Performance Of Electric Double-layer Capacitors Research Articles

Recycling of activated carbons from spent supercapacitors to refabricate improved supercapacitors

The growing market of commercial electric double-layer capacitors (EDLCs) will lead to a vast accumulation of waste as they arrive out of operation. Recycling the spent electrode activated carbon is a closed-loop solution to provide environmental and economic advantages for EDLC use. However, effectively regenerated methods of high-purity activated carbons are still lacked despite their considerable expense. Herein, a simple steam physical activation method is developed to regenerate high-class activated carbons from spent EDLCs. The process is first demonstrated that can effectively eliminate few electrolytes and complex organics as well as regenerate electrode carbon materials. The activation-recycled carbon materials preserve the physicochemical properties of fresh activated carbons and enhance their mesopore ratios, which contributes to improved EDLC performance, obviously surpassing recycled carbon materials by calcination method, including superior specific capacitance of 103.3 F g−1 at 0.5 A g−1, rate performance and cycling stability, equal to fresh activated carbons. Moreover, the assembled activation-recycled carbon material EDLC delivers low self-discharge rate (89 % of initial voltage after 72 h), wide temperature range (−40 to 85 °C), excellent cycling stability (92.2 % capacitance retention after 10,000 cycles) even at elevated voltages (3.0 V) and temperature (65 °C) test. The present work provides an effective and economic method of recycling high-grade activated carbons for sustainable EDLCs.

Laser‐Induced Formation of Fine Porous Graphitic Carbon for Eco‐Friendly Supercapacitors

An electric double‐layer capacitor (EDLC), which is one of the pivotal energy storage devices, provides rapid charge–discharge capabilities and an extended cycle life. Areal capacitance, a key indicator of EDLC performance, increases with the specific surface area of its electrodes. This study demonstrates a method for significantly increasing the specific surface area in the laser‐induced graphitization of biodegradable polymers by incorporating NaHCO3 into the composite sheet, generating not only microscale pores but also a large number of nanoscale fine pores. Furthermore, it shows that using these structures as EDLC electrodes leads to a substantial increase in areal capacitance. An increase in the number of fine pores formed in the structure and a corresponding rise in the areal capacitance of the fabricated EDLC are observed with the increase in the NaHCO3 weight ratio. Notably, the composite sheets are composed of natural‐derived, biodegradable materials, while NaHCO3 is known for its low corrosivity and biotoxicity. The proposed method thus offers a pathway for fabricating energy storage devices with minimal environmental impact, ensuring their eco‐friendly disposal post‐use.

Tuning Electrode and Separator Sizes For Enhanced Performance of Electrical Double‐Layer Capacitors

AbstractAn electrical double‐layer capacitor (EDLC) comprises two porous electrodes sandwiching an electrolyte‐permeable separator, which prevents the electrodes from short‐circuiting. While previous studies have mainly focused on electrolyte and electrode properties of EDLCs, the device configuration in terms of electrode and separator sizes received less attention, with separators often simplistically modelled as infinitely large reservoirs of ions. Herein, we investigate how the relationship between electrode and separator thicknesses impacts EDLC charging. We find that the assumption of bulk reservoir holds only under specific conditions. Moreover, we identify a tradeoff between stored energy density and pressure variations within the separator, potentially jeopardizing the EDLC durability. We also explore the influence of ionic liquid additives on EDLC charging. While prior research has shown that trace amounts of uncharged additives with strong electrode affinity can significantly enhance energy storage, we observe this effect as negligible for electrodes and separators of comparable sizes. Instead, we show how to optimize EDLC performance by fine‐tuning the concentration of additives and separator‐to‐electrode size ratio to maximize stored energy density.

Propping the Electrochemical Impedance Spectra at Different Voltages Reveals the Untapped Supercapacitive Performance of Materials

Electrochemical impedance spectroscopy (EIS) is one of the most powerful and universal techniques that provides deep insights into the nature of the electrochemical system and its performance and limitations. Electric double layer capacitors (EDLCs) are electrochemical devices that can deliver and store large amounts of energy compared to traditional capacitors. The selection of an electrolyte is critical to the overall performance of EDLCs as it strongly influences the internal resistance, operating potential window, and rate capability of the device. Herein, fullerene C76 is investigated in four different electrolyte systems (Li2SO4, Na2SO4, K2SO4, and Rb2SO4) to reveal the crucial role of the electrolyte cation on the electrochemical supercapacitive performance. The Stern layer and diffuse layer formation are theorized for the electrolytes and comprehensively analysed to identify the electrolyte that provides the best system for high capacitive output. Rb+ is found to have the best conductivity and is the most responsive to applied potentials. Specifically, monitoring the EIS of the system at different applied voltages enables the unveiling of the ongoing electrochemical processes and the charge storage mechanism in detail. This approach can be generalized for various electrochemical energy storage systems to help design efficient systems and unveil the working mechanisms.

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.

Advances in Carbon Xerogels: Structural Optimization for Enhanced EDLC Performance.

This review explores the recent progress on carbon xerogels (CXs) and highlights their development and use as efficient electrodes in organic electric double-layer capacitors (EDLCs). In addition, this work examines how the adjustment of synthesis parameters, such as pH, polymerization duration, and the reactant-to-catalyst ratio, crucially affects the structure and electrochemical properties of xerogels. The adaptability of xerogels in terms of modification of their porosity and structure plays a vital role in the improvement of EDLC applications as it directly influences the interaction between electrolyte ions and the electrode surface, which is a key factor in determining EDLC performance. The review further discusses the substantial effects of chemical activation with KOH on the improvement of the porous structure and specific surface area, which leads to notable electrochemical enhancements. This structural control facilitates improvement in ion transport and storage, which are essential for efficient EDLC charge-discharge (C-D) cycles. Compared with commercial activated carbons for EDLC electrodes, CXs attract interest for their superior surface area, lower electrical resistance, and stable performance across diverse C-D rates, which underscore their promising potential in EDLC applications. This in-depth review not only summarizes the advancements in CX research but also highlights their potential to expand and improve EDLC applications and demonstrate the critical role of their tunable porosity and structure in the evolution of next-generation energy storage systems.

One-step generation of hollow graphitic carbon nanospheres with suitable oxygenated structures as conductive additives for water-soluble binder in highly stable supercapacitor electrodes

Although the weight ratio of electron conductive additives and the binder to porous carbons in the electrode component of electric double-layer capacitors (EDLC) is minimal, however, without them the electrochemical performances of EDLC would decay quickly. In this work, we present a one-step approach for manufacturing a unique conductive carbon material that shows highly connecting hollow structures as well as approximate oxygenated surface groups to create a synergy effect with aqueous binder and the main component, porous carbon. The as-prepared carbon nanospheres (HCNs) were generated from a typical carbon black-PC20 in air (denoted as PC20HT) at mild temperatures ranging from 400 to 700 °C for 2–4 h at a massive scale of approximate 1 kg h−1. The as-prepared PC20HT displays a high specific surface area of 657 m2 g−1 (the pristine PC20 has 112 m2 g−1) with a pore volume of 0.71 cm3 g−1, and an oxygen content of 14.10 wt%. Interestingly, a premixing of PC20HT and porous carbon is required before making the electrode pastes. The well-premixed carbon materials and a water-soluble binder-polyacrylic acid are composed into highly-stable components as EDLC electrodes in an organic electrolyte. As consequently, the PC20HT-assisted electrodes demonstrate highly stable life cycles (charge-discharge behaviors), with a retention rate 82.13 % over 50,000 cycles at a narrow voltage window of 2.5–2.7 V at 0.5 A g−1 when compared to two conventional conductive carbons, BP2000 (68.45 %) and Super P (67.46 %). Thermal annealing at 900 °C in Ar reduces the oxygen content of PC20HT to 4.25 wt% (referred to as TPC20HT), increasing the retention rate of life cycle to 88.89 % after the same cycles. TPC20HT-assisted EDLC electrodes exhibit a superior energy density of 34.76 Wh kg−1, which is higher than CB-BP2000 and Super P-assisted EDLC electrodes. The transforming of structural morphologies as well as the surface oxygenated and graphitic structures of the as-prepared HCS, PC20HT, and TPC20HT were also described before and after long-term cycling. This simple route for producing conductive carbons is applicable in an aqueous binder system for preparing EDLC electrodes with a cost-effective and environmentally friendly approach.

Operando Temperature Dynamic Investigation of Electric Double‐Layer Capacitors Containing Organic Electrolytes

AbstractVariations in temperature play a substantial role in influencing the electrochemical performance of electric double‐layer capacitors (EDLCs). Lower temperatures limit the charge carriers′ transport properties, diminishing overall cell performance, while higher temperatures enhance transport properties and performance at the expense of material decomposition. Thus, investigating the temperature‐dependent behavior of supercapacitors requires numerous time‐consuming experiments while maintaining specific and consistent testing conditions. Therefore, we present a fast screening technique, named operando temperature dynamic measurements, to investigate EDLC performance under varying temperatures and voltages. The swift nature of this technique minimizes cell aging, allowing the evaluation even at harsh conditions. Utilizing temperature dynamic measurements, EDLCs containing six different organic electrolytes have been compared. Incrementally increasing temperature and cell voltage while performing electrochemical impedance spectroscopy or galvanostatic charge‐discharge measurements, metrics such as capacitance, energy, power, entropy, and enthalpy were determined. Interestingly, different performance evolutions over the temperature range are observed, allowing the identification of optimal operational conditions for each device.

Unraveling concentration-dependent solvation structures and molecular interactions in water-in-salt electrolytes for enhanced performance of electric double-layer capacitors

Water-in-salt electrolytes (WiSEs) are increasingly recognized as advanced aqueous electrolytes for energy storage due to their inherent safety, wide electrochemical stability window (ESW) beyond 1.23 V, and relatively high ionic conductivity to viscosity ratio. Despite these advantages, a comprehensive understanding of the physicochemical and structural properties of WiSEs across a broad temperature spectrum remains elusive, thus limiting potential performance enhancements. In this study, the microscopic ion-ion and ion-water interactions in WiSE with potassium acetate (KOAc) as a model electrolyte were systematically analyzed in both bulk phase and electric double-layer (EDL). Concentration-dependent solvation structures, thermal and structural behaviors, and ionic transport properties were investigated through Raman spectroscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and analytical electrochemistry measurements. The phase diagram of the aqueous KOAc solution was constructed using low-temperature XRD to identify the solidified phases, and DSC was used to determine the phase transition temperatures in a wide temperature range from -120 °C to 80 °C. We also explored the temperature-dependent electrochemical performance of various concentrations of KOAc electrolytes, finding the 5 m KOAc shows optimal performance for EDLCs at room temperature and in sub-zero temperatures. Additionally, we proposed a novel concept of an optimal cutoff voltage for maximizing power, comparing it with other commonly used metrics for setting cutoff voltage. This study validates our approach and highlights its significance in balancing key performance parameters.

High-energy storage capacity of cellulose nanofiber supercapacitors using bound water

The performance of electric double-layer capacitors and lithium-ion batteries deteriorates with increasing humidity. The desirable effect of bound water on the energy-storage properties of physically dry cellulose nanofiber (Na-ACF) supercapacitors with sodium (Na) carboxylate radicals was investigated using infrared and near-infrared spectroscopy, and nuclear magnetic resonance spectroscopy, alternating current impedance analyses, and first-principles calculations. The storage capacity decreased gradually upon heating to 423 K and reached zero upon exceeding 483 K, accompanied by increasing electrical resistance, forming a distorted semicircle in Nyquist diagram and drawing the phase angle to zero in Bode diagram. This is attributed to the water in the hydration gel bound to the Na+-ions that cross-link the cellulose chains, evaporating as the temperature increases, and finally becoming Na2O. The increased band-gap energy from the increase in bound water prevents leakage from the supercapacitor. In contrast to ordinary batteries, bound water is necessary for developing Na-ACF supercapacitors.

Impact of Preprocessing of Graphene Additive via Ultrasonication on the Electrochemical Performance of Activated Carbon‐Based Supercapacitors

This article investigates the influence of the addition method of graphene to activated carbon (AC) on the electrochemical performance of the fabricated electric double‐layer capacitors (EDLCs). To do so, the same graphene material is introduced to the other electrode slurry materials either in the form of a powder (GP) or a suspension (GL). It is discovered that the introduction of graphene already dispersed by ultrasonic in a suspension to AC has a better effect on the electrochemical performance of the fabricated EDLCs than directly employing its powder. GL EDLC achieves the highest capacitance of 54 F g−1 at a current density of 0.05 A g−1 in an organic electrolyte, which is much better than the GP EDLC (41.9 F g−1) and about 77% more than AC EDLC. Moreover, after cycling at various current densities (2000 cycles), GL EDLC maintains 28.8 F g−1 of its capacitance with 99.4% coulombic efficiency at a current density of 1 A g−1. Finally, GL EDLC exhibits 35.8 and 21.4 Wh kg−1 energy densities at 50.2 and 1102.8 W kg−1 power densities, respectively. The better performance of GL EDLC in comparison with GP EDLC is ascribed to better graphene dispersion.

EDLC performance of ammonium salt-green polymer electrolyte sandwiched in metal-free electrodes

Microplastic and metal waste from electronic industries are becoming a major threat to the environment and marine ecosystem. Green processing and employing natural derived materials are solutions to this issue. In this work, an electrical double-layer capacitor (EDLC) fabricated from green polymer electrolyte (GPE) sandwiched in between two microbial cellulose-based electrodes is characterized. The eco-friendly electrode films of interconnected cellulose and multiwalled carbon nanotube (MWCNT) are obtained via harmless, inexpensive, and simple procedure. The GPE consists of methylcellulose-potato starch blend as polymer blend and ammonium iodide (NH4I) is chosen as ion provider. Glycerol serves as plasticization agent for alternative pathways enabling ionic migration. The most optimum GPE possesses good ionic conductivity of ∼ 10−3 S/cm. Ions are the dominant charge carrier in the GPE as ion transference number shown to be close to unity. Linear sweep voltammetry (LSV) analysis illustrated that the GPE is electrochemically stable up to 2.4 V. The green EDLC stores energy through non-Faradaic mechanism and the specific capacitance from charge-discharge, Ccd is influenced by the sweep rates. The EDLC can be charged and discharged up to 2 V with a stable 1000 cyclability performance. This work implied the potential of microbial cellulose-based EDLC as ideal green-based energy storage device for low voltage applications such as smart electronic textiles.

EDLC performance of plasticized NBG electrolyte inserted with Ba(NO3)2 salt: Impedance, electrical and electrochemical properties

The great challenge in front of energy storage devices is to reduce the microplastics in the ocean and their potential harm to human health through drinking. The microplastics topic mainly results from the increasing usage of electronic devices and their components. Developing green electrolytes based on natural polymers and plasticizers is unique in overcoming microplastic issues. Natural beef gelatin (NBG) based plasticized polymer electrolyte films doped with barium nitrate (Ba(NO3)2) and glycerol (Gly) were fabricated using a solution casting process. The NBG electrolyte containing 12 wt.% Ba(NO3)2 and 50 wt.% Gly-has the maximum ionic conductivity, 4.87 × 10−5 S cm−1, at room temperature. The electrochemical performance of plasticized NBG:Ba(NO3)2 was examined utilizing impedance spectroscopy, with an emphasis on the dielectric characteristics and electric modulus parameters. With electrochemical stability up to 2.65 V, the plasticized specimen with the highest conducting demonstrated eligibility for use in energy storage devices. Transference number measurement (TNM) proved the dominancy of ions as a charge carrier in the plasticized NBG:Ba(NO3)2 system, yielding an electron transference number of 0.045 and an ion transference number of 0.955. The absence of redox peaks in the cyclic voltammograms suggests a capacitive charge accumulation in the plasticized electrolyte near to the electrode surface other than the Faradaic process. The triangle-shaped galvanostatic charge-discharge (GCD) plot shows a comparatively low drop voltage. The constructed electric double layer capacitor (EDLC) using activated carbon displays energy and power densities of 3.41 W/kg and 336.4 Wh/kg, respectively, at the first cycle. The EDLC performances of plasticized NBG:Ba(NO3)2 demonstrated the cell has potential industrial use as an energy storage device.

Stable and Efficient Dye-Sensitized Solar Cells and Supercapacitors Developed Using Ionic-Liquid-Doped Biopolymer Electrolytes.

An ionic liquid (IL) 1-ethyl, 2-methyl imidazolium thiocyanate incorporated biopolymer system is reported in this communication for applications in dual energy devices, i.e., electric double-layer capacitors (EDLCs) and dye-sensitized solar cells (DSSCs). The solution caste method has been used to synthesize ionic-liquid-incorporated biopolymer electrolyte films. The IL mixed biopolymer electrolytes achieve high ionic conductivity up to the order of 10-3 S/cm with good thermal stability above 250 °C. Electrical, structural, and optical studies of these IL-doped biopolymer electrolyte films are presented in detail. The performance of EDLCs was evaluated using low-frequency electrochemical impedance spectroscopy, cyclic voltammetry, and constant current charge-discharge, while that of DSSCs was assessed using J-V characteristics. The EDLC cells exhibited a high specific capacitance of 200 F/gram, while DSSCs delivered 1.53% efficiency under sun conditions.

Revisiting the performance of electrical double-layer capacitors implementing a sodium perchlorate water-in-salt electrolyte

Revisiting the performance of electrical double-layer capacitors implementing a sodium perchlorate water-in-salt electrolyte

The impact of physicochemical features of carbon electrodes on the capacitive performance of supercapacitors: a machine learning approach

Hybrid electric vehicles and portable electronic systems use supercapacitors for energy storage owing to their fast charging/discharging rates, long life cycle, and low maintenance. Specific capacitance is regarded as one of the most important performance-related characteristics of a supercapacitor’s electrode. In the current study, Machine Learning (ML) algorithms were used to determine the impact of various physicochemical properties of carbon-based materials on the capacitive performance of electric double-layer capacitors. Published experimental datasets from 147 references (4899 data entries) were extracted and then used to train and test the ML models, to determine the relative importance of electrode material features on specific capacitance. These features include current density, pore volume, pore size, presence of defects, potential window, specific surface area, oxygen, and nitrogen content of the carbon-based electrode material. Additionally, categorical variables as the testing method, electrolyte, and carbon structure of the electrodes are considered as well. Among five applied regression models, an extreme gradient boosting model was found to best correlate those features with the capacitive performance, highlighting that the specific surface area, the presence of nitrogen doping, and the potential window are the most significant descriptors for the specific capacitance. These findings are summarized in a modular and open-source application for estimating the capacitance of supercapacitors given, as only inputs, the features of their carbon-based electrodes, the electrolyte and testing method. In perspective, this work introduces a new wide dataset of carbon electrodes for supercapacitors extracted from the experimental literature, also giving an instance of how electrochemical technology can benefit from ML models.

Fast Ion Transfer Associated with Dehydration and Modulation of Hydration Structure in Electric Double-Layer Capacitors Using Molecular Dynamics Simulations and Experiments

Carbon materials, such as graphite and activated carbon, have been widely used as electrodes in batteries and electric double-layer capacitors (EDLCs). Graphene, which has an extremely thin sheet-like structure, is considered as a fundamental carbon material. However, it was less investigated as an electrode material than graphite and activated carbons. This is because graphene is a relatively new material and is difficult to handle. However, using graphene electrodes can enhance the performance of nanodevices. Here, the performance of EDLCs based on single-layer and bilayer graphene electrodes in LiCl, NaCl, and KCl aqueous electrolyte solutions was evaluated using cyclic voltammetry, and the charging mechanism was evaluated using molecular dynamics simulations. KCl aqueous solution provided the highest capacitance compared to LiCl and NaCl aqueous solutions in the case of single-layer graphene electrodes. In contrast, the dependence of the capacitance on the ion species was hardly observed in the case of bilayer graphene. This indicates that Li and Na ions also contributed to the capacitances. The high EDLC performance can be attributed to the fast ion transfer promoted by the dehydration and modification of the second hydration shell on the bilayer graphene because of the relatively strong interaction of ions with the bilayer graphene.

Analysis of thermal and electrochemical properties of electrical double-layer capacitors by using an in-situ simultaneous thermal analysis cell

In this work, the impact of temperature and voltage on aging of electrical double-layer capacitors (EDLCs) made with a model silica-templated carbon has been investigated by using an in-situ simultaneous thermal analysis (STA) cell. We show that the use of such a cell allows the electrochemical performance of EDLCs and the heat generation occurring within these devices to be simultaneously measured. In EDLCs containing adiponitrile (ADN) based electrolytes, the combination of high voltage (≥ 3.5 V) and elevated temperature of 60 °C leads not only to a fast decrease in capacitance retention, but also to important degradation processes causing a significant increase of heat flow within the system. For a temperature of 60 °C, the heat flow at 3.75 V is about 6.5 times higher than in an EDLC operating at 3.0 V. The correlation of the heat flow and electrochemical properties provides a helpful insight into the stability of EDLCs, and thus the in-situ STA cell appears as an interesting and novel tool for the investigation of these systems.

Effects of Oxygen-Containing Functional Groups on the Electrochemical Performance of Activated Carbon for EDLCs

Activated carbon (AC) is used in commercial electric double-layer capacitors (EDLC) as electrode active material owing to its favorable properties. However, oxygen functional groups (OFGs) present in AC reduce the lifespan of EDLCs. Thus, we investigated the correlation between the OFGs in AC and their electrochemical characteristics. Samples were prepared by heat-treating commercial AC at 300 °C-900 °C for 1 h under two gas atmospheres (N2 and 4% H2/N2 mixed gas). The textural properties were studied, and the reduction characteristics of AC under Ar and H2/Ar mixed gas atmospheres were investigated. Additionally, changes in the OFGs with respect to the heat-treatment conditions were examined via X-ray photoelectron spectroscopy. The specific surface areas of AC-N and AC-H were 2220-2040 and 2220-2090 m2/g, respectively. Importantly, the samples treated in hydrogen gas exhibited a higher yield than those treated in nitrogen while maintaining their pore characteristics. Additionally, the electrochemical performance of the AC was significantly enhanced after the reduction process; the specific capacitance increased from 62.1 F/g to 81.6 F/g (at 0.1 A/g). Thus, heat treatment in hydrogen gas improves the electrochemical performance of EDLCs without destroying the pore characteristics of AC.

Unveiling the Effects of Solvent Polarity within Graphene Based Electric Double-Layer Capacitors

Solvents have been considered to show a profound influence on the charge storage of electric double-layer capacitors (EDLCs). However, the corresponding mechanisms remain elusive and controversial. In this work, the influences of solvent dipole moment on the EDL structures, kinetic properties, and charging mechanisms of graphene-based EDLCs are investigated with atomistic simulations. Specifically, electrolyte structuring is conspicuously modulated by solvents, where a sharp increment of capacitance (~325.6%) and kinetics (~10-fold) is documented upon the slight descent of polarity by ~33.0%. Unusually, such an impressive enhancement is primarily attributed to the suppressed interfacial electric fields stimulated by strong-polarity solvents in the proximity of electrodes, which goes beyond the previously observed issues that stemmed from the competitive interplays between ions and solvents. Moreover, a distinctive polarity-dependent charging mechanism (i.e., from pure counterion adsorption to coion desorption) is identified, which for the first time delineates the pivotal role of solvent polarity in manipulating the charge storage evolutions. The as-obtained findings highlight that exploiting the solvent effects could be a promising avenue to further advance the performances of EDLCs.