plentiful naturally available energy, helping to solve both the global warming and the energy crisis [2, 3]. DSSCs are the most noticeable third generation solar cell, which exhibits several features as an encouraging economical substituent to the silicon-based solar cells in different aspects such as convenient synthetic process, low cost, higher flexibility, good plasticity, ease of building combination, and environmental friendliness [1]. The outstanding study of DSSCs by O’Regan and M. Gratzel in 1991 [2] has become a turning point in the area of research, capturing a great attention in the international research community. A typical DSSC comprises of four major parts, semiconductor oxide (photoanode), redox electrolyte, counter electrode (CE), and dye as sensitizer. Among them, CE plays a significant role to enhance the power conversion efficiency of the DSSCs. The main function of the CE is to gather the electrons from the external circuit and catalyze the iodidetriiodide (I−/I3−) redox reaction in the electrolyte [3]. A CE should possess high electrocatalytic activity towards the reduction of I 3− into I − ions, good stability towards the

J Solid State Electrochem

electrolyte used in the device, low charge resistance and low cost. These characteristics will reduce the device internal series resistance, which results in a high fill factor value of the cell. Generally, a platinum (Pt) deposited conducting glass substrate (fluorine/indium doped tin oxide) is the most commonly used CE in DSSCs due to its superior electrocatalytic activity. Conversely, Pt is considered to be a more expensive metal with low abundance in earth and dissolution of Pt in I−/ I3− redox electrolyte to produce PtI4 and H2PtI6 which limits the commercialization of DSSCs [4, 5]. These limitations promoted numerous studies to develop an alternative material in terms of reduced cost and to simultaneously maintain the effectiveness of the DSSCs. In the past decades, various CE materials which comprise carbon-based materials [2, 6], conducting polymers [6, 7], metal oxides [8], metal sulfides [9, 10], transition metal nitrides, carbides [11–13], and selenides [14] have been developed to replace Pt which showed reliable power conversion efficiencies in DSSCs. Recent reports on the use of layered transition metal sulfide, molybdenum disulfide (MoS2) [15, 16] have increasingly attracted much attention for variety of applications like supercapacitor, battery, hydrogen evolution reaction, DSSCs, etc. MoS2 has an analogous structure similar to graphene that has drawn enormous attention as a potential alternative to Pt free CE material [17–19]. In addition to electrocatalytic activity, the electrical conductivity of the CE is also a critical element that affects the performance of DSSCs. However, a pure inorganic material will limit its practical application due to poor charge carrier transport efficiency between the metal nanoparticles and the conducting glass substrate, resulting in nanoparticles aggregation [20]. Hence, many recent reports have been proposed to improve an electrical conductivity and also catalytic reduction of I3− by hybrids of carbonaceous materials and inorganic compounds. The pure MoS2 with well-distributed carbon materials can possibly provide a synergistic effect concerning the electrocatalytic regeneration of I− from I3− [21–23]. In the present investigation, a flower-like MoS2 microspheres and various carbon materials (acetylene black (AB), vulcan carbon (VC), multi-walled carbon nanotubes (CNT), carbon nanofibers (CNF), and rice husk ash (RHA)) incorporated MoS2 microspheres were prepared by simple one-step hydrothermal route. The produced materials were employed as CE material in DSSCs for the reduction of I3−, and also investigated the influence of incorporation of carbon materials as a co-catalyst with MoS2 towards the improvement of DSSCs. Herein, phthaloylchitosan (PhCh)-based polymer electrolyte was used as an electrolyte for DSSCs. In our previous work [13], we have used the optimized composition of PhCh-based polymer electrolyte to fabricate the DSSCs and investigated the electrocatalytic performances of α-Mo2C towards the reduction of I3− to I− ions. Another study by Arof et al. [24] reported the performance of an anthocyanin (An)

and chlorophyll (Chl) as natural dyes extracted from black rice and fragrant screwpine leaves towards the DSSCs utilizing an optimized PhCh polymer electrolyte. In this work, we inspected the performance of various carbon materials incorporated MoS2 as CE for DSSCs by using PhCh polymer electrolyte. Although, the higher power conversion efficiency of the DSSCs have been obtained by engaging liquid electrolytes (I−/I3− redox couple) in DSSCs. It has more disadvantages like solvent evaporation, leakage, less stability, difficulty in sealing the device, and corrosion of electrode etc., which confines their practical appliance. These issues could overcome by using polymer electrolyte [25]. Herein, we investigate the synergistic effect of various carbon materials incorporated MoS2 towards the electrocatalytic reduction of I3− in polymer electrolyte based DSSCs.

Experimental section Materials Sodium molybdate dihydrate (Na2MoO4.2H2O), thiourea (CH4N2S), N-Methyl-2-pyrrolidone (NMP), acetonitrile, and isopropanol were acquired from SDFCL, India. Acetylene black (AB) was purchased from Chevron Chemical Company, TX, USA. Multi-walled carbon nanotubes (CNT) were purchased from NanoLab, Inc., USA. CNFs (grade PR 24 LHT) were obtained from Pyrograf, USA. Vulcan XC-72 (VC) was obtained from Cabot Corporation, USA. RHA was prepared as described elsewhere [26]. Chitosan and phthalic anhydride were purchased from Merck-Germany. Carbowax w as ac qu i r ed fr om S up elc o, U SA . N 3 d ye [ c i s diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II)], ethylene carbonate (EC), tetrapropylammonium iodide (TPAI), polyvinylidine difluoride (PVDF), fluorine-doped tin oxide (FTO (R: 10 Ω/ cm2) conducting glass, lithium iodide (LiI), and lithium perchlorate (LiClO4) were obtained from Sigma Aldrich, India. Iodine (I2) was acquired from Fluka, Switzerland. N,NDimethylformamide (DMF) and absolute ethanol were acquired from Friendemann Schmidt. Super P was purchased from Timcal, USA. TiO2 nanoparticles (P25 and P90) were delivered by Degussa. Synthesis of flower-like MoS2 and MoS2/C hybrid microspheres The flower-like MoS2 and MoS2/C hybrid microspheres were synthesized by convenient single-step hydrothermal route using Na2MoO4.2H2O and CH4N2S as Mo and S sources. In a typical synthesis of MoS2/C hybrid microspheres, 30 mg of carbon material (AB/VC/CNT/CNF/RHA) was ultrasonically dispersed in 30 mL of deionized water for 30 min. Then,


1.50 g of Na2MoO4.2H2O, 2.35 g of CH4N2S, and 0.23 g of CTAB were added and agitated well for 15 min until a homogeneous solution was obtained. After that, the reaction mixture was transferred into a 200 mLTeflon-lined autoclave. The autoclave was sealed tightly and the hydrothermal reaction was performed at 220 °C for 12 h. Then, the autoclave was naturally cooled down. The black color product was obtained after filtration and several washings. Finally, it was dried at 80 °C for 5 h. The synthesized materials were denoted as MoS2, MoS2/AB, MoS2/VC, MoS2/CNT, MoS2/CNF, and MoS2/RHA. Preparation of CEs The CEs were prepared by doctor blade technique using a slurry coating on a pre-cleaned conducting FTO glass substrate. The cleaned FTO substrate was obtained by washing with detergent, distilled water, ethanol, and acetone. The FTO substrate was dried using an air dryer and then it was soaked in isopropanol solvent to remove the residues of dirt and dried again. This pre-cleaned FTO glass substrates were used for the further studies. 80 wt.% of hydrothermally synthesized catalytic material, 10 wt.% of super P, and 10 wt.% of PVDF were ground together and mixed well with NMP solvent to make a slurry. The obtained slurry was coated on the FTO substrate by doctor blade method and scotch tape was used as a spacer to obtain a uniform coating. Then, it was dried in a hot air oven at 100 °C for 1 h. The thickness of the CE film was around 10 μm and the loaded active material on the electrode is 2 mg. Preparation of polymer electrolyte An optimized phthaloylchitosan (PhCh)-based polymer electrolyte developed by Prof. A.K. Arof, Research group, Centre for Ionics University of Malaya, (Patent filed No. PI2015701146) was used for this investigation. In a typical preparation of polymer electrolyte, 1.3 wt.% of PEO, 5 wt.% of phthaloylchitosan, 31.5 wt.% of DMF, 37.8 wt.% of ethylene carbonate, and 22.7 wt.% of tetrapropylammonium iodide were taken in a closed glass container. The contents were stirred well at 90 °C until a homogeneous gel was formed. Then, the heater was stopped and the solution was allowed to cool down naturally. Iodine (1.7 wt.%) was then added to the polymer electrolyte and agitated well until a homogeneous solution was achieved. Fabrication of DSSCs The TiO2 photoanode was prepared as defined in our previous reports [27, 28]. In brief, the two layers (compact and porous TiO2 layers) of TiO2 photoelectrode were arranged on the precleaned FTO glass substrate. The TiO2 paste for the compact layer was assembled by grinding 0.5 g of TiO2 (P90) powder

with 2 mL of 0.1 M HNO3 for about 30 min in an agate mortar and pestle. The resulting TiO2 (P90) paste was spin coated (2650 rpm, 1 min) onto the FTO glass substrate and allowed for sintering at 450 °C for 30 min. The porous layer of TiO2 second layer was assembled by crushing 0.5 g of TiO2 (P25) powder with 2 mL of 0.1 M HNO3, 0.1 g of polyethylene glycol (carbowox), and 2 drops of Triton X-100 surfactant. The resulting TiO2 (P25) paste was doctor bladed on the compact layer and sintered at 450 °C for 30 min. The obtained TiO2 photoelectrode was dye sensitized by soaking in 3 mM ethanolic solution of N3 dye for 24 h in dark condition. The dye-sensitized TiO2 photoelectrode was rinsed with ethanol and dried before assembling the DSSC. A small amount of PhCh-based polymer electrolyte was evenly applied on the surface of dye-sensitized TiO2 photoelectrode. Then, the sandwich type DSSCs was fabricated by clamping the as prepared CE over the polymer electrolyte applied photoanode. The fabricated cell configuration of FTO/TiO2/N3/polymer electrolyte/MoS2-C hybrid/FTO and the photovoltaic analysis were executed in open air with an active cell area of 0.2 cm2.

Material characterization The phase formation, purity, and crystal structure of the fabricated materials were analyzed using powder X-ray diffraction (XRD). The diffraction patterns were logged using a XRD (Mini Flex II, Japan) with Cu Kα radiation (λ = 0.154 nm) at a scan rate of 3o/min. The shape and surface morphology of the samples were obtained by Hitachi, S-4800, Field-Emission Scanning Electron Microscopy (FE-SEM) at 10 kV operating voltage. High resolution transmission electron microscopy (HRTEM) of the materials was carried out using a FEI-TECNAI G2 instrument. The electrocatalytic activity of the samples towards the reduction I3− into I− ions was assessed by cyclic voltammogram (CV) measurements. The CV studied was completed in a three electrode assembly system of acetonitrile solution comprising of LiClO4 (0.1 M), LiI (10 mM), and I2 (1 mM) via electrochemical work station (CHI 608E) at a scan speed of 100 mVs−1. The Pt-wire, Ag/ AgCl, and FTO/MoS2-C electrode serves as CE, reference electrode (RE), and working electrode (WE), respectively. The electrolyte was purged with N2 gas for 30 min. The photocurrent density-voltage (J-V) features of the DSSC were examined under 1 sun (light intensity 100 mW/cm2) illumination via PEC-L01 (PECCELL Inc.,) solar simulator. The fill factor (FF) and overall power conversion efficiency (η) were estimated as described in our previous reports [14]. Electrochemical impedance spectroscopy (EIS) analysis for the fabricated DSSCs were performed in electrochemical work station (Metrohm Autolab B.V. PGSTAT 128 N). All the analysis was carried out in an applied bias of open circuit voltage (Voc) under AM 1.5.


Results and discussion

Morphology studies

XRD studies

The shape and morphology of the hydrothermally synthesized samples were investigated by FE-SEM analysis. The distinctive FE-SEM morphologies of MoS2, MoS2/AB, MoS2/VC, MoS2/CNT, MoS2/CNF, and MoS2/RHA are displayed in Fig. 2a–f. The FE-SEM micrograph of pure MoS2 (Fig. 2a) showed a flower-like microspheres. It can be observed that all MoS2/C microspheres exhibited a slightly destroyed flowerlike structure due to the addition of various carbon materials (Fig. 2b–f). However, the surface morphology of the carbon materials was found to be well distributed in the MoS2 microspheres. A well-organized surface structure with tightly packed nanocrystals was described to endorse a charge transport phenomenon from their surface to redox electrolyte in DSSCs [32]. Further, the morphology of MoS2/CNF was observed by HRTEM and the equivalent images are presented in Fig. 3a–b. This showed numerous uniformly decorated MoS2 particles on CNF. This morphology of uniformly distributed MoS2 and CNF are expected to expose more active catalytic sites for the reduction of I3− to I− ions.

The XRD patterns of the hydrothermally synthesized MoS2/C (AB, VC, CNT, CNF, and RHA) microspheres are presented in Fig. 1. The diffraction peaks of pure MoS2 (Fig. 1a) showed peaks of 2θ = 14.02o, 33.34o, 39.54o, and 58.83o, which are in consistent with the corresponding (003), (101), (104), and (110) diffraction planes of rhombohedral MoS2 (JCPDS No. 17-0744). The XRD patterns of various MoS2/AB, MoS 2/VC, MoS2/CNT, MoS2/CNF, and MoS2/RHA are displayed in Fig. 1b–f. It can be witnessed that the XRD patterns of MoS2 is unchanged due to the incorporation of carbon with no specific diffraction peaks of carbon in the MoS2/C hybrid samples which may be due to (i) very low amount of carbon present in the hybrids and (ii) dense covering of MoS2 microspheres on the surface of carbon [29, 30]. However, small changes in the peak intensity of MoS2 were observed which may be due to the added carbon atoms at the interstitial positions creating substantial contraction and expansion of the lattice constants. The average crystalline size of the hydrothermally synthesized MoS2/C hybrids is calculated using Sherrer’s equation [18, 31]. The crystalline size is designated as the diameter of the core of the nanocrystal and will not give information about thickness of the surface capping agent. The calculated average crystalline size of the pure MoS2 and MoS2/C samples are presented in Table 1 and it can be noted that the average crystalline size of the synthesized MoS2/C samples are around 4–7 nm. When compared with other samples, MoS2/CNF showed the smaller crystalline size of 4.09 nm.

Electrocatalytic activity The electrocatalytic activity of the hydrothermally synthesized MoS2, MoS2/AB, MoS2/VC, MoS2/CNT, MoS2/CNF, and MoS2/RHA CEs towards the reduction of I3− to I− ions were investigated using CV studies. The CV for the I−/I3− redox peaks at a scan rate of 100 mVs−1 are presented in Fig. 4. There are two pairs of redox peaks (Ox-1/Red-1, Ox-2/Red-2) were detected for all the electrodes. However, only one reduction peak was observed for pure MoS2. The formation of left and right redox pairs are presented in Eqs. (1, 2), respectively. I3 ‐ þ2 e‐ ⇌3I‐ ðOx‐1=Red‐1Þ ‐

‐

3 I2 þ2 e ⇌2I3 ðOx‐2=Red‐2Þ

Fig. 1 XRD patterns of (a) MoS2, (b) MoS2/AB, (c) MoS2/VC, (d) MoS2/CNT, (e) MoS2/CNF, and (f) MoS2/RHA microspheres

ð1Þ ð2Þ

The left pair of peaks Ox-1 and Red-1 is the key parameters of our analysis because of the function of CE in DSSCs is to catalyze the I3− reduction [33]. Generally, the peak current density and peak-to-peak separation (Epp) are the important factors for the evaluation of electrocatalytic activity of the CEs. The CEs with higher peak current density and smaller Epp values were reported to exhibit an excellent electrocatalytic activity [34]. The obtained Epp value for all the CEs are presented in Table 1. It can be noted that the Epp values of MoS2/CNF CE are smaller (0.44 V) than the other MoS2/C electrodes. Also, it is clear from Fig. 4 that MoS2/CNF CE showed a higher peak current density value than other MoS2/ C electrodes. This trend indicated the higher electrocatalytic activity towards the reduction of I3− ions of MoS2/CNF than

J Solid State Electrochem Table 1 Crystalline size, Epp and electrochemical active surface sites for the synthesized electrode materials

Sample

FWHM

Crystalline size (nm)

MoS2 MoS2/AB MoS2/VC MoS2/CNT MoS2/CNF MoS2/RHA

1.161 1.186 1.179 1.173 1.955 1.167

6.89 6.74 6.78 6.28 4.02 6.85

the other MoS2/C materials. Hence, MoS2/CNF offered a positive synergistic effect which is due to the results of upsurge in the active catalytic sites. The CV of MoS2/CNF CE at various scan rates for I−/I3− redox system is presented in Fig. 5a. The peak current density is found to upsurge with an increase in the scan rates with a gradual shifting of the cathodic and anodic peaks towards negative and positive directions. Figure 5b displays that the linear relationship between the peak current densities and the square root of the scan rates. Agreeing to the Langmuir isotherm principle, this linear relationship revealed that the transport of I− species on the surface of CE is affected by the diffusion limitation of the redox reaction [35]. Additionally, no significant integration between the I−/I3− and MoS2/CNF CE was also observed. The obtained CV results designate that the MoS2/CNF CE is a promising electrocatalyst which can be used towards the reduction of I3− ions than the other MoS2/C materials.

Epp (V) – 0.60 0.57 0.51 0.44 0.63

Active surface sites/cm2 9.9 × 1015 1.2 × 1016 2.8 × 1016 3.0 × 1016 3.1 × 1016 2.0 × 1016

Electrochemical active surface sites Electrochemical active surface sites per cm2 of geometric area for various MoS2/C materials were assessed by non-Faradaic capacitance measurements in 0.5 M H2SO4. In a typical experiment, non-Faradaic capacitance was measured between 0.1–0.35 V vs reversible hydrogen electrode (RHE), since beyond 0 V vs RHE, in the cathodic direction, hydrogen evolution reaction sets in [36]. Figure 6a shows CVof MoS2/CNF at various scan rates. Absence of Faradaic features like peaks, the current increasing with scan rate point to double layer charging in the potential range selected. Current density increases linear with scan rate as shown in Fig. 6b, indicating non-Faradaic nature of current [37], and the calculated nonFaradaic capacitance for MoS2/CNF is 1.6 mF/cm2. Procedure proposed by Jaramillo et al. was followed in the analysis of number o f active surface sites of various MoS 2 electrocatalysts [38]. Briefly, the procedure involves (i)

Fig. 2 Typical FE-SEM images of a MoS2, b MoS2/AB, c MoS2/VC, d MoS2/CNT, e MoS2/CNF, and f MoS2/RHA microspheres


Fig. 3 a–b HRTEM images of MoS2/CNF microsphere

estimation of non-Faradaic capacitance by measuring half of the difference between anodic and cathodic charging current at a particular potential (0.15 mV vs RHE) as shown in Fig. 6b, (ii) estimation of roughness factor (RF) from the ratio of non-Faradaic capacitance of MoS2 electrocatalyst to that of a flat standard MoS2 (60 μF/cm2), and (iii) finally surface site density of MoS2/C electrocatalysts were estimated by multiplying the respective RF with surface site density of flat standard MoS2 (1.164 × 1015 sites/cm2). The results of electrochemical active surface sites are shown in Table 1 for various MoS2/C material electrocatalysts. As expected, a direct

Fig. 5 a Cyclic voltammograms for MoS2/CNF electrode at different scan rates. b Relationship between redox peak current vs. square root of the scan rate

correlation can be seen between active surface site density and solar cell efficiency.

Photovoltaic performance of DSSCs

Fig. 4 Cyclic voltammograms for MoS2, MoS2/AB, MoS2/VC, MoS2/ CNT, MoS2/CNF, and MoS2/RHA CEs at a scan rate of 150 mVs−1 in 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 as supporting electrolyte in acetonitrile

The influence of incorporating various carbon materials into the flower-like MoS2 CE on the J-V characteristics of DSSCs under an irradiation of solar light with intensity 100 mW cm−2 are revealed in Fig. 7. The photovoltaic parameters viz., Voc, Jsc, FF, and η for the DSSCs fabricated with pure MoS2, MoS2/AB, MoS2/VC, MoS2/CNT, MoS2/CNF, and MoS2/ RHA CEs are summarized in Table 2. The photovoltaic parameter of DSSC fabricated with pure MoS2 CE was found to have a Voc value of 0.61 V, Jsc value of 8.15 mA/cm2, FF value of 0.21, and η value of 1.04 %. It can be noted that the incorporation of carbon materials into the MoS2 CE led to increase in Jsc and FF values which in turn a significantly increased the η of the DSSCs. The DSSC fabricated with MoS2/CNF CE achieved a η of 3.17 % with a Voc value of 0.58 V, Jsc value of 10.33 mA/cm2, and FF value of 0.53, which is higher than

J Solid State Electrochem Table 2 Charge transfer resistance and photovoltaic parameters of the fabricated DSSCs with various CEs CEs

Rct(Ω cm2)

Voc (V)

Jsc(mA/cm2)

FF

η (%)

MoS2 MoS2/AB MoS2/VC MoS2/CNT

190.50 98.57 11.29 17.40

0.61 0.64 0.64 0.62

08.15 10.27 10.81 12.43

0.21 0.20 0.43 0.4

1.04 1.31 2.97 3.08

MoS2/CNF MoS2/RHA

4.80 24.96

0.58 0.60

10.33 10.95

0.53 0.32

3.17 2.10

other MoS2/C materials. This can be credited to the synergistic effect of the combination of CNF and MoS2, low charge transfer resistance (Rct) in the electrode/electrolyte interface and increase in the electrocatalytic active sites for the reduction of I3− [19]. The electrocatalytic activity of the CE for the regeneration of I−/I3− redox couple may also depend upon

the morphology of the material, which benefits the efficient electron transport mechanism [39]. The experiment was repeated several times, and the average values of the cell efficiencies with an error bar are plotted in Fig. 8. The photovoltaic performance of the DSSCs significantly depends on the Rct values [1]. In order to examine the influence of various MoS2/C CEs on the interfacial charge transfer process within the fabricated DSSCs, the EIS analysis was carried out and the corresponding Nyquist plots are presented in Fig. 9. EIS is useful method to examine the charge transfer process, internal resistance and correlation between the electrocatalytic activities of the CEs [40]. The intersection of the high frequency intercept on the real axis (Z′-axis) corresponds to the series resistance (Rs), and the higher frequency region of semicircle (left side) is credited to the Rct [33, 35]. The CE material with low Rct value at the CE/electrolyte interface deliver superior electrocatalytic activity for the I3− reduction, which results in an enhancement in FF value of the DSSC [41–43]. The obtained Rct values for the fabricated DSSCs with numerous CEs are displayed in Table 2. It is noted that the fabricated DSSC with MoS2/CNF CE showed a very low

Fig. 7 Photocurrent-voltage curves of DSSCs fabricated with MoS2, MoS2/AB, MoS2/VC, MoS2/CNT, MoS2/CNF, and MoS2/RHA CEs

Fig. 8 Power conversion efficiencies of fabricated DSSCs with various CEs

Fig. 6 a Cyclic voltammograms of MoS2/CNF at various scan rates in 0.5 M H2SO4. b Current density was plotted as a function of scan rate


to the original state by receiving electrons from the redox system through the polymer electrolyte (Eq. 3). 3I‐ þ 2Dþ →I3 − þ 2DðRegeneration of N3 dyeÞ

ð3Þ

I3 − þ 2 e− →3I − ðRegeneration of iodide ionsÞ

ð4Þ

Subsequently, the oxidized redox mediator (I3−) gets reduced to its original state (I−) at the MoS2/CNF CE. The circuit is accomplished by the migration of electron between the TiO2 photoanode and CE through an external circuit. Thus, the CE is the crucial part in the DSSC and the cell performance is greatly influenced by the nature of the CEs. Fig. 9 Nyquist plots of DSSCs fabricated with MoS2, MoS2/AB, MoS2/ VC, MoS2/CNT, MoS2/CNF, and MoS2/RHA CEs

Conclusions

Rct value (24 Ω) than other CEs. The above investigation suggests that MoS2/CNF CE can show a significant electrocatalytic activity and thereby useful to improve the efficiency of DSSCs. The schematic representation of DSSC with MoS2/CNF CE under visible light illumination is shown in Fig. 10. Upon cell illumination, the adsorbed N3 dye molecule undergoes photo-excitation and the photoexcited N3 dye molecule injects its electron into the conduction band of the TiO2 photoanode. The oxidized dye gets reduced back

A variety of carbon materials (AB, VC, CNT, CNF, and RHA) incorporated flower-like MoS2 microspheres were prepared by convenient single-step hydrothermal process. The characterization studies revealed the shape, morphology, and the distribution of carbon materials in MoS2 microspheres. The synthesized samples were tested as low-cost Pt free CEs in DSSCs. The CV and EIS analysis showed high electrocatalytic activity and low Rct of carbon incorporated MoS2 electrodes. The MoS2/CNF CE exhibited higher electrocatalytic activity towards the regeneration of I− from I3−, than the other MoS2/C electrodes, which might be due to an increase in the

Fig. 10 The schematic representation of DSSC based on MoS2/CNF CE under visible light illumination


number of electrocatalytic active sites and lower Rct at the CE/ electrolyte interface. The DSSC assembled with MoS2/CNF CE and PhCh-based polymer electrolyte displayed an improved η of 3.17 %. The obtained results demonstrate that the carbon incorporated MoS2 samples can expected to be an efficient CE in DSSCs. Acknowledgments We gratefully acknowledge the financial support from the Department of Atomic Energy-Board of Research in Nuclear Sciences (DAE-BRNS) (Grant No. 2013/37P/1/BRNS/10), Mumbai, India. This study was supported by the Deanship of Scientific Research, College of Science Research Centre, King Saud University, Saudi Arabia. Also, we acknowledge Mr. Shahan Shah, Centre for Ionics University of Malaya (CIUM), University of Malaya, 50603 Kuala Lumpur, Malaysia, for his support in lab works.

14.

15.

16.

17.

18.

References 19. 1.

2.

3.

4. 5. 6.

7.

8.

9.

10.

11.

12.

13.

Lim SP, Pandikumar A, Lim YS, Huang NM, Lim HN (2014) Insitu electrochemically deposited polypyrrole nanoparticles incorporated reduced graphene oxide as an efficient counter electrode for platinum-free dye-sensitized solar cells. Sci Rep 4:5305. doi:10.1038/srep05305 Joshi P, Xie Y, Ropp M, Galipeau D, Bailey S, Qiao Q (2009) Dyesensitized solar cells based on low cost nanoscale carbol/TiO2 composite counter electrode. Energy Environ Sci 2:426–429 Jiang N, Bogoev L, Popova M, Gul S, Yano J, Sun Y (2014) Electrodeposited nickel-sulfide films as competent hydrogen evolution catalysts in neutral water. J Mater Chem A 2:19407–19414 Gratzel M (2003) Dye sensitized solar cells. J Photochem Photobiol C: Photochem Rev 4:145–153 Regan O, Gratzel M (1991) A low-cost, high efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353:737–740 Al-bahrani MR, Ahmad W, Mehnane HF, Chen Y, Cheng Z, Gao Y (2015) Enhanced electrocatalytic activity by RGO/MWCNTs/NiO counter electrode for dye-sensitized solar cells. Nano-Micro Lett 7: 298–306 Munkhbayar B, Nine MJ, Jeoun J, Ji M, Jeong H, Chung H (2013) Synthesis of a graphene–tungsten composite with improved dispersibility of graphene in an ethanol solution and its use as a counter electrode for dye-sensitised solar cells. J Power Sources 230:207–217 Zhang J, Kuang S, Nian S, Wang G (2015) The effect of carbonization temperature on the electrocatalytic performance of nitrogendoped porous carbon as counter electrode of dye-sensitized solar cells. J Mater Sci:Mater Electron 26:6913–6919 Ke CR, Chang CC, Ting JM (2015) Modified conducting polymer films having high catalytic activity for use as counter electrodes in rigid and flexible dye-sensitized solar cells. J Power Sources 284: 489–496 Saranya K, Md R, Subramania A (2015) Developments in conducting polymer based counter electrodes for dye-sensitized solar cells—an overview. Eur Polym J 66:207–227 Lan Z, Que L, Wu W, Wu J (2016) High-performance Pt-NiO nanosheet-based counter electrodes for dye-sensitized solar cells. J Solid State Electrochem 20:759–766 Chen SL, Tao J, Tao HJ, Shen YZ, AC X, Cao FX, Jiang JJ, Wang T, Pan L (2016) Rounded Cu2ZnSnS4 nanosheet networks as a costeffective counter electrode for high-efficiency dye-sensitized solar cells. Dalton Trans 45:4513–4517 Shen J, Cheng R, Luo Y, Chen Y, Chen X, Sun Z, Huang S (2015) Growth of NiS/graphene nanocomposites for enhanced

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

performance of dye sensitized solar cells. J Solid State Electrochem 19:1045–1052 Li GR, Song J, Pan GL, Gao XP (2011) Highly Pt-like electrocatalytic activity of transition metal nitrides for dye-sensitized solar cells. Energy Environ Sci 4:1680–1683 Theerthagiri J, Senthil RA, Buraidah MH, Madhavan J, Arof AK (2016) Synthesis of α-Mo2C by carburization of α-MoO3 nanowires and its electrocatalytic activity towards tri-iodide reduction for dye-sensitized solar cells. J Mater Sci Technol. doi:10.1016/j. jmst.2016.03.003 Theerthagiri J, Senthil RA, Susmitha K, Raghavender M, Madhavan J (2015) Synthesis of efficient Ni0.9X0.1Se2 (X = Cd, Co, Sn, and Zn) based ternary selenides for dye-sensitized solar cells. Mater Sci Forum 832:61–71 Yue G, Lin JY, Tai SY, Xiao Y, Wu J (2012) A catalytic composite film of MoS2/graphene flake as a counter electrode for Pt-free dyesensitized solar cells. Electrochim Acta 85:162–168 Huang KJ, Wang L, Zhang JZ, Xing K (2015) Synthesis of molybdenum disulfide/carbon aerogel composites for supercapacitors electrode material application. J Electroanal Chem 752:33–40 Lin JY, Yue G, Tai SY, Xiao Y, Cheng HM, Wang FM, Wu J (2013) Hydrothermal synthesis of graphene flake embedded nanosheetlike molybdenum sulfide hybrids as counter electrode catalysts for dye-sensitized solar cells. Mater Chem Phys 143:53–59 Hung HC, Lin YJ, Ke ZY (2016) Interface modification of MoS2: TiO2 counter electrode/electrolyte in dye-sensitized solar cells by doping with different Co contents. J Mater Sci:Mater Electron 27: 5059–5063 Yue G, Wu J, Xiao Y, Huang M, Lin J, Lin JY (2013) High performance platinum-free counter electrode of molybdenum sulfide–carbon used in dye-sensitized solar cells. J Mater Chem A 1:1495– 1501 Liao Y, Pan K, Wang L, Pan Q, Zhou W, Miao X, Jiang B, Tian C, Tian G, Wang G, Fu H (2013) Facile synthesis of high-crystallinity graphitic carbon/Fe3C nanocomposites as counter electrodes for high-efficiency dye-sensitized solar cells. ACS Appl Mater Interfaces 5:3663–3670 Tai SY, Liu CJ, Chou SW, Chien FSS, Lin JY (2012) Few-layer MoS2 nano sheets coated onto multi-walled carbon nanotubes as a low-cost and highly electrocatalytic counter electrode for dyesensitized solar cells. J Mater Chem 22:24753–24759 Zheng M, Huo J, Tu Y, Wu J, Hu L, Dai S (2015) Flowerlike molybdenum sulfide/multi-walled carbon nanotube hybrid as Ptfree counter electrode used in dye-sensitized solar cells. Electrochim Acta 173:252–259 Liu W, He S, Wang Y, Dou Y, Pan D, Feng Y, Qian G, Xu J, Miao S (2014) PEG-assisted synthesis of homogeneous carbon nanotubesMoS2- carbon as a counter electrode for dye-sensitized solar cells. Electrochim Acta 144:119–126 Shah S, Buraidah MH, Teo LP, Careem MA, Arof AK (2016) Dyesensitized solar cells with sequentially deposited anthocyanin and chlorophyll dye as sensitizers. Opt Quant Electron 48:219 Theerthagiri J, Senthil RA, Madhavan J, Maiyalagan T (2015) Recent progress in non-platinum counter electrode materials for dye-sensitized solar cells. Chem Electro Chem 2:928–945 Prabunathan P, Sethuraman K, Alagar M (2014) MnO2-doped, polyaniline-grafted rice husk ash nanocomposites and their electrochemical capacitor applications. RSC Adv 4:47726–47734 Theerthagiri J, Senthil RA, Buraidah MH, Madhavan J, Arof AK (2015) Effect of tetrabutylammonium iodide content on PVDFPMMA polymer blend electrolytes for dye-sensitized solar cells. Ionics 21:2889–2896 Theerthagiri J, Senthil RA, Buraidah MH, Madhavan J, Arof AK (2015) Studies of solvent effect on the conductivity of 2mercaptopyridine-doped solid polymer blend electrolytes and its


31.

32.

33.

34.

35.

36.

37.

application in dye-sensitized solar cells. J Appl Polym Sci 132: 42489 Huang KJ, Wang L, Zhang JZ, Wang LL, Mo YP (2014) One-step preparation of layered molybdenum disulfide/multi-walled carbon nanotube composites for enhanced performance supercapacitor. Energy 67:234–240 Jayaraman T, Raja SA, Priya A, Jagannathan M, Ashokkumar M (2015) Synthesis of a visible-light active V 2 O 5 –g-C 3 N 4 heterojunction as an efficient photocatalytic and photoelectrochemical material. New J Chem 39:1367–1374 Shankar BD, Theerthagiri J, Manivel Raja M, Panda RN (2013) Synthesis, characterization and magnetic properties of nanocrystalline FexNi80−xCo20 ternary alloys. JMagnMagn Mater 344:30–34 Dong J, Wu J, Jia J, Wu S, Zhou P, Tu Y, Lan Z (2015) Cobalt selenides nanorods used as a high efficient counter electrode for dye-sensitized solar cells. Electrochim Acta 168:69–75 Li Z, Gong F, Zhou G, Wang ZS (2013) NiS2/reduced graphene oxide nanocomposites for efficient dye-sensitized solar cells. J Phys Chem C 117:6561–6566 Zuo X, Zhang R, Yang B, Li G, Tang H, Zhang H, Wu M, Ma Y, Jin S, Zhu K (2015) NiS nanoparticles anchored on reduced graphene oxide to enhance the performance of dye-sensitized solar cells. J Mater Sci:Mater Electron 26:8176–8181 Wang W, Pan X, Liu W, Zhang B, Chen H, Fang X, Yao J, Dai S (2014) FeSe2 films with controllable morphologies as efficient

38.

39.

40.

41.

42.

43.

counter electrodes for dye-sensitized solar cells. Chem Commun 50:2618–2620 Behranginia A, Asadi M, Liu C, Yasaei P, Kumar B, Phillips P, Foroozan T, Waranius JC, Kim K, Abiade J, Klie RF, Curtiss LA, Salehi-Khojin A (2016) Highly efficient hydrogen evolution reaction using crystalline layered three dimensional molybdenum disulfides grown on graphene film. Chem Mater 28:549–555 Conway BE, Birss V, Wojtowicz J (1997) The role and utilization of pseudocapacitance for energy storage by supercapacitors. J Power Sources 66:1–14 Benck JD, Chen Z, Kuritzky LY, Forman AJ, Jaramillo TF (2012) Amorphous molybdenum sulfide catalysts for electrochemical hydrogen production: insights into the origin of their catalytic activity. ACS Catal 2:1916–1923 Vijayakumar P, Senthil Pandian M, Lim SP, Pandikumar A, Huang NM, Mukhopadhyay S, Ramasamy P (2015) Facile synthesis of tungsten carbide nanorods and its application as counter electrode in dye sensitized solar cells. Mater Sci Semicond Process 39:292– 299 Yang X, Luo J, Zhou L, Yang B, Zuo X, Li G, Tang H, Zhang H, Wu M, Ma Y, Jin S, Sun Z, Chen X (2014) A novel Pt-free counter electrode for dye-sensitized solar cells: nickel sulphide hollow spheres. Mater Lett 136:241–244 Yue G, Wu J, Lin JY, Xiao Y, Tai SY, Lin J, Huang M, Lan Z (2013) A counter electrode of multi-wall carbon nanotubes decorated with tungsten sulfide used in dye-sensitized solar cells. Carbon 55:1–9