Acid-etched defective NiCoFe layered double hydroxides for enhanced oxygen evolution reaction

Abstract

Layered double hydroxides (LDHs) have emerged as promising low-cost electrocatalysts for the oxygen evolution reaction (OER) in recent years. However, their widespread application is limited by the scarcity of intrinsic active sites. Herein, defect-rich NiCoFe–LDHs were synthesized using hydrochloric acid (HCl) etching and nitric acid (HNO₃) etching. The HCl-etched NiCoFe–LDH (HCl–NiCoFe–E2) and HNO₃-etched NiCoFe–LDH (HNO₃–NiCoFe–E2) exhibited OER overpotentials of 291 mV and 312 mV at 50 mA cm⁻², with corresponding Tafel slopes of 42 mV dec⁻¹ and 62 mV dec⁻¹, respectively, outperforming the pristine NiCoFe–LDH and commercial RuO₂ in OER performance. Material characterization showed that the enhanced electrocatalytic activity of acid-treated NiCoFe–LDHs primarily results from their abundant oxygen vacancies, metal vacancies and thinner nanosheets. The OER activity of HCl–NiCoFe–E2 is higher than that of HNO₃–NiCoFe–E2, attributed to the exchange of chloride ions for interlayer nitrate ions in NiCoFe–LDH, thereby facilitating the acid etching reaction. This work presents a facile and cost-effective strategy to optimize the OER performance of LDHs.

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1. Introduction

With the global depletion of non-renewable fossil fuels, energy shortages and environmental pollution have intensified, driving the urgent need for green, efficient and sustainable energy. Hydrogen is an ideal choice for solving the energy and environmental crises due to its high efficiency, high purity, abundant reserves, and environmental friendly nature.^1–3 Electrocatalytic water splitting is considered a convenient and cost-effective technology for hydrogen production.^4–7 The core of this technology lies in two key reactions during water electrolysis: the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. However, the sluggish kinetics of the OER has become a bottleneck in splitting water to produce hydrogen. The conventional OER catalysts are mostly precious metal-based oxides, such as ruthenium oxide (RuO₂) and iridium oxide (IrO₂). Nevertheless, their high cost and low reserves severely impede their large-scale industrial applications.^8,9 Consequently, it is extremely necessary to explore efficient non-noble metal catalysts for water splitting. Ni-, Co-, Fe-, and Mn-based catalysts have shown considerable OER activity.^10–12

LDHs, a class of two-dimensional layered materials, are typically represented by the general formula [M_1−x^IIM_x^III(OH)₂](Aⁿ⁻)_x/n·mH₂O, where M^II and M^III denote divalent and trivalent metallic cations, respectively, and Aⁿ⁻ represents intercalated charge-compensating anions. LDHs have attracted extensive interest as OER electrocatalysts due to their unique electronic structures and tunable chemical composition.^13–15 NiFe–LDH shows the highest activity among the binary LDHs. However, their practical application remains constrained by insufficient intrinsic active sites and poor electrical conductivity. Therefore, various strategies have been adopted to enhance the catalytic performance of LDHs. It has been reported that the Cu₂Se@NiFe–LDHNS (NiFe–LDH nanosheet) electrocatalyst provides more exposed edges and catalytically active sites, thus exhibiting excellent OER performance.¹⁶ B–Pt–NiFe–LDH displays superior OER performance due to active NiFe–LDH sites and the heterogeneous interface between the Pt⁰ species and LDH.¹⁷ Studies have demonstrated that embedding tertiary metal ions into the interlayers of LDHs can not only effectively regulate the microstructure and physicochemical properties of LDHs,^18–20 but also significantly enhance their intrinsic catalytic properties, thereby enabling a more efficient and stable catalyst system for water splitting. Notably, NiCoFe–LDH has emerged as an important catalyst candidate for future clean energy applications, accelerating the industrialization of water splitting technology.^21,22 Yan et al. revealed that NiCoFe–LDH nanosheets prepared via a modified coprecipitation method exhibit an overpotential of 288 mV at 10 mA cm⁻².²³ Ma et al. synthesized a N₂–NiCoFe Prussian blue analogue (PBA) catalyst rich in cyanogen vacancies by etching [Fe(CN)₆]⁴⁻ groups from the NiCoFe–PBA framework using glow-discharge nitrogen plasma. This approach effectively suppressed Fe leaching.²⁴ Sial et al. fabricated nickel cobalt iron (NiCoFe) carbides supported on nickel foam (NF) via direct laser writing. The synthesized NiCoFe carbides exhibit a distorted lattice and disordered atomic arrangement, distinct from conventional structures.²⁵ However, NiCoFe–LDH still fails to satisfy the operational requirements for the OER while maintaining desirable current density at low overpotential.

Defect engineering has been shown to be one of the most effective strategies for structural modulation of materials to enhance their OER activity.^26,27 This approach enables the modulation of electronic configurations, creates additional active sites, enhances electrical conductivity, and consequently boosts intrinsic activity.^28,29 While physical etching methods like argon plasma³⁰ and flame-sculpting³¹ methods can generate cation/anion vacancies, their industrial scalability is hampered by operational complexity and high costs. In contrast, chemical etching demonstrates greater suitability for industrial applications owing to simplified processes and cost-effective scalability.³² Acid etching demonstrates dual functionality in LDH modification: enhancing electrical conductivity, while simultaneously promoting the exposure and density of active centers. The etching process introduces abundant atomic-scale defects and vacancies within the material matrix. These unsaturated coordinated sites often optimize the adsorption energy for reaction intermediates and exhibit excellent catalytic activity.^33,34 Zhou et al. achieved a 46 mV reduction in the OER overpotential of CoFe–LDH (from 346 mV to 300 mV) through nitric acid etching. This treatment induced structural exfoliation and generated multiple atomic defects, including cobalt, iron, and oxygen vacancies.³⁵ Similarly, Peng et al. implemented a comparable chemical etching approach on NiFe LDH, enabling precise engineering of edge iron sites. The modified material demonstrated enhanced OER activity, exhibiting a reduced overpotential from 342 mV to 308 mV.³⁶ A versatile polyoxometallic acid (POM) etching approach has been reported to ingeniously reconstruct NiFe LDH. PMo12 etching can produce appropriate and controllable oxygen and metal vacancies, which subtly modulate local coordination environments and electronic structures of iron and nickel cations.³⁷

However, while most reported studies focus on the effects of acid type and concentration on OER activity during acid etching, only a few studies have investigated the influence of different acid anions at the same acid concentration. In this work, two common inorganic acids, nitric acid (HNO₃) and hydrochloric acid (HCl), were used as etching reagents. The NiCoFe–LDHs were synthesized via a hydrothermal method followed by controlled acid etching treatments. The HCl–NiCoFe–E2 and HNO₃–NiCoFe–E2 exhibit OER overpotentials of 291 mV and 312 mV at 50 mA cm⁻², respectively, outperforming commercial RuO₂ in OER performance. The enhanced electrocatalytic activity primarily results from their abundant oxygen vacancies, metal vacancies, and thinner nanosheets. The exchange of chloride ions for interlayer nitrate ions in NiCoFe–LDH facilitates acid etching, leading to the formation of thinner nanosheets and more metal and oxygen vacancies. Consequently, the OER activity of HCl–NiCoFe–E2 is higher than that of HNO₃–NiCoFe–E2.

2. Experimental

2.1. Reagents and materials

Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), sodium carbonate anhydrous (Na₂CO₃), sodium hydroxide (NaOH), HNO₃ (68%), and HCl (38%) were used. Nafion solution (5%) was purchased from KL Corporation. NF (1.7 mm in thickness) was received from KL Corporation. Deionized (DI) water (18.25 MΩ cm) was obtained from a UPH-II-10T water purification system.

2.2. Synthesis of NiCoFe–LDH

24 mmol of Ni(NO₃)₂·6H₂O, 8 mmol of Co(NO₃)₂·6H₂O and 8 mmol of Fe(NO₃)₃·9H₂O were dissolved in 20 mL of DI water and labeled as solution A. 38 mmol of NaOH and 16 mmol of Na₂CO₃ were dissolved in another 20 mL of DI water and labeled as solution B. The A and B solutions were simultaneously added to a beaker under vigorous stirring. The resulting slurry was then transferred into a Teflon-lined stainless-steel autoclave and heated at 80 °C for 24 h. After cooling down to room temperature, the precipitates were centrifuged, washed thoroughly with DI water and ethanol, and then dried at 60 °C. The NiCoFe–LDH sample was obtained.

2.3. Synthesis of acid-etched NiCoFe–LDH

0.2 g of NiCoFe–LDH was dispersed in water to obtain a uniformly dispersed suspension. Next, 1.0, 2.0, and 3.0 mL of HCl (1.0 M) were added to the NiCoFe suspension, respectively; all other conditions remained unchanged except for the acid volume, and then the mixture was stirred for 30 min at room temperature. After centrifugation and washing, the resulting products were dried at 60 °C overnight. The samples are designated as HCl–NiCoFe–Ex (x = 1.0, 2.0 and 3.0 mL). Samples etched with HNO₃ (1.0 M) were prepared following the same procedure. The samples are denoted as HNO₃–NiCoFe–Ex (x = 1.0, 2.0 and 3.0 mL).

2.4. Electrode preparation

Several pieces of NF, cut into 1 cm × 1 cm in size, were sonicated in 3.0 M HCl and in acetone for 15 minutes each, followed by ultrasonic treatment in ethanol and DI water for 10 minutes, respectively. The aim is to remove surface oxides and organic contaminants. Finally, NF was dried at 60 °C overnight. The electrode ink was prepared by mixing 20 mg of catalyst powder with 500 µL of ethanol, 450 µL of water and 50 µL of Nafion solution. After 1 h of sonication, 200 µL of the catalyst ink (containing 4 mg of catalyst) was dripped on a piece of NF, which was dried at room temperature. The RuO₂ electrode was prepared using a similar method, with RuO₂ powder replacing the LDH powder.

2.5. Characterization

The morphology of all electrocatalysts was investigated by scanning electron microscopy (SEM, Quanta 400 FEG, US FEI). Transmission electron microscopy (TEM) was performed using a Tecnai G2 F20 (FEI, USA). X-ray diffraction (XRD) patterns were recorded using an X-ray powder diffractometer (DX-2700BH). Raman measurements were performed on a Renishaw Raman spectrometer (RW2000 Renishaw, UK) with a laser excitation wavelength of 532 nm. Nitrogen adsorption isotherms were measured at 573 K using Micromeritics TriStar II 3020/3-Flex instrument, and the specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The chemical components were characterized using an X-ray photoelectron spectroscopy system (XPS, ESCALAB 250XI Thermo, US) as a monochromatic X-ray source. The oxygen vacancy signals were recorded by electron paramagnetic resonance (EPR, A300-10/12, Bruker, Germany). Before the analysis, the LDH samples were degassed at 130 °C overnight. The elements nickel, cobalt and iron were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 720, Agilent Technologies, USA).

2.6. Electrochemical measurements

All of the electrochemical measurements were performed with a CHI 760E electrochemical workstation. The test of the OER activity was performed in a three-electrode system comprising the fabricated working electrode, Hg/HgO as the reference electrode and a Pt sheet as the counter electrode in 1 M potassium hydroxide (KOH) solution. All potentials obtained were converted to the reversible hydrogen electrode (RHE) according to E_RHE = E_Hg/HgO + 0.059 pH + 0.098 = η + 1.23. Before all the tests, the catalysts were activated by cyclic voltammetry (CV), which took at least 20 cycles until a stable current density was reached. Linear sweep voltammetry (LSV) scanning at a scanning speed of 5 mV s⁻¹ was performed to obtain polarization curves. All the polarization curves were corrected by eliminating the iR of the ohmic resistance of the solution. The Tafel slopes were calculated by fitting the linear regions of Tafel plots according to the Tafel equation (η = a + b log j). Electrochemical impedance spectroscopy (EIS) was performed at a constant potential in the frequency range of 0.1–10⁵ Hz. The electrochemically active surface areas (ECSAs) of catalysts were calculated based on their electrical double-layer capacitance (C_dl) using the equation ECSA = C_dl/C_S, where C_dl was obtained from CV plots recorded in a narrow non-faradaic potential window from 0.43 to 0.53 V (vs. Hg/HgO). The measured capacitive current densities at the average potential within this potential window were plotted as a function of scan rate, and the slope of the linear fit was taken as C_dl. CV measurements were performed on working electrodes for five cycles at scanning rates of 5, 10, 20, 30, and 40 mV s⁻¹. The intrinsic activity was revealed by normalizing the current to the ECSAs to exclude the effect of surface area on catalytic performance. The OER stability test was conducted using the constant pressure method.

3. Results and discussion

3.1 Characterization results

The XRD patterns of NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2 are shown in Fig. 1. The diffraction peaks of all samples match the characteristic peaks of the hydrotalcite structure (JCPDS 40-0215), which can be indexed as (003), (006), (012), (015), (110), and (113), indicating that the backbones of NiCoFe–LDHs remain intact and retain their lamellar structure after a 2 mL acid-etching treatment. Moreover, the absence of obvious impurity peaks in the XRD patterns indicates high sample purity, which is crucial for studying catalytic performance.

Fig. 1 XRD patterns of NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2.

In comparison with the NiCoFe–LDH sample, the (003) diffraction peaks of the HCl–NiCoFe–E2 and HNO₃–NiCoFe–E2 samples become broader. Interlayer spacings are in descending order of HCl–NiCoFe–E2 > HNO₃–NiCoFe–E2 > NiCoFe–LDH (Table 1), indicating that the basal spacing of NiCoFe–LDH was enlarged after nitric acid or hydrochloric acid etching, which might benefit the OER performance of the LDHs. It has been reported that the enlarged basal spacing can provide more space for mass diffusion and exposing more inner active sites for oxygen evolution.³⁸ Meanwhile, increasing the acid amount leads to a decrease in the (003) diffraction peak intensity of the samples (Fig. S1 and S2), indicating a decline in crystallinity. This may be attributed to the partial etching effect of nitric acid or hydrochloric acid on hydroxyl groups, which results in the separation between the host layers and a reduction in the structural order of the samples. Furthermore, the analysis of the crystal domain length (CDL) using the Scherrer equation shows that the CDL both in the ab-plane and along the stacking axis (c-axis) decreases with the addition of acid (Table 1), suggesting that adding nitric acid or hydrochloric acid decreases the CDL. These results are consistent with the morphological structures observed in Fig. 2.

Table 1 Interlayer spacing, lattice parameter c, CDL and Ni/(Fe + Co) molar ratios of NiCoFe–LDH, HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2

Sample	Interlayer spacing (d003, nm)	Lattice parameter c^a (nm)	CDL^b (nm)		Ni/(Fe + Co) molar ratio^c
			ab-plane	c-axis
a Lattice parameter c is calculated as c = 3d003. b CDL is determined using the Scherrer equation, Dhkl = Rλ/β cos θ, where Dhkl is the length in a specific crystallographic direction, R is the Scherrer constant (0.89), λ is the wavelength (0.1542 nm), β is the peak width at half-maximum (rad) and θ is the Bragg angle (rad). The ab-plane is the (110) plane, and the c-axis is the (003) plane. c Ni/(Fe + Co) molar ratio is determined by ICP–OES.
NiCoFe–LDH	0.823	2.469	10.14	9.99	1.81
HNO3–NiCoFe–E2	0.856	2.568	8.86	9.04	1.34
HCl–NiCoFe–E2	0.860	2.580	8.49	8.79	1.32

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Fig. 2 SEM images of NiCoFe–LDH (a) and (d), HNO₃–NiCoFe–E2 (b) and (e), and HCl–NiCoFe–E2 (c) and (f). TEM images of NiCoFe–LDH (g), HNO₃–NiCoFe–E2 (h), and HCl–NiCoFe–E2 (i). Insets show the images of the samples dispersed in water (a)–(c). HRTEM images of HNO₃–NiCoFe–E2 (j) and HCl–NiCoFe–E2 (k).

As shown in Fig. 2a–i, SEM and TEM images were used to characterize the morphological structures of NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2. The SEM images reveal the uniform nanosheets of NiCoFe-LDH with a uniform distribution. Voids or channels between the nanosheets are observed, consistent with a typical lamellar structure. Furthermore, the TEM images (Fig. 2g–i) of the HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 materials show a nanosheet structure with a slightly diminished average thickness in comparison with the NiCoFe–LDH. The NiCoFe–LDH exhibits nanosheets with a transverse size of 9–11 nm (Fig. 2g), which are formed through heterogeneous nucleation of the nanosheets at an early stage. Partial dissolution of surface and edge regions during nitric acid etching leads to the formation of thinner nanosheets, with the lateral sizes narrowing to 5–7 nm (Fig. 2h). After hydrochloric acid etching, the surface becomes rougher (Fig. 2c and f), and a smaller thickness of 4–5 nm is obtained (Fig. 2i). Furthermore, atomic vacancies, marked by red circles in Fig. 2j and k, are clearly visible in the high-resolution transmission electron microscopy (HRTEM) images, demonstrating the successful creation of atomic vacancies via acid etching without collapsing the LDH framework. During acid etching, the ionic and hydrogen bonds in the intercalation are broken, disturbing the normal charge balance and separating the positively charged host layers. This may lead to effective stripping, which favors ion diffusion and electron conduction in the electrocatalytic reaction. More significant etching of the nanosheets is observed with increasing acid volume (Fig. S3 and S4), accompanied by a color change from brown to green. Eventually, the NiCoFe–LDHs are completely dissolved in 4.0 mL of 1.0 M nitric acid or hydrochloric acid.

The defect structure of these LDHs can also be observed using Raman spectroscopy (Fig. 3). The NiCoFe–LDH exhibits characteristic resonances at approximately 289, 455, and 524 cm⁻¹, respectively, which are attributed to the E-type, M–O (H), and M–O vibrations in the hydrotalcite structure.¹⁵ Compared to the NiCoFe–LDH sample, the resonances for HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 are broadened and less intense, suggesting a more disordered local environment for M–O(H) and M–O. An additional resonance at around 695 cm⁻¹ for the HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 samples is observed, which is attributed to the metal–oxygen (M(III)–O) vibration.^39,40 Additionally, the resonances at 1039 and 1051 cm⁻¹ are assigned to the characteristic vibrations of nitrate and carbonate, respectively. Both the nitrate and carbonate vibrations are weakened in the HCl–NiCoFe–E2 sample, probably due to the anion exchange between chloride ions and interlayer anions of NiCoFe–LDH during etching.

Fig. 3 Raman spectra of NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2.

The Ni/(Fe + Co) molar ratios in HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 are ca. 1.34 and 1.32, respectively, as determined by ICP–OES; the Ni/(Fe + Co) ratio in the NiCoFe–LDH was found to be 1.81 (Table 1). The analytical data indicate that the Ni cations are preferentially lost in the acid etching process compared to the Fe and Co cations. The specific surface areas of NiCoFe–LDH, HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 were investigated using nitrogen adsorption/desorption isotherms (Fig. 4a and b). The NiCoFe–LDH sample exhibits the highest BET-derived specific surface area (102.7 m² g⁻¹). In contrast, the surface areas of HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 were 68.2 and 43.1 m² g⁻¹ (Fig. 4a). According to IUPAC classification, all NiCoFe–LDH samples displayed Type IV isotherms with an H2-type hysteresis loop (Fig. 4b).⁴¹ The hysteresis observed at relative pressures (P/P₀) above 0.4 is attributed to capillary condensation within mesopores. The order of hysteresis loop magnitude was NiCoFe–LDH > HNO₃–NiCoFe–E2 > HCl–NiCoFe–E2, consistent with the trend observed in their pore volume. This phenomenon results from the structural collapse of certain pores during acid etching.

Fig. 4 (a) The N₂ surface area using the BET model and cumulative pore volume for NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2. (b) N₂ adsorption/desorption isotherms.

XPS experiments were performed to further understand the surface defects in the samples. The Ni 2p, Co 2p, Fe 2p and O 2p spectra of LDH samples are shown in Fig. 5a–e. Notably, the XPS spectrum of HCl–NiCoFe–E2 (Fig. 5a) exhibits two typical peaks at binding energies of 268.6 and 199.8 eV, assigned to Cl 2s and Cl 2p, respectively. Combined with Raman results (Fig. 3), this confirms an exchange of chloride ions for interlayer nitrate ions in NiCoFe–LDH after HCl etching. In comparison with NiCoFe–LDH, the Ni 2p and Co 2p photoemission peaks of both HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 shift to a higher energy direction, suggesting that Ni and Co metal cations have a higher oxidation state. Further peak-splitting analysis is obtained by deconvolution technology, the Ni 2p_3/2 main peak at 855.4–855.6 eV is accompanied by a satellite peak at 861.1–861.5 eV, and the peak at 856.0–856.4 eV along with a satellite peak at 862.7–863.0 eV, corresponding to Ni²⁺ and Ni³⁺ (Fig. S5), respectively. The Co 2p peaks at 780.8–781.1 eV and 774.8–775.0 eV associate with Co²⁺ and Co³⁺ (Fig. S6) respectively.⁴² Additionally, the Fe 2p peaks at 712.5–712.7 eV and 725.1–726.2 eV associate with Fe³⁺ 2p_3/2 and Fe³⁺ 2p_1/2 (Fig. S7), respectively. Both HNO₃–NiCoFe–E₂ and HCl–NiCoFe–E₂ exhibit a higher proportion of Ni³⁺ and Co³⁺ compared to NiCoFe–LDH (Fig. S5 and S6). Combined with the HRTEM results (Fig. 2), these findings further confirm the increased oxidation state of Ni and Co ions due to the introduction of metal vacancies. This phenomenon can be explained by the fact that metal vacancies allow the remaining metal cations to share more oxygen ions, thereby promoting the formation of higher-valence species (Ni³⁺ and Co³⁺) within the LDH framework as a result of charge compensation. These results indicate that nitric acid or hydrochloric acid treatments generate more metal vacancies.

Fig. 5 XPS spectra of NiCoFe–LDH, HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2: (a) the survey scan spectrum, (b) Ni 2p, (c) Co 2p, (d) Fe 2p, and (e) O 1s.

The O 1s photoelectron peaks of HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 are shifted toward higher binding (Fig. S8). The O 1s photoelectron spectral peaks can be deconvoluted into several components, including nitrate at 532.2 eV, carbonate at 530.8 eV, metal hydroxyl (M–O–H) at 531.3 eV, water (H–O–H) at 531.5 eV, metal–oxygen (M–O–M) at 530.7 eV, and lattice oxygen vacancies at 531.9 eV. The proportions of lattice oxygen in the HCl–NiCoFe–E2 and HNO₃–NiCoFe–E2 samples are 38.92% and 38.20%, respectively, significantly higher than that in NiCoFe–LDH (8.52%), indicating a substantial increase in oxygen vacancies after acid etching. The incorporation of metal vacancies and oxygen vacancies not only introduces additional active centers into the catalyst system, but also modulates the electronic structure of the LDH. This modification may significantly reduce the activation energy of the reaction and improve the catalytic activity.⁴²

EPR spectroscopy was employed to verify the existence of oxygen vacancy levels (Fig. 6). All catalysts exhibited a characteristic signal at g = 2.003, corresponding to structural defects induced by oxygen vacancies. The EPR signal intensities of three samples decrease in the following order: HCl–NiCoFe–E2 > HNO₃–NiCoFe–E2 > NiCoFe–LDH, indicating that the acid-etched NiCoFe–LDH samples produce more oxygen vacancies compared with NiCoFe–LDH. This is consistent with the XPS results. Specifically, the intensity of oxygen vacancies in HCl–NiCoFe–E2 is higher than that in HNO₃–NiCoFe–E2. The anion exchange order of Cl⁻ > NO₃⁻ in LDH has been confirmed, indicating that chloride ions possess a stronger anion intercalation exchange capacity.⁴³

Fig. 6 EPR spectra of NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2.

This leads to the exchange of chloride ions for interlayer nitrate ions in NiCoFe–LDH. Moreover, the radii of chloride ions and nitrate ions are 0.181 and 0.179 nm,⁴⁴ respectively, and the enlarged interlayer spacing might allow faster diffusion of H⁺ to react with host layers, which may facilitate the acid etching reaction.

3.2 Electrocatalytic performance

The electrocatalytic properties of these materials were evaluated by loading the samples onto NF. The OER performance of the samples in 1 M KOH electrolyte was investigated using a standard three-electrode system. As shown in Fig. 7a, three catalysts show a peak in the potential range of 1.35–1.45 V, corresponding to Ni²⁺/Ni³⁺ redox transition, which indicates the valence state changes of nickel ions during the electrochemical process. The OER performance of the samples was quantified by measuring the overpotentials required to achieve a current density of 50 mA cm⁻². As demonstrated in Fig. 7a–c, the bare NF electrode shows an overpotential of 525 mV with a Tafel slope of 108 mV dec⁻¹. When coated with NiCoFe–LDH, these values are greatly significantly reduced to 367 mV and 74 mV dec⁻¹, respectively. Remarkably, after acid etching, the HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 catalysts exhibit overpotentials of 312 mV and 291 mV, accompanied by Tafel slopes of 62 and 42 mV dec⁻¹, respectively. The significantly reduced overpotentials and Tafel slopes strongly demonstrate that both HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 exhibit superior catalytic activity for the OER, even surpassing the performance of commercial RuO₂ (339 mV at 50 mA cm⁻²) benchmarks. The lower Tafel slopes observed for the acid-etched samples suggest faster electrocatalytic kinetics, particularly in the rate-determining step of the OER process, implying that acid etching promotes more efficient oxygen evolution kinetics in NiCoFe–LDH. According to the SEM, TEM, XPS, and EPR results, the NiCoFe–LDH sample undergoes an acid–base reaction during acid etching, leading to exfoliation and the generation of more metal and oxygen vacancies. The improved OER performance of HNO₃–NiCoFe–E₂ and HCl–NiCoFe–E₂ can be primarily attributed to these increased vacancies and thinner LDH nanosheets. Thinner nanosheets and larger interlayer spacing benefit mass diffusion and expose more inner active sites, thereby boosting the OER performance.^18,38,42 Metal vacancies can reduce the adsorption energy of H₂O molecules, while oxygen vacancies enhance electronic conductivity and optimize the adsorption energy of OER intermediates.^45–47 Additionally, studies have reported that metal vacancies can strengthen the ability of LDH to bond with OH⁻.⁴⁸ This facilitates the formation of OH* intermediates (* denotes the active site or adsorption site on the catalyst surface). Consequently, the OER performance of acid-etched NiCoFe–LDH is further improved.

Fig. 7 (a) iR-corrected polarization curve graph in 1 M KOH at a scanning rate of 5 mV s⁻¹. (b) Overpotentials at a current density of 10 mA cm⁻² and 50 mA cm⁻². (c) Corresponding Tafel plots. (d) The polarization curves of HNO₃–NiCoFe–Ex. (e) The polarization curves of HCl–NiCoFe–Ex. (f) Nyquist plots of NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2.

In addition, we treated NiCoFe–LDHs using different volumes of HNO₃ and HCl. It can be confirmed that the OER performance improves with the increase of HNO₃ or HCl addition up to 2.0 mL. However, the OER performance declines with a further increase in acid volume. Therefore, the optimum volume of acid is 2.0 mL (Fig. 7d and e). This is because too little acid leads to fewer defects, whereas excessive acid destroys the structure of the catalysts. Both factors negatively affect OER performance.^15,35 Based on XRD and SEM results (Fig. S1–S4 and Table S1), more severe etching of the nanosheets is observed with increasing acid volume. Eventually, the NiCoFe–LDHs are completely dissolved in 4.0 mL of 1.0 M nitric acid or hydrochloric acid, and the structures of NiCoFe–LDHs were thoroughly destroyed.

EIS was used to evaluate the conductivity of the electrocatalysts. The Nyquist curves for different electrocatalysts are shown in Fig. 7f. The EIS curves were fitted with an equivalent circuit consisting of R1 (attributed to the electrolyte resistance), R2 (attributed to the charge-transfer resistance), and CPE1 (constant phase element). According to the simulation results, HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 exhibit lower charge transfer impedance than NiCoFe–LDH, indicating that these acid-treated samples possess enhanced charge transfer ability and higher electrode conductivity, which might be attributed to the formation of more oxygen vacancies (Fig. 6). It has been reported that oxygen vacancies can lower the charge transport resistances for favorable accessibility of catalytically active sites.⁴⁹ The electrochemical double layer capacitance was used to evaluate the effect of ECSA, as the capacitance is expected to be linearly proportional to the ECSA. As shown in Fig. 8a–f, the C_dl values of 34.76 mF cm⁻² for HCl–NiCoFe–E2 and 32.32 mF cm⁻² for HNO₃–NiCoFe–E2 are both higher than that of the NiCoFe–LDH (27.74 mF cm⁻²). The ECSA of each electrocatalyst was estimated from the double layer capacitance (C_dl) using the equation ECSA = C_dl/C_S. Here, a C_S value of 0.040 mF cm⁻² was applied to evaluate the ECSA in 1 M KOH.⁵⁰ ECSA increases with the C_dl value, and the relatively high specific surface area facilitates the exposure of more active centers. Compared with NiCoFe–LDH (693 cm²) and HNO₃–NiCoFe–E2 (808 cm²), HCl–NiCoFe–E2 (867 cm²) has a larger electrochemically active surface area and exhibits better catalytic performance. To further assess intrinsic activities, the area activities of catalysts were calculated through normalization by ECSA (Fig. 8e and f). HCl–NiCoFe–E2 still exhibits the best area activity among these catalysts at higher potentials. The HCl–NiCoFe–E2 and the HNO₃–NiCoFe–E2 catalysts exhibit specific activities (ECSA-normalized current density) that are 1.8-fold and 1.6-fold higher than that of NiCoFe–LDH at 1.5 V, respectively, indicating their superior intrinsic activities compared to NiCoFe–LDH.

Fig. 8 CVs of NiCoFe–LDH (a), HNO₃–NiCoFe–E2 (b), and HCl–NiCoFe–E2 (c) with different rates from 5 to 40 mV s⁻¹. Capacitive current at 1.23 V as a function of scan rate for NiCoFe–LDH, HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 (d). Corresponding ECSA-normalized LSV curves (e). Comparison of the current density normalized by ECSA at 1.5 V (vs. RHE) (f).

In addition, the long-term stability of the catalysts is a critical criterion for practical applications of electrocatalysts. To assess this property, chronopotentiometry measurements were conducted at a constant current density of 50 mA cm⁻² (Fig. 9a–c). The results indicate that the potential decay of the catalysts over 20 h is negligible, while the NiCoFe–LDH catalyst exhibits a decrease in current density of less than 2%, demonstrating that acid-treated catalysts (HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2) and the pristine NiCoFe–LDH possess exceptional operational durability under OER conditions. Fig. 9d–f shows a comparison of LSV curves before and after cycling, revealing no significant differences between the curves. Furthermore, microstructure characterization of the NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2 samples after undergoing 20 h of OER catalysis at a constant current density of 50 mA cm⁻² confirmed that the layered double hydroxide (LDH) structure was well preserved (Fig. 10a–i).

Fig. 9 Time dependence of the current density under a constant overpotential of NiCoFe–LDH (a), HNO₃–NiCoFe–E2 (b), and HCl–NiCoFe–E2 (c). LSV curves of NiCoFe–LDH (d), HNO₃–NiCoFe–E2 (e), and HCl–NiCoFe–E2 (f) before and after 20 h of reaction.

Fig. 10 SEM images of the catalysts. NiCoFe–LDH after 20 h of OER (a) and (b), HNO₃–NiCoFe–E2 after 20 h of OER (d) and (e), and HCl–NiCoFe–E2 after 20 h of OER (g) and (h); NiCoFe–LDH before the OER (c), HNO₃–E2 before the OER (f), and HCl–NiCoFe–E2 before the OER (i).

All these results indicate that the defective NiCoFe–LDHs obtained through acid-etching treatment exhibit significantly enhanced OER performance, highlighting the crucial role of acid-etching treatment in catalyst defect engineering.

4. Conclusions

Defective NiCoFe LDH electrocatalysts were synthesized via a simple hydrothermal method followed by acid-etching treatment. The OER performance of NiCoFe LDHs was further enhanced through nitric acid or hydrochloric acid etching. The resulting thinner LDH nanosheets promoted ion diffusion and electron conduction during the electrocatalytic OER process. Additionally, the etching process introduced more metal and oxygen vacancies, leading to enhanced OER catalytic activity. Electrochemical measurements revealed that HNO₃–NiCoFe–E2 and HCl–NiCoFe–E2 exhibit overpotentials of 312 mV and 291 mV at a current density of 50 mA cm⁻², with corresponding Tafel slopes of 62 mV dec⁻¹ and 42 mV dec⁻¹, respectively, surpassing both the pristine NiCoFe–LDH and commercial RuO₂. The OER activity of HCl–NiCoFe–E2 is higher than that of HNO₃–NiCoFe–E2. This might be due to the exchange of chloride ions for interlayer nitrate ions in NiCoFe–LDH, which facilitates acid etching and leads to the formation of thinner nanosheets and more metal and oxygen vacancies. Moreover, NiCoFe–LDH, HNO₃–NiCoFe–E2, and HCl–NiCoFe–E2 demonstrated excellent long-term OER stability over 20 h of continuous operation in alkaline media. This work provides an effective strategy for designing highly efficient and cost-effective OER electrocatalysts.

Author contributions

Y. X.: conceptualization, data curation, validation, methodology, formal analysis, investigation, writing – original draft, writing – review & editing, and visualization; X. P. Y.: conceptualization, methodology, formal analysis, writing – review & editing, supervision, project administration, and funding acquisition.; S. X. H.: supervision; and Y. L.: supervision and funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

All data are available in the main text or the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03431k.

Acknowledgements

This work was supported by the Scientific Research and Innovation Team Program of Sichuan University of Science and Engineering (SUSE652B008).

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