Ti mesh films as efficient counter electrodes for quantum dots sensitized solar cells

Solar Energy 178 (2019) 108–113

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PbS decorated multi-walled carbon nanotube/Ti mesh films as efficient counter electrodes for quantum dots sensitized solar cells

T

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Qingqing Peia,b, Zeng Chena, , Shuo Wangb, Di Zhangb, Pin Mab, Shengjun Lia, Xiaowen Zhoub, ⁎ Yuan Linb, a b

Henan Key Laboratory of Photovoltaic Materials, Henan University, Kaifeng 475001, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Quantum dots sensitized solar cells Counter electrodes Flexible meshes Carbon nanotubes PbS

Multi-walled carbon nanotubes (MCNTs) were doctor-bladed on Ti meshes to prepare flexible counter electrodes for quantum dots sensitized solar cells. PbS was decorated on MCNTs surfaces through successive ionic layer adsorption and reaction to improve the catalytic activities of redox reactions in polysulfide electrolytes. The electrochemical catalytic activities of obtained PbS/MCNT counter electrodes were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization curves. After optimization, the QDSCs based on PbS/MCNTs counter electrodes delivered photovoltaic conversion efficiencies reaching 6.39%, values 40% higher than those of pure MCNTs CEs.

1. Introduction Quantum dots-sensitized solar cells (QDSCs) are promising candidates for third generation solar cells. The QDSCs attracted increasing attention due to their advantages, such as high theoretical power conversion efficiency, simple preparation process, and low cost (Duan et al., 2015; Pan et al., 2018; Wang et al., 2018; Du et al., 2016). QDSCs are mainly composed of three components: photoanode, polysulfide electrolyte, and counter electrode (CE). The main role of CE is to collect electrons transferred from the external circuit to catalyze the reduction of Sn2− into S2− (Guo et al., 2016; Li et al., 2014). Over the past few years, photoanodes are found to influence not only the open circuit voltage but also short current density of the cells. However, only a few studies have been carried out on polysulfide electrolytes and CEs. Pt-based CEs have often been used in QDSCs but now mostly employed as reference electrodes due to their high cost and poor catalytic activities in polysulfide electrolytes (Wu et al., 2013). Carbon materials, such as activated carbon, carbon nanotube and graphene are widely utilized as CE materials in QDSCs due to easy preparation, high stability, and low cost (Jiao et al., 2017; Zhang et al., 2017; Gopi et al., 2016; Sahasrabudhe et al., 2016). Another kind of active materials used in QDSCs based CEs consists of metal sulfides, including PbS, Cu2S, CoS, and NiS (Jiang et al., 2014; Tachan et al., 2011; Mani et al., 2014; Gopi et al., 2015; Kim et al., 2014). The metal sulfides CEs showed elevated catalytic activity in polysulfide electrolytes. CEs composites based on

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metal sulfides decorated carbon might combine both advantages of carbon and metal sulfides (Ghosh et al., 2016; Guo et al., 2014; Gopi et al., 2017; Yang et al., 2012; Zhang et al., 2013). For instance, Ghosh et al. mixed CuxS nanoplates with graphene oxide nanoribbons to prepare composite CEs for QDSCs delivering favorable energy band alignment with redox potential of polysulfide electrolyte (Ghosh et al., 2016). Guo et al. grew cobalt sulfide nanotubes on carbon fiber to form flexible CEs for QDSCs (Guo et al., 2014). Gopi et al. deposited PbS, CuS, CoS and NiS on MCNTs by chemical bath deposition to form MCNT/NiS CEs with superior photon energy conversion efficiency (PCE) and remarkable stability (Gopi et al., 2017). Yang et al. prepared PbS/carbon black with no degraded PCE over 1000 h under room conditions (Yang et al., 2012). Their data revealed that CuInS2/ carbon CEs possessed similar stability (Zhang et al., 2013). The conductive substrate is another factor worth considering during preparation of fine flexible CEs. Ti-mesh showed many advantages as flexible substrates for CEs in QDSCs, such as low sheet resistance, fine chemical stability, and superior bending quality. Du et al. prepared Ti mesh supported mesoporous carbon CEs with improved PCE of CdSe0.65Te0.35 QDSCs to certified record of 11.16% (Du et al., 2016). They also assembled full flexible QDSCs based on Ti mesh substrates with PCE of 5% (Du et al., 2017). MCNTs are highly active materials for CEs of QDSCs owing to their superior conductivities, elevated specific surface areas, and excellent stability in polysulfide electrolytes. However, MCNTs have relatively

Corresponding authors. E-mail addresses: [email protected] (Z. Chen), [email protected] (Y. Lin).

https://doi.org/10.1016/j.solener.2018.12.022 Received 3 October 2018; Received in revised form 21 November 2018; Accepted 9 December 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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2.4. Fabrication of QDSCs

poor catalytic activities in polysulfide electrolytes (Gopi et al., 2018). By comparison, lead sulfide (PbS) shows super catalytic activity in polysulfide electrolytes. In this study, PbS decorated MCNTs composite films deposited on Ti meshes were prepared through in situ deposition of PbS quantum dots by successive ionic layer adsorption and reaction technique (SILAR). The Ti meshes could provide 3-D tunnels for electron transfer from the external circuit to CEs. In addition, since Ti meshes were ca. 100 μm, a large amount of MCNTs will be deposited on Ti mesh grid. This would provide large numbers of catalytic sites. The obtained composite CE based on PbS/MCNTs showed excellent catalytic performances due to synergistic effect between the MCNTs and PbS quantum dots. The prepared PbS/MCNT CEs were also assembled with CdS/CdSe co-sensitized TiO2 electrode to yield QDSCs, where PCE was tested under 100 mW cm−2 at AM 1.5 illumination.

The QDSCs were sandwiched with CdS/CdSe co-sensitized TiO2 films and PbS/MCNT flexible CEs. The polysulfide electrolyte included 0.5 M sulfur, 2.0 M Na2S and 0.2 M KCl all dissolved in a mixture of ethanol and DI-water (volume ratio 3:7). The polysulfide electrolyte was filled between the two electrodes. 2.5. Measurements and characterizations The morphologies and structures of PbS/MCNTs flexible CEs were identified by field emission scanning electron microscopy (FESEM, JSM-7001F) and transmission electron microscopy (TEM, JEM 2100). The electrochemical impedance spectra (EIS) were collected on a potentiostat (Solartron SI1287) connected to frequency response analyzer (Solartron 1255B) at 0 V bias potential and 20 mV amplitude over the frequency range from 0.1 Hz to 100 kHz. The Tafel polarization curves were also recorded on the same electrochemical workstation. The testing cells used for EIS and Tafel measurements were dummy cells assembled with two symmetric CEs. Cyclic voltammetry (CV) was carried out in a three-electrode system containing aqueous solution composed of 0.5 M Na2S and 0.5 M S at the scan rate of 0.01 V S−1. The photovoltaic performances of QDSCs were measured using a Keithley 2440 source meter under AM 1.5G illumination from a Newport Oriel solar simulator at intensity of 1 Sun. The incident light intensity was calibrated using a standard Si solar cell obtained from Newport Oriel. The active cell area of the assembled QDSCs was 0.25 cm2.

2. Experimental 2.1. Preparation of MCNTs films The MCNTs were purchased from Beijing DK nano technology and used without further purification. MCNTs paste was obtained by dissolving ethyl cellulose (0.2 g) in 8 mL terpineol and 0.5 mL of titanium (IV) isopropoxide. The solution was then stirred for 24 h to form the binder solution. MCNTs powder (0.1 g) was added into the above binder solution (0.8 mL) and ultrasonically dispersed for 1 h. The obtained MCNTs paste was then doctor-bladed on Ti mesh substrates (Kangwei Technology, 80 meshes, 100 μm thickness), sequentially cleaned with acetone, de-ionized water, and ethanol for 10 min. The obtained MCNTs based films were dried in an oven at 100 °C for 5 min and subsequently heated at 450 °C for 30 min in tube furnace under Ar atmosphere.

3. Results and discussion Fig. 1 shows the surface and cross-sectional images of pure MCNTs based films deposited on Ti meshes. MCNTs appeared uniformly filled into the Ti mesh grids (Fig. 1(a)). The surface of pure MCNTs film looked well-distributed to form smooth surfaces. Variable large amounts of pores in MCNTs film could be observed in Fig. 1(b). These could offer substantial room for deposition of PbS QDs and penetration of the electrolyte. Fig. 1(c) shows the cross-sectional images of pure MCNTs film. The thickness of MCNTs film was estimated to ca. 146 μm. Also, MCNTs were firmly combined with Ti mesh and did not peel off from Ti mesh when the samples were cut into small strips. Fig. 1(d) represents a photo of bent MCNTs film. No cracks were observed over the surface. In sum, MCNTs/Ti mesh should be a good candidate for flexible QDSCs. PbS was decorated on MCNTs based film surfaces through SILAR. Fig. 2 shows the morphology of PbS decorated MCNTs/Ti meshes after different treatment times. Pure MCNTs appeared smooth with strongly intertwined entanglements formed with three-dimensional (3-D) network structure (Fig. 1(b)). Compared to pure MCNTs, the surface of MCNTs decorated with PbS was unsmoothly containing well-distributed small particles. Meanwhile, no unabsorbed PbS particles were present in the samples. Hence, the adopted preparation method was suitable for coating PbS on MCNTs. The MCNTs could provide a conductive frame network for PbS particles. On the other hand, PbS particles could improve the catalytic activity of MCNTs based electrodes. Also, numbers of PbS particles increased as SILAR time rose (Fig. 2(b–e)). Fig. 3 shows the TEM images of pure MCNTs and PbS decorated MCNTs. The MCNTs used in these experiments contained significant amounts of multi-walled structures (Fig. 3(a)). Some small PbS particles appeared on the surface of MCNTs after SILAR treatment. The size of most particles lied below 10 nm. These PbS particles were firmly fixed on MCNTs surfaces and well-distributed. Cyclic voltammetry (CV) was performed to evaluate the catalytic activities of MCNTs/Ti mesh films before and after PbS decoration (Fig. 4). The CV measurements were performed in a standard threeelectrode configuration using SCE as reference, Pt as counter, and PbS decorated MCNTs/Ti film as working electrode. An aqueous solution

2.2. Preparation of counter electrodes The PbS/MCNT composite counter electrodes (CEs) were prepared by successive ionic layer adsorption and reaction (SILAR) technique. Firstly, MCNTs based films were dipped in 0.1 M Pb(CH3COO)2 aqueous solution for 1 min, then rinsed with de-ionized (DI) water and dried in air. Secondly, the samples were immersed in 0.1 M Na2S·9H2O aqueous solution for 1 min then rinsed with DI water and dried in air. This procedure was considered as the 1st cycle and the process and was repeated for 2nd, 3rd, 4th or 5th cycles to yield respective samples denoted as, PbS-1/MCNT, PbS-2/MCNT, PbS-3/MCNT, PbS-4/MCNT, and PbS-5/MCNT. For comparison, PbS CE was also prepared. Firstly, Pb metal foil was polished with sandpaper and rinsed with DI water and ethanol. The clean Pb sheets were then acidized in H2SO4 solutions (volume (H2SO4): volume (H2O) = 1:1) for 1 h at 70 °C in an oven and rinsed with deionized water and ethanol. Next, the acidized Pb sheets were vulcanized in polysulfide electrolytes at 70 °C for 1 h. Finally, the obtained Pb CE were rinsed with deionized water and stored in a desiccator. 2.3. Preparation of photoanodes Mesoporous TiO2 films were prepared by doctor-blading method according to former reports (Zhao et al., 2012). The CdS QDs were deposited on TiO2 films by means of SILAR technique. Briefly, TiO2 films were dipped in aqueous solution of Cd(Ac)2 (0.1 M) for 1 min and rinsed with DI water. Next, TiO2 films were immersed in Na2S aqueous solution (0.1 M) for 1 min then rinsed with DI water. This procedure was repeated six times. The CdSe QDs were deposited onto CdS QDs sensitized TiO2 films through chemical bath deposition (CBD) at 5 °C for 5 h and 25 °C for 24 h, respectively. Finally, ZnS passivation layers (4 cycles) were deposited onto CdS/CdSe co-sensitized TiO2 films. 109

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Fig. 1. SEM images and photos of pure MCNTs films deposited on Ti meshes: (a) low magnification, (b) high magnification, (c) cross-section image, and (d) photo of bent sample.

Fig. 2. SEM images of surface morphology of PbS decorated MCNTs films: (a) PbS-1/MCNTs, (b) PbS-2/MCNTs, (c) PbS-3/MCNTs, (d) PbS-4/ MCNTs, and (e) PbS-5/ MCNTs. 110

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Fig. 3. TEM image of MCNTs before (a) and after PbS decoration (b).

Fig. 4. Cyclic voltammetry of different PbS/MCNT counter electrodes. Fig. 5. Electrochemical impedance spectra of different PbS/MCNTs and PbS counter electrodes, the line is the fitting results of the corresponding EIS data.

composed of 0.5 M Na2S and 0.5 M S was used as the electrolyte. The CV measurements were carried out from −1.2 V to 0.6 V at the scan rate of 0.01 V S−1. The precise redox mechanisms of polysulfide electrolytes are complex and not yet well understood. It is widely accepted that oxidation of S2− and reduction of Sn2− occur at the interface of QDs/electrolyte and CE/electrolyte, respectively. The CVs of different PbS/MCNTs composite counter electrodes are shown in Fig. 4. All CEs illustrated welldefined reduction peaks with corresponding oxidation peaks. The redox current peaks were assigned to reduction of Sn2− to S2− and oxidization of S2−, respectively. The reduction peak current of various CEs decreased in the following order: MCNT < PbS-1/MCNT < PbS-2/ MCNT < PbS-3/MCNT < PbS-5/MCNT < PbS-4/MCNT. On the other hand, the peak-to-peak separation (Epp) value might be applied to evaluate the electrocatalytic performances of CEs. The Epp value obviously decreased as PbS decoration rose. Epp of PbS-4/MCNT showed the minimum value, indicating that PbS-4/MCNT possessed the best electrocatalytic properties. The reason for the decrease in electrocatalytic properties at elevated SILAR more than 4 cycles should be related to occurrence of side reactions during SILAR processes, where few white materials appeared after the 4th SILAR cycle. Furthermore, the number of white materials increased as SILAR time enhanced. The white material was confirmed through XRD testing as PbSO4 (Supplemental Materials, Fig. S1). Note that PbSO4 showed negligible electrocatalytic properties towards reduction of Sn2−. Electrochemical impendence spectroscopy (EIS) is a powerful tool for investigation of charge transfer processes. The charge transfer resistance at the electrode/electrolyte interface indicating that the catalytic activities of counter electrodes could be obtained from the fitting of EIS data. EIS was carried out using sandwich type cells comprising of two symmetric CEs placed face to face. Fig. 5 shows the EIS plots (Nyquist type) of MCNTs/Ti mesh films before and after PbS decoration. The sheet resistance (Rs) of CEs can be determined at high

frequencies around 100 KHz. According to previous work, the first semicircle in frequency regions of 103–105 Hz could possibly be related to charge-transfer resistance (Rct1) at substrate/CE interface. This process might clearly be seen in Bode plots (Supplemental Materials, Fig. S2). The second semicircle should be attributed to charge-transfer processes occurring at the counter electrode/electrolyte (Rct2). The Rs showed no significant changes before and after PbS decoration. However, Rs value was lower when compared to that of PbS electrode, suggesting better electron transfer attributed to high electro-conductivities of Ti-mesh and MCNTs. The Rct1 of various counter electrodes showed no obvious changes. The EIS plots were fitted with equivalent circuit shown in inset of Fig. 4 except the data of Pt counter electrode (the equivalent circuit was shown in Fig. S3). The Rct2 of MCNTs, PbS-1/MCNTs, PbS-2/MCNTs, PbS-3/MCNTs, PbS-4/MCNTs, PbS-5/MCNTs and PbS CEs were estimated to ca. 100.0, 62.4, 44.0, 29.5, 6.1, 28.6 and 12.4 Ω, respectively. It is worth noting that Rct2 values gradually decreased as SILAR cycles in PbS decoration rose. However, Rct2 began to increase at SILAR cycles was above 4. To further evaluate the electrochemical catalytic abilities of CEs, Tafel polarization measurements were also performed. The test equipment was based on symmetric cells similar to those used in EIS measurements. Fig. 5 gathers the Tafel curves, consisting of the logarithmic output current density (logJ) as a function of voltage. The exchange current (J0) and limiting current (Jlim) could be obtained from the Tafel curves. The intersection point of the tangent of cathodic branch in Tafel zone and equilibrium potential gives the value of LogJ0. Eq. (1) shows the relationship between J0 and Rct.

J0 =

RT nFRct

(1)

where R stands for the gas constant, T is temperature, F is the Faraday 111

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Fig. 7. Photovoltaic properties of CdSe/CdS QDSCs based on different PbS/ MCNTs and PbS counter electrodes.

Fig. 6. Tafel polarization curves of symmetric cells based on different PbS/ MCNTs and PbS counter electrodes.

SILAR. Interestingly, VOC of QDSCs also increased for unclear reasons. PCE significantly enhanced with deposition of PbS. The highest PCE was recorded as 6.39%, obtained after 4 SILAR cycles. For comparison, the PCE of QDSCs fabricated with PbS CEs and same photoelectrode yielded ca. 5.52%. Hence, PbS decorated MCNTs/Ti mesh flexible CEs possessed higher performances than MCNTs CEs and PbS CEs.(See Fig. 7)

Table 1 The photovoltaic properties obtained from the J–V curves. CEs MCNT PbS-1/ PbS-2/ PbS-3/ PbS-4/ PbS-5/ PbS-6/ PbS

MCNT MCNT MCNT MCNT MCNT MCNT

JSC/mA cm−2

VOC/mV

FF

PCE/%

17.75 17.61 17.29 17.72 17.12 17.78 17.45 17.46

455 515 525 525 545 545 545 495

0.557 0.597 0.639 0.653 0.685 0.648 0.644 0.639

4.50 5.41 5.80 6.08 6.39 6.28 6.13 5.52

4. Conclusions Uniform MCNTs films were deposited on flexible Ti Meshes with superior bending quality. PbS was homogeneously formed on MCNTs surfaces by successive ionic layer adsorption and reaction (SILAR) technique. The reduction peak current obtained from CVs improved after PbS decoration. Meanwhile, the exchange current density also significantly enhanced and electrochemical reaction resistance efficiently decreased. These data demonstrated improvements in electrocatalytic performances through PbS decoration. QDSCs were assembled using PbS decorated MCNTs/Ti mesh films and CdS/CdSe QDs co-sensitized TiO2 films. The FF and VOC of QDSCs both enhanced after deposition of PbS on MCNTs CEs. After optimization, the QDSCs based on PbS/MCNTs counter electrodes delivered photovoltaic conversion efficiencies reaching 6.39%, which were ca. 40% higher than those of pure MCNT CEs. Overall, PbS/MCNT counter electrodes based on Ti mesh look promising as efficient flexible CEs for QDSCs.

constant, and n is the number of electrons exchanged during the reaction at the electrolyte interface. According to Eq. (1), J0 was inversely proportional to Rct, so the electrochemical catalytic activity of CE increased with J0 value. Here, the J0 value obtained from tested curves decreased in the following order: PbS-4/MCNT > PbS-5/MCNT > PbS-3/MCNT > PbS > PbS2/MCNT > PbS-1/MCNT > MCNT. Hence, it can be concluded that PbS-4/MCNT possessed higher catalytic activity than other counter electrodes. The limiting diffusion current density (Jlim) is determined by diffusion of the redox couple in the electrolyte. Eq.(2) shows the relationship between diffusion coefficient of the electrolyte (D) and Jlim.

Jlim =

2nFCD l

Acknowledgements

(2)

This work was financially supported by the Natural Science Foundation of China (No. 51673204, 21502195 and 21403056), National Materials Genome Project (2016YFB0700600), Program for Science and Technology Innovation Talents in Universities of Henan Province (18HASTIT031), and the key science and technology project of Henan Province (182102210243).

where l is the spacer thickness, n stands for the number of electrons involved during the reduction of S2n−at CEs, F is the Farady constant, and C is the concentration of S2n−. The higher Jlim value might be attributed to increased concentration of S2n−, which should be related with enhanced catalytic activity towards polysulfide reduction after PbS decoration. The PbS decorated MCNTs/Ti meshes were clipped together with CdS and CdSe QDs co-sensitized TiO2 film to assemble QDSCs. Polysulfide electrolyte was injected into the space between both electrodes. J-V characteristic curves were obtained under AM1.5G simulated solar irradiation (100 mW cm−2). Fig. 6 shows the J-V curves of QDSCs devices assembled with MCNTs /Ti mesh counter electrodes after different SILAR cycles. The relevant photovoltaic parameters are listed in Table 1.. The QDSCs fabricated with pure MCNTs/Ti mesh showed power conversion efficiency (PCE) of 4.50%, with short circuit current (JSC) of 17.75 mA/cm2, open circuit potential (VOC) of 455 mV, and fill factor (FF) of 0.56. Tab. 1 revealed that FF of QDSCs significantly enhanced as PbS was deposited on MCNTs CEs. The reason for this might be related to decreased electrochemical reaction resistances on CEs. The FF enhanced from 0.56 to 0.69 after 4 cycles

Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2018.12.022. References Du, J., Du, Z.L., Hu, J.S., Pan, Z.X., Shen, Q., Sun, J.K., Long, D.H., Dong, H., Sun, L.T., Zhong, X.H., Wang, L.J., 2016. J. Am. Chem. Soc. 138, 4201–4209. Du, Z.L., Pan, Z.X., Fabregat-Santiago, F., Zhao, K., Long, D.H., Zhang, H., Zhao, Y.X., Zhong, X.H., Yu, J.S., Bisquert, J., 2016. J. Phys. Chem. Lett. 7, 3103–3111. Du, Z.L., Liu, M.D., Li, Y., Chen, Y.X., Zhong, X.H., 2017. J. Mater. Chem. A 5, 5577–5584. Duan, J.L., Zhang, H.H., Tang, Q.W., He, B.L., Yu, L.M., 2015. J. Mater. Chem. A 3, 17497–17510.

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Q. Pei et al.

2014. Sol. Energy Mater. Sol. Cells 120, 454–461. Mani, A.D., Deepa, M., Ghosal, P., Subrahmanyam, C., 2014. Electrochim. Acta 139, 365–373. Pan, Z.X., Rao, H.S., Mora-Seró, I., Bisquert, J., Zhong, X.H., 2018. Chem. Soc. Rev. 47, 7659–7702. Sahasrabudhe, A., Kapri, S., Bhattacharyya, S., 2016. Carbon 107, 395–404. Tachan, Z., Shalom, M., Hod, I., Rühle, S., Tirosh, S., Zaban, A., 2011. J. Phys. Chem. C 115, 6162–6166. Wang, W., Feng, W.L., Du, J., Xue, W.N., Zhang, L.L., Zhao, L.L., Li, Y., Zhong, X.H., 2018. Adv. Mater. 30, 170546. Wu, C.F., Wu, Z.X., Wei, J., Dong, H., Gao, Y.C., 2013. ECS Electrochem. Lett. 2, H31–H33. Yang, Y.Y., Zhu, L.F., Sun, H.C., Huang, X.M., Luo, Y.H., Li, D.M., Meng, Q.B., 2012. A.C.S. Appl. Mater. Interf. 4, 6162–6168. Zhang, X.L., Huang, X.M., Yang, Y.Y., Wang, S., Gong, Y., Luo, Y.H., Li, D.M., Meng, Q.B., 2013. A.C.S. Appl. Mater. Interf. 5, 5954–5960. Zhang, H., Yang, C., Du, Z.L., Pan, D.Y., Zhong, X.H., 2017. J. Mater. Chem. A 5, 1614–1622. Zhao, F.Y., Tang, G.S., Zhang, J.B., Lin, Y., 2012. Electrochim Acta 62, 396–401.

Ghosh, D., Halder, G., Sahasrabudhe, A., Bhattacharyya, S., 2016. Nanoscale 8, 10632–10641. Gopi, C.V.V.M., Srinivasa Rao, S., Kim, S.-K., Punnoose, D., Kim, H.J., 2015. J. Power Sour. 275, 547–556. Gopi, C.V.V.M., Venkata-Haritha, M., Kim, S.-K., Kim, H.J., 2016. J. Power Sou. 311, 111–120. Gopi, C.V.V.M., Ravi, S., Rao, S.S., Reddy, A.E., Kim, H.J., 2017. Sci. Rep. 7, 46519. Gopi, C.V.V.M., Singh, S., Eswar Reddy, A., Kim, H.J., 2018. ACS Appl Mater Interf. 10, 10036–10042. Guo, W.X., Chen, C., Ye, M.D., Lv, M.Q., Lin, C.J., 2014. Nanoscale 6, 3656–3663. Guo, W., Du, Z., Zhao, Q., Zhang, H., Zhong, X.H., 2016. J. Phys. Chem. C 120, 16500–16506. Jiang, Y., Zhang, X., Ge, Q.Q., Yu, B.B., Zou, Y.G., Jiang, W.J., Song, W.G., Wan, L.J., Hu, J.S., 2014. Nano Lett. 14, 365–372. Jiao, S., Du, J., Du, Z.L., Long, D.H., Jiang, W.Y., Pan, Z.X., Li, Y., Zhong, X.H., 2017. J. Phys. Chem. Lett. 8, 559–564. Kim, H.J., Yeo, T.B., Kim, S.K., Srinivasa Rao, S., Dennyson Savariraj, A., Prabakar Chandu, K., Gopi, V.V.M., 2014. Eur. J. Inor. Chem 26, 4281–4286. Li, D.M., Cheng, L.Y., Zhang, Y.D., Zhang, Q.X., Huang, X.M., Luo, Y.H., Meng, Q.B.,

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