Anchoring Spinel MnCo2S4 on Carbon Nanotubes as Efficient

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Anchoring Spinel MnCoS on Carbon Nanotubes as Efficient Counter Electrodes for Quantum-Dot Sensitized Solar Cells Wenhua Li, Lei He, Jing Zhang, Bin Li, Qianqiao Chen, and Qin Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04309 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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The Journal of Physical Chemistry

Anchoring Spinel MnCo2S4 on Carbon Nanotubes as Efficient Counter Electrodes for Quantum-Dot Sensitized Solar Cells

Wenhua Li, Lei He, Jing Zhang, Bin Li, Qianqiao Chen*, Qin Zhong * Corresponding author: Nanjing University of Science and Technology, Nanjing, People’s Republic of China, 210094 E-mail: [email protected] Abstract：Ternary spinel MnCo2S4 was anchored on CNTs via two-step method of precursor preparation followed by ion exchange, and employed as efficient counter electrodes for quantum dot sensitized solar cells. Electrochemical tests show that the interface charge transfer resistance (Rct) of MnCo2S4 is only 2.86 Ω and the Rct of MnCo2S4/CNTs further decrease to 1.09Ω, suggesting both the MnCo2S4 and its composite exhibit excellent catalytic activity for Sn2- reduction. The power conversion efficiencies of QDSSCs assembled CdS/CdSe QD photoanodes with MnCo2S4 and MnCo2S4/CNTs counter electrodes reach 2.98% and 4.85%, respectively. The better photovoltaic performance of the QDSSC based on MnCo2S4/CNTs is mainly ascribed to synergistic effect of the excellent electrocatalytic activity of MnCo2S4 itself and conductivity of CNTs. The addition of CNTs not only significantly reduces the size of MnCo2S4 and increases catalytic activity sites, but also forms a crosslinked conductive network to accelerate the delivery of electrons. Due to the excellent reducing ability for Sn2-, MnCo2S4/CNTs is expected to be a competitive counter electrode material for efficient QDSSCs. 1. Introduction Quantum dot sensitized solar cells (QDSSCs) employing narrow band inorganic semiconductor quantum dots (QDs) as sensitizers were developed on the basis of dyesensitized solar cells（DSSCs）with the theoretical photoelectric conversion efficiency of 44%, breaking through the Shockley-Queisser limit (31%).

1-3

As an alternative to

traditional organic sensitizers, inorganic semiconductors QDs possess the unique advantages, including high extinction coefficient, quantum confinement effect, hot electron extraction and multi-exciton effect, so it has been widely concerned by researchers. 4-6 In the past few years, extensive efforts have been devoted to improve the performance of

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QDSSCs, including material improvement of quantum dot, electrolytes and CE, and a large number of remarkable achievements has achieved.

7-9

Through the exploration of the

superior colloidal QD sensitizers and interface modification engineering, the power conversion efficiency of QDSSCs has been reached 12.75% 10. Platinum is the most efficient CE in DSSCs and other electrochemical devices owing to extremely high catalytic activity and chemical stability, but in S2-/Sn2- polysulfide electrolyte systems, platinum and other precious metal electrodes have strong bonding with S2- ions, which makes it difficult for S2- to detach from the surface of electrode, showing poor catalytic activity for Sn2-reduction in QDSSCs.

11-12

Therefore, a large number of

researches are devoted to explore CE materials with high catalytic activity and low cost CE for QDSSCs. The results show that many transition metal sulfides including Cu2S, CuS, PbS, CoS, NiS, CuInS2, Co3S4, etc., as the CE materials of QDSSCs, show good catalytic activity for S2- reduction, the open circuit voltage and short circuit current of the cell, compared with Pt CE, there is a significant improvement. 13-19 Ternary spinel-type metal sulfides (AB2S4) have attracted extensive attention due to excellent chemical and physical properties and have become one kind of important functional material in recent year. 20-21 The materials generally possess versatile structure, which is able to accommodate various transition metal cations including Zn, Co, Ni, Cu, etc. 22-25 Therefore, the manageable chemical composition may provide vast opportunities to manipulate their physico-chemical properties.

26

For example, the high conductive

NiCo2S4 urchin-like nanostructures was prepared as the electrode for pseudocapacitors, and the structure exhibits ultra-high rate, long life and high capacitance retention capability. 27

A spinel-structured MnCo2S4 nanowire array on Ti network by vulcanization of

MnCo2O4 was prepared for effective oxygen evolution reaction under alkaline conditions. 28

MnCo2S4 nanosheets are grown on carbon paper and MnCo2S4@Ni-Co-S core/shell

double-layer nanocomposites via electrodeposition followed by low temperature vulcanization for application in Li-O2 batteries 20 and supercapacitor. 29 Elshahawy et al. 30

prepared a controlled sulfadiazine MnCo2S4 nanostructure with ideal hollow structure

by vulcanization process for high performance hybrid supercapacitors. Ternary spinel sulfides exhibit superior catalytic activities toward polysulfide reduction, although there are fewer reports about the material as CEs for QDSSCs. Xiao et al. 31 firstly


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reported the preparation of hollow metal sulfide single crystalline arrays (NiCo2S4) as CEs for QDSSCs, and it achieved a power conversion efficiency of 4.22%. Then, Luo et al. 22 prepared NiCo2S4 nanoparticle CEs by a simple low-temperature hydrothermal sulfurization, and the QDSSCs based on CdS/CdSe QDs and NiCo2S4 CE obtained an efficiency of 3.30%. The manganese-cobalt sulfide prepared by Vijayakumar et al. obtained 3.22% efficiency as the CE of QDSSCs, which further indicates that manganesecobalt sulfide as CEs of QDSSCs is prospective. 21 In present work ， a MnCo2S4/CNTs composite was designed and prepared as an efficient CE for QDSSCs. Firstly, CNTs was treated with mixed acid to produce active groups such as -COOH and -OH on the surface of CNTs to promote the nucleation of MnCo oxide precursor, and then, MnCo2S4/CNTs composite was obtained from precursor by ion exchange, as shown in Fig. 1. The composite fully combines advantage of MnCo2S4 with CNTs and shows excellent reduction ability for Sn2-. Fig. 2 summarizes the charge transfer resistance parameters (Rct) of ternary spinel sulfide and photovoltaic parameters of QDSSCs based the same CdS/CdSe QDs and polysulfide with this work. 21-22, 31-32 These results reveal that the MnCo2S4/CNTs electrode of present work outperforms other electrodes in terms of four categories shown in Fig. 2, including Rct, short-circuit current density (Jsc), open-circuit voltage (Voc) and power conversion efficiency (η).

Fig. 1. Schematic illustration of the synthesis of MnCo2S4/CNTs.



Fig. 2. Comparisons of the Rct and photovoltaic parameters of MnCo2S4/CNTs with other similar electrodes reported in the literatures.

2. Experimental section If without special stated, all chemicals used in this work were purchased from Aladdin (Sigma-Aldrich) and used without any further purification. 2.1 Synthesis Mn-Co/CNTs precursor The raw multi-walled CNTs (ca. 15 nm in diameter, purchased from Shenzhen Nanotech Port Co. Ltd., China) were first refluxed in the concentrated H2SO4/HNO3 (3: 1, V: V) solution at 80 ℃ for 1h and then washed with distilled water until the pH value reached 7. At last, the functional CNTs were obtained by filtrating the above dispersion through a porous polytetraflouroethylene membrane and drying in air. 0.422 g Mn(OAc)2·4H2O, 0.858 g Co(OAc)2·4H2O, 3.000 g polyvinyl pyrrolidone and 0.128 g functional CNTs were added to 200 ml ethanol under vigorous stirring. The pink solution was then heated and refluxed for 2h at 85 ℃. After cooling down to room temperature, the black product was collected by centrifugation, and washed with deionized water and ethanol until neutral, then dried at 50 ℃ for 12 h at vacuum. No CNT was added to the precursor when pure MnCo2O4 was prepared. 2.2 Synthesis MnCo2S4/CNTs 0.08 g of above precursor was dispersed in deionized water (40 ml) by ultrasonic treatment. Then, 0.39 g of Na2S was added under stirring. After completely mixed, the


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mixture was transformed into a Teflon-lined stainless-steel autoclave and heated to 120 ℃for 6 h. After cooling down to room temperature, the black product was collected by centrifugation and washed with deionized water and ethanol until neutral, then dried at 50 ℃ for 12 h at vacuum. 2.3 Fabrication of counter electrodes The as-prepared electrode material was deposited onto FTO substrates by drop-coating. First, 20 mg of the sample was dispersed in 5 ml of ethanol by probe ultrasonication for 15 min to obtain a uniformly dispersion suspension. Then, 70 μ L of the suspension was dropwise dispersed onto the FTO glass for 5 times with FTO being heated to 60 ℃, the active area of the CE is 0.16 cm2. 2.4 Fabrication of photoanodes and assembly of QDSSCs Porous TiO2 films were fabricated from commercial P25 TiO2 nanopowder. Commercially available F-doped tin oxide (FTO) glass was used as transparent conducting substrates for the preparation of TiO2 photoanodes. TiO2 slurry consisting of P25, ethyl cellulose and terpineol was spin-coated onto FTO glass and sintered at 500 ℃ for 30 min. Then, the TiO2 film was dipped in 40 mM TiCl4 solution at 70 ℃ for 30 min and annealed at 500 ℃ for 30 min. CdS QDs and CdSe QDs were deposited by SILAR process described previously. 33-34 Briefly, TiO2 films were immersed into a solution of Cd(CH3COO)2 (0.1 M) for 1 min, rinsed with deionized water, and dried with an air gun and then dipped for 1 min into 0.1 M aqueous Na2S, rinsed with deionized water, and dried with an air gun. The process was repeated four cycles. Then, the TiO2/CdS electrodes were dipped into a solution of Cd2+ (0.1 M) for 1 min at room temperature and then immersed into 0.2 M aqueous Na2SeSO3 for 0.5 h at 50 ℃, followed by rinsing with deionized water and drying. The process was repeated up to eight cycles and TiO2/CdS/ CdSe electrodes were obtained. At last, the two layers ZnS were deposited by SILAR again to obtain TiO2/CdS/CdSe/ZnS photoanodes. The fabricated TiO2/CdS/CdSe/ZnS photoanodes and CEs were sealed to obtain sandwich QDSSCs using insulation tapes. The internal space of the cells were filled with a redox electrolyte containing 2 M Na2S, 2 M S, and 0.2 M KCl in a methanol to water volume ratio of 7:3.



2.5 Characterization A ﬁeld-emission scanning electron microscopy (FESEM, ZEISS SIGMA 500) attached energy-dispersive spectroscopy (EDS) was performed to investigate the morphology and elemental composition of the composite. The sample structure was characterized by Transmission electron microscopy (TEM, JEM2100F). The specific surface area was measured using the nitrogen adsorption–desorption technique (Micromeritics ASAP 2460). Crystallographic information of powders was studied by Xray diffraction (XRD, BrukerAXS GmbH, D8ADVANCE, Cu kα radiation). Raman spectra were obtained by Raman microscope (Sierra IM-52 of Snowy Range Instruments) using a laser wavelength of 785nm. The chemical state and surface composition were analyzed by X-ray photoelectron spectroscopy (XPS, PHI Quantera Ⅱ ) using a monochromatized Al Kα X-rays, at a base pressure of 5×10-10 Torr. Electrochemical impedance spectroscopy (EIS), Tafel polarization measurements (Tafel), cyclic voltammetry tests (CV) and photovoltaic performances were carried out using an electrochemical system (Ivium-N-Stat Multichannel potentiostat). The current–voltage characteristics (J–V curves) of QDSSCs were recorded under 1 sun illumination (AM 1.5 G, 100 mW·cm-2) using a HGILX500 technologies (China) solar simulator, which was calibrated by a commercial available standard crystalline silicon solar cell. 3. Results and discussion 3.1 Morphological and structural characterization Morphological characterizations of MnCo2S4/CNTs and MnCo2S4 are shown in the Fig. 3. CNTs form a crosslinked conductive network in the composite, and the uniform MnCo2S4 nanoparticles are anchored to the network, as shown in SEM (Fig. 3a and c) and TEM (Fig.3e-f). Compared with pure MnCo2S4 shown in Fig. 3b and d, the addition of CNTs significantly reduces the MnCo2S4 size and attenuates the aggregation of fine particles, and the sample specific surface areas increase from 36.88 m2·g-1 (MnCo2S4) to 53.38 m2·g-1 (MnCo2S4/CNTs, Fig. S1). The composite structure not only contributes to more catalytic activity sites, but also accelerates the delivery of electrons.


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Fig. 3. SEM images of (a, c) MnCo2S4/CNTs and (b, d) MnCo2S4; (e, f) TEM images of MnCo2S4/CNTs

Raman spectroscopy is a powerful tool for the characterization of material structure, molecular vibration, and rotational energy levels.

35

In the CNTs and composite spectra,

two sharp peaks (ca. 1291 and 1575 cm-1) are corresponding to D and G bands of CNTs, respectively. 36 In the MnCo2S4 and composite spectra as shown in Fig. 4b , the sharp peak located at 661 cm-1 is assigned to Mn-S lattice vibrations

37-38,

and two broad peaks at

457 and 487 cm-1 are from Co-S vibration, 20, 35 indicating all the three peaks are derived



from MnCo2S4. XRD tests were performed to identify the crystal structure of the samples. The diffraction peaks at 27.08°, 31.04°, 37.68°, 39.32° and 54.48° of MnCo2S4/CNTs are assigned to (220), (311), (400), (211) and (440) crystal facets of the MnCo2S4, 20, 23 and the weak peaks located at 26.04° and 43.52° arises from CNTs (Fig. S4b). Both Raman and XRD results indicate that the composite of MnCo2S4 and CNTs was successfully synthesized via two-step method.

Fig. 4. (a) Raman spectra of CNTs, MnCo2S4, and MnCo2S4/CNTs, (b) enlargement of MnCo2S4 and MnCo2S4/CNTs spectra.

In order to further obtain chemical composition details, the X-ray photoelectron spectroscopy (XPS) of MnCo2S4/CNTs was characterized, as shown in Fig. 5. The full spectrum of the sample shows in Fig. 5a, and five main peaks observed are corresponding to Mn, Co, S, C and O. The O 1s peak arises from the surface oxidation of MnCo2S4/CNTs due to the exposure to air. 20, 35, 39 By using the Gaussian–Lorentz mix fitting method, two spin-orbit doublets and one shake-up satellite (denoted as ‘Sat.’) can be observed in Fig. 5b. The peaks located at 641.7eV and 653.4eV are assigned to Mn2+, and peaks at 643.7eV and 654.5 eV are ascribed to Mn3+.

20, 23, 40

Similarly, the Co 2p spectrum is better fitted

into two spin-orbit doublets (Co2+ and Co3+) and two shake-up satellites, 21, 27 as shown in Fig. 5c. In the high-resolution S 2p energy spectrum (Fig. 5d), the peaks at 161.5 eV and 162.7 eV are associated with S 2p3/2 and S 2p1/2, which are the typical metal-sulfur bond in the ternary metal sulfide. The appearance of additional satellite peak located at 168.44 eV is attributed to surface sulfur with high oxide state, such as sulfate. 20, 30, 41-42 These analyses demonstrate the existence of MnCo2S4 in composite, further validating the results of Raman and XRD.


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Fig. 5. (a) XPS survey spectrum of MnCo2S4/CNTs and high-resolution XPS of (b) Mn 2p, (c) Co 2p and (d) S 2p.

3.2 Electrochemical characterization In order to better evaluate the electrocatalytic activity of electrodes, cyclic voltammetry (CV) tests were performed with a three-electrode system in an aqueous solution of 0.1 M Na2S, 0.1 M S, and 0.1 M KCl at a scan rate of 5 mV·s-1. Pt and Ag/AgCl electrode are used as counter and reference electrode, respectively. High peak current density and low peak-to-peak separation (Epp) are prominent features of high electrocatalytic capability for polysulfide reduction.

43-44

As shown in Fig.6, the reduction peak current density of

composite is the highest, moreover, the Epp value (0.60V) of MnCo2S4/CNTs is lowest. These results suggest that the addition of highly conductive CNTs greatly improves the electrocatalytic performance towards the polysulfide redox couple for QDSSCs.



Fig. 6. (a) CV of CNTs, MnCo2S4 and MnCo2S4/CNTs in polysulfide redox couples, (b) Enlarged part of the MnCo2S4 and MnCo2S4/CNTs electrodes CV plots.

An electrochemical impedance spectroscopy (EIS) experiment was carried out on a symmetric dummy cell fabricated with two identical CEs to confirm the catalytic activities of CEs, and the Nyquist plots were shown in Fig. 7a. The nonzero intersection of the semicircle with the real axis at high frequency is the series impedance (Rs), which includes the connection of the electrode material with the FTO and the ohmic impedance of the external circuit. The semicircle at middle frequency represents the charge transfer resistance (Rct) at the electrode/electrolyte, which reflects the catalytic activity of a CE. The values of Rs and Rct fitted by Zsimplewin software are summarized in Table 1. The Rct value (2.86 Ω) of pure MnCo2S4 is extremely small, indicating its good catalytic activity. However the Rs is 7.84 times of Rct value for MnCo2S4 in the study, indicating the controlling resistance is Rs rather than Rct at the side of the CE. So，it is necessary to reduce the Rs value to improve performance of CE side. The Rs of MnCo2S4/CNTs is 12.98% lower than MnCo2S4 CE, and the Rct value is further reduced to 1.09Ω, exhibited the excellent catalytic activity for Sn2- reduction. Fig. 7b represents the Bode spectra of symmetric dummy cells from CEs. The lifetimes of electrons participating in the Sn2- reduction reaction (τ1) can be represented as τ1 = 1/2πfp, 45

where fp is the peak frequency in Bode spectrum. The order of the calculated τ1 values is

CNTs >MnCo2S4>MnCo2S4/CNTs. The MnCo2S4/CNTs CE has shortest lifetime, implies that the required time for the Sn2- reduction reaction by the CE is the shortest, indicating the MnCo2S4/CNTs CE possesses the best catalytic activity.


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Fig. 7. (a) Nyquist plots and (b) Bode plots of symmetric dummy cells.

The as-prepared composite CEs were assembled into symmetrical dummy cells, and Tafel polarization tests were carried out and results are shown in Fig. 8. The exchange current density (J0) is derived as the intercept of the extrapolated linear region of the curves when overpotential is zero. The J0 values are also summarized in Table 1, which confirm the trends observed for Rct in Table 1. The higher exchange current density shows the superior electric catalysis of the MnCo2S4/CNTs compared to the other CEs. This is consistent with the EIS experimental results.

Fig. 8. Tafel polarization curves of the symmetric dummy cells. Table 1 Electrochemical parameters from EIS and Tafel tests for different CEs CEs

Rs (Ω)

Rct (Ω)

J0 (mA·cm-2)

CNTs

19.89

22.06

4.07

MnCo2S4

22.42

2.86

7.18

MnCo2S4/CNTs

19.51

1.09

12.01



3.3 Photovoltaic performance of QDSSCs The J-V curve is an important basis for judging the overall performance of QDSSCs. The cell was assembled with a TiO2/CdS/CdSe/ZnS photoanode and a CE, and was placed in a simulated sunlight of 100 mW·cm-2 for J-V curve test. The Jsc, Voc, FF, and η can be obtained from the curve, as shown in Fig. 9 and Table 2. The QDSSC based MnCo2S4 CE has a good performance with the Jsc, Voc, FF and η are 16.20mA·cm-2, 0.46V, 0.40 and 2.98%, respectively, while all these performance parameters have improved for QDSSCs based MnCo2S4/CNTs CE, with Jsc of 18.45 mA·cm-2, Voc of 0.58V, FF of 0.45 and η of 4.85% respectively. This is mainly due to the excellent reduction ability of MnCo2S4/CNTs for Sn2-, resulting in a larger concentration gradient of S2-/Sn2- between the photoanode and CE, accelerating diffusion of S2-/Sn2- in electrolyte, thus weakening recombination rate at the photoanode/electrolyte interface, which finally increases the Jsc and Voc values of MnCo2S4/CNTs-based QDSSC.

Fig. 9. J–V plots of QDSSCs based on various CEs.

Multi-cycle successive CV scanning can evaluate the electrochemical stability of CE material.

46

Fig. 10 shows 50 cycles continuous CV scanning of MnCo2S4/CNTs and

MnCo2S4 at a scan rate of 100 mV·s-1. The insets show the CV plots only for the 1st and 50th cycle. After 50 cycles of continuous scanning, the MnCo2S4/CNTs CE maintained almost the same CV curves, while the voltage and current density of pure MnCo2S4 have changed a lot. These demonstrate MnCo2S4/CNTs CE excellent electrochemical stability in the polysulfide electrolyte system and the addition of CNTs enhances the stability of MnCo2S4/CNTs.


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Fig. 10. Fifty successive CV cycles for (a) MnCo2S4/CNTs and (b) MnCo2S4 in polysulfide electrolyte. Table 2 Photovoltaic parameters and EIS results of QDSSCs with different counter electrodes under one sun. CEs

Jsc (mA · cm-2)

Voc (V)

FF

η (%)

CNTs

14.63

0.41

0.39

2.37

MnCo2S4

16.20

0.46

0.40

2.98

MnCo2S4/CNTs

18.45

0.58

0.45

4.85

4. Conclusion A ternary spinel MnCo2S4 and CNTs composite was prepared by depositing a precursor on CNTs and followed by ion exchange. The experimental results show that MnCo2S4 itself has good catalytic activity for Sn2- reduction. For MnCo2S4/CNTs CE, it exhibits lower Rs and Rct and higher J0 for Sn2- reduction due to the addition of CNTs. The power conversion efficiency of the cell with MnCo2S4/CNTs CE is the highest, reaching 4.85%, increased by 62.75% compared with the QDSSCs assembled with pure MnCo2S4 CE. This is mainly because the addition of CNTs significantly reduces the size of MnCo2S4 and aggregation of crystals particles, and the CNTs form a conductive crosslinked network, showing superior catalytic activity for Sn2- reduction. In this work, the Rs rather than the Rct is the controlling resistance of Sn2- reduction process. Reducing the Rs value, including reducing the square resistance of the FTO and improving the contact between CE material and FTO, may be the most rewarding strategy to improve the performance of CE side and further improve the photovoltaic performance of the QDSSCs. This work reveals that MnCo2S4/CNTs are expected to be highly efficient composite CEs for QDSSCs. Supporting Information



N2 adsorption-desorption isotherm curves and of MnCo2S4 and MnCo2S4/CNTs, supplementary TEM images of MnCo2S4/CNTs, XRD patterns of CNTs, MnCo2S4 and MnCo2S4/CNTs. Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest Acknowledgments This work was supported by provincial-level scientific research training for undergraduates provided by Nanjing University of Science and Technology, and the National Natural Science Foundation of China (51578288). References 1.

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TOC Graphic


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TOC Graphic



Fig. 1


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Fig. 2



Fig. 3


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Fig. 4



Fig. 5


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Fig. 6



Fig. 7


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Fig. 8



Fig. 9


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Fig. 10


Anchoring Spinel MnCo2S4 on Carbon Nanotubes as Efficient

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