Comparative Study of Reduced Graphene Oxide Aerogels and Films for Supercapacitor Electrodes

Sunghee Choi; Seulgi Kim; Seojin Woo; Dongju Lee

doi:10.4150/jpm.2024.00472

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Comparative Study of Reduced Graphene Oxide Aerogels and Films for Supercapacitor Electrodes: Sunghee Choi¹, Seulgi Kim¹, Seojin Woo¹, Dongju Lee^1,2,*; Journal of Powder Materials 2025;32(1):23-29.
DOI: https://doi.org/10.4150/jpm.2024.00472
Published online: February 28, 2025

¹Department of Urban, Energy, and Environmental Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju, Chungbuk 28644, Republic of Korea

²Department of Advanced Materials Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju, Chungbuk 28644, Republic of Korea

*Corresponding author. E-mail: dongjulee@chungbuk.ac.kr

• Received: December 5, 2024 • Revised: January 23, 2025 • Accepted: February 1, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Abstract
Graphical abstract
Introduction
Experimental section
Results and discussion
Conclusion
Article information
References

Abstract

Supercapacitors, renowned for their high power density and rapid charge-discharge rates, are limited by their low energy density. This limitation has prompted the need for advanced electrode materials. The present study investigated reduced graphene oxide (rGO) in two distinct structures, as a film and as an aerogel, for use as supercapacitor electrodes. The rGO film, prepared by vacuum filtration and thermal reduction, exhibited a compact, lamellar structure, while the aerogel, synthesized through hydrothermal treatment, was a highly porous three-dimensional network. Electrochemical analyses demonstrated the aerogel’s superior performance, as shown by a specific capacitance of 121.2 F/g at 5 mV/s, with 94% capacitance retention after 10,000 cycles. These findings emphasize the importance of structural design in optimizing ion accessibility and charge transfer. They also demonstrate the potential of rGO aerogels for increasing the energy storage efficiency of advanced supercapacitor systems.
Keywords: Reduced graphene oxide; Aerogel; Film; Supercapacitor

Graphical abstract

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This study compares reduced graphene oxide (rGO) in film and aerogel forms as supercapacitor electrodes. The porous aerogel outperformed the compact film, achieving a higher specific capacitance and excellent cycle stability. These results highlight the impact of structural design on energy storage performance and efficiency.

Introduction

With the increasing global demand for sustainable and efficient energy storage systems, improving the technologies that address diverse energy requirements has become an area of extensive research [1]. Among these technologies, lithium-ion batteries and supercapacitors are leading candidates, each one offering distinct advantages. Lithium-ion batteries are renowned for their high energy density, making them suitable for long-term energy storage, whereas supercapacitors excel in applications requiring high power density, rapid charge-discharge rates, and an exceptional cycle life [2]. Expanding the applicability of supercapacitors across a wider range of fields requires complementing their inherent advantages of high power density and long cycle life with an improvement of their energy density.

The performance of supercapacitors is largely governed by the choice of electrode materials, which directly influence energy and power capabilities. Carbon-based materials, including activated carbon, carbon nanotubes, and graphene, are extensively used due to their high specific surface area and excellent conductivity [3]. In particular, graphene has gained significant attention given its mechanical strength [4], high electrical conductivity [5], and extensive surface area [6]. These properties make graphene a highly promising candidate for electric double-layer capacitors (EDLCs) [7]. However, the strong van der Waals forces between graphene sheets often lead to aggregation, reducing the effective surface area and limiting the electrochemical performance [8]. Strategies that have been explored to prevent aggregation include the introduction of structural modifications and the incorporation of spacers [9]. Such methods are aimed at enhancing the specific surface area and improving the overall electrochemical properties of graphene.

In this study, a freestanding rGO film and a freestanding rGO aerogel were synthesized and their properties as supercapacitor electrodes were then compared. The structural difference was found to play a critical role in EDLC behavior, with the rGO aerogel demonstrating a higher specific capacitance of 121.2 F/g at 5 mV/s, outperforming the rGO film. The enhanced performance of the aerogel can be attributed to its superior surface area and efficient ion transport pathways. These findings highlight the importance of structural design in optimizing rGO electrodes for energy storage systems requiring both high energy and high power densities.

Experimental section

Synthesis of graphene oxide (GO): GO was synthesized using Hummers' method. Briefly, 1 g of graphite flakes (Bay Carbon Inc) were placed into a flask, followed by the addition of 40 mL of concentrated H₂SO₄ (SAMCHUN) under an ice bath. After the mixture had been stirred for several minutes, 3.5 g of KMnO₄ (DAEJUNG) was slowly added to the flask, followed by stirring at 35°C for 2 h. Next, 200 mL of deionized (DI) water was added gradually under continuous stirring, followed by the addition of 10 mL of H₂O₂ (Junsei). The resulting solution was washed with diluted HCl (Junsei) (DI water:HCl = 9:1) to remove residual metal ions and then rinsed repeatedly with DI water until the pH of the solution reached approximately 6. The product was dried in a vacuum oven at room temperature to obtain GO powders.

Synthesis of the reduced graphene oxide (rGO) film: The rGO film was prepared via vacuum filtration followed by thermal reduction. GO was dispersed in DI water at a concentration of 2 mg/mL and sonicated to ensure uniform dispersion. The resulting dispersion was subjected to vacuum filtration to form a compact film on a membrane. The obtained GO film was dried at room temperature before it was thermally reduced at 200°C for 2 h under atmospheric conditions.

Synthesis of the rGO aerogel: The rGO aerogel was fabricated according to the hydrothermal method. A GO dispersion (2 mg/mL) was prepared in DI water and transferred into a Teflon-lined autoclave. The autoclave was heated at 180°C for 2 h to induce the self-assembly and reduction of the gel. The resultant hydrogel was freeze-dried to obtain the rGO aerogel.

Materials characterization: The crystallographic structures of GO and rGO were investigated by X-ray diffractometry (XRD; JP/SmartLab, Rigaku). Morphology and size were characterized by transmission electron microscopy (JEM-ARM200F, JEOL), and the surface chemical composition, elemental states, and bonding environment of the materials by X-ray photoelectron spectroscopy (K-Alpha+, Thermo Fisher Scientific). The microstructures of the rGO aerogel and film were examined using field-emission scanning electron microscopy (Crossbeam 540, ZEISS). The Raman spectra were analyzed using a micro-Raman spectrometer (RAMANtouch, Nanophoton, Republic of Korea) with a 514 nm laser source. The specific surface area was calculated in Brunauer-Emmett-Teller analysis, using the nitrogen adsorption isotherm branch with nano-porosity (Miraesi, Republic of Korea).

Electrochemical measurement: The rGO aerogel was used as an electrode by compressing it at 10 MPa. Both the rGO aerogel and film electrodes were prepared with a diameter of 12.7 mm. The electrodes were tested in a two-electrode symmetric cell configuration using 1 M H₂SO₄ as the electrolyte. Electrochemical performance was evaluated using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements, performed on a potentiostat (SP-50e, Bio-Logic). The specific capacitance (C_g) was calculated using the following equation:

$Cg=2mvΔV∫IdV$

where m is the mass of the electrode, v is the scan rate, ΔV is the voltage window, and I is the discharge current. The EIS analysis was conducted at an amplitude of 20 mV and a frequency range of 100 mHz to 100 kHz.

Results and discussion

Fig. 1 shows the fabrication process of the rGO film and rGO aerogel. The rGO film was prepared by vacuum filtering a GO dispersion to form a compact film, followed by thermal reduction at high temperature to restore conductivity and reduce oxygen-containing functional groups. The rGO aerogel was prepared using a GO dispersion in a hydrothermal synthesis method. During the hydrothermal process, the rGO aerogel was reduced and self-assembled without the need for additional reductants.

Fig. 1.

A schematic illustration of the preparation process for (a) the reduced graphene oxide (rGO) film and (b) the rGO aerogel.

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The morphology of the GO and the resulting rGO was examined using TEM. Fig. 2a shows the monolayer sheet morphology, typical of well-exfoliated GO. After thermal reduction, the morphology of the rGO was similar (Fig. 2b). The lateral size of both GO and rGO was approximately 1 μm, indicating that the reduction process did not significantly alter the sheet dimensions. The structural changes resulting from the reduction of GO to rGO were analyzed using XRD and Raman spectroscopy. Fig. 2c presents the XRD patterns of GO and rGO. In the GO spectrum, the prominent peak at 10.7° corresponded to the (001) plane [10], indicating the presence of oxygen-containing functional groups and an increased interlayer spacing due to oxidation [11]. After thermal reduction, the (001) peak disappeared and a broad peak appeared at 21.2°, corresponding to the (002) plane, indicating a significant reduction in interlayer spacing, the removal of oxygen groups, and the partial restoration of the sp² carbon network [12]. These structural changes, confirmed by XRD analysis, are critical for enhancing the physical and electrochemical properties of rGO. The Raman spectra of GO and rGO (Fig. 2d) provided further insights into the structural changes. Both the GO and the rGO sample exhibited distinct D and G bands. For GO, the I_D/I_G ratio was 0.87, whereas for the thermally reduced rGO the I_D/I_G ratio was higher, 0.99. After thermal reduction, the I_D/I_G ratio increased significantly, indicating partial restoration of the graphene network. Fig. 2e, f shows the XPS spectra of GO and rGO, in which the changes in their chemical composition during the reduction process are apparent. The distinct peaks of the C 1s spectrum of GO corresponded to various carbon-oxygen bonding configurations: the C–C bond at 284.80 eV, the C–O bond at 287.20 eV, and the C=O bond at 289.50 eV [13]. These peaks confirmed the significant presence of oxygen-containing groups, such as hydroxyl, epoxide, and carbonyl. The oxygen atomic percentage of GO was 72.0 at.%, reflecting its high degree of oxidation. After thermal reduction to rGO, the oxygen content decreased significantly, to 36.3 at.%, indicating the partial removal of oxygen functional groups The C 1s spectrum of rGO had a dominant C–C peak at 284.8 eV, suggesting the preservation and partial restoration of the sp²-hybridized carbon network [13]. For the peaks corresponding to oxygen-containing groups, specifically the C–O and C=O bonds, their intensity was significantly reduced after the reduction process, suggesting partial reduction and restructuring of the oxygen functionalities and thus a transition toward a more graphene-like structure with fewer oxygen-containing groups.

Fig. 2.

Transmission electron microscopy image of (a) graphene oxide (GO) and (b) reduced graphene oxide (rGO); (c) X-ray diffraction patterns and (d) Raman spectra of GO and rGO; X-ray photoelectron spectroscopy C 1s spectra of (e) GO and (f) rGO.

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Fig. 3 shows the microstructures of the rGO aerogel and the rGO film. The highly porous, interconnected network of the rGO aerogel (Fig. 3a) provides abundant ion-accessible sites and facilitates rapid ion transport, which would enhance its electrochemical performance [14]. The rGO film (Fig. 3b), by contrast, has a more compact, layered structure with a distinct lamellar arrangement. The cross-sectional image showed stacked layers, attributable to strong π-π interactions between graphene sheets [15]. This compact layering restricts ion transfer pathways, which can negatively affect capacitance performance [16]. The N₂ adsorption/desorption isotherms of the rGO aerogel and rGO film are shown in Fig. 3c, 3d. The specific surface area of the rGO aerogel was 250.61 m²/g, significantly larger than the 50.08 m²/g of the rGO film. This difference reflects the enhanced porosity of the aerogel, which contributes to its superior ion accessibility and electrochemical properties, as compared to the more compact rGO film.

Fig. 3.

Scanning electron microscopy images of (a) reduced graphene oxide (rGO) aerogel and (b) rGO film, with digital images shown as insets. N₂ adsorption/desorption isotherms of (c) the rGO aerogel and (d) the rGO film.

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