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reduced graphene oxide counter electrodes as a new avenue for high-efficiency liquid junction photovoltaic devices
Accepted Manuscript Cost-effective CoPd alloy/reduced graphene oxide counter electrodes as a new avenue for high-efficiency liquid junction photovoltaic devices Hyo-Jun Oh, Van-Duong Dao, Ho-Suk Choi PII:
S0925-8388(17)30626-6
DOI:
10.1016/j.jallcom.2017.02.180
Reference:
JALCOM 40908
To appear in:
Journal of Alloys and Compounds
Received Date: 17 December 2016 Revised Date:
12 February 2017
Accepted Date: 18 February 2017
Please cite this article as: H.-J. Oh, V.-D. Dao, H.-S. Choi, Cost-effective CoPd alloy/reduced graphene oxide counter electrodes as a new avenue for high-efficiency liquid junction photovoltaic devices, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.180. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Cost-effective CoPd alloy/reduced graphene oxide counter
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electrodes as a new avenue for high-efficiency liquid junction
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photovoltaic devices
4
Hyo-Jun Oh, Van-Duong Dao*, Ho-Suk Choi*
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Department of Chemical Engineering & Applied Chemistry, Chungnam National University,
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Daejeon, 305-764, South Korea
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*Corresponding authors; email: [email protected]/[email protected] (V.D.D.)
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and [email protected] (H.S.C.)
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Abstract
This work presents the synthesis of CoxPd1-x alloys (0 ≤ x ≤ 1) on a reduced graphene oxide
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(RGO) surface using the dry plasma reduction method. The formation of CoPd alloys on the
12
RGO surface is confirmed through high-resolution scanning electron microscopy (HRSEM) and
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X-ray photoelectron spectroscopy (XPS) measurements, X-ray diffraction, and transmittance
14
electron microscopy (TEM). Then, the developed materials are applied as Pt-free counter
15
electrodes (CEs) in dye-sensitized solar cells (DSCs). In order to obtain efficient CEs, the
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chemical composition of the CoxPd1-x/RGO is controlled through optimizing the volume ratio of
17
the Co and Pd precursors during the synthesizing process. Due to the optimization of the charge-
18
transfer resistance (Rct) and the diffusion impedance (Zw) values of the Co0.9Pd0.1/RGO CE, the
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device using the Co0.9Pd0.1/RGO CE exhibits the highest efficiency among the fabricated cells.
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Note that the cells using Co/RGO and Pd/RGO exhibit efficiencies of 5.17% and 5.41%,
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respectively. The proposed strategy is simple and efficient; thus, it is promising for fabricating
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highly efficient and low-cost CE materials for DSCs.
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Keywords: dry plasma reduction; CoPd alloy; reduce graphene oxide; counter electrode; dye-
4
sensitized solar cell
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1. Introduction
6
Due to their low-cost fabrication, environmentally benign process, and high power energy
7
conversion efficiency (up to 14.3% [1]), a photovoltaic (PV) cell based on a liquid-junction solar
8
cell and a dye-sensitized solar cell (DSC) has significant potential to become a universal
9
environmentally friendly next-generation PV device [2]. From a scientific perspective, the DSC
10
efficiency can reach 20% and higher, and this is highly competitive with other types of
11
conventional thin film PV cells [3]. As mentioned above, the highest achieved efficiency is
12
14.3%, which is lower than the expected value (20%). It has been reported that the working
13
electrode has the greatest potential to improve the efficiency of DSCs. There are several ways to
14
improve the efficiency of DSCs through adding CdS quantum-dot in TiO2 layer [4], using Ag
15
nanoparticles for plasmonic enhancement effect [4-7], controlling the morphology of working
16
electrode to become TiO2 nanourchin structure [8], or modifying the surface of flourine-doped
17
tin oxide (FTO) and TiO2 layer though plasma treatment [9-11]. However, the counter electrode
18
(CE) is also important for the reduction of triiodide ions to iodide ions. In general, Pt is well
19
known to be an expensive material (~137.00 USD/G for chloroplatinic acid hydrate, Sigma-
20
Aldrich) with limited reserves; however, it is used as a CE in DSCs due to its high catalytic
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activity, high electrical conductivity, and high chemical stability in acid-base environments [12-
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14]. Thus, it is difficult to apply on a large scale. In order to introduce an alternative efficient
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electrode, a material with excellent catalytic activity and high electrical conductivity is required.
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Therefore, numerous studies have attempted to developed substitutes for Pt, such as carbon
3
materials, metal oxides, metal carbine, metal nitrides, polymers, and nanohybrids [15]. Among
4
them, graphene materials have attracted significant interest as a potential replacement of the
5
expensive Pt CE in DSCs due to their high electron mobility, large specific area, and excellent
6
electrochemical stability [16, 17].
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It has been reported that due to its high electrical conductivity and good electrocatalytic activity,
8
DSCs with 90 nm-thick Pd used as a CE can yield an efficiency of 4.63% [18] and 0.74% with a
9
very thin Pd nanoparticle (NP) layer CE [19]. Co is known as a typical transition metal that has
10
been used widely as an electrocatalyst candidate in alloying with other metals [20]. Based on this
11
concept, He et al. prepared a blend of PdCo alloy with poly(vinylidene fluoride) as CEs. With a
12
thickness of 5 µm, the PdCo alloy CEs displayed high electrocatalytic activity for the reduction
13
of triiodide ions to iodide ions, fast charge-transfer ability, and low cost [20]. Although the
14
charge-transfer resistance (Rct) was reduced, the diffusion impedance (Zw) remained high due to
15
the thicker layer-coated FTO CE. It has been reported that a high Zw is the primary reason for the
16
high photovoltaic performance in DSCs with Pt CEs [21]. Thus, the optimization of the Rct and
17
Zw values of the developed materials is necessary for their use as CE materials. Furthermore, it is
18
known that the alloying of the transition metals can favor the electronic perturbation of other
19
metals and, therefore, accelerate the electrocatalytic activity of the alloys [21].
20
In this work, we present the synthesis of a new type of CoPd alloy on an RGO-coated FTO
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electrode for DSC applications using the dry plasma reduction (DPR) method. It should be noted
22
that the DPR method was conducted under atmospheric pressure and low temperature, and
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without using toxic chemical reagents [22]. The optimization of the Rct and Zw values for the CEs
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is thoroughly investigated in this work. For this purpose, the co-reduction of the Co and Pd
3
precursor ions on the RGO surface via DPR is designed precisely. It is expected that the
4
developed materials have both high electrical conductivity and excellent catalytic activity for the
5
reduction of triiodide ions to iodide ions.
6
2. Experimental
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2.1. Preparation of the graphene paste and GO-coated FTO electrodes
8
The graphene oxide (GO) paste was prepared from a solution of 1 wt.% ethyl cellulose (Sigma-
9
Aldrich) in alpha-terpineol (Sigma-Aldrich) with 10 mg of graphene grade C750 (XG Science)
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powder through a three-roll miller. Then, the GO paste was coated on an FTO glass substrate
11
using the doctor blade method and dried at 300 °C for 30 min.
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2.2. Preparation of the CoPd/graphene nanohybrid CEs
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First, two precursor solutions of 10 mM cobalt(II) nitrate hexahydrate (Co(NO3)2 ·6H2O, reagent
14
grade, 98%, Sigma-Aldrich) in isopropyl alcohol (IPA) (99.5%, Sigma-Aldrich) and 10 mM
15
Palladium(II) chloride (Sigma-Aldrich) in IPA were prepared. Then, the mixture solutions of
16
both precursors with different volume ratios were prepared.
17
The PdCo alloy was synthesized and deposited on a GO-coated FTO using DPR. Briefly, 8 µl of
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the mixture solutions was dropped on the surface of the GO-coated FTO electrodes. Next, the
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samples were dried on the substrate in a 70 °C oven for 15 min. Finally, the samples were
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reduced using DPR as described in a previous study [17].
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2.3. Preparation of working electrodes and DSC assembly
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Fabrication of working electrodes: First, the FTO was cut into 2 × 2 cm2 pieces and washed with
2
acetone, ethanol, and water before use. The FTO glass was then immersed in 40 mM of TiCl4
3
solution at 70 °C for 30 min in order to formation of a blocking layer. Then, we coated a 12 µm-
4
thick mesoporous TiO2 layer (T/SP, size: 15–20 nm, 100% anatase) and a 4 µm-thick scattering
5
TiO2 layer (Dyesol 18NR-AO) on the modified FTO substrate using a screen printing method.
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After sintering the electrodes at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and
7
500 °C for 15 min, an additional blocking layer was added using a similar procedure. Finally,
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these electrodes were re-sintered at 500 °C for 30 min. After cooling, these electrodes were
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immersed in a 0.3 mM N719 dye-solution, which was prepared in a mixed solvent solution of
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acetonitrile and tert-butyl alcohol with a volume ratio of 1:1, for 24 h.
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DSC assembly: The devices were fabricated into a sandwich-type cell and sealed at 120 °C/5 min
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using a polymer film (Meltonix 1170) with a thickness of 60 µm. We used the solution of 0.6 M
13
1-methyl-3-butylimidazolium iodide, 0.03 M I2, 0.10 M guanadium thiocyanate, and 0.5 M 4-
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tert-butylpiridine in a mixed solvent of acetonitrile and valeronitrile with a volume ratio of 85:15
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as an electrolyte. Finally, the cell was completed by sealing holes on the backside of the CEs
16
which was used to inject electrolyte.
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2.4. Characterization and measurements
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The morphology of the PdCo alloy/RGO was investigated using a high resolution scanning
19
electron microscope (HRSEM; Hitachi S-4800, Hitachi), and a transmission electron microscope
20
(TEM; JEM-2100F HR, JEOL) measurements. The surface chemical state and electronic
21
structure were determined using a Multilab 2000 spectrometer equipped with monochromatic Al
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Kα (1486.65 eV) X-ray radiation under a base pressure of 10-10 Torr. The binding energy scale
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was calibrated using the binding energy positions of the Au 4f7/2 core level at 83.98 eV.
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The electrochemical catalytic activity of the developed CEs was characterized using
4
electrochemical impedance spectroscopy (EIS), Tafel measurements, and cyclic voltammetry
5
(CV). The EIS and Tafel curves were measured on equal electrode cells (i.e. symmetrical
6
dummy cells). For the CV analyses, the three-electrode configuration was used for the
7
electrochemical cells. Seven electrodes were tested as working electrodes in a three electrode
8
configuration. A Pt mesh and an Ag/AgCl electrode functioned as the CE and the reference
9
electrode, respectively. An electrolyte consisting of 10 mmol LiI, 1 mmol I2, and 1 mmol LiClO4
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was used for the measurements. The CVs were recorded in the potential range of 600 to –300
11
mV at a scan rate of 10 mVs-1. The photocurrent-voltage (J-V) characteristics were measured
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under simulated AM 1.5G sunlight at 100 mWcm-2 irradiance. A Sun 3000 solar simulator
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(ABET Technologies Inc., USA) was used for the light generation, and it was calibrated using
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the calibrated silicon reference cell PECSI01 (HS Technologies Inc., Korea).
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3. Results and Discussion
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3.1. Synthesis and characterization of the Co0.9Pd0.1/RGO nanohybrid materials.
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Fig. 1(a) and 1(b) present the HRSEM and TEM images of the Co0.9Pd0.1/RGO nanohybrid. As
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seen in Fig. 1(a), the Co0.9Pd0.1 NPs were successfully immobilized on the surface of the RGO
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without agglomeration. Note that the reduction of GO to RGO through DPR was verified by X-
20
ray photoelectron spectroscopy (XPS) and Raman in our previous publication [17]. Bimetallic
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CoPd NPs with a small size on the RGO surface are clearly visible in the TEM images in Fig.
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1(b). As seen in the figure, the size of the Co0.9Pd0.1 NPs was approximately 2 nm. The bimetallic
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Co0.9Pd0.1 were not only uniform, but were also well distributed on the RGO surface without
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aggregation. The lattice spacing was calculated to be 3.64 Å, which agrees well with the {111}
3
plane of Co0.9Pd0.1 [23].
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The XPS measurements were conducted in order to determine the surface compositions of the
5
Co0.9Pd0.1 alloy/RGO. The results are presented in Fig. 1(c). As seen in the figure, the Co and Pd
6
elements coexist on the Co0.9Pd0.1/RGO. It should be noted that the molar ratios of Co and Pd in
7
the precursor solutions cannot match the real atomic ratios of Co and Pd in the corresponding
8
bimetallic CoPd NPs, as revealed by the XPS analyses. For example, the chemical formula of
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Co0.9Pd0.1/RGO was determined through XPS to be Co0.82Pd0.18/RGO.
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In order to investigate the variation of the Co and Pd oxidation states on the surface of the CoPd
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alloys, the XPS spectra of Co2p and Pd3p were recorded; the results are presented in Fig. 1(d)
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and 1(e), respectively. The fitting was achieved through combining Shirley’s and Tougaard’s
13
backgrounds. It was found that the variations of the Pd oxidation states were Pd0 and Pd2+, which
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corresponded to the Pd@PdO structure. These results are in good agreement with our previous
15
work [24]. The variation of the Co oxidation states were Co2+ and Co3+, which are the
16
characteristic peaks of Co3O4 [25]. The results indicate that the state of the CoPd alloy on the
17
RGO surface was the oxidation state.
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3.2. Optimization of nanohybrid counter electrodes for highly efficient DSCs
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3.2.1. Scanning electron microscopy measurements
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HRSEM measurements were conducted in order to investigate the morphologies of the CoxPd1-
21
x/RGO-coated
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graphene-coated FTO electrode was also prepared and used as a reference for comparison with
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FTO CEs. The obtained HRSEM images are presented in Fig. 2. In this work, a
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the alloy electrodes. As seen in Fig. 2(a), the graphene-coated FTO electrode had a 3D network
2
structure with a flake size in the range of 100 nm to 500 nm. Fig. 2(b) to 2(h) present the
3
morphologies of the CoxPd1-x alloys/RGO prepared with different volume ratios of precursors
4
under DPR. It was found that the CoxPd1-x alloy NPs were immobilized on the graphene flake
5
surfaces for all CEs. As seen in the figures, the CoPd alloy NPs were immobilized on the RGO
6
surfaces without agglomeration. A difference in the particle sizes could not be found in this study.
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3.2.2. Cyclic voltammogram measurements
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In order to evaluate the catalytic activity of the CoPd/RGO nanohybrid CEs, CV measurements
9
were conducted; the experimental results are presented in Fig. 3(a). The catalytic activities of the
10
electrodes were estimated using the values of the reduction currents (Jred) and the peak-to-peak
11
separations (∆E). The corresponding data are listed in Table 1. It was found that the |Ired| value
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increased with the initial decrease in the volume percentage of the Co precursors in the mixture
13
solution from 100% to 90%, but it decreased with further reductions in the volume percentage of
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the Co precursors in mixture solution from 90% to 0%. The highest |Ired| value was 1.341 mA for
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the Co0.9Pdo0.1/RGO electrode. Note that there was no change in the |Ired| values when the
16
volume ratio of the Co:Pd precursor decreased from 0.5:0.5 to 0:1, as described in Table 1. The
17
results demonstrate that the highest diffusion current of ions [26], the lowest diffusion impedance
18
(Zw), and the largest surface active area [27] were obtained for the Co0.9Pdo0.1/RGO electrode.
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The results were further confirmed through the EIS measurements.
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The ∆E values were calculated from the CV curves for the CoPd/RGO CEs. The extracted ∆E
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values are listed in Table 1. Both Co1Pd0/RGO and Co0Pd1/RGO electrodes were used as
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references electrodes. The ∆E for the reference CEs were estimated to be 570 and 710 mV for
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Co1Pd0/RGO and Co0Pd1/RGO, respectively. As seen in Table 1, the ∆E values recorded for the
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developed CEs were smaller than the reference electrodes. It is known that a lower ∆E indicates
3
a faster rate of I3-/I- redox reaction with increases in the catalytic area [27]. The observations in
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the CV curves agreed well with the charge-transfer resistance (Rct) and open-circuit voltage (Voc)
5
recorded for the DSCs [28, 29]. The lowest ∆E value was 470 mV for the Co0.9Pdo0.1/RGO
6
electrode. The result indicates that the highest catalytic activity was obtained for
7
Co0.9Pdo0.1/RGO electrode [27].
8
The chemical stability of the developed materials was tested for the reduction of triiodide ions to
9
iodide ions. Note that this test is generally required for the application of CoPd alloy/RGO to
10
CEs in DSCs. For this purpose, the current-voltage curve measurements were conducted over
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100 cycles for the redox system with the Co0.9Pd0.1/RGO electrode. The extracted values of Jred
12
and the oxidation current (Joxd) from the experimental current-voltage curves recorded over 100
13
cycles are provided in Fig. 3(b) as functions of the cycle number. It is seen clearly that there
14
were no noticeable changes in the values of Jred and Joxd over 100 cycles of the Co0.9Pd0.1
15
alloy/RGO electrode, which indicates that the Co0.9Pd0.1 alloy/RGO films have high stability
16
under electrochemical reaction conditions.
17
3.2.3. Electrochemical impedance spectroscopy
18
The EIS performances were obtained in order to examine the interfacial charge-transfer
19
resistance (Rct) properties of the triiodide/iodide couple on the CE surfaces. Note that the EIS
20
measurements were conducted on different symmetrical dummy cells fabricated with two
21
identical CoxPd1-x/RGO electrodes. The Nyquist plots are presented in Fig. 4(a) and 4(b). The
22
equivalent circuit, which was used to fit the spectra with the Z-view software, is depicted in the
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inset of Fig. 4(b). The parameters of the EIS spectra obtained from the equivalent circuit are
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summarized in Table 1. It was found that the Rct values followed the sequence of Co0.9Pd0.1/RGO
3
(1.17 Ω) < Co0.7Pd0.3/RGO (2.14 Ω) < Co0.5Pd0.5/RGO (2.52 .52< Co0.3Pd0.7/RGO (3.30 .30<
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Co0.1Pd0.9/RGO (4.39 Ω) < Co0Pd1/RGO (5.78 .78< Co1Pd0/RGO (8.88 .8. A lower Rct at the
5
CE/electrolyte interface indicated a higher reaction rate of the iodide ions regeneration from the
6
triiodide ions. The electron lifetime (τ) data for the triiodide ion reduction (τ = 1/2πfmax, where
7
fmax is the peak frequency in the Bode EIS plots) and exchange current density (Jo = RT/nFRct,
8
where R, T, n, and F are the gas constant, absolute temperature, number of electrons involved in
9
the reduction of iodide electrolyte, and Faraday’s constant, respectively, in the Tafel
10
measurements) were conducted in order to further confirm the electrocatalytic activity of the
11
developed electrodes. The experimental results are presented in Fig. 4(c) for the Bode EIS plots
12
and Fig. 4(d) for the Tafel curves. Analyses of the Bode EIS plots and Tafel measurements
13
revealed that the trends of the fmax and Jo values followed the sequence of Co0.9Pd0.1/RGO >
14
Co0.7Pd0.3/RGO > Co0.5Pd0.5/RGO > Co0.3Pd0.7/RGO > Co0.1Pd0.9/RGO > Co0Pd1/RGO >
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Co1Pd0/RGO. Given that τ = 1/2πfmax and Jo = RT/nFRct, the trends in the values of τ and Jo were
16
similar to that of the Rct values. It has been reported that a shorter τ indicates a faster electron
17
transfer across the CE/electrolyte interface [19] and that a high Jo value indicates a high Jsc in
18
DSCs [30]. Table 1 also provides the Zw values of the electrodes. As seen in the table, Zw became
19
small with the initial decrease in the volume percentage of Co precursors in the mixture solution
20
from 100% to 90%, but it increased with further reductions in the volume percentage of Co
21
precursors in the mixture solution from 70% to 0%. The lowest Zw value was 16.34 Ω for the
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Co0.9Pd0.1/RGO electrode. The results agree well with those of the CV analyses, the limiting
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diffusion current density (Jlim) values, which were obtained from the Tafel measurements, and
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the SEM observations.
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3.2.4. Photovoltaic performances
4
As discussed above, an optimized balance between Rct and Zw is required in order to achieve a
5
higher DSC performance. In order to further confirm these relationships, the PV performances
6
were investigated. The J-V characteristics are described in Fig. 5(a), and the PV parameters are
7
summarized in Table 2. It was found that the efficiencies followed the sequence of
8
Co0.9Pd0.1/RGO (6.40%) > Co0.7Pd0.3/RGO (5.97%) > Co0.5Pd0.5/RGO (5.79%) > Co0.3Pd0.7/RGO
9
(5.72%) > Co0.1Pd0.9/RGO (5.48%) > Co0Pd1/RGO (5.41%) > Co1Pd0/RGO (5.14%). The highest
10
efficiency of 6.40% was obtained for the DSC with the Co0.9Pd0.1/RGO CE, while the efficiency
11
of the cells fabricated with the Co1Pd0/RGO and Co0Pd1/RGO CEs were 5.17% and 5.41%,
12
respectively. This might result from the balance effect between the Rct and Zw values [31]. The
13
enhancement in the Jsc values compared with the reference electrodes could be ascribed to the
14
enhancement in the catalytic activity of the alloy CEs [32] and the decreases in the diffusion
15
impedance in the developed CEs [33]. In order to further confirm the improvement in the Jsc
16
value, incident photon-to-current efficiency (IPCE) performance tests were conducted, as
17
depicted in Fig. 5(b). It was found that the IPCE of the device with the Co0.9Pd0.1/RGO CE was
18
higher than that of the cells assembled with the references CEs over the visible wavelength. The
19
current values calculated from the overlap integral of the IPCE spectra align with the Jsc derived
20
from the J-V characteristics. As discussed previously, the difference in the Voc values was
21
supported by the ∆E measurements [34, 35]. Therefore, this result aligns with the XPS, SEM,
22
CV, EIS, and Tafel measurements.
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4. Conclusions
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Using DPR, CoxPd1-x alloys were successfully synthesized and immobilized on RGO surfaces
3
with different volume ratios of Co and Pd precursors. The synthesized materials were
4
characterized using HRSEM, TEM, XRD, and XPS measurements. The size and distribution of
5
the CoPd alloys on the RGO surface was controlled through changing the volume percentage of
6
the Co precursor in the mixed solution. The developed materials were tested as Pt-free CEs for
7
DSCs. The electrochemical catalytic activity of the CEs was examined using CV, EIS, and Tafel
8
measurements. In the results, the catalytic activity of the electrodes followed the sequence of
9
Co0.9Pd0.1/RGO > Co0.7Pd0.3/RGO > Co0.5Pd0.5/RGO > Co0.3Pd0.7/RGO > Co0.1Pd0.9/RGO >
10
Co0Pd1/RGO > Co1Pd0/RGO. Furthermore, Zw became small with the initial decrease in the
11
volume percentage of the Co precursors in the mixture solution from 100% to 90%, but it
12
increased with further reductions in the volume percentage of Co precursors in the mixture
13
solution from 70% to 0%. Accordingly, the efficiencies had the following order: Co0.9Pd0.1/RGO
14
(6.40%) > Co0.7Pd0.3/RGO (5.97%) > Co0.5Pd0.5/RGO (5.79%) > Co0.3Pd0.7/RGO (5.72%) >
15
Co0.1Pd0.9/RGO (5.48%) > Co0Pd1/RGO (5.41%) > Co1Pd0/RGO (5.14%). Furthermore, the
16
results indicated that the developed CEs were stable in iodide electrolytes. This work can be very
17
useful for the fabrication of CoxPd1-x/RGO CEs for application in efficient DSCs, as well as other
18
fields such as methanol oxidation, oxygen reduction reaction, and so on.
19
Acknowledgements
20
This
21
(2014R1A2A2A01006994), the Korea Research Fellowship Program (2015H1D3A1061830),
22
and a Korea CCS R&D Center (KCRC) grant (2014M1A8A1049345). These are all funded by
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research
was
supported
by
a
National
12
Research
Foundation
(NRF)
grant
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the Ministry of Science, ICT and Future Planning through the National Research Foundation of
2
Korea. This research was also supported through the Creative Human Resource Development
3
Consortium for Fusion Technology of Functional Chemical/Bio Materials grant funded by the
4
Brain Korea 21 Plus grant (21A20151513147).
5
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Rep. 6 (2016) 25227-25236.
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performance by core shell Ag@ organic (organic= 2-nitroaniline, PVA, 4-choloroaniline and
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PVP): Effects of shell type on photocurrent, Electrochimi. Acta 153 (2015) 90-96.
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[6] M. Sabet, M. Salavati-Niasari, O. Amiri, Using different chemical methods for deposition of
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CdS on TiO2 surface and investigation of their influences on the dye-sensitized solar cell
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performance, Electrochimi. Acta 117 (2014) 504-520.
4
[7] V.D. Dao, H.S. Choi, Highly-efficient plasmon-enhanced dye-sensitized solar cells created
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by means of dry plasma reduction, Nanomaterials 6 (2016) 70-79.
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[8] N.T.Q. Hoa, V.D. Dao, H.S. Choi, A facile one-pot synthesis of hierarchical TiO2
7
nanourchins for highly efficient dye-sensitized solar cells, J. Electrochem. Soc. 161 (2014)
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H627-H632.
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[9] V.D. Dao, L.L. Larina, H.S. Choi, Plasma reduction of nanostructured TiO2 electrode to
10
improve photovoltaic efficiency of dye-sensitized solar cells, J. Electrochem. Soc. 161 (2014)
11
H896-H902.
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[10] V.D. Dao, L. Larina, H.S. Choi, Suppression of charge recombination in dye-sensitized
13
solar cells using the plasma treatment of fluorine-doped tin oxide substrates, J. Electrochem. Soc.
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162 (2015) H903-H909.
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[11] V.D. Dao, L. Larina, H.S. Choi, Minimizing energy losses in perovskite solar cells using
16
plasma treated transparent conducting layers, Thin Solid Films 593 (2015) 10-16.
17
[12] N. Papageorgiou, Counter-electrode function in nanocrystalline photoelectrochemical cell
18
configurations, Coord. Chem. Rev. 248 (2004)1421-1446.
19
[13] A. Hauch, A. Georg, Diffusion in the electrolyte and charge-transfer reaction at the platinum
20
electrode in dye-sensitized solar cells, Electrochimi. Acta 46 (2001) 3457-3466.
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2
aqueous and organic media, J. Electrochem. Soc. 144 (1997) 876-884.
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[15] S. Yun, A. Hagfeldt, T. Ma, Pt-Free counter electrode for dye-sensitized solar cells with
4
high efficiency, Adv. Mater. 26 (2014) 6210-6237.
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[16] L. Kavan, J.H. Yum, M. Gratzel, Graphene nanoplatelets outperforming platinum as the
6
electrocatalyst in co-bipyridine-mediated dye-sensitized solar cells, Nano Lett. 11 (2011) 5501-
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5506.
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[17] V.D. Dao, L. Larina, H. Suh, K. Hong, J.K. Lee, H.S. Choi, Optimum strategy for designing
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a graphene-based counter electrode for dye-sensitized solar cells, Carbon 77 (2014) 980-992.
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[18] Y.Y. Noh, O.S. Song, Property of palladium counter electrode for dye sensitized solar cells
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J. Kor. Inst. Met. Mater. 51 (2013) 71–76.
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[19] W.-Y. Lee, V.-D. Dao, H.-S. Choi, Shape-controlled synthesis of PtPd alloys as a low-cost
13
and efficient counter electrode for dye-sensitized solar cells, RSC Adv. 6 (2016) 38310–38314.
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[20] B. He, Q. Tang, H. Zhang, L. Yu, Counter electrode electrocatalysts from binary Pd–Co
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alloy nanoparticles for dye-sensitized solar cells, Solar Energy 124 (2016) 68–75.
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[21] G.R. Li, J. Song, G.L. Pan, X.P. Gao, Highly Pt-like electrocatalytic activity of transition
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metal nitrides for dye-sensitized solar cells, Energy Environ. Sci. 4 (2011) 1680-1683.
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[22] V.D. Dao, Q.C. Tran, S.H. Ko, H.S. Choi, Dry plasma reduction to synthesize supported
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platinum nanoparticles for flexible dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 4436-
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4443.
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Forschungszentrum Jülich, 2013.
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nanocomposite as a high-performance electrocatalyst support for formic acid oxidation, S. Afr. J.
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Chem. 66 (2013) 86–91.
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[25] Z. Zhang, Y. Chen, J. Bao, Z. Xie, J. Wei, Z. Zhou, Co3O4 hollow nanoparticles and Co
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organic complexes highly dispersed on N-doped graphene: an efficient cathode catalyst for Li-O2
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batteries, Part. Part. Syst. Charact. 32 (2015) 680-685.
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[26] Y. Xiao, G. Han, Efficient hydrothermal-processed platinum–nickel bimetallic nano-
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catalysts for use in dye-sensitized solar cells, J. Power Sources 294 (2015) 8-15.
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[27] V.D. Dao, H.S. Choi, Pt nanourchins as efficient and robust counter electrode materials for
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dye-sensitized solar cells, ACS Appl. Mater. Interfaces 8 (2016) 1004-1010.
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[28] J. Roy-Mayhew, D. Bozym, C. Punckt, I. Aksay, Functionalized graphene as a catalytic
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counter electrode in dye-sensitized solar cells, ACS Nano 4 (2010) 6203-6211.
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[29] M. Wang, A. Anghel, B. Marsan, N. Ha, N. Pootrakulchote, S. Zakeeruddin, M. Grätzel,
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CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells,
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J. Am. Chem. Soc. 131 (2009) 15976-15977.
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[30] V.D. Dao, Comment on “Energy storage via polyvinylidene fluoride dielectric on the
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counter electrode of dye-sensitized solar cells” by Jiang et al., J. Power Sources 337 (2017) 125-
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129.
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[31] T.N. Murakami, S. Ito, Q. Wang, M.K. Nazeeruddin, T. Bessho, I. Cesar, P. Liska, R.
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Humphry-Baker, P. Comte, P. Péchy, M. Grätzel. Highly efficient dye-sensitized solar cells
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based on carbon black counter electrodes, J. Electrochem. Soc. 153 (2006) A2255-A2261.
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[32] V.D. Dao, L. Larina, J.K. Lee, K.D. Jung, B.T. Huy, H.S. Choi, Graphene-based RuO2
5
nanohybrid as a highly efficient catalyst for triiodide reduction in dye-sensitized solar cells,
6
Carbon 81 (2015) 710-719.
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[33] V.D. Dao, L. Larina, K.D. Jung, J.K. Lee, H.S. Choi, Graphene-NiO nanohybrid prepared
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by dry plasma reduction as a low-cost counter electrode material for dye-sensitized solar cells,
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Nanoscale 6 (2014) 477-482.
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[34] S.Y. Jang, Y.G. Kim, D.Y. Kim, H.G. Kim, S.M. Jo, Electrodynamically sprayed thin films
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of aqueous dispersible graphene nanosheets: Highly efficient cathodes for dye-sensitized solar
12
cells, ACS Appl. Mater. Interfaces 4 (2012) 3500-3507.
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[35] V.D. Dao, S.H. Kim, H.S. Choi, J.H. Kim, H.O. Park, J.K. Lee, Efficiency enhancement of
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dye-sensitized solar cell using Pt hollow sphere counter electrode, J. Phys. Chem. C 115 (2011)
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25529-25534.
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Table 1. Counter electrode properties extracted from the CV, EIS spectra, and Tafel
2
measurements.
|Ired|
∆E
electrode
(mA)
(mV)
Co1Pd0/RGO
1.267
Co0.9Pd0.1/RGO
Rct
(Ω)
570
8.88
16.46
1.341
470
1.17
6.34
Co0.7Pd0.3/RGO
1.308
550
2.14
8.40
Co0.5Pd0.5/RGO
1.270
710
2.52
9.26
Co0.3Pd0.7/RGO
1.257
710
3.30
29.38
Co0.1Pd0.9/RGO
1.250
710
4.39
36.97
Co0Pd1/RGO
1.171
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(Ω)
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710
|Jred|: absolute value of the redox current peak (I3- + 2e- 3I-)
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∆E: peak-to-peak separation
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Rct: charge-transfer resistance
7
Zw: diffusion impedance
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5.78
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Table 2. Photovoltaic parameters of DSCs with various CoPd/RGO CEs.
Counter
Jsc -2
Voc
FF
η
(mAcm )
(mV)
(%)
(%)
Co1Pd0/RGO
12.37
620
67.38
5.17
Co0.9Pd0.1/RGO
14.06
670
67.98
6.40
Co0.7Pd0.3/RGO
13.92
630
67.53
5.97
Co0.5Pd0.5/RGO
13.59
635
67.08
5.79
Co0.3Pd0.7/RGO
13.65
640
65.44
5.72
Co0.1Pd0.9/RGO
13.43
625
Co0Pd1/RGO
13.34
625
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electrode
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65.25
5.48
64.86
5.41
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1 2
Fig. 1. Characteristics of the Co0.9Pd0.1/RGO nanohybrid: (a) HRSEM, (b) TEM, (c) XPS survey,
3
(d) P3d, and (e) Co2p narrow XPS of Co0.9Pd0.1/RGO. 20
1
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Fig. 2. SEM images of the CoPd alloy/RGO CEs: (a) GO, (b) Co1Pd0/RGO, (c) Co0.9Pd0.1/RGO,
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(d) Co0.7Pd0.3/RGO, (e) Co0.5Pd0.5/RGO, (f) Co0.3Pd0.7/RGO, (g) Co0.1Pd0.9/RGO, and (h)
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Co0Pd1/RGO. 21
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Fig. 3. (a) CV curves with different working electrodes at a scan rate of 50 mVs-1. (b)
3
Relationship between the peak current and square root of the scan rates. (c) Repeated 100-cycle
4
CV curves of the CEs in the iodide/triiodide ions redox electrolyte recorded at a scan rate of 50
5
mVs-1. (d) Plots of Jred and Jox as a function of the cycle number. 22
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Fig. 4. (a, b) Nyquist plots of the symmetrical dummy cells with two identical CEs. The inset
3
depicts the equivalent circuit diagram used to fit the EIS spectra. Rh: ohmic serial resistance; Rtrns:
4
electron transport resistance in the graphene layer; CPEtrap: constant phase element in the
5
graphene layer; Rct: charge transfer resistance at the electrode/electrolyte interface; CPEdl:
6
constant phase element at the electrode/electrolyte interface; and W: Warburg impedance. (c)
7
Bode phase plots of the symmetrical dummy cells. (d) Tafel curves of different symmetrical
8
dummy cells that are similar to those used for the EIS measurements.
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Fig. 5. (a) Characteristic current density-voltage curves of the DSCs with different CEs
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measured under standard conditions, and (b) IPCE curves of the different DSCs.
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•
Bimetallic CoPd nanoparticles/reduced graphene oxide are prepared by dry plasma reduction.
•
The electrocatalytic activity as well as electric conductivity is significantly enhanced.
•
The highest efficiency is recorded from DSC employed Co0.9Pd0.1/reduced graphene oxide counter electrode.
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nanoparticles/reduced graphene oxide.
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S0925-8388(17)30626-6
DOI:
10.1016/j.jallcom.2017.02.180
Reference:
JALCOM 40908
To appear in:
Journal of Alloys and Compounds
Received Date: 17 December 2016 Revised Date:
12 February 2017
Accepted Date: 18 February 2017
Please cite this article as: H.-J. Oh, V.-D. Dao, H.-S. Choi, Cost-effective CoPd alloy/reduced graphene oxide counter electrodes as a new avenue for high-efficiency liquid junction photovoltaic devices, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.180. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Cost-effective CoPd alloy/reduced graphene oxide counter
2
electrodes as a new avenue for high-efficiency liquid junction
3
photovoltaic devices
4
Hyo-Jun Oh, Van-Duong Dao*, Ho-Suk Choi*
5
Department of Chemical Engineering & Applied Chemistry, Chungnam National University,
6
Daejeon, 305-764, South Korea
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*Corresponding authors; email: [email protected]/[email protected] (V.D.D.)
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and [email protected] (H.S.C.)
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Abstract
This work presents the synthesis of CoxPd1-x alloys (0 ≤ x ≤ 1) on a reduced graphene oxide
11
(RGO) surface using the dry plasma reduction method. The formation of CoPd alloys on the
12
RGO surface is confirmed through high-resolution scanning electron microscopy (HRSEM) and
13
X-ray photoelectron spectroscopy (XPS) measurements, X-ray diffraction, and transmittance
14
electron microscopy (TEM). Then, the developed materials are applied as Pt-free counter
15
electrodes (CEs) in dye-sensitized solar cells (DSCs). In order to obtain efficient CEs, the
16
chemical composition of the CoxPd1-x/RGO is controlled through optimizing the volume ratio of
17
the Co and Pd precursors during the synthesizing process. Due to the optimization of the charge-
18
transfer resistance (Rct) and the diffusion impedance (Zw) values of the Co0.9Pd0.1/RGO CE, the
19
device using the Co0.9Pd0.1/RGO CE exhibits the highest efficiency among the fabricated cells.
20
Note that the cells using Co/RGO and Pd/RGO exhibit efficiencies of 5.17% and 5.41%,
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respectively. The proposed strategy is simple and efficient; thus, it is promising for fabricating
2
highly efficient and low-cost CE materials for DSCs.
3
Keywords: dry plasma reduction; CoPd alloy; reduce graphene oxide; counter electrode; dye-
4
sensitized solar cell
5
1. Introduction
6
Due to their low-cost fabrication, environmentally benign process, and high power energy
7
conversion efficiency (up to 14.3% [1]), a photovoltaic (PV) cell based on a liquid-junction solar
8
cell and a dye-sensitized solar cell (DSC) has significant potential to become a universal
9
environmentally friendly next-generation PV device [2]. From a scientific perspective, the DSC
10
efficiency can reach 20% and higher, and this is highly competitive with other types of
11
conventional thin film PV cells [3]. As mentioned above, the highest achieved efficiency is
12
14.3%, which is lower than the expected value (20%). It has been reported that the working
13
electrode has the greatest potential to improve the efficiency of DSCs. There are several ways to
14
improve the efficiency of DSCs through adding CdS quantum-dot in TiO2 layer [4], using Ag
15
nanoparticles for plasmonic enhancement effect [4-7], controlling the morphology of working
16
electrode to become TiO2 nanourchin structure [8], or modifying the surface of flourine-doped
17
tin oxide (FTO) and TiO2 layer though plasma treatment [9-11]. However, the counter electrode
18
(CE) is also important for the reduction of triiodide ions to iodide ions. In general, Pt is well
19
known to be an expensive material (~137.00 USD/G for chloroplatinic acid hydrate, Sigma-
20
Aldrich) with limited reserves; however, it is used as a CE in DSCs due to its high catalytic
21
activity, high electrical conductivity, and high chemical stability in acid-base environments [12-
22
14]. Thus, it is difficult to apply on a large scale. In order to introduce an alternative efficient
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electrode, a material with excellent catalytic activity and high electrical conductivity is required.
2
Therefore, numerous studies have attempted to developed substitutes for Pt, such as carbon
3
materials, metal oxides, metal carbine, metal nitrides, polymers, and nanohybrids [15]. Among
4
them, graphene materials have attracted significant interest as a potential replacement of the
5
expensive Pt CE in DSCs due to their high electron mobility, large specific area, and excellent
6
electrochemical stability [16, 17].
7
It has been reported that due to its high electrical conductivity and good electrocatalytic activity,
8
DSCs with 90 nm-thick Pd used as a CE can yield an efficiency of 4.63% [18] and 0.74% with a
9
very thin Pd nanoparticle (NP) layer CE [19]. Co is known as a typical transition metal that has
10
been used widely as an electrocatalyst candidate in alloying with other metals [20]. Based on this
11
concept, He et al. prepared a blend of PdCo alloy with poly(vinylidene fluoride) as CEs. With a
12
thickness of 5 µm, the PdCo alloy CEs displayed high electrocatalytic activity for the reduction
13
of triiodide ions to iodide ions, fast charge-transfer ability, and low cost [20]. Although the
14
charge-transfer resistance (Rct) was reduced, the diffusion impedance (Zw) remained high due to
15
the thicker layer-coated FTO CE. It has been reported that a high Zw is the primary reason for the
16
high photovoltaic performance in DSCs with Pt CEs [21]. Thus, the optimization of the Rct and
17
Zw values of the developed materials is necessary for their use as CE materials. Furthermore, it is
18
known that the alloying of the transition metals can favor the electronic perturbation of other
19
metals and, therefore, accelerate the electrocatalytic activity of the alloys [21].
20
In this work, we present the synthesis of a new type of CoPd alloy on an RGO-coated FTO
21
electrode for DSC applications using the dry plasma reduction (DPR) method. It should be noted
22
that the DPR method was conducted under atmospheric pressure and low temperature, and
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without using toxic chemical reagents [22]. The optimization of the Rct and Zw values for the CEs
2
is thoroughly investigated in this work. For this purpose, the co-reduction of the Co and Pd
3
precursor ions on the RGO surface via DPR is designed precisely. It is expected that the
4
developed materials have both high electrical conductivity and excellent catalytic activity for the
5
reduction of triiodide ions to iodide ions.
6
2. Experimental
7
2.1. Preparation of the graphene paste and GO-coated FTO electrodes
8
The graphene oxide (GO) paste was prepared from a solution of 1 wt.% ethyl cellulose (Sigma-
9
Aldrich) in alpha-terpineol (Sigma-Aldrich) with 10 mg of graphene grade C750 (XG Science)
10
powder through a three-roll miller. Then, the GO paste was coated on an FTO glass substrate
11
using the doctor blade method and dried at 300 °C for 30 min.
12
2.2. Preparation of the CoPd/graphene nanohybrid CEs
13
First, two precursor solutions of 10 mM cobalt(II) nitrate hexahydrate (Co(NO3)2 ·6H2O, reagent
14
grade, 98%, Sigma-Aldrich) in isopropyl alcohol (IPA) (99.5%, Sigma-Aldrich) and 10 mM
15
Palladium(II) chloride (Sigma-Aldrich) in IPA were prepared. Then, the mixture solutions of
16
both precursors with different volume ratios were prepared.
17
The PdCo alloy was synthesized and deposited on a GO-coated FTO using DPR. Briefly, 8 µl of
18
the mixture solutions was dropped on the surface of the GO-coated FTO electrodes. Next, the
19
samples were dried on the substrate in a 70 °C oven for 15 min. Finally, the samples were
20
reduced using DPR as described in a previous study [17].
21
2.3. Preparation of working electrodes and DSC assembly
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Fabrication of working electrodes: First, the FTO was cut into 2 × 2 cm2 pieces and washed with
2
acetone, ethanol, and water before use. The FTO glass was then immersed in 40 mM of TiCl4
3
solution at 70 °C for 30 min in order to formation of a blocking layer. Then, we coated a 12 µm-
4
thick mesoporous TiO2 layer (T/SP, size: 15–20 nm, 100% anatase) and a 4 µm-thick scattering
5
TiO2 layer (Dyesol 18NR-AO) on the modified FTO substrate using a screen printing method.
6
After sintering the electrodes at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and
7
500 °C for 15 min, an additional blocking layer was added using a similar procedure. Finally,
8
these electrodes were re-sintered at 500 °C for 30 min. After cooling, these electrodes were
9
immersed in a 0.3 mM N719 dye-solution, which was prepared in a mixed solvent solution of
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acetonitrile and tert-butyl alcohol with a volume ratio of 1:1, for 24 h.
11
DSC assembly: The devices were fabricated into a sandwich-type cell and sealed at 120 °C/5 min
12
using a polymer film (Meltonix 1170) with a thickness of 60 µm. We used the solution of 0.6 M
13
1-methyl-3-butylimidazolium iodide, 0.03 M I2, 0.10 M guanadium thiocyanate, and 0.5 M 4-
14
tert-butylpiridine in a mixed solvent of acetonitrile and valeronitrile with a volume ratio of 85:15
15
as an electrolyte. Finally, the cell was completed by sealing holes on the backside of the CEs
16
which was used to inject electrolyte.
17
2.4. Characterization and measurements
18
The morphology of the PdCo alloy/RGO was investigated using a high resolution scanning
19
electron microscope (HRSEM; Hitachi S-4800, Hitachi), and a transmission electron microscope
20
(TEM; JEM-2100F HR, JEOL) measurements. The surface chemical state and electronic
21
structure were determined using a Multilab 2000 spectrometer equipped with monochromatic Al
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Kα (1486.65 eV) X-ray radiation under a base pressure of 10-10 Torr. The binding energy scale
2
was calibrated using the binding energy positions of the Au 4f7/2 core level at 83.98 eV.
3
The electrochemical catalytic activity of the developed CEs was characterized using
4
electrochemical impedance spectroscopy (EIS), Tafel measurements, and cyclic voltammetry
5
(CV). The EIS and Tafel curves were measured on equal electrode cells (i.e. symmetrical
6
dummy cells). For the CV analyses, the three-electrode configuration was used for the
7
electrochemical cells. Seven electrodes were tested as working electrodes in a three electrode
8
configuration. A Pt mesh and an Ag/AgCl electrode functioned as the CE and the reference
9
electrode, respectively. An electrolyte consisting of 10 mmol LiI, 1 mmol I2, and 1 mmol LiClO4
10
was used for the measurements. The CVs were recorded in the potential range of 600 to –300
11
mV at a scan rate of 10 mVs-1. The photocurrent-voltage (J-V) characteristics were measured
12
under simulated AM 1.5G sunlight at 100 mWcm-2 irradiance. A Sun 3000 solar simulator
13
(ABET Technologies Inc., USA) was used for the light generation, and it was calibrated using
14
the calibrated silicon reference cell PECSI01 (HS Technologies Inc., Korea).
15
3. Results and Discussion
16
3.1. Synthesis and characterization of the Co0.9Pd0.1/RGO nanohybrid materials.
17
Fig. 1(a) and 1(b) present the HRSEM and TEM images of the Co0.9Pd0.1/RGO nanohybrid. As
18
seen in Fig. 1(a), the Co0.9Pd0.1 NPs were successfully immobilized on the surface of the RGO
19
without agglomeration. Note that the reduction of GO to RGO through DPR was verified by X-
20
ray photoelectron spectroscopy (XPS) and Raman in our previous publication [17]. Bimetallic
21
CoPd NPs with a small size on the RGO surface are clearly visible in the TEM images in Fig.
22
1(b). As seen in the figure, the size of the Co0.9Pd0.1 NPs was approximately 2 nm. The bimetallic
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Co0.9Pd0.1 were not only uniform, but were also well distributed on the RGO surface without
2
aggregation. The lattice spacing was calculated to be 3.64 Å, which agrees well with the {111}
3
plane of Co0.9Pd0.1 [23].
4
The XPS measurements were conducted in order to determine the surface compositions of the
5
Co0.9Pd0.1 alloy/RGO. The results are presented in Fig. 1(c). As seen in the figure, the Co and Pd
6
elements coexist on the Co0.9Pd0.1/RGO. It should be noted that the molar ratios of Co and Pd in
7
the precursor solutions cannot match the real atomic ratios of Co and Pd in the corresponding
8
bimetallic CoPd NPs, as revealed by the XPS analyses. For example, the chemical formula of
9
Co0.9Pd0.1/RGO was determined through XPS to be Co0.82Pd0.18/RGO.
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In order to investigate the variation of the Co and Pd oxidation states on the surface of the CoPd
11
alloys, the XPS spectra of Co2p and Pd3p were recorded; the results are presented in Fig. 1(d)
12
and 1(e), respectively. The fitting was achieved through combining Shirley’s and Tougaard’s
13
backgrounds. It was found that the variations of the Pd oxidation states were Pd0 and Pd2+, which
14
corresponded to the Pd@PdO structure. These results are in good agreement with our previous
15
work [24]. The variation of the Co oxidation states were Co2+ and Co3+, which are the
16
characteristic peaks of Co3O4 [25]. The results indicate that the state of the CoPd alloy on the
17
RGO surface was the oxidation state.
18
3.2. Optimization of nanohybrid counter electrodes for highly efficient DSCs
19
3.2.1. Scanning electron microscopy measurements
20
HRSEM measurements were conducted in order to investigate the morphologies of the CoxPd1-
21
x/RGO-coated
22
graphene-coated FTO electrode was also prepared and used as a reference for comparison with
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FTO CEs. The obtained HRSEM images are presented in Fig. 2. In this work, a
7
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the alloy electrodes. As seen in Fig. 2(a), the graphene-coated FTO electrode had a 3D network
2
structure with a flake size in the range of 100 nm to 500 nm. Fig. 2(b) to 2(h) present the
3
morphologies of the CoxPd1-x alloys/RGO prepared with different volume ratios of precursors
4
under DPR. It was found that the CoxPd1-x alloy NPs were immobilized on the graphene flake
5
surfaces for all CEs. As seen in the figures, the CoPd alloy NPs were immobilized on the RGO
6
surfaces without agglomeration. A difference in the particle sizes could not be found in this study.
7
3.2.2. Cyclic voltammogram measurements
8
In order to evaluate the catalytic activity of the CoPd/RGO nanohybrid CEs, CV measurements
9
were conducted; the experimental results are presented in Fig. 3(a). The catalytic activities of the
10
electrodes were estimated using the values of the reduction currents (Jred) and the peak-to-peak
11
separations (∆E). The corresponding data are listed in Table 1. It was found that the |Ired| value
12
increased with the initial decrease in the volume percentage of the Co precursors in the mixture
13
solution from 100% to 90%, but it decreased with further reductions in the volume percentage of
14
the Co precursors in mixture solution from 90% to 0%. The highest |Ired| value was 1.341 mA for
15
the Co0.9Pdo0.1/RGO electrode. Note that there was no change in the |Ired| values when the
16
volume ratio of the Co:Pd precursor decreased from 0.5:0.5 to 0:1, as described in Table 1. The
17
results demonstrate that the highest diffusion current of ions [26], the lowest diffusion impedance
18
(Zw), and the largest surface active area [27] were obtained for the Co0.9Pdo0.1/RGO electrode.
19
The results were further confirmed through the EIS measurements.
20
The ∆E values were calculated from the CV curves for the CoPd/RGO CEs. The extracted ∆E
21
values are listed in Table 1. Both Co1Pd0/RGO and Co0Pd1/RGO electrodes were used as
22
references electrodes. The ∆E for the reference CEs were estimated to be 570 and 710 mV for
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Co1Pd0/RGO and Co0Pd1/RGO, respectively. As seen in Table 1, the ∆E values recorded for the
2
developed CEs were smaller than the reference electrodes. It is known that a lower ∆E indicates
3
a faster rate of I3-/I- redox reaction with increases in the catalytic area [27]. The observations in
4
the CV curves agreed well with the charge-transfer resistance (Rct) and open-circuit voltage (Voc)
5
recorded for the DSCs [28, 29]. The lowest ∆E value was 470 mV for the Co0.9Pdo0.1/RGO
6
electrode. The result indicates that the highest catalytic activity was obtained for
7
Co0.9Pdo0.1/RGO electrode [27].
8
The chemical stability of the developed materials was tested for the reduction of triiodide ions to
9
iodide ions. Note that this test is generally required for the application of CoPd alloy/RGO to
10
CEs in DSCs. For this purpose, the current-voltage curve measurements were conducted over
11
100 cycles for the redox system with the Co0.9Pd0.1/RGO electrode. The extracted values of Jred
12
and the oxidation current (Joxd) from the experimental current-voltage curves recorded over 100
13
cycles are provided in Fig. 3(b) as functions of the cycle number. It is seen clearly that there
14
were no noticeable changes in the values of Jred and Joxd over 100 cycles of the Co0.9Pd0.1
15
alloy/RGO electrode, which indicates that the Co0.9Pd0.1 alloy/RGO films have high stability
16
under electrochemical reaction conditions.
17
3.2.3. Electrochemical impedance spectroscopy
18
The EIS performances were obtained in order to examine the interfacial charge-transfer
19
resistance (Rct) properties of the triiodide/iodide couple on the CE surfaces. Note that the EIS
20
measurements were conducted on different symmetrical dummy cells fabricated with two
21
identical CoxPd1-x/RGO electrodes. The Nyquist plots are presented in Fig. 4(a) and 4(b). The
22
equivalent circuit, which was used to fit the spectra with the Z-view software, is depicted in the
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inset of Fig. 4(b). The parameters of the EIS spectra obtained from the equivalent circuit are
2
summarized in Table 1. It was found that the Rct values followed the sequence of Co0.9Pd0.1/RGO
3
(1.17 Ω) < Co0.7Pd0.3/RGO (2.14 Ω) < Co0.5Pd0.5/RGO (2.52 .52< Co0.3Pd0.7/RGO (3.30 .30<
4
Co0.1Pd0.9/RGO (4.39 Ω) < Co0Pd1/RGO (5.78 .78< Co1Pd0/RGO (8.88 .8. A lower Rct at the
5
CE/electrolyte interface indicated a higher reaction rate of the iodide ions regeneration from the
6
triiodide ions. The electron lifetime (τ) data for the triiodide ion reduction (τ = 1/2πfmax, where
7
fmax is the peak frequency in the Bode EIS plots) and exchange current density (Jo = RT/nFRct,
8
where R, T, n, and F are the gas constant, absolute temperature, number of electrons involved in
9
the reduction of iodide electrolyte, and Faraday’s constant, respectively, in the Tafel
10
measurements) were conducted in order to further confirm the electrocatalytic activity of the
11
developed electrodes. The experimental results are presented in Fig. 4(c) for the Bode EIS plots
12
and Fig. 4(d) for the Tafel curves. Analyses of the Bode EIS plots and Tafel measurements
13
revealed that the trends of the fmax and Jo values followed the sequence of Co0.9Pd0.1/RGO >
14
Co0.7Pd0.3/RGO > Co0.5Pd0.5/RGO > Co0.3Pd0.7/RGO > Co0.1Pd0.9/RGO > Co0Pd1/RGO >
15
Co1Pd0/RGO. Given that τ = 1/2πfmax and Jo = RT/nFRct, the trends in the values of τ and Jo were
16
similar to that of the Rct values. It has been reported that a shorter τ indicates a faster electron
17
transfer across the CE/electrolyte interface [19] and that a high Jo value indicates a high Jsc in
18
DSCs [30]. Table 1 also provides the Zw values of the electrodes. As seen in the table, Zw became
19
small with the initial decrease in the volume percentage of Co precursors in the mixture solution
20
from 100% to 90%, but it increased with further reductions in the volume percentage of Co
21
precursors in the mixture solution from 70% to 0%. The lowest Zw value was 16.34 Ω for the
22
Co0.9Pd0.1/RGO electrode. The results agree well with those of the CV analyses, the limiting
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diffusion current density (Jlim) values, which were obtained from the Tafel measurements, and
2
the SEM observations.
3
3.2.4. Photovoltaic performances
4
As discussed above, an optimized balance between Rct and Zw is required in order to achieve a
5
higher DSC performance. In order to further confirm these relationships, the PV performances
6
were investigated. The J-V characteristics are described in Fig. 5(a), and the PV parameters are
7
summarized in Table 2. It was found that the efficiencies followed the sequence of
8
Co0.9Pd0.1/RGO (6.40%) > Co0.7Pd0.3/RGO (5.97%) > Co0.5Pd0.5/RGO (5.79%) > Co0.3Pd0.7/RGO
9
(5.72%) > Co0.1Pd0.9/RGO (5.48%) > Co0Pd1/RGO (5.41%) > Co1Pd0/RGO (5.14%). The highest
10
efficiency of 6.40% was obtained for the DSC with the Co0.9Pd0.1/RGO CE, while the efficiency
11
of the cells fabricated with the Co1Pd0/RGO and Co0Pd1/RGO CEs were 5.17% and 5.41%,
12
respectively. This might result from the balance effect between the Rct and Zw values [31]. The
13
enhancement in the Jsc values compared with the reference electrodes could be ascribed to the
14
enhancement in the catalytic activity of the alloy CEs [32] and the decreases in the diffusion
15
impedance in the developed CEs [33]. In order to further confirm the improvement in the Jsc
16
value, incident photon-to-current efficiency (IPCE) performance tests were conducted, as
17
depicted in Fig. 5(b). It was found that the IPCE of the device with the Co0.9Pd0.1/RGO CE was
18
higher than that of the cells assembled with the references CEs over the visible wavelength. The
19
current values calculated from the overlap integral of the IPCE spectra align with the Jsc derived
20
from the J-V characteristics. As discussed previously, the difference in the Voc values was
21
supported by the ∆E measurements [34, 35]. Therefore, this result aligns with the XPS, SEM,
22
CV, EIS, and Tafel measurements.
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4. Conclusions
2
Using DPR, CoxPd1-x alloys were successfully synthesized and immobilized on RGO surfaces
3
with different volume ratios of Co and Pd precursors. The synthesized materials were
4
characterized using HRSEM, TEM, XRD, and XPS measurements. The size and distribution of
5
the CoPd alloys on the RGO surface was controlled through changing the volume percentage of
6
the Co precursor in the mixed solution. The developed materials were tested as Pt-free CEs for
7
DSCs. The electrochemical catalytic activity of the CEs was examined using CV, EIS, and Tafel
8
measurements. In the results, the catalytic activity of the electrodes followed the sequence of
9
Co0.9Pd0.1/RGO > Co0.7Pd0.3/RGO > Co0.5Pd0.5/RGO > Co0.3Pd0.7/RGO > Co0.1Pd0.9/RGO >
10
Co0Pd1/RGO > Co1Pd0/RGO. Furthermore, Zw became small with the initial decrease in the
11
volume percentage of the Co precursors in the mixture solution from 100% to 90%, but it
12
increased with further reductions in the volume percentage of Co precursors in the mixture
13
solution from 70% to 0%. Accordingly, the efficiencies had the following order: Co0.9Pd0.1/RGO
14
(6.40%) > Co0.7Pd0.3/RGO (5.97%) > Co0.5Pd0.5/RGO (5.79%) > Co0.3Pd0.7/RGO (5.72%) >
15
Co0.1Pd0.9/RGO (5.48%) > Co0Pd1/RGO (5.41%) > Co1Pd0/RGO (5.14%). Furthermore, the
16
results indicated that the developed CEs were stable in iodide electrolytes. This work can be very
17
useful for the fabrication of CoxPd1-x/RGO CEs for application in efficient DSCs, as well as other
18
fields such as methanol oxidation, oxygen reduction reaction, and so on.
19
Acknowledgements
20
This
21
(2014R1A2A2A01006994), the Korea Research Fellowship Program (2015H1D3A1061830),
22
and a Korea CCS R&D Center (KCRC) grant (2014M1A8A1049345). These are all funded by
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research
was
supported
by
a
National
12
Research
Foundation
(NRF)
grant
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the Ministry of Science, ICT and Future Planning through the National Research Foundation of
2
Korea. This research was also supported through the Creative Human Resource Development
3
Consortium for Fusion Technology of Functional Chemical/Bio Materials grant funded by the
4
Brain Korea 21 Plus grant (21A20151513147).
5
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Table 1. Counter electrode properties extracted from the CV, EIS spectra, and Tafel
2
measurements.
|Ired|
∆E
electrode
(mA)
(mV)
Co1Pd0/RGO
1.267
Co0.9Pd0.1/RGO
Rct
(Ω)
570
8.88
16.46
1.341
470
1.17
6.34
Co0.7Pd0.3/RGO
1.308
550
2.14
8.40
Co0.5Pd0.5/RGO
1.270
710
2.52
9.26
Co0.3Pd0.7/RGO
1.257
710
3.30
29.38
Co0.1Pd0.9/RGO
1.250
710
4.39
36.97
Co0Pd1/RGO
1.171
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710
|Jred|: absolute value of the redox current peak (I3- + 2e- 3I-)
5
∆E: peak-to-peak separation
6
Rct: charge-transfer resistance
7
Zw: diffusion impedance
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5.78
44.05
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Table 2. Photovoltaic parameters of DSCs with various CoPd/RGO CEs.
Counter
Jsc -2
Voc
FF
η
(mAcm )
(mV)
(%)
(%)
Co1Pd0/RGO
12.37
620
67.38
5.17
Co0.9Pd0.1/RGO
14.06
670
67.98
6.40
Co0.7Pd0.3/RGO
13.92
630
67.53
5.97
Co0.5Pd0.5/RGO
13.59
635
67.08
5.79
Co0.3Pd0.7/RGO
13.65
640
65.44
5.72
Co0.1Pd0.9/RGO
13.43
625
Co0Pd1/RGO
13.34
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65.25
5.48
64.86
5.41
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Fig. 1. Characteristics of the Co0.9Pd0.1/RGO nanohybrid: (a) HRSEM, (b) TEM, (c) XPS survey,
3
(d) P3d, and (e) Co2p narrow XPS of Co0.9Pd0.1/RGO. 20
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Fig. 2. SEM images of the CoPd alloy/RGO CEs: (a) GO, (b) Co1Pd0/RGO, (c) Co0.9Pd0.1/RGO,
3
(d) Co0.7Pd0.3/RGO, (e) Co0.5Pd0.5/RGO, (f) Co0.3Pd0.7/RGO, (g) Co0.1Pd0.9/RGO, and (h)
4
Co0Pd1/RGO. 21
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Fig. 3. (a) CV curves with different working electrodes at a scan rate of 50 mVs-1. (b)
3
Relationship between the peak current and square root of the scan rates. (c) Repeated 100-cycle
4
CV curves of the CEs in the iodide/triiodide ions redox electrolyte recorded at a scan rate of 50
5
mVs-1. (d) Plots of Jred and Jox as a function of the cycle number. 22
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Fig. 4. (a, b) Nyquist plots of the symmetrical dummy cells with two identical CEs. The inset
3
depicts the equivalent circuit diagram used to fit the EIS spectra. Rh: ohmic serial resistance; Rtrns:
4
electron transport resistance in the graphene layer; CPEtrap: constant phase element in the
5
graphene layer; Rct: charge transfer resistance at the electrode/electrolyte interface; CPEdl:
6
constant phase element at the electrode/electrolyte interface; and W: Warburg impedance. (c)
7
Bode phase plots of the symmetrical dummy cells. (d) Tafel curves of different symmetrical
8
dummy cells that are similar to those used for the EIS measurements.
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Fig. 5. (a) Characteristic current density-voltage curves of the DSCs with different CEs
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measured under standard conditions, and (b) IPCE curves of the different DSCs.
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Bimetallic CoPd nanoparticles/reduced graphene oxide are prepared by dry plasma reduction.
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The electrocatalytic activity as well as electric conductivity is significantly enhanced.
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The highest efficiency is recorded from DSC employed Co0.9Pd0.1/reduced graphene oxide counter electrode.
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nanoparticles/reduced graphene oxide.
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The strategy is for Pt-free counter electrode in PV-device using bimetallic CoPd
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