Ni–Cu nano-crystalline alloys for efficient electrochemical hydrogen production in acid water

Abstract

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.

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1. Introduction

Hydrogen is considered as an ideal and efficient fuel for the future because it burns with zero emission of global warming gases and could be produced from abundant renewable sources.^1–3 Sustainable hydrogen production from water splitting using renewable electricity has attracted growing attention.⁴ The HER occurring during water splitting in particular is an attractive reaction that illustrates the importance of research in the field of renewable energy. The most common cathode material used for hydrogen evolution in electrolysis is platinum due to its high electrocatalytic activity.^5–8 But the high price and limited supply of Pt are serious barriers for the wider use of water electrolysis. Extensive efforts have been devoted to the search for alternative catalysts containing non-platinum elements for the HER catalysis.^9–13

Nickel and nickel alloys are well known to exhibit good electrocatalytic activity toward the hydrogen evolution reaction (HER) and play important roles in various electrochemical processes. For example, nickel and its alloys are widely used electrode materials for water electrolysis in alkaline solution.^14–16 The activity of Ni towards the HER can be further improved by alloying with appropriate elements, through an electrocatalytic synergistic effect well documented in the literature.¹⁷ One approach to enhance the electroactivity of Ni electrodes towards the HER is to form Ni-transition metal alloys, such as with Co, V, W, Fe, Mo, Zn and Cu^15–25 where the Ni-based alloy electrodes are fabricated by electrodeposition. The addition of the transition metal(s) is expected to alter the electrode reaction mechanism leading to a change in the activation energy of the HER.²⁶ The choice of the alloying metal(s) and the electrodeposition conditions influence the physical and chemical properties of the resultant Ni-based alloy electrodes, which in turn affect their electroactivity for the HER. Among the Ni-based alloy electrodes studied, Ni–Cu alloy has shown potential for use as the cathode in the alkaline HER due to the improved 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 reported yet.

The main disadvantages of alkaline water electrolysis systems are mainly related to their low efficiency and high energy consumption.²⁹ The use of acid electrolytes provides a potential alternative to this issue. In more aggressive acid media, platinum and its alloys are among the best and most extensively investigated electrocatalysts for hydrogen evolution because of their low overpotential for the HER process and good corrosion resistance in acid media. However, these materials are expensive and their extensive utilization would be hindered by a possible worldwide depletion of Pt supplies. By using Ni-based alloys, the cost for the electrocatalyst can be reduced dramatically if the corrosion resistance of Ni-based alloys can be improved.

The present work describes the electrocatalytic activity of nanoparticles Ni–Cu alloys electroplated from sulphate electrolyte on copper foil by the galvanostatic technique and ultrasound waves for the hydrogen evolution reaction in acidic medium. HER was investigated in acidic solutions with a view to possible application as cathodes in hydrogen production.

2. Experimental

2.1 Alloy preparation and material characterization

High purity copper foil (99.98% pure) was utilized as substrate cathode and 2.4 cm² platinum sheets were used as anode. The electro-deposition of the Ni coatings was performed in a Pyrex cylinder cell using TTI PL310 32V-1A PSU as the source of the direct current. An ultrasonic bath (Branson 3510, power 100 W, frequency 42 kHz), a pH and conductivity-meter (Mettler Toledo) were employed in the electro-deposition experiments. The chemicals used in the preparation of the electrolytic baths were NiSO₄·7H₂O, CuSO₄·5H₂O, H₃BO₃, sodium gluconate and cysteine all from Aldrich, Sigma and Merck. The chemicals were weighted and dissolved in 1 L of triply distilled water to have the concentrations presented in Table 1. The nano-crystalline Ni and the Ni–Cu alloys were prepared from the gluconate bath using 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. Different concentrations of cysteine between 0.05 mM and 0.1 M were used to optimize the conditions of electrodeposition. For each experiment, freshly prepared electrolyte was always used. Prior to each run, the copper foil was etched in a (1 : 1) concentrated nitric acid for 1 min to remove the native oxide layer, washed with triply distilled water, and finally rinsed with acetone and weighted. The surface morphology and chemical composition of the nanocrystalline deposited Ni or Ni–Cu alloys was examined by energy dispersive X-ray analysis, EDX, which was incorporated 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 the deposited films. The film composition was determined by EDX and the results are presented in Table 2.

Table 1 Composition of electrodeposition baths was utilized to electrodeposit Ni and Ni–Cu alloys using conventional ultrasound waves at 293 K for 1 hour

Bath	Composition					Operating conditions
	NiSO4, M	CuSO4, M	Sodium gluconate, M	H3BO3, g L^−1	Cysteine, mM	pH	Current density, A cm^−2	Conductivity, mS cm^−2
1	0.1000	—	0.1	10	0.18	4.1	0.025	11.40
2	0.0995	0.0005	0.1	10	0.18	4.0	0.025	11.31
3	0.0980	0.0020	0.1	10	0.18	4.1	0.025	11.40
4	0.0950	0.0050	0.1	10	0.18	4.1	0.025	11.80
5	0.0935	0.0065	0.1	10	0.18	4.1	0.025	11.10
6	0.0925	0.0075	0.1	10	0.18	4.1	0.025	11.17
7	0.0900	0.0100	0.1	10	0.18	4.0	0.025	11.50
8	0.0500	0.0500	0.1	10	0.18	3.8	0.025	11.75
9	—	0.1000	0.1	10	0.18	3.5	0.025	11.10

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Table 2 Chemical compositions of nano-crystalline Ni and Ni–Cu alloys obtained using conventional ultrasound waves and additives at 293 K for 1 hour from EDX analysis

Coating	Ni%	Cu%
1	99.9	—
2	81	19
3	74	27
4	56	44
5	51	49
6	41	59
7	14	86
8	6	94
9	—	99.9

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2.2 Electrochemical measurements

A conventional all glass three electrode electrochemical cell with a large area Pt counter- and saturated calomel, SCE, reference electrode was used. The electrochemical measurements were carried out in aqueous solutions, where analytical grade reagents and triply distilled water were always used. The test electrolyte was a freshly prepared 0.5 M H₂SO₄ solution. The polarization experiments and electrochemical impedance spectroscopic investigations, EIS, were performed using a Voltalab PGZ 100 “All in one” potentiostat/galvanostat. The potentials were measured against and referred to the saturated calomel reference, SCE, (0.245 V vs. the standard hydrogen electrode, SHE). All potentiodynamic polarization experiments were carried out using a scan rate of 5 mV s⁻¹ to achieve quasi-stationary condition. The impedance, Z, and phase shift, θ, were recorded in the frequency domain 0.1–10⁵ Hz. The superimposed ac-signal was 10 mV peak to peak. To achieve reproducibility, each experiment was carried out at least twice.

3. Results and discussion

3.1 Characterization of the electrodes

Pure Ni, Cu and different Ni–Cu_x alloys (x = 19, 27, 49, 59, 86 and 94 at% Cu) were prepared as nano-crystalline deposits on copper foil substrates. It was necessary to investigate the surface morphology, structure and the elemental constituents of the prepared electrodes. The SE micrographs of Fig. 1(a–d) show clearly that the electro-deposition of Ni, Cu and Ni–Cu alloys from the gluconate baths using the conventional ultrasonic waves, CUW, is convenient and easy to handle. The use of the organic substances like the gluconate salt and cysteine in addition to boric acid improves the surface morphology of the deposit. Such combination leads to fine, dense shiny granules of Ni and Ni–Cu alloys. The addition agents changed the surface morphology of Ni coating from granular to smooth and shiny. Sodium gluconate created the regular and granular nickel. Moreover, the boric acid produced the fine, dense and granular Ni. Cysteine made the dense, smooth and shiny Ni using different current density varied between 0.1 and 12 A dm⁻². Also, it was found that the surface morphology of Ni coating was slightly affected by the increase of the current densities between 0.1 and 12 A dm⁻² in the presence of CUW. The CUW eliminated pinholes from the coated Ni as compared to that obtained from silent solution. Fig. 1 shows the SEM/EDX images of the Ni–49Cu and Ni–86Cu alloys (as example) obtained from gluconate bath using a current density of 2.5 A dm⁻². However the increase of current density more than 2.5 A dm⁻² produced very rough thin film of Ni–Cu alloys.

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

The EDX spectra of Ni, Cu and Ni–Cu layer are presented in Fig. 1. The related atomic ratios of elements for the investigated alloys, as obtained from the EDX analysis, are presented in Table 2. The presented data correspond to a complete closed deposited layer. The variation of Cu content of the Ni–Cu alloys varied between 19% and 99.9% which was developed by the change of Ni²⁺ and Cu²⁺ concentrations in the electrolytes. The content of Cu in the alloys increased sharply with the increase of Cu²⁺ concentration between 0.01 M and 0.1 M in the bath. The Highest Cu content was obtained in the Ni–94Cu alloy. On the other hand, highest Ni content was obtained in the Ni–19Cu alloy. Also, the pure Ni and Pure Cu were obtained as can be seen in Table 1.

X-ray diffraction, XRD, was used for the structural characterization of the Ni and Ni–Cu catalysts. Fig. 1(e) shows the XRD patterns 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 diminutive amounts of FCC (200) structure (not shown). Moreover, boric acid increased the intensity of FCC (111) and decreased the intensity of FCC (200) structure. Additionally, cysteine increased the percentage of FCC (111), and decreased the intensity of FCC (200), forming the homogenous and dense Ni. Fig. 1(e) intimates that the crystal structure of Ni–Cu alloys depends on the percentage of Ni and Cu in the electrolyte solution. The XRD patterns demonstrated the peaks intensities related to nano-size and polycrystalline grains. The nanocrystalline Ni–Cu alloys displayed the FCC structures of two characteristic crystal planes, FCC (111) and FCC (200) of Ni-rich and Cu-rich phases. The FCC (111) and FCC (200) structures of Cu-rich phase increased with increasing % Cu for the Ni–Cu alloys which appeared obviously at 2θ of 43° and 50°, respectively. In addition, the intensity of FCC (111) and FCC (200) of Ni-rich phases decreased with increasing % Cu for the Ni–Cu alloys which appeared clearly at 2θ of 44° and 51.9°. However, the pure Cu and 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 small amounts of FCC (200) for the two phases. The increase of Cu content in the alloys led to the growth of the intensity of FCC (111) and FCC (200) of Cu-rich phases Ni–Cu alloys. The average grain size of the Ni–Cu alloys was calculated using Debye–Scherer's equation:³⁰ τ = Kλ/β cos θ where λ is the X-ray wavelength, typically 1.54 Å, β is the line broadening at half the maximum intensity in radians, K is the shape factor, and θ is the Bragg 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 size of Ni–Cu alloys with Cu-rich phases of the FCC (111) remained constant within the Cu range from 19% to 44%. Then, this grain size increased gradually from 100 nm to 120 nm within the Cu range between 44% and 99.9%. Moreover, the grain size of Ni–Cu alloys with the Cu-rich phases of the FCC (200) declined gradually with the increase of % Cu which varied between 90 nm and 62 nm. On the other hand, the grain size increased gradually between 43 nm and 48 nm for the range of Cu content of 26–49% of the Cu-rich phases. In addition, the grain size of Ni–Cu alloys with Ni-rich phases of the FCC (111) decreased sharply for the range of Ni content 74–99.99%, which lowered from 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 of Ni content 51–81%, which changed between 26 nm and 16 nm, and then the grain size changed slightly between 16 nm and 21 nm for the range of Ni content of 81–100%.

3.2 Electrocatalytic activities for hydrogen evolution in acidic electrolytes

3.2.1 Polarization measurements. Hydrogen evolution reaction was examined by the cathodic current–potential curves in 0.5 M H₂SO₄ on coated Ni–Cu surfaces and obtained results are presented in Fig. 2. For comparison the same experiments were carried out using bulk Ni, ed. Ni and ed. Cu electrodes under the same conditions and the polarization curves of these electrodes compared to that of Ni–49Cu as an example of Ni–Cu alloys are presented in Fig. 2(a). It is clear from this Figure that the electro-catalytic activity of the electrodeposited layer is much higher than bulk Ni. The cathodic hydrogen overpotential on any electrode is an important parameter which controls the HER and the rate of hydrogen evolution, presented as the recorded current density at the same potential, represents a comparison parameter for the electro-catalytic activity of the electrodes. For this reason, the steady state potential and the potential at which hydrogen starts to evolve on each electrode under the same conditions were recorded and presented in Table 3. The catalytic activity of the electrodeposited layers towards the HER is clear, and the hydrogen evolution potential on these electrodes is more positive than that of bulk electrodes (cf. Table 3). This means that the nano-crystalline structure of the electrodeposited layers enhances the electro-catalytic activity of the electrode towards the HER. It may be observed that at −1.5 V, the cathodic current density for HER increases for deposition materials (i.e. ed. Ni and ed. Cu) than the bulk electrodes. Also, the addition of Cu to Ni for formation Ni–Cu alloys increases the cathodic current density for HER as shown in 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 respectively. This evidences the fact that Ni–Cu coating exhibits highest activity for HER.

Fig. 2 (a) Cathodic polarization of bulk Ni ([thick line, graph caption]), electrodeposited Ni ([dash dash, graph caption]), electrodeposited Cu (⋯) and Ni–49Cu ([dash dot, graph caption]) in stagnant naturally aerated 0.5 M H₂SO₄ solutions at 25 °C. (b) Cathodic current potential curves of Ni–Cu coatings in 0.5 M H₂SO₄ solution at 25 °C. (c) Variation of the cathodic hydrogen overpotential with the atomic percent of Cu in the electrodeposited alloys in stagnant naturally aerated 0.5 M H₂SO₄ solutions at 25 °C.

Table 3 The steady state potential, hydrogen evolution potential and the cathodic hydrogen overpotential determined from cathodic current–potential curves for the different investigated materials in stagnant naturally aerated 0.5 M H₂SO₄ at 25 °C

Materials	Ess/mV	Ehydrogen evolution/mV	Cathodic hydrogen overpotential/mV	Potential at −30 mA/mV
Ni bulk	−184	−1640	−1456	−1950
Ni deposited	−360	−595	−235	−730
Cu deposited	−10	−623	−523	−713
Ni–94Cu	−20	−641	−621	−755
Ni–86Cu	−78	−477	−399	−551
Ni–59Cu	−320	−487	−167	−568
Ni–49Cu	−326	−466	−140	−544
Ni–27Cu	−150	−546	−396	−595
Ni–19Cu	−124	−524	−400	−635

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Electro-catalytic behavior of any material is due to presence of an active surface site, having an electron transfer pathway. Hence electrocatalysis applies to the influence of the nature of the electrode material in terms of the morphology of the electrode surfaces. Basically cathodic hydrogen evolution reaction is considered to have four steps starting from discharge of hydrogen ions till the formation of gaseous molecular hydrogen. Out of these four steps any one may be the rate determining step and is responsible for the appearance of hydrogen over potential. At the same time, it is important to know that electrocatalysis for any reactions is due to presence of such active surface site, which allows an easy electron transfer. Thus electrocatalysis is largely influenced by the nature of the electrode material and the morphology of the electrode surfaces. 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, the highest electro-catalytic activity can also be ascribed by cracks and rough surface of the deposit as may be seen in Fig. 1. The addition of Cu to Ni forming Ni–Cu alloys also fortifies the formation of porous coatings, responsible for liberation of increased hydrogen.

The cathodic current–potential curves of the Ni–Cu catalysts with different Cu content which are obtained in 0.5 M H₂SO₄ solution are given in Fig. 2(b). From the presented polarization data it is clear that the developed coatings yield remarkably high HER catalytic activity, and the Ni–Cu layer with ≈50–86% Cu content gives high current density for lower overpotential. Also, the Ni–49Cu catalyst showed a relatively more positive hydrogen overpotential (cf., Table 3 and Fig. 2(c)) compared to the other Ni–Cu alloy electrodes, indicating that the a 49% Ni content alloy enhances the electro-catalytic activity of electrodeposited Ni–Cu electrodes towards the HER, which is good for electrochemical hydrogen production. For the same potential, current density for HER on Ni–Cu alloys of 49–85% Cu is higher than on other Ni–Cu alloys. At the current density of −30.0 mA cm⁻², positive potential shift with respect to Ni–49Cu is about 50 mV (cf. Table 3 and Fig. 1(b) inset). It is evident that different kinds of cracks are formed in all the electrodes. Many smaller cracks are found on the electrodes prepared from the ≈49% Cu alloy. The formation of these cracks is vital for higher utilization because cracks, being filled with the electrolyte, render a greater part of the internal surface of the surface, accessible to electrochemical gas evolution. Numerous cracks, formed by electrodeposition, lead to sufficiently short diffusion paths of dissolved hydrogen, for the fastest release of the gas and for avoiding excessive gas accumulation and concentration polarization in the micropores.³¹ The relatively high electro-catalytic activity of the alloy with 49 at% Cu can be interpreted on the basis of the large specific surface area, high surface porosity and synergistic combination of Ni and Cu.³² The observed relationship between absorbed hydrogen and activity for HER can be tentatively explained in terms of the creation of new channels for the formation of adsorbed surface hydrogen. With increasing overpotential, the adsorption of hydrogen by the electrode reaches its saturation and the rate determining step, rds, for the HER reaction becomes the recombination of hydrogen. The excess hydrogen produced by the surface charge transfer reaction can thus be stored by hydrogen absorption, first at the surface, in a subsurface layer: MH_ads(surface) → MH_abs(surface) and then into the bulk of the metal phase: MH_abs(surface) → MH_abs(bulk). With the overpotential further increasing, the rds will be the generation of adsorbed surface hydrogen since under these conditions the electrochemical hydrogen recombination will be fast and in equilibrium.

3.2.2 Electrochemical impedance spectroscopy measurements. The electrochemical impedance spectroscopy, EIS, is a powerful technique to study the electrode/electrolyte interface and frequently employed to investigate the HER.^8,28 Charge transfer controlled, diffusion controlled and mixed kinetics phenomena can be well characterized in this method. In order to derive a physical picture of the electrode/electrolyte interface and the processes occurring at electrode surface, experimental EIS data were modeled using Z fit analysis software provided with the impedance system. The impedance data of electro-deposited Ni and Ni–49Cu layers are presented as Nyquist plots in Fig. 3. For comparison the impedance data of bulk Ni is presented as inset in the same figure. The data were recorded at a constant cathodic potential of −700 mV, where H₂ was evolved at an appreciable rate. The complex-plane impedance plots of bulk Ni (inset Fig. 3) and Ni-coated copper (curve ) displayed a capacitive loop related to the resistance capacitance (RC) network, consisting of the charge transfer resistance (R_ct) of H⁺ reduction and the corresponding capacitance (C_dl) at the electrode/electrolyte interface.^33,34 This impedance behavior is modeled by a simple electrical equivalent circuit, presented in Fig. 4(a). The impedance plots of the Ni–Cu electrodes exhibit two capacitive loops which is characteristic behavior of porous and rough surface.³⁵ The first one is small in size and observed at high frequencies with a diameter (resistance) R₁. The second one is noticed at medium- and low-frequency domains with a high resistance, R₂. The scatterings at low frequencies for the activated electrodes are most probably due to instability of impedance and vigorous hydrogen bubble evolution. The impedance responses of the Ni–Cu catalysts tested can be modeled by the equivalent circuit presented in Fig. 4(b), where the overall impedance is characterized by a parallel combination of capacitance and resistance of two charge-transfer processes. The total charge-transfer resistance, R_f, equals (R₁ + R₂). The circuit elements (C₁ and R₁) and (C₂ and R₂) are related to the high frequency and the low frequency capacitive loops, respectively.³⁶ From Fig. 3 the impedance values of the electrodeposited electrodes are much lower than those recorded for bulk Ni, which implies a different nature of the HER on the electrodeposited materials. The fitting parameters of the impedance data are listed in Table 4. Fig. 5 shows clearly how the total charge-transfer resistance, R_f, for the hydrogen evolution on the electrodeposited layer cathodes is much lower than that on the bulk Ni electrode. This can be attributed to the morphology of the deposited layers which are mainly in the nanosize. It can be seen that the electrodeposited Ni, Cu and Ni–Cu alloys are more efficient catalysts for the HER.

Fig. 3 Nyquist plots of bulk Ni (■■■) [as inset 1], electrodeposited Ni (●●●), electrodeposited Cu (▲▲▲) and Ni–49Cu (▼▼▼) at −700 mV in stagnant naturally aerated 0.5 M H₂SO₄ solutions at 25 °C.

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

Table 4 Equivalent circuit parameters obtained by fitting EIS experimental spectra recorded at −700 mV potential on bulk Ni, ed. Ni and Ni–Cu alloys in 0.5 M H₂SO₄ at 25 °C

Coatings	Rs/Ω	R1/Ω cm^2	C1/mF cm^−2	R2/Ω cm^2	C2/mF cm^−2	Rf = R1 + R2
Bulk Ni	1.59	99.2	0.16	—	—	99.2
Ed. Ni	1.23	11.7	0.014	—	—	11.7
Ed. Cu	1.2	0.40	25.54	1.00	39.8	1.4
Ni–19Cu	1.6	0.28	0.35	0.84	0.46	1.12
Ni–27Cu	1.34	0.22	0.576	0.67	1.18	0.89
Ni–49Cu	1.30	0.20	0.469	0.34	2.89	0.54
Ni–59Cu	0.86	0.21	0.296	0.52	0.972	0.73
Ni–86Cu	1.2	0.22	1.49	0.57	3.69	0.79
Ni–94Cu	1.28	0.30	2.15	1.23	8.18	1.53

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Fig. 5 Variation of the charge-transfer resistance for the hydrogen evolution on the different materials in stagnant naturally aerated 0.5 M H₂SO₄ solutions at 25 °C.

In order to investigate the effect of electrolysis on the corrosion resistances of Ni–Cu films, EIS data were recorded at OCP of each electrode in 0.5 M H₂SO₄. Fig. 6(a) presents the Nyquist plots of these measurements. The figure shows clearly that the Ni–49 at% Cu has the higher corrosion resistance i.e. the largest semicircle diameter, which reflects itself on its stability. The increase of the Cu content more than 50% leads to a decrease in the charge transfer resistance and hence deterioration in the stability and electrochemical activity could be expected. Since the Bode format enables equal presentation of impedance data and the phase angle, θ, as a sensitive parameter for any interfacial phenomena, appears explicitly, the impedance data are also presented as Bode plots in Fig. 6(b). In this figure, one can see that the phase angle curve splits into two maxima representing two time constants. In such cases the simple equivalent circuit model is not enough to fit the experimental EIS data. A more complex model, such as that presented in Fig. 4(b), is needed. The two time constants are also seen in the Nyquist format as the two deformed semicircles, the first occurs at high frequencies and the second is present at low frequencies. In this model, another combination R₂C₂ representing a passive film resistance, R₂, and a passive film capacitance, C₂, was introduced to account for the presence of a passive film. The calculated fitting parameters are presented in Table 5. According to the Nyquist and Bode plots in Fig. 6 and the electrochemical circuit parameters of Table 5, the corrosion resistances of Ni–Cu films are increases with increasing of Cu content up to ∼50% then decreases again. The highest film resistance was recorded for Ni–49Cu alloy which indicates that this electrode is more corrosion resistant. The stability of this layer can be attributed to the presence of compact surface with very few microcracks. This means that the Ni–49Cu film is more stable against corrosion and is stable enough to be applied for hydrogen evolution. The passive film disappears upon cathodic polarization and the simple model can be applied.^37,38 The values of impedance data fitting according the simple equivalent circuit model of Fig. 4(b) are presented in Table 5. The recorded total impedance, Z, under polarization in the same solution is lower compared to the recorded values under open circuit potential conditions. This can be attributed to the surface activation of the Ni–Cu films due to the hydrogen evolution and the reduction of the passive film. The highest value of the charge transfer resistance (i.e. largest semicircle) was recorded for relatively low and high Cu content alloys, and this resistance decreases as the Cu content reaches ≈50 at%. This behavior was interpreted in terms of the surface roughness,³⁹ the formation of hydrides⁴⁰ and the absorption of hydrogen.⁴¹

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

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

Coatings	Rs/Ω	R1/Ω cm^2	C1/mF cm^−2	α1	R2/Ω cm^2	C2/mF cm^−2	α2
Cu	1.3	1.0	701.5	0.90	4.2	377.6	0.99
Ni–19Cu	1.3	4.8	415.7	0.99	18.8	13.36	1.0
Ni–27Cu	1.3	2.4	2136	0.99	19.9	52.98	0.97
Ni–49Cu	1.3	2.1	599.0	0.96	63.4	7.94	1.0
Ni–94Cu	1.6	1.0	648.4	0.94	29.5	34.1	0.99

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To explain the effect of cathodic polarization on the behavior of the electrodeposited layers, EIS measurements were performed at different cathodic potentials, namely, −600, −650, −700 and −750 mV vs. SCE. Fig. 7(a and b) presents the EIS spectra recorded for the Ni–27Cu and Ni–49Cu alloys cathode in stagnant naturally aerated 0.5 M H₂SO₄ at the above mentioned potentials. It is clear that, two time constants response are present at all potentials. The EIS experimental data of the different Ni–Co alloys are fitted to theoretical data according to the equivalent circuit model presented in Fig. 4(b). The calculated values at various potentials are presented in Table 6. The variation of the charge transfer resistance with the cathodic potential for the different electrodeposited cathodes is presented in Fig. 7(c). As presented in Fig. 7(c) and Table 6, the total charge-transfer resistance, R_f, for all investigated samples decreases as the cathodic potential gets more negative. This means that the increased rate of hydrogen evolution enhances the electro-catalytic activity of the cathode material. The decrease of the charge transfer resistance is reflected in an increase in the current density i.e. an increase in the rate of hydrogen evolution. From Fig. 7 it is clear that the diameter of both semicircles considerably decreases with both the cathodic overpotential and Cu content, indicating that both semicircles are related to the electrode kinetics.^42,43 As the overpotential increases, the semicircle in the impedance plots becomes smaller at very high cathodic overpotentials. This is due to the fact that the adsorption process is facilitated and the charge transfer process dominates the impedance response as the potential and Cu content increases. Therefore, according to the results obtained from the EIS studies, one can assume that the Volmere Heyrovsky mechanism is controlling the HER on those electrodes.

Fig. 7 3D Nyquist plots of Ni–27Cu (a) and Ni–49Cu (b) coatings obtained at −600 mV (■), −650 mV (●), −700 mV (▲) and −700 mV (▼) potentials. (c) The E vs. R_f obtained by modeling of impedance spectra for various Ni–Cu samples.

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/Ω	R1/Ω cm^2	C1/mF cm^−2	R2/Ω m^2	C2/mF cm^−2	Rf = 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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4. Conclusions

Nano-crystalline Ni–Cu films were electrodeposited on copper substrates and characterized. The prepared electrodeposits were used as cathodes for hydrogen evolution. The prepared cathodes have remarkably high catalytic activity for the HER. The rate of hydrogen production increases by incorporation of Cu to Ni. Layer alloys with ≈50 atom% Cu exhibits the highest electro-catalytic activity for the HER and gives the highest rate of hydrogen evolution at any cathodic potential with lower hydrogen overpotential. The increase of Cu content more than 50 at% leads to a decrease in the rate of hydrogen production.

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