DES-Mediated Mild Synthesis of Synergistically Engineered 3D FeOOH-Co₂(OH)₃Cl/NF for Enhanced Oxygen Evolution Reaction

Abstract

Hydrogen energy is a pivotal carrier for achieving carbon neutrality, requiring green and efficient production via water electrolysis. However, the anodic oxygen evolution reaction (OER) involves a sluggish four-electron transfer process, resulting in high overpotentials, while the prohibitive cost and complex preparation of precious metal catalysts impede large-scale commercialization. In this study, we develop a FeCo-based bimetallic deep eutectic solvent (FeCo-DES) as a multifunctional reaction medium for engineering a three-dimensional (3D) coral-like FeOOH-Co₂(OH)₃Cl/NF composite via a mild one-step impregnation approach (70 °C, ambient pressure). The FeCo-DES simultaneously serves as the solvent, metal source, and redox agent, driving the controlled in situ assembly of FeOOH-Co₂(OH)₃Cl hybrids on Ni(OH)₂/NiOOH-coated nickel foam (NF). This hierarchical architecture induces synergistic enhancement through geometric structural effects combined with multi-component electronic interactions. Consequently, the FeOOH-Co₂(OH)₃Cl/NF catalyst achieves a remarkably low overpotential of 197 mV at 100 mA cm⁻² and a Tafel slope of 65.9 mV dec⁻¹, along with 98% current retention over 24 h chronopotentiometry. This study pioneers a DES-mediated strategy for designing robust composite catalysts, establishing a scalable blueprint for high-performance and low-cost OER systems.

1. Introduction

Amidst the global imperative for low-carbon energy transition, hydrogen has emerged as a critical clean energy carrier, driving intense focus on scalable green production technologies [1,2]. While conventional methods rely on fossil fuels, water electrolysis offers a sustainable pathway using water as the sole feedstock [3]. Among various electrolysis technologies, alkaline water electrolysis stands as the most mature platform for clean hydrogen generation [4,5]. However, the anodic oxygen evolution reaction (OER) involves a four-electron transfer process whose sluggish kinetics result in substantial overpotential losses, representing the primary bottleneck for efficient water splitting [6,7,8,9,10,11]. Consequently, developing highly efficient and stable OER catalysts is paramount to overcoming these thermodynamic and kinetic barriers. Although noble metal oxides (e.g., IrO₂, RuO₂) exhibit benchmark activity, their scarcity, high cost, and insufficient stability impede large-scale deployment [12,13,14,15,16].

Recently, transition metal-based catalysts have demonstrated comparable intrinsic activity to precious metals under alkaline conditions [17,18,19], spurring extensive research on Ni-, Co-, and Fe-based materials, including oxides [20,21,22], sulfides [23,24], phosphides [25,26,27,28], and hydroxides [29,30,31,32]. Among these, transition metal hydroxides stand out as promising OER candidates due to tunable electronic structures, multi-valent states, and environmental compatibility. Specifically, FeOOH is recognized as the true active species in Fe-based catalysts [33,34]. However, its practical applications are severely constrained by low intrinsic conductivity (~10⁻⁵ S/cm) and poor structural stability (>40% activity decay after cycling). Consequently, constructing composite catalysts to optimize electronic structures has emerged as an effective strategy to overcome the performance limitations of single-component systems. For instance, Gao et al. [31] synthesized a Co(OH)F/Ni(OH)₂@FeOOH core–shell heterostructure via hydrothermal methods, where interfacial electron transfer among Co(OH)F, Ni(OH)₂, and FeOOH was modulated to enhance OER performance, achieving a reduced overpotential of 270 mV at 100 mA cm⁻². Similarly, Liao et al. [35] engineered a FeOOH/Ni₃(NO₃)₂(OH)₄ composite system, where the strong chemical interaction between NO₃⁻ and Fe species effectively mitigated Fe segregation, thereby significantly enhancing the catalytic stability of Ni-Fe catalysts. Despite these advances, current composite catalyst synthesis predominantly relies on complex multi-step processes under harsh conditions (high temperature/pressure), while insufficient bonding between active components and substrates promotes structural collapse during the prolonged OER process, severely compromising durability [36]. To address these limitations, innovative strategies enabling facile, mild-condition fabrication of robust composites with strong substrate adhesion and optimized interfaces are urgently needed.

To overcome these constraints, a FeCo-based bimetallic deep eutectic solvent (FeCo-DES)-mediated strategy is proposed to engineer a FeOOH-Co₂(OH)₃Cl/NF composite for enhanced OER performance. By employing the multifunctional FeCo-DES system that simultaneously serves as the solvent, metal source, and redox agent, we achieve the controlled in situ assembly of FeOOH nanosheets and Co₂(OH)₃Cl hollow spheres directly on nickel foam (NF) under exceptionally mild conditions (70 °C, ambient pressure). The DES-mediated process not only facilitates the formation of this unique three-dimensional (3D) coral-like architecture but also induces the generation of an interfacial Ni(OH)₂/NiOOH layer through substrate etching. The resulting hierarchical architecture synergistically integrates geometric structural effects that enhance mass/charge transport with multi-component electronic interactions that optimize the reaction kinetics. Consequently, FeOOH-Co₂(OH)₃Cl/NF achieves a remarkably low overpotential and exceptional stability under an alkaline environment, providing great potential for developing high-performance and low-cost OER systems.

2. Results and Discussion

2.1. Morphological Structure Analysis

The FeOOH/NF, Co₂(OH)₃Cl/NF, and FeOOH-Co₂(OH)₃Cl/NF catalysts were synthesized through a mild one-step impregnation approach in metal-based deep eutectic solvents (M-DESs) [37]. As illustrated in Figure 1, this process involves homogenizing metal salt precursors with urea under heating before immersing pretreated NF substrates. The resulting systems exhibit distinct pH characteristics such as Fe-DES maintaining strong acidity (pH ≈ 1.5) from Fe³⁺ hydrolysis, while Co-DES shows alkalinity (pH ≈ 8.5) due to limited Co²⁺ hydrolysis. Remarkably, the FeCo-DES ternary system self-regulates to a weakly acidic pH (5.8–6.2) through Fe³⁺/Co²⁺ charge balance, creating an optimal environment for controlled growth of active species. During synthesis, DES etches the NF surface (Ni → Ni²⁺ + 2e⁻), generating a Ni²⁺ concentration gradient, while the simultaneously released OH^- facilitates metal ion deposition to form FeOOH and Co₂(OH)₃Cl, as well as a Ni(OH)₂/NiOOH layer on the NF substrate. This dual-function mechanism enables efficient in situ catalyst formation while maintaining mild reaction conditions (70 °C, ambient pressure), demonstrating the unique advantages of DES-mediated synthesis.

Surface morphological characterization of the FeOOH/NF, Co₂(OH)₃Cl/NF, and FeOOH-Co₂(OH)₃Cl/NF catalysts was performed via scanning electron microscopy (SEM), with the results presented in Figure 2a–c and Figure S1. SEM analysis reveals that FeOOH/NF exhibits a rough, irregular sheet-like architecture, whereas Co₂(OH)₃Cl/NF displays spherical particles uniformly stacked on the NF substrate. Remarkably, the FeOOH-Co₂(OH)₃Cl/NF composite demonstrates a distinctive three-dimensional coral-like architecture. Furthermore, the material’s morphology and lattice characteristics were thoroughly characterized using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM). As illustrated in Figure 2d–f, the FeOOH/NF catalyst exhibits a two-dimensional nanosheet architecture, while Co₂(OH)₃Cl/NF consists of stacked hollow nanospheres. Remarkably, FeOOH-Co₂(OH)₃Cl/NF manifests a distinctive three-dimensional coral-like morphology, showing excellent consistency with SEM observations. HR-TEM images (Figure 2g–i) reveal distinct lattice fringe spacings of 0.204, 0.210, 0.227, 0.233, and 0.242 nm in the FeOOH/NF sample, which are indexed to the (111) plane of the NF substrate (JCPDS: 87-0712); the (321), (301), and (240) planes of FeOOH (JCPDS: 75-1594); and the (104) plane of Ni(OH)₂/NiOOH (JCPDS: 89-7111). In the Co₂(OH)₃Cl/NF system, the measured lattice spacings of 0.204, 0.213, 0.220, and 0.242 nm correspond to the (111) plane of NF, the (122) and (211) planes of Co₂(OH)₃Cl (JCPDS: 73-2134), and the (104) plane of Ni(OH)₂/NiOOH, respectively. HR-TEM image of FeOOH-Co₂(OH)₃Cl/NF reveals well-defined lattice spacings of 0.204, 0.213, 0.220, 0.227, and 0.242 nm, which are unambiguously assigned to the (111) plane of NF, the (122) and (211) planes of Co₂(OH)₃Cl, the (301) plane of FeOOH, and the (104) plane of Ni(OH)₂/NiOOH. These results demonstrate that, under the unique microenvironment regulation of FeCo-DES, the Ni²⁺ ions released from etched NF substrate form in situ composite deposits with FeOOH, Co₂(OH)₃Cl, and Ni(OH)₂/NiOOH through synergistic intermolecular interactions and bimetallic effects. Preliminary analysis of the SEM and TEM results indicates that the observed morphology stems from the FeCo-DES ternary system’s modulation of the chemical environment on the NF substrate surface. It is noteworthy that the redox potential difference between Fe³⁺/Co²⁺ in the ternary DES system (E°(Fe³⁺/Fe²⁺) = 0.77 V vs. E°(Co³⁺/Co²⁺) = 1.82 V) facilitates a synergistic etching effect, ultimately yielding a coral-like architecture with three-dimensionally interconnected pore channels. The unique structural configuration combining high-curvature surfaces with interconnected pore channels not only substantially enhances the density of exposed active sites but also optimizes mass transport pathways to improve electrolyte–electrode interfacial contact efficiency. To quantitatively evaluate the structural advantages of the unique morphologies, we conducted BET surface area measurements on all three catalysts (Figure S2). The results demonstrate that FeOOH-Co₂(OH)₃Cl/NF exhibits the highest specific surface area (6.55 m² g⁻¹), significantly exceeding those of FeOOH/NF (5.42 m² g⁻¹) and Co₂(OH)₃Cl/NF (1.14 m² g⁻¹). In accordance with the 3D coral-like architecture observed by SEM/TEM, the synergistic combination of high-curvature surfaces and interconnected pore channels endows the composite with a maximized surface area, providing direct evidence for enhanced active site exposure and optimized mass transport pathways. This synergistic architecture simultaneously accelerates electron transfer kinetics and ion diffusion rates, demonstrating the cooperative advantages of the composite catalyst.

X-ray diffraction (XRD) analysis systematically characterized the crystalline phases of the synthesized catalysts (Figure 3). All samples exhibit intense diffraction peaks at 44.50°, 51.85°, and 76.38°, corresponding to the (111), (200), and (220) planes of the nickel foam substrate, respectively. The strong NF signals initially obscure the catalyst phase signatures, requiring a careful examination of magnified diffraction patterns. Through this detailed analysis, distinct but weaker peaks are identified at 21.6° and 34.2°, which are indexed to the (110) and (130) crystallographic planes of FeOOH (JCPDS: 75-1594). To further demonstrate the generation of the active component FeOOH, a Raman spectroscopy analysis was conducted on FeOOH and FeOOH-Co₂(OH)₃Cl/NF. The Raman spectra of FeOOH/NF and FeOOH-Co₂(OH)₃Cl/NF (Figure S3) show characteristic peaks at 540 cm⁻¹, which can be attributed to the asymmetric stretching vibration of Fe-OH, confirming the successful synthesis of FeOOH. Concurrently, characteristic reflections at 26.5° and 32.8° match the (101) and (102) planes of Co₂(OH)₃Cl (JCPDS: 73-2134). Furthermore, subtle diffraction features observed at 19.8° and 38.5° are attributed to a Ni(OH)₂/NiOOH transition layer resulting from surface oxidation of the NF substrate during synthesis. These collective XRD results provide definitive evidence for the successful co-deposition of FeOOH and Co₂(OH)₃Cl phases onto the NF support, with the incidental formation of a Ni(OH)₂/NiOOH interfacial layer, serving as a ternary composite architecture that establishes an optimal structural framework for enhanced electrocatalytic activity.

X-ray photoelectron spectroscopy (XPS) investigations yielded critical insights into the surface chemical states and electronic interactions within the composite system (Figure 4). The full-range survey XPS spectra (Figure 4a and Figure S4) demonstrate elemental specificity, with FeOOH/NF exhibiting characteristic Fe 2p (~711.5 eV) and Co₂(OH)₃Cl/NF showing distinct Co 2p (~780.6 eV) and Cl 2p (~198.5 eV) signatures. The XPS fitting analysis (Figures S5 and S6) confirms the coexistence of Ni²⁺ and Ni³⁺ oxidation states on the surface of all three samples, along with the successful incorporation of Cl in both the Co₂(OH)₃Cl/NF and FeOOH-Co₂(OH)₃Cl/NF catalysts. In the FeOOH-Co₂(OH)₃Cl/NF composite, the co-existence of both Fe 2p and Co 2p signals conclusively verifies bimetallic integration. High-resolution analysis of the O 1s region (Figure 4b) resolves three constituent components, including metal–oxygen bonds (M-O, 529.1 eV), hydroxyl groups (M-OH, 531.2 eV), and adsorbed water molecules (532.9 eV), confirming the expected chemical environments in these hydroxide-based materials. The Fe 2p XPS spectra (Figure 4c) display characteristic Fe³⁺ signatures at 712.7 eV (2p_3/2) and 724.9 eV (2p_1/2), with the distinctive satellite peak at 720.2 eV, all consistent with the FeOOH phase identified by XRD. Notably, the FeOOH-Co₂(OH)₃Cl/NF composite exhibits an obvious 0.6 eV negative shift in the Fe 2p_3/2 binding energy relative to pure FeOOH/NF, indicating substantial electron density redistribution. Similarly, the deconvolution of the Co 2p region (Figure 4d) reveals the coexistence of Co²⁺ (781.2 eV, 796.9 eV) and Co³⁺ (784.1 eV, 799.1 eV) oxidation states, with the composite showing analogous negative shifts of 0.4–0.8 eV across both Co 2p orbitals. These synchronous binding energy reductions in both Fe and Co species demonstrate a strong electronic coupling between the constituent phases, where electron transfer from Co/Ni species to Fe centers optimizes the local electronic structure for oxygen evolution catalysis. This interfacial electron redistribution, combined with the advantageous morphological features revealed by microscopy, creates a synergistic system that simultaneously enhances charge transfer kinetics and reduces reaction intermediate adsorption energies, which are the key factors underlying the observed OER performance enhancement.

2.2. Electrochemical Performance Evaluation

To achieve optimal catalytic performance of the FeOOH-Co₂(OH)₃Cl/NF catalyst, we focused on investigating the molar ratio of CoCl₂·6H₂O to FeCl₃·6H₂O as well as the ratio between metal salts and urea. Through controlled-variable experiments, the optimal preparation conditions are Co:Fe = 1:1 and M:U = 2:1 (Figure S7). To systematically evaluate the regulatory effects of M-DES systems on the electrocatalytic performance of NF substrates, we conducted OER measurements for a series of catalysts in 1.0 M KOH electrolyte using a standard three-electrode electrochemical workstation. As shown in the linear sweep voltammetry (LSV) curves (Figure 5a), the FeOOH-Co₂(OH)₃Cl/NF composite catalyst requires only 197 mV overpotential to achieve a current density of 100 mA cm⁻², which is substantially lower than those of its individual components: FeOOH/NF (363 mV), Co₂(OH)₃Cl/NF (426 mV), and bare NF substrate (724 mV). These performance metrics surpass those of all previously reported Ni-based OER electrocatalysts, demonstrating the unique advantages of the FeCo-DES etching strategy for constructing high-efficiency composite catalysts. The enhanced catalytic activity originates from the electronic synergy between the FeOOH-Co₂(OH)₃Cl composite interface and the Ni(OH)₂/NiOOH layer, while the three-dimensional coral-like architecture provides abundant exposed active sites and efficient mass transport pathways. The Tafel slope analysis further reveals the reaction kinetics of the catalytic system (Figure 5c). The FeOOH-Co₂(OH)₃Cl/NF composite catalyst exhibits the lowest Tafel slope of 65.9 mV dec⁻¹, which is significantly smaller than those of FeOOH/NF (121.5 mV dec⁻¹), Co₂(OH)₃Cl/NF (191.1 mV dec⁻¹), and bare NF (256.5 mV dec⁻¹). These results demonstrate that the FeOOH-Co₂(OH)₃Cl composite interface formed via FeCo-DES etching significantly enhances the intrinsic OER activity, with the kinetic improvement originating from the electronic structure reconstruction of metal active sites. Experimental evidence confirms that the multimetal synergy among Fe/Co/Ni components establishes continuous charge-transfer pathways, effectively reducing the energy barrier for hydroxide intermediate formation during the rate-determining step, aligned with the multi-component electron transfer effects revealed by XPS. To quantitatively compare the advantages of bimetallic composite catalysts, controlled electrolysis was conducted at 100 mA cm⁻² for 400 s with oxygen yield precisely measured by a self-constructed water displacement apparatus. As shown in Figures S8 and S9, FeOOH-Co₂(OH)₃Cl/NF achieves the highest Faraday efficiency (85.9%), substantially exceeding those of FeOOH/NF (68.7%) and Co₂(OH)₃Cl/NF (62.5%). This directly verifies that the Fe-Co bimetallic synergy significantly enhances four-electron oxygen evolution selectivity and efficiency via optimized interfacial charge-transfer kinetics. Meanwhile, to quantitatively elucidate the enhancement of intrinsic activity via Fe-Co bimetallic synergy, the turnover frequency (TOF) was systematically determined by integrating electrochemical active surface area (ECSA) with the actual oxygen yield. The results demonstrate that FeOOH-Co₂(OH)₃Cl/NF achieves a TOF of 2.21 s⁻¹, significantly surpassing FeOOH/NF (1.97 s⁻¹) and Co₂(OH)₃Cl/NF (1.85 s⁻¹). This kinetically verifies that the intrinsic activity superiority of the composite catalyst originates from intermetallic electron synergy. The long-term operational stability was evaluated by chronoamperometry (Figure 5d–f). After 24 h of continuous testing at a constant overpotential, the FeOOH-Co₂(OH)₃Cl/NF system retained 98% of its initial current density, substantially outperforming both FeOOH/NF (50%) and Co₂(OH)₃Cl/NF (88%). This exceptional durability originates from the 3D coral-like framework induced by FeCo-DES etching, where the interconnected porous architecture enables the uniform electrochemical activation of active sites by mitigating local current density gradients. To objectively evaluate the electrochemical performance of the FeOOH-Co₂₍OH)₃Cl/NF composite, we systematically compared it with recently reported catalysts containing Fe/Co active sites (

Table 1. OER catalytic performance comparison between FeOOH-Co₂(OH)₃Cl/NF and previously reported Fe-, Co-based metal oxide/hydroxide/oxyhydroxide.

Material	Substrate	Current Density (mA cm−2)	η (mV)	Tafel Slope (mV dec−1)	Reference
FeOOH-Co2(OH)3Cl/NF	NF	100	197	65.9	This work
NiCo2S4/FeOOH NF	NF	100	385.6	80.4	[38]
CoB2O4@FeOOH/NF	NF	100	260	116.9	[39]
Co3O4/NC	NC	100	264	80	[40]
FeCo-MOF	-	100	475	121.8	[41]
Co2P@Fe2P/NF	NF	50	267	65.0	[42]
FeOOH/NiSH/NF	NF	50	265	73.8	[43]
FeOOH/NCS/NS/NF	NF	50	285	93	[44]
Ni(OH)2-MD	-	10	300	104	[45]
P-MoO3/FeCo LDH/NF	NF	10	225	87.4	[46]
Cd-2D	-	10	236	98	[47]

). As shown in the table, FeOOH-Co₂(OH)₃Cl/NF exhibits significantly lower overpotential (197 mV) and Tafel slope (65.9 mV dec⁻¹) compared to the catalysts listed.

The electrochemical active surface area (ECSA) was quantitatively characterized through cyclic voltammetry scans within the potential window of 1.02–1.12 V vs. RHE (Figure 6a–d), employing a scan rate gradient from 20 to 120 mV s⁻¹. As shown in Figure 6e, the FeOOH-Co₂(OH)₃Cl/NF catalyst exhibits the highest double-layer capacitance (C_dl) of 3.59 mF cm⁻², significantly surpassing those of FeOOH/NF (2.75 mF cm⁻²), Co₂(OH)₃Cl/NF (2.42 mF cm⁻²), and bare NF (1.88 mF cm⁻²). These results demonstrate that the unique porous network of the 3D coral-like architecture not only substantially increases active site density (30.5–48.3% enhancement compared to single-component counterparts) but also optimizes electrolyte mass transport pathways. In addition, to elucidate the active site formation during pre-oxidation, cyclic voltammetry was performed over an extended potential window (0.6–2.1 V vs. RHE). As shown in Figure S10, within the 1.4–1.8 V region, FeOOH-Co₂(OH)₃Cl/NF exhibits a prominent oxidation peak at 1.68 V with substantially enhanced current intensity compared to FeOOH/NF and Co₂(OH)₃Cl/NF. This directly confirms the composite’s superior oxidation capability, providing critical kinetic evidence for Fe-Co bimetallic synergy in enhancing intrinsic OER activity. Consequently, the FeOOH-Co₂(OH)₃Cl/NF catalyst exhibits optimal charge transfer efficiency and reaction kinetics during OER, with the performance enhancement mechanism attributable to the synergistic reinforcement between the geometric effects induced by the coral-like structure and the multi-component electronic effects. The electrochemical impedance spectroscopy (EIS) analysis further elucidates the charge-transfer characteristics of the FeOOH-Co₂(OH)₃Cl/NF catalyst (Figure 6f and Figure S11). The Nyquist plot reveals a well-defined high-frequency semicircle for FeOOH-Co₂(OH)₃Cl/NF, with a significantly reduced semicircle diameter compared to the reference samples. Combined with the previous ECSA and Tafel analysis results, the hierarchical porosity of the 3D coral-like architecture and multi-component electronic synergy collectively enable FeOOH-Co₂(OH)₃Cl/NF to achieve the simultaneous enhancement of active site exposure and rapid charge transfer kinetics during OER. Therefore, the dual geometric–electronic mechanism provides new insights for engineering effective and durable OER electrocatalysts for hydrogen production.

3. Materials and Methods

3.1. Materials

Cobalt (II) chloride hexahydrate (CoCl₂·6H₂O, AR) and iron (III) chloride hexahydrate (FeCl₃·6H₂O, AR) were supplied by Aladdin Reagent (Shanghai, China). Urea (AR), potassium hydroxide (KOH, AR), and absolute ethanol (AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nickel foam (NF) was purchased from Suzhou Keshenghe Metal Materials Co., Ltd. (Suzhou, China). High-purity oxygen gas (>99.999%) was procured from Qingdao Tianyuan Gas Co., Ltd. (Qingdao, China). All chemicals were used as received without further purification.

3.2. Synthesis of FeOOH-Co₂(OH)₃Cl/NF Catalyst

The nickel foam (NF) was cut into 1.5 × 1.5 cm² pieces and sequentially ultrasonicated in 1.0 M HCl, deionized water, and ethanol for 15 min each to remove surface oxides and impurities. The treated NF was air-dried and stored in absolute ethanol for subsequent use. A homogeneous FeCo-based deep eutectic solvent (FeCo-DES) was prepared by mixing 5 mmol each of FeCl₃·6H₂O, CoCl₂·6H₂O, and urea in a vial with continuous stirring at 70 °C for 30 min. The pretreated NF was immersed in the FeCo-DES solution and maintained at 70 °C for 25 min. The resulting sample was then alternately rinsed with deionized water and ethanol (2–3 cycles) before vacuum drying at 60 °C for 12 h. The final product was designated as FeOOH-Co₂(OH)₃Cl/NF. For comparative studies, control samples were prepared following identical procedures using solely FeCl₃·6H₂O or CoCl₂·6H₂O, yielding FeOOH/NF and Co₂(OH)₃Cl/NF, respectively.

3.3. Characterization

The samples were comprehensively characterized using multiple analytical techniques to investigate their morphology and structure. Scanning electron microscopy (SEM, Hitachi SU8010, Hitachi High-Tech Corporation, Tokyo, Japan) was employed to examine the surface morphology through synchronized electron beam scanning, which generates various signals displayed on a fluorescent screen. Transmission electron microscopy (TEM, JEOL JEM-2100F, JEOL Ltd., Tokyo, Japan) provided ultrastructural characterization, where transmitted electrons underwent angular scattering upon interacting with atomic nuclei, revealing crystal structure and lattice spacings. X-ray diffraction (XRD, Empyeranr, Malvern Panalytical, Almelo, the Netherlands) analysis was performed by comparing the diffraction patterns with standard reference cards (JCPDS) to determine phase composition, crystallinity, and crystal structure. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) characterized surface elemental composition, chemical states, and electronic structure by measuring the kinetic energy of photoelectrons emitted under X-ray irradiation, with the subsequent peak fitting analysis of electron cloud distribution and bonding configurations.

3.4. Electrochemical Performance Testing

The prepared FeOOH/NF, Co₂(OH)₃Cl/NF, and FeOOH-Co₂(OH)₃Cl/NF samples were trimmed to 1.0 cm × 1.0 cm dimensions. OER measurements were conducted using a standard three-electrode configuration on a CH Instruments 760E electrochemical workstation (Shanghai Chenhua), with an oxygen-saturated 1.0 M KOH electrolyte, a carbon rod counter electrode, and an Ag/AgCl reference electrode. The NF-supported samples served as self-supporting working electrodes. Linear sweep voltammetry (LSV) was performed under oxygen-saturated conditions with the following parameters: potential range of 1.1–2.2 V vs. RHE, and scan rate of 5 mV s⁻¹. All potentials were converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation:

E(RHE) = E(Ag/AgCl) + 0.197 V + 0.059 × pH

(1)

Electrochemical impedance spectroscopy (EIS) measurements were conducted with a 5 mV AC amplitude under 0.5 V bias potential across a frequency range of 0.01–10⁵ Hz. Cyclic voltammetry (CV) was performed in the non-Faradaic potential region (1.02–1.12 V vs. RHE) at scan rates of 20, 40, 60, 80, 100, and 120 mV s⁻¹. The double-layer capacitance (C_dl) was determined from the slope of the linear plot of current density difference (“∆j = (janode-jcathode)/2” ) at 1.072 V vs. RHE versus scan rate. C_dl was employed to evaluate the electrochemical active surface area. Chronoamperometry tests were conducted at a fixed current density of 100 mA cm⁻² for 24 h to assess long-term stability, with the current retention rate serving as the stability indicator.

4. Conclusions

This study pioneers a novel one-step impregnation strategy mediated by an iron–cobalt bimetallic deep eutectic solvent (FeCo-DES), enabling the direct synthesis of 3D coral-like FeOOH-Co₂(OH)₃Cl/NF composite catalysts on NF substrates. The FeCo-DES system facilitates in situ coupled growth and electronic structure modulation of Fe/Co/Ni multi-active components through an etching-growth mechanism, establishing a foundation for high-performance OER catalysts. The three-dimensional coral-like architecture significantly enhances mass transport efficiency by providing abundant accessible active sites. Moreover, the electronic synergy among Fe-Co-Ni multi-components effectively modulates the electronic states of active sites, leading to reduced reaction energy barriers. As a result, the FeOOH-Co₂(OH)₃Cl/NF catalyst exhibits exceptional electrocatalytic OER performance in an alkaline environment. Specifically, it demonstrates a low overpotential of merely 197 mV at a current density of 100 mA cm⁻² and a low Tafel slope of 65.9 mV dec⁻¹, suggesting rapid charge transfer kinetics. Remarkably, the catalyst demonstrates exceptional structural stability during prolonged operation, maintaining 98% current density retention after 24 h of constant voltage testing with negligible decay. Combining geometric effects and multi-component electronic interactions, this synergistic strategy presents a novel and promising approach for constructing OER electrocatalysts with both high intrinsic activity and superior operational stability.