Electrochemistry of chloride in ambient room temperature ionic liquids: Formation of oxychloride species

Article abstract of DOI:10.1016/j.elecom.2013.07.006

The electrochemistry of chloride in water-containing hydrophobic ([Emim][NTf₂] and [BmPyrr][NTf₂]) and hydrophilic ([Emim][OAc]) ionic liquids (ILs) has been described in detail for the first time. Cyclic voltammetric studies at a glassy carbon electrode note the significant effect of ambient water on the electrochemistry of chloride, with different outcomes based upon the hydrophilicity (c.f. water content) of the hygroscopic ILs. Added hydroxide highlighted this as a reactive species. Evaluation of chloride, hypochlorite, chlorite, chlorate and perchlorate electrochemistry (chlorine oxidation states -1, +1, +3, +5 and +7) was performed. Ultimately, the electrochemically formed chlorine (Cl₂) was determined to react with water or hydroxide to yield higher oxidation state species via oxychloride intermediates (e.g. hypochlorite) through multiple EC steps, likely resulting in chlorate as the final product.

Full text of DOI:10.1016/j.elecom.2013.07.006

Electrochemistry Communications 34 (2013) 331–334
Contents lists available at ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrochemistry of chloride in ambient room temperature ionic
liquids: Formation of oxychloride species
Md. Mokarrom Hossain, Elham Hosseini Bab Anari, Leigh Aldous ⁎
School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemistry of chloride in water-containing hydrophobic ([Emim][NTf ] and [BmPyrr][NTf ]) and
2
2
Received 21 June 2013
Accepted 4 July 2013
Available online 11 July 2013
hydrophilic ([Emim][OAc]) ionic liquids (ILs) has been described in detail for the first time. Cyclic
voltammetric studies at a glassy carbon electrode note the significant effect of ambient water on the electro-
chemistry of chloride, with different outcomes based upon the hydrophilicity (c.f. water content) of the hy-
groscopic ILs. Added hydroxide highlighted this as a reactive species. Evaluation of chloride, hypochlorite,
chlorite, chlorate and perchlorate electrochemistry (chlorine oxidation states −1, +1, +3, +5 and +7)
Keywords:
Cyclic voltammetry
Chloride
2
was performed. Ultimately, the electrochemically formed chlorine (Cl ) was determined to react with
Oxychloride
Water content
water or hydroxide to yield higher oxidation state species via oxychloride intermediates (e.g. hypochlorite)
through multiple EC steps, likely resulting in chlorate as the final product.
© 2013 Elsevier B.V. All rights reserved.
1-Ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide
Acetate
1
. Introduction
Ionic liquids (ILs) have numerous applications due to their various
ILs under ‘real conditions’ for ammonia gas sensing (e.g. wet ILs
which are equilibrated with the ambient atmosphere), with resulting
interferences on Au from oxide formation [14].
electrochemical and physical properties, as well as solvating and cat-
alytic abilities [1]. The quantification and electrochemical properties
of halides in ILs (and particularly chloride) have attracted significant
interest with, as many ILs are synthesised from chloride precursors.
Chloride can therefore be present as an impurity in the final IL
post-metathesis [2], and these impurities can exert a significant influ-
ence upon the ILs' physiochemical properties such as viscosity, melt-
ing point, electrochemical window and even catalytic activity [2,3].
Electroanalytical methodologies have been developed to quantify
trace chloride in ILs [2,4] and bulk electrolysis suggested as a method
of chloride removal from ILs [5]. The reported high solubility and sta-
In this study we report the electrochemistry of chloride in three ILs
under ambient conditions (i.e. containing a significant molar ratio of am-
bient water) for the first time. Ultimately, electrochemically-generated
chlorine reacts with water or hydroxide to form various oxychloride
compounds, indicating water content needs to be considered for all
−
electrochemical processes involving Cl and Cl
2
in ILs (c.f. [2–11]),
and reported simulations [3,11] therefore only apply to anhydrous ILs.
2. Experimental
1-Ethyl-3-methylimidazolium chloride ([Emim]Cl), 1-ethyl-
3-methylimidazolium acetate ([Emim][OAc]) and 1-butyl-1-
methylpyrridinium bis(trifluoromethanesulfonyl)imide ([Bmpyrr]
2
bility of chlorine gas (Cl ) in ILs suggest potential application of ILs for
2
Cl gas sensing [6]. Chloride-based ILs are widely used to solubilise
biomass [1]. Simulation [3] and experiment [3,7–9] have demonstrat-
[NTf
methylimidazolium bis(trifluoromethanesulfonyl)imide ([Emim]
[NTf ]) was synthesized in-house via the bromide salt, according to
previously reported methods [15]. Sodium hypochlorite (12.5%w/v,
Ajax Finechem Pty Ltd.), sodium chlorite, sodium chlorate and sodi-
um perchlorate (Sigma–Aldrich, Australia) were used as received. A
solution of tetramethylammonium hydroxide in methanol ([TMA]
[OH], Sigma–Aldrich, Australia) was evaporated to dryness under
vacuum before use.
2
]) (IoLiTec, Germany) were used as received. 1-Ethyl-3-
−
−
ed the formation of [Cl
3
]
by addition of Cl
2
to Cl [3,8] or electrolysis
−
−
of Cl [2,7]. HCl gas is highly soluble in ILs [10] and leads to the [HCl
2
]
2
−
in the presence of Cl [10–12].
Almost exclusively, the studies noted above were performed using
virtually anhydrous ILs. Water is ubiquitous in IL studies unless exten-
sive efforts are made to exclude it, due to the hygroscopic nature of
ILs, and water is known to significantly change the physiochemical
properties of ILs, such as viscosity, conductivity, electrochemical win-
dow, etc. [13] Murugappan et al. have recently investigated ambient
Ambient experiments were performed using an Autolab PGSTAT101
(
Ecochemie, the Netherlands) and a conventional three-electrode sys-
tem, consisting of a glassy carbon (GC, 3 mm diameter) working elec-
trode, a coiled Pt counter and non-aqueous reference electrode kit
⁎
Corresponding author. Tel.: +61 29385 4752.
E-mail address: l.aldous@unsw.edu.au (L. Aldous).
1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.elecom.2013.07.006

332
M.M. Hossain et al. / Electrochemistry Communications 34 (2013) 331–334
(
BASI Analytical, USA) containing an Ag wire immersed in 0.01 M
in [Emim][NTf ]. The potential of the latter was determined
daily against Fc/Fc in [Emim][NTf ], and the data shifted such that
(
a) 7
AgNO
3
2
5
0
5
0
+
2
00 mV s-1
4
+
E°(Fc/Fc ) = 0 V. Experiments under vacuum were performed using
an Ag quasi-reference, as described previously [16]. Water content
was measured by Karl Fischer titration using an 831 KF Coulometer
5
2
v increases
(
Metrohm, Switzerland).
-1
3
. Result and discussion
15 mV s
3
.1. Oxidation of chloride in wet ILs
The hydrophobic IL [Emim][NTf
were equilibrated with the ambient atmosphere for 4 h, resulting in
respective water contents of 0.521 ± 0.082w/w% and 22.49 ±
2
] and hydrophilic IL [Emim][OAc]
v increases
-
25
0
.75w/w%. At inert electrodes the oxidation of chloride anions in ILs
may generate two species; chlorine (Cl
2
) and the tri-chloride anion
0.00
0.50
1.00
1.50
−
(
Cl
3
) [2];
Potential / V (vs. Fc/Fc+)
−
−
(b)
2
Cl þ 2e → Cl
ð1Þ
ð2Þ
2
2
0
5
0
5
0
−
−
-1
Cl þ Cl ↔ Cl
2
3
400 mV s
1
1
Fig. 1(a) displays cyclic voltammograms (CVs) for 10 mM [Emim]
Cl in ambient [Emim][NTf
rate. The CVs display one oxidation peak for Cl at ca. +1.0 V and a
reverse peak for Cl
2
] at a GC electrode as a function of scan
v increases
−
2
reduction at ca. −0.3 V. The diffusion
−
co-efficient (D) of Cl was calculated using the Randle–Sevcik equa-
−
7
2 −1
tion to be 4.35 × 10 cm s in ambient [Emim][NTf
perature. Such redox features are consistent with prior investigations
in various (dried) ILs [2,9]. However, at slower scan rates the Cl re-
duction feature was lost (discussed below). In order to exclude reac-
tion between the imidazolium cation and Cl , the experiment was
2
] at room tem-
5 mV s-1
1
2
2
2
repeated in the pyrrolidinium-based IL [Bmpyrr][NTf ] which
exhibited qualitatively identical responses (not shown).
−
In contrast, Cl oxidation was completely irreversible in ambient
0.00
0.25
0.50
0.75
1.00
Potential / V (vs. Fc/Fc+)
[
Emim][OAc] (Fig. 1(b)). The oxidation peak was observed at ca.
+
2
1.0 V (same as [Emim][NTf ]) and increased as a function of
root-square of scan rate, although an associated reduction feature
was entirely absent.
Fig. 1. CVs of 10 mM [Emim]Cl in ambient (a) [Emim][NTf
ν = 15, 40, 100, 200, 400 mVs− ).
2
] and (b) [Emim][OAc] (GC,
1
3.3. Chloride oxidation in ILs containing hydroxide anions
3
.2. Investigation of the influence of water on chloride oxidation in ILs
−
Basic pHs encourage the formation of the more stable anionic hy-
pochlorite ([ClO] ) species instead of the unstable hypochlorous acid
20], c.f. Eqs. (5) and (6)
Water was found to influence the Cl /Cl
Fig. 2(a) displays 5mVs
2
redox couple in the ILs.
−
−
1
scans recorded for 10 mM [Emim]Cl in
] before and after vacuum drying at room temperature
reduction feature at +0.85 V (vs. Ag)
[
[
Emim][NTf
2
for 18 h. Before drying the Cl
2
−
−
was virtually absent at this slow scan rate, while an additional reduc-
tion feature is clearly present at +0.42 V (vs. Ag). After 18 h vacuum
drying, the peak current for Cl oxidation decreased by ca. 45%, con-
sistent with an increase in viscosity with water removal [16]. Howev-
reduction peak increased indicating more Cl
the time the scan is reversed, with a decrease in the peak at +0.42 V.
Cl2 þ ½OHꢀ →ClOH þ Cl
ð4Þ
ð5Þ
−
−
−
ClOH þ ½OHꢀ →½ClOꢀ þ H O
2
−
er, the Cl
2
2
remains by
Therefore the electrochemical oxidation of Cl in the presence of
−
[OH] can lead to the process shown in Eq. (6).
−
A Cl
2
reduction peak 555 mV cathodic of the Cl oxidation peak
−
−
−
−
−
(
not shown) appeared in [Emim][OAc] only after 18 h vacuum drying
at 70 °C. These observations indicates that the water present in ambi-
ent [Emim][NTf ] and [Emim][OAc] reacts with Cl , likely along simi-
lar routes to those identified in aqueous media [17,18] which forms
hypochlorous acid (ClOH) (Eq. (3)). ClOH is unstable in (aqueous) so-
2
Cl þ 2½OHꢀ →½ClOꢀ þ Cl þ H O þ 2e
ð6Þ
2
One half of Cl− is consumed via an EC mechanism, while the other
half is regenerated by an EC` mechanism. Further oxidation and con-
2
2
−
−
sumption of Cl results in a ca. 4e oxidation process with respect to
−
−
−
lution and known to disproportionate to form [ClO
3
]
and 2HCl [19].
the initial Cl , when [ClO] is the final product.
Fig. 2(b) displays CVs of 10 mM [Emim]Cl in ambient [Emim]
[NTf ] in the absence and then with the gradual addition of
2
Cl þ H O →ClOH þ HCl
ð3Þ
2
2

M.M. Hossain et al. / Electrochemistry Communications 34 (2013) 331–334
333
−
(
a)
aqueous solution [19]). However, extended bulk electrolysis of Cl in
9
6
3
0
−
−
the presence of [OH] in [Emim][NTf
2
] resulted in consumption of Cl
but did not produce [ClO] , nor any other UV–vis active species. There-
−
−
−
fore phase-transferred [ClO] and the higher oxidation species [ClO
2
] ,
ambient (wet)
−
−
+
3
[ClO ]
4
and [ClO ]
(introduced as Na salts) were investigated
after 18 hr under
vacuum (dry)
electrochemically.
Fig. 3(a) displays the CV for 10 mM [TMA][OH] before and after
the introduction of 20 mM [ClO] (as an aqueous Na[ClO]/Na[OH] so-
lution). The increase in the [OH] oxidation feature is attributed to
additional [OH] present in the aqueous [ClO] solution, while the
broad new oxidation feature between +0.9 to +1.3 V corresponds
to the oxidation of [ClO] to a higher valence state of chlorine;
3
] in aqueous sys-
tems (Eq. (6)) [18,22]. An associated reduction feature at +0.20 V
−
−
−
−
−
−
−
[
ClO] is known to be easily over-oxidised to [ClO
+
(
vs. Fc/Fc ) is qualitatively similar to the reduction feature observed
at +0.42 V (vs. Ag) after the reaction of Cl
2
and water in Fig. 2(a).
0.0
0.5
1.0
1.5
½ClOꢀ þ 3H O →2½ClO ꢀ þ 4Cl þ 6Hþ
−
−
−
3
2
−
6
þ
O þ 6e
ð7Þ
2
3
2
Potential / V (vs. Ag wire)
(
b)
6
4
2
0
0
0
0
(
a)
-
-
1
0mM Cl + 50mM [OH]
8
0
-
Addition of [OH]
10mM
-
60
[
OH]
-
1
0mM Cl
-
-
1
0mM [OH] + 20mM [ClO]
-
4
2
0
0
0
Just 10mM [OH]
0.0
0.5
1.0
1.5
+
-0.5
0.0
0.5
1.0
1.5
Potential / V (vs. Fc/Fc )
Potential / V (vs. Fc/Fc+)
Fig. 2. (a) CVs of 10 mM [Emim]Cl in [Emim][NTf
2
] before (−−−) and after (—) 18 h
vacuum drying (GC, ν = 5 mVs− ), and (b) CVs for 10 mM [TMA][OH] (—) and
1
1
2
0 mM [Emim]Cl (−-−) in [Emim][NTf ], with successive addition of 15, 20, 28, 36
−
1
(b)
and 50 mM [TMA][OH] to the latter (GC, ν = 100 mVs ).
-
-
2
0mM [ClO3]
20mM [ClO4]
4
5
-
Just 10mM [OH]
tetramethylammonium hydroxide ([TMA][OH]). Also shown is the
−
oxidation of 10 mM [TMA][OH] in the absence of Cl . Upon addition
30
-
2
0mM [ClO2]
-
-
of the [OH] , the Cl oxidation peak increased in a manner consistent
−
−
with a change from a 2e to 4e oxidation, with the concurrent
loss of the Cl reduction peak indicating its consumption. The peak
also shifted anodically, whereas a cathodic shift with addition of
TMA][OH] might initially be expected due to the shift in equilibrium
21]; this is believed to relate to the [OH] preferentially adsorbing at
2
1
5
0
[
[
−
−
the GC surface over Cl at lower potentials, with an increasing con-
−
−
centration of [OH] displacing more Cl .
3
2
.4. Electrochemistry of various oxychloride species in [Emim][NTf ]
-
15
-
0.5
0.0
0.5
1.0
1.5
−
reacting with [OH]− in an IL, and
[
ClO] is the logical product of Cl
2
Potential / V (vs. Fc/Fc+)
−
[
ClO] could be phase transferred from a commercial hypochlorite so-
lution into [Emim][NTf
characteristic UV–vis spectra of [ClO] in [Emim][NTf
with λmax at 293 nm (c.f. λmax = 292 nm for [ClO] in 1 M Na[ClO
2
] providing the IL contained [TMA][OH]. The
] containing 10 mM [TMA][OH] (GC, ν = 100mVs−1) with
Fig. 3. CVs in [Emim][NTf
2
−
2
] was observed,
−
−
−
3
the addition of (a) 20 mM [ClO] , (b) 20 mM [ClO
2
]
, 20 mM [ClO
]
and 20 mM
−
−
4
]
[ClO
4
]
.

334
M.M. Hossain et al. / Electrochemistry Communications 34 (2013) 331–334
CVs of [ClO
Fig. 3(b) displays a representative CV of 20 mM [ClO
2
]− demonstrated it could not be a final product.
Acknowledgements
−
2
]
in the pres-
addition. The
]− were electro-
chemically formed it would be rapidly oxidised further. In both the
−
ence of 10 mM [TMA][OH] ca. 14 min after [ClO
2
]
LA acknowledges the Australian Research Council (ARC DECRA
DE130100770) for research funding.
oxidative feature at +0.62 V highlights that if any [ClO
2
−
−
presence and absence of [OH] , the [ClO
2
]
voltammetry was unstable
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