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Accepted Manuscript
Synthesis of ternary nickel cobalt phosphide nanowires through phosphorization for
use in platinum-free dye-sensitized solar cells
Lijun Su, Honggang Li, Yaoming Xiao, Gaoyi Han, Miaoli Zhu
PII:
S0925-8388(18)33015-9
DOI:
10.1016/j.jallcom.2018.08.136
Reference:
JALCOM 47224
To appear in:
Journal of Alloys and Compounds
Received Date: 4 July 2018
Revised Date:
14 August 2018
Accepted Date: 16 August 2018
Please cite this article as: L. Su, H. Li, Y. Xiao, G. Han, M. Zhu, Synthesis of ternary nickel cobalt
phosphide nanowires through phosphorization for use in platinum-free dye-sensitized solar cells, Journal
of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.08.136.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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Synthesis of ternary nickel cobalt phosphide nanowires
through
phosphorization
for
use
in
platinum-free
dye-sensitized solar cells
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Lijun Su,a,c Honggang Li,b Yaoming Xiao,a,c,* Gaoyi Han,a Miaoli Zhua
[a] Institute of Molecular Science, Key Laboratory of Chemical Biology and
Molecular Engineering of Education Ministry, Innovation Center of Chemistry and
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Molecular Science, Key Laboratory of Materials for Energy Conversion and Storage
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of Shanxi Province, Shanxi University, Taiyuan, 030006, P. R. China.
[b] School of Chemistry and Chemical Engineering, Liaocheng University,
Liaocheng, 252059, P. R. China.
[c] Fujian Key Laboratory of Functional Materials (Huaqiao University), Xiamen,
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361021, P. R. China.
[*] Corresponding authors.
E-mail: ymxiao@sxu.edu.cn (Yaoming Xiao).
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Abstract: Nickel cobalt phosphide nanowires are fabricated on titanium foil through
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phosphating reaction from their nickel and cobalt hydroxide precursors, which are
employed as the counter electrode materials for the dye-sensitized solar cell.
Electrochemical investigations demonstrate that the ternary nickel cobalt phosphide
counter electrode exhibits higher catalytic activity than that of the binary nickel
phosphide and cobalt phosphide counter electrodes, which is due to that the
introduction of two transition metals can adjust the valence electron and provide two
electron donating active sites. The dye-sensitized solar cell with the ternary nickel
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cobalt phosphide counter electrode achieves a competitive photoelectric conversion
efficiency of 8.01%, which is higher than that of the binary nickel phosphide (3.73%)
and cobalt phosphide (2.80%) counter electrodes under the same conditions and
electrode (8.69%) for the dye-sensitized solar cell.
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comparable photovoltaic performance to that using the conventional platinum counter
Keywords: metal phosphides, nanowires, Pt-free counter electrode, dye-sensitized
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solar cell, phosphorization
1. Introduction
Dye-sensitized solar cells (DSSCs) with the advantages of low manufacturing cost
and simple fabrication process have garnered broad attention since the pioneered
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work reported by Gr鋞zel group [1]. Typical DSSCs consist of three independent
components, namely, a photoanode, an electrolyte, and a counter electrode (CE) [2,3].
Among them, the CE function is to regenerate the iodide (I?) to keep the function of
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DSSC [4]. In other words, the CE development with high electrocatalytic activity
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plays an important role in DSSCs. Platinum (Pt) is traditionally used as the
electrocatalyst in the case of iodide/tri-iodide (I?/I3?) redox couple due to its
outstanding electrocatalytic performance and high conductivity [5]. However, the cost
of Pt must be addressed before their commercialization is viable [6]. Therefore, it is
urgently needed to develope a low-cost and highly efficient CE [7]. The transition
metal compound could be a candidate for replacing the noble Pt because of its
incompletely filled d-orbitals for easily giving and taking electrons [8]. In this regard,
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various Pt-free transition-metal CE materials, such as transition-metal sulfides [9-13],
nitrides [14,15], and selenides [16] have been widely reported.
Besides, transition-metal phosphides also exhibit promising electrochemical
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properties and have been extensively investigated as advanced hydrogen evolution
reaction (HER) electrocatalysts [17,18], but only a few of papers were reported on the
phosphide catalysts as the CEs for the DSSCs [19,20]. In 2012, Gao et al. introduced
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a Ni12P5/graphene composite CE in I?/I3? based DSSC, resulting in a power
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conversion efficiency (PCE) of 5.70% [19]. Ma et al. employed a Ni5P4/C material as
CE, which showed a high PCE of 4.54%, an improvement of 41% compared with the
Pt-based DSSC [20]. Moreover, compared with the binary transition-metal phosphides,
the ternary phosphides have an increase of catalytic activity due to that the
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introduction of two transition metals results in a redistribution of the valence electrons
and offers two electron donating active sites [21,22]. However, metal phosphide with
two metal species employed as the CE for the DSSC has never been investigated.
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Herein, we demonstrated the successful synthesis of ternary NiCoP with
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nanowires structure on the flexible titanium (Ti) foil substrate from its hydroxide
precursor through phosphatizing for the first time. The obtained ternary NiCoP was
directly used as a CE and displayed a competitive PCE of 8.01%, which was superior
to the binary transition metal phosphides.
2. Experiment
2.1. Materials
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All chemicals were used as received without further purification. Cobalt () nitrate
hexahydrate (Co(NO3)2�2O), nickel () nitrate hexahydrate (Ni(NO3)2�2O),
sodium hypophosphite (NaH2PO2稨2O), urea (H2NCONH2), ammonium fluoride
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(NH4F), lithium iodide, lithium pechlorate, iodine, tetrabutyl ammonium iodide,
ganidine thiocyanate, 4-tert-butyl-pyridine, and acetonitrile were purchased from
Shanghai Chemical Agent Ltd., China (Analysis purity grade). Dye N719 was
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purchased from Dyesol, Australia. Titanium (Ti) foils (> 99.98%, 0.05 mm thickness,
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purchased from Baoji Yunjie Metal Production Co., Ltd. China).
2.2. Preparation of ternary NiCoP nanowires
Ni-Co hydroxide precursor was prepared as follows [23]. A piece of Ti foil (2 cm � 3
cm) was carefully pretreated with HF (0.03 mM) for 1 min to remove impurity of
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surface and then deionized water and ethanol were used to ensure the surface of the Ti
foil was well cleaned. 4 mmol of NH4F, 7.5 mmol of urea, Ni(NO3)2�2O and
Co(NO3)2�2O with different mole ratios of Ni/Co (1/1, 0.67/1.33, 0.5/1.5 with a
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constant metal ion concentration of 2 mmol) were dissolved in 30 mL of deionized
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water to form a homogeneous solution and named as c-1, c-2, and c-3, respectively.
Then the as-prepared solution was transferred into a 50 mL Teflon-lined autoclave. Ti
foil was immersed in the solution and kept at 120 癈 for 12 h. Thereafter, the Ni-Co
hydroxide precursor on Ti foil was washed with deionized water and dried in vacuum
at 60 癈 for 8 h. To obtain ternary NiCoP nanowires, the resulting Ni-Co hydroxide
precursor and NaH2PO2 were put at two separate positions in a quartz boat of the tube
furnace (OTF-1200X Hefei Kejing Materials Co.Ltd) (Scheme 1). The sample was
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then heated to 300 癈 at a heating rate of 2 癈 min?1 for 2 h in the atmosphere of Ar.
Finally, the ternary NiCoP nanowires were successfully fabricated on Ti foil after
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cooling in the furnace.
Scheme 1 Schematic diagram of the NiCoP CE.
For further comparison, the binary Ni-P and Co-P CEs were prepared by the
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phosphorization of Ni or Co hydroxide precursor with a constant metal ion
concentration of 2 mmol, and the products were named as a and b. Moreover, the Pt
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CE was prepared by thermally decomposing H2PtCl6 isopropanol solution (0.50 wt%)
on Ti foil at 400 癈 for 30 min.
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2.3. Assembly of DSSCs
A preliminary TiO2 colloid was synthesized via a two-step hydrothermal processes
[24,25]. The TiO2 colloid was blade-coated on the FTO substrate and then sintered at
450 癈 for 30 min to obtain TiO2 film. The TiO2 film was immersed in 0.30 mM
N719 dye solution at room temperature for 24 h to obtain TiO2 photoanode. Then, a
TiO2 photoanode and an as-prepared CE was assembled. Finally, the electrolyte (0.60
M tetrabutyl ammonium iodide, 0.10 M lithium iodide, 0.10 M iodine, 0.10 M
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ganidine thiocyanate, and 0.50 M 4-tert-butyl-pyridine) was injected into the cell gap
between the two electrodes.
2.4. Physical characterizations
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X-ray diffraction (XRD) tests were conducted on BRUKER D8-ADVANCE to
analyze crystal phases of the as-prepared samples. Field emission scanning electron
microscope
(FESEM)
and
energy-dispersive
X-ray
(EDX)
spectrometry
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measurements were performed to characterize the morphology and chemical elements
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of as-obtained materials, respectively. Transmission electron microscope (TEM)
characterization was carried out on JEOL-JSM-2100 to characterize lattice fringes.
2.5. Electrochemical measurements
The cyclic voltammetry (CV) was carried out in an acetonitrile solution that contained
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0.05 M lithium iodide, 0.01 M iodine, and 0.05 M lithium pechlorate at a scan rate of
50 mV s?1 using a computer-controlled potentiostat (Autolab Type III). The resultant
CE acted as the working electrode, a Pt-foil as counter electrode and a Pt-wire as
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reference electrode. Electrochemical impedance spectroscopy (EIS) was carried out
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with a Zennium electrochemical workstation (Zahner) and the electrolyte composed
of 0.60 M tetrabutyl ammonium iodide, 0.10 M lithium iodide, 0.10 M iodine, 0.10 M
ganidine thiocyanate, and 0.50 M 4-tert-butyl-pyridine in acetonitrile. The spectra
were scanned in a frequency ranging from 0.05 Hz to 100 kHz at room temperature.
The Z-view software was used to fit the resultant impedance spectra. Tafel
polarization measurements were performed using a symmetric cell made of two
identical CEs at a scan rate of 10 mV s?1. The photovoltaic efficiency of DSSC was
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measured under 100 mW cm?2 from a solar simulator (CEL-S500, Beijing Ceaulight
Science and Technology Ltd., China).
3.1. Structural and compositional analysis
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3. Results and discussion
The crystallinity and phase of the products were characterized by X-ray diffraction.
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The peaks of the Ti substrate were so strong, which would interfere in the peaks of the
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sample, so we scraped the sample off from the Ti substrate and used the sample
powder to test the XRD. Obviously, the major diffraction peaks (Fig. 1 a) of 40.79�,
44.59�, 47.31�, and 54.23�, are respectively attributed to the planes of (1 1 1), (0 2 1),
(2 1 0), and (3 0 0) for the Ni2P, which is in agreement with its Joint Committee on
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Powder Diffraction Standards (JCPDS No. 65-3544) [26]. Moreover, some diffraction
peaks at 32.73�, 35.38�, 54.03�, 60.43�, and 63.79� are respectively attributed to the
planes of (3 1 0), (0 0 2), (5 1 0), (2 5 1), and (3 0 3) for the contribution from Ni12P5
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(JCPDS No. 65-1623) [26]. Although the phosphating reaction temperature (300 癈)
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is relatively low and the crystallinity intensity in Fig. 1 b is weak, there still exists
typical diffraction peaks at 32.02�, 48.13�, 52.29�, and 56.78�, which are respectively
indexed to the (0 0 2), (2 1 1), (1 0 3), and (3 0 1) planes of CoP by comparison with
the data from JCPDS No. 29-0497 [27,28]. The weak diffraction peaks in Fig. 1 c at
40.98�, 44.89�, 47.58�, and 54.44� could be indexed to the (1 1 1), (2 0 1), (2 1 0), and
(3 0 0) planes of NiCoP, respectively, which is consistent with NiCoP standard
pattern (JCPDS No.71-2336) [29,30].
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Fig. 1 XRD patterns of the as-prepared Ni2P/Ni12P5, CoP, and NiCoP samples, respectively.
EDS measurement was employed to confirm the elemental composition of the
as-prepared NiCoP CE (Fig. S1). The atomic ratio of Ni : Co: P is approximately 1 :
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1 : 1 on Ti foil substrate, close to the stoichiometric ratio of NiCoP, which is
corresponding well with the XRD pattern. The C and O weak peaks could be
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attributed to CO2 adsorbed by the sample and the relatively weak Ti peak might be
attributed to the sample was very thick on the Ti foil. Elemental mapping images (Fig.
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S2) display a consistent distribution of Ni, Co, and P elements, indicating that the
NiCoP was synthesized successfully.
3.2. Morphology analysis
The ternary NiCoP with nanowires structure was derived from its hydroxide precursor
via phosphorization. During the synthesis processes, the surface color of the Ti foil
changed from silver white to lavender after the hydrothermal growth of the hydroxide
precursor and further turned to black after the phosphidation (Fig. S3).
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Fig. 2 Top-view FESEM images of the (a-1 and a-2) Ni2P/Ni12P5, (b-1 and b-2) CoP, and (c-1 and
c-2) NiCoP, respectively.
Fig. 2 exhibits the surface morphology of the samples. It can be seen that the
binary Ni2P/Ni12P5 nanoplates with high density were uniformly grown on Ti foil
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substrate (Fig. 2 a-1 and a-2). These nanoplates are interconnected with each other,
forming a relatively open 3D structure. Fig. 2 b-1 and b-2 show that the binary CoP
has an uniformly distributed nanoneedles structure. Fig. 2 c-1 and c-2 present the
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FESEM images of the ternary NiCoP nanowires. The as-synthesized nanowires
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packed on the Ti foil substrate with lengths of several micrometers and diameters of
tens of nanometers. The appropriate length-diameter ratio ensures that the electron
can easily transfer from the Ti foil substrate to the surface of NiCoP nanowires for the
I3? reduction.
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Fig. 3 TEM images of the (a) Ni2P/Ni12P5, (b) CoP, (c) NiCoP samples.
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Fig. 3 a is the TEM image of the binary Ni2P/Ni12P5 nanoplate. Well-resolved
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lattice fringes can be seen, showing the crystallinity of the binary Ni2P/Ni12P5
nanoplates is very strong. The lattice fringe spacings of 0.508 nm and 0.241 nm are
attributed to the (1 0 0) plane of Ni2P [31] and the (1 1 2) plane of Ni12P5 [26],
respectively. The TEM image of Fig. 3 b is adopted to analyze the nanostructure of
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binary CoP, where lattice fringe can be seen with a lattice spacing of 0.28 nm,
corresponding to the lattice distance of the (0 0 2) plane of CoP [30]. The TEM image
(Fig. 3 c) taken from single NiCoP nanowire exhibits lattice fringes with interplanar
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distance of 0.22 nm, which can be readily indexed to the (1 1 1) of NiCoP [30]. These
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results agree well with the XRD results.
The ternary nickel cobalt phosphides with optimal electrochemical performances
were exploited via harmonizing Ni/Co molar ratios. According to the results of Fig.
S4-6 and Table S1, the optimized initial feed ratio of Ni and Co sources to get the
best NiCoP CE is 1 : 2. So the ternary (c-2) NiCoP CE was used to contrast with the
binary Ni2P/Ni12P5, binary CoP, and Pt CEs and renamed as (c) in the following text.
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Fig. 4 (A) CVs of the (a) Ni2P/Ni12P5, (b) CoP, (c) NiCoP, and Pt CEs at a scan rate 50 mV s?1,
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respectively; (B) relationships between all the redox peak currents and scan rates from Fig. S7.
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3.3. Electrochemical measurements
To estimate the electrocatalytic performances of various CEs toward I3? reduction, CV
studies at different scan rates were employed with various CE materials (Fig. S7). Fig.
4A shows the CVs of the I3?/I? system for Ni2P/Ni12P5, CoP, NiCoP, and Pt CEs in the
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potential interval of ?0.8 V to 1.0 V (vs. Pt) taken at scan rate of 50 mV s?1. The CE
in a DSSC is mainly used to regenerate I3?/I? redox couple. In other words, it is
intended to trigger the I3? reduction represented as Eq. (1)
(1)
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I3? + 2e? ? 3I?
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CV is aimed at precisely quantify the electrocatalytic ability in a DSSC using two
parameters: the peak current density (Jpc) and the peak-to-peak separation (Epp). These
parameters refer to the overall electrocatalytic ability and kinetic reduction capability
of a CE, respectively [32,33]. A larger Jpc indicates a better electrocatalytic ability,
while a smaller Epp refers to a lower overpotential to trigger I3?/I? redox reaction.
From Fig. 4A, it can be observed that the ternary NiCoP and Pt CEs exhibit two redox
pairs whereas no significant peak is observed for the binary Ni2P/Ni12P5 and CoP CEs.
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This suggests that the ternary NiCoP CE has similar electrocatalytic activity to the Pt
CE, but the binary Ni2P/Ni12P5 and CoP CEs have a poor electrocatalytic activity. The
catalytic performance of CEs is in a similar order of the Jpc, which is CoP ?
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Ni2P/Ni12P5 < NiCoP < Pt. The Epp demonstrates that the CEs have an inverse order of
the catalytic activity. The ternary NiCoP CE shows an enhanced electrocatalytic
activity compared to that of the binary Ni2P/Ni12P5 and CoP CEs, which may be due
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to that the introduction of two transition metals results in a redistribution of the
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valence electrons and offers two electron donating active sites, leading to an increase
of catalytic activity [21,22]. Meanwhile, the good linear relationship between square
root of the scan rate and peak current density is also illustrated in Fig. 4B. This
reveals the diffusion limitation of the redox reaction on the ternary NiCoP CE and no
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specific interaction between I3?/I? redox couple and NiCoP CE [11,34]. The diffusion
coefficient (Dn) can be calculated by the empirical Randles-Sevcik theory: Jred = K
n1.5 A C (Dn )0.5 ?0.5, Where Jred is the peak current density, K represents the constant
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of 2.69�5, A is the electrode area, C is the concentration of I3? species, n stands for
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the number of electrons, v is the scan rate, and Dn is the diffusion coefficient. As
listed in Table 1, the ternary NiCoP has the higher Dn value than that of the binary
Ni2P/Ni12P5 and CoP CEs, which demonstrates the ternary NiCoP CE has the higher
catalytic activity [11,34].
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Fig. 5 Nyquist plots of the (a) Ni2P/Ni12P5, (b) CoP, (c) NiCoP, and Pt CEs, respectively.
EIS measurements were used to investigate the internal resistance. The Nyquist
plots were shown in Fig. 5 and the fitted results were listed in Table 1. The
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high-frequency intercept on the real axis is defined as ohmic series resistance (Rs) of
the electrolyte and electrodes. Rs indicates electron-conducting ability, affects the fill
should
be
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factor, and the performance of DSSCs [33]. The lower value of Rs for NiCoP CE
attributed
to
the
better
electrical
transport
properties
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and electrical conductivity. The simulated RCT (charge-transfer resistance at the
electrolyte-electrode interface) value of the ternary NiCoP CE is 1.07 ? cm2, which is
smaller than that of the binary Ni2P/Ni12P5 (1.13 ? cm2) and CoP (1.24 ? cm2) CEs.
The smaller RCT suggests that the ternary NiCoP CE has better electrocatalytic activity
toward the reduction of I3? than that of the binary Ni2P/Ni12P5 and CoP CEs [11,35].
Table 1 Dn, Epp, and fitted EIS values of DSSCs based on different CEs.
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Dn
Epp
RS
RCT(CE)
RCT(TiO2)
W
(? cm2
(? cm2)
(? cm2)
(? cm2)
1.054
1.13
7.86
1.21
1.416
1.24
9.28
1.336
1.07
DSSC
(cm?2 s?1) (V)
9.71�?
-
a
7.75�?
-
7
c
3.12�?
0.982
Pt
4.79�?
1.008
5
5.4
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b
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0.861
1.01
3.2
1.42
1.16
0.52
Tafel polarization curves (Fig. 6) were measured to evaluate the catalytic kinetics
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and further studied the electrocatalytic ability [32,33]. The area of low potential in a
Tafel curve is attributed to the polarization, whereas the zone at high potential is
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assigned to the limiting diffusion. The middle potential range corresponds to the Tafel
zone. The slopes indicate the exchange current densities (J0) on the electrode, which is
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proportional to the catalytic performance about the reduction reaction toward triiodide
and are expressed by eq. (2). Based on the tangent slope of the Tafel curves, J0 values
for the fabricated dummy cells are in order of Pt?>?NiCoP >?Ni2P/Ni12P5 > CoP. The
NiCoP sample exhibits higher cathodic peak current than Ni2P/Ni12P5 and CoP and
higher electrocatalytic ability towards I3?/I? redox couples. The limiting diffusion
current density (Jlim) can be acquired according to the intersection of cathodic branch
and Y-axis, which can assess the diffusion coefficient of the I3?/I? redox couple and
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are expressed by eq. (3).
RT
nFJ 0
(2)
l
2nFC Jlim
(3)
R CT =
D=
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where R, T, n, F, D, and l symbolize the gas constant, temperature, number of
electrons transferred in the reduction reaction, Faraday constant, diffusion coefficient
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of I3?, and spacer thickness, respectively. According to eq. (2) and (3), the ternary
NiCoP CE has the higher J0 and Jlim values, which indicates that the ternary NiCoP
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CE exhibits a higher electrocatalytic activity than that of the binary Ni2P/Ni12P5 and
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CoP CEs.
Fig. 6 Tafel polarization curves of the (a) Ni2P/Ni12P5, (b) CoP, (c) NiCoP, and Pt CEs,
respectively.
Table 2 Photovoltaic performances of DSSCs based on different CEs under 100 mW cm?2 (AM
1.5 G).
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VOC (V)
JSC (mA cm?2)
FF
? (%)
a
0.641
14.74
0.395
3.73
b
0.575
13.81
0.356
2.80
c
0.776
15.16
0.681
8.01
Pt
0.794
15.96
0.686
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DSSC
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8.69
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Fig. 7 Photocurrent density-voltage characteristics of DSSCs based on the (a) Ni2P/Ni12P5, (b)
CoP, (c) NiCoP, and Pt CEs, respectively.
3.4. Photovoltaic performance
The photocurrent density-voltage (J-V) curves were tested to directly check PCE
based on different CEs under irradiation at 100 mW cm?2 (AM 1.5) and clearly
displayed in Fig. 7. The photovoltaic performance parameters were listed in Table 2.
The DSSC based on the ternary NiCoP CE exhibits a VOC of 0.776 V, a JSC of 15.16
mA cm?2, a FF of 0.681, and a high PCE of 8.01 %, which are higher than those of
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the binary Ni2P/Ni12P5 and CoP CEs. This might be owing to that the nanowires
structure with an appropriate length-diameter ratio could accelerate the electron
transmission from the Ti foil substrate to the surface of NiCoP nanowires for the I3?
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reduction, and more importantly the introduction of two transition metals could adjust
the valence electrons and provide two electron donating active sites [21,22]. Both the
high JSC and PCE indicate that the ternary NiCoP CE could be one of the promising
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alternative for the Pt-free DSSC.
4. Conclusions
The efficient ternary NiCoP CE was successfully prepared on the Ti foil substrate by
the phosphorization of its hydroxide precursor. The DSSC based on the ternary NiCoP
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CE achieved a competitive PCE of 8.01%, which was higher than that of the binary
transition metal phosphide CEs under the same conditions. This work has not only
afforded a convenient method toward Ni- or Co-based CEs but also opens new
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opportunities in designing integrated mixed-metal or doped transition metal
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phosphides with enhanced activity for electrocatalyst.
Acknowledgments
The authors appreciate funding from National Natural Science Foundation of China
(61504076, 21574076, and U1510121), Fund for Shanxi ?1331 Project? Collaborative
Innovation Center, and Fund of Fujian Key Laboratory of Photoelectric Functional
Materials (FJPFM-201502). And we are also very grateful for the test platform
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provided by Shanxi University of Scientific Instrument Center.
Competing financial interests
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The authors declare no competing financial interests.
B. O?Regan, M. Gr鋞zel, Low-cost high-efficiency solar cell based on
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[1]
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Research Highlights
NiCoP nanowires were fabricated through a low temperature phosphidation.
NiCoP nanowires were used as a counter electrode materials for the first time.
NiCoP counter electrode exhibits superior electrocatalytic activity.
Solar cell based on NiCoP counter electrode yields a competitive efficiency of 8.01%.
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