Rationally Designed PPy-Coated Fe₂O₃ Nanoneedles Anchored on N-C Nanoflakes as a High-Performance Anode for Aqueous Supercapacitors

Abstract

Flexible supercapacitors have emerged as pivotal energy storage components in wearable smart electronic systems, owing to their exceptional electrochemical performance. However, the widespread application of flexible supercapacitors in smart electronic devices is significantly hindered by the developmental bottleneck of high-performance anode materials. In this study, a novel electrode composed of surface-modified Fe₂O₃ nanoneedles uniformly coated with a polypyrrole (PPy) film and anchored on Co-MOF-derived N-C nanoflake arrays (PPy/Fe₂O₃/N-C) is designed. This composite electrode, grown in situ on carbon cloth (CC), demonstrated outstanding specific capacity, rate performance, and mechanical flexibility, attributed to its unique hierarchical 3D arrayed structure and the protective PPy layer. The fabricated PPy/Fe₂O₃/N-C@CC (P-FONC) composite electrode exhibited an excellent specific capacitance of 356.6 mF cm⁻² (143 F g⁻¹) at a current density of 2 mA cm⁻². The current density increased to 20 mA cm⁻², and the composite electrode material preserved a specific capacitance of 278 mF cm⁻² (112 F g⁻¹). Furthermore, the assembled quasi-solid-state Mn/Fe asymmetric supercapacitor, configured with P-FONC as the negative electrode and MnO₂/N-C@CC as the positive electrode, demonstrated robust chemical stability and notable mechanical flexibility. This study sheds fresh light on the creation of three-dimensional composite electrode materials for highly efficient, flexible energy storage systems.

1. Introduction

Nowadays, with the continuous depletion of traditional fossil fuels, environmental pollution and energy crises have emerged as global challenges [1,2,3]. The development of high-efficiency and environmentally friendly energy storage technologies is one of the most effective solutions [4,5]. Currently, widely used energy storage technologies primarily include electrical energy storage, hydrogen storage, and thermal storage [6,7]. Among these, electrical energy storage encompasses electrochemical energy storage, mechanical energy storage, and electromagnetic energy storage. Electrochemical energy storage systems, characterized by their high energy density, moderate response speed, and broad applicability, hold immense potential for large-scale implementation [8,9,10]. Supercapacitors, as a component of electrochemical energy storage systems, have attracted significant attention from researchers due to their unique advantages, such as high power density, rapid charge–discharge capabilities, and long life cycle [11,12,13,14]. Flexible supercapacitors, in particular, have attracted considerable interest in the fields of wearable electronics and smart textiles due to their distinctive physical, chemical, and mechanical properties [15,16,17]. However, challenges such as low energy density and the need for further improvement in cycle stability currently limit their practical application in flexible electronic devices [18]. Therefore, enhancing the energy density and cycle stability of flexible supercapacitors to make them truly suitable for portable and wearable applications remains a critical focus of current research.

Among the various components of flexible supercapacitors, flexible electrodes are one of the most critical elements, playing a pivotal role in determining the overall performance of the device [19,20]. Currently, common negative electrode materials for supercapacitors primarily include carbon-based materials, transition metal oxides, and conductive polymers [21,22,23]. Among these, transition metal oxides have attracted significant attention due to their high theoretical capacitance, excellent electrochemical reversibility, and abundant natural resources [24,25,26]. In particular, Fe-based oxides such as Fe₂O₃ have attracted considerable interest owing to their high theoretical specific capacitance and abundant availability on Earth [27,28,29,30]. However, the inadequate conductivity and poor stability of Fe₂O₃ make it challenging to use as an energy storage electrode material. To address these limitations, strategies such as structural optimization and interface engineering are commonly employed to modify and enhance the performance of Fe₂O₃-based electrodes, thereby improving their specific capacity. For instance, Chakraborty et al. successfully synthesized a three-dimensional ultramicroporous triazolite Fe-MET material [31]. The as-prepared electrode delivered a specific capacitance of 304 F g⁻¹ and an areal capacitance of 181 mF cm⁻² at a current density of 0.5 A g⁻¹. The Fe-MET-based supercapacitor maintained approximately 70% of its initial capacity and 85% of its Coulombic efficiency after 5000 cycles, showcasing exceptional electrochemical performance and outstanding cycling stability. Furthermore, Zhou’s research team developed a porous Fe-based electrode by thermally treating an Fe-based metal-organic framework (Fe-MIL-88B-NH₂) at 500 °C under a nitrogen atmosphere [32]. Additionally, by further calcining MIL-88-Fe on oxidized carbon nanotube fibers (S-Fe₂O₃@C/OCNTF) at 350 °C, they obtained spindle-shaped carbon-integrated Fe₂O₃. The resulting S-α-Fe₂O₃@C/OCNTF electrode exhibited an impressive areal capacitance of 1232.4 mF cm⁻² at a current density of 2 mA cm⁻², while retaining 63% of its capacity when the current density was increased to 20 mA cm⁻². To further improve the electrical conductivity of Fe₂O₃ materials, Jin et al. used metal–organic skeleton (MOF)-derived carbon (MDC) electrode materials for their study [33]. The carbonization of the selected MOF produced highly conductive carbon with a hierarchical pore structure, and the fabricated MDC SC exhibited significant area and volume capacitances. In an alternative approach, some conductive carbon materials, such as graphene, carbon fibers, carbon nanotubes, and carbon cloths, are usually used as conductive substrates for composites [34]. Lu et al. vertically anchored α-Fe₂O₃ nanorods onto carbon cloths using hydrothermal and calcination methods and further coated the ternary NiCo₂Mn-LDH on the surface of the α-Fe₂O₃ nanorods to obtain the sandwiched core–shell heterostructure of self-supporting flexible electrodes [35]. This construction method effectively reduces the agglomeration effect of the ternary layered double hydroxide (LDH) and improves ion transport efficiency. Ji’s team also deposited phosphate-functionalized Fe₂O₃ (P-Fe₂O₃) on reduced graphene oxide (rGO) using a solvothermal calcination strategy [36]. The synthesized P-Fe₂O₃/rGO electrode material exhibited a capacitance of up to 586.6 F g⁻¹ at a current density of 1 A g⁻¹. Moreover, enhancing the stability of Fe₂O₃ materials through composite modification, surface coating, and ion-doping modification are also popular approaches. For example, Zheng’s research team employed an electrostatic self-assembly strategy to deposit anionically doped PPy onto cationic bacterial cellulose (BCD) nanofibers [37]. The optimized PPy@BCD electrode demonstrated excellent cycling stability (its capacity retention was still as high as 100% after 10,000 charge/discharge cycles at a current density of 10 mA cm⁻²)

Therefore, to address the issues of the low capacity and poor stability of Fe₂O₃ electrodes, this study utilized Co-MOF as a precursor and synthesized Fe₂O₃-based electrode materials (Fe₂O₃/N-C@CC) through a hydrothermal + calcination method. Subsequently, a polypyrrole (PPy) protective layer was formed in situ on the surface of an Fe₂O₃/N-C@CC (FONC) electrode in an ice water bath. The Co-MOF-derived three-dimensional N-C nanoflake arrays uniformly anchored on the carbon cloth (CC), with a porous structure and excellent conductivity, served as a sub-substrate for loading the Fe₂O₃ nanoneedles, resulting in a three-dimensional hierarchical arrayed nanostructure. This novel design not only increases the active material’s loading mass, but it also ensures intimate electrolyte–electrode interfacial contact, which accelerates electron transport and ion diffusion rates. Moreover, the PPy coating not only provides the electrode material with additional capacity but also forms a protective film on the surface of the electrode material, which bolsters its structural stability. As a result, the as-prepared P-FONC electrode material exhibited a high energy density, superior rate performance, and excellent flexibility.

2. Materials and Methods

2.1. Preparation of FONC

First, 0.8925 g of Co(NO₃)₂·6H₂O and 1.970 g of 2-methylimidazole were separately dissolved in 60 mL of deionized (DI) water with stirring until complete dissolution. Subsequently, the two solutions were mixed, and a clean piece of CC (2.5 cm × 4 cm) was quickly immersed into the mixture. After standing for 4 h, the CC was removed, washed three times with DI water, and then dried at 60 °C. The resulting material was denoted Co-MOF@CC. Next, Co-MOF@CC was placed in a nitrogen atmosphere and heated to 800 °C at a rate of 4 °C/min. The sample was held at this temperature for 2 h before cooling to room temperature, and the resulting material was denoted N-C@CC. Subsequently, a mixed solution of 0.05 M FeCl₃·6H₂O and 0.05 M Na₂SO₄ (60 mL) was prepared and transferred into a 100 mL autoclave. A piece of N-C@CC was placed inside the autoclave, which was then sealed. The reaction was carried out at 120 °C for 6 h. After cooling to room temperature, the sample was removed, washed with DI water to remove impurities, and dried at 60 °C. Finally, the sample was placed in a tubular furnace and heated to 400 °C at a rate of 5 °C/min under a nitrogen atmosphere. The sample was held at this temperature for 1 h before cooling to room temperature. The final product was denoted FONC.

2.2. Synthesis of P-FONC

Initially, 0.416 g of p-toluenesulfonic acid (p-TSA) was dissolved in 20 mL of anhydrous ethanol and placed in an ice water bath. Under stirring, 0.1 mL of pyrrole was slowly added to the solution. Subsequently, a 20 mL aqueous solution containing 0.12 g of ammonium persulfate (APS) was added dropwise to the mixture. Finally, a piece of FONC was immersed in the solution and kept in the ice water bath for 6 h. To optimize the reaction time, samples with reaction durations of 4 h and 8 h in the ice water bath were also prepared. The samples were denoted P-FONC-xh (x = 4, 6, 8). Unless otherwise specified, P-FONC refers to the sample with a 6 h ice water bath reaction time.

2.3. Preparation of MnO₂@N-C

Briefly, 0.5 mL of strong hydrochloric acid was added to 35 mL of deionized water, followed by 1 mmol of potassium permanganate (KMnO₄) and 10 min of stirring. The mixed solution was then placed in a polytetrafluoroethylene liner. After immersing a piece of the previously prepared N-C/CC in the combined solution, the liner was placed in an autoclave reactor, sealed, and reacted at 85 °C for 20 min. After natural cooling, it was rinsed with deionized water and dried in a vacuum-drying oven at 60 °C. The finished product, MnO₂/N-C@CC, was thusly obtained.

2.4. Assembly of the Hybrid Supercapacitor

Based on the charge balance condition q⁺ = q⁻ and $C={\displaystyle \frac{I\times △\mathrm{t}}{S\times △V}}$, the sizes of the positive electrode material (MnO₂/N-C@CC) and the negative electrode material (P-FONC) were determined. For the aqueous device, both the positive and negative electrodes were clamped with Pt electrode holders and subjected to electrochemical performance testing using 1 M KOH solution as the electrolyte. As for the quasi-solid-state device, firstly, the PVA-KOH quasi-solid-state electrolyte was prepared following the method detailed in our previous publication [38,39]. Subsequently, P-FONC, MnO₂/N-C@CC, and a separator were individually immersed slowly in the PVA-KOH electrolyte. The components were then stacked in the order of P-FONC, separator, then MnO₂/N-C@CC and clamped between two glass plates. The assembly was left undisturbed at room temperature for 6 h to complete the process.

2.5. Electrochemical Measurements

The prepared samples, measuring 1 × 1 cm², were mounted on a Pt electrode holder in order to function as the working electrode. A Pt foil served as the counter electrode, while a Ag/AgCl electrode acted as the reference. A 1 M KOH solution was utilized as the electrolyte to evaluate the electrochemical performance of the CHI760E electrochemical workstation. The assessments included cyclic voltammetry (CV) tests at scan rates ranging from 5 to 100 mV s⁻¹ and galvanostatic charge–discharge (GCD) tests at current densities varying from 2 to 20 mA cm⁻². Furthermore, electrochemical impedance spectroscopy (EIS) was conducted across a frequency spectrum of 100 kHz to 0.01 Hz. Additionally, the cycling stability of the electrodes/devices was tested using the Landian V7.3.0.1 battery cycling system.

2.6. Materials Characterization

The surface morphology of the composites was observed using a scanning electron microscope (SEM, FEI Verios G4) (FEI Company, Columbia, MD, USA). The microstructure of the nanomaterials was further analyzed using transmission electron microscopy (TEM, FEI Talos F200X) (FEI Company, Columbia, MD, USA). The physical structure of the materials was analyzed using X-ray diffractograms (Bruker Corporation, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54 Å) as the target material and scanning angles from 10 to 80° at a tube voltage of 40 kV and a current of 40 mA with an advance-D8 X-ray diffractometer. In addition, X-ray photoelectron spectroscopy (XPS) analysis was carried out to further analyze the chemical composition and elemental state of the samples, using a PHI5400 device (PE Corp., London, UK). BET testing was performed with Micromeritics ASAP 2460 (Micromeritics Instrument Corporation, Atlanta, GA, USA).

3. Results and Discussion

3.1. Morphological and Structural Characterization

As schematically illustrated in Figure 1, the controllable synthesis of PPy/Fe₂O₃ on a Co-MOF@CC-derived N-C@CC substrate was performed via hydrothermal and ice bath procedures. Firstly, cobalt nitrate hexahydrate and dimethylimidazole were self-assembled to form a Co-MOF nanoflake precursor via a straightforward solution approach. After being calcinated under a N₂ atmosphere, the Co-MOF@CC was converted into N-C@CC nanoflake arrays [40,41]. Then, using sodium sulfate as the structure-inducing agent and ferric chloride hexahydrate as the Fe source, FONC was fabricated by hydrothermal and subsequent annealing techniques [42]. After several hours of immersion in an ice water bath solution containing a pyrrole monomer, a PPy protective layer was successfully coated on the surface of FONC, yielding P-FONC.

The chemical phases of a series of Fe₂O₃ samples were characterized by X-ray diffraction. As shown Figure 2, all exhibited clear and consistent diffraction peaks. The XRD analysis revealed that, apart from a characteristic peak at around 2θ = 26° attributed to carbon cloth, the remaining peaks corresponded to the standard PDF card of Fe₂O₃ (JCPDS No.: 33-0664), indicating that the preparation of P-FONC did not alter the original crystal structure of Fe₂O₃ [38]. To further investigate the surface chemical composition and valence bond states of the P-FONC composite, X-ray photoelectron spectroscopy (XPS) analysis was conducted. Figure 2b shows that the XPS pattern of the P-FONC sample only detected signals from Fe, O, C, and N elements, indicating the absence of other impurities. It should be noted that after high-temperature calcination of Co-MOF in an inert atmosphere, the cobalt content was converted into metallic cobalt nanoparticles, meaning that the calcined N-C@CC contained a small number of metallic cobalt nanoparticles. During the process of loading Fe₂O₃ onto N-C@CC, Fe³⁺ hydrolyzed in the aqueous environment, resulting in an acidic solution that etched the metallic cobalt nanoparticles in N-C@CC, forming a porous structure. Similarly, during the preparation of MnO₂/N-C@CC, the weakly acidic environment etched away the metallic cobalt nanoparticles. Additionally, the absence of cobalt signals in the XPS spectrum of P-FONC confirmed the complete etching of cobalt, consistent with the XRD results. Comparison of the XPS spectra of FONC and P-FONC revealed an additional N element signal peak in the latter, which was not observed in the former. This is because XPS is a surface-sensitive technique. In the FONC sample, Fe₂O₃ was attached to the N-C framework, making it difficult to detect the N element signal effectively. In contrast, the PPy layer on the surface of P-FONC, which is rich in nitrogen, allowed for clear detection of the N signal.

The microscopic morphologies of Co-MOF@CC, N-C@CC, and FONC were observed by SEM. As shown in Figure 3a–c the triangular-shaped Co-MOF@CC nanoarrays with smooth surfaces were vertically anchored on the carbon cloth fibers [43,44]. After calcination treatment, the Co-MOF flakes transformed into nitrogen-doped carbon (N-C) flakes with a rough surface but were still uniformly distributed. The flake structure of the N-C nanoarrays was well preserved and exhibited a thickness of approximately 100 nm and a lateral size of about 1.5 μm, as shown in Figure 3d–f. This structure served as an optimal secondary substrate for the deposition of active materials. During the calcination at elevated temperatures, the metal cobalt ions from Co-MOF were reduced to metal cobalt nanoparticles and partially aggregated on the carbonized N-C nanosheets, which caused the surface of the nanoflakes to become rough. Figure 3g–i present the SEM images of the FONC composite, intuitively displaying a large number of Fe₂O₃ nanowires uniformly grown on the N-C@CC nanosheet array framework, with lengths reaching up to 0.5 µm. This unique binder-free three-dimensional nanowire array structure effectively ensured a large specific surface area for FONC, facilitating the full contact between the active material and the electrolyte, thereby significantly enhancing the electrochemical reaction rate [45,46].

To further enhance the electrochemical performance of FONC, we attempted to modify it with the conductive polymer polypyrrole (PPy). Under ice water bath conditions and using ammonium persulfate, p-toluenesulfonic acid, and pyrrole as raw materials, a PPy coating was successfully deposited on the surface of FONC. As shown in Figure 4, a thin layer of PPy (about 20 nm in thickness) uniformly covered the surface of the FONC nanoneedles. The morphology of the P-FONC electrode remained largely unchanged after PPy coating, indicating that the PPy layer did not disrupt the original nanostructure of FONC. The PPy coating not only improved the electrical conductivity of the material but also enhanced its stability, thereby significantly boosting its capacitive performance [47]. To explore the optimal reaction time for the ice water bath process, samples with reaction durations of 4 h and 8 h were prepared, and their SEM images are provided in Supplementary Information Figure S1. As shown in Figure S1a–c, the morphologies of the samples with 4 h and 6 h reaction times were highly consistent. However, when the reaction time was extended to 8 h, a noticeable collapse of the Fe₂O₃ nanoneedles was observed. This phenomenon may be attributed to the prolonged ice water bath duration, which led to an excessively thick PPy coating that disrupted the arrayed structure of the iron Fe₂O₃ nanoneedles. Moreover, N₂ adsorption–desorption isotherms of P-FONC, FONC and FO were used to investigate the porosity of the samples. As shown in Figure S2, all three samples displayed type-IV isotherms, but curves of P-FONC and FONC exhibited an obvious hysteresis loop in the relative pressure range of 0.5–1.0 P/P0, indicating that the P-FONC and FONC samples possessed a larger amount of mesopores. Based on BET method, the calculated specific surface area was 266.77, 214.23, and 62.62 m² g⁻¹ for P-FONC, FONC, and FO, respectively. A high surface area means more accessible active sites and a shorter ion diffusion distance during the electrochemical processes, which endow the sample with favorable electrochemical performance.

3.2. Single-Electrode Electrochemical Performance

To evaluate the application of the P-FONC electrode in the field of energy storage, its electrochemical behaviors were investigated, and the results are presented in Figure 5. Figure 5a displays the CV curves of P-FONC-6h. As can be seen, the P-FONC-6h electrode displayed a distinct pair of redox peaks. Notably, as the scanning rate rose from 5 to 100 mV s⁻¹, the CV curves of the P-FONC-6h electrode retained their shape, indicating superior reversibility. On the other hand, Figure 5b presents the GCD curves at various current densities (2–20 mA cm^–2); all the curves exhibit quasi-triangular shapes, indicating that the electrode possessed good capacitive characteristics [18,48]. It is worth mentioning that the areal capacity of the P-FONC-6h reached 356 mF cm^–2 (143 F g⁻¹) at a current density of 2 mA cm^–2.

To further evaluate the practical feasibility of the modified P-FONC-6h electrode material, we compared it with PPy@CC (P), Fe₂O₃@CC (FO), and P-FONC electrode materials. Figure S3a displays all the related CV curves observed at a scan rate of 10 mV s⁻¹. Among all the electrode materials, the CV curve of P-FONC-6h exhibited the largest integrated area, signifying its superior areal capacitance. Moreover, the GCD curves at 5 mA cm⁻² are displayed in Figure 5d; all of the samples exhibited a quasi-triangular shape, indicating pseudocapacitance. Among them, the P-FONC-6h electrode exhibited the longest charge–discharge duration, which further confirmed its highest specific capacitance and validated the rationality and feasibility of our modification strategy. The conductivity characteristics of the different electrodes were analyzed using electrochemical impedance spectroscopy (EIS) in the frequency range of 100 kHz~0.01 Hz with an amplitude of 5 mV, as shown in Figure 5e (the small figure in the middle of the figure shows the equivalent circuit model). In the high-frequency region, the real axis intercept corresponds to the equivalent series resistance (Rs), and the semicircle radius corresponds to the indicated transfer resistance (Rct). In the low-frequency region, the slope of the line can be attributed to the Warburg resistance. A lower value of Rs for the P-FONC-6h electrode indicates a higher conductivity. This suggests that the composite electrode possessed the best electrical conductivity and that 6 h is the ideal duration of ice water bath treatment. Rate capability is another crucial parameter for evaluating the performance of electrochemical materials [49]. Figure 5f and Figure S3b present a comparison of the rate capability of several different samples. As shown in the figure, the P-FONC-6h electrode exhibited the highest area-specific capacitance at the same current density. At a current density of 3 mA cm⁻², the electrode presented a specific capacitance of 348.6 mF cm⁻² (or 140 F g⁻¹). Upon increasing the current density to 20 mA cm⁻², the electrode’s areal specific capacitance remained impressive at 278 mF cm⁻² (or 112 F g⁻¹), with a capacitance retention rate close to 80%. This indicates that the electrode possessed excellent rate performance. Notably, the samples coated with PPy exhibit a marked increase in specific capacitance compared to their uncoated counterparts, indicating that the application of a PPy coating serves as a potent method for boosting the capacitance properties of electrode materials. P-FONC-8h exhibited lower areal capacitance than P-FONC-6h, attributable to compromised structural integrity in its 3D hierarchical framework. In addition, we calculated the ‘b’ values of the prepared materials using the peak redox currents in the CV curves. In Figure S4, the calculated b values for the oxidation and reduction processes are 0.86 and 0.93. These findings suggest that the prepared materials exhibited properties closer to those of capacitively controlled charge storage processes. Meanwhile, we also compared the prepared electrode materials with other works (Table S1) and found that the as-prepared P-FNOC presented comparable electrochemical performance [38,50,51,52,53,54,55,56,57].

To match with P-FONC, a MnO₂/N-C@CC electrode was also prepared. Figure S5 displays SEM images depicting the arrays of MnO₂ nanosheets. As shown in the figure, the MnO₂ was composed of ultrathin nanosheets that were stacked and assembled together and densely adhered to the N-C@CC skeleton. The MnO₂/N-C@CC electrode’s electrochemical performance was assessed using a three-electrode setup. The CV curves maintained an approximately rectangular shape with various scan rates (Figure S6a), indicating favorable capacitive characteristics. The GCD curves showed virtually isosceles triangle-like curves over a current density range of 3–20 mA cm⁻² (Figure S6b), indicating good pseudocapacitive properties. Furthermore, the EIS of the MnO₂/N-C@CC electrode was examined (Figure S6c). The small charge transfer resistance (Rct) on the real axis and the near-vertical profile in the low-frequency region of the EIS curve indicate that the MnO₂/N-C@CC material has low ion diffusion resistance and high electronic conductivity. These results collectively demonstrate the favorable electrochemical performance of the MnO₂/N-C@CC electrode.

3.3. Electrochemical Performance of the Device

To explore the potential applications of the P-FONC, an aqueous hybrid supercapacitor was assembled using P-FONC-6h as the negative electrode, MnO₂/N-C@CC as the positive electrode, and a 1 M KOH solution as the electrolyte. Figure 6 displays the electrochemical characterization results of these aqueous hybrid supercapacitors. Figure 6a,b display CV and GCD curves in a voltage window ranging between 0–1.6 V and 0–2.6 V. The CV and GCD results show that the aqueous device operated consistently within a voltage range of 0–2.6 V, indicating that expanding the device’s working voltage to 0–2.6 V is rational. Figure 6c presents the CV curves of the hybrid supercapacitor throughout its scan rate range of 10–200 mV s⁻¹. The CV curves exhibited a certain degree of rectangularity and distinct redox peaks, suggesting that the device possessed both electric double-layer capacitance and Faradaic pseudocapacitance mechanisms. The GCD curves in Figure 6d show a typical capacitive characteristic, and based on the discharge time, the device attained a specific capacitance of 173 mF cm⁻² (70 F g⁻¹) at a density of current of 4 mA cm⁻². Specifically, the device can provide an energy density of up to 0.1625 mWh cm⁻² at a power density of 5.2 mW cm⁻². When the power density increased to 52 mW cm⁻², the energy density remained at 0.026 mWh cm⁻². These values significantly surpass those reported in many similar systems, such as Fe₂O₃NTs@PPy//MnO₂ (0.0594 mWh cm⁻², 1 mW cm⁻²) [50], UiO66/PPY (0.0128 mWh cm⁻², 2.1 mW cm⁻²) [58], NiO/Ni(OH)₂/PEDOT (0.011 mWh cm⁻², 33 mW cm⁻²) [59], Cu(OH)₂/CC (0.049 mWh cm⁻², 0.6 mW cm⁻²) [60], CF/MoO_3//CF/MnO₂ (0.0027 mWh cm⁻², 0.35 mW cm⁻²) [61], and MnO₂//Fe₂O₃@FeSe₂ (0.086 mWh cm⁻², 2 mW cm⁻²) [62]. This comparison demonstrates our aqueous device’s better performance in terms of energy and power density. Figure 6g shows the aqueous device’s long-term cycle stability. After 1000 cycles, the device kept 70% of its initial capacitance while maintaining a Coulombic efficiency close to 100%, showing that the MnO₂/N-C@CC//P-FONC has remarkable cycling stability. Furthermore, to demonstrate the potential applicability of MnO₂/N-C@CC//P-FONC in the field of flexible energy storage, quasi-solid-state supercapacitors were assembled. When two assembled quasi-solid-state devices were connected in series, they successfully powered a red light-emitting diode (LED) under both flat and bent conditions, with the LED brightness remaining constant (Figure 6h). This result highlights the potential feasibility of the assembled quasi-solid-state device as an electrochemical energy storage component for wearable applications.

4. Conclusions

In summary, this study proposed a solution to the low specific capacity of Fe₂O₃ as a supercapacitor anode material by constructing composite-structured materials. Using Co-MOF as a precursor, a three-dimensional hierarchical nanostructured array material was fabricated, and PPy was employed as a protective coating on the surface of this nanostructured electrode array. The Co-MOF-derived three-dimensional N-C nanosheets exhibited porosity and good electrical conductivity, serving as a secondary substrate for loading active materials to form a three-dimensional hierarchical nanostructured array. This design not only increased the amount of active materials that could be loaded but also facilitated full contact between the electrolyte and the electrode materials, thus accelerating electron transfer and the ion diffusion rate. Furthermore, the PPy coating not only increased capacity but also served as a protective layer, improving the structural stability of the electrode material. The P-FONC electrode displayed a remarkable specific capacitance of 356.6 mF cm⁻² at a current density of 2 mA cm⁻². Overall, this project demonstrated that the efficient ice water bath method for coating a PPy protective layer on the surface of FONC is an effective approach that can enhance the capacitance of this material. This strategy provides a viable pathway for the preparation of materials for flexible energy storage devices.