Application of ionic liquids containing tricyanomethanide [C(CN) 3]- or tetracyanoborate [B(CN)4]- anions in dye-sensitized solar cells

Article abstract of DOI:10.1021/ic201513m

A series of novel ionic liquids composed of imidazolium, pyridinium, pyrrolidinium, and ammonium cations with tricyanomethanide or tetracyanoborate anions were prepared. The ionic liquids were characterized by NMR and IR spectroscopy and ESI-mass spectrometry, and their physical properties were investigated. Solid state structures of the N-propyl-N-methylpyrrolidinium and triethylpropylammonium tetracyanoborate salts were obtained by single crystal X-ray diffraction. The salts that are liquid at room temperature were evaluated as electrolyte additives in dye-sensitized solar cells, giving rise to efficiencies 7.35 and 7.85% under 100 and 10% Sun, respectively, in combination with the standard Z907 dye.

Full text of DOI:10.1021/ic201513m

Article
pubs.acs.org/IC
Application of Ionic Liquids Containing Tricyanomethanide [C(CN)3]−
or Tetracyanoborate [B(CN)4]− Anions in Dye-Sensitized Solar Cells
Magdalena Marszalek,† Zhaofu Fei,† Dun-Ru Zhu,‡ Rosario Scopelliti,† Paul J. Dyson,*,†
Shaik Mohammed Zakeeruddin,*,† and Michael Gratzel†
̈
†Institut des Sciences et Ingen
́
ierie Chimiques, Ecole Polytechnique Fed
‡State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing
University of Technology, Nanjing 210009, P. R. China
́
er
́
ale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: A series of novel ionic liquids composed of
imidazolium, pyridinium, pyrrolidinium, and ammonium
cations with tricyanomethanide or tetracyanoborate anions
were prepared. The ionic liquids were characterized by NMR
and IR spectroscopy and ESI-mass spectrometry, and their
physical properties were investigated. Solid state structures of
the N-propyl-N-methylpyrrolidinium and triethylpropylammo-
nium tetracyanoborate salts were obtained by single crystal X-
ray diffraction. The salts that are liquid at room temperature
were evaluated as electrolyte additives in dye-sensitized solar
cells, giving rise to efficiencies 7.35 and 7.85% under 100 and
10% Sun, respectively, in combination with the standard Z907
dye.
Some of the room temperature ILs were employed as
electrolytes in dye-sensitized solar cells giving very good
efficiencies exceeding 7%.
INTRODUCTION
■
Despite the ongoing interest in ionic liquids (ILs),1,2 the
majority of the research is based on ILs containing a rather
limited set of fluorine-containing anions, notably tetrafluor-
−
−
RESULTS AND DISCUSSION
oborate (BF4 ), hexafluorophosphate (PF6 ), and bis-
(trifluoromethylsulfonyl)imide (Tf2N−).3 In contrast, ILs
based on nonfluorous anions, i.e., tricyanomethanide ([C-
(CN)3]−)4−6 and tetracyanoborate ([B(CN)4)−]),7,8 are some-
what underexplored. Indeed, it has been reported that ILs with
[C(CN)3]− and [B(CN)4]− anions have low viscosities.4,8 For
example, [EMI][C(CN)3] (where EMI = 1-ethyl-3-methyl-
imidazolium cation) has a viscosity of 18 cP at 22 °C and a
melting point of 11 °C,4 and [EMI][B(CN)4] has a viscosity of
19 cP at 21 °C,8 both being lower than that of [EMI][Tf2N],
27 cP at 20 °C.9 N,N,N′,N′-Tetramethyl-N″,N″-dipentylguani-
dinium tricyanomethanide has a viscosity of 88 cP,6 the value
being relatively low given the cation.
■
The preparation of ILs with BF4 , PF6 , and Tf2N− anions is
well established,16,17 whereas there are relatively few reports
describing the synthesis of ILs with [C(CN)3]− anions. The
synthesis of ILs with [B(CN)4]− anions is only covered in a
patent, and details of the anion exchange of the alkali metal
salts of tetracyanoborate with imidazolium or pyridinium
halides have not been reported.7 In this work, we used a
literature protocol to react 1-propyl-3-methylimidazolium
chloride [PMI]Cl with Na[C(CN)3] in aqueous solution, and
the resulting IL, [PMI][C(CN)3] (1a), was extracted with
dichloromethane. Similarly, reaction of [PMI]Cl with K[B-
(CN)4] gives the IL [PMI][B(CN)4] (1b). Using the same
approach, ILs containing N-propylpyridinium, N-propyl-N-
methylpyrrolidinium, and triethylpropylammonium cations
with the [C(CN)3]− anion (2a−4a) and the [B(CN)4]−
anion (2b−4b) were prepared (see Figure 1).
−
−
There are only a limited numbers of ILs that contain
[C(CN)3]− and [B(CN)4]− anions and their applications have
been scarcely explored, despite the high stability and interesting
electrochemical and physicochemical properties of the [B-
(CN)4]− anion.10 Indeed, in nearly all of the applications,
including electrolytes,11−13 as well as others,14,15 studies are
almost exclusively on the [EMI]+-based systems.
The [C(CN)3]− containing ILs, 1a−4a, are liquid at room
temperature, whereas of the ILs containing the [B(CN)4]−
In this paper, the synthesis and characterization of a series of
Received: July 15, 2011
Published: October 25, 2011
new ILs with [C(CN)3]− and [B(CN)4]− anions are described.
© 2011 American Chemical Society
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Inorganic Chemistry
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Figure 2. Solid state structures of 3b (top) and 4b (bottom) with the
thermal ellipsoids at 50% probability and H atoms were omitted for
clarity. Selected bond distances (Å) and angles (deg). 3b: N(1)−C(5)
1.506(4), N(1)−C(1) 1.522(5), N(2)−C(9) 1.153(5), N(3)−C(10),
1.143(5), B(1)−C(10), 1.590(6), B(1)−C(11), 1.594(7), C(10)−
B(1)−C(11) 109.7(3), C(10)−B(1)−C(12) 108.7(3). 4b: N(1)−
C(1) 1.518(3), N(1)−C(4) 1.522(3), N(2)−C(6)1.143(2), B (1)-
C(6) 1.5941(18), C(6)−B(1)−C(6A) 109.00(6).
Figure 1. Structures and numbering scheme of the ILs prepared in this
study.
anion, i.e., 1b−4b, only 1b and 2b are liquid at room
temperature. The H and 13C NMR spectra of 1a−4a, 1b, and
1
2b are recorded as ILs without being dissolved in any solvent
using a C6D6 capillary for login. Spectra of 3b and 4b were
obtained in a CD2Cl2 solution (see the Experimental Section).
In general, the signals do not differ much from those of the
precursor saltsthe variation being typical for ILs.18−20 The IR
spectra of 1a−4a contain absorptions between 2153 and 2164
cm−1 that correspond to the CN bond; the absorptions lie in
a rather smaller range compared to those observed in salts with
metal cations.21−24 In 1b−4b, the absorptions of the CN group
are observed at markedly higher wavenumbers (ca. 2222 cm−1)
and are significantly shifted from those containing metal
cations, which are found in the range of 2250 to 2290
cm−1,25−27 but nonetheless are close in value to those observed
in tetrabutylammonium28 and tetrabutylphosphonium29 salts. It
is known that the CN absorption in salts containing
[C(CN)3]− and [B(CN)4]− anions is sensitive to the structure
of the cations and is susceptible to π···π and/or π···H
noncovalent interactions.21
groups is 1.14 Å in 3b and 4b, nearly the same as those
observed in the lithium (1.14 Å)31 and potassium (1.14 Å)
salts.32 These values are in the range found in the ammonium
and phosphonium salts.28,29 In the crystal of 3b, the anions are
connected by weak hydrogen bonds forming a C−H···N−C−
B−C−N···H−C network (Figure 3). The D···A distances of the
C−H···N hydrogen bonds range from 3.40 to 3.53 Å (Figure 3
and Table 1). These distances are rather longer than those that
exist in series of imidazolium salts with other anions such as
Tf2N− and BF4 .34,35 The CH3 group of the propyl substituent
−
in 4b is disordered, but hydrogen bonds are observed between
the CH3 group of the ethyl substituents and the anion, with the
shortest (C)−H···N−C distance being 2.638 Å.
The room temperature ILs 1a/b, 2a/b, 3a, and 4a were used
to prepare electrolytes, and their viscosities and tri-iodide
diffusion coefficients were determined. Subsequently, they were
evaluated in dye-sensitized solar cells following the procedure
described for the standard eutectic melt electrolyte coded
Z952.14 Essentially, the new ILs were incorporated into the
electrolytes in place of 1-ethyl-3-methylimidazolium tricyano-
methanide [EMI][C(CN)3] at the same molar ratio, giving
electrolytes termed M-1a, M-1b, M-2a, M-2b, M-3a, and M-4a
(see the Experimental Section).
Although salts containing the tetracyanoborate anion were
reported many years ago,30 the structure of simple lithium salt
was reported only recently,31 followed by the structures of
potassium,32 sodium, rubidium, cesium, and ammonium salts;25
several transition metal salts;26,27 and somewhat more exotic
salts.33 Single crystals of 3b and 4b were obtained by slow
cooling from the liquid state from 110° to room temperature
over a period of 24 h, and their structures were determined by
X-ray diffraction (see the Experimental Section). Only very few
salts with organic cations (e.g., tetrabutylphosphonium and
tetrabutylammonium salts) have been reported,28,29 and the
structures of 3b and 4b (Figure 2) represent the first examples
of tetracyanoborate salts containing asymmetric cations.
In 3b and 4b, the anion adopts a nearly ideal tetrahedral
structure with the C−B−C angles close to 109.4° and B−C−N
angles close to 180°. The average C−N distance in the nitrile
The electrolytes with [C(CN)3]− are less viscous than their
[B(CN)4]− analogues (namely M-1a vs M-1b and M-2a vs M-
−
2b). Diffusion coefficients of I3 were obtained by cyclic
voltammetry and electrochemical impedance spectroscopy
(EIS) performed on symmetric cells, and the results obtained
by these two methods are shown in Table 2. Electrolytes M-1a
and M-2a have slightly higher tri-iodide diffusion coefficients,
presumably due to their lower viscosity.
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Table 3. Photovoltaic Characteristics of the Solar Cells
Measured under AM 1.5G, 100% Sun, and 10% Sun
sun intensity/
%
Jsc/
Voc/
mV
fill
factor
efficiency/
%
electrolyte
mA·cm−2
M-1a
100
10
13.7
1.43
13.6
1.41
12.65
1.32
12.2
1.36
14.8
1.54
12.7
1.36
725
655
710
645
725
655
725
660
685
620
700
635
0.743
0.802
0.750
0.797
0.722
0.798
0.733
0.806
0.714
0.779
0.746
0.805
7.35
7.85
7.2
M-2a
M-3a
M-4a
M-1b
M-2b
100
10
7.6
100
10
6.7
7.2
100
10
6.5
7.5
100
10
7.2
7.7
100
10
6.6
7.25
under these light conditions. The J−V curves of the device with
electrolyte M-1a are presented in Figure 4. The incident
Figure 3. Crystal packing in 3b (top) and 4b (bottom).
Table 1. Principle Hydrogen Bond Parameters in 3b
Figure 4. Current density−voltage characteristic of the device
containing the electrolyte M-1a. Incident photon-to-current con-
version efficiency spectrum of the same cell (inset).
D−H···A
D−H (Å) H···A (Å) D···A (Å) D−H···A (deg)
a
b
C4−H4B···N5
C8−H8A···N3
0.990
0.980
2.570
2.560
3.408(5)
3.536(5)
142.00
171.00
a
b
Symmetry code: x, 3/2 − y, 1/2 + z. Symmetry code: −x, 1 − y, −z.
photon-to-current efficiency (IPCE) of the device M-1a is
shown in the inset of Figure 4. The maximum IPCE value at
550 nm is more than 70%. The DSC devices exhibit rather
linear performance with an increasing intensity of irradiating
light, and only a small current drop (∼5%) was noticed at 1
Sun, presumably due to mass transport limitations of tri-iodide.
There is a clear trend between the open circuit potential of
the DSCs and the counter ion of the IL used in the electrolyte.
Electrolytes containing the [C(CN)3]− anion exhibit higher Voc
values, by ∼20 mV, compared to those with the [B(CN)4]−
anion. We employed transient photovoltage decay measure-
ments on the cells with different electrolytes to compare the
rate of interfacial recombination of electrons from the TiO2
conduction band to the oxidized form of the redox mediator,
tri-iodide. In Figure 5, the apparent electron lifetime calculated
for cells with the different electrolytes is plotted against
extracted charge, which provides a clearer comparison, because
the difference between the quasi-Fermi level and the
conduction band is kept constant for each measured point.
−
Table 2. Comparison of the Diffusion Coefficients of I3 in
Electrolytes and Their Viscosities
a
electrolyte
M-1a
M-2a
M-3a
M-1b
M-2b
−7
DI /10 cm2 s−1
CV
EIS
2.51
1.66
49.8
2.27
1.54
49.4
3.42
1.82
59.1
2.39
1.20
56.5
2.09
1.27
53.0
−
3
viscosity (cP at 20 °C)
a
Data for M-4a is not available because it is a quasi-solid substance.
The six new electrolytes have been formulated for use in
DSC devices in combination with Z907 as a sensitizer (Table
3). The best DSC device contains the electrolyte M-1a; the
open-circuit voltage (Voc), short-circuit current density (Jsc),
and fill factor (FF) are 725 mV, 13.7 mA/cm2, and 0.743,
respectively, yielding an overall photoconversion efficiency
(PCE) of 7.35% at full sunlight intensity. However, the PCE at
10% light intensity is as high as 7.85% due to diminished
problems with diffusion connected with slower cell dynamics
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Figure 5. Apparent electron lifetimes in DSCs with electrolytes M-1a,
M-1b, M-2a, and M-2b. The lines are only present to guide the eye.
Figure 6. The upward shift of the conduction band of TiO2 induced
by the different anions present in the electrolyte in a DSC.
The DSCs with electrolytes 1a, 1b, 2a, and 2b were compared.
It appears that the two anions, [C(CN)3]− and [B(CN)4]−, do
not influence the recombination rate, since the electron
lifetimes for the cells with electrolytes sharing the same cation
are practically identical. Nevertheless, the planar [C(CN)3]−
anion36 could potentially facilitate access to the TiO2 surface, as
it could adsorb on it more easily than the tetrahedral
[B(CN)4]− anion.32 Indeed, a more detailed comparison
between the [B(CN)4]− and dicyanoamide, [N(CN)2]−, anions
has been reported.37 It was found that [N(CN)2]− containing
electrolytes have higher open circuit potential than [B(CN)4]−
based electrolytes, and this difference was ascribed to
suppressed interfacial charge-transfer behavior in addition to
an increase in the conduction band edge position of TiO2 due
to the surface interaction with the [N(CN)2]− anion. Cell
capacitance corresponds to the density of states. Difference of
the open-circuit potentials at a given capacitance indicates a
shift of the conduction band. While we did not observe similar
differences in the recombination rates, the specific adsorption
of [C(CN)3]− may be responsible for the upward shift of the
conduction band of TiO2, especially as this difference
corresponds very well with the difference between Voc values
of the devices (Figure 6) at a given density of states.
General Synthetic Procedure. The appropriate imidazolium,
pyridinium, pyrrolidinium, or ammonium halide was mixed with
Na[C(CN)3] or K[B(CN)4] in a molar ratio of 1:1 in water (5 mL).
After stirring at room temperature for 24 h, the reaction mixture was
extracted with dichloromethane (3 × 10 mL). Removal of the
dichloromethane gave the desired IL that was dissolved in dry
dichloromethane and then stored at −20 °C for 24 h. The solution was
filtered to remove traces of sodium or potassium halide, and after
removal of the solvent, the product was dried under vacuum
conditions.
1a: This IL was prepared from 1-propyl-3-methylimidazolium
chloride (1.61 g, 0.01 mol) and Na[C(CN)3] (1.13 g, 0.01 mol). Yield:
95%. 1H NMR (Neat, C6D6 carpillar): 9.00 (s, 1H), 7.80 (s, 1H), 7.75
(s, 1H), 4.50 (t, 2H, J(HH) = 6.50 Hz), 4.20 (s, 3H), 2.20 (m, 2H),
1.25 (t, 3H, J(HH) = 6.50 Hz). 13C NMR (Neat, C6D6 carpillar):
136.5, 123.5, 122.5, 121.4, 51.6, 36.0, 24.5, 11.0, 6.1. IR (cm−1): 3114,
2970, 2164, 1573, 1456, 1169, 1066, 892. ESI-MS (CH3CN) (m/z),
positive ion: 125 [cation]+. Negative ion: 90 [C(CN)3]−. Anal. Calcd
for C11H13N5 (215.26): C, 61.38; H, 6.09; N, 32.53%. Found: C,
61.29; H, 6.07; N, 32.56%.
1b: This IL was prepared from 1-propyl-3-methylimidazolium
chloride (1.00 g, 0.006 mol) and K[B(CN)4] (0.96 g, 0.006 mol).
Yield: 97%. 1H NMR (neat, C6D6 carpillar): 9.00 (s, 1H), 7.96 (s, 1H),
7.92 (s, 1H), 4.70 (t, 2H, J(HH) = 6.50 Hz), 4.45 (s, 3H), 2.45 (m,
2H), 1.50 (t, 3H, J(HH) = 6.50 Hz). 13C NMR (neat, C6D6 carpillar):
137.0, 123.5, 123.0, 122.2, 51.6, 37.1, 24.5, 11.0. IR (cm−1): 3154,
3118, 2971, 2882, 2223, 1572, 1458, 1168, 931. ESI-MS (CH3CN)
(m/z), positive ion: 125 [cation]. Negative ion: 115 [B(CN)4]−. Anal.
Calcd for C11H13BN6 (240.07): C, 55.03; H, 5.46; N, 35.01%. Found:
C, 55.09; H, 5.66; N, 34.98%.
2a: This IL was prepared from N-propyl-pyridinium iodide (2.49 g,
0.01 mol) and Na[C(CN)3] (1.13 g, 0.01 mol) using the same
procedure as described above. Yield: 98%. 1H NMR (Neat, C6D6
carpillar): 9.25 (d, 2H), 8.90 (dd, 1H), 8.42 (d, 2H), 4.95 (t, 2H), 2.35
(m, 2H), 1.25 (t, 3H). 13C NMR (neat, C6D6 carpillar): 146.5, 145.5,
130.0, 121.6, 64.0, 31.0, 25.5, 11.0, 6.0. IR (cm−1): 3064, 2971, 2153,
1636, 1488, 1227, 1171, 1066. ESI-MS (CH3CN) (m/z), positive ion:
122 [cation]+. Negative ion: 90 [anion]−. Anal. Calcd for C12H12N4
(212.25): C, 67.91; H, 5.70; N, 26.40%. Found: C, 67.88; H, 5.68; N,
26.38%.
In conclusion, we have prepared and characterized a series of
new fluorous-free ILs containing imidazolium, pyridinium,
pyrrolidinium, and ammonium cations and [C(CN)3]− and
[B(CN)4]− anions. These ILs were evaluated as components in
dye-sensitized solar cells, and the influence of the anion was
found to be negligible, giving comparable results to those
containing Tf2N anions. Nevertheless, these results are likely to
generate increased interest in ILs containing these anions.
EXPERIMENTAL SECTION
■
Imidazolium, pyridinium, pyrrolidinium, and ammonium halides were
purchased from Iolitech (Germany) and used as received. Sodium
tricyanomethanide Na[C(CN)3] and potassium tetracyanoborate
K[B(CN)4] were received as a gift from Lonza AG and from Merck
KGaA, respectively. IR spectra were recorded on a Perkin-Elmer FT-
IR 2000 system. NMR spectra were measured on a Bruker DMX 400,
using Me4Si as an external standard. Electrospray ionization mass
spectra (ESI-MS) were recorded on a ThermoFinnigan LCQ Deca XP
Plus quadrupole ion trap instrument using a literature method.38
Elemental analysis was carried out at the EPFL.
2b: This IL was prepared from N-propyl-pyridinium iodide (1.25 g,
0.005 mol) and K[B(CN)4] (0.77 g, 0.005 mol) using the same
procedure as described above. Yield: 97%. 1H NMR (neat, C6D6
carpillar): 9.25 (d, 2H), 9.00 (dd, 1H), 8.52 (d, 2H), 5.05 (t, 2H), 2.65
(m, 2H), 1.50 (t, 3H). 13C NMR (neat, C6D6 carpillar): 146.5, 145.5,
130.1, 122.4, 64.5, 25.5, 11.5. IR (cm−1): 3070, 2972, 2222, 1637,
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1488, 1173, 1066, 933. ESI-MS (CH3CN) (m/z), positive ion: 122
[cation]+. Negative ion: 115 [B(CN)4]−. Anal. Calcd for C12H12BN5
(237.07): C, 60.80; H, 5.10; N, 29.54%. Found: C, 60.88; H, 5.11; N,
29.52%.
Table 4. Crystal Data and Structure Refinement Details for
3b and 4b
empirical formula
C12H18BN5 (3b)
C13H22BN5 (4b)
259.17
140(2)
3a: This IL was prepared from N-propyl-N-methylpyrrolidinium
fw
243.12
chloride (1.64 g, 0.01 mol) and Na[C(CN)3] (1.13 g, 0.01 mol) using
temp (K)
cryst size (mm3)
cryst syst
space group
a (Å)
140(2) K
1
the same procedure as described above. Yield: 95%. H NMR (neat,
0.32 × 0.22 × 0.17 0.23 × 0.20 × 0.11
monoclinic
P21/c
C6D6 carpillar): 3.47 (m, 4H), 3.21 (t, 2H), 3.00 (s, 3H), 2.27 (m,
4H), 1.82 (m, 2H), 1.06 (t, 3H). 13C NMR (neat, C6D6 carpillar):
121.8, 66.2, 65.5, 53.8, 21.7, 17.3, 10.5. IR (cm−1): 2972, 2158, 1470,
1229, 1065, 938, 892. ESI-MS (CH3CN) (m/z), positive ion: 128
[cation]+. Negative ion: 90 [anion]−. Anal. Calcd for C12H18N4
(218.30): C, 66.02; H, 8.31; N, 25.67%. Found: C, 66.28; H, 8.40;
N, 25.62%.
tetragonal
P42(1)m
̅
8.5730(17)
16.777(3)
9.822(2)
94.74(3)
1407.9(5)
4
11.9402(9)
11.9402(9)
5.5735(11)
b (Å)
c (Å)
β (deg)
V (Å3)
3b: This IL was prepared from N-propyl-N-methylpyrrolidinium
794.60(18)
2
chloride (1.30 g, 0.008 mol) and K[B(CN)4] (1.23 g, 0.008 mol) using
Z
1
the same procedure as described above. Yield: 95%, Mp. = 78 °C. H
Dcalcd (g·cm−3
F (000)
)
1.147
1.083
NMR (CH2Cl2): 3.65 (m, 4H), 3.30 (t, 2H), 3.05 (s, 3H), 2.30 (m,
4H), 1.90 (m, 2H), 1.10 (t, 3H). 13C NMR (CH2D2): 122.8, 67.0,
66.5, 49.5, 22.5, 18.0, 10.5. IR (cm−1): 2983, 2887, 2223, 1462, 1307,
1001, 970, 926. ESI-MS (CH3CN) (m/z), positive ion: 128 [cation]+.
Negative ion: 115 [B(CN)4]−. Anal. Calcd for C12H18BN5 (243.12):
C, 59.28; H, 7.46; N, 28.81%. Found: C, 59.38; H, 7.60; N, 28.62%.
4a: This IL was prepared from triethylpropyleammonium iodide
(2.71 g, 0.01 mol) and Na[C(CN)3] (1.13 g, 0.01 mol) using the same
procedure as described above. Yield: 94%. 1H NMR (neat, C6D6
carpillar): 3.65 (q, 6H), 3.45 (q, 2H), 2.05 (m, 2H), 1.65 (t, 9H), 1.35
(t, 3H). 13C NMR (neat, C6D6 carpillar): 121.4, 59.2, 53.2, 15.7, 11.0,
7.8, 5.9. IR (cm−1): 2987, 2901, 2156, 1486, 1456, 1229, 1056, 892.
ESI-MS (CH3CN) (m/z), positive ion: 144 [cation]+. Negative ion:
90 [anion]−. Anal. Calcd for C13H22N4 (234.34): C, 66.63; H, 9.46; N,
23.91%. Found: C, 66.68; H, 9.48; N, 23.88%.
520
280
abs coeff. (mm−1
θ range (deg)
h/k/l
)
0.072
0.067
2.38−24.50°
−9,9/−19,19/0,11 −14,14/−14,14/−
3.41−25.98
6,6
reflns collected
ind reflns
2301
6902
2301 [Rint = 0.0]
1502
6902 [Rint = 0.0447]
709
obsd reflns
data/restraints/params
goodness-of-fit on F2
2301/0/165
1.175
833/0/57
1.095
R, wR indices [I > 2σ(I)]
R, wR indices (all data)
largest diff. peak and hole
0.0796, 0.2079
0.1225, 0.2315
0.225, −0.257
0.0494, 0.1142
0.0583, 0.1175
0.141, −0.107
(e·Å−3
)
4b: This IL was prepared from triethylpropyleammonium iodide
(1.36 g, 0.005 mol) and K[B(CN)4] (0.77 g, 0.005 mol) using the
same procedure as described above. Yield: 93%, Mp. = 95 °C. H
Experiments and data acquisition were supported by the HAAKE
RheoWin software. A Ti cone of a diameter of 20 mm and an angle of
0.5° was used for all experiments. Samples were measured at 20 °C.
The applied shear rate was in the range 10−8000 s−1 for electrolytes
(10−2000 s−1 for pure ILs), and an equilibration time of 30 s was used
for each point. In order to obtain the values, a straight line was fitted to
a plateau in the low shear rate region for each plot. A point for lowest
applied shear rate (10 s−1) was neglected due to the applied
measurement procedure.
1
NMR (CH2Cl2): 3.30 (q, 6H), 3.10 (q, 2H), 1.75 (m, 2H), 1.45 (t,
9H), 1.15 (t, 3H). 13C NMR (CH2D2): 122.6, 59.2, 52.8, 16.0, 11.5,
8.2. IR (cm−1): 2991, 2954, 2223, 1488, 1445, 1392, 1360, 1167, 1086,
998, 927, 800, 789. ESI-MS (CH3CN) (m/z), positive ion: 144
[cation]+. Negative ion: 115 [B(CN)4]−. Anal. Calcd for C13H22BN5
(259.16): C, 60.25; H, 8.56; N, 27.02%. Found: C, 60.28; H, 8.60; N,
27.12%.
Crystallography. Crystals suitable for X-ray diffraction studies of
3b and 4b were obtained by slow cooling of the neat IL, 3b and 4b,
from 110 °C to room temperature over 24 h. Data collection was
performed on a KUMA CCD using graphite-monochromated Mo Kα
(0.71073 Å) radiation and a low temperature device [T = 140(2) K].
Data reduction was performed using CrysAlis RED.39 The structure
was solved with SHELX97.40 Refinement was performed using the
SHELX97 software package, and graphical representations of the
structures were made with Diamond.41 All structure was solved by
direct methods and successive interpretation of the difference Fourier
maps, followed by full matrix least-squares refinement (against F2). All
non-hydrogen atoms were refined anisotropically. The contribution of
the hydrogen atoms, in their calculated positions, was included in the
refinement using a riding model. Relevant details for the structure
refinements of 3b and 4b are listed in Table 4.
Electrolyte Preparation. A state of the art eutectic ionic-liquid-
based electrolyte12 was used as a model for further modifications. It
comprises 1,3-dimethylimidazolium iodide ([DMI][I])/1-ethyl-3-
methylimidazolium iodide ([EMI][I])/1-ethyl-3-methylimimdazolium
tetracyanoborate ([EMI][B(CN)4])/iodine/N-butylbenzimidazole/
guanidinium thiocyanate (6:6:8:0.83:1.67:0.33). For the purpose of
this study, [EMI][B(CN)4] in the above-mentioned electrolyte was
replaced at the same molar ratio by each of the synthesized ILs: 1a/b,
2a/b, 3a, and 4a, thus forming six new electrolytes coded as M-
(respective IL code).
Device Fabrication. Photoanodes used to make the devices
consisted of a screen-printed nanoparticulate mesoporous TiO2 layers.
A 8-μm-thick transparent layer of 20-nm-sized TiO2 particles was first
printed on the fluorine doped SnO2 (FTO) conducting glass (NSG-
10Ω/cm2, 4-mm-thick) and subsequently coated with a 5-μm-thick
second layer of 400 nm light-scattering anatase particles (CCIC,
Japan). Detailed procedures for the preparation of TiO2 nanoparticles
and pastes are reported elsewhere.42 A standard Z907 dye was used as
the sensitizer with an addition of phenylpropionic acid as a
coadsorbent (1:1 ratio) in a solvent mixture of tert-butanol and
acetonitrile (1:1 v/v ratio). The above-described double-layered TiO2
films were burnt for 30 min at 500 °C and immersed in the dye
solution for 16 h. After sensitization, they were rinsed in pure MeCN
and assembled with platinized counterelectrodes. The counter-
electrodes were prepared using conducting glass TEC 15 (resistance
15Ω/cm2, 2 mm thick) on which the drop of a solution of
hexachloroplatinate acid in n-propanol was cast. Thermal platinization
occurred while heating the electrodes twice for 15 min at 425 °C in the
air. Electrodes were then sealed with a 25-μm-thick hot-melt film
(Surlyn, Dupont) by heating the system at 100 °C. Devices were
completed by filling the space between the electrodes with electrolyte
through predrilled holes in the counterelectrodes, and the holes were
sealed with a Surlyn sheet and a thin glass cover by heating. Finally,
metal contacts were placed on both electrodes.
Photovoltaic Measurements. Photovoltaic measurements were
performed under simulated Sun irradiance (100 mW cm−1, equivalent
of 1 Sun at air mass global, AM 1.5G, at the surface of the device)
Viscosity. Viscosity measurements of new modified electrolytes
were determined using a HAAKE RheoStress 1 Rheometer (Thermo
Scientific) with a HAAKE DC50 Thermostat (Thermo Scientific).
11565
dx.doi.org/10.1021/ic201513m|Inorg. Chem. 2011, 50, 11561−11567

Inorganic Chemistry
Article
provided by a 450 W xenon light source (Oriel, USA). A Schott K113
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Yoko, T. Inorg. Chem. 2004, 43, 1458.
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Tempax sunlight filter (Prazisions Glas & Optik GmbH, Germany)
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was used to correct the spectral output of the lamp in the region 350−
750 nm. Current−voltage characteristics (in the dark and under
illumination) were obtained by applying a forward potential bias and
measuring resulting current with a Keithley 2400 digital sourcemeter
(Keithley, USA). A metal mask was used to precisely define the
irradiated surface area (0.159 cm2). Quantum efficiencies of the cells
were measured by a similar data acquisition system. A Gemini-180
double monochromator (Jobin Yvon Ltd., UK) was used to separate
and focus the probing wavelength from the white light being shone by
a 300 W xenon lamp (ILC Technology, USA). Measurements were
conducted within the UV−vis range (350−850 nm, with a 10 nm
interval).
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decays were measured under white light bias with superimposed red
light perturbation pulses (both light sources were LEDs). The voltage
dynamics were recorded by using a Keithley 2400 source meter.
Varying the intensity of white light bias allowed the recombination rate
constant (and thus apparent electron lifetime) to be estimated at
different open-circuit potentials by controlling the concentration of the
free charges in TiO2. Red perturbation pulses were adjusted to a very
low level in order to maintain single-exponential voltage decay.
Electrochemical Measurements. Cyclic voltammetry and elec-
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−
coefficients of I3 of the electrolytes. The setup consisted of a
Potentiostat/Galvanostat Model 273A (EG&G Princeton Applied
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Schlumberger, UK) controlled by CorrWare software (v. 3.2).
Symmetric cells were employed with a 20-μm-thick layer of electrolyte
encapsulated between two identical electrodes (FTO covered
conductive glass TEC15 twice platinized with a solution of H2PtCl6
in n-propanol, at 450 °C for 15 min). A Surlyn ring, which was used to
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(19) Fei, Z.; Kuang, D.; Zhao, D.; Klein, C.; Ang, W. H.;
̈
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
Zakeeruddin, S. M.; Gratzel, M.; Dyson, P. J. Inorg. Chem. 2006, 45,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.dyson@epfl.ch, shaik.zakeer@epfl.ch.
■
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ACKNOWLEDGMENTS
■
(25) Kuppers, T.; Bernhardt, E.; Willner, H.; Rohm, H. W.;
̈
We thank EPFL and Swiss National Science Foundation for
financial support. D.-R.Z. thanks the National Natural Science
Foundation of China (No. 21171093) for financial support.
M.G. thanks the Swiss National Science Foundation for the
financial support under the Indo Swiss Joint Research
Programme (ISJRP) grant. We thank Lonza AG for providing
Na[C(CN)3] and Dr. Emil F. Aust, Merck KGaA, Germany for
proving us with a sample of K[B(CN)4].We also thank Mr.
Pascal Comte for providing us the mesoporous TiO2 films.
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