Electrochemistry of Br-/Br2 Redox Couple in Acetonitrile, Methanol and Mix Media of Acetonitrile-Methanol: An Insight into Redox Behavior of Bromide on Platinum (Pt) and Gold (Au) Electrode

Article abstract of DOI:10.1515/zpch-2018-1321

Electro-oxidation of Br^- on platinum and gold electrode was studied in acetonitrile, methanol and mix media of acetonitrile-methanol. The mechanism of Br^- oxidation in these media was investigated using CV, Semi Integration Cyclic Vo

Full text of DOI:10.1515/zpch-2018-1321

Z. Phys. Chem. 2019; aop
Muhammad Tariq*
Electrochemistry of Br−/Br2 Redox Couple in
Acetonitrile, Methanol and Mix Media of
Acetonitrile–Methanol: An Insight into Redox
Behavior of Bromide on Platinum (Pt) and Gold
(Au) Electrode
https://doi.org/10.1515/zpch-2018-1321
Received October 25, 2018; accepted April 23, 2019
Abstract: Electro-oxidation of Br− on platinum and gold electrode was studied in
acetonitrile, methanol and mix media of acetonitrile–methanol. The mechanism
of Br− oxidation in these media was investigated using CV, Semi Integration Cyclic
Voltammetry, and Digital Simulation technique. Since, Br− oxidation mechanism
−
on −platinum involves the formation of Br3 as intermediate, therefore, Kstab for
Br3 formation in the mixed media was estimated using digital simulation, Nel-
son and Iwamoto method. Redox mechanism of Br− and Br2 on gold (Au) electrode
was also investigated in protic solvent such as H2O, methanol, ethanol, 1-butanol,
and formic acid. It was ascertained that Br− oxidation on gold (Au) elect−rode in
these above protic solvents involve [AuBr2]− intermediate rather than Br3
.
Keywords: cyclic voltammetry; digital simulation; protic solvents; stability con-
stant; tribromide formation.
1 Introduction
The bromine chemistry has a wide range of applications; it may be use as a counter
anion for the cations of metals and ionic liquid or as supporting electrolyte [1–3].
Particularly the electrochemistry of the Br−/Br2 couple is under continuous inves-
tigation for the last several decades because of its importance in many applica-
tions [4–12]. The unique significance of Br−/Br2 redox couple was its application
in redox flow battery (RFB) for energy storage [4, 5]. More recently, the redox
behavior of the Br−/Br2 couple was examined in room temperature ionic liquids
(RTILs) for its possible utilization as mediator in DSSC technology [13].
*Corresponding author: Muhammad Tariq, National Centre of Excellence in Physical Chemistry,
University of Peshawar, Peshawar 25120, Pakistan, Tel.: +92-91-9216766, Fax: +92-91-9216671,
e-mail: dr.muhammadtariq@uop.edu.pk, tariq_ftj@yahoo.com
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Pioneering work on Br−/Br2 redox couple was carried out by Popov and
Geske as well as Kolthoff and Coetzee in acetonitrile [14, 15]. Compton et al. fur-
ther extended it to room temperature ionic liquid, 1-butyl 1-3-methylimidazolium
bis(trifluoromethylsulfonyl) imide, ([C4mim][NTf2]), using CV and digital simula-
tion techniques [12].
These researchers proposed the following mechanism for Br−/Br2 redox cou-
ple in both acetonitrile and room temperature ionic liquid, ([C4mim][NTf2]), as
shown by equations (1–3).
3Br− ꢀ Br−3 + 2e−
2Br−3 ꢀ 3Br2 + 2e−
(1)
(2)
Tribromide (Br3−) is formed in the above reaction via eq. (3)
Kstab
Br− + Br2 ꢀ Br−3
(3)
Currently, Bennett et al. has proposed a more realistic and comprehensive
mechanism for Br−/Br2 redox couple in nitrobenzene using digital simulation
technique. They proposed the involvement of free radical formation in the ele-
mentary steps [16].
Depending on condition, electrode materials and solvents, various routes of
mechanisms have been proposed for the Br−/Br2 redox couple [8, 12, 13, 16–22].
For example, Raju et al. proposed the involvement of bromine atom free rad-
ical formation in biphasic media for the selective bromination of toluene [8].
Similarly, in our previous study, we have proposed the generation of a bromine
atom free radical as primary electro-oxidation product (PEOP) of Br− in chloro-
form and its subsequent reaction with chloroform using cyclic voltammetric and
controlled potential electrolysis (CPE) techniques [21, 22]. Bentley et al. studied
Br−/Br2 redox system in room temperature ionic liquid, [C2mim][NTf2], using CV
along with convolution voltammetric technique [13]. They proposed similar mech-
anism for Br−/Br2 redox system as previously proposed by Popov and Geske (eqs.
1–3) [23].
Numerous papers are available about Br−/Br2 redox couple in acetonitrile,
nitrobenzene, ionic liquids, and other biphasic media [8, 12, 16, 21–23]. However,
to the best of our knowledge, there is no mechanistic study on Br−/Br2 redox cou-
ple in protic solvent and in mix media of protic and aprotic solvents. Hence, the
purpose of this study is to explore the protic and aprotic solvent (or simply solva-
tion) effect on the redox behavior of Br−/Br2 redox couple using mix media of
various proportions of aprotic and protic solvent. The change in proportion of
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Redox Chemistry of Br−/Br2 in Conventional Solvents
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protic-aprotic solvents in mix media may give us the idea about the stability of
Br−, Br2 as well tribromide ion (Br3−) by the solvation effect of protic and aprotic
solvents.
The reason for choosing acetonitrile and methanol as mixed media in our
study was that both solvents have organic in nature as well as having the abil-
ity to solubilize salt of n-tetrabutylammonium bromide and supporting elec-
trolyte n-tetrabutylammonium perchlorate. Moreover, they have comparably sim-
ilar dielectric constants. Hence, the combination of these solvents was supposed
to be suitable for fabricating mix media of aprotic-protic solvents for investigating
Br− electrooxidation.
Therefore, this study demonstrates the use of Cyclic Voltammetry (at ordinary
microelectrode), SICV, and digital simulation techniques for examining Br−/Br2
redox couple in methanol (protic solvent), acetonitrile (aprotic solvent), and in a
mix media of acetonitrile–methanol in various proportions.
2 Experimental
2.1 Materials and reagents
Salt of n-tetrabutylammonium bromide (>99%) and bromine (≥99.99%) were
obtained from Sigma-Aldrich. To further purify the salt of n-tetrabutylammonium
bromide, recrystallization was carried out from benzene at 80 °C by the addition
of hot hexane and dried under vacuum at 40–50 °C for 2 days. Bromine (≥99.99%)
was obtained from Sigma-Aldrich, while, acetonitrile (anhydrous, 99.9%) and
methanol (anhydrous, 99.9%) were obtained from Scharlau. These chemicals were
used directly without further purification.
The concentration of bromine (Br2) in acetonitrile and methanol was deter-
mined using GENESYS 10 UV spectrophotometer (Thermo, USA). Since, bromine
evaporates from solution, therefore, in each experiment, freshly prepared
bromine solution was used.
2.2 Electrochemical system and procedure
CHI-700C (CH Instruments) with three electrode cell system was used for the cyclic
voltammetric and chronoamperometric experiments. Platinum (Pt) and gold (Au)
disk having 1.6 mm diameter, surrounded by solvent-resistant PCTFE plastic body
(purchased from Bioanalytical System Inc) was used as working electrode. Sat-
urated calomel electrode (SCE) as reference and custom-made gold electrode
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(A ≈ 2 cm2) as auxiliary electrode were used. Electrochemical surface area of work-
ing electrode (∼0.020 ± 0.001) cm2 was calculated with [Fe(CN)6]3−/4− system
using Randles–Sevcik equation [24].
Prior to each measurement, electrodes were polished with alumina pow-
der (0.3–0.05 µm) and rinsed with double distilled water and acetone, followed
by sonication (in distilled water) and again rinsing with distilled water and
acetone. The electrode was then allowed to dry under room temperature. n-
tetrebutylamminum perchlorate (0.1 M) was used as supporting electrolyte. First,
cyclic voltammetric experiments were carried out in 0.1 M supporting electrolyte
without Br− addition. No peak was observed in acetonitrile and methanol in the
potential window chosen for Br− oxidation and Br2 reduction. All the experiments
were performed at ambient temperature (i.e. at room temperature ∼25 °C).
2.3 Data processing through SICV and digital simulation
techniques
An efficient processing technique, semi integration was used for determination
of heterogeneous electron transfer parameters such as diffusion coefficients (D0)
and number of electrons involve in electron transfer reaction. This method was
reported by Oldham which is a subclass of techniques collectively known as con-
volution voltammetric techniques [25, 26]. Detail of this method can be found
elsewhere in the literature [24, 27, 28]. Briefly, semi integration of experimental
I-t data is given by eq. (4) [25, 26].
t
d−1/2 I(τ)
1
I(τ)dτ
t − τ
∫
√
√
M(t) =
=
(4)
dt−1/2
π
0
where I(τ) is the current at the time, τ and t is the total elapsed time. To determine
the value of semi integrated current, M(t), the above eq. (4) was programmed in
computer through an algorithm reported by Oldham and Myland [29]. The algo-
rithm was scripted/encoded in computer programming language, Visual Basic
for Application (VBA) by de Levie [30]. A Microsoft Excel macro that includes
algorithm for these processing techniques is available free online [31]. This MS
Excel macro has been used by the researchers for processing experimental I-t data
[27, 32, 33].
Fortunately, these days, a powerful technique, digital simulation of whole
CV is also available for professional and academic purposes [34–38]. In digital
simulation technique, entire CV can be generated theoretically using digital sim-
ulation software packages for evaluation of electrochemical mechanism and other
electron transfer parameters.
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Redox Chemistry of Br−/Br2 in Conventional Solvents
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In the present study, parameters such as diffusion coefficients (D0), hetero-
geneous electron transfer rate constant (k0), potential at half of the peak current,
E1/2, and the equilibrium constant (Keq) were calculated through CV and SICV for
Br−/Br2 couple were confirmed by digital simulation technique using SIM4U 2.0
software package available free online [37]. Further details for digital simulation
of CV can be found elsewhere in literature [36, 39].
3 Results and discussion
3.1 Voltammetry of Br−/Br2 on platinum (Pt) micro disk
electrode in aprotic and protic solvents
In acetonitrile (aprotic solvent), cyclic voltammogram (CV) of 5 mM Br− on plat-
inum (Pt) electrode vs. SCE was recorded and shown in Figure 1. In the CV, two
anodic peaks at 694 mV and 1.033 mV and their corresponding cathodic peaks at
886 mV and 258 mV were observed. A CV of Br− recorded at various scan rates
highlighted that the process is diffusion controlled having irreversible nature
(Figure 2) [12, 40, 41]. CV for Br2 (2.5 mM) reduction in acetonitrile was also
Fig. 1: CV of 5 mM Br− in acetonitrile on conventional platinum disk electrode (1.6 mm diam-
eter) vs. SCE electrode. n-tetra butyl ammonium perchlorate (0.1 M) was used as supporting
electrolyte. In the CV, represented by the red line (
), potential scan was reversed after the
first anodic peak (i.e. at 0.82 V), while in CV represented by the black line (
), potential scan
reversed was after the second anodic peak (i.e. at 1.2 V). Scan rate: 50 mVs−1
.
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identical in shape (Figure 2). Based on previous reports [12, 23], first peak was
assigned to eq. (1) while the second peak was assigned to eq. (2).
In methanol (protic solvent), cyclic voltammogram (CV) of 5 mM Br− and
2.5 mM Br2 on Pt electrode vs. SCE is given in Figure 3. From the analysis of these
Fig. 2: CV of 2.5 mM Br2 in acetonitrile on platinum disk electrode (1.6 mm diameter) vs. SCE in
0.1 M TBAP supporting electrolyte. Full scan CV was represented by the black line (
) While
in CV represented by the red line (
), scan was reversed at 0.7 V, scan rate: 50 mVs−1
.
Fig. 3: CV of 5 mM Br− and 2.5 mM Br2 in methanol on platinum disk electrode (1.6 mm diam-
eter) vs. SCE in 0.1 M TBAP supporting electrolyte. CV of Br− is represented by the black line
(
), while CV of Br2 is represented by the red line (
), scan rate: 50 mVs−1
.
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Redox Chemistry of Br−/Br2 in Conventional Solvents
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CVs, it might be interpreted that simple one electron quasi reversible type reaction
is involved in Br−/Br2 redox system (having ∆E ∼ 150 or even more in case of the
first CV curve), because the system did not satisfy reversibility diagnostic criteria,
shown by eqs. (5–7), strictly [42–45].
∆EP = Epa − Epc ≈ 60 mV
56 mV
(5)
(6)
Epa − Ep/2
≈
n
Ia
Ic
≈ 1
(7)
As mention previously that in acetonitrile, nitrobenzene, and room tempera-
ture ionic liquids (RTILs), electron transfer mechanism in Br−/Br2 redox system
involved the formation Br2 which produce Br3− upon reacting with Br− (eqs. 1–3)
[12, 16, 23].
3.2 CV of Br− in mixture of various proportions of methanol
and acetonitrile on platinum electrode
The complete information about mechanism of Br−/Br2 redox system might be
difficult to be obtained from just single CV curve recorded in methanol because
as mention previously that in acetonitrile, nitrobenzene, and room tempera-
ture ionic liquids (RTILs), electron transfer mechanism in Br−/Br2 redox sys-
−
tem involved the formation Br2 which produce Br3 upon reacting with Br−
(eqs. 1–3) [12, 16, 23], hence, Br− electro-oxidation was further studied in mix
medium acetonitrile–methanol which is a suitable combination of aprotic-protic
solvents. CV of 5 mM Br− in mix media of acetonitrile–methanol is shown in
Figure 4. The following changes in CV of Br− were observed in mix media of
acetonitrile–methanol (Figure 4):
– anodic peak a2, assigned to Br3− oxidation, shifts cathodically (i.e. to the low
positive potential) upon increasing the methanol fraction.
– while, peak a1 shifts anodically (i.e. to the higher positive potentials).
– hence, ∆Epa (i.e. Epa2 –Epa1) decreases upon increasing methanol proportion.
– finally, both anodic peaks (a1 and a2) merge at 1:1 (CH3OH:CH3CN) proportion.
−
The above observations might be explained in terms of Br3 stabil−ity constant
(Kstab) as shown by eq. (3). The diagrammatic represe−ntation of Br3 formation
during Br− oxidation is shown in Figure 5. Since, Br3 has high stability (Kstab
∼107) [46] in acetonitrile due to it high dielectric constant (ε = 38) while low
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Fig. 4: CV of 5 mM Br− in mix media of acetonitrile–methanol on platinum disk electrode
(1.6 mm diameter) vs. SCE in 0.1 M TBAP supporting electrolyte. Scan rate: 50 mVs−1
.
Fig. 5: Graphical representation of the electron transfer mechanism involved during Br−
electrooxidation on platinum disk electrode (1.6 mm diameter) vs. SCE.
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Redox Chemistry of Br−/Br2 in Conventional Solvents
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stability (Kstab ∼ 177) [47] in methanol due to its relatively low dielectric constant
(ε = 33), hence, upon increasing methanol proportion in mix media, the overall
dielectric constant value become lowered which cause lowering down the Kstab of
Br3−. Rao and Chhabra has also observe similar, trend for triiodide (I3−) in mix
media of water–dioxan [48]. This decrease in Br3− stability can be calculated from
the shift in ∆E1/2 (i.e. E1/2(pa2) – E1/2pa1) using Nelson and Iwamoto method (eq. (8)
[47, 49]).
⎛
⎞
√
4
DBr
2F
−
−
2
[E1/2(Br−3 /Br2) − E1/2(Br /Br3 )] + ln
. (8)
⎝
⎠
√
ln(Kstab) =
−
3RT
3
DBr− [Br ]b
3
where “b” is the bulk concentration of Br−.
As, the oxidation of B−r− in acetonitrile is a diffusion controlled (Figure 6),
a value of E1/2 for Br−/Br3 was estimated from fist anodic peak (a1),−potential
(E) at 85% of peak current ip was taken as E1/2 (eq. 9) for the Br−/Br3 process
(eq. 1) [24].
Eat 85% of peak current (i ) = E1/2
(9)
p
Similarly, E1/2 for the Br3−/Br2 was calculated using eq. (10) [39].
E
paII + EpcII
E1/2
=
(10)
2
Fig. 6: CV of 5 mM Br− in acetonitrile on platinum disk electrode (1.6 mm diameter) vs. SCE in
0.1 M TBAP at various scan rates. Inset Ip vs. v1/2 plot.
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Tab. 1: Stability constant (Kstab) values of Br3− formation in various proportions of acetonitrile–
methanol mix media using cyclic voltammetry.
−
CH3CN:CH3OH Volume
percent proportion (%)
E1/2(Br3−/Br2)
Volt, V
E1/2(Br−/Br3
)
lnKstab
Kstab (M)
Volt, V
100:0
90:10
80:20
70:30
0:100
0.64
0.73
0.75
0.76
—
0.99
0.93
0.90
0.89
—
14.66
10.9
9.47
8.95
5.31a
2.33 × 106
5.42 × 104
1.30 × 104
7.71 × 103
2.02 × 102
aValue was calculated using linear extrapolation.
−
In acetonitrile–methanol mix media, Br2 and Br3 have almost the same
diffusion coefficient values, i.e.
DBr = DBr−
(11)
2
3
−
Thus, stability constant (Kstab) for Br3 in various proportions of methanol
and acetonitrile (Table 1) was estimated using eq. (12) which is a simplified form
of eq. (8)
(
)
2F
3RT
4
ln(Kstab) =
[E1/2(Br−3 /Br2) − E1/2(Br−/Br−3 )] + ln
(12)
−
3[Br ]b
−
Kstab for Br3 in mix media having methanol proportions higher than thirty
percent (30%) were estimated through linear extrapolation method using Origin-
Pro 9, Microcal Software, Inc (Fig. S4 Supplementary material; lnKstab vs. solvent
Proportion extrapolated plot). A value of Kstab for Br3− in pure acetonitrile throug−h
eq. (12) was found to be 2.4 × 106 M−1. A value of stability constant (Kstab) for Br3
through linear extrapolation method in a pure methanol was also calculated and
found to be ∼ 202 M−1, which was in good agreement with literature value and
could be explained by solvation effect [47]. It was inferred that methanol (−a protic
solvent) stabilized Br− more efficiently (due to solvation effect) than Br3 while
in acetonitrile (aprotic solvent), the situation was vice versa.
3.3 Cyclic voltammetry of bromide on gold electrode in
methanol and other protic solvents
The mechanism of Br−/Br2 redox system in protic media was also studied using
gold micro disk electrode. CV of 5 mM Br− in methanol was recorded at 50 mV/s on
gold electrode (Figure 7a). Two distinct anodic peaks were monitored at 750 mV
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Redox Chemistry of Br−/Br2 in Conventional Solvents
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Fig. 7: CV of (a) 5 mM Br− and (b) 2.5 mM Br2 in methanol on gold disk electrode (1.6 mm diam-
eter) vs. SCE electrode. n-Tetra butyl ammonium perchlorate (0.1 M) was used as supporting
electrolyte.
and 965 mV while cathodic peak was observed at 246 mV on gold. On the basis
of above observation as well as electrochemical behavior of gold toward halide
[17–20, 32], the mechanism of Br−/Br2 system in methanol was proposed. It was
inferred that Br−/Br2 redox system on gold electrode involve two step electron
transfer process as given below:
2Br− + Au ꢀ [AuBr2]− + e−
[AuBr2]− ꢀ Br2 + Au + e−
(13)
(14)
To confirmed the formation of [AuBr2]− intermediate in the above redox
mechanism, CV of 2.5 mM Br2 were also recorded on gold electrode (Figure 7b).
The reduction peak at 734 mV (although not well defined) and at 162 mV were
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observed in CV of Br2. An anodic peak at 745 mV were also observed. The peak
at about 734 mV (±10 mV) in both CV of Br− and Br2 indicate the formation of
[AuBr2]−. Although, reduction peak at 162 mV were not examined, experimen-
tally. Perhaps, the reduction of [AuBr4]− (which produced as result of [AuBr2]−
disproportionation reaction) may occurred at this peak position (eq. 5) [20].
[AuBr4]− + 2e− ꢀ [AuBr2]− + Au + 2Br−
(15)
While the formation of [AuBr4]− through disproportionation reaction of
[AuBr2]− was given by
3[AuBr2]− ꢀ [AuBr4]− + Au + 2Br−
(16)
Similarly, the reduction peak recorded in CV of Br− at 246 mV were attributed
to the Br2 reduction of which form due the [AuBr2]− oxidation in eq. 14 (Figure 7a).
The redox behavior of Br−/Br2 system on gold electrode were also monitored in
aqueous media (Figure 8). Although, the position for Br−/Br2 system in both aque-
ous and methanol were not the same but the overall redox mechanism of Br−/Br2
system in both aqueous media and methanol seem identical. CV of Br− in other
protic solvents such as ethanol, formic acid, 1-butanol, and acetic acid were also
recorded for further assessment of redox mechanism (Figure 9). Since, two anodic
peaks were recorded for Br− oxidation in these protic media hence confirm the
involvement of two step electron transfer process on gold electrode.
Fig. 8: CV of (a) 5 mM Br− and (b) 2.5 mM Br2 in aqueous media on gold disk electrode
(1.6 mm diameter) vs. SCE electrode. n-Tetra butyl ammonium perchlorate (0.1 M) was used
as supporting electrolyte.
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Fig. 9: CV of Br− on gold (Au) micro disk (1.6 mm diameter) electrode in four protic solvents.
3.4 Spectroelectrochemistry of Br− in on platinum (Pt) and
gold (Au) electrodes
The favorable evidence regarding the proposed mechanism for Br− oxidation
in protic solvent on both platinum and gold electrode was witnessed through
spectro-electrochemical experiments which were carried out in aqueous media.
In these experiments a controlled potential electrolysis of Br− solutions carried
out on platinum and gold electrode. The spectral changes during the electroly-
sis of Br− solutions was then monitored. It was noted that UV spectra recorded,
during electrolysis of Br− on platinum (Pt) matched with the spectra of Br2
(Figure 10a). However, when the electrolysis of Br− was carried out on gold (Au)
the spectra did not match. The λmax of the product formed during electrolysis
of Br− (on gold electrode) in aqueous solution was recorded at 254 nm which
was quite different than λmax of Br2 (∼265 nm). This lends to support that Br−
oxidation on platinum and gold electrode occurred via different mechanism. On
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Fig. 10: UV-Vis spectra of recorded during electrolysis of Br− (compared with Br2) on (a)
platinum (Pt), and (b) gold (Au) electrode.
gold electrode, it was suggested that the Br− oxidation mechanism involved the
formation of Au complexes (such as [AuBr2]−) rather than Br2 or Br3− formation.
3.5 Data processing through semi integration
Experimental cyclic voltammograms of Br− on platinum disk electrode (1.6 mm
diameter) in acetonitrile, methanol, and mix media of acetonitrile–methanol
(shown in Figures 1–4) were further analyzed using semi integration methods
(Figure S-1). Prior to implementation of SICV techniques on CV, background cur-
rent was first subtracted from experimental Br− CV. Background current was
obtained by the linear extrapolation methods using OriginPro 9, Microcal Soft-
ware Inc. [28]. Diffusion coefficient, D0 (Table 2) for Br− electro-oxidation was
then determined using equation (17) [50].
√
ML = nFC0 D0
(17)
where, F and C0 are Faraday constant and initial concentration of Br−, respec-
tively. n (∼1) represents the number of electrons involved in Br− electrooxidation,
ML, represents a plateau of semi integrated voltammogram which is not sensi-
tive to uncompensated resistance (Ru) or slow heterogeneous electron transfer
kinetics. However, it is very sensitive to background current due to non-faradic
processes, i.e. adsorption phenomenon.
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Redox Chemistry of Br−/Br2 in Conventional Solvents
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Tab. 2: Diffusion coeflcient of Br− in various proportions of acetonitrile–methanol mix media.
CH3CN:CH3OH Percent
proportion (%)
D0 × 10−5 (cm2s−1
)
100:0
80:20
70:30
60:40
50:50
0:100
1.10
1.18
1.17
1.20
1.15
0.97
3.6 Digital simulation
In the present study, experimental CV of Br− in acetonitrile and in a mixture of
acetonitrile–methanol were simulated using a mechanism proposed by Bennett
et al. [16]. Initially, CV of Br− in pure acetonitrile was simulated using Keq ∼ 5.3
× 107 M−1 (much closer to li−terature value) [47]. Then by keeping all other val-
ues fixed (except Keq for Br3 formation), CV of Br− in different proportion of
methanol and acetonitrile was simulated (Figure 11). It was observed that upon
decreasing value of Keq, both anodic and cathodic peaks came close together,
eventually, peaks merged into each other in mix media. These observations sug-
−
gest that stability of Br3 decreases the upon increasing methanol proportion
Fig. 11: Digital simulation of voltammogram of 5 mM Br− in mix media of acetonitrile–methanol
(percent volume proportion of CH3CN:CH3OH; (a) 100:0 (b) 80:20 (c) 60:40) on platinum disk
electrode (1.6 mm diameter) vs. SCE in 0.1 M TBAP. Scan rate: 50 mVs−1
.
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Tab. 3: Best fitted values of stability constant (Kstab) for Br3− in various proportions of
acetonitrile and methanol using Bennett et al. mechanism in digital simulation program [13].
CH3CN:CH3OH proportion (in %)
Kstab (M−1
100:0
80:20
60:40
)
5.3 × 107
5.3 × 104
1 × 104
which highlighted the solvation effect on these anions (Br−, Br3−) by protic sol-
vent and aprotic solvent as mentioned earlier. Best fitted values for Keq were given
in Table 3 (for the complete fitting parameter, see Table S1 in Supplementary
material).
4 Conclusion
Electron transfer mechanism for Br−/Br2 redox system was investigated in ace-
tonitrile, methanol, and mix media of acetonitrile–methanol on platinum elec-
trode as well as on gold electrode in number of protic mediums such as water
(H2O), methanol (CH3OH), 1-butanol (C4H9OH), and formic acid (HCOOH). It was
concluded that on platinum (Pt) electrode, the stability constant (Kstab) of the
Br3− decrease upon increasing methanol proportion−in mix media of acetonitrile–
methanol. These results reveal that stability of Br3 depends on the solvation
effect. The reaction for Br3− formation is given as follows:
Kstab
Br− + Br2 ꢀ Br−3
Hence, with an increasing proportion of protic solvent (methanol) in mix
media of acetonitrile–methanol, the equilibrium shift toward the backward direc-
tion (or value of Kstab become smaller) because methanol stabilized the Br− by
solvation effect more than Br3−. It was further suggested that mechanism of Br−
oxidation in protic solvent on gold (Au) follow different path way. On gold (A−u)
electrode, Br− oxidation involved the [AuBr2]− intermediate rather than Br3
.
This study may have a potential application in the field of redox flow batteries
to utilize the Br−/Br2 in mixed solvents systems.
Acknowledgment: M. Tariq cordially acknowledged support from HEC Pakistan,
HEJRIC, and Prof. Dr. Mehboob Mohammad.
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Supplementary Material: The online version of this article offers supplementary material
(https://doi.org/10.1515/zpch-2018-1321).
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