Synthesis, microstructure and electrochemical properties of Ni-P-based alloy coatings for hydrogen evolution reaction in alkaline media

Alkaline water electrolysis driven by renewable energy is a promising technology for green hydrogen generation. The cathode half-cell reaction i.e., the hydrogen evolution reaction (HER) in alkaline water electrolysis suffers from slow kinetics. Ni-P-based alloys have shown to be an efficient and cost-effective electrocatalyst to accelerate the HER rate. In this study, three Ni-P alloy coatings are prepared via electrodeposition by varying the deposition currents viz. 10 mA cm-2 direct, 10 mAcm-2 and 100 mAcm-2 pulsed currents. The XRD patterns of all the Ni-P coatings exhibited the formation of crystalline deposits and confirmed the alloying of P in Ni. The SEM images suggested that the microstructures of the Ni-P alloy deposits are highly dependent on the magnitude and waveform of the applied current employed during preparation of the alloy coatings. The composition of the alloy surface is Ni-rich in all three cases but exhibited local variations as evaluated by EDX. The surface distributions of Ni and P in the pulsed deposited samples are more uniform and homogeneous. The cyclic voltammetry patterns of the Ni-P coatings in KOH media exhibit characteristic peaks due to Ni/Ni3+ redox phenomenon. The Ni2+/Ni3+ oxidation peak area is lowest for the direct deposited sample and highest for the pulsed deposited one (100 mAcm-2). The Ni-P alloy electrocatalyst deposited under pulsed mode at 100 mAcm-2 exhibits a current density of −10 mAcm-2 at 0.09 V overpotential and is most active among all samples. The remarkable electrocatalytic activity of this sample is attributed to its smaller crystallite size, better morphological characteristics and lesser resistances to charge transfer and porosity.


Introduction
Hydrogen is a promising alternative green energy carrier with a very high calorific value of ∼ 142 KJ/mole and if it can be produced using renewable energy by water splitting a completely green energy utilization option emerges [1][2][3]. Water splitting can be accomplished by a variety of ways such as electrolytic [2][3][4][5], thermochemical [6,7], photoelectrochemical [8,9], photocatalytic [10,11] etc. Among them, electrochemical water splitting or electrolysis is at present the most feasible, commercialized, and matured technology [12]. Green hydrogen generation can be achieved upon coupling water electrolysis with a PV or wind turbine system [4,5]. Alkaline water electrolysis is a hydrogen generating process at an advanced stage of development with established commercial plants. However, the electrochemical reactions at the electrodes for alkaline water electrolysis, viz. hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), suffers from Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. overpotentials due to kinetic, ohmic, and mass transfer limitations [12,13]. To reduce the kinetic barrier, the use of electrocatalysts is desirable but noble metal based electrocatalysts like Pt, Rh etc are most effective. Although both the activity and stability of these noble metal-based electrocatalysts are high, the cost and availability are significant concerns for their industrial application [3,4,13,14]. As a cheaper replacement for costly noble metal-based electrocatalysts, Ni-based materials have found promising applications in alkaline water electrolysis [14,15]. But Ni as an electrode suffers from passivation and corrosion during the electrolysis operation [14,16]. The passivity of Ni in alkaline solution is widely studied as it finds applications in corrosive and mechanically demanding environments viz. steam generators etc. The passive film on Ni is formed spontaneously, is of the order of few nm, and is generally bi-layered with inner one predicted to be NiO with point defects [17][18][19]. Studies to investigate the probable causes for the deactivation of Ni under reaction conditions, both for HER and OER have been conducted [14,16,[20][21][22][23]. Alloying Ni with other elements were attempted to address these stability aspects, and it was found that alloying in many cases have led to a change in surface morphology and yielded materials with higher electrocatalytic activity and stability [14]. A wide variety of Ni-based alloys was investigated and some promising results were obtained in many cases, viz. Ni-Mo [14,[23][24][25][26], Ni-Fe [27,28], Ni-Co [29,30], Ni-W [31,32] and Ni-P [14,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48].
Ni-P-based alloys are extensively investigated for their superior mechanical, tribological, magnetic, and electrochemical properties with a wide range of applications [35]. Ni-P alloys show improved performance in terms of electroactivity and stability for alkaline water electrolysis. In most of these studies, Ni-P alloy coatings have been prepared by electrochemical deposition method as it has several advantages like it is simple, scalable, possibility to tailor the composition and crystallinity of the deposited alloy [14,49]. Ivo Paseka reported the electrocatalytic activity of amorphous Ni-P alloys prepared by electrodeposition under galvanostatic conditions [33]. They concluded that the amorphous nature of the deposits enabled significant absorption of hydrogen, which resulted in high electrocatalytic activity. Later, Paseka correlated the increased hydrogen absorption to the stress developed in these amorphous alloys [38]. Hu and Bai studied the electrocatalytic activity of Ni-P deposits as a function of P content and inferred that 7 wt% P is optimum to achieve the highest activity [39]. Wei et al found 10 wt% P to be the most favorable Ni-P composition for HER [40]. Lu et al studied Ni-P for HER in an acidic medium and they concluded that as the P content increased, the catalytic activity decreased for HER, which they attributed to the decrease in grain boundaries with an increase in P content [43]. However, Kucernak et al proposed that the HER electroactivity for Ni-P increased with an increase in P content and that a very high P content in the alloy is beneficial [44]. From the above studies, it is established that Ni-P coatings prepared by electrodeposition are a promising electrocatalyst for HER. However, the morphology and P-content are crucial aspects that determine the performance of the deposits.
An important consideration while preparing Ni-P coatings by electrodeposition is that the properties of the deposits vary with electrodeposition operational parameters viz. composition i.e. the P-content, crystallinity, phase, morphology. Again, these properties of the NiP coatings will have a direct impact on the electrochemical properties of the coatings. The properties of the deposits defined by its composition, uniformity, porosity, and surface roughness play a crucial role. Thus, the electrodeposition operational parameters can modify the electrochemical performance of the deposits for HER. Several parameters can be varied during electrodeposition viz. current, current waveform, bath composition, additives, temperature of bath etc [35]. In this study, we report the preparation of Ni-P coatings under three different applied current conditions and investigate its effect on the nature and properties of the deposits. The current waveforms, i.e. pulsed or direct current during electrodeposition are modified and its consequent effect on structure, properties and electrochemical properties are investigated. Ni-P alloy coatings on copper substrate are prepared under direct and pulsed current waveforms at 10 mAcm −2 and also at 100 mAcm −2 current pulses. The Ni-P alloys are characterized for structure and morphology by powder-XRD and SEM-EDX techniques. The electrochemical properties of the Ni-P deposits are analyzed by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedances spectroscopy (EIS) techniques. Finally, the performances of the deposits for HER are evaluated by LSV and a morphology-electrochemical property co-relationship for the Ni-P alloy coatings is established.

Materials
Highly conductive copper foils (99.9% pure) having a thickness 0.2 mm is employed as the substrate for electrodeposition in this study. NiCl 2 .6H 2 O and NaH 2 PO 2 .6H 2 O are used as the precursors for nickel and phosphorous, respectively, and NH 4 Cl is used as an additive in the bath during electrodeposition. All the electrolyte solutions are prepared with analytical grade chemicals and nanopure distilled water. Before electrodeposition, the copper substrate is pretreated or cleaned to obtain a luminous finish. This pretreatment is performed by first polishing the surface with emery paper, washing with water, acetone, and then drying, followed by chemical treatment in 1 M HCl solution for 5 min with ultrasonication and finally washing with nanopure water.

Preparation of the Ni-P alloy coatings
The electrodeposition experiments are conducted in an electrochemical bath using a conventional threeelectrode system consisting of the copper substrate as the working electrode (2 cm 2 ), Ni-plate as the counter electrode, and Ag/AgCl as the reference electrode. Nitrogen gas purging is carried out throughout the electrodeposition process. For direct electrodeposition the bath compositions and other parameters are given in table 1. The direct electrodeposition is carried out at a constant current density of 10 mAcm −2 for 30 min at room temperature with stirring of the bath solution with a magnetic needle at 250 rpm. The chronopotentiometry technique in Biologic SP2 electrochemical workstation and EC Lab software is employed for this purpose. For pulsed deposition current pulses of 10 mAcm −2 and 100 mAcm −2 are employed using bath conditions as mentioned in table 1.

Characterization of the Ni-P alloy coatings
The electrodeposited Ni-P alloys are characterized for the crystal phase by powder XRD and for morphology and composition by FESEM, and EDX. The powder x-ray diffraction patterns of the Ni-P thin films are recorded by Philips X'Pert pro x-ray diffractometer using CuK α radiation (λ = 1.5418 Å) at 40 kV and 30 mA. The scanning electron microscope images, and the energy dispersive x-ray spectra are recorded in a Scanning Electron Microscope (Auriga 4553, Carl Zeiss).

Electrochemical property evaluation of the Ni-P alloy coatings
The electrochemical evaluation of the electrocatalysts are carried out in a Biologic VSP-2 electrochemical workstation using the Ni-P deposited Cu foil as the working electrode, Hg/HgO as the reference electrode and graphite rod as the counter electrode. The electrochemical characterizations included cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 0.1 MHz to 0.1 Hz. All the measurements are carried out in 1 M KOH.

Results and discussions
The powder-XRD patterns of the electrodeposited Ni-P films and the Cu-substrate are shown in figure 1. Peaks due to the substrate Cu appears at 2θ ∼ 43.2 and 50.3 in all the samples. Cu crystallizes in a face center cubic (fcc) type of lattice (Space Group Fm-3 m (225) ICSD 7954). Metallic Ni is iso-structural with the substrate Cu, and crystallizes in a cubic closed-packed structure (fcc lattice, Space Group Fm-3m (225), ICSD No. 260169). Earlier studies have shown that the electrodeposited Ni-P alloy films are x-ray amorphous [35,50]. However, in all our samples, a broad peak appears at 2θ ∼ 44.5 degrees. This peak is due to the reflections from the (111) plane of the cubic Ni lattice, deposited on the substrate. It directly indicates that the electrodeposited coatings form a crystalline alloy of Ni and P, as was also observed by some other authors [51]. For the pulsed electrodeposited thin films of Ni-P, this peak is broadened. Such a peak broadening signifies a reduction in crystallite size in the pulsed electrodeposited samples than that of the directly deposited one. The crystallite sizes of the deposited alloys derived using Scherrer formula are given in table 1. As, the deposition current is increases in NiPP100 the ratio of the relative intensity of the peaks due to NiP and Cu increased. The increase in thickness of the Ni-P alloy layer is responsible for this change in relative intensity of peaks.
To understand the morphological properties of the electrodeposited Ni-P alloys, field emission scanning electron microscopy (FESEM) studies are carried out, and the images are shown in figure 2. As can be seen from these images, considerable differences in the morphological features emerge for the three electrodeposited samples. The Ni-P alloy film prepared under galvanostatic conditions, i.e., NiPD10, exhibits a planer surface with some irregularities. A non-uniform distribution of the Ni-P alloy film can be seen along with the presence of micro-cracks. The images of the NiPD sample with the selected area within the red box being magnified are shown in figure 3. It can be seen from the magnified images in figure 3, the micro-cracks developed are surrounded by nodules. Additional sporadic nodules can also be seen scattered on the film surface. Thus, it can be reasonably assumed that the micro-cracks develop due to the entrapped hydrogen evolved during the deposition process. The resulting nodules surrounding these cracks further support this fact. The surface of the NiPP10, in contrast, is considerably different. The surface morphology, observed in figure 2, is compact and smooth, which, upon magnification (shown in figure 3), exhibits shallow, irregularly distributed plough lines. The surface of the pulsed deposited alloy film in sharp contrast to the direct deposited one, is characterized by the absence of any nodules. The NiPP100 sample is again entirely dissimilar to the others and resembles a grainy surface with spheroids of Ni-P of size 1-20 μm homogeneously distributed, as can be seen from figures 2 and 3. Micro-cracks are observed on the surface of NiPP100. Still, in contrast to NiPD10 these cracks are distinct, sharp and lack the nodular characteristic. These cracks are usually formed by the release of the internal tensile stress developed due to hydrogen trapped in the coatings [52]. The NiPP100 sample exhibit a spheroid deposit with cracks as at high current pulses, enhanced mass deposition occurs with increased hydrogen generation during deposition. From the above discussions, we can presume that the surface features of the Ni-P alloy deposits are highly dependent on the waveform and magnitude of the applied current during the preparation of the alloy coatings by electrodeposition. Further, to have an estimate regarding the thickness of the prepared coatings, the NIPP10 sample was investigated by FESEM along the cross-section and the image is given in Fig SI 1  (supplementary information). It can be seen that the thickness is of the order of 24 μm.
An interesting outcome evolves from the energy dispersive x-ray spectra (EDS) analysis of the samples. All the samples are investigated for the compositional distribution by recording the EDS spectra from various locations on the sample's surface and are shown in figure SI 2 (supplementary information). The NiPD10 compositional distribution is shown in figure SI 2. The micro-crack sites, the associated nodules, and the scattered nodules are specifically due to Ni or Ni-based species and deficient in P, while the planar surface is composed of the Ni-P alloy. The weight ratio of Ni and P distributed over the planar surface is of the order of ∼ 1:13. The upshot of the analysis can be represented by the elemental mapping over the surface of the sample and is shown in figure 4(a). It is evident that the Ni and P are uniformly distributed over the surface except for the crack region and the nodules, where the composition is Ni-rich. The surface site analysis by EDS for the NiPP10 sample is also shown in figure SI 2. Evidently, an enhanced homogeneous distribution of Ni and P elements throughout the surface can be seen as compared to NiPD10. However, the P to Ni ratio is not constant throughout, and local variations are observed. The summary of this result is depicted by the elemental mapping of the NiPP10 coated alloy surface in figure 4(b). Nevertheless, a much enhanced uniform distribution of the constituent elements Ni and P is evident on the coating. Finally, the surface elemental analysis at several locations of the NiPP100 sample is shown in figure SI 2, and the corresponding elemental mapping of the constituents are shown in figure 4(c). The surface distribution of Ni and P is highly uniform and, further, the P content is much higher as compared to either NiPD10 or NiPP10. Moreover, the micro-cracks are devoid of P and are composed of Ni-based species, as in the case of NiPD10. The meager presence of Cu on the surface elemental mapping points out the fact that the coating prepared at this highest current deposition condition is much thicker as compared to that in lower current conditions, augmenting our XRD findings. Thus, the composition of the Ni-P alloy coatings is also highly dependent on the current patterns employed during electrodeposition. It is evident that a uniform coating is the desired one for efficient performance for any application. The pulsed deposition technique provides a much-improved morphology in this regard. Under the pulsed electrodeposition technique, considerable improvements occur in the deposited coating morphology along with significant compositional uniformity on the surface.
The cyclic voltammetry (CV) curves of the electrodeposited Ni-P alloy coatings in 1 M KOH are shown in figure 5. Cyclic voltammetry provides important information regarding the surface redox phenomenon on the electrodes which can have a direct impact on their electrochemical properties [53,54] The CVs of the metal/ alloy coatings in hydroxide medium provide valuable information regarding the formation and reduction of surface metal hydroxide and oxide species during water electrolysis [53]. It is clear from figure 5 that for all the samples, well-defined anodic and cathodic peaks are observed in the forward and reverse scans. P-alloying in Ni by varying the galvanic conditions have resulted in considerable modifications in the CV patterns as compared to Ni-alone [28,54]. But as can be seen from figure 5, three peaks are observed during the anodic scan of the electrodeposited Ni-P sample irrespective of the method of deposition or deposition current. As the potential is steeped from low to high voltage during the anodic sweep the first peak, a minor one, appears at ∼ 0.6 V versus RHE. As the voltage is swept further high, the second prominent peak appears at ∼ 0.95 V versus RHE [28]. Before oxygen evolution, the most intense and distinct anodic peak is observed due to Ni 2+ /Ni 3+ oxidation on the surface of the electrocatalyst [28,55]. From the comparative cyclic voltammetry plots of the electrodeposited samples, it is observed that the area under the peak for the first two anodic scans is comparable for NiPD10 and NiPP10. However, the peak due to Ni 2+ /Ni 3+ oxidation is substantially higher for NiPP10 than for NiPD10. A similar effect can be observed for the reversible Ni 3+ /Ni 2+ reduction peak during the cathodic scan. Thus, as the mode of deposition is varied, the electrochemical response is also modified in terms of the electrocatalyst oxidation-reduction phenomenon on the surface. Increased peak area in NiPP10 suggests an enhanced Ni 2+ /Ni 3+ surface redox process. In other words, the electroactive surface area for the Ni 2+ /Ni 3+ surface oxidation process is more for NiPP10 than NiPD10. Among the pulse deposited samples, it can be observed that as the magnitude of the current pulse during deposition increases from NiPP10 to NiPP100, the peak during the anodic scan is shifted to a higher voltage region, and the area under the curve increases considerably. Thus, we can fairly say that the electroactive surface area increases as the deposition peak current increases. Thus, from the cyclic voltammetry studies, it is confirmed that significant variation in the electrochemical properties of the films deposited by the pulsed technique occur as compared to the one prepared by direct deposition. One of the  probable causes for such a distinct behavior is the smaller crystallite size of the Ni-P samples deposited by pulsed technique (table 1). Further, the morphological differences and enhanced uniform distribution of P played a crucial role. A higher, P/Ni weight ratio in the NiPP100 sample and better morphological features of the surface are vital for increasing the peak areas of the redox processes.
The electrocatalytic activity of the electrodeposited Ni-P alloy films for HER is shown in figure 6, which exhibits the LSV curves at 100 mVs −1 of each sample in 1 M KOH. All the samples are active for electrochemical HER in alkaline media. As can be seen from the figure, the electrochemical performances of the samples for HER increase in the order NiPD10 < NiPP10 < NiPP100. The overpotentials exhibited at a current density of −10 mAcm −2 by NiPD10, NiPP10, and NiPP100 are 0.34 V, 0.19 V, and 0.09 V respectively. The trend in the increase of electrocatalytic activity is accordance with the increase in area of the cyclic voltammetry peak due to Ni 2+ /Ni 3+ oxidation.
The electrochemical impedance (EIS) spectra are studied for all the coated Ni-P samples, and the Nyquist plots at 0.4 V overpotential are shown in figure 7. A two-time constant series model (2-TS), as shown in figure SI 3 (supplementary information) is utilized to fit the Nyquist plots for the Ni-P samples [44,56]. The values of the resistances are given in table SI 1. In the 2-TS model, the series resistance R s can be attributed to the sum of the several ohmic resistances of the system. The other two resistances, which are parallel ones to the double layer  capacitances (C d1 and C d2 ) on the electrode surface, are the charge transfer resistance (R CT ) and the resistance arising out of the porosity of the electrode surface (R P ). The charge transfer resistance progressively decreases from NiPD10 to NiPP10 to NiPP100 as the electrodeposition process is changed from direct to pulse and the pulsed current is increased. This indicates facile electrochemical charge transfer across the electrocatalyst-coated surface in the pulsed deposited samples. The trend of resistance due to porosity also followed the similar order.
The electrochemical hydrogen evolution reaction (HER) is a heterogeneous catalyzed half-cell reaction, in which the composition, structure and morphology of the electrocatalyst plays a crucial role. These properties of the electrocatalyst have a direct impact on the electrochemical surface area, conductivity of the coatings which can have a direct impact on the nature of interaction between the electrocatalyst and the electrolyte resulting in modified HER kinetics. In this study, three Ni-P alloy coatings on Cu-substrates, are prepared by electrodeposition method by varying the current waveform, and magnitude viz. 10 mA cm −2 direct deposition, 10 mAcm −2 and 100 mAcm −2 pulsed depositions. Crystalline Ni-P alloy coatings are obtained in all the three cases, but the crystallite size varies, i.e., pulsed deposition yields smaller crystallites. Smaller crystallites are obviously favorable for improving the electrocatalytic activity of the Ni-P alloy deposits. The surface morphological features differed considerably among the three samples indicating that the galvanic deposition parameters play a crucial role in the morphological evolutions of the deposited Ni-P alloy. Planar deposit with cracks and nodules is evident in NiPD10, while spheroid deposit with cracks is seen on the NiPP100 surface. The surface of NiPP10 is devoid of cracks or nodules or spheroids but relatively compact with irregularly distributed plough lines. From the morphological point of view the Ni-P alloys prepared by pulsed deposition being uniform, compact are advantageous with respect to HER. Moreover, the surface composition also exhibited significant local differences ranging from Ni/P ratio to the 2D surface compositional distribution. The cracked regions and nodules are found to be Ni-rich, while the surface showed a reasonable Ni/P ratio varying from ∼ 6 to 20. Several novel studies and strategies are emerging for the development of crack free coatings of several metals including Ni-alloys [57,58]. Higher current during deposition resulted in uniform, and thicker deposits of spheroid type with consistent Ni/P ratio (7 to 9) on the surface, reduced resistance due to porosity, low charge transfer resistance and highest electrochemical performance for hydrogen evolution reaction in alkaline media.

Conclusions
Three Ni-P alloy coatings on Cu-substrate are prepared by electrodeposition by varying the current waveform and magnitude. Crystalline Ni-P alloy coatings are obtained and the surface features varied considerably, indicating that the galvanic deposition parameters play a crucial role in morphological evolutions of the deposited Ni-P alloy. Further, the surface composition exhibited significant local differences ranging from Ni/P ratio to surface compositional homogeneity. Higher current during deposition resulted in uniform and thicker Ni-P deposits of spheroid type with consistent Ni/P ratio on the surface, reduced resistance due to charge transfer and porosity, exhibiting highest electrochemical performance for hydrogen evolution reaction in alkaline media.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.