A kinetic study on nickel electrodeposition from sulfate acid solutions: I. Experimental results and reaction path

Article abstract of DOI:10.1149/1.3147263

In this work a systematic study on the kinetics of Ni electrodeposition in acid sulfate media over a wide pH range was carried out. Two distinct electrodeposition behaviors were experimentally detected as a function of pH. Accordingly, it is suggested tha

Full text of DOI:10.1149/1.3147263

A Kinetic Study on Nickel Electrodeposition from Sulfate Acid
Solutions : I. Experimental Results and Reaction Path
Ana Isabel C. Santana, Susana L. Díaz, Oswaldo E. Barcia and Oscar R. Mattos
J. Electrochem. Soc. 2009, Volume 156, Issue 8, Pages D326-D330.
doi: 10.1149/1.3147263
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D326
Journal of The Electrochemical Society, 156 ͑8͒ D326-D330 ͑2009͒
0013-4651/2009/156͑8͒/D326/5/$25.00 © The Electrochemical Society
A Kinetic Study on Nickel Electrodeposition from Sulfate Acid
Solutions
I. Experimental Results and Reaction Path
Ana Isabel C. Santana,a Susana L. Díaz,a Oswaldo E. Barcia,a,b and
,z
Oscar R. Mattosa,
*
aLaboratório de Ensaios Não-Destrutivos, Corrosão e Soldagem, Departamento e Programa de
Engenharia Metalúrgica e de Materiais, and bInstituto de Química, Universidade Federal do Rio de
Janeiro, 21941-972 Rio de Janeiro, Brazil
In this work a systematic study on the kinetics of Ni electrodeposition in acid sulfate media over a wide pH range was carried out.
Two distinct electrodeposition behaviors were experimentally detected as a function of pH. Accordingly, it is suggested that Ni
electrodeposition takes place through different intermediates: In the lower pH range, Ni͑I͒ads species predominates at the electrode
surface, and its formation is deactivated with increasing pH, whereas in less acid solutions, Ni electrodeposition occurs via two
+
+
ads
species, ͓Ni͑OH͔͒ and ͓Ni͑OH͔͒ . The chemical nature of Ni͑I͒ads, ͓Ni͑OH͔͒ and ͓Ni͑OH͔͒ species is discussed on the
ads
ads
ads
basis of the interfacial pH behavior.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3147263͔ All rights reserved.
Manuscript submitted January 29, 2009; revised manuscript received April 20, 2009. Published June 22, 2009.
Nickel electrodeposition has been extensively investigated due to
its vast importance as a surface finishing process that satisfies most
of the required properties for decorative, magnetic, and anticorrosive
applications. Nickel coatings are normally used for the corrosion
protection of steel and brass, and their properties have been associ-
ated with the electrolyte composition and electrodeposition param-
eters. Much work has also been dedicated to the electrodeposition
mechanism of Ni as well as Ni-containing alloys. In this latter case,
the establishment of a codeposition model should be supported by
the corresponding individual deposition mechanisms.
models proposed to explain the mechanism of Ni electrodeposition.
So far, none of the available models has fully described all the
kinetic aspects, especially those associated with a wide acid pH
range. As an example, by using electrochemical impedance analysis,
Ordine et al.8 recently verified that the interfacial processes that take
place during Ni electrodeposition from sulfate solution at pH 4 gen-
erate a faradaic capacitive loop followed by an inductive one. This
behavior cannot be explained by the reaction path proposed by Epel-
boin et al.,4 the most widespread model based on electrochemical
impedance data, which describes an inductive–capacitive faradaic
relaxation sequence. In the model of Epelboin et al.,4 the inductive
The mechanism of Ni electrodeposition from acid solutions1-12
occurs through different elementary steps that depend on the elec-
trolyte composition and pH. In one of these approaches,9 two con-
secutive one-electron charge-transfer reactions with the anion par-
ticipation were considered. From this model, the formation of an
adsorbed complex intermediate for Ni electrodeposition was pro-
posed. Some other studies have indicated the participation of a Ni
monohydroxide ion, NiOH+, in charge-transfer steps in aqueous
solutions.10-12
loop was associated with the relaxation of Ni͑I͒ at the electrode
ads
surface and the low capacitive process was related to a blocking
effect due to the adsorbed hydrogen. Because this blocking effect
always gives a capacitive feature in impedance diagrams, the inver-
sion in the diagrams observed experimentally at pH 4 by Ordine et
al.8 cannot be taken into account by that model.
Therefore, the objective of the present work is to perform a sys-
tematic study on the kinetics of Ni electrodeposition in sulfate media
in pH ranging from 1 to 6. Potentiostatic polarization curves, elec-
trochemical impedance spectroscopy, and interfacial pH measure-
ments were used as the experimental tools, the results of which are
presented and discussed in Part I of this paper. The goal is to provide
a deeper insight into the electrode reaction mechanism, aiming to
give a more comprehensive model of Ni electrodeposition, whose
main features are also described here. In Part II of this paper, the
reaction model is fully described and transcribed into mathematical
expressions to permit the simulation of the experimental results.
Moreover, the Ni electrodeposition mechanism validated for a large
pH range is useful for further studies, particularly for Ni alloy elec-
trodeposition.
By using electrochemical impedance spectroscopy, Wiart,3 Epel-
boin et al.,4 and Chassaing et al.5 studied the mechanism of Ni
electrodeposition from chloride, sulfate, and Watts solutions at dif-
ferent pH values. It was verified that the type of anion strongly
influences the electrode kinetics.4 Important surface interactions be-
tween Ni2+ and H+ were also observed, particularly in sulfate solu-
tions. These interactions, together with the anion influence, were
considered to account for the impedance results. The interpretation
was carried out in terms of adsorbed intermediates as well as of the
potential dependence of growth sites. The existence of an adsorbate,
Ni͑I͒ads, which would behave as an intermediate for Ni electrodepo-
sition as well as a catalyst for the slow formation of Hءads species,
was suggested. It was also considered that Hءads acts as an inhibitor
for hydrogen evolution, and it was concluded that the electrode ac-
tive area is closely related to the degree of coverage by this species.
Efforts to describe the participation of H+ in the Ni electrodepo-
sition process have been made in many investigations.3,13-21 By in-
terfacial pH measurements during Ni and Ni alloy electro-
deposition,8,20,21 an alkalination at the vicinity of the electrode was
verified. This was generally attributed to a catalyzing effect of the Ni
species on the hydrogen reduction reaction.
Experimental
The electrolyte used in this work consisted of 1.2
M
NiSO4·6H2O solution, whose pH varied from 1 to 6. The solutions
were prepared from analytic grade chemicals and double-distilled
water, without other substances such as additives, complexing
agents, or supporting electrolyte. The pH of the solution was ad-
justed with additions of 1.2 M H2SO4. All the experiments in the pH
range 4–6 were performed with a large volume of electrolyte ͑4 L͒,
which was circulated between the containing flask and the cell by a
peristaltic pump. The solution entered the cell through a capillary
distant enough from the working electrode, thus avoiding distur-
bances to the flow pattern. This procedure was carried out to guar-
antee the stability of the solution pH during the electrochemical
measurements.
Despite many studies, there are still some significant gaps in the
*
Electrochemical Society Active Member.
z E-mail: omattos@metalmat.ufrj.br
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Journal of The Electrochemical Society, 156 ͑8͒ D326-D330 ͑2009͒
D327
Table I. OCPs of the Pt RDE in the 1.2 M NiSO4·6H2O solution
at different pH values.
Solution
pH
OCP
͑VSSE
͒
1
2
3
4
5
6
0.201
0.196
0.157
0.098
0.028
0.003
Figure 1. Potentiostatic cathodic polarization curves obtained in the 1.2 M
NiSO4·6H2O solution at different pH values: ͑᭿͒ 6, ͑b͒ 5, ͑᭡͒ 4, ͑ᮀ͒ 3, ͑᭺͒
2, and ͑᭝͒ 1.
increase that confers a noticeable change to the slope of the curves,
which starts to vary sharply with the potential. High efficiency in
metallic deposition was only detected at this branch of the curves for
all solution pH values investigated. For simplicity, hereafter in this
text the curves are divided into two parts regarding the cathodic
reaction predominance: part 1, associated with the low slope region,
wherein H+ reduction prevails, and part 2, the high slope region of
the curve, mainly related to Ni electrodeposition.
The two parts of the polarization curves are affected differently
by the solution pH. In part 1, a decrease in current values with
increasing pH is observed. The beginning of part 2 occurs at differ-
ent E–EOCP regarding the solution pH, showing two distinct tenden-
cies. Indeed, for the curves obtained in the solution with the pH
ranging from 1 to 3, the increase in pH caused a shift in part 2 to
more negative potentials. This means that Ni electrodeposition is
inhibited with increasing pH from 1 to 3. This result agrees with
those reported by Epelboin et al.4 for Ni electrodeposition in sulfate
solutions at pH 1.5 and 3. By contrast, for the curves obtained in the
solution at the pH interval 4–6, an opposite behavior is observed in
Fig. 1. The activation of the metallic deposition is verified when the
solution pH increases from 4 to 6. Note that Ni electrodeposition
kinetics in sulfate solutions at pH Ն 4 has not been effectively in-
vestigated. The results in Fig. 1 clearly indicate that this process
may occur through two different mechanisms as a function of the pH
range.
A conventional three-electrode cell ͑600 mL͒ using a Pt rotating
disk electrode ͑RDE͒ with 0.2 cm2 as the working electrode was
employed. All experiments were performed at 650 rpm of electrode
rotation speed. The counter electrode was a cylindrical Pt grid
͑300 cm2͒, and a saturated Hg/Hg2SO4 electrode ͑SSE͒ was used as
reference. The potential values in this work are always given against
this reference. All experiments were performed in an open cell and
at room temperature.
Cathodic potentiostatic polarization curves and electrochemical
impedance measurements, in both potentiostatic and galvanostatic
modes, were carried out by a model Asservissement Electronique
rotating electrode device. The polarization curves were obtained
through a model PGSTAT30 Autolab electrochemical interface using
the linear sweep polarization staircase ͑stationary current͒ method
with 30 mV of potential step. A PG19 Omnimetra potentiostat and a
Solartron SI1254 frequency response analyzer, coupled with a PC,
were used in the impedance measurements. The signals were filtered
by means of a model VBF-8 Kemo filter in low pass mode. In most
of the experiments, the impedance diagrams were obtained in the
frequency interval of 40 kHz to 1.3 mHz with the amplitude ranging
from 5 to 15 mV, always ensuring linearity. The polarization curves
were corrected from the ohmic drop through the high frequency
limit of the electrochemical impedance plots. A chemical analysis of
the deposits obtained in some polarization conditions was performed
through atomic absorption spectrometry to estimate the efficiency of
the electrodeposition process, whose calculation was done by Fara-
day’s law.
For the pH measurements at the electrode/electrolyte interface, a
working electrode of the type proposed by Deligianni and
Romankiw21 and adapted by Deslouis et al.22 was applied. It con-
sisted of a 75 mesh Pt grid ͑3.5 cm2͒ held at the end of a flat-
bottomed glass electrode ͑Orion model 8135 BN͒ using a Teflon
adaptor. The electrode was connected to a pH meter, thus measuring
the pH of the solution retained in the mesh. The assembled electrode
was motionless, and the solution was stirred to guarantee a uniform
bulk pH, as detailed elsewhere.23 Under these conditions, surface pH
and current measurements were recorded simultaneously as the elec-
trode was potentiostatically polarized.
Aiming to better understand the role of a simultaneous hydrogen
evolution during Ni electrodeposition, cathodic polarization curves
were obtained in 1.2 M Na2SO4 solution. Figure 2 depicts typical
curves obtained at pH 3 in both Ni2+-containing and Ni2+-free
͑EOCP = 0.07 VSSE͒ solutions using the Pt RDE electrode. Higher
current densities can be observed for the Ni2+-free ͑Na2SO4͒ solu-
tion at the E–EOCP range corresponding to Ϫ0.9 to Ϫ1.4 V if com-
pared to the curve for the Ni2+-containing solution. Because H+
reduction is the major reaction in this region, this behavior suggests
Results and Discussion
Figure 1 shows the cathodic polarization curves obtained in the
solution with the pH values ranging from 1 to 6, plotted against
E–EOCP. Open-circuit potentials ͑OCPs͒ ͑EOCP͒ of the Pt electrode
in the respective solutions are given in Table I. Independently of the
pH value, two distinct regions are observed as a function of the
potential ͑E–EOCP͒ in the curves of Fig. 1. In the first one, at low
values of E–EOCP, the current varies slowly with cathodic polariza-
tion. In this region, the efficiency for metallic deposition is ex-
tremely low, and, consequently, the H+ reduction prevails. With fur-
ther polarization increase, the current densities show a sudden
Figure 2. Potentiostatic cathodic polarization curves at pH 3 obtained in ͑᭿͒
1.2 M NiSO4·6H2O and ͑᭺͒ 1.2 M Na2SO4 solutions ͑OCP = 0.07 VSSE͒.
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D328
Journal of The Electrochemical Society, 156 ͑8͒ D326-D330 ͑2009͒
Figure 4. Impedance diagrams obtained in the 1.2 M NiSO4·6H2O solution
at pH 1, 2, and 3 at the potential values corresponding to part 1 of the
polarization curves in Fig. 1 ͑frequencies in Hz͒.
acid solutions, namely, 2 and 1, the interfacial pH does not exceed 4.
The solution at pH 3 shows a transitional behavior.
Impedance measurements were performed at different points of
the polarization curves in Fig. 1 and are presented in Fig. 4-8. For
each diagram, the capacitive loop at high frequency is related to the
charge-transfer resistance in parallel with the double-layer capaci-
tance. As verified by the polarization results as well as by the inter-
facial pH data, two different behaviors were also detected. Figure 4
shows the typical impedance diagrams obtained in the solution at pH
1, 2, and 3 and at potential values corresponding to part 1 of the
polarization curves. In the low frequency domain, only one capaci-
tive loop near 0.004 Hz exists, which is associated with the H+
reduction as the main cathodic reaction in this potential range. As
expected, this feature becomes less important as the solution pH
increases, given that the rate of H+ reduction decreases.4
Figure 5 presents typical impedance diagrams obtained in the
solution at pH 1, 2, and 3 at the potential range corresponding to
part 2 of the polarization curves ͑Fig. 1͒. At the beginning of this
region, independently of the pH value, the diagrams show the same
qualitative behavior: an inductive loop at about 1 Hz followed by a
capacitive loop at 0.02 Hz, as can be seen in Fig. 5a-c, respectively,
for pH 1, 2, and 3. With increasing polarization, for pH 1 and 2, this
behavior did not change, in conformity with the results of Epelboin
et al.4 However, for pH 3, at higher current densities, as shown in
Fig. 5d, there is a noticeable modification in the kinetics: The dia-
Figure 3. Potential dependency of both ͑᭺͒ interfacial pH and ͑᭿͒ current
density obtained in the 1.2 M NiSO4·6H2O solution at different pH values:
͑a͒ 6, ͑b͒ 5, ͑c͒ 4, ͑d͒ 3, ͑e͒ 2, and ͑f͒ 1.
an inhibitory effect on such reaction by the presence of Ni2+ ions in
the solution. This result is in accordance with the model proposed by
Epelboin et al.,4 as discussed below.
Figure 3 shows the potential dependence of both interfacial pH
and current density for the 1.2 M NiSO4·6H2O solution at pH rang-
ing from 1 to 6. For the solution at pH 6 ͑Fig. 3a͒, no significant
alkalination is observed at the E–EOCP range corresponding to part 1
of the polarization curve. However, the interfacial pH increases with
increasing polarization along part 2 of the curve. This indicates that
Ni2+ reduction, which is the main reaction in this potential range,
takes place with some kind of simultaneous consumption of H+ ions.
The interfacial pH obtained in the solutions at pH 5 and 4, Fig. 3b
and c, respectively, starts to increase at E–EOCP near Ϫ400 mV. Part
2 of the curve starts when the interfacial pH attains 6 for both
solutions. This situation is similar to that of the solution at pH 6
͑Fig. 3a͒, where no additional alkalination was detected at the elec-
trode surface before part 2 of the curve. Furthermore, the highest
interfacial pH verified in Fig. 3a-c is always close to 7.
For the solution at pH 3, an increase in the interfacial pH was
only noticed at E–EOCP of about Ϫ900 mV. With increasing polar-
ization, a small drop is observed, and then the interfacial pH starts
again to increase at a potential that matches the beginning of part 2
of the polarization curve, namely, wherein metallic deposition oc-
curs with high efficiency. At much higher polarizations, the local pH
reaches about 6.5.
The interfacial pH obtained in the solution at pH 2 and 1, Fig. 3e
and f, respectively, does not show significant changes along the
potential branch associated with part 1. The increase in local pH was
only observed in part 2 of the polarization curves, wherein Ni elec-
trodeposition prevails. Moreover, the maximum values obtained
were about 4 and 3 for solution pH 2 and 1, respectively, that is, a
variation of 1.0 in the interfacial pH.
The interfacial pH results corroborate the idea of the existence of
a remarkable change in the mechanisms of Ni electrodeposition
from sulfate acid solutions as a function of the pH range. For the
solution pH in the range of 4–6, there is a significant surface alka-
lination at high polarizations, reaching values near 7. For the more
Figure 5. Impedance diagrams obtained in the 1.2 M NiSO4·6H2O solution
at ͑a͒ pH 1, ͑b͒ 2, and ͑c͒ 3 at the beginning of part 2 of the corresponding
polarization curves and at ͑d͒ pH 3 at higher polarization ͑frequencies in Hz͒.
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Journal of The Electrochemical Society, 156 ͑8͒ D326-D330 ͑2009͒
D329
Figure 6. Impedance diagrams obtained in the 1.2 M NiSO4·6H2O solution
at pH 4, 5, and 6 at the potential values corresponding to part 1 of the
polarization curves in Fig. 1 ͑frequencies in Hz͒.
Figure 8. Impedance diagrams obtained in the 1.2 M NiSO4·6H2O solution
at pH 4, 5, and 6 in part 2 of the corresponding polarization curves at current
densities above 25 mA cm−2 ͑frequencies in Hz͒.
gram displays a capacitive loop at 0.13 Hz, followed by an inductive
one at 0.013 Hz in the low frequency domain. As verified by the
interfacial pH measurements ͑Fig. 3d͒, this change in the impedance
behavior is accompanied by a strong alkalination ͑pH Х 6.5͒ at the
electrode surface. This result cannot be described by the model of
Epelboin et al.4 because it only accounts for an inductive–capacitive
behavior of the type presented in Fig. 3a-c. Indeed, the capacitive
loop at lower frequencies was attributed to a blocking effect induced
by the hydrogen reduction, and this phenomenon is always capaci-
tive.
Figure 6 shows impedance diagrams obtained in part 1 of the
polarization curves ͑Fig. 1͒ at pH 4, 5, and 6. Different from what
was observed for pH 1, 2, and 3 at the same corresponding region, in
the pH conditions of Fig. 6 the behavior can be attributed to a
hydrogen reduction reaction coupled with a very low efficient me-
tallic deposition. An extremely thin metallic film obtained in such
conditions was analyzed by a chemical analysis, the results of which
revealed a Ni deposition efficiency of 0.94%.
The impedance diagrams obtained at the beginning of part 2 of
the polarization curves ͑Fig. 1͒ in the solution at pH 4, 5, and 6 are
shown in Fig. 7. These diagrams are analogous to that observed at
high current densities for the solution at pH 3 ͑Fig. 5d͒. A capacitive
loop followed by an inductive one is verified in the low frequency
domain in all the diagrams of Fig. 7. This similarity can be associ-
ated with the alkalination degree of the electrode surface during high
polarizations in the solution at pH 3 ͑Fig. 3d͒, which is comparable
to that for the solutions at pH 4–6 ͑Fig. 3a-c͒. However, the capaci-
tive loop at medium frequencies seems to be pH dependent because
its characteristic frequency changes from 0.53 Hz at pH 4 to 5.9 Hz
at pH 6. By contrast, the inductive loop appears at a fixed frequency
of about 0.02 Hz, independently of the pH in the range of 4–6. This
behavior corroborates the results of Ordine et al.8 and, as already
mentioned, cannot be described by the model of Epelboin et al.4
Higher polarizations in the solution at pH 4, 5, and 6 brought
about a significant change in the kinetics of Ni electrodeposition.
Indeed, in Fig. 8 the higher complexity of the diagrams is notewor-
thy if compared to those shown in Fig. 7, which were obtained at the
corresponding pH values at lower polarizations in part 2 of the
curves. The features observed can be related to the high interfacial
pH ͑Ϸ7͒ verified in the solution pH range 4–6 at high polarizations
͑Fig. 3a-c͒. Under these conditions, the presence of free OH− species
at the electrode surface that gives rise to a local precipitation of
oxide–hydroxide products should be considered. This could be as-
sociated with the presence of the features verified in Fig. 8.
From the above set of experimental data, the main characteristics
for the establishment of a reaction path can now be considered. The
model proposed by Epelboin et al.4 properly described the experi-
mental results obtained at pH Յ 3, i.e., the polarization curves ͑Fig.
1͒ and impedance diagrams ͑Fig. 5a-c͒. According to this model,
Ni͑I͒ is formed and the rate of its production is deactivated with
ads
increasing pH. This assumption accounts for the shift in part 2 of the
polarization curves toward more negative potentials as well as for
the inductive loop in the impedance diagrams obtained in this pH
range. However, the diagram in Fig. 5d, obtained at pH 3 and at
higher polarizations, corresponds to a transient behavior that is not
predicted by the model. As already seen, the potential in which the
diagram in Fig. 5d was obtained is associated with an interfacial
pH Х 6.5 ͑Fig. 3d͒. This behavior is analogous to those verified in
the diagrams of Fig. 7 obtained at pH 4–6. Moreover, by interfacial
pH measurements performed in this pH range ͑Fig. 3a-c͒, it was
shown that part 2 of the polarization curves, wherein highly efficient
Ni electrodeposition occurs, starts only when the interfacial pH ar-
rives at 6. This means that once this interfacial alkalination level is
attained, the production of an adsorbed intermediate of the type of
Ni͑I͒ suggested by the available model4 is no longer favored.
ads
Consequently, this model, as proposed by Epelboin et al.,4 must be
adjusted to contemplate those changes in the mechanism due to the
interfacial pH behavior. At this pH condition, the Ni͑I͒ kind of
ads
Figure 7. Impedance diagrams obtained in the 1.2 M NiSO4·6H2O solution
at pH 4, 5, and 6 at the beginning of part 2 of the corresponding polarization
curves in Fig. 1 ͑frequencies in Hz͒.
species must have OH− in its composition. Accordingly, the forma-
tion of the following adsorbed species should be considered
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D330
Journal of The Electrochemical Society, 156 ͑8͒ D326-D330 ͑2009͒
k
−1
The interfacial alkalination in part 1 of the curves was basically
+
ads
Ni2+ + H2O + eꢀ͓Ni͑OH͔͒ + H
͓1͔
due to the H+ reduction. In the high slope part of the curves, the
interfacial pH increases during Ni electrodeposition, meaning that
this occurs with a simultaneous H+ consumption. For the solution
pH from 4 to 6, the high efficiency in Ni electrodeposition only
started to occur when the interfacial pH was 6.
k
1
k
−2
+
+
͓Ni͑OH͔͒ + Ni2+ + 2e→͓Ni͑OH͔͒ + Ni
͓2͔
͓3͔
ads
ads
k
−3
Impedance diagrams obtained in the solution with a pH ranging
from 1 to 3 showed an inductive loop at 1 Hz followed by a capaci-
tive loop. In contrast, the diagrams obtained in the pH interval from
4 to 6 presented a distinct behavior, in accordance to the trend veri-
fied from the polarization curves. The diagrams depicted a capaci-
tive loop ͑1–5 Hz͒ followed by an inductive one ͑ϳ0.02 Hz͒ in the
low frequency domain.
From the proposed reaction path, different intermediates, as a
function of the solution pH, are proposed to participate in the pro-
cess. In conformity with the model of Epelboin et al.,4 between pH
+
͓Ni͑OH͔͒ + eꢀ͓Ni͑OH͔͒
ads
ads
k
3
k
−4
͓Ni͑OH͔͒ + Ni2+ + 2e→͓Ni͑OH͔͒ + Ni
In view of Reaction 1, the higher the solution pH, the higher the
rate for the ͓Ni͑OH͔͒ production would be. This means that the
͓4͔
ads
ads
+
ads
energy required for the beginning of a highly efficient metallic depo-
sition would be lower with increasing pH. These considerations give
a suitable explanation for the opposite behavior observed for the pH
dependence of part 2 of the polarization curves obtained in the so-
lution at pH 4–6 as compared to those at pH 1–3. As seen in Fig. 1,
there is a positive shift in part 2 of the polarization curves at pH
4–6, while the opposite is verified for pH 1–3. This means that the
1 and 3, Ni͑I͒ is formed and its production is deactivated with
ads
increasing pH. For the pH range 4–6, Ni electrodeposition takes
+
ads
place through two different intermediates: ͓Ni͑OH͔͒
and
͓Ni͑OH͔͒ads. The increase in both the bulk solution pH and the
interfacial pH supports the formation of these adsorbed species in
the pH range. Because the electrodeposition that occurs by these
intermediates brings about a strong local alkalination, the formation
and precipitation of hydroxide products were considered at the elec-
trode surface. Accordingly, the complexity of the impedance dia-
grams obtained at high polarizations at this pH range is then justi-
fied.
Ni͑I͒ads intermediate previously proposed by Epelboin et al.4 is eas-
+
ads
ily obtained in the presence of H+, whereas ͓Ni͑OH͔͒ ͑Reaction
1͒ can be easily obtained in the presence of OH− for higher pH
values. At a higher pH range, Ni electrodeposition no longer pro-
ceeds through the Ni͑I͒ intermediate, which should be replaced
ads
+
ads
by ͓Ni͑OH͔͒ or ͓Ni͑OH͔͒ads, adsorbed at the electrode surface.
From the impedance diagrams in Fig. 7, it can be suggested that
+
the relaxation of ͓Ni͑OH͔͒ is associated with the capacitive loop
Acknowledgments
ads
at medium frequencies ͑0.5–5.9 Hz͒, which becomes more evident
when the pH increases within the pH 4–6 interval. Although not
shown in this paper, for the solution in such pH range, this capaci-
tive loop becomes bigger with increasing polarization probably as a
result of the higher interface alkalination under these conditions
The authors are grateful to the Brazilian agencies Conselho Na-
cional de Desenvolvimento Científico e Tecnológico, Fundação de
Amparo à Pesquisa do Estado do Rio de Janeiro, Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior, Financiadora de
Estudos e Projetos, and Fundação Universitária José Bonifácio for
their support.
͑Fig. 3a-c͒. The relaxation of ͓Ni͑OH͔͒ can be related to the
ads
inductive loop at around 0.02 Hz. Different from the capacitive loop,
this inductive aspect does not show a potential dependence, as seen
Universidade Federal do Rio de Janeiro assisted in meeting the publica-
tion costs of this article.
in Fig. 7. Accordingly, Ni electrodeposition in the sulfate solution
+
ads
with pH ranging from 4 to 6 would occur by ͓Ni͑OH͔͒ and
References
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Conclusion
The polarization curves for the Ni sulfate acid solutions at the pH
range 1–6 show two distinct regions. The first is particularly poten-
tial dependent and reflects the H+ reduction. Increasing polarization
makes the current vary sharply with potential. Ni electrodeposition
with high efficiency was only detected at this branch of the curves.
Moreover, two different mechanisms as a function of the pH range
were verified. Between solution pH 1 and 3, the increase in pH
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