Carbon Nanomaterials Based Supercapacitors: Recent Trends

Abstract

The increasing demand for renewable energy sources worldwide and the predicted depletion of current fossil fuel sources need continuous energy storage and conversion technology development. The use of supercapacitors (SC) as electrical energy storage devices in consumer electronics items and alternative power sources is an interesting and potentially lucrative area of application. Therefore, continuous developments are conducted to improve SC performance using different composites and nanocomposites. Carbon materials in SC are among the most important uses of this material. This chapter provides a short communication on recent progress in supercapacitor-based carbon materials. Various fundamental carbon allotropes were presented and debated, including fullerene, carbon nanotubes, and graphene-based supercapacitors.

1. Introduction

The increasing global energy demand due to a modern technology-dependent lifestyle puts increasing pressure on traditional nonrenewable energy resources such as fossil fuels [1, 2]. The global trend to use portable, flexible electronics and the increase in global environmental awareness push toward the use and continuous development of eco-friendly, sustainable energy conversion and storage devices [3, 4]. Supercapacitors are essential devices among energy storage devices because of their quick charging and discharging processes, high power densities, extended cycle lives, minimal maintenance requirements, long lifespans, and environmental friendliness [4]. Both the electric double layer capacitor, which stores energy by electrostatic means, and the pseudocapacitor, which keeps energy through redox reactions, are subtypes of the supercapacitor. In most cases, the hybrid electrodes can store energy in electrochemical and electrostatic methods [5]. Similar to all other devices, the performance of supercapacitors is highly dependent on the characteristics of the materials they utilize. Continuous developments of carbon materials are rapidly being employed in energy storage devices because of their advantages, including the simplicity of modifying and manipulating pore structure, surface functionality, surface area, and low cost. Furthermore, the structural integrity of the carbon framework ensures that the electrode material’s cyclic stability and capacitance retention are maintained throughout time [6].

In recent years, significant progress has been made in developing carbon nanostructure composites for high-performance energy conversion and storage devices. The recent advancement of nanoscience and nanotechnology has created novel graphitic carbon nanomaterials with multi-dimensions, such as two-dimensional (2D) graphene, (1D) carbon nanotubes (CNT), and dimensionless (0D) fullerene [7]. This chapter introduces recent supercapacitor advancements based on fundamental carbon nanostructures; graphene, carbon nanotubes, and fullerene.

2. Graphene

Graphene is an emerging carbon nanomaterial with an ideal 2D structure and unique electronic properties. On the other hand, the word graphene wasn’t coined until 1986. Graphene is a single layer (2D) honeycomb-arranged carbon atom connected with sp² bonds. Graphene serves as the fundamental building block for the structure of all other carbon allotropes. Geim and Novoselov made the groundbreaking discovery in 2004 that single-layer and two to three-layer graphene nanosheets can stably survive in the environment [8, 9]. The exceptional qualities of graphene include its high electrical conductivity, high thermal conductivity (5000 W m⁻¹ K⁻¹), high intrinsic charge mobility (250,000 cm² V⁻¹ s⁻¹), and very high surface area (2630 m² g⁻¹), and high Young’s modulus. Graphene also has a very high surface area (1.0 TPa).

Due to its low mass density, extremely large surface area, great conductivity, and high flexibility, graphene has widespread use in various sectors, including energy storage and conversion, electronic devices, sensors, polymer additives, and biological applications [10, 11, 12, 13, 14, 15]. Different graphene synthesis approaches are reported as mechanical filiation, epitaxial growth, chemical vapor deposition, and reduction of graphene oxide [9]. The use of graphene and graphene composites as supercapacitor materials was the subject of numerous publications. The following are examples of recent research on graphene-based supercapacitors.

A high-performance supercapacitor was prepared based on a composite of carbonized wood cell chamber-reduced graphene oxide@PVA (CWCC-rGO@PVA) [16]. CWCC-rGO@PVA revealed a high specific capacitance of 288F g⁻¹, capacitance retention of 91%, energy density of 36 Wh kg⁻¹, and power density of 3600 W kg⁻¹.

The Co₃O₄/CoO nanoparticles were attached to reduced graphene oxide (rGO) nanosheets by microwave irradiation. The rGO@Co₃O4/CoO electrode showed excellent electrochemical performance of specific capacitance of 276.1 F g⁻¹ and 82.37% capacitance retention after 10,000 cycles [17].

As supercapacitor electrodes, 3D flower-like spheres of NiCo₂S₄@Ni-Mo layered double hydroxide (LDH) nanocomposites grown in situ on reduced graphene oxide (RGO) were developed using a simple hydrothermal method [18]. For comparison, RGO@NiCo2S4 and RGO@NiMo-LDH electrodes were also prepared. The redox peaks in CV curves for the RGO@NiCo2S4@NiMo-LDH electrode were symmetric and had identical profiles as the scanning rate increased, demonstrating excellent pseudocapacitance behavior and rate capacity of the electrode. Charge–discharge curve platforms were more pronounced at varying current densities, suggesting the presence of a Faraday redox reaction. Capacity retention was very good for the RGO@NiCo₂S₄@NiMo-LDH electrode, with specific capacitances of 1346, 1336, 1305, 1294, 1283, and 1272 F g⁻¹at 1, 2, 4, 6, 8, and 10 A g⁻¹, respectively. The rated capacity of RGO@NiCo₂S₄@NiMo-LDH was higher than that of RGO@NiCo₂S₄. RGO@NiCo₂S₄@NiMo-specific LDH’s capacitance was greater than that of RGO@NiCo₂S4 and RGO@NiMo-LDH taken separately, suggesting that the presence of several NiCo₂S4 nanosheets on graphene sheets may give more growth spots for NiMo-LDH nanosheets than a smooth graphene skeleton. It can be seen from the symmetrical charge–discharge curve that it has good electrochemical reversibility. The device can obtain a maximum energy density of 59.38 Wh kg⁻¹ at a power density of 808.19 W kg⁻¹ and maintain an energy density of 25.24 Wh kg⁻¹ at a high power density of 8055.32 W kg⁻¹. The capacitance of the RGO@NiCo₂S4@NiMo-LDH electrode retained 80% of its initial capacitance after 10,000 cycles.

In another paper, graphene/MnV₂O₆ nanocomposite was prepared using solvothermal and liquid phase exfoliation processes. A maximum specific capacitance of 348 Fg⁻¹ and capacitance retention of 88% was achieved after 3000 cycles for an optimal graphene/manganese vanadate ratio (1:8) sample [19].

A hybrid 2D platform was constructed from polypyrrole (PPy) /rGO and nickel-tungsten metal oxides. The prepared electrode showed excellent specific capacitance of 597 F.g⁻¹ with capacitance retention of 98.2% after 5000 cycles. The two-electrode device using the same electrode platform showed a specific capacitance of 361 F.g⁻¹ [20].

New hierarchical porous hybrid architecture consists of biomass-based porous carbon derived from Ganoderma lucidum residues (DDLG)/graphene composite aerogel were synthesized by chemical self-assembly and Vitamin C as a reducing agent [21]. Composites with 2.1, 3:1, 4:1, and 8:1 porous carbon ratios to GO were prepared. The large interconnected pores of DDLG were confirmed from SEM images. In addition, graphene aerogel retains the conventional three-dimensional network structure, and the sheet-like form of graphene is orientated unpredictably. Porous carbon/graphene composites feature a new three-dimensional hierarchical porous structure when the ratio of porous carbon to graphene is between 1:1 and 3:1. This ratio creates a densely packed structure. This is due to the graphene oxide sheet reduction process to conductive reduced graphene oxide resulting in forming a porous three-dimensional network structure around the BPC. When the ratio of porous carbon to graphene is exactly one to one, a system of porous carbon and graphene tightly packed together is produced.

Furthermore, the graphene self-assembled aerogel’s structure dominates throughout the self-assembly process since just a few porous carbons are exposed owing to the high graphene concentration, and graphene nanoflakes cover the porous carbon. When the porous carbon to graphene ratio approaches 4:1, there are still two different types of porous structures, and the pore structure of porous carbon becomes more visible as the percentage increases. Furthermore, when the mass ratio of porous carbon increases to 8:1, graphene is shown to be distributed evenly throughout the porous carbon.

EIS measurements were used to analyze and compare the resistance characteristics of DDLGC and DDLGC/GO8. The DDLGC and DDLGC/GO8 ESRs were 0.53 and 0.46, demonstrating that the graphene-enhanced composite aerogel had significantly improved conductivity. The DDLGC/GO8-based electrode has a lower interfacial charge transfer resistance since the semicircle has a smaller diameter. A vertical line indicates capacitive behavior near to ideal [22]. A virtually vertical line was seen in the low-frequency region, suggesting high charge storage, rapid ion transport/diffusion, and excellent electrical double layer capacitor (EDLC) properties.

CV curves of DDLGC/GO8 at 5–100 mV s ⁻¹ show the creation of EDLC with rectangular curve shapes. The electrode of DDLGC/GO8 exhibited isosceles triangle shapes in the GCD plots at different current densities, demonstrating that the material’s energy storage mechanism is a double-layer storage energy mechanism with good electrochemical reversibility. The specific capacitances of DDLGC and DDLGC/GO8 at different current densities were calculated. At a current density of 1 A g ⁻¹, the specific capacitances of DDLGC and DDLGC/GO8 were determined to be 365.6 F g⁻¹ and 366 F g⁻¹, respectively. DDLGC/GO8 has a substantially greater rate capacity at high current density than DDLGC, which may be attributed to the material’s increased electron transfer efficiency at high scan rates [23] and the bigger average pore size and higher effective surface area. These findings show that adding graphene, another carbon element, may greatly enhance the capacitance characteristics of biomass-based porous materials.

3. Carbon nanotubes

In addition to their high electrical conductivity [24], unique pore structures, and improved power density in supercapacitors, carbon nanotubes (CNTs) have outstanding thermal stability, superior mechanical qualities, and unique pore structures. Powders made from commercially available CNTs are frequently used as collectors, either in conjunction with other pseudocapacitive materials or on their own, or as pseudocapacitive electrode materials [25]. Van der Waals force is the mechanism that allows CNTs in the electrode to link to one another. This increases the electrode’s resistance leading to self-discharge as a consequence of poor adhesion. To address these risks, CNTs are grown on collectors, which can take the form of carbon cloth, graphene, stainless steel mesh, or nickel foam [26]. The following are some instances of research on the significance of CNTs in supercapacitor production.

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNT) and carboxymethylcellulose (CMC) were combined by a hydrothermal process [27]. An N-MWCNT/CMC composite had an ultrasonication-mediated solvothermal reaction to produce the material. The good electrochemical characteristics and rapid redox reaction of the composite electrode in the presence of the PVA/H₂SO₄ gel electrolyte are deduced from the approximately rectangular shape of the cyclic voltammetry (CV) curves. The N-MWCNT/CMC composite electrode displayed a more significant current than the pure N-MWCNT, demonstrating its superior electrochemical performance and the crucial role of the CMC matrix inclusion on the CNTs in enhancing the electrode’s capacitance. According to the galvanostatic charge–discharge (GCD) cyclic stability analysis performed for up to 4000 cycles at a scan rate of 2 Ag⁻¹, the N-MWCNT/CMC nanohybrid composite retained 96% of its initial capacity. The charge transfer (Rct) of the electrodes during the first and the one-thousandth cycles, as determined by the Nyquist plots, is approximately 0.9 and 35, respectively. The steady electrochemical characteristics are influenced as a result of this factor. At low frequencies, it was noticed that the phase angle for the impedance plot of the composite electrodes was greater than 45 degrees; this indicates that the composite electrodes have electrochemical capacitive capabilities.

The closed tips and fewer active sites of CNTs can limit their electrochemical performance. Therefore, Zhang and Xie [28] investigated a successful trial to open the tips of CNTs with oxygen and nitrogen functional groups by an effective chemical acid-etching method. The chemical vapor deposition (CVD) technique was used to perform the acidic treatment on the CNTs fabricated. Li⁺-based electrolyte provided the best electrochemical performance of the functionalized and tip-open CNTs (FTO-CNTs) compared to the other investigated types of ions as Na⁺ and Mg²⁺.

The areal capacitance obtained from GCD curves for the FTO-CNTs indicates improved electrochemical performance. Compared to CNT growth on carbon cloth (CCC) and carbon cloth (CC), FTO-CNTs have the highest areal capacitance due to their largest CV area as determined by CV curves at a scan rate of 20 mV/s. The functionalization and tip-opening of the CNTs may explain the higher capacitance of FTO-CNTs compared to that of CCC. The higher number of oxidation–reduction reactions is responsible for the greatest charge–charge transfer resistance between ions and electrons (R_ct) in FTO–CNTs. The movement of ions from the open tip into the interior of the CNTs may be responsible for the higher diffusion resistances (σ) exhibited by FTO-CNTs compared to those of CCC. Since there were more entrance locations for the diffusion of ions in the FTO-CNTs due to their open tips, a scan rate of 10 mV induced a greater diffusion-controlled capacitance (75%) higher than CCC (65%).

Yang et al. developed an innovative method for dealing with polymer waste and high-value-added recycling of resources [29]. In this study, the researchers investigated a great success in treating polypropylene face mask wastes, a source of environmental pollution, to be useful by carbonizing them into CNTs. Yang et al. proposed employing the manufactured waste face mask CNTs as electrode material in supercapacitors to achieve extra financial benefits. The CNTs were produced using Ni–Fe bimetallic catalysts with varying molar ratios NiFeX (X = 1 to 5 and NiFe/Al = 1).

CV curves of CNT samples appeared in approximately rectangular patterns with broad redox peaks. The development of broad redox peaks explained by the insertion of functional groups comprising nitrogen and oxygen on the surface of carbon nanotubes. The best value of the ratio capacitance was detected in the CNT-NiFe₃ sample. For the CNTs sample, the electric double-layer capacitance features were proved by results obtained from capacitance performance (CP) curves which provided an isosceles-like triangle at a current density of 1 A/g and a range from −0.8 V to zero V. After 10,000 cycles, CNT-NiFe₃ electrodes have high cycling stability with capacitance retention of 85.41% from the initial value. Also, within a current density of 1 Ag⁻¹, they attain a specific capacitance of 56.04 F/g. Due to the bamboo-like shape of the carbon nanotubes, CNT-NiFe₃ can be purified to achieve the maximum specific surface area and N-doped concentration.

A green, simple processing protocol proposed by Bathula et al. [30] utilizing mechanochemical grinding to synthesize hybrid nanostructures of cobalt oxide on nitrogen-doped multiwalled carbon nanotubes (Co3O4-NMWCNT). The NMWCNT in its original form exhibited wire-like geomorphology; however, Co₃O₄ consists of clusters of pieces, and the NMWCNT-Co₃O₄ composite includes an interconnected tube structure. The electrochemical properties of symmetric devices made with NMWCNT, and NMWCNT-Co₃O₄ electrodes were studied. CV curves of both electrodes verified the EDLC behavior and Faradic reaction, respectively. The enclosed area of the CV of the NMWCNT-Co₃O₄ device is nearly twice that of the NMWCNT device, indicating that the Co3O4 and NMWCNT have a synergy effect. Both materials have remarkable rapid charging and discharging potential. Random CV curves illustrated that the form of CV curves for Co₃O₄-NMWCNT was maintained across all cycles (indicating exceptional structural stability).

In another article, a composite of polypyrrolopyrrolethieno thiophene (PDPT) and carbon nanotube (CNT) was created by Bathula et al. [31] to test its viability as a hybrid electrode material. The structure of PDPT is based on DPP (π-conjugated polymer), which includes moieties of both sulfur and nitrogen heterocyclic. DPT accumulates a donor-acceptor (D–A) interface utilizing chemical exfoliation suggested for electron-accepting bulk. Ultrasonic vibrations caused exfoliation in this particular investigation. To obtain an intermolecular hydrogen connection and necessary D–A and p–p packing, the authors investigated a successful mixture between CNTs and bulk DPT nanofibers. Afterwards, a standardized PDPT-CNT composite suspension was produced from the accumulation of the insoluble DPT. The GCD results showed the specific capacitance of PDPT-CNT and PDPT are 126, 90, 60, 30, and 10; and 42, 26, 16, 12, and 5 F/g, detected at current densities of 0.5, 1, 2, 3, and 5 A/g, respectively. Moreover, at a power density of 450 W/kg, the PDPT-CNT device has a maximum energy density of 15.7 W.h/kg.

Zhang et al. [32] constructed a novel wire-shaped coaxial supercapacitor with exceptional performance, made of carbon wires (CW)@MnO₂/PVA-KOH/carbon nanotubes (CNTs). For the inner electrode, copper wire was utilized as a current collector to solve the problem of the low electric conductivity of MnO₂. However, carbon nanotubes generated via in-situ chemical vapor deposition (CVD) served as the outer electrode, with cobalt-based catalyst particles uniformly dispersed across the surface of SiO₂. Then, the device was created by removing the SiO₂ layer and filling it with a polyvinyl alcohol-KOH (PVA-KOH) gel electrolyte, simultaneously using the hydrothermal method. At a power density of 37 mW cm⁻³, the wire-shaped supercapacitor had the highest volumetric energy density of 0.16 mWh cm⁻³, dropping to 0.12 mWh cm⁻³ at 62.6 mW cm⁻³ power density. The asymmetrical quasi-rectangle shape of a wire-shaped supercapacitor obtained from CV curves indicates its exceptional electrochemical performance. The observed semicircle in the Nyquist curve of electrochemical impedance spectroscopy (EIS) for the wire-shaped supercapacitor at high frequencies describes the resistance of the charge transfer, which has a value of 1815 W. The equivalent internal resistance value was 22.79 W, deduced from the semicircle’s intercept with the real axis at high frequencies. The capacitance retention continued high and stable; after 4000 cycles of charging and discharging, the capacitance of the wire-shaped supercapacitor exhibits excellent retention of over 90.38%. From the Ragone plot, at a power density of 37 mW cm³, the wire-shaped supercapacitor has the greatest volumetric energy density (0.16 mWh cm³). More significantly, it may continue to be 0.12 mWh cm³ even when the power density reaches 62.6 mW cm³.

4. Fullerene

Fullerenes or Buckyballs are a novel class of carbon nanomaterials. The basic elements of fullerenes are called isomers. Their homologs range from the lower homologs that have received the most attention, such as C60 and C70, to higher fullerenes, such as C240, C540, and C720. They have shown to be valuable in various scientific domains, including separating and identifying different chemical species. Fullerene was first produced by Kroto, Curl, and Smalley via laser-induced evaporation of graphite. As a result, the discovery of Buckminsterfullerene, also known as C60, resulted from a research study that connected synthetic chemistry, microwave spectroscopy, and radio-astronomy. Fullerene was born from the search to reproduce poly acetylenes discovered in interstellar space [33].

Fullerenes have a structure composed of sp2 carbons with distinct chemical and physical characteristics and a highly symmetrical cage with varying widths (C60, C76, etc.) [34]. Thanks to their excellent electrochemical stability, small size, unique shape, and well-ordered structure [35], they enable their use in energy conversion systems.

Thanks to their excellent electrochemical stability, small size, unique shape, and well-ordered structure, they enable their use in energy conversion systems. Fullerenes’ distinctive 0D structure makes them valuable building blocks for supramolecular assemblies and micro/nano functional materials used in drug delivery [36], photovoltaic devices [37], optoelectronics [38], sensors [39], catalysis [40], and other fields.

The fullerene molecule C60, which has a structure of 60 carbon atoms, 12 pentagonal C5-C5 single bonds, and C5 = C6 double bonds (20 hexagons), is the most often employed in supercapacitors. The following are some studies on the importance of fullerene in supercapacitor manufacture.

Activated fullerene (A-C60) decorated over zinc cobaltite (A-C60-ZCO) has been synthesized by a solvothermal approach as a supercapacitor electrode [41]. The greater enclosed area of the CV and the well-defined redox peaks suggest that A-C60-ZCO has a high specific capacitance and a strong pseudo capacitive nature. It was reported that the specific capacitance value is better for the 10 wt.% of A-C60 in ZCO loading than for the 2, 5, and 15 wt.% loadings. So, a composite with 10 wt.% A-C60 loading is the best for further electrochemical studies [41]. The character of the CV curve of A-C60-ZCO stays the same, except for a shift in peak position even at a higher scan rate (100 mV/s), indicating that the as manufactured material possesses rapid and reversible faradic performance. At scan speeds of 1, 5, 10, 20, 40, 50, 70, and 100 mV/sec, the A-C60-ZCO has volumetric specific capacitances of 593.2, 554.18, 506.2, 412.3, 332.7, 296.584, 260.696, 221.76 F/g. With an increase in scan rate, specific capacitance decreases as internal resistance becomes more dominant. The pseudocapacitive character of the active material is firmly confirmed by all GCD curves, resembling separate plateau areas compared to CV curves. Because of the partial ion migration toward the core of the active material, which may be controlled by limiting the loading quantity of active material, the specific capacitance value at higher current densities shows a small decline.

Under the same current density, -C60-ZCO has the longest charge/discharge time among ZCO, A-C60, and C60, indicating that A-C60-ZCO has the highest specific capacitance. At a current density of 2 A/g, the specific capacitance of A-C60-ZCO, ZCO, A-C60, and C60 electrodes is determined to be 269.81, 124.05, 34.41, and 24.18 F/g, respectively. The synergistic impact of the pseudo capacitive ZCO and A-C60 increases specific capacitance. All of the GCD curves have plateaus, which is strong evidence that the active material is pseudo capacitive and consistent with CV curves. The calculated specific capacitance values were 269.81, 144.36, 106.53, 84.06, 33.03, 27.89, 23.11, and 19.56 F/g at a current density of 2, 3, 4, 5, 7, 8, 9, and 10 F/g respectively.

In another work, a composite from polyaniline (PANI)/fullerene derivative (PCBM) Phenyl-C60-butyric acid methyl ester was constructed and tested as supercapacitor materials [42]. By varying the ratios of PCBM, different PANI/ PCBMx (where x = 0, 2.5, 5, and 10) were prepared. It was concluded that the PANI/PCBM electrodes had a higher specific capacitance than PANI due to the synergetic effect of PANI and PCBM. Also, it was found that the PANI/PCBM5 had the highest specific capacitance of 2609 F/g compared to 1216, 1882, and 1770 F/g for pure PANI, PANI/PCBM2.5, and PANI/PCBM10. The decreasing of specific capacitance of nanocomposite electrodes with PCBM content higher than 5 wt.% is ascribed to a larger size of PCBM, which decreases surface area.

3D pore structure produced C60 molecules into graphene sheets by hydrothermal approach to enhance their electrochemical performance [42]. The CV curves of mC60/graphene composite revealed the EDLC and pseudocapacitors. The electrochemical dependence on mass ratio, temperature, and reaction time was studied. It was found that typically when the mass ratio of C60 to GO is 1:8, reaction time is 12 hr., and temperature is 150oC, the specific capacitance reaches 332.3F/g compared to 215.1 F/g for pure reduced graphene oxide. It was concluded from GCD curves that the mass ratio of C60 to GO is 1:8 is the best for optimizing the composite charge/discharge performance. C60 molecules into graphene sheets by hydrothermal approach to enhance their electrochemical performance [43]. The CV curves of mC60/graphene composite revealed the EDLC and pseudocapacitors. The electrochemical dependence on mass ratio, temperature, and reaction time was studied. It was found that typically when the mass ratio of C60 to GO is 1:8, reaction time is 12 hr., and temperature is 150°C, the specific capacitance reaches 332.3F/g compared to 215.1 F/g for pure reduced graphene oxide. It was concluded from GCD curves that the mass ratio of C60 to GO is 1:8 is the best for optimizing the composite charge/discharge performance.

A novel supercapacitor electrode was created using a carbon nano-onion(multilayer fullerene) /manganese dioxide/iron oxide (CNO/MnO₂/Fe₃O₄) nanocomposite [44]. The electrochemical performance of prepared supercapacitors composed of MnO₂, CNO, MnO₂/Fe₃O₄, and CNO/MnO₂/Fe₃O₄ nanocomposite was investigated. The rectangular shapes of CV curves of electrodes were established. The rise in super-capacitance of the CNO/MnO₂/Fe₃O₄ electrode is due to the increased surface area of the CNO and the presence of MnO₂ and Fe₃O₄, which increases the adsorption/desorption of cation and onion on the nanocomposite surface. The supercapacitive current of the CNO/MnO₂/Fe₃O₄ was higher than metal oxides electrodes due to the presence of CNO with a high surface area. It was observed that the Metal oxide electrodes have less symmetry than those containing CNO. Furthermore, The CNO/MnO₂/Fe₃O₄ nanocomposite electrode’s longer discharge duration implies improved electrode quality. The calculated specific capacitance of CNO/MnO₂/Fe₃O₄ electrodes was higher than other electrodes. At 1, 2, 3, and 4 A/g, CNO/MnO₂/Fe₃O₄ had a specific capacitance of 1130, 972.50, 900, and 730 F/g, while MnO₂/Fe₃O₄ had 571.25, 537.50, 442.50, and 400 F/g. Specific capacitances were 487.5 F/g at 1 A/g, 415 F/g at 2 A/g, 375 F/g at 3 A/g, and 340 F/g at 4 A/g for CNO. MnO₂’s capacitance at 1 to 4 A/g was 382.94, 326.14, 285.12, and 202.59 [44].

Using the stacking interactions of graphene with aromatic rings of functionalized fullerenes created and produced several unique graphene-based nanomaterials. To assure strong contacts and stable assembly of fullerenes on the surface of graphene, C60, C70, and Sc3N@C80 fullerene derivatives containing biphenyl, naphthalene, phenanthrene, or pyrene moieties were produced [45]. Graphene coated with bis-naphthalene C₇₀ fullerene malonate (G-BN7) revealed a 15% higher capacitance than graphene before modification, with a specific capacitance value of 56.15 F/g. Thus, naphthalene is the most suitable substitution for introducing fullerene derivatives on the graphene surface via π–π stacking. Additionally, compared to C60 and Sc3N@C₈₀, the C₇₀ fullerene core delivered the greatest results.

The low long-range conductivity of fullerene severely hinders the performance of supercapacitors that use this material. It is therefore anticipated that active carbons based on fullerene will have large capacitances when they are developed. By manipulating fullerene self-assembly with a cobalt tetramethoxy phenylporphyrin (CoTMPP) and pyrolysis, mesoporous carbon composites doped with varying concentrations of cobalt (Co) and nitrogen (N) were synthesized by Jiang et al. [46]. C₆₀ crystals encapsulated CoTMPP, which underwent carbonization to become actively-bound Co–N in the carbon structures. The ratio of CoTMPP in C₆₀ crystals and the distribution state in superstructures influence the concentration of Co–N. The electrochemical performance of porous carbon composite was greatly improved by Co–N. The fabricated carbon composite demonstrated an improved specific capacitance of 416.31 F g⁻¹ at 1 A g⁻¹, which is over ten times greater than that of the pristine C60, and had no activity loss after at least 5000 cycles.

Orderly mesoporous fullerene/carbon hybrids were synthesized by combining the fullerene precursor in chloronaphthalene with varying quantities of sucrose and employing mesoporous silica SBA-15 as a template [47]. Different samples MC₆₀@C-X, where X denotes the weight ratio of the fullerene C₆₀ and sucrose were prepared. The ideal EDLC behavior was observed for all samples as deduced from CV curves. By decreasing the C60/sucrose ratio from 2 to 1.33, the calculated specific capacitance increased and then reduced as the ratio fell to 0.8. The highest specific capacitance of 213 F/g at 0.5 A/g was achieved for the MC₆₀@C-1.33 electrode, which is higher than pure mesoporous fullerene prepared without sucrose molecules. Using superior textural parameters, the authors of this study demonstrated that incorporating carbon into the fullerene matrix enhanced the electrical transport and diffusion of the electrolytes. In addition, the research findings suggested that the presence of carbon layers between the fullerenes helped to strengthen the connection between the molecules of fullerene and promoted the electronic transition.

Table 1 summarizes some features of carbon-based supercapacitor electrodes that have been recently reported.

Electrode material	Specific/Volumetric capacitance	Cyclic stability	Ref
ZnFe2O4- RGO	1419 F/g	93% retention after 5000 cycles	[48]
Poly (3-hexyl-thiophene-2, 5-diyl)/ CNT	245.8 F/g	80.5% retention after 1000 cycles	[49]
Graphene/ MoS2	290 F/ cm^3	90% retention after 10,000 cycles	[50]
PANI/CNT	541 F/ g	90% retention after 25 cycles	[35]
PANI/fullerene	2201 F/g	96% retention after 1000 cycles.	[42]
Ag/Bi nanoparticle anchored CNT	1372F/g	101.3% retention after 10,000 cycles	[51]
MnO2@CNT	386 F/ g	93.6% retention after 5000 cycles	[52]
MoS2/Mn- metal organic frameworks (MOF)/CNT	862.73F /g	71.4% retention after 5000 cycles	[53]
CeO2/graphene	782 F/g	82% retention after 6000 cycles	[54]
nitrogen-doped carbon nano-onions (N-CNO)	205 F/ g	96% retention after 5000 cycles	[55]
MXene/graphene	183.5 F/ cm^3	75% retention after 3000 cycles	[56]
CNT@PANI	138F/g	86% retention after 1000 cycles	[57]
RGO/MWCNT/ZrO2	357 F/ g	98% retention after 5000 cycles	[58]
MnO2/CoWO4/ nitrogen-doped carbon nanoonions (NCNO)	536 F/g	96% retention after 3000 cycles	[59]
RuO2 quantum dots / RGO	1120 F/ g	89% retention after 10,000 cycles	[60]
Tetraaniline(TA)/porous RGO	85.6 F/g	104.7% after 10,000 cycles	[61]

Table 1.

Some latest fabricated carbon-based supercapacitor electrodes.