Synergistically engineered NiCoMn-LDH@carbon nanofibers for boosted electrochemical charge storage and high-energy supercapacitors

Abstract

Layered double hydroxides (LDHs) are emerging as advanced electrode materials for supercapacitors owing to their unique layered structures, high theoretical capacitance, and tunable metal composition. In this study, a Nickel Cobalt Manganese Layered Double Hydroxide (NiCoMn-LDH) composite with carbon nanofibers (NiCoMn-LDH@CNF) is synthesized with varying stoichiometries via a sonication-assisted method. Structural and morphological characterizations confirm the successful integration of NiCoMn-LDH with conductive CNF networks. Among the tested ratios, the 2 : 1 composition exhibits superior electrochemical performance. The enhanced behaviour of NiCoMn-LDH@CNF (2 : 1) originates from the synergistic interaction between the redox-active LDH component and the highly conductive CNF matrix, which together facilitate efficient ion diffusion and electron transport. The optimized composite demonstrates excellent conductivity and high specific capacitance during electrochemical evaluation. An asymmetric supercapacitor is further fabricated with NiCoMn-LDH@CNF as the positive electrode and activated carbon (AC) as the negative electrode. The device delivers a specific capacitance of 348 F g⁻¹ at 1 A g⁻¹, achieving an energy density of 123.8 W h kg⁻¹ at a power density of 800 W kg⁻¹. These results highlight the potential of NiCoMn-LDH@CNF as a high-performance electrode material for next-generation energy storage applications.

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1. Introduction

The global demand for supercapacitors is substantial, with market analysts predicting that the industry will reach USD 912 million by 2027.^1,2 Leading regions include North America, Asia, Europe, and the Pacific. Such market growth reflects rapid advancements in supercapacitor research, where innovations in electrodes, separators, and electrolyte materials have boosted single-cell capacitance to thousands of farads.³ However, the single-cell terminal voltage remains limited to 2.3–3.8 V per cell, typical for many commercial electrochemical double-layer capacitors (EDLCs), prompting ongoing research to overcome this challenge. Supercapacitors, with their distinct advantages such as high-power density, rapid charge–discharge capability, and excellent cycling stability, have emerged as one of the most promising energy storage systems.⁴ These properties make them suitable for applications ranging from renewable energy integration to portable electronics and electric vehicles. However, the relatively low energy density of supercapacitors compared to batteries remains a bottleneck, necessitating the exploration of novel materials and hybrid strategies to bridge this gap. Among the materials studied,^5–7 LDHs have gained prominence due to their tuneable composition, distinctive layered structure, and excellent redox activity. LDHs are anionic clays with positively charged brucite-like layers intercalated with charge-balancing anions and water molecules.⁸ The ability to incorporate multiple metal ions into the structure allows LDHs to be tailored for specific electrochemical applications. This flexibility, combined with their inherently high theoretical specific capacitance and abundant redox-active sites, positions LDHs as strong candidates for use in supercapacitors. Despite these advantages, LDHs face challenges such as limited electrical conductivity and low-rate capability, which hinder their overall performance in practical applications.⁹ To address these shortcomings, hybridizing LDHs with conductive carbonaceous materials has emerged as a highly effective strategy.¹⁰ Such hybrids combine the superior redox activity of LDHs with the excellent electrical conductivity and structural stability of carbon material,¹¹ creating a synergistic effect that significantly enhances the performance of the electrode. CNFs have been identified as one of the most effective carbon-based materials for hybridization with LDHs. CNFs possess a unique combination of properties, including high electrical conductivity, excellent mechanical strength, and a well-developed interconnected porous structure.¹² These characteristics enable CNFs to serve as a robust scaffold for LDH deposition, preventing aggregation while facilitating efficient electron and ion transport.¹³ The integration of CNFs into LDH-based electrodes ensures enhanced structural stability, even under high charge–discharge rates, thereby improving the cycling stability and rate capability of the material. The choice of metals in the LDH structure further influences its electrochemical behaviour, making it a critical factor in material design. Transition metals such as nickel (Ni), cobalt (Co), and manganese (Mn) have been widely used in LDHs for energy storage applications due to their complementary electrochemical properties.¹⁴ Ni contributes to high redox activity and specific capacitance, while Co enhances electrical conductivity and provides structural reinforcement. Mn, on the other hand, introduces additional redox-active sites, improving energy storage capacity and charge transfer efficiency.¹⁵ The incorporation of these three metals into the LDH structure not only broadens the redox window but also creates a synergistic interaction among the metal ions, resulting in enhanced electrochemical performance.¹⁶ NiCoMn-LDH, as a ternary metal hydroxide, stands out due to its ability to combine the best features of its individual components. The inclusion of Mn with Ni and Co mitigates the conductivity limitations typically observed in binary LDHs, while the unique electronic interactions among the three metals amplify the material's redox activity.¹⁷ However, to fully exploit the potential of NiCoMn-LDH for supercapacitor applications, its intrinsic conductivity and mechanical stability must be further improved, making CNFs an ideal choice for hybridization. The integration of NiCoMn-LDH with CNFs offers multiple advantages that directly address the requirements of high-performance supercapacitors.¹⁸ First, the conductive network of CNFs ensures rapid electron transport, mitigating the semiconducting nature of the LDH. Second, the porous architecture of CNFs provides ample space for ion diffusion, improving the accessibility of redox-active sites within the LDH. Third, CNFs enhance the mechanical stability of the composite, preventing material degradation during prolonged cycling.¹⁹

In this work, we report a synergistically engineered NiCoMn-LDH@CNF composite as a high-performance electrode material for advanced supercapacitors. Unlike previous LDH-carbon hybrids that primarily rely on binary metal systems or focus solely on conductivity enhancement, the present study introduces a ternary NiCoMn-LDH integrated with a conductive CNF scaffold, where each metal ion plays a distinct and complementary electrochemical role. More importantly, the composite is systematically optimized through stoichiometric tuning, enabling a rational balance between redox-active sites, electronic conductivity, and structural stability. Beyond performance metrics, this work provides explicit mechanistic insight into charge storage behaviour, employing Dunn's kinetic analysis to quantitatively separate diffusion-controlled and capacitive contributions, thereby offering direct evidence of true synergistic charge storage rather than a simple additive effect. The optimized NiCoMn-LDH@CNF electrode exhibits a diffusion-dominated faradaic response coupled with substantial surface pseudocapacitance, which together account for its exceptional capacitance, rate capability, and long-term stability. Furthermore, the successful integration of this material into an asymmetric supercapacitor device delivering a high energy density of 123.8 W h kg⁻¹ underscores its practical relevance and scalability. This study therefore establishes a design-to-mechanism framework for ternary LDH–carbon composites, advancing both the fundamental understanding and practical development of next-generation supercapacitor electrodes.

2. Experimental

2.1 Materials

Nickel nitrate hexahydrate (Ni (NO₃)₂·6H₂O), cobalt nitrate hexahydrate Co (NO₃)₂·6H₂O, manganese nitrate tetrahydrate (Mn (NO₃)₂·4H₂O), CNF, sodium carbonate (Na₂CO₃), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich, USA, and used as received without further purification. CNFs were procured and pre-treated via acid washing to remove impurities and improve surface functionality. Deionized water was used throughout the synthesis process.

2.2 Instruments and characterization

The structural properties of the synthesized sample were analysed using X-ray diffraction (XRD) with a Bruker AXS D8 diffractometer, employing Cu Kα radiation (λ = 1.54 Å), to confirm phase formation and crystallinity. Raman spectroscopy was performed using a Horiba HR 800UV confocal Raman spectrophotometer to investigate the vibrational modes and bonding characteristics of the material. The elemental composition and electronic states of the prepared sample were examined through X-ray photoelectron spectroscopy (XPS) using a PHI 5000 Versa Probe II (FEI Inc.). Transmission Electron Microscopy (TEM) analysis was conducted using a Hitachi-7600 (Japan) to further examine the internal structure, particle distribution, and morphology at the nanoscale. The surface morphology and microstructural features were studied using Scanning Electron Microscopy (SEM) with a JSM-FPlus system, which offers a resolution of 0.8 nm at 15 kV. The electrochemical studies were done using CS350M Potentiostat/Galvanostat from Corrtest instruments, China.

2.3 Synthesis of NiCoMn-LDH

NiCoMn-LDH was synthesized through a standard co-precipitation and aging route. In a typical preparation, stoichiometric amounts of Ni (NO₃)₂·6H₂O, Co (NO₃)₂·6H₂O, and Mn (NO₃)₂·4H₂O corresponding to the desired Ni : Co : Mn ratio (2 : 1 : 1) was dissolved in deionized water under continuous stirring to obtain a clear mixed-metal precursor solution. A separate solution containing Na₂CO₃ was added dropwise to initiate the formation of metal carbonate hydroxide nuclei, followed by the slow addition of 1.0 M NaOH to maintain the reaction pH in the alkaline range throughout precipitation. The resulting suspension was stirred vigorously and subsequently transferred to a sealed Teflon-lined vessel, where it was aged at 40 °C to promote the growth and ordering of the layered double hydroxide structure. After cooling, the precipitate was collected by centrifugation, washed repeatedly with deionized water until neutral pH was reached, and dried in an oven at 80 °C overnight. The obtained NiCoMn-LDH powder served as the active LDH component for preparing the NiCoMn-LDH@CNF composites described in Section 2.4.

2.4 Preparation of NiCoMn-LDH@CNF composites

NiCoMn-LDH was synthesized following the procedure described in Section 2.3, using a 50 mL reaction volume with metal precursors weighed according to the targeted Ni : Co : Mn molar ratio of 2 : 1 : 1. Specifically, 30.0 mmol of Ni(NO₃)₂·6H₂O (8.72 g), 15.0 mmol of Co(NO₃)₂·6H₂O (4.37 g), and 15.0 mmol of Mn(NO₃)₂·4H₂O (3.77 g) were dissolved in deionized water to yield a total metal content of 60 mmol. The co-precipitation, washing, and drying steps were identical to those reported earlier, resulting in the as-prepared NiCoMn-LDH. Prior to composite fabrication, the carbon nanofibers (CNFs) were pre-treated via acid functionalization to introduce oxygen-containing surface groups such as –OH and –COOH. This functionalization enhances the surface reactivity of CNFs and provides active sites for the nucleation and anchoring of LDH during composite formation. To fabricate the NiCoMn-LDH@CNF composites, a total batch mass of 200 mg was adopted for all formulations. The composites with LDH : CNF ratios of 1 : 1, 1 : 2, and 2 : 1 were obtained by weighing 100.0 mg LDH with 100.0 mg CNF, 66.7 mg LDH with 133.3 mg CNF, and 133.3 mg LDH with 66.7 mg CNF, respectively. Each mixture was dispersed in 40 mL of deionized water and ultrasonicated for 60 minutes to ensure uniform mixing and strong interfacial contact between the LDH nanosheets and carbon nanofibers. During this process, the surface functional groups on CNFs facilitate effective anchoring of LDH through electrostatic interactions and interfacial bonding.²⁰ The resulting dispersions were collected by filtration, washed thoroughly with water, and dried under vacuum at 60–80 °C to obtain homogeneous composite powders, which were designated as NiCoMn-LDH@CNF (1 : 1), NiCoMn-LDH@CNF (1 : 2), and NiCoMn-LDH@CNF (2 : 1).

2.5 Preparation of electrodes and electrochemical characterization

To prepare working electrodes for electrochemical studies, the active material (NiCoMn-LDH@CNF composites) was mixed with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 8 : 1 : 1. N-methyl-2-pyrrolidone (NMP) was used as the solvent to form a homogeneous slurry. This slurry was uniformly coated onto a cleaned nickel foam substrate (1 cm × 1 cm) and dried at 60 °C for 12 hours under vacuum to remove residual solvent. The resulting electrodes were pressed under 10 MPa to ensure good adhesion and electrical contact. The average mass loading of the active material on each electrode was approximately 2 mg. Additional details on the formulas used for calculating specific capacitance, assessing the performance of the asymmetric supercapacitor, and evaluating specific capacitance and charge storage capacity are provided in the SI to ensure a thorough understanding of the results.

Electrochemical characterization was performed in a three-electrode system using 2 M KOH as the electrolyte. The prepared electrode served as the working electrode, while a platinum wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Cyclic voltammetry (CV) was conducted at various scan rates ranging from 5 to 100 mV s⁻¹ to evaluate the pseudocapacitive behaviour of the materials. Galvanostatic charge–discharge (GCD) tests were carried out at different current densities (1–10 A g⁻¹) to calculate the specific capacitance and assess the rate capability. Electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range of 0.01 Hz to 100 kHz to investigate the charge transfer resistance and ion diffusion behaviour. Detailed calculation formulas for specific capacitance, energy density, power density, and charge storage capacity, along with a representative calculation procedure, are provided in the SI.

2.6 Device fabrication

The NiCoMn-LDH@CNF composite was assembled into an asymmetric supercapacitor using a Swagelok cell in a two-electrode configuration. The cathode was made from the NiCoMn-LDH@CNF composite coated onto nickel foam, while the anode was fabricated using activated carbon coated on carbon cloth. A separator soaked in 2 M KOH electrolyte was used to prevent short circuits between the electrodes. The Swagelok cell design ensured robust assembly and facilitated accurate evaluation of the electrochemical performance. This asymmetric configuration not only take advantage of on the high energy storage potential of the NiCoMn-LDH@CNF composite but also leverages the stable and complementary performance of activated carbon, demonstrating the feasibility of this system for practical energy storage applications.

The X-ray diffraction (XRD) analysis of the NiCoMn-LDH@CNF composite reveals important details about its crystallographic structure and phase composition, including the assignment of Miller indices to specific diffraction peaks. The NiCoMn-LDH XRD pattern (Fig. 1a) shows characteristic peaks at 2θ values of 11.3°, 22.7°, 34.4°, and 61.2°, which correspond to the (003), (006), (012), and (113) planes, respectively.²¹ These planes are characteristic of the LDH structure with hexagonal symmetry, reflecting the orderly stacking of hydroxide layers. The presence of these peaks suggests the incorporation of nickel, cobalt, and manganese oxides within the LDH phase. Additionally, the peak observed at around 2θ = 26.1° in the CNF XRD profile corresponds to the (002) plane,²² indicative of the graphitic structure of CNF. In the NiCoMn-LDH@CNF composite, the overlap of CNF peaks with those of LDH indicates the successful integration of the two components. Variations in the intensity and sharpness of the diffraction peaks in different weight ratios (1 : 1, 1 : 2, and 2 : 1) of NiCoMn-LDH and CNF suggest different levels of crystallinity and interaction between the phases. To evaluate potential impurity phases, the XRD patterns were compared with the Joint Committee on Powder Diffraction Standards (JCPDS) references for nickel hydroxide hydrate (JCPDS 00-038-0715), cobalt oxide (JCPDS 01-075-0419), and manganese oxide (JCPDS 00-004-0732). No distinct peaks corresponding to cobalt or manganese oxide phases were detected, suggesting that the synthesis largely retained the LDH hydroxide phase, with minimal formation of oxide impurities. This observation aligns with XPS results (Fig. 2), further confirming the purity of the LDH phase. The Raman spectra of NiCoMn-LDH, CNF, and NiCoMn-LDH@CNF (2 : 1) are shown in Fig. 1b. The CNF spectrum shows two prominent bands at approximately 1340 cm⁻¹ and 1580 cm⁻¹, corresponding to the D and G bands, respectively, which are characteristic of graphitic carbon.^23,24 The intensity of these bands indicates the disordered and ordered carbon content in the CNF. In contrast, the spectrum of NiCoMn-LDH displays a significant peak in the range of 450–500 cm⁻¹, which can be attributed to M-O vibrations involving the metal cations (Ni, Co, and Mn) in the LDH structure.²⁵ The absence of carbon-specific bands confirms the purity of the metal hydroxide material. The spectrum of NiCoMn-LDH@CNF (2 : 1) combines features of both the NiCoMn-LDH and CNF. The D and G bands and the M–O vibration peaks near 500 cm⁻¹ are clearly visible. This suggests a successful integration of NiCoMn-LDH onto the CNF surface, with the possible strong interactions between the two components. The enhanced intensity of the peaks in the hybrid material indicates synergistic effects, likely due to the interactions between the carbon nanofibers and the layered double hydroxide. This analysis confirms the successful integration of NiCoMn-LDH and CNF, preserving the structural integrity of both components and validating their effective interaction within the composite.

Fig. 1 (a) XRD patterns of NiCoMn-LDH@CNF composites with varying stoichiometries (1 : 1, 2 : 1, 1 : 2), along with CNF and pristine NiCoMn-LDH, (b) Raman spectra of the NiCoMn-LDH@CNF (2 : 1) composite, CNF, and pristine NiCoMn-LDH, confirming successful composite formation through the presence of characteristic bands associated with carbon frameworks and metal hydroxide structures.

Fig. 2 (a) XPS survey spectrum of NiCoMn-LDH@CNF confirming the presence of Ni, Co, Mn, O, and C elements, (b) high-resolution Ni 2p spectrum corresponding to Ni²⁺ oxidation states, (c) Co 2p spectrum showing the presence of both Co²⁺ and Co³⁺ states, (d) Mn 2p spectrum deconvoluted into Mn²⁺, Mn³⁺, and Mn⁴⁺ oxidation states, (e) O 1s spectrum with peaks corresponding to O–H, C=O, and C–O bonding environments, and (f) C 1s spectrum revealing contributions from C–C, C=O, and C–O bonds.

3. Results and discussion

The X-ray photoelectron spectroscopy (XPS) analysis of the NiCoMn-LDH@CNF (Fig. 2a) composite offers comprehensive insights into its elemental composition, oxidation states, and chemical bonding. The survey spectrum confirms the presence of Ni, Co, Mn, O, and C elements, with sharp peaks at their respective binding energies. The high-resolution Ni 2p spectrum (Fig. 2b) displays two prominent peaks at 855 eV (2p_3/2) and 873 eV (2p_1/2), characteristic of Ni²⁺, along with their corresponding satellite peaks that further verify this oxidation state. These results confirm that nickel predominantly exists as Ni²⁺ within the LDH structure, contributing to its pseudocapacitive behaviour.²⁶ The Co 2p spectrum (Fig. 2c) shows distinct peaks at 780 eV (2p_3/2) and 795 eV (2p_1/2), corresponding to Co²⁺ and Co³⁺ oxidation states, with Co³⁺ being more prominent. This mixed-valence state of cobalt enhances redox reactions, vital for efficient charge storage.²⁷ The Mn 2p spectrum (Fig. 2d) displays peaks at 642 eV (2p_3/2) and 654 eV (2p_1/2), indicating the presence of Mn⁴⁺, Mn³⁺, and Mn²⁺, with Mn⁴⁺ being dominant, further enhancing the faradaic processes in the composite.^28,29 The O 1s spectrum (Fig. 2e) shows peaks at 529.9 eV, 531.5 eV, and 533.0 eV, which correspond to metal–oxygen bonds (O=M), hydroxyl groups (O–H), and carbonate or adsorbed oxygen species (O=C), respectively, confirming the involvement of hydroxyl groups typical in LDH structures.³⁰ The C 1s spectrum (Fig. 2f) shows peaks at 284.8 eV, 286.4 eV, and 289.0 eV, corresponding to C–C/C=C, C–O/C=O, and CO₃²⁻ groups, respectively, indicating the presence of carbonate anions within the LDH interlayers, which contribute to structural stability.³¹ The multivalent states of Ni, Co, and Mn facilitate efficient redox reactions, making the composite highly suitable for supercapacitor applications. The presence of hydroxyl groups and carbonate anions enhances ionic conductivity and improves the electrochemical stability of the NiCoMn-LDH@CNF composite, leading to superior performance in energy storage systems.

The pure NiCoMn-LDH (Fig. 3a₁–a₃) sample exhibits a dense, aggregated morphology with stacked nanosheets, characteristic of LDH. At higher magnification, the sheets appear wrinkled and uneven, indicating a high surface area, which can contribute to enhanced electrochemical performance due to increased active sites for redox reactions. The CNF (Fig. 3 b₁–b₃) shows a highly porous and interconnected network with smooth surfaces, forming a conductive matrix. The fibrous structure, clearly visible at all magnifications, offers a large surface area and excellent electrical conductivity, which together facilitate efficient charge transport in electrochemical applications.³²

Fig. 3 SEM images displaying the morphological features of (a) NiCoMn-LDH, exhibiting aggregated sheet-like structures, (b) CNF, showing a well-defined fibrous network, (c) NiCoMn-LDH@CNF (1 : 1), with uniformly distributed LDH sheets on the CNF network, (d) NiCoMn-LDH@CNF (1 : 2), demonstrating increased LDH coverage over the CNF surface, and (e) NiCoMn-LDH@CNF (2 : 1), featuring enhanced LDH anchoring with distinctly layered nanosheets integrated into the CNF matrix. Each sample is presented at 10 µm, 1 µm, and 100 nm magnifications to highlight the hierarchical structural evolution and illustrate the corresponding morphological variations.

In the composite samples, the NiCoMn-LDH nanosheets are uniformly distributed across the CNF network, with notable morphological variations depending on the weight ratios. The 1 : 1 composite (Fig. 3c1–c3) exhibits well-dispersed LDH nanosheets on the CNF surface, forming a porous architecture that supports efficient ion diffusion. The 1 : 2 composite (Fig. 3d1–d3) shows a more dominant CNF network with smaller LDH clusters, indicating enhanced conductivity resulting from the higher CNF content. In contrast, the 2 : 1 composite (Fig. 3e1-e3) displays a denser coverage of NiCoMn-LDH nanosheets that extensively coat the CNF surface. Although the CNF framework is less visible, the increased LDH content provides a larger number of electroactive sites. Importantly, the CNF scaffold mitigates LDH agglomeration even at higher loadings, maintaining good conductivity and effective ion diffusion pathways.

This optimized structural configuration enables superior electrochemical performance by balancing abundant active material with adequate electrical conductivity. Among all compositions, the 2 : 1 NiCoMn-LDH@CNF composite delivers the best performance due to its higher proportion of redox-active LDH, which contributes to increased capacitance through more available active sites. Simultaneously, the CNF scaffold preserves a porous network, facilitating rapid ion and electron transport.³³ The synergistic combination of the strong pseudocapacitive response of NiCoMn-LDH and the excellent conductivity of CNF makes the 2 : 1 composite particularly well suited for high performance supercapacitor applications.

The EDS spectrum (Fig. 4a) identifies the elemental composition of the NiCoMn-LDH@CNF (2 : 1) composite material. The elemental peaks in the sample include a strong Ni peak at around 7 keV, which appears as the highest intensity peak, indicating that nickel is the most abundant element present. Co and Mn are visible with distinct peaks, though they are less abundant than Ni. O is observed at approximately 0.5 keV, and its presence relates to the hydroxide layers in the LDH structure. C at around 0.3 keV, corresponding to the carbon present in the CNF matrix. The elemental mapping images confirm the homogeneous distribution of all constituent elements within the composite. Nickel (Fig. 4b) is uniformly dispersed, reflecting its integral role in the LDH structure. Cobalt (Fig. 4c) also shows even distribution, indicating its effective incorporation into the LDH layers. Manganese (Fig. 4d) is similarly well dispersed, ensuring a balanced contribution to the redox processes. The oxygen mapping (Fig. 4e) displays uniform presence throughout the sample, verifying the proper formation of hydroxide layers. Carbon (Fig. 4f), corresponding to the CNF matrix, is consistently distributed across the material, demonstrating that the fibres are well integrated and provide robust structural support.

Fig. 4 (a) Energy dispersive X-ray spectroscopy (EDS) spectrum and corresponding compositional analysis table for NiCoMn-LDH@CNF (2 : 1), showing the weight percentages (wt%) and standard deviations (σ) of Ni, Co, Mn, O, and C in the sample, (b-f) Elemental mapping images of Ni (b, magenta), Co (c, orange), Mn (d, cyan), O (e, green), and C (f, yellow), illustrating the spatial distribution of these elements across the NiCoMn-LDH@CNF (2 : 1) sample at a scale of 1 µm.

The SEM, EDS, and elemental mapping confirmed that the NiCoMn-LDH @CNF (2 : 1) composite has been successfully synthesized with a well distributed, layered double hydroxide structure supported on a conductive CNF matrix. The uniform elemental distribution and appropriate elemental ratios indicate a material well suited for high performance supercapacitor applications. Ni, Co, and Mn contribute to the redox activity, oxygen from the hydroxide layers supports these reactions, and carbon provides structural integrity along with enhanced electrical conductivity.

The Transmission electron microscopy (TEM) images provide a detailed characterization of the NiCoMn-LDH@CNF composite, highlighting its structural and morphological features essential for supercapacitor applications. Fig. 5a shows the uniform distribution of NiCoMn-LDH nanosheets on CNFs, with the nanosheets adhering closely to the CNFs, ensuring effective charge transfer and structural stability. Fig. 5b further reveals the fluffy, porous morphology of the LDH nanosheets, promoting electrolyte infiltration and ion diffusion. Fig. 5c captures the layered structure of NiCoMn-LDH, which enhances the exposure of active sites and facilitates efficient redox reactions. The high-resolution TEM image in Fig. 5d displays lattice fringes with a d-spacing of 0.34 nm corresponding to the (002) plane, confirming the crystalline structure critical for electron transport. Fig. 5e and f present SAED patterns for CNFs and NiCoMn-LDH, respectively, highlighting the graphitic nature of CNFs and the polycrystalline nature of LDH, both are essential for high conductivity and redox activity. The pronounced synergistic interaction between the conductive CNF network and the electrochemically active NiCoMn-LDH nanosheets, together with their optimized morphology and structural characteristics, highlights the strong suitability of the composite for high-performance energy storage applications.

Fig. 5 (a–c) TEM images of NiCoMn-LDH@CNF (2 : 1) at different magnifications, showing the uniform distribution of NiCoMn-LDH nanostructures on the CNF network, (d) high resolution TEM (HRTEM) image of NiCoMn-LDH, highlighting the lattice spacing of 0.34 nm corresponding to the (002) crystallographic plane, (e) selected area electron diffraction (SAED) pattern of CNF displaying distinct diffraction rings associated with the (101) and (002) planes, and (f) SAED pattern of NiCoMn-LDH confirming its polycrystalline structure.

3.1 Electrochemical analysis of NiCoMn-LDH@CNF composites

The electrochemical behaviour of NiCoMn-LDH@CNF composites at different ratios (1 : 1, 2 : 1, and 1 : 2) was systematically evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS), and specific capacitance analysis. The CV profiles (Fig. 6a) reveal quasi-rectangular shapes with distinct redox peaks for all NiCoMn-LDH@CNF composites, confirming their pseudocapacitive nature. The electrochemical performance of the individual electrode materials (SI Fig. S1) was evaluated using CV, GCD, and EIS. The CV curves confirm efficient charge storage, while GCD profiles indicate excellent capacitance retention. EIS analysis reveals reduced charge transfer resistance in optimized materials, enhancing conductivity and overall performance. Among the studied ratios, NiCoMn-LDH@CNF (2 : 1) displayed the highest current response, indicating superior charge storage capability due to the optimal synergy between the active materials. In contrast, the pure CNF and NCM showed relatively lower current responses, highlighting the significant enhancement achieved through composite formation. The GCD curves (Fig. 6b) further validate the superior performance of NiCoMn-LDH@CNF (2 : 1), which exhibits longer discharge times compared to other ratios. The symmetric triangular shapes of the GCD curves signify excellent charge–discharge reversibility and low internal resistance. The NiCoMn-LDH@CNF (2 : 1) composite achieves the highest specific capacitance at various current densities, emphasizing its ability to store and deliver energy effectively. Specifically, at a current density of 1 A g⁻¹, NiCoMn-LDH@CNF (2 : 1) achieved the highest specific capacitance of 1783 F g⁻¹, followed by 1447 F g⁻¹ for NiCoMn-LDH@CNF (1 : 1) and 833 F g⁻¹ for NiCoMn-LDH@CNF (1 : 2). The pristine LDH displayed a much lower capacitance of 548 F g⁻¹ under the same conditions. Even at a higher current density of 5 A g⁻¹, NiCoMn-LDH@CNF (2 : 1) maintained an impressive specific capacitance of 1497 F g⁻¹, demonstrating superior rate capability. The Nyquist plots (Fig. 6c) demonstrate a small semicircle in the high frequency region and a near vertical line in the low frequency region for all samples, indicative of low charge transfer resistance (R_ct) and ideal capacitive behaviour. NiCoMn-LDH@CNF (2 : 1) exhibited the smallest R_ct, confirming its efficient charge transport kinetics. The inset of Fig. 6c shows the enlarged high frequency region, where the differences in R_ct among the samples are evident. The pure CNF and NiCoMn-LDH materials displayed higher R_ct, reflecting poorer charge transfer characteristics compared to the composites. EIS and Bode phase plots (SI Fig. S2) were used to evaluate the charge transfer resistance and stability of CNF, NiCoMn-LDH, and NiCoMn-LDH@CNF electrodes. Nyquist plots (Fig. S2a and b) show a lower R_ct for NiCoMn-LDH@CNF, which further decreases after 10 000 cycles, indicating superior charge transfer kinetics and stability. Bode phase plots (Fig. S2c and d) confirm better capacitive behaviour and sustained electrochemical performance of NiCoMn-LDH@CNF, highlighting its efficiency for long-term supercapacitor applications. Fig. 6d illustrates the specific capacitance values of the composites as a function of current density. NiCoMn-LDH@CNF (2 : 1) achieved the highest specific capacitance across all current densities, demonstrating excellent rate capability. The specific capacitance decreases with increasing current density for all samples, which is typical due to limited ion diffusion at higher charge discharge rates. However, NiCoMn-LDH@CNF (2 : 1) retained a substantial portion of its capacitance, showcasing its high efficiency and robustness under varying operational conditions.

Fig. 6 Electrochemical performance of CNF, NiCoMn-LDH, and NiCoMn-LDH@CNF composites with different ratios (1 : 1, 2 : 1, and 1 : 2) (a) CV curves at 10 mV s⁻¹ scan rate, showing enhanced redox activity of NiCoMn-LDH@CNF composites, (b) GCD profiles at 1 Ag⁻¹, highlighting the superior charge storage capacity of NiCoMn-LDH@CNF (2 : 1), (c) Nyquist plots with inset showing the high-frequency region, indicating improved charge transfer resistance for NCM@CNF composites, and (d) specific capacitance vs. current density, demonstrating the high-rate capability and capacitance retention of NiCoMn-LDH@CNF (2 : 1) compared to other samples.

To sum up, the electrochemical results highlight the superior performance of NiCoMn-LDH@CNF (2 : 1) over other ratios, attributable to its optimized composition and efficient charge storage mechanisms. This composite demonstrated excellent capacitive behaviour, high specific capacitance (1783 F g⁻¹ at 1 A g⁻¹ and 1497 F g⁻¹ at 5 A g⁻¹), low internal resistance, and robust rate capability, making it a promising candidate for high performance energy storage application. A detailed evaluation of the specific capacitance and corresponding charge storage capacity, including conversion to mA h g⁻¹, is presented in the SIn to support the electrochemical analysis.

3.2 Charge storage kinetics and mechanistic analysis

The charge-storage kinetics of the NiCoMn-LDH@CNF electrode were systematically probed via b-value analysis and Dunn's method from CV data at 5 mV s⁻¹. Fig. 7a reveals pronounced asymmetry: anodic b = 0.46 (∼0.5) signifies diffusion-controlled faradaic intercalation, where OH⁻ anions (from 2 M KOH) insert into the brucite-like layers [Ni/Co/Mn-(OH)₆]_n⁺ during oxidation (e.g., Ni²⁺ → Ni³⁺ + e⁻; Co³⁺ → Co⁴⁺ + e⁻; Mn³⁺ → Mn⁴⁺ + e⁻). This process is rate-limited by slow solid-state diffusion (D ∼10⁻⁹-10⁻¹⁰ cm² s⁻¹, typical for LDHs), interlayer spacing constraints (∼0.78 nm from XRD (003) peak at 11.3° 2θ), and lattice expansion/strain that hinders ion migration through hydrated, carbonate-intercalated galleries. Conversely, cathodic b = 0.93 (∼1.0) indicates surface-confined pseudocapacitance and EDLC charging during reduction, driven by rapid deintercalation, outer-sphere electron transfer, and reversible surface redox at edge-plane sites, accelerated by the CNF network's high conductivity (R_ct ≈ 1.7 Ω from EIS high-frequency intercept).

Fig. 7 Charge storage kinetics of the NiCoMn-LDH@CNF electrode. (a) Log–log plots of anodic and cathodic peak currents versus scan rate for b-value determination. (b) Dunn's method fitting based on i(v) = k₁v + k₂v^1/2 to separate capacitive and diffusion-controlled contributions. (c) Relative capacitive and diffusion contributions at different scan rates. (d) Potential-resolved separation of capacitive and diffusion-controlled currents at a representative scan rate.

This directional asymmetry, common in multivalent transition metal hydroxides, arises from fundamentally distinct charge-storage kinetics: oxidation demands coupled cation/anion transport and structural reorganization (activation energy E_a > 50 kJ mol⁻¹), while the reduction process is dominated by fast surface-controlled reactions, facilitated by efficient electron transport through the conductive CNF network. Dunn's method (Fig. 7b), based on the linear relationship between i/v^1/2 and v^1/2, quantitatively separates the capacitive (k₁v) and diffusion-controlled (k₂v^1/2) contributions. The results indicate that diffusion-controlled processes dominate the charge storage behaviour, contributing approximately 65–76% over the scan rate range of 15 to 5 mV s⁻¹ (Fig. 7c). This contribution is most pronounced near the redox potentials (∼0.3–0.45 V vs. Ag/AgCl), as shown in Fig. 7d, confirming the faradaic nature of the electrode. At higher scan rates, the capacitive contribution (k₁ term) increases, which can be attributed to enhanced surface-controlled processes facilitated by the porous and conductive CNF network, consistent with the hierarchical morphology observed in SEM and TEM analyses. XPS-verified mixed valences (Ni²⁺, Co²⁺/³⁺, Mn³⁺/⁴⁺) enable multi-electron faradaic processes, underpinning the hybrid mechanism: diffusion-limited bulk storage yields high capacitance (1783 F g⁻¹ at 1 A g⁻¹), while surface capacitance ensures rate capability (80% retention at 10 A g⁻¹). This rationalizes the electrode's superiority over pristine LDH/CNF, affirming synergistic LDH-CNF design for practical ASCs.^34–36

3.3 Electrochemical performance of the NiCoMn-LDH@CNF composite in an asymmetric supercapacitor (ASC)

The electrochemical performance of the ASC was evaluated using a two-electrode configuration. CV curves (Fig. 8a) recorded at various scan rates exhibit quasi-rectangular shapes, confirming excellent capacitive behaviour and efficient ion transport dynamics. The mass balancing between positive and negative electrodes was determined based on the charge balance principle (q⁺ = q⁻), and the detailed calculation is provided in the Supplementary Information. Fig. 8d showcases the CV curves, where the current response is plotted against potential. The quasi-rectangular shape of the CV curves, which becomes more prominent at higher voltages, is indicative of excellent capacitive behaviour and efficient charge storage. The steady increase in peak current with increasing voltage further confirms the scalability of energy storage with higher applied potentials, as more charge is stored within the system. The GCD profiles (Fig. 8b) show symmetrical triangular shapes, indicating low internal resistance and high energy storage capability. Specific capacitance values calculated from GCD studies at current densities of 1 A g⁻¹, 2 A g⁻¹, 3 A g⁻¹, 4 A g⁻¹, and 5 A g⁻¹ were 348 F g⁻¹, 287 F g⁻¹, 266 F g⁻¹, 253 F g⁻¹, and 244 F g⁻¹, respectively, highlighting the composite's ability to retain significant capacitance at higher current densities. Fig. 8e presents the GCD profiles at the same range of voltages. The triangular shapes of the GCD curves highlight the device's high reversibility and low resistive losses. The progressively longer charge–discharge times at higher voltages suggest an increase in energy storage capacity, as the energy stored is proportional to the square of the applied voltage. The consistent shape and spacing of the curves across the voltage range indicate stable and reliable performance. Together, these panels validate the device's efficient energy storage capabilities, scalability with applied voltage, and suitability for practical applications in energy storage systems. EIS (Fig. 8c) reveals a small semicircle in the high-frequency region, attributed to lowest charge transfer resistance (R_ct = 1.7 Ω), and a near-vertical line in the low-frequency region, indicating ideal capacitive behaviour. The solution resistance (R_s) was determined to be 3.6 Ω, demonstrating the low internal resistance of the device. Cycling stability tests (Fig. 8f) confirm outstanding durability with 80% capacitance retention over 10 000 cycles, maintaining nearly 100% coulombic efficiency. The decrease in capacitance retention over prolonged cycling could be attributed to structural degradation or electrolyte decomposition at the interface of the NiCoMn-LDH@CNF composite. The post-cycling morphological stability of the electrode was examined using SEM analysis (Fig. S5) and to evaluate structural stability after prolonged cycling, post-cycling XRD analysis was performed and depicted in Fig. S6 in the SI. Compared to the pristine electrode, the cycled sample exhibits reduced peak intensity and pronounced peak broadening, particularly in the low-angle region. These changes indicate partial loss of crystallinity, increased microstrain, and structural disorder induced by repeated ion intercalation/deintercalation.^37,38 The observed ∼80% capacitance retention after 10 000 cycles, indicating that although degradation occurs, the electrode maintains sufficient structural integrity for sustained electrochemical performance.

Fig. 8 Electrochemical performance of NiCoMn-LDH@CNF in the ASC device. (a and c) CV curves at various scan rates and voltage windows demonstrating the capacitive behaviour (b and e) GCD profiles at different current densities and voltages, (d) Nyquist plots comparing experimental and fitted impedance data, with the inset showing the equivalent circuit model, (f) long-term cycling stability, illustrating capacitance retention and coulombic efficiency over 10 000 charge–discharge cycles.

The device's ability to operate in an extended voltage window up to 1.6 V suggests that the combination of the NiCoMn-LDH@CNF composite with activated carbon in an asymmetric configuration is effective for increasing the operating voltage range, which in turn enhances the energy density. This result is significant as it highlights the synergy between the high surface area of the activated carbon and the high pseudocapacitive nature of NiCoMn-LDH@CNF, making the device suitable for high-performance energy storage applications.

The comparative analysis is presented in the Table 1 highlights the superior electrochemical performance of the NiCoMn-LDH@CNF ASC in terms of energy and power densities. ASC achieves an energy density of 123.8 W h kg⁻¹ at a power density of 800 W kg⁻¹, surpassing many reported LDH-based supercapacitor devices. Notably from Fig. 9, it outperforms NC-LDH/PNC (121.95 Wh kg⁻¹, 798.2 W kg⁻¹) and Mn–Co LDH@Carbon Dots (79 W h kg⁻¹, 666 W kg⁻¹), indicating its enhanced charge storage capability. Additionally, compared to the Mn-doped Ni/Co LDH composite (78.79 W h kg⁻¹, 1550 W kg⁻¹), the NiCoMn-LDH@CNF device achieves a significantly higher energy density while maintaining a competitive power density. Furthermore, the fabricated NiCoMn-LDH@CNF ASC exhibits a substantial improvement over conventional LDH based devices. Overall, these results validate the effectiveness of the NiCoMn-LDH@CNF composite as an advanced electrode material for next-generation supercapacitors, achieving an optimal balance between high energy and power density. The synergistic interaction among Ni, Co, and Mn within the LDH structure, together with the conductive CNF matrix, enhances charge storage capability and electrical conductivity, establishing this composite as a promising candidate for future energy storage applications.

Table 1 Comparison of energy and power densities of the NiCoMn-LDH@CNF based ASC with previously reported LDH based systems

S. no	Sample	Specific capacitance	Voltage window (V)	Cycling stability	Mass loading (mg)	Energy density (W h kg^−1)	Power density (W kg^−1)	Ref.
1	Co–Al LDH carbon nanotube	80.6 F g^−1	0–1.6	88% at 1000 cycles	—	28	444.1	39
2	NC-LDH/PNC	343 F g^−1	0–1.6	90% at 4000 cycles	—	121.95	798.2	40
3	Mn–Co LDH@carbon dots	222 F g^−1	0–1.6	90.5% at 3000 cycles	14.17	79	666	41
4	Layered NiFe-LDH/MXene	—	0–1.5	84% at 1000 cycles	60	42.4	758.27	42
5	NiMn-layered double hydroxide/porous carbon	59 F g^−1	0–1.5	84% at 3000 cycles	3.3–5.2	18	225	43
6	CoAl-LDH/Polypyrrole/graphene	—	0–1.6	90% at 10 000 cycles	—	46.8	1200	44
7	NiFe-LDH/RGO/CNFs composite	98 F g^−1	0–1.6	97% @ 2500 cycles	—	33.7	785.8	45
8	Mn3O4/NiCo-LDH@carbon nanotube	—	0–1.6	72% at 5000 cycles	17.14	16.01	69.77	46
9	Mn-doped Ni/Co LDH	656 C g^−1 at 1 A g^−1	0–1.6	82% at 8000 cycles	—	78.79	1550	47
10	NiCoMn-LDH@CNF//AC	348 F g^−1	0–1.6	80% at 10000 cycles	5 mg (NiCoMn-LDH@CNF)//15 mg (AC)	123.8	800	This work

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Fig. 9 Ragone plot illustrating the energy and power densities of the NiCoMn-LDH@CNF based supercapacitor, benchmarked against previously reported LDH based devices. The NiCoMn-LDH@CNF composite demonstrates superior energy density across a wide range of power densities, indicating excellent electrochemical performance. The inset schematic illustrates the asymmetric supercapacitor configuration, comprising a NiCoMn-LDH@CNF positive electrode, an activated carbon negative electrode, and an electrolyte-soaked separator facilitating ion transport.

The specific capacitance values of NiCoMn-LDH@CNF materials and the fabricated ASC device at various current densities is summarized in Table S2, show that the NCM@CNF (2 : 1) composite consistently outperformed other materials across all current densities at various current densities, as summarized in Table S1, At a current density of 1 A g⁻¹, NCM@CNF (2 : 1) achieved a specific capacitance of 1783 F g⁻¹, significantly higher than 548 Fg⁻¹ for NCM LDH and 27 F g⁻¹ for CNF alone. Even at a high current density of 5 A g⁻¹, NCM@CNF (2 : 1) retained 1497 Fg⁻¹, demonstrating excellent rate capability. Statistical reproducibility analysis of the electrochemical performance is provided in the SI (Fig. S3 and Table S1) to demonstrate the reliability and consistency of the measured results. This superior performance is attributed to the synergistic interaction between the conductive CNF matrix and the high capacity NCM component, which enhances ion accessibility and charge transfer. In an asymmetric device configuration with activated carbon as the counter electrode, ASC still displayed a reasonable specific capacitance of 348 F g⁻¹ at 1 A g⁻¹. The practical demonstration (Fig. S4) showcases the assembled device powering LEDs and a stopwatch, with stable energy output for over 10 minutes, validating its potential for real-world energy storage applications.

4. Conclusion

The NiCoMn-LDH@CNF composite represents a significant advancement in supercapacitor technology, addressing key challenges in energy storage through a strategically engineered synergistic architecture. The optimized 2 : 1 stoichiometry effectively combines the rich redox-active sites of NiCoMn-LDH with the excellent electrical conductivity and mechanical strength of CNFs, resulting in enhanced charge-storage capability and long-term durability. Comprehensive characterization confirms the uniform dispersion of LDH nanosheets on the CNF matrix, which promotes efficient electron transport, provides abundant electroactive sites, and ensures robust structural integrity, the features indispensable for achieving high-performance energy storage devices. The ASC device demonstrates outstanding electrochemical performance, achieving a specific capacitance of 348 F g⁻¹ at 1 A g⁻¹, an energy density of 123.8 W h kg⁻¹, and a power density of 800 W kg⁻¹, while retaining 80% of its capacitance after 10 000 cycles. Operating within a stable 1.6 V window, the device successfully powers practical applications such as LEDs, demonstrating its feasibility for scalable, real-world deployment. In summary, the integration of high redox activity with superior conductivity in the NiCoMn-LDH@CNF composite establishes a viable pathway for sustainable, high performance energy storage technologies effectively, bridging the gap between material innovation and practical application.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The datasets supporting the findings of this study, including raw and processed electrochemical measurements, characterization data (XRD, Raman, XPS, SEM, TEM), and analysis files, are available from the corresponding authors upon reasonable request. Additional methodological details and supplementary results are provided in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta09604a.

Acknowledgements

The authors gratefully acknowledge the financial support from RUSA 2.0, Madurai Kamaraj University, Materials for Energy Storage (DST-MES), New Delhi, India for providing financial assistance through the DST-MES (DST/TMD/MES/2K17/94(G)), and the SERB-ASEAN-India collaborative research project (No. CRD/2021/000482). We also extend our sincere gratitude to the Taiwan Experience Education Program (TEEP) Project number (E01-112-E039 and VL006-AG00-114), Ministry of Education (MOE), NSTC 114-2221-E-131-021, Taiwan, for its valuable support.

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