Vertically Ti₃CN@NiFe LDH Nanoflakes as Self-Standing Catalysts for Enhanced Oxygen Evolution Reaction

Abstract

Hydrogen production from water electrolysis is gaining interest as a source of renewable energy storage due to its high efficiency and low environmental impact. However, the slow kinetics of the oxygen evolution reaction (OER) limits the overall efficiency of electrolyzer systems. This study presents the synthesis and characterization of a novel electrocatalyst with a vertical structure, composed of Ti₃CN MXene-modified NiFe-layered double hydroxides (LDHs) supported on nickel foam (NF) for efficient OER applications. The 1.0-LDH/3MXNF catalyst exhibits excellent electrocatalytic activity, achieving a low overpotential of 247 mV at a current density of 100 mA cm⁻² and a favorable Tafel slope of 67.7 mV/dec. This can be attributed to the transfer of excess electrons from Ti₃CN MXene to NiFe-LDH, which reduces the oxidation states of Ni and Fe, resulting in a strong interfacial coupling between Ti₃CN MXene and NiFe-LDHs. Additionally, the electrode exhibited exceptional stability, maintaining constant performance with minimal potential degradation over prolonged operation. These findings underscore the potential of hybrid LDH-MXene systems as advanced electrocatalysts for renewable energy applications, paving the way for further innovations in energy conversion technologies.

1. Introduction

With escalating global concern over environmental pollution and energy consumption, there is an urgent need to develop clean and environmentally friendly energy storage and conversion technologies [1,2,3,4]. Electrochemical water decomposition (EWD) technology encompasses two half-reactions: a hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which are critical for grid-scale energy storage [5]. In alkaline media, the OER involves a complex and slow four-electron transfer process that significantly impacts the efficiency of the overall water electrolysis unit [6]. However, many materials used for this purpose consist of precious metals, rendering them unsuitable for large-scale applications due to their high cost and poor durability [7,8,9].

In recent years, substantial advancements have been made in the development of non-precious metal-based OER catalysts [10,11,12,13,14,15]. Among these, layered bimetallic hydroxides (LDHs), such as NiFe-LDHs [16], have demonstrated excellent OER performance in alkaline electrolytes. The coexistence of Ni²⁺ and Fe³⁺ in NiFe-LDHs, along with their synergistic effects, marks them as highly promising materials for energy conversion and storage. Moreover, the redistribution of electron density through partial charge transfer can facilitate the oxidation of Ni²⁺, thereby enhancing OER catalytic activity [17]. However, the OER activity of LDHs is limited by its inherent low conductivity and small surface area, which result from prone agglomeration. Nanostructures can be stabilized by constructing heterostructures with conductive substrates and LDHs [18]. Nevertheless, achieving strong interfacial synergy typically necessitates surface treatments to introduce functional groups, which can compromise the structural integrity and conductivity of the substrate material.

Two-dimensional layered materials, specifically MXene, have received considerable attention in the field of energy storage and conversion [19,20,21,22,23,24,25,26]. The general formula of MXene is M_n+1X_nT_x, where M represents a pre-transition metal element; A corresponds to the main group elements, primarily from group IIIA to VIA; X is the carbon and/or nitrogen elements; T_x denotes surface termination groups such as -OH, -O, -F, -Cl, etc.; and n can range from 1 to 4 [27,28,29,30]. To further improve the electrochemical performance of MXene, simple heteroatom doping (e.g., nitrogen) is an effective strategy [31]. Theoretically, nitrogen has fewer vacant electron orbitals than carbon and exhibits better catalytic activity [32]. For instance, Shi and Liu [33] successfully employed nitrogen to replace carbon in the lattice structure of Ti₃C₂T_x, resulting in increased layer spacing. Moreover, nitrogen doping modifies the electron density of Ti³⁺ by filling incomplete orbitals, which lower the energy of the valence electron system. Among various MXene materials, Ti₃CNT_x, where nitrogen replaces half of the carbon, stands out. Due to its higher electronegativity compared to carbon, the Ti-N bond is stronger than the Ti-C bond, enabling the nitrogen side to adsorb metal cations more effectively [34,35]. This characteristic facilitates strong interfacial coupling between LDH and MXene.

In this study, we employed a one-step hydrothermal method to construct a self-standing electrocatalytic electrode with a multilayer structure, leveraging the properties of Ti₃CN. A three-dimensional framework was built using Ti₃CN-modified porous nickel foam (NF), which was further interfacially coupled with NiFe-LDH nanosheets, resulting in an electrode designated as LDH/MXNF. The findings reveal a strong electrostatic interaction between the interfaces of Ti₃CN and NiFe-LDHs, enhancing the charge transfer between LDHs and MXNF. Consequently, the as-prepared LDH/MXNF self-standing electrode exhibits excellent OER performance, rapid catalytic reaction kinetics, and good stability.

2. Results and Discussion

2.1. Catalyst Synthesis and Structure Characterization

The LDH/MXNF catalyst was synthesized following the route illustrated in Figure 1. Step I: Initially, m-Ti₃CN was produced by etching off the Al layer from Ti₃AlCN using HF. This was followed by intercalation in the TMAOH solution to yield f-Ti₃CN, which was obtained through ultrasonication in deaerated water. The etching of the bulk MAX material effectively produced m-Ti₃CN (Figure S1), which showed no MAX characteristic peak at 39° (Figure S2). This result confirms the successful removal of Al layers. After exfoliation, the (002) peak of f-Ti₃CN shifted to a smaller angle (2θ = 6.0°) compared to m-Ti₃CN, indicating the successful formation of few-layered MXenes. Due to electrostatic interactions, the negatively charged f-Ti₃CN MXene coated the acid-treated porous NF surface, resulting in a uniform MXene layer wrapping around the NF surface (Figure S4). However, the MXene layer of 5MXNF exhibited cracking, likely due to its thicker structure compared to 3MXNF. Step II: In the next stage, NiFe-LDH nanosheets were grown in situ on the 3D MXNF framework by the straightforward hydrothermal synthesis method. In general, urea is highly soluble in water and its prolonged hydrolysis produces CO₃²⁻ and OH⁻ [36].

$$\begin{array}{c}\mathrm{C}\mathrm{O}{\left({\mathrm{N}\mathrm{H}}_2\right)}_2+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{N}\mathrm{H}}_3+{\mathrm{C}\mathrm{O}}_2\\ {\mathrm{N}\mathrm{H}}_3\cdot {\mathrm{H}}_2\mathrm{O}\to {\mathrm{N}\mathrm{H}}_4^{+}+{\mathrm{O}\mathrm{H}}^{-}\\ {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_3^{2-}+2{\mathrm{H}}^{+}\end{array}$$

The MXene surface, obtained through wet chemical etching and intercalation with organic bases, is characterized by negatively charged oxygen-containing functional groups. These groups can electrostatically adsorb metal cations, facilitating the precipitation and aggregation of Ni²⁺ and Fe³⁺ on the surface of the MXene-modified nickel foam. As the hydrothermal reaction proceeded, the hydrolysis of urea intensified, leading to an increased concentration of OH⁻ in the solution. This increase facilitated the crystallization of NiFe ions, gradually forming NiFe-LDH nanosheets.

The XRD patterns shown in Figure 2a and Figure S3a indicate that the NiFe-LDH/NF, NiFe-LDH/MXNF, and NiFe-LDH/MCNF samples exhibit prominent diffraction peaks corresponding to Ni monomers at 44.7°, 52.1°, and 76.6°. The MXene signal is absent, likely due to the overwhelming intensity of NF and NiFe-LDH peaks, which obscures the MXene diffraction patterns. For the 1.0-LDH/3MXNF sample, the observed diffraction peaks at 11.7°, 23.2°, 34.6°, 39.0°, 46.1°, 59.9°, and 61.2° correspond to the (003), (006), (012), (015), (018), (110), and (113) lattices planes of NiFe-LDH, respectively [37]. In contrast, the intensities and crystallinities of the 0.5-LDH/3MXNF and 0.5-LDH/5MXNF samples are weaker due to the influence of NF and a lower concentration of metal cations used in their preparation. The Fourier transform infrared spectroscopy with attenuated total reflection spectra (FTIR-ATR) in Figure 2b and Figure S3b highlight absorption peaks around 3400 cm⁻¹ and 1636 cm⁻¹, which are attributed to the stretching vibration of hydrogen-bonded hydroxyl groups and the bending vibration of water molecules, respectively [38]. In addition, the absorption peak at 1350 cm⁻¹ represents the C-O stretching vibration of the interlayer CO₃²⁻ [39]. The absorption peak at 2179 cm⁻¹ corresponds to C-O stretching [40], consistent with the layer spacing of 7.5 Å represented by the (003) peak at 11.7° in XRD results. This suggests that the CO₃²⁻ ions generated hydrothermally from urea during synthesis act as intercalation anions, balancing the positive charge carried by the layered hydroxides. An additional absorption peak at 2185 cm⁻¹ further supports the presence of CO₃²⁻ ions. Moreover, the spectral band at 640 cm⁻¹ indicates the successful preparation of NiFe-LDH, attributed to the M-O vibrations in the hydroxide layers [38].

The EDS mapping of the 1.0-LDH/3MXNF sample in Figure S5 demonstrates a uniform distribution of Fe, Ti, N, and Ni across the nickel foam substrate. This uniformity suggests the successful integration of the LDH material onto the MXNF framework. The morphology and structure of NiFe-LDH nanosheets on MXNF can be effectively tuned by varying the concentrations of Ni²⁺ and Fe³⁺ ions, as well as the amount of MXene modification (Figure 2c–e). Notably, for the LDH/3MXNF self-standing electrode, the average thicknesses of the NiFe-LDH nanosheets were measured to be 16.42 nm, 27.51 nm, and 47.97 nm for metal cation additions of 0.5 mmol, 1.0 mmol, and 2.0 mmol, respectively (Figure 2f–h). The nanosheets formed on the MXNF surface exhibited a staggered and disordered arrangement, a result of the varying crystallization directions of nuclei originating from the initial aggregation of Ni²⁺ and Fe³⁺. This unique morphology enhances the availability of active sites and facilitates OER kinetics. Moreover, MXene serves as an interfacial bridge between NiFe-LDH and NF, improving the electron transfer ability across the electrode interface. High-resolution SEM images (Figures S6 and S7) reveal that excessive MXene modification and elevated metal ion concentrations during hydrothermal reactions can lead to cracks in the in situ grown LDH, resulting from the accumulation of internal stresses. The TEM characterization revealed that NiFe-LDH nanosheets were uniformly and densely interlaced, forming stable 3D structures on the Ti₃CN MXene surface (Figure 2i). This configuration is expected to enhance catalytic efficiency and stability for OER applications.

X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition and oxidation states of elements present in the 1.0-LDH/3MXNF and 1.0- LDH/NF. To minimize potential interference from the nickel foam substrate, the sample was ultrasonically stripped from the substrate prior to characterization. All peak positions were calibrated based on the C 1s peak at 284.8 eV. The XPS spectrum analysis (Figure S8) confirms the coexistence of Ni, Fe, Ti, N, O, and C elements in the 1.0-LDH/3MXNF sample, implying effective interfacial coupling between NiFe-LDH and Ti₃CN. This finding is consistent with observations from the SEM and TEM-EDS mapping images. In the Ni 2p spectrum (Figure 3a), two characteristic peaks were identified at 856.14 eV and 873.77 eV, which were attributed to the spin-orbit doublets of Ni 2p_3/2 and 2p_1/2 of Ni²⁺, respectively, accompanied by accompanying satellite peaks (labeled as sat.) [37]. The Fe 2p spectrum (Figure 3b) displayed peaks at 711.39 eV and 724.59 eV, associated with Fe 2p_3/2 and Fe 2p_1/2, confirming the presence of iron primarily in the Fe³⁺ oxidation state [38]. Additionally, a satellite peak along with a pre-peak peak at 705.9 eV reinforces the identification of Fe³⁺ in the catalyst [41,42]. Notably, the binding energies of the Ni 2p and Fe 2p peaks for the NiFe-LDH/Ti₃CN composite shifted lower by 0.2–0.6 eV compared to the 1.0-LDH/NF sample. This shift suggests an increase in the electron density on the surface due to the presence of Ti₃CN, which likely facilitates electron transfer to the nickel and iron ions in the LDH. Consequently, this phenomenon indicates a strong chemical interaction between MXene and NiFe-LDH, whereby Ti₃CN effectively modulates the surface electron density and reduces the oxidation states of Fe³⁺ and Ni²⁺.

2.2. Electrocatalytic Performance for OER

A The LDH/MXNF self-standing electrode was directly used as an electrocatalytic electrode for the OER to eliminate the influence of the binder on the active site. The catalytic performance was evaluated using a standard three-electrode setup with 1.0 M KOH as the electrolyte. The reference electrode was calibrated in 1.0 M KOH against a reversible hydrogen electrode, and subsequent potential conversions were performed using the calibration potentials (Figure S9). To further investigate the effects of MXene-modified NF and the modulation of metal cation concentration on the electrochemical performance of the monolithic electrodes, comparisons were made with samples 1.0-LDH/NF and 1.0-LDH/3MCNF, both tested under identical conditions. The LSV compensated by iR losses was tested at a low scan rate of 5 mV/s to minimize the capacitive background, allowing for a more accurate assessment of OER performance. Sample 1.0-LDH/3MXNF (Figure 4a), which utilized 1 mmol of metal ions and NF modified by a 3 mg/mL MXene dispersion, exhibited a reduced overpotential of 247 mV at a current density of 100 mA cm⁻². This represented a more favorable performance relative to other variations in metal ion concentrations and MXene loadings, underscoring the effective catalytic activity of the LDH/MXNF catalyst under these regulated conditions (Figure S11). Additionally, the unique 3D porous structure of NF facilitated higher current densities, with the LSV curve for 1.0-LDH/3MXNF displaying smoother characteristics at elevated current densities compared to other samples. This smoothness indicates that gas bubbles are released from the electrode surface more rapidly, enhancing kinetics related to electrolyte penetration, ionic transport, and the discharge of gaseous products during electrochemical reactions. Notably, MXene alone exhibited negligible OER catalytic performance (Figure S12), indicating that NiFe-LDH serves as the primary active site within the self-standing electrode. In this setup, MXene primarily serves as a coupling agent at the interfaces between the LDH and NF, facilitating electron transfer. However, the overall catalytic activity decreased with a higher MXene content, likely due to an increased tendency for warping and detachment following hydrothermal treatment. This adversely impacts the interfacial coupling between NiFe-LDH and NF. On the contrary, an appropriate amount of MXene can promote the effective coating of NF, maintaining this effect even after the growth of NiFe-LDH on the MXene surface. The Tafel slope analysis (Figure 4b) for the 1.0-LDH/3MXNF catalyst further highlighted its superior kinetic reaction rates compared to other conditions, yielding a Tafel slope of 67.7 mV/dec. This low Tafel slope indicates that the rate-determining step is likely associated with the latter stages of the multi-electron transfer reaction, a characteristic of high-performance catalysts. In comparison to 1.0-LDH/NF (318 mV at 100 mA cm⁻² and 148.1 mV/dec) and 1.0-LDH/3MCNF (270 mV at 100 mA cm⁻² and 89.5 mV/dec), the Ti₃CN MXene-modified self-standing electrode demonstrated improved catalytic effects over electrodes featuring Ti₃C₂T_x MXene or lacking MXene modification (Figure 4g). These findings suggest that both the metal cation concentration and the extent of MXene modification significantly influence the availability of active sites and electron transport within this hybrid structure, aiding in the enhanced energy efficiency of the monolithic electrode.

The electrochemical double-layer capacitance (C_dl) of the catalysts was evaluated in the non-Faradaic region using cyclic voltammetry (CV) (Figure 4c and Figure S10). The results indicated that the vertically self-standing electrodes exhibited similar ECSAs, a finding attributed to the shared use of nickel foam as the monolithic substrate. Notably, the C_dl values for NF-based monolithic electrodes, such as 1.0-LDH/3MXNF (0.396 mF cm⁻²) and 1.0-LDH/NF (0.185 mF cm⁻²), were improved by over tenfold compared to the LDH/MXene powder catalyst (see Figure S9). The charge transfer resistance (R_ct) is a critical parameter related to electrode kinetics, with lower R_ct values indicating faster electron transfer rates. The electrochemical impedance spectroscopy (EIS) pattern presented in Figure 4d reveals that the 1.0-LDH/3MXNF sample possesses the smallest semicircle radius, corresponding to the lowest charge transfer resistance (R_ct = 0.51 Ω). This reduction suggests that the electron transfer kinetics are significantly faster for this sample compared to 1.0-LDH/NF (R_ct = 0.82 Ω) and LDH/MXene powders (R_ct = 38.1 Ω). Consequently, the 1.0-LDH/3MXNF electrode substantially reduces the potential barrier for current flow through the catalyst, thereby enhancing the OER efficiency and exhibiting optimal catalytic activity. To further assess the dynamic performance of the 1.0-LDH/3MXNF electrode, a multi-step constant-current test was conducted (Figure 4e). The current density was incrementally increased from 50 mA cm⁻² to 200 mA cm⁻², increasing by 50 mA cm⁻² per step, before returning to 50 mA cm⁻². The electrode maintained a constant current at each step for 300 s, demonstrating rapid response times without significant fluctuations. Upon returning to 50 mA cm⁻², the current could be sustained for over 10,000 s without notable degradation, indicating robust electrochemical stability and mechanical integrity even under high current density conditions. Furthermore, in a long-term chronopotential (CP) stability test (Figure 4f) conducted at a constant current density of 50 mA cm⁻², the performance of the 1.0-LDH/3MXNF electrode remained stable over a 24 h period, with no significant increase in potential observed. This outcome underscores the excellent cycling stability and resilience of the electrode in demanding electrochemical environments. Finally, when compared to other documented OER catalysts (

Table 1. OER catalysts in 1.0 M KOH that have been reported in recent years.

Catalyst	jgeo (mA cm−2)	η (mV)	Tafel Slope (mV dec−1)	Ref.
1.0-LDH/3MXNF	100	247	67.7	This Work
NiFe-LDH/NF	50	306	143.1	[43]
NiFeAu-LDH/NF	100	267	58	[44]
MXene/TiO2/NiFeCo-LDH	10	320	98.4	[45]
NiCo-LDH/MXene/NF	100	257.4	68	[46]
Mo-Ni2P@NiFe LDH/NF	40	269	44	[47]
NiCoFe-LDH/NF	50	233	29.39	[48]
CoFeV-LDH/NF	100	330	57	[49]
Fe0.05CoNi-LDH/NF	10	212	48	[50]
CoSnO3@MX/NF	100	321	101	[51]
FeCoNi-P/NF	10	239	55.87	[52]
Ni-FeOOH/NF	100	277	52	[53]
NCP-MX/NF	50	303	69.5	[54]
NiFe-LDH/rGO@NF	50	277	59.9	[55]
Cr-FeNi LDH/MXene	10	232	54.4	[2]
LDH/H-Ti3C2Tx	100	364	47	[56]
CoNi-LDH/MXene@NiMoO4/NF	100	220	84.2	[57]
S,P-CoFeLDH/MXene	10	305	39	[58]
NiFe-LDH/Ti3C2	10	334	55	[59]

), the demonstrated performance of the 1.0-LDH/3MXNF catalyst remains competitive.

3. Materials and Methods

3.1. Chemicals

Ti₃AlCN (99% purity, 300 mesh, Laizhou KaiXi Ceramic Materials Co., Ltd., Laizhou, China), 49 wt% hydrofluoric acid (HF, Aladdin, Paris, France), 25 wt% tetramethylammonium hydroxide (TMAOH, Aladdin), iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, Aladdin), nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, Aladdin), urea (Aladdin), ammonium fluoride (NH₄F, Aladdin), L-ascorbic acid (LAA), and potassium hydroxide (KOH, 99.99%, Aladdin). Nickel foam was purchased from Suzhou Kesheng metal materials Co., Ltd. (Suzhou, China).

All chemicals were used as received, without further purification.

3.2. Synthesis of Few-Layered Ti₃CN MXene

The few-layered Ti₃CN (f-Ti₃CN) MXene nanosheets were synthesized based on a previous report [60]. In brief, 1 g of Ti₃AICN (300 mesh) was slowly added to 10 mL of 30 wt% HF and allowed to react at room temperature for 18 h. After that, the mixture was washed with deoxygenated water until neutral, centrifuged at 5000 rpm for 5 min each time, and finally filtered to obtain multilayered Ti₃CN (m-Ti₃CN), which was stored in a vacuum dryer overnight. The deoxygenated water was prepared by purging deionized water (DI water) with argon gas for 1 h.

Subsequently, 0.5 g of m-Ti₃CN was intercalated in 10 mL of 1 M TMAOH for 24 h. The resulting dispersion was washed with deoxygenated water until the pH reach approximately 9. During the washing process, care was taken to ensure no shaking, with each centrifugation conducted at 3500 rpm for 5 min. The f-Ti₃CN dispersion was subsequently obtained by cold bath sonication for 1 h, followed by centrifugation at 5000 rpm for 30 min.

3.3. Synthesis of LDH/MXNF

The nickel foam was thoroughly washed with acetone, 3 M HCl, and deionized water to remove any organic residues and oxide layers on the surface. A 3 × 3 cm² piece of NF was impregnated in 3 or 5 mg/mL of f-Ti₃CN dispersion, left to stand for 30 min, and then sonicated for an additional 30 min. The NF modified by MXene was vacuum-dried at 60 °C for 4 h, and these samples were designated as 3MXNF and 5MXNF, respectively. Next, a specific amount of Fe(NO₃)₃·9H₂O and Ni(NO₃)₂·6H₂O, 1.5 g of urea, 10 mg of LAA, and 50 mg of NH₄F were dissolved in 30 mL of DI water and sonicated for 30 min. The resulting solution was then combined with the MXNF and transferred to a 50 mL Teflon-lined stainless-steel autoclave. This assembly was placed in an autoclave and heated in an air blast drying box at 120 °C for 6 h, followed by natura cooling to room temperature. The samples were then ultrasonically washed with deionized water and ethanol several times and dried under vacuum at 60 °C for 4 h. The metal cations ratios of Ni²⁺ to Fe³⁺ were maintained at a 3:1 ratio, with total metal cations of 0.5 mmol, 1.0 mmol, and 2.0 mmol, respectively. The samples were labeled as 0.5-LDH/MXNF, 1.0-LDH/MXNF, and 2.0-LDH/MXNF, respectively.

Additionally, two comparative samples, 1.0-LDH/NF and 1.0-LDH/3MCNF, were prepared under identical conditions. The 3MCNF in the 1.0-LDH/3MCNF sample represents NF treated with 3 mg/mL of f-Ti₃C₂T_x.

3.4. Structural Characterization

The crystal phase of the catalyst was analyzed using an X-ray diffractometer (XRD, ADVANCE D8, Bruker, Billerica, MA, USA) with a Cu Kα radiation source in the range of 2θ of 5°~80°. The chemical composition of the catalyst was investigated using Fourier transform infrared spectroscopy with attenuated total reflection (FTIR-ATR, Thermo Scientific, Nicolet iS50, Waltham, MA, USA). The morphology of the obtained catalyst was characterized by a thermal field emission scanning electron microscope (SEM, Zeiss Gemini Sigma 300, Goldbach, Germany) equipped with energy-dispersive spectroscopy (EDS). Transmission electron microscopy (TEM, Talos F200X, Thermo Fisher Scientific, Waltham, MA, USA) was used to identify the catalyst’s morphology and elemental mapping. X-ray photoelectron spectra (XPS) were recorded in an XPS system (Axis Ultra DLD, Kratos, Manchester, UK) with a monochromatic Al X-ray source.

3.5. Electrochemical Measurements

All electrochemical properties were assessed in a standard three-electrode system using a Shanghai Chenhua CHI 760E electrochemical workstation. A mercuric oxide electrode (Hg/HgO) was used as the reference electrode, a graphite rod as the counter electrode, and the as-prepared catalyst as the working electrode. The two-sided geometry of the self-supported electrode was 1.2 cm², with a catalyst mass loading of 0.5 mg/cm². The electrochemical performance of the catalyst was evaluated in 1 M KOH solution. All measured potentials were converted to reversible hydrogen potential (RHE) based on the Hg/HgO calibration potential and the Nernst equation. The prepared catalysts underwent linear sweep voltammetry (LSV) tests to evaluate their OER activity at a sweep rate of 5 mV/s after 30 cycles of cyclic voltammetry (CV) activation in an O₂-saturated 1.0 M KOH electrolyte. A 90% iR compensation was applied to all potentials in the LSV curve. CV measurements were carried out in the non-Faradaic zone at various sweep rates of 20, 40, 60, 80, 100, and 120 mV/s to evaluate the electrochemical double layer capacitance (C_dl) of the electrocatalysts. Electrochemical impedance spectroscopy (EIS) measurements were performed at 1.53 V vs. RHE using a 5 mV amplitude over a frequency range of 10⁵ to 0.1 Hz. Multi-step chronopotentiometric curves were obtained by altering the current densities from 50 to 200 mA cm⁻² with an increment of 50 mA cm⁻² per 300 s before finally returning to 50 mA cm⁻² for an extended period. Chronopotentiometry (CP) curves were recorded as the current densities reached 50 mA cm⁻² to evaluate catalyst stability.

The Tafel slopes were calculated using the following formula:

$$\eta =b log j+c$$

(1)

where η is the overpotential, j is the current density, b is the Tafel slope, and c is the intercept.

The Electrochemical Active Surface Area (ECSA) of the electrocatalysts was estimated from C_dl according to the following formula [61]:

$$ECSA=C_{dl}/C_s$$

(2)

where the specific capacitance (C_s) is 40 mF/cm².

4. Conclusions

In conclusion, the foam nickel loaded with Ti₃CNT_x MXene-modified NiFe-LDH, developed through electrostatic adsorption and hydrothermal methods, exhibits excellent electrocatalytic performance for the oxygen evolution reaction. By adjusting the thickness of NiFe-LDH nanosheets and the MXene content, the OER performance of the self-standing electrode can be altered. In addition, the vertical structure of the LDH nanosheets facilitates mass transfer during the catalytic process. The 1.0-LDH/3MXNF catalyst showcased a low overpotential of 247 mV at 100 mA cm⁻² and a Tafel slope of 67.7 mV/dec, attributed to the effective interfacial coupling between NiFe-LDH and MXene, which enhanced electron transfer kinetics. The electrochemical characterization revealed a significant increase in electrochemical double-layer capacitance and a reduction in charge transfer resistance, indicating a high density of active sites and efficient charge transport. Additionally, the electrode exhibited remarkable stability with minimal potential degradation over long-term operation. This study highlights the promising potential of hybrid LDH-MXene systems for advanced electrocatalyst development in renewable energy applications.