Development of Electrochemical Water Splitting with Highly Active Nanostructured NiFe Layered Double Hydroxide Catalysts: A Comprehensive Review

Abstract

Electrochemical water splitting is a feasible and effective method for attaining hydrogen, offering a mechanism for renewable energy solutions to combat the world’s energy crises due to the scarcity of fossil fuels. Evidently, the viability and stability of the electrocatalysts are fundamental to the electrochemical water-splitting process. However, the net efficiency of this process is noticeably hindered by the kinetic drawbacks related to the OER. Hence, NiFe LDH has been widely used as a highly efficient OER and HER catalyst material due to its unique nanostructure, tunable composition, and favorable electronic structure. This review offers a systematic analysis of the latest progress in the fabrication of functional NiFe LDH catalysts and associated fabrication strategies, structure optimizations, and performance improvements. Special emphasis is given to understanding the role of nanostructure engineering in increasing active site accessibility, enhancing the effectiveness of subsequent electron transfer, and boosting the intrinsic catalytic activity for HER and OER. Moreover, we discuss the influence of doping, defects, and the formation of heterostructures with other materials on the OER and HER activities of NiFe LDHs. Additional accounts of basic structures and the OER and HER catalytic activities are provided, along with an enhanced theoretical understanding based on DFT studies on the NiFe LDH. Moreover, the limitations and potential developments of the work focus on the need for existing synthesis approaches, the stability of the NiFe LDH catalysts, and their insertion into working electrochemical processes. This review is a comprehensive analysis of the current state of research and developments in the use of NiFe LDH catalysts for the electrochemical water-splitting process to foster improved development of sustainable hydrogen sources in the future.

1. Introduction

This rapid progress in energy conversion and storage technologies is partially driven by the growing need for global energy to meet rapidly increasing demand, as well as to combat climate change [1]. Hydrogen has emerged as a promising energy carrier, mainly due to its high energy density, and it offers several environmental benefits [2]. Unlike carbon-based fuels, hydrogen combustion produces only water, making it a clean alternative to conventional fossil fuels with high energy density of 142 MJ kg⁻¹ [3,4]. The urgency of transitioning to alternative energy sources is further underscored by the limited availability of hydrocarbons and coal [5]. This future resource scarcity necessitates the exploration of sustainable and scalable methods for hydrogen production to address future energy demands without exacerbating environmental concerns. Electrochemical water splitting has garnered significant attention as an efficient and sustainable method for producing high-purity hydrogen [6]. The process involves splitting water molecules into hydrogen and oxygen using electrical energy. When powered by renewable energy sources such as solar or wind, this method becomes nearly emission-free, making it a cornerstone technology for a green hydrogen economy [7]. The efficiency of water electrolysis depends on the catalysts employed, with current research focusing on developing cost-effective alternatives to precious metals like platinum and iridium, which dominate the field due to their high catalytic activity [8]. However, the developments of multifunctional nanostructured electrocatalysts are essential in boosting the efficiency and flexibility of several electrochemical reactions [9]. The future of energy technologies will be significantly affected by the demand for clean and sustainable energy, and the importance of multifunctional nanostructured electrocatalysts will be vital. Therefore, improving the efficiency of water splitting by reducing the overpotential required for driving the HER and OER relies on electrocatalysis. The performance of traditional noble-metal catalysts, such as platinum for HER and ruthenium or iridium oxides for OER, is exceptional [10,11]. However, their cost and limited availability limit their practical applications, and alternate catalysts that are cost-effective, abundant, and stable are required [12]. Layered double hydroxides (LDHs) have shown great potential as electrocatalytic materials due to their inherent structural and physicochemical features [13]. Among various LDHs, nickel–iron (NiFe) LDHs are exceptional due to their outstanding intrinsic activity, catalytic durability, and versatility for HER and OER [14]. In addition, the chemical and physical properties of 2D-layered structure NiFe LDHs are highly tunable [15]. The brucite-like structure NiFe LDHs are two-dimensional layered materials with divalent Ni²⁺ partially substituted with trivalent Fe³⁺, yielding positively charged layers [16]. Intercalated anions and water molecules complete charge balance in these layers. The structural arrangement of the NiFe LDHs not only promotes ion transport but also dramatically enhances the exposure of active sites, as this is the key to high efficiency in catalysis [17]. The electronic and structural properties of the LDHs are modified by substituting Ni²⁺ ions with Fe³⁺ for electrochemical applications, and the modified LDH exhibits better performance in comparison to unmodified ones [18]. Several factors are proposed to be responsible for the superior catalytic activity of NiFe LDHs, as their intrinsic electronic structure makes them capable of overcoming their intrinsic overpotential for water splitting, which is critical, and combined with the tunable interlayer distance and anion exchange capabilities, offering additional adaptability to particular reaction environments [19,20]. Furthermore, NiFe LDHs possess outstanding bifunctional activity for both OER and HER and, therefore, have the potential to facilitate overall water splitting [21]. Morphological engineering, heteroatom doping, and hierarchical or core–shell nanostructure development are some of the recent efforts to improve the electrocatalytic performance of NiFe LDHs [22]. Interestingly, the mapping of molybdate ions in the interlayers within NiFe LDHs has been demonstrated to enhance the number of active sites and increase catalytic efficiency [23]. Likewise, the electrical conductivity and stability of the NiFe LDH can also be improved by doping with elements such as cobalt or by including conductive carbon supports. Since Fe plays a crucial role as an active site for the OER in NiFe LDHs, it enables high valent Ni species formation, which is essential for catalytic activity [24]. The literature results have also revealed that, in the case of Ni-based catalysts, even small amounts of Fe doping can provide a drastic enhancement in OER performance, which indicates an intrinsic synergistic interaction between Ni and Fe [25]. NiFe LDHs are suitable for long-term industrial applications due to their exceptional structural and morphological properties [26]. In addition, NiFe LDHs represent an essential and significant advance for electrocatalysis and represent a cost-effective, efficient, and sustainable way to produce hydrogen through the water-splitting reaction. Furthermore, NiFe LDHs have also been fabricated using various methods such as hydrothermal synthesis, electrodeposition, and co-precipitation to realize precise control over the morphology and composition [27]. However, NiFe LDH catalysts present hurdles to long-term stability, scalability, and industrial applicability. The structural degradation of the NiFe LDH under operating conditions is exceptionally crucial. Therefore, a deeper understanding of the catalytic mechanisms is necessary to address these challenges with robust design strategies. Fe³⁺ ion incorporation into the Ni(OH)₂ matrix improves charge transfer, while brucite-like layered architecture presents a high density of active sites [28]. These features give NiFe LDHs intrinsically low overpotential and as such, they have a high turnover frequency overall in water-splitting catalysts. In recent years, the design and engineering of NiFe LDH nanostructures have shifted to manipulating morphology in creating hierarchical nanostructures and doping with heteroatoms [29]. Furthermore, modulating the interactions between the Ni and Fe in Ruddlesden–Popper (RP)-type perovskites is also an interesting way to improve the catalytic activity for oxygen evolution reactions [30]. These strategies aim to enhance the electronic structure and increase active site availability in the NiFe LDH nanostructures for optimal catalytic efficiency and stability.

In this review, we strive to present the latest progress of NiFe LDH-based electrocatalysts for electrocatalytic water-splitting applications. Hence, morphological engineering, heteroatom doping, and hierarchical structure construction are emphasized as effective approaches to improve the electrocatalytic performance of the NiFe LDH nanostructures. The potential to scale up NiFe LDH nanostructures for industrial electrolyzers and their role in the hydrogen economy is also discussed, presenting a complete picture of the industry’s potential.

2. Morphology, Structural, Optical, and Elemental Studies

The X-ray diffraction results of NiFe LDH nanosheets prepared using diverse interlayer anions demonstrate structural transformations during exposure to aqueous KOH [31]. The X-ray diffraction results show the typical layered double hydroxide structure through strong (00l) reflections. The strength of basal reflections in the XRD pattern shows the electrons in the spaces between layers change due to water content and anion variations. The XRD results show the basal spacing in NiFe LDH nanosheets directly responds to the size of halide anions. In addition, XRD patterns indicate the successful growth of the MMT@NiFe LDH [32]. The peaks from XRD match both the montmorillonite (MMT) structure and the hydrotalcite-type NiFe LDH. The diffractogram shows (003) growing wider between 7.56 Å and 7.88 Å during Fe reduction from Ni2Fe1 LDH to Ni4Fe1 LDH. The LDH showed increased layer distance because the Ni²⁺ ions (0.69 Å) replaced Fe³⁺ ions (0.55 Å) during lattice formation. In addition, NiFe LDH and NiFeIr LDH nanosheets exhibit characteristic Raman bands consistent with previously reported NiFe LDH [33]. Specifically, these Raman bands are observed at 295 cm⁻¹, 455 cm⁻¹, 533 cm⁻¹, and 588 cm⁻¹, corresponding to E-type vibrations, Ni-OH, disordered Ni-OH, and Fe³⁺-O-Fe³⁺ vibrations, respectively. These peaks indicate the presence of key functional groups and structural features in the NiFe LDH and NiFeIr LDH. Upon closer examination, a slight shift in the Raman bands of NiFeIr LDH toward lower wavenumbers is observed compared to NiFe LDH. This shift is attributed to the influence of Ir doping, which alters the local bonding and electronic structure of the NiFeIr LDH nanosheets. The modified lattice dynamics caused by Ir incorporation signify changes in the vibrational modes, likely due to electronic interactions between Ir and the surrounding Ni and Fe atoms of the NiFeIr LDH. Interestingly, the Raman spectra provide critical insights into the structural transformations of Ni-Fe layered double hydroxide (LDH) catalysts during the oxygen evolution reaction (OER) [34]. Furthermore, with an OER onset potential of 1.5 V, the Raman bands remain consistent with the pristine Ni-Fe LDH phase, indicating structural stability under these conditions. However, at a higher potential of 1.6 V, substantial changes are observed in the Raman spectra. Few new peaks emerge at 474 and 552 cm⁻¹ are detected, which are attributed to the bending and stretching vibrations of NiO in NiOOH. Additionally, a broad Raman band appears in the 900–1100 cm⁻¹ range, indicative of negatively charged active oxygen species, such as NiO⁻ or NiOO⁻. As the potential decreases back to 1.3 V, the peaks associated with NiOOH diminish while the characteristic LDH peaks reappear. This reversible transformation highlights the robustness and stability of the Ni-Fe LDH catalyst under cyclic OER conditions, which is essential for its practical application. These findings not only provide a deeper understanding of the OER mechanism but also validate the potential of advanced in situ Raman techniques for studying electrocatalytic processes with high precision.

Figure 1a–f demonstrate the detailed changes in structural and compositional transformations of NiFe LDH/MOF to NiFe LDO/C [35]. The calcination-driven structural properties of NiFe LDH/MOF to NiFe LDO/C are quantified by XRD, nitrogen adsorption and desorption isotherms, pore size distribution, and XPS results. The thermal transformation of NiFe LDH/MOF to NiFe LDO/C is confirmed by the XRD patterns in Figure 1a. Synthesis of NiFe LDH/MOF is indicated by characteristic diffraction peaks at 11.4°, 23.0°, and 34.4°, indicating the layered double hydroxide structure. Once calcinated, the peaks shift and new prominent XRD peaks are observed at 37.33°, 43.36°, and 63.0°, which is associated with the (222), (400), and (440) planes of NiFe₂O₄. The crystallinity of NiFe LDO/C has been improved to enhance ionic interaction and structural stability. The mesoporous nature of the NiFe LDH/MOF and NiFe LDO/C are shown by the nitrogen adsorption/desorption isotherms in Figure 1b and pore size distribution in Figure 1c. Nevertheless, the NiFe LDO/C (123.74 m²/g) has a much higher specific surface area compared to NiFe LDH/MOF (96.74 m²/g) and Ni-MOF (35.48 m²/g). This increase is due to removing some interlayer components on calcination, increasing porous structures, and exposing more active sites. The 2–10 nm pore size confirms that the NiFe LDH/MOF and NiFe LDO/C are mesoporous, increasing the ion-accessible surface area and charge transfer. Figure 1d–f also further elucidate the chemical states and elemental composition of the NiFe LDH/MOF and NiFe LDO/C by the XPS analysis. The Fe 2p spectra show peaks at 712.2 eV and 724.7 eV, associated with Fe³⁺, which are unaffected. In addition, the Ni 2p spectra also show shapes of Ni²⁺, Ni 2p peaks at 855.7 eV and 873.3 eV, with satellite peaks, indicating that nickel oxidation states are stable. The O 1s spectra illuminate essential differences between the NiFe LDH/MOF and NiFe LDO/C. The hydroxyl oxygen and interlayer water peaks at 531.1 eV and 532.4 eV are seen in NiFe LDH/MOF. However, in NiFe LDO/C, the interlayer water peaks disappear, and new peaks at 530.1 eV (O-Ni and O-Fe) appear, proving the transformation of its structure occurring in calcination. Interestingly, these results validate the superior properties of NiFe LDO/C due to the structural and compositional changes, which significantly increase the surface area, porosity, and electrical conductivity. In addition to stabilizing the structure, removing interlayer water and anions improves ion accessibility.

The SEM images illustrate the transition of CoCO₃ nanowires into the CoCO₃@NiFe LDH heterostructure [36]. Their uniform surface creates a reliable platform for building quality catalysts. SEM analysis shows the nanosheet distribution covering all nanowire surfaces in a three-dimensional pattern. The change created additional surfaces that increased the number of reaction sites essential for catalytic processes. The integrated structure between nanosheets and nanowires produces a resistant platform that supports efficient electron transfers while preserving structural strength. In addition, the NCD@NiFe LDH reveals a flower-like structure composed of ultrathin nanosheets [37]. The NCD@NiFe LDH nanosheets exhibit a hierarchical structure, contributing to their high surface area and abundant active edges, which are critical for catalytic activity. The ultrathin nature of these sheets facilitates enhanced charge transfer and accessibility of catalytic sites, as evidenced by their wrinkled and folded edges, indicating mechanical flexibility and porosity. Furthermore, the NiFe LDH/Fe1-N-C heterostructure illustrates hollow nanorods critical to their bifunctional electrocatalysts [38]. The NiFe LDH/Fe1-N-C exhibit a highly uniform rod-like morphology with lengths ranging from approximately 3 to 4 µm. Such a morphology is particularly advantageous for enhancing electrocatalytic performance, as the elongated shape supports efficient electron transfer and high conductivity along the nanorod structure. The structural, morphological, and compositional aspects of Cu NWs shelled with NiFe LDH nanosheets, as shown in Figure 2a–h, are extensively characterized, and their importance for bifunctional electrocatalysis in overall water splitting is emphasized [39]. Figure 2a–c show the morphological characterization, which gives the porous architecture of the Cu@NiFe LDH. Moreover, the SEM images show NiFe LDH nanosheets uniformly and vertically grow over Cu NWs, leading to a high surface area with abundant edges exposed. In addition, Cu@NiFe LDH hierarchical structure enables not only the diffusion of reactants but also the release of gaseous products during catalytic processes. Additionally, Figure 2d,e demonstrate TEM images of the Cu@NiFe LDH hierarchical structure, illustrating an interconnected Cu wire network that ensures electrolyte penetration into sites that are otherwise difficult to access and, in turn, enhances the efficiency of the catalytic reaction. Figure 2f show high-resolution TEM images highlighting the structural details of Cu@NiFe LDH with the interplanar spacing of 0.25 nm, showing clear lattice fringes corresponding to the (012) crystal plane of NiFe LDH. Interestingly, the nanosheets are maximally catalytically efficient due to maximum exposure of their active ‘edge’ sites, which are highly reactive compared to basal planes. Such an ultrathin feature offers many active sites and, thus, enhances the overall catalytic performance. Figure 2g,h provide compositional analysis based on energy-dispersive X-ray spectroscopy (EDS) to confirm the core–shell architecture. Elemental mapping shows nearly uniform distributions of nickel and iron across the shell, while subsurface results from the EDS line scan suggest a copper-rich core encased by a Cu@NiFe LDH. The core–shell Cu@NiFe LDH is critical, allowing this homogeneous distribution to achieve consistent catalytic activity and efficient charge transfer. The Cu@NiFe LDH core–shell structure provides many advantages for electrocatalysis. The Cu core is highly conductive, and electron transport to the NiFe LDH shell is swift, advancing reaction kinetics. The hierarchical structure of the Cu@NiFe LDH also allows high surface areas and many channels to release gas.

The UV–visible diffuse reflectance spectroscopy (DRS) analysis examined the optical absorption properties of pure LDH, CN, and the CNLDH composite [40]. Pure LDH exhibits two prominent absorption bands: one in the ultraviolet region (200–400 nm) and another in the near-infrared region (600–800 nm). These bands are attributed to ligand-to-metal charge transfer transitions involving the Ni²⁺ and Fe³⁺ ions in the LDH structure. In contrast, pure CN demonstrates a characteristic semiconductor-like absorption edge around 450 nm, which arises from n → π∗ transitions involving the nitrogen atoms in the triazine and heptazine rings. When CN is incorporated into the LDH framework to form CNLDH composites, the absorption extends broadly over the visible spectrum, indicating an enhanced optical response. Notably, a blue shift in the absorption edge is observed as the CN content increases, signifying quantum confinement effects and strong electronic interactions between the CN and LDH layers. The bandgap of pure LDH is calculated to be 2.20 eV, while CN exhibits a higher bandgap of 2.70 eV. For the CNLDH composites, the bandgap values range between those of the individual components, varying with the weight percentage of CN. The CNLDH10 composite, with 10 wt% CN loading, exhibits an optimal bandgap of 2.32 eV. This value reflects a balance between effective visible light absorption and efficient charge carrier dynamics, which are crucial for photocatalytic applications. The systematic tuning of the bandgap energy highlights the impact of CN loading on modulating the electronic structure of the composites. The optical bandgap properties of NiFe₂O₄ nanoparticles play a critical role in their photocatalytic applications [41]. Using Tauc plots derived from UV–visible spectroscopy, two direct bandgaps were identified for NiFe₂O₄, such as 2.36 eV and 2.83 eV. The larger bandgap (2.83 eV) is particularly significant for the Г → Г transitions, which enable efficient utilization of higher-energy photons. In addition, integrating NiFe₂O₄ nanoparticles with layered double hydroxides (LDHs) further enhances their environmental applicability. On the other hand, the combination of UV–visible NIR and EPR reveals that Fe incorporation in LDHs significantly affects electronic transitions and magnetic properties [42]. H/Ni-Fe1 demonstrates a superior interaction network, supporting efficient electron delocalization and magnetic coupling, which enhances electrocatalytic performance for oxygen evolution reactions (OERs). The strong paramagnetic and ferromagnetic signals in H/Ni-Fe1 align with its observed high catalytic activity, emphasizing the role of Fe³⁺ in enhancing the electronic properties of Ni-based LDHs. In addition, the FT-IR spectra reveal significant chemical and compositional changes in the NiFe Layered double hydroxide (LDH) electrocatalyst after the oxygen evolution reaction (OER) [43]. The pre-cycled spectrum (black line) shows minimal peaks in the range of 1350–1400 cm⁻¹, which corresponds to Cl⁻ intercalation achieved through pre-treatment. The changes reflect the instability of the NiFe LDH under OER conditions, mainly due to the preferential leaching of Fe. This phenomenon depletes active Fe species from the LDH structure, significantly affecting the catalyst’s performance and stability. Therefore, these findings emphasize the need for enhanced material designs to mitigate such degradation mechanisms. Strategies such as structural modifications, heterostructure engineering, and protective coatings may improve the stability and performance of NiFe LDH catalysts in commercial applications. Interestingly, significant changes in PL intensity were observed upon incorporating NiFe LDH into BiVO₄ [44]. Specifically, the PL intensity decreased substantially after decoration with NiFe LDH, with the NiFe-CE (cation-exchange method) exhibiting the lowest intensity among all tested samples. This reduction in PL intensity indicates a decrease in the recombination rate of photogenerated electron–hole pairs, highlighting the role of NiFe LDH in facilitating charge separation and transfer. By providing abundant catalytic active sites, NiFe LDH enhances the extraction and utilization of photogenerated charge carriers. Furthermore, the CNNG3LDH exhibited the lowest PL intensity compared to pristine NiFe LDH, CN, and CNLDH [45]. This reduction in PL intensity indicates efficient charge separation and suppressed recombination of photogenerated electron–hole pairs, a vital factor in enhancing photocatalytic activity. The low PL intensity underscores the synergistic interaction among the composite’s components, enabling superior performance in applications like pollutant degradation and water splitting.

Figure 3a shows the enhanced optical properties of the NiFe LDH, N-rGO/NiFe LDH, and N-rGO/NiFe LDH@Au heterostructure and are instrumental in understanding its functionality [46]. It presents the UV visible absorption spectra of pristine NiFe LDH, revealing that strong light absorption happens in the UV and visible regions. The absorption bands are assigned to ligand-to-metal and metal-to-metal charge transfer transitions. The N-rGO/NiFe LDH@Au heterostructure with N-rGO and Au nanoparticles is integrated, and the shift in the absorption edge to the visible region indicates enhanced light absorption. Combined surface plasmon resonance (SPR) of Au and this redshift greatly help improve charge separation, as shown in Figure 3b. In addition, Figure 3c presents the optical bandgap of the NiFe LDH, N-rGO/NiFe LDH, and N-rGO/NiFe LDH@Au heterostructure. It is observed that the bandgap values are reduced from 2.20 eV for NiFe LDH to 2.01 eV for N-rGO/NiFe LDH@Au, suggesting improved visible light-harvesting capabilities for photonic applications. The bandgap is narrowed due to the integration of Au and N-rGO, which provide defect states and extend light absorption into the visible spectrum. Additional evidence for enhanced charge carrier dynamics can be obtained from PL spectra, depicted in Figure 3d. PL intensity decreases progressively from NiFe LDH to N-rGO/NiFe LDH and N-rGO/NiFe LDH@Au heterostructure. This PL intensity reduction from NiFe LDH to N-rGO/NiFe LDH@Au heterostructure signifies the substantially improved charge separation and less recombination of photogenerated electron–hole pairs due to the plasmonic synergistic interaction between the plasmonic Au and conductive N-rGO. On the other hand, the significantly enhanced photogenerated current density and charge transport in N-rGO are facilitated by the presence of defect sites in N-rGO in N-rGO/NiFe LDH@Au heterostructure. Figure 3e illustrates the time-resolved photoluminescence spectra of the N-rGO/NiFe LDH and N-rGO/NiFe LDH@Au heterostructure, revealing a dramatic change in fluorescence lifetimes for the heterostructures. The rapid interfacial charge transfer dynamics are highlighted by the short lifetime of N-rGO/NiFe LDH@Au. Figure 3f explores the time-resolved photoluminescence response curve of the N-rGO/NiFe LDH and N-rGO/NiFe LDH@Au heterostructure, with efficient and rapid charge transport, which shows reduced fluorescence lifetime and charge transfer efficiency.

3. Catalytic Water Splitting

3.1. Hydrogen Evolution Reaction

A scientifically robust approach to improving the hydrogen evolution reaction (HER) by developing chloride ion-modified NiFePt layered double hydroxides (LDHs) has been discussed and explored [47]. A one-pot electrodeposition method enables the rapid and energy-efficient synthesis of the NiFePt_Cl LDH. Unlike traditional fabrication techniques, which are time-consuming and complex, this method allows catalysts to be prepared in tens of seconds at room temperature. This highlights its potential for scalable production, aligning with the goals of sustainable and efficient hydrogen generation. Chloride ions (Cl⁻) play a crucial role in enhancing the performance of the NiFePt_Cl LDH catalyst. The Cl⁻ ions inhibit the reduction of Pt ions, promoting the growth of nanosheets and significantly increasing the specific surface area of the NiFePt_Cl LDH catalyst. Furthermore, Cl⁻ ions facilitate the oxidation of Pt, forming Pt-O and PtO₂ bonds. These oxidized Pt species exhibit superior HER activity and durability compared to metallic Pt, addressing the limitations of traditional Pt-based catalysts. Therefore, findings demonstrate the potential of NiFePt_Cl LDH as a scalable and high-performing catalyst, paving the way for more accessible and environmentally friendly hydrogen production solutions. In addition, a bifunctional composite catalyst, N-CNTs@NiFeZr-LDH, was developed by integrating Zr-doped NiFe layered double hydroxides (LDHs) with nitrogen-doped carbon nanotubes (N-CNTs) [48]. The strategic inclusion of Zr and N significantly enhanced the electronic structure of Fe and Ni active sites, leading to improved catalytic performance for HER. The introduction of Zr into NiFe LDH and N into CNTs improved electron density around Ni active sites, facilitating efficient proton adsorption and hydrogen evolution. Therefore, it is concluded that with the heteroatom doping and synergistic catalyst design, the researchers developed a cost-effective, durable, and efficient catalyst for sustainable hydrogen production. These findings underline the potential of such advanced materials in addressing the challenges of clean energy generation through seawater electrolysis. On the other hand, a detailed investigation into optimizing nickel–iron (NiFe) alloys for the hydrogen evolution reaction (HER) focuses on the effects of varying Ni/Fe molar ratios [49]. The study demonstrates that a Ni/Fe ratio 3:1 exhibits the highest catalytic performance. A key factor contributing to the superior performance of the NiFe (3:1) alloy is its unique nanosheet-array structure, identified as NiFe Layered double hydroxide (LDH). This structure enhances the active surface area and facilitates efficient charge transfer. Surface morphology and roughness analyses revealed that the NiFe (3:1) sample had a high surface roughness (223.72 nm) and low crystallite size (54.86 nm), both of which play a critical role in improving catalytic efficiency by increasing the density of active sites. Furthermore, this study also emphasized the practicality of the electrodeposition method used to synthesize the NiFe alloys, which is cost-effective and scalable, making the NiFe (3:1) alloy a promising candidate for large-scale water-splitting applications. Interestingly, enhancing the hydrogen evolution reaction (HER) by modifying NiFe Layered double hydroxide (NiFe LDH) with Au atoms, forming NiFeAu LDH, has been reported and discussed [50]. The incorporation of Au significantly improves the catalytic performance by modulating the electronic structure of the NiFeAu LDH. The highly electronegative Au atoms act as electron acceptors, enhancing charge transfer and conductivity, which are critical for improving the HER efficiency of the NiFeAu LDH catalyst. This modification facilitates the reaction kinetics and reduces the energy barriers associated with the hydrogen adsorption and desorption steps, making the NiFeAu LDH a highly effective electrocatalyst. Moreover, the synergy between Au and the NiFe LDH framework offers valuable insights into designing cost-effective, robust, and efficient bifunctional catalysts for sustainable hydrogen production and green energy technologies. Interestingly, a novel approach combines vanadium (V) doping with hydrogen (H₂) plasma reduction [51]. This process creates a heterostructured catalyst comprising Ni nanoparticles anchored on V-doped NiFe LDH nanosheets. The V-doped NiFe LDH nanosheets are rich in oxygen and nickel vacancies (O_v and Ni_v), which enhance conductivity and increase the number of active sites for HER. The density functional theory (DFT) calculations provide mechanistic insights into the enhanced HER performance. V doping significantly reduces the energy barrier for the Volmer step, improving hydrogen adsorption. Oxygen and nickel vacancies further facilitate water molecule adsorption and dissociation, which are critical steps in the HER process. Interestingly, a novel approach to improve hydrogen evolution reaction (HER) kinetics in alkaline solutions using a modified NiFe Layered double hydroxide (NiFe LDH) electrocatalyst has been proposed [52]. The sluggish Volmer step hinders HER performance in alkaline electrolyzers, the initial water dissociation process. While NiFe LDH is highly effective for oxygen evolution reactions (OERs), its HER activity is poor due to the weak hydrogen binding properties of Fe³⁺ centers. This limitation significantly affects its viability as a bifunctional electrocatalyst for overall water splitting. Therefore, a strategy was developed to enhance water dissociation activity, partially substituting Fe with Ru atoms in the NiFe LDH structure, creating NiFeRu LDH nanosheets. Introducing Ru dopants reduced the water dissociation energy barrier [ΔG(H₂O)] from 1.02 eV in NiFe LDH to 0.5 eV in NiFeRu-LDH, accelerating the critical Volmer step. Moreover, the hydrogen adsorption free energy [ΔG(H)] was optimized, maintaining a balance between hydrogen adsorption and desorption, which is crucial for efficient HER catalysis. Furthermore, the Ru-doped NiFe LDH represents a significant advancement in electrocatalyst design for HER in alkaline conditions. By reducing kinetic barriers and maintaining structural integrity, this catalyst offers a cost-effective and earth-abundant alternative to noble metals. Remarkably, iridium (Ir) doping in nickel–iron layered double hydroxides (NiFe LDHs) has been reported and explored with a focus on the influence of Ir valence states [53]. The density functional theory (DFT) calculations further validate these results, showing that Ir³⁺ reduces water dissociation energy barriers and enhances water adsorption, accelerating the Volmer step in HER. This advantage curtails the electronic configuration of Ir³⁺, which improves charge transfer and weakens water bonds for easier dissociation. The structural and morphological changes are observed by Ir doping. With increasing Ir content, NiFe LDH transitions from nanosheets to nanobelts and nanoparticles while retaining its layered hydroxide structure. However, excessive doping reduces crystallinity, highlighting the importance of optimizing Ir content for maximum catalytic performance. The combined experimental and theoretical insights position NiFeIr³⁺-LDH among the leading materials for alkaline HER applications, offering a promising pathway for advancing clean energy technologies.

In Figure 4A–F, the electrochemical properties and stability of Ni_xFe₁Mo₁-LDH/NF towards the hydrogen evolution reaction are comprehensively studied [54]. The polarization curves in Figure 4A show the current density as a function of potential, which suggests the highest reduction in the potential for Ni₆Fe₁Mo₁-LDH/NF. Figure 4B reveals the variation in overpotential with different catalysts and requires the lowest overpotential (104 mV) for Ni₆Fe₁Mo₁-LDH/NF, which depicts superior catalytic activity over other tested catalysts and also demonstrates the importance of Mo doping in improving electrocatalytic efficiency. Also, Figure 4C clearly shows that Ni₆Fe₁Mo₁-LDH/NF can present the slightest Tafel slope (103 mV dec⁻¹) despite the small loading compared with other catalysts. The resulting kinetics provide faster HER with the Volmer Heyrovsky process governing the kinetics of the process with the Volmer step in the rate-limiting step. Interestingly, it shows that Mo is not only essential to optimizing the electronic structure, but it also drastically accelerates reaction kinetics. The Nyquist plots in Figure 4D elucidate the charge transfer resistance of various catalysts, in which the Ni₆Fe₁Mo₁-LDH/NF has the lowest value (3.82 Ω cm²), which indicates better electron transfer efficiency and conductivity and is also consistent with the synergistic effect of Mo doping, allowing more electrons to flow during the HER process. Furthermore, in Figure 4E, the electrochemical double layer capacitance (C_dl) measurements explore that Ni₆Fe₁Mo₁-LDH/NF has the highest C_dl value (75.04 mF cm²) resulting from a larger electrochemical surface area (ECSA). Chronoamperometry measurements to 48 h at 10 and 100 mA cm² are shown in Figure 4F, signifying the excellent stability of the Ni₆Fe₁Mo₁-LDH/NF. During this period, there is negligible change in current density and the unchanged HER polarization curves before and after the stability test, as depicted in the inset, which further confirms the excellent durability of the catalyst. The structural and electrochemical stability of Ni₆Fe₁Mo₁-LDH/NF position it as a robust candidate for practical HER applications.

Figure 5 illustrates the HER mechanism of the NiFe LDH/Ni(OH)₂ composite enhanced in an alkaline medium [55]. The process is governed by two primary steps: the Volmer step, which involves water dissociation, and the Heyrovsky step, which generates molecular hydrogen. Furthermore, a schematic view compares the HER mechanism of single-phase NiFe LDH and the NiFe LDH/Ni(OH)₂ composite, as depicted in Figure 5. Water dissociation is a sluggish reaction in the single-phase NiFe LDH (left side), leaving limited proton availability for the Heyrovsky step. On the other hand, the NiFe LDH/Ni(OH)₂ composite (right side) utilizes Ni(OH)₂ as a co-catalyst, greatly accelerating water dissociation. As a result, lower overpotentials and much larger current densities are achieved with the NiFe LDH/Ni(OH)₂ composite, indicating superior catalytic activity. Also, the specifically enhanced performance of the NiFe LDH/Ni(OH)₂ composite allows for efficient generation and transfer of protons and electrons, thus favoring a more efficient hydrogen evolution process.

3.2. Oxygen Evolution Reaction

Some researchers have investigated electric field control of electron redistribution in NiFebased catalysts with regard to the oxygen evolution reaction (OER) [56]. This work addressed the critical challenges of slow kinetics in OER, which demonstrates an innovative approach to modulate electron redistribution by synthesizing heterostructure catalysts of FeO(OH)/NiFe LDH and Fe₇S₈(NiS)/NiFe LDH. The improved adsorption of catalytic intermediates due to this optimization results in an impressive catalytic performance. In addition, the FeO(OH)/NiFe LDH catalyst shows a lower OER overpotential of 192 mV at 10 mA cm⁻². Also, the FeO(OH)/NiFe LDH catalyst exhibits outstanding performance over industrially relevant temperatures (60–80 °C) at which traditional catalysts are unstable. On the other hand, abundant transition metals are available, such as nickel and iron, which provide a low-cost alternative to rare and expensive catalysts, such as Ru, Ir, and Pt. Interestingly, the d-band theory is applied to understand and improve electron redistribution in future catalyst design. However, a multistep synthesis process may hinder scalability and increase complexity in industrial realization. Moreover, a general framework for designing high-performance, low-cost, and stable electrocatalysts for water splitting resolves several fundamental issues in energy conversion. Furthermore, studies have focused on the development of heterostructured NiFebased alloy reinforced on valorized carbon waste for OERs [57]. The work describes the NiFebased catalyst developed by the dynamic hydrogen bubble template method. The bifunctional electrocatalytic activity of the NiFe/WGR catalyst is demonstrated, with an overpotential of 283 mV for the OER at 10 mA cm⁻². Additionally, the stability of the NiFe/WGR catalyst is notable throughout continuous electrolysis for 24 h, with virtually unchanged performance. These outstanding OER results are due to the porous structure and unmatched surface area of the NiFe/WGR catalyst, as well as the synergetic interactions between nickel and iron. Furthermore, the costs of the NiFe/WGR catalyst are low while its efficiency is exceptionally high, with overpotential values among the lowest reported to date for these materials in alkaline media. The study also embodies principles of environmental sustainability by valorizing the carbon residue and offers a viable alternative route to renewable resource dissemination and waste minimization. Still, there are several drawbacks because the electrodeposition process is scalable but suffers from the necessity of specific conditions for mass production. Interestingly, the magnesium-stimulated fast self-rebuilding of the NiFe catalyst has been discussed and explored in OER activities [58]. The work discusses a significant leap forward in water-splitting catalyst technology. Through a simple one-step corrosion method, the study makes a case for developing the Mg-Ni/Fe₂O₃ catalyst. Remarkably, including magnesium increases the OER performance by promoting the self-reconstruction of the Ni/Fe₂O₃ catalyst surface to highly active NiOOH species. Also, theoretical calculations show that optimizing the adsorption of oxygen intermediates by magnesium doping decreases reaction energy barriers and improves OER efficiency. The Mg-Ni/Fe₂O₃ catalyst is also relatively stable, maintaining consistent activity for 100 h in anion for OER. The single-step corrosion method is simple, but its scalability for industrial use has not been explored adequately. In addition, while the exact mechanism by which magnesium improves performance is theoretically well-defined, experimental validation under varied conditions may be needed. Overall, the results of this research provide a promising pathway to efficient and durable OER catalysts that strike a balance between the scientific and practical challenges. The development of MnO₂-modified NiFe layered double hydroxide (MnO₂@NiFe LDH/NF) for the oxygen evolution reaction (OER) in alkaline seawater [59]. It addresses the challenges of chloride corrosion and insufficient catalytic activity typical of seawater electrolysis. Integrating MnO₂ nanoparticles into NiFe LDH onto nickel foam (NF) is demonstrated as a strong electrocatalyst for OER in alkaline seawater. The MnO₂@NiFe LDH/NF catalyst has several advantages, such as enhanced catalytic activity, durability, and resistance to Cl⁻ corrosion. The modified MnO₂@NiFe LDH/NF catalyst enables selective enrichment of OH⁻ ions and repulsion of detrimental chlorine chemistry. Moreover, a highly efficient catalyst for the OER in water electrolysis is introduced, consisting of a molybdenum-doped nickel–iron-based layered double hydroxide (NiFeMo LDH) catalyst [60]. The NiFeMo LDH catalyst is synthesized in situ on nickel foam and has a unique nanoarray microstructure that significantly enhances its catalytic performance. In addition, the NiFeMo LDH shows superior charge transfer efficiency and robust catalytic activity in alkaline conditions. Interestingly, significantly improved structural and electronic properties of the NiFe LDH framework are enabled by the incorporation of molybdenum. The optimized electronic structure and density of active sites on the surface of the NiFeMo-LDH catalyst occur because of the synergistic interaction between Ni, Fe, and Mo. With the nanosheet morphology of the NiFeMo LDH catalyst, the high efficiency of the OER is attributed to its large electrochemically active area. The NiFeMo LDH catalyst is also very stable, retaining functionality over an extended time for 24 h and after 1000 cycles, indicating its durability for practical usage. In addition, a novel dual-ligand strategy has been studied to optimize the NiFeMOF structure to significantly enhance its OER activities [61]. The NiFeMOF was found to be superior in catalysis by incorporating long-chain dicarboxylic acids, such as decanedioic acid. Dual-ligand modification of NiFeMOF increased its hydrophilicity and gasiphobixity and enhanced its water adsorption and bubble release kinetics, which improved its reaction kinetics. Additionally, long-chain ligands were incorporated, and oxygen vacancies were realized along with changes in the electronic structure, leading to increased intrinsic catalytic activity. This work provides a valuable understanding of MOF-based catalysts for water splitting with efficiency, sustainability, and structural adaptability. In addition, the electrochemical and structural enhancements of nickel–iron hydroxides (NiFe LDH) through doping with molybdenum (Mo) are studied to optimize oxygen evolution reaction (OER) [62]. These results indicate that Mo doping shifts the d-band center, improving the electronic structure and catalytic activity. The Mo-NiFeO_xH_y catalyst has several benefits: (i) the incorporation of Mo accelerates charge transfer and increases the density of active sites, (ii) it maintains performance under prolonged operation, reducing degradation, and (iii) it has economic viability. However, the Mo-NiFeO_xH_y catalyst has certain limitations: (i) though it is stable under controlled conditions, the dissolution of MoO₄²⁻ ions during OER may limit long-term applications, and (ii) its structural changes and electrochemical performance are in need of further exploration. Interestingly, a porous nano coral-like NiFe foam (NIF) alloy was studied for OER application [63]. The NiFe foam alloy catalyst demonstrates exceptional catalytic activity and stability because of sulfur (S) and chromium (Cr) doping. Also, the novelty lies in its in situ growth strategy, which is achieved by combining acid etching with hydrothermal S and Cr doping. Notably, the S-Cr₀.₆@NiFe/NIF catalyst highlighted its high catalytic activity and durability, which constitute a cost-effective alternative for precious metal catalysts such as Ru and Ir. Furthermore, novel developments in the field of electrocatalysis by the performance of hafnium (Hf) doped in nickel–iron layered double hydroxides (NiFe LDH) are explored for OER [64]. High valent Hf⁴⁺ is introduced to modulate the electronic structure by redistributing charges around the Ni and Fe atoms to create more active catalytic sites. This modification improves electron transport, reaction kinetics, and structural stability. Interestingly, a substantial advancement in photo-assisted oxygen evolution reaction (OER) was achieved using the microflower-like NiFe LDH@gC₃N₄ composite [65]. The NiFe LDH@gC₃N₄ is synthesized by a one-step hydrothermal process, combining the light-harvesting characteristics of graphitic carbon nitride with the catalytic activity of NiFe LDH. The stable performance of the NiFe LDH@gC₃N₄ over extended periods and multiple cycles is also examined and found outstanding because of its high surface area, abundance of active catalytic sites, and structural stability. In addition, the oxygen evolution reaction is the key step of water-splitting systems and has a significant impact on the production of hydrogen for renewable energy. Therefore, strategies such as creating vacancies, optimizing electronic structures, and increasing active surface areas are often used to enhance the electrocatalytic performance in the OER. A new method for producing sulfur vacancies (Vs) in nickel iron sulfide (NiFeS) by scalable electrochemical reduction is presented, and the scalability of the catalyst is explored [66]. The NiFeS-Vs sulfur vacancy-rich catalysts were shown to have superior OER activity. On the other hand, sulfur vacancies are introduced, thereby enhancing conductivity, promoting the exposure of active sites, optimizing adsorption energies of intermediate states, and leading to improved catalytic performance. Fe centers were found to be the primary catalyst active sites using DFT calculations. This indicates that sulfur vacancies not only enhance intrinsic activity but also permit structural adaptation to maintain performance under industrial conditions. Interestingly, this provides a scientific foundation for using vacancy engineering in transition metal sulfides to develop robust and high-performance OER catalysts.

Figure 6 offers a detailed assessment of the OER for NiFe-based electrocatalysts and highlights the benefits of Ce doping. Structural and compositional tuning using Ce doping in the NiFe-based catalysts suggests an effective method for optimizing the activity and stability of OER [67]. Figure 6a elucidates the LSV polarization curves of the NiFe, NiFe₀.₉Ce₀.₁, NiFe₀.₈Ce₀.₂, and NiFe₀.₇Ce₀.₃ catalysts, which reveals a reduction in overpotential with different amounts of Ce doping. The NiFe₀.₈Ce₀.₂ catalyst exhibited the lowest overpotential of 175 mV, outperforming both the undoped NiFe catalyst (197 mV) and commercial RuO₂ (230 mV) for OER. The reduced overpotential indicates that optimal Ce doping enhances the catalytic efficiency of OER. In addition, the Ce incorporation in the NiFe₀.₈Ce₀.₂ catalyst enhanced its OER performance due to improved surface area, active site density, and electronic structure modifications. Figure 6b displays the overpotential performance of various electrocatalysts at higher current densities, which has vital practical importance. Interestingly, the NiFe₀.₈Ce₀.₂ catalyst maintains its superior catalytic efficiency, although it requires 245 mV and 273 mV overpotentials to produce 50 mAcm⁻² and 100 mA cm⁻², respectively. On the other hand, the same current densities require 320 mV and 390 mV overpotentials for RuO₂. However, excessive Ce doping to NiFe₀.₇Ce₀.₃ is shown to lead to the formation of Ce(OH)₃, a phase that is catalytically inactive towards OER, which offers a decline in OER performance. Figure 6c measures the Tafel slopes of various prepared catalysts, which can give an idea of the rate-determining step in the OER mechanism. More favorable reaction kinetics are shown by the NiFe₀.₈Ce₀.₂ catalyst, with a Tafel slope of 59 mVdec⁻¹, which is lower than both NiFe (82 mVdec⁻¹) and other Ce-doped NiFe compositions. Fascinatingly, it is observed that the Ce doping thus reduces the Tafel slope, providing evidence of its efficacy in accelerating the proton-coupled electron transfer steps critical for the OER. Moreover, Figure 6d illustrates the charge transfer resistance (R_ct) and series resistance (R_s) at the electrode/electrolyte interface, quantified by Nyquist plots. The smallest R_ct of the tested samples is found for NiFe₀.₈Ce₀.₂, matching its better OER catalytic activity. Captivatingly, it shows that lower R_ct indicates improved electronic conductivity and faster reaction kinetics, both of which are required for high-efficiency OER. In addition, Figure 6e elucidates that the ECSA of all prepared catalysts obtained from the double-layer capacitance measurements is a direct reflection of the number of catalytically active sites on the surface of the catalysts. The highest ECSA and roughness factor is observed with NiFe₀.₈Ce₀.₂, which reflects the increased number of actively accessible sites through optimized surface morphology. The stability of the NiFe₀.₈Ce₀.₂ catalyst is also evaluated in terms of long-term by chronopotentiometry at 20 mA cm⁻² for 50 h, as shown in Figure 6f. On the other hand, the overpotential increases by just 3 mV from 1.412 V to 1.435 V, with only negligible degradation observed on the NiFe₀.₈Ce₀.₂ catalyst. The remarkable stability indicates the robustness of the Ce-doped NiFe LDH structure and its suitability for high operating stability in OER. Therefore, it is concluded that Figure 6a–f showcase the enormous transformative impact of Ce doping on the OER performance of NiFe LDH electrocatalysts. It demonstrates superior catalytic activity, reduced overpotentials, enhanced kinetics, larger ECSA, and excellent durability, making it a promising candidate for sustainable and efficient water-splitting applications. The relationship between structure and performance is then systematically investigated, which offers valuable insights into the design of next-generation electrocatalysts.

A scientifically robust mechanism for OER using NiFe@LDH catalyst in an alkaline medium (1 M KOH) is presented in Figure 7 [68]. The mechanism proceeds through a four-electron transfer with hydroxide ions (OH⁻) adsorbing onto the catalyst’s active sites. After this step, a metal hydroxide intermediate (M-OH*) is formed and deprotonated to give a metal-oxide intermediate (M = O*). Reaction with OH⁻ ions produces a critical metal peroxide (M-OOH*), which undergoes further reaction. Without freeing this intermediate, molecular oxygen (O₂) will not be released, and the cycle cannot continue until the sites are regenerated. The OER process requires synergy, and the efficiency relies on nickel (Ni²⁺) and iron (Fe³⁺) ion interaction. Catalytic efficiency is enhanced by an increase in the adsorption of intermediates at the active sites due to nickel ions. Meanwhile, ion movement is assisted by the iron ions to transfer charges that highlight the electrocatalytic performance. Several features intrinsic to the NiFe@LDH catalyst itself are ideal for OER applications. The interlaced double hydroxide structure also provides a large surface area and abundant active site, improving reaction kinetics. Moreover, the NiFe@LDH catalyst offers low resistance for electron transfer pathways that further enhance the reaction efficiency. The NiFe@LDH catalyst shows a well-defined pathway for the OER mechanism and demonstrates the importance of optimized metal sites and structural features for catalytic performance to directly support the goal of developing durable high-performance catalysts in hydrogen production via water splitting.

Figure 8A–C collectively demonstrate how vanadium doping modifies the structure and enhances the catalytic activity of NiFe LDH, particularly for the OER [69]. Figure 8A shows the optimized structural configuration of NiFe LDH. In the NiFe LDH structure before vanadium doping, the molecular arrangement is shown in this diagram, with the Fe and Ni centers coordinated with surrounding ligands. The standard environment of vanadium, absent vanadium, is not present, making this the baseline for comparison with V-doped structures. The optimized configuration of the V-NiFe LDH is revealed in Figure 8B. It emphasizes the structural changes in the V-NiFe LDH imposed by vanadium doping. Adding vanadium results in a redistribution of the coordination surroundings, and longer O around Fe is observed (2.97 Å to 3.31 Å). This distortion implies strong V-O interactions and increased unsaturation of Fe sites, resulting in enhanced catalytic performance. Figure 8C shows the OER free energy diagrams pertinent to NiFe LDH and V-NiFe LDH models. The diagram compares the energy needed for each step in the OER pathway. Both models are identified as rate-determining steps (RDSs). Interestingly, it is observed that vanadium doping strongly reduces the theoretical overpotential by strengthening intermediate adsorptions (*O adsorption). The improved energetics results in more efficient catalysis; the V-NiFe LDH configuration maximizes catalyst reaction intermediate interaction.

3.3. Overall Water Splitting

A bifunctional catalyst, NiFe LDH@Ni₃N nano-/micro-sheet arrays, was designed and studied for efficient overall water splitting [70]. The primary objective was to create a low-cost, highly efficient, and stable alternative to traditional noble metal-based catalysts such as Pt and IrO₂, which are limited by their high costs and scarcity. The proposed catalyst consists of NiFe Layered double hydroxides (LDHs) combined with metallic nickel nitride (Ni₃N), supported on nickel foam. It offers several advantages, including a large surface area, abundant active sites, and enhanced stability. On the other hand, Ni₃N provides high conductivity, anti-corrosion properties, and improved water dissociation kinetics. This synergistic interaction between Ni₃N and NiFe LDH ensures superior catalytic efficiency. Systematically, the enhanced catalytic activity can be attributed to the strong interfacial interaction between NiFe LDH and Ni₃N, which facilitates a fast charge transfer and reaction kinetics, enabling efficient H₂ release and maintaining active sites for prolonged operation. Therefore, it is concluded that the potential of NiFe LDH@Ni₃N is a scalable, noble metal-free catalyst for industrial hydrogen production. Captivatingly, a novel Co₉S₈/Ni₃S₂@NiFe LDH core–shell heterostructure catalyst was developed for efficient overall water splitting [71]. Using hydrothermal and electrodeposition methods, the novel Co₉S₈/Ni₃S₂@NiFe LDH core–shell heterostructure has been synthesized with Co₉S₈ as the core and NiFe LDHs as the shell. The Co₉S₈/Ni₃S₂@NiFe LDH core–shell architecture effectively prevents structural and chemical changes, ensuring the catalyst retains its morphology and activity during prolonged operation. The uniform growth of NiFe LDHs on the Co₉S₈ surface via electrodeposition plays a key role in maintaining stability. This idea provides a practical framework for designing next-generation bifunctional electrocatalysts for clean energy production. Interestingly, the development of a hierarchical MnCo₂O₄ nanowire@NiFe layered double hydroxide (LDH) nanosheet heterostructure on nickel foam as a highly efficient and durable bifunctional catalyst for overall water splitting has been reported and studied [72]. The catalyst design incorporates MnCo₂O₄ nanowires, which offer excellent structural and conductive properties, combined with the catalytic activity of NiFe LDH nanosheets. This combination forms a unique hierarchical structure created using a hydrothermal–annealing–hydrothermal method. By utilizing non-precious metals and scalable fabrication methods, the study presents a practical approach to designing high-performance electrocatalysts for industrial-scale water electrolysis. This research highlights the potential of combining transition metal oxides with LDH structures to address challenges of efficiency, cost, and stability in water-splitting applications. Captivatingly, the development of a highly efficient electrocatalyst for alkaline overall water splitting by incorporating thulium (Tm) into nickel–iron layered double hydroxides (Ni₃Fe LDHs) and further enhancing the material with platinum (Pt) loading was reported and discussed [73]. Ni₃Fe LDHs are promising for the oxygen evolution reaction (OER); however, they suffer from high overpotential due to their weak adsorption and desorption of oxygen intermediates. The introduction of Tm addresses these limitations through its unique 4f orbital electronic structure, which forms an electronic buffer band. This buffer band minimizes the over-oxidation of metal–oxygen (M-O) bonds, significantly improving the OER catalytic activity. Structurally, the inclusion of Tm induces lattice distortions and creates oxygen vacancies, which increase the density of active sites and enhance catalytic activity. The 4f orbitals of Tm act as electron traps, improving charge transfer efficiency. Loading Pt further boosts the electron transfer kinetics and enhances the durability of the catalyst. Interestingly, bifunctional NiFe LDH nanosheets with CoFe₂O₄@Co₃S₄ nanowires supported on carbon fiber paper (NiFeCFS/CFP) have been developed [74]. The NiFeCFS/CFP catalyst was synthesized using a two-step hydrothermal method followed by electrodeposition, resulting in a hierarchical heterostructure. This innovative design aims to enhance the active surface area, electronic conductivity, and overall catalytic performance for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The NiFeCFS/CFP hierarchical catalyst design also offers structural advantages, including high-density active sites and improved mass transport properties. Therefore, by eliminating the need for binder additives, which can limit activity and stability, the direct use of the carbon fiber substrate ensures robust mechanical support and excellent electrical conductivity. These features, combined with the intrinsic catalytic properties of the heterostructure, enable the NiFe-CFS/CFP catalyst to achieve high efficiency and long-term durability. Interestingly, a bifunctional electrocatalyst by constructing a heterostructure comprising NiFe Layered double hydroxide (NiFe LDH) coated on nickel selenide (NiSe) nanoforests supported on nickel foam (NiSe@NiFe LDH/NF) was reported and examined [75]. The design of the NiSe@NiFe LDH/NF combines the superior electrical conductivity and high surface area of NiSe with the high density of active sites and corrosion resistance offered by NiFe LDH. Such a combination allows the catalyst to exhibit exceptional performance for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In addition, the NiFe LDH shell acts as a protective layer, mitigating corrosion and maintaining the structural integrity of the NiSe core. This durability ensures prolonged catalytic activity with minimal degradation, addressing a critical challenge in water-splitting applications. The protective nature of the shell, combined with the robust core, allows the catalyst to sustain high performance under demanding conditions.

The mechanism of intermediate modulation is explored based on the interfacial synergy between Ru sub-nanoclusters and porous NiFe-Layered double hydroxide (LDH) for improved water splitting [76]. Fascinatingly, it shows that Ru species integrated into NiFe LDH facilitate the electrochemical kinetics of hydrogen evolution (HER) and oxygen evolution reactions (OERs) via favorable hydrogen and oxygen adsorption energies. The schematic drawing illustrated the possibility of the synergistic action of potential between ruthenium (Ru) and nickel–iron layered double hydroxides (NiFe LDH) for HER and OER in alkaline conditions, as shown in Figure 9A,B [74]. Figure 9A elucidates the crystallographic depiction of the HER mechanism using the Ru/NiFe LDH catalyst. The adsorption and recombination of the hydrogen intermediates are facilitated at an essential active site on the Ru/NiFe LDH catalyst for the HER. Efficient dissociation of water drives sequential hydrogen intermediates by NiFe LDH. The intermediates are adsorbed onto the Ru surface and recombined to form molecular hydrogen. This synergy decreases the energy barriers of the Volmer step (water dissociation) and, in general, increases barriers at the Tafel step (hydrogen recombination), thereby accelerating HER kinetics in alkaline media.

Furthermore, the OER mechanism is elaborated in Figure 9B using the RuO₂/NiFe OOH_x interface. Ru shifts to RuO₂, a well-established oxygen-evolving catalyst, at potentials greater than 0.04 V vs. RHE. In parallel, NiFe LDH undergoes NiFe(OOH)_x transformation to an active phase of water oxidation. This adsorption and deprotonation interface between RuO₂ and NiFe(OOH)_x optimizes the formation of OOH⁻ intermediates. These intermediates are then automatically broken down to release molecular oxygen. RuO₂ enhanced adsorption energies of key intermediates due to the interfacial interactions with NiFe(OOH)_x, which resulted in a significant reduction in OER overpotentials.

The performance and durability of the NiFe LDH/MoS₂-Ni₃S₂/NF for overall water splitting (OWS) are shown in Figure 10a–f [77]. The design of the two-electrode system, as represented by the schematic in Figure 10a, consists of catalyst-coated nickel foam as both the anode and cathode. Owing to its pragmatic design as the two-electrode system, a nickel foam (NF) substrate covered with the NiFe LDH/MoS₂/Ni₃S₂ catalyst serves both purposes as an anode and cathode for OWS. This ability to provide excellent electrical conductivity and good mechanical stability, together with sound catalytic propagation, make nickel foam an ideal substrate for supporting the NiFe LDH/MoS₂/Ni₃S₂ catalyst for efficient electron transfer. In addition, it dramatically reduces overall water-splitting complexity and cost, making it a very scalable solution for its industrial applications. The polarization curves in Figure 10b clearly illustrate the high OWS efficiency of the NiFe LDH/MoS₂-Ni₃S₂/NF catalyst. While a current density of 10 mA cm⁻² is achieved at a low cell voltage of 1.5 V, it outperforms other catalysts. The NiFe LDH/MoS₂-Ni₃S₂/NF allows synergistic electronic interactions between NiFe LDH, MoS₂, and Ni₃S₂, which enables improved charge transfer and favorable reaction kinetics. This is confirmed by the comparison in Figure 10c, in which the NiFe LDH/MoS₂-Ni₃S₂/NF catalyst exhibits excellent catalytic activities compared with other catalysts by indicating the lowest overpotential. This is due to the synergistic electronic interaction and structural features of the NiFe LDH/MoS₂-Ni₃S₂/NF heterostructure. The durability and stability of the catalyst are demonstrated in Figure 10d–f. Figure 10d shows the multistep chronopotentiometry curve of the NiFe LDH/MoS₂-Ni₃S₂/NF, showing stable voltage operation over an extended period, indicating the robustness of the NiFe LDH/MoS₂-Ni₃S₂/NF. On the other hand, the robustness of the NiFe LDH/MoS₂-Ni₃S₂/NF concerning varied currents is seen in stable voltage operation over extended periods, which supports a high-performance operation without significant degradation, indicates that it can substantiate itself as a high-performance catalyst. Figure 10e,f illustrate the validation of the stability of the NiFe LDH/MoS₂-Ni₃S₂/NF by chronoamperometric tests, which demonstrate minimal loss for 50 h of operation at 10 and 50 mA cm⁻². In contrast, the operation of the RuO₂||Pt-C/NF cell resulted in significant performance degradation over the same duration. These results highlight the scientific and practical significance of NiFe LDH/MoS₂-Ni₃S₂/NF heterostructure. Interestingly, the NiFe LDH/MoS₂-Ni₃S₂/NF heterostructure offers a combination of high current density, minimized overpotential, and excellent durability, making the material an attractive candidate for sustainable hydrogen production. It tackles the most significant challenges in OWS, including excessive energy consumption, unfavorable long-term stability, and the high material costs inherent to traditional precious metal catalysts.

A detailed analysis of adsorption geometries and interfacial electron transfer in NiFe LDH and NiCo₂S₄@NiFe LDH heterostructure is introduced in Figure 11a–c [78]. Furthermore, the adsorption geometries of intermediates (*H and *OH) on the surface of NiFe LDH are illustrated in Figure 11a. The NiFe LDH shows high chemisorption free energies of 1.32 eV and 1.56 eV for hydrogen (ΔE_H) and hydroxide (ΔE_OH), respectively. This implies poor catalytic performance, especially for the oxygen evolution reaction (OER). Significant high-energy barriers indicate the poor ability of the surface to facilitate the vital adsorption and desorption steps of efficient catalysis. The adsorption geometries of intermediates located on the NiCo₂S₄@NiFe LDH heterostructure are shown in Figure 11b. A heterostructure formation reduces the ΔE_OH from 1.56 eV (NiFe LDH) to 1.03 eV. Fascinatingly, the coupling of NiCo₂S₄ with NiFe LDH provides enhanced interfacial interactions and optimized electronic structure, leading to the reduction of chemisorption free energy. This ΔE_H is surprisingly invariant, and the catalytic performance for the hydrogen evolution reaction (HER) remains unchanged. The changes, however, indicate that NiCo₂S₄@NiFe LDH heterostructure selectively increases OER performance with limited effect on HER efficiency. Furthermore, Figure 11c studies the electron transfer at the NiCo₂S₄ and NiFe LDH interface. The electron density maps visualize charge redistribution at the interface, where yellow regions indicate charge accumulation and light blue regions indicate charge depletion. It is observed that the 0.28 electrons are transferred from the surface of NiFe LDH to the surface of NiCo₂S₄, forming an electron-rich NiCo₂S₄ surface and a hole-rich NiFe LDH surface. In addition, it alleviates OER kinetics by redistributing charges at the NiCo₂S₄@NiFe LDH heterostructure interface, improving the adsorption of oxygen intermediates. Interestingly, the NiCo₂S₄@NiFe LDH heterostructural engineering boosts catalytic activity by visualizing interfacial charge dynamics. Therefore, it is concluded that these results demonstrate the role of interface engineering in electronic structure and catalysis with the realization of a promising route to developing efficient bifunctional electrocatalysts for water-splitting applications.

Figure 12 displays the basic mechanisms explaining the electrocatalytic efficiency of the Mo-NiS_x@NiFe LDH/NF [79]. The calculated free energy profiles for the oxygen evolution reaction (OER) on the different catalytic surfaces, such as Ni(OH)₂, Fe(OH)₂, and NiFe LDH, are shown in 12a–c. It is detected that the inclusion of Fe into the NiFe LDH structure lowers the reaction energy barrier. Therefore, optimizing the catalyst leads to the adsorption and desorption of intermediates, such as *OH, *O, and *OOH, more effectively. Therefore, the Fe sites in the NiFe LDH matrix are critical for improving the catalytic activity for OER via balancing intermediate interactions. Additionally, crystal orbital Hamilton population (COHP) analyses in Figure 12d–f investigate the bonding interactions between metal and oxygen in the OER intermediates. In addition, the bonding intensity of NiFe LDH is relatively moderate, less than that of Fe(OH)₂ and Ni(OH)_2, which possess too strong bonding of NiFe LDH. This optimal bonding balance in NiFe LDH facilitates the efficient conduct of OER intermediates. Furthermore, the DOS analysis in Figure 12g indicates that NiFe LDH has superior electronic conductivity relative to the individual metal hydroxides. The enhanced conductivity is an important feature that enables charge transport, and it is strongly correlated with the results of the electrochemical impedance spectroscopy. Further, the charge density difference for the adsorption of *OH on NiFe LDH is displayed in Figure 12h. It indicates that electron transfer from the O-H bond of the adsorbed intermediate to the Fe-O bond is prominent, representing the efficiency of the dehydrogenation step. Therefore, maintaining high catalytic performance in OER is dependent on the efficient electron transfer process. In Figure 12i, free energy profiles of the proton adsorption process on Mo-doped NiS and Ni₃S₂ surfaces are analyzed for the hydrogen evolution reaction. Interestingly, it is observed that the energy profile of the Mo-doped surfaces is optimized for fast hydrogen evolution. Moreover, the electron transfer rates are enhanced by Mo doping, resulting in enhanced HER efficiency. Therefore, it is concluded that Figure 12 highlights the synergistic roles of NiFe LDH and Mo-doped NiS_x in facilitating efficient overall water splitting. Fe in NiFe LDH reduces the energy barrier for OER by providing optimal active sites, while Mo doping in NiS_x enhances HER by modulating surface electronic properties and improving hydrogen evolution. These results align with experimental observations, confirming the superior bifunctional catalytic performance of the Mo-NiS_x@NiFe LDH/NF composite.

3.4. Key Parameters for the Evaluation of Catalytic Activities

3.4.1. Reversible Hydrogen Electrode (RHE)

The following Equation (1) is applied to transform all reference potentials into RHE:

$$E_{RHE}=E_{Ag/AgCl}+0.197+0.059pH$$

(1)

3.4.2. Overpotential (η)

The overpotentials for the OER and HER are estimated by following Equation (2a,b):

$${\eta }_{OER} \left(V\right)=E_{RHE}-1.23$$

(2a)

$${\eta }_{HER} \left(V\right)=E_{RHE}-0$$

(2b)

3.4.3. Tafel Slopes (b)

The overpotential vs. logarithm of current density is represented by Tafel plots, which are derived from the LSV plots. The linear regions of the Tafel plots around a particular overpotential value are fitted using the following Equation (3) [80]:

$$\eta =b \mathrm{l}\mathrm{o}\mathrm{g}j+a$$

(3)

where η is the overpotential, b is the Tafel slope, and a is a constant.

3.4.4. Specific Capacitance (C_S)

The specific capacitance of the catalysts is calculated by the following Equation (4):

$$C_s (\mathrm{m}\mathrm{F} {\mathrm{c}\mathrm{m}}^{-2})={\displaystyle \frac{\mathrm{A}}{2VPM}}$$

(4)

where A is the area under CV curves, V is the scan rates, P is the potential window in CV curves, and M is the mass of the catalysts.

3.4.5. Double-Layer Capacitance (DLC)

The double-layer capacitance is estimated by the following Equation (5) [81]:

$$\mathrm{D}\mathrm{L}\mathrm{C}(\mathrm{m}\mathrm{F})={\displaystyle \frac{∆J}{V}}$$

(5)

where ΔJ is the difference between anodic and cathode current densities at the middle of the potential range of the CV curves, and V represents the scan rates. The slopes of the ΔJ vs. scan rate plots provide the values of the double-layer capacitance.

3.4.6. Electrochemical Active Surface Area (ECSA)

The electrochemical active surface area of the sample is evaluated by following Equation (6) [82]:

$$\mathrm{E}\mathrm{C}\mathrm{S}\mathrm{A}({\mathrm{c}\mathrm{m}}^2)={\displaystyle \frac{DLC}{C_S}}$$

(6)

where DLC is the double layer capacitance and C_S is the specific capacitance of the sample.

3.4.7. Roughness Factor (RF)

The roughness factor is evaluated using the following Equation (7) [83]:

$$\mathrm{R}\mathrm{F}={\displaystyle \frac{ECSA}{GSA}}$$

(7)

where GSA is the geometric surface area and ECSA is the electrochemical active surface area.

3.4.8. Mass Activity (MA)

The mass activity (J_mass) of the catalysts is estimated by the following Equation (8) [67]:

$$J_{mass}(\mathrm{m}\mathrm{A}{\mathrm{m}\mathrm{g}}^{-1})={\displaystyle \frac{J_{CD}}{M}}$$

(8)

where J_CD is the current density (mA cm⁻²) and M (mg cm⁻²) represents the mass loading of catalysts.

3.4.9. Active Site (AS)

The active sites (n) of the catalysts were assessed by the CV curves using the following Equation (9) [67]:

$$\mathrm{n}(\mathrm{m}\mathrm{o}\mathrm{l})={\displaystyle \frac{Q}{2F}}$$

(9)

where Q is the number of voltammetric charges, and F is the Faraday constant (96,485 C mol⁻¹).

3.4.10. Turnover Frequency (TOF)

The turnover frequency (TOF) was calculated by Equation (10) [84]:

$$\mathrm{T}\mathrm{O}\mathrm{F}={\displaystyle \frac{JS}{4Fm}}$$

(10)

where S is the active surface area, m is the amount of catalyst, J is the current density, and F is the Faraday constant.

3.4.11. Faradaic Efficiency (FE)

The Faradaic efficiency is calculated by the following Equation (11) [68]:

$$\mathrm{F}\mathrm{E}={\displaystyle \frac{x{H}_2}{\left(\frac{Q}{nF}\right)}}$$

(11)

where n is the number of electrons in the water-splitting reactions, F is the Faraday constant, Q is the total charge, and xF is the amount of oxygen or hydrogen generated using total charge Q.

3.4.12. Stability

Stability of the catalyst is a crucial factor in determining its practical applicability for long-term use in electrochemical water-splitting applications. Stability refers to the catalyst’s ability to retain its structural integrity, activity, and efficiency over time under operational conditions. Various methods are used to evaluate the stability of catalysts, including chronopotentiometry and cyclic voltammetry (CV) over a particular time.

4. Conclusions

In light of the advancement, this study emphasizes the significance of producing nanostructured nickel iron-layered double hydroxide (NiFe LDH) catalysts for electrochemical water splitting. The resulting catalysts possess outstanding performance: high turnover frequency, mass activity, and bifunctional catalytic capability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The catalytic efficiency and stability of these materials have greatly benefited from key strategies like morphology engineering, the design of hierarchical and core–shell nanostructures, and the doping of heteroatoms. Being superior in terms of intrinsic activity and longevity, NiFe LDH catalysts promise to provide scalable hydrogen production solutions to critically important global energy and environmental challenges.

Industrial-scale application lies ahead for NiFe LDH catalysts. High performance in catalysis and cost-efficient large-scale production is critical to achieving this, so optimization of synthesis methods is required for cost-effective, large-scale production. While the exposure of active sites could be further increased, advanced structural engineering paradigms such as novel hierarchical designs and the development of ultrathin nanosheets may provide possible routes for increasing efficiency. Additionally, efforts are needed to integrate NiFe LDH catalysts into advanced electrolyzer systems. If these catalysts are to be commercialized, reducing overpotentials, increasing long-term stability, and ensuring compatibility with current technologies are key objectives. Adding synergistic materials, such as carbon supports and other transition metals, combined with NiFe LDH may enhance conductivity, stability, and overall performance.

Broadening the applicability of NiFe LDH catalysts will allow their efficiency to be studied in pH-varying operational environments. Such adaptability would enable NiFe LDH-based catalysts to be used in many industrial and environmental scenarios. Moreover, computational and theoretical studies can elucidate reaction mechanisms on the atomic scale and help rationally design next-generation catalysts. Overall, NiFe LDH catalysts have great potential to change the field of sustainable energy technologies and contribute to a hydrogen-driven economy should future research and development efforts solve the challenges involved in large-scale implementation.

Moreover, incorporating dopants, defects, and heterostructures into NiFe LDH systems has enhanced their catalytic efficiency further, highlighting the importance of engineering strategies in tailoring catalyst performance. Despite these advancements, challenges such as long-term stability in alkaline and neutral electrolytes, cost-effective large-scale synthesis, and understanding the precise catalytic mechanisms remain critical for future research. Further, NiFe LDH catalysts hold great promise for boosting the efficiency of electrochemical water splitting, offering a pathway toward clean hydrogen energy. Future research focused on overcoming existing limitations and further optimizing the NiFe LDH catalysts will be pivotal in advancing hydrogen production technologies on a global scale.