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Cite this: J. Mater. Chem., 2012, 22, 11121

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High-performance phosphide/carbon counter electrode for both iodide andorganic redox couples in dye-sensitized solar cells†

Mingxing Wu,a Jin Bai,a Yudi Wang,a Anjie Wang,a Xiao Lin,a Liang Wang,a Yihua Shen,a Zeqing Wang,a

Anders Hagfeldtb and Tingli Ma*a

Received 10th February 2012, Accepted 27th March 2012

DOI: 10.1039/c2jm30832k

In the present study, molybdenum phosphide (MoP), nickel phosphide (Ni5P4), and carbon-supported

Ni5P4 (Ni5P4/C) were proposed for use as counter electrode (CE) catalysts in dye-sensitized solar cells

(DSCs) for the regeneration of both the conventional I3�/I� redox couple and a new organic T2/T

�

redox couple. For the I3�/I� redox couple, the DSCs using MoP and Ni5P4 CE yielded a power

conversion efficiency (PCE) of 4.92 and 5.71%, and the DSC using Ni5P4/C showed a high PCE of

7.54%, which was close to that of the DSC using Pt CE (7.76%). For the T2/T� redox couple, Ni5P4/C

performed much better than Pt and the DSC using Ni5P4/C CE showed a PCE of 4.75%, much higher

than the photovoltaic performance of the DSC using Pt CE (3.38%).

1. Introduction

With the increasing energy crisis, the optimization of solar

energy resources is important and necessary. As a powerful

photovoltaic device, dye-sensitized solar cells (DSCs) have ach-

ieved several advancements after two decades of research.1–6

Generally, a DSC possesses three primary components: a pho-

toanode (a dye sensitized oxide semiconductor), an electrolyte

containing a redox couple (usually I3�/I�), and a counter elec-

trode (CE).

As a crucial component, the CE is generally conductive fluo-

rine-doped tin oxide (FTO) glass deposited with Pt as catalyst,

where I3� is reduced to I� by the electrons flowing through the

external circuit. However, the high price and dissolution of Pt in

corrosive electrolytes restrict the mass production of DSCs. To

resolve this issue, several low-cost catalysts have been proposed

to replace Pt, such as carbon materials,7,8 conductive poly-

mers,9,10 and composite materials.11,12 Recently, inorganic

materials have been introduced into DSCs as CE catalysts. These

new catalysts can be divided into four classes: carbides (MoC,

WC),13,14 nitrides (Mo2N/MoN, W2N/WN),15,16 oxides (V2O5,

WO2),17,18 and sulfides (CoS, NiS, MoS2, and WS2).

19–21 More-

over, our group introduced the carbides, nitrides and oxides of

Cr, V, Ti, Nb, and Zr into DSCs and obtained ideal results.22 Lin

et al. used CZTS as counter electrode for DSCs and received

aSchool of Chemical Engineering, State Key Laboratory of Fine Chemicals,Dalian University of Technology, Dalian, 116024, ChinabDepartment of Physical and Analytical Chemistry, Uppsala University,Uppsala, Sweden

† Electronic supplementary information (ESI) available: TEM image ofmesoporous carbon, chemical structure of T2/T

� and the photovoltaicand EIS parameters for the cells using phosphides CEs and T2/T

�

redox couple. See DOI: 10.1039/c2jm30832k

This journal is ª The Royal Society of Chemistry 2012

good results.23 Gao et al. introduced a Ni12P5/graphene

composite CE in I3�/I� based DSCs, resulting in an efficiency of

5.70%.24 All of these inorganic catalysts have shown

decent catalytic activity for the regeneration of the I3�/I� redox

couple.

The electrolyte is another key component of DSCs. Recently,

two new redox couples have been used to replace I3�/I�; they are

T2/T� and Co3+/Co2+.25–28 Interestingly, several low-cost Pt-free

catalysts (such as carbon materials, carbides, oxides) are more

suitable than Pt for the regeneration of T2/T� and Co3+/Co2+

redox couples.21,22,25,29 Therefore, it is quite significant to develop

new catalysts for I3�/I�, T2/T

� or Co3+/Co2+ redox couples to

enhance the efficiency and to reduce the cost of DSCs. Compared

with carbides, nitrides, oxides, and sulfides, phosphides are less

known as catalysts.30,31

The aim of the present work is to apply the as-synthesized

MoP, Ni5P4, and carbon-supported Ni5P4 (Ni5P4/C) as CE

catalysts in the DSC systems. Based on cyclic voltammetry

(CV), electrochemical impedance spectroscopy (EIS), and

Tafel-polarization measurements, MoP and Ni5P4 showed

decent catalytic activity for the regeneration of the conven-

tional I3�/I� redox couple. Ni5P4/C performed as well as Pt for

the I3�/I� redox couple. In addition, Ni5P4 and Ni5P4/C

catalysts have been introduced into the organic redox couple

(T2/T�), and the DSC using a Ni5P4 CE achieved a power

conversion efficiency (PCE) of 3.87%, which was slightly

higher than that of the DSC using a Pt CE (3.38%). By

contrast, the DSC using a Ni5P4/C CE showed a high PCE of

4.54%, an improvement of 41% compared with the Pt-DSC.

The current work proves the feasibility of replacing the

expensive Pt by commercial phosphides, and expands the

scope of CE catalysts.

J. Mater. Chem., 2012, 22, 11121–11127 | 11121

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2. Experimental

2.1 Preparation of Ni5P4, MoP, mesoporous carbon (MC), and

Ni5P4/C

(1) Ni5P4 was prepared by adding an aqueous solution of 1.77 g

(NH4)2HPO4 in 10 mL of deionized water dropwise to a solution

of 3.90 g Ni(NO3)2$6H2O in 15 mL deionized water under stir-

ring. The resulting precipitate was stirred while the water evap-

orated to obtain a solid product, which was dried at 120 �C for

12 h and sintered at 500 �C for 3 h to obtain the final Ni5P4

catalyst. (2) MoP was prepared by dropwise addition of an

aqueous solution of 3.90 g (NH4)6Mo7O24$4H2O in 15 mL

deionized water to a solution of 3.0 g (NH4)2HPO4 in 10 mL

deionized water under stirring. The resulting precipitate was

stirred while the water was evaporated to obtain a solid product,

which was dried at 120 �C for 12 h and sintered at 500 �C for 3 h

to obtain the final MoP catalyst. (3) Mesoporous carbon (MC)

was prepared according to our previous research.32 The Ni5P4/C

was prepared as follows. A quantity of 0.8 g of MC wet-

impregnated with the prepared Ni5P4 (0.275 g) solution for 0.5 h

at room temperature. The mixture was heated to evaporate the

water, and then the obtained solid was dried at 120 �C overnight,

followed by calcination in N2 at 500�C for 3 h to obtain the final

Ni5P4/C catalyst.

2.2 Electrode fabrication and cell assembly

A 5 layer TiO2 (Solaronix D, Switzerland) nanocrystalline film

sensitized with N719 (Solaronix, Switzerland) was used as pho-

toanode. In detail, a thin layer of TiO2 was coated on FTO

conductive glass with a screen printing technique. Then the TiO2

film was sintered at 200 �C for 15 min. The above-mentioned

process was repeated four times and 5 layers of TiO2 film was

obtained. After sintering at 500 �C for 10 min, and cooling to

room temperature, the TiO2 film was treated with 40 mM TiCl4aqueous solution, and then washed with distilled water. After

sintering at 500 �C for 30 min, the mesoporous nanocrystalline

TiO2 film was completely fabricated. The thickness of the

nanocrystalline TiO2 film was around 12 mm. The TiO2 film was

pre-heated to 80 �C, and immersed in a 5 � 10�4 M solution of

N719 dye (Solaronix SA, Switzerland) in acetonitrile/tert-butyl

alcohol (1 : 1 volume ratio) for 20 h and the photoanode was

obtained. Two kinds of electrolyte were used in this research. The

first is the I3�/I� electrolyte which contains 0.06 M of LiI, 0.6 M

1-butyl-3-methylimidazolium iodide, 0.03 M I2, 0.5 M

4-tert-butyl pyridine, and 0.1 M guanidinium thiocyanate in

acetonitrile. The second is 5-mercapto-1-methyltetrazole N-tet-

ramethylammonium salt (+NMe4T�, T�)/di-5-(1-methylte-

trazole) disulfide (T2) electrolyte. The T2/T� electrolyte contains

0.4 M +NMe4T�, 0.4 M T2, 0.05 M LiClO4 and 0.5 M 4-tert-

butylpyridine (TBP) in acetonitrile–ethylene carbonate (6 : 4,

volume ratio). MC, MoP, Ni5P4, and Ni5P4/C counter electrodes

were fabricated with the spray-coating technique as follows.

200 mg of MC (MoP, Ni5P4, or Ni5P4/C) powder was dispersed

in 4 mL isopropanol. The solution was then ultrasonically

dispersed for 30 min and the paste for spraying was obtained.

The prepared paste was sprayed onto an FTO glass with an

airbrush. Subsequently, the FTO glass coated with various pastes

was sintered in an N2 atmosphere at 500 �C for 30 min and the

11122 | J. Mater. Chem., 2012, 22, 11121–11127

CE was prepared. Pt deposited on FTO conductive glass was

used as the Pt CE.33 A DSC was assembled with a photoanode

and counter electrode sandwiching the electrolyte and sealed by

double-sided insulating tape. A symmetrical cell was assembled

with two identical counter electrodes.

2.3 Measurements

XRD was carried out with an automatic X-ray powder diffrac-

tometer (D/Max 2400, Rigaku). The surface morphologies of the

phosphides were checked by scanning electron microscopy

(SEM, FEI Quanta 450 and FEI Hitachi S-4800). The meso-

porous structure of MC was obtained by transmission electron

microscopy (TEM, Tecnai, G2 Spirit). Cyclic voltammetry (CV)

for the I3�/I� redox couple was carried out in a three-electrode

system in an argon-purged acetonitrile solution which contained

0.1 M LiClO4, 10 mM LiI, and 1 mM I2 at a scan rate of

10 mV s�1 using an electrochemical analyzer (CHI630, Chenhua,

Shanghai). Pt served as a counter electrode, and Ag/Ag+ as

a reference electrode. CV for the T2/T� redox couple was carried

out in an argon-purged acetonitrile solution containing 100 mM+NMe4T

�, 10 mM T2, and 0.2 M LiClO4. The photovoltaic

performance of the DSCs was carried out under simulated AM

1.5 illumination (I ¼ 100 mW cm�2, PEC–L15, Peccell, Japan)

with a digital source meter (Keithley 2601, USA). EIS experi-

ments were tested with symmetrical cells using a computer-

controlled potentiostat (Zennium Zahner, Germany). The

measured frequency ranged from 100 mHz to 1 MHz. The

amplitude of the alternating current was set at 10 mV. Tafel-

polarization measurements were measured with an electro-

chemical workstation system (CHI630, Chenhua, Shanghai) in

a symmetrical dummy cell. The scan rate was 10 mV s�1.

3 Results and discussion

3.1 Characterization of the prepared MoP Ni5P4, mesoporous

carbon (MC), and Ni5P4/C by X-ray diffraction (XRD) and

scanning electron microscopy (SEM)

Fig. 1 shows the XRD patterns of the MoP (dotted line, 24-0771,

PDF-2 database). The diffraction peaks at 27.9, 32.0, 43.0, 57.7,

64.9, 67.6, and 74.1� can be ascribed to the crystal planes of [001],

[100], [101], [110], [111], [200], and [201]. In Ni5P4 (dashed line,

65-2075, PDF-2 database), the diffraction peaks at 15.0, 16.1,

22.1, 28.7, 30.4, 30.9, 31.4, 34.6, 36.0, 39.3, 40.7, 41.4, 44.0, 45.1,

46.3, 47.0, 47.8, 49.8, 52.9, 54.1, 76.5, and 86.9� can be ascribed

to the crystal planes of [100], [002], [102], [103], [200], [112], [201],

[202], [104], [203], [210], [211], [212], [204], [300], [301], [213],

[006], [214], [220], [112], and [202]. For the black solid line, the

broad diffraction peaks at 24.0� and 43.0� can be attributed to

the amorphous carbon (MC). The diffraction peaks of the gray

solid line can be considered to be a result of the combination of

Ni5P4 and MC and this proved that Ni5P4/C was prepared

successfully.

Fig. S1† shows the typical mesoporous structure of MC as

previously reported.32 Fig. 2 presents the SEM images of the top

view of MC, MoP, Ni5P4, and Ni5P4/C films. Fig. 2a shows that

theMC blocks have a universal size. In Fig. 2b, theMoP particles

are stuck together, and their diameter is approximately 50 nm.

Fig. 2c shows that Ni5P4 comprises irregular particles with

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 XRD patterns of the MoP, Ni5P4, MC, and Ni5P4/C.

Fig. 2 SEM morphology images of (a) MC, (b) MoP, (c) Ni5P4, and (d)

Ni5P4/C.

Fig. 3 Cyclic voltammograms of the MoP, Ni5P4, MC, Ni5P4/C, and Pt

electrodes for the I3�/I� redox couple.

Fig. 4 Current density–voltage (J–V) curves of the DSCs using MoP,

Ni5P4, MC, Ni5P4/C, and Pt counter electrodes.

This journal is ª The Royal Society of Chemistry 2012

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particle sizes from 50 nm to 2 mm. As shown in Fig. 2d, when

Ni5P4 is embedded in MC, the MC and Ni5P4 particles are

physically mixed and disorderly.

3.2 Catalytic activity of MoP, Ni5P4, MC, Ni5P4/C, and Pt for

the I3�/I� redox couple

3.2.1 Cyclic voltammetry (CV) measurements of MoP, Ni5P4,

MC, Ni5P4/C, and Pt for the I3�/I� redox couple. CV was per-

formed using MoP, Ni5P4, MC, Ni5P4/C, or Pt as working

electrode to investigate the catalytic activity for the regeneration

of I3�/I�. As shown in Fig. 3, two pairs of redox peaks were

observed for Pt, with the left pair assigned as eqn (1) and the right

pair as eqn (2). However, for MC, MoP, and Ni5P4 electrodes,

only one pair of redox peaks was observed, and this phenomenon

was observed in previous research.34 The left cathodic peak dis-

appeared for MoP and Ni5P4, whereas the right anodic peak

disappeared for MC. Clearly, phosphides and MC have their

own defects. A composite (Ni5P4/C) was designed and formed by

embedding Ni5P4 into MC, compensating for the defects of the

phosphides and MC. As expected, for the Ni5P4/C electrode, two

pairs of redox peaks were observed and this may be caused by the

synergistic catalytic effect of Ni5P4 and carbon. On the one hand,

in addition to the perfect CV profile, the cathodic current density

(eqn (1)) of Ni5P4/C at low potential was higher than that of Pt.

On the other hand, the DEp (peak to peak separation) of the left

redox peak for Ni5P4/C was smaller than that of Pt. In theory,

DEp varies inversely with the charge transfer rate (ks) and the ksvalue for Ni5P4/C can be considered as higher than that of Pt on

the whole.35,36 Based on the CV results, we can deduce that MoP

and Ni5P4 have a decent catalytic activity for the I3�/I� redox

couple while Ni5P4/C is expected to perform as effectively as Pt.

3I2 + 2e� % 2I�3 (1)

I�3 + 2e� % 3I� (2)

3.2.2 Photovoltaic performance of the DSCs using MoP,

Ni5P4, MC, Ni5P4/C, and Pt CEs. Fig. 4 shows the current

density–voltage (J–V) curves of the DSCs using MoP, Ni5P4,

MC, Ni5P4/C, and Pt CEs. The photovoltaic parameters are

J. Mater. Chem., 2012, 22, 11121–11127 | 11123

Table 1 Photovoltaic parameters of the DSCs using MoP, Ni5P4, MC, Ni5P4/C, and Pt counter electrodes and EIS parameters of the symmetrical cellsusing two identical MoP, Ni5P4, MC, Ni5P4/C, and Pt electrodes

CE Voc/V Jsc/mA cm�2 FF PCE (%) Rs/U Rct/U Cm/mF ZN/U

Pt 0.78 15.01 0.66 7.76 12.7 3.0 2.1 9.5MC 0.80 14.54 0.60 7.01 20.8 5.3 48.4 9.6Ni5P4/C 0.78 13.85 0.69 7.54 23.4 2.2 151.0 69.5Ni5P4 0.77 13.84 0.54 5.71 20.5 18.9 105.9 271.5MoP 0.76 12.79 0.51 4.92 18.9 27.4 69.3 98.8

Fig. 5 Nyquist plots of the symmetrical cells fabricated with two iden-

tical (a) Pt and MC electrodes; (b) MoP, Ni5P4, and Ni5P4/C electrodes.

Inset in image a: equivalent circuit diagram.

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summarized in Table 1. The MoP-DSC and Ni5P4-DSC yielded

power conversion efficiencies (PCEs) of 4.92% and 5.71%,

respectively. The open circuit voltage (Voc) values of the two

DSCs were 0.76 and 0.77 V, close to that of the Pt-DSC (0.78 V).

The short circuit current density (Jsc) values were slightly lower

than that of the Pt-DSC. The lower PCE can be ascribed to the

low fill factor (FF) of the MoP-DSC and the Ni5P4-DSC, which

stems from the low catalytic activities of the phosphides due to

the large particle size and particle aggregation. If the two phos-

phide catalysts can be prepared in nanoscale size without

aggregation, the performance of the DSCs may improve signifi-

cantly. As a comparative experiment, pure MC was used as the

CE in a DSC, and a PCE of 7.01% was obtained. When Ni5P4/C

was used as the CE, the DSC showed a high PCE of 7.54%, which

was close to the performance of the Pt-DSC (7.76%). An

improvement of 32% in the photovoltaic performance was

achieved for the Ni5P4/C-DSC compared with the Ni5P4-DSC.

The high performance of the Ni5P4/C-DSC may be attributed to

the synergistic catalytic effect of the composite CE. Similar

results have been achieved for Ni12P5/graphene, TiN/CNTs, and

CoS/PEDOT composite electrodes as reported in previous

studies.24,37,38

3.2.3 Electrochemical process of the MoP, Ni5P4 MC, Ni5P4/

C, and Pt by EIS and Tafel-polarization tests. As a powerful tool

to explore the electrochemical process, EIS has been widely used

in testing the catalytic activity for the regeneration of a redox

couple. In the present work, EIS was carried out with the

symmetrical cells (SCs). Fig. 5 shows the Nyquist plots of the SCs

using various CEs. Typically, the intercept on the real axis (high

frequency) can be attributed to the series resistance (Rs). The left

semicircle (middle frequency) can be assigned to the resistance

capacitance networks of the electrode/electrolyte interface,

including the charge transfer resistance (Rct) and the corre-

sponding capacitance (Cm). The right semicircle (low frequency)

can be assigned to the diffusion impedance (ZN) of the redox

couple (I3�/I�) in the electrolyte. Based on previous research,

internal resistance has a negative effect on the performance of

DSCs.39

In the current research, the Rct values of MoP and Ni5P4 were

27.4 and 18.9 U, respectively. These values were larger than that

of the Pt electrode, indicating that the two phosphides were not

as efficient as Pt for catalyzing the reduction of I3� to I�. By

contrast, the Rct for MC was 5.3 U. When Ni5P4 was incorpo-

rated intoMC, generating aRct value of 2.2U for Ni5P4/C, which

was close to that of Pt (3.0 U), indicating a high catalytic activity.

While the values of the ZN for the pure phosphide electrodes

11124 | J. Mater. Chem., 2012, 22, 11121–11127

mentioned above were both larger than that of Pt, which corre-

sponded to a slower diffusion velocity of the redox species in the

electrolyte according to eqn (3). The disadvantage of Ni5P4/C

was the relatively large ZN, the major cause of the slightly lower

efficiency of the DSC using the Ni5P4/C CE compared with that

of the DSC using the Pt CE, although the ZN value decreased

dramatically for Ni5P4/C compared with pure Ni5P4. The Cm

values of the four Pt-free electrodes were much larger than that of

the Pt electrode, indicating a large surface area.40 Based on the

comprehensive consideration of EIS data, the key issue for

the high catalytic activity of Ni5P4/C compared to Ni5P4 is the

declination of Rct value from 18.9 to 2.2 U. The EIS parameters

are summarized in Table 1.

This journal is ª The Royal Society of Chemistry 2012

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ZN ¼ Wffiffiffiffiffiiu

p tanh

� ffiffiffiffiffiffiffiiu

KN

r �(3)

where W (Warburg parameter) ¼ kT=n2e2CAffiffiffiffiD

p; KN ¼ D/d2, D

is the diffusion constant of I3�; C is the concentration of I3

�; n is

the number of electrons transferred in the reaction; k is the

Boltzmann constant; e is the elementary charge; and d is the

thickness of the diffusion layer.

Tafel-polarization measurement was used in the experiment to

confirm the catalytic activity of the phosphide CE catalysts.

Theoretically, the Tafel curve can be divided into three zones.

The curve at low potential (|U| < 120 mV) can be attributed to the

polarization zone, the curve at intermediate potential (with

a sharp slope) can be attributed to the Tafel zone, and the curve

at high potential (horizontal part) can be attributed to the

diffusion zone. In the latter two zones, information on the

exchange current density (J0) and the limiting diffusion current

density (Jlim) was indicated, which was closely related to the

catalytic activity of the catalysts.

In Fig. 6, the Tafel curves of the SCs similar to the ones used in

the EIS measurements showed the logarithmic of J as a function

of U. In the diffusion zone, the Jlim of the MoP, Ni5P4, MC,

Ni5P4/C, and Pt electrodes were observed to be of the same

magnitude. According to eqn (4), Jlim varies with the diffusion

coefficient (D). This finding indicated that similar diffusion

properties were available for these catalysts in the electrolyte. In

the Tafel zone, the curves of MoP and Ni5P4 exhibited gentler

slopes demonstrating a low J0 on the surfaces of the electrodes

compared with that on the surfaces of the Pt electrode. This

result indicated that MoP and Ni5P4 were not effective for

catalyzing the reduction of I3� compared with Pt. For Ni5P4/C,

a large J0 close to that of Pt was observed, giving a high catalytic

activity. The Tafel-polarization results were consistent with the

EIS and J–V results.

Jlim ¼ 2neDCNA

l(4)

where D is the diffusion coefficient of I3�, l is the spacer thick-

ness,C is the I3� concentration,NA is Avogadro’s constant, and e

and n have their usual meanings.

Fig. 6 Tafel-polarization curves of the symmetrical cells fabricated with

two identical MoP, Ni5P4, MC, Ni5P4/C, and Pt electrodes.

This journal is ª The Royal Society of Chemistry 2012

3.3 Catalytic activity of the MC, Ni5P4, Ni5P4/C, and Pt for

the organic T2/T� redox couple

3.3.1 Photovoltaic performance of the DSCs using MC,

Ni5P4, Ni5P4/C, and Pt CEs. I3�/I� is the dominant redox couple

in DSCs. However, I3�/I� has disadvantages, such as the

absorption of visible light, the sublimation of I2, and corrosive

effects with various metals (Ag, Au, among others). Given these

disadvantages, several new redox couples have been introduced

in DSCs to replace I3�/I�, such as 5-mercapto-1-methyltetrazole-

N-tetramethylammonium salt/di-5-(1-methyltetrazole) disulfide

(T2/T�, Fig. S2†).41

In this paper, MC, Ni5P4, and Ni5P4/C were used in the

regeneration of the new organic redox couple, T2/T�, and the

DSCs using MC, Ni5P4, and Pt CEs showed PCEs of 4.40, 3.87,

and 3.38%, respectively (Fig. 7 and Table S1†). Ni5P4 showed

a slightly higher catalytic activity for the regeneration of the

T2/T� redox couple compared with Pt. By contrast, Ni5P4/C was

more effective than Pt, as previously reported.21,25,32 The DSC

using the Ni5P4/C CE yielded a high PCE of 4.75%, an

improvement of 41% compared with the Pt-based DSC. The

advantage of Ni5P4/C for the T2/T� redox couple was more

significant than that of the I3�/I� redox couple.

Fig. 7 J–V curves of the T2/T� based DSCs using MC, Ni5P4, Ni5P4/C

and Pt counter electrodes.

Fig. 8 Nyquist plots of MC, Ni5P4, Ni5P4/C, and Pt-symmetrical cells

using T2/T� as redox couple.

J. Mater. Chem., 2012, 22, 11121–11127 | 11125

Fig. 10 Consecutive cyclic voltammogram of the Ni5P4/C electrode for

the T2/T� redox couple.

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3.3.2 Electrochemical process of MC, Ni5P4, Ni5P4/C, and Pt

by EIS for the organic T2/T� redox couple. Fig. 8 shows the

Nyquist plots of the Ni5P4, Ni5P4/C, MC, and Pt-SCs using

T2/T� as redox couple. The Rct values of the MC, Ni5P4, and Pt

were 18.7, 40.3, and 42.5 U, respectively, indicating that Ni5P4

had a slightly higher catalytic activity than Pt for T2/T�. By

contrast, the Rct value (13.5 U) of Ni5P4/C was lower than both

MC and Ni5P4. Besides, compared with Ni5P4, Ni5P4/C has

a lower ZN. The EIS data indicated that Ni5P4/C performed the

best amongst these materials and the EIS parameters are

summarized in Table S1.† In this process, compared with Pt,

these new catalysts were found to be suitable for the regeneration

of the T2/T� redox couple rather than the conventional I3

�/I�

redox couple, which agreed with the J–V results. Thus, to

develop low-cost and highly efficient DSCs, the fitness issue of

the CE catalysts and the redox couples should be considered.

3.3.3 CV measurements of MC, Ni5P4, and Ni5P4/C for the

organic T2/T� redox couple. Fig. 9 shows the cyclic voltammo-

gram of these electrodes for the T2/T� redox couple. One pair of

redox peaks was observed for all electrodes. The current density

of Ni5P4 was slightly higher than that of Pt, while the invertibility

of Ni5P4 was worse than Pt. In the case ofMC, the current density

was higher than that of Pt, and the invertibility was as good as Pt,

resulting in a higher catalytic activity. For Pt, the potentials of the

redox peaks were around�0.40 and 0.47 V, and the DEp (Pt) was

0.87 V. Compared with Pt, the cathodic peaks for Ni5P4/C shifted

towards a more positive value (�0.20 V), whereas the anodic

peaks shifted towards a more negative value (0.40 V). Thus, the

DEp for Ni5P4/C is just 0.60 V. According to the Nernst equation,

for the T2/T� reaction, if 55# DEp# 65 mV, the electrode can be

considered to be reversible. Moreover, as pointed out above, DEp

varied inversely with ks.35,36Ni5P4/C showed a larger ks value than

that of Pt, which was favorable for a high catalytic activity.

Besides, the current densities of Ni5P4/C are much higher than

those of Pt. An overall consideration of the CV results indicates

that Ni5P4 has a decent catalytic activity and Ni5P4/C is more

effective than Pt for the regeneration of the T2/T� redox couple.

The high catalytic activity of Ni5P4/C for T2/T� can be ascribed to

the following basic reasons: (1) the intrinsic high catalytic activity

Fig. 9 Cyclic voltammograms of the MC, Ni5P4, Ni5P4/C, and Pt elec-

trodes for the T2/T� redox couple.

11126 | J. Mater. Chem., 2012, 22, 11121–11127

of Ni5P4/C, (2) the synergistic catalytic effect of Ni5P4 and mes-

oporous carbon, (3) the improvement of conductivity of Ni5P4/C

by combining Ni5P4 with mesoporous carbon, (4) the large

porosity of Ni5P4/C which is beneficial for T2/T� electrolyte

diffusion. Finally, Fig. 10 showed that after 10 cycles of CV test of

the Ni5P4/C electrode for the T2/T� electrolyte, no current density

decline and peak shifts were observed. This finding demonstrates

that Ni5P4/C and T2/T� species can coexist in the long run.22

4. Conclusion

MoP, Ni5P4, and Ni5P4/C were introduced into the DSC systems

as CE catalysts for the regeneration of the I3�/I� and T2/T

� redox

couples. MoP and Ni5P4 showed decent catalytic activity for the

I3�/I� redox couple. Also, the catalytic activity can be improved

significantly when Ni5P4 and mesoporous carbon were combined

into one composite (Ni5P4/C). The advantage of Ni5P4/C for the

T2/T� redox couple was more significant than that of the I3

�/I�

redox couple. The T2/T� based DSC using a Ni5P4/C CE yielded

a high PCE of 4.75%, which was much higher than that of the

DSC using a Pt CE (3.38%). In the process of developing low-

cost but highly efficient CE catalysts, the fitness issue of the CE

catalysts and redox couples should be considered.

Acknowledgements

This research was supported by the National Natural Science

Foundation of China (Grant 50773008) and the State Key

Laboratory of Fine Chemicals of China. This work was also

supported by the National High Technology Research and

Development Program for Advanced Materials of China (Grant

2009AA03Z220).

Notes and references

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