High-performance Electric Double-layer Capacitors Research Articles

Role of Graphene Oxide and Reduced Graphene Oxide in Electric Double-Layer Capacitors: A Systematic Review

The evolution of electric double-layer capacitors (EDLCs) has significantly benefited from advancements in graphene-based materials, particularly graphene oxide (GO) and reduced graphene oxide (rGO). This systematic review consolidates and analyzes existing research on the roles of GO and rGO in enhancing the performance of EDLCs, focusing on synthesis methods, electrode fabrication, electrolytes, and performance metrics such as capacitance, energy density, and cycling stability. Following the PICOS and PRISMA frameworks, a comprehensive literature search was conducted across Scopus, Web of Science, PubMed, and IEEE Xplore, covering the period from 2010 to 2023. A total of 128 articles were initially identified, with 27 studies meeting the inclusion criteria after rigorous screening and full-text analysis. Key findings reveal that the incorporation of GO and rGO in EDLCs leads to significant improvements in specific capacitance, energy density, and cycling stability. Notable advancements include novel synthesis techniques and composite materials such as nitrogen-doped graphene, graphene/polyaniline hybrids, and various metal oxide–graphene composites, which exhibit superior electrochemical performance. However, challenges such as material scalability, environmental sustainability, and consistency in synthesis methods remain. This review stresses the great potential of GO and rGO in the development of high-performance EDLCs and highlights the need for continued research to address existing challenges and further optimize material properties and fabrication techniques.

Gamma-induced interconnected networks in microporous activated carbons from palm petiole under NaNO3 oxidizing environment towards high-performance electric double layer capacitors (EDLCs)

Activated carbons (ACs) were developed from palm petiole via a new eco-friendly method composed of highly diluted H2SO4 hydrothermal carbonization and low-concentration KOH-activating pyrolysis followed by gamma-induced surface modification under NaNO3 oxidizing environment. The prepared graphitic carbons were subsequently used as an active material for supercapacitor electrodes. The physiochemical properties of the ACs were characterized using field emission scanning electron microscope–energy dispersive X-ray spectroscopy, N2 adsorption/desorption isotherms with Brunauer–Emmett–Teller surface area analysis, Fourier transform infrared spectroscopy, X-ray diffraction and Raman spectroscopy. The electrochemical performance of the fabricated electrodes was investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Even treated with extremely low H2SO4 concentration and small KOH:hydrochar ratio, the maximum SBET of 1365 m2 g−1 for an AC was obtained after gamma irradiation. This was attributed to radiation-induced interconnected network formation generating micropores within the material structure. The supercapacitor electrodes exhibited electric double-layer capacitance giving the highest specific capacitance of 309 F g−1 as well as excellent cycle stability within 10,000 cycles. The promising results strongly ensure high possibility of the eco-friendly method application in supercapacitor material production.

Enhancement of supercapacitive performance of carboxymethyl cellulose-derived porous carbon monoliths by the addition of carbon nanotubes

In this study, a simple and eco-friendly fabrication strategy for the preparation of porous carbon monoliths with 3-dimensional conductive networks was demonstrated. Porous carbon/carbon nanotube composite monoliths (PCCMs) with large specific surface area and hierarchical pore structures were prepared via eco-friendly ice-templating followed by carbonization. The resulting PCCMs showed excellent electrochemical performance due to their well-developed hierarchical porosity and high electrical conductivity following the addition of 1-D carbon nanotubes. In a 3-electrode configuration, the PCCM-derived electrode delivered a specific capacitance of 240 F g−1 at 1 A g−1. In a practical application, the PCCM-based symmetric capacitor exhibited the specific capacitance of 47.0 F g−1, rate capability of 72.7%, power density of 9.5 kW kg−1 and a cycling stability of ∼100% after 20,000 cycles, respectively. These results suggested that the PCCMs can be applied as an active material for high-performance electrical double-layer capacitors.

Assembly of electric double-layer capacitors with hardwood kraft lignin-based electrodes and separator together with ionic liquid electrolyte

Abstract This study aimed to assemble a high-performance electric double-layer capacitor (EDLC) using a hardwood kraft lignin (HKL)-based separator and HKL-based electrodes, which were fabricated from a nonwoven mat of electrospun HKL fibers. The separator was prepared by the thermostabilization of the mat derived from a mixed dope of HKL, hexamethylenetetramine, and polyethylene glycol (1.66/0.50/0.09, w/w) for electrospinning. Although a mat-type HKL-based electrode containing conductive carbon black (CB) has been reported to be suitable for a commercial cellulosic separator, this electrode was found to be unsuitable for the HKL-based separator because of its rough surface and poor contact with the separator interface. Hence, a powder-type electrode with a smooth surface was fabricated by grinding the mat, followed by casting with a carboxymethyl cellulose aqueous solution, and its EDLC possessed high energy (49 Wh kg−1) and power densities (151 kW kg−1). Moreover, to provide a simple process for electrode fabrication, another mat-type electrode was fabricated by adding CB to the mixed dope, followed by electrospinning, carbonization, and steam activation. The resultant EDLC exhibited excellent electrochemical performance with energy (58 Wh kg−1) and power densities (55 kW kg−1).

Hygroscopic protic ionic liquids as electrolytes for electric double layer capacitors

In this work we report about the synthesis and characterization of a novel protic ionic liquid (PIL), 1-methylpyrrolidinium tetrafluoroborate (PyrH1BF4), and about its use as electrolyte in electric double layer capacitors (EDLCs). We show that PyrH1BF4 containing 0.8 percentage by weight of water is displaying very favorable transport properties and a large electrochemical stability. With the help of molecular dynamics simulations, it is revealed that at these low concentrations the water molecules are dispersed in the ionic liquid and do not form larger aggregates, while arrangements are formed where a water molecule is placed between an anion and cation, forming solvent separated ion pairs. Employing this PIL in EDLCs allows the realization of devices that display higher energy and power in the timeframe of interest for high power systems (few minutes to seconds) compared to those possible with conventional aprotic ionic liquids, even though these latter devices have much higher operative voltages. Considering these results, PyrH1BF4 can therefore be considered as a very interesting electrolyte for the realization of safe and high performance EDLCs.

Communication—Sustainable Starfish-Derived N-Doped Porous Carbon for High-Performance Electric Double Layer Capacitors

Electric double layer capacitors can be used in broad applications, including portable electronics, high power tools, and transportation. However, such systems require more sustainable materials and a boosting of their electrochemical performance. In this study, starfish, as an unusable destroyer of marine ecosystems, are transformed into functional nitrogen-doped carbon materials with high porosity toward high-performance EDLC electrodes. The synthesized starfish-derived carbon exhibits a surface area of up to 3122 m2 g−1, a specific capacitance of up to 295.7 F g−1 at a current density of 0.5 A g−1, and capacitance retention of 81.5% upon an increase in current density from 0.5 to 10 A g−1.

Carbon electrodes with ionophobic characteristics in organic electrolyte for high-performance electric double-layer capacitors

Activated carbon (AC) in organic electrolyte-based electric double-layer capacitors (EDLCs) usually suffers from low specific capacitance. Most studies on AC focus on improving its surface area and optimizing pore structures to enhance its electrochemical performance in EDLCs. Unfortunately, the interfacial microenvironment, which is composed of nanoporous carbon and the organic electrolyte confined in it, is always ignored. Herein, a simple and powerful strategy to create AC with an ionophobic surface is proposed to address the poor efficiency of the electric double-layer process. The polar C-F bonds formed in the AC material are characterized through near-edge X-ray absorption fine structure and X-ray photoelectron spectroscopy. The ionophobic characteristic of YP-F60s in an organic electrolyte is extensively studied via contact angle measurements and small-angle X-ray scattering spectroscopy. An EDLC constructed with YP-F60s as the electrode and 1 mol L−1 tetraethylammonium tetrafluoroborate/propylene carbonate as the electrolyte demonstrates high specific capacitance, low internal resistance, and excellent cycling stability. Our results successfully demonstrate the importance of the interfacial microenvironment of AC and its confined electrolyte to the electrochemical performance of EDLCs. Our work also offers new perspectives on the use of the CF4 plasma technique to fabricate low-cost superior carbon for EDLCs.

N-doped hollow carbon spheres with controllable shell numbers for high-performance electrical double-layer capacitors

Currently, hollow carbon spheres (HCS)-based supercapacitors are gained tremendous interest due to their excellent electrochemical property. However, the inferior structural stability and unsatisfactory space utilization restrict the applications for high power devices. Herein, a kind of novel N-doped HCS (N-HCS) with controllable shell numbers is constructed by layer-by-layer assembly of polydopamine (PDA) and polypyrrole (PPy) layers on carboxyl modified polystyrene (PS–COOH) spheres. Compared with single-shelled N-HCS, the optimize N-HCS with 4 shells (N-HCS-4) exhibits improved specific surface area from 43.9 m2 g−1 to 1110.2 m2 g−1, which provides a large accessible surface area for the electrolyte, thereby facilitating the ion diffusion and electron transportation. Meanwhile, the doping heteroatom endows N-HCS with the additional pseudocapacitance. Very interestingly, the significant electrochemical properties of N-HCS at the varied coating sequence and number of PDA and PPy layers indicate the formation of the optimal microstructure, which could match with the electrolyte ion size for accelerating the capacitive response. The assemble asymmetrical supercapacitor displays a high energy density of 41.18 Wh kg−1 at a power density of 250 W kg−1, and excellent cycling stability of 99.1% over 10,000 cycles due to the improved structural stability from the synergistic effect in different shells.

Simultaneous effect of fluorine impregnation on highly mesoporous activated carbon used in high-performance electrical double layer capacitors

Owing to the increase in need for high-performance energy storage devices, activated carbon has attracted considerable attention as an electrode material for electrical double-layer capacitors (EDLCs). However, the conventional manufacturing process of activated carbon is seriously limited by the depletion of raw material sources. Biomass-based activated carbon, which is currently considered as a breakthrough, exhibits performance limitations such as low specific capacitance and poor high-rate performance. In this study, we propose the use of fluorine-doped mesoporous carbon (F-MAC), which is derived from Ecklonia cava using fluorine impregnation and KOH activation, to address the above challenges. F-MAC exhibits high specific capacitance (184 F/g), excellent high-rate performance (155 F/g at a current density of 20 A/g), and superior cycle stability (82.8% capacitance retention after 2,000 cycles). These performance improvements are attributed to the increased surface area, high mesopore volume fraction, fluorine-doping effect, and high concentration of oxygen functional groups.

Effect of Ion Species on the Electrochemical Characteristics of Liquid Electrolytes for High Performance EDLCs

Electric double layer capacitors (EDLCs) have been considered one of the most promising electrochemical energy storage devices with high specific power and long cycle life. They are being used in power sources for hybrid electric vehicles and other energy storage systems. An electrolyte with high ionic conductivity and wide electrochemical window is one of the vital constituents of EDLCs and it plays an important role in EDLC performance. In particular, acetonitrile (ACN) has been widely used as an electrolyte solvent in EDLCs, because it can dissolve salts effectively and thus has high ionic conductivity.However, there are few reports on systematic studies of ion species in salt for EDLC. In this respect, we studied the correlation between ion species and electrochemical properties of ACN-based electrolytes. In this work, ammonium, pyrrolidinium and piperidinium-based salts were employed to achieve high ionic conductivity. Ionic conductivity of electrolyte consisting of smaller ions exhibited higher ionic conductivity. Among the various salts, 1-ethyl-2-methylpyrrolidinium tetrafluoroborate, N-trimethyl-N-propylammonium bis(fluorosulfonyl)imide and 1,1-dimethyl piperidinium bis(fluorosulfonyl) imide exhibited high ionic conductivity and wide electrochemical stability window.

Oxygen-Plasma Surface Treatment of an Electrode Sheet Using Carbon from Japanese Distilled Liquor Waste for Double-layer Capacitors

Oxygen-plasma treatment was performed on an activated carbon sheet obtained from shochu (Japanese distilled liquor) waste using a high-frequency plasma generator. The capacitances of the activated carbon sheet electrode subjected to surface treatment were measured using cyclic voltammetry. The best results were obtained with a processing time of 60 min, an output power of 40 W, an interelectrode distance of 13 cm and an oxygen pressure of 40 Pa. The maximum capacitance was 247 F g−1. Investigation of the surface functional groups of the activated carbon sheet revealed that the oxygen-containing functional groups modified on the surface of the activated carbon sheet electrode contributed to the improvement in the capacitance. High-performance electric double-layer capacitors can be realized using the developed electrodes.

2D porous carbon nanosheet from sulfonated pitch-based graphene quantum dots for high volumetric performance EDLCs

With the ever-growing demand of miniaturized electric double-layer capacitors (EDLCs), it is still a big challenge to well-balance the competing relationships between high specific surface area (SSA) and high packing density in porous carbons. Herein, 2D porous carbon nanosheet (PCN) with high carbon yield is synthesized by a simple interface self-assembly and subsequent activation strategy from industrial sulfonated pitch by-product, which is composed of graphene quantum dots. It is bestowed with ultrathin thickness of 20 nm, adequate SSA of 2429 m2 g−1, high packing density of 0.562 g cm−3, high e-conductivity of 89 S m−1 and high mechanical flexibility all in one, giving full play to the synergistic effect of electrons and ions. Based on the structure advantages, this PCN can be used as high-performance EDLC electrode. In pure EMIMBF4 electrolyte, it achieves high volumetric capacitances of 90.1 F cm−3 at 0.5 A g−1, excellent rate capability C20/0.05 of 71% and good high-voltage endurance lifetime of 88% after 5000 cycles at 1 A g−1. Satisfactorily, the maximum volumetric energy density can come up to 39 Wh L−1. More significantly, this work can also provide a new perspective on the architectural conception of other 2D carbon materials.

Metal framework as a novel approach for the fabrication of electric double layer capacitor device with high energy density using plasticized Poly(vinyl alcohol): Ammonium thiocyanate based polymer electrolyte

High performance electric double-layer capacitors (EDLCs) based on poly (vinyl alcohol) (PVA): ammonium thiocyanate (NH4SCN):Cu(II)-complex plasticized with glycerol (GLY) have been fabricated. The maximum DC ionic conductivity (σDC) of 2.25 × 10-3 S cm−1 is achieved at ambient temperature. The X-ray diffraction (XRD) patterns confirmed that the addition of both Cu(II)–complex and GLY enhanced the amorphous region within the samples. Through the Fourier transform infrared (FTIR) the interactions between the host polymer and other components of the prepared electrolyte are observed. The FESEM images reveal that the surface morphology of the samples showed a uniform smooth surface at high GLY concentration. This is in good agreement with the XRD and FTIR results. Transference numbers of ion (tion) and electron (tel) for the highest conducting composite polymer electrolyte (CPE) are recognized to be 0.971 and 0.029, respectively. The linear sweep voltammetry (LSV) revealed that the electrochemical stability window for the CPE is 2.15 V. These high values of tion and potential stability established the suitability of the synthesized systems for EDLC application. Cyclic voltammetry (CV) offered nearly rectangular shape with the lack of Faradaic peak. The specific capacitance and energy density of the EDLC are nearly constant within 1000 cycles at a current density of 0.5 mA/cm2 with average of 155.322F/g and 17.473 Wh/Kg, respectively. The energy density of the EDLC in the current work is in the range of battery specific energy. The EDLC performance was found to be stable over 1000 cycles. The low value of equivalent series resistance reveals that the EDLC has good electrolyte-electrode contact. The EDLC exhibited the initial high power density of 4.960 × 103 W/Kg.

High-performance electric double-layer capacitor fabricated with nanostructured carbon black-paint pigment as an electrode

Engineering the microstructure of the carbonaceous materials is a promising strategy to enhance the capacitive performance of supercapacitors. In this work, nanostructured Black Pearl (1500 BP) carbon which is a conductive carbon being commercially used in printing rolls, conductive packaging, conductive paints, etc. is analyzed for its feasibility as an electrode material for Electric Double-Layer Capacitors (EDLCs). To achieve that commercial Black Pearl (BP), carbon is treated with mild acid H3PO4 to remove the impurities and enhance the active sites by regulating the growth of agglomerates and creating micropores in the nano-pigments. Generally, the coalescence of nanoparticles owing to their intrinsic surface energy has tendency to create voids of different sizes that act like meso/micropores facilitating the diffusion of ions. The electrochemical performance of BP carbon before and after chemical activation is investigated in aqueous (H2SO4, KOH and KCl) and a non-aqueous electrolyte (1 M TEMABF4 in acetonitrile) environment employing different electrochemical techniques such as Cyclic Voltammetry (CV), Galvanostatic charge/discharge (GCD) and Electrochemical Impendence Spectroscopy (EIS). The chemically activated BP carbon delivers the highest specific capacitance of ∼156 F g−1 in an aqueous electrolyte, 6 M KOH. The highest specific power, ~ 15.3 kW kg−1 and specific energy, 14.6 Wh kg−1 are obtained with a symmetric capacitor employing non-aqueous electrolyte because of its high working potential, 2.5 V.

Ecklonia cava based mesoporous activated carbon for high-rate energy storage devices

Activated carbon has received enormous global attention as an electrode material for electrical double layer capacitors (EDLCs) due to its superior electrochemical performance. But while manufacturing high-quality activated carbon as an electrode material, overcoming limitations such as depletion of raw material resources, poor high-rate performance, and cycle stability is still a significant challenge. To address these limitations, we firstly suggest a novel mesoporous activated carbon derived from ecklonia cava (Meso-ACDE) using an acid-base reaction and KOH activation processes. The optimized Meso-ACDE electrode showed a superior specific capacitance of 182Fg−1 at the current density of 0.2Ag−1 and a high-rate capacitance of 154Fg−1 at the current density of 20.0Ag−1 with an excellent cycling stability up to 2000 cycles. The enhanced electrochemical performance of the Meso-ACDE electrode was mainly attributed to the well-developed mesoporous structure with a high concentration of oxygen related functional groups, which were acquired during the activation process. Thus, we believe that the Meso-ACDE could be a promising alternative to conventional activated carbon for high-performance EDLCs.

Fabrication of Activated Porous Carbon Using Polymer Decomposition for Electrical Double-Layer Capacitors

Because of their excellent stability and highly specific surface area, carbon based materials have received attention as electrode materials of electrical double-layer capacitors(EDLCs). Biomass based carbon materials have been studied for electrode materials of EDLCs; these materials have low capacitance and high-rate performance. We fabricated tofu based porous activated carbon by polymer dissolution reaction and KOH activation. The activated porous carbon(APC-15), which has an optimum condition of 15 wt%, has a high specific surface area(1,296.1 m2 g−1), an increased average pore diameter(2.3194 nm), and a high mesopore distribution(32.4 %), as well as increased surface functional groups. In addition, APC has a high specific capacitance(195 F g−1) at low current density of 0.1 A g−1 and excellent specific capacitance(164 F g−1) at high current density of 2.0 A g−1. Due to the increased specific surface area, volume ratio of mesopores, and surface functional groups, the specific capacitance and high-rate performance increased. Consequently, the tofu based activated porous carbon can be proposed as an electrode material for high-performance EDLCs.

Scalable green synthesis of hierarchically porous carbon microspheres by spray pyrolysis for high-performance supercapacitors

Designing carbon materials with optimized hierarchical pore structures is crucial to realize high-performance electrical double-layer capacitors (EDLCs) with enhanced energy and power densities. Therefore, the development of the suitable synthesis strategies for hierarchically porous carbon (HPC) materials with a higher productivity has attracted significant interest. Here, a facile, economic, scalable, and environmentally friendly method to synthesize hierarchical pore-structured carbon materials by spray pyrolysis without the use of heavy metal salts is presented. A spray solution containing sucrose, NaCl, and H2SO4 is aerosolized by an ultrasonic nebulizer and undergoes pyrolysis to produce NaCl-templated carbon microspheres, followed by an activation step and washing to obtain HPC. Endowed with a specific surface area up to 1704 m2 g−1 and pore volume of 1.81 cm3 g−1, the resulting carbon contains an interconnected micro/mesoporous network, which provides efficient ion transport channels with a short diffusion length and small ionic resistance. Moreover, it exhibits a specific capacitance of 102 F g−1 at a current density of 30 A g−1 with a retention of 92% over 10,000 cycles at 10 A g−1 as an electrode in an organic-electrolyte-based EDLC. In contrast, the sheer microporous carbon does not exhibit EDLC behavior at a high rate. The proposed synthesis technique paves the way for cheap scalable syntheses of carbon materials with desired pore structures by spray pyrolysis, which may inspire further studies for various applications including energy storage, where carbon materials with high specific surface areas and yields are of significance.

Carbon electrodes with double conductive networks for high-performance electrical double-layer capacitors

Tremendous efforts have been devoted to develop the active materials for supercapacitor electrodes, but very few are focused on the improvement of the electrode material/electrolyte interfaces via protonic conductors. In this study, double conductive networks are constructed in powdery carbon electrodes by using sulfated zirconia as protonic conductor and acetylene black as electric conductor. The synergetic effect of protonic and electric double transport pathways enhances the electrode material/electrolyte interfaces, reduces diffusion distance of protons, and increases the electrode specific capacitance. Compared to the control electrode without protonic conductor, the electrode shows a specific capacitance with 21.0% increment. The proposed strategy may be a promising avenue for preparation of powdery carbon electrodes used in electrical double-layer capacitors.

Preparation of N,F,B-Doped Carbon Materials for High-Performance Electric Double Layer Capacitors

The incorporating heteroatom species into carbon materials give opportunities for improving electrochemical properties. Single N-doping strategies are widely studied because of easy preparations and relative abundance in nature. However, facile synthetic methods of multiple-heteroatom-doped carbons are urgent for high-performance supercapacitors. In this study, we propose N,F,B-doped carbons for electric double layer capacitors derived from polyamic acid mixed with boric acid using pyrolysis process. Fluorinated polyamic acid is applied for a N,F-doping source and boric acid acts as a B-doping source. The synthesized carbon materials exhibit excellent electrochemical performances because of the synergetic effects of each N, F, and B atoms. The N and F atoms improve electronegativity property and also low doped B atoms improve redox reactivity in the carbon. The electrode is fabricated by direct pyrolysis process with carbon fabrics, which is an advantage for binder-free type electrodes and flexibility. The morphology and surface properties of the fabricated electrodes are studied by SEM, XPS, Raman, sheet resistance, and contact angle measurement. Furthermore, the flexible symmetric device with PVA-1M H2SO4 gel electrolyte fabricated from the N,F,B-doped carbon indicates a high volumetric capacitance and an energy density as well as mechanical stabilities under various deformations. These results are expected that the synthesized N,F,B-heteroatom-doped carbons offer great potentials as electrode materials for supercapacitor applications.

Mesoporous Control Effect of Porous Carbon Nanofibers for Electrical Double-Layer Capacitors

To improve the performance of carbon nanofibers as electrode material in electrical double-layer capacitors (EDLCs), we prepare three types of samples with different pore control by electrospinning. The speciments display different surface structures, melting behavior, and electrochemical performance according to the process. Carbon nanofibers with two complex treatment processes show improved performance over the other samples. The mesoporous carbon nanofibers (sample C), which have the optimal conditions, have a high sepecific surface area of 696 m2 g−1, a high average pore diameter of 6.28 nm, and a high mesopore volume ratio of 87.1%. In addition, the electrochemical properties have a high specific capacitance of 110.1 F g−1 at a current density of 0.1 A g−1 and an excellent cycling stability of 84.8% after 3,000 cycles at a current density of 0.1 A g−1. Thus, we explain the improved electrochemical performance by the higher reaction area due to an increased surface area and a faster diffusion path due to the increased volume fraction of the mesopores. Consequently, the mesoporous carbon nanofibers are demonstrated to be a very promising material for use as electrode materials of high-performance EDLCs.