2.1. Synthesis of NiFe-LDH/Ni₃Se₂@NF hierarchical heterostructure

NF (thickness ~1.6 mm, porosity ~95%), selenium powder (Se, ≥99.5%), hydrazine hydrate (N₂H₄·H₂O, 80%), nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, ≥98%), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, ≥98%), urea (≥99%), ammonium fluoride (NH4F, ≥98%), hydrochloric acid (HCl), ethanol, and acetone were used as received. Deionized (DI) water (18.2 MΩ·cm) was used throughout the experiment. NF sheets were cut into 1.0 × 2.0 cm pieces and ultrasonicated sequentially in 1.0 M HCl (10 min) to remove native oxides, DI water (10 min), ethanol (10 min), and acetone (10 min). The foams were dried at 60 °C and used immediately. Ni₃Se₂ was formed via a one-pot hydrothermal conversion using elemental Se and hydrazine as the selenizing/reducing system, while NF served as the Ni source and current collector [¹⁸,¹⁹]. In a 100 mL beaker, Se powder (0.16 g, ~2.0 mmol) was dispersed in DI water (40 mL) and briefly sonicated (5 min). Hydrazine hydrate (1.0 mL, 80%) was added under vigorous stirring to obtain a homogeneous, yellowish dispersion. A pretreated NF piece was immersed in the mixture and transferred to a 50 mL Teflon-lined stainless-steel autoclave. Hydrothermal conversion was performed at 160°C for 8 h. After natural cooling to room temperature, the electrode was removed, rinsed thoroughly with DI water and ethanol, and dried at 60°C. The resulting binder-free electrode is denoted as Ni₃Se₂@NF. The NiFe-LDH nanosheets were deposited via a urea-assisted hydrothermal process. An aqueous precursor (50 mL) containing Ni(NO₃)₂·6H₂O (1.2 mmol), Fe(NO₃)₃·9H₂O (0.40 mmol), urea (10 mmol), and NH4F (5 mmol) was prepared under stirring. The Ni₃Se₂@NF substrate was placed vertically in a 50 mL Teflon liner filled with the precursor and reacted at 120 °C for 6 h. The electrode was removed, rinsed with DI water, and dried at 60 °C in air. Unless otherwise stated, the Ni:Fe molar ratio in the solution was 3:1 to obtain highly active NiFe-LDH. The hierarchical heterostructured electrode was denoted NiFe-LDH/Ni₃Se₂@NF.

2.2 Characterizations

The crystal structure of the as-prepared samples was examined using an XRD (SmartLab, Rigaku, Japan) equipped with Cu Kα radiation (λ = 1.5406 Å), operating at 40 kV and 30 mA. The diffraction patterns were recorded in the 2θ range of 10°–80° with a scan rate of 3o/min. The surface morphologies and microstructures of the samples were investigated using FE-SEM (JSM-7610F, JEOL, Japan) operating at an accelerating voltage of 5–15 kV. Prior to observation, the samples were fixed on a conductive carbon tape and sputter-coated with a thin layer of Au to prevent charging during the FE-SEM measurements. The chemical composition and surface electronic states were analyzed by XPS (ESCALAB 250Xi, Thermo Scientific, USA) using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). All the binding energies were calibrated with respect to the C 1s peak at 284.8 eV. The obtained spectra were deconvoluted using Gaussian–Lorentzian functions.

All electrochemical measurements were performed in a standard three-electrode configuration using a Bio-logic VSP potentiostat. The as-prepared samples (Ni₃Se₂@NF, NiFe-LDH@NF, and NiFe-LDH/Ni₃Se₂@NF) were used as the working electrodes without adding any binder. The OER activity of the samples was evaluated using linear-sweep voltammetry (LSV) in a 1.0 M KOH electrolyte. LSV curves were recorded in a potential window of 1.0–1.8 V (vs. reversible hydrogen electrode (RHE)) at a scan rate of 5 mV·s^-1 using the as-prepared samples as the working electrode, a Pt wire as the counter electrode, and an Ag/AgCl (3.5 M KCl) electrode as the reference electrode. Prior to measurement, the electrolyte was purged with high-purity N₂ for at least 30 min to remove dissolved oxygen. The overpotential (η) was calculated by subtracting the thermodynamic potential of OER (1.23 V) from the measured potential at the desired current density. CV tests were performed in the potential window of 0.20–0.60 V vs. Ag/AgCl with a scan rate of 5 mV·s^-1 to evaluate the OER activity. The obtained potentials were converted to the RHE scale [²⁰]. All LSV curves were corrected for ohmic drop using the uncompensated resistance R obtained from the high-frequency intercept of EIS: E_corr=E_meas− iR (compensation). The overpotential was determined at a current density of 50 and 100 mA·cm^-2. EIS measurements were performed at an overpotential of 300 mV in the frequency range of 100 kHz to 0.1 Hz, with an amplitude of 5 mV. The Nyquist plots were fitted using an equivalent circuit model to evaluate the charge-transfer resistance (R_ct). The capacitive behavior was examined by CV in a non-Faradaic region (typically 0.10–0.15 V vs. Ag/AgCl) at scan rates of 20, 40, 60, 80, 100 and 120 mV·s^-1. The C_dl value was obtained from the slope of the plot of Δj (anodic current density−cathodic current density) versus the scan rate. The ECSA was estimated using the equation: ECSA = C_dl/C_s, where C_s (specific capacitance) was assumed to be 40 μF·cm² in alkaline media [²¹]. The durability was assessed by chronopotentiometry (CP) at 50 mA·cm^-2 for 100 h.

3.1. Phase, morphology, and surface characterization

Figure 1 shows a comparison of the XRD patterns of the (a) Ni₃Se₂@NF and (b) NiFe-LDH/Ni₃Se₂@NF samples. The Ni₃Se₂@NF sample displays well-defined reflections assignable to crystalline Ni₃Se₂ (JCPDS 19-0841) together with weak substrate features from the underlying NF, confirming that the foam ligaments are conformally converted rather than etched. Upon the overgrowth of NiFe-LDH (JCPDS 40-0215), the diffraction profile retains the Ni₃Se₂ reflections; however, it develops a new, intense low-angle peak centered near 2θ ≈ 11°, which is characteristic of the (003) basal reflection of hydrotalcite-like layered double hydroxides. This (003) feature corresponds to an interlayer spacing of ~0.76–0.80 nm for Cu Kα radiation, indicating the formation of a lamellar NiFe-LDH phase with long-range gallery ordering [²²]. Higher-order basal reflections are weak/broadened, as commonly observed for ultrathin NiFe-LDH nanosheets, indicating a small stacking coherence and/or turbostratic disorder. No diffraction peaks attributable to recrystallized elemental Se were detected. Moreover, the persistence of Ni₃Se₂ peaks demonstrates that the conductive selenide scaffold remains intact after the second hydrothermal step. Figure 1 unambiguously verifies the targeted heterostructure: a crystalline Ni₃Se₂ backbone coated with a layered NiFe-LDH shell with a low-angle (003) peak near 11° serving as a diagnostic fingerprint for the LDH overlayer.

FE-SEM was used to examine the hierarchical architecture (Figure 2). At both low and high magnifications, the Ni₃Se₂@NF electrode (Figure 2 (a,b)) shows continuous conformal coverage over the 3D foam ligaments with abundant nanoscale texturing, providing a high-surface-area, electronically conductive skeleton. After the NiFe-LDH growth (Figure 2 (c, d)), the surface becomes uniformly carpeted by thin LDH domains that bridge and decorate the Ni₃Se₂ backbone, yielding a more open and corrugated texture that is conducive to electrolyte penetration and gas release. EDS results (insets) are fully consistent with the intended chemistry at each step. For Ni₃Se₂@NF, the spectra reveal strong Ni and Se signals with no extraneous elements, whereas for NiFe-LDH/Ni₃Se₂@NF, distinct Ni, Fe, and O signals are detected across the surface. The Se signal from the core is still observable, but attenuated by the LDH overlayer, as expected for a conformal coating [²³].

The combined XRD/FE-SEM/EDS results indicate that the sequential hydrothermal strategy yields (i) a robust, crystalline Ni₃Se₂ current-carrying framework closely integrated with the NF, and (ii) a lamellar NiFe-LDH shell exhibiting a hallmark basal (003) reflection near 11°. This architecture is well aligned with the design principles for high-rate alkaline OER: the Ni₃Se₂ backbone supplies metal-like conductivity and short electron pathways through the 3D foam, while the high-surface-area LDH shell furnishes abundant oxyhydroxide-convertible sites with interlayer galleries accessible to OH^⁻. The preserved Ni₃Se₂ reflections after LDH growth, together with the morphological continuity observed by FE-SEM, suggest low interfacial resistance and strong mechanical interlocking, both of which are essential for sustaining large current densities without delamination. Therefore, we expect the NiFe-LDH/Ni₃Se₂@NF electrode to outperform both single-component controls (NiFe-LDH@NF and Ni₃Se₂@NF) once benchmarked electrochemically.

High-resolution XPS of the NiFe-LDH/Ni₃Se₂@NF electrode (Figure 3 (a–d)) verifies that the NiFe-LDH shell is chemically coupled to a conductive Ni₃Se₂ scaffold. The spectra show the expected spin–orbit doublets with clear shake-up satellites and chemically reasonable component assignments, which are fully consistent with our intended heterostructure. The Ni 2p of Figure 3 (a) envelope is deconvoluted into Ni²⁺ and Ni³⁺ components in both the 2p_3/2 and 2p_1/2 regions, accompanied by satellite features (“Sat.”). The coexistence of Ni²⁺/Ni³⁺ indicates the partial oxidation of Ni(OH)₂-like motifs toward NiOOH-like high-valence sites within the LDH lattice. This mixed valence is a hallmark of Fe-incorporated Ni hydroxides and indicates strong Ni–O bonding and electron redistribution at the LDH/selenide interface, which is beneficial for the OER kinetics because higher-valence Ni centers lower the barriers to *OOH formation [²⁴].

Fe appears predominantly as Fe³⁺ with a minor Fe2+ contribution, together with the characteristic Fe³⁺ satellite at a higher binding energy. The dominance of Fe³⁺ supports octahedral Fe(III) substitution in the brucite-like layers of NiFe-LDH. Coupled with the Ni³⁺ signal, this confirms the heterovalent Ni/Fe synergy, that is, Fe incorporation stabilizes high-valence Ni under ambient conditions, preconditioning the surface for rapid OER turnover. A well-resolved Se^2– (3d_5/2/3d_3/2) doublet is observed, indicating that the Ni₃Se₂ backbone remains intact after the NiFe-LDH growth [²⁵]. A weak, higher-binding-energy component is assigned to Se–O_x, arising from the slight air exposure/reconstruction of the outermost selenide surface [²⁶]. No Se⁰ signal was detected in the XPS spectra, consistent with the XRD result showing no recrystallized elemental Se. These features confirm that the conductive selenide core is preserved while participating in the interfacial coupling with the LDH overlayer.

The O 1s spectrum comprises dominant M–OH (Ni–OH/Fe–OH) contributions and a higher-binding-energy component from H–O–H (interlayer/adsorbed water), which is consistent with a hydrotalcite-like LDH shell rich in hydroxyl terminations and interlayer species. Such hydroxylated water-containing galleries facilitate OH^– transport and promote in situ evolution to catalytically active Ni(Fe)OOH during the OER [²⁷]. In summary, the Ni²⁺/Ni³⁺ and Fe³⁺ signatures in the cation cores, preserved Se^2– in Se 3d, and hydroxyl-dominated O 1s envelope corroborate our design: a NiFe-LDH shell electronically coupled to a metallic Ni₃Se₂ framework. This chemical configuration provides (i) abundant hydroxylated active motifs, (ii) stabilized high-valence Ni aided by Fe, and (iii) conductive selenide pathway features that underlie the enhanced OER performance, as demonstrated later.

3.2 Electrochemical-performance evaluation

Figure 4 compares the alkaline OER characteristics of the Ni₃Se₂@NF, NiFe-LDH@NF, and integrated NiFe-LDH/Ni₃Se₂@NF electrodes. Across all panels, the heterostructured electrode consistently outperforms the single–component controls, which matches our design logic: a conductive Ni₃Se₂ backbone for rapid electron transport and conformal NiFe-LDH shell that furnishes abundant, Fe-tuned oxyhydroxide active sites with facile OHaccess. The polarization curves (geometric area–normalized; iR-compensation, as stated in the Methods section) show the lowest overpotential and earliest OER onset for NiFe-LDH/Ni₃Se₂@NF, followed by NiFe-LDH@NF, with Ni₃Se₂@NF being the least active. The shoulder at ~1.38 V (vs RHE) is assigned to the Ni²⁺/Ni³⁺ redox transition associated with the in-situ formation of NiOOH prior to OER. Accordingly, to avoid conflating the OER metric with this redox process, catalytic performance was evaluated using the iR-corrected overpotentials at 50 and 100 mAcm^-2. At any potential in the practical range, the integrated electrode sustains the highest current density, indicating that the heterointerface successfully translates the intrinsic LDH activity into device-relevant rates. The improved slope in the mixed-kinetic region further suggests accelerated charge transfer in the heterostructure [²⁸]. In Figure 4 (b), the bars summarize the overpotential (extra voltage required to reach the target current density) at 50 and 100 mA·cm^-2. Lower values indicate a more efficient electrode. For all three benchmarks, the ordering is clear: NiFe-LDH/Ni₃Se₂@NF requires the least overpotential, NiFe-LDH@NF is intermediate, and Ni₃Se₂@NF requires the highest. Notably, the gap widens as the current density increases from 50 to 100 mA·cm^-2, indicating that the conductive Ni₃Se₂ scaffold combined with the NiFe-LDH shell suppresses resistive/interfacial losses and bubble-induced transport limitations more effectively under high-rate operation [²⁹]. Practically, the integrated electrode delivers the same current at a lower applied voltage, that is, the desired OER rate is achieved at a lower energy cost.

The Tafel analysis shows the smallest slope for NiFe-LDH/Ni₃Se₂@NF, intermediate slopes for NiFe-LDH@NF, and the largest slopes for Ni₃Se₂@NF. The reduced slope of the heterostructure indicates more favorable reaction kinetics (higher apparent reaction order/exchange current) and is consistent with the Fe-modulated NiOOH chemistry at the LDH surface coupled electronically to selenide. Mechanistically, Fe incorporation stabilizes the high-valence Ni sites and optimizes *OH/*O/*OOH adsorption, while intimate contact with Ni₃Se₂ lowers the interfacial barrier for electron transfer, both of which manifest as a lower Tafel slope [³⁰]. The Nyquist spectra (measured at a representative overpotential) display the smallest high-frequency semicircle for NiFe-LDH/Ni₃Se₂@NF, indicating the lowest charge-transfer resistance (R_ct). NiFe-LDH@NF shows a larger semicircle (limited by the poor conductivity of hydroxides and weaker current-collector contact), and Ni₃Se₂@NF is larger (fewer intrinsically active oxyhydroxide sites despite good conductivity). The more vertical low-frequency tail of the heterostructure is consistent with improved mass transport and faster bubble disengagement within an open 3D network [³¹]. The LSV, overpotential benchmarks at 50 and 100 mA·cm^-2, Tafel slopes, and EIS all indicate the same conclusion: heterointerface engineering on a conductive porous current pathway is the key performance lever. The NiFe-LDH/Ni₃Se₂@NF electrode couples (i) abundant LDH active sites (low η and small Tafel slope) with (ii) ultrafast electron transport through Ni₃Se₂ (low R_ct) and (iii) favorable mass transport in the foam architecture (better high-current behavior). This synergy explains the systematic activity gains over NiFe-LDH@NF (limited conductivity/contact) and Ni₃Se₂@NF (limited intrinsic OER activity), aligning precisely with our electrode design.

Figure 5 links architecture to usable active sites and long-term robustness. The NiFe-LDH/Ni₃Se₂@NF electrode combines a conductive, porous scaffold with a nanosheet LDH shell, and accordingly exhibits the largest capacitive response, the highest ECSA, and stable operation under galvanostatic stress. C_dl, extracted from CVs in the non-Faradaic window, is highest for NiFe-LDH/Ni₃Se₂@NF, intermediate for NiFe-LDH@NF, and lowest for Ni₃Se₂@NF. This ranking reflects how the hierarchical heterostructure maximizes the electrochemically accessible area and wetting: ultrathin NiFe-LDH domains conformally carpet the rough Ni₃Se₂ backbone across the 3D foam, suppressing nanosheet restacking and minimizing dead volume. In contrast, NiFe-LDH grown directly on NF is limited by weaker electrical/physical contact and partial stacking, while bare Ni₃Se₂@NF provides good conductivity but fewer hydroxide-type surface sites [³²]. Using a standard specific capacitance to convert C_dl, the derived ECSA mirrors the C_dl trend, with NiFe-LDH/Ni₃Se₂@NF ≫ NiFe-LDH@NF > Ni₃Se₂@NF. The large ECSA confirms that the heterostructure offers more addressable catalytic sites per geometric area, consistent with the nanosheet coverage observed in the FE-SEM results and LDH (003) signature in the XRD results. Together with the low charge-transfer resistance in Figure 4 (d), these data indicate not only more sites, but more sites that are well wired to the current collector, which is critical for sustaining high-rate OER without ohmic bottlenecks.

Under constant-current OER operation, NiFe-LDH/Ni₃Se₂@NF exhibits a nearly flat potential–time trace (minor, benign fluctuations attributable to bubble dynamics), with no abrupt spikes or drift indicative of delamination or contact loss as shown in Figure 5 (c). A brief initial conditioning is observed, consistent with the in-situ transformation to Ni(Fe)OOH-like species, after which the potential stabilizes. This durable response is attributed to (i) the mechanical interlocking of the LDH shell with the Ni₃Se₂/NF framework, (ii) metal-like conduction through the selenide backbone that mitigates localized heating and voltage decreases, and (iii) the open, microporous foam that expedites electrolyte renewal and bubble release [³³]. The C_dl/ECSA gains explain why the integrated electrode reaches the target currents at lower overpotentials (Figure 4 (b)), whereas the CP results confirm that these advantages persist underload. In summary, heterointerface engineering on a conductive foam translates the intrinsic LDH activity into a dense population of well-connected active sites that remain stable during an extended OER. Table 1 presents a benchmarking overview in alkaline electrolyte, situating the present catalyst within the performance landscape by compiling literature-reported overpotentials at predefined current densities together with Tafel slopes, thereby facilitating a methodologically consistent, side-by-side comparison. Under matched conditions, NiFe-LDH/Ni₃Se₂@NF delivers overpotentials at 50 and 100 mAcm^-2 that are comparable to representative systems, while the somewhat higher Tafel slope suggests room for kinetic improvement. Although the Tafel slope is slightly larger, the overall performance remains competitive, as reflected by the low η at high currents.