PANI Incorporated Fe-MOF: As an Electrode Material for Supercapacitor

Manish Kumar, Shubhra Sinha, Rajiv Nayan, Manmohan L. Satnami, Manas Kanti Deb, Kallol K. Ghosh, Shamsh Pervez, Kamlesh K. Shrivas, Vaibhav Dixit and Indrapal Karbhal^*

School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur (C.G.) India

Abstract

A novel composite of Fe-metal organic framework (Fe-MOF) and polyaniline (PANI) was synthesized using a cost-effective and rapid hydrothermal method. Structural characterization confirmed the successful incorporation of granular PANI over the Fe-MOF surface, which was examined through X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The Fe-MOF@PANI electrode demonstrated a high specific capacitance of 234.6 F g⁻¹ at 1 A g⁻¹ and 87% capacitance retention after 400 cycles at 10 A g⁻¹. Key features of the synthesized nanocomposite include scalability, high porosity, long-term stability, good conductivity, low charge transport resistance, and a fast charge-discharge rate. This study highlights Fe-MOF@PANI as a promising electrode material for supercapacitors.

Keywords: Metal organic framework; Energy storage; Supercapacitor, Polyaniline, Stability.

Introduction

Energy is a fundamental concept in both scientific inquiry and everyday life, with the majority of it ultimately derived from the sun and often harnessed through various chemical reactions. For instance, gasoline used to power vehicles, electricity supplied to homes, and the food consumed in diets all rely on chemical reactions to either release or generate energy. Gasoline and food undergo specific chemical processes to release energy, while electricity production, with about 50% generated by burning coal, is a result of complex chemical reactions. This intrinsic connection makes the study of energy an integral part of chemistry. In recent years, the global demand for energy has surged dramatically and is anticipated to escalate further due to a combination of population growth, economic expansion, and technological advancements. This rising demand, coupled with environmental concerns and the intricate nature of power distribution systems, has driven an urgent shift towards renewable and sustainable energy sources, such as wind and solar energy (Karbhal et al., 2023; Karbhal et al., 2022; Thakur et al., 2017). The efficient utilization of these new energy sources is crucial, given their non-constant power generation capabilities, necessitating advancements in energy storage and distribution technologies to ensure a stable and reliable energy supply (Patrike et al., 2023; Tan et al., 2023; Sadiq et al., 2022; Deshmukh et al., 2018).

Metal-organic frameworks (MOFs) have unique qualities such as increased surface area, porous structure, high thermal and mechanical stability, active redox sites, making them a promising solution for such applications (Shaheen et al., 2023). MOFs are highly structured materials made up of metal ions or clusters (nodes) connected by organic ligands (linkers), creating unique and customizable porous architectures. Using metal cores as electrodes in supercapacitors can improve energy storage through pseudo-capacitance. MOFs are characterized by their high porosity and surface area, akin to tiny sponges with up to 90% of their volume being space. Unlike some other porous materials, MOFs offer exceptional control over their pore size and functionality. By carefully selecting the metal clusters and organic linkers used in their synthesis, scientists can design MOFs with pores of specific sizes and chemical properties, allowing for targeted applications like separating specific gas molecules or capturing pollutants (Sinha et al., 2024; Niu et al., 2022; Devi et al., 2020; Wang et al., 2020). MOFs possess a highly ordered crystalline structure, contributing to their stability and predictability, which simplifies their study and engineering for specific purposes. The organic linkers in MOFs can be modified with various functional groups, enabling researchers to tailor the MOF's surface chemistry for specific applications such as catalysis or drug delivery. Many MOFs exhibit remarkable thermal stability, capable of withstanding high temperatures without decomposition, making them suitable for applications requiring harsh operating conditions. The combination of high porosity, tunable pores, and functionalizability allows scientists to design materials with a wide range of properties, making them highly versatile materials with the potential to revolutionize various fields (Sinha et al., 2024; Sinha et al., 2023). Additionally, certain MOFs possess inherent electrical and optical properties, such as conductivity, luminescence, and nonlinear optical behaviour, which have implications for applications in electronics, photonics, and optoelectronics (Zhang et al., 2017). They may have limited applicability due to their unpredictable orientation, low conductivity, and inclination to agglomerate (Hussain et al., 2022). To improve the structure and properties of MOFs, researchers have focused on developing better preparation processes. MOFs have gained tremendous research attention in recent years for their versatile applications in a wide variety of fields, including their potential use in supercapacitors due to their high surface area, tunable porosity, and unique electrical properties.

The current research aims to overcome constraints in MOFs and maximize their potential as energy storage devices. It is noteworthy that, understanding MOF materials requires analysing their crystallographic data, such as unit cell diameters, space group symmetry, and atomic positions inside the crystal lattice. MOFs physical and chemical properties are influenced by the arrangement of their atoms or molecules, as revealed by structural information. To improve supercapacitor performance, MOFs can be combined with aqueous electrolytes, doped with conductive materials, or fabricated using various processes. MOFs as electrode materials can improve supercapacitor performance, especially when paired with aqueous electrolytes (Nagaraju et al., 2022). Hence, a composite of Fe-MOF with conducting polymer, polyaniline (PANI), i.e., Fe-MOF@PANI has been synthesized in the present work and has been employed as a supercapacitor electrode material. It was observed that Fe-MOF@PANI electrode exhibited a high specific capacitance of 234.64 F g^-1 at a current density of 1 A g^-1. Moreover, it shows 87% capacitance retention after 400 cycles at a current density of 10 A g^-1.

Experimental Section

Chemicals and Reagents

Trimesic acid (95%) and aniline were purchased from Sigma Aldrich. Ammonium persulfate (APS) and ferric chloride were purchased from Qualigens, N, N-Dimethylforamide (99.80%) and acetone were purchased from Merck. Concentrated HCl and other reagents were of analytical grade and were used without any further purification. Deionized (DI) water was used during all the experiments.

Synthesis of Fe-MOF@PANI

To synthesize the Fe-MOF, solvothermal method was selected. The solvothermal synthesis process uses aqueous or non-aqueous solvents to create ferrite materials with exact control over size distribution, shape, and crystalline phases.  Initially, trimesic acid (0.420 g) and ferric chloride (0.324 g) were taken in a 100 ml beaker and 1:4 solution of DMF and water respectively were added and stirred in a magnetic stirrer for 10 mins. The obtained solution is further kept in an autoclave for heating in a vacuum oven at 130°C for 24 hr. After cooling down filtration process is performed along with washing with acetone and deionized water and dried in a vacuum oven at 90°C. Figure 1 represents the schematic representation of the above-mentioned synthesis procedure.

For the synthesis of Fe-MOF@PANI (0.1 M), Fe-MOF (100 mg) was taken in 0.1 M HCl solution in a round bottom flask then, 0.1 M aniline was added to the same. After stirring for 15 minutes, 0.125 M APS was added dropwise to the solution. It was observed that the colour of the solution slowly changed to emerald green. The synthesized product was then stored at 10°C for 24 hours to facilitate the formation of the PANI granules. Later, it was taken out and dried in a vacuum oven at 90°C (Nayan et al., 2024). The synthesis procedure has been illustrated schematically in figure 2.

Electrolyte Preparation

For the entire period of electrode testing, a 1 M H₂SO₄ solution was prepared and used as the electrolyte.

 
Figure 2. Synthesis procedure for PANI integrated Fe-MOF

Results and Discussion

SEM Analysis

The morphological characteristics of Fe- MOF and Fe- MOF@PANI were determined through FESEM micrographs, as depicted in figure 3. The figure 3 (a) and (b) are the FE-SEM images of the Fe-MOF and it can be seen the typical octahedral morphology of Fe-MOF, whereas figure 3 (c) and (d) validate the successful incorporation of PANI. The integration of PANI with provided a high surface area and more diffusion sites (Villenoisy et al., 2021).

Figure 3. (a) & (b) SEM images Fe-MOF and (c) & (d) PANI incorporated Fe-MOF

XRD Analysis

The X-ray diffractograms of the pure Fe-MOF, and PANI integrated Fe-MOF composite are displayed in Figure 4 (a). The peaks at 9-10° (002), 15° (101), and 18-20° (201) confirm the formation of pristine Fe-MOF, peaks 43-45° (110) and 65° (200), While additional peaks can be observed in the pattern of PANI-integrated Fe-MOF peaks 15° (011), 20-30° (020), 30-40° (200) and 65° (200). The supplementary peaks correspond to PANI, confirming the successful integration of PANI in Fe-MOF (Tian et al., 2021; Chu et al., 2020).

Figure 4. (a) XRD and (b) Raman spectrum of Fe-MOF and Fe-MOF@PANI

Raman Analysis

In order to carry out the structural investigation of the synthesized materials, Raman spectroscopy was employed. The Raman spectra for Fe-MOF/PANI and Fe-MOF have been displayed in figure 4 (b). The peaks around 1328 cm^-1 and 1561 cm^-1 are related to the D, and G bands respectively. Generally, the D band is associated with disordered of sp³ carbon network. It is associated with the degree of defect in the material, while the G band is a result of in-plane stretching of sp² carbon bond. Similar results were observed in case of Fe-MOF, depicting the D and G bands at 1350 cm^-1 and 1580 cm^-1, respectively (Jeong et al., 2023).

Electrochemical measurements were conducted using the CH instrument with a 1M H₂SO₄ aqueous electrolyte. Figure 5 (a) represents the galvanic charge discharge (GCD) curves of 0.1 M Fe-MOF@PANI at a current density of 1, 2, 3, 4, 5, 8, 10 A g^-1. Figure 5 (b) shows the cyclic voltammogram (CV) of and Fe-MOF@PANI (0.1 M) displays a potential range of 0.0-1.0 V at a scan rate of 200 V s^-1. Notably, the CV curve indicates that 0.1 M Fe-MOF@PANI exhibits a significantly wide rectangular shape compared to Fe-MOF suggesting superior charge storage capacitance in 0.1 M Fe-MOF@PANI. Figure 5 (c) represents the GCD graph of Fe-MOF and Fe-MOF@PANI at a current density of 1 A g^-1. 0.1 M Fe-MOF@PANI demonstrates as high as capacitance of 234.6 F g^-1 at a current density of 1 A g^-1. The enhanced performance of 0.1 M Fe-MOF@PANI is attributed to its high conductivity and the pseudocapacitive behaviour of PANI incorporated on the surface of Fe-MOF. Similarly, the CV of Fe-MOF and Fe-MOF@PANI at a scan rate of 200 V s^-1 have been depicted in the figure 5 (d). Figures 5 (e) represent the GCD curve of Fe-MOF at different current densities of 0.3, 0.5, 0.8, 1, and 2 A g^-1 with capacitance of 7.5 F g^-1, 5.5 F g^-1, 4.8 F g^-1, 4 F g^-1 and 4 F g^-1, respectively. Figure 5 (f) shows the CV curve of Fe-MOF at various scan rates (10, 20, 50, 100, 150, and 200 V s^-1). Therefore, the results underscore that 0.1 M Fe-MOF@PANI exhibits superior capacitance compared to Fe-MOF. Moreover, Figure 6 depicts the electrochemical stability of 0.1 M Fe-MOF@PANI at a current density of 10 A g^-1 for 400 cycles. In summary, the comprehensive data confirms that 0.1 M Fe-MOF@PANI exhibits a promising capacitance of 234.6 F g^-1 at a current density of 1 A g^-1 compared to Fe-MOF and Fe-MOF@PANI samples (Dixit et al., 2023; Saleem et al., Zaka et al., 2023; Vinodh et al., 2022; Karbhal et al., 2021; Lu et al., 2021; Zhou et al., 2019; Karbhal et al., 2016).

Figure 5. Electrochemical performance of Fe-MOF and Fe-MOF@PANI composite in 1 M H₂SO₄ (a) GCD curves of 0.1 M Fe-MOF@PANI at a current density of 1, 2, 3, 4, 5, 8,10 A g^-1 (b) CV curve of Fe-MOF@PANI at a scan rate of 10 to 200 V s^-1 (c) GCD curves of Fe-MOF and Fe-MOF@PANI composite at a current density of 1 A (d) CV curve of Fe-MOF and Fe-MOF@PANI at a scan rate of 200 V s^-1 (e) GCD curves of Fe-MOF at different current density (f) CV curve of Fe-MOF at a scan rate of 10 to 200 V s^-1.

Table 1 illustrates the comparative properties of different MOF-based materials for supercapacitors (Liu et al., 2024; Jeong et al., 2023; Brindha et al., 2022; Chu et al., 2020; Peng et al., 2020; Qu et al., 2020; Shi et al., 2020; Zhou et al., 2019; Zhu et al., 2018).

Table 1. Comparative properties of different MOF-based materials for supercapacitors

Table Footnote: ^aCoNi: Cobalt/Nickel; ^bCuO-MOF@CF/NiCo₂S₄@CF: Copper oxide MOF@ Nickel-Copper Sulfide Carbon Fiber; ^cNiCoMOF@NiCo CNF: Nickel copper MOF@ nickel copper carbon nanofiber; ^dFe-MOF@AC: Iron MOF@ activated carbon; ^eNi-MOF/Ti₃C₂T_X: Titanium Carbide; ^fCOFs: Covalent Organic Frameworks; ^gNF: Nickel Foam; ^hPTCDA: Perylene-Tetracarboxylic-Dianhydride

Conclusion

Fe-MOF@PANI was synthesized using hydrothermal process. FESEM characterization confirmed the successful incorporation of granular PANI over the surface of the Fe-MOF. The key role of PANI in enhancing the performance of the Fe-MOF was investigated using electrochemical characterizations. The synthesized Fe-MOF@PANI achieved a capacitance of 234.6 F g^-1 at a current density of 1 A g^-1 and exhibited excellent cyclic stability of 87% after 400 cycles at a current density of 10 A g^-1. The performance of Fe-MOF@PANI shows promising electrochemical behaviour as an electrode material for supercapacitors.

Acknowledgements

The authors are thankful to DST-PURSE (SR/PURSE/2022/145), SERB (EEQ/2022/000967) and SERB (CRG/2022/003926) projects for supporting the work with financial assistance. 

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