Unleashing and harnessing capacity performance by a diffusion dominant process of CoNi-ZIF for high energy density asymmetric supercapacitors

Abstract

We report here the synergy of tailored cobalt nickel-based zeolitic imidazole frameworks as cathode and activated carbon (AC) as anode materials, which tends to bridge the void between energy density and power density of asymmetric supercapacitors (ASCs). CoNi-ZIF exhibits a mesoporous structure along with a specific surface area of 1126 m² g⁻¹ and a pore size of 3.128 nm. The specific capacity and capacitance of CoNi-ZIF and AC electrodes are found to be 49.06 mAh g⁻¹ and 333 F g⁻¹, respectively. The CoNi-ZIF structure exhibits a mesoporous structure, which helps explore the underlying capacitance due to the presence of bulk and surface chemical states of the ZIF structure. XPS analysis reveals that the deconvoluted peaks of nitrogen and its associated organic moieties contribute to the capacitance by increasing the hydrophilicity of the framework, which causes rapid ion diffusion. A systematic electrochemical study has been performed using cyclic voltammetry (CV) to analyze electrode kinetics during the charge storage-discharge process, and Dunn's model was employed, which validates the diffusion dominance of CoNi-ZIF. This systematic approach unveils framework kinetics and diffusion dynamics of ZIF-based electrodes, which enhances the ASC device performance. The diffusion-dominant mechanism of CoNi-ZIF electrodes at low scan rates is about 85% through a pseudocapacitive process, and a double-layer capacitive percentage of about 15% presents a device capacitance of 54.9 mAh g⁻¹ and yields an energy density of 17.04 Wh kg⁻¹ at a power density of 1241.53 W kg⁻¹. The ASC device showcases a good capacity retention of about 95.2% after prolonged 10 000 cycles.

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Introduction

The demand for energy storage devices is increasing to meet the world's energy consumption and to reduce the dependence on fossil fuels.¹ Amongst electrical energy storage (EES) devices, batteries and supercapacitors are the most prevalent. Supercapacitors (SCs) deliver better power output, prolonged cycle life and moderate energy densities.^2–4 Based on their charge storage mechanisms, SCs are divided into three types: electric double layer capacitors (EDLCs), pseudo-capacitors (PSCs) and hybrid capacitors (HCs).⁵ Many materials like carbon-based EDLCs and also high energy density PSCs, such as metal oxides,⁶ sulfides,⁷ conducting polymers,⁸ transition metal carbides,⁹etc., have been utilized for energy storage devices. Even the PSCs' materials' energy density is not sufficient, and to further enhance the energy density, high porosity, large potential window, and high cyclability materials have been examined. In this context, metal organic frameworks (MOFs) are considered promising materials for PSCs. MOFs are becoming popular due to their properties, such as a highly crystalline framework with open pores with good thermal and chemical stability. Also, MOFs can act as defined precursors for preparation of porous carbon materials.^4,10–12 Zeolite imidazole frameworks (ZIFs) are one of the sub-classes of MOFs, which are similar to zeolites and possess a rigid cage-like structure.^13,14 ZIFs can be easily synthesized by altering the metal salt and its concentration to tune their energy storage properties.^1,11,15 It has been confirmed that nitrogen atoms can enhance the pseudocapacitance and wettability of MOF-derived carbon, which implies that nitrogen precursors can augment the specific capacitance of SCs by increasing the electrode–electrolyte interaction.^16–18

Different concentrations of nitrogen- and oxygen-containing groups, such as hydroxyl and epoxy groups, in graphene have been investigated, resulting in a significant increase in the quantum capacitance of graphene. The presence of nitrogen and its associated properties, such as enhanced wettability, promotes ion transport and faradaic reactions, leading to an increased charge–discharge behavior and a substantial increase in the diffusion percentage contribution, thereby confirming the PSC nature of ZIFs.¹⁹ Notably, Brezesinski et al. recently studied α-MoO₃ thin films for SC application and reported that the capacitive-controlled mechanism is dominant over diffusion due to the mesoporous structure of α-MoO_3, which is calculated by using the Dunn model and power law relationship. Dunn et al. reported a study on the charge storage mechanism and quantification of diffusive and capacitive-controlled percentages.²⁰ Soon after, Dylla et al. worked on nanostructured TiO₂ by inserting Li ions, and the results implied that a capacitive-controlled mechanism is dominant.²¹ The charge storage mechanism is studied and reported extensively based on transition metal oxides, sulfides, derived carbon, doped carbon and pristine MOFs. These studies are unable to explore the underlying capacitances, which are supported by nitrogen groups in the organic linker, which we studied extensively based on the nitrogen linker, and the wettability of the electrode surface and established the importance of nitrogen-based ZIFs in SC application.

Herein, we employed nitrogen-rich ligand 2-methyl-imidazole to enhance the unexplored capacitances of the ZIF and augment the SC performance in asymmetrical device geometry. Bimetallic CoNi-ZIF was synthesised by a one-step hydrothermal process and was electrochemically redox active and displayed a good surface area and structural and electrochemical stability compared to pristine ZIFs. CoNi-ZIF exhibited a mesoporous structure with a surface area of 1126 m² g⁻¹ and a pore volume of 3.128 nm. An asymmetric device with CoNi-ZIF as the cathode and AC as the anode material was fabricated, and the results show a capacity of about 54.9 mAh g⁻¹ with an energy density of 17.04 Wh kg⁻¹ at a power density of 1241.53 W kg⁻¹.

The combination of Co and Ni, which forms the ZIF, stabilizes the framework and helps increase the diffusion-oriented capacitance by enhancing the mobility of K⁺ and OH⁻ ions during the electrochemical studies. The charge-storage mechanism of the framework is dominated by a diffusion-controlled process. The presence of the heteroatom nitrogen-rich ligand increases the hydrophilicity, and hence better wetting of the electrolyte on the surface increases the overall capacitance of the device, leading to better stability and enhanced performance of SCs.

Materials and methods

Cobalt nitrate hexahydrate (Co (NO₃)₂·6H₂O), nickel nitrate hexahydrate (Ni (NO₃)₂·6H₂O), 2-methyl-imidazole, commercial activated carbon (AC), N, N-dimethylformamide (DMF), methanol, Nafion, potassium hydroxide (KOH) pellets and nickel foam, as a substrate for the working electrode, were used and nickel foam was cleaned with de-ionized (DI) water and ethanol.

Synthesis of electrode materials

Synthesis of Co-ZIF. 99.6 mM of cobalt (II) nitrate hexahydrate as a metal precursor was dissolved in 50 mL DMF, and 1.5 M of 2-methyl-imidazole was dissolved in 50 mL DMF in separate beakers. Both solutions were mixed and stirred for about 30 min to obtain a homogeneous mixture. The resulting solution was transferred to a Teflon-lined autoclave and heated at 140 °C for 12 h. After cooling the autoclave at room temperature, the resulting product was collected by centrifugation, thoroughly washed several times with de-ionized water, and dried under vacuum at 60 °C overnight.

Synthesis of Ni-ZIF. Ni-ZIF was synthesized following the same procedure as Co-ZIF, except that nickel(II) nitrate hexahydrate was used as the precursor under identical conditions.

Synthesis of Co–Ni-ZIF. Co–Ni-ZIF was synthesized following the same procedure as Co-ZIF, except that a mixed metal precursor consisting of cobalt(II) and nickel(II) salts was used under identical conditions.

Material characterization

X-ray diffraction (XRD) analysis was carried out to determine the crystalline structure and phase composition of the synthesized ZIFs within the 2θ range of 5°–45° using a Bruker D8 Advance, Panalytical X Pert3. The surface area and pore structure were analyzed using nitrogen adsorption–desorption isotherms using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods on a BELLSORP MAX instrument. The morphology of the samples was examined using field-emission scanning electron microscopy (FE-SEM, Thermo Fisher Scientific (FEI) Apreo 2S) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100 Plus). The elemental composition, oxidation states, and surface chemical environment of the synthesized MOFs were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha). Electrochemical measurements were performed using an Origalysis OrigaFlex-500 electrochemical workstation.

Electrochemical measurements

Capacitive behavior of the material can be assessed by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques. These techniques are helpful to analyze the material and its storage mechanism of charges during charging and discharging, which is the main principle behind the working of energy storage devices, and were carried out at the Origalysis electrochemical workstation. Through CV and GCD techniques, we calculated the specific capacitance, energy density, and power density of the cell by applying the following equations.

In a three-electrode system, the specific capacitance was calculated using eqn (1):

In a two-electrode system, specific capacitance was calculated using the below equation:

The only difference between eqn (1) and (2) lies in the consideration of the active material mass of the cathode and anode electrodes in the three-electrode and two-electrode configurations, respectively.

Due to the non-linear curves of CoNi-ZIF, it resembles a battery-type material with non-faradaic reactions, and capacitance was termed specific capacity. The specific capacity can be calculated using the formula given below:

where C_sp is specific capacitance in F g⁻¹, I is the applied current (A), Δt is the discharge time (s), m is the mass of the active material taken to coat the substrate (g), and Δ V is the potential window (V). By using a formula, we can calculate specific capacitance from GCD curves.where E is energy density in Wh kg⁻¹.P is power density in W kg⁻¹.

The asymmetric cell was assembled with CoNi-ZIF as a positive electrode and AC as a negative electrode with 3 M KOH solution as an electrolyte. Masses of both electrodes are estimated by balancing the charge with the following equation:

where m⁺ and m⁻ are the active masses of the anode and cathode.

Results and discussion

To investigate the crystal structure and phase purity of synthesized ZIFs, powder X-ray diffraction (XRD) analysis was performed, and the obtained diffraction patterns were compared with that of simulated ZIF-67 (CIF Number: 7247762).^22,23 The patterns of synthesized samples are shown in Fig. S1(a), and the majority of the peaks are matched with those of the reported ZIF-67.²⁴ Distinct diffraction peaks were observed at 2θ values of 7.35°, 10.46°, 12.78°, 14.85°, 16.49°, 18.10°, 22.20°, 24.56°, 26.74°, and 29.45° and their corresponding Miller indices (h k l) planes are (1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (2 2 0), (4 1 1), (3 3 2), (4, 3 1), and (4 4 0), respectively.^22,23 The sharp and well-defined peaks indicate the formation of highly crystalline ZIF structures.^25,26 The diffraction peaks of Co-ZIF, Ni-ZIF and CoNi-ZIF are shown in the SI in Fig. S1(b).

Fig. 1(a) illustrates the FE-SEM images of CoNi-ZIF, showing a cube-type morphology with a densely packed and hierarchical nature. Fig. 1(b–f) elucidates the energy-dispersive X-ray spectra (EDS) of the synthesized ZIFs, and the percentages are found to be 60.81%, 20.35%, 13.08%, 8.1%, and 5.66% corresponding to carbon, nitrogen, oxygen, cobalt and nickel, respectively.²⁷

Fig. 1 (a) FE-SEM images of CoNi-ZIF. (b–f) EDS of CoNi-ZIF.

XPS was carried out to analyze the chemical oxidation states and surface environment of CoNi-ZIF. Fig. 2(a) shows the survey spectrum of CoNi-ZIF, and Fig. 2(b–f) presents the high-resolution deconvoluted Co 2p, Ni 2p, C 1s, N 1s, and O 1s spectra, respectively. Fig. 2(b) depicts the cobalt 2p spectrum, which was deconvoluted into 2p_3/2 and 2p_1/2 and exhibits five characteristic peaks and their respective binding energies are 778.6, 780.5, 793.6, 795.5 eV, and 803.3 eV. The peaks at 778.6 and 793.6 eV correspond to the metallic nature of Co. The peak at 780.5 eV implies the CoO_x/CoC_xN_y, which is attributed to the Co²⁺species of 2p_3/2, which indicates that the cobalt in the sample predominantly exists in the Co²⁺ oxidized state.²⁸ The peak at 795.5 eV is difficult to distinguish from the cobalt oxidation due to partial oxide formation, and it corresponds to 2p_1/2 of cobalt, and it is accompanied by a satellite peak at 803.3 eV.^29,30Fig. 2(c) shows a high resolution Ni 2p spectrum, which was deconvoluted into two major peaks at 856 and 872.8 eV, which are associated with Ni–(OH)₂ and Ni–O, respectively. The peak at 869.3 eV is ascribed to the metallic nature of Ni.³¹ This coordination leads to a slight enhancement in conductivity and charge transfer properties of the material, which are related to 2p_3/2 and 2p_1/2 of Ni 2p.^32,33 The two major peaks of nickel are accompanied by satellite peaks at 860 and 883.3 eV.³⁴Fig. 2(d) depicts the fitted peaks of C 1s at 284.2, 285.2 and 287.4 eV, which are associated with the 2-methyl-imidazole linker.^27,35,36 Besides, the N 1s peaks at 398.8 and 400.1 eV depicted in Fig. 2(e) are related to pyridinic-N and pyrrolic-N groups, which furnish capacitance by enhancing the wettability of the electrode materials and increasing the kinetics, which accelerates the redox reaction and increases specific capacity.³⁷ The peak at 401.2 eV is due to the graphitic-N, which augments the framework conductivity.³⁸ The O 1s was deconvoluted into two peaks, as shown in Fig. 2(f) at 531 and 532.7 eV. The peak at 531 eV corresponds to surface hydroxyl groups (M–OH) and 532.7 eV corresponds to hydroxyl groups that are bonded to O atoms, respectively.³⁹

Fig. 2 (a) Survey spectrum of CoNi-ZIF. (b–f) High resolution Co 2p, Ni 2p, C 1s, N 1s, and O 1s spectra.

Electrode wettability is a crucial factor for energy storage applications, especially in SCs. XPS analysis indicated that the presence of coordinated polar heteroatom nitrogen in the organic linker enhances the wettability of the electrode. Wettability is generally evaluated by the contact angle measurements between the electrode materials and liquid, which provides insight into the hydrophilic and hydrophobic nature of the material. Hydrophilic materials exhibit improved electrolyte penetration, enhanced accessibility of active sites and increased ion transport toward the electrode surface, leading to an increase in the energy density of SCs.⁴⁰ Fig. S2(a and b) depicts the contact angle measurements for CoNi-ZIFs, with values of 37.737°,37.017°, 34.847° and 35.733°. These low contact angles provide evidence that the synthesized ZIFs have a hydrophilic nature, and this can be attributed to their surface roughness and surface energy.^41,42

Fig. 3(a) shows the TEM image that depicts the densely packed nature of ZIFs, and the study shows CoNi-ZIF crystallinity. Fig. 3(a) shows the amorphous layer of carbon, which is due to the decomposition of 2-methyl-imidazole during synthesis and the breaking down of imidazole linkers, which causes the carbon to form a wrap around the cobalt and nickel.^43,44Fig. 3(b and c) displays the interplanar distance between the lattice fringes, about 0.424 and 0.549 nm, corresponding to the planes of (310) and (411), respectively, which was calculated by using ImageJ software. Fig. 3(d) presents the selected area electron diffraction (SAED) patterns with bright and dark spots, which depict the crystallinity of CoNi-ZIF, and the above-mentioned interplanar distance of lattice fringes is in good agreement with hkl planes of simulated XRD patterns.

Fig. 3 (a) TEM image and (b and c) high-resolution TEM images of CoNi-ZIF. (d) SAED patterns of Co–Ni-ZIF with its respective hkl planes.

The surface area and pore size distribution of CoNi-ZIF were evaluated by the BET and BJH methods. The results are depicted in Fig. 4(a), which shows the nitrogen adsorption–desorption of CoNi-ZIF, which shows a type-Ⅰ isotherm. The surface area of prepared CoNi-ZIF was found to be around 1126 m² g⁻¹ with a hysteresis loop, and this indicates that CoNi-ZIF exhibits a mesoporous structure with a pore volume of 0.615 cm³ g⁻¹ and Fig. 4(b) shows an average pore diameter of 3.128 nm.^45,46 The narrow mesopores cause the impregnation of more ions and enhance the charge storage performance.^47,48 The porosity arises from the intrinsic structure and interparticle voids. CoNi-ZIF displays a decrease in porosity compared with pristine ZIF, as shown in the SI (Fig. S8), due to partial framework collapse due to insertion of bimetals.

Fig. 4 (a) N₂ adsorption–desorption isotherm of CoNi-ZIF. (b) Pore size distribution curve of CoNi-ZIF.

Electrochemical performance

Electrochemical investigation was carried out by using CV and GCD techniques, which establish the electrochemical behaviour of the electrode materials along with electrode–electrolyte kinetics. Fig. 5(a–c) presents the CV curves of Co-ZIF, Ni-ZIF and CoNi-ZIF electrodes in 3 M KOH electrolyte at different scan rates. Potential optimization was carried out between the potential window from 100 to 550 mV for all the synthesized ZIFs, as presented in the SI Fig. S3. The two redox couple peaks at formal potentials of cobalt and nickel indicate the pseudo-capacitive nature of the electrode materials, and at the varied scan rates from low to high, a minimal peak shift was observed. The minimal peak shift with the scan rate confirms that the ZIFs exhibit better electrochemical reversibility.⁴⁹ The inherent capacitive behavior, along with diffusive behavior in ZIFs, is due to the presence of carbon, nitrogen, cobalt and nickel in ZIFs, which enhances the electrode surface area, leading to more electrolyte uptake, resulting in the capacitive nature during the electrochemical process.⁵⁰ The GCD curves of Co-ZIF, Ni-ZIF and CoNi-ZIF at various current densities are shown in Fig. 5(d–f). The curves are in good agreement with CV curves of Co and Ni redox potentials.

Fig. 5 (a–c) CV curves at different scan rates and (d–f) GCD curves at different current densities of Co-ZIF, Ni-ZIF, and CoNi-ZIF.

The calculated capacitances for Co, Ni and CoNi-ZIF are tabulated in the SI (Table S1). Fig. S7 presents the specific capacity of the ASC device at different scan rates. CoNi-ZIF takes up more KOH electrolyte into the framework, showing better performance at lower current densities due to sufficient time for ions to intercalate into the framework, which has good porosity.

Capacitive and diffusion controlled contributions

It is crucial to understand the kinetics and charge-storage mechanism of the electrode material to evaluate the performance of SCs. The power law eqn (7) is employed to theoretically validate the mechanism involved using constants a and b.⁵¹ Furthermore, evaluation of the ‘b’ parameter by applying the log function on both sides of eqn (8) and plotting the obtained function of log current vs. log scan rate and linear fitting helps find the ‘b’ value using the slope. Fig. 6(a, d and g) depicts the calculated b values for all the Co, Ni, and CoNi- ZIFs, which are found to be 0.20, 0.35 and 0.23. If the ‘b’ value lies between 0 and 0.5 it represents the diffusion-controlled mechanism, and if the ‘b’ value lies between 0.5 and 1 it indicates the capacitive-controlled mechanism. The obtained values of b indicate that the dominating mechanism is a diffusion-controlled process. This implies that the faradaic redox reaction prevails over surface-controlled processes to store charges.⁵² The comparative analysis of all the CV profiles is shown in Fig. 6(a–i), which validates the charge-storage mechanism of ZIFs by distinguishing the diffusion and capacitive-controlled processes at different scan rates, and the relative dominance of each process can be quantitatively assessed by using a power-law relationship for the observed electrochemical behavior using eqn (7) and (8).

i = i_cap + i_diff = av^b (7)

log i = log a + b log(ν) (8)

where i denotes peak current density in mA g⁻¹, a and b are constants (for b the value should be between 0 and 1), and ν stands for the constant scan rate (mV s⁻¹).

Fig. 6 (a–i) Log(i) versus log(ν), and capacitive and diffusion-controlled processes at various scan rates of Co-ZIF, Ni-ZIF, and CoNi-ZIF.

To quantify the capacitive/surface-controlled and diffusive-controlled current components, we employed Dunn's eqn (9) and (10), which indicate the charge storage process in ZIF, and the results reveal that diffusion predominates, rather than EDLC.

i(ν) = k₁ν + k₂ν^1/2 (9)

log i(ν) = k₁ν^1/2 + k₂ (10)

where ν is the scan rate, k₁ν represents the surface/capacitive controlled contribution, and k₂ν represents the diffusion-controlled contribution and by rearranging the above equation, the capacitive and diffusive currents can be interpreted at each scan rate.

Fig. 6(a) depicts the log scan rate vs. log peak current; the diffusion percentage decreases linearly in all electrode materials from low to higher scan rates of 10 to 100 mV s⁻¹.^53,54 The hierarchical structure of CoNi-ZIF increases the charge transport and the electroactive sites of the framework, which causes a high diffusion rate and results in fast charge accumulation between the framework surface and OH⁻ ions. Fig. 6(a–i) exhibits a diffusion-dominant process, where it augments the overall current and enhances the charge storage mechanism of specific capacitance and energy density by rapid charging and discharging which increases the overall performance of the SCs. The two-electrode asymmetric electrochemical device was fabricated using CoNi-ZIF as the cathode and AC as the anode, with Whatman filter paper as a separator. The electrochemical studies were performed in a 3 M KOH electrolyte and analysed in a potential window of 0–1.5 V at different scan rates. The mass-charge balance was performed using eqn (6) between CoNi-ZIF and AC.^55–57

Two-electrode system

Fig. 7 depicts the mechanism of asymmetric device adsorption/desorption at the AC (anode) on the left side and redox reaction at the CoNi-ZIF (cathode) electrode, where the charges and electrons move inward and outward during the charge-storage mechanism through the porous separator. By analyzing the CV profiles of the device within a potential window of 1.5 V, as shown in Fig. 8(a), the operating potential window was optimized. CV measurements were initially performed over a range from 1.0 to 1.5 V (Fig. S4), and based on these results, 1.5 V was selected as the fixed potential window for all subsequent CV studies. By using power law eqn (7) and (8), the b values were calculated to determine the mechanism and prominent factor responsible for charge storage. The diffusion-controlled mechanism is significant during the charge–discharge process, increasing the specific capacitance and energy density of the cell.^58,59 GCD was conducted to analyze the charge–discharge performance of the ASC device, as shown in Fig. 8(b). The presence of AC as the anode contributes a small amount of surface capacitance along with diffusion capacitance. The power-law and Dunn plot clearly indicated that ZIFs exhibit an intermediate capacitive and a highly diffusive mechanism to store the charges.⁶⁰ By using eqn (3), the specific capacitance values of the two-electrode device are found to be 13.72, 11.37, 9.82, 8.81, 7.87 and 5.69 mAh g⁻¹ at current densities of 1, 2, 3, 4, 5, and 10 Ag⁻¹, respectively. By using eqn (11) and (12), energy density and power density were calculated:where ‘E’ stands for energy density (Wh kg⁻¹), ‘P’ stands for power density (W kg⁻¹), ‘V’ stands for potential window (V), ‘C’ stands for capacitance (F g⁻¹) and ‘t’ stands for discharge time (s).

Fig. 7 Schematic illustration of the mechanism of charge–discharge of the CoNi-ZIF//AC electrode.

Fig. 8 (a) CV curves of CoNi-ZIF//AC at different scan rates, and (b) GCD curves of CoNi-ZIF//AC at various current densities.

The calculated energy density values are 16.65, 17.78, 26.66, 27.72, 33.9, and 45.6 Wh kg⁻¹ at power densities of 7127.2, 3048, 3477.3, 2565.3, 1934.02, and 1198.02 W kg⁻¹, respectively. The device stability test was carried out at 3 A g⁻¹ for prolonged charge–discharge of about 10 000 cycles, and the results depict 95.2% capacity retention as presented in Fig. 9(a). The outcome of the stability tests implies that the synthesised ZIFs show long-term electrochemical stability with a capacity loss of 4.8%.

Fig. 9 (a) Cycling stability performance of CoNi-ZIF/AC at 3 Ag⁻¹ over 10 000 cycles. (b) Ragone plot comparison of various reported materials with CoNi-ZIF/AC.

Fig. 9(b) presents the comparison of energy density and power density of various reported MOF materials.^61,62 The higher performance of the current materials could be due to the presence of a combination of the mesoporous framework and activated carbon electrode in the ASC device, which helps improve the performance of ASCs.

To further evaluate the CoNi-ZIF structure after prolonged GCD cycles, SEM analysis was carried out for the electrode material along with the substrate. The image showcases the before and after stability of CoNi-ZIF. Fig. S5(a) depicts the morphology of CoNi-ZIF before stability testing, where the dense structure of ZIFs was observed in the midst of nickel foam, and Fig. S5(b) presents the structure of ZIF after 10 000 charge–discharge cycles. The particles exhibit severe morphological degradation, likely resulting from their transformation into metal hydroxide and/or oxyhydroxide phases, a phenomenon widely reported in the literature.^63–65 This degradation is attributed to the continuous inward and outward transport of the KOH electrolyte during prolonged electrochemical cycling.

Fig. S6 shows the Nyquist plots of EIS spectra for the CoNi-ZIF//AC two-electrode device. Nyquist plots describe the series resistance R_s, charge transfer resistance R_ct, electric double layer capacitance C_dl, and Warburg resistance W. Apparently, the CoNi-ZIF//AC electrode was characterized with low R_ct and R_s values before and after stability testing. R_ct values for the device in KOH electrolyte before and after stability testing were calculated. By comparing the plots before and after 10 000 charge–discharge cycles, the results indicate that electron kinetics remain fast and the ion diffusion is well preserved. This suggests that the electrode/electrolyte interface remains stable with no significant changes observed after prolonged cycling.

Conclusion

We have demonstrated the ion kinetics in CoNi-ZIF and investigated the contribution of diffusion and capacitive-controlled processes using Dunn's model. The results indicate the diffusion-dominated mechanism over the capacitive-controlled mechanism, about 85%, causes rapid faradaic reactions, augmenting a specific capacitance of 49.06 mAh g⁻¹ at a current density of 1 A g⁻¹. The capacity of the ZIF enhances the overall specific capacity of the SC due to nitrogen and its associated organic moieties, which helps increase the wettability. The fabricated asymmetric device shows a large potential window of 1.5 V and displays a good C_s value of about 54.09 mAh g⁻¹ with high energy and power densities of 17.04 Wh kg⁻¹ and 1241.53 W kg⁻¹, respectively. The prolonged charging and discharging for about 10 000 cycles evidence the capacity retention of 95.2%.

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 research article.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta08465b.

Acknowledgements

The authors thank REVA University for the Seed Money Grant: RU/SEED/CHE/2025/10 for the financial support and Department of Science and Technology, Ministry of Science and Technology, India (DST/TMD/EWO/WTI/DM/2021/236 (G)). P. W. M. greatly acknowledges support from the German Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A/B).

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