Electric Double-layer Capacitor Cell Research Articles

Towards more Durable Electrochemical Capacitors by Elucidating the Ageing Mechanisms under Different Testing Procedures.

Electrical double‐layer capacitors (EDLCs) commonly denoted supercapacitors are rechargeable energy storage devices with excellent power and energy delivery metrics intermediate to conventional capacitors and batteries. High‐voltage aqueous electrolyte based EDLCs are particularly attractive due to their high‐power capability, facile production, and environmental advantages. EDLCs should last for thousands of cycles and evaluation of future cell chemistries require long‐term and costly galvanostatic cycling. Voltage holding tests have been proposed to shorten evaluation time by accelerating cell degradation processes. Whether voltage holding can replace cycling completely remains undemonstrated. In this work, a systematic investigation of the influence of testing procedure on cell performance is presented. The state‐of‐the‐art post‐mortem and operando experimental techniques are implemented to elucidate ageing mechanisms and kinetics inside EDLC cells under different testing procedures. Carbon corrosion occurring on the positively polarized electrode leads to the lower active surface area and higher oxygen content. On the contrary, an increase of surface area and micropore volume are observed on the negatively polarized electrode. Repeated galvanostatic cycles at U<1.6 V appears to facilitate the depletion of oxygen species on the positively polarized electrode in comparison with voltage holding, which indicates a more complex degradation mechanism during cycling. Caution is advised when comparing results from different test procedures.

Zinc oxide nanofiller-based composite polymer gel electrolyte for application in EDLCs

A ZnO nanofiller-based polymer gel electrolyte was prepared using solution-cast technique by incorporating PVDF-HFP as the polymer, ethylene carbonate (EC)-propylene carbonate (PC) as plasticizers, tetraethylammonium tetrafluoroborate (TEABF4) as salt, and ZnO as nanofiller. A maximum ionic conductivity value 6.3 × 10−3 Scm−1 was obtained by adding 12 wt% of ZnO nanoparticles in complex [{PVDF-HFP (20 wt%) + {EC-PC (v/v)-TEABF4 (1.0 M)} (80 wt%)}]. From the temperature-dependent conductivity plot, the activation energy is found to be equal to Ea ~ 0.07 eV. The amorphous structure is confirmed by SEM and XRD analysis. Electrochemical potential window and ionic transference number were calculated and found to be equal to ~3.3 V and 0.84, respectively. Optimized NCPGE is utilized in the fabrication of the electrical double-layer capacitor (EDLC) cell using market parched activated charcoal as electrode material. The specific capacitance of the fabricated EDLC cell is found to be equal to 162.9 mFcm−1 and ~ 67.8 Fg−1 for the single-electrode system.

Preparation and characterization of gel electrolyte with bacterial cellulose coated with alternating layers of chitosan and alginate for electric double-layer capacitors

A novel gel electrolyte has been prepared using bacterial cellulose (BC) coated with chitosan (CTS) and alginate (Alg) layers, which contain 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4). The gel electrolyte was optimized for use in solvent-free solid-stage electric double-layer capacitors (EDLCs). The inoculated BC was oxidized and coated with alternating layers of CTS and Alg via ionic crosslinking to produce the gel electrolytes. The CTS content of the obtained gel electrolytes linearly increased with an increase in the number of CTS and Alg layers on the BC fibers. The results imply that the CTS and Alg layers successfully bound onto each BC fiber, which was consistent with Fourier transform infrared spectroscopy measurements. Based on a structural analysis, the fabricated gel electrolytes had a nanofibrous structure with a fiber diameter range of approximately 78.6–99.0 nm, a high thermal stability, and a significantly improved tensile strength compared to gel electrolytes of only CTS or Alg. In addition, EDLCs have been fabricated using the new gel electrolyte with activated carbon as the active material. The electrochemical performance of the EDLCs was determined via charge–discharge testing and alternating current impedance measurements. The EDLCs cell with a gel electrolyte of BC coated with 15 layers of CTS and Alg containing EMImBF4 showed a high discharge capacitance, implying that the high affinity of this gel electrolyte for the activated carbon electrode leads to reduced electrode/electrolyte interfacial resistance, which shows the potential of this approach compared with liquid-phase EMImBF4 to obtain high-performance, safety-oriented EDLCs.

Electrochemical studies of EDLC Cells fabricated using drum stick (DS) activated charcoal

In this article, biomass drum stick (DS) activated carbon has been synthesized and used as an electrode material for fabrication of electrical double layer capacitor (EDLC). Polymer gel electrolyte [PVdF-HFP (20 wt%) + EC-PC (1:1v/v)-TEABF4 (1.0M)} (80 wt%)], optimized for highest conductivity, is used as an electrolyte for this EDLC cell. The performance of EDLC cell with drum stick (DS) activated carbon is tested using various techniques such as impedance spectroscopy, cyclic voltammetry and charge discharge techniques. The performance of drum stick (DS) activated carbon based EDLC cell is compared with EDLC cell fabricated using market purchased activated charcoal. DS activated carbon electrode based EDLC cell shows better results compared to the market purchased activated charcoal electrode based EDLC cell. The specific capacitance of DS electrode based EDLC cell is found to be ∼178.7 mFcm-2 and ∼ 84.7 Fg-1 in a single electrode system.

Isothermal calorimeter for measurements of time-dependent heat generation rate in individual supercapacitor electrodes

Heat generation in electric double layer capacitors (EDLCs) may lead to temperature rise and reduce their lifetime and performance. This study aims to measure the time-dependent heat generation rate in individual carbon electrode of EDLCs under various charging conditions. First, the design, fabrication, and validation of an isothermal calorimeter are presented. The calorimeter consisted of two thermoelectric heat flux sensors connected to a data acquisition system, two identical and cold plates fed with a circulating coolant, and an electrochemical test section connected to a potentiostat/galvanostat system. The EDLC cells consisted of two identical activated carbon electrodes and a separator immersed in an electrolyte. Measurements were performed on three cells with different electrolytes under galvanostatic cycling for different current density and polarity. The measured time-averaged irreversible heat generation rate was in excellent agreement with predictions for Joule heating. The reversible heat generation rate in the positive electrode was exothermic during charging and endothermic during discharging. By contrast, the negative electrode featured both exothermic and endothermic heat generation during both charging and discharging. The results of this study can be used to validate existing thermal models, to develop thermal management strategies, and to gain insight into physicochemical phenomena taking place during operation.

Internal Hybrid Li-Ion Battery and Li-Ion Capacitor Energy Storage Cells

Lithium (Li)-ion battery (LIB) and electric double-layer capacitor (EDLC) are two widely used electrochemical energy storage devices. LIB is made with Li intercalated graphite anode and Li metal oxide cathode and EDLC is made with high surface activated carbon for both anode and cathode. To compare two energy storage devices, LIB has a higher specific energy (or energy density) than EDLC, but has low specific power (or power density) and poor cycle life than EDLC as shown in Fig. 1. In high energy and low power applications, the LIB is the best choice; in high power and long cycle applications, the EDLC is used; however, in many applications, the power requirement is not a constant but rather spans over a range of power levels. It has been demonstrated that when batteries and supercapacitors were externally connected to form a hybrid power source, the voltage drop during the pulsed current for the hybrid power source was less than that for the battery, and operating time of the hybrid power source was significantly extended [1,2]. However, because the maximum cell voltage of single cell EDLC is less than that of LIB; therefore, hybrid power sources could only be made with two separate battery and EDLC cells and electrically connect them with or without electric circuits. The Li-ion capacitor (LIC) is a new energy storage device which consists of an EDLC cathode and a LIB anode, between which the ions shuttle during charge and discharge processes. The LIC not only retained all the advantages of EDLC such as specific power >5 kW/kg and cycle life > 100,000 cycles; but also had higher specific energy of 15-30 Wh/kg and higher maximum cell voltage of 4.1 V than that of EDLC [3,4]. Because the potentials of anode and cathode as well as the maximum cell voltage of LIC is comparable to that of LIB, it allows the LIC and LIB to be assembled in one package as a monolithic LIB and LIC hybrid cell. Very recently, we have demonstrated a new hybrid energy storage device that combines the advantages of both the LIB and the LIC, thereby avoiding their inherent defects, while bridging the gap between the high energy densities offered by batteries and the high power densities seen in LIC as shown in Fig. 1. The energy density and power density of the hybrid cell can be designed to meet the requirements by a reasonable distribution of the ratio between LIB and LIC electrode materials in the internal hybrid cell. For example, in a LIB/LIC internal hybrid cell, made with a 20 wt.% LiFePO4 (LFP) and 80 wt.% activated carbon (AC) mixed cathode and a pre-lithiated hard carbon anode, the specific energy increased by 40% to compare with LIC. As can be seen in Fig. 2(a), the discharge profile can be divided into two parts: at cell voltage range of 2.2-3.0 V and 3.3-3.8 V, the capacity was contributed by capacity material, while at 3.0-3.3 V, it was contributed by battery material. It can also be seen from Fig. 2, that the capacity contribution by capacitance material was less sensitive than battery material when the discharge rate increased from 0.1C to 53C. Fig. 2(b) shows the charge-discharge voltage profiles measured at an interruption rate of 0.2C as a function of cycle number after the hybrid cell was cycled at 43C rate. It must be notified that cycle life of the hybrid energy storage cell was found to be prolonged significantly as shown in Fig. 2(c). After 20,000 cycles, the degradation of specific capacity for both capacitance and battery materials inside hybrid cell was less than 5%. As a comparison, the specific capacity of a battery cell using LFP cathode, as a control experiment, decreased by 40% after 2,000 cycles. Reference: [1] J.P. Zheng, S.P. Ding, and T.R. Jow, “Hybrid Power Sources for Pulse Current Applications”, IEEE Transactions on Aerospace and Electronic Systems, 37, 288 (2001). [2] D. Shin, Y. Kim, Y. Wang, N Chang, and M. Pedram, “Constant-current regulator-based battery-supercapacitor hybrid architecture for high-rate pulsed load applications”, J. Power Sources, 205, 516 (2012). [3] W.J. Cao and J.P. Zheng, “Li-ion Capacitors with Carbon Cathode and Hard Carbon/SLMP Anode Electrodes”, J. Power Sources, 213, 180 (2012). [4] W.J. Cao, J. Shih, J.P. Zheng, and T. Doung, “Development and characterization of Li-ion capacitor pouch cells”, J. Power Sources, 257, 388 (2014). Figure 1

High performance for electric double-layer capacitors based on CNT–CG composite synthesized as additive material by CVD method

Abstract Carbon nanotubes (CNTs) were successfully grown on chemically synthesized graphene at a low temperature (700 °C) under atmospheric pressure by using chemical vapor deposition (CVD) and used as novel, suitable electrode materials for electric double-layer capacitors (EDLCs) was demonstrated. The growth state of the CNT–CG sample was characterized by SEM, TEM, and Raman spectroscopy. Then, EDLC electrodes with high surface area activated carbon (YP50F) and CNT–CG were fabricated in a simple step. Slurry type EDLCs cells were assembled using the prepared carbon materials. The electrochemical performance of the carbon electrodes was measured by galvanostatic charge/discharge and cyclic voltammetry methods With more than 80% of their capacitance was retained after 30 cycles, the YCG8 samples exhibited excellent stability and reliability at high current charge/discharge cycles. The high stability of the supercapacitors at different densities suggests that these energy storage devices are suitable for fast charging applications. Herein, CNT–graphene synthesized by the CVD method is presented as a promising substitute to conventional electrode materials for EDLCs.

Electrochemical Performance of Activated Carbon Based Electric Double Layer Capacitors Using 1-Ethyl-3-Methylimidazolium Dicyanamide Ionic Liquid and Its Gel Polymer Electrolyte

The supercapacitors have emerged as an important energy storage device as they can deliver a high specific power, long cycle life, and much higher charge-discharge rates than a battery. In particular, solid-state supercapacitors have gained worldwide attention as an emerging candidate for smart and efficient energy storage devices due to the increasing demand for miniaturized electronics. Electric double layer capacitors (EDLCs) are the most widely studied class of supercapacitors which physically stores charge by forming an electric double layer at the interfaces between the high surface area carbon electrode and the electrolyte. The conventional EDLCs using high surface area activated carbon electrodes and non-aqueous liquid electrolytes represent the current state of the art. The most common cause of failure of such devices is related to the electrolyte leakage. Most of the past work on EDLCs have been carried out using the liquid electrolytes (aqueous, organic and pure ionic liquid). The gel polymer electrolytes with high ionic mobility and good dimensional/mechanical stability, offer a viable substitute for the liquid electrolytes and they are being used for flexible and solid-state electrochemical energy storage devices. Here, we report the electrochemical performance of high surface area activated carbon based EDLCs using 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA) ionic liquid and its poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) gel counterpart as electrolytes. The gel electrolyte films were prepared by immobilizing the EMImDCA ionic liquid in PVdF-HFP copolymer matrix by solution casting method. The activated carbon electrodes were prepared by mixing the activated carbon powder with conductive additive (Super C65) and PVdF-HFP binder in a ratio of 8:1:1 (w/w/w), and then the mixture was drop-casted onto a flexible thin graphite sheet which serves as current collector. The solid-state EDLC cells were fabricated by sandwiching the gel electrolyte film between two identical activated carbon electrodes whereas EDLC cells with pure IL were fabricated by using fiberglass separator soaked with IL. The EDLC cells show highly capacitive behavior as evident from the cyclic voltammogram of the cells measured at a scan rate of 20 mV s-1 which is rectangular in shape (Fig 1a). However, gel electrolyte based solid-state cell shows slightly higher resistance, obvious from slight deviation from rectangular shape at both turning potential. Typical galvanostatic charge-discharge characteristic of the EDLC cells at current density of 5 mA cm-2is shown in Fig. 1b. The linear discharge characteristics of the cell further confirms the capacitive behavior of the cells due to formation of an electric double layer at the interfaces. The EDLC cell with pure IL shows specific capacitance of ~98 F g-1 whereas IL-gel based EDLC cell shows specific capacitance of ~86 F g-1, derived from charge-discharge curves at 5 mA cm-2. A comparative performance of the EDLC cells was carried our using the impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge techniques. This paper discusses the results of these comparative studies with pure IL and its PVdF-HFP gel electrolytes. Figure 1

CVD grown graphene/CNT composite as additive material to improve the performance of electric double layer capacitors (EDLCs)

In this work, large area graphene-carbon nanotube (G-CNT) composite as additive material for EDLC performance was grown on copper foil at low temperature (500 °C) and at atmospheric pressure by using chemical vapor deposition (CVD). The growth state was characterized by field emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. EDLC electrodes based on high surface area activated carbon (YP50F) and G-CNT were then fabricated by a facile step. Coin-type EDLC cells with two symmetrical carbon electrodes were assembled using the prepared carbon materials. The electrochemical performance of G-CNT based eletrodes were examined in an electrolyte solution of tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, TEABF4) in propylene carbonate (C4H6O3, PC) and measured by galvanostatic charge/discharge and cyclic voltammetry methods.

Electrochemical performance of microporous and mesoporous activated carbons in neat and diluted 1-ethyl-3-methylimidazolium tetrafluoroborate

Abstract 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIm·BF4), neat and diluted with propylene carbonate to 1 mol L−1, have been employed as electrolytes of electrical double-layer capacitors (EDLCs). The effects of microporosity and mesoporosity in activated carbon (AC) electrodes on the capacitive and resistive performances upon the use of neat and diluted EMIm·BF4 have been explored. In addition to cyclic voltammetry and galvanostatic charge–discharge tests, electrochemical impedance spectroscopy has been performed employing Kang's equivalent circuit model consisting of three resistances, three constant phase elements, and one bounded Warburg impedance. The overall impedance of the EDLC cell was separated into components of intrinsic resistance, bulk electrolyte, diffusion layer, and Helmholtz layer. The specific capacitance and the equivalent series resistance (ESR) of mesoporous AC were found to be highly dependent on the rate of ionic transfer. Lower cell voltage was identified as being responsible for lower specific capacitance and larger ESR of mesoporous AC, which was similarly seen in the neat and diluted EMIm·BF4, and could be alleviated by increasing the cell voltage. The inferior rate performance and the cell-voltage-dependent performance of mesoporous AC, which were more distinctly observed in the neat EMIm·BF4, could be attributed to the lower mobility of EMIm+ and BF4− in mesopores.

Solid Polymer Electrolyte Based on PVDF-HFP and Ionic Liquid Embedded with TiO2 Nanoparticle for Electric Double Layer Capacitor (EDLC) Application

Solid polymer electrolytes based on PVDF-HFP polymer and 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and TiO2 nanoparticle have been prepared for different concentrations of nanoparticles. It is observed that the electrolyte containing 5 wt% of TiO2 nanoparticle exhibits highest ionic conductivity. Electric double layer capacitor (EDLC) cells have been fabricated with the prepared electrolytes and the activated carbon electrode. The cyclic voltammetric (CV) profiles of the prepared EDLC cells display nearly rectangular shape indicating their capacitive behavior. It is observed that the specific capacitance (147.66 Fg−1 at current density of 0.5 Ag−1) of the EDLC cell fabricated with 5 wt% of TiO2 nanoparticle is higher than that (85.66 F g−1 at current density of 0.5 Ag−1) of the EDLC cell without TiO2 nanoparticle. The specific energy density and power density of the EDLC cell containing 5 wt% of TiO2 nanoparticle, obtained at a current density of 1 Ag−1 in the first cycle, are 145.82 Wh/Kg and 2.9 kW/Kg respectively with 100% coulomb efficiency. The specific capacitance of the EDLC cell obtained from cyclic voltammetry is in good agreement with the value obtained from the low frequency electrochemical impedance and galvanostatic measurements. The cyclic stability of 1500 cycles of charge-discharge processes has been observed for the EDLCs.

Activated graphene oxide/reduced graphene oxide electrodes and low viscous sulfonium cation based ionic liquid incorporated flexible gel polymer electrolyte for high rate supercapacitors

High rate performance of symmetrical electrical-double layer capacitors (EDLCs) constructed with activated graphene oxide (ac-GO)/activated reduced graphene oxide (ac-RGO) electrodes and ionic liquid (triethylsulfonium bis(trifluoromethylsulfonyl)imide, incorporated plastic crystal succinonitrile (SN) based flexible gel polymer electrolytes (GPE) have been reported. The 3D architecture with predominantly meso-porous character of the ac-RGO, in combination with SN-based high ion conducting GPE, renders it a high rate electrode with high value of knee frequency, fk (∼966 Hz), low response time τ0 (∼20 ms) and high pulse power P0 (∼111 kW kg−1) as compared to the ac-GO-electrode that exhibits the corresponding values: fk (∼215 Hz), τ0 (∼60 ms) and P0 (∼58 kW kg−1). The ac-RGO based EDLCs offer higher specific power (∼118 kW kg−1) than that for ac-GO-based EDLC (∼41 kW kg−1), as evaluated from GCD studies. However, due to the additional pseudo-capacitance arising from leftover oxygen functional groups, the ac-GO-electrodes exhibit higher values of specific capacitance and specific energy (∼88–96 F g−1 and 25 Wh kg−1, respectively) as compared to the ac-RGO based EDLCs with specific capacitance ∼66–72 F g−1 and specific energy ∼18 Wh kg−1). Both the EDLC cells exhibit stable capacitive performance up to ∼11,000–13,000 GCD cycles.

Thermostable gel polymer electrolyte based on succinonitrile and ionic liquid for high-performance solid-state supercapacitors

Abstract A flexible, free-standing, thermostable gel polymer electrolyte based on plastic crystalline succinonitrile (SN) and ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) entrapped in copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) is prepared and optimized for application in solvent-free solid-state supercapacitors. The synthesized gel polymer electrolyte exhibits a high ionic conductivity over a wide temperature range (from ∼5 × 10−4 S cm−1 at −30 °C up to ∼1.5 × 10−2 S cm−1 at 80 °C) with good electrochemical stability window (−2.9 to 2.5 V). Thermal studies confirm that the SN containing gel polymer electrolyte remains stable in the same gel phase over a wide temperature range from −30 to 90 °C. The electric double layer capacitors (EDLCs) have been fabricated using activated carbon as active materials and new gel polymer electrolytes. Electrochemical performance of the EDLCs is assessed through cyclic voltammetry, galvanostatic charge-discharge cycling and impedance spectroscopy. The EDLC cells with the proper SN-containing gel polymer electrolyte has been found to give high specific capacitance 176 F g−1 at 0.18 A g−1 and 138 F g−1 at 8 A g−1. These solid-state EDLC cells show good cycling stability and the capability to retain ∼80% of the initial capacitance after 10,000 cycles.

Electric double-layer capacitors with tea waste derived activated carbon electrodes and plastic crystal based flexible gel polymer electrolytes

We report a novel configuration of symmetrical electric double-layer capacitors (EDLCs) comprising a plastic crystalline succinonitrile (SN) based flexible polymer gel electrolyte, incorporated with sodium trifluoromethane sulfonate (NaTf) immobilised in a host polymer poly (vinylidine fluoride-co-hexafluoropropylene) (PVdF-HFP). The cost-effective activated carbon powder possessing a specific surface area (SSA) of ~ 1700 m2g-1 containing a large proportion of meso-porosity has been derived from tea waste to use as supercapacitor electrodes. The high ionic conductivity (~3.6×10-3 S cm-1 at room temperature) and good electrochemical stability render the gel polymer electrolyte film a suitable candidate for the fabrication of EDLCs. The performance of the EDLCs has been tested by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge-discharge studies. The performance of the EDLC cell is found to be promising in terms of high values of specific capacitance (~270 F g-1), specific energy (~ 36 Wh kg-1), and power density (~ 33 kW kg-1).

Degradation Characteristics of Electric Double-Layer Capacitors Consisting of High Surface Area Carbon Electrodes with Organic Electrolyte Solutions

Aging characteristics of electric double-layer capacitors (EDLCs) comprising high surface area carbon electrodes with organic electrolyte solutions have been examined under high-voltage (high cell-potential) and high-temperature cycling conditions. The influences of the electrolyte components on decreasing the capacitance were investigated for cells using different electrolyte compositions. The use of acetonitrile (AN) as the electrolyte solvent in the cell led to higher cycling capacitance than those using alkyl carbonate solvents, such as propylene carbonate or ethylene carbonate mixed with dimethyl carbonate, under moderate cycling conditions. However, when the cycling was done with a higher operating voltage (>3.5V), significant capacitance degradation was observed for the cells using AN-based electrolytes. The EDLC cells using carbonate-based electrolytes showed moderate degradation behavior even at higher operating voltages. With respect to the electrolytic salt, cells using LiBF4 generally showed lower durability than cells using tetraethylammonium salt, TEABF4, especially at high operating voltages (>3.5V). The mechanism of the decreasing capacitance with repeated cycle is discussed.

Comparative Analysis of Some Parametric Model Structures Dedicated to EDLC Diagnosis

Electrical double-layer capacitors (EDLCs) are now widely used as power buffers in energy storage systems, especially for electrical transport applications. Because of their high acquisition costs, their predictive maintenance represents a key issue for the development of electrical traction systems. This paper investigates the diagnosis of EDLCs by a parametric estimation of a dynamic continuous-time model. Indeed, the physical interpretation of their parameters simplifies the supercapacitor diagnosis and prognosis. Due to the complex electrochemical phenomena occurring in EDLC cells, modeling errors are far more important than measurement noise. Therefore, we propose to use the output error minimization techniques and the relative sensitivity functions to evaluate and compare the diagnosis potential of several model structures. This allows the use of some comprehensive comparison criteria without any assumption about the statistical characteristics of the system disturbances. An experimental accelerated aging process of 200 commercial super capacitors (Nichicon, 1F, 20 weeks long aging at 65 °C, 0 and 2.5 V dc polarization) is also performed to make statistics and illustrate the methodology.

Solid-state electric double layer capacitors fabricated with plastic crystal based flexible gel polymer electrolytes: Effective role of electrolyte anions

Flexible gel polymer electrolyte (GPE) thick films incorporated with solutions of lithium trifluoromethanesulfonate (Li-triflate or LiTf) and lithium bis trifluoromethane-sulfonimide (LiTFSI) in a plastic crystal succinonitrile (SN), entrapped in poly(vinylidine fluoride-co-hexafluoropropylene) (PVdF-HFP) have been prepared and characterized. The films have been used as electrolytes in the electrical double layer capacitors (EDLCs). Coconut-shell derived activated carbon with high specific surface area (∼2100 m2 g−1) and mixed (micro- and meso-) porosity has been used as EDLC electrodes. The structural, thermal, and electrochemical characterization of the GPEs have been performed using scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), impedance measurements and cyclic voltammetry. The high ionic conductivity (∼10−3 S cm−1 at 25 °C), good electrochemical stability window (>4.0 V) and flexible nature of the free-standing films of GPEs show their competence in the fabrication of EDLCs. The EDLCs have been tested using electrochemical impedance spectroscopy, cyclic voltammetry, and charge–discharge studies. The EDLCs using LiTf based electrolyte have been found to give higher values of specific capacitance, specific energy, power density (240–280 F g−1, ∼39 Wh kg−1 and ∼19 kW kg−1, respectively) than the EDLC cell with LiTFSI based gel electrolyte. EDLCs have been found to show stable performance for ∼104 charge–discharge cycles. The comparative studies indicate the effective role of electrolyte anions on the capacitive performance of the solid-state EDLCs.

Solid-State Planar Edlc Design Enabled By Hydroxide-Conducting Polymer

Electrolytic capacitors are the current solution for 120 Hz power filtering. However such devices are bulky, which limits power electronics miniaturization. Electric double layer capacitors (EDLCs), often referred to by the product name Supercapacitor, have much higher volumetric charge storage and potentially should allow for a size reduction in the power electronics. For an EDLC to be capable of efficient AC line filtering, its impedance phase angle at 120 Hz must reach or be close to -90 degree. The very first EDLC that met this requirement was fabricated using vertically-oriented graphene with a liquid KOH electrolyte [1]. Both series resistance and distributed charge storage were minimized to reach this level of performance. Further development has led to a planar interdigitated cell design, which offers volumetric advantages and a simple approach for series-connecting cells [2]. With this design, polymer electrolytes are preferred since they can cover each planar cell without flowing to an adjacent cell. In the past, we have demonstrated a tetraethylammonium hydroxide (TEAOH)-based polymer electrolyte system that outperformed the KOH-based polymer electrolytes [3]. In this study, we leveraged the TEAOH polymer electrolyte and the vertically-oriented graphene to demonstrate solid-state planar EDLC cells. Impedance behavior of the cells at both room temperature and elevated temperatures was investigated. Two polymer electrolyte systems based on TEAOH were studied: (a) TEAOH-polyvinyl alcohol (TEAOH-XLPVA); and (b) TEAOH- polyacrylamide (TEAOH-PAM). Utilizing these polymer electrolytes, we assembled solid-state EDLC cells using vertically-oriented graphene electrodes. These solid-state devices were first tested at room temperature for aging stability and then at higher temperature for thermal stability. Figure 1 shows three plots of capacitance versus frequency for TEAOH-based electrolyte solid-state EDLCs. The capacitance values were calculated assuming a series-RC circuit model. While both TEAOH-PAM-based and TEAOH-XLPVA-based electrolyte cells showed capacitive behavior, the former exhibited higher initial capacitance than the latter (38 vs. 32 μF at 120 Hz). Although both capacitors showed slightly reduced capacitance after ca.25 days storage without packaging (Fig. 1a), both capacitors demonstrated good shelf life at room temperature. Further evaluations of the thermal stability of these capacitors at elevated temperatures were performed at temperatures up to 110 oC (Fig. 1b and 1c). Capacitance increased with increasing temperature for both solid-state electrolytes. A detailed analysis including comparisons will be presented. Capacitor equivalent series resistance and characteristic response times will be discussed.

Outstanding features of alginate-based gel electrolyte with ionic liquid for electric double layer capacitors

An alginate-based gel electrolyte with an ionic liquid (Alg/IL) is investigated for electric double-layer capacitors (EDLCs) by using physicochemical and electrochemical measurements. The Alg/EMImBF4 (EMImBF4 = 1-ethyl-3-methylimidazolium tetrafluoroborate) gel electrolyte is thermally stable up to 280 °C, where EMImBF4 decomposes. Furthermore, the EDLC with the gel electrolyte can be operated even at high temperature. The cell containing Alg/EMImBF4 is also electrochemically stable even under high voltage (∼3.5 V) operation. Thus, the alginate is a suitable host polymer for the gel electrolyte for EDLCs. According to the result of charge–discharge characteristics, the voltage drop in the charge−discharge curve for the cell with Alg/EMImBF4 gel electrolyte is considerably smaller than that with liquid-phase EMImBF4 electrolyte. To clarify the effect of Alg in contact with the activated carbon electrode, we also prepared an Alg-containing ACFC electrode (Alg + ACFC), and evaluated its EDLC characteristics in liquid EMImBF4. The results prove that the presence of Alg close to the active materials significantly reduces the internal resistance of the EDLC cell, which may be attributed to the high affinity of Alg to activated carbon.

Electric double-layer capacitors with corn starch-based biopolymer electrolytes incorporating silica as filler

Solid-state electric double-layer capacitors (EDLCs) based on corn starch biodegradable polymer electrolytes are fabricated. Silica is added into the corn starch polymer electrolytes to improve the ionic conductivity and capacitance of the EDLCs. The ionic conductivity as a function of temperature obeys the Arrhenius equation indicating ionic hopping mechanism in the polymer electrolytes. Linear sweep voltammetry shows that the prepared corn starch-based biopolymer electrolyte is stable in the range from −2.5 to 2.5 V. Electrical capacitance performances of these capacitors are studied by cyclic voltammetry, galvanostatic charge–discharge, and AC impedance spectroscopy. The discharge characteristics have been found to be almost linear, which confirms the capacitive behavior of the EDLC cell. The fabricated EDLC cells perform good cyclability up to 500 cycles with more than 90 % of coulombic efficiency.