Electrochemical deposition and characterization of NiFe coatings as electrocatalytic materials for alkaline water electrolysis

Article abstract of DOI:10.1016/j.electacta.2009.01.064

In this study, nickel (Cu/Ni), iron (Cu/Fe) and nickel-iron (Cu/NiFe) composite coatings with various chemical compositions were electrochemically deposited on a copper electrode and characterized using cyclic voltammetry (CV), atomic absorption spectrosc

Full text of DOI:10.1016/j.electacta.2009.01.064

Electrochimica Acta 54 (2009) 3726–3734
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical deposition and characterization of NiFe coatings as
electrocatalytic materials for alkaline water electrolysis
∗
Ramazan Solmaz, Gülfeza Kardas¸
Cukurova University, Science and Letters Faculty, Chemistry Department, 01330, Balcali, Adana, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, nickel (Cu/Ni), iron (Cu/Fe) and nickel–iron (Cu/NiFe) composite coatings with various
chemical compositions were electrochemically deposited on a copper electrode and characterized using
cyclic voltammetry (CV), atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) and
atomic force microscopy (AFM) techniques in view of their possible applications as electrocatalytic mate-
rials for the hydrogen evolution reaction (HER) in an alkaline medium. The electrocatalytic activity of the
coatings for the HER was studied in 1 M KOH solution using cathodic current–potential curves and electro-
chemical impedance spectroscopy (EIS) techniques. The presence of nickel along with iron increases the
electrocatalytic activity of the electrode for the HER when compared to nickel and iron coatings individ-
ually. The HER activity of the composite coatings depends on the chemical composition of the alloys. The
Received 25 November 2008
Received in revised form 22 January 2009
Accepted 22 January 2009
Available online 2 February 2009
Keywords:
NiFe coatings
Electrochemical deposition
Hydrogen evolution reaction (HER)
Atomic force microscopy (AFM)
2
+
2+
Cu/NiFe-3 electrode (with a molar concentration ratio of Ni :Fe of 4:6 in the plating bath) was found to
be the best suitable cathode material for the HER in an alkaline medium under the experimental condi-
tions studied. Furthermore, the electrocatalytic activity of the Cu/NiFe-3 electrode for the HER was tested
for extended periods of time in order to evaluate the change in the electrocatalytic activity of the electrode
with operation time. The HER was activation controlled and has not been changed after electrolysis. A con-
−2
stant current density of 100 mA cm was applied to the electrolysis system, and the corrosion behavior of
the Cu/NiFe-3 electrode was investigated after different operation times using EIS and linear polarization
resistance (LPR) techniques. For comparison, the corrosion behavior of a Cu/NiFe-3 electrode to which
current was not applied was also investigated. The corrosion tests showed that the corrosion resistance
of the Cu/NiFe-3 cathode changed when a cathodic current was applied to the electrolysis system.
©
2009 Elsevier Ltd. All rights reserved.
1
. Introduction
electrode for water electrolysis: a large active surface area, electro-
chemical stability, good electrical conductivity, low overpotential,
selectivity, low cost, and ease of use [8]. The efficiency of the elec-
trode materials can be improved by increasing the ratio between
the real and geometric surface area of the electrode or by a syn-
ergistic combination of electrocatalytic components. However, in
order to be of technological interest, the improved cathode materi-
als with high electrocatalytic activity must have a cost comparable
to that of the traditional materials used in a conventional unipolar
water electrolyzer [7]. In addition to the electrocatalytic activity, the
electrode of choice must also have high corrosion resistance. Dur-
ing shut-down electrolysis, the electrode materials can be corroded
and as a result may lose their activity as well as life time. Because
of that, the determination of the corrosion behavior of electrode
materials is very important.
Hydrogen is considered an ideal energy carrier that can be an
alternative to fossil fuels. It is a clean and fully recyclable substance
with a practically unlimited supply and has all the criteria con-
sidered for an alternative energy source [1–3]. Hydrogen can be
produced in large quantities by water electrolysis. In addition, in
some applications (e.g., fuel cell, steel reel out) hydrogen gas is
needed in a high purity form [4] that can be produced by electrolysis
[
5,6]. However, the cost and energy consumption, which is directly
proportional with cell voltage during the production of hydrogen
by water electrolysis, are currently high. The operational voltage
depends on the overpotentials for the cathodic and anodic reactions
and the internal resistance of the cell [7]. The cost of electrolytic
hydrogen production can be reduced by decreasing the overpoten-
tials of the electrode reactions as well as by selecting inexpensive
electrode materials. The following are the desired properties of an
NiFe binary composite coatings can be easily prepared by the
electrodeposition technique, which has some advantages such as
enhanced control of the alloy composition as well as coating thick-
ness and shape. Some papers have reported the use of NiFe coatings
for the HER in an alkaline medium [9–13]. It has been reported that
nickel–iron binary alloys have better electrocatalytic activity when
∗ Corresponding author. Tel.: +90 322 338 6081; fax: +90 322 338 6070.
E-mail address: gulfeza@cu.edu.tr (G. Karda s¸ ).
0
013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.01.064

R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
3727
compared to both nickel and iron [14]. However, the effects of the
bath composition and the characterization of the NiFe composite
coatings for the HER, as well as the change of electrocatalytic activ-
ity of the electrodes with operation time, have not been studied in
detail. In addition, the corrosion behavior of the electrode materials
in the operation medium has not been reported. The determina-
tion of the corrosion behavior of the prepared electrodes can give
important information for their practical applications.
The aim of this study was the electrochemical preparation and
characterization of NiFe binary composite coatings with various
chemical compositions, as well as their long-term stability and
corrosion behavior, in view of their possible applications as elec-
trocatalytic materials for the HER.
to 0.01 Hz ≤ f ≤ 1 Hz at different overpotentials (the low frequency
was selected depending on the overpotential). The amplitude was
0.005 V. The LPR measurements were carried out by recording a
potential of ± 0.010 V around an open circuit potential at a scan
−
1
rate of 0.001 V s . The cyclic voltammograms were recorded
between the hydrogen and oxygen evolution potential range from
−
1
the negative direction with a scan rate of 0.100 V s
.
The HER activity of the working electrodes was studied in an oxy-
gen free 1 M KOH (Merck) solution, which was prepared by purging
the cell electrolyte with hydrogen gas. All the test solutions were
prepared from analytical grade chemical reagents in distilled water
without further purification. For each experiment, a freshly pre-
pared electrode and solution was used. The solution temperature
was thermostatically controlled by a Nuve BM 100 type thermostat.
The chemical composition of the alloy coatings was determined
with the help of a PerkinElmer Atomic Absorption Spectrophotome-
ter model 3100 (AAS). The surface morphology of the electrodes
was examined by high resolution SEM and AFM techniques. The
SEM images were taken using a Carl Zeiss Evo 40 SEM instrument
at high vacuum and 10 kV EHT. The AFM images were taken with
Park SYSTEMS instrument using non-contact mode.
2. Experimental
The copper electrodes were cut from a cylindrical rod to a length
2
of 5 cm and coated with polyester to a surface area of 0.283 cm .
The electrical conductivity was provided by a copper wire. Before
electrodeposition, the electrode surface was polished with emery
paper (320–1000 grain size), then washed with distilled water, thor-
oughly degreased with acetone, washed once more with distilled
waterand immersed inthe bath solution. The electrodepositionwas
performed galvanostatically using a Potentiostate–galvanostate
instrument (Princeton Applied Research Model 362) with a three-
electrode configuration. A nickel electrode was used as counter
electrode, and a Ag/AgCl electrode was used as the reference elec-
trode. A mild steel anode was used during iron electrodeposition.
The bath compositions were as follows: (a) nickel plating bath: 30%
NiSO ·7H O, 1% NiCl ·6H O, 1.25% H BO (total molar concentra-
3. Results and discussion
3.1. Preparation of the coatings
The chemical and physical properties of the metal coatings
depend on the deposition potential or the deposition current
density, the bath composition, the thickness of the coating, the tem-
perature of the plating bath, the pH of the plating bath, and the
metal ion concentration. In this study, the electrodeposition of all
4
2
2
2
3
3
+
2
tion of Ni was 1.11 M), (b) iron plating bath: 29.69% FeSO ·7H O,
4
2
−2
coatings was achieved by applying a constant 50 mA cm current
+2
0
.8351% FeCl ·4H O, 1.25% H BO (total molar concentration of Fe
2
2
3
3
density at room temperature (∼298 K) under stirring conditions.
Coatings with a 50 ␮m thickness were obtained. In all cases, the
total molar concentration of metal ions was kept at 1.11 M, whereas
was 1.11 M), (c) nickel–iron plating bath: the nickel and iron salts,
which were used in the nickel and iron baths, were fixed in different
molar ratios containing 1.25% H BO , whereas the total molar con-
3
3
2−
the concentration of H BO , SO
4
−
and Cl ions were constant. The
2
+
2+
3
3
centration of Ni and Fe was constant in all plating baths (1.11 M).
pH of the nickel plating bath was 3.5. The iron was electrodeposited
at different pHs. At pH values lower than 2.0, the iron electrode-
position could not be achieved due to excess hydrogen evolution
at the cathode. At pH values higher than 3.0, a black coating with
bad physical properties was formed, and, therefore, all iron-based
deposits were carried out at pH 2.50. Iron-based coatings were
obtained in the literature at similar pH values. The NiFe alloy coat-
ings were electrodeposited at pH values of 2 [9], 3 [16] and 3.5 [10].
The NiFeP coatings were deposited at pH values of 2.4, 2.6 and 2.9
The molar ratio of Ni2+/Fe2+ in the plating bath was 8:2 (NiFe-1), 6:4
(
NiFe-2), 4:6 (NiFe-3) and 2:8 (NiFe-4). The electrodeposition was
−
2
carried out at a constant current density of 50 mA cm at room
temperature with stirring of the bath solution with a magnetic stir-
rer. The thicknesses of the NiFe composite coatings (50 ␮m) were
theoretically calculated by assuming an average alloy density and
average atomic weight [15].
The cathodic current–potential curves, electrochemical
impedance spectroscopy (EIS) and linear polarization resis-
tance (LPR) measurements were carried out using a CHI 604 A.C.
electrochemical analyzer (Serial Number 64721A) under com-
puter control. A double-wall one-compartment cell with a three-
electrode configuration was used. A platinum sheet (with 2 cm2
surface area) and Ag/AgCl (3 M KCl) electrode were used as the aux-
iliary and the reference electrodes, respectively, and all potential
values were referred to this reference electrode. During the polar-
ization and impedance measurements (for the HER activity), the
platinum counter electrode was separated from the main cell com-
partment by a glass tube using Nafion. Before the electrochemical
tests, the working electrode was firstly held at −1.80 V for 30 min
in order to reduce the oxide film existence on the electrode surface
and obtain a reproducible electrode surface. Then the potential
was started from −1.80 V to corresponding zero current potential
[
17]. The CoNiFe and NiCuFe coatings were deposited at pH values
of 3 [14], and 3.2 [18], respectively. The NiP and NiPPt coatings were
deposited at pH 3.0 [19].
3.2. Characterization of the coatings
After deposition, the composite coatings were mechanically
removed from the surface of the electrodes and dissolved in diluted
HNO3 solution. The chemical composition of the alloys was ana-
lyzed by atomic absorption spectroscopy (AAS). The percentage
metal ratios were determined as follows: NiFe-1 (32.9% Ni, 67.1%
Fe), NiFe-2 (16.9% Ni, 83.1% Fe), NiFe-3 (10.5% Ni, 89.5% Fe), and
NiFe-4 (1.9% Ni, 98.1% Fe). The AAS results showed that the chemi-
cal composition of the alloy can be changed by changing the molar
−1
2+
2+
with a scan rate of 0.005 V s . The polarization curves were poten-
tiodynamically obtained in the potential ranges between −1.80 V
and the respective zero current potential. The Tafel curves were
corrected for the IRs drop effect. The uncompensented solution
resistance values were determined from EIS measurements. The
EIS experiments were conducted in the frequency range of 100 kHz
ratio of Ni and Fe in the plating bath. The larger iron ratio in
the deposited coatings was due to the higher deposition rate of the
2+
2+
Fe ions in comparison to that of the Ni ions.
Cyclic voltammetry (CV) is an electrochemical technique suit-
able for the characterization of electrochemically deposited thin
metallic alloys. The distribution of voltammetric peaks at different

3728
R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
Fig. 1. The cyclic voltammograms of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1 (c), Cu/NiFe-2 (d), Cu/NiFe-3 (e) and Cu/NiFe-4 (f) electrodes recorded in 1 M KOH solution at 298 K (scan
−1
rate was 0.100 V s and the potential was started from −1.1 V to positive direction).
potentials is characteristic for each alloy component because the
number and the peak potentials depend only on the alloy struc-
ture [20]. The CVs of the Cu/Ni, Cu/Fe and Cu/NiFe electrodes were
obtained in 1 M KOH solution at 298 K between the hydrogen and
oxygen evolution potential range, and the diagrams obtained are
presented in Fig. 1. The CVs of the metal coatings correspond to the
well-known formation and reduction of the metal hydroxides and
oxides. It is clear from Fig. 1a that two well-defined anodic peaks
Fig. 1b shows the CV of the Cu/Fe electrode, which was charac-
terized by three anodic and two cathodic peaks (A1, A2, A3, C1 and
C2). Peak A1 corresponds to Fe/Fe2+ and A3 to Fe /Fe [25,26], as
2+
3+
given in the following equations:
−
−
Fe + 2OH ↔ Fe(OH) + 2e
(3)
(4)
2
−
−
Fe(OH) + OH ↔ FeO(OH) + H O + e
2
2
The reductions of Fe3+/Fe2+ and Fe2+/Fe were characterized by
C1 and C2, respectively. The peak current densities of the Cu/Fe
electrode were larger than those of the Cu/Ni electrode, which is
probably due to the high oxidation and reduction reaction rates of
iron. From Fig. 1c–f, it can be seen that the CVs of the NiFe binary
coatings show both the behaviors of Ni and Fe, which indicates
the successive codeposition of two metals. At high Fe contents, the
peaks of Ni overlapped with the peaks of Fe due to larger current
densities of iron.
The SEM images of the coated electrodes are given in Fig. 2.
It is clear from Fig. 2a that the nickel coating presents a compact
and porous structure that is distributed homogenously over the
copper electrode. A similar morphology was reported in the
literature [10]. On the contrary, the iron coating (Fig. 2b) showed
a smoother surface with some micro cracks. From the SEM images
of the NiFe binary coatings (Fig. 2c–f), it can be seen that different
(
A1 and A2) and two cathodic peaks (C1 and C2) were observed
in the forward and reverse scans, respectively, for the Cu/Ni elec-
trode. The A1 peak corresponds to Ni/Ni2+ according to the reaction
shown in the following equation [21,22]:
−
−
Ni + 2OH ↔ Ni(OH) + 2e
(1)
2
A1 disappeared in subsequent cycles, as also observed in another
study [23]. The transformation of ␣-Ni(OH)2 to ␤-Ni(OH)2 takes
place between the potential ranges of −0.2 to +0.3 V [22]. The peak
+
3+
(
A2) centered at +0.4 V corresponds to the Ni2 /Ni transitions as
given in the following equation [21–24]:
−
−
Ni(OH) + OH ↔ NiO(OH) + H O + e
(2)
2
2
The cathodic peaks (C1 and C2) correspond to Ni3+/Ni2+ and
Ni2+/Ni, respectively.

R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
3729
Fig. 2. The SEM images of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1 (c), Cu/NiFe-2 (d), Cu/NiFe-3 (e) and Cu/NiFe-4 (f) electrodes.
microstructures and alloy compositions are obtained for the
different compositions.
comparison to the Ni- and Fe-coated copper electrodes, whereas
their activity was dependent upon the composition of the plat-
ing bath. The HER activity increased with higher iron contents,
which can be related to the synergistic interaction between iron and
nickel. The experimental Tafel slopes on binary coatings are approx-
imately the same and not depend on the composition of the alloy
indicating the same mechanism of the HER. Tafel slopes are closed
to 0.120 V dec indicating the Volmer step is rate determining step
[10]. The overpotentials of the NiFe coatings were comparable with
the literature data. The Á100 was found to be between −0.266 and
AFM is a powerful technique used to investigative the surface
morphology at nano- to micro-scale and has become a new choice
to study surface structures. The three-dimensional AFM images of
the coatings are shown in Fig. 3. As can be seen from Fig. 3, porous
deposits were formed on the copper surface. Much deeper pores
were formed at the NiFe-3 binary coating in comparison to that
observed in other coatings.
−
1
3.3. Hydrogen evolution
Table 1
The cathodic current–potential curves of the Cu/Ni, Cu/Fe and
Cu/NiFe electrodes were performed in 1 M KOH solution, and the
recorded data are presented in Fig. 4. The cathodic Tafel slopes
Cathodic Tafel slopes, exchange current densities, overpotentials at 0.1, 1.0, 1.0 and
100 A cm current densities for coated electrodes.
−
2
−bc (V dec−1)
Io (A cm
−2
)
−Á0.1
−Á1
−Á10
−Á100
Working
electrode
(
1
bc), exchange current densities (Io), overpotentials at 0.1, 1, 10 and
(V)
(V)
(V)
(V)
−
00 mA cm current densities (Á01, Á , Á and Á100) were deter-
1 10
2
Cu/Ni
Cu/Fe
Cu/NiFe-1
Cu/NiFe-2
Cu/NiFe-3
Cu/NiFe-4
0.093
0.137
0.112
0.114
0.116
0.115
0.012
0.208
0.381
0.428
1.356
1.307
0.032
0.006
0.002
0.003
0.001
0.001
0.175
0.099
0.057
0.058
0.023
0.022
0.263
0.245
0.165
0.169
0.115
0.116
0.350
0.382
0.291
0.319
0.264
0.265
mined from the corresponding cathodic current–potential curves
and are listed in Table 1. The apparent exchange current densities
were estimated by extrapolating the Tafel lines to the correspond-
ing zero current potentials. It is clear from Table 1 and Fig. 4 that the
HER activity of the NiFe binary coatings was considerably higher in

3730
R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
Fig. 3. The AFM images of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1 (c), Cu/NiFe-2 (d), Cu/NiFe-3 (e) and Cu/NiFe-4 (f) electrodes.
−
0.341 V for NiCo in 6 M KOH [27], between −0.285 and −0.301 V
that alloying the left-hand side transition metals (e.g., Mo, W, Fe,
◦
for the NiZn and −0.268 V for the NiAl in 1 M NaOH at 25 C [28],
etc.) with the right-half transition metals (e.g., Ni, Pd, Pt, Co, etc.)
results in significant changes to their bonding strength and, conse-
quently, increased intermetallic stability, whose maximum usually
coincides with optimal d-electrons for the synergism and maxi-
mal activity in the HER. The authors explained the high efficiency
of the prepared electrodes in an acidic medium by an increase in
the surface roughness and intrinsic activity of a material. Ananth
et al. [32] deposited NiFeZn and NiFe coatings on a mild steel sub-
strate at different current densities and tested the coating for the
HER. It was reported that an increase in the deposition current den-
sity enhanced the H2 evolution at the NiFe binary coating. The
observed results were explained on the basis that iron content,
which increases with the deposition current density, is respon-
sible for the increase in H2 evolution. Hu et al. [9] studied the
◦
−
0.248 V for NiP in 5 M KOH at 25 C [29], −0.173 V for NiFe in 50%
◦
KOH at 70 C [11], and −0.382 for NiP in 30% KOH at 323 K [19].
The Á was found to be between −0.257 and −0.284 V for the NiFe,
1
−
0.065 and −0.075 V for the NiMo, and −0.085 V for the NiW binary
coatings in 0.5 M H SO solution at 298 K [10].
2
4
The HER efficiency of the electrode materials can be explained
by a synergistic combination of the electrocatalytic components
or by increasing the ratio between the real and geometric sur-
face area of the electrode. It has been previously reported [30,31]
bipolar performance of the NiFe binary coatings, deposited at
◦
5
5 C at pH 2, with different compositions by cyclic voltammetry
and polarization methods and suggested that the hydrogen evo-
lution activity of NiFe deposits is controlled by their true surface
area.
EIS is a very sensitive tool for studying electrode reactions on
porous electrodes [20,33]. It has also been proposed as the most
appropriate technique for the determination of the true surface
area in electrochemical systems. This technique is particularly use-
ful in long-term measurements, and it is expected that more reliable
results can be obtained because it does not perturb the structure of
the double layer at the metal/solution interface. In order to obtain
information about the electrocatalytic activity of the coated elec-
trodes, EIS measurements were performed at different cathodic
overpotentials that were previously determined from the cathodic
Fig. 4. Cathodic current–potential curves of working electrodes recorded in 1 M KOH
solution at 298 K.

R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
3731
Fig. 5. The Nyquist plots of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1 (c), Cu/NiFe-2 (d), Cu/NiFe-3 (e) and Cu/NiFe-4 (f) electrodes at −0.200 V overpotential in 1 M KOH solution at
98 K: experimental (frame circle) and fitted results (solid line) (inset shows related Bode diagrams).
2
current–potential curves. The data obtained at −0.200 V of overpo-
tential are presented in Fig. 5 in Nyquist and Bode representations.
As seen from Fig. 5 a slightly depressed capacitive semi-circular
shape was observed in Nyquist plots. At intermediate frequencies,
a linear dependence of log |Z| against log f (inset in Fig. 5) was
observed in Bode plots. The same diagrams were obtained at all
the overpotentials studied. The deviation from ideal semicircle was
related to the surface inhomogeneities of the coatings [7,27]. The
observation of only one loop in the Nyquist plots indicates that
the hydrogen evolution reaction is mainly controlled by a charge
transfer process [7,10].
A single semi-circle was also found by other authors for the
metal alloy electrodes [10,21,27]. However, some authors reported
two time constants for the NiFe binary coatings. Simpraga et al. [12]
studied the HER activity in 4 M KOH solution at a Ni35Fe65 binary
coating that was deposited from an acetate–sulfate bath. Accord-
ing to their experimental results, only one semi-circle appeared
beyond −0.110 V, whereas at low overpotentials (<−0.110 V), where
the adsorption of H intermediates is appreciable and the potential
is dependent, two time constants were observed. Navvaro-Flores
et al. [10] observed two time constants at NiFe coatings with
higher Ni content (Ni80Fe20), but only one time constant at a NiFe
coating with higher iron content (Ni15Fe85) in an acidic environ-
ment.
The double layer can be represented by the electrical equiva-
lent circuit diagrams to model the metal/solution interface. We
have tested three common electrical equivalent circuit diagrams
(one-time constant model, two-time constant parallel model and
two-time constant serial model) that were generally used for the
HER in order to fit the experimental results. It was found that
that the lowest error % values were formed when the one-time
constant model was used. However, in the case of the two-time
constant model, very large errors were formed. Because of that,
the electrical equivalent circuit diagram given in Fig. 6 (one-time
constant model) was used to model the metal/solution interface.
The EIS data were fitted according to Fig. 6, and the calculated
data are given in Table 2. A constant phase element (CPE) was
used in place of a double layer capacitance (Cdl) in order to give
Fig. 6. Schematic representation of the electrical equivalent circuit diagram (Rs:
solution resistance, Rct: charge transfer resistance and CPE: constant phase element).

3732
R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
Table 2
Electrochemical parameters determined from the Nyquist plots at different overpotentials (Á).
Working electrode
Á−0.100 (V)
Rs (ꢀ)
Á−0.200 (V)
Rs (ꢀ)
Á−0.300 (V)
Rs (ꢀ)
CPE (F)
n
Rct (ꢀ)
CPE (F)
n
Rct (ꢀ)
CPE (F)
n
Rct (ꢀ)
Cu/Ni
Cu/Fe
Cu/NiFe-1
Cu/NiFe-2
Cu/NiFe-3
Cu/NiFe-4
6.1
4.1
5.9
5.3
6.0
5.8
0.00086
0.0050
0.0050
0.0027
0.0089
0.0071
0.70
0.61
0.75
0.75
0.63
0.64
6000
81.3
82.1
87.3
34.9
36.2
5.7
4.4
6.0
5.5
6.2
6.3
0.000029
0.0021
0.0019
0.00091
0.0027
0.0020
0.79
0.75
0.80
0.81
0.74
0.78
456
15.9
13.8
14.9
8.8
6.2
4.5
6.0
5.7
6.4
6.3
0.000016
0.0038
0.0015
0.00078
0.0017
0.87
0.71
0.79
0.82
0.79
0.79
68
4.9
5.1
6.2
4.4
4.6
10.1
0.0020
a more accurate fit to the experimental results. Generally, the use
of a CPE is required due to the distribution of the relaxation times
as a result of inhomogeneities present at a micro- or nano-level,
such as the surface roughness/porosity, adsorption, or diffusion
equivalent circuit diagram given in Fig. 6 was used to model the
metal/solution interface. The presence of only one loop in the
Nyquist plots indicates that the hydrogen evolution reaction is
mainly controlled by a charge transfer process both after 24 and
120 h of electrolysis, and the reaction mechanism does not change
over electrolysis. By comparing the polarization resistance that was
obtained before starting the electrolysis (Table 2) at an overpo-
tential of −0.300 V, the polarization resistance was reduced from
4.4 to 3.85 ꢀ after 24 h of electrolysis. The decreasing polarization
resistance can be related to the activation of the electrode during
hydrogen gas evolution and the removal of any existing corrosion
products from the pores. After longer operation times, a slight
increase in the polarization resistance was determined (5.52 ꢀ).
The surface SEM image of Cu/NiFe-3 after 120 h of operation was
obtained and is given in Fig. 8b. For comparison the SEM micrograph
[
10]. From Fig. 5 and Table 2, it can be seen that the charge trans-
fer resistance of the binary coatings was reduced with increasing
iron content of the coatings. The Cu/NiFe-3 electrode has the lowest
charge transfer resistance, which indicates the highest electrocat-
alytic activity for the HER. This result is in good agreement with
the results of the cathodic current–potential curves. The parame-
ter n, generally accepted to be a measure of surface inhomogeneity
[34], was lower than 1.0, which indicates that the deposited coat-
ings had a porous structure. The double layer capacitance increase
in the case of the binary coatings was related to the onset of the
Faradaic reaction of the HER, which indicates the increasing electro-
catalytic activity of the coatings [7,35]. However, the double layer
capacitance values for all the coatings decreased with increasing
overpotential, which indicates a blockage of the surface, most likely
by adsorbed hydrogen [10,14,36]. The highest double layer capaci-
tance was determined at the Cu/NiFe-3 electrode, which has better
electrocatalytic activity for the HER.
The physical and electrochemical stability of the electrode mate-
rials over long periods of time is important for their practical
applications. The electrochemical stability of the coatings was eval-
uated by continuous operation tests in 1 M KOH solution at room
temperature for up to 120 h by applying a constant current density
−2
of 100 mA cm to the electrolysis system, in which the Cu/NiFe-
electrode was the cathode and Pt was the anode. After different
3
operation times, electrolysis was stopped, and impedance measure-
ments were carried out at an overpotential of −0.300 in order to
evaluate HER performances of the working electrode with opera-
tion time. The Nyquist plots at an overpotential of −0.300 V after
24 and 120 h of electrolysis are shown in Fig. 7. The electrical
Fig. 7. The Nyquist plots of Cu/NiFe-3 electrode at −0.300 V overpotential after 24 h
᭹) and 120 h (ꢀ) of electrolysis time.
Fig. 8. SEM image of the freshly prepared NiFe-3 coating (a) and after 120 h elec-
trolysis (b).
(

R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
3733
Fig. 9. The Nyquist (a) and Bode (b) plots of Cu/NiFe-3 electrode obtained after 24 h (solid circle) and 120 h (frame circle) immersion times in 1 M KOH solution.
of the freshly prepared electrode with the same magnification was
also given in Fig. 8a. As can be seen from Fig. 8 both freshly deposited
and aged electrodes have a compact and porous structures. The
difference in surface structure of aged electrode can be related to
activation of the electrode during the hydrogen gas evolution and
removal of any existence corrosion products from the pores.
3.4. Corrosion behavior of the NiFe binary coating
The Nyquist and Bode plots are given in Fig. 9a and b for the
Cu/NiFe-3 electrode in 1 M KOH solution at room temperature after
4 and 120 h of exposure (at open circuit conditions). As seen from
2
Fig. 9, only one loop with a straight line and one time constant
were observed in the Nyquist and Bode (log f–log Z) plots after 24 h
of exposure. After 120 h, a capacitive loop appeared at high fre-
quencies. The appearance of the first loop at high frequencies was
attributed to the charge transfer resistance, which corresponds to
metal dissolution under the coating. The second straight line at
low frequencies was related to the diffusion process. The straight
line at low frequencies indicates that the corrosion reaction of the
Cu/NiFe-3 electrode is diffusion controlled. The polarization resis-
tance of the Cu/NiFe-3 electrode in 1 M KOH solution was also
determined from the LPR technique and found to be 2800 and
Fig. 10. The SEM image of the NiFe-3 coating which was exposed 1 M KOH over
120 h.
The surface SEM image of the Cu/NiFe-3 electrode after 120 h of
exposure is given in Fig. 10. As can be seen from Fig. 10, uniform,
star-shaped grains that homogenously distributed over the surface
were observed. A compact and adherence surface layer can be seen
from the surface image. Based on the SEM image, the surface layer
contributes better corrosion resistance to the electrode.
3
290 ꢀ after 24 and 120 h of exposure, respectively. The increas-
ing corrosion resistance with exposure time may be related to the
growth of protective oxide and/or hydroxide products of nickel
(
NiO, Ni(OH) , NiOOH) and iron (Fe O , Fe O , FeOOH) and a pro-
2 3 4 2 3
tective barrier layer during immersion. The corrosion resistance of
nickel was reported in the literature to be improved by the rapid for-
mation of continuous Ni(OH)2 [37] and NiO [38] protective films at
surface crystalline defects and within the pores during immersion.
The open circuit potential of the Cu/NiFe-3 electrode was −0.765
and −0.238 V after 24 and 120 h of exposure, respectively. The shift
in open circuit potential in a nobler direction is due to the formation
of a passive oxide layer over the electrode surface.
In order to investigate the effect of electrolysis on the corrosion
behavior of the Cu/NiFe-3 electrode, a constant current density of
−
2
100 mA cm was applied to the electrolysis system, in which the
working electrode was the cathode and Pt was the counter elec-
trode. Electrolysis was stopped after different operation times (an
open circuit condition was imposed to the system), and the cor-
rosion behavior of the coated electrode was determined using EIS
and LPR techniques after the open circuit potential was reached.
−
2
Fig. 11. The Nyquist (a) and Bode (b) plots of Cu/NiFe-3 electrode at open circuit potential which a constant 100 mA cm cathodic current density was applied over 24 h
solid circle) and 120 h (frame circle) in 1 M KOH solution.
(

3734
R. Solmaz, G. Karda s¸ / Electrochimica Acta 54 (2009) 3726–3734
The open circuit potential of the coated electrode was −1.020 and
1.006 V after 24 and 120 h, respectively. The Nyquist and Bode
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. Conclusions
4
[
In this study, nickel, iron and nickel–iron composite coat-
2
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deposited on a copper electrode and characterized by different
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This study has been financially supported by a Cukurova Univer-
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