Recent Progress in the Synthesis of Layered Double Hydroxides and Their Surface Modification for Supercapacitor Application

Abstract

The need for energy storage and the rapid development of new electronic platforms have prompted intense research into small and secure energy storage devices, particularly supercapacitors (SCs). Layered double hydroxides (LDHs) are potential electrode materials for SCs because of their excellent physicochemical and electrical characteristics. They involve interlayer spacing, high oxidation states, simplicity of synthesis, and distinct morphologies. Despite their potential, several kinds of LDHs still face constraints, such as particle aggregation, moderate surface area, and high resistance, which limit their use in energy storage. To overcome these challenges and enhance the electrochemical performance of LDHs, they have used strategies such as anion intercalation, oxygen vacancy, heteroatom, surfactant, fluorine, and metal doping, which have been demonstrated as electrode materials for SCs. Therefore, this review discusses recent advances in different LDHs and studies comparing bare and modified LDH for three- and two-electrode systems, with an emphasis on their morphologies, surface areas, and electrical properties for SC applications. It was found that modified LDHs achieve enhanced electrochemical performance in comparison to their corresponding bare LDHs. Consequently, there are potential opportunities to modify the surface of the recently invented LDHs for electrochemical investigations, which could result in improving their performance. This review also presents future perspectives on LDH-based energy storage devices for supercapacitors.

1. Introduction

Recently, there has been a lot of interest in researching and developing energy storage technologies across the world. Because of the rising need for digital communication and electric cars, the development of alternative energy sources, such as energy storage devices, is encouraged. Electrochemical energy storage devices offer alternative solutions to meet these needs by exploring sustainable and eco-friendly energy sources [1,2,3]. Supercapacitors (SCs) have garnered significant interest among energy storage devices, owing to their high-power density (P_d), prolonged life cycles, and safe operation [4,5,6]. SCs are classified into three types according to their charge–storage mechanisms: electrochemical double-layer capacitors (EDLCs), pseudocapacitors (PCs), and hybrid supercapacitors (HSs) [7,8,9]. Figure 1a–c depicts the configuration and charge–storage mechanisms of SCs such as EDLCs, PCs, and hybrid. Charge–storage in EDLCs occurs by electrolyte ion adsorption electrostatically at the electrode–electrolyte interface, which generates an electrical double layer when voltage is applied, rather than a chemical process [10]. Carbon-based electrode materials are examples of EDLCs. Although their specific capacitance (C_s) and energy density (E_d) are low, they offer outstanding cycle stability. PCs store charge via reversible redox processes on the electrode materials’ surfaces, which include metal oxides, metal hydroxides, metal sulfides, metal nitrides, metal carbides, conducting polymers, and MXene [11]. These materials have high C_s and E_d; however, their poor cycle stability and P_d might be attributed to slow redox processes. In the case of hybrid SCs, both the mechanisms of EDLCs and PCs are applied to provide excellent electrochemical performance, such as high C_s, cycle stability, and rate capability, by mixing carbon-based materials with either metal oxide/hydroxide or conducting polymers, or both. Hybrid SCs use an EDLC-type material as the negative electrode and a PC material as the positive electrode to obtain high Ed and Pd levels while also providing rapid charge and discharge rates.

Electrode materials are a crucial component of SCs that determine electrochemical performance when compared to electrolytes and separators. Many electrode materials have been studied as credible electrodes for SC devices, such as porous carbons, activated carbons (ACs), conducting polymers (CPs), graphene oxides (GO), reduced graphene oxides (rGO), metal oxides, carbon nanotubes (CNTs), layered double hydroxides (LDHs), MXene, metal chalcogenides, etc. [13,14,15,16,17,18,19,20,21]. Layered double hydroxides (LDHs) have shown significant developments in electrochemical performance due to their low cost, ease of synthesis, eco-friendliness, high conductivity, high surface hydroxyl groups, high anion-exchange capacity, high electrochemical activity, high cycle stability, high resistance to alkaline environments, etc. [12,22,23]. Bimetallic-type hydroxides have demonstrated exceptional performance as electrode materials for SCs in comparison to single-type metal hydroxides. This advantage is attributed to their high theoretical C_s, high redox states, and high interlayer charge transfer. In comparison to other electrode materials, such as MOFs, metal oxides, and carbons, LDHs achieve a higher C_s (>3000 Fg⁻¹) as a result of their metal hydroxide structure with interlayer hydrated anions [24]. Generally, positively charged LDHs are formed by the replacement of divalent ions in hydroxide compounds with trivalent ions, and they contain metal ions, anion charge, and water molecules in their two-dimensional structure. The structural integrity of LDHs is guaranteed by the charge balance, which combines divalent and trivalent cations, rendering them adaptable to a wide range of applications. These metal ion layers can facilitate an abundance of active centers, which in turn enables quicker ion exchange and high-energy storage, as a result of their distinctive layered structures. Furthermore, LDHs are nanostructured layered materials that provide larger surface areas and a higher proportion of electroactive sites for charge–storage, making them favorable for redox reactions.

LDHs are characterized by edge-sharing hydroxyl octahedra with divalent and trivalent cations at their centers. The positive charge of LDHs, which results from partial cation replacement, is stabilized by intercalated anions between layers. Figure 1d depicts an isometric view of the LDH structure with a C-axis perspective. As a result, LDH structures are edge-sharing of hydroxyl ions, with divalent (M²⁺) and trivalent cations (M³⁺) at the center, whereas the anions in the interlayer arrangements could facilitate the transport of charged moieties during the cycle process, strengthening it compared to common ion transport channels [12]. LDHs such as NiCo, NiAl, NiMn, and CoAl are often used as electrode materials for SCs [25,26]. Their large surface area may increase electrode–electrolyte interactions, accelerating charge–storage and consequently enhancing electrochemical performance. Selecting an interlayer anion, such as monovalent, divalent, halo complexes, or metalate anions, may have a considerable impact on device performance by influencing ion intercalation, ion accessibility, and structural stability. Electrochemists enhance these parameters to increase electrochemical performance and develop next-generation LDH-based SC devices with higher charge–storage capacities. Recently, modified LDHs have shown high electrochemical performance when compared to bare LDH. Accordingly, hydrothermally synthesized flower-structured glucose-modified NiCo-LDH had a C_s of 959.0 Fg⁻¹ at 1.0 Ag⁻¹ when compared to NiCo-LDH (77.0 Fg⁻¹) [27]. Therefore, glucose could assist the LDH in improving electrochemical performance through enhancing the surface area (18.8 m²·g⁻¹), controlling the morphology (sheet to flower), and low resistance (0.58 Ω). Fluorine doping enhances the electrochemical performance of LDH through the formation of oxygen vacancies and defects in its structure, making it able to adsorb high electrolyte ions. Therefore, a one-step solution method was used to synthesize a highly porous sheet of fluorine-doped NiCo-LDH, which was shown to have a high C_s of 1988.0 Fg⁻¹ at 1.0 Ag⁻¹ when compared to NiCo-LDH 886.0 Fg⁻¹ [28]. The high performance of this LDH could be due to the doping of fluorine, which enhances the surface area (230.0 m²·g⁻¹), lowers the resistance (0.4 Ω), and develops the porosity. The hydrophilic properties of phosphate (PO₄³⁻) can facilitate the high transfer of ions through a hydrogen bond and substantially activate the metal center for redox behavior, thereby achieving high electrochemical performance [29]. Consequently, the yolk–shell-structured phosphate doped with CoNi-LDH, which was derived from Co/Ni MOF through an anion-mediated etching approach, was recently found to possess a high surface area (306.1 m²·g⁻¹) and a high C_s of 1854.6 Fg⁻¹ at 1.0 Ag⁻¹ [30]. Certainly, the phosphate could induce the hydrophilicity and porosity of the LDHs to facilitate the high adsorption of electrolyte ions, thereby achieving high-energy storage. CuCo₂O₄ is a bimetallic oxide with outstanding electrochemical performance, high conductivity, and redox reactivity due to its various oxidation states [31,32]. Therefore, CuCo₂O₄ was electrodeposited with NiCo-LDH for 500 s, resulting in the high-porosity composite CuCo₂O₄/NiCo-LDH, with an extraordinary C_s of 4824.3 Fg⁻¹ at 1.0 Ag⁻¹ [33]. In contrast, the CuCo₂O₄/Ni foam and NiCo-LDH synthesized through solvothermal and electrodeposition methods exhibited C_s values of 253.9 Fg⁻¹ and 1444.9 Fg⁻¹, respectively. Thus, the synergistic effect could be of paramount importance when contrasted with the materials acting independently. Despite the composite’s low surface area (34.4 m²·g⁻¹), its low resistance (0.6 Ω) could significantly contribute to its improved electrochemical performance. In another study, the hydrothermally synthesized MnO₂-modified NiCo-LDH/Ni foam achieved a high C_s of 2573.0 at 1.0 Ag⁻¹ in comparison to NiCo-LDH/Ni foam (482.2 Fg⁻¹) and MnO₂/Ni foam (980.1 Fg⁻¹) [34]. Clearly, the significant electrochemical properties of MnO₂ could improve the electrochemical efficiency of the NiCo-LDH. The composite form of LDH was also found to be more potent than the bare form, as observed by Tai et al. [35]. Compared to NiMn-LDH/Ni foam (502.0 Fg⁻¹), the hydrothermally synthesized Te-doped NiMn-LDH/CTAB-MXene/Ni foam demonstrated a high C_s of 1920.0 Fg⁻¹ at a current density (CD) of 2.0 Ag⁻¹. The composite electrode’s (0.67 Ω) superior performance may be attributed to its lower resistance compared to the bare NiMn-LDH/Ni foam (8.53 Ω). Similarly, hydrothermally produced N-doped mesoporous carbon sphere-modified CoNi-LDH had a C_s value of 2244.4 Fg⁻¹ at 1.0 Ag⁻¹, which could be attributed to the large surface area (155.2 m²·g⁻¹), and N-doped mesoporous carbon improved the LDH’s conductivity for high ion/electron transfer [36]. A mild solvothermal approach was employed to generate porous hexagonal-structured polyaniline-modified NiCo-LDH, which attained a C_s of 1248.0 Fg⁻¹ at 1.0 Ag⁻¹, while bare LDH had a C_s of 711.0 Fg⁻¹ [37]. As a result, a few examples from the literature show that the LDH of electrochemical performance may be increased through surface modification or composites. Accordingly, it is crucial to identify and critically evaluate the electrochemical performance of recently synthesized new LDHs, as well as bare and surface-modified LDHs for SCs. Figure 2 shows the current review summary of bare and modified LDH-based electrode materials for supercapacitor applications.

There are recently reported reviews on supercapacitor applications using asymmetric device-based LDHs, various composite-based LDHs, crystallographic phase-dependent LDHs, anion-intercalated LDHs, NiAl-LDH-based electrode materials, graphene-LDH composites, LDHs derived from MOFs, and NiCo-LDH and graphene-based composites [12,18,24,38,39,40,41,42,43,44]. These reports discuss the following: the electrochemical performance of LDHs based on synthesis methods; bare and LDH composite structures derived from MOFs; LDHs/graphene-based composites; anion-based LDHs for supercapacitors based on chemical modification; and LDHs for supercapacitors based on crystallography. However, these reported reviews are out of date or do not take into account the most recent bare and doped LDHs for supercapacitors. As a result, in this review, the electrochemical performance of recent investigations of newly invented LDHs and studies on the comparison of bare and modified LDHs were thoroughly reviewed, taking into account their morphologies, surface area, charge transfer resistance, and device performance. The literature offered an abundance of studies on new LDHs for SCs; hence, this review was structured according to the type of LDH, followed by strategy-based LDH modifications for SC applications. As a result, readers interested in LDH-based electrochemical energy storage could find this review useful.

2. Recent Advances in the Synthesis of Various LDHs for SCs

LDHs are pseudocapacitive materials with synergistic architectures that may improve electrochemical performance, owing to their distinct morphologies, decent electrical conductivity, and high oxidation states. Furthermore, the tunable metal cation configurations in their structures may allow for multiple redox states, which significantly enhances electrochemical performance. LDHs’ high interlayer space facilitates quick ion transport from the bulk solution to the electrode surface, hence accelerating charging and discharging. However, the electrochemical performance of LDHs is dependent on the combination of metal cations. As a result, this section discusses the newly synthesized various kinds of LDHs and their electrochemical performance for SC applications. Accordingly, Figure 3 depicts an overview of recently invented LDHs, including the advantages and disadvantages, as well as their suitability for electrochemical investigations.

2.1. CoCe-LDH

Recently, the urea hydrolysis approach was utilized to synthesize CoCe-LDH, which exhibits a combination of sheet and spherical morphologies that produced a larger surface area of 95.4 m²·g⁻¹ [45]. The charge transfer resistance of CoCe-LDH (13.3 Ω) was found to be low compared to single-structured Co-hydroxide (36.3 Ω) and Ce-hydroxide (1560 Ω) materials. Therefore, when single hydroxides are synergistically combined, their electrical structures may be tweaked. As a result, CoCe-LDH may achieve high conductivity and adsorb more ions. The high capacitance retention value of 96.93% after 20,000 cycles implies that this LDH has high structural stability, which might be due to the blend of sheet and spherical particles. The developed symmetric micro-supercapacitor CoCe-LDH//CoCe-LDH also showed improved cycle stability, with 88.73% capacitance retention after 10,000 cycles. Consequently, the large surface area and high structural properties of CoCe-LDH-based electrode materials make them suitable for long-term SC applications.

2.2. CoV-LDH

The electrochemical property of the CoV-LDH was studied through the influence of ratios (1:1 and 2:1) and temperatures (120 and 180 °C) over a 12 h hydrothermal process [46]. Originally, the mono-hydroxides Co(OH)₂ and V(OH)₂ were considered structured nanospheres and nanorods. However, at 120 °C, changing the ratios (1:1 for nanosheets and 2:1 for nanorods) had an effect on the morphology. However, changing the ratios of LDH precursors did not change the morphology of the microspheres at higher temperatures (180 °C). The 1:1 ratio CoV-LDH (e.g., CVL1-120) made at 120 °C had a higher CV total area and longer charge–discharge time than others because it had a nanosheet structure and lower charge transfer resistance. Due to the low surface area (15.7 m²·g⁻¹), it acquired a modest C_s value of 314.4 Fg⁻¹ at 1.0 Ag⁻¹, and after 5000 cycles, it was found to have higher capacitance retention (97%), which could mean it is more structurally stable. Additionally, the asymmetric CVL1-120//AC device had a satisfactory E_d value of 36.5 Whkg⁻¹ and a P_d value of 1208.2 Wkg⁻¹. However, no cyclic performance testing was conducted on this device for long-term use. Therefore, the nanosheet-shaped CoV-LDH might possess weaker capacitive properties due to its small surface area. However, because of their high energy density, they are durable and may be beneficial for long-term energy storage.

2.3. CoFe-LDH

Electrode materials’ morphology may exert an important effect on electrochemical performance. Therefore, the nanosphere type of CoFe-LDH-coated nickel foam (CoFe-LDH) was produced via the hydrothermal approach. This LDH shape was then transformed to a microflower type by activating cyclic voltammetry (CV) by ranges of cycles from 100 to 2000 circles at a scan rate of 50 mVs⁻¹ [47]. For example, Figure 4a depicts the hydrothermal formation of CoFe-LDH nanospheres, followed by CV activation over multiple cycles to produce microflower-structured CoFe LDH-x. CoFe’s shape gradually changed from spherical to flower-shaped as the CV cycles increased. However, the morphology disintegrated altogether at higher cycles. Evidently, Figure 4b shows SEM images of CoFe-LDH and activated CoFe-LDH at different cycles (100–2000) at a scale of 10 and 1 µm. This activation process controls not only the morphology but also the electronic structure of the LDH, transforming its native crystalline structure into a polycrystalline structure that is highly beneficial for electron transport and hence improving electrochemical performance. Increased cycles resulted in morphological alterations that vary from nanosphere to nanoflower. However, after 2000 cycles, increased electrochemical activation affects morphology, which may have an inconsequential influence in electrochemical studies. Because of the faradaic reaction, both electrodes showed pseudocapacitance behavior, and the CV integral area of 200 cycles activated LDH microflower (CoFe-LDH-200) was larger than that of the CoFe-LDH nanosphere (Figure 4c). Figure 4d depicts charge–discharge times for both electrode materials. To this degree, significant charge–discharge time was found for CoFe-LDH-200, resulting in a high C_s of 2222 Fg⁻¹ at a current density (C_d) of 1.0 Ag⁻¹. Interestingly, as demonstrated in Figure 4e, both electrode materials maintained adequate capacitance retention even after 10,000 cycles; however, CoFe-LDH-200 (133.8%) is more stable than CoFe-LDH (110%) nanosphere. In contrast to CoFe-LDH, the CoFe-LDH-200 performs better, perhaps due to higher active sites and flower structure, which absorb a large number of ions while being structurally stable across cycles. Consequently, electrochemical activation may result in the LDH exhibiting increased conductivity, structural stability, and high capacitive properties.

2.4. FeCo-LDH

Similarly, by adjusting the hydrothermal reaction time (8, 10, 12, and 14 h) at 120 °C to produce FeCo-LDHs-coated nickel foam, namely FC1, FC2, FC3, and FC4, based on the reaction periods, they achieved dense and interconnected nanoflake structures [48]. As a result, in this study, the response time had no effect on the morphological structure of LDHs. Interestingly, the surface area (41.2 to 84.7 m²·g⁻¹) grew with an increasing reaction time (8 to 12 h); however, further extending the time (14 h) that lowered the surface area (71.0 m²·g⁻¹) might be attributed to structural development over time that hinders the pores. FC3 has a larger surface area and lower charge transfer resistance (3.52 Ω), resulting in a higher C_s (1448 Fg⁻¹) at 20 mAcm⁻² compared to other electrode materials. Additionally, FC3 has a high capacitance retention rate (87%) after 5000 cycles, indicating its outstanding durability. Furthermore, the asymmetric device FC3//AC-NF achieved a high E_d value of 157 Whkg⁻¹ at a P_d value of 4210 Wkg⁻¹, as well as high cycle stability (81% capacitance retention over 5000 cycles), which could be attributed to the densely interconnected nanoflake structure, which provides a high response to electrolyte ions with minor volume changes and faster ion diffusion and redox reaction, resulting in improved electrochemical performance.

2.5. ZnCo-LDH

Additional active sites, such as oxygen vacancies, may enhance the conductivity and adsorb high hydroxyl anions on the LDH surfaces, thereby modifying the electronic structure and promoting rapid electron/ion transfer and redox kinetics for high electrochemical performance. Thus, the one-step hydrothermal process produces oxygen vacancy-rich ZnCo-LDH on carbon cloth (Ov-ZnCo-LDH/CC) for electrochemical studies, which exhibits a nanosheet structure [49]. To develop oxygen vacancies on LDH surfaces, 2-methylimidazole (2-MI) was utilized. The increasing 2-MI ratio influences the structure of the LDH, allowing the LDH nanosheet structure to cluster and collapse on CC. In contrast, increasing the 2-MI ratio (0.1 to 0.5 M) increased the surface area of the Ov-LDHs (56 to 113 m²·g⁻¹), but increasing the ratio from 0.7 to 1.0 M lowered the surface area from 73 to 37 m²·g⁻¹. However, this value is more than the as-prepared ZnCo-LDH/CC (36 m²·g⁻¹). Based on the larger surface area, the optimum ratio of 0.5 M 2-MI of the Ov-ZnCo-LDH3/CC (e.g., Ov-LDH(3)) was chosen for electrochemical investigations. As a result, the pseudocapacitive behavior of Ov-LDH(3) demonstrates a high specific capacity of 798.9 Cg⁻¹ at 1.0 Ag⁻¹ and 84.2% capacitance retention over 5000 cycles, which might be attributed to increased surface area, strong electron transfer, faster redox reaction, and superior structural stability. The asymmetric device demonstrated significant E_d (57.9 Whkg⁻¹) and P_d (849.4 Wkg⁻¹) values, as well as adequate capacitance retention (93%), across 10,000 cycles. The Ov-ZnCo-LDH//AC device may serve as a valuable electrochemical device for SC applications. As a result, the LDH with oxygen vacancy has higher electrochemical characteristics than as-prepared LDHs.

2.6. NiFe-LDH

Recently, reactants such as urea and thiourea were employed to produce carbonate ions (e.g., NFC) and sulfate ions (e.g., NFS) intercalated NiFe-LDH, respectively, using the hydrothermal technique [50]. The hexagonal plate particles in the NFC that was generated are highly agglomerated, whereas the hexagonal NFS plates did not exhibit significant agglomeration. Consequently, the reactants may be crucial for particle agglomeration. The specific capacity of NFS (149 Cg⁻¹) was significantly higher than that of NFC (46.0 Cg⁻¹) at 1.0 Ag⁻¹, as a result of the larger interface layer that receives high –OH ions. The reason for this high value is that NFS (16.4 Ω) has lower charge transfer resistance than NFC (28.4 Ω). Although the surface area of NFS (50 m²·g⁻¹) was less than that of NFC (91 m²·g⁻¹), the NFS exhibited superior performance. Consequently, the primary function of surface area in electrode materials for activity may not be as significant. In general, the intercalation of sulfate ions into the LDH structure has the potential to serve as an alternative method for energy storage in comparison to carbonate ions. The asymmetric device (NFS//AC) that was then developed exhibited pseudocapacitive behavior; however, it was capable of producing a low E_d value of 11 Whkg⁻¹ at a P_d of 5500 Wkg⁻¹. The cycle stability of the device was improved, resulting in a retention of 83% capacitance over 10,000 cycles, as a result of the combination of NFS and AC electrode materials. Additionally, this device was capable of powering a tiny portable fan, which may be suitable for practical applications. Another study investigated the effect of hydrothermal time (6, 8, 12, 16, and 24 h) on the synthesis of NiFe-LDH for electrochemical studies [51]. Figure 5a depicts a schematic representation for the hydrothermal process of NiFe LDH with various intervals (8 to 24 h), as well as SEM photos of the morphological growth of LDH at each time point. It was shown that increasing the hydrothermal reaction time between 12 and 24 h was advantageous for particle/morphological development for LDH, resulting in porous nanoflowers for the 24 h procedure, while no significant morphology was identified below 12 h. According to the CV and GCD profiles, 24 h processed LDH (NiFe-LDH-24 h) has a larger integral area and a higher charge–discharge time than lower time processed electrode materials (Figure 5b,c). As a result, among the LDHs, NiFe-LDH-24 h attained C_s values of 635.8 Fg⁻¹ at 1.0 Ag⁻¹, owing to its very low charge transfer resistance (5.0 × 10⁻⁵ Ω). However, this improved LDH may be structurally weak, or the electrode binder impedes ion transport at the electrode–electrolyte interface throughout the cycles, as seen in poor cycle stability (58.1% capacitance retention after 500 cycles). Overall, the NiFe-LDH-24 h exhibits poor electrochemical performance.

Similarly, hydrothermally produced NiFe-LDH was modified to have different molar ratios and examined for electrochemical performance [52]. The optimal 5:1 ratio of Ni and Fe to produce Ni₅Fe₁-LDH was determined to combine both spherical and sheet structures, as illustrated in Figure 5d. Depending on the molar ratio of Ni and Fe, the sheets or particles may be intensely produced. Regarding its performance, Ni₅Fe₁-LDH exhibited a high CV integral area that increased with scan rates and a long charge–discharge time that increased with current densities, which may indicate a high specific capacity and high-rate capability (Figure 5e,f). The highest specific capacity value of 114.40 mAhg⁻¹ was achieved at a C_d of 1.0 Ag⁻¹ due to the high surface area (136.9 m²·g⁻¹) that may be beneficial for adsorbing more ions. Additionally, the high specific capacity may be attributed to the fact that the charge transfer resistance (0.88 Ω) of this Ni₅Fe₁-LDH was lower than that of other NiFe-LDH ratios. The cycle stability profile of Ni₅Fe₁-LDH with respect to capacitance retention is depicted in Figure 5g. The only observed value over 3000 cycles is 66.3%, which may be attributed to its structural instability. Additionally, the asymmetric device (Ni₅Fe₁-LDH//AC) was capable of delivering moderate E_d and P_d of 35.8 Whkg⁻¹ and 800 Wkg⁻¹, respectively, which allowed it to achieve a high cycle stability of 99.7% over 3000 cycles. Based on the three-electrode findings, the NiFe-LDH requires additional development in synthesis to generate substantial morphologies, high functional groups, and active sites that achieve high performance.

2.7. NiCu-LDH

A simple co-precipitation at 0 °C was used to produce NiCu-LDH at a 4:1 ratio (L-Ni₄Cu₁-LDH), revealing a layered sheet structure [53]. Because of the use of alkaline NaOH, the co-precipitation process often does not produce significant morphology. However, the alkaline nature of the ammonia solution used in precipitation might result in considerable morphology of the LDHs. At 1.0 Ag⁻¹, the pseudocapacitive LDH achieved a high C_s value of 1656.9 Fg⁻¹, which might be attributed to the layered sheet’s high active sites and surface area, allowing for quick redox reactions and low charge transfer resistance. The developed Ni₄Cu₁LDH//AC device could offer E_d and P_d values of 45.4 Whkg⁻¹ and 800.0 Wkg⁻¹, respectively, and because of its asymmetric design, the device may be a very resilient structure in nature, demonstrating adequate capacitance retention (73.1%) over 10,000 cycles. Furthermore, by connecting these two devices in series, 88 red LEDs could glow for 6 min. Based on the findings, the L-Ni₄Cu₁-LDH shows enhanced electrochemical performance despite having no unique shape.

Figure 5. (a) Schematic illustration of the synthesis of different morphologies of NiFe-LDH via the hydrothermal technique at various times, and (b,c) CV and GCD graphs of NiFe-LDH synthesized at various times, reproduced with permission from [51], copyright 2024, John Wiley & Sons. (d) SEM image of Ni₅Fe₁-LDH, (e,f) CV and GCD graphs of Ni₅Fe₁-LDH at scan rates from 5 to 50 mVs⁻¹ and current densities ranging from 0.5 to 3.0 Ag⁻¹, respectively, and (g) cycle stability plot for Ni₅Fe₁-LDH, reproduced with permission from [52], copyright 2024, Elsevier. (h) Cyclic stability of the CuZn-LDH//AC asymmetric device and (i) two CuZn-LDH//AC devices connected in series are used to light a red LED bulb, reproduced with permission from [54], copyright 2024, Elsevier.

Figure 5. (a) Schematic illustration of the synthesis of different morphologies of NiFe-LDH via the hydrothermal technique at various times, and (b,c) CV and GCD graphs of NiFe-LDH synthesized at various times, reproduced with permission from [51], copyright 2024, John Wiley & Sons. (d) SEM image of Ni₅Fe₁-LDH, (e,f) CV and GCD graphs of Ni₅Fe₁-LDH at scan rates from 5 to 50 mVs⁻¹ and current densities ranging from 0.5 to 3.0 Ag⁻¹, respectively, and (g) cycle stability plot for Ni₅Fe₁-LDH, reproduced with permission from [52], copyright 2024, Elsevier. (h) Cyclic stability of the CuZn-LDH//AC asymmetric device and (i) two CuZn-LDH//AC devices connected in series are used to light a red LED bulb, reproduced with permission from [54], copyright 2024, Elsevier.

2.8. CuZn-LDH

Similarly, the co-precipitation employed to synthesize the flower’s petal-structured CuZn-LDH was observed to exhibit pseudocapacitive behavior [54]. Despite the low charge transfer resistance (0.72 Ω), this LDH was able to achieve a low C_s (265 Fg⁻¹) at 1 mAcm⁻². Additionally, the CuZn-LDH was used to develop the asymmetric device (CuZn-LDH//AC), which exhibited a low E_d of 7.1 Whkg⁻¹ at P_d of 121.9 Wkg⁻¹ and a 75% capacitance retention after 1000 cycles, which may not be sufficient to confirm the device’s stability (Figure 5h). Furthermore, the device’s ability to power a red LED light for 2 min and 25 s was demonstrated through testing (Figure 5i). However, the co-precipitation method employed to synthesize LDH may not be suitable for electrochemical investigations due to its low surface area, lack of considerable morphology, low functional groups, low active sites, and structural weakness.

2.9. CuCo-LDH

A highly connected nanosheet array structure, as well as oxygen-rich vacancies in the CuCo LDH-coated carbon fiber (Ov-CuCo LDH/CF) obtained via NaBH₄ reductions shows a high C_s of 1392.4 Fg⁻¹ at 1.0 Ag⁻¹ compared to CF/Co-MOF (117.4 Fg⁻¹) and CF/CuCo LDH (1031.4 Fg⁻¹) [55]. Therefore, oxygen vacancies may improve the electrochemical performance of the LDH. However, to prepare Ov-LDH, CuCo-MOF must be coated on CF first, and then the oxygen vacancy LDH structure is produced via reduction. The Ov-CuCo LDH/CF demonstrated high-rate capabilities, with C_s remaining high (1290.5 Fg⁻¹) even after 10.0 Ag⁻¹. Furthermore, Figure 6a,b show the Ov-CuCo LDH/CF//AC asymmetric device setup and their cycle stability profile, as well as an inset photo of the device illuminating white and blue LED lights. The developed asymmetric device was observed to offer better E_d and P_d values of 58.2 Whkg⁻¹ and 850.0 Wkg⁻¹, respectively, and this device was able to maintain capacitance retention of 86.7% even after 10,000 cycles, which could be attributed to the excellent structural stability of the electrode materials and highly porous structures that adsorb higher electrolyte ions. The device was proven to light up 12 white and 12 blue LED lights for several minutes, indicating that it might be useful for practical applications.

In another work, the sonochemical approach utilized to produce CuCo-LDH with varied ratios (1:1 and 2:1) reveals diverse morphologies (nanowires (CC1) and rice-straw bundle (CC2)) [56]. Accordingly, Figure 6c illustrates SEM images of CC1 and CC2 with varying resolutions. It confirms that an increase in the Cu nitrate ratio changes the morphology of the LDHs. The CV of the CC2 electrode, which has larger areas (Figure 6d,e), a longer charge–discharge time (Figure 6f,g), and a low charge transfer resistance (1.38 × 10³ Ω), resulted in a high C_s of 50.0 Fg⁻¹ at 20.0 mVs⁻¹ in comparison to CC1 (28 Fg⁻¹ and 7.75 × 10⁷ Ω). However, these capacitance values are comparatively low compared to other reported electrode materials due to their high resistance. Over 500 cycles, CC2 demonstrated 103% capacitance retention and might be structurally resilient; however, cycle stability could have required more than 5000 cycles to validate the electrode’s original capacitance retention (Figure 6h). According to the findings, sonochemically produced CuCo-LDH (CC1 or CC2) exhibits poor electrochemical performance due to high resistance, a low surface area, and limited ion transfer to electrode surfaces, which may be ascribed to a sluggish faradaic redox reaction.

2.10. MgCo-LDH

Recently, MgCo-LDH (MCL) on Ni foam was synthesized via a vertical autoclave utilizing the hydrothermal technique and modifying the reaction time, such as to 10, 12, and 14 h, resulting in samples named MCL-1, MCL-2, and MCL-3 for electrochemical experiments [57]. The morphology of LDH growth is shown to be dependent on the reaction time, and the 14 h procedure yields flower forms of LDH on Ni foam. In contrast, the 10 and 12 h process times enable the LDH to be formed only on the Ni foam, with no specified shape. As a result, time may play an important role in the morphological development of LDHs. However, LDH treated for 12 h on Ni foam performed better electrochemically than the 10 and 14 h processes. As a result, MCL-2 demonstrated pseudocapacitive behavior with a higher C_s of 1431.2 Fg⁻¹ than MCL-1 (821.7 Fg⁻¹) and MCL-3 (829.9 Fg⁻¹) at 6.0 mAcm⁻². Figure 6i suggests that MCL-2’s longer charge–discharge time than others may account for its high C_s. MCL-2’s exceptional performance might be attributed to its lower charge transfer resistance (1.22 Ω) compared to others. The cycle stability reported to be 75% after 5000 cycles may constitute a modest performance, owing to the slightly structural weakness throughout long-term operation, and this study did not conduct any device investigations.

2.11. NiAl-LDH

Similarly, Ov-rich NiAl-LDH supported NF (e.g., NiAl-LDH-60) was synthesized through the hydrothermal technique and exhibited a specific capacity of 4028.0 mFcm⁻² at 2.0 mAcm⁻² [58]. This value was found to be greater than the standard NiAl-LDH/NF (1180 mFcm⁻²). Oxygen vacancy LDH has a high specific capacity due to low charge transfer resistance (0.68 Ω), allowing for more electrolyte ion adsorption than regular LDH (0.71 Ω). Additionally, NiAl-LDH-60 has a larger surface area (160.6 m²·g⁻¹) than standard NiAl-LDH/NF (142.6 m²·g⁻¹). Over 5000 cycles, NiAl-LDH-60 has a high-capacity retention of 85.9%, indicating strong structural stability, but NiAl-LDH/NF shows only 61.2%, suggesting structural weakness with cycles. The symmetric device (NiAl-LDH-60//NiAl-LDH-60) demonstrated a high E_d of 71.3 Whkg⁻¹ at a P_d of 2400 Wkg⁻¹. Despite undergoing over 5000 cycles, the device maintained a capacitance retention of 81.6%, which could be due to structural deterioration that occurred during the extended cycles. Therefore, the use of the same two-electrode materials may contribute to the moderate cycle stability of a symmetrically structured device.

NiAl-LDH was produced on indium tin oxide (ITO) glass using the hydrothermal technique, revealing pebble morphology, rather than lamellar, on the ITO, and the film achieved a high surface area (117.4 m²·g⁻¹) [59]. At 1.0 Ag⁻¹, the ion in the solution may have sluggish redox reactions and delayed adsorption, which led to a good C_s of 589.3 Fg⁻¹, because of its high solution resistance of 15 Ω. However, the exceptional capacitance retention of 95% over 5000 cycles suggests that this NiAl-LDH/ITO type may have high structural stability. Delivered high E_d and P_d values of 216.0 Whkg⁻¹ and 5200 Wkg⁻¹, respectively, were a result of the asymmetric device NiAl-LDH/ITO//AC/ITO’s exceptional cycle stability (91% capacitance retention over 10,000 cycles). Therefore, it seems that the asymmetric device and NiAl-LDH layer on ITO have beneficial electrochemical characteristics and could have real-world uses.

The electrochemical property of NiAl-LDH coated on carbon cloth (NiAl-LDH/CC) via electrodeposition at 1.0 V for 15 min reveals an unsmooth sheet morphology, but it achieved a high surface area (437.5 m²·g⁻¹), which may be beneficial for high ion adsorption [60]. The morphology was also found to be unaffected by the use of varying potentials via electrodeposition. As a result, the pseudocapacitive NiAl-LDH/CC achieved a high C_s of 5893.8 mFcm⁻² at 0.7 mAcm⁻² and a high cycle stability of 92% over 2000 cycles, indicating that it is structurally stable and responds quickly to contact with electrolyte ions over cycles. The asymmetric device NiAl-LDH/CC//CC was able to provide E_d and P_d values of 0.465 Whcm⁻³ and 8.23 Wcm⁻³, respectively, and this device also demonstrated substantial capacitance retention (91%) after 10,000 cycles, perhaps owing to the support of CC. As a result, electrodeposited NiAl-LDH on CC might be a highly active site, high surface area, and structurally stable electrometric material for long-term energy storage applications.

2.12. ZnAl-LDHs

Chemical bath deposition was employed to form a ZnAl-LDH nanosheet layer on Al foil for flexible SC applications [61]. The C_s was determined based on the bending angle of the flexible ZnAl-LDH/Al device from 0° to 180°, which demonstrates that increasing the angle of bending decreases the C_s from 596.5 to 209.2 Fg⁻¹ at 10.0 mVs⁻¹. As a result, the C_s of the electrode material is higher in the flat mode than in the bending/folding mode. The decrease in C_s might be attributed to impaired ion transport into electrode surfaces during the bending/folding mode. However, this flexible electrode device is structurally robust owing to excellent cycle stability, which was achieved in both bending and twisting modes at 87% and 91% over 2000 cycles, respectively. However, the device achieved low E_d and high P_d in all modes, including flat (6.4 Whkg⁻¹ at 46,000 Wkg⁻¹), bending (2.2–3.7 Whkg⁻¹ at 16,000–27,000 Wkg⁻¹), and twisting (6.3 Whkg⁻¹ at 45,000 Wkg⁻¹). Overall, this device performs moderately owing to its poor ion adsorption during bending and twisting modes.

2.13. NiMn-LDHs

Recently, a thick NiMn-LDH sheet was synthesized via co-precipitation for electrochemical experiments [62]. This LDH demonstrates pseudocapacitance behavior and acquired an excellent C_s value of 905 Fg⁻¹ at 1.0 Ag⁻¹ when compared to oxide-structured materials such as NiO (429 Fg⁻¹) and Mn₂O₃ (198 Fg⁻¹). Possible characteristics of NiMn-LDH include a high active site, low charge transfer resistance (0.4 Ω), porous architectures, large surface area, and rapid redox reactions on its surfaces. Thus, LDH-structured materials outperform single-type oxide materials in terms of electrochemical performance. Furthermore, over 5000 cycles, the high capacitance retention of 94.8% observed in this LDH may indicate excellent structural stability during prolonged cycle operation.

Another work examined the electrochemical performance of NiMn-LDH synthesized at various hydrothermal temperatures ranging from 80 °C to 140 °C [63]. The developed LDHs had three-dimensional flower structures and exhibited battery-like behavior. The rising synthesis temperature from 80 °C to 100 °C increased the C_s from 1124 to 1708 Fg⁻¹, but it reduced from 1485 to 1203 Fg⁻¹ at 1.0 Ag⁻¹ when the temperature increased from 120 °C to 140 °C. As a result, 100 °C (Ni₃Mn₁-T100) was determined to be the best hydrothermal temperature for improved performance. However, the C_s values of all temperature-based LDHs are still higher in this report. The LDH’s (Ni₃Mn₁-T100) high C_s may be attributed to their exceptionally low charge transfer resistance (0.56 Ω), which facilitates the passage of high electrolyte ions. The LDH structure may be highly stable, which may account for its high capacitance retention (72.7%) even after 10,000 cycles. The asymmetric device (Ni₃Mn₁-T100//rGO) exhibited high E_d and P_d values of 60.0 Whkg⁻¹ and 965.0 Wkg⁻¹, respectively, while the device’s capacitance retention (90.4%) remained high even after 10,000 cycles. Ni₃Mn₁-T100 LDH’s 3D structured flower shape, which has high active sites, very low resistance, and excellent structural stability, might be attributed to its superior electrochemical performance.

Similarly, NiMn-LDH was grown on Ni foam by adjusting their different ratios of LDHs precursor synthesized hydrothermally, showing that morphology changes from nanosheets to microflowers and microflowers to irregular micro pebbles were observed when decreasing the molar ratio of Ni with an increasing Mn ratio, and also, the surface area was decreased correspondingly from 23.4 to 2.4 m²·g⁻¹ [64]. As a result, the Ni precursor may have a broader impact on morphologies and the surface area than the Mn precursor. Among the ratios, the pseudocapacitve microflower-structured Ni₇₅Mn₂₅-LDH/NF (23.4 m²·g⁻¹) had a high C_s of 1000.8 Fg⁻¹ at 1.0 mAcm⁻² and may have moderate structural stability due to 72.7% capacitance retention found after 3000 cycles due to volume changes during cycles that affect ion adsorption. The optimized 75:25 NiMn-LDH on NF did not produce agglomeration, which may account for the high C_s value.

Via electrodeposition, NiMn LDH was deposited on porous Ni-coated nickel foam substrates (e.g., NNM-2/NF) to obtain a high C_s of 2755.6 Fg⁻¹ at 1.0 Ag⁻¹, in contrast to Ni-coated NF (124.3 Fg⁻¹) and NiMn-coated NF (992.1 Fg⁻¹) [65]. The schematic diagram in Figure 7a illustrates the electrodeposition process of porous Ni on NF using a hydrogen bubble strategy, as well as the NiMn-LDH coating on Ni/NF by electrodeposition. Meanwhile, Figure 7b illustrates the Ni/NF and NNM-2/NF morphologies that have been generated at varying scales. The Ni/NF exhibited flower-shaped Ni particles on the porous NF, while the NiMn LDH was covered on the Ni/NF with minor morphological changes, but the flower structure remained consistent. The higher C_s of NNM-2/NFs was a result of the porous Ni, which enhanced the specific surface area of the NNM-2/NF electrode. Consequently, the increased number of electroactive sites facilitated the rapid transport of ions and electrons, resulting in more redox reactions. Despite the increase in C_d to 20 Ag⁻¹, the NNM-2/NF maintained a high level of C_s, achieving 2135.2 Fg⁻¹. The electrode’s exceptional electrochemical performance may be the reason for its remarkably low charge transfer resistance (0.3 Ω). NNM-2/NF demonstrated exceptional cycle stability, achieving a capacitance retention of 71.2% after 5000 cycles. The schematic configuration of the asymmetric device, NNM-2/NF//AC, is illustrated in Figure 7c. This device was able to deliver a moderate E_d and higher P_d values of 41.7 Whkg⁻¹ and 2700 Wkg⁻¹, respectively. The capacitance retention was significantly higher (109.8%) after 5000 cycles, as demonstrated in Figure 7d, as a result of the robust asymmetric electrode structures. Additionally, the electrolyte maintained sufficient contact with the electrode surfaces as the cycle time increased, and no volume changes occurred. Interestingly, this device’s cycle stability was superior to that of the NM/NF//AC device (73.4%). Furthermore, the NNM-2/NF//AC device was demonstrated to be appropriate for practical use after being assessed with a lighted red LED bulb for several minutes, as shown in Figure 7e.

NiMn-LDH grown on Ni foil (NiMn-LDH/Ni foil) using the reflux condensation process exhibits a pebble-like morphology without agglomeration, exhibiting pseudocapacitive behavior with a low charge transfer resistance (0.6 Ω) and a large surface area (127.9 m²·g⁻¹) [66]. It was shown to have a high cycle stability of 92.8% capacitance retention over 3000 cycles, with a moderate C_s value of 416.0 Fg⁻¹ at a C_d of 4.0 mAcm⁻². Because of its better performance, the solid-state asymmetric device NiMn-LDH/Ni foil//AC/Ni foil was built and found to offer low E_d and P_d values of 10.0 Whkg⁻¹ and 900.0 Wkg⁻¹, respectively, while achieving high cycle stability over 4000 cycles and 99.4% capacitance retention. Furthermore, the same configuration was utilized to build liquid-state asymmetric devices, which provided low E_d (4.3 Whkg⁻¹) at a P_d of 900.0 Wkg⁻¹ while maintaining high cycle stability (97.1% over 4000 cycles). It was found that solid-state asymmetric devices performed better than liquid-state asymmetric devices. However, these devices may exhibit high stability during extended cycle tests, rather than producing high density values; hence, additional study is required to increase their E_d and P_d values.

2.14. CoMn-LDH

The as-prepared CoMn-LDH on nickel foam (CoMn-LDH/NF) via electrodeposition exhibits a lamellar structure, which results in a superior electrochemical performance compared to the ratio-adjusted Co:Mn (1:1 and 1:2) during the synthesis of CoMn-LDH/NF [67]. Therefore, changing the ratio in LDHs does not increase electrochemical performance, and accordingly, CoMn-LDH had a larger area capacitance of 10,955.5 mFcm⁻² and a better capacitance retention rate (83.6%) over 7000 cycles compared to the LDHs with adjusted ratios. The outstanding performance of as-prepared CoMn-LDH/NF might be attributed to its large surface area, low resistance, high active sites, rapid redox reaction, and excellent structural stability. The developed CoMn-LDH//AC device has an E_d of 0.409 mWhcm⁻² at P_d of 0.798 mWcm⁻², and because of the device’s asymmetric construction, excellent capacitance retention (85.6%) was found after 20,000 cycles. Furthermore, a red LED was lit by connecting two devices in series, which may be useful in practical applications. As a result, the electrodeposited CoMn-LDH may be structurally stable throughout prolonged cycle experiments, making it advantageous for SC applications. The hydrothermally produced solid porous nanosphere-shaped CoMn-LDH at 120 °C for 6 h (CoMn-LDH-6) exhibited limited performance, achieving a capacitance value (C_s) of 53.5 Fg⁻¹ at a current density of 0.67 Ag⁻¹, although possessing a low charge transfer resistance of 0.56 Ω [68]. Additionally, over 1000 cycles were found with low capacitance retention observed at 56.2%, which could be due to high volume changes; therefore, it involves severe structural changes. Accordingly, CoMn-LDH produced via electrochemical or hydrothermal methods necessitates additional procedures to improve its capacitance and stability characteristics.

2.15. NiCo-LDHs

NiCo-LDHs are promising electrode materials for energy storage applications. However, their challenges, such as tendencies for agglomeration impacting ion transport, moderate electrical conductivity and cycle stability, and reduced energy density, have limited their practical use. Thus, recent efforts have focused on developing NiCo-LDH-based electrode materials to mitigate such difficulties. The impact of modifying the ratios of water and N,N-Dimethylformamide (DMF) (2:1 (NCLDH-21), 1:2 (NCLDH-12), and 1:1 (NCLDH-11)) was conducted on the synthesis of NiCo-LDH using the solvothermal technique for electrochemical investigations [69]. Consequently, thin nanosheets were identified in NCLDH-21 and NCLDH-11, whereas agglomerated thick sheets were noted in NCLDH-12. Therefore, an excess of DMF may increase the agglomeration and thickness of the LDH sheet in comparison to water. The EDLC performance of these LDHs yielded favorable C_s values at 1.0 A g⁻¹; specifically, NCLDH-21 (2054 Fg⁻¹) demonstrated superior C_s compared to NCLDH-11 (1590 Fg⁻¹) and NCLDH-12 (1259 Fg⁻¹). According to the morphologies, the thin LDHs were acquired with high C_s values. Additionally, the C_s values of NCLDH-21 and NCLDH-11 may be attributed to their higher surface areas (47 and 38 m²·g⁻¹) and low resistances (0.96 Ω and 1.14 Ω), as well as their thin nanosheets, in contrast to NCLDH-12, which exhibits a lower surface area (31 m²·g⁻¹) and higher resistance (1.29 Ω). All NCLDH-based electrode materials exhibit significant structural stability, achieving high cycle stability over 5000 cycles and demonstrating capacitance retention between 84.7% and 93.6%. Following the optimal performance of NCLDH-21, the asymmetric device NCLDH-21//AC was developed, achieving a remarkable E_d value of 67.6 Whkg⁻¹ at a P_d of 839.9 Wkg⁻¹, with exceptional cycle stability with a capacitance retention of 106.7% over 20,000 cycles. For commercial applications, three NCLDH-21//AC devices were linked in series to light the high-powered LED bulb. Overall, the solvothermal-produced NCLDH-based electrode materials and developed device exhibit exceptional performance in electrochemical experiments and may be suitable for practical use.

Similarly, ethanol and water ratios were adjusted to synthesize NiCo-LDH on nickel foam hydrothermally at 180 °C, showing that smaller amounts of ethanol and higher amounts of water ratios obtained microflower-structured morphologies, whereas higher amounts of ethanol and lower amounts of water ratios deformed the flower structures [70]. Figure 8a depicts a schematic representation of the synthesis of NiCo-LDH-18 on nickel foam using a hydrothermal method for electrochemical testing. It was found that raising the ethanol ratio while reducing the water ratios may enhance the surface area of LDHs; however, employing a larger ethanol ratio reduces surface area and collapses the morphology. As a result, Figure 8b indicates SEM images of NiCo-LDH morphologies produced at different water and ethanol ratios, demonstrating that the morphology collapses as the ethanol ratio increases. The optimal NiCo-LDH-18/NF achieved the highest surface area (372.9 m²·g⁻¹) due to the ethanol (18):water (12) ratio utilized. Based on surface area, NiCo-LDH-18/NF had higher C_s (2266.6 Fg⁻¹) at 1.0 Ag⁻¹ than other ethanol and water ratios utilized with NiCo-LDH, and excellent cycle stability was identified, with 97.4% after 10,000 cycles, indicating excellent structural stability (Figure 8c). The developed device NiCo-LDH-18//AC demonstrated higher E_d and P_d values of 104.8 Whkg⁻¹ and 3118.0 Wkg⁻¹, respectively, which might be a promising device for energy storage. Additionally, 93.1% capacitance retention was recorded across 40,000 cycles, indicating that the electrode materials are very resilient. As shown in Figure 8d, the device’s cycle stability was maintained for up to 20,000 cycles before progressively decreasing to 40,000 cycles. As a result, an ethanol ratio higher than the water used to synthesize NiCo-LDH on NF may have a higher surface area, high structural stability, high active sites, a specific flower morphology leading to more ion adsorption, a faster redox reaction attributed to high C_s, and high cycle stability due to the robust structure.

The influence of alkaline sources such as NaOH, urea, and an ammonia solution was employed to synthesize NiCo-LDH utilizing the hydrothermal technique [71]. When urea (LDH-U) and ammonia (LDH-A) were employed to synthesize LDH, they generated substantial morphology, such as flowers and nanosheets, as compared to NaOH (LDH-N), which supplied an uneven form. LDH-A’s larger CV area may result in better performance than other electrodes. As a result, the influence of ammonia solution in the NiCo-LDH was investigated for electrochemical performance; consequently, 0.3 mL of ammonia added in the LDH synthesis (e.g., LDH-A0.3) exhibits the highest C_s value of 2237.5 Fg⁻¹ at 1.0 Ag⁻¹ when compared to other larger ammonia doses added to LDHs. Therefore, increasing the amount of ammonia caused the C_s to drop. There were no cycle or device studies in this report. Recent research has investigated the impact of hydrogen peroxide (H₂O₂) (0 to 125 µL) on the hydrothermal synthesis of interconnected nanosheet NiCo-LDH-coated nickel foam [72]. This H₂O₂ has the potential to facilitate the generation of additional oxygen vacancies on the LDH, which could result in enhanced electrochemical performance through high ion adsorption. Of these, the 100 µL H₂O₂ added LDH (NiCo-LDH/NF-100) exhibited a high C_s value of 1156.0 Fg⁻¹ at 1.0 Ag⁻¹, as well as a high-rate capability (837 Fg⁻¹) even at 10.0 Ag⁻¹. Additionally, it exhibited longer charge–discharge times. Conversely, the LDH exhibited a C_s value of 628.0 Fg⁻¹ at 1.0 Ag⁻¹ in the absence of H₂O₂. The LDH observed 95.3% capacitance retention over 5000 cycles, which may be attributed to the rapid redox reaction on the surface and the high structural stability. Additionally, the NiCo-LDH/NF-100//AC device that was produced was capable of achieving a high cycle stability of 91.56%. Consequently, H₂O₂ may be a critical factor in the adsorption of more ions on the surface of LDH, thereby enhancing its electrochemical performance. In another investigation, the hydrothermal method was employed to produce nanofibrous NiCo-LDH on NF (NiCo-LDH-140) at an optimized temperature (140 °C), time (6 h), and ratio (2:1) [73]. The optimized NiCo-LDH had a higher specific capacity of 400.2 Cg⁻¹ at 1.0 Ag⁻¹ compared to other NiCo-LDH materials that had been conditioned. The shape of LDH on CC may change when the hydrothermal temperature is higher (>140 °C), the reaction time is longer (>6 h), and the ratio is higher (>2:1). This could be because low ion/electron transfer and a slow redox reaction led to poor electrochemical performance. It was found that the asymmetric device NiCo-LDH-140//AC had modest E_d (51.59 μWhcm⁻²) and P_d (1.125 mWcm⁻²) values, as well as adequate cycle stability over 10,000 cycles, which led to a 70% retention of capacitance. However, the cycle stability of this asymmetric device may be subpar in comparison to other asymmetric device-based LDHs.

The electrochemical activities of NiCo-LDH-coated nickel foam, which was activated with KOH at different concentrations (1, 2, and 4 M), as well as that which was not activated (e.g., NiCo-LDH-0), were recently investigated [74]. LDH was synthesized hydrothermally prior to electrochemical activation. Accordingly, Figure 9a shows a simple diagram of NiCo-LDH before (NiCo-LDH-0) and after electrochemical activation (NiCo-LDH-2) using 10.0 CV cycles in 2.0 M KOH. This figure shows that after electrochemical activation, the LDH’s shape altered, its sheet reduced in size, and they were densely packed together. In comparison to unactivated LDH, KOH-activated LDH exhibits better morphological growth and a reduction in LDH size. Interestingly, KOH activation resulted in the transformation of the sheet structure into a flower. As shown in Figure 9b, the LDH sheet was larger and thicker in 0 M KOH; however, it was decreased in size and tightly packed together to form a flowery structure when the concentration was increased from 1 M to 2 M. Yet, upon activation at 4 M KOH, the LDH sheet became much larger and thinner. As a result, electrochemical activation may positively modify the morphology of the electrode materials, making it a valuable approach for analyzing electrochemical performance. As evidenced by the flower morphology acquired through 2.0 M KOH activation, NiCo-LDH-2 exhibits a higher specific capacity (2890.0 Cg⁻¹) at 1.0 Ag⁻¹ than other KOH-activated LDHs. While NiCo-LDH-2 demonstrated considerable cycle stability with 72.7% capacitance retention over 3000 cycles, there may be minor volume variations throughout cycles. However, this cycle stability was higher than that of other NiCo-LDHs activated with 1.0 or 4.0 M KOH. The higher capacity might be attributed to the nano-sized LDH, which has more active sites, a quicker redox reaction, and better ion transport. The asymmetric device (NiCo-LDH-2//carbon) attained moderate E_d and P_d values of 36.0 Whkg⁻¹ and 732.0 Wkg⁻¹, respectively, and cycle stability was reported to be 78% even after 1000 cycles. Overall, KOH-activated LDH has modest electrochemical performance.

Similarly, electrodeposited NiCo-LDH on carbon cloth (e.g., P-LDH) was electrochemically activated via CV at 1.0 mVs⁻¹ (e.g., AS-LDH), 10.0 mVs⁻¹ (e.g., AM-LDH), and 100.0 mVs⁻¹ (e.g., AL-LDH) at 1 M KOH to yield hydrogen vacancy-rich LDHs for electrochemical measurement [75]. To this degree, all activated LDHs exhibit dense interconnected nanoflake structures, resulting in high-porosity LDHs; however, when the scan speed increased, the size of the flake decreased, and more interconnected flakes were detected (AL-LDH). As a result, higher hydrogen vacancy-richer AL-LDH had a much higher specific capacity (1001.0 Cg⁻¹) at 1.0 Ag⁻¹ than lower hydrogen vacancy-richer AS, AM-LDHs, and P-LDH. An Mg-based hybrid device was developed for practical application. This device was constructed using the AL-LDH cathode and VS₂ anode in an MgSO₄ electrolyte. It exhibited a high E_d of 48.4 Whkg⁻¹ at a P_d of 937.4 Wkg⁻¹. These devices, which are configured in electrochemical systems to illuminate the white and red LEDs in real time, have the potential to be used in commercial applications. However, the electrode material in three- and two-electrode systems does not exhibit significant cycle stability over 500 cycles. Accordingly, the specific capacity was found to be swiftly decreased as the cycle numbers increased, which could suggest high volume variations in the AL-LDH. Consequently, the specific capacity of this alkaline-activated LDH may be high through CV, rather than cycle performance. In another work, electrodeposition was utilized to construct binder-free NiCo-LDH film over a stainless steel (SS) mesh, and the morphology was found to be wrinkled thin nanosheets [76]. The NiCo-LDH/SS film exhibited battery-type behavior, with a high C_s value of 1406.7 Fg⁻¹ at high C_d (3.0 Ag⁻¹) and 83% capacitance retention over 5000 cycles. The high C_s and cycle stability may be attributed to a porous and robust structure, low charge transfer resistance (0.2 Ω), and a high surface area and many active sites for ion adsorption. Because of its high C_s and cycle stability, this flexible binder-free LDH-coated SS mesh may be suitable for flexible SC applications.

Similarly, NiCo-LDH grown on Ni foam through the hydrothermal technique with varying reaction times (6, 8, and 10 h) demonstrates that the 10 h procedure was sufficient for growing complete nanopetals on Ni foam [77]. Accordingly, Figure 10a illustrates a schematic representation of the growth of LDH petals over nickel foam under varying hydrothermal times. It was found that a reaction time of 10 h was sufficient to produce mature LDH petals, as well as a high degree of fine particles. The NiCo-LDH/NF nanopetals that have been processed for 10 h may possess a high surface area, a thin thickness, and a high number of active sites, which could enable them to adsorb a significant amount of electrolyte ions, thereby facilitating the storage of energy. It was determined that the morphological growth was facilitated by an increase in the process time from 6 h to 10 h, which resulted in a surface area increase from 30.9 to 56.1 m²·g⁻¹. The prepared LDHs exhibit battery-type behavior, with NiCo-LDH-10/NF (10 h reaction time) exhibiting a larger integral CV area and a high charge–discharge time in comparison to the 6 h and 8 h processed NiCo-LDHs/NF, as shown in Figure 10b. In contrast, the Ni foam exhibited a negligible CV area and charge–discharge time, which served as evidence of its subpar performance. As a result, the specific capacity of NiCo-LDH-10/NF was significantly higher (210.5 mAhg⁻¹) than that of NiCo-LDHs-6/NF and NiCo-LDHs-8/NF (181.2 and 127.9 mAhg⁻¹) at a C_d of 2.5 mAcm⁻². The structural stability of NiCo-LDH-10 may be high, as evidenced by the 82.3% capacitance retention that was observed over 5000 cycles. Figure 10c shows a schematic depiction of the Ni-Co-LDH-10//graphene asymmetric device setup and its charge–discharge function in KOH electrolyte. The developed Ni-Co-LDH-10//graphene device was capable of achieving a moderate E_d of 22.6 Whkg⁻¹ at a P_d of 169.9 Wkg⁻¹, and a capacitance retention of 86.8% over 10,000 cycles. Furthermore, in this report, the developed 3D-printed NiCo-LDH-10//graphene was substantially demonstrated as a humidity sensor and as a toy vehicle operator for real-time purposes, suggesting that it may be a viable option for commercial applications.

The hollow three-dimensional NiCo LDH nanocages were synthesized using chemical reduction and subsequent etching to minimize ion diffusion distance and mitigate structural changes [78]. The pseudocapacitive property of the NiCo LDH-1 nanocage was prepared using a 1:1 ratio, which resulted in nanocube morphologies (Figure 11a). At 1.0 Ag⁻¹, the C_s was recorded at 1671 Fg⁻¹, which was higher than that of other ratios, including common NiCo LDH and single hydroxide materials. The capacitance retention of the nanocage NiCo and common NiCo LDHs was 67.9% and 63.7%, respectively, after 10,000 cycles. These stability values indicate that structural stability remains a concern during cycles. Furthermore, the asymmetric device (NiCo LDH-1//AC) that was developed demonstrated superior E_d and P_d values of 59.0 Whkg⁻¹ and 935.7 Wkg⁻¹, respectively. In the measurement of stability, Figure 11b illustrates that 87.1% capacitance retention was achieved after 10,000 cycles. Furthermore, the red LED lights were illuminated for 12 min after the two connected NiCo LDH-1//AC devices were charged, which may be viable for practical use (Figure 11c). Another experiment investigated the electrochemical performance of NiCo-LDH (Ni-Co-CH-x) produced using Cetyltrimethylammonium Bromide (CTAB) and urea as reactants in a medium of ethylene glycol and water, varying the solvothermal temperature from 120 °C to 200 °C for 12 h [79]. The morphology was initially generated as nanorods at 180 °C, and it transitioned to microplates at 200 °C. Therefore, the temperature may be a critical factor in the control of the morphologies. The pseudocapacitive behavior was observed in all LDHs; however, the CV curve and charge–discharge time of the 180 °C processed LDH (e.g., NiCo-CH-180) were significantly larger than those of the other temperature-processed LDHs. The NiCo-CH-180 exhibited a C_s value of 762.0 Fg⁻¹ at 1.0 Ag⁻¹, which is higher than that of other materials. This characteristic is attributed to their low resistance and high surface area (52.6 m²·g⁻¹). The developed asymmetric device (NiCo-CH-180//AC) had a high E_d and moderate P_d of 52.0 Whkg⁻¹ and 1500.0 Wkg⁻¹, respectively, and moderate cycle stability, with 76.2% capacitance retention over 5000 cycles. This moderate stability could be due to the volume changes that occurred significantly during the prolonged cycle time, impairing ion adsorption on the electrode surfaces. Additionally, the three 1.5 V red LEDs were powered up for 150 s after being charged for 3 min using this device, making it feasible for practical applications. Consequently, the NiCo-CH-180 LDH may be an appropriate electrode material for SC applications, as evidenced by its cycle stability, device performance, and C_s value.

The development of hierarchical 3D flower structures results in interconnected nanosheets that allow for excellent ion transport and electrochemical performance. To this extent, the active sites of the LDH nanosheets may make contact with ions, promoting ion transport and effective electron transfer. Accordingly, the 3D hierarchical flowers Ni_0.7Co_0.3-LDH were synthesized via solvothermal and acquired a high C_s of 2052.0 Fg⁻¹ at 1.0 Ag⁻¹ [80]. This LDH demonstrated high-rate capability, as the C_s remained at 1269.0 Fg⁻¹ at 50.0 Ag⁻¹. However, structural stability may be modest, since capacitance retention was found to be 72% after 6000 cycles. The asymmetric device (Ni_0.7Co_0.3-LDH//AC) that was constructed demonstrated high E_d and low P_d, with 54.0 Whkg⁻¹ and 750.0 Wkg⁻¹, respectively. Additionally, the device demonstrated high cycle stability, with a 90.8% capacitance retention after 5000 cycles, which may be a more impressive result.

The solvothermal method employed to synthesize NiCo-LDH (e.g., Ni₃Co₁-LDH) exhibited a high C_s value of 1978.2 Fg⁻¹ at 1.0 Ag⁻¹ [81]. This study found that higher Ni content in the composition with Co increased electrochemical performance more than higher Co content in the composition with Ni. Accordingly, the Ni₁Co₂-LDH has a C_s of 1071.0 Fg⁻¹. Therefore, the Ni₃Co₁-LDH’s high C_s may be attributed to its low charge transfer resistance of 0.48 Ω, which allows for high electrolyte ion adsorption and electrochemical performance. This LDH demonstrated resilient cycle stability, with 70.6% over 10,000 cycles. Furthermore, the asymmetric device (Ni₃Co₁-LDH//AC) demonstrated a high E_d of 54.8 Whkg⁻¹ at a P_d of 779.4 Wkg⁻¹, indicating a promising device for practical usage. Over 3000 cycles, this device demonstrated high cycle stability (96.5% capacitance retention) and may have high structural stability. However, a cycle test of less than 5000 may be insufficient to establish device functioning. Furthermore, two asymmetric devices were connected to light different colors of LED bulbs, and measuring their discharge time revealed that the time remained constant even after 20 cycles. Figure 11d,e illustrate the cycle stability of two SCs in series at 10.0 Ag⁻¹, as well as a photograph of various LED colors lighted by two asymmetric devices. According to Figure 11d, the discharge time graph showed no variation after 20 cycles of LED operation between two devices connected in series. As a result, the device may be highly durable and suitable for practical applications.

Similarly, solvothermal synthesized nanosphere Ni₁Co₄-LDH achieved a C_s of 1300.0 Fg⁻¹ at 1.0 Ag⁻¹, and 90% capacitance retention was found over 1000 cycles [82]. The morphology and resistance may have an important impact in electrochemical performance. As a result, increasing the Ni content on the NiCo-LDH morphology resulted in no petals in the nanosphere (Figure 11f), and higher Co content with low Ni had a lower charge transfer resistance than higher Ni content with low Co. As a result, Co may be improving the conductivity and morphology of NiCo-LDH. Figure 11g shows that the CV curve of Ni₁Co₄-LDH was larger than that of other NiCo-LDH compositions, indicating that they may have superior electrochemical performance. Ni₁Co₄–LDH retained 90% of its capacitance after 1000 cycles, outperforming other Ni₁Co₁-LDH (86%) and Ni₄Co₁-LDH (83%) (Figure 11h). Furthermore, TEM images before and after cycle stability of Ni₁Co₄-LDH reveal a slight change in morphology over 1000 cycles (Figure 11i). Therefore, structural modifications to the LDHs could potentially influence their cycling performance.

Figure 11. (a) SEM image of NiCo-LDH-1, (b) cycle stability profile of the NiCo-LDH-1//AC asymmetric device at 10 Ag⁻¹ and (c) photograph of a red LED illuminated by two NiCo-LDH-1//AC devices that are serially connected, reproduced with permission from [78], copyright 2023, Royal Society of Chemistry. (d) Cycling stability profile of serially connected two Ni₃Co₁-LDH//AC devices at 10 A/g and (e) photographs of different LED colors lit by serially coupled two Ni₃Co₁-LDH//AC devices, reproduced with permission from [81], copyright 2024. (f) SEM images of various Ni_xCo_y-LDHs at 100 nm scale, (g) CV profiles of Ni_xCo_y-LDHs and ZIF-67 at a scan rate of 10 mVs⁻¹, (h) cyclic stability profile of Ni_xCo_y-LDHs at 10 Ag⁻¹, and (i) TEM image of Ni₁Co₄-LDH before and after the cyclic stability test, reproduced with permission from [82], copyright 2024, Elsevier.

Figure 11. (a) SEM image of NiCo-LDH-1, (b) cycle stability profile of the NiCo-LDH-1//AC asymmetric device at 10 Ag⁻¹ and (c) photograph of a red LED illuminated by two NiCo-LDH-1//AC devices that are serially connected, reproduced with permission from [78], copyright 2023, Royal Society of Chemistry. (d) Cycling stability profile of serially connected two Ni₃Co₁-LDH//AC devices at 10 A/g and (e) photographs of different LED colors lit by serially coupled two Ni₃Co₁-LDH//AC devices, reproduced with permission from [81], copyright 2024. (f) SEM images of various Ni_xCo_y-LDHs at 100 nm scale, (g) CV profiles of Ni_xCo_y-LDHs and ZIF-67 at a scan rate of 10 mVs⁻¹, (h) cyclic stability profile of Ni_xCo_y-LDHs at 10 Ag⁻¹, and (i) TEM image of Ni₁Co₄-LDH before and after the cyclic stability test, reproduced with permission from [82], copyright 2024, Elsevier.

Recently, adjusting the Ni:Co ratio (4:6, 5:5, and 6:4) to synthesize NiCo-LDHs using microwave radiation has revealed that all LDHs are 3D porous nanosphere with mild agglomeration [83]. Therefore, changing the precursor ratio had no influence on the morphology. Furthermore, NiCo-LDH-5:5 (38.3 m²·g⁻¹) had a higher surface area than NiCo-LDH-4:6 (36.7 m²·g⁻¹) or NiCo-LDH-6:4 (20.7 m²·g⁻¹). Therefore, increasing the Ni precursor ratio may reduce the surface area, rather than morphological changes. NiCo-LDH-5:5 has the highest CV curve and charge–discharge time (Figure 12a,b), resulting in a high C_s value of 2156.0 Fg⁻¹ at 1.0 Ag⁻¹ when compared to other ratios of LDH materials. NiCo-LDH-5:5’s high C_s value may be attributed to its lower charge transfer resistance (0.13 Ω) compared to others (0.2 Ω and 0.28 Ω). Furthermore, the developed NiCo-LDH-5:5//AC device was able to deliver high E_d and P_d values of 41.6 Whkg⁻¹ and 8000 Wkg⁻¹, respectively, and high capacitance retention was observed over 6000 cycles; however, volume changes in prolonging the cycle time may have resulted in the 81.5% capacitance retention. Based on the 3D porous nanosphere, high surface area, low resistance, and high structural stability that could be ascribed to quick faradaic reaction and fast ion transfer during redox reaction, NiCo-LDH-5:5 acquired strong electrochemical performance and outstanding device results.

Similarly, the ammonia diffusion approach was used to synthesize NiCo-LDH, with it modifying various ratios (4:6, 5:5, 6:4, and 8:2) observed to increase the ratio of Ni that allows the formation of flower morphology with petals; however, at a higher ratio (8:2), it generated the nanosphere, which indicates that petals may be amalgamated [84]. As a result, a 6:4 ratio of NiCo-LDH (e.g., NiCo-LDH0.6) yielded a 3D nanoflower structure. Based on morphology, NiCo-LDH0.6 has a higher C_s value of 1463.2 Fg⁻¹ at 1.0 Ag⁻¹, as well as strong cycle stability (87.9% capacitance retention over 2000 cycles). However, 2000 cycles may not be sufficient to establish the electrode materials’ integrity. The developed NiCo-LDH0.6//AC device was able to provide a low E_d value of 23.4 Whkg⁻¹ at a P_d of 900.0 Wkg⁻¹, and because of the AC in the device, the device demonstrated remarkable cycle stability of 73.4% over 5000 cycles. The significant performance of ammonia-diffused NiCo-LDH0.6 may be attributed to its large surface area (132.8 m²·g⁻¹), 3D nanoflower structure, rapid redox reactions, and shorter pathways for electrolyte ions, which enhance adsorption and structural stability.

Hydrothermally produced 3D hierarchical flower-structured NiCo-LDH (e.g., NCDH-1) showed superior electrochemical performance, achieving a high C_s value of 2199 Fg⁻¹ at 1.0 Ag⁻¹ despite its resistance of 1.24 Ω [85]. The remarkable performance might be attributed to the 3D flower-like appearance with aligned sheet morphologies, large surface area (47.3 m²·g⁻¹), and plenty of active sites. Figure 12c–e show SEM images of NiCo-LDH at different scales, revealing that the architecture of LDH was a hierarchical 3D flower/sphere structure with thin, high cloud petals that could adsorb a large number of electrolyte ions for high-energy storage, with no agglomeration seen. The produced asymmetric device (NCDH-1//AC) significantly generates an E_d value of 72.5 Whkg⁻¹ at a P_d of 837.0 Wkg⁻¹, and strong cycle stability was reported to be 106% of its capacitance after 10,000 cycles (Figure 12f). This might be because the electrode materials are stable throughout cycles in the electrolyte solution.

In another investigation, a 1.5:1 ratio of Ni and Co precursor was employed to produce a NiCo-LDH thin film on stainless steel hydrothermally, and this structure achieved C_s of 700.4 Fg⁻¹ at 1.0 Ag⁻¹ [86]. However, this thin-film, porously structured LDH exhibited low cycle stability, with 56.9% capacitance retention after 2000 cycles, which might be ascribed to low electrochemical performance through weak structural stability, low active sites, and possibly a longer ion path. As a result, thin film-structured electrode materials may have poor structural stability, which contributes to low cycle stability. In another study, electrodeposition was utilized to produce NiCo-LDH on the NF with a single layer (1)-to-multilayer (2, 3, 4, 5, and 6) coating for electrochemical investigations [87]. Increase the coating layer to cover the entire NF in comparison to a single coating. In contrast, the six-layer coating was smooth and free of fractures, while the other layers were not smooth and exhibited defects. The single-layer coated NF exhibits a maximum surface area of 2.8 m²·g⁻¹, while the highest LDH layer-coated NF has a maximum surface area of 5.7 m²·g⁻¹. The highest coating of NiCo-LDH-6 resulted in a larger CV and a longer charge–discharge time than other layers of coated LDHs. Consequently, the C_s was determined to be 953.0 Fg⁻¹ at 3.0 mAcm⁻². As a result of their low cycle stability (37.5% capacitance retention) over 5000 cycles, the thin film-based NiCo-LDH-6 coated NF (NiCo-LDH-6/NF) may be structurally deficient. However, this value exceeded NiCo-LDH-1/NF’s capacitance retention of 29.8%. The NiCo-LDH film-coated NF exhibits a high C_s value, as indicated by the results, rather than high cycle stability performance. Nickel foam (5.0 Scm⁻¹) is a high-conductivity material whose surface could be covered with electrode material, and then oxygen vacancies could be added to the electrode material’s surface to improve its electrochemical performance. An attempt was made to use electrodeposition to deposit NiCo-LDH on nickel foam, and then oxygen vacancy was generated on the LDH/NF (NiCo-LDH-Vo/NF) by treating it with H₂O₂ [88]. The morphology was discovered to be a network of nano-wall-array-like shapes made of highly porous NiAl-LDH on NF. However, this film only had a surface area of 4.0 m²·g⁻¹. In general, the surface area was higher for powder-based materials compared to film-based materials. However, there was no significant improvement in surface area before (3.8 m²·g⁻¹) or after oxygen vacancy on the NiCo-LDH/NF. Interestingly, the oxygen vacancy on the LDH/NF achieved higher conductivity (3.5 Scm⁻¹) than the as-prepared LDH/NF (2.9 Scm⁻¹). As a result, oxygen vacancies on the materials’ surfaces play an important role in increasing conductivity. As a consequence, the NiCo-LDH-Vo/NF demonstrated a higher specific capacity (771.8 Cg⁻¹) at 1.0 Ag⁻¹ than the as-prepared sample. The film type of NiCo-LDH-Vo/NF may have excellent structural stability because of its connected nano-array walls, with 91.8% capacitance retention over 5000 cycles compared to as-prepared NiCo-LDH/NF (80.3%). After 10,000 cycles, the constructed NiCo-LDH-Vo//AC device retained 92.78% capacitance, with high E_d and P_d values of 56.46 Whkg⁻¹ and 800.0 Wkg⁻¹, respectively. As a result, the developing oxygen vacancy strategy on the LDH surface could attract more OH− ions, resulting in high electrochemical performance and a fast redox reaction. Additionally, these film-based electrode materials could be robust structures useful for high cycle life and high energy density.

NiCo-LDH was deposited onto Ni-coated carbon cloth via electrodeposition (NiCo-LDH/Ni/CC), and to assess the influence of substrate materials, NiCo-LDH was also deposited independently on nickel foam (NiCo-LDH/NF) and carbon cloth (NiCo-LDH/CC) [89]. The morphology of the NiCo-LDH was influenced by the substrates; as a result, a combination of nanoflower and nanosphere NiCo-LDH was generated on NF. Meanwhile, thin and thick nanosheets of NiCo-LDH were produced on CC- and Ni-coated CC (Ni/CC), respectively (Figure 12g). However, the NiCo-LDH films produced on all substrates had highly porous architectures. Despite its highly porous surface, the NiCo-LDH/Ni/CC exhibited a low surface area of 2.71 m²·g⁻¹. Because of the high CV, wider area, and longer charge–discharge time (Figure 12h,i), the battery-like behavior of NiCo-LDH/Ni/CC exhibited a higher C_s (1545.3 Fg⁻¹) than NiCo-LDH/CC (1470.9 Fg⁻¹) and NiCo-LDH/NF (1034.2 Fg⁻¹) at C_d of 3.0 mAcm⁻². It was observed that NiCo-LDH/Ni/CC had higher capacitive characteristics than others, which might be related to their low resistance. All electrode materials exhibit remarkable cycle stability, with almost 100% capacitance retention, perhaps due to their strong structure; however, NiCo-LDH/Ni/CC demonstrated outstanding capacitance retention of 180.6% over 5000 cycles (Figure 12j). Furthermore, the developed NiCo-LDH/Ni/CC//AC/Ni/CC hybrid device demonstrated a high E_d value of 49.3 Whkg⁻¹ at a P_d of 750.0 Wkg⁻¹, as well as strong capacitance retention (97.3%) over 5000 cycles, indicating that there may be no volume changes during prolonged operation. Overall, the electrodeposited NiCo-LDH/Ni/CC film might be a promising material for SC applications because of its high-porosity structure, low resistance, and durable structure, which allow for high electrochemical performance.

Figure 12. (a,b) CV and GCD profiles of different NiCo-LDH ratios at 30 mVs⁻¹ and 1.0 Ag⁻¹, respectively, reproduced from [83], copyright 2024, MDPI. (c–e) FE-SEM images of 3D flower-like NCDH-1 at various scales, and (f) cycle stability profile of the NCDH-1//AC asymmetric device, reproduced with permission from [85], copyright 2024, Elsevier. (g) SEM images of NiCo-LDH on NF, CC, and Ni//CC at 2.0 µm and 200 nm scales, (h,i) NiCo-LDH’s CV and GCD profiles on Ni/CC, CC, and NF at 5.0 mVs⁻¹ and 3 mAcm⁻², respectively, and (j) cycle stability of NiCo-LDH/Ni/CC and others, reproduced with permission from [89], copyright 2025, Elsevier.

Figure 12. (a,b) CV and GCD profiles of different NiCo-LDH ratios at 30 mVs⁻¹ and 1.0 Ag⁻¹, respectively, reproduced from [83], copyright 2024, MDPI. (c–e) FE-SEM images of 3D flower-like NCDH-1 at various scales, and (f) cycle stability profile of the NCDH-1//AC asymmetric device, reproduced with permission from [85], copyright 2024, Elsevier. (g) SEM images of NiCo-LDH on NF, CC, and Ni//CC at 2.0 µm and 200 nm scales, (h,i) NiCo-LDH’s CV and GCD profiles on Ni/CC, CC, and NF at 5.0 mVs⁻¹ and 3 mAcm⁻², respectively, and (j) cycle stability of NiCo-LDH/Ni/CC and others, reproduced with permission from [89], copyright 2025, Elsevier.

Electrodeposition of bio-inspired NiCo-LDH nanosheets on carbon felt (CF/CRBI-NiCo-LDH) resulted in high C_s of 1661.6 Fg⁻¹ at 1.0 Ag⁻¹ and 96.7% capacitance retention over 3000 cycles. The highly porous structures, moderate surface area (14.5 m²·g⁻¹), and low charge transfer resistance (0.118 Ω) of the CF/CRBI-NiCo-LDH could be attributed to good capacitive properties [90]. Additionally, the developed CF/CRBI-NiCo-LDH//AC/CF demonstrated high E_d and moderate P_d values of 57.2 Whkg⁻¹ and 820.0 Wkg⁻¹, respectively, and high capacitance retention of 89% over 3000 cycles, indicating strong structural stability due to high ion adsorption on electrode materials surfaces. The electrodeposited electrode materials and asymmetric-based devices have high C_s and high cycle stability, owing to their porous architectures and structural stability, and it was found that these LDH materials are held on the carbon felt surfaces even after a prolonged cycle test.

2.16. CoNi-LDH

Thin sheet-like petals linked with one another to form 3D nanoflower (NF) structured CoNi-LDH, produced via solvothermal, demonstrated battery-type performance [91]. This flower LDH has a specific capacity of 768.3 Cg⁻¹ at 1.0 Ag⁻¹, and this study also developed nanosheet (NS) CoNi-LDH with a capacity of 669.3 Cg⁻¹. As a result, the 3D flower’s structure may be a useful morphology for enhancing electroactivity. Flower-structured LDH (0.13 Ω) may have lower charge transfer resistance than nanosheet-structured LDH (0.17 Ω) because of its high specific capacity. Based on the low resistance of both LDHs, asymmetric devices were constructed to produce CoNi-LDH NF//AC and CoNi-LDH NS//AC, which provided moderate E_d (37.1 and 31.0 Whkg⁻¹) and P_d (748.0 Wkg⁻¹ and -). In terms of cycle stability, CoNi-LDH NS//AC retained 96.9% of capacitance after 4000 cycles, compared to 93.7% for CoNi-LDH NF//AC. Nanosheets may have more structural stability throughout cycle tests, resulting in higher cycle performance than nanoflower-structured LDH. However, flower-structured LDH outperformed nanosheet-structured LDH in terms of surface area, resistance, and active sites, allowing for higher ion adsorption and energy storage. A simple sonochemical approach was employed to synthesize CoNi-LDH with pH adjustments of 7.5 and 8.5, which resulted in nanopetal- and nanoflake-structured LDH, respectively [92]. As a result, pH could influence the morphological changes of LDHs. However, increasing the pH in the procedure reduces the surface area of the Co-Ni LDHs. Thus, the surface area of this LDH produced at pH 7.5 and 8.5 was 365.2 and 257.50 m²·g⁻¹, respectively. As a result, nanopetals (pH 7.5) may have more active sites than nanoflakes (pH 8.5). Accordingly, CoNi-LDH7.5 (740.8 Cg⁻¹) had a higher specific capacity than CoNi-LDH8.5 (668.1 Cg⁻¹) at 1.0 Ag⁻¹. The high specific capacity of these LDHs may be attributable to higher surface areas and low charge transfer resistance (0.30 to 0.35 Ω). Over 5000 cycles, the produced asymmetric devices CoNi-LDH7.5//AC and CoNi-LDH8.5//AC exhibit higher cycle stability (96.8% and 97.5% capacitance retention, respectively), and both devices achieve almost identical Ed values of 35.6 and 33.8 Whkg⁻¹. As a result, the CoNi-LDHs exhibit superior electrochemical performance; however, changing pH did not provide significant performance.

The hydrothermal approach utilized to synthesize CoNi-LDH grown on carbon cloth (CC) revealed a nanoneedle structure, and this electrode material was activated electrochemically using chronoamperometry (i-t) activation at different durations to yield crystalline-amorphous LDH (CA-CoNi-LDH/CC) [93]. It was found that the original morphology of the LDH on CC was changed based on the electrochemical activation. For instance, at 0 s, the LDH nanoneedle became a broccoli structure at 10,000 s, and a granular structure was observed at 12,000 s. The changes in morphology at different activation times may be attributed to structural degradation caused by 6 KOH. Based on the better morphology, 10,000 s-activated CA-CoNi-LDH/CC was chosen, and the charge transfer resistance of the electrode materials (0.24 Ω) was lower than that of the as-prepared CoNi-LDH/CC (1.9 Ω). Furthermore, the surface area of activated LDH (45.5 m²·g⁻¹) was significantly higher than that of as-prepared LDH (12.5 m²·g⁻¹). Consequently, the C_s was determined to be higher for pseudocapacitive CA-CoNi-LDH/CC (695.2 Fg⁻¹) than for CoNi-LDH/CC (341.0 Fg⁻¹) at 5.0 mAcm⁻². Additionally, the capacitance retention was high, with 103.1% and 86.4%, respectively, over 5000 cycles. Consequently, the electrochemically activated LDH/CC exhibits a higher level of ion adsorption and structural stability than the as-prepared LDH/CC. Subsequently, the asymmetric CA-CoNi-LDHs@CC//AC cloth demonstrated low E_d and P_d values of 37.2 Whkg⁻¹ and 206.8 Wkg⁻¹, respectively. However, this device was capable of maintaining a 91.6% capacitance over 5000 cycles. For practical purposes, these two devices were connected in series to illuminate a green LED for 25 min, which could be a prospective asymmetric device for SC application. Consequently, the electrochemically activated CoNi-LDH may possess superior electrochemical properties in comparison to the as-prepared CoNi-LDH.

For bare LDHs, the majority of the studies employed synthesis techniques such as hydrothermal, co-precipitation, solvothermal, and electrodeposition, as indicated in Table 1. However, the electrodeposition technique is distinct from other methods in that it generates film-based LDHs, whereas the remaining techniques are powder- or particle-based. In this regard, the most straightforward procedures for LDH preparation may be in the following order: co-precipitation > hydrothermal > solvothermal > electrodeposition. In this case, co-precipitation and hydrothermal are environmentally benign or low-toxic procedures that involve water. Likewise, the chemical-based procedures involved in solvothermal and electrodeposition render them environmentally unfriendly. The electrode material sheath on nickel foam (NF) or carbon cloth (CC) may be effective when using electrodeposition; however, this technique primarily produces dense coatings or fractured structures instead of significant morphologies. Electrodeposition is used to generate film-based electrode materials, which are structurally robust, resulting in high cycle stability, rather than high capacitive properties. LDH-coated NF or CC was shown to offer higher electrochemical performance than LDH-coated SS material. Additionally, hydrothermal or solvothermal processes may be employed to encapsulate the electrode material on the nickel foam, resulting in significant morphologies. However, this process necessitates a high temperature, extended duration, and non-uniformity. Co-precipitation may be a simple method for preparing LDHs, and it may be able to encapsulate material on the nickel foam or CC. However, it may not be effective, and it may be challenging to achieve significant morphologies and/or the possibility of agglomeration on substrates. The electrode materials synthesized using hydrothermal and solvothermal methods exhibited a larger surface area than those synthesized using other methods. The utilization of morphology controllers at specific temperatures and times may be the reason these techniques enable the production of materials with distinctive morphologies, including nano/micro-flowers, nanosheets/flakes/pebbles, and highly porous structures with nano-sized particles. Electrodeposition is often employed to generate LDHs with sheet morphologies and low surface area; such behavior might be due to a low surface-to-volume ratio, less porous architectures, and the possibility of bulk sheet development, all of which can impair electrochemical performance. Rarely, electrodeposition-generated NiAl-LDH/CC was found to have a large surface area (437.5 m²·g⁻¹), while other LDHs made using this technique had a low surface area.

Table 1. Recently invented LDHs demonstrate their electrochemical performance in three- and two-electrode systems.

Electrode Materials	Synthesis	Morphology	Three Electrodes		Two Electrodes			EL	Ref
			Cs at Cd	Cycle Stability	Devices	Ed	Pd
CoCe-LDH
CoCe-LDH	Urea hydrolysis	Sheet/spherical (95.4 m2·g−1)	-	96.93%, after 20,000 cycles	CoCe-LDH// CoCe-LDH	2.98 μWhcm−2	375.1 μWcm−2	3 M KOH	[45]
CoV-LDH
CoV-LDH (CVL1-120)	Hydrothermal	Nanosheets (15.7 m2·g−1)	314.4 at 1.0	97%, after 5000 cycles	CVL-1-120//AC	36.5	1208.2	1 M KOH	[46]
CoFe-LDH
CoFe-LDH-200	Hydrothermal	Microflowers	2222.0 at 1.0	100%, after 10,000 cycles	-	-	-	3 M KOH	[47]
FeCo-LDH
FeCo-LDH /NF (FC3)	Hydrothermal	Dense nanoflakes (84.7 m2·g−1)	1448.0 at 20 mAcm−2	87%, after 5000 cycles	FC3//AC-NF	157.0	4210.0	1 M KOH	[48]
ZnCo-LDH
Ov-ZnCo-LDH/CC (Ov-LDH(3))	Hydrothermal	Nanosheets (113 m2·g−1)	-	84.2%, after 5000 cycles	Ov-LDH(3)//AC	57.9	849.4	3 M KOH	[49]
NiFe-LDH
NiFe-LDH (NFS)	Hydrothermal	Hexagonal plates (50 m2·g−1)	-	-	NFS//AC	11.0	5500.0	6 M KOH	[50]
NiFe-LDH- 24 h	Hydrothermal	Porous nanoflowers	635.8 at 1.0	58%, after 500 cycles	-	-	-	2 M KOH	[51]
Ni5Fe1-LDH	Hydrothermal	Combined sheets and particles (136.9 m2·g−1)	-	66.3%, after 3000 cycles	Ni5Fe1-LDH//AC	35.8	800.0	6 M KOH	[52]
NiCu-LDH
L-Ni4Cu-LDH	Co-precipitation	Layered sheets	1656.9 at 1.0	-	L-Ni4Cu-LDH//AC	45.4	800.0	2 M KOH	[53]
CuZn-LDH
CuZn-LDH	Co-precipitation	Flower petals	265.0 at 1.0 mAcm−2	-	CuZn-LDH//AC	7.1	121.9	3 M KOH	[54]
CuCo LDH
Ov-CuCo LDH/CF	Solution method/NaBH4 reduction	Nanosheet arrays	1392.4 at 1.0	-	Ov-CuCo LDH/CF//AC	58.2	850.0	-	[55]
CuCo-LDH (CC2)	Sonochemical	Bundled rice straw	50.0 at 20 mVs−1	103%, after 500 cycles	-	-	-	6 M KOH	[56]
MgCo-LDH
MgCo-LDH/Ni (MCL-2)	Vertical autoclave hydrothermal	Sheet	1431.2 at 6.0 mAcm−2	75%, after 5000 cycles	-	-	-	1 M KOH	[57]
NiAl-LDH
NiAl-LDH-60	Hydrothermal	Nanoflakes (160.6 m2·g−1)	-	85.9%, after 5000 cycles	NiAl-LDH-60//NiAl-LDH-60	71.3	2400.0	3 M KOH	[58]
NiAl-LDH/ITO film	Hydrothermal	Micropebbles (117.4 m2·g−1)	589.3 at 1.0	95%, after 5000 cycles	NiAl-LDH/ITO// AC/ITO	216.0	5200.0	1 M KOH	[59]
NiAl-LDH/CC	Electrodeposition	Microsheets (437.5 m2·g−1)	-	92%, after 2000 cycles	NiAl-LDH/CC//CC	0.465 Whcm−3	8.23 Wcm−3	1 M KOH	[60]
ZnAl-LDH
ZnAl-LDH/Al flexible device	Chemical bath deposition	Nanosheets	596.5 at 10 mVs−1 (Flat mode)	-	-	6.4	46,000.0	2 M H2SO4	[61]
NiMn-LDH
NiMn-LDH	Co-precipitation	Thick sheets	905.0 at 1.0	94.8%, after 5000 cycles	-	-	-	1 M KOH	[62]
Ni3Mn1-T100	Hydrothermal	3D Flowers (36.2 m2·g−1)	1708.0 at 1.0	72.7%, after 10,000 cycles	Ni3Mn1-T100//rGO	60.0	965.0	3 M KOH	[63]
Ni75Mn25-LDH/NF	Hydrothermal	3D Microflower (23.4 m2·g−1)	1008.0 at 1.0 mAcm−2	72.7%, after 3000 cycles	-	-	-	1 M KOH	[64]
NNM-2/NF	Electrodeposition	3D Flowers	2755.6 at 1.0	71.2%, after 5000 cycles	NNM-2/NF//AC	41.7	2700.0	6 M KOH	[65]
NiMn-LDH/Ni foil	Reflux condensation method	Pebbles (127.9 m2·g−1)	416.0 at 4.0 mAcm−2	92.8%, after 3000 cycles	NiMn-LDH/Ni foil//AC/Ni foil	10.0	900.0	2 M KOH	[66]
CoMn-LDH
CoMn-LDH/NF	Electrodeposition	Lamellar	-	83.6%, over 7000 cycles	CoMn-LDH/NF//AC	0.409 mWh cm−2	0.798 mW cm−2	2 M KOH	[67]
CoMn-LDH-6	Hydrothermal	Porous solid nanosphere	53.5 at 0.67	56.2%, after 1000 cycles	-	-	-	6 M KOH	[68]
NiCo-LDH
NiCo-LDH (NCLDH-21)	Solvothermal	Thin nanosheets (47 m2·g−1)	2054.0 at 1.0	93.6%, after 5000 cycles	NCLDH-21//AC	67.6	839.9	6 M KOH	[69]
NiCo-LDH-18/NF	Hydrothermal	Microflower (372.9 m2·g−1)	2266.6 at 1.0	97.4% after 10,000 cycles	NiCo-LDH-18//AC	104.8	3118.0	6 M KOH	[70]
NiCo-LDH (LDH-A0.3)	Hydrothermal	Nanosheets	2237.5 at 1.0	-	-	-	-	6 M KOH	[71]
NiCo-LDH/NF-100	Hydrothermal	Microporous nanosheets	1156.0 at 1.0	95.3%, after 5000 cycles	NiCo-LDH/NF-100//AC	-	-	2 M KOH	[72]
NiCo-LDH/NF-140	Hydrothermal	Nanofibrous	-	-	NiCo-LDH/NF-140//AC	51.5 μWh cm−2	1.125 mW cm−2	3 M KOH	[73]
NiCo-LDH-2	Hydrothermal	Nanoflowers	-	72.7%, after 3000 cycles	NiCo-LDH-2// carbon	36.0	732.0	1 M KOH	[74]
NiCo-LDH (AL-LDH)	Electrodeposition	Interconnected nanoflakes	-	Poor stability over 500 cycles	Mg-Hybrid SC (Al-LDH/VS2)	48.4	937.4	1 M KOH or MgSO4	[75]
Binder-free NiCo-LDH/SS film	Electrodeposition	Wrinkled nanosheets	1406.7 at 3.0	83%, over 5000 cycles	-	-	-	1 M KOH	[76]
NiCo-LDH-1	Reduction/ etching	Nano cubes (90.4 m2·g−1)	1671.0 at 1.0	67.9%, after 10,000 cycles	NiCo-LDH-1//AC	59.0	935.7	1 M KOH	[78]
NiCo-CH-180	Solvothermal	Nanorods (52.6 m2·g−1)	762.0 at 1.0	-	NiCo-CH-180//AC	52.0	1500.0	2 M KOH	[79]
NiCo-LDH-10	Hydrothermal	Nanopetals (56.1 m2·g−1)	-	-	NiCo-LDH-10//graphene	22.6	169.9	3 M KOH	[77]
Ni0.7Co0.3-LDH	Solvothermal	3D hierarchical Flowers (132 m2·g−1)	2052.0 at 1.0	72%, after 6000 cycles	Ni0.7Co0.3-LDH//AC	54.0	750.0	6 M KOH	[80]
Ni3Co1-LDH	Solvothermal	3D microflowers (143.8 m2·g−1)	1978.2 at 1.0	70.6%, after 10,000 cycles	Ni3Co1-LDH//AC	54.8	779.4	3 M KOH	[81]
Ni1Co4-LDH	Solvothermal	3D nanosphere	1300.0 at 1.0	90.0%, after 1000 cycles	-	-	-	3 M KOH	[82]
NiCo-LDH-5:5	Microwave radiation	3D nanospheres (38.3 m2·g−1)	2156.0 at 1.0	-	NiCo-LDH-5:5//AC	41.6	8000.0	2 M KOH	[83]
NiCo-LDH0.6	Ammonia diffusion method	3D nanoflower	1463.2 at 1.0	87.9%, after 2000 cycles	NiCo-LDH0.6//AC	23.4	900.0	2 M KOH	[84]
NiCo-LDH (NCDH-1)	Hydrothermal	3D hierarchical Flowers (47.3 m2·g−1)	2199.0 at 1.0	-	NCDH-1//AC	72.5	837.0	6 M KOH	[85]
NiCo-LDH film	Hydrothermal	Network porous flakes (23.3 m2·g−1)	700.4 at 1.0	56.9%, after 2000 cycles	-	-	-	1 M KOH	[86]
NiCo-LDH-6/NF	Electrodeposition	Thick films (5.7 m2·g−1)	953.0 at 3 mAcm−2	37.5%, after 5000 cycles	-	-	-	1 M KOH	[87]
NiCo-LDH-Vo/NF	Electrodeposition	Interconnected nano-wall arrays (4.0 m2·g−1)	-	91.8%, after 5000 cycles	NiCo-LDH-Vo//AC	56.46	800.0	1 M KOH	[88]
NiCo-LDH/Ni/CC	Electrodeposition	Thick, porous films (2.71 m2·g−1)	1545.3 at 3 mAcm−2	180.6%, over 5000 cycles	NiCo-LDH/Ni/ CC//AC/Ni/CC	49.3	750	3 M KOH	[89]
CF/CRBI-NiCo-LDH	Electrodeposition	Nanosheets (14.5 m2·g−1)	1661.6 at 1.0	96.7%, after 3000 cycles	CF/CRBI-NiCo-LDH// AC/CF	57.2	820.0	6 M KOH	[90]
CoNi-LDH
CoNi-LDH nanoflower	Solvothermal	3D Nanoflowers (101.3 m2·g−1)	-	-	CoNi-LDH NF//AC	37.1	748.0	2 M KOH	[91]
CoNi-LDH7.5	Sonochemical	Nanopetals	-	-	CoNi-LDH7.5// AC	35.6	781.1	2 M KOH	[92]
CA-CoNi-LDH/CC	Hydrothermal/ electrochemical activation	Broccoli structures (45.5 m2·g−1)	695.2 at 5 mAcm−2	103.1%, after 5000 cycles	CA-CoNi-LDH/CC// AC cloth	37.2	206.8	6 M KOH	[93]

Specific capacitance, C_s (Fg⁻¹); current density, C_d (Ag⁻¹); energy density, E_d (Whkg⁻¹); power density, P_d (Wkg⁻¹); EL (electrolyte).

Table 1. Recently invented LDHs demonstrate their electrochemical performance in three- and two-electrode systems.

Electrode Materials	Synthesis	Morphology	Three Electrodes		Two Electrodes			EL	Ref
			Cs at Cd	Cycle Stability	Devices	Ed	Pd
CoCe-LDH
CoCe-LDH	Urea hydrolysis	Sheet/spherical (95.4 m2·g−1)	-	96.93%, after 20,000 cycles	CoCe-LDH// CoCe-LDH	2.98 μWhcm−2	375.1 μWcm−2	3 M KOH	[45]
CoV-LDH
CoV-LDH (CVL1-120)	Hydrothermal	Nanosheets (15.7 m2·g−1)	314.4 at 1.0	97%, after 5000 cycles	CVL-1-120//AC	36.5	1208.2	1 M KOH	[46]
CoFe-LDH
CoFe-LDH-200	Hydrothermal	Microflowers	2222.0 at 1.0	100%, after 10,000 cycles	-	-	-	3 M KOH	[47]
FeCo-LDH
FeCo-LDH /NF (FC3)	Hydrothermal	Dense nanoflakes (84.7 m2·g−1)	1448.0 at 20 mAcm−2	87%, after 5000 cycles	FC3//AC-NF	157.0	4210.0	1 M KOH	[48]
ZnCo-LDH
Ov-ZnCo-LDH/CC (Ov-LDH(3))	Hydrothermal	Nanosheets (113 m2·g−1)	-	84.2%, after 5000 cycles	Ov-LDH(3)//AC	57.9	849.4	3 M KOH	[49]
NiFe-LDH
NiFe-LDH (NFS)	Hydrothermal	Hexagonal plates (50 m2·g−1)	-	-	NFS//AC	11.0	5500.0	6 M KOH	[50]
NiFe-LDH- 24 h	Hydrothermal	Porous nanoflowers	635.8 at 1.0	58%, after 500 cycles	-	-	-	2 M KOH	[51]
Ni5Fe1-LDH	Hydrothermal	Combined sheets and particles (136.9 m2·g−1)	-	66.3%, after 3000 cycles	Ni5Fe1-LDH//AC	35.8	800.0	6 M KOH	[52]
NiCu-LDH
L-Ni4Cu-LDH	Co-precipitation	Layered sheets	1656.9 at 1.0	-	L-Ni4Cu-LDH//AC	45.4	800.0	2 M KOH	[53]
CuZn-LDH
CuZn-LDH	Co-precipitation	Flower petals	265.0 at 1.0 mAcm−2	-	CuZn-LDH//AC	7.1	121.9	3 M KOH	[54]
CuCo LDH
Ov-CuCo LDH/CF	Solution method/NaBH4 reduction	Nanosheet arrays	1392.4 at 1.0	-	Ov-CuCo LDH/CF//AC	58.2	850.0	-	[55]
CuCo-LDH (CC2)	Sonochemical	Bundled rice straw	50.0 at 20 mVs−1	103%, after 500 cycles	-	-	-	6 M KOH	[56]
MgCo-LDH
MgCo-LDH/Ni (MCL-2)	Vertical autoclave hydrothermal	Sheet	1431.2 at 6.0 mAcm−2	75%, after 5000 cycles	-	-	-	1 M KOH	[57]
NiAl-LDH
NiAl-LDH-60	Hydrothermal	Nanoflakes (160.6 m2·g−1)	-	85.9%, after 5000 cycles	NiAl-LDH-60//NiAl-LDH-60	71.3	2400.0	3 M KOH	[58]
NiAl-LDH/ITO film	Hydrothermal	Micropebbles (117.4 m2·g−1)	589.3 at 1.0	95%, after 5000 cycles	NiAl-LDH/ITO// AC/ITO	216.0	5200.0	1 M KOH	[59]
NiAl-LDH/CC	Electrodeposition	Microsheets (437.5 m2·g−1)	-	92%, after 2000 cycles	NiAl-LDH/CC//CC	0.465 Whcm−3	8.23 Wcm−3	1 M KOH	[60]
ZnAl-LDH
ZnAl-LDH/Al flexible device	Chemical bath deposition	Nanosheets	596.5 at 10 mVs−1 (Flat mode)	-	-	6.4	46,000.0	2 M H2SO4	[61]
NiMn-LDH
NiMn-LDH	Co-precipitation	Thick sheets	905.0 at 1.0	94.8%, after 5000 cycles	-	-	-	1 M KOH	[62]
Ni3Mn1-T100	Hydrothermal	3D Flowers (36.2 m2·g−1)	1708.0 at 1.0	72.7%, after 10,000 cycles	Ni3Mn1-T100//rGO	60.0	965.0	3 M KOH	[63]
Ni75Mn25-LDH/NF	Hydrothermal	3D Microflower (23.4 m2·g−1)	1008.0 at 1.0 mAcm−2	72.7%, after 3000 cycles	-	-	-	1 M KOH	[64]
NNM-2/NF	Electrodeposition	3D Flowers	2755.6 at 1.0	71.2%, after 5000 cycles	NNM-2/NF//AC	41.7	2700.0	6 M KOH	[65]
NiMn-LDH/Ni foil	Reflux condensation method	Pebbles (127.9 m2·g−1)	416.0 at 4.0 mAcm−2	92.8%, after 3000 cycles	NiMn-LDH/Ni foil//AC/Ni foil	10.0	900.0	2 M KOH	[66]
CoMn-LDH
CoMn-LDH/NF	Electrodeposition	Lamellar	-	83.6%, over 7000 cycles	CoMn-LDH/NF//AC	0.409 mWh cm−2	0.798 mW cm−2	2 M KOH	[67]
CoMn-LDH-6	Hydrothermal	Porous solid nanosphere	53.5 at 0.67	56.2%, after 1000 cycles	-	-	-	6 M KOH	[68]
NiCo-LDH
NiCo-LDH (NCLDH-21)	Solvothermal	Thin nanosheets (47 m2·g−1)	2054.0 at 1.0	93.6%, after 5000 cycles	NCLDH-21//AC	67.6	839.9	6 M KOH	[69]
NiCo-LDH-18/NF	Hydrothermal	Microflower (372.9 m2·g−1)	2266.6 at 1.0	97.4% after 10,000 cycles	NiCo-LDH-18//AC	104.8	3118.0	6 M KOH	[70]
NiCo-LDH (LDH-A0.3)	Hydrothermal	Nanosheets	2237.5 at 1.0	-	-	-	-	6 M KOH	[71]
NiCo-LDH/NF-100	Hydrothermal	Microporous nanosheets	1156.0 at 1.0	95.3%, after 5000 cycles	NiCo-LDH/NF-100//AC	-	-	2 M KOH	[72]
NiCo-LDH/NF-140	Hydrothermal	Nanofibrous	-	-	NiCo-LDH/NF-140//AC	51.5 μWh cm−2	1.125 mW cm−2	3 M KOH	[73]
NiCo-LDH-2	Hydrothermal	Nanoflowers	-	72.7%, after 3000 cycles	NiCo-LDH-2// carbon	36.0	732.0	1 M KOH	[74]
NiCo-LDH (AL-LDH)	Electrodeposition	Interconnected nanoflakes	-	Poor stability over 500 cycles	Mg-Hybrid SC (Al-LDH/VS2)	48.4	937.4	1 M KOH or MgSO4	[75]
Binder-free NiCo-LDH/SS film	Electrodeposition	Wrinkled nanosheets	1406.7 at 3.0	83%, over 5000 cycles	-	-	-	1 M KOH	[76]
NiCo-LDH-1	Reduction/ etching	Nano cubes (90.4 m2·g−1)	1671.0 at 1.0	67.9%, after 10,000 cycles	NiCo-LDH-1//AC	59.0	935.7	1 M KOH	[78]
NiCo-CH-180	Solvothermal	Nanorods (52.6 m2·g−1)	762.0 at 1.0	-	NiCo-CH-180//AC	52.0	1500.0	2 M KOH	[79]
NiCo-LDH-10	Hydrothermal	Nanopetals (56.1 m2·g−1)	-	-	NiCo-LDH-10//graphene	22.6	169.9	3 M KOH	[77]
Ni0.7Co0.3-LDH	Solvothermal	3D hierarchical Flowers (132 m2·g−1)	2052.0 at 1.0	72%, after 6000 cycles	Ni0.7Co0.3-LDH//AC	54.0	750.0	6 M KOH	[80]
Ni3Co1-LDH	Solvothermal	3D microflowers (143.8 m2·g−1)	1978.2 at 1.0	70.6%, after 10,000 cycles	Ni3Co1-LDH//AC	54.8	779.4	3 M KOH	[81]
Ni1Co4-LDH	Solvothermal	3D nanosphere	1300.0 at 1.0	90.0%, after 1000 cycles	-	-	-	3 M KOH	[82]
NiCo-LDH-5:5	Microwave radiation	3D nanospheres (38.3 m2·g−1)	2156.0 at 1.0	-	NiCo-LDH-5:5//AC	41.6	8000.0	2 M KOH	[83]
NiCo-LDH0.6	Ammonia diffusion method	3D nanoflower	1463.2 at 1.0	87.9%, after 2000 cycles	NiCo-LDH0.6//AC	23.4	900.0	2 M KOH	[84]
NiCo-LDH (NCDH-1)	Hydrothermal	3D hierarchical Flowers (47.3 m2·g−1)	2199.0 at 1.0	-	NCDH-1//AC	72.5	837.0	6 M KOH	[85]
NiCo-LDH film	Hydrothermal	Network porous flakes (23.3 m2·g−1)	700.4 at 1.0	56.9%, after 2000 cycles	-	-	-	1 M KOH	[86]
NiCo-LDH-6/NF	Electrodeposition	Thick films (5.7 m2·g−1)	953.0 at 3 mAcm−2	37.5%, after 5000 cycles	-	-	-	1 M KOH	[87]
NiCo-LDH-Vo/NF	Electrodeposition	Interconnected nano-wall arrays (4.0 m2·g−1)	-	91.8%, after 5000 cycles	NiCo-LDH-Vo//AC	56.46	800.0	1 M KOH	[88]
NiCo-LDH/Ni/CC	Electrodeposition	Thick, porous films (2.71 m2·g−1)	1545.3 at 3 mAcm−2	180.6%, over 5000 cycles	NiCo-LDH/Ni/ CC//AC/Ni/CC	49.3	750	3 M KOH	[89]
CF/CRBI-NiCo-LDH	Electrodeposition	Nanosheets (14.5 m2·g−1)	1661.6 at 1.0	96.7%, after 3000 cycles	CF/CRBI-NiCo-LDH// AC/CF	57.2	820.0	6 M KOH	[90]
CoNi-LDH
CoNi-LDH nanoflower	Solvothermal	3D Nanoflowers (101.3 m2·g−1)	-	-	CoNi-LDH NF//AC	37.1	748.0	2 M KOH	[91]
CoNi-LDH7.5	Sonochemical	Nanopetals	-	-	CoNi-LDH7.5// AC	35.6	781.1	2 M KOH	[92]
CA-CoNi-LDH/CC	Hydrothermal/ electrochemical activation	Broccoli structures (45.5 m2·g−1)	695.2 at 5 mAcm−2	103.1%, after 5000 cycles	CA-CoNi-LDH/CC// AC cloth	37.2	206.8	6 M KOH	[93]

Specific capacitance, C_s (Fg⁻¹); current density, C_d (Ag⁻¹); energy density, E_d (Whkg⁻¹); power density, P_d (Wkg⁻¹); EL (electrolyte).

All LDHs are exceptional electrode materials for supercapacitors due to their high surface area, unique morphologies, pseudocapacitance behavior, good conductivity, and high cycle stability, as was demonstrated. The electronic structure of monotype hydroxides may change when they are in the LDH state, which is associated with high conductivity. Electrochemical activation could alter the morphology of LDHs, their electronic structure, and structural stability, resulting in higher electrochemical performance than without activated LDH. Interestingly, oxygen vacancy techniques on the LDH may boost surface area, cycle stability, and electrochemical performance. Among the LDHs, the high C_s values of CoFe-LDH-200, NiMn-LDH (NNM-2/NF), NiCo-LDH (NCLDH-21, NiCo-LDH-18/NF, LDH-A0.3, Ni_0.7Co_0.3-LDH, NiCo-LDH-5:5, and NCDH-1) were observed. This improvement may be attributed to their morphologies, which include micro/nanoflower and nanosheets/spheres, which have highly porous structures that facilitate high ion adsorption. Additionally, the low charge transfer resistance may facilitate high electron transfer. It is possible that high cycle stability is a result of high structural stability. Consequently, materials such as CoCe-LDH, CoV-LDH, CoFe-LDH, FeCo-LDH, NiAl-LDH, NiMn-LDH, NiCo-LDH (NCLDH-21, NiCo-LDH-18/NF, NiCo-LDH/NF-100, Ni-Co-LDH-Vo/NF, NiCo-LDH/Ni/CC), and CA-CoNi-LDH/CC perform well. Therefore, LDHs are electrode materials that are characterized by their high mechanical strength. Devices with high energy density may possess a high-energy storage capacity; thus, all devices demonstrated high E_d values. Accordingly, devices such as FC3//AC-NF, NiAl-LDH/ITO//AC/ITO, Ni-Co-LDH-18//AC, NCDH-1//AC, NCLDH-21//AC, and Ni₃Mn₁-T100//rGO demonstrated the potential for practical application of high-energy storage devices. The specific capacitances of the CoFe-LDH, NiCu-LDH, NiMn-LDH, and NiCo-LDH-based electrode materials were found to exhibit exceptional electrochemical performance among the LDHs.

3. Strategies for Modifying LDHs and Their Electrochemical Performance

The unique morphologies, tunable metal cations, and high surface functional properties of LDHs have made them the focus of both industrial and research applications. LDHs have been investigated as a potential candidate for the next-generation capacitive materials; however, their efficacy is restricted due to their relatively low conductivity. It is advised that the electrochemical performance of LDHs be enhanced through the functionalization or modification of conductive materials. Subsequently, numerous functionalization strategies have been implemented in recent years to enhance performance. The electrochemical performance of LDHs can be significantly enhanced through the functionalization or modification of their surface properties, morphologies, and sizes to stimulate them. Additionally, the introduction of other chemical species or surface modification on LDHs can be used to develop new functionalities. Consequently, this section addresses the strategies for modifying the structure or surfaces of LDHs, including the addition of metal cations, the doping of anions, chemical modification, heteroatom doping, fluorine doping, oxygen vacancy formation, surfactant treatment, and rGO modification, all of which are relevant to electrochemical studies. Figure 13 provides a comprehensive overview of the various synthesis methods for LDH modification, as well as their physiochemical properties, modification strategies, and electrochemical performance, in order to facilitate comprehension of this section.

3.1. Sulfurized LDHs

The sulfurization of metal hydroxides may boost their conductivity, hence improving energy storage. Therefore, agglomerated sheet-structured CoNi-LDH was sulfurized (CoNiS) using an ion-exchange method for electrochemical investigations [94]. In the absence of sulfurization, the flower-like structure of CoNi-LDH was seen; however, this structure transitioned to a sheet configuration, with an increased likelihood of agglomeration as the degree of sulfurization increased. Furthermore, the surface area (58.0–4.0 m²·g⁻¹) decreased with increased sulfurization, which could block the pores of the LDH surfaces. The study revealed that sulfurization did not influence electrochemical performance, as the naked CoNi-LDH exhibited a capacitance of 1096 Fg⁻¹, outperforming that of sulfurized CoNi-LDHs, which ranged from 954 to 732 Fg⁻¹ at a CD of 1.0 Ag⁻¹. As a result, increasing the degree of sulfurization could increase the charge transfer resistance, which diminishes the specific capacitance of the LDH. Consequently, the resistance of the electrode materials was not a significant factor in this investigation. Moreover, the constructed asymmetric device (CoNiS-50//AC) demonstrated exceptional cycle stability with a capacitance retention of 92.2% after 10,000 cycles, while achieving elevated E_d and P_d values of 37.8 Whkg⁻¹ and 750 Wkg⁻¹, respectively. Although the specific capacitance diminishes with a rise in the degree of sulfurization in LDH, it may still maintain high values. Based on device findings, sulfurization may be an effective method for preserving the structural stability, which enhances the cycle performance. Similarly, the electrochemically synthesized heterostructure sulfurized CoMn-LDH/NF (CoMn-LDH/CoMn-S/NF) may serve as a superior electrode material for energy storage, characterized by an exceptionally low resistance of 0.0035 Ω, battery-type behavior, and cross-linked nanosheet morphologies, which contribute to a high specific capacity of 792.4 Cg⁻¹ at 1.0 Ag⁻¹ [95]. Furthermore, the higher E_d and P_d values were determined to be 82.6 Whkg⁻¹ and 985 Wkg⁻¹, respectively, as provided via the asymmetric device CoMn-LDH/CoMn-S/NF//AC. This device has the potential to function as a high-energy storage device, demonstrating 94% capacitance retention over 6000 cycles and indicating its suitability for long-term use. Additionally, the device was believed to be practical since it illuminated a green LED for 22 min. Consequently, the sulfurization process via electrochemical methods may serve as a viable strategy to improve the conductivity and structural stability of LDH for superior electrochemical performance.

3.2. Heteroatom-Doped LDHs

Electrodeposition was employed to obtain the sulfur-doped NiCo-LDH on stainless steel (SS) for electrochemical studies [96]. The electrochemical performance was optimized by adjusting the quantities of Ni and Co, revealing that an equivalent proportion of the materials yielded superior performance (S-Ni_0.5Co_0.5-LDH). The electrodeposited S-doped LDH on the SS plate in this investigation was found to exhibit substantially fractured and thick nanosheets. The maximal specific capacity of S-Ni_0.5CO_0.5-LDH was 488.0 Cg⁻¹ at 1.0 Ag⁻¹, which was higher than S-Ni(OH)₂ and S-Co(OH)₂. As a result, the combination of these has a more significant impact on electrochemical performance than bare hydroxide structures. Because of its low resistance, it may account for the rapid ion/charge transport in S-doped NiCo-LDH structures. Due to the fractured and thick sheet structure, cycle stability was limited, with 56.6% capacitance retention after 10,000 cycles. Poor E_d (4.9 Whkg⁻¹) and P_d (83.2 Wkg⁻¹) values were recorded despite the device’s asymmetric design (S-Ni_0.5CO_0.5-LDH//AC), and no cycle stability data for this device was available. Based on the findings, further research may be needed on sulfur-doped NiCo-LDH thick cracked sheets that are coated on stainless steel plates via electrodeposition to improve high and long-term energy storage for SCs due to their poor electrochemical performance.

In another investigation, hydrothermally produced sulfide-doped CoMo-LDH-coated on nickel foam (Co-MoS/NF) indicates C_s of 1655.1 Fg⁻¹ at 10.0 Ag⁻¹, which may be modest, given the high C_d value [97]. The low surface area of this electrode material (7.3 m²·g⁻¹) may account for its reduced capacitance, exhibiting a low charge transfer resistance of 0.404 Ω. Furthermore, sulfide doping on CoMo-LDH showed superior electrochemical performance when compared to bromide-doped CoMo-LDH obtained via the soaking technique and naked LDH prepared using the hydrothermal approach. The sulfide may provide superior conductivity and structural stability to the LDH in comparison to bromide. The designed asymmetric device CoMoS-LDH//rGO demonstrated moderate E_d and P_d values of 37.1 Whkg⁻¹ and 700.0 Wkg⁻¹, respectively, along with a high cycle stability of 93% over 7500 cycles, indicating potential structural stability.

3.3. Fluorine-Doped LDH

The high C_s (683.0 Fg⁻¹) found on the solvothermal-produced fluorine (F)-doped NiCo(OH)₂ coated on carbon cloth (2F-NiCo(OH)₂) might be attributed to low charge resistance and a succulent shape, which allow for significant ion adsorption [98]. Figure 14a depicts a schematic depiction of the solvothermal synthesis of F-NiCo(OH)₂ over 6 h at 120 °C. It was found that changing the fluorine ratio from low to high causes a change in the morphology of LDHs, as well as a possibility of increased agglomeration (Figure 14b). As a result, fluorine might affect the morphology of LDHs. Figure 14c,d clearly indicate that 2F-NiCo(OH)₂ has a larger CV integral area and charge–discharge time than 1F- and 3F-NiCo(OH)₂. The higher performance of 2F-NiCo(OH)₂ might be attributed to its succulent morphology, large surface area, and low resistance compared to others. The developed asymmetric device has the potential for substantial energy storage, owing to high E_d and Pd values of 67.0 Whkg⁻¹ and 10,666.0 Wkg⁻¹, respectively, as well as strong cycle stability, as seen with capacitance retention of 95% after 13,000 cycles. Furthermore, the device’s dependability was tested by illuminating a red LED when two devices were connected, and it was also capable of powering the calculator, indicating that it may be a useful device (Figure 14e). As a result, fluorine might be a promising dopant for succulent-shaped Ni-Co-LDH because it improves conductivity, structural stability, allows for quick redox reactions, and has high ion adsorption for high-energy storage. Similarly, hydrothermally produced, flower-structured, highly conductive fluorine-doped NiFe-LDH (F-NiFe-LDH-24 h) may be a suitable electrode material, owing to its high C_s (1942 Fg⁻¹) at 1.0 Ag⁻¹ and good cycle stability (91.55% over 1000 cycles) [51]. As a result, fluorine could enhance the conductivity and structural stability of NiFe-LDH, demonstrating that it is a promising option for energy storage applications.

3.4. Biological Molecule-Modified LDH

Hydrothermally synthesized flower-structured 0.05 g glucose-modified NiCo-LDH (NiCoLDH-0.05) was found to have high C_s (959.0 Fg⁻¹) at 1.0 Ag⁻¹, though it has a low surface area (18.8 m²·g⁻¹) [27]. However, its low charge transfer resistance (0.58 Ω) allows for strong electrochemical performance. Interestingly, increasing the concentration of glucose in the LDH was shown to change the morphology from rod to flower structures and to increase the surface area, decrease the resistance, and increase the specific capacitance. The developed device NiCoLDH-0.05//AC was able to deliver adequate E_d and P_d values of 32.7 Whkg⁻¹ and 400.0 Wkg⁻¹, respectively, and significant cycle stability was found to be 91.0% over 10,000 cycles, indicating that it is a structurally strong device and that it can be used to illuminate red LEDs for commercial energy storage applications. As a result, glucose might be a low-cost dopant for fine-tuning the morphologies and electrochemical characteristics of NiCo-LDH to achieve excellent performance.

3.5. Oxygen Vacancy-Rich Modified LDH

Oxygen vacancy-rich Mn-doped NiCo-LDH on NF (e.g., Ov-NCM) produced hydrothermally demonstrates a remarkable electrochemical performance, correspondingly determined to be Cs of 4253.0 Fg⁻¹ at 1.0 Ag⁻¹, may be a high-performance electrode material [99]. The morphology was shown to be spherical protrusions on clouds of nanopetals capable of adsorbing high ions for high-energy storage, while NiCo-LDH was revealed to be simply nanopetals. The oxygen vacancy on the LDH electrode material may increase conductivity when compared to bare LDH and Mn-doped LDHs. The Ov-NCM has a lowered charge transfer resistance (0.6 Ω) compared to others. The oxygen vacancy and Mn-doping method on the LDH may result in high structural stability, as shown by the remarkable stability of 89% capacity retention over 5000 cycles. Higher E_d and P_d values of 71.0 Whkg⁻¹ and 800.0 Wkg⁻¹, respectively, given by the Ov-NCM//AC device, and 93.6% capacitance retention over 10,000 cycles, may indicate a high-energy storage material with long-term benefits. Overall, oxygen vacancy-rich Mn-doped NiCo-LDH on NF may be a promising choice for high-energy storage, owing to their low resistance, distinctive spherical protrusions with nanopetal morphology, excellent structural stability, strong ion/charge transfer, and quicker redox reaction.

3.6. Anion-Incorporated LDH

Electrodeposited anion-intercalated NiCo-LDH was coated on carbon paper and tested for electrochemical characteristics at 1.0 M KOH [100]. Accordingly, PO₄³⁻ ion intercalated NiCo-LDH (NiCo-LDH-P) attained a high C_s of 2070.0 Fg⁻¹ at 1.0 Ag⁻¹, possibly because of its thin nanosheet structure, large surface area (113.6 m²·g⁻¹), and low charge transfer resistance (0.028 Ω). The designed asymmetric NiCo-LDH-P//AC device achieved a high E_d of 49.2 Whkg⁻¹ at a P_d of 375.0 Wkg⁻¹, as well as good cycle stability (92.8% capacitance retention over 10,000 cycles), which may be attributed to significant ion adsorption. Furthermore, this device was tested to light up the green LED for more than 15 min and may be suitable for SC applications. Based on the findings, electrochemically deposited anion intercalation into NiCo-LDH on carbon paper might be a promising material for high-energy storage.

Similarly, the solvothermal synthesized anion-incorporated oxygen vacancy-rich NiCo-LDH (NiCo-LDH-ClO₃⁻) microflower exhibits a superior surface area (38.5 m²·g⁻¹) and minimal charge transfer, resulting in a high specific capacity (229.0 mAh g⁻¹ at 1.0 Ag⁻¹) when compared to other anion-incorporated (BrO₃⁻, HPO₄²⁻, SO₄²⁻, and IO₃⁻) oxygen vacancy-rich NiCo-LDH [101]. The incorporation of the anion ClO₃⁻ into LDH exhibited superior electrochemical performance compared to other anion-LDHs, likely attributable to its extensive interlayer spacing, abundant electroactive sites resulting from oxygen vacancies, and the elevated surface area associated with its flower-like morphology, which facilitates enhanced ion diffusion. However, this category of electrode materials may exhibit limited structural stability, as shown by just 60% capacitance retention after 1000 cycles. The NiCo-LDH-ClO₃⁻//AC device exhibited moderate E_d and P_d values of 15.0 Whkg⁻¹ and 1910.0 Wkg⁻¹, respectively, indicating modest energy storage capacity. Additionally, the device demonstrated moderate cycle stability, with a capacitance retention of 62.1% after 5000 cycles. The findings indicate that solvothermal synthesized oxygen vacancy-rich NiCo-LDH-ClO₃⁻ materials may serve as a modest energy storage medium, but their limited surface area, microstructure, and structural stability render them ineffective for prolonged usage. Consequently, this electrode material requires further investigation to enhance specific capacitance and cycle stability.

3.7. Metal Cation-Doped LDHs

Doping metal ions into the electrode material often occupies places within the crystal structure, resulting in changes to the electronic structure, internal charge density, and band gap; hence, it facilitates a charge transfer that enhances conductivity. Furthermore, metal doping may be enlarging the interlayer spacing of LDHs, facilitating the development of layered structured nanosheets, and accelerating ion/electron transport. Recently, Al-doped NiCo-LDH was synthesized on CC (NiCoAl_0.1) using hydrothermal/solvothermal methods [102]. The morphology of the NiCo LDH is varied after Al doping, as it is observed to be interconnected thin nanosheets on CC. Conversely, the bare NiCo-LDH on CC is a hybrid of nanosheets with nanowire arrays. In general, the ordered porous structures of NiCoAl_0.1, which are formed by interconnected nanosheets, have the potential to be effective for the adsorption of high concentrations of ions. The specific capacity of pseudocapacitive NiCoAl_0.1 was 683.2 Cg⁻¹ at 10.0 mAcm⁻², and the high cycle performance was observed to be 108% capacitive retention over 40,000 cycles. In contrast, NiCo-LDH was achieved at 84% over 10,000 cycles. Consequently, the high-order porous structure of NiCoAl_0.1 may exhibit structural stability, rapid redox reaction, and rapid ion/charge transport during the electrochemical reaction. Additionally, the low performance of NiCo LDH nanoarrays may be attributed to their stacked and agglomerated structure, which leads to a reduction in surface area and minor volume changes. Furthermore, the NiCoAl_0.1 LDH//ACC asymmetric device attained an E_d of 0.84 mWhcm⁻² at a P_d of 10.00 mWcm⁻² and a capacitance retention of 135% over 150,000 cycles, making it potentially superior to LDH-based devices. This device has the potential to be an excellent choice for supercapacitor applications, as it is capable of operating a small fan and lighting up 21 red LEDs.

The Mo-doped NiCo-LDH (e.g., NiCo-LDH-Mo0.5) microsphere has a high surface area of 91.7 m²·g⁻¹ and retains its cauliflower shape despite an increase in the molar ratio of the Mo precursor during hydrothermal synthesis [103]. However, the particle size increased when the molar ratio of Mo-doping on LDH increased, as demonstrated in Figure 15a–e. It was found that Mo-doping could not be a suitable approach to NiCo-LDH since the highest C_s value was observed to be 160.5 Fg⁻¹, which was higher than bare NiCo-LDH (83.8 Fg⁻¹) at 1.0 Ag⁻¹. Over 1500 cycles, the NiCo-LDH-Mo0.5 demonstrated 82.7% capacitance retention due to volume variations over cycles; however, this percentage was not as high for the shorter cycle tests. Furthermore, the NiCoDH-Mo0.50//AC device was able to produce a high E_d of 41.3 Whkg⁻¹ at a P_d of 850.0 Wkg⁻¹, with 84% capacitance retention obtained over 8000 cycles, indicating that it might be a decent energy storage and structurally robust device. Interestingly, Mo (0.05 mmol)-doped NiCo-LDH cross-linked nanosheet (MoNiCo-LDH-0.05/CC)-coated carbon cloth using a microwave showed battery-type behavior and achieved a high C_s of 3002.0 Fg⁻¹ at 1.0 Ag⁻¹, which could be due to a high surface area, high charge transport, and a fast redox reaction, as well as high structural stability because 90.7% capacitance retention was maintained over 3000 cycles [104]. It is possible that MoNiCo-LDH-0.05/CC functions effectively because it has low charge transfer resistance (0.34 Ω), while NiCo-LDH/CC has high resistance (1.46 Ω). Additionally, the MoNiCo-LDH-0.05/CC//AC device performed better, achieving a high E_d of 103.3 Whkg⁻¹ at a P_d of 750.0 Wkg⁻¹. It also showed good cycle stability, with 85.2% capacitance retention over 5000 cycles, which may indicate good structural stability. For practical purposes, the two asymmetrical devices that were set up to light up the 32-LED bulb and were connected in a series could be suitable for a supercapacitor. Mo might be a good doping material for NiCo-LDH for high-energy storage because it has low resistance, thin cross-linked nanosheet shapes, and a high surface area (178.8 m²·g⁻¹) that makes it good at adsorbing ions.

In a further investigation, the process of materials can be employed to regulate morphology. Therefore, the two-step hydrothermal process was capable of producing a flower-structured morphology, whereas the single-step hydrothermal process was demonstrated to produce a pure spherical morphology. The two-step procedure was determined to be advantageous for the improvement of the electrochemical performance and surface properties of electrode materials. Thus, the agglomerated nanoflower-structured morphology, high surface area (670.9 m²·g⁻¹), and low charge transfer resistance (0.14 Ω) of the Zn-doped glycerate NiCo-LDH (ZnNiCo-G-H-0.5) synthesized using a two-step hydrothermal method exhibit superior electrochemical performance [105]. Therefore, the pseudocapacitance behavior of these electrode materials was observed to be high, with a C_s of 1145 Fg⁻¹ at 1.0 Ag⁻¹. This may be attributed to the rapid redox reaction, high ion adsorption, and charge transport. However, the moderate capacitance retention of 75.6% over 5000 cycles may result in volume changes during the cycle test. Additionally, the ZnNi-Co−G−H-0.5//AC asymmetric device that was developed has the potential to be a highly effective energy storage device. This is due to the fact that the E_d and P_d values were high, at 40.0 Whkg⁻¹ and 825.0 Wkg⁻¹, respectively. Additionally, the device may be structurally stable, as evidenced by the capacitance retention of 93.8% over 5000 cycles. Consequently, the high surface area, nanoflower structure, low resistance, and structural stability of Zn-doped glycerate NiCo-LDH by two-step hydrothermal processing make it a potential candidate for high-energy storage material. Similarly, the scandium-doped NiCo-LDH with intercalated bromine (e.g., NiCoSc-LDH-Br1) nanoplates, electrochemically produced on carbon cloth, exhibited a remarkable capacitance of 1994 Fg⁻¹ at a current density of 1.0 Ag⁻¹, likely attributable to its extensive surface area (211.4 m²·g⁻¹) and the cross-linked morphology of the nanoplates [106]. However, this material exhibited an elevated charge transfer resistance (12.7 Ω), which did not influence the electrochemical performance. The structural stability may be high, since cycle stability demonstrated 83% capacitance retention after 30,000 cycles. Furthermore, the developed NCS-LDH-Br1//AC device demonstrated moderate E_d and P_d values of 31.4 Whkg⁻¹ and 360 Wkg⁻¹, respectively, with a capacitance retention of 82.3% over 30,000 cycles, indicating its suitability for long-term energy storage applications. The single-pot electrodeposited Sc-doped NiCo-LDH-Br1 on CC may serve as a promising electrode material for high-energy storage and durability, owing to its elevated surface area facilitating substantial ion adsorption and cross-linked nanoplates providing enhanced ion adsorption and structural stability.

3.8. Surfactant-Modified LDH

Poly(vinylpyrrolidone) (PVP) is a surfactant that can regulate particle size and shape. As a result, using PVP-treated Ni foam as a substrate to coat NiCo-LDH (NiCo-LDH-PVP) via electrodeposition might result in high-energy storage [107]. As a result, Figure 15f–h of SEM images demonstrate that NiCo-LDH-PVP had smaller particles than NiCo-LDH-CTAB and NiCo-LDH-SDS. NiCo-LDH-PVP outperformed other surfactants, including SDS (Sodium dodecyl sulfate) and CTAB-treated Ni foam/NiCo-LDHs. It is possible that the PVP had a lower resistance than SDS and CTAB, resulting in a large CV integral area and charge–discharge time for Ni-Co-LDH-PVP (Figure 15i,j). However, since this electrode material was poorly structured, it demonstrated limited cycle stability (57% capacitance retention) after 2000 cycles. Figure 15k indicates that an increase in the number of cycles correlates with a decline in capacitance retention; however, NiCo-LDH-PVP exhibited superior performance compared to the other materials. As a result, surfactant-modified Ni foam/NiCo-LDH may not be suitable for long-term energy storage applications. Furthermore, the designed asymmetric device Ni-Co-LDH-PVP//AC exhibited high E_d and P_d values of 741 μWhcm⁻² and 800 μWcm⁻², respectively, and a high cycle reliability (71.3% capacitance retention over 3000 cycles) can be observed in Figure 15l. Overall, the surfactant-modified Ni foam/NiCo-LDH via electrodeposition demonstrates decent electrochemical performance, owing to high resistance and structural weakness.

Figure 15. (a–e) SEM images of Mo-doped NiCoDH at varying ratios (0.0–1.0), reproduced with permission from [103], copyright 2024, Elsevier. SEM image of NiCo-LDH coated (f) PVP-treated NF, (g) CTAB-treated NF, and (h) SDS-treated NF, (i–k) CV, GCD, and cycle stability profiles of NiCo-LDH modified with various surfactants (PVP, CTAB, and SDS) and bare NiCo-LDH, and (l) cycle stability of NiCoLDH-PVP//AC device, reproduced with permission from [107], copyright 2024, American Chemical Society.

Figure 15. (a–e) SEM images of Mo-doped NiCoDH at varying ratios (0.0–1.0), reproduced with permission from [103], copyright 2024, Elsevier. SEM image of NiCo-LDH coated (f) PVP-treated NF, (g) CTAB-treated NF, and (h) SDS-treated NF, (i–k) CV, GCD, and cycle stability profiles of NiCo-LDH modified with various surfactants (PVP, CTAB, and SDS) and bare NiCo-LDH, and (l) cycle stability of NiCoLDH-PVP//AC device, reproduced with permission from [107], copyright 2024, American Chemical Society.

3.9. Reduced Graphene Oxide-Modified LDH

Reduced graphene oxide (rGO), with its large surface area, conductivity, and chemical and mechanical reliability, may be beneficial for energy storage applications. On the other hand, LDH offers excellent physiochemical characteristics and electrochemical performance. However, it has low electrical conductivity, which impedes excellent electrochemical performance. Given the distinct features of rGO and LDH sheets, the development of a hybrid or composite electrode material based on rGO and LDH nanosheets could present novel possibilities for improving the performance of SCs. Accordingly, hydrothermally synthesized rGO-modified NiCo-LDH (rGO/NiCo-LDH) showed a surface area of 124.8 m²·g⁻¹, and the sheet shape was detected in the modified LDH, but LDH could not be recognized on the rGO, owing to both being in sheet form [108]. At 1.0 Ag⁻¹, the modified LDH had a C_s of 1975.0 Fg⁻¹, which was higher than the bare NiCo-LDH (720.0 Fg⁻¹). It is possible that this variation is the cause of the lower R_ct of rGO/NiCo-LDH (1.41 Ω) in comparison to NiCo-LDH (1.67 Ω). The improved LDH demonstrated significant capacitance retention (98.5% over 10,000 cycles), which might indicate strong electrochemical stability. Furthermore, the constructed asymmetric device, rGO/NiCo-LDH//AC, was able to provide E_d and P_d values of 54.1 Whkg⁻¹ and 789.0 Wkg⁻¹, respectively. This device was also shown to offer remarkable cycle stability, with 95% capacitance retention over 10,000 cycles. Similarly, 10 mg rGO was composited with NiCo-LDH-coated Ni foam (rGO/NiCo-LDH/Ni foam) using the hydrothermal technique, showing outstanding electrochemical performance [109]. As a result, the porous structured honeycomb-type composite surface area was 42.5 m²·g⁻¹, which was more than that of bare NiCo-LDH (32.1 m²·g⁻¹). The charge transfer resistance of rGO/NiCo-LDH/Ni foam was determined to be 0.394 Ω, lower than NiCo-LDH (0.428 Ω). Therefore, rGO improved the conductivity of the NiCo-LDH, increasing the C_s values by 2702.0 Fg⁻¹ as compared to bare NiCo-LDH (1035.0 Fg⁻¹). After 10,000 cycles, a capacitance retention of 108% suggests that the rGO/NiCo-LDH/Ni foam has strong structural stability. However, the asymmetric device composed of NiCo-LDH/rGO/Ni foam and AC/Ni foam did not perform well, exhibiting E_d and P_d values of 48.5 Whkg⁻¹ and 200.0 Wkg⁻¹, respectively; however, it demonstrated excellent structural stability with a reported capacitance retention of 97% over 5000 cycles. Similarly, hydrothermally produced rGO-modified NiCo-LDH (e.g., NCR)-coated Ni foam was investigated for its electrochemical activities in 1.0 M KOH [110]. For comparative purposes, the wt% (10–40%) of rGO was varied during synthesis to alter the LDH (e.g., NCR1-NCR4). Figure 16a illustrates the hydrothermal method’s detailed synthesis of rGO and its various wt% modified NiCo-LDH deposits on Ni foam. Increasing the wt% of rGO from 10 to 20 resulted in an increase in composite surface area from 60.3 to 70.2 m²·g⁻¹, followed by a drop (66.6 to 64.6 m²·g⁻¹) when the wt% climbed from 30 to 40. As a result, the excess wt% of rGO that blocked the LDH pore lowered the surface area. As shown in Figure 16b, the electrode materials NCR1 through NCR4’s morphology was found to be microflowers with very thin petals and no substantial changes. However, increasing the wt% of rGO influences the size of the microflower, resulting in larger petals at 40% rGO. The R_ct values of NCR decrease with increasing wt% of rGO up to 20%, attributed to an increase in surface area; however, R_ct values thereafter rise with wt% of rGO from 30% to 40%, owing to a reduction in surface area. Consequently, resistance may be reduced by increasing the surface area, whereas it will be increased by decreasing the surface area. The CV profile (Figure 16c) indicates that NCR2 (Ni-Co-LDH/rGO20%) exhibited a larger integral area than the other composites. Consequently, the C_s of the NCR2 (2823.0 Fg⁻¹ at 4.0 mAcm⁻²) was elevated, which was likely attributable to the low R_ct (2.6 Ω) in comparison to others. Furthermore, the NCR2 has a high structural stability, with 96.2% capacitance retention over 10,000 cycles, as shown in Figure 16d. The proposed device (NiCo-LDH/rGO/Ni foam//AC/Ni foam), shown in Figure 16e, offers significant energy storage capabilities, achieving a high E_d of 63.9 Whkg⁻¹ at a P_d of 7080.0 Wkg⁻¹. However, this report did not investigate the electrochemical activity of bare NiCo-LDH for comparative purposes.

In the case of bacterial cellulose incorporated with 0.75 wt% of rGO-modified NiCo-LDH (BC/rGO0.75/NiCo-LDH) using the reflux method, it was revealed that the low electrochemical performance (311.0 Fg⁻¹ at 1 mVs⁻¹) [111]. This result may be due to either the high resistance of the electrode material or the low transfer of electrons and ions from the electrolyte solution to the electrode material. It was found that this study does not undertake cycle stability and device performance tests. In another work, 30 mg of rGO produced from aluminum electric spent cathode carbon was composited with NiCo-LDH (NiCo-LDH/rGO-SCC₃₀) via a microwave-assisted approach, demonstrating that a blend of nanoflower and nanosheet morphologies may have a high ion path for energy storage [112]. As a result, this report produced a high C_s value of 2818.0 Fg⁻¹ at 1.0 Ag⁻¹, which may indicate low resistance, a large surface area, and outstanding morphologies. The constructed device, NiCo-LDH/rGO-SCC//AC, may have a reasonable energy storage property, according to an E_d value of 35.9 Whkg⁻¹ at a P_d of 375.0 Wkg⁻¹; however, its structural stability may be limited due to 85.2% capacity retention over 1000 cycles. Similarly, hydrothermal method was employed to produce sheets/spherical-shaped rGO/ZnMgAl-LDH with adjusted GO and ZnMgAl-LDH molar ratios of 1:0.5, 1:1, and 1:2 for electrochemical studies in 1.0 M KOH [113]. It was found that spherical-shaped LDHs integrate with rGO sheets. However, this kind of composition did not improve electrochemical performance since the highest C_s value achieved with a 1:1 ratio of rGO/ZnMgAl-LDH (e.g., ZMAG2) was 656.6 Fg⁻¹ at 1.0 Ag⁻¹ when compared to other ratios. However, this value was larger than that of the bare ZnMgAl-LDH (397.9 Fg⁻¹). These electrode materials’ low C_s values might be attributed to their high resistance. Furthermore, ZMAG2 demonstrated strong cycle stability (81% capacitance retention over 10,000 cycles), which might indicate high structural stability in the electrolyte over lengthy cycles. The designed asymmetric device, ZMAG2//AC, demonstrated excellent cycle stability (89% capacitance retention) across 15,000 cycles, with specific E_d and P_d values of 31.7 Whkg⁻¹ and 784.0 Wkg⁻¹, respectively. Based on the findings, ZMAG2, both as an electrode and in device form, may achieve high structural stability in an electrolyte over lengthy cycles.

Recently, rGO was modified on F-doped NiFe-LDH using a hydrothermal technique for electrochemical studies in 2.0 M KOH [114]. Figure 17a shows a schematic illustration of the hydrothermal synthesis of rGO-modified F-doped NiFe-LDH (rGO/F-NiFe-LDH). In brief, the ultrasonicated GO was added to the LDH precursor solution, along with NH₄F, urea, and other chemicals. After stirring, the whole solution was transferred to an autoclave for hydrothermal treatment at 120 °C for 24 h to produce rGO/F-NiFe-LDH. The nanoflower shape of LDH was perfectly formed, depending on the amount of GO sheets added (5–20 mg) to the F-doped LDH. As shown in Figure 17b(i–iv), increasing GO for assisting LDH flower growth may be more effective than adding chemical agents. According to the CV investigations (Figure 17c), rGO-15/F-NiFe-LDH may achieve good electrochemical performance, owing to its large integral area in comparison to other compositions. At 1.0 Ag⁻¹, rGO-15/F-NiFe-LDH achieved a high C_s of 2860.0 Fg⁻¹. The high value of the modified LDH could account for the low R_ct (0.12 Ω). Furthermore, this composition has shown remarkable cycle stability, with 102% capacitance retention over 1000 cycles. The designed F-NiFe-LDH/rGO//AC device was able to offer a high Ed value of 110.0 Whkg⁻¹ at a P_d of 1250.0 Wkg⁻¹, indicating that it could serve as a high-energy storage device suitable for commercial applications. As a result, the addition of rGO and fluorine to NiFe-LDH could significantly boost its electrochemical performance, owing to their high conductivity and cycle stability. Figure 17d compares the electrolyte ion (OH−) adsorption on bare and rGO-modified F-NiFe-LDH. It was found that rGO may increase the active sites and surface area of the F-doped LDH, resulting in increased ion paths and improved electrochemical performance when compared to F-NiFe-LDH, which had low active sites and low paths. Despite the composite material’s highly porous surface, the C_s (916.0 Fg⁻¹) value was low at 1.0 Ag⁻¹, and the cycle stability was low at 96.1% over 4000 cycles in the case of chemical reduction used to synthesize 16 mg of rGO composite with B-doped NiMn-LDH (NiMnB/rGO-16) [115]. Consequently, the device (NiMnB/rGO-16//AC) was also found to have low cycle stability (86.3% over 8000 cycles) and low E_d and P_d values of 50.2 Whkg⁻¹ and 800.4 Wkg⁻¹, respectively. Thus, the low electrochemical performance of core–shell and porously structured NiMnB/rGO-16 may be attributed to its low structural stability and low resistance.

NiMn-LDH/Ni foam was synthesized using the hydrothermal method, and rGO was subsequently modified through the dip coating of the NiMn-LDH/Ni foam (rGO/NiMn-LDH/Ni foam) for electrochemical studies in 3.0 M KOH [116]. The prepared electrode materials appeared to be porous nanoflake LDH that was embedded with rGO sheets, which may have the potential for ion adsorption for energy storage. Interestingly, the rGO-modified LDH exhibited a larger surface area (99.5 m²·g⁻¹) than the NiMn-LDH/Ni foam (43.5 m²·g⁻¹). The charge transfer resistance of 0.15 Ω in this rGO-modified LDH is lower than that of naked NiMn-LDH/Ni foam (0.33 Ω), indicating higher conductivity. Consequently, the utilization of high-energy storage may be facilitated by the ability of rGO to reduce the resistance and increase the surface area of the LDH. As a result, the modified LDH exhibited a high C_s of 1090.0 at 1.0 Ag⁻¹ and excellent cycle stability, with 92.3% capacitance retention over 10,000 cycles, outperforming bare NiMn-LDH/Ni foam (87.5%). The developed device, which is NiMn-LDH/rGO//AC, may offer decent energy storage properties due to its high E_d value of 45.8 Whkg⁻¹ at a P_d of 623.0 Wkg⁻¹. As a result of its low resistance and good cycle stability, rGO-modified NiMn-LDH could pose as a promising choice for supercapacitor applications. Another study found that rGO/NiMn-LDH/Ni foam//AC achieved strong cycle stability, with 99.5% capacitance retention over 12,000 cycles and a moderate E_d of 0.95 mWhcm⁻² at a P_d of 0.83 mWcm⁻², making it a promising device for extended cycle life [117].

According to Table 2, the most common approaches for doping LDHs are the hydrothermal approach and electrodeposition, which may be the simplest and most straightforward processes. However, the hydrothermal approach might be deemed more successful than electrodeposition since it is a low-cost procedure in a closed pot at a high temperature for a longer period of time, as well as being environmentally benign. The majority of the doped LDH morphologies were identified as flowers or sheet-like structures. Therefore, these findings indicated that LDHs often exhibit flower or sheet morphologies. In general, doping or surface modification can decrease the electrode material’s surface area by blocking the surface pores. However, modified LDHs such as NiCo-LDH-Mo0.5, MoNiCo-LDH-0.05/CC, ZnNiCo-G-H-0.5, NCS-LDH-Br1, rGO/NiCo-LDH, and rGO/NiMn-LDH/Ni foam continue to exhibit larger surface areas. The hydrothermal process, electrodeposition, the microwave technique, and dip coating have been shown to generate larger surface areas of electrode materials, which may include a high-porosity formation on their surfaces. With the exception of NiCo-LDH-Mo0.5 and NiCo-LDH-PVP, almost all modified LDHs showed a high specific capacitance, for example, F-NiFe-LDH-24 h, CoMoS/NF, MoNiCo-LDH-0.05/CC, Ov-NCM, NCS-LDH-Br1, NiCo-LDH-P, NiCo-LDH/NF, rGO/NiCo-LDH/Ni foam, rGO-15/F-NiFe-LDH, and NiCoLDH/rGO-SCC₃₀. Doped LDHs exhibit low charge transfer resistance and high specific capacitance values; their morphologies may range from sheets to spheres or even flowers. With the exception of S-Ni_0.5Co_0.5-LDH, NiCoLDH-ClO₃⁻, and NiCo-LDH-PVP, all doped LDHs achieve excellent cycle stability and potentially high structural stability. On the other hand, rGO-modified LDHs have shown remarkable electrochemical performance, including high specific capacitance and cycle stability, as well as significant energy storage qualities, owing to their high conductivity and mechanical robustness. It follows that LDHs may not benefit from sulfurization, PVP, or anions in terms of long-term cycle stability. When the LDHs were doped with Al, Mo, Zn, Mn, Sc, or F, their structural stability would be improved, making them more suitable for long-term usage. As far as cycle stability was concerned, Al-doped LDH performed well. The energy storage characteristics of all devices that included doped LDH were significantly improved. The results indicated that devices with CoMn-LDH/CoMn-S/NF//AC, 2F-NiCo(OH)₂//AC, MoN-iCo-LDH-0.05/CC//AC, Ov-NCM //AC, rGO/NiCo-LDH/Ni foam//AC/Ni foam, and F-NiFe-LDH/rGO//AC exhibited improved energy storage capacities, most likely due to the doping and surface modification on LDHs, which resulted in outstanding conductivity and structural stability. The most effective modified LDHs for supercapacitor applications include metal cation-doped, oxygen vacancy-rich, fluorine-doped, anion-incorporated, and rGO-modified LDHs.

Table 2. Electrochemical performance of modified LDHs using various strategies in three- and two-electrode systems.

Electrode Materials	Synthesis	Morphology	Three Electrodes		Two Electrodes			EL	Ref
			Cs at Cd	Cycle Stability	Devices	Ed	Pd
Sulfurized LDHs
CoNiS-10	Ion exchange	Agglomerated sheets (30.0 m2·g−1)	954.0 at 1.0	-	CoNiS-50//AC	37.8	750.0	6 M KOH	[94]
CoMn-LDH/CoMn-S/NF	Chronoamperometric	Cross-linked nanosheets	-	-	CoMn-LDH/CoMn-S/NF//AC	82.6	985	3 M KOH	[95]
Heteroatom-doped LDHs
S-Ni0.5Co0.5-LDH	Electrodeposition	Thick cracked nanosheet	-	56.6%, after 10,000 cycles	S-Ni0.5Co0.5-LDH //AC	4.9	83.2	1 M KOH	[96]
CoMoS/NF	Hydrothermal	Leaf structure (7.3 m2·g−1)	1655.1 at 10.0	-	CoMoS/NF//rGO	37.14	700.0	3 M KOH	[97]
Fluorine-doped LDH
2F-NiCo(OH)2	Solvothermal	Succulent	683.0 at 1.0 mAcm−2	-	2F-NiCo(OH)2//AC	67.0	10,666.0	1 M KOH	[98]
F-NiFe-LDH-24 h	Hydrothermal	Flower structure	1942.0 at 1.0	91.55%, after 1000 cycles	-	-	-	2 M KOH	[51]
Biological molecule-modified LDH
NiCoLDH-0.05	Hydrothermal	Flower structure (18.8 m2·g−1)	959.0 at 1.0	-	NiCoLDH-0.05//AC	32.7	400	6 M KOH	[27]
Oxygen-vacancy-rich modified LDH
Ov-NCM	Hydrothermal	Spherical protrusions/ nanopetals	4253.0 at 1.0	89%, after 5000 cycles	Ov-NCM //AC	71.0	801	2 M KOH	[99]
Anion-incorporated LDH
NiCo-LDH-P	Electrodeposition	Nanosheets	2070.0 at 1.0	-	NiCo-LDH-P//AC	49.2	375.0	1 M KOH	[100]
NiCoLDH-ClO3−	Solvothermal	Microflower (25.6 m2·g−1)	-	60%, after 1000 cycles	NiCoLDH-ClO3−//AC	15.06	1910.0	1 M KOH	[101]
Metal cation-doped LDHs
NiCoAl0.1	Hydrothermal/ solvothermal	Interconnected nanosheets (69.06 m2·g−1)	-	108%, after 40,000 cycles	NiCoAl0.1//ACC	0.84 mWh cm−2	10.0 mWcm−2	2 M KOH	[102]
NiCo-LDH-Mo0.5	Hydrothermal	Cauliflower structured microsphere (91.7 m2·g−1)	160.5 at 1.0	82.7%, after 1500 cycles	NiCo-LDH-Mo0.5//AC	41.3	850.0	6 M KOH	[103]
MoNiCo-LDH-0.05/CC	Microwave	Cross-liked nanosheets (178.8 m2·g−1)	3002.0 at 1.0	90.7%, after 3000 cycles	MoNiCo-LDH-0.05/ CC//AC	103.3	750.0	2 M KOH	[104]
ZnNiCo−G−H-0.5	Two-step hydrothermal	Flower structures (670.9 m2·g−1)	1145.0 at 1.0	75.68%, after 5000 cycles	ZnNiCo−G−H-0.5//AC	40.0	825.0	6 M KOH	[105]
NCS-LDH-Br1	Electrodeposition	Cross-linked nanoplates (211.4 m2·g−1)	1994.0 at 1.0	70.4%, after 15,000 cycles	NCS-LDH-Br1//AC	31.4	360.0	3 M KOH	[106]
Surfactant-modified LDH
NiCo-LDH-PVP	Electrodeposition	Nanoflower	-	57.0%, after 2000 cycles	NiCo-LDH-PVP//AC	741.0 μWhcm−2	800.0 μWcm−2	3 M KOH	[107]
rGO-modified LDH
rGO/NiCo-LDH	Hydrothermal	Sheets (124.8 m2·g−1)	1975.0 at 1.0	98.5%, after 10,000 cycles	rGO/NiCo-LDH//AC	54.1	789.0	3 M KOH	[108]
rGO/NiCo-LDH/Ni foam	Hydrothermal	Honeycomb structure (42.5 m2·g−1)	2702.0 at 0.25	108%, after 10,000 cycles	NiCo-LDH/rGO/Ni foam//AC/Ni foam	48.5	200.0	2 M KOH	[109]
rGO/NiCo-LDH/Ni foam (NCR2)	Hydrothermal	Microflower (70.2 m2·g−1)	2823.0 at 4.0 mAcm−2	96.2%, after 10,000 cycles	rGO/NiCo-LDH/Ni foam//AC/Ni foam	63.9	7080.0	1 M KOH	[110]
BC/rGO0.75-NiCo-LDH	Reflux	Spherical particle embedded sheet	-	-	-	-	-	1 M H2SO4	[111]
NiCoLDH/ rGO-SCC30	Microwave- assisted hydrothermal	Nanoflower modified sheet	2818.0 at 1.0	-	NiCoLOH/rGO-SCC//AC	35.9	375.0	6 M KOH	[112]
rGO/ZnMgAl-LDH (1:1)	Hydrothermal	Sheets/spherical (78.5 m2·g−1)	656.6 at 1.0	81.0%, after 10,000 cycles	rGO/ZnMgAl-LDH//AC	31.7	784.0	1 M KOH	[113]
rGO-15/F-NiFe-LDH	Hydrothermal	Nanoflower/sheets	2860.0 at 1.0	102%, after 1000 cycles	F-NiFe-LDH/rGO//AC	110.0	1250.0	2 M KOH	[114]
rGO/NiMnB-16	Chemical reduction	Core–shell porous structured particles	916.0 at 1.0	96.1%, after 4000 cycles	NiMnB/rGO-16//AC	50.2	800.4	6 M KOH	[115]
rGO/NiMn-LDH/Ni foam	Dip coating	Porous nanoflake/sheet (99.5 m2·g−1)	1090.0 at 1.0	92.3%, after 10,000 cycles	NiMn-LDH/Ni foam/rGO//AC	45.8	623.0	3 M KOH	[116]

Specific capacitance, C_s (Fg⁻¹); current density, C_d (Ag⁻¹); energy density, E_d (Whkg⁻¹); power density, P_d (Wkg⁻¹); EL (electrolyte).

Table 2. Electrochemical performance of modified LDHs using various strategies in three- and two-electrode systems.

Electrode Materials	Synthesis	Morphology	Three Electrodes		Two Electrodes			EL	Ref
			Cs at Cd	Cycle Stability	Devices	Ed	Pd
Sulfurized LDHs
CoNiS-10	Ion exchange	Agglomerated sheets (30.0 m2·g−1)	954.0 at 1.0	-	CoNiS-50//AC	37.8	750.0	6 M KOH	[94]
CoMn-LDH/CoMn-S/NF	Chronoamperometric	Cross-linked nanosheets	-	-	CoMn-LDH/CoMn-S/NF//AC	82.6	985	3 M KOH	[95]
Heteroatom-doped LDHs
S-Ni0.5Co0.5-LDH	Electrodeposition	Thick cracked nanosheet	-	56.6%, after 10,000 cycles	S-Ni0.5Co0.5-LDH //AC	4.9	83.2	1 M KOH	[96]
CoMoS/NF	Hydrothermal	Leaf structure (7.3 m2·g−1)	1655.1 at 10.0	-	CoMoS/NF//rGO	37.14	700.0	3 M KOH	[97]
Fluorine-doped LDH
2F-NiCo(OH)2	Solvothermal	Succulent	683.0 at 1.0 mAcm−2	-	2F-NiCo(OH)2//AC	67.0	10,666.0	1 M KOH	[98]
F-NiFe-LDH-24 h	Hydrothermal	Flower structure	1942.0 at 1.0	91.55%, after 1000 cycles	-	-	-	2 M KOH	[51]
Biological molecule-modified LDH
NiCoLDH-0.05	Hydrothermal	Flower structure (18.8 m2·g−1)	959.0 at 1.0	-	NiCoLDH-0.05//AC	32.7	400	6 M KOH	[27]
Oxygen-vacancy-rich modified LDH
Ov-NCM	Hydrothermal	Spherical protrusions/ nanopetals	4253.0 at 1.0	89%, after 5000 cycles	Ov-NCM //AC	71.0	801	2 M KOH	[99]
Anion-incorporated LDH
NiCo-LDH-P	Electrodeposition	Nanosheets	2070.0 at 1.0	-	NiCo-LDH-P//AC	49.2	375.0	1 M KOH	[100]
NiCoLDH-ClO3−	Solvothermal	Microflower (25.6 m2·g−1)	-	60%, after 1000 cycles	NiCoLDH-ClO3−//AC	15.06	1910.0	1 M KOH	[101]
Metal cation-doped LDHs
NiCoAl0.1	Hydrothermal/ solvothermal	Interconnected nanosheets (69.06 m2·g−1)	-	108%, after 40,000 cycles	NiCoAl0.1//ACC	0.84 mWh cm−2	10.0 mWcm−2	2 M KOH	[102]
NiCo-LDH-Mo0.5	Hydrothermal	Cauliflower structured microsphere (91.7 m2·g−1)	160.5 at 1.0	82.7%, after 1500 cycles	NiCo-LDH-Mo0.5//AC	41.3	850.0	6 M KOH	[103]
MoNiCo-LDH-0.05/CC	Microwave	Cross-liked nanosheets (178.8 m2·g−1)	3002.0 at 1.0	90.7%, after 3000 cycles	MoNiCo-LDH-0.05/ CC//AC	103.3	750.0	2 M KOH	[104]
ZnNiCo−G−H-0.5	Two-step hydrothermal	Flower structures (670.9 m2·g−1)	1145.0 at 1.0	75.68%, after 5000 cycles	ZnNiCo−G−H-0.5//AC	40.0	825.0	6 M KOH	[105]
NCS-LDH-Br1	Electrodeposition	Cross-linked nanoplates (211.4 m2·g−1)	1994.0 at 1.0	70.4%, after 15,000 cycles	NCS-LDH-Br1//AC	31.4	360.0	3 M KOH	[106]
Surfactant-modified LDH
NiCo-LDH-PVP	Electrodeposition	Nanoflower	-	57.0%, after 2000 cycles	NiCo-LDH-PVP//AC	741.0 μWhcm−2	800.0 μWcm−2	3 M KOH	[107]
rGO-modified LDH
rGO/NiCo-LDH	Hydrothermal	Sheets (124.8 m2·g−1)	1975.0 at 1.0	98.5%, after 10,000 cycles	rGO/NiCo-LDH//AC	54.1	789.0	3 M KOH	[108]
rGO/NiCo-LDH/Ni foam	Hydrothermal	Honeycomb structure (42.5 m2·g−1)	2702.0 at 0.25	108%, after 10,000 cycles	NiCo-LDH/rGO/Ni foam//AC/Ni foam	48.5	200.0	2 M KOH	[109]
rGO/NiCo-LDH/Ni foam (NCR2)	Hydrothermal	Microflower (70.2 m2·g−1)	2823.0 at 4.0 mAcm−2	96.2%, after 10,000 cycles	rGO/NiCo-LDH/Ni foam//AC/Ni foam	63.9	7080.0	1 M KOH	[110]
BC/rGO0.75-NiCo-LDH	Reflux	Spherical particle embedded sheet	-	-	-	-	-	1 M H2SO4	[111]
NiCoLDH/ rGO-SCC30	Microwave- assisted hydrothermal	Nanoflower modified sheet	2818.0 at 1.0	-	NiCoLOH/rGO-SCC//AC	35.9	375.0	6 M KOH	[112]
rGO/ZnMgAl-LDH (1:1)	Hydrothermal	Sheets/spherical (78.5 m2·g−1)	656.6 at 1.0	81.0%, after 10,000 cycles	rGO/ZnMgAl-LDH//AC	31.7	784.0	1 M KOH	[113]
rGO-15/F-NiFe-LDH	Hydrothermal	Nanoflower/sheets	2860.0 at 1.0	102%, after 1000 cycles	F-NiFe-LDH/rGO//AC	110.0	1250.0	2 M KOH	[114]
rGO/NiMnB-16	Chemical reduction	Core–shell porous structured particles	916.0 at 1.0	96.1%, after 4000 cycles	NiMnB/rGO-16//AC	50.2	800.4	6 M KOH	[115]
rGO/NiMn-LDH/Ni foam	Dip coating	Porous nanoflake/sheet (99.5 m2·g−1)	1090.0 at 1.0	92.3%, after 10,000 cycles	NiMn-LDH/Ni foam/rGO//AC	45.8	623.0	3 M KOH	[116]

Specific capacitance, C_s (Fg⁻¹); current density, C_d (Ag⁻¹); energy density, E_d (Whkg⁻¹); power density, P_d (Wkg⁻¹); EL (electrolyte).

According to Table 1 and Table 2, both newly synthesized LDHs and modified LDHs performed well in the electrochemical experiments. Interestingly, the electrochemical performance of modified LDHs is better than their corresponding bare LDH. Mostly, modified LDHs achieved outstanding electrochemical performance; moreover, a large number of newly synthesized LDHs were shown to be the best for electrochemical research. Applying surface modification or doping to these newly synthesized LDHs could potentially enhance their energy storage properties. The literature has demonstrated that NiCo-LDHs-based LDHs and their modified forms are more effective in energy storage. Additionally, producing LDHs for supercapacitor applications and focusing on low-cost synthesis methodologies may be preferable to synthesizing doping or surface modification on LDHs for electrochemical studies, which might minimize process cost and time.

4. Advantages and Disadvantages of Three- and Two-Electrode Systems

Prior to assessing the electrochemical activities and stabilities of electrode materials, it may be necessary to develop an electrochemical system with consistent electrochemical cells. As a consequence, electrochemical cell design, electrode selection and preparation, and electrolyte choice should all be carefully considered. In electrochemistry, two- and three-electrode systems perform different functions, each with its own set of advantages and disadvantages. Accordingly, (i) a three-electrode system is commonly utilized to investigate the electrochemical properties of electrode materials, while a two-electrode system is employed for device-based testing, either symmetric or asymmetric [118,119,120]. (ii) The two-electrode system is less expensive, having simply a working and a counter electrode, and easier to build for electrochemical experiments. A three-electrode system may be expensive since it needs working and counter electrodes, as well as a reference electrode and a potentiostat. (iii) The three-electrode arrangement can be used for the fundamental electrochemical characterization of specific electrode materials [118,121]. This system facilitates the regulation of potential and offers an in-depth understanding of the behavior of electrode materials. Nevertheless, this system does not accurately represent the actual device outcomes, and the results it generates regarding electrode materials may be inaccurate, as it evaluates the behavior of individual electrodes [122]. In contrast, the two-electrode system was designed to mimic real-world device scenarios, with both electrodes contributing to the total capacitance, which may provide an accurate result. This system measures a device’s practical performance, including E_d (Whkg⁻¹), P_d (Wkg⁻¹), R (Ω), and cycle performance. (iv) However, the two-electrode system can provide more accurate results; still, it lacks precise control over individual electrode materials, making it unsuitable for fundamental electrochemical experiments. However, they are unable to precisely measure the potential of the working electrode because of the lack of a reference electrode, which may result in oversights in the findings [118]. In contrast, the three-electrode system is appropriate for preliminary materials testing and electrochemical characterization, whereas the two-electrode system is primarily used to determine practical applicability and device-level performance. (v) In a two-electrode system, polarization effects may grow due to the current flowing through the integrated counter/reference electrode, altering electrochemical measurements and resulting in inaccuracies [123]. In a three-electrode configuration, polarization effects are decreased by the distance of the counter electrode from the reference electrode, allowing for higher measurement accuracy [121,124]. (vi) The experiment’s specific needs determine whether a two- or three-electrode setup is used. Accordingly, a two-electrode setup is simple and inexpensive, but a three-electrode configuration provides more control, accuracy, and diversity, making it preferred for several electrochemical experiments that need reliable results. Moreover, a two-electrode system produces the specific capacitance of the whole cell, while a three-electrode configuration determines the specific capacitance of the active material of the electrode. (vii) Furthermore, a three-electrode system enables precise regulation of the working electrode potential, resulting in better reproducibility of the results [118]. On the other hand, a two-electrode configuration may result in unpredictability and challenges in reproducing results due to variations in the counter electrode and solution resistance [125].

5. Conclusions and Future Perspectives

LDHs are considered the most promising electrode materials for energy storage applications due to their cheap cost, simplicity of synthesis, and high specific capacitance. Specifically, their tunable host metal ions and exchangeable interlayer anions make them a high-conductivity material for supercapacitors. As a result, in this review, the most recent advances that improve the capacitive and device performance of different bare and modified LDHs for supercapacitors were critically reviewed. It was found that three vital characteristics, including a large surface area, a nanoflower or nanosheet shape, and low charge transfer resistance, exert an important influence on the electrochemical performance of LDHs. The electrode materials might be coated directly on the substrate, such as nickel foam or carbon cloth, to provide binder-free electrodes that minimize internal resistance and increase charge transfer. Furthermore, altering the electronic structure of LDHs via oxygen vacancy techniques might improve charge transfer and electrochemical performance. Several studies have shown that intercalation of anions, metal cations, fluorine, and rGO into LDH structures may increase interlayer spacing and conductivity due to their capacitive characteristics. Modified LDHs demonstrated superior capacitive properties for electrochemical research. However, newly synthesized plain LDHs found in the literature seem to have a favorable response in electrochemical performance, which may exhibit high capacitive qualities and cycle stability because of their outstanding flower and sheet morphologies, large surface area, and high ion path. Additionally, changing the metal category in the LDH structure may dramatically increase electrochemical performance. As a result, further attention may be needed to focus on the surface modification or doping strategies of recently developed LDHs that achieve high electrochemical performance. NiCo-LDHs and devices based on them were determined to be the best electrode materials, owing to their high specific capacitance, cycle stability, and energy density. An asymmetric supercapacitor device that combines LDH-based materials and activated carbons achieves high energy density.

One of the most significant challenges in LDH synthesis is the development of large-scale preparation. Therefore, a future study should concentrate on the possibilities of large-scale techniques. In terms of application, it is challenging to determine the structural properties of LDHs, which are necessary for justifying and evaluating the relationships between their structures and electrochemical performances. Focusing on low-cost synthesis methods combined with modern instrumentation techniques is critical for achieving the desired structural characteristics for industrial manufacturing. Furthermore, the physicochemical and conductivity properties of LDHs are determined by their structural characteristics, which affect their electrochemical performance. Therefore, future research should concentrate on enhancing the surface and electrical properties of LDHs for the development of high-energy storage supercapacitors. To improve surge ion approachability and structural stability, effective surface doping and/or modification should be researched. Derived LDHs and magnetic property-based LDHs may have effective surface and electronic properties, resulting in high capacitive characteristics; hence, future research is predicted to concentrate on low-cost derived LDHs and magnetic-field-aided LDHs. Integrating LDHs into wearable and flexible supercapacitor devices is a promising future advance. It is expected that computational tools will aid in the discovery of efficient LDH structures and optimal attributes for high-energy storage. Cost is an important issue in the commercialization of LDHs; hence, synthesis processes should be inexpensive, with the potential of quick approaches. It is expected that, due to the efforts of many researchers, LDHs will improve in the future and contribute more to the advancement of energy storage-based systems.