Efficient bifunctional water splitting catalysts enabled by crystalline–amorphous Ni_xS_y@NiFe LDH heterojunctions

Abstract

Developing crystalline–amorphous heterojunctions presents a promising pathway to enhance the electrocatalytic performance of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). This study reports the exploration of novel crystalline–amorphous Ni_xS_y@NiFe LDH heterostructures for efficient HER and OER, which are synthesized via in situ growth of NiFe-LDH on nickel sulfide (Ni_xS_y) nanowires. The resultant three-dimensional core–shell structures remarkably increase the active sites, enhance the charge transfer, and facilitate the gas release during the catalytic process. In situ Raman spectroscopy and density functional theory (DFT) calculations verify that the introduced Fe could boost the OER activity by promoting the structural reconstruction to form the disorder of NiOOH@NiFeOOH species, and reducing the energy barrier for conversion of oxygen-containing intermediates. The heterojunction interface in Ni_xS_y@NiFe LDH modifies the electron distribution, thus significantly lowering the Gibbs free energy of hydrogen adsorption (ΔG_H* = 0.1 eV) compared to that of Ni_xS_y. Correspondingly, the Ni_xS_y@NiFe LDH exhibits superior bifunctional performance for the HER and OER in alkaline solution, delivering high current densities of −100 and 200 mA cm⁻² at low overpotentials of 159 and 250 mV for the HER and OER, respectively, as well as an excellent stability against operation over 250 h at 200 mA cm⁻², implying its promise toward commercial applications.

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1. Introduction

With the acceleration of industrialization and the widespread use of fossil fuels, the problem of the greenhouse effect has become increasingly severe. As a result, reducing carbon emissions has become one of the most urgent global challenges. Hydrogen, as a clean energy source, is regarded as an ideal substitute for fossil fuels, and the electrolysis of water to produce hydrogen is considered a practical method for achieving this transition.^1–4 Over the years, extensive research has been conducted on electrochemical water splitting. The cathodic process involves a two-electron hydrogen evolution reaction, while the oxygen evolution reaction (OER) at the anode, which is a four-electron coupled proton transfer process, represents a significant barrier to efficient water splitting, ultimately limiting overall performance.^5,6 Currently, commercially available catalysts predominantly consist of precious metal catalysts, such as Pt- and Ru/Ir-based catalysts. However, their limited availability and high costs pose significant challenges for large-scale application. Therefore, the development of resourceful, cost-effective and high-performance bifunctional catalysts is crucial for achieving large-scale water splitting.^7,8 Consequently, researchers have explored various low-cost alternatives for the HER and OER including transition metal sulfides,^9,10 phosphides,^11,12 nitrides,^13,14 carbides^14,15 and metal hydroxides.^16–20 To minimize material costs and improve economic viability, researchers are focusing on the development of bifunctional catalysts with high catalytic activity and exceptional durability for use in electrolytic cells.

Layered double hydroxides (LDHs) are a class of materials with layered structures, where the main layers are composed of two different valence metal cations (such as Ni²⁺ and Fe³⁺) connected by oxygen bridges, and interlayer spaces are occupied by exchangeable anions (such as carbonate and nitrate) and water molecules.²¹ In 2013, Dai and his team first applied LDHs to the electrocatalytic oxygen evolution reaction (OER), sparking widespread attention in the field.^22–24 However, the intrinsic low electrical conductivity and limited exposure of active sites in LDHs have limited their catalytic performance. To overcome these limitations, researchers have employed various strategies to optimize the performance of LDHs, for example, doping, exfoliation, morphology modulation, and construction of heterojunctions. Guo et al. developed an efficient NiFeOOH–V_Zn catalyst through in situ leaching of Zn from the NiFeZn LDH.²⁵ This leaching process effectively lowers the formation potential of NiFeOOH active species, thereby significantly enhancing catalytic activity for the OER. Zhou et al. reported a defect-rich few-layered NiFe-LDH nanosheet with in situ borate modified by gas-exfoliation using NaBH₄ as a tri-functional reductant. The introduction of borate elevated the oxidation state of Ni, narrowed the band gap, and upshifted the d band center of the catalyst, resulting in a reduced energy barrier for the OER.²⁶ Compared with other structural adjustment strategies, the construction of conductive core and amorphous shell heterojunction interfaces is a particularly effective strategy for enhancing electrocatalytic performance.^27,28 This strategy offers several notable advantages: (1) overcoming the inherently low conductivity of LDHs; (2) maximizing the exposure of active sites to improve catalytic efficiency; (3) regulating the interfacial charge distribution and optimizing the electron structure; and (4) integrating diverse active species onto a single electrode, thereby achieving a multifunctional catalyst. The synergistic interaction of these factors not only significantly enhances catalytic performance but also imparts multifunctional catalytic properties to the electrode. For example, Lv and his team fabricated a Co₃O₄@NiFe LDH heterojunction for the OER, whose interface provides abundant active sites, thereby facilitating rapid charge transfer.²⁹ He et al. developed a NiTe@NiCo LDH heterojunction for the OER. The heterojunction significantly enhances the efficiency of electron transfer and catalytic activity through the significant interface effect, showing lower overpotential and superior stability compared to single-component materials.³⁰

In parallel, transition metal sulfides such as Ni₃S₂ have garnered attention as promising HER catalysts due to their excellent metallic conductivity and stability. For example, Zhang et al. demonstrated that copper doping in Ni₃S₂ significantly enhanced its HER performance by enhancing water adsorption and dissociation.³¹ Peng and his colleagues developed porous Ni/Ni₃S₂ micro-sheets on nickel foam, achieving excellent HER and OER activity.³² Ni₃S₂ also exhibits intrinsic OER catalytic activity and excellent conductivity, making it an ideal candidate for constructing efficient heterojunctions to simultaneously enhance both the OER and HER performance. Consequently, integrating LDH and Ni₃S₂ into a single electrocatalyst represents an efficient strategy for constructing highly effective bifunctional catalysts for both the OER and the HER. However, while several studies have reported bifunctional electrocatalysts based on LDH and Ni₃S₂, the rational design of a structure that maximizes the exposure of active species and enhances conductivity remains an important yet challenging task. Furthermore, systematically uncovering the synergistic interactions between LDH and Ni₃S₂ is critical for the theoretical design and development of high-performance catalysts.

Herein, we developed a crystalline–amorphous Ni_xS_y@NiFe LDH heterojunction bifunctional catalyst, characterized by crystalline Ni_xS_y nanowires serving as the core and amorphous NiFe LDH nanosheets forming the shell. The dynamic surface reconstruction mechanism during the OER process was unveiled through in situ Raman spectroscopy. The results reveal that the Ni_xS_y@NiFe LDH heterojunction undergoes a surface reconstruction process, leading to the formation of a Ni_xS_y@NiOOH@NiFeOOH heterojunction with an increased degree of structural disorder. Theoretical calculations further demonstrated that the reconstructed NiOOH@NiFeOOH active species possesses lower Gibbs free energies for the rate-limiting step in the conversion from *OH to *O, enhancing the OER performance. For the HER, theoretical calculations were employed to investigate the active sites in Ni_xS_y@NiFe LDH, including H adsorption on Ni_x*S_y@NiFe LDH and Ni_xS_y@Ni*Fe LDH, as well as Ni_x*S_y. The calculations confirmed that the Ni* sites in Ni*_xS_y@NiFe LDH exhibit an extremely low adsorption Gibbs free energy (ΔG_H* = 0.1 eV), significantly lower than those in Ni*_xS_y (0.32 eV) and Ni_xS_y@Ni*Fe LDH (0.71 eV). These findings indicate the heterojunction interface between Ni_xS_y and NiFe LDH efficiently regulates the electronic structure, thus improving the catalytic activity for the HER. As a result, the Ni_xS_y@NiFe LDH exhibited excellent catalytic activity and stability for both the OER and the HER. Specifically, it required an overpotential of 159 mV at −100 mA cm⁻² for the HER and 250 mV at 200 mA cm⁻² for the OER, while maintaining stable operation for over 250 hours. This innovative design not only achieves highly efficient and durable bifunctional electrocatalysts but also provides valuable insights into the synergistic catalytic mechanisms of heterojunction catalysts, paving the way for the development of advanced electrocatalysts for sustainable energy applications.

2. Results and discussion

2.1 Synthesis and characterization

The synthesis route for Ni_xS_y@NiFe LDH is illustrated in Fig. 1a. Ni_xS_y nanowire arrays (Fig. 1b) with an average diameter of approximately 70 nm were first synthesized on smooth nickel foam (Fig. S1†) using a hydrothermal method. The control group produced Ni_xS_y precursor catalysts with varying morphologies by precisely controlling the amount of sulfur powder added, as shown in Fig. S2a–f.† Subsequently, Ni_xS_y@NiFe LDH nanowire heterojunctions were obtained through chemical soaking in a FeCl₃·6H₂O aqueous solution at room temperature for 30 min. Following soaking with FeCl₃·6H₂O solution, numerous nanosheets were clearly observed to grow uniformly and vertically on the nanowires, forming a unique 3D core–shell nanostructure with a very rough surface (Fig. 1c and d). In the control group, different morphologies of Ni_xS_y@NiFe LDH catalysts (Fig. S3a–j†) were achieved by carefully regulating the soaking time. The Ni_xS_y nanowire arrays on nickel foam provide plenty of active sites for the growth of NiFe LDH nanosheets. Furthermore, these nanowires also function as excellent electron-conducting pathways. This novel hierarchical three-dimensional core–shell nanostructure not only offers abundant catalytically active sites but also facilitates contact with the electrolyte and promotes rapid release of generated gas bubbles.³³

Fig. 1 (a) Schematic illustration of the formation of Ni_xS_y@NiFe LDH nanowires. (b) SEM image of Ni_xS_y nanowires. (c) SEM image of Ni_xS_y@NiFe LDH nanowires. (d) TEM image of the Ni_xS_y@NiFe LDH nanowire. (e) SAED pattern of the Ni_xS_y@NiFe LDH nanowire. (f–f2) HR TEM images of the Ni_xS_y@NiFe LDH nanowire. (g and h) The EDS mapping and its EDS line scan results of Fe, S, Ni and O elements in the Ni_xS_y@NiFe LDH sample.

The TEM image in Fig. 1d further illustrates the core–shell nanostructures with a 50 nm core of Ni_xS_y nanorods and a 20 nm shell of NiFe LDH nanosheets. The interconnected NiFe LDH nanosheets provided abundant channels for rapid electrolyte diffusion and efficient gas release, and the interface between crystalline Ni_xS_y and amorphous NiFe LDH generates rich defects that modulate the electronic structure and facilitate charge redistribution at the boundary, thereby enhancing HER and OER performance. Fig. 1e displays the selected area electron diffraction (SAED) pattern of Ni_xS_y@NiFe LDH. The bright, evenly distributed diffraction spots are assigned to the (211) face of Ni₉S₈ and the (12̄3̄) and (300) faces of Ni₃S₂, indicating that the Ni_xS_y core exhibits a high degree of crystallinity. And the diffraction rings without distinct bright spots are associated with the amorphous (1̄21) and (011) faces of Ni(OH)₂ and the (150) face of Fe(OH)₃, confirming the amorphous nature of the NiFe-LDH shell. This combination of crystalline and amorphous structures highlights the unique core–shell architecture of Ni_xS_y@NiFe LDH. The HR-TEM images of Ni_xS_y@NiFe LDH (Fig. 1f) reveal lattice spacings of 0.19 nm for the (333) crystal face of Ni₉S₈ (Fig. 1f1) and 0.18 nm for the (300) crystal face of Ni₃S₂ (Fig. 1f2). The NiFe LDH nanosheets in Ni_xS_y@NiFe LDH exhibited numerous defects, as indicated by the arrows in Fig. S4.† These disordered defects distributed along the lattice edges of the NiFe LDH nanosheets, suggesting the presence of a large number of dangling bonds at the surface of these materials.³⁴ This can increase the number of exposed active sites and alter the electronic properties of the interface, thereby significantly enhancing the catalytic activity. The successful synthesis of Ni_xS_y@NiFe LDH core–shell nanostructures was further confirmed using the elemental mapping image (Fig. 1g) and Energy Dispersive X-ray (EDX) line scanning results (Fig. S5 and S2h†), clearly indicating the presence of Ni_xS_y in the central region and NiFe LDH in the outer region. The Ni and Fe contents in the fabricated samples were quantitatively analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES), as summarized in Table S1.† The Ni contents in Ni_xS_y@NiFe LDH, Ni_xS_y and NiFe LDH are 67.3%, 68.7% and 65.9%, respectively, while the Fe contents in Ni_xS_y@NiFe LDH and NiFe LDH are 3.5% and 3.3%, respectively. Additionally, we investigated the morphological changes of NiFe LDH at varying soaking times. The SEM images are presented in Fig. S6.† As the soaking time increases, the soaking patterns on the smooth nickel foam surface become more pronounced.

The structure of the as-prepared samples was further analyzed using X-ray diffraction (XRD). As shown in Fig. 2a, the diffraction peaks at 21.7°, 31.1°, 38.2°, 50.1° and 55.3° correspond to the (101), (110), (021), (211) and (300) crystal planes of Ni₃S₂ (JCPDS no. 44-1418), while the diffraction peaks at 18.3°, 27.3° and 51.3° belong to the (021), (131) and (423) crystal planes of Ni₉S₈ (JCPDS no. 07-0778). Following the modification of NiFe LDH nanosheets, no new diffraction peaks were detected, which can be attributed to the amorphous state of the NiFe LDH nanosheets consistent with the TEM results.³⁵ The prepared samples were further characterized using Raman spectroscopy. As shown in Fig. 2b, a prominent characteristic peak at 377 cm⁻¹ associated with a non-vulcanized Ni–S bond was observed in the Ni_xS_y sample.³⁶ The Raman peaks at 178 and 282 cm⁻¹ correspond to characteristic vibrations of Ni–S and Fe–O bonds, while the broad bands detected at 452 cm⁻¹ and 535 cm⁻¹ are attributed to the presence of disordered Ni(OH)₂ clusters. Additionally, the peak at 658 cm⁻¹ corresponds to the Fe–O bond within disordered FeOOH nanosheets, which were observed in Ni_xS_y@NiFe LDH. These results also verify the successful preparation of Ni_xS_y@NiFe LDH heterojunctions.^37,38

Fig. 2 (a) XRD patterns of Ni_xS_y and Ni_xS_y@NiFe LDH, (b) Raman spectra of Ni_xS_y and Ni_xS_y@NiFe LDH. High-resolution XPS spectra of (c) Ni 2p, (d) Fe 2p, (e) S 2p and (f) O 1s of Ni_xS_y and Ni_xS_y@NiFe LDH samples.

X-ray photoelectron spectroscopy (XPS) was employed for further analyzing the elemental composition and chemical valence states of the as-fabricated samples. The XPS survey scan shown in Fig. S7† reveals the presence of O, S, Fe, and Ni elements in Ni_xS_y@NiFe LDH, while the characteristic signals associated with Fe are absent in the Ni_xS_y sample. In the high-resolution Ni 2p spectra depicted in Fig. 2c, the Ni⁰ characteristic peak at 851.5 eV is only observed in Ni_xS_y,³⁹ and the Ni⁰ peak disappears in Ni_xS_y@NiFe LDH, possibly due to signal attenuation and oxidation of high-valence metal ions from the NiFe LDH layer. Nevertheless, the binding energies at 854.78 eV (Ni 2p_1/2) and 872.83 eV (Ni 2p_3/2) for Ni_xS_y@NiFe LDH are shifted toward higher values compared to those of Ni_xS_y, indicating a significant electron interaction between the Ni_xS_y core and the NiFe LDH shell.⁴⁰ In the Fe 2p spectra presented in Fig. 2d, the Fe 2p_3/2 and 2p_1/2 peaks of Fe²⁺ are located at 708.7 and 722.3 eV, and Fe 2p_3/2 and 2p_1/2 peaks of Fe³⁺ are located at 711.2 and 724.5 eV, respectively. Fe²⁺ arises from the reduction of trivalent iron ions to bivalent ions during sulfide soaking with trivalent iron salts, while a peak at approximately 704.8 eV is attributed to Fe^δ+ associated with an Fe–S bond,⁴¹ suggesting that the Fe–S bonds are formed during the soaking process. Peak fitting of the S 2p spectrum of Ni_xS_y (Fig. 2e) reveals a pair of peaks at 160.73 and 162.03 eV, along with a broad peak at 167.07 eV. The former peaks correspond to the presence of a M–S bond, while the latter peak indicates the existence of a surface sulfur–oxygen bond.⁴² In the Ni_xS_y@NiFe LDH sample, the shift towards lower binding energy in the sulfur–nickel metallic bond signals confirms electron transfer between Ni–S–Fe. The peaks centered at 532.19, 530.60, and 529.50 eV in the O 1s spectrum correspond to surface-adsorbed molecular water (Fig. 2f), hydroxyl groups, and metal oxide species, respectively.⁴³ The content of M–O in Ni_xS_y@NiFe LDH is significantly increased due to the formation of Ni–O and Fe–O bonds in the soaking process. These findings demonstrate that oxygen-containing nickel and iron species are successfully associated with binding Ni_xS_y during the soaking process, leading to alterations in electronic states and thus enhancing the catalytic activity.

2.2 Electrocatalytic performance

The OER and HER performances of the as-fabricated electrodes were investigated and compared with those of RuO₂ and Pt/C coated nickel foam (RuO₂/NF and Pt/C/NF) in 1.0 M KOH electrolyte. In order to obtain the best performance for the OER and HER, Ni_xS_y precursors and Ni_xS_y@NiFe LDH were evaluated by varying the amount of S powder (Fig. S8†) and the soaking time (Fig. S9†). It is worth noting that the Ni_xS_y precursor and Ni_xS_y@NiFe LDH electrode exhibit superior OER and HER performance when using a S powder dosage of 60 mg and a soaking time of 30 min, respectively.

2.2.1 Electrocatalytic performance for alkaline OER. As previously mentioned, the NiFe LDH exhibits commendable catalytic activity for the OER in alkaline electrolytes. The LSV curves shown in Fig. 3a and the corresponding overpotentials at the current densities of 150, 250, and 300 mA cm⁻² shown in Fig. 3b indicate that Ni_xS_y@NiFe LDH exhibits excellent OER performance. At the current density of 200 mA cm⁻², an overpotential of only 250 mV is required, markedly lower than that of Ni_xS_y (390 mV) and NiFe LDH (370 mV), and significantly outperforming the commercial catalyst RuO₂ (470 mV). The oxidation peak observed in the LSV curves corresponds to the transition of nickel from a bivalent state to a trivalent state, leading to the formation of NiOOH, which has been confirmed as the active site in nickel-based catalysts.⁴⁴ It is obvious that the oxidation potential of Ni_xS_y@NiFe LDH is lower than that of Ni_xS_y, confirming that the introduction of Fe facilitates the reconstruction process and enhances the OER catalytic activity. Compared to the most previously reported OER catalysts, Ni_xS_y@NiFe LDH also exhibits superior catalytic activity, as shown in Table. S2.† Furthermore, we extracted the Tafel slope from the polarization curves to evaluate the OER kinetics. As illustrated in Fig. 3c, the Ni_xS_y@NiFe LDH exhibits the smallest Tafel slope of 39.1 mV dec⁻¹ compared to those of NiFe LDH (47.9 mV dec⁻¹), Ni_xS_y (129.7 mV dec⁻¹), and RuO₂ (139.1 mV dec⁻¹), indicating its superior catalytic kinetics. These results signify that the Ni_xS_y@NiFe LDH catalyst possesses remarkable intrinsic activity for the OER. Electrochemical active surface area (ECSA) is positively correlated with the number of exposed active sites of the materials, which can be evaluated from double-layer capacitance. Fig. 3d depicts the ECSA derived from the corresponding C_dl (Fig. S10†). The Ni_xS_y@NiFe LDH electrode demonstrates the maximum ECSA value of 899, which is 4.4 and 11 times greater than that of Ni_xS_y (202) and NiFe LDH (82), respectively. These results underscore that the rational design of 3D core–shell nanostructures can effectively enhance ECSA and maximize the exposure of active sites.

Fig. 3 (a) IR-corrected polarization curves of the fabricated samples for the OER in 1 M KOH. (b) The overpotential histogram at 150, 200, and 300 mA cm⁻². (c) The Tafel slopes for the corresponding samples. (d) Plots of the C_dl. (e) EIS spectra of Ni_xS_y@NiFe LDH and other control samples. (f) The chronopotentiometric curves of Ni_xS_y@NiFe LDH and RuO₂-NF at 200 mA cm⁻².

Turnover frequency (TOF) is one of the important parameters for evaluating the kinetics of electrocatalysts. According to previous studies, TOF was measured by electrochemical methods.⁴⁵ Fig. S11† shows the linear relationship between the reduction peaks of the fabricated samples at different scan rates, with the detailed calculation process described in the TOF calculation section. As shown in Fig. S12a,† the Ni_xS_y@NiFe LDH catalyst exhibited the highest TOF value. And at an overpotential of 250 mV, the TOF value of Ni_xS_y@NiFe LDH (0.82) was significantly higher than that of NiFe LDH (0.01) and Ni_xS_y (0.02), as shown in Fig. S12b.† To investigate the intrinsic OER catalytic activity of the fabricated catalysts, the OER kinetic current was normalized using their corresponding ECSA and roughness factor (R_f). As shown in Fig. S13a,† the Ni_xS_y@NiFe LDH electrode exhibited an overpotential of 267 mV at a current density of 0.5 mA cm⁻²_ECSA, which was 53 and 83 mV lower than those of NiFe LDH and Ni_xS_y electrodes, respectively. By standardizing the OER kinetic current using the R_f,¹⁹ the specific activity (j_s) of the electrode was evaluated at 1.48 V vs. RHE. As shown in Fig. S13b,† the j_s of Ni_xS_y@NiFe LDH was 0.21 mA cm⁻²_Rf, significantly higher than those of NiFe LDH (0.064 mA cm⁻²_Rf) and Ni_xS_y (0.0047 mA cm⁻²_Rf).

The electrochemical impedance spectroscopy (EIS) results indicate that Ni_xS_y@NiFe LDH exhibits the lowest charge-transfer resistance among the catalysts (R_ct = 0.91 Ω, Table. S3†). This highlights its superior ability to facilitate electron transfer and accelerate kinetics during the OER process, which is attributed to the crystalline Ni_xS_y core (Fig. 3e). In addition, durability is another critical parameter for OER catalysts. As demonstrated in Fig. 3f, the Ni_xS_y@NiFe LDH electrocatalyst shows excellent long-term stability by retaining 96.8% performance at a current density of 200 mA cm⁻² over a duration of 250 h.

2.2.2 Electrocatalytic performance for alkaline HER and overall water splitting performance. Fig. 4a presents the polarization curves for the as-fabricated materials and commercial Pt/C for the HER in 1 M KOH. Notably, the Ni_xS_y@NiFe LDH demonstrates excellent HER activity, surpassing those of Ni_xS_y, NiFe LDH, and bare NF; however, its performance is slightly inferior to that of commercial Pt/C at low current densities. For a comprehensive comparison, Fig. 4b displays overpotentials for different catalysts at current densities of 10, 100, and 200 mA cm⁻². In particular, the overpotential of the Ni_xS_y@NiFe LDH is significantly reduced to only 82 mV, which is 158 and 13 mV lower than that of NiFe LDH and Ni_xS_y at 10 mA cm⁻², respectively. Furthermore, this catalyst exhibits excellent performance at high current densities, with a minimum overpotential of 159 mV at 100 mA cm⁻². In contrast, pure NiFe LDH and Ni_xS_y require higher overpotentials to achieve the same current densities; specifically, values are recorded as follows: 360 mV for NiFe LDH and 208 mV for Ni_xS_y. The Ni_xS_y@NiFe LDH catalyst also exhibits superior HER catalytic activity compared with most reported materials summarized in Table S4.† The Tafel slope was determined from polarization curves to evaluate the HER kinetics. As shown in Fig. 4c, the Tafel slope values for Pt/C-NF, Ni_xS_y@NiFe LDH, NiFe LDH, and Ni_xS_y are 64.4, 84.5, 125.2, and 105.7 mV dec⁻¹, respectively. This result suggests that the Ni_xS_y@NiFe LDH heterojunctions favor accelerated HER kinetics. A larger electrochemical active surface area (ECSA) facilitates water molecule adsorption, promotes close contact with the electrolyte, and provides abundant catalytic reaction sites, which undoubtedly contributes to improved activity. Remarkably, as shown in Fig. 4d, the C_dl values of Ni_xS_y@NiFe LDH, Ni_xS_y, and NiFe LDH are 28.26, 21.33 and 2.31 mF cm⁻², respectively. The corresponding ECSA values shown in Fig. S14d† demonstrated that Ni_xS_y@NiFe LDH exhibited a maximum ECSA value of 706, surpassing that of Ni_xS_y (533) by 1.3 times and NiFe LDH (58) by an impressive factor of 12 times. These findings highlight that the three-dimensional core–shell nanostructures can significantly enhance ECSA in terms of active site exposure, thereby potentially contributing to the observed improvements in HER activity. Fig. 4e and Table S5† show that the R_ct value of Ni_xS_y@NiFe LDH (5.23 Ω) is smaller than those of Ni_xS_y (8.64 Ω) and NiFe LDH (15.63 Ω), suggesting that the Ni_xS_y@NiFe LDH electrode has low charge transfer resistance and rapid electrocatalytic kinetics. The intrinsic HER catalytic activity of the catalysts was also normalized using their corresponding ECSA and R_f. As shown in Fig. S15a,† the voltage of the Ni_xS_y@NiFe LDH electrode at −0.4 mA cm⁻²_ECSA is −0.195 V_RHE, which is 51 and 102 mV lower than those of the Ni_xS_y and NiFe LDH electrodes, respectively. By standardizing the HER kinetic current using the R_f, the j_s of the electrode was assessed at −0.17 V vs. RHE. As depicted in Fig. S15b,† the j_s of Ni_xS_y@NiFe LDH reached −0.16 mA cm⁻²_Rf, markedly higher than those of NiFe LDH (−0.034 mA cm⁻²_Rf) and Ni_xS_y (−0.083 mA cm⁻²_Rf), underscoring its enhanced intrinsic HER performance. The stability was evaluated at a constant current density of −200 mA cm⁻² (Fig. 4f). The results show that the Ni_xS_y@NiFe LDH electrode can retain 85% of its catalytic activity after 250 h, which is significantly higher than that of Pt/C-NF and superior to those of other TMS-based electrocatalysts, as shown in Table. S4.†

Fig. 4 (a) IR-corrected polarization curves of the fabricated samples for the HER in 1 M KOH. (b) The overpotential histogram at −10, −100, and −200 mA cm⁻². (c) Tafel slopes. (d) Plots of the C_dl. (e) EIS spectra of Ni_xS_y@NiFe LDH and other control samples. (f) The chronopotentiometric curves of Ni_xS_y@NiFe LDH and Pt/C at −200 mA cm⁻². (g) The overall water splitting performance of coupled Ni_xS_y@NiFe LDH‖Ni_xS_y@NiFe LDH and Pt/C-NF‖RuO₂-NF electrolyzers. (h) The corresponding chronopotentiometric curves of the electrolyzers at 50 mA cm⁻² in 1 M KOH. (i) Comparison of experimental and theoretical amounts of generated H₂ and O₂ over 400 min using the Ni_xS_y@NiFe LDH ‖ Ni_xS_y@NiFe LDH electrolyzer.

Based on the results derived from the aforementioned OER and HER analyses, it can be concluded that the bifunctional Ni_xS_y@NiFe LDH catalyst demonstrates exceptional efficiency in water electrocatalysis within alkaline solutions. Subsequently, we assembled an overall water electrolysis cell based on Ni_xS_y@NiFe LDH as both the anode and the cathode. Concurrently, the cell based on the noble metal Pt/C-NF‖RuO₂-NF was tested under the same conditions. As depicted in Fig. 4g, the cell voltage for Ni_xS_y@NiFe LDH at a current density of 10 mA cm⁻² was measured to be 1.48 V, which is lower than that observed for the cell based on commercial Pt/C-NF‖RuO₂-NF (1.59 V). A comprehensive comparison of the overall water electrolysis activity of Ni_xS_y@NiFe LDH with other reported catalysts is presented in Table S6.† Notably, Ni_xS_y@NiFe LDH surpasses most of the existing overall water electrolysis catalysts. The long-term stability of overall water electrolysis at a high current density of 50 mA cm⁻² was investigated (Fig. 4h). Ni_xS_y@NiFe LDH also exhibited excellent stability without obvious decay after 120 h of stability testing. Thus, the Ni_xS_y@NiFe LDH bifunctional catalyst exhibits large ECSA, excellent conductivity, and superior stability—factors that strongly support efficient water electrolysis. At a constant current density of 20 mA cm⁻², gas products generated during water electrolysis were collected utilizing a self-constructed gas collection system (Fig. 4i and S16†). Following this collection period, it was determined that the ratio of H₂ to O₂ produced was maintained at approximately 2 : 1. These measured values align closely with theoretical expectations, indicating Faraday efficiency approaching nearly 100%. Fig. S17† reveals that a working voltage of 1.5 V can successfully drive an overall water splitting cell assembled with Ni_xS_y@NiFe LDH, suggesting that the Ni_xS_y@NiFe LDH exhibits broad prospects in practical applications.

2.3 Further analysis of the catalytic mechanism of Ni_xS_y@NiFe LDH

2.3.1 The catalytic mechanism for the OER. To investigate the relationship between the structure of Ni_xS_y@NiFe LDH and its OER performance, the structural evolution of Ni_xS_y and Ni_xS_y@NiFe LDH during the OER process was studied using in situ Raman spectroscopy (Fig. 5a and b). The peak at 200–400 cm⁻¹ was attributed to the Ni–S bond in Ni_xS_y, and new peaks appeared until the voltage reached 1.63 V in both Ni_xS_y and Ni_xS_y@NiFe LDH samples. When the applied voltage was further increased, two new vibrational peaks were observed at approximately 472 and 556 cm⁻¹, which corresponded to the vibrational modes of Ni–O in γ-NiOOH. As the potential increases, the peaks of Ni_xS_y in the Ni_xS_y sample disappear, and new NiOOH peaks are formed, which is attributed to the transformation of Ni_xS_y to NiOOH during the OER process. In contrast, in the Ni_xS_y@NiFe LDH sample, except for the transformation of Ni_xS_y to NiOOH, two peaks of Ni_xS_y@NiFe LDH at 452 and 535 cm⁻¹ were blue-shifted to 472 and 556 cm⁻¹, respectively, corresponding to the transformation of NiFe(OH)₂ to NiFeOOH during the OER process. Meanwhile, the peak intensity of Ni_xS_y@NiFe LDH at 1.63 V is significantly higher than that of Ni_xS_y, indicating that Fe promotes structural evolution, forms high-activity sites, and also leads to lattice disorder.⁴⁶ Additionally, the peak areas corresponding to the various vibrational modes of NiOOH provide insight into the disorder present within the Ni–O lattice. We analyzed the interpeak area of the NiOOH peak generated at 1.63 and 1.68 V, as shown in Fig. 5c and d. The A₅₅₆/A₄₇₂ ratio is an indicator of the disorder degree in the NiOOH structure, with a higher ratio indicating stronger catalytic activity. It is worth noting that the A₅₅₆/A₄₇₂ ratio of Ni_xS_y@NiFe LDH at 1.63 V and 1.68 V was 0.76 and 0.68, respectively, exceeding the values of 0.51 and 0.56 for Ni_xS_y. This indicates that the reconstituted NiOOH in Ni_xS_y@NiFe LDH has a higher degree of disorder, thus endowing it with excellent catalytic activity. Overall, the improved OER performance of the Ni_xS_y@NiFe LDH composite material can be primarily attributed to the increased phase transition and structural disorder in NiOOH.

Fig. 5 (a and b) In situ Raman spectra of Ni_xS_y and Ni_xS_y/NiFe LDH; (c and d) peak areas of Ni_xS_y and Ni_xS_y/NiFe LDH at potentials of 1.63 and 1.68 V. (e) Raman spectra of Ni_xS_y@NiFe LDH after the 250 h stability test; (f) XRD spectra of Ni_xS_y@NiFe LDH after the OER stability test for 250 h; (g and h) TEM images of the Ni_xS_y@NiFe LDH after the OER stability test. High-resolution XPS spectra of (i) Ni 2p and O 1s (j) of Ni_xS_y@NiFeOOH.

To gain a deeper understanding of the reaction mechanism, the systematic characterization of the Ni_xS_y@NiFe LDH electrode after the OER was carried out using Raman spectroscopy, XRD, SEM, TEM, and XPS. As shown in Fig. 5e, following the OER test, two distinct peaks at 472 and 556 cm⁻¹ attributed to NiOOH emerged in the Raman spectrum, indicating that the surface of Ni_xS_y and NiFe LDH was converted to NiOOH and NiFeOOH, respectively. The XRD pattern (Fig. 5f) shows a reduction in the overall intensity of the Ni_xS_y diffraction peaks, while the SEM images demonstrate that the nanowire morphology of Ni_xS_y@NiFe LDH was well preserved (Fig. S18†). The TEM image shown in Fig. 5g further confirms the morphology of the nanowires after the OER test. The SAED results show the (013), (1̄21), and (005) crystal planes of γ-NiOOH with poor crystallinity, as well as the (300) face of Ni₃S₂ that retains its crystal structure, which is consistent with the results of HRTEM, as shown in Fig. 5h. During the long-term stability test, the lattice fringes of Ni₃S₂ were still clearly visible, with a 0.16 nm lattice fringe clearly corresponding to the (300) plane of Ni₃S₂. Following the OER stability assessment, the STEM image and corresponding element mapping of Ni_xS_y@NiFe LDH revealed the coexistence of Fe, Ni, O, and S elements, as demonstrated in Fig. S19a.† From the Ni 2p spectrum analysis, it was observed that the Ni 2p peak (Fig. 5i) exhibits an overall blue shift, indicating an increase in the nickel oxidation state, with high-valent nickel typically being considered the active species for the OER. This corresponds to the in situ Raman results. In the O1s spectrum shown in Fig. 5j, we identified metal hydroxide as the predominant oxygen component, which increased from an initial value of 36.33% to 46.05% after prolonged testing. Furthermore, according to the ESI data (Fig. S5 and S19b†), the oxygen content increased from 40.96% to 56.72%, indicating the formation of a thin metal hydroxide film on the heterogeneous surface. As shown in Fig. S20a,† there was an overall blue shift observed for the S 2p peak along with its transformation into a higher oxidation state. However, as depicted in Fig. 1h, S5, S19b and c,† following the OER stability test, the sulfur content in the Ni_xS_y@NiFe LDH nanowire core decreased significantly, which can be attributed to the desulfurization oxidation of sulfur-containing substances and the difference in depth of penetration between XPS and EDS mapping techniques.⁴⁷ Additionally, a similar blue shift was noted in the Fe 2p spectra (Fig. S20b†).

To further investigate the mechanism for the high performance of Ni_xS_y@NiFe LDH, DFT calculations were carried out separately for NiOOH and NiOOH@NiFeOOH, where the reconstructed structure was used as a computational model, as shown in Fig. S21a and b.† The alkaline OER consists of four basic steps: first, adsorption of OH⁻ to form *OH, followed by the generation of *O and *OOH intermediates, and finally the desorption of O₂, as demonstrated in Fig. 6a. The Gibbs free energies of the four steps shown in Fig. 6b suggest that the rate-determining step (RDS) for both NiOOH and NiOOH@NiFeOOH is the deprotonation process of *OH to form *O. Compared with NiOOH, the ΔG_(*OH) of NiOOH@NiFeOOH is 1.6 eV, lower than that of NiOOH (2.15 eV)(Fig. S22†). The decrease of energy barriers in the RDS for NiOOH@NiFeOOH is beneficial to facilitate the desorption of OH and improve the OER catalytic activity. The above results confirmed that the reconstructed NiOOH@NiFeOOH is the real active species for the OER. The introduction of Fe into the materials promotes the structural transition and increases the degree of structural disorder. As a result, the Ni_xS_y@NiFe LDH exhibits remarkable OER catalytic activity.

Fig. 6 (a) Structural model and schematic illustration of the OER pathway of NiOOH@NiFeOOH. Color scheme for chemical representation: grey for Ni, brown for Fe, red for O, yellow for S, and pink for H atoms, respectively; (b) free energy diagram of the OER process for NiOOH and NiOOH@NiFeOOH; (c) structural model and schematic illustration of the HER pathway of Ni_x*S_y@NiFe LDH; free energy diagram of the transition state of H₂O dissociation (d) and free energy diagram of H₂ adsorption (e) for Ni_x*S_y@NiFe LDH, Ni_xS_y@Ni*Fe LDH and Ni_x*S_y.

2.3.2 The catalytic mechanism for the HER. Meanwhile, the Ni_xS_y@NiFe LDH electrode after the HER stability test was characterized using XRD, Raman spectroscopy, SEM, TEM, and XPS. The XRD pattern (Fig. S26†) shows that the Ni_xS_y crystal structure remained essentially unchanged. The SEM results indicate that the morphology of the Ni_xS_y@NiFe LDH nanowire structure (Fig. S27†) remained largely unchanged before and after the HER test. This suggests that the structure of the Ni_xS_y nanowire and NiFe LDH nanosheet is stable under the conditions of the HER process. Furthermore, in the HRTEM (Fig. S28†) images, the (333) and (300) planes of Ni₉S₈ and Ni₃S₂, as well as clear core–shell structural features, were observed. In the EDS mapping, the elements of Fe, Ni, O, and S are evenly distributed throughout the interior of the Ni_xS_y@NiFe LDH nanowires (Fig. S29†). It is worth noting that the S element is only present in the core part of the core–shell structure, consistent with the initial state. Additionally, we observed that the content of the O element in the EDS spectrum and line scan analysis significantly decreased after the HER (Fig. S29b and c†). Furthermore, after long-term HER testing, the Ni_xS_y@NiFe LDH XPS spectrum (Fig. S30†) exhibited a high degree of similarity to the one before the stability test. However, the peak of SO₄²⁻ in the S 2p region was significantly reduced, which is consistent with the previous EDS results. Meanwhile, the Fe–O peak observed in the Raman spectrum after the HER test also showed a weakening trend (Fig. S31†). To delve deeper into the catalytic mechanisms of the materials fabricated for the HER, we conducted in situ Raman spectroscopy tests (Fig. S32†) on Ni_xS_y and Ni_xS_y@NiFe LDH, and the results showed that no new substances were formed during the test.

Given that no redox reaction occurred during the HER process and Ni₃S₂ was still present after the reaction, we constructed a model similar to the Ni₃S₂@NiFe LDH and Ni₃S₂ structures (Fig. S21c and d†), and all possible active sites in both Ni₃S₂ and Ni₃S₂@NiFe LDH were screened for better explain the relationship between the activity and structure. Activity towards the HER is strongly correlated with the energy barrier of the transition state of water dissociation. Fig. 6c and d demonstrate that the energy barrier of the transition state of water dissociation on Ni₃*S₂@NiFe LDH is 1.04 eV, which is significantly lower than those of Ni₃S₂@Ni*Fe LDH (1.56 eV, Fig. S23†) and Ni₃*S₂ (1.14 eV, Fig. S24†). As for H adsorption, the corresponding adsorption model is shown in Fig. S25.† The calculation results showed that Ni₃*S₂@NiFe LDH exhibits the lowest hydrogen adsorption energy of 0.1 eV (Fig. 6e), which is much lower than the values for Ni₃S₂@Ni*Fe LDH (0.71 eV) and Ni₃*S₂ (0.32 eV). These results confirm that the active sites for water dissociation and hydrogen adsorption in Ni₃S₂@NiFe LDH are primarily located in the Ni₃*S₂@NiFe LDH rather than in the Ni₃S₂@Ni*Fe LDH, indicating that the interfacial interaction between Ni_xS_y and NiFe LDH endows the Ni_xS_y@NiFe LDH with excellent catalytic activity.

3. Conclusion

In summary, the Ni_xS_y@NiFe LDH bifunctional electrocatalyst, featuring a three-dimensional crystalline–amorphous core–shell structure, demonstrated excellent performance for both the OER and the HER in alkaline solutions. It exhibited impressive electrocatalytic performance, with overpotentials of 159 mV for the HER at −100 mA cm⁻² and 250 mV for the OER at 200 mA cm⁻², alongside outstanding long-term stability for over 250 h at 200 mA cm⁻². Additionally, the electrolyzer utilizing this material as both the cathode and the anode operated at a low potential of 1.48 V at a current density of 10 mA cm⁻² and retained robust performance in long-term stability tests. The exceptional bifunctional catalytic properties are attributed to its structural properties: (1) the crystalline–amorphous structure enhances the maximization of catalytically active sites and promotes efficient electron/proton transport, (2) the heterojunction interface changes interfacial charge distribution and optimizes the free energy of adsorption for reactive intermediates. As for the OER, the incorporation of iron into the structure enhances the catalytic activity by promoting structural transformation and increasing the degree of disorder of NiOOH active species. In terms of the HER, the unique interface between the Ni_xS_y core and NiFe LDH shell optimizes hydrogen adsorption, significantly improving HER activity.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

Shaobo Ye: conceptualization, investigation, experimental analysis, methodology, and writing original draft. Yong Xu: conceptualization, methodology, and data curation. Xiaoyu Bai: methodology and experimental analysis. Zhao Liang: methodology. Qiao Liu: methodology. Qiliang Wei: experimental analysis. Dongjiang Yang: methodology. Weiyou Yang: conceptualization, funding, and review & editing. Fengmei Gao: conceptualization, funding, and review & editing. Qing Shi: conceptualization, funding, and review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 52302144, 52372063, and 52202061), the Scientific Research Starting Foundation of Ningbo University of Technology (2022KQ39), the Natural Science Foundation of Shanxi Province (202101D121119), and the Natural Science Foundation of Hunan Province (Grant No. 2022JJ40068).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta00601e

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