Facile Synthesis of Sulfate-Intercalated CoFe LDH Nanosheets Derived from Two-Dimensional ZIF-9(III) for Promoted Oxygen Evolution Reaction

Abstract

Layered double hydroxide (LDH) has emerged as a promising electrocatalyst; however, the synthetic method usually requires high temperature and high pressure, and sulfate-intercalated LDH is rarely reported. Herein, the sulfate-intercalated CoFe LDH nanosheets were successfully fabricated at ambient temperature via a facile strategy, using two-dimensional ZIF-9(III) as a template and FeSO₄ as both etchant and iron source. When the as-prepared sulfate-intercalated CoFe LDH acts as an electrocatalyst, it presents superior electrocatalytic performance for the oxygen evolution reaction (OER), requiring low overpotential (η@10 mA cm⁻² = 218 mV) with a small Tafel slope of 59.9 mV dec⁻¹ in 1.0 M KOH, which compares favorably with commercial RuO₂ and most reported transition-metal electrocatalysts. The high catalytic activity of CoFe LDH might be ascribed to the large interlayer space distance originating from special SO₄²⁻ ions and the strong synergistic effects between Fe and Co. This work provides a novel and feasible approach to designing highly efficient electrocatalysts based on advanced LDH materials for OER.

1. Introduction

Hydrogen energy has been extensively explored as an ideal green alternative to traditional fossil fuels because of its high energy density, perfect conversion efficiency, and zero carbon emissions [1]. Water electrolysis provides a potential path to produce high-purity hydrogen. Nonetheless, the efficiency of water electrolysis is greatly hampered by the sluggish kinetics of the oxygen evolution reaction (OER), which involves a complex multielectron and multiproton process. Currently, noble metal oxides (e.g., RuO₂ and IrO₂) are the benchmark of OER electrocatalysts, but their scarcity, prohibitively high cost, and unsatisfactory stability have severely limited their widespread commercial application [2,3]. Therefore, exploiting the high efficiency, large reserves, and low costs of non-precious-metal-based catalysts to accelerate the reaction rate of OER has become an imperative and urgent matter.

Transition-metal (e.g., Co, Fe and Ni) layered double hydroxides (LDH) have emerged as promising electrocatalysts for OER because of their flexible open layer structures, tunable chemical composition, and cost effectiveness [4,5,6]. In particular, the OER activity of bimetallic CoFe LDH is dramatically improved compared to the individual Co and Fe components due to the modulated electronic structures, enhanced charge transfer, and synergistic interactions between Co and Fe [7,8]. However, the bulk CoFe LDH still suffers from both intrinsically weak electronic conductivity and serious aggregation problems. To acquire high-performance CoFe LDH electrocatalysts, various strategies have been adopted, including heteroatom doping, the design of nanostructures, combining CoFe LDH with other active materials, and the exfoliation of CoFe LDH et al. [9,10,11,12]. For example, Wang’s group developed a plasma-enabled exfoliation strategy to obtain ultrathin CoFe-LDH nanosheets, which exhibited superior catalytic activity compared to the pristine samples because they were exposed to more active sites [12]. It is well known that LDH nanosheets have abundant active sites, rapid mass transport, and superior electron transfer ability, thereby greatly promoting catalytic properties. On the other hand, expanding the interlayer distance by changing interlayer anions is also proved to be an efficient strategy to create more active sites and improve electrocatalytic activity [13,14,15]. For example, Sun’s group found that more intercalated NO₃⁻ instead of CO₃²⁻ could enlarge the interlayer space from 7.69 Å to 8.04 Å, thus reducing the charge-transfer resistance and enhancing the catalytic activity of NiFe LDH [16]. Furthermore, Lang’s group reported ultrathin sulfate-intercalated NiFe-LDH nanosheets that show excellent stability and great electrocatalytic performance in OER [17]. Therefore, developing a facile synthetic method to fabricate sulfate-intercalated CoFe LDH nanosheets with a large interlayer space is highly desirable.

Recently, due to their high surface area, dispersed metal centers, and abundant pore structures, metal–organic frameworks (MOFs) have been suggested as one of the best precursors for preparing the metal oxides/hydroxide or porous carbon materials through pyrolysis or etching [18,19]. Moreover, numerous studies have demonstrated that these MOFs derivatives are promising candidates for OER electrocatalysts [20,21,22]. In particular, most MOFs-derived LDHs exhibit excellent OER electrocatalytic performances [23,24,25]. It is worth noting that the Co-N bond in ZIF-67 structure can be easily broken in acidic conditions and that the released Co²⁺ ions can be used as metal precursors to build MOF-derived LDHs [26,27]. However, the microstructure of the involved precursor MOFs usually consists of three-dimensional (3D) nanomaterials and the transformation from 3D MOFs into 2D LDH is a high-energy-consuming and complicated synthetic process, leading to a high cost [28]. In our previous work, a novel 2D ZIF-9(III)/Co LDH was successfully synthesized via the controllable phase transition of 3D ZIF-9(I) and exhibited an efficient OER electrocatalytic performance [29]. Thus, developing a facile, surfactant-free, high-yield, and economically feasible synthetic route based on 2D ZIF-9 to fabricate CoFe LDHs with large interlayer space is a considerable challenge.

Herein, a facile method of etching–coprecipitation is developed to drive the conversion of 2D ZIF-9(III) into sulfate-intercalated CoFe LDH nanosheets. The as-prepared CoFe LDH not only retains the 2D morphology of the ZIF-9(III) precursor, but also exhibits a hierarchical structure composed of small crimped nanosheets. Benefiting from the larger interlayer spacing with sulfate intercalation, abundant active sites, and synergistic effect between Co and Fe, the CoFe LDH shows extraordinary mass transport and excellent activities for OER in alkaline media.

2. Results and Discussion

The synthetic process of the CoFe LDH nanosheets is schematically illustrated in Figure 1. Firstly, uniform 2D ZIF-9(III) nanosheets were fabricated using a simple precipitation method at room temperature. Then, sulfate-intercalated CoFe LDHs were successfully prepared through a gentle etching–coprecipitation method using FeSO₄·7H₂O as the etchant and 2D ZIF-9(III) as the precursor. During the chemical transformation, the protons generated from the hydrolysis of Fe²⁺ gradually attacked the Co-N bond of ZIF-9(III) to release the Co²⁺ ions into the solution. At the same time, Fe(OH)₂ from the hydrolysis of Fe²⁺ was oxidized to form Fe(OH)₃ in the air. Afterwards, Co²⁺ and Fe(OH)₃ coprecipitated homogeneously to form CoFe LDH nanosheets on the outer layer of the ZIF-9 precursors, while the SO₄²⁻ was reserved as intercalation anions. Finally, with the gradual dissolution of the precursor, hierarchical CoFe LDH composed of ultrathin nanosheets was obtained.

X-ray diffraction (XRD) analysis was used to investigate the composition and structural parameters of the as-synthetic samples. The characteristic XRD pattern of ZIF-9(III) was identified as the simulated XRD pattern of ZIF-9(III) (CCDC-988184, Figure 2a) [30], which indicates that the precursor was successfully prepared. As shown in Figure 2b, the XRD pattern of the CoFe LDH nanosheets could be assigned to a typical hydrotalcite-like structure [31], demonstrating the complete conversion of the ZIF-9(III) into CoFe LDH using the facile method. The four peaks were observed at the 2θ values of 10.0°, 20.3°, 33.4°, and 59.7°, which can be attributed to the (003), (006), (012), and (110) planes of LDH (JCPDS No. 52-0552), respectively. The LDH interlayer spacing was calculated using Bragg’s law: d = nλ/2sinθ. It should be pointed out that the selected 2θ values correspond to the (003) crystal plane from the structure of the LDH [15]. Thus, the interlayer spacing was calculated to be 0.89 nm from the XRD result (Figure 2b), which is larger than that of previously reported LDH [10,11,12]. As we know, increasing the interlayer spacing could accelerate the electrocatalytic activity for the water oxidation reaction [11,14].

To investigate a fingerprint of molecular vibrations of the bonding structure from the as-prepared samples, Fourier transform infrared spectroscopy (FTIR) was utilized. As shown in Figure 2c, the broad band located at 3430 cm⁻¹ was attributed to the hydroxyl groups of the adsorbed water molecules. A broad band ranging from 500–650 cm⁻¹ in the FTIR spectrum of ZIF-9(III) related to the stretching vibration of the Co-N bonds. The absorption at 1606 cm⁻¹ was attributed to the C=N stretching mode [32]. The peaks at about 745 cm⁻¹ and 1461 cm⁻¹ corresponded to the C-H out-of-plane bending vibration and the ring vibrations, respectively [33]. The above results demonstrate the successful preparation of ZIF-9(III). For CoFe LDH, the presence of an obvious and wide-ranging band at around 3430 cm⁻¹ demonstrates the stretching vibration of hydrogen-bonded hydroxyl groups from both the brucite-like layers and the intercalated H₂O molecules. The sharp peak at around 1629 cm⁻¹ might originate from the bending vibration of water [34]. The weak bands below 800 cm⁻¹ may belong to the characteristic bending vibration of Fe-O and Co-O. The bands at 1113, 1066, and 618 cm⁻¹ [17,35,36] are ascribed to the vibrations of S-O, demonstrating that the main interlayer anion in the LDH is SO₄²⁻. The bivalent SO₄²⁻ can be intercalated more easily into the interlayer of LDH due to the stronger electron affinity between the SO₄²⁻ anions and the positively charged metal-OH host layers [15]. The Raman spectra of the ZIF-9(III) and CoFe-LDH are displayed in Figure 2d. All the bands of 2D ZIF-9(III) are found in the same position as in previously reported work [37]. In addition, the two obvious peaks at 446 and 526 cm⁻¹ for CoFe LDH are ascribed to the M-O vibration [38]. The strongest intensity band at 981 cm⁻¹ can be observed in the Raman spectrum of the CoFe LDH, which is assigned to the symmetric stretching of SO₄²⁻ tetrahedra [39], in agreement with the results of FTIR.

Surface morphologies of nanostructured ZIF-9(III) and CoFe LDH were characterized using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). As displayed in Figure 3a, the sheet-like morphology of ZIF-9(III) can be observed. The corrugations of some wrinkles in the sample indicate that the ZIF-9(III) is a multilayered structure (Figure S1). Moreover, the CoFe LDH maintains the size and morphology of the precursor in Figure 3b. However, the flat surface is covered by substantial and densely packed nanosheets with smaller lateral size and thickness. As shown in the high-magnification FE-SEM images (Figure 3c,d and Figure S2), the CoFe LDH is composed of many ultrathin nanosheets, indicating that the material displays a hierarchical structure. Furthermore, the structure of CoFe LDH is analyzed using TEM and HRTEM. As shown in Figure 3e,f, the CoFe LDH is composed of ultrathin nanosheets. The folded edges or wrinkles of nanosheets illustrate the ultrathin character. The atomic force microscopy (AFM) results (Figure S3) demonstrate that the average thickness of the ZIF-9(III) and CoFe LDH nanosheets is about 1.75 nm and 1 nm, respectively, and the lateral size is about 1.5 μm and 100–200 nm, respectively. Such thin nanosheets could provide rich redox-reaction sites to improve the electrochemical performance of CoFe LDH [2,12]. Figure 3g and Figure S4 demonstrate HRTEM images of the nanosheet, and the spacing of 0.890 nm, 0.265 nm, and 0.445 nm correspond to the (003), (012), and (006) plane of the LDH structure, respectively, in accordance with the above XRD data. The corresponding EDX mappings confirm the uniform distribution of Co, Fe, O, and S on the nanosheets (Figure 3h). In addition, the BET surface area and porosity of the CoFe LDH are characterized using N₂ isothermal adsorption–desorption measurements (Figure S5). The surface area of CoFe LDH is 57.0 m² g⁻¹. From the corresponding Barrett–Joyner–Halenda (BJH) pore-size distributions, CoFe LDH exhibits the mesoporous property.

The surface compositions and the detailed valence states of samples were investigated using X-ray photoelectron spectroscopy (XPS) measurements. The binding energy corrections on high-resolution scans were calibrated by referencing the C 1s peak (284.8 eV). The survey spectra of CoFe LDH confirms the coexistence of Co 2p, Fe 2p, O1s, and S 2p (Figure 4a). As can be seen from the Co 2p spectrum (Figure 4b), the two peaks located at 781.2 and 797.3 eV are assigned to the Co 2p_3/2 and Co 2p_1/2 spin-orbit peaks, respectively. According to the peak-fitting analysis, the characteristic peaks of binding energy at 782.7 and 798.6 eV are ascribed to Co²⁺, while the other two peaks at 781.1 and 797.1 eV are attributed to Co³⁺ [13]. Additionally, the presence of the peaks at 786.8 and 803.4 eV are assigned to the typical satellite peaks of 2p_3/2 and 2p_1/2 spin orbits (denoted as “sat.”), respectively. Interestingly, compared with our previously reported Co 2p for Co LDH, peaks of both Co 2p_3/2 and Co 2p_1/2 for CoFe LDH have a slightly positive shift [29]. The result indicates that the electron cloud arrangement of the Co atom could be regulated by incorporating the Fe atom and the synergistic effect of the two atoms could be further justified. For the Fe 2p spectrum in Figure 4c, two prominent peaks located at 726.0 eV and 712.5 eV are attributed to Fe 2p_1/2 and Fe 2p_3/2, respectively, confirming that Fe²⁺ ions are oxidized to Fe³⁺ ions during the reaction process [10]. In the C 1s spectrum, peaks associated with adventitious carbon (284.8 eV), C-O (286 eV), and C=O (288.9 eV) moieties are observed. As shown in Figure 4e, it can be clearly seen that the high-resolution O 1s XPS spectrum can be divided into four peaks. The peaks at 530.3, 531.5, 532.2, and 533.13eV are typically attributed to metal–oxygen bonds, carbon–oxygen bonds, sulfur–oxygen bonds, and oxygen in hydroxyl groups, respectively [13]. The peaks at 168.7 eV and 169.9 eV in the S 2p spectrum indicate the presence of SO₄²⁻ anions in LDH (Figure 4f) [17]. SO₄²⁻ displays a larger size when compared with other intercalated anions (Cl⁻, CO₃²⁻, and NO₃⁻), which may result in increased spacing between the hydroxide layers, thus increasing the electrochemically active surface area of the catalyst, as well as its catalytic property [6,16]. Moreover, SO₄²⁻ processes the higher negative charge and the higher electron-donating ability, which might also help facilitate the OER activity of the corresponding LDH materials [17].

The OER catalytic performance of ZIF-9(III), CoFe LDH, and commercial RuO₂ catalysts were studied in a standard three-electrode setup using a 1.0 M KOH solution as an electrolyte. As seen in Figure 5a, CoFe LDH nanosheets exhibit a lower overpotential of 218 mV at 10 mA cm⁻² than those of ZIF-9(III) (373 mV) and commercial RuO₂ (285 mV). It is widely accepted that Fe³⁺ ions play an essential role in improving the electrochemical performance of Co LDH OER catalysts. For example, the strong electronic interactions between the Co and Fe elements have been demonstrated from the fact that Fe incorporation correlates with an anodic shift in the Co²⁺/Co³⁺ redox wave [8]. In addition, Fe³⁺ can stabilize the LDH structure due to the similar ionic radii of the Co and Fe ions in the hydroxide layer [40]. Thus, the enhanced catalytic performance can be attributed to the hierarchical structure composed of ultrathin nanosheets and the synergistic effect of Co and Fe [41]. In addition, the low value of the Tafel slope indicates that the rate-determining step is the final part of the multiple electron reactions, which is commonly a sign of great OER activity for catalysts [4]. As can be seen from Figure 5b, the slope of the Tafel plot of the CoFe LDH (59.9 mV dec⁻¹) is lower than ZIF-9 (69.1 mV dec⁻¹), and commercial RuO₂ (85.8 mV dec⁻¹). Correspondingly, the Tafel slope of CoFe LDH suggests that the rate-determining step is the third electron transfer step, a step-generating MOOH intermediate [42]. Significantly, CoFe LDH performs competitively when compared with most of the catalysts reported recently (Figure 5c and Table S1).

To further investigate charge-transport kinetics, the electrochemical impedance spectroscopy (EIS) of electrodes was tested. The Nyquist plots of as-synthetized electrodes and the relevant equivalent circuit model in the form of R_s, (Q₁, R_o) and (Q₂, R_ct) are displayed in Figure 5d. R_s denotes the solution resistance, and R_o denotes the oxide film resistance, which elaborate the electron transport of the electrode material/catalyst interface and the catalyst/electrolyte interface, respectively [43]. The charge-transfer impedance (R_ct) indicates the charge-transfer kinetics of the OER catalytic process and further evaluates the OER activity of the electrocatalyst. The CoFe LDH exhibits a smaller R_ct value (1.3 Ω cm²) than ZIF-9(III) (7.94 Ω cm²), indicating the low mass-transfer resistance and fast charge-transfer rate for the CoFe LDH. The electrochemical active surface areas (ECSA) of the as-prepared samples were evaluated by calculating the double-layer capacitance (C_dl), according to cyclic voltammograms (CV) at different scan rates (Figure S5). As seen from Figure 5e, CoFe LDH has a higher C_dl of 26.3 mF cm⁻², which is superior to ZIF-9(III) (5.6 mF cm⁻²), suggesting that more exposed active sites in the CoFe LDH arise. Indeed, the turnover frequency (TOF) is an important parameter for catalysts. Generally, there are two approaches to determine the TOF, denoted by TOF total metal (TOF_tm) and TOF_redox [44,45]. As displayed in Figure S7, the TOF change trends for CoFe LDH and ZIF-9(III) catalysts are positively correlated with overpotential. Moreover, the TOF value of CoFe LDH is much higher than that of ZIF-9(III) at the same overpotential, confirming its superior intrinsic activity. Specifically, CoFe LDH nanosheets exhibit a higher TOF value, of 0.2716 s^–1 at an overpotential of 300 mV, than that of ZIF-9(III) (0.0009 s^–1). The molar number (n) is calculated based on the inductively coupled plasma-mass spectrometry (ICP-MS) results [13,46]. For CoFe LDH, the content of Co and Fe is 37.23 wt. % and 27.60 wt. %, respectively. For ZIF-9(III), the content of Co and Fe is 39.98 wt. % and 2.05 wt. %, respectively.

The catalytic stability of samples is another crucial parameter for OER. In Figure 5f, the durability of the CoFe LDH electrode is observed using chronopotentiometric measurements at a current density of 10 mA cm⁻² within 55 h. Therefore, the catalytic stability of CoFe LDH is demonstrated in the alkaline electrochemical conditions for the OER. In order to further justify the structural stability of hierarchical CoFe LDH, the XRD pattern and SEM image for the CoFe LDH were provided after the OER tests. As shown in Figure S8a, the structure of catalysts can be ascribed to that of LDH [9]. Meanwhile, the catalyst still maintains hierarchical nanosheets after the OER process (Figure S8b). The results suggest that the hierarchical structure can improve the structural stability of LDH.

The outstanding OER performance of CoFe LDH can be attributed to the following aspects. Firstly, the co-existence of Co and Fe ions in the host layers can provide abundant redox reactions during electrochemical processes. Secondly, the large interlayer distance created using intercalated SO₄²⁻ anions can give the advantage of exposure of more active sites. Thirdly, the hierarchical structure can improve the structural stability of LDH.

3. Experimental Procedure and Methods

3.1. Materials

Benzimidazole (H-PhIM; 98%), cobalt nitrate hexahydrate (Co(NO₃)₂⋅6H₂O; 99%), ferrous sulfate heptahydrate (FeSO₄·7H₂O, 99%), potassium hydroxide (KOH; 90%), and dehydrated ethanol (EtOH; 99.7%) were used from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Nafion solution (5 wt. %) was supplied by DuPont Co., Ltd. (Circleville, OH, USA). The above chemicals were of analytical grade and were used directly without further purification. Ruthenium (IV) oxide (RuO₂) was synthesized via directly calcining Ruthenium(III) chloride anhydrous (RuCl₃, Ru content 45–55%) in the air at 400 °C.

3.2. Synthesis of Compounds

3.2.1. Preparation of 2D ZIF-9(III)

Two-dimensional ZIF-9(III) was prepared using a facile precipitation method with slight modifications, according to our previous report [29]. Firstly, 4 mmol H-PhIM and 2 mmol Co (NO₃)₂⋅6H₂O were both dissolved in 100 mL deionized (DI) water under constant stirring at ambient temperature. Then, 20 mL ammonia water was added into the above solution after 5 min, and the resulting solution was continuously stirred for 45 min. After the solution was left to stand for 6 h, the solid powder was collected using centrifugation, washed with ethanol three times, and finally vacuum dried at 60 °C.

3.2.2. Synthesis of Hierarchical CoFe LDH Nanosheets

Firstly, the obtained 80 mg 2D ZIF-9(III) precursor was dispersed in a mixed solution of ethanol and DI water (the volume ratio of ethanol and water was 9:1). Then, the 50 mg FeSO₄·7H₂O was added into the above solution and stirred for 1 h. Finally, the resulting brown powder was washed three times with ethanol and dried in an oven at 60 °C overnight.

3.3. Physicochemical Characterization

The powder X-ray diffraction (XRD) data of as-prepared samples were collected from a Rigaku D/Max-3c X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a range between 5 and 70° at a scan step of 5°/min. The morphologies and elemental compositions were obtained using field emission scanning electron microscopy (FE-SEM; SU8220) and Transmission Electron Microscopy (TEM; Model JEM-2011, JEOL) equipped with a Rontec energy-dispersive X-ray (EDX) system. The X-ray photoelectron spectrometer (XPS; ESCA-LAB250) with Al Kα radiation was used to characterize the chemical states (elementary composition and the valence state characteristics) of the catalysts, and binding energy corrections on high-resolution scans were calibrated by referencing the C 1s peak (284.8 eV). The functional groups of as-synthesized samples were analyzed at room temperature using Fourier transform infrared spectroscopy (FTIR; PerkinElmer) using KBr as the reference sample. Raman spectra of catalysts were obtained using inVia Quotation Evolution (Renishaw) equipment with laser source of 514 nm. The thickness of the as-fabricated nanosheets was determined using atomic force microscopy (AFM; Bruker). The N₂ adsorption/desorption measurements were performed on a Micromeritics ASAP 2020 analyzer (GA, USA) at the liquid nitrogen temperature (77 K), after degassing the samples for 12 h under vacuum at 120 °C. The specific surface areas and pore size analysis of the samples were calculated from the N₂-sorption isotherms via the Brunauer-Emmett-Teller (BET) and Barrett–Joyner–Halenda (BJH) method, respectively. The Co and Fe contents were tested using inductively coupled plasma-mass spectrometry (ICP-MS).

3.4. Electrochemical Measurements

All electrochemical properties of OER were demonstrated in 1.0 M KOH electrolyte (pH = 13.6) in a conventional three-electrode system at room temperature using a CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). Graphite rod and Ag/AgCl with 1.0 M KOH (0.1989 V vs. RHE) were used as the counter electrode and the reference electrode, respectively. The carbon paper (CP) electrode (surface = 0.5 cm²) modified with catalysts was used as the working electrode. Prior to the test, the CP electrode was cleaned in a solution of diluted hydrochloric acid, followed by rinsing with deionized water. To obtain the uniformly distributed catalyst ink solution, catalyst (2 mg) and Nafion solution (25 μL, 5 wt. %) were dispersed in 250 μL mixture solution of DI water and absolute ethanol (1:1). Homogeneous ink was obtained using 30 min of ultrasonication, dropping on the surface of a carbon paper electrode with a load of 0.3 mg cm⁻², and then drying at room temperature.

The measured potentials were converted according to the equation: E_{vs. RHE} = (E_{vs. Ag/AgCl} + 0.059 × pH + 0.1989) V. The overpotential was calculated as: η = E_RHE − 1.23 V. Tafel slopes were derived from the linear sweep voltammetry (LSV) curves and computed as η = blogj + a, while the η, b, and j represented overpotential, Tafel slope, and current density, respectively. The LSV polarization curves were conducted at a scan rate of 5 mV/s from 0.2 V to 0.8 V (vs. Ag/AgCl) with 95% ohmic potential drop (iR) correction. The OER stability was tested using chronopotentiometry (CP) at a constant current density of 10 mA cm⁻² for 55 h without iR-compensation. Electrochemical impedance spectroscopy (EIS) was carried out over a frequency range of 10⁵–10⁻² Hz at potential 1.52 V (vs. RHE) for CoFe LDH and 1.62 V (vs. RHE) for ZIF-9(III). The electrochemical double-layer capacitance (C_dl) curves of catalysts were measured using cyclic voltammetry in a non-Faradaic region (1.10-1.20 V vs. RHE) at varied scan rates of 10, 20, 30, 40, 50, and 60 mV s⁻¹ based on the following equation: C_dl =(j_a − j_c)/(2 $\times$ v), where j_a, j_c, and v correspond to the current density of anode, cathode, and scan rate, respectively. The TOF was calculated using the following equation: TOF = jA/(4nF), where j (A cm⁻²) is current density, A is surface area (0.5 cm²), F is the Faraday constant of 96,485 C mol⁻¹, and n is the molar number of total metal ions.

4. Conclusions

To summarize, we developed a facile etching–coprecipitation method to drive the conversion of 2D ZIF-9(III) into sulfate-intercalated hierarchical CoFe LDH nanosheets. The CoFe LDH exhibits excellent electrocatalytic performance and a high stability for OER in 1.0 M KOH. Specifically, the CoFe LDH can achieve a current density of 10 mA cm⁻² at a low overpotential of 218 mV with a small Tafel slope of 59.9 mV dec⁻¹. The excellent electrocatalytic performance of CoFe LDH can be mainly attributed to the hierarchical structure, the synergistic effect between Co and Fe ions, and the large interlayer space distance originating from SO₄²⁻ ions. The current work provides a novel method for the rational design of LDH catalysts with desirable nanostructure and improved activity. Moreover, the CoFe LDH catalyst can be incorporated in an industrially relevant scale-type electrolyzer due to the high electrocatalytic activity, stability, and low cost.