Physicochemical properties of phosphonium-based and ammonium-based protic ionic liquids

Article abstract of DOI:10.1039/c2jm34359b

Trioctylphosphonium triflate, trioctylammonium triflate, triphenylphosphonium triflate and triphenylammonium triflate were synthesized and characterized. It was found that phosphonium-based protic ionic liquids (PILs) exhibit higher thermal stability and

Full text of DOI:10.1039/c2jm34359b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 20574
www.rsc.org/materials
PAPER
Physicochemical properties of phosphonium-based and ammonium-based
protic ionic liquids†
c
a
Jiangshui Luo,ab Olaf Conrad and Ivo F. J. Vankelecom
*
*
Received 4th July 2012, Accepted 6th August 2012
DOI: 10.1039/c2jm34359b
Trioctylphosphonium triflate, trioctylammonium triflate, triphenylphosphonium triflate and
triphenylammonium triflate were synthesized and characterized. It was found that phosphonium-based
protic ionic liquids (PILs) exhibit higher thermal stability and ionic conductivity than the
corresponding ammonium-based PILs, no matter whether the P and N center atoms are bonded to the
electron-donating octyl groups or the electron-withdrawing phenyl groups. The ion conduction
behavior of the PILs can be adequately described by the Vogel–Fulcher–Tamman (VFT) equation. The
higher ionic conductivity of phosphonium-based PILs may be attributed to their weaker hydrogen
bond and Coulombic interactions as well as higher carrier ion concentrations, indicated by infrared
analysis, lattice potential energy estimation and VFT fitting results. Interestingly, the stronger
hydrogen bonds inside trioctylammonium triflate may lead to a much decreased melting point.
Furthermore, compared with electron-withdrawing phenyl, electron-donating octyl enhanced the
thermal stability of the PILs.
stability.14–20 For example, Tsunashima and Sugiya16 reported
1. Introduction
that AILs based on triethylalkylphosphonium cations and the
ꢀ
Ionic liquids include subgroups of protic (PILs) and aprotic ionic
liquids (AILs).1,2 PILs are formed by the stoichiometric combi-
nation of a Brønsted acid and a Brønsted base, while AILs are
formed by transfer of an alkyl group (or a group of comparable
complexity) to the same site occupied by a proton in a PIL.3 So
far, AILs have received much more attention than PILs.
Nevertheless, due to their protic nature, PILs have been inten-
sively studied as proton-conducting electrolytes in various fields
such as fuel cells and supercapacitors.1–11 Indeed, the key prop-
erty that distinguishes PILs from AILs is the proton transfer
from the acid to the base, leading to the presence of proton-
donor and proton-acceptor sites, which can be used to build up a
hydrogen-bonded network.12,13
N(SO2CF3)2 anion showed excellent electrochemical and
thermal stabilities as well as quite low viscosities, when compared
with the corresponding ammonium AILs.
However, while a great deal of attention has been given to
ammonium-based PILs, phosphonium-based PILs have received
far less attention as electrolytes,11,21,22 although they have found
some applications in catalysis.23–25 Recently, Rana et al.21 showed
that PILs based on a tributylphosphonium cation exhibit higher
acidities, thermal stabilities and conductivities as well as lower
viscosities over the corresponding ammonium analogues. In
addition, Anouti et al. used a solution of tributylphosphonium
tetrafluoroborate in acetonitrile as an electrolyte for carbon-
based supercapacitors, which showed capacities comparable to
conventional aqueous electrolytes and a combination of good
cycling abilities and a large operating temperature range.11
PILs prepared from oxoacids tend to exhibit low over-
potentials for H2 oxidation and O2 reduction.4,26,27 Among the
oxoacids, triflic acid (CF3SO3H) is a superacid with a pKa of
around ꢀ14.28 Appleby and Baker29 reported that the oxygen
reduction reaction at a Pt surface in 1.1 mol Lꢀ1 CF3SO3H
solution showed apparently greater kinetics than that in 85%
H3PO4 solution at 1 atm and ordinary temperature. Further-
more, CF3SO3H is resistant to both oxidation and reduction with
a high thermal stability and a relatively low viscosity.30,31
Therefore, it can be used in various protonation reactions.
The present work compares the physicochemical properties of
phosphonium-based and ammonium-based PILs with either
Phosphonium-based AILs are of considerable interest with
regard to electrochemical aspects and have been reported to be
superior to their ammonium counterparts in terms of thermal
stability, viscosity, ionic conductivity and electrochemical
aCentre for Surface Chemistry and Catalysis, Faculty of Bioscience
Engineering, KU Leuven, Kasteelpark Arenberg 23, Box 2461, Leuven,
3001, Belgium. E-mail: ivo.vankelecom@biw.kuleuven.be; Fax: +32 16
321998; Tel: +32 16 321594
b
NEXT ENERGY $ EWE-Forschungszentrum fur Energietechnologie e.V.,
€
Carl-von-Ossietzky-Str. 15, D-26129 Oldenburg, Germany
cHySA/Catalysis, Dept. of Chemical Engineering, University of Cape
Town, Private Bag X3, Rondebosch, Cape Town, 7701, South Africa.
E-mail: olaf.conrad@uct.ac.za; Tel: +27 21 650 4366
† Electronic supplementary information (ESI) available: Pictures of
as-synthesized PILs and the emulsion of [Oct3NH][TfO] dispersed in
water; detailed IR analysis. See DOI: 10.1039/c2jm34359b
electron-donating
or
electron-withdrawing
groups:
20574 | J. Mater. Chem., 2012, 22, 20574–20579
This journal is ª The Royal Society of Chemistry 2012

View Article Online
trioctylphosphonium
triflate
([Oct3PH][TfO]),
tri-
Isothermal chronogravimetric experiments were conducted on
the same instrument under N2 atmosphere (30 mL minꢀ1) at
250 ꢁC for [Oct3PH][TfO] and [Oct3NH][TfO], respectively. Each
sample weight was about 22 g.
octylammonium triflate ([Oct3NH][TfO]), triphenylphospho-
nium triflate ([Ph3PH][TfO]) and triphenylammonium triflate
([Ph3NH][TfO]).‡ The phosphonium-based PILs showed
attractive advantages in terms of thermal stability and ionic
conductivity over their ammonium analogues as non-aqueous
electrolytes.
Differential scanning calorimetry (DSC) measurements were
performed on a Netzsch DSC 204 F1 Phoenix apparatus under
N2 atmosphere. Samples around 10 mg each were tightly sealed
in Al pans. The samples were heated from room temperature to
200 ꢁC and then cooled to ꢀ150 ꢁC and heated again to 200 ꢁC at
cooling and heating rates of 10 K minꢀ1. Glass transition
temperature (Tg, onset temperature of the heat capacity change)
and melting temperature (Tm, onset temperature of the corre-
sponding endothermic peak) were determined from the DSC
thermograms recorded during the reheating scans.
2. Experimental
2.1 Chemicals
Tri-n-octylphosphine ([CH3(CH2)7]3P, 97%, Aldrich), tri-n-
octylamine ([CH3(CH2)7]3N, 98%, Acros Organics), triphenyl-
phosphine ((C6H5)3P, 99%, Alfa Aesar), triphenylamine
((C6H5)3N, $99.0%, Acros Organics) and CF3SO3H (98%,
Sigma-Aldrich) were used as received. [Oct3PH][TfO], [Oct3NH]
[TfO], [Ph3PH][TfO] and [Ph3NH][TfO] were prepared in a dry
glove box by equimolar combination of the respective Brønsted
bases and CF3SO3H. All the mixtures were heated above the
melting point for 5 h to promote the formation of the PILs. All
the synthesized PILs were found to be oily and hydrophobic,
which should be closely related to the octyl or phenyl groups.†
The chemical structures are displayed in Fig. 1.
2.3 Fourier transform infrared (FT-IR) spectra
Fourier transform infrared (FT-IR) spectra were recorded at
room temperature on a PerkinElmer Spectrum 100 FT-IR
spectrometer with universal ATR accessory at a spectral reso-
lution of 4 cmꢀ1
.
2.4 Ionic conductivity
The ionic conductivities of each PIL were measured above the
respective melting points from low temperature to high temper-
ature by complex impedance spectroscopy. The temperature was
controlled at 10 K interval (ꢂ0.01 K). The experimental details
have been described previously.9
2.2 Thermal analysis
Thermogravimetric analysis (TGA) for each PIL was recorded
on a thermogravimetric analyzer (TGA 4000, PerkinElmer) in a
stream of N2 (30 mL minꢀ1) with an open Al2O3 pan. The
heating rate was 10 K minꢀ1. The temperature corresponding to
10% mass loss during the heating scan was used as the decom-
position temperature (Td). Thermal stability in O2 was studied by
heating each PIL in a stream of O2 (30 mL minꢀ1) at a heating
rate of 10 K minꢀ1 using the above apparatus. All the above
sample weights were controlled to be between 12 and 15 mg.
2.5 Ion volume calculation
The cation volume (Vcation) and anion volume (Vanion) for each
PIL were computed at the B3LYP/aug-cc-pVDZ//B3LYP/cc-
pVDZ level of theory with Gaussian 09 package (Gaussian,
Inc.).32
3. Results and discussion
3.1 Thermal analysis
Fig. 2a shows the TG curves of each PIL in N2 at a heating rate of
10 K minꢀ1. The TG curves of [Oct3PH][TfO] and [Oct3NH]
[TfO] both exhibit a one-step weight loss process. The values of
Td are listed in Table 1. Obviously, while the Td of [Oct3PH][TfO]
is only slightly higher than that of [Oct3NH][TfO], [Ph3PH][TfO]
has a much higher Td than [Ph3NH][TfO]. In particular, the TG
curve of [Ph3NH][TfO] even shows an initial weight loss event
from the starting temperature of 30 ꢁC. This should be attributed
to the weight loss of free CF3SO3H as revealed by FT-IR analysis
below. Furthermore, Ph3N has an estimated pKa < 0 and rarely
forms salts.33 (Nevertheless, triphenylammonium fluoroborate, a
pale green oil, has been reported in the literature.34) In the solid
state, the Ph3N molecule is almost planar at the central N atom,
allowing for maximum orbital overlap with the p-system.34–36 In
contrast, the Ph3P molecule has a pyramidal configuration
around the P atom in the solid state and has a weaker pp
conjugation between the lone electron pair of the central P atom
and p electrons of the adjoining phenyl group and thus should
have much more basic character.35,37 Therefore, the driving force
Fig. 1 Chemical structures of [Oct3PH][TfO], [Oct3NH][TfO], [Ph3PH]
[TfO] and [Ph3NH][TfO].
‡ Although the melting points of the protic salts should be lower than 100
ꢁC to qualify the liquids as protic ionic liquids (PILs) by the familiar
criterion, PILs are used here to refer to all protic molten salts for
convenience.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 20574–20579 | 20575

View Article Online
for proton transfer from CF3SO3H to Ph3N is relatively
weaker, resulting in a ‘‘poor’’ ionic liquid.38 This has also been
indicated by the much lower thermal stability of [Ph3NH][TfO]
with a Td of 208 C. Watanabe et al.39 have reported a similar
ꢁ
Brønsted acid–base complex, which was prepared from the
combination of 2,5-diphenyl-1,3,4-oxadiazole (DOD) and bis-
(trifluoromethanesulfonyl)imide (HTFSI), which is also
a
superacid. The poor thermal stability of the [DOD]/[HTFSI] ¼
5/5 complex was due to the elimination of free HTFSI from it.39
The isothermal weight loss curves depicted in Fig. 2b further
reveal the superior thermal stability of [Oct3PH][TfO] compared
with [Oct3NH][TfO], indicating that phosphonium-based PILs
are thermally more stable than their ammonium analogues, no
matter whether the P and N center atoms are bonded to electron-
donating octyl or electron-withdrawing phenyl. Furthermore,
Fig. 2c demonstrates that these PILs all have good thermal
stability in O2 except [Ph3NH][TfO].
The electron-withdrawing phenyl seems to weaken the thermal
stability of both phosphonium and ammonium based PILs
compared with electron-donating octyl. This might be attributed
to the induced lower electron density of P and N atoms by
phenyl, leading to lower basicity of the Brønsted bases and hence
weaker thermal stability of the PILs. It has been shown that PILs
prepared from stronger Brønsted bases tend to have higher
thermal stability.28,40
3.2 FT-IR analysis
The observed FT-IR spectra demonstrate the formation of the
proton transfer salts. As shown in Fig. 3a, the absence of O–H
stretching bands between 3400 and 3800 cmꢀ1 confirms quali-
tatively the very low water contents of the PILs.
For ammonium-based PILs, the region of 3200–2400 cmꢀ1 is
characteristic of ammonium structures.9,41–43 For example, the
bands at 3042 and 2812 cmꢀ1 are attributed to the N–H
stretching vibration for [Oct3NH][TfO].43 For [Ph3NH][TfO], the
N–H frequencies are split and appear at 2926, 2843, 2735 and
2555 cmꢀ1, whose position and appearance provide spectro-
scopic evidence for the presence of hydrogen bonding between
cation and anion in the salt.34 According to Reed et al.,43 for
trialkylammonium salts with weakly basic anions Aꢀ such as
CF3SO3ꢀ, weaker NH/Aꢀ hydrogen bond strength means
higher N–H stretching frequencies. Compared with the split
broad N–H stretching bands of [Ph3NH][TfO], the higher N–H
stretching frequencies of [Oct3NH][TfO] confirm that Oct3N is
more basic than Ph3N and indicate that the NH/Aꢀ hydrogen
bonds inside [Oct3NH][TfO] may be relatively weaker than those
inside [Ph3NH][TfO].
Fig. 2 (a) TG curves in N2 (10 K minꢀ1), (b) isothermal chronogravi-
metric plots for [Oct3PH][TfO] and [Oct3NH][TfO] at 250 ꢁC, and (c) TG
curves in O2 (10 K minꢀ1).
Table 1 Thermal properties of each PIL
For [Oct3PH][TfO], the P–H stretching bands may be over-
lapped with C–H vibrational bands in the region of 3000–2800
cmꢀ1 when the frequency is lower than 3000 cmꢀ1,† resembling
the case of the N–H stretching frequencies.43 For [Ph3PH][TfO],
compared with the IR spectra of CF3SO3H and Ph3P,44–46 a new,
broad and medium strong peak appearing at 2405 cmꢀ1 is very
likely due to the P–H stretching vibration.
b
c
PIL
Tga/K
Tm /ꢁC
Td /ꢁC
[Oct3NH][TfO]
[Oct3PH][TfO]
[Ph3NH][TfO]
[Ph3PH][TfO]
186
185
261
277
9
51
117
125d
363
371
208
292
a
Onset temperature of the heat capacity change for glass transition (Tg).
Onset temperature of the corresponding endothermic peak (Tm).
Temperature of 10% mass loss in the TGA experiment (Td). The
observed Tm of [Ph3PH][TfO] during slow heating was slightly lower
than 120 ꢁC.
ꢀ
b
c
As displayed in Fig. 3b, the formation of CF3SO3 anions in
d
each PIL is confirmed by the following bands: 1289, 1235, 1222,
1159, 1027 and 757 cmꢀ1 for [Oct3NH][TfO]; 1255, 1223, 1155,
1031 and 755 cmꢀ1 for [Oct3PH][TfO]; 1275, 1230, 1155 and 1023
20576 | J. Mater. Chem., 2012, 22, 20574–20579
This journal is ª The Royal Society of Chemistry 2012

View Article Online
in it as revealed by the peak at 1212 and 923 cmꢀ1 associated with
free CF3SO3H.44,52,53 This again showed the relatively weak
complex between Ph3NH and CF3SO3H.
In addition to inter-ionic interactions such as Coulombic
interactions and van der Waals forces seen in aprotic ionic
liquids, Watanabe et al.54 have experimentally established that
strong hydrogen bonds between the protonated cation and anion
are present in PILs through N–H bonds. The broadening of N–H
stretching vibration peaks of [Oct3NH][TfO] (e.g. at 3042 and
2812 cmꢀ1) and [Ph3NH][TfO] (e.g. at 2555 cmꢀ1) in the range of
3200–2400 cmꢀ1 indicates that the ammonium-based PILs exist
in a strongly hydrogen-bonded network.9,34,41,42,55 However, for
[Oct3PH][TfO], no obvious broadening is observed, suggesting
weaker hydrogen bonds inside it.
On the one hand, the covalent radius of P (110.7 pm) is
reported to be larger than that of N (73.4 pm).56 Table 2 further
shows that [Oct3PH][TfO] and [Ph3PH][TfO] have larger calcu-
lated Vcation than [Oct3NH][TfO] and [Ph3NH][TfO], respec-
tively, indicating weaker Coulombic interactions between cations
and anions inside the corresponding phosphonium-based PILs.
Matsumiya et al. have shown that the Coulombic interaction
between the cation and the anion in phosphonium-based AILs is
weaker than that in ammonium counterparts.17 It should be
noted that in their reported AILs the inter-ionic interactions are
mainly Coulombic interactions and van der Waals forces.
Furthermore, the lattice potential energy (UPOT) of these PILs
may be estimated using the following empirical equation:57
!
117:3
1=3
Vm
UPOT=kJ molꢀ1 ¼ 2
þ 51:9
(1)
where Vm is the molecular volume in units of nm3 (Vm ¼ Vcation
+
Vanion) and UPOT < 5000 kJ molꢀ1. As shown in Table 2, the
calculated values of UPOT of phosphonium-based PILs are lower
than those of their ammonium analogues, suggesting weaker
Coulombic interactions inside the former. As the Coulombic
interactions between the cation and the anion of the PIL tend to
cause a decrease in mobility,1 phosphonium-based PILs are
expected to have higher ionic conductivities than their ammo-
nium counterparts.
On the other hand, as discussed above, the Coulombic inter-
actions inside [Oct3PH][TfO] should be weaker than those inside
[Oct3NH][TfO] and hence a lower Tm for the former would be
expected. However, as shown in Table 1, Tm of [Oct3PH][TfO] is
about 42 K higher than Tm of [Oct3NH][TfO]. Since the molec-
ular structures of [Oct3PH][TfO] and [Oct3NH][TfO] are highly
similar and the molecular weight of [Oct3PH][TfO] is only
slightly higher, the relatively weak van der Waals forces should
Fig. 3 FT-IR spectra of the PILs at room temperature in the region of
(a) 2200–4000 cmꢀ1 and (b) 650–1700 cmꢀ1
.
cmꢀ1 for [Ph3NH][TfO]; 1273, 1251, 1225, 1150 and 1029 cmꢀ1
for [Ph3PH][TfO].47,48 In addition, the octyl functional groups in
[Oct3NH][TfO] and [Oct3PH][TfO] are evidenced by the peaks at
2956, 2926, 2857, 1468, 1379, 724 cmꢀ1 and 2958, 2926, 2857,
1461, 1379, 722 cmꢀ1, respectively.49 The mono-substituted
benzene rings of [Ph3NH][TfO] and [Ph3PH][TfO] are marked by
the bands at 3062, 3034, 1584, 1482, 1461, 1445, 747, 690 cmꢀ1
Table 2 Cation volume (Vcation), molecular volume (Vm) and lattice
potential energy (UPOT) of each PIL
PIL
Vcationa/nm3
Vmb/nm3
UPOT/kJ molꢀ1
and 3088, 3065, 2997, 1587, 1481, 1442, 1437, 747, 688 cmꢀ1
,
respectively.50,51 The detailed analysis of these bands is given in
[Oct3NH][TfO]
[Oct3PH][TfO]
[Ph3NH][TfO]
[Ph3PH][TfO]
0.512
0.618
0.311
0.346
0.632
0.738
0.431
0.466
377
363
414
406
the ESI.†
Particularly, for [Ph3NH][TfO], while the sharp bands at 1328,
1172 and 894 cmꢀ1 indicate the existence of some Ph3N in
[Ph3NH][TfO] (i.e. the equimolar combination of Ph3NH and
CF3SO3H),34 a certain amount of CF3SO3H seems to also persist
The calculated triflate anion volume is 0.120 nm3. Vm ¼ Vcation
+
a
b
Vanion
.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 20574–20579 | 20577

View Article Online
Table 3 VFT fitting parameters for the ionic conductivity
A/S K1/2 cmꢀ1 B/K
Particularly, [Oct3NH][TfO] is a room temperature ionic liquid
with an ionic conductivity of 3.03 ꢃ 10ꢀ5 S cmꢀ1 at 25 ꢁC. As the
temperature dependence of ionic conductivity exhibits deviation
from a simple Arrhenius behavior, the Vogel–Fulcher–Tamman
(VFT) equation is used to describe the ion conduction behavior:60
PIL
T0/K
R2a
[Oct3NH][TfO] 5.018 ꢂ 0.306
[Oct3PH][TfO] 9.752 ꢂ 0.447
1340 ꢂ 26
1231 ꢂ 20
151.8 ꢂ 2.0 0.9999
173.5 ꢂ 1.8 0.9999
[Ph3NH][TfO]
[Ph3PH][TfO]
3.453 ꢂ 0.499
11.43 ꢂ 0.316
758.5 ꢂ 53.4 242.4 ꢂ 6.4 0.9999
717.3 ꢂ 10.6 235.2 ꢂ 1.4 0.9999
ꢀ
ꢁ
A
ꢀB
pffiffiffiffi
sðTÞ ¼
exp
(2)
T ꢀ T0
T
R2: correlation coefficient.
a
where A is proportional to the concentration of carrier ions, B is
the pseudoactivation energy for ion conduction, and T0 is the
ideal glass transition temperature.9,61 A, B and T0 were all
adjustable parameters. The best-fit parameters are listed in Table
3. The VFT plots are demonstrated to be straight lines in Fig. 4b.
Clearly, the ion conduction behavior of the as-studied PILs
obeys the VFT equation very well. The difference between Tg
obtained from DSC measurements and T0 calculated from the
VFT equation (namely Tg ꢀ T0) lies in a reasonable range of 10–
50 K.61 [Oct3PH][TfO] and [Ph3PH][TfO] are found to have much
higher values of A than [Oct3NH][TfO] and [Ph3NH][TfO],
respectively. This may also explain the higher ionic conductivity
of phosphonium-based PILs. As Table 2 shows that phospho-
nium-based PILs have a comparable Vm with their ammonium
counterparts, higher concentrations of carrier ions (effective
ionic concentrations) for phosphonium-based PILs may indicate
their higher ionicity.62–64 Actually, it is reported that tributyl
phosphonium PILs exhibit higher degree of ionicity compared
with their ammonium analogues.21 One reason for why phos-
phonium-based PILs exhibit higher ionic conductivity than their
ammonium analogues might be as follows: the stronger
hydrogen bond interactions in the ammonium-based PILs herein
may lead to associative diffusion in which the ion pair likely
diffuses together as a noncharged species and does not contribute
to the direct current ionic conduction,65 while weaker hydrogen
bond interactions in the corresponding phosphonium-based
PILs make the individual ions diffuse more freely, thus resulting
in higher ionic conductivity for phosphonium-based PILs. This
consideration is supported by higher concentrations of carrier
ions of phosphonium-based PILs shown in Table 3.
be comparable. Therefore, the obvious difference in Tm should
be attributed to the strength of hydrogen bonding. Opposite to
the behavior of hydrogen bonds in liquids and solutions con-
sisting purely of neutral molecules, Ludwig et al.58,59 reported
that strong hydrogen bonds formed between cations and anions
introduce ‘‘defects’’ into the Coulomb network of ILs and
increase the dynamics of the cations and anions, resulting in
decreased melting points. Similarly, the stronger hydrogen bond
interactions inside [Oct3NH][TfO] may fluidize the PIL and lead
to a remarkably lower Tm than that of [Oct3PH][TfO].
3.3 Ion conduction behavior
Fig. 4a confirms that phosphonium-based PILs have higher ionic
conductivity than the corresponding ammonium-based PILs.
4. Conclusions
Trioctylphosphonium triflate, trioctylammonium triflate, tri-
phenylphosphonium triflate and triphenylammonium triflate
were compared with respect to their physicochemical properties.
Phosphonium-based PILs exhibited higher thermal stability and
ionic conductivity than their ammonium analogues, no matter
whether the P and N center atoms are bonded to the electron-
donating octyl groups or the electron-withdrawing phenyl
groups. The temperature dependence of the ionic conductivity of
the PILs obeys the Vogel–Fulcher–Tamman (VFT) equation
very well. The higher ionic conductivity of phosphonium-based
PILs may be due to their weaker hydrogen bond and Coulombic
interactions as well as higher carrier ion concentrations. In
addition, compared with the electron-withdrawing phenyl group,
the electron-donating octyl group is advantageous for the
thermal stability of both phosphonium-based and ammonium-
based PILs. The phosphonium-based PILs are expected to be
promising non-aqueous electrolytes for electrochemical
applications.
Fig. 4 (a) Arrhenius plots and (b) VFT plots for the ionic conductivity
of phosphonium-based and ammonium-based protic ionic liquids.
20578 | J. Mater. Chem., 2012, 22, 20574–20579
This journal is ª The Royal Society of Chemistry 2012

View Article Online
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark,
J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,
R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega,
J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken,
C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
Acknowledgements
J.L. and I.F.J.V. acknowledge IOF-KP 10/005 of KU Leuven for
financial support. J.L. deeply thanks Prof. Dr Carsten Agert and
Dr Alexander Dyck (NEXT ENERGY) for their financial
support and the opportunity to conduct the experimental work
at NEXT ENERGY. We appreciate Prof. Dr Tianying Yan
(Nankai University) for the help of ion volume calculation. We
are grateful to Prof. Dr Koen Binnemans (KU Leuven) for
technical discussions.
€
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian
09, Revision A.2, Gaussian, Inc., Wallingford, CT, 2009.
33 T. Manifar and S. Rohani, Can. J. Chem. Eng., 2004, 82, 323.
34 R. D. W. Kemmitt, R. H. Nuttall and D. W. A. Sharp, J. Chem. Soc.,
1960, 46.
35 A. N. Sobolev, V. K. Belsky, I. P. Romm, N. Y. Chernikova and
E. N. Guryanova, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 1985, 41, 967.
Notes and references
1 T. L. Greaves and C. J. Drummond, Chem. Rev., 2008, 108, 206.
2 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati,
Nat. Mater., 2009, 8, 621.
3 W. Xu and C. A. Angell, Science, 2003, 302, 422.
4 H. Nakamoto and M. Watanabe, Chem. Commun., 2007, 2539.
5 T. Mizumo, T. Watanabe and H. Ohno, Polym. J., 2008, 40, 1099.
6 F. Yan, S. Yu, X. Zhang, L. Qiu, F. Chu, J. You and J. Lu, Chem.
Mater., 2009, 21, 1480.
36 X. Wu, A. P. Davis, P. C. Lambert, L. K. Steffen, O. Toy and
A. J. Fry, Tetrahedron, 2009, 65, 2408.
37 J. J. Daly, J. Chem. Soc., 1964, 3799.
38 M. Yoshizawa, W. Xu and C. A. Angell, J. Am. Chem. Soc., 2003,
125, 15411.
39 H. Matsuoka, H. Nakamoto, M. A. B. H. Susan and M. Watanabe,
Electrochim. Acta, 2005, 50, 4015.
40 H. Luo, G. A. Baker, J. S. Lee, R. M. Pagni and S. Dai, J. Phys.
Chem. B, 2009, 113, 4181.
41 K. Mori, S. Hashimoto, T. Yuzuri and K. Sakakibara, Bull. Chem.
Soc. Jpn., 2010, 83, 328.
ꢀ
42 V. H. Alvarez, S. Mattedi, M. Martin-Pastor, M. Aznar and
M. Iglesias, Fluid Phase Equilib., 2010, 299, 42.
43 E. S. Stoyanov, K.-C. Kim and C. A. Reed, J. Am. Chem. Soc., 2006,
128, 8500.
44 P. Laflamme, A. Beaudoin, T. Chapaton, C. Spino and A. Soldera, J.
Comput. Chem., 2012, 33, 1190.
7 Y. S. Ye, M. Y. Cheng, J. Y. Tseng, G.-W. Liang, J. Rick,
Y. J. Huang, F. C. Chang and B. J. Hwang, J. Mater. Chem., 2011,
21, 2723.
8 L. Wang, S. G. Advani and A. K. Prasad, Electrochem. Solid-State
Lett., 2012, 15, B44.
9 J. Luo, J. Hu, W. Saak, R. Beckhaus, G. Wittstock,
I. F. J. Vankelecom, C. Agert and O. Conrad, J. Mater. Chem.,
2011, 21, 10426.
10 L. Timperman, P. Skowron, A. Boisset, H. Galiano, D. Lemordant,
ꢀ
E. Frackowiak, F. Beguin and M. Anouti, Phys. Chem. Chem.
Phys., 2012, 14, 8199.
11 L. Timperman, H. Galiano, D. Lemordant and M. Anouti,
Electrochem. Commun., 2011, 13, 1112.
12 B. Nuthakki, T. L. Greaves, I. Krodkiewska, A. Weerawardena,
M. I. Burgar, R. J. Mulder and C. J. Drummond, Aust. J. Chem.,
2007, 60, 21.
45 S. P. Gejji, K. Hermansson and J. Lindgren, J. Phys. Chem., 1993, 97,
6986.
46 R. J. H. Clark, C. D. Flint and A. J. Hempleman, Spectrochim. Acta,
Part A, 1987, 43, 805.
13 K. Fumino, A. Wulf and R. Ludwig, Angew. Chem., Int. Ed., 2009, 48,
3184.
47 C. M. Burba, N. M. Rocher, R. Frech and D. R. Powell, J. Phys.
Chem. B, 2008, 112, 2991.
14 K. J. Fraser and D. R. MacFarlane, Aust. J. Chem., 2009, 62, 309.
15 C. J. Bradaric, A. Downard, C. Kennedy, A. J. Robertson and
Y. Zhou, Green Chem., 2003, 5, 143.
16 K. Tsunashima and M. Sugiya, Electrochem. Commun., 2007, 9, 2353.
17 M. Matsumiya, S. Suda, K. Tsunashima, M. Sugiya, S. Kishioka and
H. Matsuura, J. Electroanal. Chem., 2008, 622, 129.
18 K. Tsunashima, S. Kodama, M. Sugiya and Y. Kunugi, Electrochim.
Acta, 2010, 56, 762.
48 A. Bernson and J. Lindgren, Solid State Ionics, 1993, 60, 37.
49 W. Abdallah and U. Yilmazer, Thermochim. Acta, 2011, 525, 129.
50 J. Tang, X. Jing, B. Wang and F. Wang, Synth. Met., 1988, 24, 231.
51 D. L. Ou and A. B. Seddon, J. Non-Cryst. Solids, 1997, 210, 187.
52 C. M. Burba, N. M. Rocher and R. Frech, J. Phys. Chem. B, 2009,
113, 11453.
53 Y. Katsuhara, R. M. Hammaker and D. D. DesMarteau, Inorg.
Chem., 1980, 19, 607.
54 M. S. Miran, H. Kinoshita, T. Yasuda, M. A. B. H. Susan and
M. Watanabe, Chem. Commun., 2011, 47, 12676.
55 J. Luo, T. V. Tan, O. Conrad and I. F. J. Vankelecom, Phys. Chem.
Chem. Phys., 2012, 14, 11441.
56 R. T. Sanderson, J. Am. Chem. Soc., 1983, 105, 2259.
57 H. D. B. Jenkins, D. Tudela and L. Glasser, Inorg. Chem., 2002, 41,
2364.
58 K. Fumino, A. Wulf and R. Ludwig, Angew. Chem., Int. Ed., 2008, 47,
8731.
19 S. Seki, K. Hayamizu, S. Tsuzuki, K. Fujii, Y. Umebayashi,
T. Mitsugi, T. Kobayashi, Y. Ohno, Y. Kobayashi, Y. Mita,
H. Miyashiro and S. Ishiguro, Phys. Chem. Chem. Phys., 2009, 11,
3509.
20 N. W. Duffy and A. M. Bond, Electrochem. Commun., 2006, 8, 892.
21 U. A. Rana, R. Vijayaraghavan, M. Walther, J. Sun,
A. A. J. Torriero, M. Forsyth and D. R. MacFarlane, Chem.
Commun., 2011, 47, 11612.
22 L. Timperman and M. Anouti, Ind. Eng. Chem. Res., 2012, 51, 3170.
23 M. Picquet, S. Stutzmann, I. Tkatchenko, I. Tommasi,
J. Zimmermann and P. Wasserscheid, Green Chem., 2003, 5, 153.
24 B. Mariampillai, J. Alliot, M. Li and M. Lautens, J. Am. Chem. Soc.,
2007, 129, 15372.
ꢀ
59 K. Fumino, T. Peppel, M. Geppert-Rybczynska, D. H. Zaitsau,
J. K. Lehmann, S. P. Verevkin, M. Kockerling and R. Ludwig,
€
Phys. Chem. Chem. Phys., 2011, 13, 14064.
60 C. A. Angell, J. Phys. Chem., 1964, 68, 1917.
25 P. Thansandote, D. G. Hulcoop, M. Langer and M. Lautens, J. Org.
Chem., 2009, 74, 1673.
61 S. Seki, M. A. B. H. Susan, T. Kaneko, H. Tokuda, A. Noda and
M. Watanabe, J. Phys. Chem. B, 2005, 109, 3886.
26 J. Thomson, P. Dunn, L. Holmes, J.-P. Belieres, C. A. Angell and
D. Gervasio, ECS Trans., 2008, 13, 21.
62 H. Tokuda, S. Tsuzuki, M. A. B. H. Susan, K. Hayamizu and
M. Watanabe, J. Phys. Chem. B, 2006, 110, 19593.
27 T. Yasuda, A. Ogawa, M. Kanno, K. Mori, K. Sakakibara and
M. Watanabe, Chem. Lett., 2009, 38, 692.
63 K. Ueno, H. Tokuda and M. Watanabe, Phys. Chem. Chem. Phys.,
2010, 12, 1649.
64 D. R. MacFarlane, M. Forsyth, E. I. Izgorodina, A. P. Abbott,
G. Annat and K. Fraser, Phys. Chem. Chem. Phys., 2009, 11, 4962.
65 H. Tokuda, K. Ishii, M. A. B. H. Susan, S. Tsuzuki, K. Hayamizu and
M. Watanabe, J. Phys. Chem. B, 2006, 110, 2833.
28 J.-P. Belieres and C. A. Angell, J. Phys. Chem. B, 2007, 111, 4926.
29 A. J. Appleby and B. S. Baker, J. Electrochem. Soc., 1978, 125, 404.
30 R. D. Howells and J. D. Mc Cown, Chem. Rev., 1977, 77, 69.
31 P. J. Stang and M. R. White, Aldrichimica Acta, 1983, 16, 15.
32 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 20574–20579 | 20579