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Ni–Cu nano-crys

aChemistry Department, Faculty of Science, F

nhm00@fayoum.edu.eg; Tel: +20 10698781bChemistry Department, Faculty of Science &

Arabia. E-mail: nhmahmod@ju.edu.sa; Tel:

Cite this: RSC Adv., 2016, 6, 51111

Received 1st April 2016Accepted 17th May 2016

DOI: 10.1039/c6ra08348j

www.rsc.org/advances

This journal is © The Royal Society of C

talline alloys for efficientelectrochemical hydrogen production in acid water

H. Nady*ab and M. Negema

In this work, Ni, Cu and Ni–Cu nano-crystalline alloys were electrochemically deposited on a Cu electrode

(Cu/Ni–Cu) by the galvanostatic technique and ultrasound waves in view of their possible applications as

electrocatalytic materials for the hydrogen evolution reaction (HER). The obtained materials were

characterized morphologically and chemically by XRD and scanning electron microscopy, SEM, coupled

with EDX analysis. The HER activity of the prepared electrodes was studied in acidic media (0.5 M H2SO4

solution) by the polarization measurements and EIS technique. It was shown that the electrodeposited

Ni–Cu alloy coatings possess high catalytic activity for hydrogen evolution in the acidic solutions. The

electrocatalytic activity of the prepared electrodes depended on the morphology and the

microstructure. Ni–Cu surfaces exhibited an enhanced catalysis for HER with respect to the Ni and Cu

cathode, which is mainly attributed to the high surface area of the developed electrode. Ni–Cu deposits

with a Cu content of 49 at% manifests the highest intrinsic activity for HER as a consequence of the

synergetic combination of Ni and Cu. The experimental impedance data were fitted to theoretical data

according to a proposed model for the electrode/electrolyte interface.

1. Introduction

Hydrogen is considered as an ideal and efficient fuel for thefuture because it burns with zero emission of global warminggases and could be produced from abundant renewable sour-ces.1–3 Sustainable hydrogen production from water splittingusing renewable electricity has attracted growing attention.4

The HER occurring during water splitting in particular is anattractive reaction that illustrates the importance of research inthe eld of renewable energy. The most common cathodematerial used for hydrogen evolution in electrolysis is platinumdue to its high electrocatalytic activity.5–8 But the high price andlimited supply of Pt are serious barriers for the wider use ofwater electrolysis. Extensive efforts have been devoted to thesearch for alternative catalysts containing non-platinumelements for the HER catalysis.9–13

Nickel and nickel alloys are well known to exhibit goodelectrocatalytic activity toward the hydrogen evolution reaction(HER) and play important roles in various electrochemicalprocesses. For example, nickel and its alloys are widely usedelectrode materials for water electrolysis in alkaline solu-tion.14–16 The activity of Ni towards the HER can be furtherimproved by alloying with appropriate elements, through anelectrocatalytic synergistic effect well documented in the

ayoum University, Fayoum, Egypt. E-mail:

04

Arts in Qurayat, Al-Jouf University, Saudi

+966 535589807

hemistry 2016

literature.17 One approach to enhance the electroactivity of Nielectrodes towards the HER is to form Ni-transition metalalloys, such as with Co, V, W, Fe, Mo, Zn and Cu15–25 where theNi-based alloy electrodes are fabricated by electrodeposition.The addition of the transition metal(s) is expected to alter theelectrode reaction mechanism leading to a change in the acti-vation energy of the HER.26 The choice of the alloying metal(s)and the electrodeposition conditions inuence the physical andchemical properties of the resultant Ni-based alloy electrodes,which in turn affect their electroactivity for the HER. Among theNi-based alloy electrodes studied, Ni–Cu alloy has shownpotential for use as the cathode in the alkaline HER due to theimproved electrocatalytic activity,23 high corrosion resistance27

and good stability.28 But, the electrocatalytic activity of the Ni–Cu coatings for the HER in acidic medium has not been re-ported yet.

The main disadvantages of alkaline water electrolysissystems are mainly related to their low efficiency and highenergy consumption.29 The use of acid electrolytes providesa potential alternative to this issue. In more aggressive acidmedia, platinum and its alloys are among the best and mostextensively investigated electrocatalysts for hydrogen evolutionbecause of their low overpotential for the HER process and goodcorrosion resistance in acid media. However, these materialsare expensive and their extensive utilization would be hinderedby a possible worldwide depletion of Pt supplies. By using Ni-based alloys, the cost for the electrocatalyst can be reduceddramatically if the corrosion resistance of Ni-based alloys can beimproved.

RSC Adv., 2016, 6, 51111–51119 | 51111

Table 2 Chemical compositions of nano-crystalline Ni and Ni–Cualloys obtained using conventional ultrasound waves and additives at293 K for 1 hour from EDX analysis

Coating Ni% Cu%

1 99.9 —

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The present work describes the electrocatalytic activity ofnanoparticles Ni–Cu alloys electroplated from sulphate elec-trolyte on copper foil by the galvanostatic technique and ultra-sound waves for the hydrogen evolution reaction in acidicmedium. HER was investigated in acidic solutions with a view topossible application as cathodes in hydrogen production.

2 81 193 74 274 56 445 51 496 41 597 14 868 6 949 — 99.9

2. Experimental2.1 Alloy preparation and material characterization

High purity copper foil (99.98% pure) was utilized as substratecathode and 2.4 cm2 platinum sheets were used as anode. Theelectro-deposition of the Ni coatings was performed in a Pyrexcylinder cell using TTI PL310 32V-1A PSU as the source of thedirect current. An ultrasonic bath (Branson 3510, power 100 W,frequency 42 kHz), a pH and conductivity-meter (MettlerToledo) were employed in the electro-deposition experiments.The chemicals used in the preparation of the electrolytic bathswere NiSO4$7H2O, CuSO4$5H2O, H3BO3, sodium gluconate andcysteine all from Aldrich, Sigma and Merck. The chemicals wereweighted and dissolved in 1 L of triply distilled water to have theconcentrations presented in Table 1. The nano-crystalline Niand the Ni–Cu alloys were prepared from the gluconate bathusing current density of 2.5 A dm�2 at pH 4. The electro-deposition time was 1 h at 293 K on the copper foil. Differentconcentrations of cysteine between 0.05 mM and 0.1 M wereused to optimize the conditions of electrodeposition. For eachexperiment, freshly prepared electrolyte was always used. Priorto each run, the copper foil was etched in a (1 : 1) concentratednitric acid for 1 min to remove the native oxide layer, washedwith triply distilled water, and nally rinsed with acetone andweighted. The surface morphology and chemical compositionof the nanocrystalline deposited Ni or Ni–Cu alloys was exam-ined by energy dispersive X-ray analysis, EDX, which wasincorporated with scanning electron microscope (JOEL JSM-5300 LV, at 25 kV under high vacuum). X-ray diffraction, XRD,was also used to specify the structural characteristics of thedeposited lms. The lm composition was determined by EDXand the results are presented in Table 2.

Table 1 Composition of electrodeposition baths was utilized to electrodK for 1 hour

Bath

Composition

NiSO4, M CuSO4, MSodiumgluconate, M

H3BO3,g L�1

1 0.1000 — 0.1 102 0.0995 0.0005 0.1 103 0.0980 0.0020 0.1 104 0.0950 0.0050 0.1 105 0.0935 0.0065 0.1 106 0.0925 0.0075 0.1 107 0.0900 0.0100 0.1 108 0.0500 0.0500 0.1 109 — 0.1000 0.1 10

51112 | RSC Adv., 2016, 6, 51111–51119

2.2 Electrochemical measurements

A conventional all glass three electrode electrochemical cellwith a large area Pt counter- and saturated calomel, SCE,reference electrode was used. The electrochemical measure-ments were carried out in aqueous solutions, where analyticalgrade reagents and triply distilled water were always used. Thetest electrolyte was a freshly prepared 0.5 MH2SO4 solution. Thepolarization experiments and electrochemical impedancespectroscopic investigations, EIS, were performed using a Vol-talab PGZ 100 “All in one” potentiostat/galvanostat. Thepotentials were measured against and referred to the saturatedcalomel reference, SCE, (0.245 V vs. the standard hydrogenelectrode, SHE). All potentiodynamic polarization experimentswere carried out using a scan rate of 5 mV s�1 to achieve quasi-stationary condition. The impedance, Z, and phase shi, q, wererecorded in the frequency domain 0.1–105 Hz. The super-imposed ac-signal was 10 mV peak to peak. To achieve repro-ducibility, each experiment was carried out at least twice.

3. Results and discussion3.1 Characterization of the electrodes

Pure Ni, Cu and different Ni–Cux alloys (x ¼ 19, 27, 49, 59, 86and 94 at% Cu) were prepared as nano-crystalline deposits oncopper foil substrates. It was necessary to investigate the surface

eposit Ni and Ni–Cu alloys using conventional ultrasound waves at 293

Operating conditions

Cysteine,mM pH

Current density,A cm�2

Conductivity,mS cm�2

0.18 4.1 0.025 11.400.18 4.0 0.025 11.310.18 4.1 0.025 11.400.18 4.1 0.025 11.800.18 4.1 0.025 11.100.18 4.1 0.025 11.170.18 4.0 0.025 11.500.18 3.8 0.025 11.750.18 3.5 0.025 11.10

This journal is © The Royal Society of Chemistry 2016

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morphology, structure and the elemental constituents of theprepared electrodes. The SE micrographs of Fig. 1(a–d) showclearly that the electro-deposition of Ni, Cu and Ni–Cu alloysfrom the gluconate baths using the conventional ultrasonicwaves, CUW, is convenient and easy to handle. The use of theorganic substances like the gluconate salt and cysteine in

Fig. 1 Morphological structure, SEM, and EDX results of electro-deposited layers of nano-crystalline Ni (a), Ni–49Cu (b), Ni–86Cu (c)and pure Cu (d) alloys produced using conventional ultrasonic wavesfor 1 h at 293 K. (e) X-ray diffraction, XRD, patterns of nano-crystallineNi–Cu alloys obtained from gluconate bath using conventionalultrasound waves.

This journal is © The Royal Society of Chemistry 2016

addition to boric acid improves the surface morphology of thedeposit. Such combination leads to ne, dense shiny granulesof Ni and Ni–Cu alloys. The addition agents changed the surfacemorphology of Ni coating from granular to smooth and shiny.Sodium gluconate created the regular and granular nickel.Moreover, the boric acid produced the ne, dense and granularNi. Cysteine made the dense, smooth and shiny Ni usingdifferent current density varied between 0.1 and 12 A dm�2.Also, it was found that the surface morphology of Ni coating wasslightly affected by the increase of the current densities between0.1 and 12 A dm�2 in the presence of CUW. The CUW elimi-nated pinholes from the coated Ni as compared to that obtainedfrom silent solution. Fig. 1 shows the SEM/EDX images of theNi–49Cu and Ni–86Cu alloys (as example) obtained fromgluconate bath using a current density of 2.5 A dm�2. Howeverthe increase of current density more than 2.5 A dm�2 producedvery rough thin lm of Ni–Cu alloys.

The EDX spectra of Ni, Cu and Ni–Cu layer are presented inFig. 1. The related atomic ratios of elements for the investigatedalloys, as obtained from the EDX analysis, are presented inTable 2. The presented data correspond to a complete closeddeposited layer. The variation of Cu content of the Ni–Cu alloysvaried between 19% and 99.9% which was developed by thechange of Ni2+ and Cu2+ concentrations in the electrolytes. Thecontent of Cu in the alloys increased sharply with the increase ofCu2+ concentration between 0.01 M and 0.1 M in the bath. TheHighest Cu content was obtained in the Ni–94Cu alloy. On theother hand, highest Ni content was obtained in the Ni–19Cualloy. Also, the pure Ni and Pure Cu were obtained as can beseen in Table 1.

X-ray diffraction, XRD, was used for the structural charac-terization of the Ni and Ni–Cu catalysts. Fig. 1(e) shows the XRDpatterns of the Ni–Cu alloys electroplated from gluconate bath.The Ni displayed besides the peaks of the Cu substrate,considerable amount of FCC (111) structure and diminutiveamounts of FCC (200) structure (not shown). Moreover, boricacid increased the intensity of FCC (111) and decreased theintensity of FCC (200) structure. Additionally, cysteine increasedthe percentage of FCC (111), and decreased the intensity of FCC(200), forming the homogenous and dense Ni. Fig. 1(e) inti-mates that the crystal structure of Ni–Cu alloys depends on thepercentage of Ni and Cu in the electrolyte solution. The XRDpatterns demonstrated the peaks intensities related to nano-size and polycrystalline grains. The nanocrystalline Ni–Cualloys displayed the FCC structures of two characteristic crystalplanes, FCC (111) and FCC (200) of Ni-rich and Cu-rich phases.The FCC (111) and FCC (200) structures of Cu-rich phaseincreased with increasing % Cu for the Ni–Cu alloys whichappeared obviously at 2q of 43� and 50�, respectively. In addi-tion, the intensity of FCC (111) and FCC (200) of Ni-rich phasesdecreased with increasing % Cu for the Ni–Cu alloys whichappeared clearly at 2q of 44� and 51.9�. However, the pure Cuand Ni–94Cu alloy showed only the Cu-rich phases of FCC (111)and FCC (200). The Ni–19Cu, Ni–26Cu and Ni–44.5Cu and Ni–49Cu displayed notable amounts of FCC (111) and smallamounts of FCC (200) for the two phases. The increase of Cucontent in the alloys led to the growth of the intensity of FCC

RSC Adv., 2016, 6, 51111–51119 | 51113

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(111) and FCC (200) of Cu-rich phases Ni–Cu alloys. The averagegrain size of the Ni–Cu alloys was calculated using Debye–Scherer's equation:30 s ¼ Kl/b cos q where l is the X-ray wave-length, typically 1.54 A, b is the line broadening at half themaximum intensity in radians, K is the shape factor, and q is theBragg angle; is the mean size of the ordered (crystalline)domains. By calculating the grain size of Ni, the FCC (111) and(222) were almost 18 nm and 42 nm, respectively. The grain sizeof Ni–Cu alloys with Cu-rich phases of the FCC (111) remainedconstant within the Cu range from 19% to 44%. Then, this grainsize increased gradually from 100 nm to 120 nm within the Curange between 44% and 99.9%. Moreover, the grain size of Ni–Cu alloys with the Cu-rich phases of the FCC (200) declinedgradually with the increase of % Cu which varied between 90nm and 62 nm. On the other hand, the grain size increasedgradually between 43 nm and 48 nm for the range of Cu contentof 26–49% of the Cu-rich phases. In addition, the grain size ofNi–Cu alloys with Ni-rich phases of the FCC (111) decreasedsharply for the range of Ni content 74–99.99%, which loweredfrom 47 nm to 13 nm. The grain size of Ni–Cu alloys with the Ni-rich phase of the FCC (200) decreased gradually for the range ofNi content 51–81%, which changed between 26 nm and 16 nm,and then the grain size changed slightly between 16 nm and 21nm for the range of Ni content of 81–100%.

Fig. 2 (a) Cathodic polarization of bulk Ni (—), electrodeposited Ni(----), electrodeposited Cu (/) and Ni–49Cu (--.-) in stagnant naturallyaerated 0.5 M H2SO4 solutions at 25 �C. (b) Cathodic current potentialcurves of Ni–Cu coatings in 0.5 MH2SO4 solution at 25 �C. (c) Variationof the cathodic hydrogen overpotential with the atomic percent of Cuin the electrodeposited alloys in stagnant naturally aerated 0.5 MH2SO4 solutions at 25 �C.

3.2 Electrocatalytic activities for hydrogen evolution inacidic electrolytes

3.2.1 Polarization measurements. Hydrogen evolutionreaction was examined by the cathodic current–potential curvesin 0.5 M H2SO4 on coated Ni–Cu surfaces and obtained resultsare presented in Fig. 2. For comparison the same experimentswere carried out using bulk Ni, ed. Ni and ed. Cu electrodesunder the same conditions and the polarization curves of theseelectrodes compared to that of Ni–49Cu as an example of Ni–Cualloys are presented in Fig. 2(a). It is clear from this Figure thatthe electro-catalytic activity of the electrodeposited layer ismuch higher than bulk Ni. The cathodic hydrogen overpotentialon any electrode is an important parameter which controls theHER and the rate of hydrogen evolution, presented as therecorded current density at the same potential, representsa comparison parameter for the electro-catalytic activity of theelectrodes. For this reason, the steady state potential and thepotential at which hydrogen starts to evolve on each electrodeunder the same conditions were recorded and presented inTable 3. The catalytic activity of the electrodeposited layerstowards the HER is clear, and the hydrogen evolution potentialon these electrodes is more positive than that of bulk electrodes(cf. Table 3). This means that the nano-crystalline structure ofthe electrodeposited layers enhances the electro-catalyticactivity of the electrode towards the HER. It may be observedthat at �1.5 V, the cathodic current density for HER increasesfor deposition materials (i.e. ed. Ni and ed. Cu) than the bulkelectrodes. Also, the addition of Cu to Ni for formation Ni–Cualloys increases the cathodic current density for HER as shownin Fig. 2(a), where to the cathodic current density for HER at�1.5 V for ed. Ni, ed. Cu and Ni–49Cu are 460, 500 and 600 mA

51114 | RSC Adv., 2016, 6, 51111–51119

respectively. This evidences the fact that Ni–Cu coating exhibitshighest activity for HER.

Electro-catalytic behavior of any material is due to presenceof an active surface site, having an electron transfer pathway.Hence electrocatalysis applies to the inuence of the nature ofthe electrode material in terms of the morphology of the

This journal is © The Royal Society of Chemistry 2016

Table 3 The steady state potential, hydrogen evolution potential andthe cathodic hydrogen overpotential determined from cathodiccurrent–potential curves for the different investigated materials instagnant naturally aerated 0.5 M H2SO4 at 25 �C

MaterialsEss/mV

Ehydrogen evolution/mV

Cathodic hydrogenoverpotential/mV

Potential at�30 mA/mV

Ni bulk �184 �1640 �1456 �1950Ni deposited �360 �595 �235 �730Cu deposited �10 �623 �523 �713Ni–94Cu �20 �641 �621 �755Ni–86Cu �78 �477 �399 �551Ni–59Cu �320 �487 �167 �568Ni–49Cu �326 �466 �140 �544Ni–27Cu �150 �546 �396 �595Ni–19Cu �124 �524 �400 �635

Fig. 3 Nyquist plots of bulk Ni (---) [as inset 1], electrodepositedNi (CCC), electrodeposited Cu (:::) and Ni–49Cu (;;;) at

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electrode surfaces. Basically cathodic hydrogen evolution reac-tion is considered to have four steps starting from discharge ofhydrogen ions till the formation of gaseous molecularhydrogen. Out of these four steps any one may be the ratedetermining step and is responsible for the appearance ofhydrogen over potential. At the same time, it is important toknow that electrocatalysis for any reactions is due to presence ofsuch active surface site, which allows an easy electron transfer.Thus electrocatalysis is largely inuenced by the nature of theelectrode material and the morphology of the electrodesurfaces. Hence, observed high cathodic current density of Ni–Cu alloy coatings can be attributed to increased surface area,consequent to changed composition of the alloys. Further, thehighest electro-catalytic activity can also be ascribed by cracksand rough surface of the deposit as may be seen in Fig. 1. Theaddition of Cu to Ni forming Ni–Cu alloys also forties theformation of porous coatings, responsible for liberation ofincreased hydrogen.

The cathodic current–potential curves of the Ni–Cu catalystswith different Cu content which are obtained in 0.5 M H2SO4

solution are given in Fig. 2(b). From the presented polarizationdata it is clear that the developed coatings yield remarkablyhigh HER catalytic activity, and the Ni–Cu layer with z50–86%Cu content gives high current density for lower overpotential.Also, the Ni–49Cu catalyst showed a relatively more positivehydrogen overpotential (cf., Table 3 and Fig. 2(c)) compared tothe other Ni–Cu alloy electrodes, indicating that the a 49% Nicontent alloy enhances the electro-catalytic activity of electro-deposited Ni–Cu electrodes towards the HER, which is good forelectrochemical hydrogen production. For the same potential,current density for HER on Ni–Cu alloys of 49–85% Cu is higherthan on other Ni–Cu alloys. At the current density of �30.0 mAcm�2, positive potential shi with respect to Ni–49Cu is about50 mV (cf. Table 3 and Fig. 1(b) inset). It is evident that differentkinds of cracks are formed in all the electrodes. Many smallercracks are found on the electrodes prepared from thez49% Cualloy. The formation of these cracks is vital for higher utilizationbecause cracks, being lled with the electrolyte, render a greaterpart of the internal surface of the surface, accessible to

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electrochemical gas evolution. Numerous cracks, formed byelectrodeposition, lead to sufficiently short diffusion paths ofdissolved hydrogen, for the fastest release of the gas and foravoiding excessive gas accumulation and concentration polari-zation in the micropores.31 The relatively high electro-catalyticactivity of the alloy with 49 at% Cu can be interpreted on thebasis of the large specic surface area, high surface porosity andsynergistic combination of Ni and Cu.32 The observed relation-ship between absorbed hydrogen and activity for HER can betentatively explained in terms of the creation of new channelsfor the formation of adsorbed surface hydrogen. Withincreasing overpotential, the adsorption of hydrogen by theelectrode reaches its saturation and the rate determining step,rds, for the HER reaction becomes the recombination ofhydrogen. The excess hydrogen produced by the surface chargetransfer reaction can thus be stored by hydrogen absorption,rst at the surface, in a subsurface layer: MHads(surface) /

MHabs(surface) and then into the bulk of the metal phase:MHabs(surface) / MHabs(bulk). With the overpotential furtherincreasing, the rds will be the generation of adsorbed surfacehydrogen since under these conditions the electrochemicalhydrogen recombination will be fast and in equilibrium.

3.2.2 Electrochemical impedance spectroscopy measure-ments. The electrochemical impedance spectroscopy, EIS, isa powerful technique to study the electrode/electrolyte interfaceand frequently employed to investigate the HER.8,28 Chargetransfer controlled, diffusion controlled and mixed kineticsphenomena can be well characterized in this method. In orderto derive a physical picture of the electrode/electrolyte interfaceand the processes occurring at electrode surface, experimentalEIS data were modeled using Z t analysis soware providedwith the impedance system. The impedance data of electro-deposited Ni and Ni–49Cu layers are presented as Nyquistplots in Fig. 3. For comparison the impedance data of bulk Ni ispresented as inset in the same gure. The data were recorded ata constant cathodic potential of�700mV, where H2 was evolvedat an appreciable rate. The complex-plane impedance plots ofbulk Ni (inset Fig. 3) and Ni-coated copper (curve ) displayed

�700mV in stagnant naturally aerated 0.5 M H2SO4 solutions at 25 �C.

RSC Adv., 2016, 6, 51111–51119 | 51115

Fig. 5 Variation of the charge-transfer resistance for the hydrogenevolution on the different materials in stagnant naturally aerated 0.5 MH2SO4 solutions at 25 �C.

Fig. 4 Equivalent circuits used for modeling of EIS spectra for Ni andNi–Cu samples.

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a capacitive loop related to the resistance capacitance (RC)network, consisting of the charge transfer resistance (Rct) of H

+

reduction and the corresponding capacitance (Cdl) at theelectrode/electrolyte interface.33,34 This impedance behavior ismodeled by a simple electrical equivalent circuit, presented inFig. 4(a). The impedance plots of the Ni–Cu electrodes exhibittwo capacitive loops which is characteristic behavior of porousand rough surface.35 The rst one is small in size and observedat high frequencies with a diameter (resistance) R1. The secondone is noticed at medium- and low-frequency domains witha high resistance, R2. The scatterings at low frequencies for theactivated electrodes are most probably due to instability ofimpedance and vigorous hydrogen bubble evolution. Theimpedance responses of the Ni–Cu catalysts tested can bemodeled by the equivalent circuit presented in Fig. 4(b), wherethe overall impedance is characterized by a parallel combina-tion of capacitance and resistance of two charge-transferprocesses. The total charge-transfer resistance, Rf, equals (R1 +R2). The circuit elements (C1 and R1) and (C2 and R2) are relatedto the high frequency and the low frequency capacitive loops,respectively.36 From Fig. 3 the impedance values of the elec-trodeposited electrodes are much lower than those recorded forbulk Ni, which implies a different nature of the HER on theelectrodeposited materials. The tting parameters of theimpedance data are listed in Table 4. Fig. 5 shows clearly howthe total charge-transfer resistance, Rf, for the hydrogen evolu-tion on the electrodeposited layer cathodes is much lower thanthat on the bulk Ni electrode. This can be attributed to themorphology of the deposited layers which are mainly in thenanosize. It can be seen that the electrodeposited Ni, Cu andNi–Cu alloys are more efficient catalysts for the HER.

Table 4 Equivalent circuit parameters obtained by fitting EIS experimentalloys in 0.5 M H2SO4 at 25 �C

Coatings Rs/U R1/U cm2 C1/mF cm�2

Bulk Ni 1.59 99.2 0.16Ed. Ni 1.23 11.7 0.014Ed. Cu 1.2 0.40 25.54Ni–19Cu 1.6 0.28 0.35Ni–27Cu 1.34 0.22 0.576Ni–49Cu 1.30 0.20 0.469Ni–59Cu 0.86 0.21 0.296Ni–86Cu 1.2 0.22 1.49Ni–94Cu 1.28 0.30 2.15

51116 | RSC Adv., 2016, 6, 51111–51119

In order to investigate the effect of electrolysis on thecorrosion resistances of Ni–Cu lms, EIS data were recorded atOCP of each electrode in 0.5 M H2SO4. Fig. 6(a) presents theNyquist plots of these measurements. The gure shows clearlythat the Ni–49 at% Cu has the higher corrosion resistance i.e.the largest semicircle diameter, which reects itself on itsstability. The increase of the Cu content more than 50% leads toa decrease in the charge transfer resistance and hence deteri-oration in the stability and electrochemical activity could beexpected. Since the Bode format enables equal presentation ofimpedance data and the phase angle, q, as a sensitive parameterfor any interfacial phenomena, appears explicitly, the imped-ance data are also presented as Bode plots in Fig. 6(b). In thisgure, one can see that the phase angle curve splits into twomaxima representing two time constants. In such cases thesimple equivalent circuit model is not enough to t the exper-imental EIS data. A more complex model, such as that pre-sented in Fig. 4(b), is needed. The two time constants are alsoseen in the Nyquist format as the two deformed semicircles, therst occurs at high frequencies and the second is present at lowfrequencies. In this model, another combination R2C2 repre-senting a passive lm resistance, R2, and a passive lm capac-itance, C2, was introduced to account for the presence ofa passive lm. The calculated tting parameters are presentedin Table 5. According to the Nyquist and Bode plots in Fig. 6 andthe electrochemical circuit parameters of Table 5, the corrosion

al spectra recorded at �700 mV potential on bulk Ni, ed. Ni and Ni–Cu

R2/U cm2 C2/mF cm�2 Rf ¼ R1 + R2

— — 99.2— — 11.71.00 39.8 1.40.84 0.46 1.120.67 1.18 0.890.34 2.89 0.540.52 0.972 0.730.57 3.69 0.791.23 8.18 1.53

This journal is © The Royal Society of Chemistry 2016

Fig. 6 (a) Nyquist plots of electrodeposited Cu ( ), Ni–27Cu ( ),Ni–49Cu ( ) and Ni–94Cu ( ) at the open circuit potential in0.5 M H2SO4 solutions at 25 �C. (b) Bode plots of electrodeposited Cu( ), Ni–27Cu ( ), Ni–49Cu ( ) and Ni–94Cu ( ) at theopen circuit potential in 0.5 M H2SO4 solutions at 25 �C.

Fig. 7 3D Nyquist plots of Ni–27Cu (a) and Ni–49Cu (b) coatingsobtained at �600 mV (-), �650 mV (C), �700 mV (:) and �700 mV(;) potentials. (c) The E vs. Rf obtained by modeling of impedancespectra for various Ni–Cu samples.

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resistances of Ni–Cu lms are increases with increasing of Cucontent up to �50% then decreases again. The highest lmresistance was recorded for Ni–49Cu alloy which indicates thatthis electrode is more corrosion resistant. The stability of thislayer can be attributed to the presence of compact surface withvery few microcracks. This means that the Ni–49Cu lm is morestable against corrosion and is stable enough to be applied forhydrogen evolution. The passive lm disappears upon cathodicpolarization and the simple model can be applied.37,38 Thevalues of impedance data tting according the simple equiva-lent circuit model of Fig. 4(b) are presented in Table 5. Therecorded total impedance, Z, under polarization in the samesolution is lower compared to the recorded values under opencircuit potential conditions. This can be attributed to thesurface activation of the Ni–Cu lms due to the hydrogenevolution and the reduction of the passive lm. The highestvalue of the charge transfer resistance (i.e. largest semicircle)was recorded for relatively low and high Cu content alloys, and

Table 5 Equivalent circuit parameters obtained by fitting the experimental EIS recorded at the open circuit potential on the investigatedelectrodeposited coatings in stagnant naturally aerated 0.5 M H2SO4 at a 25 �C, according to the equivalent circuit of Fig. 4b

Coatings Rs/U R1/U cm2 C1/mF cm�2 a1 R2/U cm2 C2/mF cm�2 a2

Cu 1.3 1.0 701.5 0.90 4.2 377.6 0.99Ni–19Cu 1.3 4.8 415.7 0.99 18.8 13.36 1.0Ni–27Cu 1.3 2.4 2136 0.99 19.9 52.98 0.97Ni–49Cu 1.3 2.1 599.0 0.96 63.4 7.94 1.0Ni–94Cu 1.6 1.0 648.4 0.94 29.5 34.1 0.99

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 51111–51119 | 51117

Table 6 Equivalent circuit parameters obtained by fitting EIS experimental spectra recorded at various potential on Ni–27Cu, Ni–49Cu and Ni–86Cu alloy coatings in stagnant naturally aerated 0.5 M H2SO4 at a 25 �C according to the model of Fig. 4(b)

Coatings E/mV Rs/U R1/U cm2 C1/mF cm�2 R2/U m2 C2/mF cm�2 Rf ¼ R1 + R2

Ni–27Cu �600 1.30 0.68 0.296 1.4 1.13 2.08�650 1.31 0.35 0.260 0.89 0.89 1.24�700 1.34 0.22 0.576 0.67 1.18 0.89�750 1.32 0.21 0.776 0.45 1.43 0.66

Ni–49Cu �600 1.27 0.30 0.664 0.67 5.90 0.97�650 1.28 0.21 1.167 0.50 5.54 0.71�700 1.30 0.20 0.469 0.34 2.89 0.54�750 1.27 0.10 0.992 0.31 0.51 0.41

Ni–86Cu �600 1.2 0.56 1.79 0.72 8.87 1.28�650 1.2 0.23 3.10 0.64 4.95 0.87�700 1.2 0.22 1.49 0.57 3.69 0.79�750 1.18 0.19 1.45 0.39 4.41 0.58

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this resistance decreases as the Cu content reaches z50 at%.This behavior was interpreted in terms of the surface rough-ness,39 the formation of hydrides40 and the absorption ofhydrogen.41

To explain the effect of cathodic polarization on the behaviorof the electrodeposited layers, EIS measurements were per-formed at different cathodic potentials, namely, �600, �650,�700 and �750 mV vs. SCE. Fig. 7(a and b) presents the EISspectra recorded for the Ni–27Cu and Ni–49Cu alloys cathode instagnant naturally aerated 0.5 M H2SO4 at the above mentionedpotentials. It is clear that, two time constants response arepresent at all potentials. The EIS experimental data of thedifferent Ni–Co alloys are tted to theoretical data according tothe equivalent circuit model presented in Fig. 4(b). The calcu-lated values at various potentials are presented in Table 6. Thevariation of the charge transfer resistance with the cathodicpotential for the different electrodeposited cathodes is presentedin Fig. 7(c). As presented in Fig. 7(c) and Table 6, the total charge-transfer resistance, Rf, for all investigated samples decreases asthe cathodic potential gets more negative. This means that theincreased rate of hydrogen evolution enhances the electro-catalytic activity of the cathode material. The decrease of thecharge transfer resistance is reected in an increase in thecurrent density i.e. an increase in the rate of hydrogen evolution.From Fig. 7 it is clear that the diameter of both semicirclesconsiderably decreases with both the cathodic overpotential andCu content, indicating that both semicircles are related to theelectrode kinetics.42,43 As the overpotential increases, the semi-circle in the impedance plots becomes smaller at very highcathodic overpotentials. This is due to the fact that the adsorp-tion process is facilitated and the charge transfer processdominates the impedance response as the potential and Cucontent increases. Therefore, according to the results obtainedfrom the EIS studies, one can assume that the Volmere Heyr-ovsky mechanism is controlling the HER on those electrodes.

4. Conclusions

Nano-crystalline Ni–Cu lms were electrodeposited on coppersubstrates and characterized. The prepared electrodeposits

51118 | RSC Adv., 2016, 6, 51111–51119

were used as cathodes for hydrogen evolution. The preparedcathodes have remarkably high catalytic activity for the HER.The rate of hydrogen production increases by incorporation ofCu to Ni. Layer alloys with z50 atom% Cu exhibits the highestelectro-catalytic activity for the HER and gives the highest rate ofhydrogen evolution at any cathodic potential with lowerhydrogen overpotential. The increase of Cu content more than50 at% leads to a decrease in the rate of hydrogen production.

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