Investigation of Supercapacitor Electrodes Based on MIL-101(Fe) Metal-Organic Framework: Evaluating Electrochemical Performance through Hydrothermal and Microwave-Assisted Synthesis

Abstract

Supercapacitors have garnered substantial interest owing to their capacity to deliver power effectively for short-term applications. However, current supercapacitors suffer from limited stability and low-capacity storage. Metal-organic frameworks (MOFs) have emerged as a promising solution due to their high surface area and abundant active redox sites. MOF-based electrodes combined with aqueous based electrolytes have shown potential to enhance supercapacitor performance. While there is limited literature on MIL-101(Fe) MOF-based electrodes, a comparative study was conducted to investigate the supercapacitor performance of MIL-101(Fe) electrodes synthesized using hydrothermal and microwave-assisted processes. Processing parameters, such as the method used, alter the microstructure, morphology, and uniformity of supramolecular chemistry, impacting electrochemical characteristics. This study aimed to determine the active redox reactions, chemical stability, surface area, adsorption characteristics, and electrochemical characteristics of the electrodes. The electrodes from hydrothermal synthesis [MF(ht)] exhibited excellent electrochemical activity in comparison to the microwave-assisted [MF(m)] electrodes in the three-electrode configuration. At a high current density of 7 A/g, the MF(ht) electrode displayed a remarkable specific capacitance of 775.6 F/g and a good cyclic stability (82% @ 10 A/g) after 5000 galvanostatic charge–discharge cycles. At a current density of 1 A/g, the two-electrode configuration of MF(ht) yielded a high energy density of 74.7 Wh/kg at a power density of 2160 W/kg and a decent cyclic stability after 5000 cycles. The results suggest that the MF(ht) electrodes possess remarkable electrochemical properties that make them a promising candidate for advanced applications in energy storage.

1. Introduction

As modern society shifts towards a future powered by renewable energy sources, the focus of global energy consumption is moving away from rapid utilization of fossil fuels and towards the use of renewable sources such as solar, wind, hydraulic, and ocean energy [1,2]. This shift has intensified the research efforts towards the development of advanced sustainable energy storage devices [3]. Supercapacitors (SCs) are among the most promising energy storage devices due to their high power density [4,5,6,7,8]. However, their low energy density presents a major challenge to their commercialization. To overcome the energy density limitation, researchers have explored various electrode materials for SCs, including metal oxides and carbon-based materials [9]. While these materials have shown some improvements in energy density, they often suffer from poor stability and low capacitance.

Metal-organic frameworks (MOFs) have emerged as a solution to these challenges, as they possess intriguing properties such as a greater surface area, porous structure, and abundant active redox sites [10,11,12]. MOFs are highly structured materials composed of metal ions or clusters (nodes) linked by organic ligands (linkers), resulting in unique and customizable porous structures [13]. When used as electrodes for supercapacitors, the presence of metal centers may enhance their ability to store energy through pseudocapacitance. However, the unpredictable orientation of the MOFs, as well as their low conductivity and tendency to aggregate, may limit their potential applications [14]. Therefore, significant attention has been paid to developing improved MOF preparation techniques to enhance their structure and properties. These efforts aim to overcome the limitations associated with MOFs and unlock their full potential as energy storage devices. To gain a fundamental understanding of MOF materials, it is imperative to explore their crystallographic data, including unit cell dimensions, space group symmetry, and atomic coordinates within their crystal lattice. This structural information provides valuable insights into the arrangement of atoms or molecules in MOFs, which, in turn, influences their physical and chemical properties.

Performance is a critical characteristic of supercapacitors, and there are several methods to enhance the conductivity of MOFs and to optimize their performance with techniques such as combining MOFs with aqueous electrolytes, doping with conductive materials, and fabrication techniques. MOFs, as electrode materials, offer the potential for enhancing supercapacitor performance, particularly when combined with aqueous electrolytes [15,16]. However, it has been observed that the I-V characteristics of MOF-based electrodes often exhibit multiple broad peaks, indicating the occurrence of surface redox reactions. To make these redox peaks more distinctive, increasing the surface area of the MOF-based electrodes is essential [17,18,19,20]. This can be achieved through a combination of different materials or by adopting suitable processing methodologies. Researchers have been actively exploring the development of MOF-based or MOF-derived active materials to investigate their suitability for energy storage applications. For example, in a study conducted by Guo et al., a MOF-polyaniline sandwich-like composite was prepared using a successive oil bath method, carbonization, and polymerization. The resulting material exhibited an impressive maximum specific capacitance of 477 F/g at a current density of 1 A/g [21]. Similarly, Cao et al. synthesized a cobalt-doped Cu-MOF/Cu₂₊₁O hybrid material and investigated its performance as a supercapacitor. The hybrid material demonstrated a remarkable specific capacitance of 518.5 F/g at a current density of 2 mA/cm² [22]. In another study, Shao et al. achieved a high specific capacitance of 414.50 F/g at 0.50 A/g using nitrogen (N)-contained Co-MOF nanorods [23]. Several other MOF-based or MOF-derived active materials have been extensively investigated for their potential in supercapacitor applications, including Co-based MOFs [24], Zn-based MOFs [25], Ni-based MOFs [26], Cd-based MOFs [27], Mn-based MOFs [28], and bimetallic MOFs such as Co/Zn [29], among others. The literature suggests that processing parameters play a crucial role in achieving desired properties for electrode materials. The choice of processing method has a significant impact on the microstructure, including particle shape and size, material porosity, and the uniformity of supramolecular chemistry, ultimately affecting the electrochemical characteristics. In the aforementioned electrode materials, the most commonly employed synthesis method is solvent-based hydrothermal processing, which offers the advantage of tailoring the morphology by adjusting the pressure based on the vapor pressure of the main components involved in the reaction [30,31]. Additionally, microwave synthesis, although a chemical solution-based approach, provides uniform heating of the particles, enabling the attainment of desired characteristics with greater uniformity. Furthermore, microwave synthesis offers the advantages of faster and more homogeneous heating of precursor materials [32,33]. These considerations highlight the importance of selecting the appropriate synthesis method to achieve the desired properties and enhance the electrochemical performance of electrode materials for supercapacitor applications.

In light of the aforementioned justifications and motivations, we have undertaken the synthesis of MIL-101 (Fe) using two distinct methodologies, namely hydrothermal [designated as MF(ht)] and microwave-assisted [designated as MF(m)]. Our objective was to manipulate the morphology of MOF structures by subjecting them to identical temperature conditions (120 °C). Subsequently, the synthesized samples were evaluated under both three-electrode and two-electrode configurations to discern the explicit impact of morphology on the electrochemical performance of the supercapacitor. Surprisingly, MF(ht) exhibited superior characteristics in terms of high-performance supercapacitor behavior compared to MF(m). The underlying reasons for this disparity will be comprehensively discussed in the subsequent sections. Nevertheless, it is noteworthy that the sample prepared via microwave-assisted synthesis technique demonstrated comparable or even superior electrochemical properties compared to the hydrothermal synthesis technique employed by other researchers [34]. Specifically, both MF(ht) and MF(m) electrodes exhibited remarkable specific capacitances of 775.6 F/g and 403 F/g, respectively, even at a high current density of 7 A/g. Furthermore, they demonstrated excellent cycling rate capabilities with retention rates of 82% and 71% after 5000 charge–discharge cycles, respectively. Additionally, a two-electrode configuration was assembled using MF(ht) electrodes, yielding impressive energy and power densities of 74.7 Wh/kg and 2160 W/kg, respectively, alongside decent cycling stability after 5000 cycles. Consequently, the methodology employed in this study offers a transformative blueprint for the conceptualization of cutting-edge nanoarchitectures, heralding captivating prospects for fabricating unprecedented nanostructures dedicated to forthcoming energy storage endeavors.

2. Experimental Section

2.1. Material Synthesis

A total of 0.427 g (2.57 mmol) of Terephthalic acid and 1.461 g (5.4 mmol) of FeCl₃·6H₂O were dissolved in 30 mL of N,N-dimethylformamide (DMF) (solution A). The same chemical proportions were used to form solution B. Both of these solutions were stirred for 1 h individually to form two homogeneous solutions. Solution A was transferred into a Teflon-lined container and placed inside an Analytik Jena (Top-wave-CX-100, Jena, Germany) microwave digestive reactor. The container was then heated to a temperature of 120 °C for a period of 40 min, with a gradual increase in temperature at a rate of 3 °C/min. Similarly, solution A was transferred to Teflon-lined autoclave and processed hydrothermally at 120 °C for 24 h. After allowing both solutions A and B to cool naturally, they underwent a similar process of washing and drying. The resulting precipitates from solution A and solution B, obtained through centrifugation, were individually subjected to multiple washings using deionized water and ethanol. Subsequently, the precipitates from both solutions were subjected to overnight desiccation within a hot air oven at a temperature of 80 °C. The final powder obtained from hydrothermal synthesis and microwave-assisted synthesis are designated as MF(ht) and MF(m), respectively. The schematic for the synthesis procedure is given in Figure 1.

2.2. Characterization Techniques

The crystal structures of MF(m) and MF(ht) were thoroughly examined using powder X-ray diffraction (XRD) analysis. The XRD patterns were recorded using a PANalytical X’pert PRO instrument (Almelo, The Netherlands) equipped with X’Pert Industry software for accurate Bragg measurements. The X-ray source utilized Cu Kα radiation (λ = 1.54056 Å) and was operated at 40 kV and 30 mA. The morphologies of the samples were investigated using a field emission scanning electron microscope (FESEM) model Hitachi S4800 (Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM) model Tecnai G² F30 S-Twin. This high-resolution imaging technique allowed for a detailed characterization of the sample’s surface and morphology. Raman spectra of materials were recorded using a Compact Raman Microscope model NS200-Raman/633 nm (Nanoscope Systems, Daejeon, Republic of Korea). For assessing the chemical states of the samples, X-ray photoelectron spectroscopy (XPS) was employed. The XPS analysis was performed using a Thermo Fisher Scientific MultiLab 2000 instrument (Manchester, MA, USA). The X-ray monochromatic Al Kα radiation (1486.6 eV) was used to probe the chemical compositions and states of the samples. To determine the Brunauer–Emmett–Teller (BET) surface area of the samples, the micromeritics ASAP 2020 Plus instrument (Santa Clara, CA, USA) was employed. The BET surface area provides valuable information about the specific surface area and porosity of the samples.

2.3. Preparation of Electrodes

The working electrode was fabricated through a drop-casting technique, wherein a precisely crafted slurry was meticulously administered onto a substrate composed of Ni foam. The slurry itself was concocted by amalgamating the active material with carbon spheres, serving as an electrically conductive supplement, in a proportionate weight ratio of 8:2. In light of achieving a uniform amalgamation, the slurry was blended with ethanol. Following this, the slurry was meticulously deposited onto a Ni foam substrate that was thoroughly cleaned and weighed beforehand, and which possessed an active surface area measuring 1 × 1 cm². Lastly, the manufactured electrodes were subjected to an overnight drying process in an oven, maintaining a stable temperature of 80 °C.

2.4. Electrochemical Characterizations

The three-electrode configuration utilized in this study consisted of several components. The Ag/AgCl reference electrode was employed, while a platinum wire served as the counter electrode. The working electrode involved the drop-casting of an active material (such as MF(ht) and MF(m)) onto a Ni foam substrate. The working electrode was submerged within an electrolytic solution comprising 3.0 M KOH. The mass loading of the active material on the working electrode was approximately 1 mg. To explore the electrochemical attributes of the produced electrodes, a sequence of examinations was carried out employing the SP-200 Bio-Logic instrument (Seyssinet-Pariset, France). The chosen techniques for this investigation included cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). These electrochemical methods provided valuable insights into the performance and behavior of the fabricated electrodes.

2.5. Symmetric Supercapacitor Preparation and Testing

The symmetric supercapacitor (SSC) device configuration involved the utilization of MF(ht) drop-cast electrodes on Ni foam substrates. These electrodes were submerged within an electrolytic solution comprising 3.0 M KOH. Each electrode was loaded with an approximate active mass of 0.5 mg. To thoroughly assess the performance of the SSC device, comprehensive measurements were conducted. Cyclic voltammetry (CV) measurements were executed within a potential window ranging from 0 to 1.2 V, employing a range of scan rates, specifically varying from 5 to 80 mV/s. Galvanostatic charge–discharge assessments were conducted across a spectrum of current densities, ranging from 1 to 1.75 A/g, encompassing a voltage range of 0 to 1.2 V. Additionally, electrochemical impedance spectroscopy (EIS) measurements were executed to delve into the electrochemical characteristics of the SSC device. These measurements were conducted at an open-circuit voltage (OCV) across a frequency range spanning from 20 kHz to 0.1 Hz.

To determine the specific capacitance, Galvanostatic charge–discharge (GCD) characterizations were carried out for both configurations (three-electrode and device). The specific capacitance was calculated utilizing the following formula:

$$C_{sp}=\int \frac{I·dt }{m·dV}$$

(1)

where the specific capacitance ($C_{sp}$) is expressed in farads per gram (F·g⁻¹), while the applied current (I) is measured in amperes (A). The mass of the active material (m) is denoted in grams (g), the discharge time (dt) is quantified in seconds (s), and the range of the operating voltage (dV) is given in volts (V).

The values corresponding to the energy (E_d) and power (P_d) densities were obtained by utilizing the subsequent equations:

$$E_d=\frac{1}{2}{C}_{sp}{\left(dV\right)}^2$$

(2)

$$P_d=\frac{E_d}{dt}$$

(3)

The energy and power densities were expressed in Wh·kg⁻¹ and W·kg⁻¹, respectively.

3. Results and Discussion

3.1. Morphological, Structural, and Chemical Characteristics

The crystallinity of MF(m) and MF(ht) samples were analyzed using X-ray diffraction (XRD) analysis, as displayed in Figure 2a. The recorded patterns from XRD exhibited normal characterization peaks at 8.9°, 10.0°, and 12.5° for Fe-based MIL at high intensity, indicating the high crystallinity of MF(m) and MF(ht) samples [35,36]. All of the peaks formed for MF(m) and MF(ht) confirm the well-formed nature of the crystals, supported by CCDC number 605510 [37,38].

The morphological characteristics of MF(m) and MF(ht) were determined through FESEM micrographs, as depicted in Figure 2b,c. The SEM images reveal the typical octahedral morphology of MF(m) and MF(ht). Notably, the samples synthesized using microwaves display a significantly higher degree of porosity within their octahedral structures. The highly porous MF(m) and non-porous MF(ht) can also be seen in the schematic in Figure 2d. The porosity of the MF(m) sample suggests that, during the synthesis process, the nucleation sites in the MF(m) structure are activated at a quicker rate and with a greater frequency compared to the MF(ht) samples, leading to the creation of a material characterized by increased porosity [39]. Moreover, a comprehensive assessment of the surface morphology of both MF(m) and MF(ht) was conducted using high-resolution transmission electron microscopy (HR-TEM). Figure S1a shows the morphology of MF(m) with further magnification presented in Figure S1b. These images show distorted octahedral morphology. Figure S1b shows MF(ht) with octahedral morphology, as confirmed in SEM. Further magnifications reveal that the octahedral structure of MF(ht) is formed through a stacking of individual layers. In addition to the identification of MIL-101(Fe) through Raman spectroscopy, the vibrational bands observed at 1607 and 2327.1 cm⁻¹, as depicted in Figure S1e, provide critical insights into the material’s structural characteristics. These specific bands can be attributed to the distinctive vibrational modes of MIL-101(Fe), shedding light on its composition and confirming its presence in the analyzed sample [40].

The porosity and specific surface area were further characterized using the BET (Brunauer–Emmett–Teller) analysis method (Figure 3). The nitrogen adsorption–desorption isotherm of the samples exhibits a characteristic Type IV hysteresis, featuring a loop at a relative pressure range of 0.9 to 1.0, indicating the presence of mesopores, as shown in Figure 3a. This mesoporous structure facilitates the reduction in electron transport distance and enhances the efficiency of the electron transport process [41]. As given in Figure 3b, the pore diameter of MF(m) and MF(ht) is 128.814 Å and 49.005 Å, respectively. These results are also consistent with the FESEM results. It can be observed that the BET specific surface area of MF(ht) (545.4 cm²/g) is 3.7 times higher than that of the MF(m) (147.2 cm²/g) sample (see Figure 3c). This is due to the fact that the temperature-induced phase may have been attributed towards the growth of the pore diameter and hence resulted in a decreased surface area for the MF(m) sample.

Further, the chemical states of the MIL-101(Fe) nanostructures were investigated using X-ray photoelectron spectroscopy (XPS) characterization, which can be seen in Figure 4. The survey spectrum of the XPS in Figure 4a displays the C, O, and Fe elements that existed in both the MF(m) and MF(ht) samples. The high-resolution spectra revealed consistent features in both samples, specifically in the Fe2p region of MIL-101(Fe) (Figure 4b,c). The Fe2p_3/2 and Fe2p_1/2 peaks were observed at energy values of 710.6 eV and 723.9 eV, respectively, with a separation of 13 eV. The Fe2p_3/2 peak exhibited three sub-peaks at 710.6 eV, 714.3 eV, and 718.1 eV, while the Fe2p_1/2 peak showed two sub-peaks at 723.9 eV and 728.2 eV. These peaks were assigned to Fe³⁺ within the MIL-101(Fe) structure. Additionally, a distinct peak at approximately 718 eV was attributed to Fe²⁺, indicating the presence of divalent iron species. The presence of Fe²⁺ in MIL-101(Fe) contributes to enhanced catalytic activity in heterogeneous reactions and potentially improves the performance of energy storage devices [42]. In the spectra of O1s (Figure 4d,e), three distinct peaks were observed for both MF(m) and MF(ht) at energy values of 529.6 eV, 530.8 eV, and 531.9 eV. These peaks were attributed to Fe–O, –COOH, and –OH functional groups, respectively, which is congruent with the previously reported findings [43]. The spectra of C1s exhibited three distinctive peaks at energy values of 284.6 eV, 285.8 eV, and 288.4 eV. These peaks were assigned to different carbon species (C–H/C–C, C–O, and O–C=O, respectively), as depicted in the accompanying Figure 4f,g. The peak at 284.6 eV corresponds to carbon atoms bonded to hydrogen or carbon atoms in hydrocarbon chains. The peak at 285.8 eV signifies the presence of carbon atoms bonded to oxygen atoms in various functional groups, such as carbonyl (C=O) or hydroxyl (C–OH) groups. The peak at 288.4 eV indicates the presence of carbon atoms in carboxylate (O–C=O) or ester (O–C–O) functional groups. These assignments are consistent with the existing literature and provide insights into the chemical composition and bonding environment of the samples [43].

3.2. Supercapacitor Electrode Performance Evaluation

The electrochemical behavior of MF(m) and MF(ht) electrodes was systematically studied under ambient conditions employing cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). These investigations were conducted in a three-electrode system, utilizing a 3.0 M KOH aqueous electrolyte solution.

Figure 5a,d shows the CV profiles of MF(m) and MF(ht) samples at various scan rates from 5 to 100 mV/s in a voltage range of 0.1 to 0.46 V, revealing a shift in the cathode and anode peaks towards negative and positive potentials, respectively. The observed separation between the cathodic and anodic peaks in the electrochemical response can be ascribed to the inherent nature of the quasi-reversible process, which arises due to electrode polarization. This phenomenon is indicative of the complex interplay between the electrochemical reactions occurring at the electrode–electrolyte interface and the associated polarization effects [6]. A comprehensive examination of the electrochemical mechanism of the synthesized materials was undertaken to further corroborate their attributes governed by diffusion-controlled kinetic characteristics. Figure 5b,e illustrates the oxidative and reductive peak currents as functions of scan rates for the MF(m) and MF(ht) samples, respectively. The observed linear relationship in the oxidative and reductive peak currents of these samples signifies a surface-controlled electrochemical behavior resulting from the combined influences of pseudocapacitance and electric double-layer capacitance (EDLC) effects [44]. To further validate this, we employed the power law (Equation (4)) and the modified power law (Equation (5)), as depicted in Figure 5c,f.

$$I=a{v}^b$$

(4)

In accordance with the power law (where I = peak current in A, ν = applied scan rate in V/s, and a, b = variable parameters), we calculated the ‘b’ values for the prepared materials using the redox peak currents from the CV curves. The ‘b’ value in this formula typically falls within the range of 0.5 to 1. For a, the ‘b’ value falling around 0.5 signifies a dominant mechanism of the diffusion-type process, whereas it suggests a dominant surface-controlled charge storage method for a value of 1. To discern the nature of the charge storage mechanism, we constructed linear curves representing log(I) vs. log(ν) for both MF(m) and MF(ht) samples. In Figure 5c, the calculated b values during the oxidation and reduction processes were 0.56 and 0.66 for MF(m); in Figure 5f, they were 0.65 and 0.80 for MF(ht). These findings suggest that the prepared materials demonstrate characteristics more closely aligned with dominant diffusion-controlled charge storage processes.

$$I=k_{1}v+k_2{v}^{1/2}$$

(5)

Furthermore, we modified the power law to quantitatively assess the amount of charge accumulated by capacitive-type ($k_{1}v$) and diffusion-type ($k_2{v}^{1/2}$) controlled processes. The comprehensive solution to the provided power law equations is elaborated upon elsewhere [45]. For scan rates ranging from 5 to 100 mV/s, the contribution of diffusion to the overall current for the MF(m) electrode, as illustrated in Figure 5g, was found to be 95%, 92%, 90%, 86%, 83%, 81%, and 79%, respectively. Likewise, for the MF(ht) electrode, as depicted in Figure 5h, the diffusion contribution at scan rates of 5, 10, 20, 40, 60, 80, and 100 mV/s amounted to 85%, 80%, 73%, 66%, 61%, 58%, and 55% of the total current. These findings provide strong evidence that the diffusion-controlled mechanism significantly governs the charge storage performance of both the MF(m) and MF(ht) electrodes. Following this, an extensive series of 5000 cyclic voltammetry (CV) cycles (as depicted in Figure S2) were executed to scrutinize potential variations in the redox reactions as the number of cycles progressed. This meticulous examination was carried out to attain a profound understanding of the enduring stability and enduring performance of both electrodes. Strikingly, no discernible deviations or fluctuations were observed in the redox reactions throughout the course of numerous CV cycles, signifying the electrodes’ steadfast and unwavering electrochemical behaviors over an extended duration.

The response times pertaining to the charge and discharge processes of the electrodes were rigorously examined through the utilization of GCD measurements. These measurements are comprehensively illustrated in Figure 6a,b. The GCD profiles of the electrodes were systematically analyzed across a spectrum of current densities, ranging from 7 to 10 A/g, while maintaining a consistent voltage window within the range of 0.1 to 0.46 V. The rapid charge and discharge capabilities are a highly desirable feature of electrochemical capacitors. Consequently, the MF(m) and MF(ht) electrodes are subjected to varying current densities, as illustrated in Figure 6a,b. The non-linear GCD profiles, evident in both the MF(m) and MF(ht) electrodes, offer compelling corroboration of the concurrent kinetics of faradaic oxidation and reduction. This alignment closely corresponds with the observed redox behavior showcased in the CV curves. Notably, the MF(ht) electrode demonstrated a prolonged GCD duration when compared with the MF(m) electrode. This enhanced GCD performance can be attributed to the non-porous octahedral morphology and the augmented surface area inherent to the MF(ht) electrode. Conversely, the diminished GCD values observed for the MF(m) electrode may be attributed to its higher porosity, which subsequently diminishes the overall surface area available for charge storage. Leveraging the derived discharge times, the specific capacitance is calculated and compared, as depicted in Figure 6c. As expected, the MF(ht) electrode exhibited higher specific capacitance of 775.6 F/g at a current density of 7 A/g when compared with the MF(m) electrode which exhibited a specific capacitance of 403 F/g. The MF(ht) electrode exhibited specific capacitances of 691.9, 647.7, and 614.5 F/g at current densities of 8, 9, and 10 A/g, respectively, as opposed to the MF(m) electrode which exhibited lower specific capacitances of 383.5, 356.6, and 336.0 F/g. In order to gain insights into the short-term stability of the active materials, a series of five cycles were administered to the electrodes at each specific current density, as illustrated in Figure 6d. It is evident that the electrodes are stable for short-term cycling stability and show approximately the same specific capacitances for 5 cycles at the respective current densities. However, to comprehensively assess the long-term cycling stability, it becomes imperative to subject the system to an extensive number of GCD cycles. In this particular instance, a total of 5000 GCD cycles were meticulously executed, all at a consistent current density of 10 A/g (Figure 6e). Both electrodes consistently demonstrate a coulombic efficiency of 100% throughout the course of 5000 GCD cycles. Even after an extended cycling period, it is noteworthy that the MF(ht) electrode displayed a remarkable capacitance retention of 82%, corresponding to 492 F/g, outperforming the MF(m) electrode, which exhibited a capacitance retention of 71%, equivalent to 233 F/g (Figure S3).

The investigation into the charge transfer kinetics of the fabricated MF(m) and MF(ht) electrodes was carried out using electrochemical impedance spectroscopy (EIS), as elucidated in Figure 6f and Figure S4. EIS analyses were conducted on both the pristine electrodes and those subjected to 5000 GCD cycles, as demonstrated in Figure S4a,b. The impedance plots were effectively fitted and are illustrated in Figure S4c, with corresponding parameters meticulously detailed in

Table 1. The fitted impedance parameters of MF(m) and MF(ht) electrodes tested in three-electrode configuration.

Electrode	R1 (Ω)	R2 (Ω)	Q2 (mF·s(α−1))	C3 (mF)
MF(m)	0.63	39.4	3.8	5
MF(ht)	0.42	4.5	3.4	15

. To model the Nyquist plots of all of the electrodes, an equivalent circuit model was employed encompassing various elements, including the internal resistance (R1), charge transfer resistance (R2) at the electrode–electrolyte interface, the constant phase element representing the double-layer capacitance (Q2), Warburg impedance (W2), and pseudocapacitance (C3). The determined R1 values stood at 0.63 Ω for MF(m) and 0.42 Ω for MF(ht), implying favorable interactions between the electrolyte, nickel foam, and the active electrode material. Th charge transfer resistance (R2) for MF(m) was found to be 39.4 Ω, which is notably higher than that of MF(ht) (4.5 Ω). This discrepancy can be attributed to the highly porous octahedral structure of MF(m), which is less conducive to ion motion compared to the non-porous octahedral configuration of MF(ht). In terms of double-layer capacitance (Q2), MF(m) exhibited a value of 3.8 mF·s^(α−1), while MF(ht) showed a value of 3.4 mF·s^(α−1). This parameter highlights the temporary insulating layer formed at the solid–liquid interface. It effectively isolates the charged electrode from the electrolyte, thereby aiding in the accumulation of charge within the double layer. Additionally, the pseudocapacitance (C3) values for the MF(m) and MF(ht) electrodes were determined as 5 mF and 15 mF, respectively. The superior supercapacitive properties observed for MF(ht), as indicated by the EIS analysis, closely align with the findings from the GCD and CV data, further substantiating the enhanced performance of the MF(ht) electrodes.

The main difference in evaluating the electrochemical performance between these two methods can be summarized as follows:

(a.)

Morphological Differences: The most prominent difference between the two synthesis methods lies in the resulting morphologies of the MIL-101(Fe) MOFs. MF(ht) exhibited a non-porous octahedral morphology, while MF(m) featured a highly porous structure. This distinction in morphology significantly influenced the electrochemical behavior of the electrodes.

(b.)

Charge Storage Mechanism: Through our electrochemical analysis, we found that both MF(ht) and MF(m) electrodes demonstrated characteristics closely aligned with a diffusion-controlled charge storage mechanism. This indicates that the charge storage in both electrodes is primarily governed by the kinetics of ion diffusion. However, the degree of dominance of the diffusion-controlled mechanism varied slightly between the two methods.

(c.)

Performance Discrepancies: Despite both synthesis methods showing diffusion-controlled charge storage, MF(ht) outperformed MF(m) in terms of electrochemical performance. MF(ht) exhibited higher specific capacitance, more extended charge–discharge durations, and better capacitance retention during extended cycling. These differences can be attributed to the non-porous octahedral morphology of MF(ht) and its superior surface area.

(d.)

Kinetics of Faradaic Reactions: Both MF(m) and MF(ht) electrodes displayed non-linear GCD profiles, indicating concurrent kinetics of faradaic oxidation and reduction. The prolonged GCD duration observed for MF(ht) is associated with its non-porous octahedral morphology and augmented surface area, which promotes charge storage.

(e.)

Impedance Analysis: EIS analysis further substantiated the superior performance of the MF(ht) electrodes. The charge transfer resistance (R2) for MF(m) was notably higher than that of MF(ht), indicating less conducive ion motion in the highly porous MF(m) structure

(f.)

Pseudocapacitance: Both electrodes exhibited pseudocapacitance. However, the value for MF(ht) was higher than that of MF(m), reflecting the enhanced charge accumulation within the double layer at the solid–liquid interface of the former electrode.

(g.)

Extended Cycling Stability: MF(ht) demonstrated remarkable capacitance retention even after 5000 charge–discharge cycles, surpassing the performance of MF(m) in terms of capacitance retention.

3.3. Supercapacitor Two-Electrode Device Performance Evaluation

To provide a precise assessment of the electrochemical performance demonstrated by the MF(ht) electrode material, a symmetric supercapacitor (SSC) device was meticulously constructed, employing MIL-101(Fe) that was synthesized through hydrothermal means as the electrode material. The cyclic voltammetry (CV) profiles for the developed SSC are meticulously illustrated in Figure 7a, spanning a voltage range from 0 to 1.2 V and incorporating scanning rates ranging from 5 to 80 mV/s. It is noteworthy that the CV plots manifest distinct rectangular profiles characterized by well-defined redox peaks, indicative of the presence of electric double-layer capacitance (EDLC) behavior. The existence of both reduction and oxidation peaks further signifies the occurrence of pseudocapacitive behavior within the SSC. Remarkably, even at higher scanning rates, these CV profiles maintain their rectangular shapes, underscoring the superior rate capability of the assembled SSC. The preservation of these rectangular profiles underlines the SSC’s capacity to uphold its electrochemical performance at elevated scanning rates, emphasizing its potential suitability for high-rate applications [46]. Figure 7b portrays the oxidative and reductive peak currents in relation to varying scan rates for the symmetric supercapacitor (SSC) device. The discerned linear relationship observed in the oxidative and reductive peak currents within these datasets indicates a surface-controlled electrochemical behavior, arising from the combined effects of pseudocapacitance and the EDLC phenomena. Figure 7c presents the GCD plot of the SSC device using MF(ht) electrodes, covering a voltage range spanning from 0 to 1.2 V. This comprehensive analysis spans four distinct current densities (1, 1.25, 1.5, and 1.75 A/g). Across this range, noteworthy discharge durations are evident, affirming the favorable performance of the SSC device and underscoring its potential for exceptional performance in energy storage applications [47]. At a current density of 1 A/g, the developed MF(ht) SSC device achieved a reliable specific capacitance of 103.8 F/g, as vividly depicted in Figure 7d. However, with an increase in the current density to 1.75 A/g, a corresponding reduction in specific capacitance to 19.8 F/g was observed. These results distinctly highlight the direct correlation between specific capacitance and the applied current density. To evaluate the long-term capacitance retention of the fabricated MF(ht) SSCD, a GCD test was conducted over an extensive duration spanning 5000 cycles, while maintaining a consistent current density of 1.75 A/g. The outcomes, showcased in Figure 7e, substantiate that even after subjecting the fabricated SSCD to 5000 cycles of GCD, the capacitance retention remained robust at 63%. This observation underscores the satisfactory cyclic stability inherent in the fabricated SSCD, reaffirming its potential for long-term energy storage applications. Figure 7f provides the Ragone plot, illustrating the calculated relationship between the energy and power densities of the fabricated SSCD by employing Equations (2) and (3). Remarkably, the MF(ht) SSC device attains an impressive peak energy density of 74.7 Wh/kg at a power density of 3780 W/kg, surpassing previously reported values for both energy and power densities [6,48,49,50]. For an extensive comparative analysis of these parameters with those of other symmetric supercapacitor devices, please refer to

Table 2. Performance comparison of BWSq with various symmetric supercapacitors.

Electrodes	Operating Potential (V)	Electrolyte	Specific Capacitance (F·g−1)	Energy Density (Wh·kg−1)	Power Density (W·kg−1)	Capacitance Retention	Ref.
Fe3O4-V2O5	0–0.85	3 M KOH	93	13	1530	84% after 5000 cycles	[6]
PW12@MIL-101/PPy-0.15	0–1	1 M Li2SO4	149	20.7	277.6	83.7% after 2000 cycles	[48]
BPC//MIL-53(Cr) ASCD	0–1	1 M aqueous CSA	70	9.7	1250	85% after 10,000 cycles	[49]
Mn-BDC MOF	0–1.5	PVA-1 M Na2SO4	64.5	4.3	171.6	98% after 2000 cycles	[50]
MIL-101(Fe)	0–1.2	3 M KOH	103	14.2	3780	63% after 5000 cycles	This work

. The electrochemical characterization of the meticulously crafted SSC device was conducted through EIS. In Figure 7g, the Nyquist plot offers a comparative view of the SSC both before and after enduring 5000 cycles of GCD, spanning a frequency range from 20 kHz to 0.1 Hz. Significantly, both curves exhibit similar characteristics even after enduring extensive cycling, manifesting minor intercepts on the real axis, measuring at 0.36 Ω and 0.39 Ω, respectively (as detailed in Table S1). This parity implies a reduced charge transfer resistance at the interface connecting the electrode material and electrolyte, subsequently enhancing specific power capabilities. Furthermore, the repeated cycling sequences encouraged the accessibility of previously inaccessible pore sites, fostering heightened interactions between these sites and ions. The observed minimal charge transfer resistance value of 19 Ω, coupled with the improved double-layer capacitance, underscores the exceptional supercapacitive performance of the developed MF(ht) SSC device. These findings resoundingly emphasize the potential of MF(ht)-based symmetric supercapacitor devices for advanced energy storage applications.

4. Conclusions

A meticulous comparative analysis was conducted, delving into the synthesis of MIL-101 (Fe) MOF-based electrodes for supercapacitor applications through hydrothermal and microwave-assisted methods. While both approaches yielded an octahedral morphology, the microwave-assisted synthesis uniquely bestowed a mesoporous structure. However, it is noteworthy that the hydrothermal-assisted synthesis [MF(ht)] exhibited notably superior supercapacitive properties in contrast to its microwave-assisted counterpart [MF(m)]. Furthermore, cyclic stability was also scrutinized, revealing a superior performance for the hydrothermal synthesis method when compared to the microwave-assisted alternative. The disparities in performance and stability may likely be attributed to the shorter reaction time in the microwave synthesis, where rapid temperature escalation might not provide sufficient time for crystal phases to settle. Despite the presence of a mesoporous structure, the uneven distribution of porosity in the microwave-synthesized materials could lead to erratic conductivity, ultimately resulting in lower performance. Porous electrodes immersed in electrolyte solutions generally exhibit reduced cyclic stability. Hence, meticulously tailoring temperature and pressure becomes an imperative parameter in achieving a uniform mesopore structure with consistent pore radii across all crystal planes. In conclusion, at a current density of 1 A/g, the two-electrode configuration employing MF(ht) demonstrated an impressive energy density of 74.7 Wh/kg at a power density of 2160 W/kg, while maintaining commendable cyclic stability even after 5000 cycles. These outcomes underscore the exceptional electrochemical properties of MF(ht), positioning them as promising candidates for advanced applications in energy storage. As we move forward, further research should aim to explore methods to optimize the microwave-assisted synthesis process, potentially mitigating the challenges we observed. Additionally, the utilization of advanced characterization techniques and in-depth structural studies can aid in a more comprehensive understanding of the material’s behavior, offering opportunities for further improvements. The outcomes of this study open avenues for future research directions in the field of advanced energy storage materials.