Multistep electrochemical deposition of hierarchical platinum alloy counter electrodes for dye-sensitized solar cells

Journal of Power Sources 303 (2016) 243e249

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Multistep electrochemical deposition of hierarchical platinum alloy counter electrodes for dye-sensitized solar cells Junjun Zhang a, Mingming Ma a, Qunwei Tang a, *, Liangmin Yu b, c, ** a

Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China c Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao 266100, China b

h i g h l i g h t s
PtM (M ¼ Ni, Fe, Co) alloy CEs are synthesized by a multistep electrodeposition.
The resultant PtM alloy CEs are fabricated into DSSCs.
The PtM alloy CEs have markedly enhanced catalytic activity.
The optimized DSSC yields a maximum efficiency of 8.65%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 October 2015 Received in revised form 2 November 2015 Accepted 4 November 2015 Available online xxx

The preferred platinum counter electrode (CE) has been a burden for commercialization of dye-sensitized solar cell (DSSC) due to high expense and chemical corrosion by liquid electrolyte. In the current study, we have successfully realized the multistep deposition of platinum alloy CEs including PtNi, PtFe, and PtCo for liquid-junction DSSC applications. The preliminary results demonstrate that the enhanced electrochemical activities are attributable to high charge-transfer ability and matching work functions of the PtM (M ¼ Ni, Fe, Co) alloy CEs to redox potential of I/I 3 electrolyte. The resultant DSSCs yield impressive power conversion efficiencies of 8.65%, 7.48%, and 7.08% with PtNi, PtFe, and PtCo CEs, respectively. On behalf of the competitive reactions between transition metals with liquid electrolyte, the PtM alloy CEs display enhanced long-term stability. © 2015 Elsevier B.V. All rights reserved.

Keywords: Dye-sensitized solar cell Counter electrode Alloy electrocatalyst Electrochemical deposition

1. Introduction The nowaday's economy development requires growing demands for energy consumption, however the utilization of fossil fuels have emerged considerable crises in energy depletion, environmental pollution, and ecological damage. Therefore, more attentions have been placed on creating green and renewable power sources including solar cells, a class of devices converting solar energy into electricity by complicated photoelectrochemical re€tzel [2], dyeactions [1]. Since the first prototype reported by Gra sensitized solar cells (DSSCs) [3,4] have attracted widespread interests on behalf of the superiorities in simple fabrication,

* Corresponding author. ** Corresponding author. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China. E-mail addresses: [email protected] (Q. Tang), [email protected] (L. Yu). http://dx.doi.org/10.1016/j.jpowsour.2015.11.012 0378-7753/© 2015 Elsevier B.V. All rights reserved.

environmental friendliness, and high power conversion efficiency [5,6]. As a platform of collecting reflux electrons from external  circuit and reducing I 3 into I ions, an efficient counter electrode (CE) is expected to have merits in good electron-conduction ability, high electrocatalytic activity, reasonable dissolution resistance, and cost-effectiveness [7,8]. Although preferred Pt species can provide promising performances as a CE material, the high cost has been a burden for DSSC commercialization. One of the solutions to this dilemma is to reduce Pt dosage or to replace Pt by other alternatives. Recently, we have systematically studied the possibility of utilizing conductive polymers [9], carbonaceous materials [10] as well as their hybrids [11,12] as CE candidates. The altered molecular structure of conductive polymers and electrochemical corrosion of carbonaceous materials seem to be unsolved technological issues. More recently, the CEs with alloyed electrocatalysts by alloying Pt with transition metals [13], alloying transition metals with nonmetals [14], or alloying between transition metals [15] have been

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created in our group. Due to good matching of the work functions of alloy CEs to redox potential of I/I 3 and redistribute the electronic structure on the Pt surface, the electrocatalytic activity has been markedly enhanced, yielding impressive power conversion efficiencies in their liquid-junction DSSCs. In this fashion, pursuit of cost-effective CEs without sacrificing photovoltaic performances is a persistent objective for DSSCs. Although the study on alloy CEs is in their initial stage, the primary results have demonstrated them to be good CE alternatives to traditional Pt electrode. In searching for other efficient alloy CEs, in the current work, we launch an experimental realization of Pt alloy CEs including PtNi, PtFe, and PtCo by a multistep electrochemical deposition strategy. This control over good electrocatalytic behaviours and therefore photovoltaic performances recommends the hierarchical alloy nanostructures to manufacture advanced DSSC platforms. 2. Experimental 2.1. Preparation of binary alloy CEs The feasibility of this strategy was confirmed by following experimental procedures: A mixing aqueous solution was composed of 0.08 mL of H2PtCl6, 0.105 g of NiSO4$6H2O (or 0.111 g of FeSO4$7H2O or 0.095 g of CoCl2$6H2O) and 40 mL of deionized water by controlling the molar rate of Pt: Ni (Fe, Co) to be 1: 1. Subsequently, the fabrication of PtM (M ¼ Ni, Fe, Co) alloy CEs was carried out on an electrochemical workstation (CHI660E): a cleaned FTO glass substrates (sheet resistance 12 U square1, purchased from Hartford Glass Co, USA) was used as a work electrode, a Pt plate was a counter electrode, and an Ag/AgCl was a reference electrode. A multistep electrochemical deposition method was realized by depositing Ni at 0.8 V (Fe at 0.5 V, Co at 0.68 V) for 30 s and Pt at 0.1 V for 3 s in each cycle. After five cycles, the alloy CEs were thoroughly rinsed with deionized water and dried by nitrogen. 2.2. Assembly of DSSCs The TiO2 nanocrystal anode films with an average thickness of 10 mm were prepared according to previous procedures [16]. Subsequently, the resultant TiO2 anodes with an active area of 0.25 cm2 were sensitized by immersing in a 50 mM ethanol solution of N719 dye (purchased from DYESOL LTD) for 24 h. The DSSC comprises of a sandwiched dye-sensitized TiO2 anode/I/I 3 redox electrolyte/CE architecture. Each DSSC was assembled using a dye-sensitized TiO2 anode and alloy CE and subsequently sealed with a hot-melt film (30 mm) through hot-pressing for 1 min. The pressure and temperature were controlled at 0.2 MPa and 110  C, respectively. Finally, the I/I 3 redox electrolyte was injected into the space between the photoanode and the CE through a hole from the CE side. After covering the hole, the resultant DSSC device could be utilized to characterize the photovoltaic performances. 2.3. Electrochemical characterizations The electrochemical performances were recorded on a conventional CHI660E setup (purchased from Shanghai Chenhua Device Company, China) with an Ag/AgCl reference electrode, a CE of platinum sheet, and a working electrode of FTO glass supported PtM alloy. The cyclic voltammetry (CV) curves were recorded by scanning from 0.4e1.2 V and back to 0.4 V in a supporting electrolyte of 50 mM LiI, 10 mM I2, and 500 mM LiClO4 in acetonitrile. Electrochemical impedance spectroscopy (EIS) measurements for symmetric dummy cells with a sandwiched structure of alloy CEjredox electrolytejbinary alloy CE were carried out on the

electrochemical workstation in a frequency range of 0.01 Hz ~ 106 kHz and an ac amplitude of 10 mV at room temperature. Tafel polarization curves were was performed on the same symmetric dummy cells used for EIS measurements at a scan rate of 10 mV s1. 2.4. Photovoltaic measurements The photocurrent-voltage (JeV) curves of DSSCs were recorded on the CHI600E workstation under simulated solar irradiation from a 100 W xenon-mercury arc lamp. The light intensity was controlled at 100 mW cm2 (air mass 1.5 global, AM1.5). A black mask with an aperture area of around 0.25 cm2 was applied on the surface of DSSCs to avoid stray light. In order to control the efficiency deviations within experimental error range, each DSSC device was repeatedly measured at least ten times to control the experimental errors within ±5%. Start-stop switch measurement was conducted by alternatively turning on and off the illumination at an interval of 25 s. 2.5. Other characterizations The surface morphologies of alloy CEs were determined on a scanning electron microscope (SEM, S4800). X-ray photoelectron spectroscopy (XPS) experiment was carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with MgKa radiation (hn ¼ 1253.6 eV). The compositions of the alloy CEs were determined by inductively coupled plasma-atomic emission spectra (ICP-AES). Prior to ICP measurements, the alloy CEs were immersed in concentrated nitric acid to dissolve the FTO glass substrate. 3. Results and discussion 3.1. Structure and morphology characterizations The binary PtM alloys are subjected to XPS measurements to determine the chemical structures. As shown in Fig. 1, the binding energy of Pt4f locates at 70.63 eV, confirming the metallic nature of Pt in alloy CEs [17], while the binding energies of Ni2p at 855.87 eV, Fe2p at 716.13 eV, and Co2p at 780.43 eV also indicate metallic phases of the transition metals [18]. The metallic bonds between transition metals (Ni, Fe and CO) can result in a rapid charge transfer from transition metals to Pt and therefore electronic redistribution on Pt surface, leading to a shift of Ni2p, Fe2p and Co2p to a higher binding energy. The real stoichiometries of alloy CEs are determined by ICP-AES equipment, yielding atomic ratios of 1.000: 0.986, 1.000: 1.875, and 1.000: 1.104 for PtNi, PtFe, and PtCo, respectively. The measured atomic ratios are close to the stoichiometries of PtM alloy CEs. Therefore, the chemical formulae of the alloy CEs can be expressed according to their stoichiometric ratios. SEM photographs in Fig. 2 suggest a high surface coverage of PtM alloys on FTO glass substrate without any aggregations. Deep examination shows a homogeneous nanoparticle topology having average diameter of around several tens of nanometers and relative loose structure [19] This loose architecture is beneficial to the I 3 ions across the alloy layer [20,21], allowing for accelerating I 3 reduction process and therefore cell performances. 3.2. Electrochemical behaviours of CEs CV characterization is an efficient technique to study the electrochemical reactions at CE/electrolyte interface, therefore, we compare the CV curves of the pure Pt, PtNi, PtFe, and PtCo CEs recorded at a scan rate of 50 mV s1. As shown in Fig. 3, all the CV curves have two pairs of redox reactions, which can be assigned to

J. Zhang et al. / Journal of Power Sources 303 (2016) 243e249

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a Intensity (a.u.)

PtFe

PtNi

PtCo

1000

800

600

400

200

0

Binding Energy (E) eV

b

c

855.87 eV

70.73 eV Pt4f

73.84 eV

Intensity (a.u.)

Intensity (a.u.)

Ni2p

860

855

850

85

845

80

d

75

70

65

Binding Energy (E) eV

Binding Energy (E) eV 716.13 eV

e

71.28 eV

Pt4f

74.58 eV

Intensity (a.u.)

Intensity (a.u.)

Fe2p

740

730

720

710

700

Binding Energy (E) eV

f

85

80

g

780.24 eV

75

Co2p

74.32 eV

65

71.02 eV

Pt4f

Intensity (a.u.)

Intensity (a.u.)

70

Binding Energy (E) eV

790

785

780

Binding Energy (E) eV

775

770

85

80

75

70

65

Binding Energy (E) eV

Fig. 1. (a) XPS spectra of the PtNi, PtFe, and PtCo alloy CEs. XPS spectra of (b) Ni2p with (c) Fe4f, (d) Fe2p with (e) Fe4f and (f) Co2p with (g) Fe4f.

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Fig. 2. Top-view SEM images of (a, b) PtNi, (c, d) PtFe, and PtCo alloy CEs.

    Red1/Ox1 (I 3 þ 2e 4 3I ) and Red2/Ox2 (3I2 þ 2e 4 2I3 ). The peak potential for Red1 reaction (ERed1), peak current density of the Red1 (JRed1) and peak-to-peak separation (Epp) between Red1 and Ox1 are crucial parameters for assessing the catalytic activity of a CE. As shown in Fig. 3, the JRed1 of PtNi alloy CE is 3.84 mA cm2, which is higher than 3.81 mA cm2 for pure Pt, 3.19 mA cm2 for PtFe, and 2.99 mA cm2 for PtCo CE. Moreover, the PtNi alloy CE has the smallest ERed1 and Epp values among the four CEs, demonstrating that the PtNi alloy CE has superior catalytic activity. The stacking CV curves of PtNi alloy CE in liquid electrolyte recorded at different scan rates are shown in Fig. 4a. The linear relationships between the peak current density and square root of scan rate, as shown in Fig. 4b, illustrates that the diffusion layer becomes thinner and the electrochemical polarization degree becomes larger at increased scan rates. After suffering 100-cycles in liquid electrolyte, as shown in Fig. 4c, no obvious reduction in peak current density is detected for PtNi alloy CE (Fig. 4d), suggesting

that the PtNi alloy CE is reasonable stable under long-term operation [22]. The electrochemical impedance spectroscopy (EIS) of symmetric dummy cells with two identical CEs is used to explore the charge-transfer process and evaluate the catalytic activity of a CE. The Nyquist EIS plots for various CEs are shown in Fig. 5a and the corresponding electrochemical parameters are summarized in Table 1. The high-frequency semicircle corresponding to chargetransfer resistance (Rct) at CE/electrolyte interface is considered as a crucial parameter in evaluating the charge-transfer ability of a CE. In general, a lower Rct value illustrates high charge-transfer ability and therefore enhanced electrocatalytic activity for I 3 reduction. The Rct value of PtNi alloy CE is 2.989 U cm2 by fitting the Nyquist EIS plot with an equivalent circuit, which is smaller than 7.619 U cm2 for pure Pt, 15.410 U cm2 for PtFe, and 41.19 U cm2 for PtCo. This result demonstrates that PtNi electrode has superior electrocatalytic ability, which is in good agreement with CV

J. Zhang et al. / Journal of Power Sources 303 (2016) 243e249

6

Ox1, 3I -2e=I3 -2

Current density (mA cm )

current density (J0 ¼ RT/nFRct, R is the gas constant, F represent Faraday's constant, T is the absolute temperature, n is the number of electrons for I 3 reduction) or limiting diffusion current density (Jlim ¼ 2nFCDn/l, l is the distance between the CEs in a symmetric dummy cell, C is I 3 concentration, Dn is diffusion coefficient and can be calculated by the RandleseSevcik theory [24]) suggests an electrocatalytic activity evolution of PtNi > Pt > PtFe > PtCo.

-

Ox2, 2I3 -2e=3I2

-

-

247

3

Epp 0 -

-3

-

3.3. Photovoltaic performances of DSSCs

PtCo PtFe Pt PtNi

Red2,3I2+2e=2I3

The JeV characteristics of DSSCs based on PtNi, PtFe, PtCo, and pure Pt CEs are shown in Fig. 6a and the photovoltaic parameters are summarized in Table 1. All the photovoltaic parameters follow an order of PtNi > Pt > PtFe > PtCo. Notably, the DSSC with PtNi alloy CE yields a remarkable power conversion efficiency (h) of 8.65% (Jsc ¼ 17.6 mA cm2, Voc ¼ 0.702 V, and FF ¼ 0.701) in comparison to 7.86% for pristine Pt CE based solar cell. A possible mechanism behind the photovoltaic performance is the electrocatalytic activity enhancement of PtNi alloy CE. The real opencircuit voltage (Voc) measured in a DSSC device is lower than the difference between the Fermi energy of TiO2 and the redox potential of electrolyte because of the backward reaction between electrons at TiO2 nanocrystallites and I 3 species. The Jsc value depends on the electron density in the percolating TiO2 injected from excited N719 dyes [25]. Due to a fact that the PtNi alloy CE has the highest electrocatalytic activity toward I/I 3 redox couples, the  rapid conversion from I 3 to I allows for the accelerated regeneration of N719 dye molecules by I species. The dark currents created by the backward recombination reaction between electrons and I 3 species [26] are shown in Fig. 6b. The smaller dark current

-

Red1,I +2e=3I -6 -0.4

0.0

0.4

0.8

1.2

Potential (V vs Ag/AgCl) Fig. 3. CV curves of various CEs in liquid electrolyte recorded at a scan rate of 50 mV s1 and room temperature.

characterization. This conclusion can be cross-checked by the electron lifetime [t ¼ 1/(2pfp), fp is the peak frequency in the Bode EIS plots (Fig. 5b)] at CE/electrolyte interface [23]. The calculated t follows an order of PtNi (74 ms) < Pt (89 ms) < PtFe (107 ms) < PtCo (108 ms). A low t means that the electrons at CE/electrolyte inter face can rapidly participate in the reduction reaction of I 3 into I ions. The Tafel polarization curves are also used to demonstrate the electrocatalytic activity of CEs, as shown in Fig. 5c. Either exchange

b

9

Ox1 Ox2 Red1

-2

25mV 50mV 75mV 100mV 125mV 150mV

0

-3

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Red2

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1 10 20 30 40 50 60 70 80 90 100

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number of circle (n)

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Current density (mA cm )

Current density (mA cm )

3

-6 -0.4

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Current density (mA cm )

a

0.8

1.2

Red1 Ox1

0

-3

-6 0

20

40

60

80

100

Circle number (n)

Fig. 4. (a) CV curves of PtNi alloy CE recorded at various scan rates. (b) The plots of peak current density as a function of cycle number. (c) The stacking 100-cycle CV curves for PtNi CE recorded at a scan rate of 50 mV s1. (d) Relationship between peak current density and cycle number.

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b

50

CPE

-2

-Z'' (ohm cm )

40

PtCo PtFe Pt PtNi

30

RS

Rct

20

60

PtCo PtFe PtNi Pt

50 40

-Phase(degree)

a

W

30 20 10

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0 0

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3

J

2

J

1

Pt PtFe PtCo PtNi

0

-1

-2 -1.0

-0.5

0.0

0.5

1.0

Voltage V Fig. 5. (a) Nyquist and (b) Bode EIS plots and (c) Tafel polarization curves of the symmetric dummy cells with two identical CEs. The insert gives an equivalent circuit for symmetric dummy cells.

Table 1 Photovoltaic parameters of DSSCs with varied CEs and the simulated data from EIS plots. CEs

Jsc (mA cm2)

Voc (V)

FF

h (%)

Rs (U cm2)

Rct (U cm2)

W (U cm2)

t (ms)

PtNi Pt PtFe PtCo

17.6 16.6 15.7 15.68

0.702 0.701 0.698 0.697

0.701 0.683 0.681 0.652

8.65 7.86 7.48 7.08

3.324 3.341 3.370 4.986

2.989 7.619 15.410 41.190

0.76 0.80 0.84 1.12

74 89 107 108

density for PtNi based solar cell indicates that the recombination reaction between I 3 ions in liquid electrolyte and the electrons at conduction band of TiO2 nanocrystallites is retarded [27], which is

also beneficial to enhanced Voc and Jsc. Additionally, FF is a parameter inversely proportional to the internal resistance of a DSSC device. Generally, the internal resistance includes Ranode,

a

b Current density (mA cm )

15

-2

-2

Current density (mA cm )

20

10

5

PtNi Pt PtFe PtCo

0 0.0

0

-10

-20

-30

PtNi Pt PtFe PtCo

-40

0.2

0.4

Volage (V)

0.6

0.8

0.0

0.2

0.4

0.6

0.8

Volage (V)

Fig. 6. The JeV characteristics of the resultant DSSC device recorded under (a) AM1.5G irradiation from photoanode and (b) in the dark.

J. Zhang et al. / Journal of Power Sources 303 (2016) 243e249

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Fig. 7. (a) The on-off switches and (b) photocurrent stabilities of the DSSCs.

Ranode/electrolyte, Relectrolyte, Relectrolyte/CE, RCE. The same TiO2 anode and liquid electrolyte are utilized for different devices, therefore, the internal resistance depends on Relectrolyte/CE (Rct) and RCE (Rs). As shown in Table 1, the total value of Rct and Rs follows an order of PtNi < Pt < PtFe < PtCo, leading to FF of PtNi > Pt > PtFe > PtCo. When applied as solar panels for windows, and roof panels, and portable sources, the DSSC devices are expected to have merits on fast start-up, multiple on/off capability, and photocurrent stability. As shown in Fig. 7a, the photocurrent density abruptly increases at “light on” state, suggesting a fast start-up of solar cell under AM1.5 irradiation. This rapid start-up process is beneficial to the high electrocatalytic activity of alloy CE for I 3 reduction. After ten on/off cycles, there is only a little reduction in photocurrent density, which is the requirement for a durable solar cell. Further studies on photocurrent stability under persistent irradiation over 3000 s (Fig. 7b) reveal that 94.9% of the photocurrent density is kept for PtNi alloy CE based DSSC in comparison to 91.4%, 86.5%, and 84.1% for the devices with Pt, PtFe, and PtCo, respectively. In our previous report [28], we have carefully described the chemical dissolution of alloy CE in a real liquid-junction DSSC device. Briefly, the Pt species will suffer dissolution in I/I 3 redox electrolyte, mainly from  Pt(s) þ 2I2(aq) ¼ PtI4(s) and Pt(s) þ 2I 3 (aq) ¼ PtI4(s) þ 2I (aq). The thermodynamical Gibbs free energy (DrGm,25oC) are 78.3 and 45.9 kJ mol1, respectively. The negative values of DrGm mean that the reactions between Pt and I2 or I 3 ions are spontaneous process, which will lead to the corrosion of Pt species. However, the corresponding DrGm,25oC values for the dissolution reactions of Ni, Fe, or Co are more negative in comparison with that of Pt. The competitive reactions of transition metals with I2/I 3 can restrict the dissolution of Pt species and therefore protect the high catalytic activity of CEs. 4. Conclusions

photovoltaic performances demonstrate the resultant PtNi alloys are good CE candidates for advanced DSSC platforms.

Acknowledgements The authors would like to acknowledge financial supports from National Natural Science Foundation of China (21503202, U1037604).

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In summary, we demonstrate here the experimental realization of PtM (M ¼ Ni, Fe, Co) alloy CEs by a multistep electrochemical deposition method for liquid-junction DSSC applications. Due to the alloying effect of transition metals with Pt, the electrocatalytic activity and charge-transfer ability have been markedly enhanced in comparison with pristine Pt. The optimized DSSC device with PtNi alloy electrode yields an impressive power conversion efficiency of 8.56% in comparable to 7.08% for pristine Pt based solar cell. Additionally, the DSSCs with PtNi alloy CE display other advantages such as multiple on-off switches and long-term stability under persistent irradiation. The simple synthesis process, low dosages of precious Pt, and promising electrochemical and

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