Zinc Electrodeposition in the Presence of an Aqueous Electrolyte Containing 1-Ethylpyridinium Bromide: Unexpected Oddities

Article abstract of DOI:10.1071/CH17281

The reversible electrodeposition of zinc was investigated in an aqueous electrolyte containing zinc bromide (50mM) and 1-ethylpyridinium bromide ([C2Py]Br, 50mM) by cyclic voltammetry, chronoamperometry, and scanning electron microscopy. Unusual voltammetric behaviour for the Zn/ZnII redox couple was observed in the presence of [C2Py]Br. Passivation of the redox couple was observed after a single deposition-stripping cycle at switching potentials more negative than-1.25V versus Ag/AgCl. This unusual behaviour was attributed to the reduction of 1-ethylpyridinium cations to pyridyl radicals and their follow-up reactions, which influenced the zinc electrochemistry. This behaviour was further seen to modify the nucleation process of electrodeposition, which altered the morphology of zinc electrodeposits.

Full text of DOI:10.1071/CH17281

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2017, 70, 1025–1031
Full Paper
https://doi.org/10.1071/CH17281
Zinc Electrodeposition in the Presence of an Aqueous
Electrolyte Containing 1-Ethylpyridinium Bromide:
Unexpected Oddities*
A
A
A
Max E. Easton, Lisa C. Player, Anthony F. Masters,
A B
,
and Thomas Maschmeyer
A
The Laboratory for Advanced Catalysis for Sustainability, School of Chemistry F11,
University of Sydney, Sydney, NSW 2006, Australia.
B
Corresponding author. Email: thomas.maschmeyer@sydney.edu.au
The reversible electrodeposition of zinc was investigated in an aqueous electrolyte containing zinc bromide (50 mM) and
1-ethylpyridinium bromide ([C2Py]Br, 50 mM) by cyclic voltammetry, chronoamperometry, and scanning electron
microscopy. Unusual voltammetric behaviour for the Zn/ZnII redox couple was observed in the presence of [C2Py]Br.
Passivation of the redox couple was observed after a single deposition–stripping cycle at switching potentials more
negative than ꢀ1.25 V versus Ag/AgCl. This unusual behaviour was attributed to the reduction of 1-ethylpyridinium
cations to pyridyl radicals and their follow-up reactions, which influenced the zinc electrochemistry. This behaviour was
further seen to modify the nucleation process of electrodeposition, which altered the morphology of zinc electrodeposits.
Manuscript received: 23 May 2017.
Manuscript accepted: 18 July 2017.
Published online: 3 August 2017.
Introduction
Results and Discussion
The electrodeposition of metals commonly utilizes a variety of
supporting electrolytes and additives to assist in the efficiency
and quality of the electrodeposit. These various additives may be
used to alter physical properties of the solution or to control the
speciation of metal ions in order to influence deposition mor-
phology.[1–3] In the case of reversible electrodeposition for
rechargeable flow batteries, these additives may enhance re-
oxidation of the metal to metal ions,[4,5] or their functions may
be extended to non-electrochemical functions that influence
other chemistries in the cell.[6–8]
An increasingly prevalent group of additives is a class of
compounds called ionic liquids (IL), which are defined as salts
that melt below 1008C.[9] IL bromide salts, for example, are
traditionally used in the zinc and vanadium bromide flow
batteries, with 1-ethyl-1-methylpyrrolidinium bromide conven-
tionally applied as a bromine sequestration agent.[10–12] ILs have
also been successfully applied as compounds capable of forming
favourable complexes with metal ions to influence electrode-
posit morphology.[13,14] They can also possess wide electro-
chemical windows[15] to allow the deposition of metals with low
Nernst potentials, such as aluminium[16] or magnesium.[17]
Pyridinium-based ILs have shown promise as components of
dye-sensitized solar cells and conductive polymers, and as
additives for rechargeable battery cells.[18,19] In the present
work, we demonstrate their limits for use as electrolyte additives
in zinc-containing cells. These insights are likely to extend to
other metal-containing redox couples.
Cyclic Voltammetry
Fig. 1a shows cyclic voltammograms (CVs) for the 0.2 M KCl
background electrolyte and for 50 mM 1-ethylpyridinium bro-
mide ([C2Py]Br) in the absence of zinc bromide at two different
switching potentials. At a switching potential of ꢀ1.4 V, it is
clear that a reduction of the [C2Py]Br occurs (onset at ꢀ1.25 V).
This CV matches the electrochemical signal observed for the
second and third CV cycle seen in the inset of Fig. 1b. Extending
the switching potential for the [C2Py]Br electrolyte reveals a
peak current at ꢀ1.5 V, which is ascribed to the irreversible
reduction of the 1-ethylpyridinium cation to a radical
(Scheme 1), observed previously in aqueous solutions of pyri-
dine.[20] The electrochemical reduction of pyridine to pyridyl
radicals is known to occur for neat alkylpyridinium ILs at
ꢀ1.25 V versus Ag/AgCl,[15,21] and has also been described in
aqueous solutions of pyridine via a pyridinium intermedi-
ate.[20,22–24] In the present case, a direct and irreversible one-
electron reduction of the pyridinium cation’s p-system is likely,
as is observed for neat ILs.[21] The observed crossover current in
Fig. 1a is likely due to the grafting of the pyridical radical
species on the electrode surface. The absence of a reoxidation
process on the reverse scans is consistent with a rapid (on the
cyclic voltammetric time scale) reaction of the radical, which is
known to dimerize.[25,26]
Fig. 1b shows three consecutive CVs for the Zn/ZnII redox
couple observed in equimolar (50 mM) solutions of zinc
*Dedicated to Professor Alan Bond on the occasion of his 70th birthday – over 40 years of collaborations, distinguished and generous colleague and friend.
Journal compilation Ó CSIRO 2017
www.publish.csiro.au/journals/ajc

1026
M. E. Easton et al.
(a)
(b)
0.05
0
0.4
0.3
0
0.02
0.04
0.2
Scan direction
Ϫ1.4 Ϫ1.2 Ϫ1.0 Ϫ0.8 Ϫ0.6
Ϫ0.05
Ϫ0.10
Ϫ0.15
0.1
0
Scan direction
0.2 M KCl
Ϫ0.1
Ϫ0.2
Ϫ0.3
50 mM [C2Py]Br (to −1.40 V)
50 mM [C2Py]Br (to −1.50 V)
50 mM [C2Py]Br (to −1.70 V)
Cycle 1
Cycle 2
Cycle 3
Ϫ1.6
Ϫ1.4
Ϫ1.2
Ϫ1.0
Ϫ0.8
Ϫ0.6
Ϫ1.4
Ϫ1.2
Ϫ1.0
Ϫ0.8
Ϫ0.6
Potential [V vs Ag/AgCl]
Potential [V vs Ag/AgCl]
Fig. 1. Cyclic voltammograms performed at 100 mV sꢀ1 on a glassy carbon working electrode in aqueous solutions of 0.2 M KCl supporting electrolyte: (a) in
the absence of zinc bromide, showing the background scan for KCl and the irreversible reduction of 1-ethylpyridinium bromide onset at ꢀ1.25 V. (b) Zinc
electrodeposition and stripping from an electrolyte containing zinc bromide (50 mM) and 1-ethylpyridinium bromide (50 mM) (inset is the expanded view of
cycles 2 and 3).
ꢀ0.84 V is also observed. This demonstrates that the two distinct
N
ϩ
N
zinc redox processes are a result of the concurrent reduction of
[C2Py]þ cations. On the third CV cycle, the previously
described passivation of the zinc’s electroactivity is seen only
when zinc deposition occurs at potentials that also generate the
proposed 1-ethylpyridyl radicals (Scheme 1).
ϩ eϪ
(1a)
(I)
(II)
Scheme 1. One-electron reduction of 1-ethylpyridinium (I, [C2Py]þ)
cations to 1-ethylpyridyl radicals (II, [C2Py]ꢁ)
The manner in which the formation of 1-ethylpyridyl radicals
([C2Py]ꢁ) affects zinc electrodeposition may be surmised from
the experimental data. The additional deposition currents
observed at ꢀ1.30 V occur only at potentials at which [C2Py]ꢁ
radicals have been formed. The lack of Zn2þ reduction observed
on subsequent scans may be due to numerous mechanisms. Most
likely, the radical species generated can graft onto the electrode
surface,[27,28] preventing further Zn2þ reduction on subsequent
cycles. The radical generated may also direct itself as a ligand
for the divalent zinc ions in solution, forming, in the first
instance, what could be assumed to be a complexed [Znꢀ
([C2Py]ꢁ)n]2þ species. The stability of zinc complexes with
radical ligands is well established.[29–32] Any such species with
presumed reduced charge density is likely to diffuse more
slowly to the negatively polarized electrode (at reducing poten-
tials) than do the divalent Zn2þ ions. This could explain the
resultant second deposition signal that follows the direct reduc-
tion of ZnII ions. After scanning to further reducing potentials
and returning to oxidizing potentials, a sufficient amount of
[C2Py]ꢁ radicals would have been generated to complex any zinc
ions that were not electrodeposited on the time-scale of the CV
experiment. The complexation of the zinc means the Zn2þ
centre is thus unlikely to be reduced to electrodeposited metal
at the potentials of these experiments, resulting in the passiv-
ation of zinc ions, as is observed in the CV study. However,
[C2Py]ꢁ radicals are not the only complexing species that may be
present.
bromide and [C2Py]Br in 0.2 M KCl supporting electrolyte. The
onset of zinc electrodeposition can be seen at ꢀ1.12 V, with a
peak current of ꢀ0.25 mA reached at a potential of ꢀ1.20 V.
Unexpectedly, two further reduction processes can be observed
to begin at ꢀ1.30 and ꢀ1.38 V, before the switching potential of
ꢀ1.40 V. On the return sweep to oxidizing potentials, zinc
oxidation (stripping) begins at ꢀ1.03 V and two distinct oxida-
tion processes can be observed with maxima at ꢀ0.95 and
ꢀ0.84 V respectively. This suggests two distinct steps for the
deposition and subsequent stripping of zinc in this system.
Subsequent potential cycling results in a near-complete disap-
pearance of signals assigned to the Zn/ZnII redox couple (inset
of Fig. 1b), with only the reduction at ꢀ1.4 V observed.
To further elucidate the role of reduced 1-ethylpyridinium
cations in the voltammetric response observed in Fig. 1b, the
switching potential was varied for a series of CV experiments
(Fig. 2). When the first CV scan is reversed at ꢀ1.15, ꢀ1.20, and
ꢀ1.25 V versus Ag/AgCl, conventional CV patterns for revers-
ible metal electrodeposition are observed for both the first
(Fig. 2a) and third (Fig. 2b) CV cycle. For those voltammograms
recorded with switching potentials more negative than ꢀ1.30 V
(beyond the observed onset potential for [C2Py]þ reduction,
Fig. 1a), hydrogen generation and the development of a blue hue
to the solution is noted. The zinc electrodeposition pattern is
altered, with an additional reduction process observed. A shift in
the onset of the oxidation voltage of zinc is observed (Fig. 2a) at
switching potentials more negative than ꢀ1.30 V. This voltage
shift is likely to be due to the grafting of the radical species onto
the electrode surface, inhibiting the stripping of the electrode-
posited zinc. A small oxidation process at a slightly more
negative potential than that of the oxidative peak current at
Scheme 2 demonstrates possible reaction outcomes that may
follow the electroreduction of 1-ethylpyridinium bromide in the
investigated solutions. The noted colour change and evidence of
hydrogen evolution suggest the dimerization of pyridyl radicals
(II) to form the known neutral N,N0-diethyltetrahydro-4,40-
bipyridine compound (III)[25,26] capable of coordinating metal
ions through the lone pair electrons of one or both of its nitrogen
atoms. In the presence of water, a dicationic viologen-type

Zinc Electrodeposition from Aqueous Electrolytes
1027
(a)
(b)
Ϫ1.4
Ϫ1.2
Ϫ1.0
Ϫ0.8
Ϫ0.6
Ϫ1.4
Ϫ1.2
Ϫ1.0
Ϫ0.8
Ϫ0.6
Potential [V vs Ag/AgCl]
Potential [V vs Ag/AgCl]
Fig. 2. Cyclic voltammograms performed at 100 mV sꢀ1 on a glassy carbon working electrode in aqueous solutions of zinc bromide
(50 mM) and 1-ethylpyridinium bromide (50 mM) with KCl (0.2 M) as supporting electrolyte. The influence of switching potential on the
Zn/ZnII redox couple in the presence of 1-ethylpyridinium bromide is illustrated for: (a) the first CV cycle; (b) the third CV cycle. The
switching potentials of the CV experiment are (top to bottom): ꢀ1.15, ꢀ1.20, ꢀ1.25, ꢀ1.30, ꢀ1.35, and ꢀ1.40 V versus Ag/AgCl.
0 V
N
(2a)
(2b)
Ϫ 1.0 V
Ϫ 1.2 V
Ϫ 1.3 V
Ϫ 1.4 V
N
N
N
N
2
(III)
(IV)
(II)
ϩ
ϩ
(III)
2H2O
ϩ
2eϪ
(V)
N
N
N
N
(IV)
(IV)
ϩ
ϩ
(2c)
(2d)
(V)
ϩ
2
25000
30000
35000
40000
Wavenumber [cmϪ1
]
(VI)
Scheme 2. Possible reactions following the electrochemical formation of
[C2Py]ꢁ (II, from Scheme 1) including: (2a) dimerization of two pyridyl
radicals; (2b) formation of dicationic viologen-type dimer; (2c) further
reduction of the dication to form vinylidene compound; and (2d) combina-
tion of IV and V to form cation–radical pair.
Fig. 3. Spectral progression of [C2Py]Br (15 mM) at different potentials
versus Ag/AgCl in KCl (0.1 mM) solution. The arrow indicates the direction
of change of the spectral feature at more negative potentials.
ions from the starting bromide salt are common ligands for zinc,
and are likely to be associated with zinc ions as bromozincates of
the order of, for example, ZnBrꢀ3 ;[34,35] however, this was not
explored in the present work.
UV-vis spectroelectrochemistry was used to probe the for-
mation of the proposed pyridine-containing complexes at dif-
ferent potentials (Fig. 3). At all potentials, an intense absorption
at ,40000 cmꢀ1 was observed, which is ascribed to the pyr-
idinium ring transition.[36] Applying a negative potential of
ꢀ1.0 V showed no change in the spectra. However, changing
the negative potential to ꢀ1.2 V showed the formation of an
additional absorption at ,31000 cmꢀ1. The intensity of this
absorption band increased with the change in the applied
product is also capable of being formed (IV); however, owing to
its positively charged nitrogen atoms, IV is less likely to
coordinate Zn2þ ions. Instead, this compound can be further
reduced to a neutral pyrilidene-type dimer V that may also act as
a ligand in the complexation of zinc ions in a similar arrange-
ment to that previously reported for the 1,10-dihydro-4,40-
bipyridyl complex.[32] The dicationic species IV has been
previously reported to subsequently react with V to form two
molecules of the cation–radical compound VI, which may
complex ZnII via the radical site in a manner similar to that
previously reported.[33] It is also important to note that the Brꢀ

1028
M. E. Easton et al.
0
Ϫ100
Ϫ200
Ϫ300
Ϫ400
(a)
1.0
0.8
0.6
0.4
0.2
0
Ϫ1.20 V
Instantaneous model
Progressive model
Ϫ1.20 V
Ϫ1.25 V
Ϫ1.30 V
0
1
2
3
4
5
0
1
2
3
4
5
6
7
t/tm
t [s]
(b)
1.0
0.8
0.6
0.4
0.2
0
Fig. 4. Chronoamperometric response for the potentiostatic deposition of
zinc at various deposition potentials from an aqueous electrolyte compris-
ing: zinc bromide (50 mM), 1-ethylpyridinium bromide (50 mM), and KCl
(0.20 M).
Ϫ1.25 V
Instantaneous model
Progressive model
potential from ꢀ1.2 to ꢀ1.4 V. The formation of this additional
absorption is similar to that observed in the work of Wang
et al.,[36] who ascribed this absorption to the formation of a
neutral pyrilidene-type dimer, as suggested in Scheme 2 (V). We
also suggest that the pyridine-containing ligand may vary during
the time-scale of the reducing potential scan.
0
1
2
3
4
5
Chronoamperometry
t/tm
Fig. 4 shows the chronoamperometric response for the potentio-
static electrodepositionof zinc undervariousdepositionpotentials
near the peak reductive current identified by CV. The influence of
the presenceor absenceofpyridylradicalformation canbe clearly
seen in these chronoamperograms. On depositing at a constant
potential of ꢀ1.20 V, the current response rapidly approaches a
maximum of ꢀ250 mA, before a diffusion-controlled current
decay for the short life time of the experiment (7 s). For poten-
tiostatic depositions at ꢀ1.25 and ꢀ1.30 V, the maximum current
is observed to be higher (owing to the greater overpotential
delivered), reaching maximum currents of ꢀ370 and ꢀ420 mA
respectively. Following the chronoamperometric current decay,
additional steps in the current response can be observed at 4
and 2 s respectively. This is consistent with the proposed addi-
tional deposition process, from, for example, the possible
(Znꢀ([C2Py]ꢁ)n)2þ complex(es) or of complexes between Zn2þ
and the products of radical dimerization, which are formed from
the reduction of [C2Py]þ during electrodeposition at these higher
overpotentials.
(c)
1.0
0.8
0.6
0.4
0.2
0
Ϫ1.30 V
Instantaneous model
Progressive model
0
2
4
6
8
10 12 14 16 18 20
t/tm
Fig. 5. Normalized current responses and simulations using Scharifker–
Hills nucleation models for the chronoamperograms of Fig. 3 at deposition
potentials of: (a) ꢀ1.20 V; (b) ꢀ1.25 V; (c) ꢀ1.30 V.
The appearance of chronoamperometric transients are a con-
sequence of a nucleation-controlled initial process, followed by a
current decay consistent with the diffusion of ions to the surface of
the growing electrodeposit. Nucleation of metal sites can proceed
by either an instantaneous or progressive mechanism of three-
dimensional island growth.[37,38] For instantaneous nucleation, all
nucleation sites are activated rapidly, resulting in evenly distrib-
utedgrowthsites. Progressive nucleation differsbya moregradual
activation of nucleation sites, where clusters of metal deposits
grow as other nucleation sites are activated. These theories of
nucleation have been described by Scharifker and Hills,[39] who
derived mathematical models for both instantaneous (Eqn 1) and
progressive nucleation mechanisms (Eqn 2). Although this model
has limitations when used for complicated system such as that
outlined here, it provides a qualitative method for investigating the
nucleation processes occurring in this system.
ꢀ
ꢁ
2
i
1:9542
t=tm
2
¼
f1 ꢀ exp½ꢀ1:2564ðt=tmÞꢂg
ð1Þ
ð2Þ
im
ꢀ
ꢁ
2
n
h
io
2
i
1:2254
t=tm
2
¼
1 ꢀ exp ꢀ2:3367ðt=tmÞ
im
By normalizing the experimental chronoamperogram to its
current and time maxima (i.e. im and tm), it can be compared with
the theoretical transients defined by the relationships defined in

Zinc Electrodeposition from Aqueous Electrolytes
1029
(a)
10 µm
2 µm
2 µm
2 µm
WD ϭ 3.0 mm EHT ϭ 5.00 kV Mag ϭ 9.72 K X Signal A ϭ SE2
WD ϭ 3.0 mm EHT ϭ 5.00 kV Mag ϭ 1.92 K X Signal A ϭ SE2
(b)
10 µm
WD ϭ 5.1 mm EHT ϭ 5.00 kV Mag ϭ 1.93 K X Signal A ϭ SE2
WD ϭ 5.1 mm EHT ϭ 5.00 kV Mag ϭ 9.40 K X Signal A ϭ SE2
(c)
10 µm
WD ϭ 5.1 mm EHT ϭ 5.00 kV Mag ϭ 1.93 K X Signal A ϭ SE2
WD ϭ 5.1 mm EHT ϭ 5.00 kV Mag ϭ 9.40 K X Signal A ϭ SE2
Fig. 6. SEM images for the electrodeposition of zinc (zinc bromide, 50 mM) in the presence of 1-ethylpyridinium bromide (50 mM) for 3 min
on FTO glass, showing the influence of deposition potential on the resulting electrodeposit. Deposition potentials are: (a) ꢀ1.20 V; (b) ꢀ1.25 V;
(c) ꢀ1.30 V.
Eqns 1 and 2. These mathematical models have been routinely
utilized for the analysis of zinc electrodeposition.[40–42] Fig. 5
shows the experimentally derived dimensionless plots of (i/im)2
versus t/tm compared with the theoretical transients for the
diagnostic analysis of nucleation processes. When deposited at
ꢀ1.20 V, a very good match to the predicted current transient for
instantaneous nucleation is observed. This suggests that deposi-
tion at this potential will result in an evenly distributed zinc
electrodeposit, a postulate that was explored by scanning elec-
tron microscopy (SEM; see the section Deposition Morpholo-
gies below). At the more negative reducing potential of
ꢀ1.25 V, however (Fig. 4b), the dimensionless chronoampero-
gram is observed to closely match the theoretical transient for
progressive nucleation. Electrodeposition at ꢀ1.30 V is also
consistent with a progressive nucleation process; however, the
experimental transient fits the simulation of the simplified

1030
M. E. Easton et al.
mathematical model (which assumes, for example, spherical
nucleation sites[37]) less well.
isopropanol, and deionized water sequentially. The treated FTO
glass was dried under flowing nitrogen at room temperature
before use.
The dimensionless chronoamperograms demonstrate that
electrodeposition at more negative reducing potentials (concur-
rent with [C2Py]ꢁ formation) alters the nucleation process from
an instantaneous to a progressive process, which gives a further
indication of a secondary process of electrodeposition. Addi-
tional zinc electrodeposition processes have been previously
observed in electrolyte media capable of complexing metal ions,
such as ILs and related electrolytes.[43–46]
Voltammetric Methods
All voltammetric experiments were performed on an edaq ER466
Integrated Potentiostat System (edaq, Australia) using a con-
ventional three-electrode configuration. Cyclic voltammograms
were obtained using a 1-mm diameter glassy carbon working
electrode (edaq) with a Pt wire as counter-electrode and a leakless
Ag/AgCl reference electrode (edaq). Potentials are reported rel-
ative to the Ag/AgCl reference electrode. Working electrodes
were polished on a succession of alumina slurries (1 and 0.3 mM)
before each experiment and treated with oxidizing potentials
following each experiment to remove any deposited zinc.
UV-vis spectroelectrochemistry was performed over the
range of 20000–45000 cmꢀ1 using a Cary5000 spectrophotom-
eter. The absorption spectra of the electrogenerated species was
obtained in situ using a spectroelectrochemical cell mounted in
the beam pathway of the spectrophotometer, with platinum
mesh working and counter-electrodes and a Ag/AgCl reference
electrode. Potentials were applied using an edaq ER466 Inte-
grated Potentiostat System. Solutions were composed of [C2Py]
Br (15 mM) and a supporting electrolyte of KCl (0.1 M).
Chronoamperometry was performed in refreshed electrolyte
solutions of identical compositions to those used in the cyclic
voltammetric studies. Current-time transients were collected via
two-step chronoamperometry, with 3-s equilibration time at
ꢀ0.6 V preceding reduction at the required potential. Zinc
electrodeposits were prepared on an FTO glass working elec-
trode substrate by the same methodology. These electrodeposits
were rinsed with deionized water and then acetone, before
drying under flowing nitrogen at room temperature and mount-
ing for SEM analysis. SEM was performed with a Zeiss
ULTRAþ operating at 5.0 kV.
Deposition Morphologies
In order to further examine the consequence of the differing
current responses, electrodeposits were prepared via potentio-
static electrodeposition on a fluorine-doped tin oxide (FTO)
glass substrate for 3 min. The zinc deposit morphology was
examined by SEM, as seen in Fig. 6. Good uniform electro-
deposits with deposits ,1 mm in diameter are obtained when zinc
is electrodeposited at ꢀ1.20 V. This is to be expected of an
instantaneousnucleationprocessfollowedby diffusion-controlled
growth. However, when zinc is deposited at ꢀ1.25 V sꢀ1, disc-
like deposition morphology is observed in addition to the
evenly distributed zinc, with features as large as 2 mm. This
additional deposition morphology is consistent with the inter-
pretations of the CV and chronoamperometric data, which
indicate the presence of a second deposition process at signifi-
cantly reducing potentials. As is to be expected, on depositing at
ꢀ1.30 V, these disc-like features become much more prominent.
The disc-like morphologies are seen to be larger (,4 mm) and
more widespread, perhaps due to the generation of a greater
proportion of the possible intermediate species such as
[Znꢀ([C2Py]ꢁ)n]2þ] and/or [Zn(III)n]2þ, [Zn(V)m]2þ, etc.
Conclusion
The unfavourable voltammetric behaviour observed for the Zn/
ZnII redox couple in the presence of 1-ethylpyridinium bromide
was ascribed to the influence of 1-ethylpyridyl radicals and/or
their dimerization products that are formed electrochemically at
potentials more negative than ꢀ1.25 V versus Ag/AgCl.
Deposition of zinc at potentials more negative than this results in
the predominance of a second, slow-nucleating deposition
process that was observed to result in zinc deposits with large,
disc-like morphologies by SEM. The complete passivation of
zinc electroactivity on subsequent sweeps is proposed to be the
result of either or both passivation of the electrode and com-
plexation of Zn2þ ions by the reduction products of the IL cation.
Thus, although the promise of alkylpyridinium IL salts has been
outlined elsewhere, we have demonstrated a reducing potential
limit of ꢀ1.25 V versus Ag/AgCl for metal deposition in the
presence of such cations.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgements
The authors would like to thank the ARC research grant (DP160104905) and
the Henry Bertie and Florence Mabel Gritton Research Scholarship for
financial support for this research. We would also like to acknowledge
Dr Alex Yuen for valuable discussions.
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Zinc bromide dihydrate (ZnBr2.2H2O) was used as received
(Aldrich). 1-Ethylpyridinium bromide ([C2Py]Br) was prepared
by quaternization of pyridine (Alfa-Aesar) with bromoethane
(BDH) by standard literature methods.[47] The resulting white
solid was recrystallized from its acetonitrile solution by addition
of ethyl acetate and was dried under vacuum at 608C before use.
FTO glass (TEC-15 glass plate, Dyesol Industries) was pre-
treated by three rinses in a sonicating bath with acetone,

Zinc Electrodeposition from Aqueous Electrolytes
1031
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