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• Electrochemical activation (EA) is employed to improve the specific capacitance. • Various EA methods and important factors are critically reviewed. • Effects of carbons, electrolytes and binders on the EA behaviour are discussed. • Some future research topics are recommended to improve the EA method. The electrical double layer capacitors (EDLCs) have been commercially available and useful in the daily lives. Nevertheless, conventional EDLCs have reached their limit in terms of cell capacitance and cell voltage. The resultant low energy density urges researchers to improve either the maximum cell voltage or the specific capacitance to obtain a high-energy-density supercapacitor. In a fixed voltage window, the electrochemical activation (EA) method has been demonstrated to effectively enhance the specific capacitance of certain carbons with small surface areas. Progress that has been made since the introduction of this EA method in 1998 will be systematically discussed. The choice of (1) carbon materials with different properties, (2) electrolytes composed of various salts/solvents, and (3) binders that have diverse chemical/mechanical properties, proves influential in determining the resultant capacitive performance after the EA process. Although the EA mechanism has been intensively investigated and described as the irreversible intercalation of ions into graphitic regions in initial charge-discharge steps, the interplay amongst carbon, electrolyte and binder has not been fully understood. Therefore, in this article, a few research topics have been recommended to inspire further research interest in order to improve the EA method for practical applications. [Display omitted] [ABSTRACT FROM AUTHOR]

This study demonstrates the criteria for determining the working potential window of commercial activated carbons (ACs) in the propylene carbonate-based (TEABF 4 /PC) electrolyte for electrical double-layer capacitors (EDLCs). Through a common potential-floating examination, the working potential window of ACs in 1 M TEABF 4 /PC is between − 2.0 and 0.3 V (vs. Ag/AgNO 3 ). However, an EDLC consisting of the above ACs in 1 M TEABF 4 /PC with charge/discharge in this potential window for 1152 cycles shows 27% capacitance loss. The electrochemical impedance spectroscopic (EIS) analyses show the presence of an obvious impedance arc behind the high-frequency semicircle on both positive and negative electrodes after the charge–discharge (CD) cycling. Consequently, a combination of a potential-floating test and EIS analyses precisely determines the working potential window between − 1.9 and 0.2 V, which is a reliable method to efficiently define the working potential window. In this potential range, an EDLC exhibits about 6% capacitance loss during the CD test for 10,000 cycles. The EIS results also reveal an acceptable capacitor behavior after the 10,000-cycle CD test. [ABSTRACT FROM AUTHOR]

The upper limit of the double-layer potential window for the propylene carbonate/carbon black interface is effectively extended from 0.1 to 1 V (vs. Ag/AgNO 3 ), compared with activated carbon. Commercial carbon black with low defects and activated carbon are respectively employed as positive and negative electrode materials to construct an advanced asymmetric supercapacitor with cell voltage reaching 3 V in propylene carbonate. This asymmetric supercapacitor of the double-layer type with a cell voltage of 3 V is very stable in 10,000-cycle charge-discharge tests. [ABSTRACT FROM AUTHOR]

Nanoporous carbons (NPCs) have large specific surface areas, good electrical and thermal conductivity, and both chemical and mechanical stability, which facilitate their use in energy storage device applications. In the present study, highly graphitized NPCs are synthesized by one-step direct carbonization of cobalt-containing zeolitic imidazolate framework-67 (ZIF-67). After chemical etching, the deposited Co content can be completely removed to prepare pure NPCs with high specific surface area, large pore volume, and intrinsic electrical conductivity (high content of sp2-bonded carbons). A detailed electrochemical study is performed using cyclic voltammetry and galvanostatic charge-discharge measurements. Our NPC is very promising for efficient electrodes for high-performance supercapacitor applications. A maximum specific capacitance of 238 F g−1 is observed at a scan rate of 20 mV s−1. This value is very high compared to previous works on carbon-based electric double layer capacitors. [ABSTRACT FROM AUTHOR]

Abstract: This work employs a simple one-step method, anodic composite deposition, to fabricate hydrous RuO2–TiO2 (denoted as RuO2·xH2O–TiO2) composite films from an aqueous bath containing RuCl3·xH2O and TiO2 nanoflowers for the electrochemical capacitor (EC) application. The composition of composites can be simply controlled by varying the concentration of TiO2 nanoflowers of the rutile phase in the deposition solution. The highly porous morphologies of various RuO2·xH2O–TiO2 composites are characterized with a field emission scanning electron microscope (FE-SEM). The pseudocapacitive performances (e.g., electrochemical reversibility, specific capacitance, electrochemical impedance spectroscopic responses) of relatively thick RuO2·xH2O–TiO2 composites (0.9±0.1mgcm−2) are systematically investigated. The high-power capacitive behavior of RuO2·xH2O–TiO2 composites, high utilization of RuO2·xH2O and the simple one-step method for electrode preparation indicate the unique merits of anodic composite deposition. [Copyright &y& Elsevier]

Through hydrogen-bonding between choline chloride (CC) and hydrogen bond donors, the binary systems, liquidized at their eutectic point (deep eutectic solvents, DESs), can be considered as organic salts in the liquid state (a kind of ionic liquids). Such cheap and biocompatible ionic liquids were developed for electrolytes of electrical double-layer capacitors (EDLCs). This work employs glycerol (G), malonic acid (M), and urea (U) to form DESs (GCC, UCC, and MCC). The stable potential window of these DESs is about 2 V while only GCC shows a nearly rectangular CV curve at room temperature at 1 mV/s. The ionic conductivity of GCC is equal to ca. 1.3 and 10.0 mS/cm at 25 and 75 °C, respectively. γ-Butyrolactone (GBL), working as a good solvent to disperse GCC, significantly reduces the viscosity of GCC, increases the ionic conductivity, and enhances the performances of GCC-based EDLCs. This GCC-GBL system shows the promising applicability in EDLCs at 10 mV/s at 25 °C. In this work, viscosity of DESs is found to dominate their performance in EDLCs while the mesoporous structure of reduced graphene oxide (RGO) further reduces the loss of electrolyte-accessible area at higher scan rates, emphasizing the important influence of material pore structure on the capacitive performances. [ABSTRACT FROM AUTHOR]

Abstract: Crystalline tungsten oxide mixtures, WO3–WO3·0.5H2O, prepared by microwave-assisted hydrothermal (MAH) synthesis at 180°C for various periods, show capacitive-like behavior at 200mVs−1 and C S ≈290Fg−1 at 25mVs−1 in 0.5M H2SO4 between −0.6 and 0.2V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75h while WO3·0.5H2O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO3 and WO3·0.5H2O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures ≤400°C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO3–WO3·0.5H2O anode and a RuO2·xH2O cathode, with a potential window of 1.6V is demonstrated in this work, which shows the device energy and power densities of 23.4Wkg−1 and 5.2kWkg−1, respectively. [ABSTRACT FROM AUTHOR]

Abstract: A new type vanadium oxide deposit with a porous, three-dimensional (3-D) network architecture plated at 0.7V (vs. Ag/AgCl) from 25mM VOSO4 with 5mM H2O2 shows capacitive-like behavior at 250mVs−1 and C S ≈167Fg−1 at 25mVs−1 in 3M KCl for pseudocapacitor applications. This work also emphasizes that anodic deposition of vanadium oxide does occur at a potential much more negative to oxygen evolution from aqueous VOSO4 solutions due to the presence of V5+ by adding H2O2. Through the X-ray photoelectron spectroscopic analyses, this oxide deposit, mainly consisting of V5+ with 11mol.% V4+, shows a hydrous nature. The unique power characteristics of this hydrous vanadium oxide are reasonably attributed to its intrinsic porous and crystalline structure produced by means of this novel anodic deposition process at a potential much more negative to the oxygen evolution reaction. [Copyright &y& Elsevier]

Abstract: A two-step hydrothermal process was developed to synthesize hydrous 30RuO2–70SnO2 composites with much better capacitive performances than those fabricated through the normal hydrothermal process, co-annealing method, or modified sol–gel procedure. A very high specific capacitance of RuO2 (C S,Ru), ca. 1150Fg−1, was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150°C for 2h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500mVs−1), demonstrating its excellent power property. From Raman, UV–vis spectroscopic and TEM analyses, the reduction in mean particulate size is clearly found for this two-step oxide composite, attributable to the co-precipitation of (RuδSn1−δ)O2·xH2O onto partially dissolved SnO2·xH2O and the formation of (RuδSn1−δ)O2·xH2O crystallites in the second step. This effect significantly promotes the utilization of RuO2 (i.e., very high C S,Ru). The excellent capacitive performances, very similar to that of RuO2·xH2O, suggest the deposition of RuO2-enriched (RuδSn1−δ)O2·xH2O onto SnO2·xH2O seeds as well as the individual formation of (RuδSn1−δ)O2·xH2O crystallites in the second hydrothermal step. [Copyright &y& Elsevier]

The utilization of electrode materials could be greatly promoted by the employment of a mesh-type stainless steel (SS) current collector in comparison with a plate-type one. The capacitive behavior of activated carbon (AC) and AC–ruthenium (AC–Ru) composites fabricated by the wet impregnation were systematically investigated in 0.1 M H2SO4 by cyclic voltammetry (CV) and chronopotentiometry (CP). The specific capacitance of AC reached a maximum (ca. 117 F g−1) through a combination of the impregnation of ruthenium precursors and the chemical conversion of the chloride precursors into hydroxides in an ultrasonic bath of NaOH for 30 min (AC–Ru(OH)x). The stability of AC–Ru(OH)x composites was greatly promoted by annealing in air at 240 °C although a significant loss in total capacitance was found. The crystalline information and chemical environment of the impregnated oxy-hydroxyl-Ru species with annealing was, respectively, obtained from the X-ray diffraction (XRD) patterns and X-ray photoelectron spectra (XPS). The specific surface area and pore size distribution of these composites were analyzed by the Brunauer–Emmett–Teller (BET) method. [Copyright &y& Elsevier]

The capacitive characteristics of activated carbon fabrics (ACFs) coated on the graphite substrates were systematically investigated by means of cyclic voltammetry and the galvanostatic charge–discharge technique. Effects of the PVDF contents in the electronically conductive binder, electrochemical pretreatments, and the electrolytes on the capacitive performance of ACFs were compared in aqueous media. These ACF-pasted electrodes showed the more ideally capacitive responses in 1 M NaNO3 with a specific capacitance of 76 F g−1 when the electronically conductive binder contained 40 wt.% PVDF. The specific capacitance of ACF-pasted electrodes reached a maximum in 0.5 M H2SO4 (about 153 F g−1 measured at 25 mV s−1), due to the presence of a suitable density of oxygen-containing functional groups, when they were subjected to the potentiostatic polarization at 1.8 V (versus reversible hydrogen electrode (RHE)) or potentio-dynamic polarization between 1.3 and 1.8 V in NaNO3 for 20 min. The oxygen-containing functional groups within the electrochemically pretreated ACFs were identified by means of X-ray photoelectron spectroscopy (XPS). [Copyright &y& Elsevier]

The high specific capacitance of ruthenium oxide (denoted as RuOx) nanoparticles prepared by a modified sol–gel method with annealing in air for supercapacitors was demonstrated in this work. The specific capacitance of activated carbon (denoted as AC) measured at 5 mA/cm2 is significantly increased from 26.8 to 38.7 F/g by the adsorption of RuOx nanoparticles with ultrasonic weltering in 1 M NaOH for 30 min. This method is a promising tool in improving the performance of carbon-based double-layer capacitors. The total specific capacitance of a composite composed of 90 wt.% AC and 10 wt.% RuOx measured at 25 mV/s is about 62.8 F/g, which is increased up to ca. 111.7 F/g when RuOx has been previously annealed in air at 200 °C for 2 h. The specific capacitance of RuOx nanoparticles was promoted from 470 to 980 F/g by annealing in air at 200 °C for 2 h. The nanostructure of RuOx was examined from the transmission electron microscopic (TEM) morphology. [Copyright &y& Elsevier]

The optimal deposition conditions for plating the amorphous manganese oxide (denoted as a-MnOx·nH2O) with a maximum specific capacitance were approached by using the experimental strategies, including the fractional factorial design (FFD), the path of steepest ascent and the central composite design (CCD) coupled with the response surface methodology (RSM). The anodic deposition variables such as current density of deposition, pH, MnSO4 concentration and temperature of the plating bath were found to be the key factors influencing the specific capacitance of a-MnOx·nH2O in the FFD study. These variables were subjected to the path of the steepest ascent study to approach the vicinity of optimal conditions for plating the a-MnOx·nH2O deposit with a maximum specific capacitance. Effects of the current density and the MnSO4 concentration were further examined using a regression model in the CCD study. This model, represented as response surface contour plots, showed that a-MnOx·nH2O deposited under pH of 5.6, a current density of 3.7 mA cm−2, a passed charge of 0.3 C cm−2, a MnSO4 concentration of 0.16 M and a deposition temperature of 24 °C exhibited the highest specific capacitance of ca. 220 F g−1. [Copyright &y& Elsevier]

Abstract: This study discusses and demonstrates how the open-circuit potential and charges stored in the working potential window on positive and negative electrodes affect the cell voltage of carbon-based electrical double-layer capacitors (EDLCs) in aqueous electrolytes. An EDLC consisting of two activated carbon electrodes is employed as the model system for identifying these key parameters although the potential window of water decomposition can be simply determined by voltammetric methods. First, the capacitive performances of an EDLC with the same charge on positive and negative electrodes are evaluated by cyclic voltammetric, charge–discharge, electrochemical impedance spectroscopic (EIS) analyses, and inductance–capacitance–resistance meter (LCR meter). The principles for obtaining the highest acceptable cell voltage of such symmetric ECs with excellent reversibility and capacitor-like behaviour are proposed. Aqueous charge-balanced EDLCs can be operated as high as 2.0 V with high energy efficiency (about 90%) and only 4% capacitance loss after the 600-cycle stability checking. The necessity of charge balance (but not capacitance balance) for positive and negative electrodes is substantiated from the lower acceptable cell voltage of charge-unbalanced EDLCs. [Copyright &y& Elsevier]

Electrochemical pre-lithiation stands as a practical and widely adopted technique for precisely controlling the degree of pre-lithiation, but the process is usually time-consuming. Herein, we introduce a novel three-stage electrochemical pre-lithiation method, comprising an initial stage of a high current density, a second stage of a low current density, and an additional stage of constant voltage to ensure the complete formation of solid electrolyte interphase (SEI) on the amorphous carbons: soft carbon (SC) and hard carbon (HC). Lithium-ion capacitors (LICs) are configured with the pre-lithiated SC or HC as the negative electrode and activated carbon as the positive electrode to assess the efficacy and adaptability of this three-stage pre-lithiation approach. Our findings demonstrate that this method can reduce the pre-lithiation time from 1114 to 604 min for SC, and from 1913 to 1080 min for HC, achieving nearly 46 % and 44 % decreases, respectively. Furthermore, the three-stage pre-lithiation method can simultaneously increase reversible Li-ion storage capacity within the same operating potential window, thereby enhancing the electrochemical performance of the LIC cells. Specifically, the LIC cell employing the three-stage pre-lithiated SC exhibits an energy density of 96.5 Wh kg-1 at 0.15 kW kg-1 and 76.5 Wh kg-1 at 5.98 kW kg-1. These cells demonstrate remarkable reversibility, retaining 89.6 % charge capacity after 10,000 cycles. Similarly, the LIC with the three-stage pre-lithiated HC achieves even higher energy densities of 105.3 Wh kg-1 at 0.15 kW kg-1 and 86.42 Wh kg-1 at 5.98 kW kg-1 with a retention of 85.2 % after 5000 cycles. The successful implementation and validation of this three-stage pre-lithiation method pave the way for further development of LICs. [ABSTRACT FROM AUTHOR]

Poly(vinylidene fluoride) (PVdF), carboxymethyl cellulose (CMC), and half-sodiated poly(acrylic acid) (PAANa) are typically selected to examine the influences of polymeric binders on the charge storage performances of expanded mesocarbon microbeads (EMCMB) as the positive electrode in the lithium-ion capacitors (LICs). The surface composition of various EMCMB-coated electrodes before and after electrochemical activation are examined by X-ray photoelectron spectroscopy (XPS). The enhanced and reversible capacitance of EMCMB electrodes after electrochemical activation is examined by various electrochemical tests. In comparison with the electrode using PVdF, electrochemically activated EMCMB electrodes with the aqueous binders show a lower onset potential of electrochemical activation and a lower contact resistance. Both aqueous binders can increase the micro-porosity and specific capacitance of EMCMBs, confirmed by the electrochemical impedance spectroscopic (EIS) test although the electrode using PVdF shows better capacitance retention in the 1000-cycle test. • EMCMB could be electrochemically activated in LiPF 6 /EC:DMC electrolyte. • The performance of EMCMB would be adjusted by binder types. • The schemes showing the influences of binders and electrode potentials are provided. [ABSTRACT FROM AUTHOR]

Abstract: Mesoporous binary nickel–cobalt (Ni–Co) oxy-hydroxides have been obtained through the microwave-assisted hydrothermal annealing (MAHA) method. The porosity control of the nanostructured Ni–Co oxy-hydroxide nanoparticles is achieved through adding the pluronic triblock copolymer F127 as the surfactant. The structural and electrochemical properties of porous Ni–Co oxy-hydroxide nanostructures are characterized by means of X-ray diffraction (XRD), Brumauer–Emmett–Teller (BET) analysis, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and cyclic voltammetry (CV). The electrochemical measurement demonstrates that the Ni–Co oxy-hydroxides calcined at 200°C are able to deliver a specific capacitance of 636Fg−1 in 1M NaOH, suggesting their high potential as a novel electrode material of pseudocapacitors with good electrochemical reversibility. In the asymmetric supercapacitor test, the positive electrode is Ni–Co oxy-hydroxide and negative electrode is activated carbon. The specific energy and power, measured at 2Ag−1, for this asymmetric combination are equal to ca. 17Whkg−1 and 1.6kWkg−1, respectively. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Functionalised polyimide (PI) separators, with high electrolyte wettability and good thermo-dimensional stability, in combination with electrolytes provide ionic conductivities > 1 mS for electrical double-layer capacitors (EDLCs) and lithium-ion batteries (LIBs). The mechanical strength of PI separators can be further improved by a factor of 3 via the optimisation of ratio of soft and hard segments, and the introduction of 5% SiO 2 nanoparticles (NPs). The electrolyte uptake capability and ionic conductivity of this copolymerized PI separator is also promoted by the addition of SiO 2 NPs, favouring the high-rate performances of EDLCs and LIBs. The cyclic voltammetry (CV) curves of an EDLC with this separator show the rectangle shape of a typical capacitor, indicating the low ionic resistance of this copolymerized PI separator. For the usage in the LIBs, on the other hand, a coating of low-density polyethylene (LDPE) onto this PI-SiO 2 (PI–SiO 2 @PE) separator enables the thermal shutdown function of a LIB without sacrificing cell performance. There is no significant change in morphology for this PI-SiO 2 @PE separator after the 50-cycle full cell test, indicating its promising application potential in LIBs. The study has demonstrated the ability to tailor the multi functionality of the polyimide separator to optimise it properties for specific applications. Image 1 • Electrospun PI-copolymers significantly improve mechanical strength. • Cells used Co-PI-5/5-SiO 2 separators have better electrochemical performance. • Thermal shutdown function is added by a LDPE coating. • Co-PI-5/5-SiO 2 @PE separators have good rate capability and cycle stability. [ABSTRACT FROM AUTHOR]