Electric Double-layer Capacitor Cell Research Articles

Development and Characterization of a New Solid Polymer Electrolyte for Supercapacitor Device

In this study, solid polymer electrolytes (SPEs) are based on methylcellulose (MC) used as a polymer host and sodium iodide (NaI) as a dopant. The SPE films are developed using different contents of ethyl carbonate (EC) as a plasticizer to enhance their properties via a solution casting method. The surface morphology of SPE films is shown using polarized optical microscopy (POM), which indicates the existence of amorphous patches due to the plasticizing effect of EC. The creation of a complex between MC, NaI, and EC was confirmed by Fourier transform infrared (FTIR) spectra. A tiny amount of EC applied to the MC-NaI polymer salt matrix increases the number of charge carriers and improves ionic conductivity. The ionic conductivity of the generated polymer electrolytes is examined using electrochemical impedance spectroscopy (EIS). The high-ion conducting PE of 5.06 × 10−3 S·cm−1 was found with the mixture MC + 50 wt% NaI + 10 wt% EC (room temperature). The linear speed voltammetry (LSV) test shows that the optimized polymer electrolyte can withstand decomposition up to 2.5 V. The optimized sample transmission numbers were calculated using a TNM (transference number measurement) approach, and the results show that 99% of the ions contribute to the conductivity, compared to only 1% of the electrons. A solid-state electrical double-layer capacitor (EDLC) was fabricated using the highest ion-conductive polymer electrolyte and graphene oxide (GO)-based electrodes. The galvanostatic charge-discharge (GCD) technique was performed, and the GCD graph shows the behavior of an ideal capacitor with a less Faradic process and a low ESR value. The GO-based cell’s columbic efficiency is 100%, and the system delivers the charge for a long duration. The EDLC cell demonstrates outstanding cyclability. The specific capacitance of the EDLC cell incorporated with MC + 50 wt. % NaI + 10 wt. % EC was found to be 154.66 F/g.

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.

Ionic Liquid Encapsulated Poly (Methyl Methacrylate) Electrolyte Film in Electrical Double Layer Capacitor

One of the main components of electrical double layer capacitor (EDLC) is the electrolyte. Gel-type electrolyte has shown good performance in EDLC. However, not much information is available on film-type electrolytes, which are known to provide better mechanical stability than the gel-type electrolyte. In present study, we have reported the performance of film-type poly(methyl methacrylate (PMMA) as electrolyte in electrical double layer capacitor (EDLC) systems and compared with the gel-type PMMA electrolytes. This film-type PMMA electrolyte is modified by encapsulating 1-methyl-3-pentamethyldisiloxymethylimidazolium bis (trifluoromethylsulfonyl)imide, [(SiOSi)C1C1 im][NTf2 ], an ionic liquid (IL), into the PMMA matrix, PMMAIL. Doping this PMMAIL film with 30% lithium triflate salt (LiTf), LiTfPMMAIL, provides an EDLC cell with higher breakdown voltage (3.2 V) and higher specific discharge capacity (Csp) (6.59 Fg-1 ) and energy density (0.92 Whkg-1 ) than the selected PMMA-based gel electrolyte systems. These values exceed the minimum requirements for a working supercapacitor. However, this LiTF-PMMAIL cell exhibited lower power density (23.04 Wkg-1 ) than the selected EDLC cells due to the more congested system of LiTF-PMMAIL. Thus, the performance of this LiTF-PMMAIL cell could be improved by adjusting the amount of doping salt and using different types of carbon electrodes. Keywords—capacitors, charging/discharging, polymer electrolyte films, poly (methyl methacrylate) electrolytes, solid electrolytes.

Structural, thermal, and electrochemical studies of biodegradable gel polymer electrolyte for electric double layer capacitor

A quasi-solid-state supercapacitor is fabricated using a biodegradable gel polymer electrolyte (GPE), and graphene nano-platelets (GNPs) capacitive electrodes. The GPE film comprises biodegradable polymer cellulose acetate (CA), ionic liquid, 1-ethyl-3-methylimidazolium thiocyanate (EMImSCN) and dopant salt potassium thiocyanate (KSCN). The polymeric gel films are prepared using the standard “solution cast technique” and characterized by using X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Thermogravemetric analysis (TGA), Impedance spectroscopy, and Linear sweep voltammetry techniques. The synthesized GPE exhibits maximum room temperature ionic conductivity ∼1.2 x 10−3 S cm−1 and an electrochemical stability window of ∼2.4 V. The electrical double layer capacitor (EDLC) is configured by sandwiching the GPE film between two graphene nanoplatelets (GNPs) electrodes. The performance of the EDLC cell is evaluated in terms of specific capacitance, rate, and pulse power by using cyclic voltammetry, and electrochemical impedance measurement techniques.

Multi-Stacked Superbuck Converter-Based Single-Switch Charger Integrating Cell Voltage Equalizer for Series-Connected Energy Storage Cells

Voltages of series-connected energy storage cells, such as electric double-layer capacitors (EDLCs) and lithium-ion batteries, need to be equalized to ensure years of safe operation. However, to this end, a voltage equalizer is necessary in addition to a charger, increasing the system complexity and cost. This paper proposes a family of transformerless single-switch integrated chargers that merge a charger and equalizer into a single unit, achieving a simplified system and circuit. Proposed integrated chargers are derived by stacking multiple conventional pulse width modulation (PWM) converters, such as a superbuck converter, that contain two inductors and one energy transfer capacitor. Detailed operation analyses, including an investigation on the impact of component tolerance on voltage equalization performance, are also performed. Experimental charging tests using a 12-W prototype were performed for four EDLC cells. All cells were charged with eliminating voltage imbalance and demonstrating the charging and equalization performance of the proposed integrated charger.

All-solid-state electric double layer supercapacitors using Li1.3Al0.3Ti1.7(PO4)3 reinforced solid polymer electrolyte

In this work, we report the fabrication and characterization of all-solid-state supercapacitors based on a conductive filler dispersed solid polymer electrolyte. Fast ionic Li1.3Al0.3Ti1.7(PO4)3 (LATP) reinforced PEO-PEG-LITFSI composite solid polymer electrolyte (CSPE) membranes are prepared by milling assisted route. The electric double-layer capacitor (EDLC) cells, using a hot-roll lamination technique, are fabricated using a CSPE membrane as electrolyte and activated charcoal (surface area ∼ 817m2g−1) on graphite sheet as electrode. The EDLCs display appreciable areal capacitance of ∼ 12 Fcm−2 and ∼40 Fcm−2 at 40 °C and 80 °C, respectively at ∼0.65 mAcm2 and 2 V. These solid-state EDLCs at 40 °C exhibit stability up to ∼16,000 cycles. Further, the electrode-electrolyte (solid-solid) interface remains quite stable after the charge-discharge cycling. The electrical conductivity of the CSPE membranes correlates well with the EDLC performance. The LATP content in the CSPE membranes play important role in enhancing the capacitance. The present investigation suggests that CSPE membranes with conductivity between ∼10−4–10−5 Scm−1 are useful for low-power EDLC applications. The EDLCs cells with LATP dispersed CSPE exhibit better stability during thermal cycling between 40 °C-80 °C.

Design of polyimide-based separators for effective suppression of self-discharge in non-aqueous electrical double layer capacitors

The self-discharge phenomenon is an unavoidable response of electrical double layer capacitors (EDLCs) when the applied voltage is interrupted. How to alleviate this natural behavior is a challenge in the practical usage of EDLCs, especially such a self-discharge phenomenon is demonstrated to be induced by the presence of trace water in the organic electrolyte. This research focuses on the alleviation of the water-induced self-discharge process in non-aqueous EDLCs through the utilization of a polyimide (PI) separator coated with the polyurethane-poly(acrylic acid) (PUPAA) copolymer (denoted as PI@PUPAA). The trace water in the non-aqueous electrolytes is effectively absorbed by the copolymer, leading to the obvious suppression of self-discharge in the EDLC cells. The introduction of Al 2 O 3 nanoparticles into PI to form a composite separator (PI + Al 2 O 3 @PUPAA) substantially improves the electrochemical responses of EDLCs though the ionic conductivity of the PI@PUPAA separator is reduced by the PUPAA coating. This PI + Al 2 O 3 @PUPAA composite has been demonstrated to be a promising separator significantly decelerating the self-discharge process of an EDLC with a comparable charge/discharge performance in comparison with the same cell utilizing the commercial TF4030 separator. • The higher water content of electrolyte is, the more self-discharge of EDLCs is found. • PUPAA absorbing H 2 O from electrolytes suppresses water-induced self-discharge of EDLCs. • PI + Al 2 O 3 @PUPAA is a cost-effective separator reducing self-discharge of EDLCs.

Highly mesoporous carbons derived from corn silks as high performance electrode materials of supercapacitors and zinc ion capacitors

An agricultural waste, corn silks, is used as raw material to prepare mesoporous carbons for electrochemical energy storage applications. The natural hollow tubular structure of corn silks leads to adequate mixing of the KOH activating reagent with the carbon precursor and results in highly mesoporous carbons. When tested as active materials of both electrical double layer capacitors (EDLCs) and zinc ion capacitors (ZICs), the mesoporous carbons exhibit very encouraging energy storage properties. The EDLC cell using 1 M TEABF4/PC (propylene carbonate) electrolyte delivers an energy density of 25 Wh kg−1 even at a high power density of 23,070 W kg−1, while the ZIC cell with 1 M Zn(CF3SO3)2/DMF (N, N-Dimethylformamide) electrolyte displays a high specific energy of 38.5 Wh kg−1 at 14,741 W kg−1. The high specific surface area and large mesopore volume of the corn silks-derived mesoporous carbons might have provided more adsorption sites for the formation of electrical double layer and also facilitated the ion diffusion kinetics, which have synergistically improved the energy-power outputs in both EDLCs and Zn-ion capacitors.

Effect of pore size in activated carbon on the response characteristic of electric double layer capacitor

The effect of pore size on the response characteristic of an electric double layer capacitor (EDLC) was closely examined. A series of phenol-resin-based activated carbon (AC) samples was prepared as average pore size from 0.94 to 1.68 nm by KOH activation by varying the activation temperature and KOH/carbonized phenol-resin ratio. The impedance properties of pouch-type EDLC cells prepared using AC samples were evaluated by applying an alternating current at 3 V between 10 mHz and 100 kHz for confirming the response characteristic. The cell based on the AC with a largest pore size (1.68 nm) showed fast response frequency, and had a high dielectric relaxation time constant as calculated from the response frequency value. The AC with the largest pore size, which consisted of both micropores (>1 nm) and mesopores (2–4 nm), was confirmed to facilitate extremely low electrolyte-diffusion resistance during the formation of the electric double layer, implying that the large pores lead to fast and stable response frequency. The presented findings suggest that AC with a largest pore size as the electrode leads to superior capacitance and response frequency characteristics in the alternating current than those of an AC with a smaller pore size.

Enhanced Electrochemical Performance of Supercapacitors via Atomic Layer Deposition of ZnO on the Activated Carbon Electrode Material.

Fabricating electrical double-layer capacitors (EDLCs) with high energy density for various applications has been of great interest in recent years. However, activated carbon (AC) electrodes are restricted to a lower operating voltage because they suffer from instability above a threshold potential window. Thus, they are limited in their energy storage. The deposition of inorganic compounds’ atomic layer deposition (ALD) aiming to enhance cycling performance of supercapacitors and battery electrodes can be applied to the AC electrode materials. Here, we report on the investigation of zinc oxide (ZnO) coating strategy in terms of different pulse times of precursors, ALD cycles, and deposition temperatures to ensure high electrical conductivity and capacitance retention without blocking the micropores of the AC electrode. Crystalline ZnO phase with its optimal forming condition is obtained preferably using a longer precursor pulse time. Supercapacitors comprising AC electrodes coated with 20 cycles of ALD ZnO at 70 °C and operated in TEABF4/acetonitrile organic electrolyte show a specific capacitance of 23.13 F g−1 at 5 mA cm−2 and enhanced capacitance retention at 3.2 V, which well exceeds the normal working voltage of a commercial EDLC product (2.7 V). This work delivers an additional feasible approach of using ZnO ALD modification of AC materials, enhancing and promoting stable EDLC cells under high working voltages.

Dead Ashoka (Saraca asoca) leaves–derived porous activated carbons and flexible iongel polymer electrolyte for high-energy-density electric double-layer capacitors

In this work, porous activated carbons (ACs) are synthesized from dead Ashoka leaves by a modified chemical activation method where KOH and K2CO3 are used as activating agent in different weight percentages. The irregular surface morphology, high surface area, and the substantial amount of micropores and mesopores volumes simultaneously have been found in the synthesized ACs. The surface morphology and porosity parameters of ACs have been found significant with changing the incorporated activating agents in the different weight percentages. These porous ACs derived from biowaste material have been tested as electrode material in the electric double-layer capacitors (EDLCs) with iongel polymer electrolyte (iPE). A series of iPEs have been prepared using PVdF-HFP polymer host, 1-ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([PYR12][TFSI]) ionic liquid, and mixed organic carbonate-based plasticizers. EDLCs have been fabricated using AC-based electrodes and flexible iPE30 solid electrolyte. The high ionic conductivity (~6.11 × 10−3 S.cm−1 at 303 K), good electrochemical stability window (~ −2.74 V to +3.14 V vs. stainless steel electrodes), and flexible nature of the freestanding film of iPE30 provided outstanding compatibility with bio-derived AC electrodes. Cyclic voltammetry and galvanostatic charge–discharge studies have been performed, which show ideal and reversible capacitive features together with good cyclability of the fabricated EDLC cells.

The Study of Plasticized Solid Polymer Blend Electrolytes Based on Natural Polymers and Their Application for Energy Storage EDLC Devices.

In this work, plasticized magnesium ion-conducting polymer blend electrolytes based on chitosan:methylcellulose (CS:MC) were prepared using a solution cast technique. Magnesium acetate [Mg(CH3COO)2] was used as a source of the ions. Nickel metal-complex [Ni(II)-complex)] was employed to expand the amorphous phase. For the ions dissociation enhancement, glycerol plasticizer was also engaged. Incorporating 42 wt% of the glycerol into the electrolyte system has been shown to improve the conductivity to 1.02 × 10−4 S cm−1. X-ray diffraction (XRD) analysis showed that the electrolyte with the highest conductivity has a minimum crystallinity degree. The ionic transference number was estimated to be more than the electronic transference number. It is concluded that in CS:MC:Mg(CH3COO)2:Ni(II)-complex:glycerol, ions are the primary charge carriers. Results from linear sweep voltammetry (LSV) showed electrochemical stability to be 2.48 V. An electric double-layer capacitor (EDLC) based on activated carbon electrode and a prepared solid polymer electrolyte was constructed. The EDLC cell was then analyzed by cyclic voltammetry (CV) and galvanostatic charge–discharge methods. The CV test disclosed rectangular shapes with slight distortion, and there was no appearance of any redox currents on both anodic and cathodic parts, signifying a typical behavior of EDLC. The EDLC cell indicated a good cyclability of about (95%) for throughout of 200 cycles with a specific capacitance of 47.4 F/g.

From Cellulose, Shrimp and Crab Shells to Energy Storage EDLC Cells: The Study of Structural and Electrochemical Properties of Proton Conducting Chitosan-Based Biopolymer Blend Electrolytes.

In this study, solid polymer blend electrolytes (SPBEs) based on chitosan (CS) and methylcellulose (MC) incorporated with different concentrations of ammonium fluoride (NH4F) salt were synthesized using a solution cast technique. Both Fourier transformation infrared spectroscopy (FTIR) and X-ray diffraction (XRD) results confirmed a strong interaction and dispersion of the amorphous region within the CS:MC system in the presence of NH4F. To gain better insights into the electrical properties of the samples, the results of electrochemical impedance spectroscopy (EIS) were analyzed by electrical equivalent circuit (EEC) modeling. The highest conductivity of 2.96 × 10−3 S cm−1 was recorded for the sample incorporated with 40 wt.% of NH4F. Through transference number measurement (TNM) analysis, the fraction of ions was specified. The electrochemical stability of the electrolyte sample was found to be up to 2.3 V via the linear sweep voltammetry (LSV) study. The value of specific capacitance was determined to be around 58.3 F/g. The stability test showed that the electrical double layer capacitor (EDLC) system can be recharged and discharged for up to 100 cycles with an average specific capacitance of 64.1 F/g. The synthesized EDLC cell was found to exhibit high efficiency (90%). In the 1st cycle, the values of internal resistance, energy density and power density of the EDLC cell were determined to be 65 Ω, 9.3 Wh/kg and 1282 W/kg, respectively.

Protonic EDLC cell based on chitosan (CS): methylcellulose (MC) solid polymer blend electrolytes

In this work, preparation and application of solid polymer blend electrolytes (SPBEs) based on chitosan: methylcellulose (CS:MC) incorporated with various amounts of ammonium iodide (NH4I) salt are described. Solution cast technique was adapted for the preparation of the SPBE films. Structural investigation and extent of interaction of the NH4I salt with the CS:MC film surface were conducted through X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy measurements. Electrical DC conductivity of the prepared samples was studied by electrochemical impedance spectroscopy. It is confirmed through transference number (TNM) analysis that ions are the dominant charge carrier in the polymer electrolyte system, in which the ionic (tion) and electron (tel) transference numbers were found to be 0.934 and 0.036, respectively. Through the use of linear sweep voltammetry (LSV), the CS:MC:NH4I system is found to experience electrolyte degradation at 2.1 V. The polymer blend electrolyte sample with highest DC conductivity property was also used as separator in electrical double-layer capacitor (EDLC) cell. Two identical activated carbon electrodes were used to sandwich the electrolyte film. Cyclic voltammetry (CV) was used to show the capacitive behavior of the fabricated EDLC cell, in which no redox peaks were observed. The EDLC was examined for 100 cycles at current density of 0.2 mA/cm2. The values of specific capacitance and equivalent series resistance were determined to be 1.76 F/g, and 650 to 1050 Ω, respectively.

Effect of temperature on irreversible and reversible heat generation rates in ionic liquid-based electric double layer capacitors

This study reports for the first time, isothermal calorimetric measurements of the instantaneous heat generation rate at each electrode of ionic liquid-based electric double layer capacitors (EDLCs) at different temperatures. Indeed, EDLCs generate reversible and irreversible heat during normal operation and the properties of ionic liquids are known to depend strongly on temperature. Here, the EDLC cell consisted of two identical activated carbon electrodes separated by a mesh separator submerged in ionic liquid-based electrolyte consisting of 1 M N-butyl-n-methylpyrrolidinium bis(trifluoromethane sulfonyl)imide (Pyr14TFSI) dissolved in propylene carbonate (PC). The instantaneous heat generation rate at each electrode was measured at 20 °C, 40 °C, and 60 °C under galvanostatic cycling using an in operando calorimeter. The potential window was limited to 1 V to compare with results from similar devices using aqueous or organic electrolytes. The time-averaged irreversible heat generation was attributed to Joule heating and decreased with increasing temperature due to the associated decrease in internal resistance. Furthermore, the reversible heat generation rates at the positive and negative electrodes were mostly exothermic during charging due to ion adsorption and endothermic during discharging due to ion desorption. They increased slightly with increasing temperature as a result of increasing charge storage. The reversible heat generation rate at the positive electrode was slightly larger than that at the negative electrode. In fact, an endothermic dip was observed in the reversible heat generation rate at the negative electrode at the beginning of charging caused by overscreening effect and/or desolvation of Pyr14+ cations as they enter the activated carbon pores.

Fabrication of supercapacitor using banyan leaves-based activated carbon electrode and formic acid-based polymer electrolyte

In the present studies, activated carbon as an electrode material and polyvinyl- alcohol (PVA) as a polymer. Formic acid (FA) and potassium iodide-based electrolyte system have been synthesized for the fabrication of energy storage devices, supercapacitors. The diluted formic acid (CH2O2) in the double-distilled (DD) water has been optimized so that it exhibits maximum conductivity. The electrolyte system has also been prepared with PVA, redox additive (KI) with optimized formic acid (4.0 M). The redox additive concentration has also been identified that can have maximum conductivity. The selected system (PVA + FA + KI) shows the highest conductivity in the range ∼10−1 Scm−1. The banyan leaves (biomass) based carbon electrode (BAC) synthesized as an electrode material. The electric double layer capacitor (EDLC) has been fabricated using BAC as electrodes and (PVA + FA + KI) as electrolyte material. The result of (BAC) based EDLC cell has been compared with the cell fabricated using commercially available activated carbon (AC). The performance of cells has been determined by impedance spectroscopy (IS), Cyclic voltammetry (CV) and charge–discharge (CD) techniques. The overall capacitance of BAC / (PVA + FA + KI) / BAC and AC / FA (4.0 M) / AC have been found to be ∼356 mFcm−2 and ∼100 mFcm−2.

Synthesis and characterization of modified chitosan membranes for applications in electrochemical capacitor

Chitosan-based membranes were formed by a casting method from a CS (chitosan) acetic acid solution, and then modified. Sodium hydroxide (NaOH) and glutaraldehyde (GA) solutions were used as covalent or ionic crosslinking agents respectively. The physicochemical properties of crosslinked CS membranes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Changes in surface hydrophilicity as a result of the crosslinking process were investigated by means of contact angle measurements. In addition, for both modified and unmodified CS membranes, tensile and electrolyte solution swelling ratio tests were performed. All of the obtained chitosan-based membranes were used in electric double layer capacitor (EDLC) cells to study their applicability as gel electrolytes and membranes. 2 M LiOAc (lithium acetate) water solution was used as an electrolyte in the EDLC cells. Electrochemical characteristics of EDLC cells containing chitosan membranes were determined by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) methods. The results show that the CS crosslinking agent type may improve the mechanical properties and limit electrolyte absorption by the membranes. All tested EDLC cells with CS membranes exhibited very good electrochemical performance. The specific capacitances for EDLC cells with modified chitosan (CS) membranes are equal 106 F g−1 after the 10 000th discharge cycle for both CS/GA and CS/NaOH capacitor cells.

Pinecone-derived porous activated carbon for high performance all-solid-state electrical double layer capacitors fabricated with flexible gel polymer electrolytes

We report a novel configuration of electrical double layer capacitors (EDLCs), fabricated with porous activated carbon electrodes derived from bio-waste pinecone and plastic crystals-based gel polymer electrolytes. Different activated carbons have been produced by activating raw pinecone powder employing different amount of activating agent ZnCl2. Optimization of morphology, structure and porosity parameters of activated carbons has been performed by scanning electron microscopy, x-ray diffraction, Raman and porosity analyses. Mixed (micro- and meso-) porous interiors in activated carbon show optimum characteristics of EDLCs. Gel polymer electrolyte comprising mixture of non-ionic plastic crystal and organic ionic plastic crystal (succinonitrile and 1-ethyl-1-methyl pyrrolidinium bis(trifluoromethyl sulfonyl)imide, respectively) entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) has been found suitable to fabricate EDLCs owing to their flexible nature, high ionic conductivity (σ ∼1.53 × 10−3 S cm−1 at room temperature) and wide enough electrochemical stability window, ESW (∼3.1 V versus Ag). Addition of Li-salt in the gel polymer electrolyte improves the electrochemical properties (σ ∼2.87 × 10−3 S cm−1 and ESW ∼3.8 V) and found to influence the performance of the EDLCs, particularly, the specific power, significantly. The EDLC cell with Li-salt containing electrolyte offers higher values of specific capacitance, energy and power (∼255 F g−1, ∼20 Wh kg−1 and ∼55.7 kW kg−1, respectively) than the cell with electrolyte without Li-salt (∼244 F g−1, ∼19 Wh kg−1 and ∼39.3 kW kg−1, respectively). The EDLC (with Li-salt electrolyte) exhibits 96–100% Coulombic efficiency and almost stable specific capacitance for ∼20,000 charge-discharge cycles after only ∼11% initial fading. The capacitor gives stable performance for a wide temperature range from −30 to 90 °C.

Preparation of polyvinylidene fluoride-co-hexafluoropropylene-based polymer gel electrolyte and its performance evaluation for application in EDLCs

Polymer gel electrolyte (PGE) film is prepared by incorporating polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) as polymer, ethylene carbonate (EC)-propylene carbonate (PC) as plasticizers and tetraethylammonium tetrafluoroborate ( $$\hbox {TEABF}_{4}$$ ) as salt using solution cast technique. By varying weight percentages of EC-PC- $$\hbox {TEABF}_{4}$$ into a host polymer, different samples of PGE were prepared, and the concentration of EC-PC- $$\hbox {TEABF}_{4}$$ was optimized for maximum conductivity, and stability of the prepared film. The maximum ionic conductivity of the order of $$4.9\times 10^{-3}\,\hbox {S cm}^{-1}$$ was determined for 80 wt% of [EC-PC (1:1 v/v)- $$\hbox {TEABF}_{4}$$ (1.0 M)]. From the conductivity as a function of temperature, activation energy $${E}_{\mathrm{a}}$$ was calculated and it is about 0.06 eV. Overall, the amorphous structure was confirmed by X-ray diffraction analysis. Dielectric and impedance spectroscopic analysis was also carried out to understand the electrical behaviour of electrolyte using modified Debye’s function. An ionic character of prepared electrolyte was confirmed from DC polarization method. The ionic transference number of 0.91 was calculated from the data while the electrical potential stability window was found to be 3.8 V. The electrical performance of prepared PGE was examined by fabricating electrical double-layer capacitor (EDLC). In a supercapacitor, commercially procured activated carbon electrodes were employed. The specific capacitance of EDLC cell is found to be $$\sim $$ 60 mF cm $$^{-2}$$ , and equivalent single-electrode capacitance is about 39 F g $$^{-1}$$ .

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.