Temperature effects on the electrodeposition of zinc

Article abstract of DOI:10.1149/1.1525269

The temperature dependence of the cyclic voltammetric profiles for the reduction of zinc into a static mercury drop electrode and the electrocrystallization of zinc onto a glassy carbon (GC) electrode have been examined using cyclic voltammetry and chronoamperometry, respectively. The quasi-two-dimensional growth and the microstructure of the deposited zinc film have also been characterized by video camera and scanning electron microscopy (SEM). The kinetic parameters D, α, and k for the Zn²⁺ + 2e^- = Zn(Hg) reaction were calculated at three temperatures. Their values increase with increasing temperature. According to the slope, d ln k/d(1/T), we determined the activation energy of the Zn²⁺ + 2e^- = Zn(Hg) reaction to be -39.662 kJ mol^-1. On the other hand, the chronoamperometric results showed that the nucleation density increases when the temperature increases. Moreover, the nucleation rate constant, A, is always very large at different temperatures, about 1.41 × 10⁹ s^-1. This indicates that the mechanism of zinc electrocrystallization onto a GC electrode follows a three-dimensional (3-D) instantaneous nucleation and growth model within the controlled temperature range. As for the quasi-2-D growth forms and the microstructure of the zinc deposit, the experimental results showed that the fractal morphology of zinc exhibits isotropic growth with more and thicker branches at 273 K. In contrast, the fractal morphology of zinc exhibits anisotropic growth with fewer and thinner branches at 323 K. Similarly, the results obtained for the microstructure of the zinc deposit showed that plate-like crystals grow unintegrally like irregular hexagonal crystallites at 273 K, but the grains develop integrally and adequately at 323 K.

Full text of DOI:10.1149/1.1525269

Journal of The Electrochemical Society, 150 ͑1͒ C19-C23 ͑2003͒
C19
0013-4651/2002/150͑1͒/C19/5/$7.00 © The Electrochemical Society, Inc.
Temperature Effects on the Electrodeposition of Zinc
Jingxian Yu,z Lei Wang, Lei Su, Xinping Ai, and Hanxi Yang
Research Center of Electrochemical Engineering, Department of Chemistry, Wuhan University,
Wuhan 430072, China
The temperature dependence of the cyclic voltammetric profiles for the reduction of zinc into a static mercury drop electrode and
the electrocrystallization of zinc onto a glassy carbon ͑GC͒ electrode have been examined using cyclic voltammetry and chrono-
amperometry, respectively. The quasi-two-dimensional growth and the microstructure of the deposited zinc film have also been
characterized by video camera and scanning electron microscopy ͑SEM͒. The kinetic parameters D, ␣, and k for the Zn2ϩ
ϩ 2eϪ ϭ Zn͑Hg) reaction were calculated at three temperatures. Their values increase with increasing temperature. According to
the slope, d ln k/d(1/T), we determined the activation energy of the Zn2ϩ ϩ 2eϪ ϭ Zn͑Hg) reaction to be Ϫ39.662 kJ molϪ1. On
the other hand, the chronoamperometric results showed that the nucleation density increases when the temperature increases.
Moreover, the nucleation rate constant, A, is always very large at different temperatures, about 1.41 ϫ 109 sϪ1. This indicates that
the mechanism of zinc electrocrystallization onto a GC electrode follows a three-dimensional ͑3-D͒ instantaneous nucleation and
growth model within the controlled temperature range. As for the quasi-2-D growth forms and the microstructure of the zinc
deposit, the experimental results showed that the fractal morphology of zinc exhibits isotropic growth with more and thicker
branches at 273 K. In contrast, the fractal morphology of zinc exhibits anisotropic growth with fewer and thinner branches at 323
K. Similarly, the results obtained for the microstructure of the zinc deposit showed that plate-like crystals grow unintegrally like
irregular hexagonal crystallites at 273 K, but the grains develop integrally and adequately at 323 K.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1525269͔ All rights reserved.
Manuscript submitted October 11, 2001; revised manuscript received July 15, 2002. Available electronicallyNovember 21, 2002.
The electrodeposition of zinc has gained much attention in the
past twenty years, because it is strongly related to the properties of
electrodeposited zinc surface and the lifetime of secondary zinc
electrodes. Our earlier studies1-3 showed that organic additives can
not only change the electrocrystallization mechanism of zinc, but
also affect the morphology and texture of the deposited film and
give rise to crystalline defects. By the use of selected organic addi-
tives, Zhou and his co-workers4 prepared zinc deposits with a highly
preferred orientation. Hibbert et al.5 investigated the chemical ef-
fects of several polymer supported membranes on morphology of
the two-dimensional ͑2-D͒ growth. Although the zinc electrodeposi-
tion mechanisms are well known, the temperature effects on the
electrodeposition are seldom reported.
In this paper, the temperature dependence of the cyclic voltam-
metric profiles for the reduction of zinc into a static mercury drop
electrode was determined at different temperatures. The electrodepo-
sition of zinc onto a glassy carbon ͑GC͒ electrode was examined
using chronoamperometry. Meanwhile the quasi-2-D growth forms
and the microstructure of the deposited zinc film were characterized
by a video camera and scanning electron microscopy ͑SEM͒.
a thermostat. The water level in the bath was maintained above the
solution in the cell. The experiments were conducted at 273, 298,
and 323 K, respectively.
In order to investigate the quasi-2-D zinc growth forms, we
adopted the apparatus reported in Ref. 5. Two pieces of Perspex, 7.0
cm in diam, were secured by the anode zinc ring. A piece of cellu-
lose filter paper was placed between two pieces of Perspex. The
cathode was a Pt wire of 0.2 mm diam and electrolyte was 1 mol
LϪ1 Zn͑Ac)2 . Under a constant potential of 2.0 V, the quasi-2-D
zinc layers grown at 273 and 323 K were recorded by a video
camera.
SEM samples of the zinc electrodeposited layer were prepared
on carefully polished copper sheets 0.2 mm thick and of 1.5
ϫ 1.5 cm area. In the solution of 0.1 mol LϪ1 Zn͑Ac)2 with 0.5
mol LϪ1 NaAc, the copper sheets were electroplated for 15 min with
a constant current of 80 mA and agitation at 298 and 323 K, respec-
tively, and the cell voltages were recorded simultaneously. The sur-
face morphology of the deposited zinc film was observed by ISI-
SX-40-type SEM.
All solutions were prepared from chemicals of analytical grade
and triply distilled water, and deaerated by bubbling with N2 for 10
min before use. A potentiostat ͑RDE4, Pine Instrument Co.͒ was
used in all experiments.
Experimental
All of the cyclic voltammograms ͑CVs͒ were carried out using a
static mercury drop working electrode. The electrolyte was an aque-
ous solution of 0.1 mol LϪ1 Zn͑Ac)2 with 0.5 mol LϪ1 NaAc ͑Ac is
acetate͒. The secondary electrode was a Pt wire with a large surface
area. All potentials were reported with respect to the saturated
calomel electrode ͑SCE͒. All of the chronoamperometric experi-
ments were performed in the solution of 0.1 mol LϪ1 Zn͑Ac)2 with
0.5 mol LϪ1 NaAc. The working electrode was a Teflon-shrouded
GC rod, 2 mm in diam. A 99.9% pure zinc sheet was used as the
counter electrode. Prior to each experiment, the working electrode
was polished to a mirror finish with successively finer grades of
alumina powders, eventually to 0.05 ␮m, and then ultrasonically
cleaned in acetone, dilute aqueous HCl, and distilled water. The step
potentials were provided and corresponding current transients were
recorded simultaneously by computer. The initial potential for all
chronoamperometric experiments was set to Ϫ1.00 V. During the
experiments, the electrochemical cells were immersed in a water
bath, and the temperature was controlled within Ϯ1.0 K by means of
Results and Discussion
Voltammetric experiments at different temperatures.—The CVs
of a static mercury drop electrode in the solution of 0.1 mol LϪ1
Zn͑Ac)2 with 0.5 mol LϪ1 NaAc at 273, 298, and 323 K are shown
in Fig. 1, respectively. The potential was swept from Ϫ0.8 V going
in the cathodic direction. The anodic part is ignored in this figure.
The distinct feature of the CV curves in Fig. 1 is a single cathodic
peak from Ϫ0.8 to Ϫ1.3 V. It indicates that the reduction of Zn2ϩ
ions is a simple reaction at three temperatures and that the second
electron reduction exhibits a very fast step. Under the condition of
the half-infinite linear diffusion, the peak current ip can be described
by the following equation6-8
1/2
i ϭ K c
͓1͔
v
p
D
where c is the concentration of the active cation ͑mol cmϪ3͒, the
v
z E-mail: jxyu@whu.edu.cn
scan rate ͑V sϪ1͒, and KD is a constant related to the properties of
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Journal of The Electrochemical Society, 150 ͑1͒ C19-C23 ͑2003͒
Figure 1. Cyclic voltammograms for Zn2ϩ ions on a static mercury drop electrode immersed in a solution of 0.1 mol LϪ1 Zn͑Ac)2 with 0.5 mol LϪ1 NaAc at
different temperatures. The scan rates are labeled on each curve. ͑a͒ 273 K, ͑b͒ 298 K, and ͑c͒ 323 K.
and k at different temperatures were calculated, as listed in Table I.
From this table, it can be seen that the values of D, ␣, and k increase
with the temperature. The parameters obtained at 298 K are analo-
gous to the values reported by other workers.9 Figure 4 shows the
conditional standard rate constant as a function of temperature.
These results reveal that an increase in temperature leads to a higher
k and that the relation between log k and 1/T is linear. Based on the
Arrhenius equation,10,11 the slope allows us to calculate the activa-
tion energy of the process using the following equation
the reaction system. For an irreversible reation, KD is equal to
2.99 ϫ 105n(␣n␣)1/2AD1/2 and the relationship between ip and the
peak potential Ep is given by6
␣n␣F
i ϭ 0.227nFAck0 exp Ϫ
E Ϫ E0͒
͓2͔
͑
ͫ
ͩ
ͪ
ͬ
p
p
RT
where k is the conditional standard rate constant and E0 is the equi-
librium potential.
As shown in Fig. 2, the plot of i vs. 1/2 is a straight line with a
v
p
zero intercept on the current axis at each temperature. Moreover, an
d ln k
⌬HÞ ϭ 2.303R
͓3͔
increase in temperature leads to a larger slope. This suggests that the
Zn2ϩ reduction process is controlled by mass transfer. As depicted in
Fig. 3, the plot of ln ip vs. Epc Ϫ E0 is linear at each temperature and
the intercept on the ln ip axis increases with temperature. According
to the slopes of these lines shown in Fig. 2 and 3, the values of D, ␣,
d 1/T͒
͑
where ⌬HÞ is the activation energy of the process. According to the
1/2
Figure 2. Plot of ip vs.
for the reduction of Zn2ϩ ions in a solution of
Figure 3. The plot of ln ip vs. Epc Ϫ E0 for the reduction of Zn2ϩ ions at
different temperatures.
v
0.1 mol LϪ1 Zn͑Ac)2 with 0.5 mol LϪ1 NaAc at different temperatures.
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Journal of The Electrochemical Society, 150 ͑1͒ C19-C23 ͑2003͒
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Table I. The values of D, ␣, and k obtained at different tempera-
tures.
T
͑K͒
D
k
͑cm2 sϪ1
͒
␣
͑cm sϪ1
͒
1.93 ϫ 10Ϫ6
5.71 ϫ 10Ϫ6
9.54 ϫ 10Ϫ6
2.45 ϫ 10Ϫ4
1.02 ϫ 10Ϫ3
3.66 ϫ 10Ϫ3
273
298
323
0.231
0.237
0.243
slope, we determine the activation energy of the Zn2ϩ ϩ 2eϪ
ϭ Zn͑Hg) reaction to be Ϫ39.662 kJ molϪ1, which is comparable
to the value reported in Ref. 9-11.
Chronoamperometric experiments at different tempera-
tures.—Figure 5 shows three families of potentiostatic current/time
transients for zinc electrodeposition onto GC electrode from a solu-
tion of 0.1 mol LϪ1 Zn͑Ac)2 with 0.5 mol LϪ1 NaAc at different
temperatures. When a step potential E was employed, the current
density first increased sharply to reach a maximum value Im , at the
time, tm , and then gradually decayed with time as described by the
Cottrell equation. At a constant temperature, the larger the amplitude
of the potential step, the larger was Im . For a given E, Im grew with
increasing temperature. In consideration of the overlapping of the
diffusion fields of randomly growing nuclei, Scharifker and
Mostany12 deduced the following expression to describe the general
potentiostatic current/time relations for nucleation and growth of an
electrodeposit
zFD1/2c
1 Ϫ exp ϪAt͒
͓
͑
͔
I ϭ
1 Ϫ exp ϪN ␲kD t Ϫ
ͫ
ͩ
ͪ
ͬ
ͭ
ͮ
0
␲
1/2t1/2
A
͓4͔
1/2
k ϵ 8␲cM/␳͒
͑
where z is the number of equivalents, D is the diffusion coefficient,
c is the concentration of active cation, M is the atomic weight of the
metal, ␳ is the density of the deposit, N0 is the nucleation density
͑cmϪ2͒, and A is the nucleation rate constant ͑cmϪ2 sϪ1͒.
In previous research, we have proposed a genetic algorithm to
deal with the nucleation problem,13 where Eq. 4 can be regarded as
the basic model structure, with A, N0 , and D as estimated param-
eters. By using this algorithm to fit the whole current transient at
different temperatures, we can optimize those parameters easily by
choosing 500 data points as samples in the corresponding curves.
Because it is a stochastic algorithm, 10 runs were conducted inde-
Figure 5. Potentiostatic current/time curves for zinc electrodeposition on a
GC electrode from a solution of 0.1 mol LϪ1 Zn͑Ac)2 with 0.5 mol LϪ1
NaAc at three temperatures: ͑a͒ 273 K, ͑b͒ 298 K, and ͑c͒ 323 K. For each
curve, the potential pulse starts from Ϫ1.0 V to the indicated potential.
pendently for each experimental current/time transient. We also cal-
culate the square of the correlation coefficient r2 to assess the fitting
quality by the following formula
m
2
ˆ
I Ϫ I ͒
͑
i
i
͚
iϭ1
r2 ϭ 1 Ϫ
͓5͔
m
2
¯
Figure 4. Relation between the conditional standard rate constant and the
temperature.
I Ϫ I͒
͑
i
͚
iϭ1
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Journal of The Electrochemical Society, 150 ͑1͒ C19-C23 ͑2003͒
Table II. The values of D, A, N0 , the fitting error, and the mean runtime at different temperatures.
T
͑K͒
E
͑V͒
D
N0
A
͑cm2 sϪ1
͒
͑cmϪ2
͒
͑sϪ1
͒
Fitting error
r2
1.185 ϫ 10Ϫ6
1.123 ϫ 10Ϫ6
1.161 ϫ 10Ϫ6
1.121 ϫ 10Ϫ6
1.124 ϫ 10Ϫ6
2.109 ϫ 10Ϫ6
2.230 ϫ 10Ϫ6
1.924 ϫ 10Ϫ6
2.010 ϫ 10Ϫ6
2.040 ϫ 10Ϫ6
4.328 ϫ 10Ϫ6
3.326 ϫ 10Ϫ6
3.224 ϫ 10Ϫ6
3.394 ϫ 10Ϫ6
3.234 ϫ 10Ϫ6
1.02 ϫ 106
1.63 ϫ 106
1.85 ϫ 106
2.37 ϫ 106
2.89 ϫ 106
2.33 ϫ 106
2.72 ϫ 106
5.44 ϫ 106
5.50 ϫ 106
1.09 ϫ 107
1.19 ϫ 106
2.88 ϫ 106
4.51 ϫ 106
5.84 ϫ 106
7.60 ϫ 106
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
1.41 ϫ 109
8.9 ϫ 10Ϫ6
1.05 ϫ 10Ϫ5
1.05 ϫ 10Ϫ5
1.18 ϫ 10Ϫ5
9.65 ϫ 10Ϫ6
1.42 ϫ 10Ϫ4
3.10 ϫ 10Ϫ4
5.48 ϫ 10Ϫ4
3.59 ϫ 10Ϫ4
9.82 ϫ 10Ϫ4
1.45 ϫ 10Ϫ4
2.27 ϫ 10Ϫ4
8.78 ϫ 10Ϫ4
2.87 ϫ 10Ϫ4
2.11 ϫ 10Ϫ4
273
273
273
273
273
298
298
298
298
298
323
323
323
323
323
Ϫ1.24
Ϫ1.26
Ϫ1.28
Ϫ1.30
Ϫ1.32
Ϫ1.24
Ϫ1.26
Ϫ1.28
Ϫ1.30
Ϫ1.35
Ϫ1.20
Ϫ1.22
Ϫ1.24
Ϫ1.26
Ϫ1.28
0.99
0.99
0.99
0.99
0.99
0.97
0.95
0.95
0.95
0.94
0.97
0.96
0.94
0.95
0.96
m
¯
As shown in Table II, the nucleation rate constant, A, is very
large at all three temperatures, about 1.41 ϫ 109 sϪ1. According to
the first order nucleation rate equation, the 3-D nucleation can be
expressed as
where I ϭ 1/m͚iϭ1Ii . Table II lists the calculated values of D, A,
r2, N0 , and the fitting error for different step potentials.
As shown in Table II, the fitting errors for all of the experimental
current/time transients are very small, and their r2 values are all
greater than 0.94. This indicates that the fitting curves coincide with
the experimental current/time transients very well. Furthermore, the
diffusion coefficients obtained for different step potentials are very
consistent under the same temperature. The mean values of the dif-
fusion coefficients at 273, 298, and 323 K are 1.14 ϫ 10Ϫ6, 2.10
ϫ 10Ϫ6, and 3.50 ϫ 10Ϫ6 cm2 sϪ1, respectively. Obviously the
diffusion coefficients increase slowly with an increase in tempera-
ture. It can also be seen that the D value obtained by chronoamper-
ometry is lower than that obtained by the cyclic voltammetry at the
same temperature. A similar phenomenon was observed by other
researchers,14 but the reason for this result is not clear yet.
Using the values of N0 listed in Table II, the dependence of
log N0 vs. overpotential, ␩, is plotted in Fig. 6. From this figure, it is
clear that N0 has a strong dependence on ␩ and T. That is, an
increase in ␩ or temperature leads to a higher value for N0 . At a
given ␩, N0 at 298 K is two times larger than that at 273 K. This fact
indicates that an increase in temperature facilitates the formation of
new nuclei.
N t͒ ϭ N 1 Ϫ exp ϪAt͒
͔
͓6͔
͑
͓
͑
s
where Ns is the number of active sites on the substrate. This equa-
tion has two extreme cases. One case is N(t) ϭ Ns for a large A
value which corresponds to instantaneous nucleation. The other is
N(t) ϭ ANst for a small A value which corresponds to progressive
nucleation. The large A value indicates that the mechanism of zinc
electrocrystallization onto GC electrode at all three temperatures
follows a 3-D instantaneous nucleation and growth model.
Fractal morphology of a quasi-2-D zinc deposit at different
temperatures.—Under the constant cell potential of 2.0 V, the mor-
phology of the quasi-2-D zinc deposition at 273 and 323 K were
monitored, respectively, as shown in Fig. 7. From this figure, the
random fractal morphology of zinc at 273 K exhibits isotropic
growth with larger and thicker branches. In contrast, the fractal mor-
phology of zinc at 323 K exhibits anisotropic growth with less and
thinner branches. According to the diffusion-limited aggregation
͑DLA͒ model, in an electric field, the Zn2ϩ ions in the solution
diffuse and discharge randomly once they encounter the growing
cluster. They first form adatoms by obtaining electrons and then are
trapped immediately with no further diffusion. This model can ac-
count for the ramified structure of zinc growth, but cannot explain
the variation of branch width. In fact, the 2-D zinc growth in a
15
¨
practical solution is much more complicated. Roder demonstrated
that the adatoms attached to the growing cluster can continue to
Figure 6. Plot of log N0 vs. ␩ for zinc electrodeposition on a GC electrode at
different temperatures: ͑a͒ 273 K, ͑b͒ 298 K, and ͑c͒ 323 K.
Figure 7. The fractal morphology of quasi-2-D zinc deposition at different
temperatures at a constant potential of 2.0 V. ͑a͒ 273 K, ͑b͒ 323 K.
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Journal of The Electrochemical Society, 150 ͑1͒ C19-C23 ͑2003͒
C23
paper, and because the diffusion rate of surface adatoms is slow at
273 K, a part of the adatoms cannot diffuse and reach the stable sites
simultaneously. This causes the earlier adatoms to be encased by
new adatoms and to form new kinks. Thus, plate-like crystals grow
unintegrally. On the contrary, because the diffusion rate of surface
adatoms accelerates exponentially at 323 K, the production rate of
adatoms is relatively slow. Thus, the grains develop integrally and
adequately.
Conclusion
From the results of cyclic voltammetry experiments at different
temperatures, we calculated the kinetic parameters D, ␣, and k for
the Zn2ϩ ϩ 2eϪ ϭ Zn͑Hg) reaction. Based on the slope of
d ln k/d(1/T), we determined the activation energy of this reaction to
be Ϫ39.662 kJ molϪ1
.
Chronoamperometric experiments showed that the nucleation
density increases when the temperature increases. Moreover, the
nucleation rate constant is always very large at all temperatures,
Figure 8. Scanning electron micrographs for zinc films deposited from a
solution of 0.1 mol LϪ1 Zn͑Ac)2 with 0.5 mol LϪ1 NaAc onto 1.5ϫ1.5 cm
copper sheets. ͑a͒ 273 K, ͑b͒ 323 K.
about 1.41 ϫ 109 sϪ1
.
The fractal morphology of zinc exhibits isotropic growth with
more and thicker branches at 273 K. In contrast, the fractal morphol-
ogy of zinc exhibits anisotropic growth with fewer and thinner
branches at 323 K. Similarly, the microstructure of zinc deposits
shows that plate-like crystals grow unintegrally like irregular hex-
agonal crystallites at 273 K, but the grains develop integrally and
adequately at 323 K.
migrate and reach the stable sites with more coordination points.
According to the previous chronoamperometric experiments, the dif-
fusion coefficient of Zn2ϩ ions and the nucleation density increase
with the increasing of temperature. It means that Zn2ϩ ions obtain
electrons and form adatoms more quickly at a higher temperature.
On the other hand, the edge diffusion rate of adatoms grows expo-
nentially when the temperature increases. At 273 K, because the
edge diffusion rate is low, a large amount of adatoms are produced
before the earlier adatoms migrate and reach the stable sites. This
makes the mobile adatoms attach mutually and form plenty of new
kinks. Thus, the fractal morphology of a zinc deposit exhibits iso-
tropic growth with more and thicker branches. In contrast, at 323 K,
because the edge diffusion of adatoms accelerates exponentially, the
production rate of adatoms is relatively slow. This makes the ada-
toms more likely to diffuse and reach the stable sites. Thus, the
fractal morphology of these zinc deposits exhibits anisotropic
growth with fewer and thinner branches.
Acknowledgments
This work was partially supported by the Chinese National Natu-
ral Science Foundation ͑no. 29833090͒. The authors would like to
thank Professor Charles L. Hussey for his helpful comments and
efforts in improving the English and style of this paper.
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