Novel Biochar-Modified ZIF-8 Metal–Organic Frameworks as a Potential Material for Optoelectronic and Electrochemical Energy Storage Applications

Abstract

Herein, we report the preparation of nanocomposites using activated biochar derived from rice husk (RHBC) by doping with a metal–organic framework, namely the zeolitic imidazolate framework (ZIF-8). The morphological and structural characterization of the prepared nanocomposite was performed using SEM, BET, XRD, FTIR, TGA, and UV–Vis spectroscopy. The average particle sizes as observed from SEM micrographs for ZIF-8 and ZIF-8@RHBC were 67 nm and 78 nm, respectively. The BET surface analysis of the ZIF-8@RHBC composite showed a value of 308 m²/g and a pore diameter of about 42.56 A°. The inclusion of RHBC in ZIF-8 resulted in a 4% increase in the optical band gap and a 5% increase in the optical conductivity. The electrochemical properties of this nanocomposite were investigated through cyclic voltammetry, and it was observed that ZIF-8@RHBC showed improved CV curves in comparison to bare ZIF-8. The specific capacitance of ZIF-8@RHBC was significantly enhanced from 348 F/g to 452 F/g at a 1 A/g current density after incorporating ZIF-8 into the RHBC matrix. The formation of a mesoporous structure in the ZIF-8@RHBC composite contributed to the improved diffusion rate at the electrode surface, resulting in excellent electrochemical features in the composite. Furthermore, the EIS studies confirmed the reduced charge transfer resistance and increased conduction at the electrode surface in the case of the ZIF-8@RHBC composite. Owing to the ease of its green synthesis and its excellent structural and morphological features and optical and electrochemical properties, this ZIF@RHBC nanocomposite could represent a novel multifunctional material to be used in optoelectronics and energy storage applications.

1. Introduction

The problems of pollution and climate change have recently emerged as the main challenges facing research organizations and nations. The management of agricultural waste has a significant impact on the environment. For example, burning rice husks can be dangerous due to the production of toxic carbon monoxide [1,2,3]. Using green biomaterials has numerous benefits, both for the environment and for the economy [4,5]. Biomaterials are sustainable, as they can be continually and indefinitely replenished, leading to reduced waste and resources used. They are also much more efficient and cost-effective, as they can be used to create products without the need for additional energy or resources [6]. Furthermore, their use helps to reduce emissions, as they do not emit harmful carbon dioxide or other greenhouse gases. These characteristics offer large potential for physical and/or chemical adsorption techniques.

Biochar can be obtained by the pyrolysis of different waste materials, such as food waste, agricultural waste, and algae [7]. This enables waste management and the creation of inexpensive, ecologically friendly, and durable materials. Nevertheless, a few experimental and technical variables—notably the pyrolysis temperature and the raw material used—can affect the characteristics of the biochar [8]. Wang et al. synthesized graphene CNTs from rice husk biochar and used them as electrochemical electrodes due to their high capacitance and surface area [9]. Rice husk biochar was found to have various beneficial properties, such as a large surface area, high porosity, and a high adsorption capacity for organic compounds and heavy metals. Using a chemical process, ambient heat and furnace heating were used by Sharma et al. to create silica- and carbon-based nanomaterials from rice husk ash [10].

Metal–organic frameworks have high porosity, allowing them to be used for the storage of gases, such as hydrogen or carbon dioxide. These MOFs are small, nanoscale, porous structures composed of metal ions or clusters connected to organic molecules. They are highly versatile materials with numerous applications in energy, healthcare, electronics, and engineering. Imidazolate frameworks (IFs) are a type of metal–organic framework (MOF) that have gained significant attention due to their unique properties and potential applications [11,12]. ZIF-8 is a promising material for different applications, including gas detachment and capacity, catalysis, and energy storage [13,14]. One vital benefit of ZIF-8 is its tunable, controllable porosity, as well as its adsorption properties, large surface region, and high gas uptake limit, making it a flexible material for gas capacity applications [15,16,17]. The zeolite imidazolate framework (ZIF-8) is a four-ringed metal–organic framework (MOF) with 2-methylimidazole and Zn²⁺ ions as the imidazole linkages [18]. Tian et al. proved the high porosity and chemical stability of cadmium imidazolate frameworks [19]. ZIF-8 exhibits interesting optical and electronic properties, such as luminescence and conductivity [20]. These properties make it attractive for applications such as sensing, imaging, and photovoltaics. Dong et al. studied the broad-band optical nonlinearity for rapid photonics using ZIF-8 [21]. The relation between the change in the refractive index of a ZIF-8 thin film and the optical quality was investigated by Keppler et al. [22]. Overall, ZIF-8 is considered a promising material with a wide range of potential applications [23]. In addition, apart from metal–organic frameworks, covalent triazine frameworks with improved electrochemical properties have been widely investigated for their energy storage applications, such as in high-performance supercapacitors in modern technologies [24,25].

In recent years, research in materials science has undergone a significant paradigm shift, driven by the emergence of nanotechnology. Doping materials with biochar extracted from various natural resources has great potential in the industry [26,27]. Herein, we report the doping of biochar extracted from rice husk with ZIF-8 and the investigation of its optical and electrochemical properties, which is an intriguing research area with significant scope and novelty. By investigating the effects of doping biochar derived from rice husk, researchers can explore its potential applications in various domains, such as energy storage, environmental remediation, and electronic devices. This research not only contributes to the development of sustainable materials but also presents an opportunity to enhance the functionality and versatility of biochar. Further, the raw material needed to prepare the activated biochar is abundant in nature; hence, the green synthesis of activated biochar and the formation of composites with ZIF-8 offer eco-friendly, low-cost materials for the fabrication of devices. Additionally, examining the optical and electrical properties within the framework of ZIF-8 adds a novel dimension to this study. The combination of biochar doping and the unique characteristics of ZIF-8 opens up new avenues for the understanding of the interactions between materials and their performance in optical and electrical systems. Overall, this research holds promise in advancing our understanding of biochar doping and its potential applications, while simultaneously uncovering new insights within the realm of materials. This research describes, for the first time, the preparation of novel composites based on activated biochar and ZIF-8, which could potentially be used in electrochemical energy storage and optoelectronic applications.

In this study, we examined the use of biochar extracted from rice husks for the biogenic production of ZIF-8@RHBC. The structural, linear optical, nonlinear optical, and electrical properties of ZIF-8 and ZIF-8@RHBC were studied and interpreted. Apart from this, the prepared ZIF-8@RHBC composite electrodes were tested for their electrochemical behavior. The specific capacitance of the ZIF-8@RHBC composites was investigated to use them as electrode materials in supercapacitor applications. Due to their easy and eco-friendly synthesis and improved electrical, optical, and electrochemical properties, these novel ZIF-8@RHBC composites could be used in optoelectronics and electrochemical energy storage applications.

2. Results and Discussion

2.1. FTIR Analysis

The infrared spectra of the functional groups of solids, liquids, and gases can be obtained using the Fourier transform infrared (FTIR) method. The FTIR results of ZIF-8 and ZIF-8@RHBC are shown in Figure 1. As seen in Figure 1, the characteristic peaks corresponding to ZIF functional bonds such as the carboxyl group appeared at 1394 cm⁻¹ and the ligand benzene ring resulted in a C=C stretching peak at 1637 cm⁻¹ [28]. It can be seen from the FTIR spectra of ZIF-8@RHBC that the addition of RHBC to ZIF-8 results in a peak at 1639 cm⁻¹, which is related to the N-H bending vibrations of the amide group, and a peak at 1064 cm⁻¹, representing the C-N stretching peak of the amine group, which is a significant functional group for BC. The peak at 1384 cm⁻¹ resulted from the original O-H bending of the carboxyl group. The change in the peak intensity was adjusted with the N-H group of the carboxyl group, forming a new group such as O=C-N [29]. The FTIR results indicate the presence of RHBC, which can result in improved optical and electrical properties of ZIF-8@RHB. Due to the presence of polar groups C-N and O-H, when these two groups are present in a molecule, they can interact with each other through hydrogen bonding or dipole–dipole interactions. This can lead to changes in the electronic transitions and energy levels of the molecule, which can affect its optical properties.

2.2. XRD Analysis

X-ray diffraction (XRD) may provide valuable information on the crystalline phases of materials and help in obtaining the particle and grain sizes. The XRD spectra of ZIF-8 and ZIF-8@RHBC are shown in Figure 2. As seen from Figure 2, ZIF-8 had characteristic peaks (110), (002), (022), (113), (004), (115), and (224) found at 2θ angles equal to 5°, 12°, 13°, 14°, 17°, 22°, and 24°, respectively. The patterns were consistent with previous reports in the literature [24,25]. The (002) plane reflection is a specific peak in the X-ray diffraction pattern that is used to confirm the presence of ZIF-8 in the sample [27]. The XRD pattern of ZIF-8@RHBC has two significant peaks at 17° and 27°, which are related to (111) and (002), respectively. These two peaks tend to increase the crystallinity of ZIF-8@RHBC. The presence of a peak at 27° is attributed to the (002) basal plane reflection of graphitic carbon [26].

The degree of crystallinity presented in the synthesized ZIF-8@RHBC can be calculated using the relation mentioned in the previous literature [30].

(1)$X_c\left(\%\right)={\displaystyle \frac{A_c}{A_c+A_a}}\times 100$

where A_c and A_a stand for the total crystalline region and the total amorphous region, respectively. The degree of crystallinity obtained using the above-mentioned relation for ZIF-8 and ZIF-8@RHBC is 56% and 67%, respectively. By applying Scherrer’s equation [31] to all planes demonstrated above, the average crystallite size of ZIF-8 and ZIF-8@RHBC is found to be 78 and 81 nm, respectively.

2.3. SEM Analysis

Scanning electron microscopy (SEM) is the most effective tool used to investigate the surface morphology of a material. The surface morphologies of ZIF-8 and ZIF-8@RHBC obtained from SEM are depicted in the micrographs in Figure 3. The micrograph of the bare ZIF (Figure 3) shows spherical-shaped agglomerated particles. The addition of RHBC into ZIF facilitates the creation of a highly mesoporous structure in ZIF-8@RHBC. Further, the RHBC is uniformly and homogeneously distributed in the ZIF matrix. The creation of a mesoporous structure and increased porosity results in a decrease in the conductivity value of the composite, but, at the same time, such a mesoporous structure facilitates improved electrochemical performance. The presence of mesopores in the composite structure helps the diffusion of electrolyte ions and improves the reaction kinetics at the electrode surface. The average particle sizes as observed from the SEM micrographs for ZIF-8 and ZIF-8@RHBC are 67 nm and 78 nm, respectively. Hence, the SEM analysis confirms the particle size for both ZIF and ZIF-8@RHBC, as well as confirming the presence of a mesoporous structure in the composite.

2.4. TGA Analysis

The thermal stability of the synthesized nanocomposites can be analyzed through thermogravimetric analysis (TGA). The weight loss of the nanocomposites as a function of the temperature was computed using TGA analysis. Figure 4 illustrates the weight loss for both ZIF-8 and the ZIF-8@RHBC nanocomposite as a function of the temperature. Both the ZIF and the ZIF-8@RHBC nanocomposite show a three-step weight loss. The first phase of weight loss, between 0 and 300 °C, occurs mainly due to the loss of water and chemicals used during the synthesis process. A significant loss of mass was observed in phase 2 between 300 and 500 °C, mainly due to the decomposition of the functional groups and the volatilization of ZIF-8 [32,33]. The final mass loss in phase 3, between the temperatures of 500 and 700 °C, is due to the structural failure of ZIF-8. The TGA spectra suggest that, at the end of the period at 700 °C, the residual mass in the case of ZIF-8 is about 2.5 mg, whereas the ZIF-8@RHBC composite shows a residual mass of about 7.5 mg. Hence, the TGA analysis indicates that the ZIF-8@RHBC composite is thermally more stable than bare ZIF-8; this could be mainly due to the presence of activated biochar in the composite’s structure.

2.5. BET Analysis

The sample was subjected to gassing and degassing with nitrogen at a temperature of 70 °C for 10 h before analysis. The BET surface analysis of the ZIF-8@RHBC composite showed a value of 308 m²/g. The cumulative surface pore volume was measured to be 0.0819 mL/g; this value suggests an increase in the available pore space within the material. This can potentially enhance its adsorption capacity for various substances, such as heavy metals or organic pollutants. The pore diameter was found to be 42.56 A°, which indicates that the material contains pores of a relatively large size. Such pores can allow for efficient mass transfer and provide accessibility for larger molecules. These results indicate that the biochar’s surface contains a significant number of micropores.

2.6. Optical Properties

Transmission and reflection are two important optical properties that can provide valuable information about the structure and composition of organic materials such as MOFs. By analyzing the transmission and reflection spectra of MOFs, one can gain insights into their electronic and optical properties, such as the band gap, refractive index, and absorption coefficient. Figure 5A depicts the relation between optical transmission and reflection for ZIF-8 and the ZIF-8@RHBC composite. One can find, from Figure 5A, that the transmission of light within ZIF-8@RHBC increased by about 8% in comparison to that in ZIF-8; this may be attributed to the increase in the porosity of our prepared material. This increase in MOF transmission could be useful in optical sensors as well as photovoltaic applications [33]. The addition of T and R is approximately equal to unity, which indicates that the material has very negligible scattering. This ensures that our biobased composite has a uniform structure.

According to Tauc’s theory, the optical band gap (E_g) of a thin film can be evaluated using the relation mentioned below [34]:

(αhv)^1/2 = C(hv − E_g)(2)

The optical energy band gap is one of the most important characteristic features of materials for electronic device fabrication and optoelectronic applications. The optical band gap values of the ZIF-8 and ZIF-8@RHBC composite samples were calculated from the Tauc plots using the data obtained from the UV–visible characterization technique and are depicted in Figure 5B, which shows the relation with the photon energy for ZIF-8 and the ZIF-8@RHBC composite. According to Equation (2), the values of E_g can be obtained by extrapolating the linear portion of (αhv)^½; thus, the evaluated energy band gaps were 2.6 and 2.68 eV for ZIF-8 and ZIF-8@RHBC, respectively. Scheme 1 shows the energy gap of ZIF-8@RHBC. When activated biochar is added to ZIF-8, the pore size and the pore volume in the composite structure are significantly enhanced; hence, the optical band gap values for the composite were found to increase marginally. This is because the local electric field created around the pores creates a higher barrier for photon movement, resulting in a larger energy gap [34].

The refractive index spectra for ZIF-8 and ZIF-8@RHBC are shown in Figure 6A. For wavelengths greater than 500 nm, one can conclude that the values of n decreased due to the addition of the activated biochar. In the case of ZIF-8 doped with biochar, the lower refractive index of the composite material causes the light to bend less than it would in pure ZIF-8. Meanwhile, at a wavelength less than 500 nm, several peaks appear due to the scattering of light within the materials. This is known as Mie scattering, which occurs when the size of the scattering particles is comparable to the wavelength of the incident light. In the case of ZIF-8 doped with biochar, the scattering peaks may be related to the size and distribution of the biochar particles in the material. The presence of biochar particles can cause light to scatter in different directions, leading to interference patterns that can produce peaks in the scattering spectrum.

The absorption index spectra for ZIF-8 and ZIF-8@RHBC are depicted in Figure 6B. Activated biochar is known to have a large surface area and pore volume, which can enhance its adsorption capacity for various molecules and ions. When biochar is added to ZIF-8, the resulting composite material has significant pores as well as an increased volume; these pores result in an increased surface area and adsorption capacity. Hence, the absorptive power of the ZIF-8@RHBC composite was found to increase in comparison to that of bare ZIF-8.

Investigating the natural logarithm of the optical conductivity (ln(α)) as a function of the photon energy (E) is a common method to analyze the optical properties of materials. This is because the optical conductivity is related to the absorption coefficient (α) of the material, which describes the ability of the material to absorb light at a given frequency or photon energy. The absorption coefficient, α, is related to the imaginary part of the dielectric function (ε₂) by the following equation:

α = 2ωε₀ε₂/c(3)

where ω is the angular frequency of light, ε₀ is the permittivity of free space, ε₂ is the imaginary part of the dielectric constant, and c is the speed of light.

Figure 7A shows the relation of ln(α) (optical conductivity) as a function of the photon energy (E) for ZIF-8 and the ZIF-8@RHBC composite. The increase in the optical conductivity of ZIF-8@RHBC can be related to the increase in the number of charge carriers or decrease in the amount of scattering of light within the material. This increase in conductivity can have important implications for the material’s properties and potential applications, such as the production of solar cells and other optoelectronic devices.

The essential optical parameters, such as the dielectric constant (n_o), the linear susceptibility (χ⁽¹⁾), the third-order susceptibility (χ⁽³⁾), and the nonlinear refractive index (n₂), can be calculated using the measurements of n [35]:

(4)${\epsilon }_0=n_0^2$

(5)${\chi }^{\left(1\right)}=\sqrt[4]{{\displaystyle \frac{{\chi }^{\left(3\right)}}{B}}}$

(6)$n_2={\displaystyle \frac{12\pi }{n_o}}{\chi }^{\left(3\right)}$

(7)$n_2={\displaystyle \frac{12\pi }{n_o}}{\chi }^{\left(3\right)}$

where B = 1.7 × 10⁻¹⁰ for χ⁽³⁾ in the electrostatic system of units (esu). The specific values of these parameters for ZIF-8 and the ZIF-8@RHBC composite were calculated and are reported in Table 1. In the case of ZIF-8 and the ZIF-8@RHBC nanocomposite, a numerical value of 0 is observed between 5.47 and 5.52, and their change in behavior resembles that of n₀.

In this section, we discuss two important parameters that are influenced by λ or hν, which can be deduced from the obtained optical parameters. The optical surface resistance (R_s) and the thermal emissivity (ε_th) as a function of λ are given by [36]

(8)$R_s=-{\displaystyle \frac{4\pi }{c}}\times {\displaystyle \frac{1}{n\times ln\left(T\right)}}$

(9)${\epsilon }_{\mathrm{th}}=1-{\displaystyle \frac{1}{{\left[1+\left(2{\epsilon }_{0}c\times R_s\right)\right]}^2}}$

where c is the speed of light in a vacuum, and n is the refractive index of the material being studied. ε_th represents the effect of photonic stimulation on the electrons on the surface of the films and describes how well a material’s surface can emit radiation. In Figure 7B, the graph shows the relationship between the sheet resistance ε_th and wavelength λ for ZIF-8 and the ZIF-8@RHBC nanocomposite when exposed to different levels of radiation. It is observed that ε_th remains constant for wavelengths less than 340 nm but then significantly increases (by over 10 times), before becoming constant again for the remaining range of wavelengths. It is widely acknowledged that factors like the temperature, measurement direction, and others can influence the value of ε_th. Therefore, adjusting parameters like the measurement direction and temperature can change the estimated value of ε_th.

The presence of activated biochar in ZIF-8 increases the crystallites, as observed from the XRD analysis, which is due to the interactions between the phases of RHBC and ZIF-8. The strong interactions between the C-N, C=C, and C-H bonds in the composite act as trapping centers, thereby improving the optical properties. The increased number of trapping centers facilitates a lower energy transition. All of the optical parameters studied in this work were found to be enhanced significantly with the addition of RHBC in ZIF-8, as RHBC causes the pronounced alteration of the crystallite regions in the composite.

2.7. Electrochemical Properties

The electrochemical performance in terms of CV and specific capacitance was investigated for both ZIF-8 and the ZIF-8@RHBC nanocomposite to understand the energy storage performance of these materials as supercapacitors. The cyclic voltammetry results of ZIF-8 and the ZIF-8@RHBC nanocomposite are shown in Figure 8A. The CV curve of the ZIF-8@RHBC nanocomposite shows improved CV characteristics in comparison to ZIF-8; this may be attributed to the improved porosity in the nanocomposite. The as-prepared ZIF-8 shows a denser morphological structure, which hinders electrolyte diffusion into the electrode surface, resulting in a small CV curve. The in situ growth of ZIF-8 on RHBC results in the formation of a mesoporous structure. The mesopores present in the composite material facilitate the diffusion of electrolyte ions and improve the electrochemical reactions at the surface of the electrode [37]. Apart from this, these mesopores in the composite material act as active sites to facilitate interactions between the surface atoms of the electrode and the electrolytic ions, due to the improved charge density in the composite structure; this facilitates higher charge transfer at the surface electrode. In conclusion, the improved diffusion rate and pore structures could be responsible for the better CV performance of the ZIF-8@RHBC nanocomposite.

The specific capacitances for both ZIF-8 and the ZIF-8@RHBC nanocomposite at different scan rates and at a constant current density of 0.1 A g⁻¹ are depicted in Figure 8B. At lower scan rates of around 5 mVs⁻¹, ZIF-8 and the ZIF-8@RHBC nanocomposite show a specific capacity of 897 F/g and 1195 F/g, respectively. The high specific capacity for ZIF-8@RHBC results from the improved CV performance due to the enhanced diffusion rate of electrolytes at the electrode surface. Ma et al. [38] have reported that the unique mesoporous structure of biochar improves the surface area for activation with KOH and results in higher specific capacitance. The specific capacity is found to decrease with higher scan rates for both ZIF-8 and the ZIF-8@RHBC composite as, at higher scan rates, the creation of a locally charged field around the pores prevents the easy diffusion of the electrolyte into the electrode surface. For the scan rates of 50 mVs⁻¹ and above, both ZIF-8 and the ZIF-8@RHBC composite show almost the same specific capacitance. Further, the dependence of the specific capacitance on the current density for both ZIF-8 and the ZIF-8@RHBC composite is shown in Figure 9A.

The specific capacitance of ZIF-8 and the ZIF-8@RHBC composite shows a value of 348 F/g and 452 F/g, respectively, at a current density of 0.5 Ag⁻¹, which further reduces with an increasing current density for both materials. The specific capacitance of the ZIF-8@RHBC composite offers higher specific capacitance in comparison to the specific capacitances reported in the literature based on biochar-based composites, as reported in Table 2. The doping of activated BC in ZIF-8 offers synergetic reactions between both phases and improves the electrochemical performance due to the following reasons: (i) the specific structural features of ZIF-8 supported by activated BC improve the specific area for the reaction of electrolytes; (ii) the bonding between ZIF-8 and activated BC creates active centers that facilitate electrolyte diffusion and enhance the reaction kinetics, thereby improving the electrochemical performance and, in particular, the specific capacitance of the composite. The combination of the structural features of ZIF-8 and activated RHBC leads to a highly efficient material for energy storage in comparison to other composites based on BC and reported earlier in the literature (as indicated in Table 2). The higher specific capacitance at low current densities for the ZIF-8@RHBC composite in comparison to ZIF-8 makes it a superior material as a supercapacitor in energy storage applications. The electrochemical impedance spectroscopy (EIS) for ZIF-8 and the ZIF-8@RHBC composite is shown in Figure 9B. The real and imaginary parts of the impedance results in the form of Nyquist plots show characteristic semicircular behavior for both materials. The area under the semicircle is found to be smaller for the ZIF-8@RHBC composite in comparison to ZIF-8, suggesting improved conduction in the composite. The reduced area under the semicircle for the composite suggests a decrease in the charge transfer resistance of the composite and an enhanced conduction process at the electrode surface. The doping of biochar into ZIF-8 results in enhanced charge density at the electrode surface, which facilitates charge diffusion and charge transfer at the electrode surface. The EIS results confirm the formation of a better conducting network in the composite and strengthen the evidence of the improved conductivity and electrochemical performance of the ZIF-8@RHBC composite. This work reports a novel synthesis method for the development of biocomposites based on ZIF-8@RHBC composites for advanced optical and electrochemical applications. Further tailoring the synthesis methods and improving the composite’s structural formation could lead to even better properties. Hence, there is a further need to prepare these composites with varying concentrations of ZIF-8 in the RHBC matrix, to identify the best possible concentrations of these phases that could be utilized in advanced applications.

Table 2 compares the present and other published electrochemical results for doped biochar materials. The specific capacitance of 452 F/gm observed in the present material, ZIF-8 doped with biochar, is an impressive result that demonstrates the potential for high energy storage capabilities in this composite material. This value indicates a significant capacity for charge storage, which is essential for applications such as supercapacitors and energy storage devices. Achieving such high specific capacitance suggests that the ZIF-8–biochar composite has promising prospects for use in advanced energy storage systems.

Figure 10 illustrates the galvanostatic charge–discharge (GCD) of activated ZIF-8 and ZIF@RHBC. The curves show a triangular shape for all current densities. This indicates that the material has capacitive properties under a high current density. Figure 11 shows the GCD results under 3000 cycles, reflecting the cyclic stability of (a) ZIF-8 and (b) ZIF@RHBC. As we can see, the ZIF@RHBC electrode has the best cycle stability compared to ZIF.

3. Experimental Methods

3.1. Chemicals and Materials

Zinc nitrate hydroxide (99% purity), 2-methylimidazole (99% purity), 6M KOH solution (99% purity), HNO₃ (99% purity), HCl (98% purity), and H₂SO₄ (98% purity) were procured from Sigma-Aldrich (Saint Louis, MO, USA) and used without further purification. Carbon black, a separator (Celgard (Charlotte, NC, USA) 2325), styrene butadiene rubber (SBP), and carboxymethyl cellulose (CMC) were used as received from Sigma-Aldrich.

3.2. Material Characterization Techniques

The ZIF-8@RHBC thin films were prepared using an HHV Auto 306 coating system and thermal evaporation in a vacuum of around 1.3 × 10⁻⁵ Pa. The evaporation procedure was conducted at a fixed evaporation rate (5 gm/s). The surface morphologies of ZIF-8 and ZIF-8@RHBC were investigated using a scanning electron microscope (TESCAN-VEGA, Brno, Czech Republic). The crystal structure analysis of ZIF-8 and ZIF-8@RHBC was carried out using an X-ray diffractometer (XRD) (Ultima IV, Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.540598 Å). The functional groups present in ZIF-8 and ZIF-8@RHBC were investigated using a Fourier transform infrared spectrometer (FTIR) (Bruker, Billerica, MA, USA, ISS-88). The thermal stability of the samples in terms of TGA analysis was assessed using a NETSCH (Exton, PA, USA) STA-409PC thermal analyzer. The surface area and pore size analysis of the synthesized composites was conducted using a Nova 2200e surface area and pore analyzer (Quantichrome Instruments). A UV–vis–NIR spectrophotometer (JASCO, Tokyo, Japan, V-570) was used to study the optical spectra of the ZIF-8@RHBC thin films. At normal incidence, the transmittance (T) and reflectance (R) coefficients were calculated in the wavelength range of 200–2500 nm.

3.3. Synthesis of ZIF-8

In a typical method, 0.30 g of zinc nitrate hexahydrate was added to 15 mL of DI (deionized) water and stirred continuously for 20 min; then, 5 g of 2-methylimidazole was added to 100 mL of DI water and stirred for 25 min. Further, the solutions of zinc nitrate and 2-methylimidazole were mixed and stirred for about 10 min. This mixed solution was then transferred to a Teflon autoclave for hydrothermal synthesis at 110 °C for 8 h. The final product was collected through centrifugation, washed several times with water and methanol, and dried in a vacuum overnight at 80 °C to obtain the final white-colored ZIF-8 powder.

3.4. Synthesis of Rice Husk-Activated Biochar (RHBC) and ZIF-8@RHBC

Rice husks were gathered and treated with methanol and distilled water to remove dust, dirt, and impurities. After drying the rice husk samples for 24 h in an oven at 100 °C, they were ground into a fine powder. The finely ground rice husk powder, weighing 10 g, was dispersed in 6M KOH (99% purity) to form a solution and refluxed at 70 °C for 8 h. The obtained solution was subjected to slow pyrolysis in a muffle furnace at a temperature of 600 °C for 3 h. The resulting biochar was treated with HCl (98% purity) to remove the unreacted KOH. Further, this obtained biochar was functionally modified via the reflux process using HNO₃ (99% purity) and H₂SO₄ (98% purity) (at a ratio of 3:1 at 70 °C for 5 h) to obtain functional biochar. To prepare the composite, the as-prepared ZIF-8 in a volume of about 0.25 g was stirred with 50 mL ethanol for 25 min at room temperature. The activated biochar (0.5 g) was mixed with a solution containing the MOF precursor and left to settle for a few hours to allow the MOF to grow on the biochar. Finally, the reaction mixture was transferred to an autoclave for the hydrothermal synthesis of ZIF-8 on RHBC at 150 °C for 6 h. This led to the formation of the final composite known as ZIF-8@RHBC. The final composite product in the form of a carbon material was washed with a diluted solution of a combination of nitric acid and sulfuric acid with a volume ratio of 3:1 to remove the contaminants from the resulting black powder. A schematic representation of the synthesis and work carried out in this study is shown in Scheme 2. The chemical structures of ZIF-8 and the activated biochar are illustrated in Scheme 3.

3.5. ZIF-8@RHBC-Modified GCE Electrodes and Electrochemical Characterization

To fabricate ZIF-8@RHBC-modified anode electrodes (working electrode), functionalized ZIF-8@RHBC (70%), conductive carbon black (20%), CMC (5%), and SBR (5%) were mixed to form a uniform slurry. The slurry was coated on Cu foil using the doctor blade method. The Cu foil was dried in a vacuum oven at 100 °C for 12 h, and circular electrode sheets of this Cu foil (with a diameter of about 10 mm) were obtained as working electrodes. The electrochemical performance of the prepared samples was investigated through cyclic voltammetry (CV). The electrochemical tests were performed using a three-electrode cell configuration with ZIF-8 and ZIF-8@RHBC as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode with 1 M LiPF₆ electrolytic solution. The cyclic voltammetry tests were performed on an electrochemical workstation (Chenhua, Shanghai, China, CHI 660C) at a rate of 2 mV/s in the range of −0.6 to 0.6 V.

4. Conclusions

The biogenic synthesis of biochar-based ZIF-8 (MOF) provides new pathways for the environmentally friendly synthesis of innovative materials for optoelectronics and energy storage applications. Herein, we have successfully synthesized a rice husk-based biochar nanocomposite with ZIF-8. The structural characterization of the synthesized materials was carried out in terms of SEM, XRD, BET, FTIR, TGA, and UV–vis spectroscopy. The addition of biochar to ZIF-8 significantly modified the composite into a highly mesoporous structure. An increase in the porosity of ZIF-8@RHBC beyond that in ZIF-8 has been observed by SEM. The ZIF-8@RHBC composite shows a considerable improvement in its optical and electrochemical properties in comparison to bare ZIF-8. The optical conductivity for the ZIF-8@RHBC composite is found to increase by about 5% in comparison to ZIF-8, which can be related to the increase in the number of charge carriers or decrease in the amount of scattering of light within the material. Further, the ZIF-8@RHBC composite shows excellent electrochemical performance with improved CV curves and specific capacitance due to its mesoporous structure, which supports a higher diffusion rate. The specific capacitance is enhanced from 374 F/g (ZIF-8) to 451 F/g for the ZIF-8@RHBC composite as a function of the current density. Further, the specific capacitance is enhanced from 892 F/g (ZIF-8) to 1195 F/g for the ZIF-8@RHBC composite. This indicates the importance of the synthesized material for possible supercapacitor applications. The EIS studies suggest a decrease in charge transfer resistance for the ZIF-8@RHBC composite compared to ZIF-8, which provides support for the fact that biochar doping improves the conduction mechanism at the electrode surface. Due to its biosynthesis, superior structural features, and excellent electrical conductivity and optical and electrochemical properties, this novel ZIF-8@RHBC composite could be potentially used in optoelectronics, as well as energy storage applications.

Author Contributions

Conceptualization, S.K., T.A.H. and M.M.A.; methodology, S.A.-a., S.K., T.A.H. and S.A.A.-G.; synthesis, S.A.-a., M.M.A. and T.A.H.; materials characterization, T.A.H.; software, A.M.A.; data analysis, S.K., A.M.A. and S.A.A.-G.; writing—review and editing, S.K. and T.A.H.; fund acquisition, M.M.A. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The authors confirm that the datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors extend their appreciation to the Deanship of Research and Graduate Studies at the University of Tabuk for funding this work through research no. S-1444-0019.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures, Schemes and Tables

Figure 1. Functional groups for ZIF-8 and ZIF-8@RHBC using FTIR spectra.

Figure 2. XRD for ZIF-8 and ZIF-8@RHBC.

Figure 3. SEM micrographs of ZIF-8 and ZIF-8@RHBC.

Figure 4. TGA analysis for ZIF and ZIF-8@RHBC.

Figure 5. (A) The optical transmission and reflection of ZIF-8 and ZIF-8@RHBC. (B) The dependence of (αE)1/2 on the photon energy (E) for ZIF-8 and ZIF-8@RHBC.

Scheme 1. The energy gap of ZIF-8@RHBC.

Figure 6. (A) Refractive index spectra and (B) absorption spectra for ZIF-8 and ZIF-8@RHBC (B).

Figure 7. (A) Plot of ln(α) (optical conductivity) as a function of photon energy (E) for ZIF-8 and ZIF-8@RHBC. (B) Dependence of thermal emissivity (εth) on wavelength (λ) for ZIF-8 and ZIF-8@RHBC.

Figure 8. (A) Cyclic voltammetry (CV) curves for ZIF-8 and ZIF-8@RHBC at a scanning rate of 2 mVs−1 (in aqueous 1M LiPF6 electrolyte). (B) Specific capacitance as a function of the scan rate at a constant current density of 0.1 Ag−1 for ZIF-8 and ZIF-8@RHB.

Figure 9. (A) Specific capacitance as a function of the current density for ZIF-8 and ZIF-8@RHBC. (B) Nyquist plot for ZIF-8 and ZIF-8@RHBC.

Figure 10. Galvanostatic charge–discharge (GCD) of activated ZIF-8 and ZIF@RHBC.

Figure 11. GCD results under 3000 cycles: cyclic stability of (a) ZIF-8 and (b) ZIF@RHBC.

Scheme 2. The experimental setup used in the present study.

Scheme 3. The chemical structures of (a) ZIF-8 and (b) activated biochar.

Table 1

Optical band gap ($E_g$), linear refractive index ($n_o$), linear susceptibility (χ⁽¹⁾), nonlinear susceptibility (χ⁽³⁾), and nonlinear refractive ($n_2$) for ZIF-8 and ZIF-8@RHBC nanocomposite.

	ZIF-8	ZIF-8@RHBC
Eg (eV)	2.60	2.68
$n_o$	2.45	2.52
${\epsilon }_0$	5.47	5.48
χ(1) (esu)	0.711	0.714
χ(3) × 10−11(esu)	4.35	4.41
$n_2$ × 10−10(esu)	7.02	7.10

Table 2

Comparison between the present and other published electrochemical results for doped biochar materials.

	BC Extracted Source	Method	Specific Capacitance Fg−1	Ref.
NPCP/BC	Cornstalks	Thermochemical process	203.5	[39]
Native sulfur/BC	Camellia japonica flowers	Chemical processs	125.42	[40]
N/BC	Kapok (Ceiba insignis)	Microwave-assisted solvothermal method	283	[41]
ZIF-@/BC	Rice husk	Hydrothermal carbonization method	452	Our work

References

1. El Safety, A.M.K. Environmental and health impact of open burning rice straw. Egypt. J. Occup. Med.; 2020; 44, pp. 679-708.

2. Shamsollahi, Z.; Partovinia, A. Recent advances on pollutants removal by rice husk as a bio-based adsorbent: A critical review. J. Environ. Manag.; 2019; 246, pp. 314-323. [DOI: https://dx.doi.org/10.1016/j.jenvman.2019.05.145] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31185318]

3. Khasim, S.; Dastager, S.G.; Alahmdi, M.I.; Hamdalla, T.A.; Panneerselvam, C.; Makandar, M.B. Novel biogenic synthesis of Pd/TiO@ BC as an electrocatalytic and possible energy storage materials. Ceram. Int.; 2023; 49, pp. 15874-15883. [DOI: https://dx.doi.org/10.1016/j.ceramint.2023.01.182]

4. Alsharif, M.A.; Alatawi, A.; Hamdalla, T.A.; Alfadhli, S.; Darwish, A.A.A. CuO nanoparticles mixed with activated BC extracted from algae as promising material for supercapacitor electrodes. Sci. Rep.; 2023; 13, 22321. [DOI: https://dx.doi.org/10.1038/s41598-023-49760-4] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38102239]

5. Hamdalla, T.A.; Alfadhli, S.; Khasim, S.; Darwish, A.A.A.; ElZaidia, E.F.M.; Al-Ghamdi, S.A.; Aljohani, M.M.; Mahmoud, M.E.; Seleim, S.M. Synthesis of novel Cu/Fe based benzene Dicarboxylate (BDC) metal organic frameworks and investigations into their optical and electrochemical properties. Heliyon; 2024; 10, 1235. [DOI: https://dx.doi.org/10.1016/j.heliyon.2024.e25065] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38317972]

6. Reshmy, R.; Philip, E.; Madhavan, A.; Sindhu, R.; Pugazhendhi, A.; Binod, P.; Sirohi, R.; Awasthi, M.K.; Tarafdar, A.; Pandey, A. Advanced biomaterials for sustainable applications in the food industry: Updates and challenges. Environ. Pollut.; 2021; 283, 117071.

7. Kwapinski, W.; Byrne, C.M.P.; Kryachko, E.; Wolfram, P.; Adley, C.; Leahy, J.J.; Novotny, E.H.; Hayes, M.H.B. Biochar from biomass and waste. Waste Biomass Valorization; 2010; 1, pp. 177-189. [DOI: https://dx.doi.org/10.1007/s12649-010-9024-8]

8. Zhang, X.; Zhang, P.; Yuan, X.; Li, Y.; Han, L. Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar. Bioresour. Technol.; 2020; 296, 122318. [DOI: https://dx.doi.org/10.1016/j.biortech.2019.122318]

9. Wang, Z.; Ogata, H.; Morimoto, S.; Ortiz-Medina, J.; Fujishige, M.; Takeuchi, K.; Muramatsu, H.; Hayashi, T.; Terrones, M.; Hashimoto, Y. et al. Nanocarbons from rice husk by microwave plasma irradiation: From graphene and carbon nanotubes to graphenated carbon nanotube hybrids. Carbon; 2015; 94, pp. 479-484. [DOI: https://dx.doi.org/10.1016/j.carbon.2015.07.037]

10. Sharma, S.K.; Sharma, G.; Sharma, A.; Bhardwaj, K.; Preeti, K.; Singh, K.; Kumar, A.; Pal, V.K.; Choi, E.H.; Singh, S.P. et al. Synthesis of silica and carbon-based nanomaterials from rice husk ash by ambient fiery and furnace sweltering using a chemical method. Appl. Surf. Sci. Adv.; 2022; 8, 100225. [DOI: https://dx.doi.org/10.1016/j.apsadv.2022.100225]

11. Phan, A.; Doonan, C.J.; Uribe-Romo, F.J.; Knobler, C.B.; O’keeffe, M.; Yaghi, O.M. Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks. Acc. Chem. Res.; 2010; 43, pp. 58-67. [DOI: https://dx.doi.org/10.1021/ar900116g] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/19877580]

12. Khan, U.A.; Iqbal, N.; Noor, T.; Ahmad, R.; Ahmad, A.; Gao, J.; Amjad, Z.; Wahab, A. Cerium based metal organic framework derived composite with reduced graphene oxide as efficient supercapacitor electrode. J. Energy Storage; 2021; 41, 102999. [DOI: https://dx.doi.org/10.1016/j.est.2021.102999]

13. Wang, J.-R.; Wan, F.; Lü, Q.-F.; Chen, F.; Lin, Q. Self-nitrogen-doped porous biochar derived from kapok (Ceiba insignis) fibers: Effect of pyrolysis temperature and high electrochemical performance. J. Mater. Sci. Technol.; 2018; 34, pp. 1959-1968. [DOI: https://dx.doi.org/10.1016/j.jmst.2018.01.005]

14. Ahmad, R.; Khan, U.A.; Iqbal, N.; Noor, T. Zeolitic imidazolate framework (ZIF)-derived porous carbon materials for supercapacitors: An overview. RSC Adv.; 2020; 10, pp. 43733-43750. [DOI: https://dx.doi.org/10.1039/D0RA08560J] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35519688]

15. Ahmad, R.; Iqbal, N.; Baig, M.M.; Noor, T.; Ali, G.; Gul, I.H. ZIF-67 derived nitrogen doped CNTs decorated with sulfur and Ni (OH)₂ as potential electrode material for high-performance supercapacitors. Electrochim. Acta; 2020; 364, 137147. [DOI: https://dx.doi.org/10.1016/j.electacta.2020.137147]

16. Ahmad, R.; Iqbal, N.; Noor, T. Development of ZIF-derived nanoporous carbon and cobalt sulfide-based electrode material for supercapacitor. Materials; 2019; 12, 2940. [DOI: https://dx.doi.org/10.3390/ma12182940] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31514366]

17. Raza, M.A.; Wahab, A.; Bhatti, A.H.U.; Ahmad, A.; Ahmad, R.; Iqbal, N.; Ali, G. CoS₂/MnS₂ co-doped ZIF-derived nitrogen doped high surface area carbon-based electrode for high-performance supercapacitors. Electrochim. Acta; 2022; 407, 139914. [DOI: https://dx.doi.org/10.1016/j.electacta.2022.139914]

18. Zhou, K.; Mousavi, B.; Luo, Z.; Phatanasri, S.; Chaemchuen, S.; Verpoort, F. Characterization and properties of Zn/Co zeolitic imidazolate frameworks vs. ZIF-8 and ZIF-67. J. Mater. Chem. A; 2017; 5, pp. 952-957. [DOI: https://dx.doi.org/10.1039/C6TA07860E]

19. Tian, Y.-Q.; Yao, S.-Y.; Gu, D.; Cui, K.-H.; Guo, D.-W.; Zhang, G.; Chen, Z.-X.; Zhao, D.-Y. Cadmium imidazolate frameworks with polymorphism, high thermal stability, and a large surface area. Chem.-Eur. J.; 2010; 16, pp. 1137-1141. [DOI: https://dx.doi.org/10.1002/chem.200902729] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/20033974]

20. Van Cleuvenbergen, S.; Stassen, I.; Gobechiya, E.; Zhang, Y.; Markey, K.; De Vos, D.E.; Kirschhock, C.; Champagne, B.; Verbiest, T.; Van Der Veen, M.A. ZIF-8 as nonlinear optical material: Influence of structure and synthesis. Chem. Mater.; 2016; 28, pp. 3203-3209. [DOI: https://dx.doi.org/10.1021/acs.chemmater.6b01087]

21. Dong, L.; Chu, H.; Li, Y.; Zhao, S.; Li, D. Broadband optical nonlinearity of zeolitic imidazolate framework-8 (ZIF-8) for ultrafast photonics. J. Mater. Chem. C; 2021; 9, pp. 8912-8919. [DOI: https://dx.doi.org/10.1039/D1TC01665B]

22. Keppler, N.C.; Hindricks, K.D.J.; Behrens, P. Large refractive index changes in ZIF-8 thin films of optical quality. RSC Adv.; 2022; 12, pp. 5807-5815. [DOI: https://dx.doi.org/10.1039/D1RA08531J] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35424585]

23. Schernikau, M.; Sablowski, J.; Martinez, I.G.G.; Unz, S.; Kaskel, S.; Mikhailova, D. Preparation and application of ZIF-8 thin layers. Appl. Sci.; 2021; 11, 4041. [DOI: https://dx.doi.org/10.3390/app11094041]

24. Lin, M.-C.; Kuo, S.-W.; Mohamed, M.G. High-performance anthracene-linked covalent triazine frameworks with dual functions for CO₂ capture and supercapacitor applications. Mater. Adv.; 2024; 5, pp. 6222-6233. [DOI: https://dx.doi.org/10.1039/D4MA00565A]

25. Lebedeva, O.; Zakharov, V.; Kuznetsova, I.; Kultin, D.; Chernyshev, V.; Kalmykov, K.; Savilov, S.; Dunaev, S.; Aslanov, L.; Kustov, L. Green Synthesis of the Triazine Derivatives and their Application for the Benign Electrocatalytic Reaction of Nitrate Reduction to Ammonia. Chem.-Eur. J.; 2024; e202402075. [DOI: https://dx.doi.org/10.1002/chem.202402075]

26. Gong, X.; Wang, Y.; Kuang, T. ZIF-8-based membranes for carbon dioxide capture and separation. ACS Sustain. Chem. Eng.; 2017; 5, pp. 11204-11214. [DOI: https://dx.doi.org/10.1021/acssuschemeng.7b03613]

27. Qin, P.; Song, H.; Ruan, Q.; Huang, Z.; Xu, Y.; Huang, C. Direct observation of dynamic surface reconstruction and active phases on honeycomb Ni₃N−Co₃N/CC for oxygen evolution reaction. Sci. China Mater.; 2022; 65, pp. 2445-2452. [DOI: https://dx.doi.org/10.1007/s40843-021-1995-4]

28. Wang, Z.; Lai, C.; Qin, L.; Fu, Y.; He, J.; Huang, D.; Li, B.; Zhang, M.; Liu, S.; Li, L. et al. ZIF-8-modified MnFe₂O₄ with high crystallinity and superior photo-Fenton catalytic activity by Zn-O-Fe structure for TC degradation. Chem. Eng. J.; 2020; 392, 124851. [DOI: https://dx.doi.org/10.1016/j.cej.2020.124851]

29. Li, R.; Li, W.; Jin, C.; He, Q.; Wang, Y. Fabrication of ZIF-8@TiO₂ micron composite via hydrothermal method with enhanced absorption an photocatalytic activities in tetracycline degradation. J. Alloys Compd.; 2020; 825, 154008. [DOI: https://dx.doi.org/10.1016/j.jallcom.2020.154008]

30. Hu, Y.; Kazemian, H.; Rohani, S.; Huang, Y.; Song, Y. In situ high pressure study of ZIF-8 by FTIR spectroscopy. Chem. Commun.; 2011; 47, pp. 12694-12696. [DOI: https://dx.doi.org/10.1039/c1cc15525c] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/22037659]

31. Alfadhli, S.; Darwish, A.A.A.; Soliman, S.; El-Zaidia, E.F.M.; Yahia, I.S.; Laariedh, F.; Alatawi, A.; Bahamran, A.; Alatawi, N.M.; Hamdalla, T.A. Structural characterizations and photoelectric performance of non-crystalline boron subphthalocyanine chloride films/FTO for photodiode applications. J. Non-Cryst. Solids; 2023; 601, 122044. [DOI: https://dx.doi.org/10.1016/j.jnoncrysol.2022.122044]

32. Al-Ghamdi, S.A.; Hamdalla, T.A.; El-Zaidia, E.F.M.; Alzahrani, A.O.M.; Alghamdi, N.; Khasim, S.; Yahia, I.S.; Darwish, A.A.A. Structural, electronic, and optoelectronic characteristics of GaClPc/n-Si heterojunction for photodiode device. Mater. Sci. Semicond. Process.; 2022; 147, 106704. [DOI: https://dx.doi.org/10.1016/j.mssp.2022.106704]

33. Liu, Y.; Cao, S.; Xi, C.; Su, H.; Chen, Z. A new nanocomposite assembled with metal organic framework and magnetic biochar derived from pomelo peels: A highly efficient adsorbent for ketamine in wastewater. J. Environ. Chem. Eng.; 2021; 9, 106207. [DOI: https://dx.doi.org/10.1016/j.jece.2021.106207]

34. Al-Ghamdi, S.A.; Darwish, A.A.A.; Hamdalla, T.A.; Alzahrani, A.O.M.; Khasim, S.; Qashou, S.I.; El-Rahman, K.F.A. Preparation of TlInSe₂ thin films using substrate temperature: Characterization, optical and electrical properties. Opt. Mater.; 2022; 129, 112514. [DOI: https://dx.doi.org/10.1016/j.optmat.2022.112514]

35. Dissanayake, P.D.; You, S.; Igalavithana, A.D.; Xia, Y.; Bhatnagar, A.; Gupta, S.; Kua, H.W.; Kim, S.; Kwon, J.-H.; Tsang, D.C.W. et al. Biochar-based adsorbents for carbon dioxide capture: A critical review. Renew. Sustain. Energy Rev.; 2020; 119, 109582. [DOI: https://dx.doi.org/10.1016/j.rser.2019.109582]

36. Al-Harbi, F.F.; Darwish, A.A.; Al-Ghamdi, S.A.; Khasim, S.; El-Shazly, E.A.; El-Rahman, K.F.A.; Hamdalla, T.A. Impedance spectroscopy, electronic transport, and dielectric relaxation studies in methylene green organic semiconductor for photovoltaic application. Phys. Scr.; 2022; 97, 085810. [DOI: https://dx.doi.org/10.1088/1402-4896/ac7d15]

37. Khasim, S.; Misra, Y.K.; Badi, N.; Pasha, A.; Lakshmi, M. Fabrication of high performance flexible supercapacitors using secondary doped PEDOT-PSS: Graphene nanocomposite films for large area solid state devices. RSC Adv.; 2020; 10, pp. 10526-10539. [DOI: https://dx.doi.org/10.1039/D0RA01116A]

38. Gao, M.; Lu, M.; Zhang, X.; Luo, Z.; Xiao, J. Application of Fiber Biochar–MOF Matrix Composites in Electrochemical Energy Storage. Polymers; 2022; 14, 2419. [DOI: https://dx.doi.org/10.3390/polym14122419] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35745995]

39. Ma, Z.W.; Liu, H.Q.; Lü, Q.F. Porous biochar derived from tea saponin for supercapacitor electrode: Effect of preparation technique. J. Energy Storage; 2021; 40, 102773. [DOI: https://dx.doi.org/10.1016/j.est.2021.102773]

40. Yuan, X.; Xiao, J.; Yılmaz, M.; Zhang, T.C.; Yuan, S. N, P Co-doped porous biochar derived from cornstalk for high performance CO₂ adsorption and electrochemical energy storage. Sep. Purif. Technol.; 2022; 299, 121719. [DOI: https://dx.doi.org/10.1016/j.seppur.2022.121719]

41. Xia, C.; Surendran, S.; Ji, S.; Kim, D.; Chae, Y.; Kim, J.; Je, M.; Han, M.K.; Choe, W.S.; Choi, C.H. et al. A sulfur self-doped multifunctional biochar catalyst for overall water splitting and a supercapacitor from Camellia japonica flowers. Carbon Energy; 2022; 4, pp. 491-505. [DOI: https://dx.doi.org/10.1002/cey2.207]