Introduction

Charge storage is a crucial aspect of modern technology, enabling the use of various electronic devices such as batteries, supercapacitors, and electrochemical sensors. However, the performance of these devices heavily depends on the materials used for charge storage1. Traditional materials such as carbon2 and metal oxides3 have limited surface area and poor conductivity, leading to low energy density and a slow charging rate. Carbon-based materials usually exhibit high electrochemical performances, thermal stability, and electrical conductivity. These materials with positive curvature are suitable for high-performance supercapacitors because they can store electrolyte ions on the carbon surface during the charging process. However, reaching high energy density with chemical stability is still a challenge4. To overcome these limitations, the development of novel materials with high surface area and tunable properties has become a significant research focus. Metal-organic frameworks (MOFs) are a class of porous materials that have shown promise for charge storage applications due to their high surface area, tunable pore size, and chemical diversity5,6,7. MOFs possess designable flexible structures with high porosity reaching the cutting edges standard in current technologies. Hybrid materials known as MOFs are emerging as versatile substances that offer a range of crystal structures while maintaining the integrity of their crystal morphology. This variability allows for the development of materials with desirable physical properties. By carefully selecting ligands and metal centers, these materials can serve as building blocks that exhibit exceptional electrical conductivity and redox activity within their framework8. However, their synthesis typically involves harsh conditions and lengthy processes, resulting in low yields and poor reproducibility. This has hindered their practical applications and motivated the search for new methods of MOF synthesis8.

Chronoamperometry electrodeposition has emerged as a novel technique to fabricate MOFs with high electroactive sites and good conductivity, which are crucial properties for efficient charge storage. This technique involves the controlled reduction of metal ions in the presence of organic ligands, resulting in the formation of a porous MOF film on the electrode surface. By adjusting the electrodeposition conditions, such as the potential and deposition time, the morphology and properties of the resulting MOF can be tailored to meet specific requirements. In this research article, we present a study on the electrodeposition of porous MOFs for efficient charge storage. We demonstrate the successful synthesis of a highly conductive MOF film with a large surface area and tunable pore size. We also investigate the charge storage performance of the electrodeposited MOF film in a supercapacitor device, and show that it exhibits excellent capacitance and cycling stability. Our results demonstrate the potential of electrodeposition as a promising technique for the scalable synthesis of MOFs for energy storage applications.

Results

The electrodeposition technique was employed to deposit Co-MOFs on a nickel foam substrate using Co2+ metal ions and 1,2 benzene dicarboxylic acid in a DMF: water. The deposition was carried out using the chronoamperometry technique, which is a commonly used technique to deposit metal species onto a conducting substrate. Figure 1 illustrates schematically the process used to create the Co-MOF-based electrode material.

Fig. 1: Schematic representation of the preparation of Co-MOF/NF through electrodeposition technique.
figure 1

a Electrochemical deposition measurement with chronoamperometric technique. b Initial three dimensional growth of MOF crystals over a nickel foam current collector, (c) development of two dimensional structure of MOF.

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The MOF was prepared by taking Co2+ metal ions and 1,2 benzene dicarboxylic acid in the DMF: water. The electrodeposition technique is performed to deposit chemical species over the substrate. The schematic representation of the preparation of MOFs-based electrode material is shown in Fig. 1. The nucleation and growth kinetics of the prepared MOF film were analyzed using the current-time transients’ curve at an applied potential, as shown in Fig. 2a–c. The I-t curves showed a classic diffusion-controlled nucleation process, which was divided into three parts. The first portion of the plot at the near longitudinal axis corresponds to the formation of the electrochemical double layer on the surface of the substrate. The second section shows a trend that increases gradually until it reaches its maximum value with current density, suggesting a typical crystal nucleation and growth process. Finally, at constant current density, the diffusion of deposit ions from the solution to the surface of the current collector is observed9. In region Fig 2a, the electrode potential is maintained at a constant value before electroreduction is initiated. This region is important to ensure that any electroactive species that may interfere with the electroreduction of the Co-MOFs are removed from the solution. Region Fig. 2b corresponds to the onset of electroreduction, as the Co-MOFs start to deposit onto the electrode surface. This region is characterized by a high rate of nucleation and growth of Co-MOFs on the electrode surface. Region Fig. 2c represents the steady state of electroreduction, where the current density stabilizes as the deposition rate of Co-MOFs becomes constant. At this point, the growth of Co-MOFs is limited by the rate of diffusion of the precursor ions to the electrode surface.

Fig. 2: The experimental curve of chronoamperometry, which is a technique used to study the electrochemical behavior of materials.
figure 2

The curve is divided into three regions: (a) the justified region of prior electroreduction, (b) the state at the onset of electroreduction, (c) the steady state of electroreduction, (d) the dimensionless analysis of the prepared Co-MOF electrodeposition fitted with experimental results.

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To evaluate the nucleation and growth mechanism of Co-MOF deposition, a chronoamperometry I-t transient curve was analyzed by the SH model. Here, the nucleation and growth mechanism were examined by I max which occurs at t max. Based on the nucleation theory, two possible nucleation mechanisms can be expected: instantaneous nucleation and progressive nucleation. Here the nucleation and growth mechanism were examined by I max that occurs at t max. the following equations expressed the mathematical expression of the theoretical model:

$${\left[\frac{I}{{I}_{\max }}\right]}^{2}= \frac{1.9542}{\frac{t}{{t}_{\max }}}{\left\{1-\exp \left[-1.2564\left(\frac{{{{\rm{t}}}}}{{{{{\rm{t}}}}}_{\max }}\right)\right]\right\}}^{2}\\ ({{{\rm{Instantaneous\; Nucleation}}}})$$
(1)
$${\left[\frac{I}{{I}_{\max }}\right]}^{2}= \frac{1.2254}{\frac{t}{{t}_{\max }}}{\left\{1-\exp \left[-2.3367{\left(\frac{{{{\rm{t}}}}}{{{{{\rm{t}}}}}_{\max }}\right)}^{2}\right]\right\}}^{2} \\ ({{{\rm{Progressive\; Nucleation}}}})$$
(2)

Where, I max and t max were the maximum current at the maximum time obtained from the chronoamperometry response of I -t transients.

Scharifker and Hills (SH) model were used to analyze the I-t transient curve in Fig. 2d after it was obtained from chronoamperometry of Co-MOF deposition at a potential of -1.57 V10. I max and t max is used to validate the mathematical model. Figure 2d shows the dimensionless analysis of the prepared Co-MOF electrodeposition, which is fitted with the experimental results. The dimensionless analysis allows for the comparison of the electrochemical behavior of Co-MOFs. The fitted curve shows a good agreement between the experimental results and the theoretical model, which confirms the reliability and reproducibility of the electrochemical deposition method used to prepare the Co-MOFs. Instantaneous nucleation and the progressive nucleation model came after the experimental transient curve’s initial portion (I-t transient). The Fig demonstrates instantaneous nucleation to have a smaller deviation than progressive nucleation. Moreover, (I/Imax)2 data appear to depart from the theoretical model when t/t max > 1. Although this is related to the nucleation and growth process at smooth surfaces, the substrate’s surface will also contain several dislocations and active sites that promote extra-active nucleation11. The deposition of Co-MOF is thought to be reasonably close to the instantaneous nucleation model since it deviates less from the experimental transient curve, according to the study of the I-t transient using Imax and t max. The side reaction from the electrolyte is however indicated by the substantial deviation in Fig. 2d after the t maximum12.

In essence, the aqueous solution containing metal ions reduces and recombines with deprotonated ligands during the electrodeposition process. Four phases make up the mechanism of the metal-organic framework created by electrodeposition: initial nucleation, growth of the MOF on the substrate, intergrowth of the MOF crystals, and detachment13. The metal ion-containing solution begins to nucleate at the beginning of the process and attaches to the conducting substrate. This nucleation begins at the defective surface; after the nucleation process, the flat surface may stop the nucleation of the crystals into almost isolated mono-dispersion crystals. After the nuclei are formed, a brand-new crystal grows precisely next to them, creating an intergrown structure13. The growth process is influenced by several factors, including the concentration of metal ions, the presence of ligands, and the electrochemical potential of the system. As the MOF crystals continue to grow, they begin to interlock with each other, leading to the formation of an intergrown structure. The intergrowth of MOF crystals is a crucial step in the electrodeposition process, as it determines the final morphology of the MOF. The intergrown structure arises due to the directional growth of MOF crystals, which align themselves along specific crystallographic directions. The interlocking of these crystals leads to the formation of a three-dimensional network with well-defined pores and channels. The last phase of the MOF growth is crystal detachment; this process is not favorable during the growth of crystals during electrodeposition because the process exposes instantaneous nucleation with empty spots over the substrate and hence is avoided during the high-quality deposition process. The reaction mechanism of Co2+ metal ion and bidentate ligand 1, 2 benzene dicarboxylic acid (BDC) is given below11:

$${{{{\rm{Co}}}}}^{2+}+{2{{{\rm{e}}}}}^{-}\,\leftrightarrow {{{{\rm{Co}}}}}^{0}$$
(3)
$${{{{{\rm{NO}}}}}_{3}}^{2+}+{4{{{\rm{H}}}}}^{-}+{4{{{\rm{e}}}}}^{-}\leftrightarrow {{{{\rm{NO}}}}2}^{-}+{{{{\rm{H}}}}}_{2}{{{\rm{O}}}}$$
(4)
$${{{{\rm{NO}}}}}_{{2}^{-}}+{3{{{\rm{H}}}}}_{2}{{{\rm{O}}}}\leftrightarrow {{{{\rm{NO}}}}}_{{3}^{-}}+{2{{{\rm{OH}}}}}^{-}+{2{{{\rm{H}}}}}_{2}$$
(5)
$${2{{{\rm{OH}}}}}^{-}+{{{\rm{BDC}}}}\leftrightarrow {{{{\rm{BDC}}}}}^{-}+{{{{\rm{H}}}}}_{2}{{{\rm{O}}}}$$
(6)
$${{{{\rm{BDC}}}}}^{-}+{{{{\rm{Co}}}}}^{2+}\leftrightarrow [{{{{\rm{Co}}}}}_{3}{({{{\rm{BDC}}}})}_{2}]$$
(7)

At potential ranges greater than –1.5 V, the electrochemical deposition process converts metal ions containing nitrates to hydroxide ions. The crystallization of the MOF occurs when anionic ligands react with metal ions that contain nitrate, which takes place when the potential is applied. In this case, the generation of MOF was dependent upon several factors, including (1) the base generation of hydroxide ions or cathodic ions, (2) the deprotonation of hydroxide ions to form a crystalline metal-organic framework, and (3) the crystallization of the MOF. The production of MOF crystals at the surface of the conducting current collector is depicted in the schematic of Fig. 1 along with the deprotonation of ligands. Moreover, (Fig S1) shows a two-dimensional perspective of Co-MOF creation and an ordered interconnected network.

The X-ray diffraction (XRD) analysis revealed the presence of Co-MOF, as evidenced by diffraction peaks situated at different 2ϴ values, as shown in Fig. 3a. The sharp peaks indicate the formation of a highly crystalline metal-organic framework. The XRD pattern of the prepared material matches well with the existing single crystal data of (C20H16CoN2O4) (CCDC 4501569), indicating the presence of Co, C, H, and O in the Co-MOF. The crystallographic data suggests that the prepared crystal of MOF is associated with a triclinic system with a space group of 2: P-1 with two formula units in one unit cell. This demonstrates the successful electrodeposition synthesis of Co-MOF. Furthermore, the elemental composition of the prepared Co-MOF was analyzed using energy dispersive spectroscopy (EDS), as depicted in Fig S2. The EDS data showed that the prepared Co-MOF contains 40.8 wt % of cobalt, 17.4 wt % of carbon, and 8.9 wt % of oxygen. This was further confirmed by Fourier transform infrared spectroscopy (FTIR), as shown in Fig. 3b. The FTIR spectrum shows additional functional groups resulting from the bonding of metal ions with 1,2 benzene dicarboxylic acid. The spectrum also shows stretching vibration of OH-, COO-, and a para-atomic group of CH stretching, providing further evidence of the successful synthesis of Co-MOF14.

Fig. 3: Structural and morphological characterization of prepared MOF.
figure 3

a XRD analysis of Co –MOF material with very well-matched peaks. b FTIR measurement of prepared Co-MOF material, (c–e) SEM analysis of directly grown Co-MOF/NF.

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The Co-MOF material’s FTIR spectrum exhibits several peaks that are associated with the stretching vibrations of various functional groups. Around 3500–3000 cm–1, a single sharp peak is seen, and this peak corresponds to the stretching vibrations of the (–OH) groups found in the phthalate ions15. Another peak, corresponding to the stretching vibration of (C = C) groups brought on by the interaction of phthalate ions with metal cations, is seen at 1687 cm–1 and 1400 cm–1. In Co-MOFs, the metal-carboxylate bonds also contribute to the FTIR spectra. The bands in the range of 1600–1400 cm–1 correspond to the asymmetric and symmetric stretching vibrations of the metal-carboxylate bonds, while the bands in the range of 1300–1000 cm–1 are attributed to the bending vibrations of these bonds. The presence of these bands in the FTIR spectra confirms the coordination of the metal ions with the carboxylic acid ligands. These functional groups are crucial for maintaining the material’s electrical conductivity while enhancing the selectivity of metal ions. The presence of these functional groups in the Co-MOF material can have significant implications for its performance as a charge storage material. Firstly, the (C-OH) groups present in the phthalate ions can act as proton donors and acceptors, which can enhance the charge transfer process between the electrode and the electrolyte. This can lead to a higher capacitance, as the material can store more charge. Secondly, the (C = O) groups resulting from the interaction of phthalate ions with metal cations can also contribute to the charge storage capacity of the material. These functional groups can participate in redox reactions, where electrons are transferred between the electrode and the electrolyte, leading to an increase in the amount of charge stored. Furthermore, the selectivity of metal ions provided by these functional groups can help to improve the stability of the material during charge-discharge cycles. This is because unwanted metal ions that can cause degradation of the material can be selectively removed, while the desired metal ions that contribute to charge storage can be retained.

Cobalt-metal organic frameworks (Co-MOFs) typically contain cobalt ions coordinated with carboxylic acid ligands and organic linkers. The FTIR spectra of Co-MOFs can provide insights into the different types of chemical bonds present in the material. Some possible FTIR bonds in Co-MOFs are:

  1. 1.

    Cobalt-carboxylate bond: The stretching vibrations of the cobalt-carboxylate bond can be observed in the FTIR spectra in the range of 500–700 cm–1. The asymmetric and symmetric stretching vibrations of this bond are typically observed as two separate peaks.

  2. 2.

    Organic linker vibrations: The FTIR spectra of Co-MOFs typically show bands in the range of 1000–1100 cm–1, which correspond to the stretching vibrations of C-O bonds in the organic linker.

  3. 3.

    Cobalt-oxide bond: The stretching vibrations of the cobalt-oxide bond can be observed in the FTIR spectra in the range of 400–700 cm–1. This bond is typically weaker than the cobalt-carboxylate bond and may not always be observed in the spectra.

  4. 4.

    Carboxylic acid vibrations: The carboxyl group (C = O) in the carboxylic acid ligands can be observed in the FTIR spectra in the range of 2700–2800 cm–1. Further, the stretching vibrations C ≡ N-stretch can also be observed in the range of 2340 cm–1.

  5. 5.

    Hydrogen bonding: In some Co-MOFs, the organic linkers may form hydroxyl groups. This can be observed as broad bands in the FTIR spectra in the range of 3500–3000 cm–1.

The SEM images of Co-MOF shown in Fig. 3c–e the images show a large number of agglomerated structures covered over Ni foam in the range of several micrometers. This means that the material is made up of clusters of smaller particles that have fused. The agglomerated structure can provide a high surface area for electrochemical reactions, making the MOF-based electrode a promising material for improved charge storage. The nanostructure gives a chance of high capacitance and stability. Moreover, the interaction of water molecules with the layers of the metal-organic framework gives a highly connected supermolecular structure16. This interaction of water molecules with interspaces favors high diffusion and storage through electrolyte solutions. The colloidal structure provides a highly connected framework that supports the pseudocapacitive reaction with controlled diffusion.

Electrochemical characterization

The electrochemical properties of the Co-MOF/NF electrode were investigated using a three-electrode cell configuration. The Co-MOF grown directly on the Ni foam served as the working electrode, while a platinum plate and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. A 1 M KOH electrolyte was employed for all electrochemical measurements. Cyclic voltammetry (CV) analysis was performed over a potential range of 0.0 to 0.5 V at different scan rates (10–100 mV/s) to study the electrochemical behavior of the Co-MOF/NF electrode. The CV measurement (Fig. 4a) displayed a pair of redox peaks at 0.25 V and 0.4 V, indicating the presence of well-defined redox active sites within the electrode material. These redox peaks are consistent with previous studies on Co(OH)2-based electrodes for supercapacitors17,18. The observed redox reactions can be attributed to the following processes14,15:

$${{{\rm{Co}}}}({{{\rm{II}}}})+{{{{\rm{OH}}}}}^{-}\leftrightarrow {{{\rm{Co}}}}({{{\rm{II}}}}){{{\rm{OH}}}}+{{{{\rm{e}}}}}^{-}$$
(8)
$${{{\rm{Co}}}}({{{\rm{II}}}}){{{{\rm{OH}}}}}^{-}+{{{{\rm{e}}}}}^{-}\leftrightarrow {{{\rm{Co}}}}({{{\rm{III}}}})({{{\rm{OH}}}})+{{{{\rm{e}}}}}^{-}$$
(9)
Fig. 4: Electrochemical performance analysis of Co-MOF/NF electrode in 1 M KOH electrolyte.
figure 4

a Cyclic voltammetry (CV) analysis of Co-MOF/NF in a three-electrode cell measurement with 1 M KOH electrolyte at different scan rates. b Galvanostatic charge-discharge measurement of the prepared Co-MOF/NF electrode in 1 M KOH electrolyte at different current densities, demonstrating its charge storage and release capabilities under various operating conditions. c Charge storage kinetics analysis by a power law, revealing the value of b and characterizing the charge storage behavior of the Co-MOF electrode material. d Percentage contribution of diffusion and capacitive charge storage kinetics in the Co-MOF electrode. e Cyclic stability plot of the Co-MOF electrode in 1 M KOH electrolyte, showcasing its long-term cyclic performance and stability.

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These redox reactions indicate the occurrence of pseudocapacitance, where charge storage is facilitated through reversible redox processes. The galvanostatic charge-discharge (GCD) curve (Fig. 4b) exhibited a slight deviation from linearity with a distinct plateau, suggesting the presence of reversible pseudocapacitance in the Co-MOF material. The deviation from linearity is characteristic of pure pseudocapacitive behavior, which arises from the interaction between ions at the electrode-electrolyte interfaces during redox reactions. The GCD results were in good agreement with the CV curve. The specific capacitance of the Co-MOF/NF electrode was calculated from the charge-discharge measurements at different current densities (4–20 A/g). The electrode demonstrated a maximum specific capacitance of 1618.56 F/g at a current density of 4 A/g. Additionally, the Co-MOF/NF electrode exhibited a high energy density of 56.2 Wh/kg at a power density of 1000 W/kg, which was retained at 28.94 Wh/kg at a higher power density of 4999.12 W/kg. These values are comparable to those reported for other metal-organic framework-based electrodes materials19,20. Detailed electrochemical performance data are provided in Supplementary Table T1, while a comparison of the electrochemical properties with literature reports is presented in Table 1. The table highlights key parameters such as specific capacitance, voltage window, energy density, power density, and cyclic stability. This comparison provides valuable insights into the performance of the Co-MOF electrode material and its potential for use in high-performance energy storage devices.

Table 1 It presents a comprehensive comparison of the electrochemical performances of the prepared Co-MOF electrode with those reported in the literature
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The electrochemical deposition of MOF films using the electrodeposition method offers advantages over solvothermal and hydrothermal synthesis methods, as it provides better control over film thickness and functionality. The Co-MOF/NF electrode material demonstrated excellent electrochemical performance, comparable to Co (OH)2 and other synthesized cobalt-based MOFs with controlled thickness and tuned functionality 21,22. Furthermore, the cyclic stability of the Co-MOF/NF electrode was evaluated through 5000 charge-discharge cycles at a high current density of 6 A/g within the potential range of 0.0 to 0.5 V. The electrode exhibited remarkable cyclic stability, with 97% capacitance retention (Fig. 4e). The before and after cyclic performance CV plots are shown in Fig S4, indicating the electrode’s ability to maintain capacitance after cyclic testing. The Co-MOF material demonstrated uniform response even at high current densities, indicating reversible pseudocapacitive behavior and excellent capacitance, energy density, power density, and cyclic stability.

The analysis of the total capacitance contribution of the Co-MOF/NF electrode material involves assessing the capacitive and diffusion-controlled contributions. These contributions are related to the interaction of ions at the outer and inner surfaces of the material. Capacitive contribution is particularly important as it determines the overall charge stored by the material through mechanisms such as electrochemical double-layer formation and pseudocapacitance. The charge storage mechanism of pseudocapacitive materials allows for ion interaction through redox reactions and intercalation/deintercalation processes at the electrode-electrolyte interfaces. To understand the electrochemical kinetics of the charge storage mechanism, the current-voltage relationship is analyzed. The effect of capacitive contribution for the Co-MOF/NF electrode material is determined by studying the relationship between the peak current (ip) and the square root of the scan rate (v) according to the equation16:

$${{{{\rm{i}}}}}_{{{{\rm{p}}}}}={{{{\rm{av}}}}}^{{{{\rm{b}}}}}$$
(10)

Taking the logarithm of both sides, the equation becomes:

$${{{\rm{Log}}}}{{{{\rm{I}}}}}_{{{{\rm{p}}}}}={{{\rm{b}}}}\; {{{\rm{Log}}}}\; {{{\rm{V}}}}+{{{\rm{Log}}}}\; {{{\rm{a}}}}$$
(11)

In this equation, a and b are adjustable parameters, and v is the scan rate at a constant applied voltage. A b value of 0.5 indicates diffusion-controlled contribution, while a b value of 1 suggests the capacitive contribution of the material. In the case of the prepared electrode, the obtained b value was 1.02 (Fig. 4c), indicating the capacitive contribution of the electrode material.

Furthermore, Dunn’s method is employed to determine the percentage of capacitive and diffusion-controlled contributions at different scan rates. The total contribution is determined by the equation23:

$${{{\rm{i}}}}({{{\rm{v}}}})={{{{\rm{k}}}}}_{1}{{{\rm{v}}}}+{{{{\rm{k}}}}}_{2}\,{{{{\rm{v}}}}}^{1/2}$$
(12)

Dividing both sides of the equation by v1/2, we have:

$${{{\rm{i}}}}({{{\rm{v}}}})/{{{{\rm{v}}}}}^{1/2}={{{\rm{k}}}}_{1}{{{{\rm{v}}}}}^{1/2}+{{{{\rm{k}}}}}_{2}$$
(13)

Here, k1 and k2 are constants that can be determined by analyzing the linearity of i(v)/v1/2 versus v1/2. The gradient and intercept values provide the capacitive and diffusion-controlled contributions, respectively. For the lowest scan rate of 10 mV/s, the Co-MOF material exhibits 53% diffusion-controlled and 47% capacitive contribution (Fig S3). As the scan rate increases from 10 to 100 mV/s, the capacitive contribution of the Co-MOF/NF electrode material dominates, increasing from 47% to 82% (Fig. 4d). This evidence indicates that the capacitive behavior of the electrode material is more prominent in high-power applications.

To investigate the resistive dynamics involved in the electrochemical process, electrochemical impedance spectroscopy (EIS) was conducted on the Co-MOF/NF electrode material in a 1 M KOH electrolyte. The EIS measurement was performed over a frequency range of 0.01 Hz to 10000 Hz. The directly grown MOF-based electrode material exhibited low equivalent series resistance (ESR) and charge transfer resistance (Rct), indicating its high conductivity due to the three-dimensional porous framework. In the Nyquist plot, the higher frequency region showed a dominant capacitive behavior of the electrode material, as evidenced by the line being nearly parallel to the y-axis (Fig S5). The Bode phase and modulus plots of the Co-MOF-based electrode material (Fig. 5a) revealed a phase angle greater than 45 degrees, indicating its high capacitive behavior at fast current rates. The phase angle of the Co-MOF material was measured to be 80 degrees, making it suitable for low leakage current applications. Additionally, the material exhibited a shorter semicircle at higher frequency regions, indicating reduced charge transfer resistance, high electrical conductivity, and decreased resistance. The total system can be identified as a supercapacitor, and by analogy with the imaginary capacitance, it can be expressed as Z = R(ω) + [1/(jC(ω)ω)]. The complex impedance Z(ω) can be written as Z(ω) = Z′(ω) + jZ″(ω), and the real and imaginary capacitance C(ω) can be obtained as follows24:

$${{{\rm{C}}}}^{\prime} ({{\omega }})=(-{{{\rm{Z}}}}^{\prime \prime} ({{\omega }}))/({{\omega }}|{{{\rm{Z}}}}\left({{\omega }}\right)|^2)$$
(14)
$${{{\rm{C}}}}^{\prime \prime} ({{\omega }})=({{{\rm{Z}}}}^{\prime} ({{\omega }}))/({{\omega }}|{{{\rm{Z}}}}\left({{\omega }}\right)|^2)$$
(15)
Fig. 5: Electrochemical impedance spectroscopy analysis of Co-MOF electrode.
figure 5

a Bode plot of the prepared Co-MOF electrode in 1 M KOH electrolyte, illustrating the frequency response of the electrode material and providing valuable information about its electrical behavior. b Bode phase angle plot at different frequency ranges of the Co-MOF electrode material and demonstrating its high capacitive behavior at fast current rates. c The plot of real and imaginary capacitance against different frequency ranges for the Co-MOF electrode, (d) percentage normalized active power P/S and reactive power Q/S against the frequency of the Co-MOF electrode.

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In Fig. 5b, the low-frequency value of C′(ω) represents the capacitance of the cell measured at constant-current discharge, and C′(ω) decreases as the frequency increases, which is characteristic of the electrode material at the electrode-electrolyte interface. C″(ω) represents the imaginary part of the capacitance C′(ω) and contributes to energy dissipation through an irreversible reaction, resulting in hysteresis. The relaxation time constant was determined by analyzing Fig. 5c, d, which exhibited a Bode phase angle greater than 45 degrees, indicating a good frequency response for an ideal capacitor. From the C′(ω) plot, the relaxation time was calculated as half of the full capacitance (1/τ). The relaxation time constant for the directly grown Co-MOF material was found to be 3.44 seconds, indicating a capacitive electrode material suitable for high-power applications with shorter durations. This fast relaxation time corresponds to rapid energy storage and delivery performance of the electrode material.

The impedance behavior of the supercapacitor varies from pure resistive behavior at high frequencies to pure capacitive behavior at low frequencies (Fig. 5c). The complex normalized power imaginary part |Q | / | S| and real part |P | / | S| versus frequency plots provide complementary presentations of the impedance response of the supercapacitor electrode. The plots reveal the frequency-dependent behavior of the electrode material, ranging from pure resistance at high frequencies to pure capacitance at low frequencies. At high frequencies, all the power is dissipated (P = 100%), and the supercapacitor behaves like a pure resistor (R), while in a pure capacitance, the real part of the complex power P dissipated is zero (phase angle = 90°)25. The normalized imaginary part of the power |Q | / | S| increases as the frequency decreases. The maximum value of |Q | / | S| is reached at low frequencies when the supercapacitor behaves like a pure capacitor. The crossing of the two plots occurs at |P | = |Q| which corresponds to the time constant defining the partition between pure resistive and capacitive behavior of the electrode material. The relaxation time is calculated from the frequency of the crossing of the two plots, indicating that very little relaxation time is required to discharge all the energy with over 50% efficiency. The electrodeposited Co-MOF-based electrode material exhibited a very low relaxation time, indicating its capability for rapid charge storage and delivery.

An asymmetric supercapacitor design based on Co- MOF

In the practical application of directly grown Co-MOF, an asymmetric supercapacitor was designed using activated carbon cloth (ACC) as the negative electrode material and directly grown Co-MOF as the positive electrode material (Fig. 6a). To achieve mass balancing in the asymmetric device, the mass ratio of electroactive species in both electrodes was determined based on the charge balancing theory, where the charge stored by the positive electrode must be equal to that of the negative electrode. The mass balancing equation for asymmetric devices is given by 26:

$$\frac{{M}_{{Co}-{MOF}}}{{M}_{{AC}}}=\frac{{C}_{-}{\Delta V}_{-}}{{C}_{+}{\Delta V}_{+}}$$
(16)
Fig. 6: Performance analysis of Co-MOF/NF/ACC asymmetric supercapacitor.
figure 6

a Schematic representation of Co-MOF/NF/ACC asymmetric cell, highlighting the arrangement of components and their respective roles in the supercapacitor system. b Cyclic voltammetry analysis of Co-MOF/NF/ACC in 1 M KOH electrolyte, revealing the potential window of operation from 0.0 to 1.2 V. c Galvanostatic charge-discharge process of Co-MOF/NF/ACC in 1 M KOH electrolyte at different current densities. d Comparative Ragone plot of energy density vs. power density of Co-MOF/NF/ACC with previously reported literature37,38,39,40,41,42,43,44. e Cyclic stability measurement through the Galvanostatic charge-discharge process of Co-MOF/NF/ACC. f LED indicator of the extended Co-MOF/NF/ACC device, illustrating its practical application by powering a cluster of yellow LEDs.

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Here, M Co-MOF and MACC represent the mass loading of the positive and negative electrode materials, respectively. The specific capacitance (Fg–1), voltage window (V), and mass of the electrodes (m) are parameters that need to have equal responses for mass balancing.

The cyclic voltammetry (CV) analysis of the asymmetric device showed ideal behavior with redox peaks, and a high potential window of 0 to 1.2 V was obtained (Fig. 6b). The CV curve exhibited similar characteristics at different scan rates, indicating a good response of the two-electrode cell. The CV curve at 50 mV/s is shown in Fig S6. The curve demonstrated a synergistic response of electric double-layer capacitance (EDLC) and pseudocapacitive behavior.

Furthermore, galvanostatic charge-discharge measurements were conducted at different current densities to evaluate the specific capacitance and rate performance of the two-electrode cell (Fig. 6c). The Co-MOF/NF/ACC device exhibited a high specific capacitance of 315.33 Fg–1 at 0.2 Ag–1 and a high energy density of 63.06 Whkg–1 at 479.94 Wkg–1 power density. These results were compared with other literature reports in Table 2, demonstrating the favorable electrochemical performance of the Co-MOF/NF/ACC device. Table 2 presents the set parameters used for analyzing the characteristics of asymmetric supercapacitor cells. These parameters are crucial in evaluating and understanding the performance of the cells. The table includes parameters such as cell component, cell voltage, cell capacitance (Fg–1), specific energy and power, rate test, internal resistance, and capacitance retention. Additional electrochemical performance data of the two-electrode cell can be found in Supplementary Table T2.

Table 2 Set parameters for analyzing characteristics of asymmetric supercapacitor cells
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The electrochemical impedance spectroscopy (EIS) analysis of Co-MOF/NF/ACC, represented by the Nyquist plot (Fig S7), exhibited well-fitted behavior with low resistances of 1.36 Ω and a charge transfer resistance of 5.42 Ω. The asymmetric device demonstrated a high energy density, consistent with previous reports on MOF-based asymmetric devices. A comparative Ragone plot of energy density versus power density (Fig. 6d) showed the performance of the prepared Co-MOF//ACC device in other literature reports.

To assess the cyclic stability, the asymmetric device was tested over 50,000 cycles using a LABstation battery testing system. The Co-MOF//ACC configuration exhibited a high capacitance retention of 99.6% (Fig. 6e). To demonstrate the charge storage performance, three identical ASCs were connected in series and connected to an LED frame, providing a voltage of 3 V. Remarkably, the device was able to power 37 yellow LEDs in the EMDL (Energy Material and Devices Laboratory) (Fig. 6f). The device charged quickly at 3 V, highlighting the potential applicability of the assembled ASC device and the efficiency of directly grown Co-MOF as an electrode material in terms of its cell performance.

Discussion

In summary, the present work showcases a remarkable advancement in the field of supercapacitor technology through the successful fabrication and characterization of a Co-MOF binder-free electrode. This elegant and innovative electrode design realized on a three-dimensional conducting Ni foam substrate, exhibits an exquisite morphology and surface architecture. By harnessing the synergistic effects of metal ions and organic ligands within the Co-MOF structure, the electrode surpasses conventional counterparts in terms of electrolytic diffusion and charge transfer at the critical electrode-electrolyte interfaces. This asymmetric configuration exhibits a notable energy density of 63.06 Whkg–1 at a power density of 479.94 Wkg–1, surpassing previously reported values for MOF-based asymmetric devices. The remarkable cyclic stability of the asymmetric cell, demonstrated by a capacitance retention of 99.6% over 50,000 cycles, underscores the potential of the Co-MOF/NF electrode in enabling long-lasting and high-performance energy storage systems. The findings of this study not only contribute to the fundamental understanding of electrode materials and their electrochemical behavior but also pave the way for the development of advanced supercapacitors with superior energy storage capabilities. The exceptional performance, coupled with the elegant design and innovative synthesis strategy employed in this work, positions the Co-MOF binder-free electrode as a promising candidate for a wide range of applications, including portable electronics, electric vehicles, and renewable energy storage. Overall, this study represents a significant leap forward in the quest for high-performance energy storage devices, bringing us closer to the realization of efficient and sustainable energy solutions. The innovative design, impressive electrochemical performance, and outstanding cyclic stability showcased by the Co-MOF binder-free electrode highlight its transformative potential in advancing the field of supercapacitor technology and shaping the future of energy storage.

Methods

Materials

Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), BDC (1,2-benzene dicarboxylic acid), DMF (N, N-dimethylformamide), DDH2O. All chemicals were purchased from Merck Pvt. Ltd. and used without further purification.

Electrodeposition of Co-MOF electrode

The Co-MOF/NF electrode was prepared using an electrodeposition technique, which allows for the controlled growth of metal-organic framework (MOF) architectures on a conductive substrate. In this study, a nickel foam (NF) substrate was selected as it offers a high surface area and good electrical conductivity, making it suitable for efficient charge storage applications. To ensure the cleanliness and quality of the NF substrate, a combination of mechanical and chemical cleaning methods was employed. The mechanical cleaning process involved rinsing the NF substrate with deionized water and gently scrubbing it with a soft-bristled brush to remove any loose debris or particles. This step is crucial to eliminate any potential contaminants that may interfere with the electrodeposition process and the subsequent electrochemical performance.

Following the mechanical cleaning, the NF substrate underwent a chemical cleaning step. It was ultrasonicated for 10 min in a solution of 0.1 M hydrochloric acid (HCl) to remove any remaining organic or inorganic impurities. The acid solution effectively dissolves and removes contaminants and NiO layer from the substrate’s surface. After the acid treatment, the substrate was thoroughly rinsed with deionized water to ensure the removal of any residual acid or dissolved impurities. Finally, the cleaned substrate was dried under vacuum conditions at 80°C to achieve a pristine surface.

The Co-MOF/NF electrode was fabricated by immersing the cleaned NF substrate into a precursor solution. The precursor solution was prepared by dissolving Co (NO3)2.6H2O (15 mM) and (BDC) 1,2 benzene dicarboxylic acid (10 mM) in a mixture of DMF and DDH2O (1:1). The solution was carefully stirred for 10 minutes to ensure complete dissolution and homogeneity. Under controlled experimental conditions, the NF substrate was subjected to electrodeposition. The electrodeposition process was carried out in a three-electrode system, with the NF substrate serving as the working electrode, a platinum foil as the counter electrode, and an Ag/AgCl electrode as the reference electrode. To maintain consistent deposition conditions, a controlled potential of –1.57 V vs Ag/AgCl was applied during the electrodeposition process for 300 s27. This potential ensures the desired electrochemical reactions and the controlled growth of the MOF architecture on the NF substrate. By utilizing this electrodeposition technique and following the proper cleaning protocols, the Co-MOF/NF electrode was successfully synthesized, offering a promising platform for efficient charge storage applications. The electrode’s reproducibility and electrochemical performance were enhanced by ensuring the cleanliness and quality of the NF substrate, as residual contaminants or impurities can adversely affect the morphology, structure, and electrochemical properties of the deposited material.

Two-electrode cell assembly for charge storage evaluation

The charge storage performance of the Co-MOF material was evaluated using a two-electrode testing configuration. The assembly involved the utilization of a Co-MOF/NF electrode as the positive electrode material and an Activated Carbon Cloth (ACC) as the negative electrode material. Both electrodes were attached to a nickel foam current collector to facilitate electrical conductivity and mechanical stability. To ensure proper separation and prevent short circuits, a Celgard 3501 separator was inserted between the two electrodes. The preparation of the Co-MOF/NF electrode involved the direct growth of Co-MOF on the nickel foam substrate. The Co-MOF was synthesized via the electrodeposition method, as described earlier. Subsequently, the resulting Co-MOF material was pressed onto the nickel foam current collector at a pressure of 10 MPa for a duration of 10 s. Care was taken to avoid exceeding this duration to maintain the integrity and performance of the Co-MOF/NF electrode. For the negative electrode, the ACC electrode was cut into dimensions of 0.5 × 0.5 cm without the inclusion of any additives such as binders or conducting carbon. This choice aimed to assess the charge storage performance of the Co-MOF material in a practical energy storage system. The two-electrode cell was assembled by combining the Co-MOF/NF electrode as the positive electrode and the ACC electrode as the negative electrode. The Celgard 3501 separator was positioned between the two electrodes to prevent direct contact and minimize the possibility of short circuits. This two-electrode cell configuration allowed for the evaluation of the charge storage performance of the Co-MOF material in a practical energy storage system, highlighting its suitability and effectiveness in real-world applications.

Materials characterization

Materials characterization techniques are used for analyzing and evaluating the properties and characteristics of a material to understand its structure and behavior. In the case of Co-MOF grown over Ni foam, several techniques were employed to investigate its surface morphology, elemental composition, crystal structure, functional groups, and electrochemical behavior. The surface morphology of the Co-MOF grown over Ni foam was examined using SEM (scanning electron microscopy) at JEOL JSM 7617 F SEM equipped with energy-dispersive spectroscopy (EDS) from Oxford Instruments. EDS mapping was also carried out at low magnification over a coated area of the substrate to determine the detailed elemental composition present in the prepared Co-MOF/NF. To determine the crystal structure of the Co-MOF, XRD (X-ray diffraction) measurements were done using the RIGAKU Miniflex 300/600 instrument. FTIR (Fourier transform infrared) analysis was carried out at room temperature using the Bruker instrument to identify the functional groups present in the Co-MOF. All electrochemical characterization was done using the Metrohm 128 N AUTOLAB Potentiostat from the Netherlands in 1 M KOH electrolyte. The electrochemical measurements provide information about the material’s electrochemical behavior, including its redox reactions and charge storage capacity. The cyclic testing of the asymmetric device was done in the LABstation Battery testing system. Cyclic testing involves repeated charging and discharging of the device to evaluate its long-term performance and stability.