Electrochemical studies of Zn underpotential/overpotential deposition on a nickel electrode from non-cyanide alkaline solution containing glycine

Article abstract of DOI:10.1016/j.electacta.2011.02.106

In this work we present an electrochemical study of the underpotential deposition (UPD) and overpotential deposition (OPD) of zinc onto nickel electrode (NE) from a non-cyanide alkaline solution containing glycine. The studied parameters were zinc concentration, glycine concentration and scanning rate. The analysis of the experimental data clearly showed the presence of UPD and OPD processes that started at -0.8 V vs. SCE and -1.4 V vs. SCE, respectively. The voltammetric studies also indicate diffusion control of the zinc UPD and OPD processes onto the NE. From the potentiostatic transients we found instantaneous nucleation (2D) mechanisms, which agree to that observed in the AFM study. In order to compare the effect of zinc/glycine concentration, we calculate thermodynamic parameters for the OPD process.

Full text of DOI:10.1016/j.electacta.2011.02.106

Electrochimica Acta 56 (2011) 5443–5451
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical studies of Zn underpotential/overpotential deposition on a
nickel electrode from non-cyanide alkaline solution containing glycine
J.C. Ballesterosa,b, E. Chaînet , P. Ozil , G. Trejo , Y. Meas
b
b
a,∗
a,∗
a
Centro de Investigación y Desarrollo Tecnológico en Electroquímica (CIDETEQ), Parque Tecnológico Sanfandila, Pedro Escobedo, Querétaro, C.P. 76703 Querétaro, Mexico
Laboratoire d’Electrochimie et de Physicochimie des Matériaux et des Interfaces, LEPMI, UMR 5631 CNRS-INPG-UJF, BP 75, 38402 Saint-Martin d’Heres Cedex, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work we present an electrochemical study of the underpotential deposition (UPD) and overpoten-
tial deposition (OPD) of zinc onto nickel electrode (NE) from a non-cyanide alkaline solution containing
glycine. The studied parameters were zinc concentration, glycine concentration and scanning rate. The
analysis of the experimental data clearly showed the presence of UPD and OPD processes that started at
Received 15 November 2010
Received in revised form 24 February 2011
Accepted 25 February 2011
Available online 24 March 2011
−
0.8 V vs. SCE and −1.4 V vs. SCE, respectively. The voltammetric studies also indicate diffusion control of
the zinc UPD and OPD processes onto the NE. From the potentiostatic transients we found instantaneous
nucleation (2D) mechanisms, which agree to that observed in the AFM study. In order to compare the
effect of zinc/glycine concentration, we calculate thermodynamic parameters for the OPD process.
Keywords:
Electrodeposition
Zinc
©
2011 Elsevier Ltd. All rights reserved.
Nickel
Glycine
UPD
Nucleation
1
. Introduction
underpotential deposition occurs, prior to the formation of a
bulk deposit [17]. When the work function of a metal being
electrodeposited is lower than that of the substrate metal, the
electrodeposition may occur at a potential more positive than
the equilibrium potential, a phenomenon called underpotential
deposition (UPD) [17]. The region where this process occurs on
polycrystalline substrates is defined by the Kolb–Gerischer equa-
tion [18,19]:
Zinc electroplating and its alloys have been widely used in
the steel industry for the protection of steel products in corro-
sive environments [1]. Several factors such as zinc concentration
[
2], complexing agents [3], anions [4,5] and additives [6] play
fundamental roles in zinc electrodeposition. These factors may
modify the texture and morphology of zinc electrodeposited coat-
ing [7–10].
Alkaline non-cyanide zinc baths is the outcome of the efforts to
produce a non-toxic cyanide free zinc electrolyte. Formerly, it was
thought that these baths can produce only dark, spongy or powdery
deposits and the addition of complexing agents like EDTA, glu-
conate, tartrate and triethanolamine in relatively large quantities
can improve the zinc deposit quality [11].
The glycine has been used as a complexing agent in the elec-
trodeposition of Zn–Ni [12], Cu–Co [13], Zn–Co [14], Zn–Co–Cu
alloys [15] and more recently by our research group to obtain Zn–Co
alloy [16]. These studies show that the deposits obtained from alka-
line bath containing glycine are of high quality.
ꢀE = 0.5 ꢀ∅
(1)
where (ꢀE) is the underpotential shift in V and (ꢀ ∅) is the dif-
ference in the work function of the electron for both metals in
eV.
On the basis of Eq. (1) and data published by Trasatti [18] on
∅
for different metals, it is possible to calculate approximately the
underpotential shift(ꢀE) forthe metal couplenickelsubstrate–zinc
adsorbate, equal to (ꢀE) = 0.585 V. This, in turn, provides grounds
for presuming that underpotential adsorption of zinc onto nickel
electrode is possible.
Several investigators [20–37] have studied the effect of several
factors on the zinc UPD process, such as: adanions, organic com-
pounds, pH of the solution, hydrogen adsorption, and resulting
morphology of the deposit have been investigated extensively at
noble metal electrodes using electrochemical techniques coupled
with other techniques.
On the other hand, it is well known that during the elec-
trocrystallization of metals on foreign substrates, very often an
∗ Corresponding authors. Tel.: +52 442 211 6070; fax: +52 442 211 6001.
E-mail addresses: gtrejo@cideteq.mx (G. Trejo), yunnymeas@cideteq.mx
Despite these many important contributions to the study of elec-
trochemical reduction of zinc UPD, a study of this phenomenon
(
Y. Meas).
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.02.106

5
444
J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
Table 1
which are characterized by the equilibrium constants:
Chemical composition of solutions used in this work.
+
[
HG][H ]
[Zn2+] (mol L−1
)
[Cl ] (mol L
−
−1
)
[G ] (mol L
−
−1
)
Solution
Ka1 =
[
(3)
+
H G ]
2
S0
S1
S2
0.2
0.4
0.6
0.4
0.8
1.2
1.6
2.2
2.8
−
+
[G ][H ]
Ka2
=
(4)
[
HG]
◦
The pK’s values of glycine are: pKa1 = 2.07 and pKa2 = 9.97 at 25 C
[
38–40].
on a nickel substrate and from zinc–glycine complexes in alkaline
solution is still not done.
Depending on the zinc concentration, the glycine concentra-
tion and the pH, glycine molecules can successively replace water
molecules in zinc’s coordination octahedron, forming zinc–glycine
complexes. An important point is to understand the deposition
solution chemistry and specially the zinc species involved in the
deposition process. In aqueous solutions, Zn(II) soluble species
are Zn2+, hydroxide complexes, chloride complexes and glycine
The aim of the present work is threefold. In a first part, we
present a thermodynamic study on the zinc–glycine–water system
by means of species distribution and potential–pH diagrams with
the view to better understand the effect of the zinc/glycine concen-
tration on the zinc electroreduction. In a second part, we present
an electrochemical study in order to obtain information on the
UPD and OPD processes from non-cyanide alkaline bath containing
glycine. Finally, in a third part, we present an AFM morphological
study of the zinc UPD processes.
2
+
complexes. The concentration of the latter are related to Zn con-
centration by the equilibrium reaction:
Zn2 + iL− ↔ ZnLi(2−i)
each complex is characterized by its stability constant ˇL
+
(5)
(6)
−
.
i
2
. Experimental
[ZnL(
2−i)
]
−
iZn
L
i
ˇ
=
Zinc deposition onto NE was carried out in a conventional three-
−
2+
[
L ][Zn
]
◦
electrode cell from the solutions shown in Table 1 at 25 C. All
solutions were prepared using analytic grade reagents (provided by
Sigma–Aldrich Company) with ultra pure water (Millipore-Q sys-
−
−
−
−
where L represents a ligand, OH , Cl or G in the present work,
and i the coordination number. The values of ˇ
from literature [38–40] and are reported in Table 2.
In addition, when the pH increases, there is an increasing ten-
dency for the formation of metal hydroxide. With zinc cations, this
equilibrium can be expressed by the following equilibriums:
−
iZn
L
were obtained
tem) and were deoxygenated by bubbling N for 20 min before each
2
experiment. The working electrode was a nickel electrode (NE) pro-
vided by Sigma–Aldrich, made of a nickel rod embedded in Teflon.
2
The exposed surface area (0.726 cm ) was polished to mirror finish
with different grades of alumina down to 0.05 ␮m and ultrasoni-
cally cleaned before experiments. A graphite bar with an exposed
area greater than the working electrode was used as counter elec-
trode. A saturated calomel electrode (SCE) was used as reference,
and all potentials refer to this scale. The electrochemical experi-
ments were carried out with a potentiostat/galvanostat PAR273A
Zn(OH)2(s) ⇔ Zn2+ + 2OH−
(7)
characterized by the solubility constant Ks.
2
+
−
2
Ks = [Zn ][OH ]
(8)
−
iZn
From the values of ˇL and K , it is possible to calculate the
s
(
EG&G) connected to a personal computer running the general pur-
ratio between a soluble/insoluble zinc species and total zinc con-
centration in solution. The species repartition–pH diagrams were
computed using the definition of complexation coefficient of Zn(II),
pose electrochemical system software (GPES) (Eco Chemie B.V.)
for control of experiments and data acquisition. The conditions for
electrochemical experiments are chosen in where the ohmic drop
is considered negligible. The solution temperature was controlled
and made constant with a circulating water thermostat (Bioblock
Scientific). AFM (Digital Instruments, Mod. Nanoscope E) in con-
tact mode was used to obtain images of the UPD zinc deposit on
the nickel electrode. Coating morphology was evaluated using a
scanning electron microscope (Jeol Mod. DSM-5400LV).
2
+
˛
Zn(L=G−,OH ,Cl
−
−
[41]. It is defined as: the ratio cZn/[Zn ], where
)
cZn is the total concentration of zinc including all complexes, [Zn2+]
and [Zn(OH)2(s)]; and [Zn ] is the concentration of free zinc cations
2+
2+
Zn
.
ꢀ
(2−i)
[ZnLi
cZn
2+
[
Zn2+] +
] + [Zn(OH)2(s)
]
˛
−
Zn(L=G−,OH ,Cl−
=
=
(9)
)
2+
[Zn
]
[Zn
]
Combining Eqs. (6), (8) and (9), ˛
expressed as:
−
Zn(L=G−,OH ,Cl−
can be
)
3
. Results and discussion
2
− 3
L
−
L
−
L
˛
Zn(L=G−,OH ,Cl−
−
= 1 + ˇ [G ] + ˇ [G ] + ˇ [G ]
3.1. Zinc–glycine chemistry
)
1Zn
2Zn
3Zn
Cl
1Zn
−
Cl
2Zn
−
2
Cl
3Zn
− 3
Cl
4Zn
− 4
+
ˇ
[Cl ] + ˇ [Cl ] + ˇ [Cl ] + ˇ [Cl ]
Zinc ions in aqueous solutions have an octahedral coordination
that allows for the accommodation of six ligands in its first hydra-
tion shell. For each of the solution compositions in this study, the
speciation of zinc in glycine solution was examined by construct-
ing the species repartition and potential–pH diagrams. Glycine,
H NCH COOH, is the simplest amino acid, which exists under
Kw)2
− 4
(
OH
+ ˇ4 [OH ] + Ks
(10)
Zn
2
+
[
H ]
Using this last equation, one can easily calculate the distribution
of different species:
2
2
three different forms in aqueous solutions depending on the pH
value: +H NCH COOH (cation), H NCH COO (zwitterion), and
+
−
100
3
2
3
2
2+
%
%
%
Zn = ˛
(11)
(12)
(13)
−
+
H NCH COO (anion). These species are denoted as: H G , HG, and
G , respectively, for brevity. The equilibria between these may be
depicted as [38–40]:
−
Zn(L=G−,OH ,Cl−
2
−
2
2
)
−
ZnL(
2−i)
= ˇ [L ] %Zn
L
−
2+
i
iZn
2
+
+
− K
−
s w
K (K )
H NCH COOH K H NCH COO
H NCH COO
Zn(OH)2(s) = %Zn2+
3
2
a1
3
2
a2
2
2
⇔
⇔
(2)
+
−
2
+
(
H G )
2
(HG)
(G )
[H ]

J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
5445
Table 2
−
Equilibrium constants for the overall formation of complexes in the zinc–glycine–OH system and the hydroxide solubility product obtained from the literature [38–40],
ˇ iL = [MLi
(2−i)
2+
−
i
]/[M ][L ] .
−
−
−
L = G
L = OH
L = Cl
pKs
i
1
2
3
4
1
2
3
4
1
2
3
4.4
11.3
13.7
16.6
0.43
0.61
0.53
0.20
4.88
9.06
11.56
16.92
As a result, the key point of the calculation is the determination
of [G ]. In a first step, we can establish the glycine protonation
The conditional (formal) potential for the reaction (17) is given
by:
−
coefficient ˛G
−
, defined as:
o
ꢀ
ꢀ
ꢀ
−
E
(ZnG /Zn(0)) = −1.346 − 0.089 log[G ] + 0.0296 log[Zn ]
eq
3
−
+
+
+ 2
[H ]
= [
G ] + [HG] + [H G ]
= 1 +
[H ]
+
Ka1Ka2
(14)
2
(18)
˛G
−
−
[
G ]
Ka1
ꢀ
−
where Eo (ZnG /Zn(0)) represents the conditional (formal) poten-
tial, in volts vs. SCE, for the system ZnG3 /Zn(0). The notation
generalized Zn represents the free zinc ions solution, the soluble
zinc–glycine complexes, or the metal hydroxide in either the sol-
uble or insoluble form; in this diagram G corresponds to the free
concentration of glycine.
eq
3
It must be pointed out that under our experimental conditions
differs from the cG
concentration, assuming that a significant part of glycine ions is
involved in the formation of complexes. Then, this ratio can be
expressed as:
−
−
˛G
−
−
/[G ] ratio, where c
G
−
is the overall-glycine
ꢀ
ꢀ
+
−
Eq. (18) allows us to predict a displacement in the value of equi-
librium potential when there is a change in glycine and/or zinc ions
cG
−
[ZnG + 2[ZnG2]] + 3[ZnG ]
3
=
˛G
−
+
(15)
(16)
−
−
[
G ]
[G ]
ꢀ
o
−
concentration. Table 3 contains E (ZnG /Zn(0)) values obtained
eq
3
leading to a third degree equation:
from Eq. (18), which correspond to the conditions used in the solu-
tions S0 (−1.384 V vs. SCE), S1 (−1.388 V vs. SCE), S2 (−1.392 V vs.
SCE).
−
Zn
cZn
3
−
2Zn
cZn
2
G
−
G
−
3
ˇ
[G ] + 2ˇ
[G ]
3
˛
Zn(L=G−,OH ,Cl
−
−
˛
−
Zn(L=G−,OH ,Cl−
)
)
ꢁ
ꢂ
−
1Zn
cCu
3.2. Voltammetric study
G
−
+
˛G
−
+ ˇ
[G ] − c
−
= 0
G
˛
−
Zn(L=G−,OH ,Cl−
)
Cyclic voltammetry study was performed in the potential range
−
0 to −1.65 V vs. SCE onto NE from solutions S , S and S . The poten-
The determination of [G ] can be achieved by an iterative
method involving Eqs. (10), (14) and (16). Then, using Relations
11)–(13), one can easily obtain the distribution diagram for the
0
1
2
tial scan was initiated in the negative direction from the open circuit
−
1
potential (EOCP) at scan rate (ꢁ) of 10 mV s
.
(
It is possible to define the region of the UPD process from previ-
zinc/glycine system in aqueous solution. Three such diagrams are
presented in Fig. 1(a)–(c).
−
ous thermodynamic study. The potential of the ZnG3 /Zn(0) couple
under our experimental conditions was previously determined as
Fig. 1 shows the species repartition diagrams obtained from
Relations (11)–(13) and considering the conditions displayed in
Table 1. In these diagrams it is possible to observe four charac-
teristic pH regions. The first region is characterized by the free zinc
ion and zinc–chloride complexes as the most stable species (pH
region 0–4). In the second region, between pH 4 and pH 8, the pre-
−
1.388 V vs. SCE (average value). Therefore, the deposition of zinc
that takes place at potentials more positive than −1.388 V vs. SCE
will be defined as occurring by the UPD process, and zinc deposi-
tion that occurs at more negative potential values is defined as the
OPD process.
+
Typical cyclic voltammograms obtained for zinc are presented in
Fig. 3. During the direct scan it is possible to note that all voltammo-
grams present the following characteristics: a negligible cathodic
current from the EOCP to −0.8 V vs. SCE that indicates none reaction
occurring in this potential range; the formation of two cathodic
peaks, A and B, at the peak potentials −1.30 V vs. SCE and −1.55 V
vs. SCE, respectively (this potential value depends on the composi-
tion of the solution). The peak A appears at more positive potential
values than the equilibrium potential predicted from Eq. (18) and
this start at −0.8 V vs. SCE, which is a value 0.6 V before that the
predicted from Eq. (18). Is important to note that this value is very
close to the calculated (ꢀE = 0.585 V) from Eq. (1) for the underpo-
tential shift. The peak B is located at more negative potential values
dominant zinc species are ZnG and ZnG2 complexes. In the third
region, from pH 8 to pH 12, the predominant zinc species are ZnG3
−
complexes. The fourth region, above pH 12, is characterized by two
zinc hydroxides; between pH 12 and pH 14, the predominant zinc
hydroxide is the insoluble species Zn(OH)2(s) and for pH > 14 the
predominant zinc species is the soluble hydroxide Zn(OH)42
−
.
These results clearly indicate that under our experimental con-
ditions the anionic 1:3 ZnG3 complex predominates in three
solutions mentioned in Table 1 for pH ranging from 8 to 12.
Based on the information provided by the species repartition
diagrams, we have constructed the E–pH diagram for values of pH
above 7. Fig. 2 shows the E–pH diagram for the system with 0.2 M
−
2+
−
−
Zn + 0.4 M Cl + 1.6 M G .
−
than the equilibrium potential of the ZnG3 /Zn(0) couple and this
On the basis of these results, we therefore assume that under
our experimental conditions the overall reaction of zinc electrode-
position process at pH 10 will be the following:
start at E = −1.4 V vs. SCE. On the basis of these results, we asso-
0
ciate the peak A to the zinc UPD deposition onto the NE and the
−
peak B to the zinc OPD deposition of the chemical species ZnG3 to
−
−
0
−
ZnG3 + 2e ⇔ Zn + 3G
(17)
Zn(0), as is proposed in Eq. (17).
Table 3
Voltammetric parameters of zinc deposition on NE from solutions S0, S1 and S2.
ꢀ
−
o
iPB (mA cm2)
Solution
E
(ZnG3 /Zn(0)) (V vs. SCE)
EPB (V vs. SCE)
EPB
ꢀ
(V vs. SCE)
EPB/2 (V vs. SCE)
ꢀ
(EPB + EPB )/2 (V vs. SCE)
eq
S0
S1
S2
−1.384
−1.388
−1.392
−1.537
−1.564
−1.574
−1.215
−1.251
−1.259
−1.451
−1.479
−1.480
−1.376
−1.389
−1.417
−23.541
−30.601
−41.939

5
446
J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
-
-
-
-
1,0
1,1
1,2
1,3
-
3
ZnG2
Zn(OH) 24 -
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
ZnG
ZnG-
3
Zn(OH)2(s)
ZnG2
Zn(OH)2
+
+
-1,4
ZnCl
ZnG
2
+
Zn
Zn
-1,5
-
-1,6
ZnCl
3
7
8
9
10
11
12
13
14
2
-
ZnCl4
pH
0
2
4
6
8
10
12
14
ꢀ
Fig. 2. Potential–pH diagram for the reduction of Zn(II) species. The conditions
considered in the computation of this diagram correspond with the given for the
solution S0.
pH
1
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
-
3
ZnG
associated with the dissolution of the zinc UPD deposition. Similar
behavior has been reported by Ortiz-Aparicio [16] for zinc–glycine
complexes reduction at pH 11.
ZnG2
Zn(OH)2(s)
ꢀ
In our experiments, the difference between the anodic B and
cathodic B peak potentials (ꢀEP = EPB − E
ꢀ
), for cyclic voltammo-
PB
grams in Fig. 3, are 0.322, 0.349 and 0.315 V vs. SCE for S0, S1
and S solutions, respectively. The obtained values for ꢀEP signif-
2
icantly exceed the quantity characteristic of a reversible process,
+
◦
ZnCl
ꢀE = 0.059/n at 25 C and n = 2 [42]. Therefore, we could assume
P
+
-
3
ZnG
6
that a totally irreversible reaction might be the case, from the
cyclic voltammograms recorded (Fig. 3) and then from Fig. 3(a)
one can determine the magnitude of the product ˛ n , where ˛ is
ZnCl
2
+
Znn
2
4
-
c
˛
c
ZnCl
the cathodic transfer coefficient and n˛ is the number of electrons
transferred up to, and including, the rate determining steps. With
this purpose, such a parameter of the mentioned cyclic voltammo-
grams as the shape factor |EPB − EPB/2| can be applied [42]:
0
2
4
8
10
12
14
pH
100
4
8
-
3
◦
ZnG
|EPB − EPB/2| =
c ˛
˛ n
mV at 25 C
(19)
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Zn(OH)2(s)
where EPB/2 is the potential of a half-peak. It was found that
ZnG2
˛
cn˛ ≈ 0.558, if the reaction (17) is considered as totally irre-
versible.
40
30
B'
-
3
ZnCl
2
0
0
0
+
2
-
2+
ZnG
6
ZnCl 4
Zn
1
+
A'
ZnCl
2
A
0
4
8
10
12
14
-
-
10
20
pH
Fig. 1. Species distribution diagrams as a function of the solution pH for the
zinc(II)–glycine–water system. These diagrams were constructed under the con-
ditions mentioned in Table 1: (a) solution S0, (b) solution S1 and (c) solution S2.
B
-30
-
-
40
50
When the scan direction was reversed in the switching potential
-
1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2
0.0
0.2
E = −1.65 V vs. SCE, three anodic current density peaks appeared at
ꢂ
E vs SCE / V
potentials of −1.45, −1.26 and −0.41 V vs. SCE. The peaks at −1.45 V
ꢀ
vs. SCE (which appears as a shoulder) and B could be due to the
Fig. 3. Cyclic voltammetry of zinc as a function of zinc chloride and glycine con-
centration at pH 10 and 25 C. (a) Solution S0, (b) solution S1 and (c) solution S2.
Scanning rate: 10 mV s
oxidation of different zinc phases. At potentials more positive than
◦
ꢀ
ꢀ
−1
peak B region, an anodic peak A is observed; this peak could be
.

J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
60
5447
It has also been known that on the basis of diffusion-controlled
a
cyclic voltammograms a formal standard redox potential can
be approximated by the mid-point potential of the anodic and
B'
4
2
0
0
−
cathodic peak potentials [42]. Regarding ZnG3 /Zn(0) equilibrium,
it should be noted that the theoretical value of this equilibrium
is known of the thermodynamic study. From the cyclic voltam-
mograms recorded, the (EPB + E
ꢀ
)/2 value of the average peak
PB
−
potential for the ZnG3 /Zn(0) couple were found to be ca. −1.376,
0
-20
-40
−
1.389 and −1.417 V vs. SCE for S , S and S solutions, respectively.
0
1
2
A
Differences of approximately 8, 1 and 21 mV, between experimen-
ꢀ
−
tal and theoretical potentials Eo (ZnG /Zn(0)), were found for S ,
-1
eq
3
0
10 mV s
B
S1 and S2 solutions, respectively. These differences can be associ-
ated to adsorption process or formation of chemical species in the
interphase. On the basis of these results, it is possible to propose
-1
3
0 mV s
-
1
5
0 mV s
-1
ꢀ
−
70 mV s
o
that E (ZnG /Zn(0)) corresponds to the equilibrium potential of
-1
eq
3
90 mV s
-60
−
ZnG3 /Zn(0) couple.
In Fig. 3, it is clearly seen that the peak B potential (EP) and the
peak B current maximum density (iP) depend on the composition
of the solution, see Table 3. Note that the magnitude of the cathodic
peak B diminished proportionally to the decrease of Zn(II) concen-
tration in solution, as expected. The concentration of chloride and
glycine in solution affect the position of the cathodic peak B men-
tioned, which is negatively shifted with the increase of glycine and
chloride concentration.
-
2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
E vs SCE / V
100
b
B'
8
0
0
6
The effect of scanning rate on cyclic voltammograms from
solutions S , S and S2 is given in Fig. 4(a)–(c), respectively. The
40
0
1
2
0
voltammograms, all initiated from the EOCP, were scanned in the
−
1
negative direction at rates ranging from 10 to 100 mV s
.
0
According to Fig. 4(a)–(c), the voltammograms are character-
A
-
20
ꢀ
ꢀ
ized by two cathodic (A and B) and anodic (A and B ) peaks. The
cathodic peak maximum currents (ic) were found to increase and
the cathodic peak potentials (EP) shifted negatively when ꢁ was
increased during the scan.
In either case, the maximum current value of the peak B were
found to be proportional to the square root of the sweep rate within
the range of sweep rates applied (Fig. 5(a)), indicating a diffusion-
controlled process [42].
The cathodic peak potentials EP shift in the negative direction, as
mentioned above, and are nearly linear with respect to the decimal
logarithm of the sweep rate (Fig. 5b), suggesting that the reaction
16) is not reversible and tentatively may be considered as irre-
versible. Then, from the variation of EP with log ꢁ given by the
B
0 mV s-1
1
-
40
60
80
1
3
0 mV s-
-
-
-1
5
0 mV s
70 mV s-
1
-1
1
00 mV s
-100
-
2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 - 0.6 - 0.4 -0.2 0.0 0.2
E vs SCE / V
100
c
(
B'
8
6
4
0
0
0
following equation [42]:
ꢃ
ꢄ
2
.3RT
2
0
EP = K −
log ꢁ
(20)
2
˛cnF
0
A
-
20
where
-
1
-
40
B
10 mV s
ꢅ
ꢁ
ꢂꢆ
ꢃ
ꢄ
ꢃ
ꢄ
-1
RT
2.3
2
˛cn˛FD
-60
-80
30 mV s
o
K = E −
−0.78 −
log
(21)
2
-1
˛
cn˛F
K RT
s
50 mV s
-
1
7
0 mV s
-
-
100
120
-
1
Ks is the standard rate constant, and the other terms have their
usual significance, it should be expected that EP shifts by 30/˛cn˛
mV for each decade increase in ꢁ. However, as is evident from
Fig. 5b, the experimental shift of EP for the S , S and S solutions
with increasing ꢁ from 10 to 100 mV s is equal to 122, 154 and
66 mV, respectively. Quite apparently, such values of the negative
90 mV s
-
2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
E vs SCE / V
0
1
2
−
1
Fig. 4. Voltammetric curves obtained onto NE at different scan rates from solutions:
a) S0, (b) S1 and (c) S2.
1
(
shift of EP for each decade increase in ꢁ are in a considerable excess
of that expected for the totally irreversible reaction [42]. These fea-
tures of cyclic voltammograms recorded in this work clearly show
that the diagnostic criteria for a totally irreversible system reported
in the literature [42] are not wholly satisfied. Consequently, the
reaction (16) could not be considered as a simple irreversible reac-
tion.
3.3. Study of zinc UPD process onto nickel electrode
Fig. 6 shows the cyclic voltammograms displaying the effects
of scan rate on zinc electrodeposition in UPD deposition region in
solution S . Note that the Fig. 6 shows the dependences of the peak
1

5
448
J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
a -1.4
a
-120
S2
-
100
-
1.2
1.0
A
-80
-60
-40
-20
0
S1
S0
-
-0.8
-
0.6
0.4
A'
-
0.10
0.15
0.20
0.25
0.30
0.03
0.04
0.05
0.06
0.07
1
/2
1/2 -1/2
1
/2
1/2 -1/2
υ
υ
/
,
V
V
s
s
1/2
/2
1/2 -1/2
1
1/2 -1/2
υ
υ
/
,
V
V
s
s
b
1200
-1,80
-1,75
-1,70
-1,65
-1,60
-1,55
-1,50
b
6
00
S0
S1
S2
A'
0
-
600
-
-
-
1200
1800
2400
A
12
16
20
24
28
32
1,0
1,2
1,4
1,6
1,8
2,0
-
1/2
-1 -1/2
-
1/2
-1/2 1/2
υ
υ
/ V s
, (V s )
-1
-1
g ( mV s )
lol og (υ / mV s )
v,
ꢀ
Fig. 7. Current density maxima jP and potentials EP for peaks A and A in the cyclic
ꢀ
Fig. 5. Variation of current density maxima iP and potentials EP for peaks B and B
1/2
voltammograms as a function of the square root of scan rate. (a) EP vs. ꢁ and (b)
1/2
in the cyclic voltammograms as a function of the scan rate. (a) jPB vs. ꢁ and (b) EP
vs. log ꢁ.
−1/2
jP vs. ꢁ
.
ꢀ
A and the peak A on scan rate. However, the potential values asso-
ciated with these peaks do not change significantly as expected for
UPD processes, which occurs trough adsorption, in sum a linear
1
500
000
1/2
A'
relationship is found between E vs. ꢁ
(see Fig. 7(a)).
P
1
In Fig. 6 is possible to observe that during the anodic scan a
second cathodic peak is observed. We expected that the cathodic
peak A occurs only at the fraction of active sites that are not blocked
by chloride ions and/or glycine molecules and/or HER. Thus, during
the anodic scan occurs the desorption of chloride ions from the
nickel surface, allowing the reduction of Zn(II) ions to take place at
the active sites vacated by the chloride ions. However, additional
studies should be realized to support this hypothesis.
5
00
0
-
500
-
1
-1000
-1500
-2000
1
mV s
-1
3
mVs
In order to determine the type of nucleation that occurs dur-
A
ing the zinc UPD deposition, the maximum current density value
-1
5
mV s
−1/2
(
(
iP) associated with the peak A was plotted as a function of ꢁ
Fig. 7b) and both EP vs. iP (not shown). A linear relationship was
-
2500
found for both cases indicating an instantaneous nucleation process
[42].
-
3000
-
1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
The former of the peak A could be associated with incipient
hydrogen reduction followed by zinc UPD deposition [36],
E vs SCE / V
Fig. 6. Voltammetric curves obtained from the solution S1 at different scan rates in
the zinc UPD region.
−
−
o
2
H O + 2e ⇔ H + 2OH
E = −1.069 Vvs.SCE
(22)
2
2

J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
5449
and
10
Zn2 + OH + 2e ⇔ Zn(OH)
+
−
−
−
E = −1.131 Vvs.SCE
o
(23)
ads
8
6
4
2
0
Previous studies reported that the inhibition of HER (hydrogen
evolution reaction) in presence of zinc ions in alkaline [43–46] and
acid [47] solutions is due to the formation of sub-monolayer of
Zn on the substrate. It is proposed that zinc UPD competes with
hydrogen evolution reaction [16,33].
(
(
(
(
1)
2)
3)
4)
-1,15 V
-
1,20 V
-1,25 V
-1,30 V
ꢀ
The cathodic charges associated with the peak A and A were
obtained by integration from the voltammetric curves (Fig. 6). The
magnitude of the integrated charges was found to be approximately
-0,62 V
− −2 ꢀ
2
1
95 ␮C cm and 165 ␮C cm for peak A and A , respectively. The
(
4)
difference between both values can be attributed to the hydrogen
evolution reaction, which occurs simultaneously with the zinc UPD
process. The theoretical charge for a zinc monolayer corresponds
(3)
(2)
−
2
(1)
to 367 ␮C cm , which was determined from the exposed surface
2
area (0.726 cm ) of the NE. This value is very close to predicted by
0
20
40
60
80
−
2
Kolb [17] for polycrystalline surfaces, 200(n) ␮C cm , where n is
the electron number involved in the electrochemical reaction. The
degree of coverage by zinc ꢃZn-UPD = 0.45, which was determinate
t / s
Fig. 8. Set of experimental current transients recorded during the zinc UPD process
of the ratio: QA
ꢀ
/367. This result indicates that the zinc UPD cor-
in the solution S . In all cases, the starting potential of −0.62 V vs. SCE was applied
1
responds to the formation of sub-monolayer of zinc and that this
occurs simultaneously with HER onto the NE.
to the NE surface and t = 80 s.
When the scan direction was reversed in the switching potential
mechanism and the second is that diffusion is the rate control-
ling step of the zinc UPD process, however an additional process is
involved. The theoretical description of similar potentiostatic cur-
rent transients as shown in Fig. 8 has been predicted adequately by
Mendoza-Huizar [50]:
ꢀ
E = −1.41 V vs. SCE, an anodic peak (peak A ) is observed; this peak
ꢂ
was previously associated with the dissolution of zinc UPD deposi-
tion (peak A). Fig. 7(a) shows the peak potentials (EP) as a function
1
/2
ꢀ
of ꢁ
associated with the peak A . Fig. 7(b) shows the maximum
ꢀ
current density value (iP) associated with the peak A , which was
plotted as a function of ꢁ
−1/2
.
jtotal(t) = jads(t) + j2Di-dc(t)
(24)
Fig. 7(a) and (b) shows the linear dependence of the peak poten-
ꢀ
1/2
−1/2
,
where
tials and current maxima for peaks A and A on ꢁ
and ꢁ
respectively. In the literature [48,49] has been shown that this
behavior is characteristic of an instantaneous nucleation process,
as was mentioned above. It is important to mention that this test
is valid for small sweep rate.
jads(t) = k1 exp(−k t)
(25)
2
and
j2Di-dc(t) = k3 exp(−k t)
(26)
4
3.3.1. Potentiostatic study
jads(t) is the current density for
a
Lagmuir type
Chronoamperometry was used as a supporting experimen-
adsorption–desorption process and j2Di-dc is the current den-
tal technique to further elucidate the nucleation mechanisms of
zinc onto NE. Current transients in Fig. 8 were measured by the
sity associated to an instantaneous two-dimensional nucleation
mechanism. k = k Q and Qads is the charge density due to the
1
2
ads
double-potential step technique from solution S . The experiments
adsorption process. The potential dependence of k2 is assumed
1
2
2
consisted in application of an initial potential of −0.62 V vs. SCE,
where zinc deposition had not started yet. After application of this
initial potential, a second potential step was applied to the elec-
trode surface for 80 s. The second potential values correspond with
the zinc UPD region (peak A in Fig. 6).
to obey the Butler–Volmer relation. k = qmonꢄS D, k = ꢄS DN .
3 4 0
In these equations, qmon is the charge density associated with
formation of the monolayer, S is a constant controlled by the
potential, D is the diffusion coefficient of the metal ion, and N0 is
the density number of active sites. Note that k = k Q and Qnucl
3
4
nucl
Fig. 8 shows a set of current transients recorded at different
is the charge density due to the 2D nucleation process.
potentials from solution S . The shape of these transients is inde-
pendent of the final potential. In sum, the shape of these transients
is quite similar to those reported by Holzle et al. [49] and Mendoza-
Fig. 9 shows a comparison between the experimental results for
the transient obtained at −1.15 V vs. SCE during zinc UPD and the
nonlinear fit of Eq. (24). It is clear from this figure that the nucleation
of zinc UPD conforms to the 2Di-dc model. Table 4 lists the values
of the parameters pertaining to Eq. (24) that best fitted the exper-
imental data. From these parameters it is possible to estimate the
charge density due to both nucleation (Qnucl) and adsorption (Qads)
1
Huizar [50]. Linearization of the transients by either the Cotrell
√
−
kt
equation (j ≈ 1/ t) or an exponential law (j ≈ e ) has been real-
ized and was found failed; this indicates that there is no pure
diffusion control.
−
2
The previous results obtained from voltammetry cyclic indi-
cated two features of the zinc UPD process on NE; the first feature
is that it involves an instantaneous two-dimensional nucleation
processes. From Table 4, Qnucl = 20.6 ␮C cm
indicating that the
degree of coverage by zinc ꢃZn-UPD = 0.06. It is interesting to observe
that the contribution due to the nucleation process is 76% indicating
Table 4
Best fit parameters obtained from non-linear adjustment of Eq. (22) to the experimental current density transient recorded during zinc UPD on NE.
E vs. SCE (V)
jads
jnucl
Qads (%)
Qnucl (%)
k1 (␮A cm 2
−
)
k2 (s
−1
)
k3 (␮A cm
−2
)
k4 (s
−1
)
−
1.15
7.98 ± 0.02
1.11 ± 0.01
1.65 ± 0.03
0.08 ± 0.02
24
76

5
450
J.C. Ballesteros et al. / Electrochimica Acta 56 (2011) 5443–5451
6
5
4
3
2
1
0
solution for the conditions used in this work and that from these
complexes the electrodeposition of zinc occurs.
Analysis of the experimental data clearly shows that a zinc
adlayer is formed during application of potential in the UPD region
and which corresponds to the formation of a submonolayer. In
sum, also was founded that this zinc submonolayer involves the
simultaneous presence of both adsorption and 2D nucleation under
a mass-transfer controlled process. The morphology of zinc UPD
obtained by AFM confirmed the formation of a submonolayer onto
the nickel electrode.
The experimental data obtained by cyclic voltammetry confirms
unambiguously that the electrodeposition of zinc onto nickel elec-
trode occurs simultaneously with the HER, but that this last is
diminished by effect of the formation of the zinc submonolayer.
iexperimental
itotal
iads
i2Di-dc
Acknowledgments
0
20
40
60
80
This work was supported by the Consejo Nacional de Ciencia y
Tecnología (CONACyT-PCP, México-France). J.C. Ballesteros thanks
CONACyT-PCP-México-France for scholarship support.
t / s
Fig. 9. Comparison of an experimental current density transient recorded during
zinc electrodeposition at −1.15 V vs. SCE onto a polycrystalline nickel electrode from
the solution S1 and a theoretical transient obtained by non-linear fit of Eq. (21).
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