Non-Coalesced ZIF‑8 MOF-Derived Hierarchical Porous Carbon Nanofibers for Energy Storage Applications

Abstract

The development of a nitrogen-doped carbon nanofiber-incorporated 3D MOF structure as an electrode for an energy storage device has attracted significant attention for advanced electrochemical performance due to its high energy and power density. Herein, ZIF-8 was deliberately grown in situ on PAN/PVP nanofiber using an electrospinning technique to attain a hierarchically porous architecture. The integration of ZIF-8 templates and the thermal decomposition of PVP created a network of interconnected micropores and mesopores, yielding a Brunauer–Emmett–Teller (BET) specific surface area of 373.18 m² g^–1 and a maximum pore volume of 0.28 cm² g^–1. X-ray photoelectron spectroscopy (XPS) confirmed successful nitrogen doping, which significantly enhanced pseudocapacitive behavior, surface wettability, and charge-transfer kinetics in an acidic medium. The dominant surface-controlled capacitive storage mechanism (65–94%) makes the C-ZNF electrode exhibit a high specific capacitance of 278 F g^–1 at 1 A g^–1 in 1 M H₂SO₄. This unique architecture facilitates effective ion transport pathways and improves the charge storage mechanism by reducing internal resistance at the electrode–electrolyte interface. Furthermore, the symmetric coin cell-assembled device exhibits higher power density and energy density of 1000 W kg^–1 and 34 Wh kg^–1, respectively. The device exhibited a maximum specific capacitance of 340 F g^–1 at a current density of 0.3 A g^–1 (C-ZNF // C-ZNF). The device retained 84% of its initial capacitance after 5000 charge–discharge cycles. These findings demonstrate the potential of ZIF-8-derived carbon nanofiber as a promising electrode material for high-performance supercapacitors.

1. Introduction

The rapid growth of the global economy has led to a surge in energy consumption from hydrocarbon sources, which, in turn, has escalated pollution and greenhouse gas emissions. , It is imperative that fossil fuel-powered vehicles and mobile electronics transition to electric power within a few decades in order to combat the risks associated with ongoing fossil fuel consumption, which is no longer debatable. − In the contemporary condition, the consumption and storage of energy are crucial for fulfilling the economic and global needs of humanity. However, the intermittent nature of renewable energy resources can be addressed with integrated energy storage devices, making energy storage technologies essential for sustainable energy systems. , There is a need for the creation of robust and effective energy storage technologies that can capture energy from renewable resources despite variations in weather conditions. Among various energy storage devices, including lithium-ion batteries and fuel cells, supercapacitors outperform due to their fast charge–discharge rates, high cyclic stability, good rate performance, scalability, and eco-friendly solutions, facilitating global energy systems to be environmentally sustainable. With the rapid expansion of scientific and technological innovations, it is crucial to enhance the energy density and the practical scalability for commercial utilization purposes. , Recent research has, therefore, focused on tailoring the electrode materials, and engineering their architecture plays a crucial role in the electrochemical performance of the supercapacitor.

Commercial supercapacitor electrodes primarily consist of carbon-based materials like graphene, biomass-derived activated carbon, carbon nanotubes, etc. These materials are dispersed for electrode coating purposes, which aggregates, consequently inhibiting the active sites. Carbon nanofibers (CNFs) , with 1D structures facilitate rapid ion transport channels; improving their electrochemical properties makes them better-performing electrode materials. Such electrodes offer many advantages such as enhanced electrical conductivity, improved mechanical stability, reduced structural weight, and efficient electrolyte accessibility. Recent studies have demonstrated promising progress in this direction. For instance, Imran Shafi et al. fabricated a self-standing carbon membrane using carbon nanostructures (CNS) and microcrystalline cellulose, which were cast with cross-linking agents of glutaraldehyde and H₂SO₄, exhibiting a specific capacitance of 292 F g^–1 at 0.5 A g^–1. Similarly, Wenbo Liu et al. developed a binder-free porous carbon composite PPC@Co-NCNT electrode using the chemical vapor deposition technique, achieving a specific capacitance of 1260 mF cm^–2 at 1 A cm^–2 in PVA/Na₂SO₄ gel. Furthermore, a self-standing 3D porous P-Mxene/CNT-KOH film fabricated using the filtration process shows a high capacitance of 708.7 F g^–1 at 1 A g^–1, where the composite film was developed by introducing poly­(methyl methacrylate) as a sacrificial template and CNT to enhance the surface area and interlayer spacing for better interaction with electrolytic ions. These studies highlight the significance of designing hierarchical porous architectures to increase electrochemical activity and electrolyte ion transport. , CNFs have emerged as promising candidates for electrode materials due to their excellent electrical conductivity, large surface area, and better electrochemical properties. Electrospinning , is widely recognized as an effective technique for producing uniform nanofibers with controllable morphology and porosity. The fiber morphology obtained through electrospinning depends on several parameters, including polymer solution properties and physical conditions such as feed rate, applied voltage, and collector distance. However, the resultant fiber morphology is influenced by an interplay of these systems and process parameters.

Several studies have explained the process of electrospinning and its myriad applications, , underscoring the critical significance of fabricating fibers characterized by negligible diameters, elevated specific surface areas, improved permeation properties, and precisely regulated pore dimensions as essential prerequisites. In the context of energy conversion and storage applications, these nanofibers, owing to their porous architecture, enhance discharge current and capacity by facilitating ion transport. Mucahid Ozcan et al. prepared ZnO incorporated and codoped with Al and N CNFs synthesized using a polyacrylonitrile (PAN) polymer as a precursor via electrospinning, which exhibited a specific capacitance of 206.28 F g^–1 and energy density of 61.827 Whkg^-1 at 1 kWkg^-1. PAN is most widely used as a precursor for CNFs due to its mechanical strength after carbonization and high carbon yield. The incorporation of polyvinylpyrrolidone (PVP) can introduce defect sites and improve surface wettability that facilitate charge storage. , In addition, metal–organic frameworks (MOFs) , have gained significant interest as sacrificial templates for the fabrication of porous carbon structures, owing to their ordered 3D architecture and the tunability of pore sizes. , Zeolitic imidazolate framework-8 (ZIF-8) , stands out as particularly intriguing due to its substantial surface area, controllable particle size, and stability at elevated temperatures. , Yongfeng Bu et al. reported size-controlled ZIF-8 structures that exhibited a volumetric capacitance of 195 F cm^–3 with exceptionally high cyclic stability. Employing ZIF-8 has the advantage of size-controlled growth that can be tailored according to the size of electrolytic ions, consequently acting as an impeccable template for porous CNFs. So far, the reported ZIF-8-derived CNF electrodes have issues like structural stability during high-temperature carbonization, insufficient control over pore architecture, electrical conductivity associated with excessive microporosity, nanoparticle agglomeration, limited ion diffusion pathways, and inadequate mechanical stability for long-term electrochemical cycles. This has been addressed in this work by improving the structure stability even after long charge–discharge cycles, employing the synergistic effect of PAN/PVP and ZIF-8 post carbonization.

The conventional electrospinning method involves preparing a homogeneous polymeric solution with the nanomaterial incorporated for nanofiber formation. , This may limit the accessible surface area and porous surface with the electrolytic ions due to the polymeric coating over the porous material. However, the surface-embedded growth of ZIF-8 over the nanofiber prevents agglomeration of the nanomaterial, with a subsequent increase in the portable surface area with enhanced specific capacitive performance. This favors electron transportation induced by the formation of interconnected channels. The carbonization process converts ZIF-8 into a porous template, creating a porous surface membrane favorable for electric double-layer formation (EDLC) behavior during the charge storage process. In the current work, the electrospun porous CNF membrane has been developed through electrospinning of PAN/PVP polymer blends, followed by in situ growth of ZIF-8 nanoparticles. Further carbonization resulted in the formation of ZIF-8-derived porous CNFs under a N₂ environment (C-ZNF). An in-depth study of electrochemical performance as a result of PAN/PVP with ZIF-8 has been explored. This significantly showed an enhanced surface area and interconnected conductive pathways. The structural and morphological properties of the synthesized materials were studied to confirm the formation of porous architecture. The scope of the study is to compare the electrochemical performance of C-ZNF and pristine ZIF-8-derived carbon electrodes in an aqueous acidic electrolytic environment and their reaction mechanism. The electrochemical performance of the C-ZNF electrode was explored for its high specific capacitance, making it more suitable for supercapacitor applications and in high-performance energy storage devices. Furthermore, the challenges in employing symmetric coin cell device testing with C-ZNF//C-ZNF have been discussed.

2. Materials and Methods

2.1. Reagents

Zinc acetate dihydrate (Zn­(OAc)₂·2H₂O), 2-methylimidazole, PAN (M _w = 150 000), PVP (M _w = 3,60,000), N,N-dimethylformamide (DMF, ≥99.9%), cetyltrimethylammonium bromide (CTAB), Nafion, N-methylene propylene (NMP), and Milli-Q water were used in all experiments.

2.2. Preparation of ZIF-8 Nanocrystals

The synthesis process of ZIF-8 nanocrystals involves dispersion of zinc acetate dihydrate (1.51 g) in 25 mL of aqueous medium with uniformly dispersed 2-methylimidazole (5.582 g) and CTAB (7 mg) in 25 mL of aqueous medium. This is followed by sonication for 3 min and dropwise addition to the former solution, which turns the transparent solution milky white. The reaction is maintained for 6 h to form a white precipitate and is separated by centrifugation for 3 times in deionized water, followed by vacuum drying for 6 h at 70 °C that results in white flakes of ZIF-8 nanocrystals.

2.3. Fabrication of Zn­(OAc)₂·2H₂O-Incorporated Nanofibers

PAN and PVP are taken in the weight percentage ratio of 4:1 and are dissolved in 15 mL of DMF containing uniformly dispersed 1 g of Zn­(OAc)₂.2H₂O. The solution was continuously stirred for 12 h, forming the precursor for the electrospinning device. The nanofiber was fabricated by applying an electrostatic potential of 13 kV with a feed rate of 1 mL/h. The distance between the needle tip and the mandrel was maintained at 18 cm.

2.4. Fabrication of In Situ-Grown ZIF-8 Nanofiber and C-ZIF-8-Derived Nanofiber

The prepared PAN/PVP nanofiber was added with 5.582 g (2.7 mol L^–1) of 2-methylimidazole and 7 mg of CTAB in aqueous medium. The mixture was subjected to orbital shaking for 6 h at 100 rpm at room temperature and washed thrice in deionized water and dried at 60 °C for 12 h. The synthesized, in situ-grown ZIF-8 PAN nanofiber was carbonized at 900 °C for 3 h at a ramping rate of 3 °C min^–1. The direct growth of ZIF-8 crystals was subjected to the same condition.

2.5. Electrochemical Measurements

Electrochemical measurements were done with a CHI760F electrochemical workstation under ambient conditions.

2.5.1. Electrode Designing Process

The working electrode was prepared by the doctor blade coating method. To attain uniform coating, a homogeneous electrode slurry was prepared containing 85% active materials, 10% carbon black, and 5% PVDF in N-methyl-2-pyrrolidone (NMP). The prepared slurry was coated onto a 1.2 cm² graphite electrode. The working electrode was then vacuum-dried at 60 °C overnight. The electrochemical testing was conducted using a three-electrode cell arrangement consisting of a Pt counter electrode, Ag/AgCl reference electrode, and working electrode in 1 M H₂SO₄ electrolyte for supercapacitor testing.

2.5.2. Configuration of a Symmetric Coin Cell Device

The electrode for coin cell CR2032 was fabricated by preparing a slurry with the same ratio employed for three-electrode testing. Graphite sheet acts as a current collector. Subsequently, 1 M H₂SO₄ was used as the electrolyte and was added dropwise over the Whatman filter paper as a separator in negligible quantity to favor ionic transport and to maintain the stability of the device. The coin cell assembly was completed by sandwiching electrodes, a separator, and a spacer on the other side of the active components. Electrochemical studies have been demonstrated to understand the potential utility of the energy storage device. In supercapacitors, the electrochemical performance of the electrode material can be evaluated using certain crucial parameters. The specific capacitance can be calculated from the galvanostatic charge–discharge (GCD) curve by employing the discharging time. The capacitance has been calculated based on the mass normalization method for both the three-electrode system and the device.

$$C_{\mathrm{s}\mathrm{p}}=\frac{I\times \mathrm{\Delta }t}{m\times \mathrm{\Delta }V}$$ 1

where C _sp denotes the specific capacitance, Δt, and I represents the current density (A g^–1), s is the discharge duration, and ΔV is the potential window. Subsequently, the energy density (Whkg^-1) and the power density (Wkg^-1) , can be calculated for C-ZNF//C-ZNF symmetric cell testing from eq below

$$E_d=\frac{Csp\times \mathrm{\Delta }{V}^2}{2\times 3.6}$$ 2

$$P_d=\frac{E_d\times 3600}{\mathrm{\Delta }T}$$ 3

where C _sp is the specific capacitance of the cell (Fg^-1), ΔV is working potential window (V), and ΔT is the discharge time (s).

2.6. Material Characterization

The crystallographic structures of ZIF-8 were analyzed using an X-ray diffractometer (D8 Advance from Bruker, Cu Kα radiation) with a monochromatic copper source (Cu Kα, λ = 1.5406 Å). The surface chemical analysis was determined by the model K-alpha X-ray photoelectron spectrometer (Thermo Scientific, USA) and analyzed by X-ray photoelectron spectroscopy (XPS). The surface area was calculated based on the Brunauer–Emmett–Teller (BET) method. The N₂ sorption isotherms and pore size distribution were analyzed using Autosorb, Quanta Chrome–BET. The defect density of the composite was investigated by Raman spectroscopy (HORIBA-512 nm laser). The morphological analysis was performed using FE-SEM (Thermo Fisher Quanta 250 FEG). All the electrochemical measurements were carried out with a CHI instrument (CHI-760F workstation).

3. Results and Discussion

The 1D nanofibers derived from ZIF-8 were synthesized via the in situ growth of 3-D ZIF-8 structures on electrospun PAN/PVP nanofibers. The synthesis process of ZIF-8 was done using Zn²⁺ ions and methylimidazole anions, which facilitated the controlled nucleation and growth on the surface of the nanofibers. FESEM analysis was used to study the morphology of ZIF-8, the electrospun nanofibers integrated with ZIF-8 nanoparticles, and the carbonized ZIF-8 nanofibers (C-ZNF). As given in Figure , a uniform deposition of ZIF-8 nanoparticles with an average diameter of 170.6 ± 17.4 nm was successfully achieved, indicating a process of homogeneous nucleation. The ZIF-8 nanoparticles were uniformly distributed over the 1D nanofiber surface, which exhibited an average fiber diameter of 1037.6 ± 196.2 nm (Figure c,d), thereby leading to the development of ZIF-8 nanoparticles on the nanofibers. This observation proves the successful in situ growth of ZIF-8 nanoparticles upon the PAN/PVP nanofibers. Following carbonization, the resulting C-ZNF nanofibers demonstrated a rough and porous surface morphology, which facilitated the growth of efficient ionic transport channels and enhanced the utilization of electrochemically active sites. Moreover, the incorporation of PVP was determined to be crucial in the formation of defective sites post carbonization, consequently enhancing the accessible surface area and effectively mitigating the challenge of underutilized active sites in electrochemical energy storage applications. The elemental maps reveal the homogeneous distribution of Zn, N, O, and C throughout the nanofiber network, confirming the uniform growth of ZIF-8 nanoparticles on the electrospun fibers without noticeable agglomeration. The uniform distribution of ZIF-8 over the nanofiber can be confirmed by comparing the C-ZNF precarbonization shown in Figure S1.

1.

FESEM images of (a,b) in situ-grown ZIF-8 crystals over polymer nanofiber and their size distribution, (c) carbonized ZIF-8 nanofiber, and (d,e) synthesized ZIF-8 nanocrystals and their size distribution.

The prepared CNF contains ZIF-8 particles electrospun with PAN and PVP polymers, where PAN improves the carbon yield in the nanofiber membrane when annealed at 900 °C and, by incorporating PVP, improves the pore accessibility of the fiber. The XRD spectrum of ZIF-8 and ZNF is shown in Figure a,b, where the diffraction spectra with an incident wavelength of 1.546 Å show characteristic peaks at 2θ = 7.71°, 10.50°, 12.88°, 14.88°, 16.50°, 18.60°, 19.6°, 22.2°, 24.6°, 25.9°, 29.9°, 31.0°, and 32.7°, with the corresponding Miller indices at (011), (002), (112), (022), (013), (222), (123), (114), (233), (114), (233), (134), (044), (244), and (235), in good agreement with the standard pure ZIF-8. These planes match the CCDC card number602542 with space group I4̅3m and crystal system as simple cubic (α = β = γ = 90°). The XRD peaks of in situ-grown ZIF-8 crystals over PAN/PVP fibers well match the former, indicating the confirmation of particle growth and distribution throughout the nanofiber. The observable shift in the peak intensity to 7.52° indicates an increase in the interplanar distance due to hydrogen bond formation as a result of polymer interaction. As shown in Figure b, the characteristic peak of ZIF-8 disappears with an increase in carbonization temperature. As the carbonization temperature reaches 900 °C, it leads to the partial evaporation of Zn metal, thereby creating defects and facilitating the formation of porous carbon. The two broad peaks at 2θ = 24° and 44° are due to amorphous carbon that tends to form graphitic carbon corresponding to planes (002) and (100). The calculated interlayer distance of d ₍₀₀₂₎ is 3.56 Å, which is larger than that of pure graphite. There is an observable peak broadening in the diffraction spectra of carbonized fiber at 24°, due to the reduced crystalline size via increased microstrain influenced by the nitrogen doping in C-ZNF. This shows that the interaction of PAN and PVP with ZIF-8 during carbonization under a N₂ atmosphere favors nitrogen doping.

2.

XRD pattern of (a) in situ-grown ZIF-8 over nanofiber (ZNF), ZIF-8, and standard ZIF-8 (CCDC no; 602542) and (b) carbonized ZIF-8 and nanofiber.

Raman spectroscopy was used to evaluate the structural arrangement and defect characteristics of carbonized nanofibers and the carbon materials originating from C-ZIF-8. The spectra (Figure a) revealed two prominent bands at 1340 cm^–1 (D band) and around 1577 cm^–1 (G band). The G band corresponds to the E _2g vibrational mode of sp²-hybridized carbon, signifying the presence of graphitic domains, and the D band corresponds to the symmetric A_1g mode with asymmetric vibrational mode of CC species in the carbonized materials, which are associated with structural imperfections and lattice disorder. , The broad peak in the region of 2800 cm^–1 attributes to first order signals, and the G + D combinational peak is well pronounced due to the disordered nature of the graphitic lattice at 900 °C. The defects created are due to the evacuation of Zn metal at that temperature. The I _D/I _G ratio functions as an indicator of the degree of graphitic disorder. The high ratio observed for CNFs denotes a higher concentration in smaller sp² domains. Conversely, the carbon produced from C-ZIF-8 shows a lower I _D/I _G ratio, indicating relatively improved graphite formation, which can be attributed to the carbonization process from the MOF and the removal of PVP at that temperature, consequently decreasing the sp² domain and the evaporation of zinc species that promotes the development of porous carbon.

3.

(a) Raman Spectra of both Z-CNF and C-ZIF and (b) thermogravimetric analysis of Z-CNF.

The thermogravimetric analysis of C-ZNF is demonstrated under a N₂ atmosphere in Figure b. In the region under 270 °C, a weight loss of 3% was observed due to the evaporation of absorbed solvent molecules. There exist guest molecules in the cavities and unreacted species of ZIF-8 during the synthesis process that evaporate at 330 °C, experiencing a weight loss of 25% in the range of around 330–420 °C. The sharp reduction in weight loss at 420 °C is due to the decomposition of the organic linker, and PVP starts to degrade at that temperature until 700 °C with 45% weight loss. This creates a pathway for ionic diffusion channels, thereby leaving a porous structure that eases the accessibility of active sites. Despite the volume reduction of the electrospun fiber after carbonization, the structure remains stable, creating a template structure with suitable characteristics for electrode materials.

The N₂ adsorption/desorption isotherms were determined to study the porous properties of the C-ZNF and ZNF. The isotherm curves exhibit a type-IV profile with a broader hysteresis loop than ZNF due to the effect of carbonization, indicating the existence of a greater mesoporous structure. The isotherm exhibits a distinct adsorption knee in the low-pressure region (P/P ₀ < 0.1), indicating monolayer formation on the pore walls and delineating the BET fitting region, indicating the presence of micropores. An incremental rise in adsorbed volume corresponding to increasing relative pressure signifies the advancement of multilayer adsorption. A pronounced hysteresis loop is observed in the medium- to high-pressure region, which can be attributed to capillary condensation occurring within mesopores. The presence of a hysteresis loop at intermediate to high relative pressures confirms the presence of mesopores (Figure a). The BET surface area of C-ZNF is observed to be 373.18 m²/g. Such a hierarchical pore architecture provides abundant electrochemically active surface area through micropores and efficient electrolyte diffusion pathways through mesopores, thereby enhancing electrochemical performance. This is comparable to or higher than most of the recently reported ZIF-8-derived porous CNFs prepared under similar carbonization conditions shown in Table S2. The significant increase observed as saturation approaches (P/P ₀ → 1) reinforces the existence of larger mesoporous and interparticle porosity, suggesting a relatively wide pore-size distribution. Similarly, the surface area for ZNF is 118.43 m²/g, as given in Figure b. It is observed that the increase in surface area and pore volume is because of polymer decomposition and partial Zn volatilization at 900 °C post carbonization creates active sites for effective ion accessibility. In addition, the micropores interconnected to form an ion transport channel consequently widen the pores. The existence of larger mesopores facilitates a shorter path for electrolytic ions, thereby improving the charge transportation kinetics. The broader pore size distribution creates active sites for effective ion accessibility. As shown in Table S3, C-ZNF possesses a maximum pore volume of 0.284936 cm³/g and a micropore volume of 0.172860 cm³/g, which reveals the contribution of significantly large mesoporous volume. These characteristic features are a prerequisite for the EDLC performance of the supercapacitor device.

4.

(a,b) The nitrogen adsorption/desorption isotherm curve of C-ZNF and ZNF.

The surface elemental composition and the chemical bonding nature of Z-NF and Z-CNF have been investigated. , The survey spectra (Figure a,d) with binding energies in the range of 0–1300 eV evidence the presence of C, O, N, and Zn elements in the synthesized material. This confirms the presence of ZIF-8 over the CNF. To further analyze the chemical bonds from the survey in detail, the peaks are deconvoluted. The higher resolution of the C 1s spectrum for C-ZNF possesses peaks at 284.54 eV (CC), 286.06 eV (–CN), and 288.0 eV (O–C–O). Notably, the deconvoluted C 1s spectrum (Figure b,e) of C-ZIF exhibits a shift toward higher binding energies for C-ZNF, which represents the nitrogen doping in Z-CNF due to interaction between PAN and ZIF-8. The higher electronegativity of nitrogen than carbon corresponds to the shift in the binding energies of Z-CNF. This improves the pseudocapacitive behavior and charge transfer kinetics. Figure c,f shows the higher resolution N 1s spectrum of C-ZNF, where the peaks correspond to pyridinic (398.37 eV), pyrrolic (399.65 eV), and graphitic nitrogen (401.9 eV). The presence of pyridinic nitrogen increases the electron-donating ability of the material. The observed peak at 401.9 eV confirms nitrogen doping in the CNF. This favors the ion transport rate and the adsorption of electrolytic ions, consequently enhancing the electrochemical performance of C-ZNF compared to the pristine material. It is observed that C-ZNF is the better-performing material due to the contribution of nitrogen active sites, in addition to which fibers are likely to facilitate rapid electron transfer, thus increasing the efficiency to access the active sites. Figure h,j shows the plotted O 1s peak corresponding to M-O (531.12 eV) and C–O (532.15 eV), indicating the successful growth of ZIF-8 over CNF. The deconvoluted metal peaks of Zn 2p spectra from Figure i,k exhibit a doublet peak due to spin–orbital splitting. For Zn 2p, the peaks correspond to Zn 2p_1/2 (1021.98 eV) and Zn 2p_3/2 (1045.05 eV). The difference in peaks obtained was observed to be 23 eV, which validates the presence of the Zn²⁺ oxidation state. , The deconvoluted N 1s spectrum of C-ZNF revealed the presence of pyridinic N, pyrrolic N, and graphitic N species. Pyridinic and pyrrolic nitrogen are generally considered electrochemically active sites that contribute additional pseudocapacitance through reversible faradaic reactions due to the properties of quinone-type groups in an aqueous acidic electrolyte and improve electrolyte wettability, while the presence of graphitic nitrogen in C-ZNF due to the influence of the nitrile group in PAN enhances the conductivity of the carbon matrix. Figure g represents the high-resolution N 1s spectrum with the percentage distribution of C-ZNF. The relative contents of pyridinic N, pyrrolic N, and graphitic N were determined to be 48%, 35%, and 15%, respectively. The high proportion of pyridinic and pyrrolic nitrogen provides abundant redox-active sites, contributing significantly to the observed capacitance enhancement, whereas graphitic nitrogen facilitates rapid electron transport and improves rate capability. Therefore, the synergistic effect of these nitrogen species is responsible for improved electrochemical performance.

5.

X-ray photoelectron spectroscopy (XPS) analysis of (a) C-ZNF and (d) C-ZIF, (b,e) C 1s spectrum of C-ZNF and C-ZIF, (c,f) N 1s spectrum of C-ZNF and C-ZIF, (g) C-ZNF high-resolution N 1s spectrum with the percentage distribution of each nitrogen species, (h,j) O 1s spectrum of C-ZNF and C-ZIF, and (i,k) Zn 2p spectrum of C-ZNF and C-ZIF.

3.1. Electrochemical Analysis

The evaluation of electrochemical and capacitance performance of the prepared C-ZIF and C-ZNF electrodes was studied using cyclic voltammetry (CV) and GCD studies in 1 M H₂SO₄ aqueous solution at an optimized potential window of 0.1–0.9 V (Vs Ag/AgCl). Figure a,b shows the CV curves of the electrodes at various scan rates from 10 to 100 mV/s, which indicates the charge storage process is predominantly electric double layer capacitive behavior. As the scan rate increases, the enclosed area of the CV curve gradually elevates while retaining the behavior, suggesting excellent rate capability and rapid ion transport within the porous carbon framework. The good retention of the quasi–rectangular profile at high scan rates suggests low polarization and efficient electrolyte accessibility to the active sites. From Figure c, the comparative CV curves of C-ZIF and Z-CNF at 100 mV/s reveal that the area is significantly higher for Z-CNF, implying a better capacitance value. The good rate performance can be attributed to the porous architecture of the carbon electrode, in which micropores provide abundant active sites for charge accumulation, while mesopores facilitate rapid electrolyte penetration and shorten ion diffusion pathways. For comparison, the CV curve of PAN/PVP CNF is shown in Figure S7. This shows improved electrochemical performance of Z-CNF compared to bare CNF, which might be due to the improved porous structure post carbonization with the influence of ZIF-8. The enhanced electrochemical performance is influenced by several factors like hydrophilicity, surface area, conductivity, along with the type and concentration of nitrogen species present in the carbon framework. A detailed analysis of the CV curves was done at different scan rates for comparison of the charge storage kinetics between C-ZNF and C-ZIF within the potential window of 0.1 to 0.9 V, from 1 to 100 mV s^–1, as given in Figure d, e. Consequently, the graphs depict the relationship between the peak current (i) and the scan rates (ν) during the anodic and cathodic processes that were constructed utilizing the power law equation, thereby facilitating the differentiation between capacitive and diffusion-controlled mechanisms present within the CV curves.

$$i=a{v}^b$$ 4

$$\log (i)=b\log (v)+\log (a)$$ 5

6.

Schematic showing the reaction mechanism of pyridinic-N and pyrrolic-N, which exhibits pseudo-capacitance through redox reaction in the presence of protons.

Herein, the variables a and b delineate whether the underlying mechanism is governed by capacitance or diffusion. The redox reaction predominantly exhibits diffusion control when the b value approaches 0.5, whereas it demonstrates capacitive control when the b value attains 1. The b values were ascertained through the analysis of the slope derived from log (i) versus log (ν) plots corresponding to various anodic and cathodic peaks. − From Figure d, the b values for C-ZNF and C-ZIF have been calculated to be 0.96 and 0.90, indicating the dominant capacitance behavior. The reaction kinetics for C-ZNF demonstrates a higher b value that implies higher capacitive controlled mechanism. This contributes to the higher surface area, pore volume, and the graphitic nature because of PAN/PVP with ZIF at a higher carbonization temperature. Subsequently, the high proportion of pyridinic and pyrrolic nitrogen provides abundant redox-active sites, contributing significantly to the observed capacitance enhancement, whereas graphitic nitrogen facilitates rapid electron transport and improves rate capability. Therefore, the synergistic effect of these nitrogen species is responsible for the improved electrochemical performance of C-ZNF. The electron transfer kinetics improves with the increase in the percentage of capacitive behavior. To further calculate the quantitative contribution of the charge storage mechanism for C-ZNF and C-ZIF, it was separated into diffusion control and capacitive contribution. The total current response at a specific voltage (V) is given by the sum of capacitive contribution (k ₁ v) and diffusive contribution (k ₂ v ^1/2)­

$$i(V)=k_{1}v+k_2{v}^{1/2}$$ 6

7.

Electrochemical studies of C-ZIF and C-ZNF in 1 M H₂SO₄ electrolyte with the potential range from 0.1 to 0.9 (V vs Ag/AgCl). (a,b) The cyclic voltammetry curves at different scan rates from 10 to 100 mV/s, (c) the comparative CV profile of C-ZIF and C-ZNF at the scan rate of 100 mV/s, (d) the current response versus scan rate at 0.4 V, (e) the linear plot between I _a/V ^1/2 versus V ^1/2 at 0.4 V, (f,g) the bar graphs expressing the percentage of the capacitive and diffusive contributions at different scan rates for C-ZNF and C-ZIF, respectively, (h,i) galvanostatic charge–discharge curves at different current densities from 1 A/g to 5 A/g, and (j,k) the Nyquist plot and fitting of C-ZNF and C-ZIF.

The above equation can be rearranged to be written as

$$\frac{i(V)}{v^{1/2}}=k_1{v}^{1/2}+k_2$$ 7

where K ₁ and K ₂ are constants from the slope and the intercepts obtained from the plot $\frac{i(V)}{v^{1/2}}$ Vs v ^1/2 respectively Figure d. , The capacitive contribution has been studied for different scan rates from 1 to 80 mV/s. As shown in Figure f, relatively higher capacitive percentage is observed for C-ZNF ranging from 65 to 94% than the pseudo-behavior, evidencing the influence of graphitic N after carbonization. As shown in Figure g, C-ZIF displays a relatively lower capacitive contribution, particularly at lower scan rates, accompanied by a higher diffusion-controlled percentage. Although the capacitive contribution increases with scan rate, the diffusion-controlled process remains more pronounced because of pyridinic and pyrrolic nitrogen. This indicates relatively slower ion transport kinetics occurring at lower scan rates that favors pseudo-capacitance. In comparison with C-ZIF, the formation of graphitic nitrogen with the influence of the nitrile group from PAN facilitates improved specific capacitance with sight pseudo-behavior. This mechanism is well consistent with XPS and findings from other reports that pyridinic and pyrrolic nitrogen involving protons exhibits pseudocapacitive behavior. The possible reaction from the literature is given in Figure . Consequently, C-ZNF demonstrates surface-controlled charge storage, leading to enhanced rate capability and better capacitive performance. In accordance with the hard–soft acid–base (HSAB) principle, H⁺ ions, which behave as hard Lewis acids, preferentially coordinate with pyridinic nitrogen, acting as hard Lewis base sites. The XPS results (Figure g) reveal that C-ZIF contains a significantly higher pyridinic nitrogen content (52%) than C-ZNF (37%). This higher concentration of pyridinic nitrogen enhances the affinity of the electrode toward H⁺ ions, promoting improved ion adsorption and charge storage kinetics.

The capacitance can also be studied using GCD studies at various current densities. Figure h,i shows the GCD profiles of C-ZIF and Z-CNF with minimal IR drop of the discharge curve, attributed to the good conductivity of the electrode materials, and the observed triangular-like shape evidences the good EDLC property of the C-ZNF and C-ZIF, indicating lower equivalent series resistance that ensures efficient electron transfer during repeated charge–discharge cycles. The specific capacitance of the materials can be calculated from eq , where C (F g^–1) is the specific capacitance, I (A) refers to the discharge current, Δt (s) represents the discharge time with the change in potential ΔV (V), and m (g) corresponds to the mass loading of the material on the electrode. The specific capacitance of C-ZIF and Z-CNF is observed to be 189.12 F g^–1 and 278 F g^–1 at 1 A g^1–. Notably, Z-CNF shows a better-performing material with a higher capacitance value. This results from the effect of nitrogen doping with the interaction between ZIF-8 and PAN at a certain temperature. In addition to this, PVP acts as a sacrificial material that favors the accessibility of active sites in the nanofiber structure for rapid ion transmission and diffusion during the electrochemical process. From the GCD profile of bare CNF, the specific capacitance has been observed to be 125 F g^–1 at 1 A g^–1 (Figure S7b). Notably, Z-CNF shows a better electrochemical performance with a higher capacitance value due to the influence of ZIF-8 post carbonization.

Subsequently, the high proportion of pyridinic and pyrrolic nitrogen provides abundant redox-active sites, contributing significantly to the observed capacitance enhancement, whereas graphitic nitrogen facilitates rapid electron transport and improves rate capability. Therefore, the synergistic effect of these nitrogen species is responsible for the improved electrochemical performance of C-ZNF. As shown in Figure j,k, the electrochemical impedance spectrum profile has been investigated and fitted with an equivalent circuit for C-ZNF and C-ZIF. In the high-frequency region, the Nyquist plot shows a very small semicircle, indicating a low charge-transfer resistance (R _ct) at the electrode/electrolyte interface. The intercept of the impedance spectrum with the real axis (Ź) corresponds to the equivalent series resistance (R _s), which includes the intrinsic resistance of the electrode material, electrolyte resistance, and contact resistance between the current collector and the active material. The relatively low Rs value demonstrates good electrical conductivity and efficient current collection. C-ZNF shows lower ohmic (R _s ∼ 1.9 Ω) and charge transfer resistance values (R _ct ∼ 0.15 Ω) and exhibits good conductivity and improved electron transmission kinetics. In the low-frequency region, C-ZNF shows a straight line at 45°, which is due to the Warburg impedance caused by ion diffusion between the porous material and the electrolyte, confirming the excellent electrical conductivity, rapid charge-transfer kinetics, and efficient ion diffusion characteristics of the electrode. For comparison, the electrochemical measurements have been performed in a neutral electrolyte (1 M Na₂SO₄) to evaluate the influence of the electrolyte type on the electrochemical behavior of the prepared electrode, as shown in Figure S5. The current response and enclosed CV area were comparatively lower than those obtained in 1 M H₂SO₄ due to the lower ionic conductivity and reduced mobility of hydrated Na⁺ ions (Figure S5c). Furthermore, the EIS analysis revealed an equivalent series resistance of 4.86 Ω and a charge-transfer resistance of 0.99 Ω in 1 M Na₂SO₄, reflecting comparatively slower ion transport kinetics, as given in Figure S5d. These electrochemical features are consistent with the micro-/mesoporous structure with enhanced surface area and conductive carbon framework, which collectively contribute to the superior capacitive performance.

4. Coin Cell Device

The constructed CR2032 coin cell device testing was performed to explore the potential feasibility for real-time utilization of the prepared electrodes by employing their electrochemical activities. A symmetric supercapacitor using a coin cell was designed to study its electrochemical properties. The electrode preparation process has been mentioned in Section 2.7. For the conductive substrate, a graphite sheet was employed, similar to the three-electrode system due to its good conductivity and to explore the intrinsic electrochemical activity of the electrode material. For the separator, Whatman filter paper was saturated with 1 M H₂SO₄, which facilitates ion diffusion. All the components were assembled and sandwiched with a CR2032 coin cell case via a hydraulic crimper. Extensive electrochemical properties of the coin cell were tested by performing CV, GCD, and EIS. From Figure a, the CV profile predominantly shows EDLC at lower scan rates. As the scan rate increases, slightly pseudocapacitive performance has been observed due to negligible interfacial resistance that occurs during rapid charge transfer processes with the influence of the separator. The ion diffusion occurs primarily at the outer regions at higher scan rates, while the electrolytic ions are accessible for both inner and outer regions at lower scan rates. In the GCD, the capacitance of C-ZNF//C-ZNF increases with the range of current densities from 2 A g^–1 to 0.3 A g^–1. As shown in Figure b, the constructed symmetric cell is tested in the 1 M H₂SO₄ electrolyte, which exhibits a maximum capacitance of 384 F g^–1 at a current density of 0.3 A g^–1. The specific capacitance decreases with an increase in applied current density. Due to the consequences of limited time availability at higher current densities, it hinders the movement of electrolytic ions into the surface of the electrode. The pores favor charge storage better due to increased pore volume from the effect of PVP and Zn ions on the nanofiber post carbonization. This promotes the permeation of electrolytic ions with a shorter diffusion path. To further verify the results, the GCD curves of the coin cell and the three-electrode system at the same current density were compared, and the discharge times were found to be consistent, as given in Figure S4. The cyclic stability of the C-ZNF//C-ZNF-based coin cell device was investigated for 5000 cycles. The coin cell electrode material was observed to retain 84% of its initial capacitance at 2 Ag^–1. Figure c shows the charge transport and conductivity at the electrode/electrolyte interface, as investigated by EIS before and after 5000 charge/discharge cycles. The EIS spectrum is composed of two regions: the high-frequency domain featuring a semicircle, whereas the inclined straight line appears in the low-frequency domain. In the plot of Z′ (real) versus Z″ (imaginary), the semicircle diameter corresponds to the ohmic charge-transfer resistance of the electrode material, and the intercept of the high-frequency response on the x-axis reflects the electrolyte resistance. , Moreover, the presence of a semicircle is primarily attributed to faradaic reactions and the double-layer capacitance, while the straight line is associated with Warburg impedance, which arises because of ionic diffusion between the electrolyte and the pores in the electroactive material. The electrolyte resistance includes both ionic and electronic contributions as it moves through small pores. From EIS plot, the solution resistance before and after 5000 charge/discharge cycles was observed to be 7.515 Ω and 7.071 Ω, respectively, with similar values. The charge transfer resistance was calculated to be 16.06 Ω and 34.02 Ω, respectively. ,, The significant increase in the diameter of the high-frequency semicircle after prolonged cycles can be attributed to slight changes in fiber morphology due to several charge/discharge cycles that can be observed from Figure S6a,b. In addition, the electrolyte decomposition products or negligible contact degradation between the active material and current collector might be the reason. In the low frequency region, a linear component is observed due to Warburg impedance, which involves diffusion between the electrolyte and the active material. Ionic resistance contributes to the internal resistance and the contact resistance with the current collector. Therefore, the conductivity and ionic transfer were well established, showing an enhanced electrochemical performance of the coin cell electrode. , The energy density E (Whkg^-1) and power density P (Wkg^-1) of the fabricated device were calculated from the specific capacitance at various current densities using eqs and . The maximum power density obtained for the symmetric coin cell is 1000 Wkg^-1 at an energy density of 7.4 Whkg^-1. The maximum energy density calculated was 34 Whkg^-1 at a power density of 120 Wkg^-1. From the Ragone plot, it can be seen that the calculated energy density and power density were comparatively better than those reported in other studies for symmetric ZIF-derived carbon devices.

8.

Electrochemical performance of the symmetric coin cell device of C-ZNF//C-ZNF in 1 M H₂SO₄ electrolyte. (a) The cyclic voltammetry curves at different scan rates from 10 to 100 mV/s, (b) galvanostatic charge–discharge curves at different current densities from 0.3 A/g to 2 A/g, (c) the Nyquist plots tested from 0.01 Hz to 10⁵ Hz for the constructed symmetric coin cell before and after 5000 charge/discharge cycles, (d) the Ragone plot of the C-ZNF symmetric coin cell supercapacitor, (e) the cyclic stability plot of C-ZNF // C-ZNF for 5000 charge/discharge cycles, and (f) photographic image of the C-ZNF symmetric coin cell supercapacitor assembly (C-ZNF // C-ZNF).

5. Conclusion

A high-performance supercapacitor was successfully fabricated using ZIF-8 N-doped CNFs (C-ZNFs) with a unique porous structure. The N-doped CNF with hierarchical porous structural formation was due to the template structure of ZIF-8 after carbonization, which acts as a pivotal factor for pore formation. In C-ZNF, the degradation of PVP in the CNF facilitates shorter diffusion channels for ion transport and eases access to active sites. This simple and scalable synthesis procedure highlights its potential for large-scale production that can be commercialized. The uniform growth of ZIF-8 over the surface of the nanofiber was achieved, which prevents aggregation, resulting in enhanced electrochemical performance. The symmetric coin cell device shows a maximum specific capacitance of 384 F g^–1 at a current density of 0.3 Ag^-1. Overall, this work evaluates a promising approach for developing efficient MOF-derived CNF electrode materials for coin cell energy storage devices. This study demonstrates the intrinsic electrochemical characteristics of the C-ZNF electrode in a conventional symmetric configuration.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c05089.

• Additional characterization and analysis, elemental mapping, surface area comparison with recent reports, BET parameter analysis before and after carbonization, CP graph comparison for device and three-electrode system, electrochemical studies of C-ZNF in neutral electrolyte, post-mortem analysis of coin cell, and electrochemical analysis of bare CNF (PDF)

The authors declare no competing financial interest.