Rice Straw-Derived Carbon Integrated with PANI: As an Electrode Material for High-performance Supercapacitor

Vaibhav Dixit, Rajiv Nayan, Shubhra Sinha, Suryakant Manikpuri, Manmohan L. Satnami, Kallol K. Ghosh, Manas Kanti Deb, Shamsh Pervez, Indrapal Karbhal*

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

Abstract

A novel composite electrode material has been synthesized using in-situ chemical polymerization of aniline over the surface of rice straw-derived carbon (RSC). The detailed structural characterization validates the effective incorporation of granular polyaniline (PANI) over the RSC surface. The supercapacitor performance of the RSC@PANI electrode was systematically investigated and achieved as high as specific capacitance of 408 Fg^-1 at a current density of 1 Ag^-1. Moreover, RSC@PANI shows 93% capacitance retention after 1000 cycles at a current density 10 Ag^-1. Apart from its outstanding electrochemical performance, the resulting RSC@PANI electrode exhibits exceptional characteristics such as scalability and lightweight properties. This study contributes valuable insights into the synthesis and characterization of RSC@PANI as a promising electrode material for supercapacitors.

Keywords: Rice Straw, Polyaniline, RSC@PANI, Supercapacitor, Scalability, Low-cost.

Introduction

The growing energy consumption, depletion of natural energy resources, drastic climate changes and environmental concerns paradigm shift towards sustainable energy sources with green and clean energy storage systems. As the world strives for a transition to cleaner and more efficient energy sources, energy storage technologies play a key role in ensuring the reliability and sustainability of power systems for the next generation (Li et al., 2014; Olabi et al., 2022; Kasprzak et al., 2023). Among the various available energy storage devices, supercapacitors (SCs), also known as electrochemical capacitors or ultracapacitors, stand out as promising electrochemical devices that show higher power density than traditional batteries. Although SCs exhibit remarkable characteristics such as high power density and long cycle life but its large-scale energy storage system is hindered due to low energy density (Guan et al., 2012; Benoy et al., 2022; Karbhal et al., 2022). Despite this drawback, the exceptionally high power density of SCs renders them promising for integration in hybrid and heavy electric vehicles for high power supply. Due to its inherent characteristics, researchers are actively exploring diverse approaches to improve the energy density of SCs by employing a variety of electrode materials (Karbhal et al., 2021; Behzadi pour et al., 2023).

In general, carbon-based materials, encompassing activated carbon (AC) (Zhi et al., 2014), graphene (He et al., 2013), carbon nanotube (CNT) (Huang et al., 2023), carbon fiber (CF) (Yu et al., 2014), templated porous carbon (Yang et al., 2020) and fullerene (Tang et al., 2017) are widely employed as electrode materials in electrochemical double-layer capacitors (EDLCs). Broadly, enhancing the specific capacitance and energy density involves adjusting the surface area and conductivity of carbon-based materials. Recent efforts have concentrated on boosting the energy density of SCs through the combination of EDLC materials with pseudo-capacitive materials like metal oxides (MnO₂, RuO₂, NiO, Co₃O₄ etc.) and conductive polymers like polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), etc. However, in comparison with the EDLCs, pseudocapacitive electrode materials have appreciable energy densities but exhibit intrinsic structural instability and poor cycle life. Additionally, metal oxide-based electrodes corrode when dipped in acidic electrolytes resulting in poor cycle life. Moreover, the structural rigidity and low electrical conductivity make them less likely to be used as electrode materials. This brings up the need for the development of hybrid electrodes to achieve enhanced energy density as well as electrochemical stability (Karbhal et al., 2020; Abhishek et al., 2023; Jayakumar et al., 2023). Furthermore, there have been various endeavours to modify the carbon network by introducing pseudo-capacitive heteroatoms, such as boron, nitrogen and sulphur have been introduced on carbon matrices to improve the overall performance of carbon-based materials (Karbhal et al., 2022; Sinha et al., 2023; Yang et al., 2023).

It is noteworthy that, there is active exploration into the synthesis of carbon materials derived from diverse organic wastes and biomass, driven by their potential applications in energy storage. This interest stems from their abundant availability and environmentally friendly characteristics (Vijayakumar et al., 2019; Liao et al., 2023). Due to its diverse agricultural practices and abundant biomass resources, India has various biowaste materials that hold potential sources for energy storage application. Some of the major biowaste materials include, cotton stalks, rice husk, rice straw, coconut shell, sugarcane bagasse, bamboo residues, coffee husk, tea waste etc. Among such biowaste materials, rice straw (Oryza sativa) is one of the most common materials from the agriculture sector. This is because, as one of the leading rice-producing countries, India plays a key role in the production of rice straw and produces approximately 112 million tons of milled rice and ~140 million tons of rice paddy straw every year (Singh et al., 2016; Singh & Arya, 2021). However, the dark side of the picture lies in the fact that, after the harvest of rice grains, the fibrous stalks are left behind and traditional disposal methods like burning lead to air pollution. (Sudhan et al., 2016; Chaudhary et al., 2024). Recognizing the intrinsic value within this waste is crucial for sustainable resource management. With unique physical and chemical properties, rice straw emerges as a compelling material for repurposing, particularly in technological applications for mass production of carbon-based materials for energy storage devices such as SCs and battery. Such properties not only offer advantageous electrochemical properties but also serve as structurally sound and scalable substrate for designing of composites with pseudocapacitive materials such as conducting polymers and metal oxides. The exploration of conducting polymers, particularly PANI, adds layer of complexity to the development of advanced materials with unique electrical properties (Thakur et al., 2017; Yazar et al., 2022). PANI, a conducting polymer, has garnered attention not only for its remarkable electrical conductivity but also due to its ease of synthesis and tunable properties. The chemical structure of PANI allows for the reversible protonation and deprotonation of nitrogen sites, resulting in distinctive redox behavior (Das et al., 2015; Fei et al., 2023). The ability of polyaniline to undergo reversible redox reactions enhances its overall performance of an electrode material (Eftekhari et al., 2017; Suresh et al., 2023).

In this context, the current study reports, a simple and scalable synthesis of PANI-decorated RSC (RSC@PANI) and the fabrication of electrodes for symmetric supercapacitor. The electrochemical performance has been tested in 1 M H₂SO₄ aqueous electrolyte and RSC@PANI achieved as high as capacitance of 408 Fg^-1 at a current density of 1 Ag^-1 and 93% retention capacity after 1000 cycles at current density of 10 Ag^-1. This indicates the RSC@PANI shows the promising electrode material for supercapacitor. Furthermore, this study opens up a new window to design and synthesis of carbon-based material from waste followed by its composites.

 Experimental Section

Reagents and Materials

Rice straw (RS) was collected from a field near Raipur, Chhattisgarh, India. Aniline (C₆H₅NH₂), N-methyl-2-pyrrolidone (NMP), Polyvinylidene fluoride (PVDF), and Sulphuric acid (H₂SO₄) were purchased from Sigma Aldrich. All the chemicals were of analytical grade and hence used as received without further purification.

Synthesis of RSC@PANI

Figure 1. Schematic representation of the synthesis of RSC@PANI.

Electrode Fabrication

To fabricate the electrode, 80% of the as-synthesized RSC@PANI composite, 15% of conducting carbon and 5% of PVDF as binder were mixed by adding NMP dropwise to make a homogeneous slurry for suitable coating. The prepared slurry was evenly applied on a carbon electrode and dried overnight in a vacuum oven (Rumjit et al., 2022; Pandya et al., 2023).

Characterization Techniques

The surface morphology of both the synthesized materials and the composite was characterized using a Carl Zeiss UHR FESEM (field-emission scanning electron microscopy) Model (Gemini SEM 500 KMAT) scanning electron microscope. Powder X-ray diffraction patterns were obtained on a Bruker D8 Advanced A25 diffractometer with Cu-Kα radiation (λ= 0.15418 nm) within the scan range (2q) of 10° and 80°. Raman analysis was carried out using a Laser Micro Raman System, Lab Ram HR from Horiba Jobin Yvon spectrometer, operating with a 532 nm laser. Fourier-transform infrared spectrometry measurements were performed using a Bruker 300 Hyperion Microscope with the Vertex 80 FTIR system.

Electrochemical Measurements

To assess the performance of the supercapacitor electrode in terms of specific capacity, cyclic stability, Galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV), measurements were conducted in a 1.0 M H₂SO₄ electrolyte using CH instruments.

Equation (1) was used to evaluate the specific capacitance of a two-electrode system based on GCD data (Karbhal et al., 2016).

                                                         C = 2[I/(mDV/Dt)]                                                          ……..(1)

Here,      I = applied current [A], 

       DV/Dt = slope of the discharge curve,

              m = average mass of active material per electrode,

    Factor 2 = the addition of two equivalent single-electrode capacitors in a series.

Results and Discussion

The surface morphologies of RSC, PANI and RSC@PANI were investigated by FESEM. Figure 2 (a) shows FESEM image of RSC, which depicts a microporous honeycomb-like structure, confirming the inner structure of RSC. Figure 2 (b) shows the FESEM images of PANI which appears as granular particles. Figure 2 (c, d) represents FESEM images of RSC@PANI, where, it can be seen clearly that the granular particles of PANI are embedded on the surface of RSC, which helps to provide framework support for polymerization (Zhou et al., 2020).

To examine the polymerization of aniline and the formation of the RSC@PANI composite, FTIR analysis was carried out, the results of which are depicted in Figure 3 (a). In the FTIR spectrum of PANI, the bands at 1556 cm⁻¹ and 1454 cm⁻¹ correspond to the stretching vibrations of the C=C bond in the quinoid range and the benzenoid ring, respectively (Das et al., 2015). Additionally, the low-intensity peaks observed in the 3500-3000 cm⁻¹ and 3000-2800 cm⁻¹ signify the stretching vibrations of N-H and C-H bonds, respectively not only in PANI but also present in RSC@PANI composite. An additional characteristic peak around 1285 cm⁻¹ and 1236 cm⁻¹ could be attributed to the C-N stretching mode in the quinoid-benzenoid-quinoid unit and the benzenoid ring, respectively (Oraon et al., 2015).  Furthermore, the presence of peaks at 1110 cm⁻¹ and 790 cm⁻¹ indicates the in-plane bending mode of C-H stretching in the quinoid ring and the out-of-plane C-H bending vibration of the quinoid ring, facilitating the identification of substitute benzene types. The RSC@PANI composite also exhibits a peak at 780-790 cm⁻¹, confirming the C-H bending vibration of the quinoid ring (Thakur et al., 2017). The FTIR spectrum of the RSC sample displays multiple peaks at 3450-3520 cm⁻¹, 1820- 1750cm⁻¹, and 1470-1400 cm⁻¹ are corresponds to the various functional groups, including -OH, -C=O, and -C-H, respectively (Sudhan et al., 2016).

To investigate the crystal structure of the synthesized electrode material, XRD patterns for the RSC, PANI and RSC@PANI were recorded and the results are shown in Figure 3 (b). The XRD pattern of PANI exhibited three characteristics peaks at 2q value of 13.9°, 20°, 25°-30° assigned as (011), (020) and (200) crystal planes of PANI in emeraldine salt form, respectively (Kumar et al., 2012; Thakur et al., 2017). RSC@PANI composite also showed semicrystalline behaviour as like as PANI. Moreover, the XRD pattern of RSC shows two peaks at 2θ value of 21.7° and 43° corresponds to (002) and (100) planes which clearly indicate the formation of amorphous carbon.

                    Figure 3. (a) FTIR spectra and (b) XRD spectra of  RSC, PANI and RSC@PANI

In the Raman spectra of PANI, the characteristic bands were observed at 1391 cm^-1, 1475 cm^-1, and 1580 cm^-1 are assigned to the C-N⁺ stretching vibration, out-of-plane stretching of C=C bonds in the quinoid ring and stretching of C=C bonds in the benzenoid ring, respectively (Das et al., 2015). In the Raman spectra of RSC, the peaks at 1321 cm^-1 were identified as the D-band, also known as the defective band, while the peak at 1582 cm^-1 was attributed to sp² carbon in RSC, referred to as the G-band. Similarly, in the Raman spectra of the RSC@PANI composite, the presence of RSC in the composite was confirmed by the appearance of peaks at 1333 cm^-1 (D-band) and 1587 cm^-1 (G-band) (Jiang et al., 2023).

Electrochemical measurements were conducted using the CH instrument with a 1M H₂SO₄ aqueous electrolyte. Figure 5 (a), presents the cyclic voltammogram of RSC and different concentrations of RSC@PANI (0.01, 0.2 and 0.1 M), displays a potential range of 0.0-1.0 V at a scan rate of 20 mVs^-1. Notably, the CV curve indicates that 0.1 M RSC@PANI exhibits a significantly wide rectangular shape compared to RSC and (0.2 M and 0.01 M) RSC@PANI, suggesting superior charge storage capacitance in 0.1 M RSC@PANI. Figure 5 (b) represents the GCD graph of RSC and various concentrations of RSC@PANI at a current density of             1 Ag^-1. 0.1 M RSC@PANI demonstrates as high as capacitance of 408 Fg^-1 at a current density of 1 Ag^-1 whereas both 0.2 M RSC@PANI and 0.01 M RSC@PANI shows the capacitance of 256 Fg^-1 and 71.8 Fg^-1 respectively. While only RSC achieves the capacitance of 32.2 Fg^-1 at   1 Ag^-1. The enhanced performance of 0.1 M RSC@PANI is attributed to its high conductivity and the pseudocapacitive behavior of PANI incorporated on the surface of RSC (Elgrishi et al., 2017). Figure 5 (c) shows the CV curve of RSC@PANI at various scan rates (10, 20, 40, 50, 100 and 150 mVs^-1). In contrast, Figure 5 (d) represents the GCD curve of 0.1 M RSC@PANI, at different current densities of 1, 2, 3, 4, 5, 8 and 10 Ag^-1, with capacitance of 408, 392, 386.4, 381.6, 374, 360 and 340 Fg^-1 respectively. (Ruan et al., 2023; Thejas Prasannakumar et al., 2024).

Figure 5 (e) illustrates the capacitance behavior of RSC and different concentrations of RSC@PANI at various current densities (1, 2, 3, 4, 5, 8, and 10 Ag^-1). The results underscore that 0.1 M RSC@PANI exhibits superior capacitance compared to all other prepared samples.  In Figure 5 (f), the Nyquist plot of electrochemical impedance spectroscopy (EIS) of RSC and different concentrations of (0.1, 0.2 and 0.01) M of RSC@PANI revealed interesting characteristics across a frequency range of 0.1 Hz to 10 kHz is depicted. The 0.1 M RSC@PANI composite displayed significantly lower resistance compared to the other composites, indicating enhanced electrical conductivity. These exceptional EIS data points highlight the superior electrochemical properties of the 0.1 M RSC@PANI (Salinas-Torres et al., 2013; Wu et al., 2023). Moreover, Figure 6 depicts the electrochemical stability of 0.1 M RSC@PANI at a current density of 10 Ag^-1 for 1000 cycles and maintains 93% capacitance retention (Zhou et al., 2011; Yu et al., 2015). In summary, the comprehensive data confirms that 0.1 M RSC@PANI exhibits promising capacitance of 408 Fg^-1 at a current density of            1 Ag^-1, as compared to RSC and other RSC@PANI samples.

Figure 5. Electrochemical performance of RSC and RSC@PANI composite in 1 M H₂SO₄ (a) Cyclic voltammograms of RSC, (0.01, 0.2 and 0.1) M RSC@PANI at a scan rate of 20 mVs^-1 (b) GCD curves of RSC, (0.01, 0.2 and 0.1) M RSC@PANI at a current density of 1 Ag^-1  (c) Cyclic voltammograms of 0.1 M RSC@PANI at a scan rate of 10, 20, 50, 100, 150 mVs^-1 (d) GCD curves of 0.1 M RSC@PANI at a current density of 1, 2, 3, 4, 5, 8,10 Ag^-1 (e) Specific capacitance of RSC, (0.01, 0.2 and 0.1) M RSC@PANI of active material at different current densities 1, 2, 3, 4, 5, 8,10 Ag^-1 (f) Electrochemical impedance spectra of RSC, (0.01, 0.2 and 0.1) M RSC@PANI.

Conclusion

RSC@PANI was synthesized using in-situ chemical oxidative polymerization. FESEM characterization confirmed the successful incorporation of granular PANI over the surface of the RSC. The key role of PANI in enhancing the performance of the RSC, was investigated using electrochemical characterizations. The synthesized RSC@PANI achieved a capacitance of 408 Fg^-1 at a current density of 1 Ag^-1 and exhibited excellent cyclic stability of 93% after 1000 cycles at a current density of 10 Ag^-1. The performance of RSC@PANI shows promising electrochemical behaviour as an electrode material for supercapacitors. The current work open up a new window for the next generation to use waste materials like rice straw and its composites as a carbon source for supercapacitors and other applications.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

One of the authors (VD) would like to acknowledge DST-INSPIRE (Ref. No. DST/INSPIRE Fellowship/2021/IF210328) fellowship for providing financial assistance. The authors are thankful to DST-PURSE (SR/PURSE/2022/145), SERB (CRG/2022/003926) and SERB (EEQ/2022/000967) projects for supporting the work with financial assistance. 

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