Influence of deposition conditions on performance of Ni₃S₂ as the bifunctional electrocatalyst in alkaline solutions by galvanostatic deposition

Abstract

The electrodeposition method is a popular synthesis method due to its low cost, simplicity, and short synthesis time. In addition, this synthesis route results in the preparation of a self-supporting electrocatalyst, which eliminates the use of binders and ultimately facilitates the durability as well as the activity of the catalyst. In this work, a series of Ni₃S₂/Ni mesh electrodes are prepared by galvanostatic deposition at different deposition current densities and times. The morphology, microstructure, and elemental composition distribution of these obtained electrodes are characterized, and the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance of the series of Ni₃S₂/Ni meshes are tested. The results show that the Ni₃S₂/Ni mesh electrodes electrodeposited at 30 mA cm⁻² for 1200 s have superior electrochemical performance for HER and OER. The overpotentials of Ni₃S₂/Ni mesh 30 mA cm⁻²–1200 s are 236 and 244 mV for HER and OER, respectively, at a current density of 10 mA cm⁻². In addition, the Tafel slopes for HER and OER are 113 mV dec⁻¹ and 176 mV dec⁻¹, respectively. This research provides some valuable insights into the use of the electrodeposition method for the fabrication of electrocatalysts.

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1. Introduction

The global energy crisis and greenhouse gas emissions have resulted in significant impacts, including climate changes and rising sea levels.^1–4 Therefore, it is crucial to conduct research on renewable energy as a matter of urgency.^5,6 Hydrogen is a promising secondary energy source with a high calorific value in new energy systems.^7,8 It can be stored as a gas or liquid under high pressure in tanks or reversibly absorbed and released by solid hydrogen storage materials.⁹ In addition, hydrogen has a strong potential for grid applications.¹⁰ Renewable energy sources, including wind and solar photovoltaic, have seasonal, intermittent, and regional limitations. To optimize energy storage and transportation, such sources can be converted into hydrogen energy.^8,11,12

Hydrogen has recently been widely considered as a potential renewable energy source. However, the majority of hydrogen is currently produced from natural gas and coal, which are fossil fuels.^13–15 To fulfill the Paris Agreement's objective, hydrogen production must transition effectively from a traditional fossil fuel-based approach to one that employs renewable sources.¹⁶ In this context, electrolytic hydrogen production from renewable energy sources is highly anticipated due to its eco-friendliness, abundant water resources, and high hydrogen purity (≈99.999%).^17,18 Additionally, it can integrate other renewable energy sources into the grid and create a significant downstream market.¹⁹ Depending on the electrolytes and operating temperatures involved, water electrolysis technology can be classified into four categories: alkaline water electrolysis, proton exchange membrane water electrolysis, anion exchange membrane water electrolysis, and solid oxide water electrolysis.²⁰ Among these, alkaline water electrolysis is known to be the optimal choice for large-scale hydrogen synthesis due to the utilization of non-precious metal catalysts.²¹ This technology is widely accepted by suppliers as the most commercially viable and applicable process.²²

The process of water electrolysis includes two sluggish half-reactions, namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).^23–25 In acidic media, precious-metal-based materials, such as Pt for HER and Ir/Ru compounds for OER, are typically employed as electrocatalysts to enhance slow electrochemical activity.²⁶ However, the use of scarce and expensive catalysts is not preferred as it significantly raises the cost of hydrogen production and impedes growth.^27–29 Non-precious metal electrocatalysts comprising alloys, oxides, hydroxides, sulfides, carbides, nitrides, and other materials are presently undergoing rapid development.^30–32 Among them, nickel sulfides are excellent options for OER and HER electrocatalysts,^33–40 and Ni₃S₂ is theoretically considered to be an excellent dual-function electrocatalyst because Ni₃S₂ not only promotes H₂ production in HER but also promotes OOH_ads formation in OER.^41,42 On the one hand, multiple valence states of Ni existing in Ni₃S₂ could promote the adsorption and dissociation of water on the surface to generate the intermediate (OOH*), which is favorable for OER.⁴³ In addition, due to the higher nickel content, Ni₃S₂ surfaces are easily coated with nickel (oxygen) hydroxides with catalytic activity, thus improving the OER performance.⁴⁴ On the other hand, the Ni–S bond has a strong chemisorption capability for hydrogen due to the small free energy, which is beneficial for HER.⁴⁵ Furthermore, the modulation of the surface atomic configuration of Ni₃S₂ is convenient for balancing the absorption/desorption of the targeted intermediates (OH⁻ and H*), thereby improving the kinetics of HER.^46,47 Experimental and theoretical calculations have attributed the good catalytic performance of Ni₃S₂ to its inherent metal conductivity, numerous active sites, and suitable Gibbs free energy of the catalytic site.^26,42 The low cost, abundant resources, easy preparation, and excellent catalytic activity make Ni₃S₂ popular in non-precious metal electrocatalysts.⁴⁸ In recent years, Ni₃S₂ has garnered significant attention as an economical and high-activity catalyst for alkaline water electrolysis.^41,49 DFT calculations suggest that the primary interaction and contribution of Ni₃S₂ is attributed to the d-orbital of Ni, with the s-orbital of S playing a minor role.⁵⁰ Ni₃S₂ displays metallic properties as most of its orbitals cross the Fermi level. Foam nickel and nickel mesh are commonly utilized as electrode substrates in research, as well as in industrial electrolysis procedures for large-scale hydrogen generation. Ni₃S₂ catalyst was prepared by high current density deposition and in situ vulcanization, resulting in a highly exposed active site and a unique 3D heterogeneous interface.⁵¹ This paper specifically centers on the Ni₃S₂ material for the creation of high-performance Ni₃S₂ electrocatalysts that can be used in alkaline water electrolysis based on these advantages.

The hydrothermal and electrodeposition methods are widely accepted techniques for preparation due to their low cost, simple synthesis process, and wide range of applications.^52,53 These methods can produce electrocatalysts with high purity, good crystallinity, and regular morphology.⁵⁴ The core of the hydrothermal method is to select appropriate precursors and reaction conditions. The electrodeposition method employs electricity to facilitate the deposition process of the catalyst on the substrate, making it ideal for creating self-supporting electrodes.⁵⁵ This method can synthesize various products through the regulation of the precursor solution, deposition time, and deposition current density.^52,56–58 As a result, the electrodeposition method has broad applications in developing microstructured electrocatalysts based on non-precious metals.^59,60 However, there is less discussion regarding the effect of electrodeposition condition parameters, such as deposition time and deposition current, on the catalysts' performance.

Depositing Ni₃S₂ on nickel mesh and foam nickel exhibits lower overpotential and better electrocatalytic performance.⁴² This study utilized an electrodeposition method based on nickel mesh and foam to prepare the Ni₃S₂/Ni mesh electrode. The impact of alterations to electrodeposition condition parameters on electrode morphology and catalytic performance was investigated by varying the deposition current density and deposition time. The electrodes' composition and morphology were analyzed using various methods, including scanning electron microscopy (SEM), Raman spectroscopy, transmission electron microscopy (TEM), X-ray diffraction patterns (XRD), and X-ray photoelectron spectroscopy (XPS). Additionally, cyclic voltammetry curves, polarization curves, and electrochemical impedance spectroscopy (EIS) were utilized to evaluate the impact of the synthesized electrodes on catalytic activity for HER and OER. First, this research combines the electrodeposition condition parameters with the electrodes' morphology and electrochemical performance to discuss the influence of electrodeposition condition parameters on Ni₃S₂/Ni mesh electrodes.

2. Results and discussion

First, the in situ generation of Ni₃S₂ electrocatalysts on Ni mesh was synthesized via electrodeposition for 1200 s under the current density of 30 mA cm⁻². The sample of Ni₃S₂/Ni mesh 30 mA cm⁻²–1200 s was chosen to conduct the XRD, Raman, and XPS analysis. Fig. 1a shows the XRD pattern of the Ni₃S₂/Ni mesh sample. A series of XRD diffraction peaks at 2θ = 21.7°, 31.1°, 37.8°, 38.2°, 44.3°, 50.1°, and 55.1° are attributed to the (101), (110), (003), (021), (202), (211), and (122) facets of the Ni₃S₂ crystal (JCPDS 44-1418),⁶¹ respectively. There are distinct diffraction peaks at 2θ = 44.8°, 52.0° and 76.4° from the Ni mesh matrix (JCPDS 04-0850).⁶² To verify the successful synthesis of Ni₃S₂/Ni mesh composites, the synthesized samples were further analyzed using Raman spectroscopy. As shown in Fig. 1b, the Raman shifts at 206, 302, and 352 cm⁻¹ are consistent with the Ni–S bond of Ni₃S₂, strongly confirming the formation of Ni₃S₂/Ni mesh.^63–65

Fig. 1 (a) XRD patterns of Ni₃S₂/Ni mesh; (b) Raman spectra of Ni₃S₂/Ni mesh.

To investigate the structure and morphology of the Ni₃S₂/Ni mesh in detail, the electrode material was carefully peeled off the Ni mesh for TEM analysis. The results showed clusters of Ni₃S₂ of different sizes, as shown in Fig. 2a, with a diameter of about 200 nm and, in Fig. 2b, with a diameter of about 100 nm each. HRTEM images of Ni₃S₂ are shown in Fig. 3c. The lattice stripe spacing of 0.21 nm observed on the nanosheets indicates the presence of the (202) crystal plane of Ni₃S₂. Similarly, the lattice stripe spacing of 0.24 nm corresponds to the (003) crystal plane of Ni₃S₂. The TEM and HRTEM images are in agreement with the XRD and Raman results.

Fig. 2 (a and b). TEM images of clusters of Ni₃S₂ 3–12; (c) HRTEM images of clusters of Ni₃S₂ 3–12.

Fig. 3 XPS survey spectra for Ni₃S₂/Ni mesh 30 mA cm⁻²–1200 s (a); XPS results of different elements. (b) Ni 2p and (c) S 2p.

The surface composition and valence state of the Ni₃S₂/Ni mesh 30 mA cm⁻²–1200 s (hereinafter referred to as Ni₃S₂/Ni 3–12) crystal structure were measured by X-ray photoelectron spectroscopy (XPS). The survey spectra Fig. 3a of the Ni₃S₂/Ni 3–12 show the peaks of Ni and S with unexpected signals of C and O elements due to the contamination/surface oxidation of the product. In the Ni 2p spectrum (Fig. 3b), the two main peaks at 855.5 eV and 873.6 eV, with two satellite peaks at 861.9 and 879.7 eV, could be attributed to Ni 2p3/2 and Ni 2p1/2 from Ni₃S₂.^66,67 The two peaks at 855.5 eV and 873.6 eV represent Ni⁺ and Ni²⁺ respectively. In the S 2p spectrum (Fig. 3c), there are two peaks at 161.6 eV and 163.0 eV corresponding to S 2p_3/2 and S 2p_1/2 of S, respectively, while the peak at 168.3 eV indicates a partial oxidation state of the S element on the sample surface.⁶⁸

To examine how diverse electrodeposition current densities impact electrode morphology, Ni₃S₂/Ni mesh electrodes were electrodeposited at the current densities of 20, 30, 40, and 50 mA cm⁻² for 1200 s, respectively. Fig. 4 displays SEM images of the synthesized electrodes at different deposition current densities. As shown in Fig. 4, the electrodeposition current density increased from 20 mA cm⁻² to 50 mA cm⁻², resulting in a gradual increase in Ni₃S₂ deposition on the Ni mesh. Ni₃S₂ was observed to exist in the form of clusters on the Ni mesh, which is in agreement with the TEM observations. As the deposition current density increased, the clusters' growth sites gradually expanded, resulting in denser clusters on the electrode surface. This can increase the electrode's active substance loading to some degree, thereby improving its catalytic performance.^57,69 However, an excessive number of growth sites results in the dense dispersion of active substances on the substrate. These substances tend to accumulate upwards, perpendicular to the electrode surface. Fig. 4 provides a clearer visualization of this phenomenon. Increasing the current density to 40 mA cm⁻² shows that Ni₃S₂ exhibits a distinct forest-like growth pattern on the Ni mesh. The dense, forest-like morphology may hinder the improvement of the electrode's electric active surface area and the rapid release of gas bubbles. Additionally, it may degrade the electrode performance, which is in line with the subsequent electrochemical test results.⁷⁰

Fig. 4 (a) SEM image of Ni₃S₂/Ni mesh with a current density of 20 mA cm⁻² for 1200 s; (b) SEM image of Ni₃S₂/Ni mesh with a current density of 30 mA cm⁻² for 1200 s; (c) SEM image of Ni₃S₂/Ni mesh with a current density of 40 mA cm⁻² for 1200 s; (d) SEM image of Ni₃S₂/Ni mesh with a current density of 50 mA cm⁻² for 1200 s.

The electrocatalytic activity of Ni₃S₂/Ni mesh electrodes synthesized at various current densities was experimentally examined at ambient temperature. The LSV and Tafel slope curves of Ni₃S₂/Ni mesh electrodes for HER and OER are presented in Fig. 5. The Ni₃S₂/Ni mesh electrodes, synthesized through deposition at a current density of 30 mA cm⁻², displayed a lower overpotential for both HER and OER when contrasted with electrodes synthesized at other current densities. Their overpotentials at a current density of 10 mA cm⁻² were 230 and 245 mV for HER and OER, respectively. As for the HER catalyst, the overpotentials of Ni₃S₂/Ni mesh 30 mA cm⁻²–1200 s were 268, 333, and 430 mV at 20, 50, and 100 mA cm⁻². For the OER, the overpotentials of Ni₃S₂/Ni 3–12 were 470 mV, 572 mV, and 691 mV at 20 mA cm⁻², 50 mA cm⁻², and 100 mA cm⁻², respectively. Above 100 mA cm⁻², Ni₃S₂/Ni 3–12 demonstrated a lower HER and OER overpotential. In particular, the Tafel slopes of 119 mV dec⁻¹ and 121 mV dec⁻¹ for HER and OER of Ni₃S₂/Ni 3–12 were lower than the electrodes synthesized at other current densities of the same series, presenting better reaction kinetics.

Fig. 5 HER polarisation curves (a) and OER polarisation curves (c) for Ni₃S₂/Ni mesh electrodes synthesized in 1.0 M KOH at deposition current densities of 20, 30, 40, and 50 mA cm⁻²; Tafel plots for HER (b) and OER (d) for Ni₃S₂/Ni mesh electrodes synthesized in 1.0 M KOH at deposition current densities of 20, 30, 40, and 50 mA cm⁻².

To investigate the effect of different deposition times on the catalytic performance of the electrodes, a series of Ni₃S₂/Ni mesh electrodes with different deposition times were further deposited at 30 mA cm⁻² deposition current density for 300, 600, 1200, and 1800 s in the experiments. To investigate the effect of different electrodeposition times on the morphology of the electrodes, SEM tests were carried out on Ni₃S₂/Ni mesh electrodes deposited for 600, 900, 1200, and 1800 s in this experiment. Fig. 6 shows the SEM images of the electrodes with different deposition times. From the images, it can be seen that as the deposition time increases, the active materials deposited on the Ni mesh surface gradually increase and grow outward in a tree-like manner, the active material loading increases, and the size of the clusters tends to increase, and the pores between the clusters are further reduced. When the deposition time reached 1200 s, a forest-like morphology was formed on the substrate surface, and the catalytic performance was improved by more active substance loading. As the deposition time increases further, the active substance covering the substrate grows further. When the deposition time reaches 1800 s, the excessively deposited actives cause a decrease in the specific surface area, and the decrease in the gaps between the clusters also affects the rapid release of gas bubbles, which results in a decrease in the catalytic performance of the electrode, which is further verified by subsequent electrochemical performance tests.⁷¹

Fig. 6 (a–c) SEM image of Ni₃S₂/Ni mesh 30 mA cm⁻²–600 s;(d–f) SEM image Ni₃S₂/Ni mesh 30 mA cm⁻²–900 s;(g–i) SEM image of Ni₃S₂/Ni mesh 30 mA cm⁻²–1200 s;(j–l) SEM image of Ni₃S₂/Ni mesh 30 mA cm⁻²–1800 s.

Fig. 7 shows the LSV and Tafel slope curves for HER and OER of Ni₃S₂/Ni mesh electrodes with different deposition times. The electrodes deposited for 1200 s for HER and OER show smaller overpotentials compared to the electrodes synthesized at other current densities. Their overpotentials are 236 and 244 mV for HER and OER, respectively, at a current density of 10 mA cm⁻². For HER, the overpotentials of Ni₃S₂/Ni 3–12 electrocatalysts are 271, 332, and 433 mV at 20, 50, and 100 mA cm⁻², respectively. For OER, the overpotentials of Ni₃S₂/Ni 3–12 electrocatalysts are 471 and 570 mV at 20 and 50 mA cm⁻², respectively. The overpotential of the electrocatalyst is 471 and 570 mV at 20 and 50 mA cm⁻², respectively. In the higher current region above 100 mA cm⁻², Ni₃S₂/Ni 3–12 shows lower HER and OER overpotentials. In addition, the Tafel slopes of 113 mV dec⁻¹ and 176 mV dec⁻¹ for HER and OER, respectively, are in general agreement with the performance of Ni₃S₂/Ni 3–12 in Fig. 5b and d, showing good reaction kinetics. This suggests that the optimal performance is achieved by increasing the active substance loading at a deposition time of 1200 s without overdepositing to the extent that it adversely affects the contrasting surface area and the rapid release of gas bubbles.^52,53 Despite its basis in a nickel mesh, the catalyst described in this paper demonstrates comparable performance to that of Ni₃S₂/NF as listed in Table 1.⁷⁴ In addition, a comparison with the noble metal catalyst Pt/C and IrO₂ is listed in Table 2.⁷⁷ Compared to the noble metal catalyst, the HER performance of the Ni₃S₂ is weak, while the OER performance is good due to the higher nickel content.

Fig. 7 (a) Polarization curves of HER for Ni₃S₂/Ni mesh electrodes that were deposited for different time durations (600, 900, 1200, and 1800 s) at a deposition current density of 30 mA cm⁻²; (b) the corresponding Tafel plots of HER polarization curves; (c) polarization curves of OER for Ni₃S₂/Ni mesh electrodes that were deposited for different time durations (600, 900, 1200, and 1800 s) at a deposition current density of 30 mA cm⁻²; (d) the corresponding Tafel plots of OER polarization curves.

Table 1 The performance of Ni₃S₂/Ni mesh compared with other prepared Ni₃S₂ catalysts reported in the literature

Catalyst	Water electrolysis test	Electrolyte	Overpotential [mV]@current density [mA cm^−2]	Tafel slopes	Reference
Ni3S2/Ni 3–12	HER	1 M KOH	236@10	113 mV dec^−1	This work
	OER		244@10	176 mV dec^−1	This work
Ni3S2/AT-Ni foam	HER	1 M KOH	200@10	107 mV dec^−1	72
	OER		217@10	163 mV dec^−1	72
Ni3S2–FeS–CoS/PNFCF	HER	1 M KOH	82@10	68 mV dec^−1	73
	OER		170@10	76 mV dec^−1	73
Ni(OH)2/Ni3S2	HER	1 M KOH	50@10	49 mV dec^−1	43
	OER		210@10	72 mV dec^−1	43
Ni3S2/NF	HER	1 M KOH	223@10	—	74
	OER		260@10	—	74
Co–Ni3S2/NF	OER	1 M KOH	120@10	38.4 mV dec^−1	75
DSO-Ni3S2/NF	OER	1 M KOH	241@20	40	76

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Table 2 The performance of Ni₃S₂/Ni mesh compared with commercial noble metal catalysts

Catalyst	Water electrolysis test	Electrolyte	Overpotential [mV]@current density [mA cm^−2]	Tafel slopes	Reference
Pt/C	HER	1 mol L^−1 KOH	25@10	42 mV dec^−1	77
IrO2	OER		360@10	139 mV dec^−1	77
Ce–NiVS	HER	1 M KOH and 0.5 M urea mixed alkaline solution	141@10	114.67 mV dec^−1	78
CuO@NiCo LDH/CF	OER	1 M KOH	256@20	91 mV dec^−1	79
CuCo2O4	HER	1 M KOH and 0.5 M urea mixed alkaline solution	176@10	85.56 mV dec^−1	80

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The electrochemical impedance spectra (EIS) in Fig. 8a demonstrate that the samples deposited at a current density of 30 mA cm⁻² have a smaller charge transfer resistance. Fig. 8b displays the EIS results for electrodes prepared by depositing the sample at a current density of 30 mA cm⁻² for different durations. And Table 3 show the EIS parameters of all samples. It indicates that the Ni₃S₂/Ni mesh electrodes deposited for 1200 s exhibit better charge transfer compared to the other electrodes. The improved performance of the Ni₃S₂/Ni 3–12 electrode could be attributed to the specific morphology of the catalyst layer. The forest-like morphology of the Ni₃S₂/Ni mesh created under these conditions leads to a moderate amount of active material loading, which could guarantee not only a high electrochemically active surface area of the Ni₃S₂ catalyst layer but also rapid bubble release. Therefore, the Ni₃S₂/Ni 3–12 is deemed optimal.^57,81

Fig. 8 (a) EIS spectra of Ni₃S₂/Ni mesh electrodes synthesized at deposition current densities of 20, 30, 40, and 50 mA cm⁻²; (b) EIS spectra of Ni₃S₂/Ni mesh electrodes deposited at 30 mA cm⁻² for 600, 900, 1200, and 1800 s, respectively.

Table 3 EIS parameters of all samples

Samples	Rct (Ω)@ −0.2 V
Ni3S2/Ni 2–12	252
Ni3S2/Ni 3–12	196.6
Ni3S2/Ni 4–12	210.1
Ni3S2/Ni 5–12	555
Ni3S2/Ni 3–18	221.4
Ni3S2/Ni 3–16	435.5
Ni3S2/Ni 3–9	235.3

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The electrocatalytic activity of Ni₃S₂/Ni mesh for OER and HER is dependent on active site exposure⁸² and fast gas diffusion.^70,71,83 As the deposition current density increases while keeping the electrodeposition time constant, the growth sites of the clusters gradually expand. This, in turn, results in denser clusters on the electrode surface, leading to an increase in the electrode's active substance loading and, consequently, an improvement in its catalytic performance. However, an overabundance of growth sites leads to a crowded distribution of active substances on the substrate. At a right angle, these substances accumulate vertically to the electrode surface. The formation of a new layer not only envelops the pre-existing active sites but also elevates the impedance due to the creation of fresh crystal interfaces.⁸⁴ In addition, the new formation layer suppresses the gas diffusion. The same is true when the deposition current density is fixed: as the deposition current density increases, the amount of active substance on the electrode surface and the active sites also increase. By increasing the deposition time to 1200 s, Ni₃S₂/Ni 3–12 demonstrated the greatest OER and HER properties. However, further increases in deposition time led to a decrease in electrode activity, likely caused by the formation of a new layer covering pre-existing active sites and suppressing gas diffusion, resulting in escalated impedance during electrodeposition.⁸⁵ The electrochemical impedance spectra (EIS) results correspond with the kinetic parameters, including the Tafel slope curves and LSV curves.

3. Materials and methods

3.1. Catalyst fabrication

This study involves the pretreatment of commercial nickel mesh. Nickel mesh in 100 mesh size is supplied by Hebei Kangwei. Initially, the mesh was cut into a 1 cm × 2 cm square. Next, the nickel mesh underwent 20 min of sonication in acetone, followed by sonication for 10 min in a 1.0 M hydrochloric acid solution to eliminate the oxide layer present on the surface of the material. Finally, the nickel mesh was washed using deionized water and anhydrous ethanol and then dried under vacuum at 60 °C for one hour.

In this experiment, after processing the nickel mesh, 0.1 M thiourea (H₂NCSNH₂) and 0.02 M nickel sulfate hexahydrate (Ni(NO₃)₂·6H₂O) were placed in a beaker containing 60 mL of deionized water. The beaker was then placed in a heating jacket and the heating temperature was controlled at 70 °C until the drug was dissolved in the beaker and the solution appeared as a transparent dark green color. Then, the experiment was carried out using a CHI660E electrochemical workstation. The Ni mesh was the working electrode, and Pt foil was the counter electrode. The reference electrode was a saturated Ag/AgCl electrode. The electrodeposition was carried out on the pretreated Ni mesh using the prepared solution as an electrolyte. Finally, the Ni₃S₂/Ni mesh electrode was dried in a 60 °C oven for eight hours. In this experiment, the preparation of Ni₃S₂/Ni mesh electrodes was controlled by adjusting the current density and deposition time of electroplating. The electrodeposition current densities were 20, 30, 40, and 50 mA cm⁻², and deposition times were 600, 900, 1200, and 1800 s. The electrodeposition current density and deposition time were adjusted to study the effect of different electroplating parameters on electrode morphology and catalytic performance. For convenience, they are referred to as Ni₃S₂/Ni mesh X mA cm⁻²-Y, where X is the plating current density, and Y is the deposition time.

3.2. Characterization

The composition and crystal structure of the prepared electrode were analyzed in this experiment using a Bruker D8 ADVANCE X-ray diffractometer produced in Germany. The diffractometer utilized Cu Kα radiation as the diffraction source, with a working voltage of 30 kV/40 mA, a testing angle range of 10°–90°, and a scanning speed of 5° min⁻¹. In addition, the X-ray photoelectron spectroscopy (XPS) spectra were measured on an ESCALAB 250 Xi with Al Kα as the source. The electrode was deposited directly onto the substrate using the electrodeposition method. It was then cut into a square sample measuring 0.5 cm × 0.5 cm for testing. Raman spectra were taken using a HORIBA Raman microscope (Horiba LabRAM HR Evolution).

Furthermore, the electrode's composition and crystal structure were observed using transmission electron microscopy (FEI F20). Firstly, the sample of Ni₃S₂/Ni 3–12 was sonicated to uniformly disperse it in an ethanol solution. Subsequently, the resulting suspension was deposited onto a copper mesh. The sample was subjected to an acceleration voltage of 100 kV, and its morphologies were characterized using a Zeiss Sigma 300 (the manufacturer is Zeiss, the equipment was sourced from Oberkochen, German. For obtaining scanning electron microscopy (SEM) images of Ni₃S₂, the electrode was prepared by cutting it into square samples with a side length of about 0.3 cm and then directly testing them by sticking on the conductive adhesive. This method allowed us to achieve the desired SEM images.

3.3. Electrochemical measurement

To study the electrocatalytic properties of Ni₃S₂ for OER and HER, a three-electrode setup was used for all electrochemical tests. The three-electrode system consists of a working electrode, a counter electrode, and a reference electrode. During the test, current flows from the working electrode to the counter electrode, and the reference electrode provides only a reference potential. In this experiment, an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd, Shanghai, China) was used for electrochemical testing. The prepared electrode is used as the working electrode, the graphite electrode as the auxiliary electrode, and the saturated calomel electrode as the reference electrode. The electrolyte is 1.0 M KOH.

Linear sweep voltammetry (LSV) records the electrolytic current of the electrode by applying a linearly changing voltage across the electrode. In the study of electrocatalytic performance of materials, linear sweep voltammetry is mainly used to test the polarization process of materials to obtain the overpotential of electrocatalysis at a certain current density. For OER, the LSV voltage range is 0.92–1.95 V vs. reversible hydrogen electrode (RHE), and the scan rate is 5 mV s⁻¹; for HER, the LSV voltage range is 0.92–1.95 V vs. RHE, and the scan rate is 5 mV s⁻¹. In addition, this experiment uses an electrochemical workstation to perform EIS tests on synthesized electrodes, which are usually performed directly at open circuit voltage with automatic compensation set to 95%. The AC impedance has a high frequency of 100 kHz and a low frequency of 0.01 kHz. The amplitude and pause time are set to the default value.

4. Conclusions

The work presents the preparation of high-performance composite bifunctional electrodes by electrodepositing high-active Ni₃S₂ electrocatalysts on Ni mesh. Influences of electrodeposition conditions on microstructure and electrochemical performance of electrodes are studied here. Experiments were designed to synthesize Ni₃S₂/Ni mesh electrocatalysts by means of electrodeposition while optimizing the preparation process. A range of Ni₃S₂/Ni mesh catalysts deposited at 20, 30, 40, and 50 mA cm⁻² were obtained by regulating current density and controlling the deposition time. Following subsequent electrochemical testing, it was determined that the Ni₃S₂/Ni mesh electrocatalysts deposited at 30 mA cm⁻² exhibited optimal electrocatalytic characteristics. As for the HER catalyst, the overpotentials of Ni₃S₂/Ni 3–12 were 430 mV at 100 mA cm⁻². For the OER, the overpotentials of Ni₃S₂/Ni 3–12 were 691 mV at 100 mA cm⁻², respectively, and the Tafel slopes for HER and OER were 119 mV dec⁻¹ and 121 mV dec⁻¹. Ni₃S₂/Ni mesh electrodes were prepared by maintaining a current density of 30 mA cm⁻² and regulating the deposition time for 600, 900, 1200, and 1800 s. Electrochemical tests indicated that the electrode deposited for 1200 s displayed the most significant electrocatalytic performance. The overpotentials of Ni₃S₂/Ni mesh 3–12 were 236 and 244 mV for HER and OER, respectively, at a current density of 10 mA cm⁻². In addition, the Tafel slopes for HER and OER were 113 mV dec⁻¹ and 176 mV dec⁻¹, respectively. When the deposition current density was excessively high and the deposition time was overly prolonged, the additional active substance tended to accumulate in a forest-like morphology, protruding upwards perpendicularly to the electrode surface. The new formation layer covered the existing active sites, leading to the new structure with fewer voids and holes than the other ones. The new structure may have fewer active sites and hinder the rapid release of gas bubbles, ultimately resulting in suboptimal electrode performance. The superior electrocatalytic performance of the Ni₃S₂/Ni mesh 3–12 electrode results from the even distribution of the catalyst on the surface with a moderate thickness. This ensures adequate active material and avoids a reduction in the contact area between the active material and the electrolyte due to excessive clustering. The results indicate that electrodeposition conditions containing current density and deposition time have significant effects on the performance of the Ni₃S₂/Ni mesh electrode. It is believed that the present work provides valuable insights into fabricating advanced noble-metal-free catalysts by electrodeposition for alkaline water electrolysis.

Data availability

All relevant data are within the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by State Grid Zhejiang Electric Power Company Taizhou Power Supply Company (No. 5211TZ220003).

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