Localization of high-entropy metal oxide in iron oxide nanoparticles for efficient oxygen evolution reaction

Abstract

The development of efficient, durable, and cost-effective oxygen evolution reaction (OER) electrocatalysts is essential for advancing renewable energy technologies. Herein, a novel strategy is reported that spatially confines a nanoscale localized high-entropy oxide (LHEO) consisting of Fe, Co, Ni, Zn, and Mn within α-Fe₂O₃, forming a unique α-Fe₂O₃@LHEO nanoarchitecture. The catalyst exhibits outstanding OER performance for alkaline water splitting, achieving a current density of 10 mA/cm² at a low overpotential of 229 mV with a small Tafel slope of 34.4 mV/dec, significantly outperforming commercial RuO₂ (326 mV, 118.8 mV/dec). It also shows excellent long-term stability over 1000 h at 100 mA/cm² without notable activity degradation. Applied in rechargeable zinc–air batteries with natural seawater, the α-Fe₂O₃@LHEO cathode delivers a high power density of 88.3 mW/cm² and stable operation over 600 cycles, substantially surpassing the benchmark Ru-Pt electrocatalyst (74.7 mW/cm², 170 cycles). Combined experimental and theoretical studies reveal that LHEO induces lattice strain in α-Fe₂O₃, modulates its electronic structure, and lowers the crystal field splitting energy to stabilize high-spin Fe³⁺. These effects enhance metallic character for efficient charge transfer and optimize the adsorption/desorption of key oxygen reaction intermediates, thus shifting the OER pathway from the conventional adsorbate evolution mechanism (AEM) to the more energetically favorable lattice oxygen mechanism (LOM) with the energy barrier reduced from 1.85 to 1.71 eV. Overall, this work proposes a novel localized high-entropy engineering approach that overcomes key bottlenecks in designing efficient and durable OER electrocatalysts based on earth-abundant materials.

Graphical abstract

Keywords

localized high-entropy engineering / earth-abundant oxygen evolution reaction electrocatalysts / lattice oxygen mechanism / seawater splitting / Zn–air battery

1 Introduction

The relentless depletion of fossil fuels underscores an urgent need to transition toward a renewable energy system [1–3]. Central to this transition is the oxygen evolution reaction (OER), a fundamental process in many renewable energy technologies, including water electrolysis for green hydrogen production and metal–air batteries for energy storage [4–7]. However, OER is inherently hampered by sluggish kinetics, involving a complex four-electron transfer with high energy barriers (2H₂O → O₂ + 4H⁺ + 4e⁻) [8,9]. Precious metal oxides such as IrO₂ and RuO₂ remain the benchmark OER catalysts to overcome these issues. Despite their high activity, their extreme scarcity and unaffordable price—for instance, the global iridium yield is only about 7–8 tons per year—prevent their deployment at the terawatt scales [10,11]. In this context, it is highly desirable to develop high-performance noble-metal-free OER catalysts to break the bottleneck in the renewable energy revolution [12].

Among a variety of non-precious metals, iron-based catalysts have garnered considerable interest owing to their natural abundance, low toxicity, and rich redox chemistry [13–15]. The versatile oxidation states of iron (Fe²⁺/Fe³⁺) support tunable electronic structures well-suited for multi-step reactions such as OER [16]. The adsorption of oxygen-containing intermediates on the catalyst surface is closely related to the spin state of the active metal [17]. Fe³⁺ exists in three distinct spin states: low-spin (LS, t_2g⁵ e_g⁰), intermediate-spin (IS, t_2g⁴ e_g¹), and high-spin (HS, t_2g³ e_g²) [18,19]. High-spin Fe³⁺ often plays a significant role in OER reactions [20]. On the one hand, turning the spin-electronic configuration of Fe centers to the high-spin state enables electron penetration into the antibonding π-orbitals of adsorbed species, thereby modulating intermediate adsorption strength [21,22]. On the other hand, high-spin ions can serve as spin channels, enhancing selective extraction of spin-oriented electrons and facilitating the formation of triplet oxygen molecules [23]. However, conventional iron-based catalysts suffer from inherent limitations, including poor electrical conductivity (~10^–14 S/cm for α-Fe₂O₃), suboptimal active-site configurations, and the difficulty in stabilizing high-spin Fe³⁺ under operating conditions, all of which hinder the efficient adsorption/desorption of key reaction intermediates and impose significant energy barriers, thereby restricting overall catalytic performance [24]. Recent studies show that tailoring the local chemical environment via doping, defect engineering, and interface structuring can partially enhance catalytic properties [25–27]. Nevertheless, iron-based OER catalysts remain far from practical application.

High-entropy alloys (HEAs), composed of five or more principal elements in near-equimolar ratios, exhibit high configurational entropy and unique electronic and catalytic properties [28–30]. HEA engineering offers a promising route to improve iron-based materials by simultaneously tuning composition, strain, and defects [31–33]. Specifically, lattice distortion induced by atomic size mismatch generates abundant surface defects and oxygen vacancies, promoting OER pathway toward the lattice oxygen mechanism (LOM) [34,35]. Moreover, HEAs tune the electronic structure and d-band center of iron active sites, optimizing adsorption energies and reducing activation energy barriers [36,37]. Furthermore, the cocktail effect arising from synergistic interactions among multiple elements further enables the integration of complementary functionalities, helping to overcome the activity-stability trade-off that plagues conventional iron-based catalysts [38].

In this study, a localized high-entropy engineering strategy is proposed to improve iron-based OER performance (Fig. 1). A localized high-entropy oxide (LHEO) composed of Fe, Co, Ni, Zn, and Mn is spatially confined within ɑ-Fe₂O₃ (α-Fe₂O₃@LHEO) for OER catalysis. During water electrolysis, α-Fe₂O₃@LHEO delivers a low overpotential of 229 mV at 10 mA/cm², with a distinct Tafel slope of 34.4 mV/dec in alkaline electrolyte, substantially outperforming α-Fe₂O₃ (299 mV, 61.2 mV/dec) and commercial RuO₂ (326 mV, 118.8 mV/dec). It exhibits outstanding stability for over 1000 h at 100 mA/cm² without significant activity degradation. When applied in a rechargeable Zn–air battery using natural seawater, a high peak power density of 88.3 mW/cm² and stable operation over 600 cycles are achieved, surpassing a commercial Ru-Pt catalyst (74.7 mW/cm², 170 cycles). Combined spectroscopic analysis and density functional theory (DFT) calculations reveal that LHEO induces lattice strain, electronic reconfiguration, and stabilization of high spin Fe³⁺ in ɑ-Fe₂O₃. This facilitates charge transfer and optimizes intermediate adsorption energies. Meanwhile, the reaction pathway shifts from the adsorbate evolution mechanism (AEM) to the more energetically favorable LOM, with a dramatically reduced energy barrier. This study introduces a nanoscale localized high-entropy engineering strategy for developing efficient, durable, and cost-effective OER electrocatalysts based on earth-abundant materials.

2 Method

2.1 Working electrode preparation

For the three-electrode configuration, nickel foam (NF) was used as the substrate, with a LHEO/α-Fe₂O₃/carbon nanotubes (CNTs) composite catalytic layer grown in situ via a one-step hydrothermal route. Briefly, Fe₂(SO₄)₃, ZnCl₂, CoCl₂, MnSO₄, NiCl₂, and CNTs were weighed at a target molar ratio, dispersed in deionized water (DIW), and stirred until fully dissolved to form a homogeneous precursor solution, followed by ultrasonication for 30 min to ensure uniform dispersion.

Commercial NF substrates (2 × 1 cm²) were pretreated by solvent degreasing in acetone (30 min, 40 °C, ultrasonic), acid etching in 3 mol/L HCl (20 min, ultrasonic) to remove native oxides, sequential rinsing with DIW and anhydrous ethanol, and dehydration in a vacuum oven. The pretreated NF was then immersed in the precursor solution, transferred to a Teflon-lined hydrothermal autoclave, sealed, and maintained at 150 °C for 120 min to achieve in situ growth of the composite layer (Fig. S1). After natural cooling to ambient temperature, the NF was retrieved, and the residual solution was centrifuged to collect precipitate. Finally, the NF was rinsed alternately with DIW and ethanol to remove residual salts, air-dried at room temperature, and directly used as the working electrode for OER tests in the three-electrode system.

2.2 Pretreatment of natural seawater

Raw natural seawater was collected from the coastal area of Beihai, Guangxi Zhuang Autonomous Region, China (21.4254° N, 109.0992° E), and vacuum-filtered through Whatman Grade 1 qualitative filter paper (11 μm pore size) to remove large, suspended solids and insoluble impurities, yielding pretreated seawater.

For three-electrode electrochemical tests, the pH of 1 mol/L KOH reference solution was calibrated to 13.6 at 26 °C. Analytical grade KOH was added to pretreated seawater under stirring with real-time pH monitoring until stabilization at pH 13.6. The solution was then centrifuged at 9000 r/min (25 °C, 10 min) to remove insoluble metal hydroxides, and the supernatant was stored in a polyethylene container.

For ZAB tests, analytical grade KOH and zinc acetate dihydrate were dissolved in pretreated seawater to final concentrations of 6 and 0.2 mol/L, respectively. After 30 min of stirring, the solution was centrifuged at 10000 r/min (25 °C, 15 min), and the clear supernatant was collected as the ZAB electrolyte.

2.3 Preparation of cathode for ZAB

The ZAB cathode was fabricated on a three-layer composite substrate. Briefly, 4 mg of catalyst powder was dispersed in ethanol and ultrasonicated for 30 min to form homogeneous ink. The ink was uniformly drop-cast onto the substrate and oven-dried to obtain a catalytically active cathode.

A Zn sheet was used as the anode after sanding, ultrasonicly cleaning in ethanol, and drying. The electrolyte consisted of 6 mol/L KOH and 0.2 mol/L zinc acetate in natural seawater. The cathode, electrolyte, and anode were sequentially assembled into a rechargeable ZAB. For comparison, an IrO₂-Pt/C (1:1 mass ratio) cathode with the same loading was prepared.

For the flexible ZAB, the same procedure was followed. A dry polyacrylic acid (PAA) gel was immersed in the electrolyte for 24 h and used as the electrolyte. A well-polished Zn foil served as the anode and was sealed in a plastic bag. After standing for 1 h, electrochemical tests were conducted.

3 Characterization

Scanning electron microscopy (SEM) images were obtained on a Quattro ESEM (FEI). Scanning transmission electron microscopy (STEM) imaging was performed on a Thermo Fisher Scientific Talos F200X S/TEM at 200 kV with a Super-energy-dispersive X-ray spectroscopy (EDS) detector. Transmission electron microscopy (TEM) images were obtained using a JEOL 2100F microscope. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker A300 instrument (Germany). X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance diffractometer. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB 250xi system equipped with a non-monochromatic aluminum anode, with the C 1s peak at 284.8 eV used for calibration. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were conducted using an AGILENT 730 instrument. Vibrating Sample Magnetometer (VSM) were performed using MPMS-VSM at 300 K. Raman spectra were recorded on a HORIBA LabRAM HR Evolution spectrometer, which has a focal length of 800 mm and covers a spectral range from 200 to 3200 nm.

3.1 Electrochemical characterization

All electrochemical tests were performed on a CHI660E electrochemical workstation in a conventional three-electrode system with 1.0 mol/L KOH or 1.0 mol/L KOH + seawater electrolyte (pH 13.6) at 25 ± 1 °C. The working electrode was catalyst-loaded NF, the reference electrode was Hg/HgO (1 mol/L KOH), and the counter electrode was Pt mesh (99.99% purity).

All measured potentials were converted to the reversible hydrogen electrode (RHE) scale using:

(1)E_{\mathrm{R}\mathrm{H}\mathrm{E}}=E_{\mathrm{H}\mathrm{g}/\mathrm{H}\mathrm{g}\mathrm{O}}+0.0591\ast \mathrm{p}\mathrm{H}+0.098\mathrm{V}.

Linear sweep voltammetry (LSV) was conducted at 10 mV/s from 0.2 to 0.8 V versus RHE. iR compensation (85%) was applied. Tafel slopes were derived using

(2)\eta =b\log \left(j\right)+a.

where η represents the overpotential, j is the current density, and b denotes the Tafel slope.

The electrochemical double-layer capacitances (C_dl) within the non-Faradaic potential region (1.024–1.224 V versus RHE) were calculated using CV curves recorded at scan rates of 20, 40, 60, 80, 100, and 120 mV/s, and the C_dl values were obtained by linear fitting the capacitive current differences (Δ_j/2) versus scan rate. Electrochemical impedance spectra (EIS) of the catalyst’s tests were performed at 10 mA/cm² over a frequency window of 10⁵–0.1 Hz. Long-term catalyst durability was assessed via chronopotentiometry (CP) at various current densities.

After assembly, the ZAB was allowed to stand for 12 h. The instantaneous open-circuit potential (OCP) is measured using a multimeter and recorded. Subsequently, a CHI660E workstation was employed to conduct activity and stability tests. The OCP sampling interval was set at 60 s, and the data were monitored and recorded for 10 h. The charge–discharge curves were measured by the LSV method, with a charging range of 1.5–3.2 V, a discharging range of 1.5–0 V, and a scan rate of 10 mV/s. The power density was calculated by the product of the voltage and current of the discharge curve (P = U × I). The specific capacity was determined by constant-current discharging (0.005 A) until battery failure. After recording the mass of the consumed zinc sheet, it was calculated according to C = It/m. For the cycle stability test, the charge and discharge currents were both set at 5 mA, with each charge and discharge step lasting 600 s. The cycling process was monitored and the data were recorded.

3.2 H₂O₂ detection

The H₂O₂ produced during anodic water oxidation was quantified via UV-Vis spectroscopy (Shimadzu UV-3600). Electrocatalytic water oxidation was run in a three-electrode setup at a fixed potential of 1.6 V vs RHE for 30 min. Post-reaction, 1 mL of 1 mol/L HCl was mixed with 3 mL of the reaction solution, then 1 mL of 0.1% OPD solution (chromogenic agent) was added. After complete mixing, the absorption spectrum of the solution was acquired from 300 to 600 nm. The characteristic 438 nm absorption peak of the OPD-H₂O₂ complex confirmed H₂O₂ production from water oxidation.

3.3 In situ Raman spectroscopy monitoring protocol

Electrochemical Raman spectroscopy (ERS) measurements were conducted using a conventional three-electrode setup under potentiostatic regulation. The active working electrode was sequentially polarized at potentials ranging from the open-circuit potential (OCP) to 0.8 V versus RHE in 0.1 V increments. For every applied potential, chronoamperometric (i–t) measurements were recorded for 300 s. Raman spectra were acquired after a 5-min stabilization period to minimize transient interfacial effects. Each potential step was replicated 1–2 times to mitigate the adverse impacts of random signal fluctuations and gas bubble formation during testing.

3.4 Computational detail

DFT [39] calculations were performed using the Vienna Ab Initio Simulation Package (VASP) [40,41]. The interaction between valence electrons and ionic cores was described by the projector-augmented wave (PAW) method [42,43]. Exchange-correlation effects within the Kohn-Sham framework were treated using the Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) [44]. A plane-wave basis set with a kinetic energy cutoff of 400 eV was used to expand the wave functions. Due to the relatively large system and sampling size, only the Γ k-point is sampled in the Brillouin zone [45].

Geometry optimization was performed with convergence criteria of 0.02 eV/Å for atomic forces and 10⁻⁴ eV for total energy, while a stricter criterion of 10⁻⁶ eV was applied for precise energy calculations. During structural relaxation, the bottom two atomic layers were fixed at their bulk positions in the slab models. To avoid interactions between periodic images, a vacuum layer of at least 25 Å was introduced perpendicular to the surface for all slabs and in all three spatial dimensions for nanoclusters.

The Gibbs free energy of adsorption (ΔG) was calculated using

(3)\mathrm{\Delta }G=E_{\mathrm{a}\mathrm{d}}+\mathrm{\Delta }ZPE-T\mathrm{\Delta }S,

where E_ad is the adsorption energy defined by

(4)E_{\mathrm{a}\mathrm{d}}=E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}+\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}}-E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}-E_{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}}.

ΔZPE and ΔS are the changes in zero-point energy and entropy during adsorption [46,47].

4 Results and discussion

A LHEO composed of Fe, Co, Ni, Zn, and Mn was incorporated into α-Fe₂O₃ via a one-step hydrothermal method (see Experimental Section and Fig. S1). SEM shows that the α-Fe₂O₃@LHEO nanoarchitecture is firmly anchored on the nickel foam substrate (Fig. S2). Contact angle measurements indicate a transition from hydrophobic to hydrophilic behavior with gas-repellent properties (Fig. S3), facilitating rapid bubble release during water electrolysis.

STEM reveals α-Fe₂O₃@LHEO nanoparticles with an average size of approximately 50 nm (Fig. 2(a)). EDX mapping confirms homogeneous dispersion of the LHEO phase within α-Fe₂O₃ (Fig. 2(b)). ICP-AES indicates Fe as the main component, while the other four elements exhibit similar contents (Fig. S4). Fast Fourier transform (FFT) analysis identifies the [001] zone axis, corresponding to the exposed (001) plane of α-Fe₂O₃ (Fig. 2(c)) [48,49].

Aberration-corrected TEM (AC-TEM) further confirms the coexistence of LHEO and α-Fe₂O₃ (Figs. 2(d) and 2(e)). Notably, the LHEO region exhibits higher 3D intensity, attributed to severe lattice distortion induced by the high-entropy effect. Well-defined lattice fringes with a spacing of 0.252 nm correspond to the (110) plane of α-Fe₂O₃, while a reduced lattice spacing of 0.247 nm in the LHEO region indicates lattice contraction (Figs. 2(e) and 2(f)). EPR spectra reveal a significantly enhanced oxygen vacancy signal, evidencing that the LHEO induces substantial lattice strain and promotes oxygen vacancy formation (Fig. 2(g)).

XRD confirms that α-Fe₂O₃@LHEO retains the hematite R3c structure (α-Fe₂O₃, a = b = 5.037 Å, c = 13.771 Å, α = β = 90°, γ = 120°) (JCPDS 89-0596), with no additional peaks observed (Fig. S5), indicating phase compatibility. XPS reveals a positive shift of 0.81 eV in the Fe 2p in α-Fe₂O₃@LHEO compared to pristine α-Fe₂O₃ and an increased Fe³⁺ (Figs. 2(h), S6). Additionally, the O 1s peak of pristine α-Fe₂O₃ shifts from 529.3 to 530.2 eV after LHEO incorporation (Fig. 2(i)). The presence of Co, Cr, Zn, and Mn is also confirmed (Fig. S7). These results verified successful nanoscale localization of high-entropy oxide in α-Fe₂O₃, and strong electronic interactions with α-Fe₂O₃.

DFT calculations were conducted to study the structural and electronic properties of the electrocatalysts. The introduction of LHEO induces lattice distortion in α-Fe₂O₃, as reflected by a reduced average Fe–O bond length (1.91 Å in α-Fe₂O₃@LHEO versus 1.93 Å in pristine α-Fe₂O₃; Fig. 3(a)). Charge density difference and Bader charge analysis (Fig. 3(b) and Table S1) reveal electron transfer from α-Fe₂O₃ to LHEO, lowering the average Bader charge on both Fe and O sites in α-Fe₂O₃@LHEO. This charge redistribution signifies an increased oxidation state of Fe, which is corroborated by the electronic density of states (DOS). The total DOS shows an elevated number of occupied states near the Fermi level in α-Fe₂O₃@LHEO, indicating enhanced electrical conductivity, which is conducive to rapid electron transfer during electrocatalysis (Fig. S8).

More critically, projection of the DOS onto the Fe-3d orbitals reveals a downshift of the Fe d-band center from −1.51 eV in pristine α-Fe₂O₃ to −1.63 eV in α-Fe₂O₃@LHEO (Fig. 3(c)). This shift results from increased nuclear attraction upon electron loss, which contracts the d orbitals. This lowered d-band center is favorable for weakening the adsorption strength of oxygenated intermediates on the catalyst surface. It thereby lowers the energy barrier of the potential-determining step in the OER, as further discussed below. Crystal field theory provides additional insight into this electronic regulation. The crystal field splitting energy (Δ₀) decreases significantly from Δ₀ = 0.106 eV in α-Fe₂O₃ to 0.087 eV in α-Fe₂O₃@LHEO (Figs. 3(d), S9). A smaller Δ₀ favors a high-spin state of Fe³⁺ ions (Fig. 3(e)) by promoting electron occupation of higher-energy orbitals rather than spin pairing [50]. This is supported by room-temperature magnetic hysteresis (M-H) loop combined with EPR results (Figs. 2(g), S10) [51–53]. This high-spin state results in a larger ionic radius of Fe³⁺, providing a fundamental explanation for the observed lattice contraction [54–56].

Overall, localized high-entropy engineering not only downshifts the Fe d-band center to optimize intermediate adsorption but also introduces defects that significantly enhance the intrinsic conductivity of the material, thereby synergistically contributing to the exceptional OER catalytic performance, as discussed below.

4.1 Electrocatalytic water splitting performance of α-Fe₂O₃@LHEO

The OER performance of water electrolysis over α-Fe₂O₃@LHEO was first evaluated in 1.0 mol/L KOH aqueous solution using a standard three-electrode configuration (Figs. S11 and S12). The iR-corrected LSV curves were collected at a scan rate of 10 mV/s. As shown in Fig. 4(a), the overpotential (η) of α-Fe₂O₃@LHEO is markedly lower than that of commercial RuO₂. Specifically, the overpotentials of α-Fe₂O₃@LHEO at current densities of 10 and 100 mA/cm² were measured to be 229 and 306 mV, respectively. Meanwhile, the corresponding Tafel slope of α-Fe₂O₃@LHEO is as low as 34.4 mV/dec, substantially lower than that of RuO₂ (118.8 mV/dec), indicating accelerated OER kinetics (Fig. 4(b)).

To elucidate the origin of the exceptional performance, the electrochemical active surface area (ECSA) was determined via C_dl measurements in the non-Faradaic region (1.024–1.224 V versus RHE; Fig. S13). The C_dl of α-Fe₂O₃@LHEO is 6.8 mF/cm², significantly higher than those of α-Fe₂O₃ (4.7 mF/cm²) and RuO₂ (4.2 mF/cm²) (Fig. 4(c)), corresponding to an ECSA of 85 cm² (Table S2). This large active area provides abundant reaction sites for catalysis. Furthermore, ECSA-normalized polarization curves (Fig. S14) show that, at a given overpotential, the current density of α-Fe₂O₃@LHEO is significantly higher than that of α-Fe₂O₃. This result confirms that the enhanced catalytic performance of α-Fe₂O₃@LHEO arises not only from increased active sites but also from enhanced intrinsic activity per site. EIS measurements reveal a lower charge-transfer resistance for α-Fe₂O₃@LHEO compared to α-Fe₂O₃ and RuO₂ (Fig. 4(d)). This finding is in excellent agreement with the DFT results above, suggesting facilitated charge transfer upon the introduction of LHEO.

Compared with recently reported high-entropy OER electrocatalysts, the alkaline OER catalyst presented in this work demonstrates superior performance (Fig. 4(e) and Table S3). Notably, α-Fe₂O₃@LHEO suppresses the formation of H₂O₂ compared to α-Fe₂O₃, as revealed by UV-Vis spectroscopy (Fig. S15 and S16). In addition, α-Fe₂O₃@LHEO exhibits excellent stability, maintaining operation for nearly 1000 h at a high current density of 100 mA/cm², highlighting its potential for storing renewable electricity as green H₂ without noble metals (Fig. 4(f)). After the stability test, no collapse of the NF framework or detachment of α-Fe₂O₃@LHEO was observed (Figs. S17–S19).

To broaden its applicability, the catalyst was also evaluated for OER in natural seawater electrolyte (Fig. S20 and Table S4). As illustrated in Figs. 4(g) and S21, α-Fe₂O₃@LHEO maintains a substantial overpotential advantage over RuO₂ and exhibits faster OER kinetics in natural seawater, as reflected by a lower Tafel slope (49.5 mV/dec; Fig. S22). The measured C_dl value of α-Fe₂O₃@LHEO (5.2 mF/cm²) is 1.79 times that of RuO₂ (2.9 mF/cm²) (Figs. S23 and S24). Meanwhile, α-Fe₂O₃@LHEO shows more efficient charge transfer in seawater (Fig. S25). These results demonstrate that localized high-entropy oxides in α-Fe₂O₃ serve as a promising alternative to noble metals as efficient and stable OER electrocatalysts for both water and seawater electrolysis.

To further investigate the origin of the exceptional catalytic performance, the pH-dependent OER activity was measured (Fig. 5(a)) [57]. The significantly stronger pH dependence of the OER rate (higher j versus pH slope) for α-Fe₂O₃@LHEO is consistent with the proton-electron non-concerted transfer feature of the LOM pathway, which is distinctly different from the AEM behavior of pristine α-Fe₂O₃. The OER performance of α-Fe₂O₃@LHEO was further tested in 1 mol/L TMAOH (Fig. S26), showing significantly reduced OER activity and slower kinetics compared to those in 1 mol/L KOH (Fig. 5(b)) [58]. This is attributed to TMA⁺ consuming surface O₂²⁻/O²⁻ active species, thereby suppressing the LOM-OER process. These results confirm that the reaction pathway shifts from AEM to LOM upon the introduction of localized high-entropy oxides in α-Fe₂O₃.

Surface Raman spectroscopy (SRS) characterization was performed to gain deeper insight into the reaction mechanism using ¹⁸O-labeled H₂O. As illustrated in Fig. 5(c), the characteristic peak at 1050–1100 cm⁻¹ is assigned to the metal-oxygen (M–O) stretching vibration. Notably, this peak in the isotopically labeled H₂¹⁸O electrolyte exhibits a negative shift from 1077.12 to 1051.21 cm⁻¹ compared to H₂O. This shift is attributed to the substitution of lattice M–¹⁶O with M–¹⁸O [59]. These observations provide strong evidence that the LOM dominates OER catalysis over α-Fe₂O₃@LHEO.

Further investigation was performed using in situ Raman spectroscopy under varying applied potentials (Fig. 5(d)). In the transverse optical (TO) region, one spectral peak at ~490 cm⁻¹ is assigned to the symmetric A_1g mode, while two additional peaks at ~410 and ~610 cm⁻¹ correspond to doubly degenerate E_g modes. An additional peak at 660 cm⁻¹ is observed, which can be ascribed to the relaxation of longitudinal optical (LO) phonon selection rules induced by oxygen vacancies [60]. The intensity ratio of the LO mode to the E_g mode (I[LO]/I[E_g]) was employed to quantify variation in oxygen vacancy content [61]. As illustrated in Fig. S27, the oxygen vacancy concentration exhibits a fluctuating upward trend with increasing applied potential, indicating a dynamic generation-consumption cycle during the OER. Additionally, the peak located at ~1070 cm⁻¹ is assigned to bridging peroxide species (M–O₂–M), whose intensity increases progressively with increasing applied potential. Combined with the EPR results (Fig. 2(g)), these findings indicate that the reaction pathway shifts from AEM in α-Fe₂O₃ to LOM in α-Fe₂O₃@LHEO (Fig. 5(e)).

DFT calculations were further conducted to elucidate the reaction mechanism at the atomic scale. Since the characteristic step of AEM involves OH adsorption on metal sites, a potential energy surface based on OH adsorption free energies was constructed for α-Fe₂O₃@LHEO (Figs. 5(f) and S28). As shown in Fig. 5(g), OH adsorption on pristine α-Fe₂O₃ exhibits negative adsorption energy, indicating that the formation of Fe-OH intermediates is energetically favorable in the absence of LHEO. This further supports the role of LHEO in shifting the mechanism from AEM to LOM.

The OER pathways over α-Fe₂O₃@LHEO and α-Fe₂O₃ were further calculated (Figs. 5(h), S29 and S30). The reaction initiates at abundant surface oxygen vacancies, where a solution-phase OH^- fills an oxygen vacancy and undergoes deprotonation. Subsequently, OH is adsorbed onto a lattice oxygen site to form a Fe-OOH intermediate, followed by deprotonation of OOH and regeneration of oxygen vacancy. The OOH deprotonation step (O-Fe-OOH + OH⁻→ O-Fe-OO + H₂O + e⁻) is identified as the rate-determining step (RDS). Notably, the introduction of LHEO lowers the energy barrier of the RDS from 1.85 to 1.71 eV.

4.2 Rechargeable ZAB

To broaden the application, α-Fe₂O₃@LHEO was employed as the air electrode (catalyst loading: 0.25 mg/cm²) in a rechargeable ZAB, with commercial Pt/C serving as the cathode (Fig. S31). The battery was assembled in a layered configuration (Fig. S32) and tested in a 6 mol/L KOH-natural seawater electrolyte containing 0.20 mol/L Zn(Ac)₂, using a 0.3 mm thick zinc plate as the anode. The α-Fe₂O₃@LHEO + Pt/C based ZAB achieved an open-circuit potential of 1.415 V, surpassing that of RuO₂ + Pt/C (1.349 V) (Fig. 6(a)). As shown in Fig. 6(b), it also delivered a higher peak power density of 88.3 mW/cm², compared to 74.7 mW/cm² for the RuO₂ + Pt/C counterpart. At a current density of 5 mA/cm², the specific discharge capacity of α-Fe₂O₃@LHEO + Pt/C reached 819.9 mAh/g_Zn (Figs. 6(c) and S33), exceeding the 790.9 mAh/g_Zn obtained with RuO₂ + Pt/C. Long-time cycling tests at 5 mA/cm² further confirmed the superior stability of the α-Fe₂O₃@LHEO + Pt/C system (Figs. 6(d) and 6(e)).

It exhibited initial charge/discharge voltages of 2.08 V and 1.14 V, respectively, yielding a voltage gap (ΔE) of 0.93 V, and a corresponding round-trip efficiency of 54.8%. After 600 cycles, the voltage gap increased by only 1.07 mV. In contrast, the RuO₂ + Pt/C battery, although showing a slightly higher initial efficiency of 56.7% (charge: 2.06 V, discharge: 1.17 V), suffered from rapid degradation, with the voltage gap widening to 1.30 V after only 170 cycles. These results highlight the markedly improved energy conversion efficiency and cycling durability of the α-Fe₂O₃@LHEO-based electrode.

Given the growing promise of all-solid-state ZAB as efficient and sustainable power sources, a flexible all-solid-state ZAB (F-ZAB) was further fabricated. The device consists of a nickel-foam-supported α-Fe₂O₃@LHEO + Pt/C integrated electrode as the air cathode, a natural-seawater-soaked alkaline polyacrylic acid (PAA) gel as the electrolyte, and a Zn foil as the anode (Fig. 6(f)). The assembled F-ZAB achieved an open-circuit voltage of 1.36 V (Fig. 6(g)) and delivered a high power density of 36.4 mW/cm² (Fig. S34). During galvanostatic cycling at 5.0 mA/cm², it exhibited stable charge and discharge voltages of 1.96 and 1.11 V, respectively (Fig. 6(h)). The resulting ΔE of 0.85 V corresponds to a round-trip efficiency of 56.6%, and the battery maintained stable operation for over 50 cycles. The flexibility of the device was also validated, as it retained a consistent open-circuit voltage of ~1.32 V under bending angles of 120°, 90°, and 60° (Fig. S35). Compared to conventional liquid-electrolyte ZABs, this rechargeable all-solid-state design offers distinct advantages, including compact design, mechanical flexibility, high power density, and improved operational safety.

5 Conclusions

This work develops a novel localized high-entropy engineering strategy to construct a high-performance α-Fe₂O₃@LHEO electrocatalyst via spatially confining nanoscale Fe-Co-Ni-Zn-Mn high-entropy oxide within the α-Fe₂O₃ lattice. This unique design breaks the intrinsic bottlenecks of conventional iron-based OER electrocatalysts at the electronic and atomic levels, delivering exceptional alkaline OER activity with greatly accelerated reaction kinetics, ultra-long operational stability under high current density, and outstanding performance in natural seawater-based rechargeable Zn-air batteries, far outperforming commercial noble metal benchmark catalysts.

Combined experimental characterizations and theoretical calculations reveal that the core of this performance breakthrough lies in the localized high-entropy effect: it induces lattice strain in α-Fe₂O₃, tailors the electronic structure, and reduces the crystal field splitting energy to stabilize high-spin Fe³⁺. These synergistic effects not only enhance charge transfer efficiency and optimize the adsorption/desorption of key oxygenated intermediates, but also switch the OER pathway from the conventional AEM to the more energetically favorable LOM, with the rate-determining step energy barrier reduced from 1.85 to 1.71 eV.

More importantly, the nanoscale localized high-entropy engineering proposed in this work breaks through the limitations of traditional bulk high-entropy material design and single modification strategies, establishing a new paradigm for the rational design of efficient and durable OER electrocatalysts using earth-abundant elements.

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