English

• 0.   Introduction

  Dye sensitized solar cells (DSSCs) directly convert solar energy into electricity to improve energy effici-ency and protect the environment^[1-3].DSSCs consist of a transparent photoanode, electrolyte, and a counter electrode (CE).The CE is an important component of DSSCs and plays an important role in collecting electrons from an external circuit and in catalyzing the regeneration of the redox couple at the CE/electrolyte interface^[4-5].Generally, Pt is widely used as a catalytic material on CEs because of its high catalytic activity and high efficiency^[6-8].However, Pt is not only expensive and rare but can also be readily corroded by the I₃^-/I^- electrolyte.Development of Pt-free catalysts is considered to be one of the crucial steps toward improved energy conversion efficiency and low-cost alternatives of DSSCs.

  A significant amount of Pt-free catalytic materials for the CE in DSSCs has been reported, and these materials have included carbon materials (activated carbon, multiwall carbon nanotubes (MWCNTs), graphite, nanotubes, graphene, and C₆₀)^[9-12], conducting and doped polymers (polypyrrole (PPy), polyaniline (PAn), polythiophene (PTh), polyphenylene (PPP), and poly-(3, 4-ethylene dioxythiophene) (PEDOT))^[13-15], and transition metal compounds (oxides, nitrides, carbides, sulfides, and selenides)^[16-25].Among the transition metal compounds, La-Mo compounds and their corresponding composites might be essential in various electro-chemical devices because of their great flexibility in structure and morphology, convenience in synthesis, and high electrocatalytic activity toward the electr-olyte^[26]. Wu et al.^[27] synthesized molybdenum carbide microspheres (Mo₂C-Ms) and nanorods (Mo₂C-Nr) as CE catalysts.The DSSCs resulted in power conversion efficiencies (PCEs) of 5.50% and 4.86% for Mo₂C-Ms and Mo₂C-Nr, respectively.The performance of single catalysts can be further improved through the use of conductive carbon and carbon derivative materials.Previously, Jin et al.^[28] successfully synthesized nano-composites of the perovskite-phase La_0.65Sr_0.35MnO₃ with reduced graphene oxide (LSMO@RGO).Areerob et al.^[29] fabricated DSSCs using La/TiO₂-graphene nanocomposite CEs and their DSSCs yielded a PCE of 6.75%. La/TiO₂-graphene CEs exhibited efficient electrocatalytic ability because the catalytic La particles were uniformly distributed on the surface of graphene.

  In our preliminary experiments, we have synthesized a series of solid solution and intermediate compounds of La₂O₃-MoO₃, with the different stoichio-metric ratios from 0:1 to 1:0, according to the phase diagram of La₂O₃-MoO₃^[30].The performance of La₂Mo₂O₇ as a CE catalyst is superior to other materials for the reduction of I₃^-/I^- in DSSCs.Herein, we combined La₂Mo₂O₇ with MWCNTs to synthesize La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs electrodes to improve the performance of DSSCs.Carbon composites have attracted research attention for enhancing the value of products and for reducing production costs, which has become increasingly important.MWCNTs is a kind of carbon material that has a significant amount of void spaces, a large specific surface area, and acts as a carrier and catalyst^[31].La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs were investigated in iodine electrolyte, and they exhibited high PCEs in DSSCs.The composites have both the increased active surface area of the MWCNTs and the coupling effects between La₂Mo₂O₇ and MWCNTs^[19].La₂Mo₂O₇/MWCNTs comp-osite catalytic materials exhibited high electrocatalytic activity and low charge transfer resistance.

  1.   Experimental

  1.1   Synthesis of the composites

  Lanthanum acetate hydrate (La(CH₃COO)₃·xH₂O, 99.9%, molecular weight:316.04) was supplied by Aladdin, Shanghai.Hexaammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, ≥99.0%, molecular weight:1 235.86) was supplied by Sinopharm Chemical Reagent Co., Ltd.MWCNTs was supplied by Aladdin, Shanghai.All reagents and solvents were analytical grade and were used without further purification.La(CH₃COO)₃·xH₂O and ((NH₄)₆Mo₇O₂₄·4H₂O) were mixed according to their stoichiometric ratio and ground in an agate mortar with 3 mL of ethanol.The mixture was then placed in an oven to dry at 150 ℃ and pressed at 8 MPa into pellets that had a 10 mm diameter.Then, La₂Mo₂O₇ (La₂O₃-2MoO₂) was obtained from the solid precursor via a high-temperature solid phase reaction at 800 ℃ for 8 h in air.The synthesis of La₂Mo₂O₇@MWCNTs was performed by doping 0.125 0 g of La₂Mo₂O₇ on 0.025 0 g of MWCNT_S in isopropanol solvent using ultrasonic dispersion for 15 min.The mixture pastes of La₂Mo₂O₇@MWCNTs were obtained for La₂Mo₂O₇ and MWCNTs with mass ratios of 5:1.

  1.2   Cell fabrication

  Pt, pure La₂Mo₂O₇, La₂Mo₂O₇@MWCNTs and La₂Mo₂O₇/MWCNTs composites were used as CEs and prepared as follows:An FTO glass substrate piece was cleaned with distilled water, then cleaned with a mixture of ethanol and acetone in an ultrasonic bath, and dried under a hot dryer.Then, 100 mg of the prepared La₂Mo₂O₇, La₂Mo₂O₇@MWCNTs and the purchased MWCNTs were respectively dispersed in 2 mL of isopropanol.These mixture pastes were obtained via ball-milling for 50 min and directly sprayed on each FTO glass using an airbrush.The thickness of the electrode was about (12±1) μm.The preparation of La₂Mo₂O₇/MWCNTs was first sprayed with 6 μm of MWCNTs on the FTO glass and then sprayed with 6 μm of La₂Mo₂O₇ onto the surface of the MWCNTs.La₂Mo₂O₇ and MWCNTs show upper and lower layers on the FTO glass.Followed by next step, the FTO glass, which was coated with active material paste of La₂Mo₂O₇, La₂Mo₂O₇@MWCNTs, MWCNTs and La₂Mo₂O₇/MWCNTs, was sintered at 500 ℃ under N₂ flow for 30 min, and the CEs were obtained.Preparation of the Pt CE was carried out using a chemical reduction method.A mass fraction of 5% chloroplatinic acid isopropanol was sprayed onto the ITO substrate and dried at 120 ℃ for 1 h.Then, the ITO substrate was rapidly immersed in an aqueous solution of 30 mmol·L^-1 NaBH₄ for 3 min.The ITO substrate was then removed and rinsed with water and absolute ethanol and dried at 120 ℃ for 1 h.The photoanode used in the DSSCs was a 12-mm thick TiO₂ film sensitized with N719 dye.The electrolyte I₃^-/I^- contained 0.1 mol·L^-1 LiI, 0.6 mol·L^-1 1-propyl-3-methylimidazolium iodide, 0.07 mol·L^-1 I₂, 0.5 mol·L^-1 4-tert-butyl pyridine, and 0.1 mol·L^-1 guanidinium thiocyanate in 3-methoxypropionitrile (MPN).The active area of the electrode was about 0.4 cm×0.4 cm.For electrochemical measurements, the symmetrical cell had a sandwich configuration and was fabricated with two identical CEs clipping the electrolyte together using hot-melt surlyn film.

  1.3   Measurements

  The XRD pattern of La₂Mo₂O₇, MWCNTs and La₂Mo₂O₇@MWCNTs material powders was taken by a D8 Bruker X-ray diffractometer (λ=0.154 nm) using Cu Kα radiation (Ni filter) at 2°·min^-1 with 2θ ranging from 10° to 70°.The voltage is 40 kV and the current is 40 mA.Scanning electron microscopy (SEM) was performed with an S-4800 SEM instrument (Hitachi, Japan) at an accelera-tion voltage of 3 kV.The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization tests were performed using a CHI 660E (SHANGHAI, CHEN HUA) electrochemical analyzer.The CV experiments were carried out in a two-compartment glass cell with a three-electrode configuration at 25 ℃ in an acetonitrile solution containing 1 mmol·L^-1 iodine (I₂), 0.1 mol·L^-1 LiClO₄ (lithium perchlorate), and 10 mmol·L^-1 LiI (lithium iodate).The as-fabricated CEs acted as the working electrode, the Pt sheet (4 cm²) as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode.All potentials were presented on the SCE scale.EIS was carried out in the frequency range of 0.01~10⁵ Hz at 0 V bias voltage with perturbation amplitude of 5 mV in the dark.Tafel polarization was measured at a scan rate of 10 mV·s^-1 from -0.7 to 0.7 V.The assembled DSSCs were illuminated under AM 1.5 illumination (I=100 mW·cm^-2, PEC-L01, Peccell, Yokohama, Japan) with a digital source meter (Keithley 2601, Cleveland, OH).

  2.   Results and discussion

  2.1   Characterization of the as-prepared composites

  The XRD patterns for MWCNTs, La₂Mo₂O₇, and La₂Mo₂O₇@MWCNTs were recorded at room tempera-ture and are presented in Fig. 1.In the diffraction pattern of the powder sample, the most intense diffra-ction peak values of La₂Mo₂O₇ are at 15.96°, 27.65°, 28.14°, 31.06°, 33.50°, 39.83°, 46.49°, 48.76°, 52.90°, 55.66°, and 64.35°, which could be well identified as (210), (310), (011), (111), (120), (301), (002), (610), (421), (430), and (422) and matched well with pristine La₂Mo₂O₇ (PDF No.84-1234).Fig. 1 shows a typical XRD pattern of MWCNTs subjected to N₂ annealing at 500 ℃.It is known that MWCNTs have a strong and sharp characteristic peak at 2θ=26.59°, and this peak is attributed to diffraction of the single carbon atom.The diffraction patterns of La₂Mo₂O₇@ MWCNTs are similar to that of pure La₂Mo₂O₇.The amorphous peak of MWCNTs is relatively weak in the La₂Mo₂O₇@MWCNTs sample, and this result indicates that MWCNTs was barely physically combined with La₂Mo₂O₇ as catalyst carriers.

  Figure 1

  

  Figure 1.  XRD patterns of the prepared MWCNTs, La₂Mo₂O₇ and La₂Mo₂O₇@MWCNTs

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  Fig. 2 shows the morphologies of the MWCNTs, La₂Mo₂O₇, La₂Mo₂O₇@MWCNTs, and La₂Mo₂O₇/MWCNTs.The MWCNTs had attained a clear shape of nanofibers with smooth, uniform and beads-free surface (Fig. 2(a)).There are some void spaces, observed around the MWCNTs.The particle size of La₂Mo₂O₇ is the smallest and more evenly distributed, as seen in Fig. 2(b). Fig. 2(c) shows that La₂Mo₂O₇ was fully combined with MWCNTs to construct La₂Mo₂O₇@ MWCNTs; Fig. 2(d) shows sectional view morphology that the small area La₂Mo₂O₇ coating was scraped away from the surface of the La₂Mo₂O₇/MWCNTs electrode using a blade.The La₂Mo₂O₇/MWCNTs electrode presents upper and lower layers.The intact part of the electrode La₂Mo₂O₇ is uniformly distributed over the MWCNTs.

  Figure 2

  

  Figure 2.  SEM images of the prepared MWCNTs (a), La₂Mo₂O₇ (b), La₂Mo₂O₇@MWCNTs (c) and La₂Mo₂O₇/MWCNTs (d)

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  2.2   Application of the as-prepared composites

  Fig. 3 presents photocurrent density-voltage (J-V) curves of the DSSCs using CEs based on Pt, MWCNTs, La₂Mo₂O₇, La₂Mo₂O₇/MWCNTs, and La₂Mo₂O₇@ MWCNTs.The photovoltaic parameters are presented in Table 1, where V_oc presents the open-circuit voltage.Fig. 3 shows that PCEs of 6.09% and 4.84% were obtained for La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@ MWCNTs, respectively, and these values were superior to the PCEs of Pt (4.54%), MWCNTs (3.94%), and La₂Mo₂O₇ (0.87%) CE-based DSSCs.The short-circuit current density (J_SC) of the four materials were in the order of: La₂Mo₂O₇/MWCNTs (12.19 mA·cm^-2) >La₂Mo₂O₇@MWCNTs (10.88 mA·cm^-2) > MWCNTs (9.75 mA·cm^-2) > La₂Mo₂O₇ (8.88 mA·cm^-2).The significantly enhanced J_SC values contribute to the improved performance of the DSSCs and reveal that the rate of pore recovery at the La₂Mo₂O₇@MWCNTs and La₂Mo₂O₇/MWCNTs electrode-electrolyte interface were faster than that at the electrode-electrolyte interface of the other CEs.Furthermore, fill factor (FF) values of the La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@ MWCNTs cathodes were as high as 0.64 and 0.63, which were higher than the FF of 0.13 indicated by the La₂Mo₂O₇ CE-based DSSC.FF mainly reflects the internal resistance of the electrode/electrolyte in a cell.Thus, the higher J_SC and FF values for the four materials used as CEs are ascribed to the consid-erable enhancement in charge transfer at the CE/electrolyte interface and to catalytic ability, which lowers the internal resistances, the concentration gradients in the electrolyte, and the recombination rates^[32]. The high catalytic activity of each of the La₂Mo₂O₇@MWCNTs and La₂Mo₂O₇/MWCNTs used in the DSSCs could be because of the MWCNTs increased catalytic surface area of the electrode films.

  Figure 3

  

  Figure 3.  J-V characteristics of DSSCs with different samples as CEs under a light intensity of 100 mW·cm^-2

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  Table 1

  

  Table 1.  Photovoltaic parameters for the DSSCs assembled with various CEs

  下载: 导出CSV

  CE	Voc / mV	JSC / (mA·cm-2)	FF	PCE / %
  Pt	779	11.63	0.50	4.54
  MWCNTs	643	9.75	0.63	3.94
  La2Mo2O7	777	8.88	0.13	0.87
  La2Mo2O7@MWCNTs	710	10.88	0.63	4.84
  La2Mo2O7/MWCNTs	786	12.19	0.64	6.09

  2.3   Electrocatalytic process of electrodes

  The CV measurements were conducted at a scan rate of 20 mV·s^-1 to determine the electrocatalytic kinetics of five materials toward the reduction couple of I₃^-/I^- in 3-methoxypropionitrile solution, which consisted of 1 mmol·L^-1 I₂, 10 mmol·L^-1 LiI and 0.1 mol·L^-1 LiClO₄.The CV curves are presented in Fig. 4.It is noteworthy that two oxidation-reduction couples were observed in the CV curves of all five samples.

  Figure 4

  

  Figure 4.  CV curves of various CE electrodes for the I^-/I₃^- electrolyte

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  The reduction peaks in the negative potential could be associated with the reaction:I₃^-+2_e^- → 3I^-; the oxidation peaks in the negative potential correspond to the reaction:3I^- → I₃^-+2_e^{- [33]}.Two main parameters from the CV curve demonstrated the entire electrocatalytic abilities, and these parameters were the peak-to-peak separation (ΔE_p) and the cathodic peak current density (I_P) at more negative potential.In the case of composites, the ΔE_p values of La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs were 0.222 and 0.237 V, respectively.The values of ΔE_p are lower than that of the CE assembled with the La₂Mo₂O₇ (0.377 V) electrode, and this indicates the enhanced electrocatalytic activity and reversibility of the La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs CEs.The ΔE_p value is attributed to the redox ability of the counter electrode to electric pair in electrolytes.The lower ΔE_p value corresponds to the higher redox capability of CE for I₃^-/I^-.The I_P values from the CV profile also demonstrated the electrocatalytic activity of CE.The I_P values of the La₂Mo₂O₇/MWCNTs (1.255 mA·cm^-2) and La₂Mo₂O₇@MWCNTs (0.702 3 mA·cm^-2) composite CEs are higher than that of the CEs with the La₂Mo₂O₇ (0.211 0 mA·cm^-2), MWCNTS (0.412 0 mA·cm^-2), and Pt (0.363 0 mA·cm^-2) electrodes.The higher I_P values indicated the excellent electrocatal-ytic activity of the two La₂Mo₂O₇-MWCNTs compo-sites, which are promising as Pt-free efficient CEs in DSSCs.Moreover, the catalytic reaction is a kinetic process, which is associated with the electron transfer rate constant and the number of active sites.The ntegral area of the negative redox peak for each of the two composite electrodes was bigger than that for La₂Mo₂O₇ (La₂O₃-2MoO₂) and MWCNTs, which indicateed that the La₂Mo₂O₇-MWCNTs composite structure provides more catalytic reaction sites than the La₂Mo₂O₇ and MWCNTs CEs.

  EIS was carried out to assess the electrical conductivity and electrochemical catalytic activities of the CEs.Fig. 5 presents the Nyquist plots of the symmetric cells using different materials as the CEs.The charge transfer resistance (R_ct) of the CE can be taken as half the value of the real semicircles in the high-frequency region.The R_ct values of Pt, MWCNTs, La₂Mo₂O₇, and the La₂Mo₂O₇@MWCNTs and La₂Mo₂O₇/MWCNTs composites were 54.0, 634.4, 2 007.3, 328.4, and 262.2 Ω, respectively.The data indicate that the R_ct value of the La₂Mo₂O₇ electrode is much larger and the La₂Mo₂O₇/MWCNTs composite electrode is smaller than those of the other CE-based cells.The La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs CEs demonstrated better redox capacity and high electrocatalytic ability than the La₂Mo₂O₇ and MWCNTs CE.This is attributed to the lower conductivity of the composites and the accelerated high electron transmission at the CE/electrolyte interface, which thus result in higher J_sc and FF.

  Figure 5

  

  Figure 5.  Nyquist plots for symmetrical cells based on various CEs

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  Tafel polarization curves were recorded at a scan rate of 10 mV·s^-1 from -0.7 to 0.7 V to estimate the electrocatalytic activity of different CEs for I₃^- reduction.The exchange current density (J₀) of DSSCs can be regarded as the intercept of the extrapolated linear region of the anodic and cathodic branches when the potential is zero.The anodic and cathodic branches of the Tafel curves for La₂Mo₂O₇/MWCNTs and the composite La₂Mo₂O₇@MWCNTs CEs show larger slopes than that for the MWCNTs and La₂Mo₂O₇ CEs, exhibiting a higher exchange current density (J₀) on CE in Fig. 6^[30]. Both the La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs CEs demonstrated higher J₀ values than the La₂Mo₂O₇ and MWCNTs electrodes, and a maximal J₀ value for the La₂Mo₂O₇/MWCNTs CE was observed.Obviously, the order of J₀ is La₂Mo₂O₇/MWCNTs (0.416 4 mA·cm^-2) > La₂Mo₂O₇@MWCNTs (0.199 1 mA·cm^-2) > MWCNTs (0.158 6 mA·cm^-2) >La₂Mo₂O₇ (0.019 96 mA·cm^-2). This order indicates that the La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs composites possessed higher diffusion velocities and better electrocatalytic activities for the reduction of I₃^- than the MWCNTs and La₂Mo₂O₇ CEs.This is consistent with the trend of the cathodic peak current shown in the CV curve.The J₀ value can be directly calculated using Eq.(1):

  $ {J_0} = \frac{{RT}}{{nF{R_{{\rm{ct}}}}}} $

  (1)

  Figure 6

  

  Figure 6.  Tafel plots of symmetrical cells between -0.7~0.7 V with a scan rate of 10 mV·s^-1 at room temperature

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  Where R is the gas constant, T is the absolute temperature (K), n is the number of electrons related to the electrochemical reduction reaction in the reduction of I₃^-.The J₀ value is inversely proportional to the R_ct value.A larger J₀ value indicates more electrons migrating through the CE/electrolyte interface, and this thereby indicates that the catalytic film has a faster electron transfer capability.Tafel polarization results match well with EIS and CV results.The experimental results clearly reveal that the composite of La₂Mo₂O₇ and MWCNTs obtained the higher charge transfer between I₃^- ions and the La₂Mo₂O₇/MWCNTs surfaces and the rapid redox transfer reaction of I₃^-/I^- in a dye-sensitized solar cell system.

  The La₂Mo₂O₇/MWCNTs CEs demonstrated higher PCE, J_sc and V_oc of CEs values than that of Pt, La₂Mo₂O₇, La₂Mo₂O₇@MWCNTs and MWCNTs CEs from J-V curves.The maximal PCE value was obtained for La₂Mo₂O₇/MWCNTs composite, which was 6.09%.The CV results were in good agreement with the PCE results from J-V measurements.From the CV curves, La₂Mo₂O₇/MWCNTs had shown the lowest ΔE_p and highest I_P.The Tafel polarization results indicated that La₂Mo₂O₇/MWCNTs CEs obtained the higher J₀, as compared to other CEs, which indicated a higher catalytic activity of composite CEs.According to Eq.(1), the R_ct was inversely proportional to J₀.We can deduce that R_ct of composite CEs was higher than that of La₂Mo₂O₇ and MWCNTs, which is in good agreement with the experimentally measured R_ct from EIS.Compared with La₂Mo₂O₇@MWCNTs, La₂Mo₂O₇/MWCNTs enhanced catalytic active sites, increased exchange current density and load resistance reduction at CE/electrolyte interface.

  The CV, EIS, Tafel, and J-V characterizations demonstrated that the La₂Mo₂O₇/MWCNTs CEs could catalyze the regeneration of the I₃^-/I^- couple, as effec-tively as Pt.In addition, a desirable catalyst should possess robust stability along with high catalytic activity.Thus, herein, we checked the electrochemical stability of the La₂Mo₂O₇/MWCNTs CEs via conse-cutive CV cycling (Fig. 7).After 10 consecutive CV cycles, minor current density attenuation or no peak position shift was observed, this demonstrates that La₂Mo₂O₇/MWCNTs was stable and coexist with the I₃^-/I^- redox couples.

  Figure 7

  

  Figure 7.  CVs of La₂Mo₂O₇/MWCNTs CE for the I^-/I₃^- electrolyte 1st and 10th cycle

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  3.   Conclusions

  Composite materials of La₂Mo₂O₇ with MWCNTs were successfully synthesized using a high-temper-ature solid-state reaction.La₂Mo₂O₇ was modified on the surface of MWCNTs to synthesize La₂Mo₂O₇/MWCNTs and doped in MWCNTs to synthesized La₂Mo₂O₇@MWCNTs.Both composite materials served as Pt-free catalytic materials for CEs for efficient DSSCs.MWCNTs and La₂Mo₂O₇ combined to expand the surface area, increase the exchange current density of the composite materials, and reduce the load resistance at the counter electrode/electrolyte interface.MWCNTs act as both a carrier and a catalyst.Power conversion efficiency values of 6.09% and 4.84% were obtained for La₂Mo₂O₇/MWCNTs and La₂Mo₂O₇@MWCNTs, respectively, using each as a counter electrode, and these values are superior to those of Pt (4.54%), La₂Mo₂O₇ (0.87%), and MWCNTs (3.94%) toward the reduction of I₃^-/I^- ions.Thus, after an integrated analysis of the CV, EIS, and Tafel polarization results, we conclude that the La₂Mo₂O₇/MWCNTs hybrids are potential catalysts for replacing Pt for future large-scale fabrication of DSSCs.

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  Figure 1  XRD patterns of the prepared MWCNTs, La₂Mo₂O₇ and La₂Mo₂O₇@MWCNTs

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  Figure 2  SEM images of the prepared MWCNTs (a), La₂Mo₂O₇ (b), La₂Mo₂O₇@MWCNTs (c) and La₂Mo₂O₇/MWCNTs (d)

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  Figure 3  J-V characteristics of DSSCs with different samples as CEs under a light intensity of 100 mW·cm^-2

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  Figure 4  CV curves of various CE electrodes for the I^-/I₃^- electrolyte

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  Figure 5  Nyquist plots for symmetrical cells based on various CEs

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  Figure 6  Tafel plots of symmetrical cells between -0.7~0.7 V with a scan rate of 10 mV·s^-1 at room temperature

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  Figure 7  CVs of La₂Mo₂O₇/MWCNTs CE for the I^-/I₃^- electrolyte 1st and 10th cycle

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  Table 1.  Photovoltaic parameters for the DSSCs assembled with various CEs

  CE	Voc / mV	JSC / (mA·cm-2)	FF	PCE / %
  Pt	779	11.63	0.50	4.54
  MWCNTs	643	9.75	0.63	3.94
  La2Mo2O7	777	8.88	0.13	0.87
  La2Mo2O7@MWCNTs	710	10.88	0.63	4.84
  La2Mo2O7/MWCNTs	786	12.19	0.64	6.09

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