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RGO-loaded flower-like ZnCo2O4 nanohybrid as counter electrode for dye-sensitized solar cells
Materials Letters 225 (2018) 5–8
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
RGO-loaded flower-like ZnCo2O4 nanohybrid as counter electrode for dye-sensitized solar cells Haigang Hou, Haicheng Shao, Xiangzhao Zhang, Guiwu Liu ⇑, Shahid Hussain, Guanjun Qiao ⇑ School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 10 March 2018 Received in revised form 23 April 2018 Accepted 25 April 2018 Available online 26 April 2018 Keywords: Nanocomposites Solar energy materials Semiconductors
a b s t r a c t Reduced graphene oxide (RGO)-loaded flower-like ZnCo2O4 nanohybrid (ZnCo2O4/RGO) was prepared and then used as counter electrode (CE) in dye-sensitized solar cells (DSSCs). Scanning electron microscopy showed that the flower-like structure of ZnCo2O4 was made up of a large number of thin nanosheets and that the RGO sheets reunited with ZnCo2O4 together commendably. The results and analysis illustrated that the distinctive ZnCo2O4 structure provided numberous catalytic reaction sites and that the RGO sheets served as a guide way to accelerate the transmission of electrons. Benefited from the synergetic effect of ZnCo2O4 and RGO, the ZnCo2O4/RGO nanohybrid exhibited excellent electrocatalytic performance. Finally, a competitive power conversion efficiency (PCE) of 7.22% was achieved for the DSSCs based on the ZnCo2O4/RGO nanohybrid CE. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs), which usually comprise photoanode (dye-sensitized TiO2 nanocrystalline), electrolyte (containing I /I3 redox couple) and counter electrode (CE), are considered as a promising alternative to traditional solar cells due to relatively easy fabrication process, respectable energy conversion efficiency and environmental friendliness [1,2]. Although the new perovskite solar cell has attracted more attention nowadays, it has some essential restrictions, such as toxicity, harsh preparation process and poor stability [3–5]. So, it is still necessary to investigate the DSSCs in the new energy field. The CE, as a crucial role in the photoelectric conversion process, is responsible for catalyzing the reduction of I3 and collecting and transmitting electron in the circuit [6,7]. Generally, the noble metal platinum (Pt) is the preferred counter electrode material, which possesses excellent catalytic activity, high electrical conductivity and good stability [8]. However, the low abundance and expensive of Pt will severely limit its long-term development and application [9]. Therefore, it is vital to develop non-Pt electrodes to make the DSSCs more competitive for future economical deployment. Ideal CE materials should be provided with good electrical conductivity, electrocatalytic activity and feasibility of mass production. Recently, inexpensive multiple oxide materials such as NiCo2O4, CuCo2O4 and CoFe2O4, have been proved to have ⇑ Corresponding authors. E-mail addresses: [email protected] (G. Liu), [email protected] (G. Qiao). https://doi.org/10.1016/j.matlet.2018.04.103 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
electrocatalytic performance [10–12]. Meanwhile, as a kind of carbon material, reduced graphene oxide (RGO) has aroused special interests owing to its large specific surface area and good electron conduction properties [13]. Herein, the ZnCo2O4/RGO nanohybrid was prepared and then applied as CE in DSSCs, and the electrocatalytic performances of ZnCo2O4/RGO nanohybrid, ZnCo2O4 and Pt were investigated comparatively.
2. Experimental process Flower-like ZnCo2O4 was synthesized by annealing treatment after a simple solvothermal process as follows: firstly, 0.595 g of Zn(NO3)26H2O, 0.164 g of Co(NO3)26H2O and 0.84 g of C6H12N4 were added in a mixed liquor of ethanol (30 mL) and distilled water (5 mL). After stirring, the resulting transparent solution was put into a Teflon-lined autoclave of 50 mL, and then heated to 180 °C and held for 16 h. Subsequently, the precipitate (precursor) was collected and washed by a centrifugal process. Finally, the flower-like ZnCo2O4 was obtained after annealing of the precursor at 400 °C for 1 h. Graphene oxide (GO) alcohol suspension was obtained by applying the common Hummers method [14]. The ZnCo2O4/RGO nanohybrid was prepared via a hydrothermal method combined with freeze-drying. Typically, 0.08 g of flower-like ZnCo2O4 and 5 mL of GO alcohol suspension (7.28 mgmL 1) were dispersed into ethanol solution of 30 mL, and stirred for 2 h. Then, the mixture was treated by the hydrothermal reaction in an autoclave at
6
H. Hou et al. / Materials Letters 225 (2018) 5–8
160 °C for 5 h. Finally, the ZnCo2O4/RGO nanohybrid was acquired by using freeze-drying method. The sandwich DSSCs devices were configured by assembling a TiO2 photoanode (sensitized by dye), redox electrolyte and CEs, respectively. Here, the Pt CE was purchased commercially, while the other CEs were prepared by coating a blended paste on fluorine-doped tin oxide (FTO) conducting glass to form a 0.5 0. 5 cm2 exposed area, and then annealed at 400 °C for 60 min under Ar atmosphere after drying naturally. The blended paste was obtained by grinding 30 mg as-prepared powder (ZnCo2O4, RGO or ZnCo2O4/RGO), 10 mg polyethylene glycol powder and 2 mL ethanol. The various samples were characterized by X-ray diffraction (XRD, equipped with Cu Ka radiation) and Raman spectrum (an Invia-Reflex spectrometer with a laser of 532 nm). The microstructures of samples were observed by using scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS), Tafel polarization and CV curves of various as-prepared CEs were measured on an electrochemical workstation (ZAHNER ZENNIUM CIMPS-1, Germany). The photocurrent density voltage curves of various cells assembled by different CEs were tested by applying a solar light simulator with light intensity of 100 mWcm 2. 3. Results and discussion The XRD patterns of ZnCo2O4/RGO nanohybrid and flower-like ZnCo2O4 are shown in Fig. 1a. The reflection peaks (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) can be indexed to JCPDS card (NO. 23-1390) clearly [15]. There was no characteristic peak of other substances, indicating that the pure ZnCo2O4 was synthesized. Meanwhile, the relatively few quantity of RGO made no obvious difference between the two patterns [16]. Moreover, compared with the two Raman spectra (Fig. 1b), the ZnCo2O4
and ZnCo2O4/RGO displayed the same stretching vibration peaks at 178.7, 465.8, 507.8, and 681.3 cm 1. The four peaks corresponded to F2, Eg, F2g and A1g modes of the crystalline ZnCo2O4 phase, respectively. Meanwhile, the ZnCo2O4/RGO spectrum gave the both related peaks of ZnCo2O4 and RGO, which confirmed the presence of RGO [17]. Fig. 1c and d shows the morphologies of flower-like ZnCo2O4 and ZnCo2O4/RGO nanohybrid, respectively. To the pure ZnCo2O4, it can be observed that the specific flower-like structure was made up of a large number of thin nanosheets, which can contribute to increase the active reaction sites. For the ZnCo2O4/RGO nanohybrid, the presence of RGO cannot change the flower-like morphology of ZnCo2O4. In the hydrothermal reduction stage, the RGO reunited with the ZnCo2O4 together commendably. Here, the formation of ZnCo2O4/RGO nanohybrid can be dependent on the interplay of carboxyl, epoxy couple and hydroxyl on the GO nanosheets. The hydrothermal reaction produced the unpaired p electrons by removing the functional groups, which can make it more easily combine the ZnCo2O4 [18–20]. In the composite structure, the RGO sheets served as a guide way to conduct electrons, which made for the catalytic reaction. EIS measurement data of Pt, ZnCo2O4/RGO and pure ZnCo2O4 CEs are shown in Fig. 2a. The results can estimate the ability of electronic transmission and mass diffusion. In each curve, the intersection of the left on the horizontal axis comes from substrate resistance and lead resistances of the unit, which is named as the series resistance (Rs). Followed by a semicircle, its radius was an important parameter originated from the charge-transfer resistance (Rct) between the CE and electrolyte interface. In contrast, the pure ZnCo2O4 CE gave the biggest Rct of 236.97 X. The ZnCo2O4/RGO nanohybrid CE had a superior Rct of 0.54 X, which was merely bigger than that of Pt CE (0.33 X), indicating that the nanohybrid possessed a considerable electronic conduction ability.
Fig. 1. (a) XRD patterns and (b) Raman spectra of ZnCo2O4/RGO and ZnCo2O4; SEM images of (c) flower-like ZnCo2O4 and (d) ZnCo2O4/RGO nanohybrid.
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H. Hou et al. / Materials Letters 225 (2018) 5–8
Fig. 2. (a) EIS, (b) Tafel and (c) CV curves based on Pt, ZnCo2O4/RGO nanohybrid and flower-like ZnCo2O4.
The high electronic mobility of RGO sheets can provide a path for electronic transmission and made the nanohybrid system more capable of catalytic reaction. Tafel polarization data of Pt, ZnCo2O4/RGO and pure ZnCo2O4 CEs are presented in Fig. 2b. The ZnCo2O4/RGO nanohybrid CE had a similar slope of the Tafel curve to the Pt CE, suggesting that the nanohybrid possessed a relatively large exchange current density (J0). Meanwhile, the intersection of the left branch on the vertical axis is represented by limiting diffusion current density (Jlim), whose value can be employed to assess the diffusion ability of I / I3 . Obviously, the nanohybrid had the largest Jlim, indicating an excellent diffusion level for the redox couple [21]. CV measurement is another means to explore the catalytic performance of various CEs, which was accomplished in a three-electrode system: CE used by Pt, working electrode employed by various CEs and reference electrode adopted by calomel electrode. The obvious difference can be observed between the ZnCo2O4/RGO and pure ZnCo2O4 CEs, that is the ZnCo2O4/RGO nanohybrid CE gave a relatively small peak-to-peak separation (Epp) value which was comparable to that of Pt CE (Fig. 2c), meaning that the ZnCo2O4/RGO nanohybrid possessed a competitive catalytic activity. Fig. 3 shows J-V curves of DSSCs equipped with the Pt, ZnCo2O4/ RGO, ZnCo2O4 and RGO CEs. The corresponding parameter values are listed in Table 1. Pure RGO sheets CE gave a small shortcircuit current density (Jsc) of 12.90 mAcm 2, low open-circuit voltage (Voc) of 0.75 V and a little fill factor (FF) of 22.93%, resulting in a PCE of only 2.22%. By comparison, the pure ZnCo2O4 CE had a similar Jsc (12.40 mAcm 2) and Voc (0.77 V) to the RGO CE, but the ZnCo2O4 CE exhibited a relatively high FF of 40.95% due to the
Table 1 Photovoltaic parameters of various as-prepared CEs. CEs
Jsc (mAcm
Pt ZnCo2O4/RGO ZnCo2O4 RGO
16.11 16.55 12.40 12.90
2
)
Voc (V)
FF
PCE
0.78 0.78 0.77 0.75
63.03% 55.93% 40.95% 22.95%
7.92% 7.22% 3.91% 2.22%
relatively good catalytic ability, which produced a PCE of 3.91%. However, by recombining the RGO sheets and flower-like ZnCo2O4, the ZnCo2O4/RGO nanohybrid CE showed different level of promotion in the electrochemical parameters: Jsc = 16.55 mAcm 2, Voc = 0.78 V and FF = 55.93%, resulting in a higher PCE of 7.22%, which was close to that of Pt CE (7.92%). 4. Conclusions In summary, RGO-loaded flower-like ZnCo2O4 nanohybrid was prepared and then assembled in DSSCs as CE. In the nanohybrid system, the ZnCo2O4 and RGO sheets can provide catalytic reaction sites and act as paths for electronic transmission, respectively. Benefited from the synergetic effect of ZnCo2O4 and RGO, the cell based on the ZnCo2O4/RGO CE exhibited a competitive PCE of 7.22%, which was comparable to that of the Pt CE (7.92%) and much higher than those of pure ZnCo2O4 (3.91%) and RGO (2.22%) CE. Acknowledgements This study is supported by the Six Talent Peaks Project of Jiangsu Province (2014-XCL-002, TD-XCL-004), the Innovation/ Entrepreneurship Program of Jiangsu Province ([2015]26) and the Qing Lan Project ([2016]15). References
Fig. 3. PCE for DSSCs equipped with Pt, ZnCo2O4/RGO, ZnCo2O4 and RGO CEs.
[1] A. Yella, H. Lee, H. Tsao, C. Yi, A. Chandiran, M.K. Nazeeruddin, E.D.C. Yeh, E. Diau, S. Zakeeruddin, M. Gratzel, Science 334 (2011) 629–634. [2] A. Halgfeldt, G. Boschloo, L. Sun, Chem. Rev. 110 (2010) 6595–6663. [3] X. Chen, S. Yang, Y.C. Zheng, Y. Chen, Y. Hou, X.H. Yang, H.G. Yang, Adv. Sci. 2 (2015) 1500105. [4] X. Chen, L.J. Tang, S. Yang, Y. Hou, H.G. Yang, J. Mater. Chem. A 4 (2016) 6521– 6526. [5] P. Gao, M. Grätzel, M.K. Nazeeruddin, Energy Environ. Sci. 7 (2014) 2448–2463. [6] W.Y. Zhou, X.T. Jia, L.Y. Chen, Z. Yin, Z. Zhang, G.D. Gao, Mater. Lett. 163 (2016) 1–3. [7] Y.R. Huang, T.W. Huang, T.H. Wang, Y.C. Tsai, J. Colloid Interface Sci. 428 (2014) 128–132. [8] Z. Zheng, J. Chen, Y. Hu, W. Wu, H. Tian, J. Mater. Chem. A 2 (2014) 8497–8500. [9] E. Bi, H. Chen, X. Yang, W. Peng, M. Grätzel, L. Han, Energy Environ. Sci. 7 (2014) 2637–2641. [10] F. Du, X.Q. Zuo, Q. Yang, B. Yang, G. Li, H.B. Tang, H.J. Zhang, M.Z. Wu, Y.Q. Ma, Sol. Energy Mater. Sol. Cells 149 (2016) 9–14.
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[11] K.W. Xiong, W.J. Nie, P. Yu, L.H. Zhu, X. Xiao, Mater. Lett. 204 (2017) 69–72. [12] H. Yuan, Q.Z. Jiao, S.L. Zhang, Y. Zhao, Q. Wu, H.S. Li, J. Power Sources 325 (2016) 417–426. [13] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, Science 332 (2011) 1537–1541. [14] M. Hirata, T. Gotou, S. Horiuchi, M. Fujiwara, M. Ohba, Carbon 42 (2004) 2929– 2937. [15] S.G. Mohamed, T.F. Hung, C.J. Chen, C.K. Chen, S.F. Hu, R.S. Liu, K.C. Wang, X.K. Xing, H.M. Liu, A.S. Liu, M.H. Hsieh, B.J. Lee, RSC Adv. 3 (2013) 20143–20149.
[16] H. Wang, Y. Liang, Y. Li, H. Dai, Angew. Chem. Int. Ed. 50 (2011) 10969–10972. [17] A.C. Ferrari, D.M. Basko, Nat. Nanotechnol. 8 (2013) 235–246. [18] S. Khamlich, T. Khamliche, M.S. Dhlamini, M. Khenfouch, B.M. Mothudi, M. Maaza, J. Colloid Interface Sci. 493 (2017) 130–137. [19] R. Hatel, M. Khenfouch, M. Baitoul, M. Maaza, Appl. Phys. A 120 (2015) 1069– 1074. [20] S. Khamlich, Z. Abdullaeva, J.V. Kennedy, M. Maaza, Appl. Surf. Sci. 405 (2017) 329–336. [21] C. Liu, S. Tai, S. Chou, Y. Yu, K. Chang, J. Mater. Chem. 22 (2012) 21057–21062.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
RGO-loaded flower-like ZnCo2O4 nanohybrid as counter electrode for dye-sensitized solar cells Haigang Hou, Haicheng Shao, Xiangzhao Zhang, Guiwu Liu ⇑, Shahid Hussain, Guanjun Qiao ⇑ School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 10 March 2018 Received in revised form 23 April 2018 Accepted 25 April 2018 Available online 26 April 2018 Keywords: Nanocomposites Solar energy materials Semiconductors
a b s t r a c t Reduced graphene oxide (RGO)-loaded flower-like ZnCo2O4 nanohybrid (ZnCo2O4/RGO) was prepared and then used as counter electrode (CE) in dye-sensitized solar cells (DSSCs). Scanning electron microscopy showed that the flower-like structure of ZnCo2O4 was made up of a large number of thin nanosheets and that the RGO sheets reunited with ZnCo2O4 together commendably. The results and analysis illustrated that the distinctive ZnCo2O4 structure provided numberous catalytic reaction sites and that the RGO sheets served as a guide way to accelerate the transmission of electrons. Benefited from the synergetic effect of ZnCo2O4 and RGO, the ZnCo2O4/RGO nanohybrid exhibited excellent electrocatalytic performance. Finally, a competitive power conversion efficiency (PCE) of 7.22% was achieved for the DSSCs based on the ZnCo2O4/RGO nanohybrid CE. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs), which usually comprise photoanode (dye-sensitized TiO2 nanocrystalline), electrolyte (containing I /I3 redox couple) and counter electrode (CE), are considered as a promising alternative to traditional solar cells due to relatively easy fabrication process, respectable energy conversion efficiency and environmental friendliness [1,2]. Although the new perovskite solar cell has attracted more attention nowadays, it has some essential restrictions, such as toxicity, harsh preparation process and poor stability [3–5]. So, it is still necessary to investigate the DSSCs in the new energy field. The CE, as a crucial role in the photoelectric conversion process, is responsible for catalyzing the reduction of I3 and collecting and transmitting electron in the circuit [6,7]. Generally, the noble metal platinum (Pt) is the preferred counter electrode material, which possesses excellent catalytic activity, high electrical conductivity and good stability [8]. However, the low abundance and expensive of Pt will severely limit its long-term development and application [9]. Therefore, it is vital to develop non-Pt electrodes to make the DSSCs more competitive for future economical deployment. Ideal CE materials should be provided with good electrical conductivity, electrocatalytic activity and feasibility of mass production. Recently, inexpensive multiple oxide materials such as NiCo2O4, CuCo2O4 and CoFe2O4, have been proved to have ⇑ Corresponding authors. E-mail addresses: [email protected] (G. Liu), [email protected] (G. Qiao). https://doi.org/10.1016/j.matlet.2018.04.103 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
electrocatalytic performance [10–12]. Meanwhile, as a kind of carbon material, reduced graphene oxide (RGO) has aroused special interests owing to its large specific surface area and good electron conduction properties [13]. Herein, the ZnCo2O4/RGO nanohybrid was prepared and then applied as CE in DSSCs, and the electrocatalytic performances of ZnCo2O4/RGO nanohybrid, ZnCo2O4 and Pt were investigated comparatively.
2. Experimental process Flower-like ZnCo2O4 was synthesized by annealing treatment after a simple solvothermal process as follows: firstly, 0.595 g of Zn(NO3)26H2O, 0.164 g of Co(NO3)26H2O and 0.84 g of C6H12N4 were added in a mixed liquor of ethanol (30 mL) and distilled water (5 mL). After stirring, the resulting transparent solution was put into a Teflon-lined autoclave of 50 mL, and then heated to 180 °C and held for 16 h. Subsequently, the precipitate (precursor) was collected and washed by a centrifugal process. Finally, the flower-like ZnCo2O4 was obtained after annealing of the precursor at 400 °C for 1 h. Graphene oxide (GO) alcohol suspension was obtained by applying the common Hummers method [14]. The ZnCo2O4/RGO nanohybrid was prepared via a hydrothermal method combined with freeze-drying. Typically, 0.08 g of flower-like ZnCo2O4 and 5 mL of GO alcohol suspension (7.28 mgmL 1) were dispersed into ethanol solution of 30 mL, and stirred for 2 h. Then, the mixture was treated by the hydrothermal reaction in an autoclave at
6
H. Hou et al. / Materials Letters 225 (2018) 5–8
160 °C for 5 h. Finally, the ZnCo2O4/RGO nanohybrid was acquired by using freeze-drying method. The sandwich DSSCs devices were configured by assembling a TiO2 photoanode (sensitized by dye), redox electrolyte and CEs, respectively. Here, the Pt CE was purchased commercially, while the other CEs were prepared by coating a blended paste on fluorine-doped tin oxide (FTO) conducting glass to form a 0.5 0. 5 cm2 exposed area, and then annealed at 400 °C for 60 min under Ar atmosphere after drying naturally. The blended paste was obtained by grinding 30 mg as-prepared powder (ZnCo2O4, RGO or ZnCo2O4/RGO), 10 mg polyethylene glycol powder and 2 mL ethanol. The various samples were characterized by X-ray diffraction (XRD, equipped with Cu Ka radiation) and Raman spectrum (an Invia-Reflex spectrometer with a laser of 532 nm). The microstructures of samples were observed by using scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS), Tafel polarization and CV curves of various as-prepared CEs were measured on an electrochemical workstation (ZAHNER ZENNIUM CIMPS-1, Germany). The photocurrent density voltage curves of various cells assembled by different CEs were tested by applying a solar light simulator with light intensity of 100 mWcm 2. 3. Results and discussion The XRD patterns of ZnCo2O4/RGO nanohybrid and flower-like ZnCo2O4 are shown in Fig. 1a. The reflection peaks (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) can be indexed to JCPDS card (NO. 23-1390) clearly [15]. There was no characteristic peak of other substances, indicating that the pure ZnCo2O4 was synthesized. Meanwhile, the relatively few quantity of RGO made no obvious difference between the two patterns [16]. Moreover, compared with the two Raman spectra (Fig. 1b), the ZnCo2O4
and ZnCo2O4/RGO displayed the same stretching vibration peaks at 178.7, 465.8, 507.8, and 681.3 cm 1. The four peaks corresponded to F2, Eg, F2g and A1g modes of the crystalline ZnCo2O4 phase, respectively. Meanwhile, the ZnCo2O4/RGO spectrum gave the both related peaks of ZnCo2O4 and RGO, which confirmed the presence of RGO [17]. Fig. 1c and d shows the morphologies of flower-like ZnCo2O4 and ZnCo2O4/RGO nanohybrid, respectively. To the pure ZnCo2O4, it can be observed that the specific flower-like structure was made up of a large number of thin nanosheets, which can contribute to increase the active reaction sites. For the ZnCo2O4/RGO nanohybrid, the presence of RGO cannot change the flower-like morphology of ZnCo2O4. In the hydrothermal reduction stage, the RGO reunited with the ZnCo2O4 together commendably. Here, the formation of ZnCo2O4/RGO nanohybrid can be dependent on the interplay of carboxyl, epoxy couple and hydroxyl on the GO nanosheets. The hydrothermal reaction produced the unpaired p electrons by removing the functional groups, which can make it more easily combine the ZnCo2O4 [18–20]. In the composite structure, the RGO sheets served as a guide way to conduct electrons, which made for the catalytic reaction. EIS measurement data of Pt, ZnCo2O4/RGO and pure ZnCo2O4 CEs are shown in Fig. 2a. The results can estimate the ability of electronic transmission and mass diffusion. In each curve, the intersection of the left on the horizontal axis comes from substrate resistance and lead resistances of the unit, which is named as the series resistance (Rs). Followed by a semicircle, its radius was an important parameter originated from the charge-transfer resistance (Rct) between the CE and electrolyte interface. In contrast, the pure ZnCo2O4 CE gave the biggest Rct of 236.97 X. The ZnCo2O4/RGO nanohybrid CE had a superior Rct of 0.54 X, which was merely bigger than that of Pt CE (0.33 X), indicating that the nanohybrid possessed a considerable electronic conduction ability.
Fig. 1. (a) XRD patterns and (b) Raman spectra of ZnCo2O4/RGO and ZnCo2O4; SEM images of (c) flower-like ZnCo2O4 and (d) ZnCo2O4/RGO nanohybrid.
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H. Hou et al. / Materials Letters 225 (2018) 5–8
Fig. 2. (a) EIS, (b) Tafel and (c) CV curves based on Pt, ZnCo2O4/RGO nanohybrid and flower-like ZnCo2O4.
The high electronic mobility of RGO sheets can provide a path for electronic transmission and made the nanohybrid system more capable of catalytic reaction. Tafel polarization data of Pt, ZnCo2O4/RGO and pure ZnCo2O4 CEs are presented in Fig. 2b. The ZnCo2O4/RGO nanohybrid CE had a similar slope of the Tafel curve to the Pt CE, suggesting that the nanohybrid possessed a relatively large exchange current density (J0). Meanwhile, the intersection of the left branch on the vertical axis is represented by limiting diffusion current density (Jlim), whose value can be employed to assess the diffusion ability of I / I3 . Obviously, the nanohybrid had the largest Jlim, indicating an excellent diffusion level for the redox couple [21]. CV measurement is another means to explore the catalytic performance of various CEs, which was accomplished in a three-electrode system: CE used by Pt, working electrode employed by various CEs and reference electrode adopted by calomel electrode. The obvious difference can be observed between the ZnCo2O4/RGO and pure ZnCo2O4 CEs, that is the ZnCo2O4/RGO nanohybrid CE gave a relatively small peak-to-peak separation (Epp) value which was comparable to that of Pt CE (Fig. 2c), meaning that the ZnCo2O4/RGO nanohybrid possessed a competitive catalytic activity. Fig. 3 shows J-V curves of DSSCs equipped with the Pt, ZnCo2O4/ RGO, ZnCo2O4 and RGO CEs. The corresponding parameter values are listed in Table 1. Pure RGO sheets CE gave a small shortcircuit current density (Jsc) of 12.90 mAcm 2, low open-circuit voltage (Voc) of 0.75 V and a little fill factor (FF) of 22.93%, resulting in a PCE of only 2.22%. By comparison, the pure ZnCo2O4 CE had a similar Jsc (12.40 mAcm 2) and Voc (0.77 V) to the RGO CE, but the ZnCo2O4 CE exhibited a relatively high FF of 40.95% due to the
Table 1 Photovoltaic parameters of various as-prepared CEs. CEs
Jsc (mAcm
Pt ZnCo2O4/RGO ZnCo2O4 RGO
16.11 16.55 12.40 12.90
2
)
Voc (V)
FF
PCE
0.78 0.78 0.77 0.75
63.03% 55.93% 40.95% 22.95%
7.92% 7.22% 3.91% 2.22%
relatively good catalytic ability, which produced a PCE of 3.91%. However, by recombining the RGO sheets and flower-like ZnCo2O4, the ZnCo2O4/RGO nanohybrid CE showed different level of promotion in the electrochemical parameters: Jsc = 16.55 mAcm 2, Voc = 0.78 V and FF = 55.93%, resulting in a higher PCE of 7.22%, which was close to that of Pt CE (7.92%). 4. Conclusions In summary, RGO-loaded flower-like ZnCo2O4 nanohybrid was prepared and then assembled in DSSCs as CE. In the nanohybrid system, the ZnCo2O4 and RGO sheets can provide catalytic reaction sites and act as paths for electronic transmission, respectively. Benefited from the synergetic effect of ZnCo2O4 and RGO, the cell based on the ZnCo2O4/RGO CE exhibited a competitive PCE of 7.22%, which was comparable to that of the Pt CE (7.92%) and much higher than those of pure ZnCo2O4 (3.91%) and RGO (2.22%) CE. Acknowledgements This study is supported by the Six Talent Peaks Project of Jiangsu Province (2014-XCL-002, TD-XCL-004), the Innovation/ Entrepreneurship Program of Jiangsu Province ([2015]26) and the Qing Lan Project ([2016]15). References
Fig. 3. PCE for DSSCs equipped with Pt, ZnCo2O4/RGO, ZnCo2O4 and RGO CEs.
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