Metal–Organic Framework–Spinel Fe‐ZIF‐8@NiAl₂O₄ Heterostructures Enabling Efficient and Durable Alkaline Water Splitting

Abstract

Spinel metal oxides such as NiAl₂O₄ are attractive electrocatalysts for alkaline water splitting but are often limited by low surface area, poor conductivity, and sluggish reaction kinetics. Herein, we report a bifunctional MOF–spinel heterostructured catalyst, Fe‐ZIF‐8@NiAl₂O₄, constructed by integrating solvothermally synthesized spinel NiAl₂O₄ with iron‐doped ZIF‐8 without post‐synthetic pyrolysis. Benefiting from the porous, nitrogen‐rich MOF scaffold and the structurally robust spinel backbone, the composite delivers low overpotentials of 231 mV for the oxygen evolution reaction (OER) and 109 mV for the hydrogen evolution reaction (HER) at 10 mA cm⁻², together with Tafel slopes of 97.45 and 239.29 mV dec⁻¹, respectively. The enhanced activity originates from synergistic Fe–Ni interfacial interactions that facilitate electronic redistribution, accelerate charge transfer kinetics, and promote the in situ formation of catalytically active NiFe (oxy)hydroxide species during operation. Importantly, Fe‐ZIF‐8@NiAl₂O₄ exhibits excellent durability and stable overall water splitting performance, retaining bulk crystallinity while undergoing beneficial surface reconstruction under prolonged electrolysis. This work establishes MOF hybridization as an effective strategy to activate Al‐based spinel oxides and provides practical design insights for developing cost‐effective, noble‐metal‐free electrocatalysts for sustainable hydrogen production.

Fe–Ni interfacial coupling in a nonpyrolyzed MOF–spinel heterostructure activates an Al‐based spinel for efficient alkaline water splitting. Surface reconstruction to NiFe (oxy)hydroxide ensures high activity and durability

1. Introduction

The continuous increase in global energy demand, driven by population growth and industrialization, is inevitable [1, 2]. Overreliance on fossil fuels has led to severe environmental issues, including sea‐level rise, drought, greenhouse gas emissions, and global warming. To mitigate these effects, the development of clean, reliable, and renewable energy sources is essential for long‐term socioeconomic sustainability [3]. In particular, hydrogen is a promising energy carrier due to its zero carbon emissions at the point of use, high gravimetric energy density, long‐term storage capability, and versatile applications [4, 5]. However, its production remains challenging, as most hydrogen is currently derived from fossil fuels through steam reforming, which generates significant CO₂ emissions [6]. Thus, sustainable hydrogen production requires alternatives beyond fossil‐fuel reforming. In this context, water electrolysis, involving the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode, offers an efficient and environmentally friendly pathway for large‐scale hydrogen production [7, 8].

The overall water‐splitting reaction has a thermodynamic potential of 1.23 V; however, sluggish multistep proton‐coupled electron‐transfer kinetics and the associated overpotentials markedly reduce the energy efficiency of practical electrolysis systems [9]. This limitation necessitates the rational design of efficient electrocatalysts to lower overpotentials [10, 11]. Ru/Ir‐based catalysts remain the benchmark for OER, while Pt‐based systems show excellent HER activity. However, their high cost, scarcity, and limited stability hinder large‐scale applications [12]. Consequently, recent research has focused on developing low‐cost, abundant, and stable alternatives, particularly transition‐metal‐based catalysts [13, 14]. These include phosphides, sulfides, oxides, and alloys, which have been investigated as substitutes for noble‐metal systems in HER, OER, and overall water splitting due to their earth abundance, low cost, and versatile redox properties [15, 16]. In particular, Ni‐, Co‐, and Fe‐based catalysts receive considerable attention for their high activity, structural stability, and facile synthesis. These features highlight not only their fundamental catalytic versatility but also their practical promise as scalable alternatives to precious metals in industrial hydrogen production. Their electron transport and catalytic performance can be further enhanced through synergistic effects in multimetallic systems (e.g., FeCo, NiCo, NiCoFe) or by forming spinels and composite structures [17, 18].

Spinels, consisting of divalent A‐site and trivalent B‐site cations in tetrahedral and octahedral coordination, enable electron hopping between transition‐metal ions, conferring redox activity, structural versatility, and tunable composition [19]. These properties make them attractive electrocatalysts for water splitting in alkaline media [20]. Their OER activity is primarily governed by crystal field orbital occupancy, cation distribution, and transition‐metal oxidation states. For instance, optimal activity requires orbital occupancy close to one, while incorporating metals with multiple valence states (e.g., Fe, Co, Mn) enhances redox flexibility and oxygen evolution rates [21]. Representative examples include Co₃O₄, which exhibits long‐term stability with moderate overpotential [22]. In addition, CoFe₂O₄ demonstrates improved activity when Fe substitution reduces the overpotential to 320 mV in 1 M KOH by modulating electronic structure and conductivity [23]. NiFe₂O₄ also shows superior performance over its monometallic counterparts due to Ni–Fe synergistic effects, achieving 310 mV at 10 mA cm⁻² [24], while MnCo₂O₄ combines stability and conductivity for bifunctional OER/ORR catalysis [25]. Spinel oxides can be synthesized via sol–gel, coprecipitation, hydrothermal, or solid‐state routes, with morphology, particle size, and crystallinity controlled accordingly [26, 27]. Performance is further optimized by strategies such as dopant incorporation, morphological tuning, and hybridization with conductive supports (e.g., graphene, carbon nanotubes) [28, 29].

This study focuses on Al‐based spinel oxides, which remain relatively underexplored as bifunctional electrocatalysts for alkaline water splitting despite their low cost, elemental abundance, and structural robustness. In NiAl₂O₄, catalytically active Ni²⁺ occupies tetrahedral sites, whereas electrochemically inactive Al³⁺ resides in octahedral site, resulting in inferior catalytic activity compared with NiFe₂O₄ or NiCo₂O₄ [30]. Its electrocatalytic performance is further limited by poor conductivity and particle aggregation under alkaline conditions [31]. Consequently, rational hybridization strategies are required to overcome these intrinsic limitations and unlock the practical potential of Al‐based spinels as cost‐effective bifunctional catalysts. Hybrid architectures have been widely explored to enhance spinel oxide activity. For example, Nagarani et al. [32] reported improved OER performance for CoFe₂O₄ supported on M/S‐doped reduced graphene oxide, whereas Kwon et al. [33] demonstrated that FeCo alloy nanoparticles anchored on N‐‍doped carbon nanotubes outperformed commercial RuO₂ under alkaline conditions. However, most previously reported metal–organic framework (MOF)‐derived electrocatalysts rely on high‐temperature pyrolysis to generate conductive carbon frameworks, frequently accompanied by partial collapse of the original MOF structure. In contrast, the present work adopts a nonpyrolyzed MOF–spinel heterostructure strategy through direct integration of Fe‐doped ZIF‐8 with spinel NiAl₂O₄, thereby preserving the intrinsic porous framework and coordination environment of the MOF while enabling intimate Fe–Ni interfacial coupling.

In this work, Fe‐doped zeolitic imidazolate framework‐8 (Fe‐ZIF‐‍8) was integrated with spinel NiAl₂O₄ to construct a bifunctional Fe‐ZIF‐8@NiAl₂O₄ electrocatalyst for alkaline OER and HER. The nitrogen‐rich porous framework of Fe‐ZIF‐8 provides abundant coordination sites and facilitates charge redistribution, while the spinel phase contributes structural robustness and electrochemical stability [34, 37]. Fe–N _x coordination environments introduced by Fe‐ZIF‐8 cooperate with Ni centers to promote the formation of catalytically active Ni³⁺/Ni⁴⁺ species and in‐situ reconstructed NiFe (oxy)hydroxide active sites during operation [38, 40]. In addition, the direct MOF–spinel interfacial interaction facilitates charge‐transfer kinetics and improves electrochemical accessibility without relying on carbonization‐derived conductive phases. As a results, the Fe‐ZIF‐8@NiAl₂O₄ heterostructure provides an effective and scalable platform for efficient alkaline water splitting and offers a viable design strategy for integrating MOFs with spinel oxides toward sustainable energy conversion.

2. Experimental Section

2.1. Materials and Reagents

Aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), 2‐methylimidazole, and polyvinylpyrrolidone (PVP) were purchased from Sigma‐Aldrich (analytical grade). Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), sodium hydroxide (NaOH), anhydrous ethanol (99.5%), and methanol (99.8%) were obtained from Duksan, South Korea (reagent grade). Vulcan XC‐72 carbon and commercial 20 wt% Ir/C and 20 wt% Pt/C catalysts were supplied by Premetek. Milli‐Q water (18 MΩ cm) was used in all solution preparations.

2.2. Synthesis of Spinel NiAl₂O₄

The spinel NiAl₂O₄ was synthesized by a solvothermal method followed by pyrolysis. Typically, 1.16 g of Ni(NO₃)₂·6H₂O and 3.00 g of Al(NO₃)₃·9H₂O were dissolved in anhydrous ethanol and sonicated until completely dissolved. While stirring, 2 mL of 2 M NaOH was added dropwise to adjust the pH to 9–10. After 30 min, the mixture was transferred into a Teflon‐lined autoclave (filled to three‐quarters of its volume) and heated at 180 °C for 18 h. The resulting precipitate was collected by centrifugation, washed with ethanol and deionized water, and dried in a freeze dryer overnight. To improve crystallinity and remove residual precursors, the dried sample was placed in a porcelain boat and calcined in a tube furnace at 800 °C for 4 h under N₂ flow, with a heating rate of 5 °C min⁻¹.

2.3. Preparation of Catalyst Fe‐ZIF‐8@NiAl₂O₄

The composite was synthesized following a typical synthetic procedure. In the first beaker, 6.2 g of 2‐methylimidazole was dissolved in 60 mL of methanol. In another beaker, 2.668 g of Zn(NO₃)₂·6H₂O, 190.69 mg of Fe(NO₃)₃·9H₂O, and 1.51 g of PVP were dissolved in 60 mL of methanol. The solution from the second beaker was then quickly poured into the first beaker and stirred for 30 min. Subsequently, 50 mg of the as‐prepared spinel NiAl₂O₄ was added, and the mixture was stirred continuously for 24 h. The resulting product was collected by centrifugation, washed with ethanol and deionized water, and dried overnight in a freeze dryer. For comparison, Fe‐ZIF‐8 was prepared using the same procedure without adding NiAl₂O₄. Spinel NiAl₂O₄ was prepared via solvothermal treatment and subsequent pyrolysis, while Fe‐doped ZIF‐8 was formed from Zn²⁺ and 2‐methylimidazole in the presence of Fe³⁺. The final composite was obtained by incorporating presynthesized NiAl₂O₄ into the Fe‐ZIF‐8 precursor solution, followed by aging and crystallization. The formation of the Fe‐ZIF‐8@NiAl₂O₄ heterostructure is considered to proceed through coordination‐driven ZIF‐8 crystallization in the presence of preformed NiAl₂O₄ particles. During the aging process, Zn²⁺ and Fe³⁺ ions coordinate with 2‐methylimidazole to generate Fe‐doped ZIF‐8 domains, while PVP regulates nucleation and suppresses excessive particle aggregation. Simultaneously, the surface oxygen/hydroxyl groups of NiAl₂O₄ provide anchoring sites for the growing Fe‐ZIF‐8 framework, promoting intimate interfacial coupling between the MOF and spinel phases rather than simple physical mixing. As a result, Fe‐ZIF‐8 domains are gradually assembled on the NiAl₂O₄ surface, leading to the formation of a coupled MOF–spinel heterostructure.

2.4. Physicochemical Characterization

The crystal pattern and lattice parameters of the synthesized catalysts were analyzed by powder X‐ray diffraction (XRD, Diatome MPD) using Cu Kα radiation (λ = 1.5406 Å). The particle size and surface morphology of Fe‐ZIF‐8, NiAl₂O₄, and Fe‐ZIF‐8@NiAl₂O₄ were examined by field‐emission scanning electron microscopy (FE‐SEM, Hitachi S‐4800) and high‐resolution transmission electron microscopy (HR‐TEM, Hitachi HF‐3300, 300 kV), equipped with energy‐dispersive X‐ray spectroscopy (EDS) for elemental analysis. Additional structural information was obtained from selected‐area electron diffraction (SAED), high‐angle annular dark field (HAADF) imaging, and STEM elemental mapping. X‐ray photoelectron spectroscopy (XPS) was performed to investigate the surface chemistry and composition of the as‐synthesized catalysts, using a monochromatic Al Kα X‐ray source (1486.3 eV, 14.5 kV, 30.5 mA). A pass energy of 200 eV was used to achieve higher energy resolution. Charging effects were compensated with a flood gun, and binding energies were calibrated to the C 1s peak at 284.5 eV. The specific surface area was determined by Brunauer–Emmett–Teller (BET) analysis using a BELSORP‐mini II instrument (Microtrac‐BEL) from N₂ adsorption–desorption isotherms.

2.5. Electrochemical Characterization

All electrocatalytic measurements for the OER, HER, and overall water splitting were performed in KOH electrolyte using an electrochemical workstation (CHI 750E) with a standard three‐electrode setup at 298 K. The electrolyte was purged with high‐purity N₂ for 20 min prior to each measurement. A Hg/HgO electrode was used as the reference electrode, a Pt wire as the counter electrode, and Ni foam (0.5 × 0.5 cm) loaded with catalyst as the working electrode. Ni foam was pretreated by sonication in 1 M HCl, ethanol, and deionized water for 15 min, followed by drying. Catalyst inks were prepared by dispersing 5 mg of catalyst in 0.99 mL of ethanol with 10 μL of Nafion solution (0.05 wt%), followed by sonication for 30 min. Then, 30 μL of the ink was drop‐cast onto Ni foam (0.25 cm²) to achieve a mass loading of 0.6 mg cm⁻² and dried in an oven at 50 °C for 2 h. For comparison, Ir/C and Pt/C were used as benchmarks for OER and HER, respectively. Linear sweep voltammetry (LSV) was performed from 0.924 to 1.674 V (vs. reversible hydrogen electrode, RHE) at a scan rate of 10 mV s⁻¹. The resulting polarization curves were used to obtain Tafel slopes. Cyclic voltammetry (CV) was conducted in the nonfaradaic region at scan rates of 20–100 mV s⁻¹ to estimate the electrochemical double‐layer capacitance (C _dl) and calculate the electrochemically active surface area (ECSA). Chronoamperometry (CA) was carried out at 10 mA cm⁻² for 40 h to evaluate catalyst stability. Electrochemical impedance spectroscopy (EIS) was performed with an AC amplitude of 5 mV at a selected potential (vs. RHE) over a frequency range of 1 Hz–100 kHz. Overall water splitting was evaluated in a two‐electrode system with Ni foam electrodes loaded with catalyst. All measured potentials were converted to the RHE scale according to E_RHE= E_Hg/HgO + 0.05916 × pH + 0.098, without iR correction, and the overpotential (η) was obtained from LSV data as η = E (V vs. RHE) – 1.23 V, unless otherwise stated.

3. Results and Discussion

3.1. Structural and Morphological Characterization of Fe‐ZIF‐8@NiAl₂O₄

MOF‐derived composites represent one of the most effective strategies for enhancing the electrocatalytic activity of catalyst. When MOF precursors are combined with additives, efficient composite catalysts can be obtained [41]. In such hybrid designs, the MOF provides a well‐defined surface structure and porosity, which facilitates the incorporation of additional precursors [42]. The spinel NiAl₂O₄ was synthesized via a solvothermal process and subsequently pyrolyzed to remove residual organics and solvents, improve crystallinity, ensure phase transformation to the spinel structure, and control morphology (Figure 1). For composite formation, ZIF‐8 precursors and an iron source were first dispersed in methanol and stirred prior to the addition of the as‐synthesized NiAl₂O₄. The mixture was aged for 24 h to allow ZIF‐8 crystallization and effective Fe incorporation. Since pure Fe‐based ZIFs are unstable and generally do not exist, ZIF‐8 forms from Zn²⁺, serving as a template for the incorporation of other favorable metals. Part of the Fe is doped into the ZIF structure, while the remainder interacts synergistically with Ni from the spinel. Mechanistically, the presynthesized NiAl₂O₄ spinel acts as a structurally robust oxide scaffold, while Zn²⁺/Fe³⁺ ions coordinate with 2‐methylimidazole to promote heterogeneous nucleation and growth of Fe‐ZIF‐8 domains on the spinel surface. During this process, interfacial interactions between the imidazolate ligands, metal ions, and oxygen‐containing surface sites of NiAl₂O₄ facilitate the anchoring of Fe‐ZIF‐8 onto the spinel particles. This coordination‐assisted assembly leads to intimate MOF–spinel interfacial contact and effective electronic coupling between Fe and Ni species. The spinel particles anchored on the ZIF surface provide additional active sites and enhance the corrosion resistance of the catalyst [43].

FIGURE 1.

Schematic illustration of the synthesis of Fe‐ZIF‐8@NiAl₂O₄ composite catalyst.

FE‐SEM, TEM, and elemental mapping were employed to investigate the morphology, structure, and elemental distribution of the as‐prepared catalysts. As shown in Figures 2a and S1a, FE‐SEM images of Fe‐doped ZIF‐8 at different magnifications display well‐defined polyhedral particles with smooth surfaces and negligible aggregation, with an average diameter of ∼300 nm. The homogeneous morphology and uniform particle size can be attributed to the presence of PVP during synthesis, which plays a key role in regulating nucleation and particle growth. In contrast, FE‐SEM images of spinel NiAl₂O₄ (Figure 2b) reveal small, irregularly shaped particles. Pyrolysis of the Ni and Al oxide precursors resulted in the formation of crystalline NiAl₂O₄ spinel. This is consistent with the typical morphology of thermally treated mixed metal oxides. For the composite catalyst (Fe‐ZIF‐8@NiAl₂O₄), FE‐SEM images (Figures 2c and S1b) show rhombic dodecahedral structures with spinel particles anchored on their surfaces. The deposition of NiAl₂O₄ significantly altered the surface morphology and increased the particle size of Fe‐doped ZIF‐8 from ∼300 nm to nearly 700 nm, suggesting effective interfacial coupling between the ZIF framework and the spinel phase. The surface‐deposited spinel particles are expected to provide additional active sites and enhance electrochemical performance, by facilitating charge transfer and improving reactant accessibility.

FIGURE 2.

FE‐SEM images of (a) Fe‐doped ZIF‐8, (b) spinel NiAl₂O₄, and (c) Fe‐ZIF‐8@NiAl₂O₄ composite. (d) TEM image of Fe‐ZIF‐8@NiAl₂O₄, (e) HR‐TEM image of Fe‐ZIF‐8@NiAl₂O₄, (f,g) lattice fringes pattern of Fe‐ZIF‐8@NiAl₂O₄, and (h) STEM high‐angle annular dark‐field (HAADF) image, TEM‐EDS elemental mappings, and SAED image of Fe‐ZIF‐8@NiAl₂O₄.

TEM images in Figures 2d and S2 provide further evidence supporting the SEM observations. The formation and random decoration of spinel particles on the surface of the ZIF‐8 framework result in an electrochemically modified composite catalyst with enhanced performance. HR‐TEM images of Fe‐ZIF‐8@NiAl₂O₄ (Figures 2e and S3) clearly resolve lattice fringes with interplanar distances of 0.214 and 0.216 nm (Figure 2f,g), corresponding to the (311) crystal planes of NiAl₂O₄. These lattice fringes are observed in close proximity to the ZIF‐8 matrix, indicating the successful integration of NiAl₂O₄ into the MOF framework. The contrast difference between the crystalline spinel particles and the relatively low‐contrast ZIF‐8 framework further supports the formation of the composite structure. The EDS elemental analysis (Figure S5) further verifies that the Ni:Al ratio in the spinel NiAl₂O₄ and the elemental composition of Fe‐doped ZIF‐8 are consistent with their expected values, indicating the successful synthesis of both components. The corresponding quantitative elemental compositions of Fe‐ZIF‐8@NiAl₂O₄ obtained from EDS measurements are summarized in Table S1. The HAADF‐STEM image of Fe‐ZIF‐8@NiAl₂O₄ (Figure 2h) and its corresponding elemental mappings confirm that Fe, N, O, C, Ni, Zn, and Al are uniformly distributed throughout the composite catalyst, with intensity variations reflecting their relative concentrations. The SAED pattern of Fe‐ZIF‐8@NiAl₂O₄ (inset of Figure 2h) further verifies the high crystallinity and polycrystalline nature of the material, as evidenced by concentric diffraction rings with discrete bright spots. A closer inspection of the diffraction rings reveals that they correspond to the (011) plane of Fe‐ZIF‐8 and the (400) and (533) planes of spinel NiAl₂O₄ [44], while the diffuse features are attributed to the ZIF domains, indicating successful integration of NiAl₂O₄ nanoparticles into the ZIF framework. N₂ adsorption–desorption isotherms provide additional insights into porosity. Fe‐‍ZIF‐‍8 exhibits a typical type I isotherm (Figure S4a), with a steep uptake at low relative pressure (P/P_o) indicative of micropores. The framework and microporous nature of ZIF‐8 are preserved after Fe doping, as shown by a narrow pore distribution below 2 nm in BJH analysis (Figure S4b). In contrast, Fe‐ZIF‐8@NiAl₂O₄ displays a mixed type I/IV isotherm with a pronounced hysteresis loop at high pressure (Figure S4c), indicating mesopore formation. The pore size distribution (Figure S4d) confirms the presence of both micropores and mesopores, suggesting a hierarchical structure that facilitates electrolyte penetration and improves accessibility to active sites. The composite catalyst exhibites improved textural properties compared to pristine NiAl₂O₄, whose N₂ adsorption–desorption isotherm and pore size distribution are shown in Figure S4e,f, respectively. The BET specific surface areas of NiAl₂O₄, Fe‐ZIF‐8, and Fe‐ZIF‐8@NiAl₂O₄ were determined to be 151.9, 1600.3, and 1365 m²g⁻¹, respectively (Table S2). The substantially higher surface area of Fe‐ZIF‐8@NiAl₂O₄ compared to pristine NiAl₂O₄ can be attributed to the incorporation of the porous Fe‐ZIF‐8 framework. The increased surface area is expected to enhance the exposure of active sites and facilitate electrolyte diffusion within the hierarchical pore network, thereby contributing to the improved electrocatalytic performance of the composite catalyst.

The XRD patterns of the as‐synthesized powder catalysts and their JCPDS are shown in Figure 3a. Spinel NiAl₂O₄ displayed the characteristic peaks at 36.7°, 44.5°, and 65.1° corresponding to the (311), (400), and (440) planes (JCPDS 10‐0399) [11]. Additional reflections at 18.8°, 31.2°, 55.3°, and 59.8° are indexed to the (111), (220), (422), and (511) planes, while weaker peaks at 37.6° and 64.2º arise from the (533) and (444) planes, confirming the formation of highly ordered cubic NiAl₂O₄. For Fe‐ZIF‐8, the diffraction peaks match well with ZIF‐8 (JCPDS 62‐1030) [45, 47], with slight peak shifts due to Fe incorporation. Prominent reflections appear at 7.6°, 10.2°, 14.5°, 16.4°, 18.2°, 22.3°, 24.6°, 26.5°, and 29.4°, assigned to the (011), (002), (022), (013), (222), (114), (233), (134), and (044) planes. The composite (Fe‐ZIF‐8@NiAl₂O₄) shows a diffraction profile nearly identical to Fe‐ZIF‐8, as the high ZIF‐8 content masks the contribution of NiAl₂O₄. Nevertheless, the presence of spinel particles is clearly evidenced by SEM and contributes significantly to the enhanced electrocatalytic activity. These results confirm that Fe‐ZIF‐8@NiAl₂O₄ is a crystalline composite containing multiple well‐defined phases, consistent with TEM observations.

FIGURE 3.

(a) XRD patterns of Fe‐ZIF‐8, NiAl₂O₄, Fe‐ZIF‐8@NiAl₂O₄, and the corresponding JCPDS patterns. The high‐resolution XPS deconvoluted spectra of Fe‐ZIF‐8, NiAl₂O₄, and Fe‐ZIF‐8@NiAl₂O₄ for (b) Ni 2p, (c) Fe 2p, (d) Zn 2p, (e) Al 2p, and (f) O 1s.

The surface electronic structure and chemical composition of Fe‐ZIF‐8, NiAl₂O₄, and the Fe‐ZIF‐8@NiAl₂O₄ composite were investigated by XPS. The XPS survey spectrum of Fe‐ZIF‐8@NiAl₂O₄ (Figure S6a) confirms the presence of all constituent elements (Fe, Ni, Zn, Al, O, C, and N), verifying successful composite formation without detectable surface contamination. Furthermore, high‐resolution C 1s and N 1s spectra of Fe‐ZIF‐8 and Fe‐ZIF‐8@NiAl₂O₄ (Figure S6b,c) indicate that the carbon framework and nitrogen coordination environment of the ZIF‐derived scaffold are largely preserved after composite formation. In the Ni 2p spectrum of pure NiAl₂O₄ (Figure 3b), the dominant peak at 854.5 eV, accompanied by its satellite, corresponds to Ni²⁺ species. In the composite, a modest electron transfer from Ni to adjacent Fe centers induced a slight 0.3 eV shift toward lower binding energy. This shift aligns with literature reports on Fe–Ni mixed oxides, where Fe incorporation stabilizes higher‐valent Ni species (e.g., Ni³⁺), enabling their participation in OER through Ni–OH/NiOOH redox cycles [6]. The Fe 2p spectrum from Fe‐ZIF‐8 (Figure 3c) shows two main peaks at ∼709.1 and 711.4 eV, corresponding to Fe²⁺ and Fe³⁺, with shakeup satellites between 719 and 722 eV indicating high‐spin Fe³⁺ with partially localized d electrons [6, 13]. The coexistence of Fe²⁺/Fe³⁺ facilitates electron transfer between neighboring metal sites, thereby accelerating OER kinetics. In the Fe‐ZIF‐8@NiAl₂O₄ composite, the Fe 2p binding energies remain essentially unchanged, confirming that Fe retains its mixed‐valence state after hybridization.

In Fe‐ZIF‐8, the Zn 2p spectrum (Figure 3d) shows two peaks at 1021.6 and 1044.7 eV, corresponding to Zn 2p_3/2 and Zn 2p_1/2, respectively, confirming that Zn remains chemically stable after Fe doping and composite formation. These results confirm that the MOF–spinel interface plays a critical role in tuning the surface electronic states. In the nonpyrolyzed Fe‐ZIF‐8@NiAl₂O₄ composite, electronic redistribution occurs primarily at the Fe/Ni interface, rather than being mediated by conductive carbon matrices typically generated during pyrolysis [48]. The Al 2p spectrum (Figure 3e) of pure NiAl₂O₄ shows a peak at ∼74 eV, which also remains unchanged in the composite, indicating that Al³⁺ does not participate in redox processes and the spinel lattice remains intact [11]. In contrast, the O 1s spectrum (Figure 3f) exhibits peaks at 529.8 and 531.4 eV, assigned to lattice oxygen and hydroxyl oxygen, respectively. Compared with Fe‐ZIF‐8 and NiAl₂O₄, the composite shows enhanced intensity for both peaks, suggesting stronger metal–oxygen bonding and a higher density of surface M–OH groups. Since hydroxyl oxygen is directly involved in NiOOH formation and water adsorption at active sites, while lattice oxygen reflects Fe–Ni interfacial interactions, both contribute significantly to improved OER performance. Overall, the XPS results demonstrate strong interfacial interactions between Fe centers in the MOF and Ni species in the spinel. This synergy promotes partial electron transfer, stabilizes higher‐valent Ni species, and enhances redox activity. Because the composite was not pyrolyzed, the observed catalytic enhancement arises mainly from Fe–Ni interfacial interactions and the intrinsic chemical structures of the MOF and spinel, explaining the superior OER and HER performance compared with the individual components.

3.2. OER and HER Performance and Kinetics

The OER performance of Fe‐ZIF‐8, NiAl₂O₄, and Fe‐ZIF‐8@NiAl₂O₄ is compared in Figure 4a. Among all catalysts, Fe‐ZIF‐8@NiAl₂O₄ exhibits the lowest overpotential of 231 mV at 10 mA cm⁻², surpassing Fe‐ZIF‐8(288 mV), NiAl₂O₄ (316 mV), and even commercial Ir/C (266 mV). This performance advantage is retained at higher current densities (25 and 50 mA cm⁻²; Figure 4a, inset), indicating robust catalytic activity under practical operating conditions. The enhanced activity is attributed to rapid surface activation, enlarged electrochemically accessible surface area, and efficient bubble detachment enabled by the porous MOF–spinel architecture. Consistently, Tafel analysis (Figure 4b) shows that Fe‐ZIF‐8@NiAl₂O₄ delivers the smallest Tafel slope (97.45 mV dec⁻¹), reflecting faster reaction kinetics and a reduced activation barrier compared with the individual components and Ir/C. The ECSA was estimated from the C _dl obtained from CV measurements recorded in the nonfaradaic region (Figure S7a). The corresponding roughness factors (RFs) and ECSA values of Fe‐ZIF‐8@NiAl₂O₄, Fe‐ZIF‐8, and NiAl₂O₄ are summarized in Table S3, further confirming the substantially increased electrochemically accessible surface area of the composite catalyst. As summarized in Figure 4c, Fe‐ZIF‐8@NiAl₂O₄ exhibits a significantly larger C _dl (5.2 mF cm⁻²) than Fe‐ZIF‐8 (2.7 mF cm⁻²) and NiAl₂O₄ (2.6 mF cm⁻²), indicating a higher density of electrochemically accessible active sites enabled by effective interfacial coupling and hierarchical porosity [23, 49, 50]. To decouple surface‐area effects from intrinsic activity, ECSA‐normalized OER polarization curves were further analyzed (Figure S7b), which confirm that the superior activity of Fe‐ZIF‐8@NiAl₂O₄ is preserved after normalization, evidencing intrinsically more active catalytic sites. Moreover, iR‐corrected LSV curves and overpotential comparisons (Figure S7c,d) show that the activity trend remains unchanged after resistance correction, validating the intrinsic catalytic advantage of the composite. A comparison of OER overpotentials with representative reported catalysts is summarized in Table S4, demonstrating that Fe‐ZIF‐8@NiAl₂O₄ exhibits competitive or superior activity among recently reported non‐noble‐metal OER electrocatalysts under alkaline conditions. In particular, the comparison includes representative MOF‐derived and spinel‐based systems, highlighting that the present catalyst achieves high OER activity without relying on pyrolysis‐derived conductive carbon frameworks.

FIGURE 4.

Electrochemical characterizations of (A) Fe‐ZIF‐8@NiAl₂O₄, (B) Fe‐ZIF‐8, (C) NiAl₂O₄, (D) bare Ni foam, and (E) Ir/C for OER. (a) LSV polarization curves (10 mV s⁻¹, 1.0 M KOH); inset: overpotentials at 10 and 50 mA cm⁻². (b) Tafel plots, (c) C _dl from nonfaradaic region CV plots at 0.874 V versus RHE, (d) Nyquist plots, (e) Bode plots, (f) mass and specific activities at 1.60 V versus RHE, (g,h) Nyquist and corresponding Bode plots of Fe‐ZIF‐8@NiAl₂O₄ at different potential. (i) LSV polarization curves of Fe‐ZIF‐8@NiAl₂O₄ before and after long‐term stability testing; insert: chronoamperometry.

EIS provides further insight into the charge‐transfer properties of the catalysts. As shown in Figure 4d, Fe‐ZIF‐8@NiAl₂O₄ displays a markedly smaller charge‐transfer resistance (R _ct = 16.01 Ω) than Fe‐ZIF‐8(44.53 Ω) and NiAl₂O₄ (38.61 Ω), confirming accelerated interfacial electron transport. The corresponding Bode plots (Figure 4e) reveal a higher phase angle and broader capacitive region for the composite, indicative of enhanced charge accumulation and interfacial homogeneity. Quantitative evaluation of catalytic efficiency (Figure 4f) shows that Fe‐ZIF‐8@NiAl₂O₄ achieves superior mass activity (165.1 mA mg⁻¹) and specific activity (0.762 mA cm⁻²), exceeding those of Fe‐ZIF‐8 and NiAl₂O₄, which reflects improved intrinsic activity and effective utilization of active sites. Potential‐dependent EIS measurements (Figure 4g,h) further demonstrate a continuous decrease in R _ct and expansion of the capacitive response with increasing potential, highlighting progressively faster charge transfer kinetics under OER‐relevant conditions [51]. Finally, long‐term durability testing (Figure 4i) shows nearly overlapping polarization curves before and after 2,000 cycles, along with stable current retention over 40 h, confirming excellent structural and electrochemical stability. The sustained performance originates from strong Fe–Ni interfacial interactions, the mechanically robust spinel backbone, and the formation of catalytically active Fe/Ni (oxy)hydroxide species under anodic bias [52, 53].

In the HER region (Figure 5a), Fe‐ZIF‐8@NiAl₂O₄ exhibits superior activity compared with its individual components, consistent with the OER trend. The composite requires an overpotential of only 109 mV to achieve 10 mA cm⁻², which is significantly lower than those of NiAl₂O₄ (177 mV) and Fe‐ZIF‐8 (207 mV), as summarized in Figure 5b. This enhanced HER performance originates from the synergistic combination of Lewis‐acidic Ni/Al sites that facilitate water dissociation and the porous ZIF‐8 framework that promotes efficient mass transport [54]. Recent studies have similarly demonstrated that interfacial engineering in NiFe₂O₄‐based spinel systems can effectively enhance alkaline HER activity [55]. This synergistic site cooperation effectively lowers the kinetic barrier for the Volmer step, thereby accelerating subsequent hydrogen evolution under alkaline conditions. Tafel analysis (Figure 5c) further reveals the lowest Tafel slope for Fe‐ZIF‐8@NiAl₂O₄ among the investigated samples, indicating relatively accelerated charge transfer kinetics and a more favorable Volmer–Heyrovsky HER pathway under alkaline conditions. However, the absolute Tafel slope value remains relatively high, suggesting that the HER process is still partially limited by sluggish water dissociation kinetics associated with the Volmer step, which is a common characteristic of nonprecious‐metal catalysts operating in alkaline media [56, 57]. In addition, surface heterogeneity and possible mass‐transport limitations at higher current densities may also contribute to the observed kinetic behavior. Overall, these results indicate that although the composite significantly improves HER activity compared with the individual components, further optimization of the interfacial electronic structure and active sites is still required to achieve faster HER kinetics comparable to those of noble‐metal‐based catalysts. EIS (Figure 5d) shows minimal charge transfer resistance for the composite, confirming rapid electron transport enabled by intimate interfacial coupling between NiAl₂O₄ and the porous ZIF‐8 framework, a behavior commonly observed in MOF‐oxide hybrid systems [14]. Consistently, Bode phase plots (Figure 5e) display enhanced capacitive response and interfacial charge accumulation for Fe‐ZIF‐8@NiAl₂O₄, in agreement with the Tafel and EIS results. The HER durability of the composite was evaluated by CA (Figure 5f), which shows stable current retention with negligible decay, and the post‐test polarization curve closely overlaps the initial LSV. This robustness surpasses that of commercial Pt/C, which typically suffers from agglomeration and activity loss under alkaline cycling. The excellent stability is attributed to the combined effects of Fe incorporation, the porous ZIF‐8 scaffold, and the mechanically robust spinel backbone, which together enhance corrosion resistance and structural integrity [58]. A comparison of HER performance with representative recently reported electrocatalysts is summarized in Table S5, demonstrating the competitive catalytic HER activity of Fe‐ZIF‐8@NiAl₂O₄ among related MOF‐derived and spinel‐based electrocatalysts.

FIGURE 5.

HER electrochemical characterization of (A) Fe‐ZIF‐8@NiAl₂O₄, (B) Fe‐ZIF‐8, (C) NiAl₂O₄, (D) bare Ni foam, and (E) Pt/C. (a) LSV curves in 1.0 M KOH, (b) overpotentials at 10 mA cm⁻², (c) Tafel plots, (d) Nyquist plots, (e) Bode plots, and (f) stability test under HER conditions.

3.3. Overall Water Splitting Performance and Poststability Analysis

Overall water splitting performance was evaluated using Fe‐ZIF‐8@NiAl₂O₄ coated on Ni foam as both the anode and cathode. As shown in Figure 6a, the two‐electrode cell delivers a current density of 10 mA cm⁻² at a low cell voltage of 1.61 V, indicating efficient bifunctional activity. The stable cell voltage maintained during prolonged operation (Figure 6a, inset) further confirms excellent operational durability [59]. The water‐splitting configuration and charge‐flow pathway are schematically illustrated in Figure 6b, highlighting the bifunctional role of Fe‐ZIF‐8@NiAl₂O₄ in driving both hydrogen and oxygen evolution in alkaline media. A comparison of OER overpotentials with representative reference catalysts is summarized in Figure 6c, demonstrating that Fe‐ZIF‐8@NiAl₂O₄ exhibits competitive or superior performance at 10 mA cm⁻² among recently reported systems.

FIGURE 6.

Overall water splitting performance and poststability characterization of Fe‐ZIF‐8@NiAl₂O₄. (a) LSV curve of Fe‐ZIF‐8@NiAl₂O₄ used as both cathode and anode in 1.0 M KOH; inset: long‐term stability under overall water splitting conditions. (b) Schematic illustration of interfacial electron‐transfer pathways during overall water splitting. (c) Comparison of OER overpotentials at 10 mA cm⁻² with reported reference catalysts. (d) XRD patterns before and after 40 h stability testing. (e–i) XPS spectra before and after stability testing for Ni 2p, Fe 2p, Zn 2p, N 1s, and C 1s.

Poststability structural and surface chemical evolution of Fe‐ZIF‐8@NiAl₂O₄ was examined by XRD and XPS to elucidate the activation and durability mechanisms. The XRD patterns before and after stability testing (Figure 6d) show no noticeable changes in peak positions, confirming the preservation of the ZIF‐8 and spinel crystal frameworks without the formation of additional crystalline phases, and thus indicating high structural robustness under alkaline conditions [60]. Although no new diffraction peaks are observed, the generation of OER‐active NiOOH/NiFeOOH species is expected to occur within an amorphous or ultrathin surface layer below the XRD detection limit, which is a well‐established feature of electrochemical reconstruction in Ni‐based oxides and spinels [13]. XPS analysis further supports this surface reconstruction. The Ni 2p spectra (Figure 6e) exhibit peak shifts and enhanced contributions from higher‐valent Ni³⁺/Ni⁴⁺ species after stability testing, indicative of NiOOH formation [50]. Under anodic OER conditions, Ni species from NiAl₂O₄ are oxidized to form NiOOH, while Fe sites derived from the Fe‐ZIF‐8 framework undergo transformation into FeOx(OH)y species. The interfacial interaction between Ni and Fe likely promotes the formation of a NiFeOOH‐like active phase. It should be noted that this transformation is inferred from ex‐situ XPS analysis, and in‐situ characterization would be required to directly monitor the dynamic surface reconstruction. Concurrently, the Fe 2p spectra (Figure 6f) show intensity redistribution and binding‐energy shifts consistent with Fe incorporation into the reconstructed NiFeOOH phase, which is recognized as the catalytically active OER species in alkaline media [58, 59]. The Zn 2p signals (Figure 6g) remain detectable after stability testing with reduced intensity and minor shifts, suggesting partial surface shielding by the oxyhydroxide layer rather than collapse of the ZIF framework [60]. Meanwhile, the pronounced attenuation of the N 1s signal (Figure 6h) reflects the leaching of labile nitrogen species, whereas framework‐bound nitrogen remains intact, as corroborated by the preserved XRD features. Partial oxidation of the organic‐derived carbon framework is further evidenced by the broadened C 1s spectra (Figure 6i). Overall, Fe‐ZIF‐8@NiAl₂O₄ retains its bulk crystalline integrity while undergoing favorable surface reconstruction to form catalytically active NiFeOOH species, underpinning its high activity and long‐term stability during overall water splitting.

4. Conclusions

In summary, a bifunctional MOF–spinel heterostructured electrocatalyst, Fe‐ZIF‐8@NiAl₂O₄, was successfully developed for efficient alkaline water splitting. By integrating Fe‐doped ZIF‐8 with spinel NiAl₂O₄, the composite combines a porous, nitrogen‐rich MOF scaffold with a structurally robust oxide backbone, leading to enhanced electrochemical accessibility, accelerated charge transfer kinetics, and improved intrinsic catalytic activity. The synergistic Fe–Ni interfacial interactions facilitate favorable electronic redistribution and promote the in situ formation of catalytically active NiFe (oxy)hydroxide species during operation. As a result, Fe‐ZIF‐8@NiAl₂O₄ exhibits markedly improved OER and HER performance, together with excellent durability under prolonged electrolysis. Importantly, the composite retains its bulk crystalline integrity while undergoing beneficial surface reconstruction, accounting for its stable bifunctional activity in overall water splitting. This work highlights an effective and scalable strategy for activating Al‐based spinel oxides through MOF hybridization and provides practical design insights for developing cost‐effective, noble‐metal‐free electrocatalysts for sustainable hydrogen production.

Author Contributions

Emily Sixberth Hhayuma: conceptualization, supervision, funding acquisition, project administration, formal analysis, writing – review & editing. Jun Ho Shim: conceptualization, supervision, funding acquisition, project administration, formal analysis, writing – review & editing.

Funding

This work was supported by the National Research Foundation of Korea (NRF) (RS‐2024‐00339411) and the Daegu University (2025‐0141).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Data Availability Statement

Data available on request from the author.