Lattice Oxygen–Mediated Water Oxidation on Reconstructed Ni₃S₂/NiFeOOH Heterointerfaces

ABSTRACT

Developing cost‐effective oxygen evolution reaction (OER) catalysts for industrial current densities remains challenging, hindered by adsorbate evolution mechanism (AEM) scaling limitations and reconstruction instability. Here, we report a heterointerface‐engineered Ni₃S₂/NiFeOOH catalyst formed via in‐situ electrochemical reconstruction of Ni₃S₂/NiFe‐LDH. Interfacial charge redistribution stabilizes high‐valent Ni³⁺/Fe³⁺ species and generates coordinatively unsaturated lattice oxygen sites, thereby activating the lattice oxygen mechanism (LOM). Combined operando spectroscopy, pH‐dependent kinetics, and isotope labeling confirm LOM dominance. Further theoretical analyses reveal that the heterointerface downshifts the metal d‐band center and upshifts the O 2p‐band center, optimized intermediate adsorption free energy. Benefiting from the compatible multi‐mechanism, the reconstructed catalyst demonstrates outstanding OER performance, only requires overpotentials of 196/305 mV to drive current densities of 10/1000 mA cm⁻² in alkaline media, with robust stability for over 500 h. This work clarifies how interfacial electronic modulation connects pre‐catalyst reconstruction to LOM activation, providing a scalable design strategy for high‐current‐density OER electrocatalysts.

Upon electrochemical activation, Ni₃S₂/NiFe‐LDH catalysts undergo transformation to Ni₃S₂/NiFeOOH. This structural reconstruction not only exposes additional catalytic active sites but also activates lattice oxygen species, thereby triggering the lattice oxygen reaction pathway. By circumventing the thermodynamic constraints inherent to the adsorbate mechanism, the reconstructed catalyst achieves a reduced overpotential, ultimately enhancing its oxygen evolution reaction (OER) catalytic performance.

1. Introduction

Hydrogen is increasingly regarded as a sustainable alternative to fossil fuels owing to its high energy density, cleanliness, and efficiency [1, 2, 3, 4, 5, 6]. Among hydrogen production methods, electrochemical water splitting is particularly attractive, but its efficiency is limited by the sluggish kinetics of the OER [7]. Noble metal catalysts such as Ir and Ru exhibit excellent OER activity, yet their scarcity and high cost hinder large‐scale deployment [8, 9]. This challenge has motivated the search for cost‐effective, high‐performance alternatives. NiFe layered double hydroxides (NiFe‐LDHs) have emerged as promising nonprecious alternatives due to their high intrinsic activity [10, 11, 12, 13, 14]. Heteroatom doping, heterojunction interface regulation, defect engineering, and other methods are employed to modify NiFe‐LDH, optimizing its adsorption capacity for oxygen intermediates and reducing the reaction energy barrier [15, 16, 17]. However, their OER performance is restricted by the AEM, which imposes a thermodynamic overpotential limit of 0.37 V [18, 19, 20, 21, 22]. Furthermore, uncontrolled metal atom migration during reconstruction compromises long‐term stability. Achieving precise reconstruction while steering the reaction pathway from AEM to LOM remains a critical challenge in catalyst design. Unlike AEM, it leverages lattice oxygen to generate intermediates, bypassing large energy barriers and overcoming the thermodynamic limitations of AEM. It is noteworthy that heterogeneous interface engineering has been demonstrated to regulate the binding ability of metal sites to oxygen species. Transition metal sites have been observed to oxidize readily to hydroxyl oxidizing species during the pre‐oxidation process of OER [23, 24, 25, 26]. This observation suggests the potential for these materials to follow the lattice oxygen mechanism, thereby achieving an exceptionally low overpotential. NiFe‐LDH undergoes autonomous reconstruction during electrolysis, and hydroxides are transformed under voltage to form hydroxyl oxides [27, 28]. Promoting the reconstitution of NiFe‐LDH into hydroxyl oxides facilitates the LOM reaction of the catalyst and enhances the catalytic activity. In previous reports, strategies such as heteroatom doping and defect engineering have been used to optimize the 3d orbitals of metals and 2p orbitals of oxygen, thereby enhancing the oxygen evolution reaction (OER) activity of hydroxyl oxides [15, 29]. Interfacial engineering can effectively modulate the O 2p‐band of catalysts by altering their electronic structure, thus improving their performance [30, 31]. Strong heterojunction connections make it more likely that it will operate long‐term and stably. This strategy represents an effective approach to promoting catalyst reconstruction through the construction of heterogeneous interfaces. Within sulfide structures, the metal sites of nickel, in conjunction with the coordination environment of sulfur, have been observed to generate a synergistic catalytic effect [32]. For instance, in the crystal structure of Ni₃S₂, the valence states of nickel (Ni²⁺, Ni³⁺) and sulfur vacancy defects have been shown to regulate electron transfer, optimize the adsorption energy of reactants, and enhance catalytic efficiency.

Herein, we demonstrate a heterointerface‐engineered Ni₃S₂/NiFeOOH catalyst that undergoes in‐situ reconstruction to activate the LOM. The Ni₃S₂/NiFeOOH heterostructure enables: (i) accelerated interfacial electron transfer that stabilizes high‐valent Ni/Fe species, (ii) lattice reorganization to generate unsaturated oxygen sites for OER, and (iii) dual‐band modulation, with downshifted metal d‐band centers and upshifted O 2p‐band centers, favoring lattice oxygen participation. This design achieves low overpotentials (196 mV at 10 mA cm⁻²; 305 mV at 1000 mA cm⁻²) and exceptional durability (500 mA cm⁻² for 500 h), establishing a scalable strategy for industrial water oxidation catalysts.

2. Results and Discussion

The lattice oxygen mechanism bypasses *OOH intermediate formation and avoids thermodynamic limits, making it an effective strategy to boost OER activity. Herein, a dual‐band modulation strategy was employed by downshifting the metal d‐band center and upshifting the O 2p‐band center, which was realized through heterointerface engineering. A weakening of the adsorbate‐metal interaction and inhibition of the adsorbate pathway are caused by the downward shift of the d‐band center of the metal. An upward shift of the 2p‐band center of oxygen results in a more delocalized oxygen atom electron and reduces the energy required for oxygen to break away from the lattice, thus facilitating the lattice oxygen pathway (Figure 1a). The synthesis of Ni₃S₂/NiFeOOH heterostructure nanosheets is shown in Figure S1. First, Ni₃S₂ nanosheet arrays were grown on nickel foam. Then, a thin NiFe‐LDH layer was electrodeposited onto the Ni₃S₂ surface. The Ni₃S₂/NiFeOOH active was synthesized by anodic cyclic voltammetry activation. In the process of OER, when the voltage is increased at both ends of the electrode, the divalent Ni and Fe atoms will lose electrons at the anode and convert into trivalent Ni and Fe atoms (Figure 1b). At the same time, the layered hydroxide also loses the hydrogen atoms on the surface layer and converts into hydroxyl oxides (Figure 1c).

FIGURE 1.

(a) Band diagrams illustrating the dual‐band modulation strategy achieved through heterointerface engineering. (b) Schematic representation of electrochemical reconstruction during water electrolysis. (c) DFT schematic of hydrogen migration and lattice reorganization. (d) Comparison of adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM). (e) Calculated energy requirements for hydrogen removal, highlighting the reduced barrier in the heterostructure.

Constructing the Ni₃S₂/NiFe‐LDH heterojunction changes the catalyst's electron distribution. This redistribution reduces the energy barrier for converting layered hydroxides to hydroxyl oxides and accelerates the conversion process. Catalyst shifts from favoring the metal adsorption pathway to favoring the lattice oxygen pathway by exposing more oxygen sites (Figure 1d). Density functional theory calculations were performed to explore the feasibility of phase regulation of Ni₃S₂. Atomic models of NiFe‐LDH and Ni₃S₂/NiFe‐LDH composites were constructed to study the reconstruction of NiFe‐LDH. The energy required for hydrogen atom breaking during heterojunction formation is lower for Ni₃S₂/NiFe‐LDH (2.87 eV/atom) than for pure NiFe‐LDH (3.02 eV/atom). This reduced energy barrier facilitates the conversion of Ni₃S₂/NiFe‐LDH into hydroxyl oxide compared to NiFe‐LDH (Figure 1e). DFT calculations were further carried out to gain a deeper understanding of the connection between the OER mechanism and the activity optimization. The actual OER active species, such as NiFe‐LDH, NiFeOOH, Ni₃S₂/NiFe‐LDH, and Ni₃S₂/NiFeOOH were chosen as computational models. Differential charge density analysis shows NiFe‐LDH losing electrons significantly, with Ni₃S₂ gaining them. The loss of electrons promotes Ni and Fe in NiFe‐LDH toward oxidation from low‐valence to high‐valence states. The differential charge calculation suggested that this heterojunction was more favorable for converting Ni and Fe to the high‐valence state to form reactive hydroxyl oxides. The H‐atom exhibits significantly higher electron density, promoting its activation for deprotonation and hydroxyl oxide formation (Figure 2a). The lost electrons are transferred to the lattice oxygen, which promotes the activity of the lattice oxygen. A thorough calculation of the differential charge distribution of the reconfigured heterojunction reveals a rearrangement of electrons clustered on the H atoms (Figure S2). This finding further corroborates the hypothesis that the material exhibits a marked preference for reconfiguring in a hydroxyl oxide configuration.

FIGURE 2.

(a) Differential charge density of Ni₃S₂/NiFe‐LDH showing electron gain/loss regions. (b) Energy required for hydrogen atom removal in NiFe‐LDH and Ni₃S₂/NiFe‐LDH. Projected density of states for the (c) Ni 3d‐band and (d) O 2p‐band, comparing Ni₃S₂/NiFe‐LDH and Ni₃S₂/NiFeOOH. (e) Crystal Orbital Hamilton Population (COHP) analysis of Ni–O bonds. (f–g) Gibbs free energy diagrams for AEM and LOM pathways at 0 V. (h) Comparison of rate‐determining step barriers for AEM and LOM.

Meanwhile, the Hirschfeld charge average value of Ni (0.140) and Fe (0.130) in Ni₃S₂/NiFe‐LDH is higher than Ni (0.136) and Fe (0.110) in NiFe‐LDH (Figure 2b). This conclusion is consistent with the in‐situ Raman results of the reconstructed structure under voltage. After that, the band positions of the reconstructed Ni₃S₂/NiFeOOH model and the reconstructed Ni₃S₂/NiFe‐LDH are compared. The d‐band center of the metal of the reconstructed structure is obviously shifting downward, while the 2p‐band center of O is obviously shifting upward. The result is consistent with the predicted structure of Figure 2c, d, and it is easier to trigger the lattice oxygen reaction mechanism. For Fe³⁺ (t_2g ³e_g ²), the absence of intra‐orbital pairing eliminates pairing repulsion, yet increased electron density enhances inter‐orbital Coulomb interactions relative to Fe²⁺. In Ni³⁺(t_2g ⁶e_g ¹), the fully occupied t_2g orbitals compress paired electrons, producing stronger intra‐orbital repulsion that outweighs the reduction in inter‐orbital contributions. In both cases, reinforced d–d Coulomb interactions drive a downward shift of the d‐band center [33]. The density of states of the O 2p orbital was calculated to reflect the lattice oxygen activity in Figure S3. The O 2p‐band center (‐3.89 eV) of Ni₃S₂/NiFeOOH and (‐4.24 eV) of NiFeOOH exhibits an upshift relative to the Fermi level (E_F) compared to Ni₃S₂/NiFe‐LDH (‐5.58 eV) and NiFe‐LDH (‐5.61 eV), which is beneficial for electron removal from oxygen sites, revealing the effective activation of lattice oxygen by Ni₃S₂ modulation and hydroxyl oxide transformation. Crystal orbital Hamilton populations (COHP) were calculated to evaluate the metal‐oxygen bond strength. In Figure 2e, the quantified Ni‐O bond strength by integrating ‐COHP up to the Fermi level (‐ICOHP) was determined to be 1.662 and 1.191 for Ni₃S₂/NiFe‐LDH and Ni₃S₂/NiFeOOH, respectively. The reason may be that in the oxyhydroxide, the hydrogen bond anchoring is weakened, and the ionic bond polarity of Ni³⁺‐O²⁻ is weakened. The lower value of ‐ICOHP for Ni₃S₂/NiFeOOH suggests that the reconfigured hydroxyl oxides allow more electrons to populate the anti‐bonding orbitals, leading to a weaker Ni‐O bond and facilitating the involvement of lattice oxygen and the formation of oxygen vacancies in the LOM pathway [34, 35]. The Gibbs adsorption free energy diagram of NiFeOOH and Ni₃S₂/NiFeOOH was also computed to illustrate the AEM and LOM pathways (Figures S4 and S5). The Gibbs free energy diagram of the AEM and LOM is exhibited in Figure 2f, g, the formation of *OOH is determined as the RDS for Ni₃S₂/NiFeOOH and NiFeOOH, with energy barriers of 1.72 eV, 1.85 eV, 2.13 eV, and 2.21 eV (Figure 2h), respectively. The results demonstrate that the reaction energy barriers are reduced for the reconstructed hydroxyl oxides. The energy barrier associated with the adsorbate pathway is higher than that of the lattice oxygen pathway, thereby substantiating the hypothesis that LOM is more probable. Reconstruction combined with heterostructure construction proves more effective in activating the LOM pathway. The heterostructure component plays a crucial role by modulating surface electron distribution, thereby optimizing the adsorption and desorption processes of intermediates.

Operando EIS measurements were conducted to probe the behavior at the catalyst/electrolyte interface and the adsorption of intermediates on active sites. Figure 3a, b shows the resulting Bode plots for Ni₃S₂/NiFe‐LDH and Ni₃S₂/NiFeOOH, acquired between 1.30 and 1.60 V. In general, the low‐frequency and high‐frequency phase angle peaks are caused by surface charge conduction and inner‐layer electron transfer, respectively. At the potential of 1.38 V, the phase angle of Ni₃S₂/NiFe‐LDH suddenly decreases due to the uneven charge distribution induced by the adsorption of the intermediates. A new smaller peak emerges at 1.38 V in the low‐frequency region, shifting to lower frequencies with increasing potential. This characteristic behavior signifies the onset of a vigorous OER process. For the Ni₃S₂/NiFeOOH, the OER process occurs at 1.38 V. In the high‐frequency potential window, the phase angle of Ni₃S₂/NiFeOOH drops faster from 1.32 V to 1.48 V, indicating a vigorous electrooxidation process with enhanced charge transport efficiency within the electrocatalyst. Then, as the potential windows gradually increase, the peak of the low‐frequency region gradually weakens until it disappears. Figure 3c summarizes the variation of the maximum phase angles of Ni₃S₂/NiFeOOH and Ni₃S₂/NiFe‐LDH with applied potential. The maximum phase angle of Ni₃S₂/NiFeOOH decreases more rapidly than that of Ni₃S₂/NiFe‐LDH, corresponding to reduced resistance and faster kinetics. In general, a faster change in the phase angle indicates more intense adsorption/desorption kinetics of oxygen‐containing intermediates and weaker adsorption stability; this is consistent with the “weakly adsorbed intermediates” characteristic of the lattice oxygen‐mediated LOM mechanism (since lattice oxygen directly participates in the reaction, reducing the reliance on adsorbed intermediates). For the Ni₃S₂ and NiFe‐LDH (Figure S6), in the high frequency region, the phase angle of Ni₃S₂ and NiFe‐LDH drops sharply in the potential range of 1.32 to 1.44 V, and still decreases in the potential range of 1.44 to 1.52 V, indicating a strong adsorption process of *OH intermediates on the catalyst surface. The results show that the Ni₃S₂/NiFe‐LDH composite has a tendency to self‐reconstruct into hydroxyl oxides. Conversion of hydroxyl oxides also favors the adsorption of the *OH intermediate.

FIGURE 3.

Bode phase plots of (a) Ni₃S₂/NiFe‐LDH and (b) Ni₃S₂/NiFeOOH at different applied potentials. (c) Maximum phase angle values versus potential, showing enhanced charge‐transfer kinetics in Ni₃S₂/NiFeOOH. Operando Raman spectra of (d) NiFe‐LDH and (e) Ni₃S₂/NiFe‐LDH, indicating reconstruction to NiFeOOH.

The catalysts Ni₃S₂/NiFe‐LDH and NiFe‐LDH were tested with the in‐situ Raman technique at different voltages. Raman peaks at 450 and 528 cm⁻¹ were observed according to Figure 3d, e, assigned to the vibration of δ(Ni^II‐O) and ν(Ni^II‐O), respectively. Meanwhile, peaks in the 200–300 cm⁻¹ range were attributed to Ni₃S₂. With the increase of voltage, the peak position shifted to 472 and 550 cm⁻¹, which were attributed to the corresponding to the E_g and A_1g vibration mode of Ni³⁺‐O, revealing the generation of NiOOH material, respectively, indicating that the catalyst was reconstructed and NiFe‐LDH was transformed into NiFeOOH. The reconstruction voltage of the target sample Ni₃S₂/NiFe‐LDH is lower than that of NiFe‐LDH, which is more conducive to catalyst reconstruction. This result is consistent with the DFT calculations that Ni₃S₂/NiFe‐LDH has a lower deprotonation energy, which is more favorable for catalyst reconstruction.

The morphologies of these synthesized catalysts were first surveyed via scanning electron microscope tests. As depicted in Figure S7a, the interconnected Ni₃S₂ nanowire arrays were vertically grown on NF with a plicated surface. On this basis, Ni₃S₂ nanowire arrays were then employed to carry out the deposition process to produce the Ni₃S₂/NiFe‐LDH. Compared with the pure NiFe‐LDH (Figure S7b), the nanoflower structure is larger, probably because the nanoarray is more conducive to the growth of the structure as the substrate. The nanoarray disappeared, and the nanosheet structure appeared at the same time, and the nanoflower structure was presented under the action of intermolecular force (Figure S7c, d). Meanwhile, both SEM and transmission electron microscopes confirm that the morphology of Ni₃S₂/NiFe‐LDH remains essentially unchanged after the reaction (Figure S8a, b, and Figure S9a, b). From the high‐resolution TEM (HRTEM) in Figure 4a, a clear heterojunction interface can be seen, and the well‐determined lattice fringes of 0.202 nm (Figure S10) and 0.229 nm (Figure S11) can be attributed to the (202) plane of Ni₃S₂ and the (015) plane of NiFe‐LDH. Subsequently, the selected area electron diffraction (SAED) pattern of Ni₃S₂/NiFe‐LDH in Figure S12a can clearly observe the (202) crystal plane of Ni₃S₂ and (015) planes of NiFe‐LDH. This directly proves that the Ni₃S₂ crystalline phase coexists with the crystalline NiFe‐LDH coexist and forms heterojunctions. HRTEM imaging of the post‐reaction catalyst reveals a distinct heterojunction interface, with well‐defined 0.223 nm (Figure S13) lattice fringes corresponding to the (101) plane of NiFeOOH (Figure 4b). Adjacent regions exhibit extensive amorphous/low‐crystalline domains, which likely serve as precursors for crystalline NiFeOOH formation. The SEAD‐blurred diffraction ring also confirms this argument (Figure S12b). Moreover, elemental mapping spectra in Figure 4c, d further reveal an even distribution of Fe, Ni, O, and S elements. The catalyst was tested by XPS, and the overview of each catalyst is shown in Figures S14 and S15. The significant positive shift (0.36 eV) of the M‐O peak in Figure 4e indicates participation of oxidized lattice oxygen (M‐O^(2‐δ)) in the electrochemical activation process. Concurrently, oxygen vacancy (O_v) content decreases as OH⁻ species fill these sites during electrolysis [23]. In Figure S16, the 2p species of S is partially converted into the characteristic peak of the S‐O species after the reaction, indicating that the S anion is leached out [36]. ICP‐OES test was carried out to test the change of S element in the solution before and after the reaction (Table S1). The content of S element in the solution increased significantly after the reaction, indicating that anion leaching occurred during the catalyst reconstruction process. However, by comparing the sulfur (S) content in the electrolytes of Ni₃S₂ and Ni₃S₂/NiFeOOH after 12 h of reaction, it is found that the S content in Ni₃S₂/NiFeOOH is significantly lower than that in Ni₃S₂. Additionally, this is also consistent with the XPS spectrum of the S 2p orbital after the stability test presented later in the manuscript. These results indicate that the heterojunction interface helps to mitigate anion leaching, thereby enhancing the stability of the catalyst. In Figure 4f, g, the Fe and Ni signals shift to higher (0.21 eV and 0.12 eV) binding energy, indicating that the metals lose electrons and convert to a higher valence state, specifically from divalent NiFe‐LDH to trivalent NiFeOOH [37, 38]. Notably, the binding energies of Ni 2p and Fe 2p for NiFe‐LDH were positively shifted compared with those of Ni₃S₂/NiFe‐LDH, indicating the partial reduction of Ni and Fe species [32]. These results prove the formation of a strongly coupled heterojunction between Ni₃S₂ and NiFe‐LDH, which can modify the local charge distribution for high electrocatalytic activities. The Raman spectrum corroborates the resulting higher valence state/oxidation state of the metals.

FIGURE 4.

High‐resolution TEM images of (a) Ni₃S₂/NiFe‐LDH and (b) Ni₃S₂/NiFeOOH, confirming heterointerface formation and reconstruction. Elemental mapping of (c) Ni₃S₂/NiFe‐LDH and (d) Ni₃S₂/NiFeOOH, showing uniform distribution of Ni, Fe, O, and S. High‐resolution XPS spectra of (e) O 1s, (f) Fe 2p, and (g) Ni 2p, revealing lattice oxygen participation and metal oxidation‐state changes.

XRD was utilized to probe the crystal structure of the synthesized catalysts, as displayed in Figure S17a. Besides the typical diffraction peaks attributed to Ni structure (PDF#04‐0850) of the NF substrate, the products obtained by the first‐step hydrothermal reaction correspond to Ni₃S₂ (PDF#44‐1418), and the products obtained by the second‐step electrodeposition reaction correspond to NiFe‐LDH (PDF#40‐0215), and the result is consistent with high‐resolution TEM tests. However, due to the low crystallinity of the electrodeposition, the XRD diffraction peak intensity of NiFe‐LDH is weakened [39]. At the same time, the Ni₃S₂/NiFe‐LDH before and after the reaction was observed (Figure S17b), and it was found that the XRD diffraction peak did not change significantly. Thus, the Raman spectra of Ni₃S₂, NiFe‐LDH, and Ni₃S₂/NiFe‐LDH before and after reaction were tested (Figure S18a, b). The Raman spectrum shows the divalent Ni and Fe peaks of Ni₃S₂/NiFe‐LDH (450 and 520 cm⁻¹) shifting to 470 and 550 cm⁻¹ after reaction, positions associated with trivalent Ni and Fe. This spectral change provides direct evidence that the divalent NiFe‐LDH structure has transformed into the trivalent NiFeOOH structure. In other words, Ni₃S₂/NiFe‐LDH is converted into Ni₃S₂/NiFeOOH.

LSV tests were performed at different pH values (pH = 12.5, 13.0, 13.5, 14.0), and the results are presented in Figure 5a, b. This design is based on the fundamental difference between the two OER pathways: the adsorption evolution mechanism (AEM) involves proton‐coupled electron transfer (PCET) and thus exhibits pH dependence, while the lattice oxygen mechanism (LOM) is a non‐PCET process with weak pH sensitivity. By investigating the pH‐dependent catalytic activity, we aim to distinguish the dominant OER pathway of the catalysts. Figure S19 shows the LSV of Ni₃S₂ and NiFe‐LDH compared at different pH values. The LOM process involves a proton‐coupled electron transfer mode. The LOM process strongly depends on the pH value of the electrolyte. The higher the pH value, the more favorable the deprotonation of OH⁻ and the faster the reaction rate. The reaction order ρ^RHE reflects its correlation with pH. The better the correlation is, the more it is biased toward the LOM process. In Figure 5c, it can be observed that the reaction order ρ^RHE of the target catalyst is 0.962, which is more biased toward 1, and higher than the 0.856, 0.755, and 0.566 of the contrast samples Ni₃S₂/NiFe‐LDH, NiFe‐LDH, and Ni₃S₂. This result directly confirms that the heterostructure‐modulated reconstructed catalyst (Ni₃S₂/NiFeOOH) tends to follow the LOM pathway, while Ni₃S₂/NiFe‐LDH mainly relies on the AEM pathway—providing key experimental evidence for our proposed reaction mechanism.

FIGURE 5.

pH‐dependent OER polarization curves of (a) Ni₃S₂/NiFe‐LDH and (b) Ni₃S₂/NiFeOOH. (c) Relationship between current density and pH, indicating stronger LOM contribution for Ni₃S₂/NiFeOOH. (d–e) LSV curves in KOH and TMAOH electrolytes, demonstrating LOM suppression by TMA⁺. (f) DEMS signals of ¹⁶O₂ and ¹⁸O‐labeled oxygen species, confirming lattice oxygen involvement.

The TMA⁺ probe in Figure 5d, e is easy to combine with the O₂ ²⁻intermediate in the LOM process, thereby inhibiting the LOM reaction process. The catalyst was placed in 1 M KOH solution and 1 M TMAOH solution, respectively, and the LSV curve was tested. The target sample Ni₃S₂/NiFeOOH exhibits the largest overpotential increase at an identical current density. This observation reveals the strongest inhibition of its LOM pathway during OER processes. Figure 5f is the electrochemical differential mass spectrometry test. The catalyst was placed in a 1 M KOH solution, replaced by heavy oxygen water, and 30 cycles of CV reaction were performed at 0‐0.8 V vs. RHE for ¹⁸O isotope calibration. The catalyst was washed with ethanol and water 6 times. The catalyst was placed in a two‐chamber electrolytic cell for LSV test. The test voltage was 1.2–2.0 V, and ³²O₂ and ³⁴O₂ signals were obtained, indicating that the ¹⁸O isotope was calibrated in the catalyst. That is to say, the lattice oxygen reaction mechanism of the catalyst occurred. Different from the ordered layered structure of NiFe‐LDH, the transformed NiFeOOH exhibits a disordered structure with a large number of oxygen vacancies and coordination‐unsaturated Ni/Fe sites. This structure endows the coordination environment of lattice oxygen with higher flexibility, significantly weakens the M‐O bond strength, and facilitates the activation of lattice oxygen to directly participate in the reaction. The heterojunction interface formed between Ni₃S₂ and NiFeOOH enables electron transfer, which further increases the proportion of high‐valent Ni/Fe in NiFeOOH. Through the polarization effect of the “metal‐oxygen” bond, the high‐valent metal ions further reduce the binding energy between lattice oxygen and metals, lowering the energy barrier of the rate‐determining step (RDS) in the lattice oxygen mechanism (LOM) and making it the dominant reaction pathway.

The electrocatalytic OER activity of different electrodes was investigated by applying a standard three‐electrode system, and their LSV curves are shown in Figure 6 Ni₃S₂/NiFeOOH demonstrates superior catalytic activity among all samples, achieving the highest current density at the lowest overpotential (Figure 6a). It can be observed that the performance of precatalyst N continuously improves through cyclic voltammetry (CV) activation, and stabilizes after the fourth cycle (Figure S20). The Ni₃S₂/NiFeOOH electrode requires overpotentials of only 196 mV to achieve 10 mA⋅cm⁻² current densities. This is significantly better than Ni₃S₂/NiFe‐LDH (229 mV), NiFe‐LDH (243 mV), Ni₃S₂ (399 mV), and RuO₂ (278 mV) (Figure 6b). Ni₃S₂/NiFeOOH also shows the smallest Tafel slope value of 48.2 mV⋅dec⁻¹, significantly lower than that of Ni₃S₂/NiFe‐LDH (64.3 mV·dec⁻¹), NiFe‐LDH (81.5 mV·dec⁻¹) and Ni₃S₂ (158.1 mV·dec⁻¹), demonstrating faster electrochemical reaction dynamics (Figure 6c). Compared with the Tafel values of catalysts reported in previous literature, the Tafel value of the catalyst in this work is at a relatively high level, indicating that the catalyst possesses fast reaction kinetics (Table S2). The electrodeposition time affects the catalytic performance, with a deposition duration of 900 s yielding the optimal performance (Figure S21). The electrochemical double‐layer capacitance (C_dl) value and electrochemically active surface area value were determined from the cyclic voltammetry curves (Figure S22). The largest value of C_dl and ECSA of Ni₃S₂/NiFeOOH is 12.88 mF·cm⁻² and 322.0 cm⁻² compared to Ni₃S₂/NiFe‐LDH (2.83 mF·cm⁻² and 70.8 cm⁻²), NiFe‐LDH (0.60 mF·cm⁻² and 16 cm⁻²), and Ni₃S₂ (1.74 mF·cm⁻² and 43.5 cm⁻²), illustrating the most accessible active sites for Ni₃S₂/NiFeOOH (Figure 6d, e). According to impedance spectroscopy (Figure 6f), Ni₃S₂/NiFeOOH possesses a notably smaller charge transfer resistance (R_ct) value (0.98 Ω) compared with Ni₃S₂/NiFe‐LDH (1.71 Ω), NiFe‐LDH (3.34 Ω) and Ni₃S₂ (2.67 Ω), demonstrating faster interfacial charge transfer and superior OER kinetics. The catalyst maintains exceptional performance at high current density, requiring only 305 mV overpotential to achieve 1000 mA·cm⁻². This significantly outperforms benchmark samples Ni₃S₂/NiFe‐LDH (330 mV) NiFe‐LDH (370 mV) and Ni₃S₂ (620 mV) at identical current density. The exceptional OER activity of the Ni₃S₂/NiFeOOH electrode is further benchmarked against recently reported nonprecious metal‐based self‐supporting anodes (Table S2). Ni₃S₂/NiFeOOH catalyst has a minimum overpotential at 10 mA·cm⁻² current density.

FIGURE 6.

Electrocatalytic OER performance comparison of Ni₃S₂, NiFe‐LDH, Ni₃S₂/NiFe‐LDH, and Ni₃S₂/NiFeOOH: (a) polarization curves, (b) overpotential at 10 mA·cm⁻², and (c) Tafel slopes. (d) Capacitive current–scan rate plots and (e) ECSA values. (f) Nyquist plots showing charge‐transfer resistance. (g) High‐current‐density polarization curves. (h) Long‐term stability comparison with RuO₂ and IrO₂, highlighting industrial durability.

The overall water splitting (OWS) activity of a two‐electrode electrolyzer employing Ni₃S₂/NiFeOOH (RuO₂) electrodes as the anode and Pt/C as the cathode was evaluated in an alkaline aqueous electrolyte containing 1.0 M KOH. The results demonstrate that the Pt/C || Ni₃S₂/NiFeOOH electrolyzer exerts a substantial influence on enhancing the OWS activity. The tank voltage of the assembled electrolyzer was found to require a mere 1.69 V to achieve a current density of 100 mA·cm⁻². In comparison, the Pt/C || RuO₂ electrolyzer necessitated a significantly higher voltage of 1.92 V. Remarkably, even at an industrial current density of 500 mA·cm⁻², the required voltage remained modest at 1.86 V (Figure S23). Furthermore, the stability of the catalyst was examined through the utilization of the constant current method. In this procedure, Ni₃S₂/NiFeOOH (RuO₂, IrO₂) and Pt/C served as the anode and cathode, respectively. The stability of the material was verified through a two‐electrode electrolysis of water in a 1 M KOH solution at 25°C with a constant current density of 500 mA·cm⁻² and a test duration of 500 h. Upon completion of the test, the catalyst commenced with a voltage of 1.866 V, at the conclusion of the test the voltage increased to 1.921 V. The overpotential decay rate of the catalyst was determined to be 0.11 mV·h⁻¹, sufficient for industrial applications. The stability of the commercial IrO₂ and RuO₂ catalysts were also tested. The initial voltages of the noble metal catalysts IrO₂ and RuO₂ were higher than those of the target sample catalysts in this paper, reaching 2.153 V and 2.263 V, respectively. After 50 h of testing, its voltage reached 2.202 V and 2.312 V, and the overpotential decay rate were 0.98 mV·h⁻¹ (Figure 6h). The current‐step test (50, 100, 150, 200 mA cm⁻²; 10 h per step, total 70 h) was carried out to demonstrate dynamic stability, and the data are provided in Figure S25. This shows the catalyst's adaptability to sudden current changes (no sharp activity drops during switching), indicating excellent reversibility of surface structure, electron conduction, and intermediate adsorption/desorption, while revealing potential issues like high‐current lattice oxygen consumption or low‐current reconstruction reversal. As reported in previous literature, S species in sulfide‐based catalysts may be oxidized to sulfate (SO₄ ²⁻) during OER, which could affect the reaction pathway and catalyst stability. However, our experimental results confirm that this phenomenon does not occur in our system: No characteristic peaks of sulfate (typically around 980 cm⁻¹ for SO₄ ²⁻ stretching vibration) were detected in the Raman spectra of Ni₃S₂/NiFeOOH. The XPS O 1s, S 2p, Ni 2p, and Fe 2p spectra of the catalysts before and after long‐term electrolysis only show almost no changes(Figure S25). The O 1s spectra show that the relative intensities of lattice oxygen and ‐OH peaks exhibit minimal shift before and after the reaction, and a well‐retained lattice oxygen peak indicates the preservation of the active phase structure. It can be found that the 2p spectra of Ni and Fe show almost no changes, indicating the structural stability of the catalyst. It can be observed from the S 2p spectra that no new relevant peaks appear before and after the reaction, indicating that the heterojunction interface effectively suppresses the leaching, oxidation, and dissolution of sulfur, thereby enhancing the structural stability of the catalyst. Combined with the above characterizations and performance tests, we propose that the slight decrease in catalyst stability may be attributed to the leaching of S element in the catalyst: the leaching of S could lead to partial collapse of the catalyst structure, which in turn impairs the catalytic activity of the catalyst and ultimately results in a slight decline in its stability—this inference is also supported by the detection of S leaching via ICP‐OES. Furthermore, LSV curves measured at different temperatures indicate that the catalytic performance improves with increasing temperature (Figure S26). It has been demonstrated that the catalyst exhibits consistent high performance under industrial‐grade reaction conditions.

3. Conclusion

In summary, we developed a heterointerface‐engineered Ni₃S₂/NiFeOOH catalyst that activates the LOM through in‐situ reconstruction. Interfacial charge redistribution stabilizes Ni³⁺/Fe³⁺ species, lowers the d‐band center, and promotes the formation of coordinatively unsaturated oxygen sites, and tailors the electronic band structure, thereby reducing the energy barrier of the rate‐determining step. Supported by DFT calculations, isotope labeling, and operando spectroscopy, we confirm that LOM predominates in the OER process of this system. The optimized catalyst achieves low overpotential, high current density operation, and long‐term durability, outperforming most reported nonprecious OER catalysts. These results provide mechanistic insights into interfacial electronic modulation and offer a scalable strategy for designing next‐generation catalysts capable of sustaining industrial current densities for water electrolysis.

4. Experimental section

4.1. Materials

Nickel foam (NF), Thiourea (CH₄N₂S), Nickel nitrate hexahydrate (Ni (NO₃)₂·6H₂O), Iron (III) nitrate nonahydrate (Fe(NO)₃·9H₂O), Hydrochloric acid (HCl), Potassium hydroxide (KOH), Ruthenium dioxide (RuO₂), Iridium dioxide(IrO₂), Nafion (20wt%), Ethyl alcohol (C₂H₅OH), and Deionized water (H₂O) were purchased from Aladdin Industrial Co., Ltd. Electrolytic cells were purchased from Lambert Scientific Instrument Ltd. (China Beijing).

4.2. Preparation of Ni₃S₂ Nanosheets

The 3 × 4 cm² nickel foam was cut and sonicated with 3 M HCl solution for 0.5 h. The nickel foam was immersed in anhydrous ethanol and deionized water, respectively, and sonicated three times, 3 min each time. Then the cleaned nickel foam was vacuum dried at 60°C for 8 h. The dried nickel foam was put into a 100 mL Teflon autoclave, and then 1.370 g (18 mmol) thiourea was weighed, put into a beaker, and 60 mL deionized water was added. After dissolution, it was added to a 100 mL Teflon autoclave, and then the autoclave was put into an oven at 160°C for 6 h. After the reaction, it was obtained by washing with deionized water and vacuum drying overnight at 60°C.

4.3. Preparation of Ni₃S₂/NiFe‐LDH Composite Structures as a Pre‐catalyst

The constant potential deposition method was used to prepare Ni₃S₂/NiFe‐LDH. Specifically, the counter electrode and the reference electrode were a platinum sheet and Hg/HgCl₂ electrode, respectively. And the previously prepared Ni₃S₂ nanosheet was the working electrode. The electrolyte contains 0.3 M Ni (NO₃)₂·6H₂O and 0.3 M Fe (NO)₃·9H₂O aqueous solutions. Subsequently, the electrodeposition process was run for 900 s at a voltage of ‐1.1 V vs. Hg/HgCl₂ by chronoamperometry, and then, Ni₃S₂/NiFe‐LDH was collected. NiFe‐LDH was deposited directly on NF under the same conditions.

4.4. Preparation of NiFeOOH and Ni₃S₂/NiFeOOH Composite Structures

By putting the prepared Ni₃S₂/NiFe‐LDH catalyst into 1 M KOH solution and reacting in a three‐electrode system (the working electrode is Ni₃S₂/NiFe‐LDH, the counter electrode is graphite rod, and the reference electrode is Hg/HgO), CV activation was performed in the potential range of 0‐0.8 V. The scan rate is set to 5 mV s⁻¹, and the activation time is 0.5 h, to obtain Ni₃S₂/NiFeOOH. NiFeOOH was prepared similarly by replacing the Ni₃S₂/NiFe‐LDH working electrode with NiFe‐LDH.

4.5. Preparation of RuO₂ and IrO₂

Commercial RuO₂ ink was loaded onto NF by a drip coating manner as follows: 20 mg commercial RuO₂ were dispersed in 1 mL ethanol, 1 mL deionized water, and 40 µL Nafion mixed solution, and treated under ultrasonic environment for 1.5 h. 500 µL of ink was measured and dropped on NF with an effective area of 1 × 1 cm², and then dried in an oven at 40 °C. The mass loading of catalysts on NF was about 5 mg cm⁻². The preparation of IrO₂ is the same as above.

4.6. Material Characterization

X‐ray diffraction (XRD) was used to study the phase composition and crystal structure by a multifunctional horizontal X‐ray diffractometer. Tescan Mira4 scanning electron microscopy (SEM) was applied to check the morphology of the samples. Transmission electron microscopy (TEM) images were acquired with Talos F200X. Thermo ESCALAB 250XI X‐ray photoelectron spectrometer was used to study the surface electronic states of all samples. All binding energies were calibrated by using the position peak of C 1 s (284.63 eV). In‐situ Raman was performed on the X plora Plus‐type microscopic Raman spectrometer with excitation wavelength of 532 nm and spectral resolution of 0.34 cm⁻¹. In‐situ Raman measurements were performed with the help of a Renishaw in via Raman Microscope instrument at 532 nm excitation wavelength in 1.0 M KOH. The spectra were collected over the range of 100–1200 cm⁻¹ with a spectral resolution of 1 cm⁻¹ at different applied potentials. The operando electrochemical impedance spectroscopy was obtained by the DH7001_B workstation under different application potential conditions, and EIS data were collected in the frequency range of 0.01 to 10⁵ Hz.

4.7. Electrochemical Measurements

All electrochemical results were recorded using a CHI760E electrochemical workstation and a CS310M electrochemical workstation in a standard three‐electrode configuration, graphite rod and Hg/HgO electrodes, respectively. The linear sweep voltammetry (LSV) curves were collected with 2 mV⋅s⁻¹ scanning rate. All Tafel slopes were calculated from the corresponding LSV curves. Cyclic voltammetry (CV) curves under non‐Faradaic region were performed at scan rates of 20–140 mV s⁻¹ to evaluate double‐layer capacitance (C_dl). Electrochemical active surface area (ECSA) was calculated based on the equation:

$$ECSA=\frac{C_{dl}}{C_s}\left(C_S=0.04mFc{m}^{-2}\right)$$

Electrochemical impedance spectroscopy (EIS) was performed at frequencies ranging from 0.01 to 10⁶ Hz. Stability was tested by the chronoamperometry method. All measured potentials were 90% iR‐corrected if not explicitly specified.

4.8. Calculation Methods

The charge density difference and free energy diagram were calculated based on the Cambridge sequential total energy package (CASTEP) module in Materials Studio software using the generalized gradient approximation (GGA) with the Perdew‐Burke‐Ernzerhof (PBE) exchange correlation functional. The energy cutoff of plane wave expansions was set to 450 eV. The Monkhorst‐Pack scheme with a k‐point separate on length of 0.05 Å⁻¹was utilized for sampling the first Brillouin zone, setting as 3×3×1. The vacuum space is adopted 15 Å above the models to separate periodic interactions during the whole calculations. The convergence energy threshold of the model optimization self‐consistent calculation and atomic force threshold are 10⁻⁴ eV and 0.05 eV/Å. The Crystal Orbital Hamilton Population was calculated based on Lobster.

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting file 1: The authors have cited additional references within the Supporting Information [40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50]. The Figures S1–S26 of SEM images, DFT moderns, TEM images, SAED patterns, XPS spectra, XRD spectra, Raman spectra, pH‐dependent test, CV curve, OWS test, and Tables S1 and S2 of ICP‐OES and OER catalysts overpotential comparison are included.

Data Availability Statement

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