Magnetoelectrodeposition of Ni–W alloy coatings for enhanced hydrogen evolution reaction

Abstract

The electrocatalytic efficiency of electrodeposited (ED) Ni–W alloy coatings for the hydrogen evolution reaction (HER) has been improved drastically through magnetoelectrodeposition (MED) approach. Ni–W alloy coatings have been developed under different conditions of magnetic field intensity ‘B’ (applied perpendicular in the range of 0.1–0.4 T), and their electrocatalytic activity for the HER has been tested using cyclic voltammetry (CV) and chronopotentiometry (CP) techniques in 1.0 M KOH solution. A drastic improvement in the electrocatalytic behavior of the MED coating, represented as (Ni–W)_{B=0.2 T} was found as compared to its conventional Ni–W alloy coatings. Improved performance of the MED coatings was explained on the basis of differences in the process of electrocrystallization affected due to the applied magnetic field, supported by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. A magnetic field has been used advantageously for the first time to increase the W content of the alloy. Increased activity of the MED coatings was attributed to the increased W content in the alloy, characterized by the unique (220) reflection, explained by the magnetohydrodynamic (MHD) effect due to Lorentz force.

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Introduction

Hydrogen is considered as a clean and fully recyclable energy carrier that can be used as an alternative to fossil fuels.^1,2 Renewable energy sources, like sun and wind, cannot produce energy all the time. But they could be used for the production of energy carriers like hydrogen and electricity, which can be stored until its requirement.^3,4 The construction of an energy infrastructure, which connects a host of energy sources to diverse ends, through hydrogen as a primary carrier may provide secure and clean energy for the future.^3–6 Alkaline water electrolysis might be an ideal technique for the production of high purity hydrogen to meet our present requirements.^6,7 However, the high overpotential (i.e., high energy consumption) for hydrogen evolution reaction (HER) constrained its widespread practical applications.^8,9 The development of an efficient non-noble electrode material, composed of earth-abundant elements, with good electrical conductivity, low overvoltage, electrochemical stability, low cost and ease of use might be quite attractive for cost-competitive hydrogen production.^3–10 In this regard, magnetoelectrodeposition (MED) technique i.e., electrodeposition under induced magnetic field (B) plays an important role in increasing the intrinsic activity of coatings, and to be used as promising electrode materials for HER.

MED refers to the electrodeposition using direct current (DC), parallel to the effect of externally imposed magnetic field (applied either parallel or perpendicular to the path of ions mobility). The applied magnetic field is considered to alter the mass transport rates towards cathode significantly, compared to those in conventional electrodeposition (ED).^11–14 It is attributed to the magnetohydrodynamic (MHD) effect introduced by Lorentz force acting on the charged moving ions under electromagnetic field.^15–17 Consequently, the properties of electrodeposits, like composition, crystalline structure, morphology and thereby the intrinsic activity can be tailored to the requirement by superimposition of magnetic field during deposition.^15–17

The combination of hardest and high melting metal, tungsten (W), with metal having noble appearance, nickel (Ni), leads to high quality alloy coatings with better properties.¹⁸ The interesting induced codeposition of reluctant metal (metal which cannot be deposited alone from aqueous solution), namely W in the presence of inducing metal (Ni) has been reported in our earlier studies.^19,20 Conventional electrodeposition enabled us to develop Ni–W alloy coatings with excellent corrosion resistance²⁰ and electrocatalytic activity.¹⁹ Wherein, due to inherent nature of induced codeposition and low limiting current density (i_L), it was not able to develop W rich coatings.²⁰ Recently, MED method has been used extensively in induced codeposition for achieving the compositional variations in reluctant metal content.^12,15,21 Many investigators have already reported the effect of applied magnetic field on various aqueous electrolytic process.^{15,17,21–23}

In continuation of the search for good electrocatalytic materials for HER,^19,24 and being inspired by the appealing appearance, better corrosion resistance and good electrocatalytic activity of Ni–W alloy coatings, an effort has been made to develop a W rich alloy coating using the advent of MHD effect for increased electrocatalytic activity towards HER. The effect of magnetic field on electrocatalytic activity of Ni–W alloy coatings have been studied, in relation to their changed composition, morphology and phase structures. To the best of author's knowledge, we are reporting first time to use the magnetic field effect advantageously to improve the electrocatalytic behavior of Ni–W alloy for HER by increasing the W content of the alloy.

Experimental

Magnetoelectrodeposition

The aqueous electrolyte containing NiSO₄·6H₂O, Na₂WO₄·2H₂O, C₆H₅Na₃O₇·2H₂O, NH₄Cl and C₃H₈O₃ was optimized for the development of electroactive Ni–W alloy coatings as explained elsewhere.²⁰ The MED coatings were deposited on a copper rod of 1 cm² exposed surface area from the optimal bath, under applied magnetic field (applied perpendicular to the direction of movement of ions. i.e., perpendicular to direction of applied current density) of varying intensity (B) (from 0.1 to 0.4 T), at an applied current density (c.d.) of 4.0 A dm⁻². The optimal c.d. of 4.0 A dm⁻², for highest HER, was arrived on the basis of our earlier study.¹⁹ An electromagnet (Polytronics, Model: EM 100) and a power source (DC Power Analyzer, Agilent Technologies, Model: N6705) were used as source of magnetic and electric field, respectively for the development of MED coatings. All depositions were carried out in a custom made glass setup of 400 mL capacity using pre-cleaned copper rod as cathode and Ni as anode for the same duration (600 s) at room temperature (303 K). The experimental setup used for MED is shown schematically in Fig. 1. The electrodeposited coatings were rinsed several times with distilled water, dried and desiccated till further analysis.

Fig. 1 Schematic of experimental setup used for the development of magnetoelectrodeposited (MED) Ni–W alloy coatings.

Characterization

The MED Ni–W alloy coatings were analyzed for their morphology, elemental composition, microstructure, phase structure, and are related to their electrochemical behavior towards alkaline HER. The surface appearance and elemental composition of the coatings were obtained through Scanning Electron Microscopy (SEM, JSM-7610F from JEOL, USA) and Energy Dispersive Spectroscopy (EDS) attached with SEM, respectively. The phase structure characterization of MED coatings was accomplished through XRD analysis using Rigaku Miniflex 600 X-Ray Diffractometer, with CuK_λ radiation (λ = 1.5418 Å) as the X-ray source. Further, the crystalline size and microstructure of the MED coatings were analyzed using Transmission Electron Microscopy (TEM, JEOL, JEM-2100). A detailed analysis of the surface chemistry of as-deposited alloy coating was made using X-ray photoelectron spectroscopy (XPS, Kratos Analytical, U.K.). The thickness of the coatings were measured using Digital Thickness Tester (Coatmeasure M & C, ISO-17025). Further, the obtained values were verified through theoretical calculations from Faraday's law and SEM cross-sectional analysis. The microhardness of the coatings was also measured, using Micro Hardness Tester (CLEMEX, CMT. HD, Canada).

Electrochemical characterization

The electrochemical analyses of the ED and MED coatings (test electrodes for HER) was carried out in a custom made three electrode tubular setup as reported in our earlier study.¹⁹ Cyclic voltammetry (CV) and chronopotentiometry (CP) analyses were used to assess the HER activity and stability of the test electrodes in alkaline medium. The electrochemical responses of the test electrodes (working electrode), in 1.0 M KOH medium, were monitored using platinized Pt as counter and saturated calomel electrode (SCE) as reference electrodes. The CV study was performed within a potential window of 0 to −1.6 V, for 50 cycles at a scan rate of 50 mV s⁻¹. CP responses of the test electrodes under a constant applied cathodic current density of −300 mA cm⁻², was recorded for 1800 s. Further, the practical utility of MED Ni–W alloy coatings, as electrodes for alkaline HER was evaluated by quantifying the amount of H₂ gas evolved from each test electrodes, obtained under different conditions of applied magnetic field.

Results and discussion

Magnetoelectrodeposited Ni–W alloy coatings

Basically, in the present study MED approach was used to increase the W content of the alloy coatings, and thereby to enhance its electrocatalytic character for HER. Hence, MED coatings were first developed at different magnetic field intensity (B) (0.1 T to 0.4 T), at an applied c.d. of 4.0 A dm⁻² from the optimal bath. The composition and thickness of the MED coatings in comparison with the conventional ED coating (all at optimal c.d. of 4.0 A dm⁻²) are given in Table 1. It may be noted that in the present study, Ni–W alloy coatings deposited under natural convection (ED) and forced convection (MED) are conveniently represented, respectively as (Ni–W)_{B=0 T} and (Ni–W)_{B=0.1 T} depending on B applied perpendicular to the direction of flow of ions. From the data given in Table 1, it is clear that the composition and thickness of the coatings are greatly influenced by the forced convection, resulted from the superimposition of the magnetic field during deposition. The development of Ni–W alloy coating with W content more than 12.4 wt% was found almost impossible from our earlier study using conventional ED methods (by using simple DC).¹⁹ However, through magnetoelectrodeposition method, we could able to achieve a coating with the highest W content of 32.4 wt%, at an applied magnetic field strength of 0.2 T.

Table 1 Effect of magnetic field intensity (B) on the composition and physical properties of magnetoelectrodeposited (MED) Ni–W alloy coatings, deposited from optimized bath at 4.0 A dm⁻²

Ni–W coating configuration	Wt% of W	Thickness (μm)	Vicker's microhardness V100 (GPa)	Nature of deposit
(Ni–W)B=0 T	12.4	15.9	3.2	Bright
(Ni–W)B=0.1 T	27.1	16.4	4.1	Bright
(Ni–W)B=0.2 T	32.4	16.8	4.3	Bright
(Ni–W)B=0.3 T	20.3	15.6	3.7	Semi-bright
(Ni–W)B=0.4 T	18.1	15.2	3.5	Semi-bright

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It may be noted that in MED Ni–W alloy coatings, W content, thickness and microhardness of coatings were observed to be increased only up to B = 0.2 T, and then decreased. This decrease in W content towards higher applied B may be attributed to an increase of hydrogen evolution during deposition with electroactive W content in the deposit, which leads to reduced cathode current efficiency.^25–27 The visual appearances of the coatings were also found to be semi-bright at higher B, compared to those at lower B.

SEM study

Variation in the surface morphology of MED coatings with applied B was studied using SEM analysis, given in Fig. 2. A significant difference in the surface morphology of the coatings with B was found. It is important to note that MED Ni–W alloy coatings are found to be cracks-free, with porous and granular structure (Fig. 2b–e) compared to that deposited under natural convection (Fig. 2a).^19,20 Further, it was found that as B increased, the granular structure covered homogeneously to give nodular surface. The increased porosity of the MED coating at higher applied magnetic field strengths is attributed to the increased evolution of H₂ during deposition. Thus, the experimental observations revealed that Ni–W alloy coating developed at B = 0.2 T as the best coating in terms of appearance and highest W content of the alloy.

Fig. 2 The SEM images showing variation in morphology of the MED Ni–W alloy coatings deposited at 4.0 A dm⁻² under different applied magnetic field strengths; (a) (Ni–W)_{B=0 T}, (b) (Ni–W)_{B=0.1 T}, (c) (Ni–W)_{B=0.2 T}, (d) (Ni–W)_{B=0.3 T} and (e) (Ni–W)_{B=0.4 T}.

Further, the variation in the surface morphology of MED coatings with magnetic field strength can be related to the variation in limiting current density (i_L), consequent to the changed mass transport process under applied magnetic field.¹¹ The i_L was found to attain its maximum value at B = 0.2 T, and it decreased with further increase of magnetic field strength. This decrease of i_L values at higher magnetic field strength is attributed to the increased evolution of H₂ gas. Thus, the deformation of nodular growths into grains with uneven nodules (Fig. 2(e)), at higher values of B is attributed to the increased H₂ evolution.

XRD and TEM study

The phase structures of MED Ni–W alloy coatings was also found to show striking variation with B. The XRD patterns of MED Ni–W alloy coatings under different conditions of B is shown in Fig. 3, against to that of ED Ni–W alloy coating.

Fig. 3 The XRD pattern showing the variation in phase structure of Ni–W alloy coatings developed in the presence and absence of applied magnetic field.

The XRD patterns, shown in Fig. 3 clearly indicates the formation of a new phase (220), due to the variation in the direction of crystal growth in case of MED coatings, compared to that in conventional ED coating. The intensity of prominent peaks corresponding to (111), (200) and (311) planes, observed in ED coating was found to be decreased in case of MED coatings.¹⁹ It was found that in MED coatings, few peaks have completely disappeared ((402), (530), (092) and (222)), and some new peaks ((211), (130) and (220)) have appeared. The intensity of the reflections corresponding to the newly formed phases in the MED coatings was observed to be increased with W content of the alloy.^25,28,29 Contrary to this, the intensities of already existing reflections of ED coating were found to be reduced, under the influence of applied magnetic field, as shown in Fig. 3. The crystalline size of the coatings was determined from the XRD data, using Scherrer formula. The crystalline size of the MED coatings was found to be decreased up to B = 0.2 T from 21 nm to 18 nm and then slightly increased to reach 23 nm at higher magnetic field strength.

Further, the microstructure and the crystalline size of Ni–W alloy coating under optimal condition (developed at 0.2 T with maximum W content), was established through TEM analysis. The obtained TEM microstructure of MED (Ni–W)_{B=0.2 T} is shown in the Fig. 4. The average crystallite size obtained from XRD data was further confirmed to be around 20 nm from TEM analysis.

Fig. 4 TEM microstructure of MED Ni–W alloy coating developed at an optimal condition of B = 0.2 T.

XPS analysis

The formation and surface chemistry of MED Ni–W alloy coating (developed at an applied B = 0.2 T), required for alkaline HER was examined through XPS analysis. The wide spectrum showing the elemental survey on the alloy surface, with a detailed analysis of Ni and W species are given in Fig. 5. The elemental survey shows the presence of W, Ni, C and O in the as-deposited alloy thin film as shown in Fig. 5a. The presence of O suggests the existence of metals in their oxide form on the coating surface. Whereas, the C content may be attributed to the presence of trace amount of W in the form of tungsten carbide, or may be resulted from the XPS sample preparation methods.^30,31

Fig. 5 X-ray photoelectron spectrum of the MED Ni–W alloy coating deposited under optimal condition of B = 0.2 T, (a) wide spectrum of deposit showing the elements on the surface, (b) deconvoluted spectra of W 4f, and (c) deconvoluted spectra of Ni 2p.

The deconvoluted spectra of the individual elements, Ni and W, gives a more detailed idea about the surface composition and thereby the electrochemical character of the coating. The XPS spectra show the coatings mainly consist of W (31.2 and 33.6 eV) and Ni (852.6 eV), with NiWO₄ Ni(OH)₂ and WO₃ in trace amounts.^30–32 The presence of metallic W on the coating surface is evident from the high-intensity peaks marked as 4f_7/2 W and 4f_5/2 W in Fig. 5b. At the same time, the low-intensity peaks at high binding energy (B.E.) in the deconvoluted spectra of W shows the presence of NiWO₄ and WO₃ as secondary phases.³² Further, the Ni 2p_3/2 spectra of the alloy coating show the presence of metallic Ni and trace of Ni(OH)₂ on the surface (Fig. 5c).^33,34

Effect of magnetic field

The increase in W content of MED coatings was attributed to the enhanced mass transport process, effected due to applied magnetic field, explained through magnetohydrodynamic (MHD) effect along with Lorentz force.^11,15 In the presence of a superimposed magnetic field (applied in the perpendicular direction), the direction of Lorentz force acting on the charged particles is perpendicular to both electric and magnetic field, given by right-hand rule.¹¹ Lorentz force is maximum when the magnetic field is applied in a perpendicular direction,^11–15 i.e., when the angle between magnetic field and c.d. is 90° (θ = 90°) according to the equation; F_L = qvB sin θ.¹¹ In the presence of induced B, charged particles can move in circular path, affected from the Lorentz force, leading to an increased mass transport with respect to the applied magnetic field intensity.^12,15,23 The enhanced convection may lead to the reduction in double layer thickness and thereby an increase in i_L.²² The schematic representation explaining the effect of magnetic field on electrodeposition process is shown in Fig. 6, where the magnetic field is applied perpendicular to the movement of ions.

Fig. 6 The schematic representation showing the magnetoelectrodeposition of Ni–W alloy coating influenced by MHD effect, responsible for increasing the limiting current density (i_L) of W by reducing the double layer thickness.

The schematic (Fig. 6a) showing the MED process, where B is applied perpendicular to the applied c.d., making the ions to move in circular path near the electrode surface leading to an increased mass transport at the interface of cathode and electrolyte, i.e. at the electrical double layer.^11,12,16 The mass transport process under natural convection (absence of B) is less at the double layer and therefore the double layer thickness (δ_ED) is more with low limiting c.d. (i_L). Whereas, during MED, the mass transport process at double layer increases due to MHD effect leading to high i_L with reduced double layer thickness (Fig. 6b). Thus, during MED of Ni–W alloy coating, the W content of the alloy increased, i.e., by preferential deposition of the reluctant metal (W) by increasing the limiting c.d. of W, or by decreasing the thickness of electrical double layer as required by relation given in eqn (1).

Where, n is the valency of the metal ions, F is the Faraday constant (96 500 C), and D is the diffusion coefficient of the depositing species.

Electrocatalytic activity of MED Ni–W alloy coatings

Cyclic voltammetry study. The efficiency and stability of alloy electrodes can be assessed from the onset potential and cathodic peak current density (i_pc). The obtained CV curves for MED coatings in comparison with the optimal ED coating are shown in Fig. 7, and the corresponding HER parameters are given in Table 2. From the obtained results, it is clear that the W rich MED coatings are electrocatalytically more active than the conventional ED coatings. The coating developed at an applied magnetic field intensity of 0.2 T was found to be the best for HER with supreme i_pc value (−0.97 A cm⁻²) and least onset potential for hydrogen evolution (−0.99 V).

Fig. 7 Cyclic voltammetric responses of MED Ni–W alloy coatings in comparison with that of ED coating in 1.0 M KOH medium.

Table 2 The HER parameters of the MED coatings developed using different applied magnetic field intensity, at an optimal c.d. of 4.0 A dm⁻²

Coating configuration	Cathodic peak c.d. (A cm^−2)	Onset potential for H2 evolution (V vs. SCE)	Volume of H2 evolved in 300 s (cm^3)
(Ni–W)B=0 T	−0.66	−1.21	14.8
(Ni–W)B=0.1 T	−0.85	−1.02	18.8
(Ni–W)B=0.2 T	−0.97	−0.99	19.9
(Ni–W)B=0.3 T	−0.80	−1.04	17.9
(Ni–W)B=0.4 T	−0.76	−1.05	17.5

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Chronopotentiometry study. The catalytic stability of the electrode materials developed through magnetoelectrolysis has been established from its ‘V–t’ (voltage–time) characteristics at a constant applied cathodic current density. The practicality in HER efficiency of the test electrodes, in alkaline KOH medium, has been assessed by quantifying the amount of H₂ gas evolved during the analysis. The obtained chronopotentiograms for MED Ni–W alloy coatings in comparison with the optimal ED coating, along with the amount of H₂ evolved in 300 s are shown in Fig. 8. The corresponding CP data are given in Table 2. The obtained results confirm that the coating with configuration (Ni–W)_{B=0.2 T} is the robust electrode material with maximum amount of H₂ evolved. The enhanced HER efficiency of MED coatings is ascribed to its highest W content, and hence it's changed phase structure and surface morphology.

Fig. 8 Chronopotentiometry curves of the MED Ni–W alloy coatings in comparison with that of ED coating in 1.0 M KOH medium, with corresponding volumes of hydrogen evolved in 300 s, shown in the inset.

It may be noted that despite the high porosity of MED Ni–W alloy coatings, deposited at high B, the highest electrocatalytic activity of (Ni–W)_{B=0.2 T} for HER reveals that the electrocatalytic efficiency is more a function of W content than its surface morphology. It is due to the fact that the effect due to porosity in the coatings was overtaken by the effect due to composition of the metal (W),^35–37 with low hydrogen overvoltage. The surface analysis obtained from XPS data confirms the presence of W in its elemental state rather in oxide form. Apart from that the less formation of Ni(OH)₂,³⁸ which favors oxygen evolution reaction (OER) more than HER, also supports the enhanced HER on the MED coatings. Thus the experimental results revealed the formation of W rich coating resulted in vibrant H₂ bubble disentanglement and increased catalytic c.d. (−0.97 A cm⁻²) with low onset potential (−0.99 V) for MED (Ni–W)_{B=0.2 T}, to be used as an electrode for alkaline HER.

Conclusions

The following conclusions were made after analyzing the obtained experimental results of improved HER efficiency of MED Ni–W alloy coatings in alkaline 1.0 M KOH medium.

1. The electrocatalytic efficiency of the conventional ED coating can be enhanced through the MED approach at optimal deposition c.d., using the same bath.

2. A drastic improvement in the electrocatalytic efficiency of MED Ni–W alloy coatings is due to the change in composition, surface morphology and phase structure, compared to its conventional alloy coating, evidenced by EDX, SEM, TEM, XRD and XPS study.

3. An enhanced electrocatalytic activity of MED Ni–W alloy coatings was attributed to the unique (220) phase structure, due to an increase in its W content.

4. The electrocatalytic efficiency of MED Ni–W coatings was found to be more a function of W content than its surface morphology. It is due to the fact that the effect of porosity in the coatings was overtaken by the effect due to composition of the metal (W), having low hydrogen overvoltage.

5. The MED coatings with coating configuration (Ni–W)_{B=0.2 T} was found to be the best electrode material with vibrant H₂ bubble disentanglement and increased catalytic c.d. (−0.97 A cm⁻²) with low onset potential (−0.99 V) for HER.

Acknowledgements

Liju Elias is thankful to National Institute of Technology Karnataka (NITK), Surathkal, India for supporting this research in the form of Institute Research Fellowship. The authors thank Dr Udaya Bhat K. and Mr Prashant Huilgol, Dept of Metallurgical and Materials Engineering, NITK for TEM studies.

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