Mechanistic, kinetic and electroanalytical aspects of quinone-hydroquinone redox system in N-alkylimidazolium based room temperature ionic liquids

Article abstract of DOI:10.1016/j.electacta.2012.07.059

Voltammetric investigations of quinone-hydroquinone redox couple in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) room temperature ionic liquids (RTILs)

Full text of DOI:10.1016/j.electacta.2012.07.059

Electrochimica Acta 81 (2012) 275–282
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Electrochimica Acta
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Mechanistic, kinetic and electroanalytical aspects of quinone–hydroquinone
redox system in N-alkylimidazolium based room temperature ionic liquids
∗
Mohsin Ahmad Bhat
Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2012
Received in revised form 16 July 2012
Accepted 16 July 2012
Available online 27 July 2012
Voltammetric investigations of quinone–hydroquinone redox couple in 1-butyl-3-methylimidazolium
tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])
room temperature ionic liquids (RTILs) are reported. In dry RTILs, the mechanism is found to be electron
transfer followed by electron transfer (EE) type, while in presence of proton donor/acceptor the electron
transfer is followed by proton transfer (ECEC). Dependence of the thermodynamic and kinetic aspects of
electron transfer on the nature of RTIL is explained in light of the viscosity and stabilization of electrogen-
erated species. The electroanalytical utility of quinone/hydroquinone electrochemistry for estimation of
base concentration in RTILs is also presented.
Keywords:
Cyclic voltammetry
ECEC
Room temperature ionic liquid
Quinone–hydroquinone redox
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
redox behavior of quinones demands a comprehensive exploration
of environmental impact and RTILs present unique media to test
the role of charge, size, intermolecular interactions and polarizib-
lity of ions on their redox characteristics without the complication
of solvent effects. In this regard few reports with a limited empha-
sis on kinetic and mechanistic aspects of electron transfer related
to the electrochemistry of quinones in RTILs have been published
recently [14,15]. Pertinent to mention these reported studies have
been carried over metal electrodes (Pt, Au) and it has been well
Room temperature ionic liquids (RTILs) continue to receive
considerable attention from the scientific and industrial commu-
nities in general and electrochemists in particular [1–4]. A recent
attraction to the RTILs is the structural organization prevailing
in their bulk and RTILs confined to interfaces [5,6], that makes
them suitable solvent systems with potential to mimic circum-
ambiency of electron transfer sites in biological systems. Though
features like tunable physicochemical characteristics, high thermal
stability, ability to dissolve wide range of organic and inorganic
substrates, stereo-selective interactions of constituent ions with
electrogenerated species, and above all the eco-green characteris-
tics of RTILs, have attracted considerable attention [3,4,7–10], their
use for understanding mechanism of electron transfers in living
systems remains under explored. Electrochemistry of quinones,
which are well represented in biological systems: playing key roles
in the photosynthetic reaction center, mitochondrial ATP synthe-
sis and as essential part of vitamins [11–13] etc. seems to be a
good choice to be investigated in this regard. Quinones in many of
their biological roles act as electron gates in structured hygroscopic
environments, thus exploration of their electrochemical behavior
in low water-content, structured solvents like RTILs is expected
to be very useful for comprehending many unexplored aspects
related to bio-electron transfers. Complete understanding of the
established that both BQ and H Q show significant adsorption
2
over these metal surfaces [16–18] that largely affects the elec-
trode kinetics and mechanism. Hence a clear understanding about
impact of RTIL specific effects on the redox behavior of BQ/H Q
2
system demand comprehensive voltammetric investigations of the
said system over electrode surfaces wherein minimal adsorption
effects are expected. In view of the reported electrochemical char-
acteristics of glassy carbon electrode (GC) [19], GC seems to be an
excellent choice for such studies. In addition to the above cited
reasons, the industrial [20,21], synthetic [22] and pharmaceutical
[23] uses of quinones, besides their role as model redox systems
for broad understanding of organic redox chemistry and envi-
ronmental impact on heterogeneous electron transfer [24,25,17],
motivated us to explore their redox behavior over GC electrode in
RTILs.
Cyclic voltammetry (CV), chronoamperometry (CA), chrono-
coulometry (CC) and differential pulse voltammetry (DPV)
were used to probe mechanistic, kinetic and electroanalytical
aspects of quinone/hydroquinone redox system in the 1-butyl-3-
∗ Present address: Department of Chemistry, University of Kashmir, Srinagar
methylimidazolium tetrafluoroborate ([BMIM][BF ]) and 1-butyl-
4
1
0
90006, India. Tel: +91 194 2424900x27; fax: +91 194 2421357.
3
-methylimidazolium hexafluorophosphate ([BMIM][PF ]).
E-mail addresses: mohsinch@kashmiruniversity.ac.in, mohsin skb@yahoo.co.in
6
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.07.059

2
76
M.A. Bhat / Electrochimica Acta 81 (2012) 275–282
2
. Experimental
2.1. Chemicals
Chemicals used were of analytical (AR) grade. Hydro-
quinone (H Q), acetonitrile (ACN) and ethyl acetate (EA), HPLC
2
grade dichlomethane (DCM), Sodium bicarbonate (NaHCO ) and
3
sodium sulfate (Na SO ) were purchased from Merck, India.
2
4
1
-methylimidazole, 1-chlorobutane, tetrafluoroboric acid (42%
aqueous solution of HBF ) and hexafluorophosphoric acid (62%
4
aqueous solution of HPF ) were procured from Spectrochem India.
6
1
,4-benzoquinone (BQ) was synthesized in the laboratory as per the
published procedure [26]. Briefly, 3.3 g (33 mmol) of H Q was dis-
2
solved in 15 mL of 60% acetic acid, the mixture was cooled to about
◦
0
–5 C in an ice bath. 4.2 g (42 mmol) CrO was separately dissolved
3
in 7 mL of water and 3 mL of glacial acetic acid. The resulting solu-
tion was added to the chilled hydroquinone solution slowly with
constant stirring to prevent temperature rise above 5 C. Reaction
Fig. 1. Cyclic voltammogram recorded on GC electrode (2 mm diameter) for 20 mM
−
1
BQ in [BMIM][BF4] at 298 K. The scan rate was 100 mV s
.
◦
mass was stirred further for 70 min at the same temperature. The
product was filtered, washed with 2–3 mL ice cold water several
times. Bright yellow crystals of BQ were obtained (yield 60%) and
tested for purity by the melting point measurement. The crystals
being light sensitive were stored in dark colored sample vials. Fresh
samples of RTILs were synthesized, purified and dried for electro-
chemical investigations, the details of the procedures are reported
elsewhere [9,10].
2.2. Electrochemical measurements
All electrochemical measurements were performed in specially
designed jacket glass cell (2 mL capacity) and having a provision for
vacuum drying and maintenance of inert atmosphere by purging
high purity Ar gas. Temperature of the cell was controlled with an
accuracy of ± 0.1 C through circulation of water from water bath.
Scheme 1. 3
quinone/hydoquinone redox couple.
×
3 square scheme depicting possible electron pathways for
◦
Electrochemical measurements were performed with Metrohm
PGSTAT100 Potentiostat/Galvanostat using 2 mm diameter glassy
carbon (GC) as working, Pt mesh as counter and Ag wire as quasi-
reference (QRE) electrode. For the sake of reliable and reproducible
measurements, all care was taken in cleaning and maintenance of
inert thermostatted conditions within the electrochemical setup.
Inbuilt positive feed back circuitry in the electrochemical work sta-
tion was used to evaluate and compensate the solution resistance.
Measurements were performed at relatively high electroanalyte
concentrations (≥20 mM) in view of high viscosity of RTILs. Though
at −0.76 V and −1.11 V (marked as C1, C2) and two anodic peaks
at −0.69 V and −1.01 V (marked as A1, A2). From Peak clipping
experiments (Fig. S1 in Supporting information) it was established
that while peak A1 is correlated to peak C1, the peak A2 arises
on account of species generated at C2. This behavior is typical for
quinones in dry, neutral and aprotic media [25]. To establish the
nature of intermediates formed in the electron transfer process,
quinone–hydroquinone redox couple was explored. The electron
transfer reactions of quinone and hydroquinone, depending upon
the prevailing conditions, can occur through one of the pathways
represented in a 3 × 3, nine species array [27–29] as in Scheme 1.
both BQ and H Q have appreciable solubility in the used RTILs,
2
their dissolution kinetics was observed to be very slow. An indirect
procedure was followed for the preparation of desired concen-
+
2+
The reported pKa values for QH (−7) and QH (≤ −7) [30], do
2
trated solutions of BQ and H Q in the RTILs. Initially an accurately
2
not support the formation of protonated forms of BQ as interme-
diates even in presence of trace amounts of water in RTIL. Hence,
there remain only four possible pathways to account for the two
electron reduction voltammogram in Fig. 1. To identify the path
followed, CVs for BQ in presence of varying amounts of proton
weighed amount of BQ and H Q was dissolved in DCM this was
2
followed by addition of a specific volume from this solution to
known volume of RTIL and the removal of DCM under vacuum.
All cyclic voltammetric data were background corrected prior to
the analysis. Details related to the electrochemical measurements
are discussed elsewhere [9,10]. Numerical calculations, convolu-
tion analysis and data fitting was performed through codes written
in, Origin 6.0 (Microcal Software Inc.) and MATLAB 7.0.4. Digital
simulations were carried out with GPES software version 4.9 from
Eco Chemie B. V. Ultrecht, Netherland.
donor (propanoic acid) and for H Q in presence varying amounts of
2
proton acceptor (ammonia), were recorded and the same are pre-
sented in Fig. S2 in the Supporting information. Representative CVs
from these voltammogramms to depict the voltammetric response
for BQ in absence/presence of proton donor and for H Q in pres-
2
ence/absence of proton acceptor are presented in Fig. 2. As clear
from Fig. 2, the peaks of BQ (in absence of propanoic acid-trace C)
3
. Results and discussion
and H Q (in absence of ammonia-trace A) do not match each other
2
implying that different intermediates are produced in their redox
cycle. However, comparison of CV for BQ in absence (trace C) and in
presence of propanoic acid (trace-D) reveals that, in presence of a
proton donor, a single pair of peaks instead of two pairs is observed.
Comparison of traces C and D in Fig. 2 also reveals that, though
3
.1. Mechanistic aspects of BQ/H Q redox couple
2
Fig. 1 represents a typical CV recorded at GC electrode for 20 mM
BQ in pre-dried [BMIM][BF ]. The CV shows two cathodic peaks
4

M.A. Bhat / Electrochimica Acta 81 (2012) 275–282
277
Fig. 2. Cyclic voltammograms recorded on GC electrode for (A) H2Q (B) H2Q in pres-
ence of ammonia (30 mM) (C) BQ (D) BQ in presence of propanoic acid (100 mM), in
−
1
[BMIM][BF4]. The CVs were recorded at 298 K and the scan rate was 100 mV s
.
Scheme 2. Mechanistic pathways for; hydroquinone and quinone redox couple in
[
BMIM][BF4] and [BMIM][PF6].
cathodic peak shifts negatively in presence of propanoic acid, the
peak separation between the two is much greater than in absence
of propanoic acid. Thus, even though E1/2 value is still more positive
to peak potential, the peak separation is very large. From thermo-
dynamic and kinetic perspectives, these comparative features in CV
imply that, though the electron transfer in presence of propanoic
acid is thermodynamically easier, the kinetics of the overall pro-
cess is quite sluggish. Such sluggish nature of the electron transfer
in the light of Laviron’s analysis [29,30] implies that in presence
of propanoic acid, electron transfer in BQ is associated with pro-
ton transfer from the acid to the electron transfer products. The
proton transfer shifts the equilibrium potential for the overall reac-
tion positively and the equilibrium requirement as such demands
sets of peaks (trace B) instead of the one set that is observed in
its absence (trace A). Interestingly the position of this new set of
peaks (generated for H Q in presence of ammonia) matches well
2
the peak set for BQ in presence of proton donor and not with that of
BQ alone. In presence of excess ammonia (concentration equal to
or more than twice the concentration of H Q) only one set of peaks
2
that matches the CV peaks for BQ in presence of proton acceptor
were observed. Thus, it may be argued that in presence of proton
acceptor for H Q and proton donor for BQ same intermediates are
2
involved in the redox cycle, while in their absence a different set
of intermediates is involved. To sum up, while the redox behavior
of BQ in presence of proton donor matches that of H Q in presence
2
0
current flow at potentials prior to E , for which facile kinetics of
of proton acceptor, they follow a different redox path in absence of
proton donor and proton acceptor respectively.
electron transfer is a prerequisite. Since the electron transfer pro-
cess for quinones is inherently slow, current does not flow until
The kinetic features associated with the CVs, i.e., shifting of the
oxidative peak positively for quinone (thermodynamically easier),
but with an increase of peak to peak separation (kinetically slug-
gish) is a signature of electron transfer followed by the proton
transfer [29,30]. These features in the CV make us to propose that
in presence of proton donor in case of BQ and proton acceptor in
0
the potential is significantly negative to E . The resulting significant
over potential due to rapid protonation equilibria, will hence lead
to sluggish kinetics of electron transfer in presence of propanoic
acid as is observed in the recorded CVs. As expected, addition of
electrons to quinone structure will increase the electron density
on the oxygen atoms thereby increasing the basicity of the overall
molecule. Thus pKa values of p-benzoquinone will be higher than
that for semiquinone. In the RTIL C2-H proton is considered most
acidic but it has a very high pKa value of 23.0 [31,32], that rules out
any proton transfer by RTIL to the electrogenerated species from
p-benzoquinone. Hence it may be argued that while in RTIL the
overall electron transfer follows EE mechanism (top horizontal row
of Scheme 1), in presence of propanoic acid the electron transfers
are followed by proton transfer(ECEC). Thus in presence of proton
case of H Q it is not the CE mechanism as proposed earlier [33],
2
but EC mechanism that operates. The presence of proton accep-
tor for H Q and proton donor for BQ leads to their conversion
2
to BQ and H Q respectively during redox cycling through ECEC
2
mechanism. Thus we propose that, in pure and dry RTILs, BQ and
H Q follow purely electron transfer pathways, i.e., top and bottom
2
pathway of Scheme 1, and electron transfer associated with pro-
ton transfer in presence and absence of proton donor and acceptor
respectively as depicted in Scheme 2. Similar behavior has been ear-
•
−
donor, after single electron reduction of Q to Q , the later is pro-
lier reported for BQ/H Q couple in buffered as well as unbuffered
2
•
−
tonated to form QH , which is reduced at the potential of the first
aqueous media wherein the proton donor/acceptor concentration
wave to produce QH and which in turn is protonated to form QH2.
is greater than [BQ] and [H Q] [34]. After establishing the mech-
2
Thus the overall two electron–two proton transfer in presence of
proton donor, proceeds through the scheme:
anism of the electron transfer, our next aim was to unravel the
kinetic and thermodynamic aspects of the oxidative and reductive
electron transfers for hydroquinone and quinone in the RTILs.
Q + e− ꢀ Q•−
(1)
(2)
(3)
(4)
•
−
+ H+ ꢀ QH•
3.2. Hydroquinone in [BMIM][PF ] and [BMIM][BF ]
Q
6
4
•
−
−
QH + e ꢀ QH
Fig.
3 shows CV recorded for 25 mM Hydroquinone in
QH + H+ ꢀ QH2
−
[BMIM][PF6] at the GC electrode. A gap of 530 mV on potential scale
between the anodic peak A1 and its associated cathodic peak C1
and other CV characteristics are an indication of an electrochem-
ically irreversible electron transfer [35,36]. Similar behavior has
•
Ease of reduction of QH is responsible for a single reductive peak
trace D) rather than a pair (trace C) as in absence of proton donor.
(
This justification was further confirmed by observations in the
been reported for H Q in acetonitrile [37,38] and propylene carbon-
2
redox behavior of H Q. In presence of ammonia(at concentration
ate [39]. In the segment a–b, the rate constant of electron transfer
is very small and consequently a capacitive current is observed. In
2
less than twice the concentration of H Q) the CV of H Q shows two
2
2

2
78
M.A. Bhat / Electrochimica Acta 81 (2012) 275–282
Fig. 3. Cyclic voltammogram recorded on GC electrode (2 mm diameter) for 25 mM
−1
H2Q in [BMIM][PF6] at 298 K. The scan rate was 100 mV s
.
Fig. 5. Anson plot for double potential step chronocoulometric data recorded on GC
electrode (2 mm diameter) for 25 mM H2Q in [BMIM][PF6] at 298 K.
Fig. 4. Cyclic voltammograms at changing scan rates recorded on GC electrode
2 mm diameter) for 25 mM H2Q in [BMIM][PF6] at 298 K. The scan rate range is
from 50 to 1000 mV s . The inset shows Ip vs. square root of scan rate for the anodic
Fig. 6. Linear sweep voltammogram (LSV) and its convoluted form recorded on GC
(
electrode (2 mm diameter) for 25 mM H2Q in [BMIM][PF6] at 298 K, the scan rate
−1
−1
was 100 mV s . Inset of the figure shows convoluted LSVs recorded at changing
and cathodic peaks of the recorded CVs.
scan rates.
the segment b–A1, the rate constant varies exponentially with the
potential and so does the current which in the region A1-c falls due
to increase in diffusion layer thickness. In the region c–d the oxida-
tive current falls slowly on account of progressive exhaustion of
can be attributed to high viscosity of the RTIL (ꢀ[BMIM][PF6] = 207 cP
[
41]) that leads to slower mass transport. The diffusion coefficient
2+
of H Q is almost three times less than that of the H Q, perhaps due
2
2
to its electrostatic interactions with the RTIL anions. The chrono-
H Q concentration, while in the region d–e the fall in rate constant
2
coulometric data recorded for H Q in [BMIM][BF ] using Cottrell
due to potential decrease leads to decrease in the current. In the CV,
the regions b–A1-c and e–C1-f represent the the potential/diffusion
dependent electron transfer for processes;
2
4
−
06
2
−1
equation gives a value of 1.51 × 10
cm s for diffusion coef-
ficient of H Q, an order of magnitude higher than that estimated
2
in [BMIM][PF ] which can be attributed to higher viscosity of the
6
2
+
H Q − 2e → H Q
(5)
(6)
latter (ꢀ[BMIM][BF = 34 cP [42]). From the slope of the anodic peak
2
2
4]
potential vs. log of scan rate plot the magnitude of the product of
electron transfer coefficient and number of electrons involved in
the rate determining step (˛ ×3n) was found to be 1.08, a value
well expected [36] from the shape of the voltammogram (Fig. 3).
For the present case this magnitude of ˛ ×n can be treated as an
evidence for the second electron transfer being the rate determin-
ing step. For estimation of heterogeneous electron transfer rate
H Q2 + 2e → H Q
+
2
2
respectively. CVs were recorded at changing scan rates from
−1
5
0–1000 mVs , and the same are presented in Fig. 4. Linear depen-
dence of peak current in the CVs on the square root of scan rate for
both peaks over the entire scan rate range investigated (inset Fig. 4),
implies a diffusion controlled process. Similar features were seen in
the CVs recorded for H Q in [BMIM][BF ], however the peak posi-
constant (k ) the LSVs recorded at changing scan rates were convo-
0
2
4
luted using the algorithm suggested by Lawson and Maloy [43]. In
the convoluted current (I(t)) vs. potential plots (Convoluted LSVs)
that are presented in Fig. 6, the limiting current was found to be
independent of the scan rate. This confirms the absence of preced-
ing and succeeding chemical reactions associated with the electron
transfer. Using the standard equations valid for the current and the
convoluted current of LSVs in irreversible electron transfer [35], the
tions were shifted more positive to those observed in [BMIM][PF ].
6
Double potential step chronocoulograms were recorded to esti-
mate the diffusion coefficients and to look into adsorption if any of
2+
H Q and its oxidized form H Q and the Anson plot for recorded
2
2
data is shown in Fig. 5. As clear from the intercepts there seems to
2+
be no adsorption of H Q or H Q during the redox process. The
2
2
Cotrell plot for the forward and reverse steps led to diffusion coef-
−
06
−1
−
07
−07
2
−1
k value was found to be 1.06 × 10
cm s almost similar to the
ficient values of 6.72 × 10
and 2.6 × 10
cm s for H Q and
0
2
2+
value reported by Wang et al.[15] in 1-ethy-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide.
H Q respectively. The diffusion coefficient is about two orders
2
of magnitude less than that reported for H Q in ACN [40] which
2

M.A. Bhat / Electrochimica Acta 81 (2012) 275–282
279
Fig. 8. Comparison of experimentally recorded cyclic voltammogram recorded on
−
1
GC electrode for 20 mM BQ in [BMIM][BF4] (black), at the scan rate of 100 mV s
Fig. 7. Cyclic voltammogram recorded on GC electrode (2 mm diameter) for 20 mM
to the one theoretically generated (red) with the approximation of ErEq mechanism
for two electron transfer. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of the article.)
BQ in [BMIM][BF4] (red) and [BMIM][PF6] (black) at 298 K. The scan rate was
−
1
1
00 mV s . (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
3
.3. Benzoquinone in [BMIM][PF ] and [BMIM][BF ]
6 4
groups for quinones in aprotic media [46,49]. Dismutation of rad-
ical anion to regenerate BQ [51], smaller diffusion coefficient of
the electron accepting specie for peak C2 which is generated in
the peak span of C1 [52,53] and complexation of dianion with BQ
have been proposed to be responsible for the said feature [46].
Qualitative analysis of CVs using the peak width measurements
Fig. 7 shows CVs recorded for 20 mM BQ in [BMIM][BF ] and
4
[
BMIM][PF ] at the GC electrode, exhibiting similar features. The
6
CVs show two cathodic peaks C1 and C2 and two anodic peaks A1
and A2 with Ep–Ep/2 in the range ≈ 60 mV. The observed differ-
ences in peak heights can be attributed to the higher viscosity of
(Ep–Ep/2), for C1–A1 peaks – 60 mV (close to 58 mV as required
[
BMIM][PF ] in comparison to that of [BMIM][BF ]. In view of the
6
4
for a reversible electron transfer), and for C2–A2 – greater than
80 mV (as expected for a quasireversible electron transfer reaction)
establishes the reversible and quasireversible redox characteristics
respectively of successive electron transfers. Semiderivative convo-
lution of voltammograms depicted in Fig. 7, consist of two mirror
image peaks corresponding to C1–A1 couple, while two peaks with
slight offset in peak potentials for C2–A2 couple respectively. Both
these features imply that the voltammograms in Fig. 7 correspond
to two successive electron transfer reactions in which the first elec-
tron transfer is thermodynamically as well as kinetically more facile
than second one. Similar findings have been reported earlier for
benzoquinone in nonaqueous aprotic media [46]. Fig. 8 shows a
comparison between the experimentally recorded CV and the one
generated theoretically on ErEq mechanism approximation. Though
there is a good match between the experimental and theoretical
CVs, some minor nonidealities are visible in the experimental one.
The experimental CV shows an extra current at the foot of the
first cathodic peak besides extra current between the two succes-
sive cathodic peaks. A variety of reasons, viz., weak adsorption of
reactants, edge diffusion, unequal diffusion coefficients of quinone
radical anion and dianion, reduction of weakly adsorbed radical
anion to strongly adsorbed dianion [54–56] have been put forth to
be responsible for such nonideality.
previously published reports [14], the values observed for Ep–Ep/2
and the discussion in Section 3.1, it can be safely argued that the
CVs presented in Fig. 7 correspond to a two-step two-electron path-
•
−
way for reduction of Q to Q at peak C1 which is subsequently
2−
2−
•−
reduced to Q at peak C2, while Q is oxidized back to Q at
peak A2 which is further oxidized to Q at peak A1. From the CVs in
Fig. 7, it appears (more negative peak potential, greater peak to peak
separation, and broader peaks in case of [BMIM][PF ]) that both
6
kinetically and thermodynamically the reduction steps in BQ espe-
cially the second one is easier in [BMIM][BF ] than [BMIM][PF ].
4
6
As has been reported earlier that both thermodynamics and kinet-
ics of electron transfer in quinones is strongly dependent on the
solvent polarity [44], nature and concentration of the supporting
electrolyte [45] and presence of acidic additives [46]. Considering
the established relationship between electron affinities and E1/2
[
47] values for quinones, viz.,
E1/2 = −Em+1 + C
(7)
where Em+1 is the energy of lowest unoccupied molecular orbital of
quinone and C a measure of difference in solvation energy between
the oxidized and reduced forms. In the present case where the
polarity of the two RTILs does not differ much (ꢁ[BMIM][BF = 11.7
4]
and ꢁ[BMIM][PF6] = 11.4) [48], we ascribe the variations in E
to
To have a clear understanding of the overall voltammetric
1/2
−
1
solvent specific solvation energies on account of ion pairing. This
is further supported by the extent of stabilization of electrogen-
erated charged species in the two RTILs, which differs more for
peak C2 than C1. The observed trend suggests that the imidazolium
cations are more effective in stabilizing the intermediates of the
behavior, CVs at changing scan rates from 100 to 1000 mV s were
recorded and the same are presented in Fig. 9. At all scan rates
it was found that for peak C1, the peak potential remains almost
unchanged, while Ep–Ep/2 value remains in the range as expected
for reversible electron transfer for the entire scan rate range inves-
tigated. However, for the peak C2, the peak position significantly
moves cathodically with increasing scan rate with Ep–Ep/2 value in
the range of quasireversible electron transfer process. Quantifica-
tion of peak characteristic parameters in the light of established
theories of heterogeneous electron transfer was undertaken to
unravel the associated kinetic and thermodynamic aspects. Thus
it was found that for peaks C1 and A1 in Fig. 9 the peak current
exhibits a linear dependence (inset Fig. 9) on the square root of
scan rate. This implies diffusion control on the shape of C1–A1 peak
pair. However, interestingly for the peak C1, the current function
−
−
redox process, in presence of [BF ] , than [PF ] and the dianion
4
6
is stabilized to greater extent than the monoanion. Similar trend
in ion pair stabilization of quinone radical anion and dianion by
imidazolium RTILs is evident from the results published by Islam
and Ohsaka [49]. This RTIL specific stabilization of electrogenerated
species has been attributed to the RTIL specific multilayer structural
organization prevailing across electrode/RTIL interface [50].
As evident from Fig. 7, the peak height corresponding to sec-
ond electron transfer (C2) in both RTILs is lesser in comparison
to the first electron transfer, a fact also reported earlier by many

2
80
M.A. Bhat / Electrochimica Acta 81 (2012) 275–282
07
−
−06
cm2s−1 for benzoquinone in [BMIM][PF6]
1
0
and 1.37 × 10
and [BMIM][BF ] respectively. The Anson plots for the coulomet-
4
ric data gave similar intercepts for forward and reverse potential
steps of C1–A1 couple establishing the absence of BQ or its radical
anion adsorption. The formal redox potentials of the two elec-
tron transfers were calculated from the average of peak potentials
corresponding to peaks C1–A1 and C2–A2 of the semideriva-
tive convoluted voltammograms. The E 10 and E 20 were respectively
found to be −0.325 and −0.666 V respectively in [BMIM][BF ], a
4
separation of about 341 mV between the two. With this much sep-
aration the electrode processes for two electron transfer are always
more complicated than the appearance of the voltammogram. In
such cases, both heterogeneous and homogeneous electron trans-
fer occurs and hence disproportionation or comproportionation
reactions make the simple EE mechanism more complicated. The
equilibrium constant for disproportionation reaction, viz.,
Fig. 9. Cyclic voltammograms at changing scan rates recorded on GC electrode
(
5
2 mm diameter) for 20 mM BQ in [BMIM][BF4] at 298 K. The scan rate range is from
0 to 1000 mV s . The inset shows Ip vs. square root of scan rate for the peaks C1
−
1
+ Q•− → Q2− + Q
•−
Q
(11)
and A1 of the recorded CVs.
calculated through equation,
ꢀ
ꢁ
nF(E0 − E )
0
2
RT
1
Kdis = exp
(12)
−
06
−07
comes out to be 1.70 × 10
and 5.28 × 10
in [BMIM][BF ] and
4
[
BMIM][PF ] respectively. The larger magnitude in former may be
6
on account of its lesser viscosity and greater ability to stabilize the
dianion than the later. In both cases the magnitude of dispropor-
tionation constant is greater than the one reported in acetonitrile
and dimethylsulfoxide [49] indicating greater stabilization of dian-
ion by RTILs especially by the [BMIM][BF ]. All these observations
4
make us to presume that the anamolies in CV current and potential
characteristics are on account of dismutation of electrogenerated
product from peak C1 and weak binding of radical anion whose
reduction gives strongly bound dianion [54]. Using Nicholson’s
method [59], from the values of peak separations on potential axis
for respective couples the k0 values for the reduction of quinone
to its radical anion and the reduction of radical anion to dianion
Fig. 10. Current function (Ip/ꢂ1/2) vs. scan rate for peaks C1 and C2 of the CVs
recorded on GC electrode (2 mm diameter) for 20 mM BQ in [BMIM][BF4] at 298 K.
−1
The scan rate range is from 50 to 1000 mV s
.
−
03
in [BMIM][BF ], were calculated to be 6.47 × 10
and 3.59 ×
4
−
04
−1
Ip/ꢂ1/2) was found to be scan rate dependent as depicted in Fig. 10,
10
cm s respectively. The values for the heterogeneous rate
(
constants are two orders of magnitude lesser than that reported in
ACN and seems low in light of the reversibility characteristics of CV
especially for C1–A1 couple. Similar features about heterogeneous
electron transfer in RTILs have been reported earlier [60,61] and
it is proposed that slow solvent relaxation dynamics of RTILs and
their high viscosity is responsible for reversibility characteristics of
CV with low magnitudes of rate constants through solvent dynamic
control on the charge transfer.
showing a decrease with increase of scan rate. Similar analysis for
peak C2 reveals an initial decrease followed by almost constancy
of current function with increase of scan rate. Though the peak
potential for C1 remains almost constant, for peak C2, it shows an
initial cathodic shift followed by constancy with increase of scan
rate. When we calculated the peak current ratio IpC2/IpC1, the ratio
shows an initial decrease followed by constancy with scan rate,
the trend being similar to the variation of peak potential for peak
C2. At all scan rates investigated the ratio of C1/A1 peak currents
was found to be very close to unity, negating the possibility of EC
mechanism for peak C1. Similar observations have been reported
for napthoquinone derivatives [57] and for vitamin K [58] in dry
aprotic solvents. Such variations in current functions, we assume,
can be an outcome of complexation reaction of dianion with the
starting material [46] and the countercation [45] in the RTILs as per
the reactions:
3
.4. Electroanalytic utility of H Q/BQ redox
2
An interesting feature, viz presence of an additional oxida-
tive peak (A2) in the CV prior to original peak (A1), as shown
in Fig. 2(trace B) and Fig. S2(B) (Supporting information), was
observed in the CVs recorded to explore the impact of base con-
centration on the H Q redox behavior in the investigated RTILs.
2
The height of this new peak was observed to increase while that
of the original peak decrease with increasing concentration of the
base added (Fig. S2(B)). It was found that the height of this new
peak increases linearly with the increase in concentration of the
base added. In light of the discussion in section 3.1, it is proposed
Q2 + Q ꢀ (Q)2
2−
−
(8)
(9)
2
Q)2 ꢀ Q + Q•−
−
•−
(
and
Q2− + 2[BMIM]+ −→ Q2−([BMIM] )2
+
that while peak A1 in the CVs (Fig. 2 (trace B), Fig. S2(B) – Support-
(10)
2+
ing information) arises due to oxidation of H Q to H Q , the new
2
2
Double potential step chronocoulograms were recorded to esti-
mate the diffusion coefficients and to look into adsorption, if any,
of BQ and its radical anion during potential scan. The resulting
data using cottrell equation gave diffusion coefficients as 5.34 ×
peak (A2) (observed in CVs recorded in presence of base) arises
on account of oxidation of H Q to BQ in presence of the added
2
base that acts as proton acceptor. In view of the considerable non-
faradaic currents in the recorded CVs, the impact of added base

M.A. Bhat / Electrochimica Acta 81 (2012) 275–282
281
in upgrading the manuscript. Author acknowledges Department of
science and Technology (DST), Govt. of India for the financial assis-
tance to participate in the summer school on electrochemistry, at
School of Chemistry, University of Southampton, UK.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2
012.07.059.
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MAB would like to thank, University authorities-especially Vice
Chancellor, University of Kashmir, and Head, Department of Chem-
istry, University of Kashmir, for sanction of study leave in his favor
and Dr. S. K. Haram (University of Pune) for providing the neces-
sary facilities and support to carry out the presented work. Author
wishes to thank Dr. Vijayamohanan K Pillai (NCL, India) and Prof.
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