Introduction

One of the most important challenges today is finding additional energy resources with low-cost options for generating sustainable energy which is the main focus of the researchers [1,2,3]. Electrochemical water splitting is a low-cost, simple, and environment-friendly method for producing clean energy [4, 5]. In the long run, it can replace fossil fuels, making it a successful and necessary strategy. The oxygen evolution reaction (OER), which involves the transfer of four electrons and the production of different substances, gives electrochemical conversion cycles that use chemical fuels and renewable electricity the electrons they need. Electrical catalysts are important for making and breaking chemical bonds because they let electrons pass through [6, 7] due to the slow kinetics and simultaneous involvement of four electrons [8, 9]. OER catalysts are difficult to design due to sluggish kinetics; therefore, the researcher focus on advance, inexpensive, highly active electrocatalysts with small overpotential [10,11,12]. Hence, electrochemical water splitting electrodes were fabricated from various metal oxides and metal sulfides to achieve low overpotential and small Tafel slope [13,14,15]. Previously, industrial H2 was produced using natural gas, which consumed a large amount of nonrenewable energy and released carbon dioxide into the atmosphere. In recent years, electrochemical water separation has emerged as a feasible promise for low-cost, pollution-free, and energy-sustaining H2 generation. As a result, researchers worldwide have been attempting to create cost-effective and efficient electrochemical water splitting techniques such as bipolar water electrolysis, photovoltaic water splitting, solar cells, thermoelectric devices, and nano-turboelectric generators [16]. The focus of this study was the interaction of strongly polarized electronegative sulfur (such as CeS) with highly electropositive metals sulfides to achieve better efficiency [16, 17]. It will assist in removing electropositive and harmful ion residues during an electrochemical operation [18, 19]. Nickel- and cerium-based sulfides are inexpensive, safe, and effective electrocatalysts [20, 21] but both sulfides as individual has low chemical stability as well as low conductivity. By combining these two materials, i.e., NiS and CeS, it can improve the chemical stability as well as the conductivity due to their covalent interaction with sulfide and metal. The solvothermal technique, which maintains an adequate pressure and temperature balance, produces materials with exceptional quality having a wide range of morphologies [22,23,24,25,26]. Hence, the electrochemical stability of the system could be improved by mixing Ni-based sulfide with Ce-based derivatives due to their synergistic effect [27, 28]. Current research looks at the intriguing possibility of morphology-dependent electrochemical activity to increase electrochemical performance [29,30,31,32]. Hence, these issues can be solved by combining transition metal sulfides with other appropriate materials to form nanocomposites [33]. Zhang et al. [34] created tubular spheres of (Fe0.2Ni0.8)0.96S on Ni substrate to explore the complete kinetics of water splitting in both HER and OER. While working on iron-doped NiS nanoarrays, Qi et al. [35] and Sun et al. [36] employed this as an effective electrode for water splitting and described its long-term durability. Zhang et al. [37] published (Ni,Fe)S2@MoS2 electrocatalysts, and recognized the role of heterojunction in water splitting. Gong and his co-workers [38] explored water splitting in the OER and HER processes using manganese-doped Cu7.2S4@NiS2@NiS/NF electrocatalyst. According to Zheng’s group, the electrocatalytic water splitting reaction of nickel sulfide nanostructures is phase-dependent. Srinivas et al. [39] and Wang et al. [40] developed FeS2/C nanocomposite as a general-purpose water splitting nanocomposite. Zhang et al. [41] also performed a study on water splitting electrocatalyst with a NiS2 nanowire shape. To make thin Ni0.8Fe0.2 films, Nakayasu et al. [42] and Jing et al. [43] used hydrothermal electrodeposition and explained the process of OER. All the above-mentioned metal sulfide composites showed excellent stability as well as remarkable efficiency. From the above motivating concerns, we decided to prepare a one-step solvothermal synthesis for Ni- and Ce-based sulfide nanocomposite and compare the OER activity of NiS/SSS, CeS/SSS, and their nanocomposite electrodes already reported in literature.

Here in the present work, the used SSS is a common chemical engineering substrate due to its exceptional physical and chemical endurance in both basic and acidic environments making it a suitable choice for the present investigation. Additionally, such type of material has an extremely low electrical resistivity. The synthesized NiS/CeS/SSS was examined using XRD, SEM, TEM, BET, and XPS to determine its structure, morphology, surface area, and chemical composition, respectively. The cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometric techniques were employed to evaluate the catalytic characteristics of the materials, and also the mechanism was studied using Tafel plots.

Experimental section

Materials

The stainless steel strip (SSS) was provided by Kunshan Jiayisheng Electronics Co., Ltd. (thickness: 2000 mm, bulk density). Acetone, ethyl alcohol, nitric acid (HNO3), cerium nitrate hexahydrate Ce(NO3)3.6H2O, nickel nitrate, hydrochloric acid (HCl), and thiourea (H2NCSNH2) were provided by Sigma-Aldrich and Alfa Aesar. All chemicals were of interpretive grade, and the water utilized in sample preparation was ultrapure (deionized water) to ensure the quality of the produced materials.

Synthesis of NiS/CeS/SSS nanocomposite

The stainless steel strip (1*2 cm2) pieces were ultrasonically cleaned with acetone and hydrochloric acid, and it was necessary to rinse with an ethanol–water solution three times. A hydrothermal technique was used to fabricate NiS/CeS/SSS samples to achieve good morphology. For this purpose, the precursors like 0.5 mM Ce(NO3)3.6H2O, 0.5 mM Ni(NO3)2, 6H2O, and 0.7 mM thiourea in 35 ml of deionized water were prepared and then stirred for 30 min. The resultant mixture and the pretreated SSS pieces were then poured into a 100 mL Teflon-lined stainless steel autoclave and reserved for heating at 160 °C for 12 h. After 12 h of treatment, the resulting system’s temperature was naturally reduced to normal levels. It was continuously washed with deionized water and dehydrated in an oven overnight.

Physical analysis

The PANanalytical X’Pert Pro device using Cu-Kα radiations was used to characterize the crystalline state and formation of the required phase of the synthesized nanostructured material. The morphology and size of the synthesized materials were studied using scanning electron microscopy with JSM-5600LV equipment at an accelerating voltage of about 80 kV. The electrochemical performance of the fabricated product was evaluated using a Mtrohm Autolab (PGSTAT-204) with a three-electrode cell setup. The working electrode was a synthesized material deposited on a stainless steel strip (SSS), with the counter and reference electrodes being an Ag/AgCl and a Pt filament, accordingly. The working electrode substrate was cleansed via micro-cloth pads to remove the impurities present on the surface of the stainless steel strip, and then the SSS was subsequently cleaned with water and ethanol. The cleaned electrode was dried at 60 °C in a drying oven for 20 min, fabricated directly during hydrothermal treatment, and then employed for electrochemical experiments. The production, fabrication, and application of the nanocomposite nanostructure are depicted in Scheme 1.

Scheme 1
scheme 1

Schematic illustration of the experimental setup

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All electrochemical studies, including linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronopotentiometry, were executed by using a three-electrode electrochemical setup. The reference and counter electrodes for the catalyst-coated SSS working electrode were Ag/AgCl and platinum wire, respectively. We employed 1.0 M of potassium hydroxide (KOH) as an electrolyte during all electrochemical performances. A reversible hydrogen electrode with V = 0.197 + 0.059 × pH was used to set all potentials. At scanning rates of 5 mV s−1, all the polarization curves of the electrocatalysts were investigated. The CV and LSV scans were used to calculate the Tafel slope by plotting the log of current density (j) versus overpotential (η). EIS was done between 100 kHz and 100 MHz at an applied potential of 0.5 V via a three-electrode system using Metrohm PGSTAT AUTOLAB-204.

Result and discussion

Structural, morphological, and textural analysis

Figure 1 represents the X-ray diffraction pattern of NiS, CeS, and NiS/CeS nanocomposite. Three strong peaks of NiS (Fig. 1) at around 2θ = 30.192°, 37.87°, and 69.80° were observed, which corresponds to the (300), (220), and (511) crystals plane indexed with the standard card JCPDS# 00–001-1286. Cell software was utilized to calculate the lattice parameters and found as a = 9.815 Å and c = 2.853 Å. The XRD pattern of the CeS has been well indexed with the standard card JCPDS # 00–004-0688, representing the diffraction peaks at 26.062°, 34.134°, 44.70°, 53.21°, 56.73°, 64.19°, and 73.74° that corresponds to the (111), (200), (311), (222), (400), and (420) planes. On the other hand, the NiS/CeS composite has diffraction peaks of NiS and CeS with smaller fluctuations in intensity and 2-theta which may be due to the interaction between CeS and NiS, demonstrating that the composite material was synthesized successfully.

Fig. 1
figure 1

XRD pattern of all the synthesized products

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The valance states of the nanocomposite were evaluated using the XPS technique. The resultant nanocomposites survey spectrum revealed typical peaks for Ce, S, C, O, and Ni components as shown in Fig. 2a, demonstrating that there is no other elemental impurity. At 248.7 eV, there was a clear C1s peak used to calibrate the position of the peaks present in the nanocomposite as shown in Fig. 2b. Figure 2c represents the O1s XPS spectrum where the peaks appeared at 533 and 531.5 eV which confirmed that it is the environmental oxygen which was adsorbed on the surface of the material from the environment. The XPS S2p spectrum is given in Fig. 2d and the peak that appeared at 169.7 eV confirmed that the sulfer is in a − 2 oxidation state. Figure 2e depicts the bonding states of Ce3+ and Ce4+ coexist in this high-resolution spectrum. There are two deconvoluted peaks present in Ce3d5/2 at 886.7 eV and 905.01 eV confirming the Ce3+, and the convolution peaks with binding energies at 900.1 eV, 906.9 eV, and 888.8 eV are used to represent the Ce4+ states. The XPS Ni 2p spectrum has a good fit with the Ni2+ valence state which is confirmed by their spin-orbital doublets (854.2, 873.3, and 879.1 eV) and shake-up satellites (860.9 and 879.78 eV). Deconvoluted Ni2p3/2 XPS reveals nickel peaks in two distinct chemical states. There is a small peak at 852.79 eV, which corresponds to metallic Ni, while the two larger peaks at 855.5 and 856.7 eV, respectively, represent Ni2+ as shown in Fig. 2f.

Fig. 2
figure 2

XPS. (a) Survey spectrum of composite, (b) = C 1 s, (c) = O 1 s, (d) = S 2p, (e) = Ce 3d, and f = Ni 2p

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Scanning electron microscopy revealed a wide diversity of morphologies and architectures of all the synthesized products. Figure 3a depicts the morphological structures of CeS, indicating self-assembled nanowires. Figure 3b shows the NiS morphological architecture which is folded into two-dimensional sheets during the hydrothermal treatment. Finally, Fig. 3c represents the flower-like features with enhanced active sites and solid polyhedral nanostructures of the NiS/CeS nanocomposite. Figure 3d represents the transmission electron microscopy (TEM) to confirm the analysis for the internal textural formation of the nanocomposite (NiS/CeS), indicating that both phases were present in the prepared nanocomposite. The improved flower-like morphology with abundant active sites was responsible for the remarkable oxygen evolution reaction efficiency.

Fig. 3
figure 3

SEM images of a CeS, b NiS, c NiS/CeS, d TEM images of NiS/CeS, and e–g BET isotherm of NiS, CeS, and NiS/CeS nanocomposite

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Nitrogen adsorption/desorption analysis was used to determine the surface area of the NiS, CeS, and NiS/CeS nanocomposites, as depicted in Fig. 3e–g. The specific surface areas of NiS, CeS, and NiS/CeS nanocomposite were computed using desorption curves and found to be 12.2 m2 g−1, 8.9 m2 g−1, and 18.14 m2 g−1, respectively. The higher the BET surface area of the nanocomposite responses, the greater efficiency of the synthesized materials due to enhanced surface-active sites of the electrocatalyst.

Electrochemical performance

For electrochemical oxygen evolution reaction (OER) performance, the electrocatalytic efficiency of all the synthesized products has been investigated, and found that the composite materials worked exceptionally well. The best results were obtained with a 1:1 molar ratio of NiS/SSS and CeS/SSS in (1.0 M KOH) basic media at a scan rate of 5 mV s−1. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) curves are shown in Fig. 4a, b. The sweep scans from numerous tries were collected to guarantee that the performed data was reproducible in 1.0 M KOH electrolyte in the potential range of 1 to − 1 V vs. RHE. The synthesized NiS/CeS/SSS electrocatalyst performed well with the lowest overpotential of 289 mV at 10 mA cm−2, confirmed via cyclic and linear sweep voltammetry. Hence, the NiS/CeS/SSS electrocatalyst was better than the NiS (319 mV) and CeS (309 mV), and the recently reported materials. The comparative study of the previously reported results has been tabulated in Table 1. Tafel plots obtained from CV and LSV curves were used to explore the kinetics and processes of OER for all the synthesized materials like NiS/SSS, CeS/SSS, and NiS/CeS/SSS nanocomposite. All the materials have different kinetic rates, i.e., the pure sample having the dynamic rates of 78 mV dec−1 (NiS), 72 mV dec−1 (CeS), and the nanocomposite having the highest kinetic rates of 44 mV dec−1 as shown in Fig. 4c. In OER processes with four electrons, theoretical Tafel slopes of 40 mV dec−1 correspond to a third charge transfer rate regulating phase. The decrease in the Tafel slope indicates that the OER surface-adsorbed species are still dominating, while the rise in the Tafel slope presents that the surface species formed in the step immediately preceding the rate-determining phase is decreasing. NiS/CeS/SSS nanocomposite has a lower Tafel slope (44 mV dec−1), indicating that the reactant (OH) is more readily adsorbed, implying a higher OER rate. The decrease in the Tafel slope and the overpotential values could be due to the synergistic effect between NiS/SSS and CeS/SSS. Figure 4d represents the overpotential comparison study of all the synthesized products. The four-step mechanism for the electrochemical OER process was discussed as:

Fig. 4
figure 4

a CV, b LSV curves, c Tafel slope, and d comparison of overpotential of all the synthesized electrocatalysts

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Table 1 Comparative study of already reported results with the present study
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$$\mathrm{OH}^-+\ast\rightarrow\mathrm{OH}\ast+\mathrm e^-$$
(1)
$$\mathrm{OH}\ast+\mathrm{OH}-\rightarrow\mathrm O\ast+{\mathrm H}_2\mathrm O+\mathrm e^-$$
(2)
$$\mathrm O\ast+\mathrm{OH}-\rightarrow\mathrm{OOH}\ast+\mathrm e^-$$
(3)
$$\mathrm{OOH}\ast+\mathrm{OH}^-\rightarrow\ast+{\mathrm O}_2+{\mathrm H}_2\mathrm O+\mathrm e^-$$
(4)

Cyclic voltammetry curves were used to assess the electrochemically active surface area of electrocatalysts to investigate their improved water oxidation activity, as depicted in Fig. 5a–c. A rough surface and many active sites influence the reaction’s catalytic activity, resulting in an enhanced electrochemically active surface area (ECSA). The calculated Cdl values for all the fabricated materials such as pure NiS/SSS, CeS/SSS, and NiS/CeS/SSS nanocomposite were 64 mF cm−2, 50 mF cm−2, and 77.5 mF cm−2, respectively, as shown in Fig. 5d–f. To calculate the ECSA, we normalized the Cdl to the previously used specific capacitance (Cs) of 0.04 mF cm−2 for a flat surface [62]. According to this study, the flower looks of the nanocomposite aids in the creation of a large number of active sites, which boosts the OER efficiency.

Fig. 5
figure 5

a–c CV curves, d–f linear plot of change in current density vs. scan rate, respectively

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To better understand the charge transport process within the electrodes, electrochemical impedance spectra (EIS) were employed to analyze the synthesized samples. A semicircle follows a near-linear line in the high-frequency domain while in the low-frequency domain that bends toward the x-axis, as seen in Fig. 6a–d. Figure 6a represents the combined Nyquist plot of all the synthesized products. As shown in Fig. 6 (inset of b and d), an equivalent circuit can handle these designs. In most applications, the series resistance, abbreviated as Rs, is created by combining the contact resistance and the electrolyte resistance. The resistance to charge transfer (Rct) at the electrode/electrolyte interface in the OER process is referred to as Rct. Due to the unequal distribution of active catalytic sites on the surface, constant phase elements are favored over the simple electrochemical process in actual EIS modeling circumstances. The resultant Nyquist plots with Rs and Rct values are depicted in Fig. 6b–d. Among all products, the nanocomposite has a more negligible charge transfer resistance. The smaller the Rct value, the higher will be the conductivity of the electrocatalyst.

Fig. 6
figure 6

a Combined Nyquist plot, b–d Nyquist plot with their fitted circuit for NiS/SSS, CeS/SSS, and NiS/CeS/SSS nanocomposite

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Figure 7 depicts the stability tests of the NiS/CeS/SSS nanocomposite confirmed via cyclic voltammetry cycles (Fig. 7a) and chronoamperometry (Fig. 7b). After 1500 cycles, the CV curves show little change in the current density between the 1st and 1500th cycles, confirming the long-term stability. The chronoamperometric experiment also tested the electrocatalyst enduring. The electrode system that catalyzes the electrolysis of water undergoes a 40-h stability test. The electrode’s excellent catalytic stability, which shows its superior anti-corrosion qualities, allows it to be used during water electrolysis. Finally, the XRD pattern of the used electrocatalyst was also performed, to confirm the stability in the structure of the NiS/CeS/SSS nanocomposite as shown in Fig. 7c. The resultant pattern showed that there are no phase changes after long-term stability in alkaline media which again confirmed its stability in terms of its structure.

Fig. 7
figure 7

a Stability cycles, b chronoamperometry of the synthesized NiS/CeS/SSS nanocomposite, and c comparison of XRD before and after stability

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Conclusion

A simple hydrothermal procedure was used to successfully create NiS/SSS, CeS/SSS, and NiS/CeS/SSS nanocomposite. All the fabricated materials are analyzed via different analytical techniques to confirm their formation. The present study describes an oxygen evolution reaction using all the synthesized materials as working electrodes performing effectively in alkaline media. To attain a current density of 10 mA cm−2, a low overpotential of 289 mV is needed with a low Tafel slope of 44 mV dec−1 for easy electron transfer reaction. The extraordinary performance of water electrolysis is due to the flower-like morphology, high surface area, and enhanced electrochemical surface-active sites of the samples.