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Cite this: Chem. Commun., 2014, 50, 7356 Received 30th April 2014, Accepted 21st May 2014
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A high efficiency CoCr2O4/carbon nanotubes nanocomposite electrocatalyst for dye-sensitised solar cells† Mingxing Guo,*a Beibei Tang,a Haimin Zhang,*b Shuhui Yin,*c Wei Jiang,a Yiming Zhang,a Mengying Li,a Hui Wangd and Liqi Jiaod
DOI: 10.1039/c4cc03221g www.rsc.org/chemcomm
A CoCr2O4/carbon nanotubes (CoCr2O4/CNTs) nanocomposite was successfully synthesised by a facile solution route, and used as an electrocatalyst for dye-sensitised solar cells (DSSCs) for the first time, exhibiting a comparable power conversion efficiency of 8.40% to Pt-based DSSCs (g = 8.68%) owing to the superior electrocatalytic activity of the nanocomposite.
As a key component of dye-sensitised solar cells (DSSCs), counter electrode (CE) plays a critically important role in catalysing the reduction reaction of I3 to I to regenerate the sensitiser.1–6 Due to the high cost and the scarcity of Pt-based electrocatalysts in DSSCs, exploration of low cost, resource abundant and high efficiency electrocatalysts as CE materials for DSSCs has aroused great research interest in the fields of science and technology.2–9 To date, varieties of low-cost and earth abundant materials, such as metal carbides/nitrides/chalcogenides, carbonaceous materials and conducting polymers,1,2,5,7–14 have been developed and investigated as electrocatalysts for DSSCs, exhibiting promising photovoltaic performance in comparison with Pt-based DSSCs. To obtain high efficiency DSSCs with Pt-free electrocatalysts, search for more low-cost and earth abundant electrocatalysts with high electrocatalytic activity still remains as a huge challenge. As a new type of catalyst, spinel-type cobalt chromite (CoCr2O4) has exhibited superior catalytic activity and great potential in water gas shift reaction, combustion catalysis, dehydrogenation and hydrogenation.15–17 A further modification (e.g., Au, Li+ and Zr4+) results in more extensive applications of the fabricated CoCr2O4 catalysts, such as oxidation of CO, C2 and C3 hydrocarbons and a
College of Environment Science and Engineering, Dalian Maritime University, Dalian 116026, China. E-mail: [email protected]; Fax: +86-411-84727675; Tel: +86-411-84724342 b Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus, QLD 4222, Australia. E-mail: haimin.zhang@griffith.edu.au c Department of Physics, Dalian Maritime University, Dalian 116026, China. E-mail: [email protected] d Navigation College, Dalian Maritime University, Dalian 116026, China † Electronic supplementary information (ESI) available: Experimental details, SEM images, and other characterisation results. See DOI: 10.1039/c4cc03221g
7356 | Chem. Commun., 2014, 50, 7356--7358
methane combustion.17,18 Although tremendous efforts have demonstrated that the catalytic activity of the CoCr2O4 catalyst in catalytic reactions is possibly related to the unique activity of chromium in the catalyst, the nature of active sites of chromium species in catalytic reactions is still under debate.15 Exploring the possibility of more applications of the CoCr2O4 catalyst (e.g., as an electrocatalyst for DSSCs) and the catalytically active mechanism is highly desired. To date, no reports have addressed the use of CoCr2O4 material as an electrocatalyst for catalysing the I3 /I reduction reaction in DSSCs. Herein, we report a facile solution route to fabricate CoCr2O4/ carbon nanotubes (CoCr2O4/CNTs) nanocomposite (see Experimental section for details, ESI†). For comparison, Cr2O3/CNTs and CoxOy/CNTs were also prepared (ESI†). The carbon nanotubes in the nanocomposite play the important role of a catalyst support and improve the electrical conductivity of the nanocomposite. The fabricated nanocomposite was used as a counter electrode (CE) material for DSSCs, exhibiting an overall light conversion efficiency of 8.40%, comparable to Pt-based DSSCs (8.68%) and significantly higher than those of Cr2O3/CNTs and CoxOy/CNTs based DSSCs. The high performance can be ascribed to the superior electrocatalytic activity of nanocrystalline CoCr2O4 in the nanocomposite, which has been confirmed by a series of electrochemical characterisations. To the best of our knowledge, this is for the first time that CoCr2O4/CNT nanocomposites have been used as electrocatalysts for DSSCs. Fig. 1A shows the XRD patterns of all investigated samples calcined at 500 1C for 30 min in N2 (detailed information in ESI†). As shown, the diffraction peak at 26.311 for three investigated samples can be assigned to the (111) plane of graphitic carbon nanotubes (JCPDS No. 75-0444). For chromium oxide/CNTs, all new diffraction peaks can be indexed as nanocrystalline Cr2O3, while all new diffraction peaks of cobalt oxide/CNTs indicate that the product is a mixture of CoO and Co2O3 (denoted as CoxOy) (JCPDS No. 70-2856 and JCPDS No. 73-1701). When the reaction precursors containing cobalt and chromium sources are mixed together with CNTs in this work (ESI†), all new diffraction peaks of the fabricated product after calcination can be indexed as
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Fig. 2 I–V characteristics of the DSSCs made of CoCr2O4/CNTs, Cr2O3/ CNTs, CoxOy/CNTs and Pt counter electrodes.
Fig. 1 (A) XRD patterns of Cr2O3/CNTs, CoxOy/CNTs and CoCr2O4/CNTs. (B) and (C) Low and high magnification TEM images of CoCr2O4/CNTs. (D) High resolution TEM image of CoCr2O4/CNTs.
spinel-type CoCr2O4 (JCPDS No. 80-1668). Fig. 1B and C shows low and high magnification TEM images of CoCr2O4/CNTs after calcination. Obviously, CoCr2O4 nanoparticles disperse uniformly on the surface of carbon nanotubes. Also, Cr2O3/CNTs display uniform nanoparticle/nanotube morphology (Fig. S1A, ESI†), while some CoxOy nanoparticle aggregates can be clearly observed for CoxOy/CNTs (Fig. S1B, ESI†). Furthermore, high resolution TEM analysis demonstrates that CoCr2O4 nanoparticles on CNTs possess a size distribution from 4 nm to 12 nm, and the fringe spacing of 0.24 nm is consistent with the d value of the (222) plane of the cubic CoCr2O4 (Fig. 1D and inset). As electrocatalysts, all fabricated products were used as counter electrode (CE) materials for DSSC measurements. Fig. 2 shows the photocurrent–photovoltage (I–V) curves of DSSCs assembled with CoCr2O4/CNTs, Cr2O3/CNTs, CoxOy/CNTs and Pt CEs under the standard AM 1.5 simulated sunlight (100 mW cm 2). It should be noted that the used CoCr2O4/CNTs sample here was fabricated with cobalt and chromium precursor molar ratio of 1 : 1. In this work, we also prepared CoxCryO4/CNT samples with different molar ratios of cobalt and chromium precursors as CEs for DSSCs. The results indicate that the DSSCs made of CoCr2O4/CNTs with cobalt and chromium precursor molar ratio of 1 : 1 give the best photovoltaic performance among all investigated CoxCryO4/CNT CEs (Fig. S2 and Table S1, ESI†), implying that excess cobalt or chromium precursor in the synthetic process could be unfavourable for the formation of spinel-type CoCr2O4 with high solar cell performance. As shown in Fig. 2, the DSSCs made of CoCr2O4/ CNTs with cobalt and chromium precursor molar ratio of 1 : 1 exhibit a short-circuit current density (Jsc) of 18.98 mA cm 2, an open-circuit voltage (Voc) of 0.75 V, and a fill factor (FF) of 0.59, resulting in an overall light conversion efficiency (Z) of 8.40% (Table 1). This solar cell efficiency is comparable to Pt-based DSSCs (Z = 8.68%), and significantly higher than that of DSSCs assembled with Cr2O3/CNTs (Z = 6.03%) or CoxOy/CNTs (Z = 5.22%).
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Table 1 Photovoltaic parameters and charge transfer resistance of CoCr2O4/CNTs, Cr2O3/CNTs, CoxOy/CNTs and Pt counter electrodes
Counter electrodes Voc (V) Jsc (mA cm 2) FF
Z (%) Rct (O cm2)
CoCr2O4/CNTs Cr2O3/CNTs CoxOy/CNTs Pt
8.40 6.03 5.22 8.68
0.75 0.71 0.71 0.76
18.98 16.34 15.64 18.73
0.59 0.52 0.47 0.61
1.12 1.16 1.36 0.96
Obviously, the CoCr2O4/CNTs show a comparable Voc value to Pt-based DSSCs, possibly owing to an intrinsic nature of spineltype CoCr2O4 as the electrocatalyst. It should be noted that to obtain meaningful comparison, we prepared simultaneously three parallel electrode samples under each experimental condition for solar cell measurements in this work. The data shown in Table 1 are close to the average photovoltaic parameters of DSSCs under each experimental condition (Table S2, ESI†). The above photovoltaic results were further confirmed by incident photon to current conversion efficiency (IPCE) spectra of the DSSCs assembled with all investigated CEs (Fig. S3, ESI†). It is known that the performance of DSSCs is highly dependent on the electrocatalytic activity of the CE material.1,2,5 In this work, the electrocatalytic activities of CoCr2O4/CNTs, Cr2O3/CNTs, CoxOy/ CNTs and Pt counter electrodes were evaluated by a series of electrochemical techniques.2,19,20 Fig. 3A shows the cyclic voltammetry (CV) curves of the investigated CEs.19 Obviously, all measured CEs exhibit two pairs of redox peaks. The relative positive pair is due to the redox reaction of I2/I3 , and the relative negative one belongs to the reaction of I3 /I .20 Apparently, the CoCr2O4/CNTs electrode displays the highest cathodic peak current (Ipc) value among all measured CEs and a comparable peak to peak potential separation (Epp) value (0.137 V) to the Pt electrode (Epp = 0.133 V) (Epp = 0.294 V for Cr2O3/CNTs and Epp = 0.350 V for CoxOy/CNTs), which result in the superior electrocatalytic activity of the CoCr2O4/CNTs electrode.21 Fig. 3B shows the electrochemical impedance spectra (EIS) of the investigated CEs. All measured CEs display two semicircles: one in the highfrequency region corresponds to the charge-transfer process (Rct) of the electrolyte/electrode interface and another in the lowfrequency region is assigned to the finite layer Nernst diffusion impedance within the electrolyte.2 A smaller Rct value indicates
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further confirm that the CoCr2O4/CNTs fabricated with cobalt and chromium precursor molar ratio of 1 : 1 possess the highest electrocatalytic activity among all investigated CoxCryO4/CNT samples with different cobalt and chromium precursor molar ratios (Fig. S5 and Table S1, ESI†), thus the highest photovoltaic performance. The superior electrocatalytic activity of the CoCr2O4/ CNTs could be ascribed to the presence of Cr3+ species with excellent activity in CoCr2O4,15 which deserves a further investigation in the future work. In summary, the CoCr2O4/carbon nanotubes (CoCr2O4/ CNTs) nanocomposite was successfully synthesised by a facile solution route. After calcination, CoCr2O4/CNTs was used as a counter electrode material for DSSCs for the first time, exhibiting an overall light conversion efficiency of 8.40%, comparable to that of Pt-based DSSCs (Z = 8.68%) owing to the superior electrocatalytic activity of the nanocomposite. This work was financially supported by the National Natural Science Foundation of China (Grant No. 11204022) and the Fundamental Research Funds for the Central Universities (Grant No. 2012QN066).
Notes and references Fig. 3 CV curves (A) and Nyquist plots (B) of the CoCr2O4/CNTs, Cr2O3/ CNTs, CoxOy/CNTs and Pt counter electrodes.
a higher electrocatalytic activity of the investigated counter electrode.2 The high frequency intercept on the real axis represents the Ohmic series resistance (Rs), which usually decides the adhesion property of the electrocatalyst on the FTO conductive substrate.2 Among all investigated CEs, the commercial Pt electrode exhibits the smallest Rs value, and CoCr2O4/CNT and Cr2O3/CNT CEs show close Rs values to the Pt electrode, indicating better adhesion property of CoCr2O4/CNTs and Cr2O3/CNTs on the FTO substrate compared to the CoxOy/CNTs electrode. Compared to Cr2O3/CNT and CoxOy/CNT CEs, the smaller Rct value (1.12 O cm2) obtained from the CoCr2O4/CNTs electrode is very close to the Rct value (0.96 O cm2) obtained from the Pt electrode (Table 1), indicating superior electrocatalytic activity of the CoCr2O4/CNTs electrode. This is critically important for improving the performance of CoCr2O4/CNT based DSSCs. Tafel polarisation curves (Fig. S4, ESI†) indicate that the order of electrocatalytic activity of the measured CEs is Pt 4 CoCr2O4/CNTs 4 Cr2O3/CNTs 4 CoxOy/CNTs, which is consistent with CV and EIS results.19 The above electrochemical characterisation results demonstrate that commercial Pt CEs possess the highest electrocatalytic activity (and superior electrical conductivity, Fig. 3B) among all investigated CEs, thus the highest DSSCs efficiency. However, it should be noted that the loading amount per unit area (ca. 8.0 mg cm 2) of commercial Pt on the FTO substrate is much higher than that (1.4 mg cm 2) of CoCr2O4/CNTs on the FTO substrate, implying that CoCr2O4/ CNTs possess higher specific mass activity, thus solar cell performance. Also, the EIS and Tafel polarisation characterisations
7358 | Chem. Commun., 2014, 50, 7356--7358
1 M. Wang, A. M. Anghel, B. Marsan, N.-L. Cevey Ha, N. Pootrakulchote, S. M. Zakeeruddin and M. Gratzel, J. Am. Chem. Soc., 2009, 131, 15976–15977. 2 M. Wu, X. Lin, Y. Wang, L. Wang, W. Guo, D. Qi, X. Peng, A. Hagfeldt, M. Gratzel and T. Ma, J. Am. Chem. Soc., 2012, 134, 3419–3428. 3 X. Zheng, J. Guo, Y. Shi, F. Xiong, W.-H. Zhang, T. Ma and C. Li, Chem. Commun., 2013, 49, 9645–9647. 4 B. Winther-Jensen and D. R. MacFarlane, Energy Environ. Sci., 2011, 4, 2790–2798. 5 Y. Hou, D. Wang, X. H. Yang, W. Q. Fang, B. Zhang, H. F. Wang, G. Z. Lu, P. Hu, H. J. Zhao and H. G. Yang, Nat. Commun., 2013, 4, 1583. 6 S. Ahmad, E. Guillen, L. Kavan, M. Graetzel and M. K. Nazeeruddin, Energy Environ. Sci., 2013, 6, 3439–3466. 7 X. Chen, Y. Hou, B. Zhang, X. H. Yang and H. G. Yang, Chem. Commun., 2013, 49, 5793–5795. 8 Y. Xue, J. Liu, H. Chen, R. Wang, D. Li, J. Qu and L. Dai, Angew. Chem., Int. Ed., 2012, 51, 12124–12127. 9 F. Gong, X. Xu, Z. Li, G. Zhou and Z.-S. Wang, Chem. Commun., 2013, 49, 1437–1439. 10 J. Guo, Y. Shi, Y. Chu and T. Ma, Chem. Commun., 2013, 49, 10157–10159. 11 Y. Li, H. Wang, H. Zhang, P. Liu, Y. Wang, W. Fang, H. Yang, Y. Li and H. Zhao, Chem. Commun., 2014, 50, 5569–5571. 12 Z. Wen, S. Cui, H. Pu, S. Mao, K. Yu, X. Feng and J. Chen, Adv. Mater., 2011, 23, 5445–5450. 13 E. Ramasamy and J. Lee, Chem. Commun., 2010, 46, 2136–2138. 14 K. S. Lee, H. K. Lee, D. H. Wang, N.-G. Park, J. Y. Lee, O. O. Park and J. H. Park, Chem. Commun., 2010, 46, 4505–4507. 15 D.-C. Kim and S.-K. Ihm, Environ. Sci. Technol., 2001, 35, 222–226. 16 D. Fino, N. Russo, G. Saracco and V. Specchia, J. Catal., 2006, 242, 38–47. 17 B. Grzybowska-Swierkosz, M. Ruszel, R. Grabowski, L. Kepinski, M. A. Malecka and J. Sobczak, React. Kinet., Mech. Catal., 2012, 105, 69–78. 18 J. Chen, W. Shi, X. Zhang, H. Arandiyan, D. Li and J. Li, Environ. Sci. Technol., 2011, 45, 8491–8497. 19 M. Wu, X. Lin, A. Hagfeldt and T. Ma, Angew. Chem., Int. Ed., 2011, 50, 3520–3524. 20 N.-Q. Fu, Y.-Y. Fang, Y.-D. Duan, X.-W. Zhou, X.-R. Xiao and Y. Lin, ACS Nano, 2012, 6, 9596–9605. 21 J. D. Roy-Mayhew, D. J. Bozym, C. Punckt and I. A. Aksay, ACS Nano, 2010, 4, 6203–6211.
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Cite this: Chem. Commun., 2014, 50, 7356 Received 30th April 2014, Accepted 21st May 2014
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A high efficiency CoCr2O4/carbon nanotubes nanocomposite electrocatalyst for dye-sensitised solar cells† Mingxing Guo,*a Beibei Tang,a Haimin Zhang,*b Shuhui Yin,*c Wei Jiang,a Yiming Zhang,a Mengying Li,a Hui Wangd and Liqi Jiaod
DOI: 10.1039/c4cc03221g www.rsc.org/chemcomm
A CoCr2O4/carbon nanotubes (CoCr2O4/CNTs) nanocomposite was successfully synthesised by a facile solution route, and used as an electrocatalyst for dye-sensitised solar cells (DSSCs) for the first time, exhibiting a comparable power conversion efficiency of 8.40% to Pt-based DSSCs (g = 8.68%) owing to the superior electrocatalytic activity of the nanocomposite.
As a key component of dye-sensitised solar cells (DSSCs), counter electrode (CE) plays a critically important role in catalysing the reduction reaction of I3 to I to regenerate the sensitiser.1–6 Due to the high cost and the scarcity of Pt-based electrocatalysts in DSSCs, exploration of low cost, resource abundant and high efficiency electrocatalysts as CE materials for DSSCs has aroused great research interest in the fields of science and technology.2–9 To date, varieties of low-cost and earth abundant materials, such as metal carbides/nitrides/chalcogenides, carbonaceous materials and conducting polymers,1,2,5,7–14 have been developed and investigated as electrocatalysts for DSSCs, exhibiting promising photovoltaic performance in comparison with Pt-based DSSCs. To obtain high efficiency DSSCs with Pt-free electrocatalysts, search for more low-cost and earth abundant electrocatalysts with high electrocatalytic activity still remains as a huge challenge. As a new type of catalyst, spinel-type cobalt chromite (CoCr2O4) has exhibited superior catalytic activity and great potential in water gas shift reaction, combustion catalysis, dehydrogenation and hydrogenation.15–17 A further modification (e.g., Au, Li+ and Zr4+) results in more extensive applications of the fabricated CoCr2O4 catalysts, such as oxidation of CO, C2 and C3 hydrocarbons and a
College of Environment Science and Engineering, Dalian Maritime University, Dalian 116026, China. E-mail: [email protected]; Fax: +86-411-84727675; Tel: +86-411-84724342 b Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus, QLD 4222, Australia. E-mail: haimin.zhang@griffith.edu.au c Department of Physics, Dalian Maritime University, Dalian 116026, China. E-mail: [email protected] d Navigation College, Dalian Maritime University, Dalian 116026, China † Electronic supplementary information (ESI) available: Experimental details, SEM images, and other characterisation results. See DOI: 10.1039/c4cc03221g
7356 | Chem. Commun., 2014, 50, 7356--7358
methane combustion.17,18 Although tremendous efforts have demonstrated that the catalytic activity of the CoCr2O4 catalyst in catalytic reactions is possibly related to the unique activity of chromium in the catalyst, the nature of active sites of chromium species in catalytic reactions is still under debate.15 Exploring the possibility of more applications of the CoCr2O4 catalyst (e.g., as an electrocatalyst for DSSCs) and the catalytically active mechanism is highly desired. To date, no reports have addressed the use of CoCr2O4 material as an electrocatalyst for catalysing the I3 /I reduction reaction in DSSCs. Herein, we report a facile solution route to fabricate CoCr2O4/ carbon nanotubes (CoCr2O4/CNTs) nanocomposite (see Experimental section for details, ESI†). For comparison, Cr2O3/CNTs and CoxOy/CNTs were also prepared (ESI†). The carbon nanotubes in the nanocomposite play the important role of a catalyst support and improve the electrical conductivity of the nanocomposite. The fabricated nanocomposite was used as a counter electrode (CE) material for DSSCs, exhibiting an overall light conversion efficiency of 8.40%, comparable to Pt-based DSSCs (8.68%) and significantly higher than those of Cr2O3/CNTs and CoxOy/CNTs based DSSCs. The high performance can be ascribed to the superior electrocatalytic activity of nanocrystalline CoCr2O4 in the nanocomposite, which has been confirmed by a series of electrochemical characterisations. To the best of our knowledge, this is for the first time that CoCr2O4/CNT nanocomposites have been used as electrocatalysts for DSSCs. Fig. 1A shows the XRD patterns of all investigated samples calcined at 500 1C for 30 min in N2 (detailed information in ESI†). As shown, the diffraction peak at 26.311 for three investigated samples can be assigned to the (111) plane of graphitic carbon nanotubes (JCPDS No. 75-0444). For chromium oxide/CNTs, all new diffraction peaks can be indexed as nanocrystalline Cr2O3, while all new diffraction peaks of cobalt oxide/CNTs indicate that the product is a mixture of CoO and Co2O3 (denoted as CoxOy) (JCPDS No. 70-2856 and JCPDS No. 73-1701). When the reaction precursors containing cobalt and chromium sources are mixed together with CNTs in this work (ESI†), all new diffraction peaks of the fabricated product after calcination can be indexed as
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Fig. 2 I–V characteristics of the DSSCs made of CoCr2O4/CNTs, Cr2O3/ CNTs, CoxOy/CNTs and Pt counter electrodes.
Fig. 1 (A) XRD patterns of Cr2O3/CNTs, CoxOy/CNTs and CoCr2O4/CNTs. (B) and (C) Low and high magnification TEM images of CoCr2O4/CNTs. (D) High resolution TEM image of CoCr2O4/CNTs.
spinel-type CoCr2O4 (JCPDS No. 80-1668). Fig. 1B and C shows low and high magnification TEM images of CoCr2O4/CNTs after calcination. Obviously, CoCr2O4 nanoparticles disperse uniformly on the surface of carbon nanotubes. Also, Cr2O3/CNTs display uniform nanoparticle/nanotube morphology (Fig. S1A, ESI†), while some CoxOy nanoparticle aggregates can be clearly observed for CoxOy/CNTs (Fig. S1B, ESI†). Furthermore, high resolution TEM analysis demonstrates that CoCr2O4 nanoparticles on CNTs possess a size distribution from 4 nm to 12 nm, and the fringe spacing of 0.24 nm is consistent with the d value of the (222) plane of the cubic CoCr2O4 (Fig. 1D and inset). As electrocatalysts, all fabricated products were used as counter electrode (CE) materials for DSSC measurements. Fig. 2 shows the photocurrent–photovoltage (I–V) curves of DSSCs assembled with CoCr2O4/CNTs, Cr2O3/CNTs, CoxOy/CNTs and Pt CEs under the standard AM 1.5 simulated sunlight (100 mW cm 2). It should be noted that the used CoCr2O4/CNTs sample here was fabricated with cobalt and chromium precursor molar ratio of 1 : 1. In this work, we also prepared CoxCryO4/CNT samples with different molar ratios of cobalt and chromium precursors as CEs for DSSCs. The results indicate that the DSSCs made of CoCr2O4/CNTs with cobalt and chromium precursor molar ratio of 1 : 1 give the best photovoltaic performance among all investigated CoxCryO4/CNT CEs (Fig. S2 and Table S1, ESI†), implying that excess cobalt or chromium precursor in the synthetic process could be unfavourable for the formation of spinel-type CoCr2O4 with high solar cell performance. As shown in Fig. 2, the DSSCs made of CoCr2O4/ CNTs with cobalt and chromium precursor molar ratio of 1 : 1 exhibit a short-circuit current density (Jsc) of 18.98 mA cm 2, an open-circuit voltage (Voc) of 0.75 V, and a fill factor (FF) of 0.59, resulting in an overall light conversion efficiency (Z) of 8.40% (Table 1). This solar cell efficiency is comparable to Pt-based DSSCs (Z = 8.68%), and significantly higher than that of DSSCs assembled with Cr2O3/CNTs (Z = 6.03%) or CoxOy/CNTs (Z = 5.22%).
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Table 1 Photovoltaic parameters and charge transfer resistance of CoCr2O4/CNTs, Cr2O3/CNTs, CoxOy/CNTs and Pt counter electrodes
Counter electrodes Voc (V) Jsc (mA cm 2) FF
Z (%) Rct (O cm2)
CoCr2O4/CNTs Cr2O3/CNTs CoxOy/CNTs Pt
8.40 6.03 5.22 8.68
0.75 0.71 0.71 0.76
18.98 16.34 15.64 18.73
0.59 0.52 0.47 0.61
1.12 1.16 1.36 0.96
Obviously, the CoCr2O4/CNTs show a comparable Voc value to Pt-based DSSCs, possibly owing to an intrinsic nature of spineltype CoCr2O4 as the electrocatalyst. It should be noted that to obtain meaningful comparison, we prepared simultaneously three parallel electrode samples under each experimental condition for solar cell measurements in this work. The data shown in Table 1 are close to the average photovoltaic parameters of DSSCs under each experimental condition (Table S2, ESI†). The above photovoltaic results were further confirmed by incident photon to current conversion efficiency (IPCE) spectra of the DSSCs assembled with all investigated CEs (Fig. S3, ESI†). It is known that the performance of DSSCs is highly dependent on the electrocatalytic activity of the CE material.1,2,5 In this work, the electrocatalytic activities of CoCr2O4/CNTs, Cr2O3/CNTs, CoxOy/ CNTs and Pt counter electrodes were evaluated by a series of electrochemical techniques.2,19,20 Fig. 3A shows the cyclic voltammetry (CV) curves of the investigated CEs.19 Obviously, all measured CEs exhibit two pairs of redox peaks. The relative positive pair is due to the redox reaction of I2/I3 , and the relative negative one belongs to the reaction of I3 /I .20 Apparently, the CoCr2O4/CNTs electrode displays the highest cathodic peak current (Ipc) value among all measured CEs and a comparable peak to peak potential separation (Epp) value (0.137 V) to the Pt electrode (Epp = 0.133 V) (Epp = 0.294 V for Cr2O3/CNTs and Epp = 0.350 V for CoxOy/CNTs), which result in the superior electrocatalytic activity of the CoCr2O4/CNTs electrode.21 Fig. 3B shows the electrochemical impedance spectra (EIS) of the investigated CEs. All measured CEs display two semicircles: one in the highfrequency region corresponds to the charge-transfer process (Rct) of the electrolyte/electrode interface and another in the lowfrequency region is assigned to the finite layer Nernst diffusion impedance within the electrolyte.2 A smaller Rct value indicates
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further confirm that the CoCr2O4/CNTs fabricated with cobalt and chromium precursor molar ratio of 1 : 1 possess the highest electrocatalytic activity among all investigated CoxCryO4/CNT samples with different cobalt and chromium precursor molar ratios (Fig. S5 and Table S1, ESI†), thus the highest photovoltaic performance. The superior electrocatalytic activity of the CoCr2O4/ CNTs could be ascribed to the presence of Cr3+ species with excellent activity in CoCr2O4,15 which deserves a further investigation in the future work. In summary, the CoCr2O4/carbon nanotubes (CoCr2O4/ CNTs) nanocomposite was successfully synthesised by a facile solution route. After calcination, CoCr2O4/CNTs was used as a counter electrode material for DSSCs for the first time, exhibiting an overall light conversion efficiency of 8.40%, comparable to that of Pt-based DSSCs (Z = 8.68%) owing to the superior electrocatalytic activity of the nanocomposite. This work was financially supported by the National Natural Science Foundation of China (Grant No. 11204022) and the Fundamental Research Funds for the Central Universities (Grant No. 2012QN066).
Notes and references Fig. 3 CV curves (A) and Nyquist plots (B) of the CoCr2O4/CNTs, Cr2O3/ CNTs, CoxOy/CNTs and Pt counter electrodes.
a higher electrocatalytic activity of the investigated counter electrode.2 The high frequency intercept on the real axis represents the Ohmic series resistance (Rs), which usually decides the adhesion property of the electrocatalyst on the FTO conductive substrate.2 Among all investigated CEs, the commercial Pt electrode exhibits the smallest Rs value, and CoCr2O4/CNT and Cr2O3/CNT CEs show close Rs values to the Pt electrode, indicating better adhesion property of CoCr2O4/CNTs and Cr2O3/CNTs on the FTO substrate compared to the CoxOy/CNTs electrode. Compared to Cr2O3/CNT and CoxOy/CNT CEs, the smaller Rct value (1.12 O cm2) obtained from the CoCr2O4/CNTs electrode is very close to the Rct value (0.96 O cm2) obtained from the Pt electrode (Table 1), indicating superior electrocatalytic activity of the CoCr2O4/CNTs electrode. This is critically important for improving the performance of CoCr2O4/CNT based DSSCs. Tafel polarisation curves (Fig. S4, ESI†) indicate that the order of electrocatalytic activity of the measured CEs is Pt 4 CoCr2O4/CNTs 4 Cr2O3/CNTs 4 CoxOy/CNTs, which is consistent with CV and EIS results.19 The above electrochemical characterisation results demonstrate that commercial Pt CEs possess the highest electrocatalytic activity (and superior electrical conductivity, Fig. 3B) among all investigated CEs, thus the highest DSSCs efficiency. However, it should be noted that the loading amount per unit area (ca. 8.0 mg cm 2) of commercial Pt on the FTO substrate is much higher than that (1.4 mg cm 2) of CoCr2O4/CNTs on the FTO substrate, implying that CoCr2O4/ CNTs possess higher specific mass activity, thus solar cell performance. Also, the EIS and Tafel polarisation characterisations
7358 | Chem. Commun., 2014, 50, 7356--7358
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