Ordered Co/Ni oxide nanostructures from MOFs: enhancing efficiency in hybrid asymmetric energy devices

Abstract

MOF-derived metal oxides are promising electrode materials for energy storage due to their tunable porosity, large surface area, and versatile structures, which enhance electrochemical performance. However, their practical use is currently limited by poor conductivity and structural instability, requiring advanced modifications to circumvent this issue. This research investigates the synthesis of Co₃O₄/NiO nanostructures (MD-Co/Ni) derived from bimetallic metal–organic frameworks (Co/Ni-MOFs) with varying Co : Ni ratios (0.25 : 1, 0.5 : 1, 0.75 : 1, and 1 : 1) through thermal decomposition. The primary objective is to enhance energy storage efficiency. The study also examines how different Co : Ni ratios influence the electrochemical performance of the resulting nanostructures. The Co/Ni-MOF precursor was synthesized via a straightforward solvothermal method using trimesic acid (TA) as the ligand and polyvinylpyrrolidone (PVP) as a stabilizer. XRD analysis confirmed the high crystallinity of MD-Co/Ni nanostructures, while FE-SEM revealed its nanorod-/nanosheet-like morphology featuring rod-shaped nanoparticles. Electrochemical evaluations demonstrated that MD-Co/Ni achieved a superior specific capacitance (C_sp) of 2836 F g⁻¹ at 1 A g⁻¹, surpassing its pure bimetallic MOF counterparts. This improvement is credited to the synergistic effects of its bimetallic oxide composition, increased surface area from the meso-porous structure, and enhanced electron/ion transport pathways. Furthermore, a hybrid asymmetric supercapacitor (HS) was fabricated using MD-Co/Ni as the positive electrode. The device exhibited an exceptional energy density (E_d) of 27.28 W h kg⁻¹ at a power density (P_d) of 380 W kg⁻¹ and an outstanding working stability, retaining 80% capacitance and achieving a coulombic efficiency of 99.53% after 9500 cycles. These findings highlight the significant potential of thermally derived MOF-based nanostructures for futuristic energy storage systems.

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1. Introduction

Supercapacitors (SCs) are at the forefront of energy storage technology, known for their fast charging and discharging characteristics, high P_d, and outstanding rate capacity.^1,2 The quest for improved SC performance has mostly focused on electrode materials, which are acknowledged as critical components in these devices.^3,4 Transition metal oxides^5–9 and hydroxides^10,11 have received extensive attention because of their rapid and reversible redox processes, which provide specific capacitances that exceed those of carbon-based electric double-layer capacitors (EDLCs).^12,13 Cobalt oxide (Co₃O₄) has been widely explored due to its high theoretical supercapacitors (3600 F g⁻¹) and electrical conductivity.^14,15 Despite these characteristics, the use of Co₃O₄ is frequently restricted by poor rate performance and cycling stability, which are attributed to significant microstructural changes that may lead to complete structural disintegration due to continuing redox activity during charge–discharge cycles.¹⁶ As a result, developing robust porous materials capable of mitigating volumetric changes during electrochemical cycling has become a critical area of research interest.

Metal–organic frameworks (MOFs), which are composed of organic ligands and metal ions coupled via coordination bonds, have emerged as reliable precursors to the development of sophisticated porous materials with complex structures since their initial synthesis by O. M. Yaghi in 1995.¹⁷ Because of their huge surface areas and ample supply of active oxidation sites, these materials have been implemented across extensive applications, including electrochemical energy storage.^18,19 Lee et al. found that a single-phase, terephthalic acid (TPA)-based red cobalt MOF had a specific capacitance of 206.76 F g⁻¹ and an E_D of 7.18 W h kg⁻¹, as well as remarkable cycling stability.²⁰ Metal nodes in MOFs contribute to pseudo capacitance effects through redox processes, hence metal node selection is critical for the electrochemical implementation of MOF-based supercapacitors.^21,22 Accordingly, adding multi-metal nodes to MOFs can greatly improve capacitance performance. The multi-metal nodes enable the development of multiphase nanostructures within MOFs, allowing the components to synergistically reinforce or change one another, imbuing the MOFs with distinct features.^22–24 For instance, Wang et al. used a solvothermal method to create a Ni–Co bimetallic MOF (Ni–Co MOF) with the ligand 4,4′-biphthalic acid (BPDC). Despite increasing the current density (C_d) to 20 A g⁻¹, this framework maintained a high capacitance of 405 F g⁻¹.²⁵ Wang et al. reported the preparation of a two-phase Ni–Co MOF in a single-step process using the TPA precursor, which has shown high capacitance of 1300 F g⁻¹ at 1 A g⁻¹, when employed in asymmetric SCs.²⁶ Raissa et al. (2024) demonstrated that cobalt content and ligand exchange significantly impact the electrochemical performance of Ni–Co MOFs, making them promising materials for supercapacitors.²⁷ These findings highlight the potential for bimetallic nodes in MOFs to greatly improve electrochemical activity during energy storage procedures.

Transition metal oxides derived from MOFs have sparked widespread interest for use in energy storage and conversion.²⁸ The synthesis of bimetallic mixtures from MOFs produces materials with complex structures due to the concentration of metal ions with various functions within the MOF framework.^29,30 This complexity is increased when one type of metal ion is replaced with another, resulting in novel functionalities dependent on the newly added metal ion. The literature indicates that bimetallic nickel–cobalt compound electrodes exhibit superior electrochemical activity and stability in comparison to electrodes composed of single metal compounds due to the synergistic effect between the two ions.^31,32 Furthermore, researchers have developed innovative structures with hollow polyhedrons using Ni–Co bimetallic hydroxide precursors derived from ZIF-67. These advanced materials exhibit high capacitance and exceptional energy density (E_d), making them highly suitable for SC applications.^33,34 Lee et al. reported the synthesis of hollow cage Ni_xCo_3−xO₄ structures derived from ZIF-67 and Ni(NO₃)₂ precursors through a thermal treatment technique. These structures exhibited a remarkable specific capacitance of 1931 F g⁻¹ at 1 A g⁻¹.³⁵ Zhao et al. used direct calcination to create a carbon-incorporated NiO@Co₃O₄, resulting in a specific capacitance of 208.5 F g⁻¹.³⁶ Qing et al. demonstrated that the oriented growth of NiCo-MOF nanosheets on ternary mixed metal oxides significantly enhances electrochemical performance for energy storage applications, achieving a high capacitance of 1724 F g⁻¹ at a C_d of 1 A g⁻¹.³⁷ Jianning et al. created Co₃O₄/NiCo₂O₄ nanosheets from ZIF-67, resulting in a specific capacitance of 846 F g⁻¹.³⁸ This highlights the potential of MOF-derived bimetallic combinations in improving energy storage devices.

Consequently, we synthesized a Co₃O₄/NiO nanostructure (MD-Co/Ni) via the thermal treatment of a MOF and investigated the effects induced by these nanostructures on their corresponding MOF structures for their application in SCs. The Co, Ni bimetallic MOF structures were prepared in a one-pot solvothermal procedure using polyvinylpyrrolidone (PVP) as a stabilizer (Co/Ni-MOF). The MOF-derived hybrid exhibited a superior C_sp of 2836 A g⁻¹ at a C_d of 1 A g⁻¹. Furthermore, a Co₃O₄/NiO nanostructure electrode-based hybrid coin-cell-type asymmetric supercapacitor (HS) device was fabricated, demonstrating an E_d of 27.28 W h kg⁻¹ at a power density (P_d) of 380 W kg⁻¹ and a long cycle life, retaining 80% capacitance over 9500 cycles.

2. Experimental section

2.1 Synthetic procedure for Ni-MOF, Co-MOF, Co/Ni-MOF, MD-CoO, and MD-Co/Ni nanostructure

The synthesis commenced with the preparation of a 210 mL solvent mixture, composed of equal parts deionized (DI) water, DMF, and ethanol. A total of 80 mL of this mixture was used to dissolve 137 mg of Ni(NO₃)₂·6H₂O, 152 mg of Co(NO₃)₂·6H₂O, and 400 mg of polyvinylpyrrolidone (PVP), followed by stirring at 200 rpm for 10 minutes. Separately, 103 mg of trimesic acid (TA) was dissolved in the remaining 130 mL of solvent mixture. This TA-containing solution was then gradually added dropwise to the previously prepared metal precursor solution under continuous stirring. The resulting mixture was transferred to a Teflon-lined autoclave and subjected to hydrothermal treatment at 160 °C for 4 hours. The obtained precipitate was collected via centrifugation at 7000 rpm, thoroughly washed multiple times with ethanol and methanol, and dried at 100 °C for 4 hours to yield the Co/Ni-MOF. To investigate the effect of varying metal ratios, Co/Ni-MOFs were synthesized using different Co : Ni molar ratios (0.25 : 1, 0.5 : 1, 0.75 : 1, and 1 : 1), and the resulting samples were designated as Co/Ni-MOF-1, Co/Ni-MOF-2, Co/Ni-MOF-3, and Co/Ni-MOF-4, respectively. The synthesis of Ni-MOF and Co-MOF followed the same procedure, with the exclusion of either the cobalt or nickel precursor. For the preparation of MD-CoO and different MD-Co/Ni samples, the Co-MOF and Co/Ni-MOFs were subjected to thermal annealing in a muffle furnace at 400 °C for 3 hours in air, resulting in their corresponding metal oxide derivatives. The schematic synthetic procedure and function of the MD-Co/Ni nanostructure are illustrated in Fig. 1. The characterization details, electrochemical testing procedures, and device fabrication methods are discussed in the ESI.†

Fig. 1 Schematic synthetic procedure and function of MD-Co/Ni nanostructure.

3. Results and discussion

Fig. 2(a)–(f) presents the morphological characteristics of Ni-MOF, Co-MOF, Co/Ni-MOFs, MD-NiO, MD-CoO, and various MD-Co/Ni nanostructures, highlighting the structural evolution of unary and bimetallic Co/Ni-MOFs. The Ni-MOF, Co-MOF, and Co/Ni-MOF samples exhibit self-assembled rod-like structures, as seen in Fig. 2(a–f(i, ii)). These rods are formed through a controlled self-assembly mechanism specific to MOFs. Notably, the Co/Ni-MOF-4 nanostructure (Fig. 2(f(i, ii))) demonstrates a well-defined spherical or flower-like morphology, characteristic of MOFs owing to their highly porous crystalline structure. This formation results in an ordered porous network, enhancing the surface area of the material. However, after undergoing thermal treatment, significant morphological transformations are observed. The MD-NiO sample (Fig. 2(a-iii, iv)) displays spherical or flower-like structures with protruding spikes along their periphery, indicating partial structural reorganization. By contrast, the MD-CoO sample (Fig. 2(b-iii, iv)) experiences decomposition of the rod-like formations into smaller, irregularly shaped particles due to thermal degradation. The MD-Co/Ni nanostructures undergo further changes, where the emergence of hollow and fragmented structures signifies the breakdown of MOF networks as a result of ligand removal and heat-induced collapse. Among these transformations, the MD-Co/Ni-4 nanostructure (Fig. 2(f-iii, iv)) stands out with a more porous and interconnected network, suggesting the development of hierarchical metal oxide nanostructures with enhanced textural properties. This can be attributed to the conformation endorsing the release of volatile organic ligands throughout its structural evolution, leading to the formation of a porous metal oxide framework. The thermal energy supplied during the heating process disrupts the cohesive forces responsible for maintaining the original rod-like structures, initiating structural reorganization and phase transitions. As a result, the rod-like morphology disintegrates into smaller particles, achieving a more thermodynamically stable configuration. In the case of MD-Co/Ni nanostructures (Fig. 2(f-iii, iv)), a nanorod/nanosheet-like morphology emerges, characterized by tiny rod-shaped nanoparticles of uniform particle size and a well-developed porous nature, making them potentially suitable for applications requiring high surface area and structural stability. The structural differences observed in SEM images arise from the intrinsic coordination behavior and growth mechanisms of Ni²⁺ and Co²⁺, influencing crystal morphology, porosity, and electrochemical activity. These variations play a crucial role in enhancing electrochemical performance by facilitating ion diffusion, promoting redox activity, and optimizing charge transfer dynamics, ultimately contributing to improved efficiency and stability.

Fig. 2 FESEM images of fresh MOFs: (a-i, ii) Ni-MOF, (b-i, ii) Co-MOF, (c-i, ii) Co/Ni-MOF-1, (d-i, ii) Co/Ni-MOF-2, (e-i, ii) Co/Ni-MOF-3, and (f-i, ii) Co/Ni-MOF-4. Heated MOFs: (a-iii, iv) MD-NiO, (b-iii, iv) MD-CoO, (c-iii, iv) MD-Co/Ni-1, (d-iii, iv) MD-Co/Ni-2, (e-iii, iv) MD-Co/Ni-3, and (f-iii, iv) MD-Co/Ni-4 nanostructures. (g) XRD patterns of Ni-MOF, Co-MOF, and various Co/Ni-MOFs. (h) XRD patterns of MD-CoO and different MD-Co/Ni nanostructures. (i) Deconvolution fitting curves of MD-CoO and MD-Co/Ni-4 nanostructures, with standard JCPDS reference lines.

The XRD spectra of the as-prepared Ni-MOF, Co-MOF, and Co/Ni-MOFs (Fig. 2(g)), along with the thermally treated MD-CoO, MD-Co/Ni-1, MD-Co/Ni-2, MD-Co/Ni-3, and MD-Co/Ni-4 nanostructures (Fig. 2(h)), were systematically analyzed. Additionally, the XRD deconvolution fitting curves of MD-CoO and MD-Co/Ni-4, along with the corresponding standard XRD patterns, are presented in Fig. 2(i). Fig. 2(i) illustrates the XRD patterns of Ni-MOF, Co-MOF, and Co/Ni-MOFs before annealing, which align well with previously reported literature.^38,39 For Ni-MOF, the diffraction peaks observed at approximately 11.9°, 23.9°, and 33.5° correspond to the (11−1), (02−4), and (025) planes, respectively, consistent with reported Ni-MOF structures.⁴⁰ Similarly, the Co-MOF exhibits distinct diffraction peaks at 10.6°, 12.6°, 17.6°, and 20.2°, which match the reported crystallographic patterns of Co-MOF, as referenced in the CCDC-1274034 database.^41,42 For the Co/Ni-MOFs, the XRD patterns display characteristic peaks of Ni-MOF and Co-MOF. Notably, as the cobalt content increases, the intensity of the Co-MOF-associated peaks becomes more prominent, confirming the successful incorporation of cobalt into the bimetallic framework. Fig. 2(h) and (i) depict the XRD patterns of the calcined samples, along with their deconvolution fitting curves and standard JCPDS reference patterns for Co₃O₄ and NiO. After annealing, the MD-CoO sample exhibits five well-defined diffraction peaks at approximately 19.1°, 31.3°, 36.9°, 59.4°, and 65.3°, which can be indexed to the (111), (220), (311), (511), and (440) planes of spinel-phase Co₃O₄ (JCPDS Card No. 01-078-1970), confirming its cubic crystal structure. Similarly, the XRD pattern of MD-Co/Ni-4 closely resembles that of MD-CoO, with the addition of characteristic NiO peaks at 37.2°, 43.3°, and 62.8°, corresponding to the (001), (200), and (111) planes, respectively, as per the JCPDS card no. 03-065-6920, indicating the formation of a monoclinic NiO phase. These results confirm the successful formation of NiO and Co₃O₄ in the annealed nanostructures. The absence of additional diffraction peaks suggests that the synthesized materials are highly pure and crystalline.

The HRTEM images exhibit a well-structured nanostructure characterized by a highly porous and interconnected framework. Variations in contrast between dark and light regions indicate a heterogeneous phase distribution, which may be linked to differences in elemental composition. In Fig. 3(a), a broad-field view reveals an irregular morphology with a rough surface texture. Fig. 3(b), captured at higher magnification, clearly displays distinct lattice fringes, signifying a well-crystallized structure. The well-ordered atomic arrangement suggests robust structural stability. Furthermore, in Fig. 3(c) and (d), uniformly dispersed nanoparticles are evident throughout the matrix, reinforcing the material's structural integrity and compositional uniformity, which aligns with the characteristics of a nanorod/nanosheet-like morphology that indicates high crystallinity. The high-magnification HRTEM image and selected area electron diffraction (SAED) pattern in Fig. 3(e) and (f) provide further validation, displaying distinct diffraction spots associated with Co₃O₄ and NiO phases, signifying a composite crystalline structure. Moreover, the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) mapping in Fig. 3(g)–(l) reveals a uniform distribution of Ni, Co, O, C, and N, confirming the successful synthesis of the Co₃O₄/NiO nanostructure.

Fig. 3 (a)–(e) High-magnification HRTEM images; (f) SAED pattern; (g) HRTEM-EDX combined mapping; and individual elemental mappings of (h) Co, (i) Ni, (j) O, (k) C, and (l) N for the MD-Co/Ni-4 nanostructure.

The chemical states of the elements present in the MD-Co/Ni-4 nanostructure were investigated using XPS analysis and the obtained results are displayed in Fig. 4(a)–(e). Fig. 4(a) illustrates the survey scan spectra of the MD-Co/Ni-4 nanostructure, endorsing the successful formation of a mixed oxide composite consisting of Co₃O₄ and NiO. The C1s spectrum (Fig. 4(b)) was resolved into three components at 284.3, 285.8, and 288.1 eV, corresponding to carbon in C=C, C–O, and C=O bonding environments. These findings are essential for understanding the electrochemical properties and catalytic behavior of the MD-Co/Ni nanostructure.⁴³ The O 1s spectrum (Fig. 4(c)) of the MD-Co/Ni nanostructure deconvoluted into three peaks at 529.3, 530.01, and 531.1 eV, corresponding to metal–oxygen bonds, defect-rich oxygen sites on the material's surface, and surface-adsorbed water molecules, respectively.³⁶ The Co 2p XPS spectrum (Fig. 4(d)) revealed spin–orbit doublets and satellite features. The peaks at 794.5 and 779.2 eV, corresponding to Co 2p_1/2 and Co 2p_3/2, indicate the presence of the Co²⁺ oxidation state. Additionally, peaks at 788.8 and 803.3 eV are associated with Co³⁺.⁴⁴ The Ni 2p XPS spectrum (Fig. 4(e)) also displayed spin–orbit doublets and satellite features, with binding energies of 872.1 and 853.9 eV for Ni 2p_1/2 and Ni 2p_3/2, confirming the Ni²⁺ oxidation state, while peaks at 879.5 and 860.9 eV suggest the presence of Ni³⁺.⁴⁵

Fig. 4 High-resolution XPS spectra of (a) survey scan (b) Co 2p, (b) C 1s, (c) O 1s, (d) Co 2p, and (e) Ni 2p of MD-Co/Ni nanostructure, (f) FTIR spectra of MD-CoO and MD-Co/Ni-4 nanostructure, (g) N₂ adsorption–desorption isotherms, and (h) pore size distribution determined by the BJH method of MD-CoO and MD-Co/Ni-4 nanostructure.

Infrared spectroscopy was employed to investigate the bonding properties of the produced materials. Fig. 4(f) depicts the infrared absorption spectra for MD-Co and MD-Co/Ni nanostructures. Metal oxides typically have vibrational peaks below 1000 cm⁻¹ due to internal atomic vibrations.⁴⁶ The peaks located at 551 cm⁻¹ indicate vibrational stretching of Ni–O and Co–O bonds, while peaks at 653 cm⁻¹ suggest Co₃O₄ and NiO production via O–Co–O and O–Ni–O connections. Furthermore, the peaks that appeared at 2031 and 2158 cm⁻¹ are likely to be associated with cyanide stretching vibrations.^47,48 The presence of these peaks suggests the formation of nitrogen-containing functional groups or cyanide species during the thermal decomposition process of the MOF. This insight into the chemical bonding and structural changes occurring in the nanocomposite is essential for understanding its properties and potential applications.

Fig. 4(g) presents the N₂ adsorption–desorption isotherms for MD-CoO and MD-Co/Ni-4 nanostructures, classified as Type IV isotherms with evident hysteresis loops, signifying their mesoporous characteristics. The BET surface area of MD-CoO is determined to be 5.15 m² g⁻¹, while MD-Co/Ni-4 exhibits a notably advanced surface area of 20.8 m² g⁻¹, offering more active sites that can enhance electrochemical performance. Fig. 4(h) depicts the pore size distribution, with average pore diameters of 1.65 nm for MD-CoO and 1.22 nm for MD-Co/Ni-4. The smaller pore size and increased surface area of MD-Co/Ni-4 indicate a more intricate porous structure, promoting better ion transport and accessibility, which are essential for superior electrochemical performance.

To evaluate the comparative energy storage capacity of Co/Ni-MOF-4, MD-CoO, and different MD-Co/Ni nanostructures, CV was implemented using a three-electrode arrangement. Fig. 5(a)–(f) demonstrates the alteration in redox potentials of Co/Ni-MOF-4, MD-CoO and different MD-Co/Ni nanostructures across variable scan rates (SRs) from 0.5 to 20 mV s⁻¹ within a potentials region of 0.0–0.6 V. With an increase in SR, the CV curves retained the pair of redox peaks, with a positive shift in the oxidation peaks and a negative shift in the reduction peaks, which indicates typical pseudocapacitive behavior.³⁸ Each CV profile showed a more obvious peak current for MD-Co/Ni nanostructures compared to Co/Ni-MOF-4, and MD-CoO. Fig. 5(g) presents a comparative analysis of various electrodes, demonstrating that the MD-Co/Ni-4 nanostructure electrode exhibits a larger integrated CV area than MD-CoO and Co/Ni-MOF-4 at 1 mV s⁻¹. The optimized bimetallic ratio of Co : Ni (1 : 1) contributes to an expanded electroactive surface area and improved electron transfer efficiency, which are key factors in its superior electrochemical performance. These findings suggest that the MD-Co/Ni nanostructures hold great potential as electrode materials. To understand the charge storage mechanism of the synthesized electrodes, the relationship between peak current (i) and scan rate (ν) was analyzed using the following eqn (1) and (2):

i(ν) = a·νb, (1)

log(i) = log(a) + b log(ν). (2)

Fig. 5 CV profiles of (a) Co/Ni-MOF-4, (b) MD-CoO, (c) MD-Co/Ni-1, (d) MD-Co/Ni-2, (e) MD-Co/Ni-3, and (f) MD-Co/Ni-4 nanostructure electrodes, (g) comparative CV curves of different electrodes at a scan rate of 1 mV s⁻¹, (h) logarithmic plot of scan rate versus peak current, and (i) schematic of the electrochemical charge transfer mechanism.

The b-values, obtained from the slopes of the log(i) versus log(ν) plots, provide insight into the dominant charge storage mechanism in the electrode materials (Fig. 5h). A b-value of 1.0 suggests a capacitive-dominated behavior, which may arise from either electrical double-layer capacitance (EDLC) or pseudocapacitance. Conversely, a b-value of 0.5 indicates a diffusion-controlled charge storage process, characteristic of a battery-like mechanism. If the b-value falls between 0.5 and 1.0, it suggests a combination of capacitive and diffusion-controlled processes. To further quantify the pseudocapacitive contribution, CV profiles were analyzed using the Cottrell eqn (3) and (4):

i(ν) = k₁ν + k₂ν^0.5, (3)

In these equations, i(ν) represents the total current, where kν^0.5 corresponds to the diffusion-controlled charge storage contribution, while kν accounts for the capacitive charge storage component. Here, k₁ and k₂ are proportionality constants. Fig. 5(h) displays the anodic peak b-values for different electrodes: Co/Ni-MOF-4 (0.86), MD-CoO (0.79), MD-Co/Ni-1 (0.68), MD-Co/Ni-2 (0.66), MD-Co/Ni-3 (0.63), and MD-Co/Ni-4 (0.58). These values suggest that the charge storage behavior of the electrodes is governed by a hybrid mechanism, incorporating pseudocapacitive and diffusion-controlled processes, with the degree of contribution varying among the different nanostructures. Fig. 5(i) represents the schematic illustration of the electrochemical charge transfer mechanism. The energy storage mechanism for the anodic and cathodic peaks is credited to the surface redox reactions of the Co²⁺/Co³⁺/Co⁴⁺ and Ni²⁺/Ni³⁺ redox couples in 1 M KOH electrolyte.^38,49 The reactions may be proposed as follows:^50,51

NiO + OH⁻ ↔ NiOOH + e⁻ (oxidation), (5)

NiOOH + e⁻ + H₂O ↔ Ni(OH)₂ + OH⁻ (reduction), (6)

Co₃O₄ + OH⁻ + H₂O ↔ 3CoOOH + e⁻ (Co²⁺ to Co³⁺), (7)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (Co³⁺ to Co⁴⁺). (8)

The occurrence of these redox-active species within the MD-Co/Ni nanostructures facilitates efficient charge storage and discharge, further contributing to the material's large capacitance and energy density (E_d). Fig. 6(a)–(c), (g)–(i) presents the distribution of charge storage contributions from capacitive and diffusion-controlled mechanisms, providing insights into the balance between surface-driven redox reactions and ion diffusion-dependent storage. In addition, Fig. 6(d)–(f) and (j)–(l) showcases CV curves, where the capacitive and diffusive contributions are distinctly separated, enabling a clearer understanding of their influence on the overall electrochemical performance. Among the studied materials, Co/Ni-MOF-4 predominantly exhibits capacitive behavior, whereas MD-CoO demonstrates advanced diffusive contribution (19%) compared to Co/Ni-MOF-4 (12.5%). This suggests that the oxide phase enhances the diffusive contribution. The proportion of pseudocapacitive contributions for the MD-Co/Ni-4 electrode is depicted in Fig. 6(i) and (l). The diffusive contributions for the MD-Co/Ni electrodes were recorded as 35.7%, 28.1%, 21.7%, 18.4%, 14.9%, and 13.8% at SRs of 0.5, 1, 2, 3, 5, and 6 mV s⁻¹, respectively. These values indicate that as the scan rate increases, capacitive behavior becomes more dominant. This trend suggests that at higher SRs, rapid ion intercalation and deintercalation processes improve electrochemical reversibility and enhance rate capability. Specifically, the MD-Co/Ni-4 electrode exhibits a 35.7% diffusion-controlled and 64.3% capacitive charge storage contribution at a scan rate of 0.5 mV s⁻¹. Furthermore, the Cottrell equation was applied to analyze the CV data for all electrodes (Fig. 6(d)–(f) and (j)–(l)), revealing a strong correlation with the charge storage behavior of the MD-Co/Ni-4 nanostructure. The findings confirm that pseudocapacitive contributions play a dominant role, highlighting the material's potential for efficient charge storage applications.

Fig. 6 (a)–(c) and (g)–(i) Representation of capacitive and diffusion-controlled charge storage contributions at various SRs (0.5–6 mV s⁻¹). (d)–(f) and (j)–(l) Comparative analysis of diffusion-controlled and capacitive charge storage through CV curves for all electrodes at a scan rate of 0.5 mV s⁻¹.

The GCD studies were conducted to evaluate the capacitive performance of the synthesized Co/Ni-MOF-4, MD-CoO, and various MD-Co/Ni nanostructures across C_d of 1–5 A g⁻¹. As illustrated in Fig. 7(a)–(f), all electrodes exhibited non-linear, quasi-symmetric charge–discharge profiles. This non-linearity aligns with the CV results, confirming the occurrence of redox reactions, including Co(II) to Co(III), Co(III) to Co(IV), and Ni(II) to Ni(III). Additionally, Fig. 7(g) presents a comparative analysis of the GCD curves for Co/Ni-MOF-4, MD-CoO, and MD-Co/Ni nanostructures at 1 A g⁻¹, revealing that the MD-Co/Ni-4 nanostructure exhibited the longest discharge duration. This extended discharge time highlights the strong faradaic behavior and superior rate capability of the MD-Co/Ni-4 electrode. The C_sp of the synthesized electrodes at different current densities (1–5 A g⁻¹) are depicted in Fig. 7(h). The estimated C_sp values for Co/Ni-MOF-4, MD-CoO, MD-Co/Ni-1, MD-Co/Ni-2, MD-Co/Ni-3, and MD-Co/Ni-4 at 1 A g⁻¹ were 534, 1242, 2318, 2482, 2804, and 2836 F g⁻¹, respectively. Furthermore, Fig. 7(i) illustrates the relationship between specific capacitance/capacity and SRs for the MD-Co/Ni-4 electrode, where a specific capacity of 1418 C g⁻¹ at 1 A g⁻¹ was observed. The MD-Co/Ni-4 nanostructure demonstrated enhanced energy storage capability due to the calcination process, which optimized the Co : Ni ratio (1 : 1). The gradual decline in specific capacity with increasing C_d suggests stable electrochemical behavior, further confirming the reliability of the MD-Co/Ni-4 electrode for high-performance energy storage uses.

Fig. 7 GCD profiles of nanostructured electrodes: (a) Co/Ni-MOF-4, (b) MD-CoO, (c) MD-Co/Ni-1, (d) MD-Co/Ni-2, (e) MD-Co/Ni-3, and (f) MD-Co/Ni-4; (g) presents a comparative analysis of GCD curves for these electrodes at 1 A g⁻¹; (h) illustrates the relationship between C_d and C_sp for each electrode; and (i) depicts the variation of specific capacitance/capacity of the MD-Co/Ni-4 nanostructure across different C_ds.

The Nyquist plots for Co/Ni-MOF-4, MD-CoO, and the series of MD-Co/Ni nanostructured electrodes (MD-Co/Ni-1 through MD-Co/Ni-4) are depicted in Fig. 8(a) and (b). The corresponding equivalent circuit model is presented in Fig. S1 (ESI†), with the extracted parameters summarized in Table 1. These Nyquist plots typically feature a semicircular arc in the high-frequency domain, transitioning to a linear segment at lower frequencies. The diameter of the semicircle at high frequencies corresponds to the R_ct at the electrode–electrolyte interface, while the linear portion at low frequencies indicates the Warburg impedance, reflecting ion diffusion within the electrode material.⁵² Analysis of the impedance data reveals that the MD-Co/Ni-4 electrode exhibits an R_ct of 26.73 Ω, which is significantly lower than that of MD-CoO (54.58 Ω) and Co/Ni-MOF-4 (65.65 Ω). This reduction in R_ct for MD-Co/Ni-4 indicates enhanced charge transfer efficiency at its interface. The R_s values range from 0.818 Ω for MD-Co/Ni-4 to 2.558 Ω for Co/Ni-MOF-4, signifying relatively low and consistent resistances across all samples. Variations in the CPE values among the samples reflect differences in their capacitive behavior and surface characteristics. Collectively, these EIS parameters indicate that the MD-Co/Ni-4 electrode possesses superior electrochemical properties, as evidenced by its lower R_s and R_ct values, which are typically associated with enhanced capacitive performance. To understand the stability of the constructed device, 11 000 cycles of consecutive GCD tests at 10 A g⁻¹ were performed and the obtained cyclic stability and coulombic efficiency results are described in Fig. 9(a). The MD-Co/Ni-4 nanostructure showed outstanding cycling stability, having maintained 96.5% of its initial specific capacitance and 99% of coulombic efficiency after 11 000 cycles of GCD analysis. This remarkable degree of retention demonstrates the capability of MD-Co/Ni-4 nanostructure for long-term energy storage. Moreover, the performance of the MD-Co/Ni-4 nanostructure electrode compared favorably against previously reported electrodes as shown in Table 2. The superior electrochemical performance of the Co : Ni (1 : 1) electrode arises from the optimized balance between redox-active sites, electronic conductivity, and structural stability. The synergy between Co and Ni oxides enhances pseudocapacitive and faradaic charge storage mechanisms, leading to improved specific capacitance and long-term cycling durability. Future work may explore further optimization of the metal ratio and hybrid materials to push the electrochemical performance limits further.

Fig. 8 (a) EIS spectra for all electrodes, (b) enlarged view of the semicircular region in the EIS spectra, and (c) cycling stability and coulombic efficiency of the MD-Co/Ni-4 nanostructure.

Table 1 EIS spectra analysis of different electrodes

Samples	R s (ohm)	R ct (ohm)	CPE (F)
Co/Ni-MOF-4	2.558	65.65	0. 348 × 10^−3
MD-CoO	2.319	54.58	1.437 × 10^−3
MD-Co/Ni-1	2.229	38.52	0.727 × 10^−3
MD-Co/Ni-2	1.519	37.63	0.104 × 10^−3
MD-Co/Ni-3	0.836	28.79	2.51 × 10^−3
MD-Co/Ni-4	0.818	26.73	5.25 × 10^−3

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Fig. 9 MD-Co/Ni-4//AC device performance: (a) HS device construction details, (b) comparative CV of AC and MD-Co/Ni-4, (c) CV profiles at various potentials, (d) CV profiles with various SRs, (e) GCD curves of MD-Co/Ni-4//AC at various potential windows at C_d of 1A g⁻¹, and (f) at various current densities (C_d).

Table 2 Comparison between electrochemical performances of MD-Ni/Co-4 nanostructure electrode and recently reported electrodes

Sample	Test solution	Specific capacitance (F g^−1)	Current density (A g^−1)	Ref.
Co3O4/NiCo2O4	6 M KOH	846	0.5	38
Co3O4/NiCo2O4	2 M KOH	430	1.0	52
NiCo-MOF/rGO	—	1320	0.2	53
Ni–Co-MOF/GO	6 M KOH	447.2	0.5	54
Ni/Co MOF	3 M KOH	82.5	0.5	55
PPNF@CoNi-MOF	2 M KOH	1096.2	1.0	56
MD-Co/Ni-4	1 M KOH	2836	1.0	This work

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A hybrid coin-cell asymmetric supercapacitor (HS) was fabricated to assess the practical application of the MD-Co/Ni-4 nanostructure, with the findings presented in Fig. 9. Fig. 9(a) shows the HS device construction details. The comparative CV profiles of activated carbon (AC) and MD-Co/Ni-4 electrodes were analyzed and illustrated in Fig. 9(b) at a SR of 20 mV s⁻¹. This analysis highlights the electric double-layer capacitor (EDLC) behavior of AC and the pseudocapacitive behavior of the MD-Co/Ni electrodes. To determine the stable potential window of the MD Co/Ni//AC device, voltages ranging from 0.4 to 1.5 V were employed (Fig. 9(c)). The increase in current response with an expanding potential window indicates the broad operational potential of the HS device. The HS device potential window was fixed to 0.0–1.5 V, and the CV profile of the device at differing SRs (from 2 to 200 mV s⁻¹) was studied, as shown in Fig. 9(d). The result illustrates that the current response increases with increasing the SR without changing the shape of the CV curve, suggesting that the fabricated HS device has fast redox activities and low internal resistance. Moreover, the obtained CV curves are in quasi-rectangular shape at various SRs further endorsing the collective effect of faradaic pseudocapacitance (MD-Co/Ni-4 nanostructure) and EDLC (from AC). Fig. 9(e) displays the GCD profiles of the fabricated HS device across various potential windows from 0.8 to 1.5 V to determine the optimal potential range. The established potential window aligns with the results from the CV analysis, found to be 1.5 V at a C_d of 1 A g⁻¹. This analysis highlights the EDLC behavior of AC and the pseudocapacitive behavior of the MD-Co/Ni-4 electrodes. Furthermore, the fabricated MD-Co/Ni-4//AC device was utilized for GCD analysis, with C_d varying from 0.8 to 1.5 A g⁻¹ as shown in Fig. 9(f). The GCD results exhibit a non-linear and quasi-symmetric profile, indicating efficient faradaic charge storage and excellent reversibility during charge–discharge cycles.

The C_sp of the HS device, calculated from GCD curves at various C_d, is displayed in Fig. 10(a). The capacitance values were found to be 80, 70, 65, and 55.0 F g⁻¹ at C_d of 0.5, 1.0, 2.0 and 3.0 A g⁻¹, respectively. Higher capacitance at lower C_d indicates that the device stores more energy during slower charge–discharge processes. Conversely, the decrease in specific capacitance at higher C_d suggests reduced efficiency due to rapid charge–discharge cycles. To evaluate the energy storage capability of the device for practical applications, E_d and P_d are critical parameters. Fig. 10(b) and (c) display the relationships between E_d, P_d, and discharge time, along with Ragone plots for the fabricated HS device. Notably, the MD-Co/Ni//AC device achieved an energy density of 27.28 W h kg⁻¹ at a power density of 380 W kg⁻¹ and maintained an energy density of 21.59 W h kg⁻¹ at a power density of 2045.46 W kg⁻¹, while operating at a potential of 1.5 V. In Fig. 10(d), the MD-Co/Ni//AC device electrode exhibited remarkable performance, retaining 79.8% of its initial capacitance and achieving a coulombic efficiency of 99.53%. This performance underscores the excellent reversibility and durability of the electrode through repeated charge–discharge cycles, confirming its potential for long-term applications. A comparison with existing technologies described in Table 3 highlights the superior capabilities of the HS device in terms of efficiency, stability, and functionality, underscoring its advantages over existing technologies.^49–51,57 Specifically, the HS device demonstrates enhanced efficiency, stability, and overall functionality, making it a standout option in the field. This comparison underscores the advancements made with the HS device, showcasing its potential to outperform existing technologies in various applications. The EIS data for MD-Co/Ni-4//AC device was acquired at an open-circuit voltage, and the EIS plot is shown in Fig. 10(e). The obtained R_ct value of MD Co/Ni//AC was 31.54 ohms. Based on the abovementioned results, the electrochemical charge transfer mechanism in MD-Co/Ni-4//AC was proposed, as illustrated in Fig. 5(i). Fig. 10(f) shows a demonstration of the MD-Co/Ni-4//AC device powering a red LED, successfully providing continuous illumination for three minutes. This demonstration highlights the device's potential for use in real-world energy storage, particularly in scenarios requiring short-term power delivery. This demonstration not only emphasizes the device's energy storage capabilities but also its rapid charge–discharge proficiency, making it a promising candidate for uses in emergency lighting, portable electronics, and other fields where quick energy deployment is essential.

Fig. 10 MD-Co/Ni-4//AC device performance: (a) specific capacitance at various C_d, (b) Ragone plot, (c) electrochemical performance plot E_dvs. P_dvs. discharge time, (d) cycling retention, (e) EIS with fitting curve and (f) demonstration of the MD-Co/Ni-4//AC device powering a red LED for 3 minutes, showcasing its practical application potential.

Table 3 Comparison of electrochemical performance of our asymmetric device MD-Co/Ni//AC with the reported literature

Asymmetric device	Potential window (V)	Stability capacitance retention (%)	Energy density (W h kg^−1)	Power density (W kg^−1)	Ref.
NiCo/GO-10//ASC	0–1.8	90.6% (5000 cycles)	24.4	291	53
NCMO@NiCo-MOF	0–1.8	68.2% (10 000 cycles)	44.5	824.8	37
KNiCoPO4/AC	0–1.4	99% (5000 cycles)	22	363	27
NiCoMOF//activated carbon	0–1.7	81.6% (10 000th cycles)	49.4	562.5	58
ZNCO@rGOsingle bondNF//rGO-MDPC	0–1.5	94% (500 cycles)	61.25	750	59
rGONi-Mn-Co oxide//N-rGO	0–1.4	77.2% (10 000th cycles)	35.6	699.9	60
MD-Co/Ni-4//AC	0–1.5	80% (9500 cycles)	27.28	380	This work

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To assess the stability of the MD-Co/Ni-4 nanostructure, XRD and FESEM analyses were performed following the cycling test. The XRD pattern presented in Fig. S2 (ESI†) indicates that the primary diffraction peaks remained nearly unchanged, signifying durable crystallinity retention. Furthermore, FESEM images captured before and after the cycling test (Fig. S3(a) and (b), ESI†) provide insights into any morphological changes. Although slight surface alterations were observed, the overall nanostructure was preserved, confirming its high stability during cycling. These findings highlight the MD-Co/Ni-4 nanostructure's exceptional structural and morphological robustness, reinforcing its potential for long-term applications.

4. Conclusion

In summary, this study illustrates the formation of hybrid Co₃O₄/NiO nanostructures (MD-Co/Ni) through the MOF thermal decomposition method composed of Co and Ni. The precursor Co/Ni-MOF was synthesized via a single-step hydrothermal method. PVP was strategically employed to enhance the crystallinity of the nanostructures, which were distinguished by their elongated or sheet-like structures and robust crystallinity, as confirmed by XRD and FE-SEM analyses. Herein, Co₃O₄/NiO nanostructures (MD-Co/Ni) were synthesized through thermal decomposition of bimetallic (Co/Ni-MOFs) with varying Co : Ni ratios (0.25 : 1, 0.5 : 1, 0.75 : 1, and 1 : 1). The impact of different Co : Ni ratios on the electrochemical performance of the resulting nanostructures was investigated. The MD-Co/Ni-4 nanostructure demonstrated an enhanced C_sp of 2836 F g⁻¹ at 1 A g⁻¹ compared to the corresponding MOF structure. The enhanced performance can be credited to the synergistic interaction between Co₃O₄ and NiO within the hybrid structures and the morphology that facilitates efficient electron transport. The practical utility of the current findings was further demonstrated through the fabrication of a HS employing the MD-Co/Ni-4 nanostructure. This device exhibited an exceptional E_d of 27.28 W h kg⁻¹ and a P_d of 380 W kg⁻¹, in addition to remarkable working stability, maintaining 80% of its capacitance even after 9600 cycles. The findings of the current study not only demonstrate the effectiveness of bimetallic oxides derived from MOFs in improving electrochemical performance but also present novel opportunities for the advancement of energy storage devices with exceptional performance. The observed electrochemical properties and synthetic strategy of the MD-Co/Ni nanostructures indicate considerable potential for future research and practical implementation in the domain of energy storage.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

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.

Acknowledgements

Authors are grateful to the Researchers Supporting Project Number (RSP2024R448), King Saud University, Riyadh, Saudi Arabia. This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS2023-00280665), the Natural Science Foundation of Hubei Province (no. 2024AFB376), Science and Technology Research Project of Education Department of Hubei Province (Q20232701), and Xiaogan City Natural Science Plan Project (XGKJ2023010056).

References

1. Y. Liu, G. Li, Y. Guo, Y. Ying and X. Peng, ACS Appl. Mater. Interfaces, 2017, 9, 14043–14050.
2. W. Raza, F. Ali, N. Raza, Y. Luo, K.-H. Kim, J. Yang, S. Kumar, A. Mehmood and E. E. Kwon, Nano Energy, 2018, 52, 441–473.
3. Y. Wang, Q. Lu, F. Li, D. Guan and Y. Bu, Adv. Funct. Mater., 2023, 33, 2213523.
4. Y. Song, H. Kim, J.-H. Jang, W. Bai, C. Ye, J. Gu and Y. Bu, Adv. Energy Mater., 2023, 13, 2302384.
5. Y. Jiang, L. Chen, H. Zhang, Q. Zhang, W. Chen, J. Zhu and D. Song, Chem. Eng. J., 2016, 292, 1–12.
6. L. Abbasi and M. Arvand, Appl. Surf. Sci., 2018, 445, 272–280.
7. Y. Wang, X. Ge, Q. Lu, W. Bai, C. Ye, Z. Shao and Y. Bu, Nat. Commun., 2023, 14, 6968.
8. S. Tanwar, N. Singh, A. K. Vijayan and A. L. Sharma, Surf. Interfaces, 2023, 42, 103504.
9. F. Ahmad, A. Shahzad, M. Danish, M. Fatima, M. Adnan, S. Atiq, M. Asim, M. A. Khan, Q. U. Ain and R. Perveen, JEnergy Storage, 2024, 81, 110430.
10. X. Zheng, X. Han, X. Zhao, J. Qi, Q. Ma, K. Tao and L. Han, Mater. Res. Bull., 2018, 106, 243–249.
11. P. Sivakumar, C. J. Raj, A. D. Savariraj, R. Manikandan and H. Jung, Surf. Interfaces, 2024, 51, 104641.
12. M. Harilal and S. G. Krishnan, Supercapacitors, Elsevier, 2024, pp. 17–43.
13. T. Tene, S. Bellucci, M. Guevara, P. Romero, A. Guapi, L. Gahramanli, S. Straface, L. S. Caputi and C. Vacacela Gomez, Batteries, 2024, 10, 256.
14. W. Sheng, W. Hanbo, P. Dongyu, W. Ziming, F. Zhitian, Y. Mingrui, L. Kechang and L. Haiyan, Surf. Interfaces, 2024, 46, 104049.
15. C. R. Babu, A. Avani, S. Shaji and E. Anila, J. Solid State Electrochem., 2024, 28, 2203–2210.
16. X. Guo, Y. Liu, L. Feng, P. Xu, Y. Yang, X. Gao, H. Sui, X. Wang, X. Yin and X. Ma, Surf. Interfaces, 2024, 53, 105048.
17. O. M. Yaghi and H. Li, J. Am. Chem. Soc., 1995, 117, 10401–10402.
18. M. Pooriraj, M. Moradi and S. Hajati, Solid State Ionics, 2021, 368, 115697.
19. B. Chettiannan, E. Dhandapani, G. Arumugam, R. Rajendran and M. Selvaraj, Coord. Chem. Rev., 2024, 518, 216048.
20. D. Y. Lee, S. J. Yoon, N. K. Shrestha, S.-H. Lee, H. Ahn and S.-H. Han, Microporous Mesoporous Mater., 2012, 153, 163–165.
21. C. Li, L. Zhang, J. Chen, X. Li, J. Sun, J. Zhu, X. Wang and Y. Fu, Nanoscale, 2021, 13, 485–509.
22. M. Zhao and S. Tong, Energy Fuels, 2024, 38, 13796–13818.
23. S. Li, Y. Duan, Y. Teng, N. Fan and Y. Huo, Appl. Surf. Sci., 2019, 478, 247–254.
24. F. Parsapour, M. Pooriraj, M. Moradi, V. Safarifard and S. Hajati, Synth. Met., 2023, 292, 117234.
25. X. Wang, Q. Li, N. Yang, Y. Yang, F. He, J. Chu, M. Gong, B. Wu, R. Zhang and S. Xiong, J. Solid State Chem., 2019, 270, 370–378.
26. J. Wang, Q. Zhong, Y. Xiong, D. Cheng, Y. Zeng and Y. Bu, Appl. Surf. Sci., 2019, 483, 1158–1165.
27. R. Raissa, N. L. Wulan Septiani, S. Wustoni, F. Failamani, N. Wehbe, M. Eguchi, H. Nara, S. Inal, V. Suendo and B. Yuliarto, J. Power Sources, 2024, 603, 234423.
28. D. Cai, Z. Yang, R. Tong, H. Huang, C. Zhang and Y. Xia, Small, 2024, 20, 2305778.
29. D. Sun, F. Sun, X. Deng and Z. Li, Inorg. Chem., 2015, 54, 8639–8643.
30. M. Wasi Ahmad, S. Anand, A. Syed, A. H. Bahkali, L. S. Wong, A. Shrivastava, A. Choudhury and D.-J. Yang, Surf. Interfaces, 2024, 44, 103834.
31. S. Dai, Y. Bai, W. Shen, S. Zhang, H. Hu, J. Fu, X. Wang, C. Hu and M. Liu, J. Power Sources, 2021, 482, 228915.
32. D. Yue, P. Rosaiah, K. Mallikarjuna, M. R. Karim, J. S. Alnawmasi, T. J. Ko and G. P. Nunna, Polyhedron, 2024, 260, 117062.
33. G. Qu, X. Zhang, G. Xiang, Y. Wei, J. Yin, Z. Wang, X. Zhang and X. Xu, Chin. Chem. Lett., 2020, 31, 2007–2012.
34. L. Zhao and F. Ran, Chem. Commun., 2023, 59, 6969–6986.
35. A. Jayakumar, R. P. Antony, R. Wang and J. M. Lee, Small, 2017, 13, 1603102.
36. X. Wei, Y. Zhang, H. He, D. Gao, J. Hu, H. Peng, L. Peng, S. Xiao and P. Xiao, Chem. Commun., 2019, 55, 6515–6518.
37. X. Qing, C. Zhang, Y. Wang, S. Wang, C. Xiang, F. Xu, L. Sun and Y. Zou, J. Alloys Compd., 2024, 1005, 176107.
38. J. Li, C. Zhang, Y. Wen, Y. Zhao, Y. Zhang, L. Shu and H. Qin, J. Chem. Res., 2021, 45, 983–991.
39. L. Wu, K. Zhang, J. Shi, F. Wu, X. Zhu, W. Dong and A. Xie, Composites, Part B, 2022, 228, 109424.
40. M. Radhika, B. Gopalakrishna, K. Chaitra, L. K. G. Bhatta, K. Venkatesh, M. S. Kamath and N. Kathyayini, Mater. Res. Express, 2020, 7, 054003.
41. M. H. Sayahi, M. Ghomi, S. M. Hamad, M. R. Ganjali, M. Aghazadeh, M. Mahdavi and S. Bahadorikhalili, J. Chin. Chem. Soc., 2021, 68, 620–629.
42. F. T. Alshorifi, S. M. El Dafrawy and A. Ahmed, ACS Omega, 2022, 7, 23421–23444.
43. X. Wei, Y. Li, H. Peng, D. Gao, Y. Ou, Y. Yang, J. Hu, Y. Zhang and P. Xiao, Chem. Eng. J., 2019, 355, 336–340.
44. C. T. Thanh Thu, H. J. Jo, G. Koyyada, D.-H. Kim and J. H. Kim, Materials, 2023, 16, 5461.
45. C. Mahala and M. Basu, ACS Omega, 2017, 2, 7559–7567.
46. R. Ramasamy, K. Ramachandran, G. G. Philip, R. Ramachandran, H. A. Therese and G. Gnana kumar, RSC Adv., 2015, 5, 76538–76547.
47. M. Shao, M.-X. Li, Z.-X. Wang, X. He and H.-H. Zhang, Cryst. Growth Des., 2017, 17, 6281–6290.
48. Q. Zhang, M. Chen, L. Zhong, Q. Ye, S. Jiang and Z. Huang, Molecules, 2018, 23, 2086.
49. S. Gao, Y. Sui, F. Wei, J. Qi, Q. Meng, Y. Ren and Y. He, J. Colloid Interface Sci., 2018, 531, 83–90.
50. A. Zhang, W. Zheng, Z. Yuan, J. Tian, L. Yue, R. Zheng, D. Wei and J. Liu, Chem. Eng. J., 2020, 380, 122486.
51. S. Sun, M. Huang, P. Wang and M. Lu, J. Electrochem. Soc., 2019, 166, A1799.
52. P. Sennu, V. Aravindan and Y.-S. Lee, J. Power Sources, 2016, 306, 248–257.
53. A. D. Salunkhe, P. S. Pawar, P. K. Pagare and A. P. Torane, Electrochim. Acta, 2024, 477, 143745.
54. J. Hong, S.-J. Park and S. Kim, Electrochim. Acta, 2019, 311, 62–71.
55. Y. Xiao, W. Wei, M. Zhang, S. Jiao, Y. Shi and S. Ding, ACS Appl. Energy Mater., 2019, 2, 2169–2177.
56. J. P. Cheng, B. Q. Wang, S. H. Gong, X. C. Wang, Q. S. Sun and F. Liu, Ceram. Int., 2021, 47, 32727–32735.
57. J. Wang, Q. Zhong, Y. Zeng, D. Cheng, Y. Xiong and Y. Bu, J. Colloid Interface Sci., 2019, 555, 42–52.
58. Q. Cheng, K. Tao, X. Han, Y. Yang, Z. Yang, Q. Ma and L. Han, Dalton Trans., 2019, 48, 4119–4123.
59. J. Acharya, B. Pant, G. P. Ojha, H.-S. Kong and M. Park, J. Colloid Interface Sci., 2021, 602, 573–589.
60. C. Wu, J. Cai, Y. Zhu and K. Zhang, ACS Appl. Mater. Interfaces, 2017, 9, 19114–19123.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00922g

‡ These authors contributed equally to this work.

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