Highly efficient catalysts for Co(II/III) redox couples in dye-sensitized solar cells

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2600 Chem. Commun., 2012,48, 2600–2602 This journal is c The Royal Society of Chemistry 2012

Cite this:

Chem. Commun

., 2012, 48, 2600–2602

Highly eﬃcient catalysts for Co(II/III) redox couples in dye-sensitized

solar cells

Liang Wang,

a

Eric Wei-Guang Diau,

b

Mingxing Wu,

a

Hsueh-Pei Lu

b

and Tingli Ma*

a Received 26th November 2011, Accepted 10th January 2012

DOI: 10.1039/c2cc17389a

We developed several low-cost catalysts with high catalytic activity, which were used as counter electrodes in dye-sensitized solar cells (DSCs). They showed higher eﬃciencies than that of Pt. The eﬃciencies were improved by 18–42% for the DSCs composed of active carbon, niobium dioxide, ordered mesoporous carbon and commercial titanium carbide.

In the past two decades, the dye-sensitized solar cell (DSC) has been widely investigated by researchers from all over the world due to its high eﬃciency, low cost and simple procedure since the breakthrough in 1991.1So far, the conversion eﬃciency of DSCs has reached 11% based on conventional iodine electrolytes.2 However, some problems arise from the use of I/I3redox

couple, such as sublimation, the corrosion of metal counter electrodes (CEs) and absorption of visible light.3,4_{Therefore, it is}

essential to develop alternative redox couples to replace the traditional redox couple. Recently, various organic redox couples including TEMPO/TEMPO+,5 TMTU/TMFDS2+,6 thiolate/ disulﬁde,7 ferrocene0/+,8 copper(I/II),9–11 and cobalt(II/III)12–17

were proposed. One of the redox couples mentioned above, cobalt(II/III), showed an exciting result with D-p-A organic dye

reported by the Gra¨tzel group. The conversion eﬃciency has reached 12.3%.17This is a promising pathway to improve the performance of the DSC and overcome the drawback of I/I3.

However, the volume of cobalt complexes is larger than those of other redox couples. This limited the charge transfer rate and mass transport of the cobalt redox couple in the electrolyte. Therefore, it is necessary to select the thin TiO2 ﬁlm and the

highly catalytically active CE. In recent years, several types of materials were extensively studied as CEs for DSCs.18–26Simple preparation and comparable catalytic activity with Pt for I/I3

attract the interest of researchers. However, there are few reports on improving the catalytic activity of CEs for DSCs containing cobalt(II/III).

Based on our previous work on non-Pt catalysts in DSCs,27–30 intense eﬀorts have been made to seek highly eﬃcient catalysts for cobalt(II/III). One purpose is to improve the catalytic activity of the cobalt redox couple, and the other

is to decrease the cost of catalysts. In this communication, we developed several low-cost materials as CEs to replace Pt in catalyzing cobalt(II/III). It is interesting to see that the highest

conversion eﬃciency of 4.13% has been achieved by employing low-cost commercial titanium carbide (TiC) CEs, higher than the eﬃciency of Pt by 42%. The synthesized niobium dioxide (NbO2), ordered mesoporous carbon (OMC) and common active

carbon (Ca) have also shown higher conversion eﬃciencies than that of Pt.

NbO2 was synthesized according to the literature method

developed by our group.31 In brief, NbCl5 (1.35 g) was

dissolved in 60 mL anhydrous ethanol, followed by the addition of 0.3 g urea. The solution was then vigorously stirred for 2 h, and allowed to stand overnight in an open Petri dish at 50 1C, and then a gel was obtained. The as-prepared gel was annealed at 1100 1C for 3 h under nitrogen. The OMC was prepared using a modiﬁed method developed by our group.29As for TiC and Ca, they are commercially available. The slurry of CEs was constructed with catalytic materials of CEs (50 mg) and isopropanol (3 mL), all of which needed to be homogenized for 4 h. The CE based on FTO glass was prepared with a simple spray-coating technique.32They were then heated at 500 1C for 0.5 h under a nitrogen atmosphere. The thickness of the non-Pt CEs was about 15 mm.

The DSCs were fabricated using the following process. Firstly, the FTO (Nippon Sheet Glass, Co., Ltd.) substrate was treated by TiCl4 aqueous solution.33,34Secondly, commercially

available TiO2paste (Solaronix T/SP, Solaronix) and 160 nm

scattering paste were used to fabricate porous TiO2 ﬁlms

(ca. 3 mm active layer and 5 mm scattering layer) by the screen printing method on the FTO substrate. The obtained TiO2

electrodes were dipped into ethanol containing 0.2 mM YD-2 for 1.5 h.35 The dye-sensitized TiO2 electrodes and CEs of

diﬀerent catalytic materials or Pt CEs were assembled to form DSCs by sandwiching a redox couple (cobalt(II/III) tris(4,40

-di-tert-butyl-2,20-bipyridine), [Co(dtb)3]2+/3+) in an electrolyte solution

composed of 0.22 M cobalt(II), 0.055 M cobalt(III), 0.2 M

4-tert-butylpyridine and 0.1 M LiOCl4in acetonitrile. The cobalt redox

couple was synthesized by the literature method.15

Fig. 1 shows the photovoltaic performance of the DSCs based on CEs of TiC, OMC, NbO2, Ca and Pt at 1 sun (AM 1.5 G,

100 mW cm2) and 0 sun illumination. As shown in the ﬁgure, the values of open-circuit photovoltage (Voc), short-circuit

photocurrent density (Jsc) and ﬁll factor (FF) of the DSCs based

a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, China. E-mail: [email protected]; Fax: +86-411-84986230; Tel: +86-411-84986237

b

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, 300, Taiwan

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012,48, 2600–2602 2601

on TiC CEs are 0.64 V, 9.77 mA cm2, and 0.66, respectively, yielding an overall conversion eﬃciency (Z) of 4.13%. Those values (Voc, Jsc, FF and Z) for the Pt CE are 0.65 V, 7.23 mA cm2,

0.62 and 2.91%, respectively. The Jsc and eﬃciency of DSC

with TiC CEs increased by 35% and 42%, respectively, in comparison to that of Pt. The results demonstrate the existence of excellent catalytic activity for cobalt(III) reduction in this system obtained by using TiC CEs. In addition, the photovoltaic results also show the advantage of the other non-Pt catalysts in reducing cobalt(III) to cobalt(II). The corresponding performance

parameters of the DSCs are summarized in Table 1.

As we know, electrochemical impedance spectroscopy (EIS) is an important method to study the reaction mechanism. Therefore, in order to ﬁnd out the reason for the improvement of eﬃciency of DSCs, EIS was employed. Several symmetrical cells were constructed, each cell using two identical electrodes from the following list: TiC, OMC, NbO2, Ca and Pt. Fig. 2

shows EIS plots and the equivalent circuit. There are two semicircles in the EIS Nyquist plots of the symmetrical cells. The two semicircles are divided into charge transfer resistance of cobalt(II/III) on the CE (left) and diﬀusion of cobalt(II/III) in

the electrolyte (right). Compared with CEs based on Pt, a larger series resistance (Rs) and a smaller charge transfer

resistance (Rct) on the non-Pt CEs can be observed clearly.

A larger arc was observed in the high frequency region in the Pt CE system than that of the non-Pt catalysts system. With respect to photovoltaic performance and EIS ﬁtting data, one can deduce that Rct of CEs is a major factor in the performance of DSCs.

A smaller Rctsuggests higher catalytic activity in the reduction of

cobalt(III) to cobalt(II). Due to large volume of the cobalt redox

couple, many catalytically active positions on the non-Pt catalyst

surface should be responsible for the high catalytic activity. The chemical capacitance (C) indicates that the non-Pt catalysts of TiC, OMC, NbO2and Ca have a larger surface area than Pt.

In the diﬀusion region, we can see the high diﬀusion resistance of the cobalt redox couple in the electrolyte clearly. In conclusion, non-Pt catalysts help to decrease the Rctand increase catalytic

activity signiﬁcantly. The EIS ﬁtting data are summarized in Table 1.

To further understand the reason for achieving the high eﬃciency, we measured the Tafel polarization curves for diﬀerent symmetrical cells. Fig. 3 shows the Tafel curves, which show the logarithmic current density (log J) as a function of voltage. The exchange current density, J0, is obtained as the intercept of

the extrapolated linear region of the curve when the overpotential is zero. In the curve at high potential (horizontal part), we can obtain the limiting diﬀusion current density (Jlim). The J0 and

the Jlimare closely connected with the activity of the catalysts.

A higher cathodic slope was found in the symmetrical cells of non-Pt catalysts as compared to that of Pt, as shown in Fig. 3. This indicates a high J0on the electrode surface of the non-Pt

catalysts. This means non-Pt catalysts can catalyze the reduction

Fig. 1 Photocurrent–voltage curves of the DSCs based on non-Pt and Pt CEs.

Table 1 Photocurrent–voltage and EIS parameters of DSCs with diﬀerent CEs under 1 sun illumination (AM 1.5 G, 100 mW cm2₎

Samples Voc/V Jsc/mA cm2 FF Z/% Rs/O Rct/O Cd1/mF TiC 0.64 9.77 0.66 4.13 15.5 0.7 49.1 OMC 0.64 9.59 0.66 4.05 16.2 1.1 30.5 NbO2 0.61 9.14 0.65 3.62 18.4 2.8 42.5 Ca 0.63 8.38 0.65 3.43 17.8 7.0 21.6 Pt 0.65 7.23 0.62 2.91 1.6 113.3 2.0

Fig. 2 Typical Nyquist plots from impedance measurements on the symmetrical cells, CE//electrolyte//CE.

Fig. 3 Tafel curves of the symmetrical cells fabricated with two identical CEs.

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2602 Chem. Commun., 2012,48, 2600–2602 This journal is c The Royal Society of Chemistry 2012 of cobalt(III) to cobalt(II) better than Pt. Furthermore, the J0can

be obtained according to the following eqn (1): J0¼

RT nFRct

ð1Þ where Rct can be obtained from the EIS. R, T, n and F are

constants. J0varies inversely with Rct. The introduction of non-Pt

catalysts caused an increase in the exchange current density. This demonstrates that the catalytic activity of non-Pt for cobalt(II/III)

is higher than that of Pt. The Tafel results are in good agreement with EIS data.

In conclusion, we developed several highly catalytic non-Pt catalysts for the reduction of cobalt(II/III). The DSCs based on

commercial TiC CEs showed the highest eﬃciency of 4.13%, 42% higher than that based on Pt CE. The other non-Pt catalysts also displayed excellent photovoltaic performance. Based on the EIS and Tafel-polarization studies, the non-Pt catalysts showed higher catalytic activity than that of the Pt. Our results demonstrated that non-Pt catalysts were potential alternatives to the expensive and scarce Pt for low-cost DSCs. It is also expected that low-cost carbides are promising catalysts for the organic redox couples in DSCs.

This work is supported by the National High Technology Research and Development Program for Advanced Materials of China (Grant 2009AA03Z220) and the National Natural Science Foundation of China (Grant No. 50773008). This research is also supported by the State Key Laboratory of Fine Chemicals of China.

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