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Electrochimica Acta 54 (2009) 3726–3734

Contents lists available at ScienceDirect

Electrochimica Acta

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lectrochemical deposition and characterization of NiFe coatings aslectrocatalytic materials for alkaline water electrolysis

amazan Solmaz, Gülfeza Kardaş ∗

ukurova University, Science and Letters Faculty, Chemistry Department, 01330, Balcali, Adana, Turkey

r t i c l e i n f o

rticle history:eceived 25 November 2008eceived in revised form 22 January 2009ccepted 22 January 2009vailable online 2 February 2009

eywords:iFe coatingslectrochemical depositionydrogen evolution reaction (HER)tomic force microscopy (AFM)

a b s t r a c t

In this study, nickel (Cu/Ni), iron (Cu/Fe) and nickel–iron (Cu/NiFe) composite coatings with variouschemical compositions were electrochemically deposited on a copper electrode and characterized usingcyclic voltammetry (CV), atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) andatomic 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 thecoatings 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 theelectrocatalytic 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. TheCu/NiFe-3 electrode (with a molar concentration ratio of Ni2+:Fe2+ of 4:6 in the plating bath) was found tobe 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 testedfor extended periods of time in order to evaluate the change in the electrocatalytic activity of the electrodewith operation time. The HER was activation controlled and has not been changed after electrolysis. A con-stant current density of 100 mA cm−2 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 polarizationresistance (LPR) techniques. For comparison, the corrosion behavior of a Cu/NiFe-3 electrode to whichcurrent was not applied was also investigated. The corrosion tests showed that the corrosion resistance

chan

of the Cu/NiFe-3 cathode

. Introduction

Hydrogen is considered an ideal energy carrier that can be anlternative to fossil fuels. It is a clean and fully recyclable substanceith a practically unlimited supply and has all the criteria con-

idered for an alternative energy source [1–3]. Hydrogen can beroduced in large quantities by water electrolysis. In addition, inome applications (e.g., fuel cell, steel reel out) hydrogen gas iseeded in a high purity form [4] that can be produced by electrolysis5,6]. However, the cost and energy consumption, which is directlyroportional with cell voltage during the production of hydrogeny water electrolysis, are currently high. The operational voltageepends on the overpotentials for the cathodic and anodic reactions

nd the internal resistance of the cell [7]. The cost of electrolyticydrogen production can be reduced by decreasing the overpoten-ials of the electrode reactions as well as by selecting inexpensivelectrode materials. The following are the desired properties of an

∗ Corresponding author. Tel.: +90 322 338 6081; fax: +90 322 338 6070.E-mail address: gulfeza@cu.edu.tr (G. Kardaş).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.01.064

ged when a cathodic current was applied to the electrolysis system.© 2009 Elsevier Ltd. All rights reserved.

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 betweenthe real and geometric surface area of the electrode or by a syn-ergistic combination of electrocatalytic components. However, inorder to be of technological interest, the improved cathode materi-als with high electrocatalytic activity must have a cost comparableto that of the traditional materials used in a conventional unipolarwater electrolyzer [7]. In addition to the electrocatalytic activity, theelectrode of choice must also have high corrosion resistance. Dur-ing shut-down electrolysis, the electrode materials can be corrodedand as a result may lose their activity as well as life time. Becauseof that, the determination of the corrosion behavior of electrodematerials is very important.

NiFe binary composite coatings can be easily prepared by the

electrodeposition technique, which has some advantages such asenhanced control of the alloy composition as well as coating thick-ness and shape. Some papers have reported the use of NiFe coatingsfor the HER in an alkaline medium [9–13]. It has been reported thatnickel–iron binary alloys have better electrocatalytic activity when

http://www.sciencedirect.com/science/journal/00134686http://www.elsevier.com/locate/electactamailto:gulfeza@cu.edu.trdx.doi.org/10.1016/j.electacta.2009.01.064

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R. Solmaz, G. Kardaş / Electro

ompared to both nickel and iron [14]. However, the effects of theath composition and the characterization of the NiFe compositeoatings for the HER, as well as the change of electrocatalytic activ-ty of the electrodes with operation time, have not been studied inetail. In addition, the corrosion behavior of the electrode materials

n the operation medium has not been reported. The determina-ion of the corrosion behavior of the prepared electrodes can givemportant information for their practical applications.

The aim of this study was the electrochemical preparation andharacterization of NiFe binary composite coatings with varioushemical compositions, as well as their long-term stability andorrosion behavior, in view of their possible applications as elec-rocatalytic materials for the HER.

. Experimental

The copper electrodes were cut from a cylindrical rod to a lengthf 5 cm and coated with polyester to a surface area of 0.283 cm2.he electrical conductivity was provided by a copper wire. Beforelectrodeposition, the electrode surface was polished with emeryaper (320–1000 grain size), then washed with distilled water, thor-ughly degreased with acetone, washed once more with distilledater and immersed in the bath solution. The electrodeposition waserformed galvanostatically using a Potentiostate–galvanostate

nstrument (Princeton Applied Research Model 362) with a three-lectrode configuration. A nickel electrode was used as counterlectrode, and a Ag/AgCl electrode was used as the reference elec-rode. A mild steel anode was used during iron electrodeposition.he bath compositions were as follows: (a) nickel plating bath: 30%iSO4·7H2O, 1% NiCl2·6H2O, 1.25% H3BO3 (total molar concentra-

ion of Ni+2 was 1.11 M), (b) iron plating bath: 29.69% FeSO4·7H2O,.8351% FeCl2·4H2O, 1.25% H3BO3 (total molar concentration of Fe+2as 1.11 M), (c) nickel–iron plating bath: the nickel and iron salts,hich were used in the nickel and iron baths, were fixed in differentolar ratios containing 1.25% H3BO3, whereas the total molar con-

entration of Ni2+ and Fe2+ was constant in all plating baths (1.11 M).he molar ratio of Ni2+/Fe2+ in the plating bath was 8:2 (NiFe-1), 6:4NiFe-2), 4:6 (NiFe-3) and 2:8 (NiFe-4). The electrodeposition wasarried out at a constant current density of 50 mA cm−2 at roomemperature with stirring of the bath solution with a magnetic stir-er. The thicknesses of the NiFe composite coatings (50 �m) wereheoretically calculated by assuming an average alloy density andverage atomic weight [15].

The cathodic current–potential curves, electrochemicalmpedance spectroscopy (EIS) and linear polarization resis-ance (LPR) measurements were carried out using a CHI 604 A.C.lectrochemical analyzer (Serial Number 64721A) under com-uter control. A double-wall one-compartment cell with a three-lectrode configuration was used. A platinum sheet (with 2 cm2

urface area) and Ag/AgCl (3 M KCl) electrode were used as the aux-liary and the reference electrodes, respectively, and all potentialalues were referred to this reference electrode. During the polar-zation and impedance measurements (for the HER activity), thelatinum counter electrode was separated from the main cell com-artment by a glass tube using Nafion. Before the electrochemicalests, the working electrode was firstly held at −1.80 V for 30 minn order to reduce the oxide film existence on the electrode surfacend obtain a reproducible electrode surface. Then the potentialas started from −1.80 V to corresponding zero current potentialith a scan rate of 0.005 V s−1. The polarization curves were poten-

iodynamically obtained in the potential ranges between −1.80 Vnd the respective zero current potential. The Tafel curves wereorrected for the IRs drop effect. The uncompensented solutionesistance values were determined from EIS measurements. TheIS experiments were conducted in the frequency range of 100 kHz

a Acta 54 (2009) 3726–3734 3727

to 0.01 Hz ≤ f ≤ 1 Hz at different overpotentials (the low frequencywas selected depending on the overpotential). The amplitude was0.005 V. The LPR measurements were carried out by recording apotential of ±0.010 V around an open circuit potential at a scanrate of 0.001 V s−1. The cyclic voltammograms were recordedbetween the hydrogen and oxygen evolution potential range fromthe negative direction with a scan rate of 0.100 V s−1.

The HER activity of the working electrodes was studied in an oxy-gen free 1 M KOH (Merck) solution, which was prepared by purgingthe cell electrolyte with hydrogen gas. All the test solutions wereprepared from analytical grade chemical reagents in distilled waterwithout further purification. For each experiment, a freshly pre-pared electrode and solution was used. The solution temperaturewas thermostatically controlled by a Nuve BM 100 type thermostat.

The chemical composition of the alloy coatings was determinedwith the help of a PerkinElmer Atomic Absorption Spectrophotome-ter model 3100 (AAS). The surface morphology of the electrodeswas examined by high resolution SEM and AFM techniques. TheSEM images were taken using a Carl Zeiss Evo 40 SEM instrumentat high vacuum and 10 kV EHT. The AFM images were taken withPark SYSTEMS instrument using non-contact mode.

3. Results and discussion

3.1. Preparation of the coatings

The chemical and physical properties of the metal coatingsdepend on the deposition potential or the deposition currentdensity, the bath composition, the thickness of the coating, the tem-perature of the plating bath, the pH of the plating bath, and themetal ion concentration. In this study, the electrodeposition of allcoatings was achieved by applying a constant 50 mA cm−2 currentdensity at room temperature (∼298 K) under stirring conditions.Coatings with a 50 �m thickness were obtained. In all cases, thetotal molar concentration of metal ions was kept at 1.11 M, whereasthe concentration of H3BO3, SO42− and Cl− ions were constant. ThepH of the nickel plating bath was 3.5. The iron was electrodepositedat different pHs. At pH values lower than 2.0, the iron electrode-position could not be achieved due to excess hydrogen evolutionat the cathode. At pH values higher than 3.0, a black coating withbad physical properties was formed, and, therefore, all iron-baseddeposits were carried out at pH 2.50. Iron-based coatings wereobtained 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[17]. The CoNiFe and NiCuFe coatings were deposited at pH valuesof 3 [14], and 3.2 [18], respectively. The NiP and NiPPt coatings weredeposited at pH 3.0 [19].

3.2. Characterization of the coatings

After deposition, the composite coatings were mechanicallyremoved from the surface of the electrodes and dissolved in dilutedHNO3 solution. The chemical composition of the alloys was ana-lyzed by atomic absorption spectroscopy (AAS). The percentagemetal 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), andNiFe-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 molarratio of Ni2+ and Fe2+ in the plating bath. The larger iron ratio in

the deposited coatings was due to the higher deposition rate of theFe2+ ions in comparison to that of the Ni2+ ions.

Cyclic voltammetry (CV) is an electrochemical technique suit-able for the characterization of electrochemically deposited thinmetallic alloys. The distribution of voltammetric peaks at different

3728 R. Solmaz, G. Kardaş / Electrochimica Acta 54 (2009) 3726–3734

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ig. 1. The cyclic voltammograms of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1 (c), Cu/NiFe-2 (date was 0.100 V s−1 and the potential was started from −1.1 V to positive direction)

otentials is characteristic for each alloy component because theumber and the peak potentials depend only on the alloy struc-ure [20]. The CVs of the Cu/Ni, Cu/Fe and Cu/NiFe electrodes werebtained in 1 M KOH solution at 298 K between the hydrogen andxygen evolution potential range, and the diagrams obtained areresented in Fig. 1. The CVs of the metal coatings correspond to theell-known formation and reduction of the metal hydroxides and

xides. It is clear from Fig. 1a that two well-defined anodic peaksA1 and A2) and two cathodic peaks (C1 and C2) were observedn the forward and reverse scans, respectively, for the Cu/Ni elec-rode. The A1 peak corresponds to Ni/Ni2+ according to the reactionhown in the following equation [21,22]:

i + 2OH− ↔ Ni(OH)2 + 2e− (1)

A1 disappeared in subsequent cycles, as also observed in anothertudy [23]. The transformation of �-Ni(OH)2 to �-Ni(OH)2 takeslace between the potential ranges of −0.2 to +0.3 V [22]. The peakA2) centered at +0.4 V corresponds to the Ni2+/Ni3+ transitions as

iven in the following equation [21–24]:

i(OH)2 + OH− ↔ NiO(OH) + H2O + e− (2)

The cathodic peaks (C1 and C2) correspond to Ni3+/Ni2+ andi2+/Ni, respectively.

NiFe-3 (e) and Cu/NiFe-4 (f) electrodes recorded in 1 M KOH solution at 298 K (scan

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 andC2). Peak A1 corresponds to Fe/Fe2+ and A3 to Fe2+/Fe3+ [25,26], asgiven in the following equations:

Fe + 2OH− ↔ Fe(OH)2 + 2e− (3)Fe(OH)2 + OH− ↔ FeO(OH) + H2O + e− (4)

The reductions of Fe3+/Fe2+ and Fe2+/Fe were characterized byC1 and C2, respectively. The peak current densities of the Cu/Feelectrode were larger than those of the Cu/Ni electrode, which isprobably due to the high oxidation and reduction reaction rates ofiron. From Fig. 1c–f, it can be seen that the CVs of the NiFe binarycoatings show both the behaviors of Ni and Fe, which indicatesthe successive codeposition of two metals. At high Fe contents, thepeaks of Ni overlapped with the peaks of Fe due to larger currentdensities 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 thecopper electrode. A similar morphology was reported in theliterature [10]. On the contrary, the iron coating (Fig. 2b) showeda smoother surface with some micro cracks. From the SEM imagesof the NiFe binary coatings (Fig. 2c–f), it can be seen that different

R. Solmaz, G. Kardaş / Electrochimica Acta 54 (2009) 3726–3734 3729

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indicating the same mechanism of the HER. Tafel slopes are closedto 0.120 V dec−1 indicating the Volmer step is rate determining step[10]. The overpotentials of the NiFe coatings were comparable withthe literature data. The �100 was found to be between −0.266 and

Table 1Cathodic Tafel slopes, exchange current densities, overpotentials at 0.1, 1.0, 1.0 and100 A cm−2 current densities for coated electrodes.

Workingelectrode

−bc (V dec−1) Io (A cm−2) −�0.1(V)

−�1(V)

−�10(V)

−�100(V)

Cu/Ni 0.093 0.012 0.032 0.175 0.263 0.350

Fig. 2. The SEM images of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1

icrostructures and alloy compositions are obtained for theifferent compositions.

AFM is a powerful technique used to investigative the surfaceorphology at nano- to micro-scale and has become a new choice

o study surface structures. The three-dimensional AFM images ofhe coatings are shown in Fig. 3. As can be seen from Fig. 3, porouseposits were formed on the copper surface. Much deeper poresere formed at the NiFe-3 binary coating in comparison to that

bserved in other coatings.

.3. Hydrogen evolution

The cathodic current–potential curves of the Cu/Ni, Cu/Fe andu/NiFe electrodes were performed in 1 M KOH solution, and theecorded data are presented in Fig. 4. The cathodic Tafel slopesbc), exchange current densities (Io), overpotentials at 0.1, 1, 10 and00 mA cm−2 current densities (�01, �1, �10 and �100) were deter-

ined from the corresponding cathodic current–potential curves

nd are listed in Table 1. The apparent exchange current densitiesere estimated by extrapolating the Tafel lines to the correspond-

ng zero current potentials. It is clear from Table 1 and Fig. 4 that theER activity of the NiFe binary coatings was considerably higher in

u/NiFe-2 (d), Cu/NiFe-3 (e) and Cu/NiFe-4 (f) electrodes.

comparison to the Ni- and Fe-coated copper electrodes, whereastheir 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 andnickel. The experimental Tafel slopes on binary coatings are approx-imately the same and not depend on the composition of the alloy

Cu/Fe 0.137 0.208 0.006 0.099 0.245 0.382Cu/NiFe-1 0.112 0.381 0.002 0.057 0.165 0.291Cu/NiFe-2 0.114 0.428 0.003 0.058 0.169 0.319Cu/NiFe-3 0.116 1.356 0.001 0.023 0.115 0.264Cu/NiFe-4 0.115 1.307 0.001 0.022 0.116 0.265

3730 R. Solmaz, G. Kardaş / Electrochimica Acta 54 (2009) 3726–3734

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Fig. 3. The AFM images of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1

0.341 V for NiCo in 6 M KOH [27], between −0.285 and −0.301 Vor the NiZn and −0.268 V for the NiAl in 1 M NaOH at 25 ◦C [28],0.248 V for NiP in 5 M KOH at 25 ◦C [29], −0.173 V for NiFe in 50%OH at 70 ◦C [11], and −0.382 for NiP in 30% KOH at 323 K [19].he �1 was found to be between −0.257 and −0.284 V for the NiFe,0.065 and −0.075 V for the NiMo, and −0.085 V for the NiW binary

oatings in 0.5 M H2SO4 solution at 298 K [10].

The HER efficiency of the electrode materials can be explained

y a synergistic combination of the electrocatalytic componentsr by increasing the ratio between the real and geometric sur-ace area of the electrode. It has been previously reported [30,31]

ig. 4. Cathodic current–potential curves of working electrodes recorded in 1 M KOHolution at 298 K.

u/NiFe-2 (d), Cu/NiFe-3 (e) and Cu/NiFe-4 (f) electrodes.

that alloying the left-hand side transition metals (e.g., Mo, W, Fe,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 usuallycoincides with optimal d-electrons for the synergism and maxi-mal activity in the HER. The authors explained the high efficiencyof the prepared electrodes in an acidic medium by an increase inthe surface roughness and intrinsic activity of a material. Ananthet al. [32] deposited NiFeZn and NiFe coatings on a mild steel sub-strate at different current densities and tested the coating for theHER. It was reported that an increase in the deposition current den-sity enhanced the H2 evolution at the NiFe binary coating. Theobserved 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 thebipolar performance of the NiFe binary coatings, deposited at55 ◦C at pH 2, with different compositions by cyclic voltammetryand polarization methods and suggested that the hydrogen evo-lution activity of NiFe deposits is controlled by their true surfacearea.

EIS is a very sensitive tool for studying electrode reactions onporous electrodes [20,33]. It has also been proposed as the mostappropriate technique for the determination of the true surfacearea 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 ofthe double layer at the metal/solution interface. In order to obtaininformation about the electrocatalytic activity of the coated elec-trodes, EIS measurements were performed at different cathodicoverpotentials that were previously determined from the cathodic

R. Solmaz, G. Kardaş / Electrochimica Acta 54 (2009) 3726–3734 3731

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the electrical equivalent circuit diagram given in Fig. 6 (one-timeconstant model) was used to model the metal/solution interface.The EIS data were fitted according to Fig. 6, and the calculateddata are given in Table 2. A constant phase element (CPE) wasused in place of a double layer capacitance (Cdl) in order to give

ig. 5. The Nyquist plots of Cu/Ni (a), Cu/Fe (b), Cu/NiFe-1 (c), Cu/NiFe-2 (d), Cu/N98 K: experimental (frame circle) and fitted results (solid line) (inset shows related

urrent–potential curves. The data obtained at −0.200 V of overpo-ential are presented in Fig. 5 in Nyquist and Bode representations.s seen from Fig. 5 a slightly depressed capacitive semi-circularhape was observed in Nyquist plots. At intermediate frequencies,linear dependence of log |Z| against log f (inset in Fig. 5) was

bserved in Bode plots. The same diagrams were obtained at allhe overpotentials studied. The deviation from ideal semicircle waselated to the surface inhomogeneities of the coatings [7,27]. Thebservation of only one loop in the Nyquist plots indicates thathe hydrogen evolution reaction is mainly controlled by a chargeransfer process [7,10].

A single semi-circle was also found by other authors for theetal alloy electrodes [10,21,27]. However, some authors reported

wo time constants for the NiFe binary coatings. Simpraga et al. [12]tudied the HER activity in 4 M KOH solution at a Ni35Fe65 binaryoating that was deposited from an acetate–sulfate bath. Accord-ng to their experimental results, only one semi-circle appearedeyond −0.110 V, whereas at low overpotentials (

3732 R. Solmaz, G. Kardaş / Electrochimica Acta 54 (2009) 3726–3734

Table 2Electrochemical parameters determined from the Nyquist plots at different overpotentials (�).

Working electrode �−0.100 (V) �−0.200 (V) �−0.300 (V)

Rs (�) CPE (F) n Rct (�) Rs (�) CPE (F) n Rct (�) Rs (�) CPE (F) n Rct (�)

Cu/Ni 6.1 0.00086 0.70 6000 5.7 0.000029 0.79 456 6.2 0.000016 0.87 68Cu/Fe 4.1 0.0050 0.61 81.3 4.4 0.0021 0.75 15.9 4.5 0.0038 0.71 4.9C 0.0C 0.0C 0.0C 0.0

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products from the pores. After longer operation times, a slightincrease in the polarization resistance was determined (5.52 �).The surface SEM image of Cu/NiFe-3 after 120 h of operation wasobtained and is given in Fig. 8b. For comparison the SEM micrograph

u/NiFe-1 5.9 0.0050 0.75 82.1 6.0u/NiFe-2 5.3 0.0027 0.75 87.3 5.5u/NiFe-3 6.0 0.0089 0.63 34.9 6.2u/NiFe-4 5.8 0.0071 0.64 36.2 6.3

more accurate fit to the experimental results. Generally, the usef a CPE is required due to the distribution of the relaxation timess a result of inhomogeneities present at a micro- or nano-level,uch as the surface roughness/porosity, adsorption, or diffusion10]. From Fig. 5 and Table 2, it can be seen that the charge trans-er resistance of the binary coatings was reduced with increasingron content of the coatings. The Cu/NiFe-3 electrode has the lowestharge transfer resistance, which indicates the highest electrocat-lytic activity for the HER. This result is in good agreement withhe results of the cathodic current–potential curves. The parame-er n, generally accepted to be a measure of surface inhomogeneity34], was lower than 1.0, which indicates that the deposited coat-ngs had a porous structure. The double layer capacitance increasen the case of the binary coatings was related to the onset of thearadaic reaction of the HER, which indicates the increasing electro-atalytic activity of the coatings [7,35]. However, the double layerapacitance values for all the coatings decreased with increasingverpotential, which indicates a blockage of the surface, most likelyy adsorbed hydrogen [10,14,36]. The highest double layer capaci-ance was determined at the Cu/NiFe-3 electrode, which has betterlectrocatalytic activity for the HER.

The physical and electrochemical stability of the electrode mate-ials over long periods of time is important for their practicalpplications. The electrochemical stability of the coatings was eval-ated by continuous operation tests in 1 M KOH solution at roomemperature for up to 120 h by applying a constant current densityf 100 mA cm−2 to the electrolysis system, in which the Cu/NiFe-electrode was the cathode and Pt was the anode. After different

peration times, electrolysis was stopped, and impedance measure-

ents were carried out at an overpotential of −0.300 in order to

valuate HER performances of the working electrode with opera-ion time. The Nyquist plots at an overpotential of −0.300 V after4 and 120 h of electrolysis are shown in Fig. 7. The electrical

ig. 7. The Nyquist plots of Cu/NiFe-3 electrode at −0.300 V overpotential after 24 h�) and 120 h (©) of electrolysis time.

019 0.80 13.8 6.0 0.0015 0.79 5.10091 0.81 14.9 5.7 0.00078 0.82 6.2027 0.74 8.8 6.4 0.0017 0.79 4.4020 0.78 10.1 6.3 0.0020 0.79 4.6

equivalent circuit diagram given in Fig. 6 was used to model themetal/solution interface. The presence of only one loop in theNyquist plots indicates that the hydrogen evolution reaction ismainly controlled by a charge transfer process both after 24 and120 h of electrolysis, and the reaction mechanism does not changeover electrolysis. By comparing the polarization resistance that wasobtained before starting the electrolysis (Table 2) at an overpo-tential of −0.300 V, the polarization resistance was reduced from4.4 to 3.85 � after 24 h of electrolysis. The decreasing polarizationresistance can be related to the activation of the electrode duringhydrogen gas evolution and the removal of any existing corrosion

Fig. 8. SEM image of the freshly prepared NiFe-3 coating (a) and after 120 h elec-trolysis (b).

R. Solmaz, G. Kardaş / Electrochimica Acta 54 (2009) 3726–3734 3733

er 24 h (solid circle) and 120 h (frame circle) immersion times in 1 M KOH solution.

oaadar

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Fig. 9. The Nyquist (a) and Bode (b) plots of Cu/NiFe-3 electrode obtained aft

f the freshly prepared electrode with the same magnification waslso given in Fig. 8a. As can be seen from Fig. 8 both freshly depositednd aged electrodes have a compact and porous structures. Theifference in surface structure of aged electrode can be related toctivation of the electrode during the hydrogen gas evolution andemoval of any existence corrosion products from the pores.

.4. Corrosion behavior of the NiFe binary coating

The Nyquist and Bode plots are given in Fig. 9a and b for theu/NiFe-3 electrode in 1 M KOH solution at room temperature after4 and 120 h of exposure (at open circuit conditions). As seen fromig. 9, only one loop with a straight line and one time constantere observed in the Nyquist and Bode (log f–log Z) plots after 24 h

f exposure. After 120 h, a capacitive loop appeared at high fre-uencies. The appearance of the first loop at high frequencies wasttributed to the charge transfer resistance, which corresponds toetal dissolution under the coating. The second straight line at

ow frequencies was related to the diffusion process. The straightine at low frequencies indicates that the corrosion reaction of theu/NiFe-3 electrode is diffusion controlled. The polarization resis-ance of the Cu/NiFe-3 electrode in 1 M KOH solution was alsoetermined from the LPR technique and found to be 2800 and290 � after 24 and 120 h of exposure, respectively. The increas-

ng corrosion resistance with exposure time may be related to therowth of protective oxide and/or hydroxide products of nickelNiO, Ni(OH)2, NiOOH) and iron (Fe3O4, Fe2O3, FeOOH) and a pro-ective barrier layer during immersion. The corrosion resistance ofickel was reported in the literature to be improved by the rapid for-ation of continuous Ni(OH)2 [37] and NiO [38] protective films at

urface crystalline defects and within the pores during immersion.he open circuit potential of the Cu/NiFe-3 electrode was −0.765nd −0.238 V after 24 and 120 h of exposure, respectively. The shiftn open circuit potential in a nobler direction is due to the formationf a passive oxide layer over the electrode surface.

ig. 11. The Nyquist (a) and Bode (b) plots of Cu/NiFe-3 electrode at open circuit potentsolid circle) and 120 h (frame circle) in 1 M KOH solution.

Fig. 10. The SEM image of the NiFe-3 coating which was exposed 1 M KOH over120 h.

The surface SEM image of the Cu/NiFe-3 electrode after 120 h ofexposure is given in Fig. 10. As can be seen from Fig. 10, uniform,star-shaped grains that homogenously distributed over the surfacewere observed. A compact and adherence surface layer can be seenfrom the surface image. Based on the SEM image, the surface layercontributes better corrosion resistance to the electrode.

In order to investigate the effect of electrolysis on the corrosionbehavior of the Cu/NiFe-3 electrode, a constant current density of100 mA cm−2 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 (anopen circuit condition was imposed to the system), and the cor-rosion behavior of the coated electrode was determined using EISand LPR techniques after the open circuit potential was reached.

ial which a constant 100 mA cm−2 cathodic current density was applied over 24 h

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he open circuit potential of the coated electrode was −1.020 and1.006 V after 24 and 120 h, respectively. The Nyquist and Bodelots of the Cu/NiFe-3 electrode obtained after 24 and 120 h oflectrolysis are given in Fig. 11a and b, respectively. As can be seenrom Fig. 11a, the Nyquist plots contain a capacitive loop at a highrequency region and a second loop with a straight line at a low fre-uency region. Two time constants were observed in the Bode plotFig. 11b). Such straight lines at low frequencies indicate a diffusion-ontrolled corrosion mechanism [37]. The polarization resistancesf the coated electrode after 24 and 120 h of electrolysis wereetermined from LPR measurements to be 60 and 105 �, respec-ively. When compared to the polarization resistance of Cu/NiFe-3o which no current was applied, the corrosion resistance of theoated electrode was sharply reduced after electrolysis due to thebsence of any corrosion products of nickel and iron over the surfacef the electrode, as can be clearly seen from the SEM image (Fig. 8b).

. Conclusions

In this study, nickel, iron and nickel–iron composite coat-ngs with various chemical compositions were electrochemicallyeposited on a copper electrode and characterized by differentechniques in view of their possible applications as electrocat-lytic materials for the HER in alkaline medium. The effect oflectrolysis on the corrosion behavior of the coated electrodes waslso reported. It was found that the presence of nickel with ironncreases the electrocatalytic activity of the coating for the HERn 1 M KOH solution when compared to nickel and iron coatings,nd that the HER activity of the coatings depends on the chemicalomposition of the composite coatings. Cu/NiFe-3 (the molar con-entration ratio of Ni2+:Fe2+ in the plating bath was 4:6) was foundo be the best suitable cathode material for the HER in an alka-ine medium under the experimental conditions studied. The HERas activation controlled and has not been changed over electroly-

is. The corrosion tests showed that the corrosion resistance of theu/NiFe-3 cathode changed when a cathodic current was appliedfter a certain electrolysis time.

cknowledgements

This study has been financially supported by a Cukurova Univer-ity research fund (Project Number: FEF2006D8) and The Scientificnd Technical Research Council of Turkey (TUBITAK) (Project Num-er: 106T542). The authors are thankful to the Cukurova Universityesearch fund and TUBITAK for their financial support.

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a Acta 54 (2009) 3726–3734

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Electrochemical deposition and characterization of NiFe coatings as electrocatalytic materials for alkaline water electrolysisIntroductionExperimentalResults and discussionPreparation of the coatingsCharacterization of the coatingsHydrogen evolutionCorrosion behavior of the NiFe binary coating

ConclusionsAcknowledgementsReferences

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