Carbon-based Electrical Double-layer Capacitors Research Articles

Tremella-like hierarchical carbon derived from carbon dots through sponge-templated synthesis for supercapacitor electrodes

Carbon-based electrical double-layer capacitors have considerable attention due to their long cycle life and high power density. However, it remains a challenge to prepare desirable electrode materials for electrical double layer capacitors through simplified methods. Herein, a tremella-like hierarchical porous carbon material is synthesized via a simple, versatile, and eco-friendly template method using ultra-small and low-cost carbon dots as the carbon source and the sponge as the scaffold. Due to the high specific surface area (2144.6 m2/g), unique structure, and abundant structural defects, the resulting tremella-like hierarchical porous carbon show a high specific capacitance of 148 F/g at 1 A/g and long-term cycling stability with 86.2% of the initial capacitance retention after 10,000 cycles at 5 A/g. The study provides a facile and efficient strategy for the construction of electrode materials in supercapacitors.

AC/Ni(OH)2 as a porous electrode material for supercapacitors with high-performance

Commonly used nickel-based compounds as electrode materials have low rate capability and poor long-term cycling stability, which limit their practical applications. In this case, it may be an idea to synthesize a composite combining carbon-based electric double layer capacitors and pseudocapacitors. In this study, a novel porous AC/Ni(OH)2 composite was prepared by hydrothermal method, which was used as an electrode for a supercapacitor for the first time with a specific capacity of 183.7 mAh g−1 at 1 A g−1 and a cycling stability of 90.2% after 3000 cycles. Moreover, there is a hybrid supercapacitor (HSC) device assembled by using AC/Ni(OH)2 and active carbon (AC). On the condition of having a power density of 801.5 W kg−1 with the voltage window of 1.6 V, the HSC produces a high energy density of 37.58 Wh kg−1, maintaining an outstanding capacitance retention of 88.5% after 3000 cycles at current density of 8 A g−1. Therefore, AC/Ni(OH)2 electrode material helps to improve the performance of the HSC device and can be used as an electrode material for the development of energy storage devices in the future.

Polymerization-Pyrolysis Assisted Construction of Multiscale Porous Carbon for High-Performance Supercapacitors

High energy density combined with rapid mass transport is highly desired for carbon-based electrical double-layer capacitors. Here, multiscale porous carbon has been constructed by an efficient polymerization-pyrolysis strategy. The resorcinol-formaldehyde polymer anchored with Fe3+ is firstly prepared, and the in situ formed Fe3O4 nanoparticles act as mesoporous template during the pyrolysis process. The resultant hierarchically porous carbon achieves an extended surface area of 2260.3 m2 g−1 and wide pore size distributions including micro-, meso-, and macropores. The synergism of large surface area, high conductivity, and interconnected ion transport channels leads to superior energy storage performances of prepared multiscale porous carbon electrode. It delivers a high specific capacitance of 271.7 F g−1 at 0.5 A g−1 in KOH electrolyte, accompanied with a prominent capacitance retention of 88.5% when the current density is 10.0 A g−1. Besides, the assembled symmetric supercapacitor using organic electrolyte exhibits a maximum energy density of 54.0 Wh kg−1 at the power density of 750.0 W kg−1, as well as the superior cyclic stability with a capacitance retention of 88.2% after 10000 cycles.

Dielectric gel electrolytes for safe charge storage from −20 to 80°C by double-layer capacitors

BackgroundSafety and workability under harsh conditions are two of the major challenges for carbon-based electric double layer capacitors (EDLCs). MethodsGel polymer electrolytes (GPEs), comprising a graphene oxide (GO)-decorated polymer blend of poly(acrylonitrile-co-methyl acrylate) and poly(ethylene glycol) integrated into a liquid electrolyte (LE), are developed. Significant findingsUnder firing, the polymer entraps solvent molecules and the GO facilitates the charring of the GO-decorated GPE (GPEG), resulting in low flammability. The GO-polymer framework enhances the dissociation of counter-ion pairs and solvent−ion clusters to increase the ionic conductivity (to a value higher than that of the LE) and reduce the dielectric loss. The GPEG–EDLC outperforms EDLCs assembled using the LE and GO-free GPE with respect to capacitance, rate capability, and cycling stability. The high dissociation of the counter-ion pairs and solvent−ion clusters in the GPEG facilitates ion diffusion into the carbon micropores, thus improving the capacitive performance. The GPEG–EDLC presents excellent performance at −20°C due to the solvent−ion cluster dissociation. Its stable performance at 80°C is ascribable to the low dielectric loss, which minimizes the chemical damage to the system. Our study demonstrates the use of a GO-decorated dielectric polymer to address issues of safety and workability at extreme-temperatures.

Facile method to produce sub-1 nm pore-rich carbon from biomass wastes for high performance supercapacitors

Sub-1 nm pores can lead to an anomalous increase in the supercapacitive performance [1], but it still faces great challenges from its relatively low sub-1 nm pore content, complicated preparation process, low yield and high cost. Here we successfully prepared a sub-1 nm pore-rich carbon from biomass wastes using a facile method by pre-treating walnut shell powder at 380 ℃ in air for different times to delicately tailor carbon defects, followed by KOH activation at 700 ℃. The as-prepared optimal material delivers the highest sub-1 nm pore content (Vsub-1 nm = 0.57 cm3 g−1, Vsub-1 nm/Vt = 58.4 %) among all reported porous carbons. A supercapacitor made from the material accomplishes an ultrahigh specific capacitance of 298.7F g−1 at 1 A g−1 in a two-electrode device, excellent rate capability (78.8 % retention from 1 to 10 A g−1) and long-cyclic life (94 % retention after 10,000 cycles at 10 A g−1) in KOH. Even in Et4NBF4 electrolyte that is often used in commercial supercapacitors, a high energy density of 82.8 Wh kg−1 at 7 kW kg−1 and excellent cycling performance (90 % retention after 10,000 cycles at 5 A g−1) can be achieved, ranking the best among all reported carbon-based electrical double layer capacitors tested in the same electrolyte. More importantly, it drives a light-emitting-diode (LED) to operate for as long as 20 min, vividly demonstrating the great potential of sub-1 nm pore-rich carbon-based high performance supercapacitors in practical applications.

Large-Pore Ordered Mesoporous Turbostratic Carbon Films Prepared Using Rapid Thermal Annealing for High-Performance Micro-pseudocapacitors.

Carbonization by rapid thermal annealing (RTA) of precursor films structured by a brush block copolymer-mediated self-assembly enabled the preparation of large-pore (40 nm) ordered mesoporous carbon (MPC)-based micro-supercapacitors within minutes. The large pore size of the fabricated films facilitates both rapid electrolyte diffusion for carbon-based electric double-layer capacitors and conformal deposition of V2O5 without pore blockage for pseudocapacitors. The pores were templated using bottlebrush block copolymers (BBCPs) via cooperative assembly of phenol-formaldehyde resin to produce microphase-segregated carbon precursor films on a variety of substrates. Ultrafast RTA processing (∼50 °C/s) at elevated temperatures (up to 1000 °C) then generated stable, conductive, turbostratic MPC films, resolving a significant bottleneck in rapid fabrication. MPC prepared on stainless steel at 900 °C demonstrated exceptionally high areal and volumetric capacitances of 6.3 mF/cm2 and 126 F/cm3 (at 0.8 mA/cm2 using 6 M KOH as the electrolyte), respectively, and 91% capacitance retention after 10,000 galvanostatic charge/discharge cycles. Post-RTA conformal V2O5 deposition yielded pseudocapacitors with 10-fold increase in energy density (20 μW h cm-2 μm-1) without adversely affecting the high power density (450 μW cm-2 μm-1). The use of RTA coupled with BBCP templating opens avenues for scalable, rapid fabrication of high-performance carbon-based micro-pseudocapacitors.

Fabrication of highly-conductive porous capacitor electrodes by the insertion of Cu-nanoparticles into N-doped flocculated carbon catalysts

In carbon-based electric double-layer capacitors (EDLC), an ideal electrode should have convenient mass transport, ensuring rich porosity and rapid electron transfer, guaranteeing the electrode bulk's high conductivity. In this study, ultrafine Cu nanoparticles inserted carbon flocculation is formed on carbon cloth using polydopamine and cupric chloride precursors via pyrolysis and electrochemical oxidation reaction. As a result, the obtained electrode has a large surface area of 55.5 m2g−1 and high conductivity of 48.7 S/mm, which shows excellent charge storage capability with high specific capacitance of 3546 mF cm−2 at a current density of 1 mA cm−2. Moreover, when the as-prepared electrode is used as electrodes in symmetric EDLC, it can provide a high energy density of 23 mWh cm−3 with a power density of 179 mW cm−3, making it as a promising carbon electrode for practical EDLCs.

Towards understanding the impact of operating voltage on the stability of adiponitrile-based electrical double-layer capacitors

In this work, we report about the impact of operating voltage on the electrochemical performance of carbon-based electrical double-layer capacitors (EDLCs) containing 1.75 mol L−1 1-butyl-1-methylpyrrolidinium tetrafluoroborate (Pyr14BF4) in adiponitrile (ADN) as electrolytic medium. The stability of EDLCs containing this electrolyte has been investigated at maximum voltage ranging from 3.0 V to 3.7 V at room temperature (RT) and at 60 °C. At RT, the devices displayed remarkable cycling stability, and after 100,000 galvanostatic charge discharge cycles (at 5 A g−1) up to 3.7 V they retained 81% of their initial capacitance. After 10,000 galvanostatic charge discharge cycles (at 5 A g−1) up to 3.4 V and 60 °C the EDLCs displayed a capacitance retention of 83%. After 300 h of floating at 3.0 V the cells kept 80% of their initial capacitance. Post-mortem experiments by gas adsorption and temperature programmed desorption (TPD) on the aged electrodes revealed that after long-term operation, the porous texture and surface chemistry of the positive electrode is more affected than the negative one. The results of this study reveal that 1.75 mol L−1 Pyr14BF4 in ADN is a promising electrolyte for the realization of high voltage EDLCs.

Dual-doping activated carbon with hierarchical pore structure derived from polymeric porous monolith for high performance EDLC

The fabrication of dual-doped activated carbon with a hierarchical pore structure is an effective way for the preparation of the high performance carbon-based electric double layer capacitor. Herein, nitrogen and sulfur dual-doped activated carbon (NSC) with hierarchical pore structure was obtained by directly carbonizing/activating polymer-based blend monolith. The as-obtained NSC samples exhibit a high specific surface area (1570–2550 cm2 g−1), a developed hierarchically pore structure, and have an appropriate nitrogen and sulfur content. Furthermore, originating from the synergistic effect by pore structure, heteroatom, and proper graphitized crystal structure, the optimized NSC-based electrode exhibits a high specific capacitance of 330.5 F g−1 at a current density of 0.5 A g−1, superior rate capability of 66% at 20 A g−1 and acceptable cycling stability in the three-electrode system. Meanwhile, the assembled carbon-based symmetric capacitor exhibits an acceptable energy density and a remarkable power density of 11 kW kg−1 with 6 mol L−1 KOH as the aqueous electrolyte. These results demonstrate that the as-resulting NSC could be a competitive candidate for carbon-based electrode materials, and this work would be a valuable exploration that as a facile synthetic strategy to prepared dual-doped activated carbon for electric double layer capacitor.

Achieving Ultrahigh Volumetric Energy Storage by Compressing Nitrogen and Sulfur Dual-Doped Carbon Nanocages via Capillarity.

High volumetric performance is a challenging issue for carbon-based electrical double-layer capacitors (EDLCs). Herein, collapsed N,S dual-doped carbon nanocages (cNS-CNC) are constructed by simple capillary compression, which eliminates the surplus meso- and macropores, leading to a much increased density only at the slight expense of specific surface area. The N,S dual-doping induces strong polarity of the carbon surface, and thus much improves the wettability and charge transfer. The synergism of the high density, large ion-accessible surface area, and fast charge transfer leads to state-of-the-art volumetric performance under the premise of high rate capability. At a current density of 50 A g-1 , the optimized cNS-CNC delivers a high volumetric capacitance of 243 and 199 F cm-3 in KOH and EMIMBF4 electrolyte, with high energy density of 7.9 and 93.4Wh L-1 , respectively. A top-level stack volumetric energy density of 75.3Wh L-1 (at power density of 0.7kW L-1 ) and a maximal stack volumetric power density of 112kW L-1 (at energy density of 18.8Wh L-1 ) are achieved in EMIMBF4 , comparable to the lead-acid battery in energy density but better in power density with 2-3 orders. This study demonstrates an efficient strategy to design carbon-based materials for high-volumetric-performance EDLCs with wide practical applications.

Insights on the Behavior of Imidazolium Ionic Liquids as Electrolytes in Carbon-Based Supercapacitors: An Applied Electrochemical Approach

This study aims to increase the knowledge on the interactions that occur at the electrode/electrolyte interface in carbon-based electric double-layer capacitors (EDLCs) when solvent-free ionic liqu...

In Operando Calorimetric Measurements for Activated Carbon Electrodes in Ionic Liquid Electrolytes under Large Potential Windows.

This study aims to investigate the effect of the potential window on heat generation in carbon-based electrical double layer capacitors (EDLCs) with ionic-liquid (IL)-based electrolytes using in operando calorimetry. The EDLCs consisted of two identical activated-carbon electrodes with either neat 1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide ([Pyr14 ][TFSI]) electrolyte or 1.0 m [Pyr14 ][TFSI] in propylene carbonate (PC) as electrolyte. The instantaneous heat generation rate at each electrode was measured under galvanostatic cycling for different potential windows ranging from 1 to 4 V. First, the heat generation rates at the positive and negative electrodes differed significantly in neat IL owing to the differences in the ion sizes and diffusion coefficients. However, these differences were minimized when the IL was diluted in PC. Second, for EDLC in neat [Pyr14 ][TFSI] at high potential window (4 V), a pronounced endothermic peak was observed at the beginning of the charging step at the positive electrode owing to TFSI- intercalation in the activated carbon. On the other hand, for EDLC in 1.0 m [Pyr14 ][TFSI] in PC at potential window above 3 V, an endothermic peak was observed only at the negative electrode owing to the decomposition of PC. Third, for both neat and diluted [Pyr14 ][TFSI] electrolytes, the irreversible heat generation rate increased with increasing potential window and exceeded Joule heating. This was attributed to the effect of potential-dependent charge redistribution resistance. A further increase in the irreversible heat generation rate was observed for the largest potential windows owing to the degradation of the PC solvent. Finally, for both types of electrolyte, the reversible heat generation rate increased with increasing potential window because of the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.

Capacitive performance of carbon-based supercapacitors using sulfonated poly(ether ether ketone) membranes as separators: influences of casting solvents

Sulfonated poly(ether ether ketone) (SPEEK) membranes are prepared by various casting solvents. They are adopted as separators for carbon-based electrical double-layer capacitors (EDLCs). The cyclic voltammetric curves, galvanostatic charge/discharge curves, electrochemical impedance spectroscopy and cycling performance of EDLCs are evaluated. It is found that the residual solvents in SPEEK membranes have important effects on the performances of EDLCs with SPEEK membranes due to the different interactions between residual solvents and -SO3H groups of SPEEK. After SPEEK membranes are treated in 60 °C water, the EDLC with an optimum SPEEK membrane shows a specific capacitance of 37.5 F g−1 at a current density of 1 A g−1, higher than that (30 F g−1 @ 1 A g−1) of the EDLC with a porous polypropylene membrane. Furthermore, the EDLC reveals an excellent cycling stability with capacitance retention of ∼100% after 5000 cycles at a current density of 1 A g−1. These results suggest that SPEEK membranes prepared by suitable casting solvent and activation processing can serve as a class of promising compact separators for EDLCs.

Carbon-encapsulated niobium carbonitride with high volumetric capacitance and wide potential windows in aqueous pseudocapacitors

Electrochemical capacitors deliver high power and have long operational lives; however, their energy densities are limited. Compared to carbon-based electrical double-layer capacitors, pseudocapacitors with an aqueous electrolyte are predicated to provide both high gravimetric and volumetric capacitance while being cost-effective and eco-friendly. However, their narrow operational potential windows limit the practical delivery of energy. In this study, we synthesized novel NbCxN1-x nanoparticles encapsulated in a uniform carbon layer. This pseudocapacitive material exhibits a high volumetric capacitance, a wide potential window stability (∼345 F cm−3, 2.1 V, in 1 M of Li2SO4), and a long cycling life (>10 000 cycles). Moreover, the symmetric cells (2.2 V, in 1 M of Li2SO4) exhibit high energy density as well as excellent cyclability and rate performances. These findings provide new opportunities for the design of aqueous energy storage devices having a wide applied potential difference and excellent performance.

Predicting the capacitance of carbon-based electric double layer capacitors by machine learning.

Machine learning (ML) methods were applied to predict the capacitance of carbon-based supercapacitors. Hundreds of published experimental datasets are collected for training ML models to identify the relative importance of seven electrode features. This present method could be used to predict and screen better carbon electrode materials.

Supercapacitors Based on Iron Decorated Carbon Nanofibers with Excellent Cyclability

Supercapacitors are attractive energy storage devices due to their high specific power and excellent cyclability. The supercapacitor field is currently dominated by porous carbon-based electric double layer capacitors (EDLC) that lack energy density. The energy density of porous carbon materials can be effectively improved by fabricating composites of carbon (for EDLC) with metal oxides/hydroxides (for pseudocapacitance). Among the metal oxides, iron oxide shows great promise owing to its high theoretical capacitance, negligible environmental impact and natural abundance. In this work, a facile methodology for obtaining freestanding, binder-free, iron decorated porous carbon nanofibers (Fe-CNF) for supercapacitor application is outlined. Briefly, blend of iron precursor and polyacrylonitrile (PAN) was electrospun into non-woven, freestanding nanofiber mats and subsequently subjected to heat treatment at 1000 degree Celsius in an inert atmosphere. Besides this thermal treatment, no additional chemical or physical activation process was conducted. The morphology of the nanofiber mat was characterized using scanning electron microscopy. The fibers exhibit diameters in the range of 150-200 nm. Electrochemical activity of the electrode was studied using cyclic voltammetry and galvanostatic charge-discharge techniques. Electrochemical measurements showed a large specific gravimetric capacitance of ~260 F/g at 20 mV/s in 6 M KOH aqueous electrolyte between 0 to -1.3 V range. The resulted synergistic effect between iron nanoparticles (pseudocapacitance) and carbon nanofibers (EDLC and conductivity) leads to such high capacitance value. The electrodes retain ~82% capacitance after 6500 cycles indicating excellent cyclic stability of these electrodes. Further, Fe-CNF’s electrodes were extensively characterized at various stages of charge-discharge using X-ray diffraction, high resolution transmission electron microscopy and X-ray photoelectron spectroscopy to understand the redox reactions of the fabricated electrode material. Figure 1

Ab-Initio approach to Model Impedance and Charge-Discharge Characteristics of Electrical Double Layer Capacitors

Electrical double layer capacitors (EDLCs) are energy storage devices which primarily utilize ionic adsorption and high surface area electrodes to store electrostatic charge at electrode/electrolyte interface. In order to describe and correlate electrode architecture to the energy and power performance of capacitors, first principle models that can explain both the ac and dc response of the capacitor need to be developed. In this work, we describe an ab initio mathematical model that take into account both the double layer and interfacial resistance contributions at the electrode/electrolyte interface. The model was tested, and validated using experimental data of various carbon-based EDLCs using both electrochemical impedance spectroscopy (EIS) and galvanostatic discharge data. Physical parameters of the model were obtained through non-linear regression of frequency-domain EIS data, which was then transformed into time domain using numerical inverse Laplace transform to predict the potential-time discharge response of the capacitor. The experimental data was in excellent agreement with the model predictions. Figure 1

High Specific Capacitance Based on N-Doped Microporous Carbon in [EMIm]AlxCly Ionic Liquid Electrolyte

Carbon-based electrical double-layer capacitors (EDLCs) store charges through the physisorption of electrolyte ions onto porous carbon electrodes and subsequently can provide high electrical power. The control over morphology and microstructure of carbon-based electrode materials is an effective strategy to provide high surface area and efficient paths for ions diffusion. Here, we report on the fabrication of N-doped microporous carbons through a two-step carbonization method using an Al-based porous coordination polymer (Al-PCP) as template. The unique 3D pore structure of N-doped microporous carbon (PCP-1600) gives rise to a high BET surface area (up to 1892.38 m2 g−1) and a corresponding pore volume of 1.26 cc g−1. The carbon-based electrodes exhibit specific capacitance of ∼280 F g−1 and good cycling stability in ionic liquid electrolyte. These performance can be assigned to the unique pore structure of N-doped microporous carbons and make them potentially promising electrodes for energy storage devices.

Pseudocapacitive materials for electrochemical capacitors: from rational synthesis to capacitance optimization

Abstract Among various energy-storage devices, electrochemical capacitors (ECs) are prominent power provision but show relatively low energy density. One way to increase the energy density of ECs is to move from carbon-based electric double-layer capacitors to pseudocapacitors, which manifest much higher capacitance. However, compared with carbon materials, the pseudocapacitive electrodes suffer from high resistance for electron and/or ion transfer, significantly restricting their capacity, rate capability and cyclability. Rational design of electrode materials offers opportunities to optimize their electrochemical performance, leading to devices with high energy density while maintaining high power density. This paper reviews the different approaches of electrodes striving to advance the energy and power density of ECs.

The synthesis of large area graphene/carbon nanotubes as additive material and their enhanced specific capacitance

Two-dimensional large area graphene/carbon nanotubes (LAG–CNTs) nanocomposites as additional material for carbon-based electric double layer capacitor (EDLC) performance have been fabricated by using chemical vapor deposition method. The as-prepared LAG–CNTs are characterized with Raman, field emission scanning electron microscope, transmission electron microscope, high-resolution transmission electron microscopy and atomic force microscopy images. Finally, EDLC electrodes based with high surface area activated carbon (YP50F) and LAG–CNTs were fabricated by a facile step. The prepared electrodes (8LAG–CNTs/AC) exhibit a specific capacitance of 15.5 F cc−1 at 0.5 mA cm−2, excellent cycling stability (96.1 % retention after 100 cycles) and high rate capability. These excellent capacitive performances may make the as-prepared nanocomposites as conductive material for preparation of promising electrode materials of EDLC.