Abstract
This paper aims to discover a novel composite material that has great potential for manufacturing high-performance supercapacitors suitable for diverse applications, such as electric vehicles, portable electronics, and stationary energy storage systems. Zeolitic imidazolate framework-8 (ZIF-8) doped by different concentrations up to 5 wt.% of nanosized Fe2O3 have been prepared (ZIF-8/Fe2O3). The effect of doping ratio 1, 3, and 5 wt.% on the structural and electrochemical properties of ZIF-8/Fe2O3 has been investigated. The structural characterization has been carried out using TGA, BET, XRD, and FTIR. The XRD analysis revealed that the crystalline size of our sample increased by approximately 16% as a result of doping ZIF-8 with 5 wt.% of Fe2O3. The structural analysis of the doped samples revealed that the material exhibited enhanced thermal stability and porosity, with an increase of approximately 105 m2/g. The introduction of doped nanometal oxides improved the capacitance value of ZIF-8 by significantly increasing its surface area. Additionally, the electron transport efficiency within ZIF-8/5 wt.% Fe2O3/electrode is increased. The Nyquist plot decreases as the doping of Fe2O3 increases. This indicates a decrease in the charge transfer resistance at the electrode–electrolyte interface, which is desired in applications such as batteries, fuel cells, or electrochemical sensors where faster electron transfer is needed for improved performance.
1. Introduction
The progress in photovoltaic and energy storage technology largely depends upon the discovery and development of innovative materials with enhanced structural, photoelectric, and electrochemical properties [1, 2]. Specifically, metal–organic frameworks (MOFs) have demonstrated considerable promise, especially zeolitic imidazolate framework-8 (ZIF-8), due to their numerous attractive properties, such as high porosity, chemical stability, and customizable functionalities [3, 4]. The molecular structure of ZIF-8 is shown in Scheme 1. Among these, ZIF-8 has emerged as a promising candidate owing to its unique structure and intriguing material properties. Renowned for its high porosity, chemical stability, and customizable functionalities, ZIF-8 has the potential to revolutionize the capabilities of energy devices. Moreover, when paired with nanosized metal oxide nanoparticles particles, the structural, photoelectric, and electrochemical properties of ZIF-8 are significantly optimized [5]. This enhancement facilitates an even broader range of applications in photovoltaic and energy storage technology.
ZIF-8 can be synthesized at room temperature using an aqueous solution and is known for its exceptional thermal and chemical stabilities [6]. ZIF-8 has diverse applications in multiple catalytic fields, including photocatalysis, due to its high photocatalytic efficiency [7]. It has been shown to efficiently degrade reactive black KN-B dye under UV irradiation [8]. ZIF-8 also has applications in environmental remediation, as it exhibits high photostability and effective reusability [9]. Additionally, it has applications in various fields, including the removal of heavy metals, wastewater treatment, gas separation, drug delivery, heterogeneous catalysis, electrochemical sensors, and electrode materials for batteries [10]. Furthermore, ZIF-8 can be incorporated into Cu-based precursor solutions to control the phase structure and morphology of composites, leading to enhanced photocatalytic efficiency and cycle stability [8].
Fe2O3 is used in different applications such as photocatalytic activity, which arises from its narrow band gap and ability to absorb visible light effectively. Fe2O3 nanoparticles possess high electron mobility and good electrochemical activity [11]. These properties can promote charge transport in ZIF-8 and assist in its electrochemical performance, like enhancing charge storage capacity and stability of the material, essential parameters for energy storage devices. Furthermore, Fe2O3 nanoparticles exhibit robust chemical stability and can improve the resistance of ZIF-8 to degradation under operational conditions, thereby enhancing the device’s longevity. The small size and high surface area of nanosized Fe2O3 allow for homogeneous distribution within the material framework, ensuring consistent enhancement of properties throughout the ZIF-8 structure. To assess the efficiency and potential limitations of a specific electrochemical system, various measurements can be conducted. (1) Current–voltage (I–V) curves can aid in determining the system’s efficiency, charge/discharge characteristics, and any limitations. (2) Impedance spectroscopy can offer insights into the system’s internal resistance, charge transfer resistance, and overall performance. (3) Cyclic voltammetry (CV) can offer information on the system’s electrochemical behavior, including its redox processes, capacitive behavior, and kinetic limitations.
Here, the structural and electrochemical properties of different doping ratios of Fe2O3 within ZIF-8 have been investigated. This research will improve our understanding and capacity to optimize ZIF-8 for various energy applications. Scheme 2 outlines the experimental procedure used in our manuscript. We are confident that our ZIF-8/Fe2O3 could be advantageous in a range of energy applications.
2. Experimental Procedure
2.1. ZIF-8/1, 3, 5 Wt.% Nanosize Fe2O3 Samples Fabrication
All the used chemicals have been purchased from Sigma–Aldrich. For preparing an as-prepared sample of ZIF-8, we dissolved 1 g of ZIF-8 in a specified 10 ml of ethanol in a beaker. To ensure complete dissolution, we use a magnetic stirrer. For preparing ZIF-8/1 wt.% Fe2O3, 0.99 g of ZIF-8, and 0.01 g of nanosize Fe2O3 have been dissolved in 10 ml ethanol, while constantly stirring to avoid the formation of clumps or precipitates. For the preparation of ZIF-8/3 wt.% Fe2O3, 0.97 g of ZIF-8 and 0.03 g of nanosized Fe2O3 were dissolved in 10 ml of ethanol. Finally, ZIF-8/5 wt.% Fe2O3, 0.95 g of ZIF-8, and 0.05 g of nanosize Fe2O3 have been dissolved in 10 ml ethanol. After adding all the nanosize Fe2O3, we use a sonicator to ensure a homogeneous distribution of ZIF-8 and Fe2O3.
2.2. Preparation of ZIF-8/Fe2O3 NPs Modified GCE
To examine the electrochemical properties of the ZIF-8/Fe2O3, a glassy carbon electrode (GCE) was utilized. The GCE was cleaned by spraying the water and diluted hydrochloric acid over the electrodes and then drying it. ZIF-8/Fe2O3 in the colloidal NPs solution was inserted properly in the GCE. The three-electrode system consisted of Pt, GC, and SC (saturated calomel acting as a reference). Within the potential; between −0.4 and 1.4 V, the CV measurement was conducted in KOH (0.1 M) solution as the electrolyte, and at a rate of scanning of about 50 mV/s. The ZIF-8/Fe2O3 GCE was dried in an oven for about 3 hr.
2.3. Instrumentation
The XRD for our prepared samples was done using a Shimadzu X-ray diffractometer (CuKα −1.541 Å) with a scan rate of 2°/m and the scans were performed at 2θ angles between 10° and 80°. The XRD model is FRINGE CLASS Benchtop X-ray diffractometer (Lan scientific). The thin film structure was examined using FTIR (Shimadzu Japan, FT-IR 8400S Spectro-photometer using the Kbr pellet method) in the 400–4,000 cm−1 spectral region. An electrochemical system (CS-300, 150) was used to measure the electrochemical characteristics. The electrochemical system contains a potentiostat (Model CS350M, Corrtest) to control the potential state and three electrodes—a working electrode, a counter electrode, and a reference electrode. These components work together to accurately measure and control electrochemical reactions within the system.
3. Results and Discussion
3.1. Structural Characterization
3.1.1. XRD
XRD is widely used in investigating the crystallinity of organic materials. Figure 1 shows the XRD spectra for (a) as-prepared (ZIF-8), (b) 1 wt.% Fe2O3, (c) 3 wt.% Fe2O3, and (d) 5 wt.% Fe2O3. As we can see from the figure that the doping ratio of Fe2O3 leads to changes in the crystal structure of the ZIF-8 framework. As shown in the figure, the Miller indices of ZIF-8 are (011), (002), (012), and (022) [12]. The Fe2O3 diffusion act to increase the intensities of the miller indices related to ZIF-8 such as (011), (002), and (012) at 7.3°, 12.6°, and 14.6°, respectively. The doping of Fe2O3 within ZIF-8 introduced XRD peaks associated with the crystal planes of Fe2O3 as the peaks around 2θ° = 33°, 41°, and 54° which correspond to (012), (113), and (024), respectively [13, 14].
Scherrer’s equation was used to evaluate the crystallite size (D) [15]:where λ is the X-ray wavelength, K is Scherrer’s constant, η is the full width at half maximum, and θ is the Bragg’s angle. The average crystalline size of for (a) as-prepared (ZIF-8), (b) 1 wt.% Fe2O3, (c) 3 wt.% Fe2O3, and (d) 5 wt.% Fe2O3 are 49, 52, 56, and 57 nm. The crystalline size of our sample increased by about 16% due to the doping of Fe2O3 by 5 wt.%. This can be beneficial for electrochemical applications where efficient charge transfer is required. The increase in the grain size can lead to electron transport improving within the material, resulting in enhanced electrical conductivity [16]. This can be beneficial for electrochemical applications where efficient charge transfer is required. The dislocation density, the microstrain, and crystallite number are calculated, respectively [17]:
The calculated crystal parameters of ZIf-8/0, 1, 3, and 5 wt.% Fe2O3 samples are listed in Table 1.
3.1.2. FTIR Analysis
Figure 2 shows the FTIR spectra of ZIf-8/0, 1, 3, and 5 wt.% Fe2O3 in the wavenumber region 4,000–400 cm−1. According to the figure, broad and strong absorption bands could appear around 3,430and 1,630 cm−1, which correspond to the O─H stretching vibration and bending vibrations, respectively, due to absorbed water in Fe2O3. The peak around 540 cm−1 is typically attributed to Fe─O stretching vibration modes in Fe2O3. The peak at 1,576 cm−1 represents the stretching vibrations of C═N in the imidazole ring. The peak found typically at 758 cm−1 portrays the C─H out-of-plane bending mode of the imidazole ring. A peak around 1,150 cm−1 represents the C─N stretching vibrations in the imidazole ring. The FTIR peaks correspond with the previously published work [18, 19].
3.1.3. TGA Analysis
Thermal gravimetric analysis (TGA) can potentially be used to examine the weight loss and thermal stability for novel organic composites. Figure 3 shows the TGA analysis for different concentrations (0, 1, 3, and 5 wt.%) of Fe2O3 in ZIF-8. The decrease in mass of ZIF-8/Fe2O3 NPs was measured at different temperatures up to 780°C. As the temperature reaches 320°C, the mass loss of ZIF-8/0, 1, 3, and 5 wt.% Fe2O3 was about 32%, 18%, 14%, and 11%, respectively. This observation assured that Fe2O3 NPs improve the thermal resistance and decrease weight loss. And ZIF-8/5 wt.% Fe2O3 was the sample with the highest thermal stability. The addition of Fe2O3 NPs to ZIF-8 could improve the dispersion of ZIF-8 particles within the composite material. This improved dispersion may result in better thermal conductivity and reduced mass transport during TGA analysis, leading to a decrease in weight loss. The improved thermal stability provided by the addition of Fe2O3 NPs to ZIF-8 can be advantageous for energy storage devices [20]. Energy storage systems often involve high temperatures or thermal cycling, and the increased thermal stability can help maintain the structural integrity of the composite material, enhancing its lifespan and performance.
3.1.4. BET Analysis
The analysis of surface area was performed using surface area and pore analyzer from Quantichrome Instruments (Nova 2200e). The BET surface analysis showed a value of 1,048, 1,085, 1,105, and 1,198 m2/g for ZIF-8/0, 1, 3, and 5 wt.% Fe2O3, respectively. The pore diameter of ZIF-8/0, 1, 3, and 5 wt.% Fe2O3 was found to be 52.56, 61.43, 65.00, and 67.23 Å, respectively. This could be explained because the addition of nanosized Fe2O3 may introduce additional porosity or modify the existing porosity of ZIF-8. Higher porosity provides a larger surface area within the material. Figure 4 shows the BET analysis for ZIf-8/0, 1, 3, and 5 wt.% Fe2O3. As we can see from the figure, the increase in the specific area and pore diameter of our samples are 14% and 27%, respectively. This increased surface area allows for more active sites where electrochemical reactions can occur, enhancing the overall energy storage capacity and efficiency [21]. Furthermore, the nanoparticles may also prevent the aggregation of ZIF-8 particles. This helps maintain a larger surface area, which can potentially improve its ability to absorb different substances, including heavy metals and organic pollutants.
3.2. Electrochemical Studies
3.2.1. Cyclic Voltammetry
CV is a widely used electrochemical technique that measures the current response of an electrode material as a function of applied potential. In the case of ZIF-8 doped with nano-Fe2O3, CV can provide valuable information about its electrochemical behavior and energy storage capabilities. Figure 5 shows the CV curves of ZIF-8/0, 1, 3, and 5 wt.% Fe2O3 electrodes at 20–100 mV/s. Figure 5 displays curves that have a quasirectangular shape. These curves show redox peaks, which indicate the behavior of an electrical double-layer capacitor (EDLC) with fast charging and discharging capabilities of the materials. This indicates a superior capacitance capability that could generate a higher current density.
Figure 6 shows the charge and discharge for the highest doped sample (ZIF-8/5 wt.% Fe2O3). As we can see, it can be observed that there is a different distribution in this electrode. This is due to its ability to improve the faradic property, which can be seen in the redox peaks of the curve. As a result, this electrode is valuable for creating an exceptional EDLC. The detailed analysis of the intensity and shape of redox peaks in the cyclic voltammogram can indicate the behavior of an EDLC, in which the potential and the current intensity at which the redox peaks occur in the CV results for our investigated material are 1.1 eV and 2 mA [22]. The doped nanometal oxides improve ZIF-8 capacitance value due to the higher electron transport efficiency within ZIF-8/5 wt.% Fe2O3/GCE [23]. The uneven distribution observed in the ZIF-8/5 wt.% Fe2O3 electrode, as mentioned in Figure 6, is attributed to the unique structure of the composite material. The presence of ZIF-8 and Fe2O3 nanoparticles in the electrode creates localized regions with varying concentrations, leading to nonuniform current distribution during charge–discharge cycles. This nonuniformity can enhance the faradic properties of the electrode by promoting heterogeneous reactions at different active sites within the electrode material. This spatial variation in reactivity can facilitate improved utilization of active materials, enhanced electron transport, and overall battery performance.
The current’s magnitude in CV measurements, commonly called the peak current, can be determined by utilizing the Randles–Sevcik equation. This equation serves as a mathematical model in the field of electrochemistry of the peak current (ip) as in the Randles–Sevcik equation [24]:where ip, n, A, and D are the peak current, electron numbers involved in the redox reaction, the working electrode area (cm2), and the analyte diffusion coefficient (cm2/s), respectively. Figure 7 shows the cathodic peak current (ip) and the square root of the scan rate (v1/2) for ZIF-8/0, 1, 3, and 5 wt.% Fe2O3 electrodes. Electrode reactions in pure and doped electrodes all show a linear connection between ip and v1/2. Based on our findings, the 5% Fe2O3-doped ZIF-8 electrode material exhibited the highest proton transfer coefficient.
3.2.2. Specific Capacitance
The capacitance, C, of our doped MOF can be determined by using the following equation [17]:where I, V, and m are the applied current applied, the rate of the potential in (mV/s), and m is load mass, respectively. The specific capacitance (F/g) is given by the following equation:where I and m are the discharge current and the weight of deposited material, respectively. Figure 8 depicts the specific capacitance and scan voltage rate for ZIF-8/0, 1, 3, and 5 wt.% Fe2O3 electrodes. It is clear from the figure that the capacitance decreases with current density increases for activated pure ZIF-8 and the doped ZIF-8 and this is due to the ions diffusing from the electrolyte toward the electrode material’s pores. These curves show redox peaks, which indicate the behavior of an EDLC with fast charging and discharging capabilities of the materials with an average capacitance value of 220 F/g can be considered relatively high and indicative of a good performance, as it suggests a high charge storage capacity per unit mass [25, 26]. Table 2 shows the specific capacitance at different scanning rates for (a) as-prepared (ZIF-8), (b) 1 wt.% Fe2O3, (c) 3 wt.% Fe2O3, and (d) 5 wt.% Fe2O3.
3.2.3. Impedance Spectroscopy (Nyquist Plot)
Nyquist plot is a graphical representation used in electrochemical impedance spectroscopy (EIS) to analyze and interpret the electrical behavior of a system. EIS is important for investigating the electrochemical properties for optimizing performance for energy storage applications. Figure 9 shows the Z′ plotted against Z″ for ZIF-8/0, 1, 3, and 5 wt.% Fe2O3 electrodes. It is clear from the figure that bulk resistance decreased by incorporating the nanometal oxide from 1,920 to 1,440 kΩ. The decrease in impedance observed in the Nyquist plot can be due to several factors. For instance, it may indicate a reduction in the charge transfer resistance at the electrode–electrolyte interface, suggesting improved kinetics of the electrochemical reactions. This is often desired in applications such as batteries, fuel cells, or electrochemical sensors where faster electron transfer is desired for enhanced performance.
4. Conclusion
In this paper, we delve into the transformative role of nanosized Fe2O3 in augmenting the properties of ZIF-8, thereby charting new courses in the development of highly efficient photovoltaic and energy storage devices. The TGA analysis of our fabricated samples showed that as the temperature reaches 320°, the mass loss of ZIF-8 at 0, 1, 3, and 5 weight percent Fe2O3 was about 32%, 18%, 14%, and 11%, respectively, and the increased thermal stability can help to maintain the structural integrity of the composite material, enhancing its lifespan and performance. The BET results confirmed that the porosity of our composite increased when nonmetallic oxides were added, and the addition of 5 wt.% Fe2O3 to ZIF-8 resulted in a pore diameter of approximately 67.56 A. The Nyquist plot decreases as the doping of Fe2O3 increases. This indicates a decrease in the charge transfer resistance at the electrode–electrolyte interface, which is desired in applications such as batteries, fuel cells, or electrochemical sensors where faster electron transfer is needed for improved performance.
Data Availability
The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that there are no conflicts of interest in the manuscript.
Acknowledgments
The authors extend their appreciation to the Deanship of Research and Graduate Studies at University of Tabuk for funding this work through Research no. 009-1444-S.