Symmetric Electric Double-layer Capacitors Research Articles

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.

Meso-pore dominant activated carbon from spent coffee grounds for high-performance electrochemical capacitors in organic electrolyte

Conventionally, conductive additives and metallic current collectors are two major decisive factors to determine the electrochemical performances of electrode materials in supercapacitors (SCs). Without those, most reported materials for SCs are far beyond the standard performances. Herein, we develop a pore-size tunable and highly-pure nanosheet-like activated carbon (AC) form spent coffee grounds (SCGs) with high specific-surface-area (2497 m2 g−1) and large pore volume (1.28 cm3 g−1). The meso-pore/total-pore volume proportions (r) of the as-prepared samples, AC1200 (r = 10.25%), AC1800 (r = 20.10%), and AC2500 (67.34%), are simply adjusted at desired temperatures (750–900 ºC) using physical activation with CO2. The SCG-based electrodes for symmetric electric double-layer capacitors (EDLC) were prepared using free-standing carbon films on cellulose separator, not on Al or Cu foils, without adding any conductive additives. Interestingly, the AC2500-based electrode, a narrow mesopore-rich (mainly 2–4 nm) AC, presents superior capacitance of 222.4 F g−1 at 0.5 A g−1 in 1.5 M LiClO4/propylene carbonate. The capacitance is superior to those of micropore-rich samples (AC1200 and AC1800) including of two commercial products (YP-50 and ACS20). Furthermore, the AC2500-based EDLC exhibits a remarkable energy density of 48.3 Wh kg−1 at 627 W kg−1, and maintains a high energy density of 33.8 Wh kg−1 at 3733 W kg−1. In present work, we demonstrate that narrow meso-pore (2–4 nm) dominant and nanosheet-like carbons would conduct high-performance electrochemical behaviors without the assistance of conductive additives. Moreover, we provide a promising preparation route for pore-size-tunable ACs from bio-mass wastes with remarkable performance for energy storage applications.

Carbon nanocomposite electrodes for electrical double layer capacitor

Abstract Activated carbons (ACs) have been used as electrode material in commercial electric double-layer capacitors (EDLC) due to their low cost, high specific surface area, and chemical stability. Unfortunately, the supercapacitive performance of AC is not satisfactory due to their low conductivity.On the other hand, carbon nanofibers (CNFs) based EDLC exhibit low equivalent series resistance (ESR) but not such energy storage values as AC ones. Here in this paper, we report on the synthesis of new nanocomposites based on a mixture of ACs and CNFs using a simple powder processing route and evaluating their electrochemical behavior as electrode materials in symmetric EDLC. The EDLC, with 10 wt. % CNFs in AC/CNFs as the electrodes have displayed a high conductivity and surface area resulting in the enhancement of the energy and power delivers in aqueous electrolyte. On the contrary, with 90 wt.% CNFs in AC/CNFs as the electrodes, higher specific power, low ESR, and low value of relaxation time constant are achieved. The highest value of specific capacitance estimated is around 207 F g−1 at 5 mV s−1 and the highest specific energy is 13.2 Wh kg−1 at 1 A g−1 for the 10 wt.% CNFs in AC/CNFs.

Nanozirconia polymer composite porous membrane prepared by sustainable immersion precipitation method for use as electrolyte in flexible supercapacitors

We report ZrO2-polymer nanocomposite porous membranes prepared by one step phase inversion method with the aim to study a) the effect of ZrO2 nanofiller on the physiochemical and electrochemical properties of the membranes, and b) the viability of the optimized membrane as an electrolyte in energy storage devices. The interaction of ZrO2 nanoparticles with the polymer and liquid electrolyte has been investigated in details using scanning electron microscopy, x-ray diffraction, and Fourier transform infrared spectroscopy. The microporous structure, liquid electrolyte uptake and ionic conductivity of the nanocomposite membranes have been found to be affected by presence of ZrO2 nanoparticles. The electrolyte membrane with 3 wt.% ZrO2 shows the highest ionic conductivity of ∼4.4 mS cm−1 with a liquid electrolyte retention of ∼375 % and electrochemical stability of ∼4.25 V when soaked in a sodium-ion conducting organic electrolyte. The performance of the highest conducting nanocomposite membrane in symmetric electrical double-layer capacitor (EDLC) has been observed to be better as compared to that of un-doped electrolyte membranes. The EDLC fabricated using the optimized electrolyte membrane and activated carbon electrodes exhibits specific capacitance ∼103 F g−1 and is stable up to 50,000 charge–discharge cycles with specific energy ∼10.2 Wh kg−1, specific power ∼14.2 kW kg−1, and Coulombic efficiency ∼ 99.8 %.

Mesophase Pitch-Derived Carbons with High Electronic and Ionic Conductivity Levels for Electric Double-Layer Capacitors.

We develop a temperature-programmed pretreatment strategy for converting aliphatic-rich petroleum pitch into a mesophase framework, which can then be activated using KOH to produce high-performance carbons for electric double-layer capacitors (EDLCs). In the pretreatment of pitch at an optimal temperature, both the temperature ramp and holding time influence the mesophase structure, which governs the pore structure and crystallinity of the resulting activated carbon. High carbon microporosity is beneficial to capacitance maximization but detrimental to ion transport. To resolve this problem, we develop a multistep ramp incorporating aliphatic species into the aromatic framework during mesophase formation. This incorporation process produces a mesophase framework that can be activated to form carbons with high crystallinity, thereby enhancing electronic conductivity and hierarchical porosity, which improves ionic conductivity. The resulting carbon electrode is used to assemble a symmetric EDLC, which exhibits a capacitance of 160 F g–1 and excellent high-rate retention in a propylene carbonate solution of N,N-diethyl-N-methylethanaminium tetrafluoroborate. The EDLC delivers a superior specific energy of 40 Wh kg–1 (based on the total carbon mass) within a voltage range of 0–2.7 V and sustained a high energy of 24 Wh kg–1 at a high power of 50 kW kg–1. The findings of this study demonstrate that incorporating aliphatic species into aromatic mesophase frameworks plays a crucial role in regulating the crystallinity and pore structure of pitch-derived carbons for charge storage.

Plasticized solid polymer electrolyte based on natural polymer blend incorporated with lithium perchlorate for electrical double-layer capacitor fabrication

A plasticized solid polymer electrolyte system is prepared using polymer blend of methyl cellulose–potato starch with lithium perchlorate (LiClO4) as dopant salt and glycerol as plasticizer. Transport properties of the electrolytes are investigated using electrical impedance spectroscopy (EIS). By applying a method proposed by Arof et al. which was found to be suitable for both Arrhenius and Vogel–Tammann–Fulcher (VTF) type of electrolytes, the number density (n), diffusion coefficient (D), and mobility (μ) of ions are found to be influenced by the concentration of glycerol. From ion and electron transference number analysis, it is verified that ions are the main charge carriers. Linear sweep voltammetry (LSV) verifies the suitability of the most conductive electrolyte to be employed in the carbon-based symmetric electrical double-layer capacitor (EDLC) fabrication. The EDLC has been tested using the galvanostatic charge–discharge and cyclic voltammetry (CV) techniques. The specific capacitance (Csp) of the electrode using CV at a sweep rate of 2 mV s−1 is found to be 61.58 F g−1. The EDLC has been tested for 1000 charge–discharge cycles with the highest Csp value of 28.04 F g−1.

S-doping coupled with pore-structure modulation to conducting carbon black: Toward high mass loading electrical double-layer capacitor

The conductive carbon black (CB) was modified by the pore-structure modulation and S doping in the presence of pore-modulation agent and S dopant via the one-step calcination methodology, resulting in the S-doped and pore-structure modified CB (labeled as SPCB). The conductivity and contact angle tests prove that the function of S doping can not only increase the electrical conductivity but also enhance the hydrophilicity. As electrode materials for symmetric electrical double-layer capacitor (EDLC), the influence of different mass loading of the electrode on the capacitive behavior is investigated. The areal mass loading of active materials for each electrode in the symmetric EDLC can reach up to 10 mg cm−2, comparable to the values of commercial devices. Without obvious sacrifice of the gravimetric capacitance is observed with the increase of mass loading, illustrating the small electrolyte ion transport resistance in electrode material and quick charge transfer property as well as the great compatibility for the gravimetric, areal and total capacitances. Additionally, the symmetric EDLC configured with SPCB affords the larger capacitance, better rate capability and higher capacitance retention as compared to the commercial activated carbon.

Hierarchically Structured Multidimensional Carbon Composite Anchored to a Polymer Mat for a Superflexible Supercapacitor

A carbon electrode was designed to guarantee flexibility of symmetric electric double layer capacitors (EDLCs) based on its architecture. Three different dimensional carbon materials were combined ...

Excellent Compatibility of the Gravimetric and Areal Capacitances of an Electric-Double-Layer Capacitor Configured with S-Doped Activated Carbon.

Coffee grounds were converted into S-doped activated carbon (SAC) in the presence of an active agent and S dopant through a one-step synthesis approach. Carbonization, activation, and S doping was achieved through this one-step methodology. The SAC was used as an electrode material for the preparation of a symmetric electrical-double-layer capacitor (EDLC), and the influence of the loading mass of the active materials on the capacitive behaviors was investigated. The assembled SAC-based symmetric EDLC not only yielded a high capacitance but it also afforded a satisfactory capacitance retention. The symmetric EDLC constructed with loading mass SAC of 7.5 mg cm-2 was capable of delivering a maximum gravimetric and areal capacitance of 200 F g-1 and 1.5 F cm-2 , respectively. The compatibility of the gravimetric and areal capacitances of SAC was mainly attributed to the high abundance of interconnected pore channels, which were beneficial for the increased contact area between electrode and electrolyte ions, fast charge transfer, and fast diffusion of the electrolyte ions. In addition to the well-developed porous networks, the introduction of S into the carbon frameworks significantly enhanced the electrical conductivity, storage capacity, and rate capability. The developed one-step synthesis provides a facile and effective route for obtaining high-performance capacitive electrode materials and realizing high value-added utilization of biomass.

High performance electrospun Li+-functionalized sulfonated poly(ether ether ketone)/PVA based nanocomposite gel polymer electrolyte for solid-state electric double layer capacitors

The synthesis of a nanocomposite gel polymer electrolytes (NGPEs) using Li+-functionalized SPEEK (SPEEK(Li)) nanofiber membrane as a matrix, cross-linked PVA as a gelled agent and LiClO4 as an electrolytic salt for electric double layer capacitors (EDLCs) were reported in this work. Compared with conventional PVA based gel electrolyte, the “SO3−Li+” functional groups and the pores structure of the SPEEK(Li) nanofiber membrane improves the ionic conductivity as well as mechanical stability, which made the prepared NGPEs exhibited a high ionic conductivity of 8.9 × 10−3 S cm−1 at ambient temperature and a large voltage stability window range from 0 to 2.0 V. The fabricated symmetric EDLCs showed a specific capacitance of 146.96 F g−1 at a current density of 0.5 A g−1, with a maximum energy density of 18.26 W h kg−1. Meanwhile, the EDLCs showed outstanding cyclical stability of 98% specific capacitance retention after 3000 cycles GCD testing, all of these results indicated that the NGPEs should be appropriate to EDLCs and next-generation energy storage systems.

(Invited) Gel Electrolyte with Low Dielectric Loss for Electric Double Layer Capacitors

This paper proposes a cyanide-containing polymer as the framework of gel electrolytes for electric double layer capacitors (EDLCs). Dielectric studies in a frequency range of 101-105 Hz demonstrate the fast polarization and low dielectric loss features of the gel electrolyte that entraps 1-M TEABF4/PC. The swift polarization of the gel electrolyte can be attributed to the high dipole moment of the contained cyanide functionality on the polymer chains. When supported with separator membranes, the ionic conductivity of the gel electrolyte is one order of magnitude higher than that of the 1-M TEMABF4/PC liquid electrolyte (10-2 S cm-2 vs. 10-3 S cm-2 at room temperature) owning to the presence of polarizable cyanide groups, which facilitate ion motion under the action of an applied electric field. In addition, the polymer framework improves the dissociation of ion-complexes to promote ion transport. Symmetric EDLCs assembled using the gel electrolyte exhibits specific capacitances of 199 and 130 F g -1 (within 0-2.7 V) at galvanostatic discharge rates of 0.5 and 100 A g-1, respectively, based on the carbon mass. The corresponding capacitances for liquid-electrolyte EDLCs are 172 and 109 F g-1. The gel EDLCs exhibit high stability, retaining ~95% of their initial capacitance after 20,000 cycles. An important feature of this gel electrolyte is its controllable gelling time. The electrolyte can be used in a manner similar to that of an ordinary liquid electrolyte during assembly and auto-gelation of the electrolyte subsequently takes place in the assembled EDLC. The implemented auto-gelation greatly enhances the durability of the resulting devices and is applicable in a wide range of energy storage systems. Acknowledgments: This research was supported by the Ministry of Science and Technology, Taiwan (104-2221-E-006-231-MY3, 104-2221-E-006-234-MY3, and 106-3113-E-006-009) and by the Ministry of Education’s Aim for the Top University Project at National Cheng Kung University, Taiwan.

Solid polymer electrolyte membranes based on quaternized polysulfone and solvent-free fluid as separators for electrical double-layer capacitors

A room temperature ionic liquid is immobilized on the surfaces of layered double hydroxides to form a solvent-free fluid via a silylation reaction. A series of solid polymer electrolyte membranes based on quaternized polysulfone and the solvent-free fluid are fabricated by solution casting method, which are tough and flexible and display moderate KOH aqueous solution uptakes. A symmetric electrical double-layer capacitor is assembled by using an optimized quaternized polysulfone-based solid polymer electrolyte membrane with KOH solution and two activated carbon electrodes. The single carbon electrode delivers a specific capacitance of 114.0 F g−1 at a current density of 1 A g−1 and outstanding cycling stability, indicating that this novel solid polymer electrolyte membrane may be appropriate for electrical double-layer capacitors.

Biomass Based Carbon Composites As Negative Electrodes for Asymmetric Supercapacitors

Nowadays, the role of electrochemical storage devices such as supercapacitors and batteries are very prominent in our modern society since they are used in various applications including for the hybrid electric vehicles. Compared to batteries, supercapacitors have some advantages such as longer cycle life,higher charge and discharge rate and higher power density. However, most of the supercapacitor technology still relies on the symmetric electric double layer capacitors (EDLCs) which are known as symmetric supercapacitors. Based on the formation of electric double layer layer and high surface electrodes,this type of capacitors demonstrates a high power density but it possesses a low energy density due to the low specific capacitance and voltage window. Hence, it is quite urgent to develop new type of supercapacitors which possess also a high energy density as well as high power density and stable cycle profiles. Asymmetric supercapacitors, consisting of two different electrodes (electric double layer and redox reaction based electrodes), can be regarded as alternative system for the next generation of supercapacitors. For the positive electrodes of asymmetric supercapacitors, most of the metal oxides and hydroxides are commonly investigated. such as NiO. Various carbon materials are commonly used as negative electrodes for supercapacitors such as activated carbons, graphenes, carbon nanotubes (CNT), carbon nanosheets, electrospun carbon , reduced graphene oxides and CNT composites. Among all of them, activated carbons are categorized as preferable negative electrodes for supercapacitors due to high surface area and their availability. Activated carbons used as negative electrodes in supercapacitors can be made from various sources of biomass such as salacca peel. In this present work, nano carbon composites were prepared by two sequential methods, Initially, the activated carbons were synthesized by KOH activation method. It was then followed by the deposition of nano carbons on the surface of activated carbon using nebulized spray pyrolysis. The waste cooking oil and ferrocene were used as carbon precursor and metal catalyst, respectively. Nebulized spray pyrolysis has been applied for the synthesis of carbon nanospheres derived from Kerosene and Turpentine oil. The carbon composite products were then employed as negative electrodes coupled with NiO as positive electrodes and KOH as electrolytes. The nano carbon composites were then characterized by scanning electron microscope (SEM), nitrogen adsorption-desorption, x-ray diffraction (XRD) and Raman spectroscopy. SEM observations showed that mixtures of nano carbons consist of carbon nanotubes and carbon nanospheres were formed on the surface of activated carbons. The carbon composites were then tested as negative electrodes for asymmetric supercapacitors coupled with nickel oxides (NiO) as positive electrodes and KOH as electrolyte. The electrochemical characteristics were studied by cyclic voltammetry (CV), galvanostatic charge discharge and electrochemical impedance spectroscopy. The electrochemical performance of nano carbon composites was dependent on the preparation conditions in the nebulized spray pyrolysis method such as catalyst concentration and deposition temperature. The specific capacitance of nano carbon composites can achieve 31.02 F/g from the CV measurement at scan rate of 2 mV/s.

Graphene-based lithium ion capacitor with high gravimetric energy and power densities

Hybrid capacitor configurations are now of increasing interest to overcome the current energy limitations of supercapacitors. In this work, we report a lithium ion capacitor (LIC) entirely based on graphene. On the one hand, the negative –battery-type- electrode consists of a self-standing, binder-free 3D macroporous foam formed by reduced graphene oxide and decorated with tin oxide nanoparticles (SnO2-rGO). On the other hand, the positive –capacitor-type- electrode is based on a thermally expanded and physically activated reduced graphene oxide (a-TEGO). For comparison purposes, a symmetric electrical double layer capacitor (EDLC) using the same activated graphene in 1.5 M Et4NBF4/ACN electrolyte is also assembled. Built in 1 M LiPF6 EC:DMC, the graphene-based LIC shows an outstanding, 10-fold increase in energy density with respect to its EDLC counterpart at low discharge rates (up to 200 Wh kg−1). Furthermore, it is still capable to deliver double the energy in the high power region, within a discharge time of few seconds.

The Effects of Self-Discharge on the Performance of Symmetric Electric Double-Layer Capacitors and Active Electrolyte-Enhanced Supercapacitors: Insights from Modeling and Simulation

The effects of self-discharge on the performance of symmetric electric double-layer capacitors (EDLCs) and active electrolyte-enhanced supercapacitors were examined by incorporating self-discharge into electrochemical capacitor models during charging and discharging. The sources of self-discharge in capacitors were side reactions or redox reactions and several impurities and electric double-layer (EDL) instability. The effects of self-discharge during capacitor storage was negligible since it took a fully charged capacitor a minimum of 14.0 days to be entirely discharged by self-discharge in all conditions studied, hence self-discharge in storage condition can be ignored. The first and second charge–discharge cycle energy efficiencies \( \eta_{{{\rm{E}}1}} \) and \( \eta_{{{\rm{E}}2}} \) of a capacitor of electrode effective conductivity \( \alpha_{1} \) = 0.05 S/cm with only EDL instability self-discharge with current density \( J_{\rm{VR}} \) = 1.25 × 10−3 A/cm2 were 72.33% and 72.34%, respectively. Also, energy efficiencies \( \eta_{{{\rm{E}}1}} \) and \( \eta_{{{\rm{E}}2}} \) of a similar capacitor with both side reactions and redox reactions and EDL instability self-discharges with current densities \( J_{\rm{VR}} \) = 0.00125 A/cm2 and \( J_{{{\rm{VR}}1}} \) = 0.0032 A/cm2 were 38.13% and 38.14% respectively, compared with 84.24% and 84.25% in a similar capacitor without self-discharge. A capacitor with only EDL instability self-discharge and that with both side reactions and redox reactions and EDL instability self-discharge lost 9.73 Wh and 28.38 Wh of energy, respectively, through self-discharge during charging and discharging. Hence, EDLCs charging and discharging time is significantly dependent on the self-discharge rate which are too large to be ignored.

Electrode Mass Balancing as an Inexpensive and Simple Method to Increase the Capacitance of Electric Double-Layer Capacitors.

Symmetric electric double-layer capacitors (EDLCs) have equal masses of the same active material in both electrodes. However, having equal electrode masses may prevent the EDLC to have the largest possible specific capacitance if the sizes of the hydrated anions and cations in the electrolyte differ because the electrodes and the electrolyte may not be completely utilized. Here we demonstrate how this issue can be resolved by mass balancing. If the electrode masses are adjusted according to the size of the ions, one can easily increase an EDLC’s specific capacitance. To that end, we performed galvanostatic cycling to measure the capacitances of symmetric EDLCs with different electrode mass ratios using four aqueous electrolytes— Na2SO4, H2SO4, NaOH, and KOH (all with a concentration of 1 M)—and compared these to the theoretical optimal electrode mass ratio that we calculated using the sizes of the hydrated ions. Both the theoretical and experimental values revealed lower-than-1 optimal electrode ratios for all electrolytes except KOH. The largest increase in capacitance was obtained for EDLCs with NaOH as electrolyte. Specifically, we demonstrate an increase of the specific capacitance by 8.6% by adjusting the electrode mass ratio from 1 to 0.86. Our findings demonstrate that electrode mass balancing is a simple and inexpensive method to increase the capacitance of EDLCs. Furthermore, our results imply that one can reduce the amount of unused material in EDLCs and thus decrease their weight, volume and cost.

Strategy for improved frequency response of electric double-layer capacitors

We propose a strategy for improving the response speed of electric double-layer capacitors (EDLCs) and electric double-layer transistors (EDLTs), based on an asymmetric structure with differently sized active materials and gate electrodes. We validate the strategy analytically by a classical calculation and experimentally by fabricating EDLCs with asymmetric Au electrodes (1:50 area ratio and 7.5 μm gap distance). The performance of the EDLCs is compared with that of conventional symmetric EDLCs. Our strategy dramatically improved the cut-off frequency from 14 to 93 kHz and this improvement is explained by fast charging of smaller electrodes. Therefore, this approach is particularly suitable to EDLTs, potentially expanding the applicability to medium speed (kHz–MHz) devices.

The Effect of Self-Discharge on the Performance of Symmetric Electric Double Layer Capacitors: Insights from Mathematical Modeling and Simulation

Varieties of symmetric electrochemical capacitors ECs models have been presented in the literature [1-12], but none was derived and solved subject to a particular mechanisms or amalgamation of distinct mechanisms of self-discharge. All the existing models were built on the assumption that the electrodes or the capacitor does not have self-discharge, in order to either simplify the models or to facilitate a solution of the models. Motivated by this knowledge gap, we have developed a generalized mathematical model of symmetric electric double layer capacitors (EDLCs) that incorporates the self-discharge effect during charging and discharging of the electrochemical device. This model is generic and applies to any symmetric EDLC using specific self-discharge mechanism or a combination of different self-discharge mechanisms applicable in EDLCs. Analysis and simulations were carried out on the model with self-discharge effects and the model without self-discharge. It was found from the analysis that the model with self-discharge took longer time to charge the capacitor to the designed voltage than the model without self-discharge. Also, the model with self-discharge discharges the stored energy faster than the model without self-discharges. The self-discharge current Iselfdis and energy loss due to self-discharge Eselfdis during charge and discharge processes of the capacitor with electrode’s effective conductivity α1=0.05 S/cm are 0.786 A and 9.73 Wh, respectively. A fully charged capacitor with self-discharge and electrodes effective conductivity α1=0.05 S/cm on storage takes 7.2 days for the capacitors to be fully discharged by self-discharge process. The energy efficiency of the first and second charge–discharge cycle and for the capacitor with self-discharge and with electrodes effective conductivity α1=0.05 S/cm are 64.62 % and 64.63 % while those for same capacitor without self-discharge are 84.241 % and 84.244 %, respectively. In summary, we prove from our models that the charging, discharging and storage time of the symmetric ECs significantly depend on self-discharge. In this presentation, we will show that the effect of self-discharge is so enormous to be ignored as exemplified in the presentations, where the potential profile φne/pe for the electrodes with and without self-discharge effect as a function of position after charging and discharging by constant current with different parameters of α1 and α2.

Fast Response, Vertically Oriented Graphene Nanosheet Electric Double Layer Capacitors Synthesized from C2H2

The growth and electrical characteristics of vertically oriented graphene nanosheets grown by radio frequency plasma-enhanced chemical vapor deposition from C2H2 feedstock on nickel substrates and used as electrodes in symmetric electric double layer capacitors (EDLC) are presented. The nanosheets exhibited 2.7 times faster growth rate and much greater specific capacitance for a given growth time than CH4 synthesized films. Raman spectra showed that the intensity ratio of the D band to G band versus temperature initially decreased to a minimum value of 0.45 at a growth temperature of 750 °C, but increased rapidly with further temperature increase (1.15 at 850 °C). The AC specific capacitance at 120 Hz of these EDLC devices increased in a linear fashion with growth temperature, up to 265 μF/cm(2) (2 μm high film, 850 °C with 10 min growth). These devices exhibited ultrafast frequency response: the frequency response at -45° phase angle reached over 20 kHz. Consistent with the increase in D band to G band ratio, the morphology of the films became less vertical, less crystalline, and disordered at substrate temperatures of 800 °C and above. This deterioration in morphology resulted in an increase in graphene surface area and defect density, which, in turn, contributed to the increased capacitance, as well as a slight decrease in frequency response. The low equivalent series resistance varied from 0.07 to 0.08 Ω and was attributed to the significant carbon incorporation into the Ni substrate.

An analysis of ethylene glycol-aqueous based electrolyte system for supercapacitor applications

In order to improve energy density of a capacitor, an operative potential limit extension is a simple way. Based on this concept, we have studied 2-electrode symmetric electrical double layer capacitor (EDLC) of PICACIFF-activated carbon electrodes in 4 mol dm−3of sodium nitrate using ethylene glycol (EG)-aqueous solution as an electrolyte media. Infrared spectroscopy was used to characterize the interactions of the electrolyte components. The electrochemical performance of the capacitor was experimented using cyclic voltammetry, galvanostatic charge–discharge cycling and impedance spectroscopy techniques at 0 °C, 25 °C and 60 °C through the aqueous and EG-aqueous electrolytes. The electrochemical performances of the aqueous cells were significantly enhanced by the EG addition. In such cases, a 1.5–2.0 V potential window and a range of 12–16 Wh kg−1 energy density were obtainable. This energy density value is two times higher than a usual aqueous cell. Moreover, a little percentile of aqueous to EG electrolyte changes the conductivity and its power density drastically. We conclude that this kind of electrolyte might be a suitable solution in between the aqueous and organic electrolytes. Also, it can achieve much higher energy density than the aqueous just a partial substitution of an organic medium.