Symmetric Electrochemical Capacitor Research Articles

Supercapacitor with Carbon/MoS2 Composites

Transition metal dichalcogenides (TMDs) with a two-dimensional character are promising electrode materials for an electrochemical capacitor (EC) owing to their unique crystallographic structure, available specific surface area, and large variety of compounds. TMDs combine the capacitive and faradaic contribution in the electrochemical response. However, due to the fact that the TMDs have a strong catalytic effect of promoting hydrogen and oxygen evolution reaction (HER and OER), their usage in aqueous ECs is questioned. Our study shows a hydrothermal l-cysteine–assisted synthesis of two composites based on different carbon materials—multiwalled carbon nanotubes (NTs) and carbon black (Black Pearl-BP2000)—on which MoS2 nanolayers were deposited. The samples were subjected to physicochemical characterization such as X-ray diffraction and Raman spectroscopy which proved that the expected materials were obtained. Scanning electron microscopy coupled with electron dispersive spectroscopy (SEM/EDS) as well as transmission electron microscopy images confirmed vertical position of few-layered MoS2 structures deposited on the carbon supports. The synthetized samples were employed as electrode materials in symmetric ECs, and their electrochemical performance was evaluated and compared to their pure carbon supports. Among the composites, NTs/MoS2 demonstrated the best electrochemical metrics considering the conductivity and capacitance (150 Fg−1), whereas BP2000/MoS2 reached 110 Fg−1 at a current load of 0.2 Ag−1. The composites were also employed in a two-electrode cell equipped with an additional reference electrode to monitor the potential range of both electrodes during voltage extension. It has been shown that the active edge sites of MoS2 catalyze the hydrogen evolution, and this limits the EC operational voltage below 1 V. Additional tests with linear sweep voltammetry allowed to determine the operational working voltage for the cells with all materials. It has been proven that the MoS2/carbon composites possess limited operating voltage, that is, comparable to a pure MoS2 material.

New insight into ion dynamics in nanoporous carbon materials: An application of the step potential electrochemical spectroscopy (SPECS) technique and electrochemical dilatometry

This paper reports on the charging mechanism of an activated carbon electrode in a symmetric electrochemical capacitor operating in an ionic liquid electrolyte (EMIm+TFSI−). With the application of step potential electrochemical spectroscopy (SPECS) and electrochemical dilatometry, it is determined that the electrical double-layer formation mechanism strongly depends on the porous structure of the electrode material. Furthermore, SPECS calculations allow us to separate and quantify the charge component responsible for the ionic movement.

Peculiar role of the electrolyte viscosity in the electrochemical capacitor performance

This paper reports on the electrochemical performance of symmetric electrochemical capacitors operating in an aqueous electrolyte (1 mol L−1 Li2SO4 solution) whose viscosity is modified by various amounts of carboxymethyl cellulose additive.

Fabrication on the Flexible Supercapacitor Based on the Polypyrrole Deposited on Polyethylene/Polypropylene Non-Woven Film

Non-woven films of polypropylene/polyethylene (PP/PE) are usually used as filter membrane or diaphragm because of their excellent mechanical strength and chemical stability. Here, by using PP/PE as substrates, the composite films of FeTS/(PP/PE) are prepared via drop-coating iron p-toluenesulfonate (FeTS) on the PP/PE. Polypyrrole (PPy) is then deposited on PP/PE non-woven films to form the PPy/(PP/PE) composites by reacting the pyrrole vapor with oxidant of FeTS. The structure of PPy/(PP/PE) prepared under different conditions have been characterized in detail. Using H3PO4/polyvinyl alcohol (PVA) as the gel electrolyte, the flexible symmetric electrochemical capacitors (FEC) of PPy/(PP/PE)//PPy (PP/PE) are prepared and the properties of the devices are determined. The results show that the optimized FEC exhibits relatively high areal specific capacitance (246.6 mF cm–2 at 2 mV s–1 scan rate), excellent cyclic stability (capacitance retention is 92.5% after 20 000 cycles), and excellent flexibility (the capacitance retention is 90.0% after 2000 and 81.3% after 3000 repeated bending cycles), which indicate that this kind of FEC may have potential application in flexible electronic device.

Symmetric electrochemical capacitor based on PPy obtained via MnO2 reactive template synthesis

The present work explores the synthesis of sulphate-polypyrrole (PPy/SO42−) using MnO2 as a reactive template. Samples were characterized by different techniques, including SEM, CHNS, XRD, and XPS, which confirmed that MnO2 acted as a reactive template during PPy synthesis. The materials obtained were characterized by cyclic voltammetry and electrochemical impedance spectroscopy techniques to evaluate their electrochemical properties as electrodes for electrochemical capacitors (ECs electrodes). Py50/M50 electrode exhibited the highest specific capacitance value being 1215 mF cm−2 (or 125 F g−1) at 5 mV s−1. According to PEIS analysis, the enhancement of capacitive performance is attributed to the elongated shape morphology of Py50/M50 electrode that improves the ionic diffusion and reduces the charge transfer resistance on its surface. Furthermore, a symmetric ECs assembly using two identical pieces of Py50/M50 electrodes exhibited a maximum areal energy and power density of 13.5 mW h cm −2 and 324 mW cm−2, respectively. These results suggest a new route of synthesis of PPy/SO42− with elongated shape morphology to produce electrodes for ECs with high electrochemical performance.

Eco-friendly and scalable synthesis of micro-/mesoporous carbon sub-microspheres as competitive electrodes for supercapacitors and sodium-ion batteries

Cost-efficiency utilization of carbon sub-microspheres is greatly significant for their versatile energy-related applications. However, eco-friendly synthesis and moderate activation protocols for enhancing their electrochemical performances are still challenging. Herein, we explore a facile two-step methodology, i.e., hydrothermal carbonization and subsequent mild activation, for bottom-up fabrication of KHCO3 activated micro-/mesoporous carbon microspheres (MCMS-K) as competitive electrodes towards electrochemical capacitors (ECs) and sodium-ion batteries (SIBs) by using the soluble starch as a low-cost precursor. Benefitting from its high specific surface area (~1972.8 m2 g−1), abundant micropores (~88.8%), and high graphitization degree, the MCMS-K electrode obtains a specific capacitance of ~369.8 F g−1 (~266.3 F cm−3) at 1 A g−1 in 1 M H2SO4, higher than that in 6 M KOH, thanks to the surface oxygen functional groups involved pseudocapacitance. More remarkably, the MCMS-K based organic symmetrical ECs deliver a high-rate energy density of 34.4 Wh kg−1 at 12.5 kW kg−1, and ultra-long cycling stability. Besides, the MCMS-K anode renders a high-rate capacity of ~78.6 F g−1 at 5.0 A g−1via Na+ surface adsorption and intercalation accompanying with excellent cycling behaviors for SIBs. It strongly highlights the promising applications of our MCMS-K product in ECs and SIBs.

Pseudo-capacitive behavior of multi-walled carbon nanotubes decorated with nickel and manganese (hydr)oxides nanoparticles

We report on the synthesis, characterization of nickel hydroxide and manganese oxide nanoparticles decorating multiwalled carbon nanotubes composite material, and electrochemical performance for energy storage processes in symmetric electrochemical capacitors. A large pseudocapacitive voltage range of 1.7 V is presented using a coin cell containing a 1.0 M Li2SO4 aqueous solution, resulting in a maximum specific capacitance of ~ 420 F g−1 verified at 1.67 A g−1 during the discharge process. The symmetric coin cell was highly stable with a very high coulombic efficiency of ~ 99% even after 70,000 cycles of charge-discharge. The high electrochemical stability of coin cell was attributed to a synergism between the nanostructured carbon support and metal (hydr)oxides nanoparticles, i.e., the transport of electrons and ions across the porous electrode structure enables with a high degree of reversibility for the solid-state redox transitions reactions, which is the main contribution to the whole electrode specific capacitance. Near-surface structural changes of the electrodes were monitored during dynamic polarization by Raman and XRD synchrotron measurements. The presence of the active Ni–O stretching mode was verified during the charge-discharge processes according to the reversible solid-state redox process. The Raman shifts were also correlated with the reversible intercalation-deintercalation processes of Li+-ions into the MnOx host material, as well as reversible adsorption of solvated ions on the surface defects of carbon nanostructures.

Ultra-thick wood biochar monoliths with hierarchically porous structure from cotton rose for electrochemical capacitor electrodes

The inspiration from biomass resources allows an efficient route to produce novel carbon materials for sustainable development of energy storage. The carbon monoliths are prepared by directly pyrolyzing cotton rose wood in an inert atmosphere. Their physicochemical properties and pore structure are controlled by altering the pyrolysis temperature. The obtained wood biochar monoliths (WBMs) inherit the hierarchically porous morphology of pristine wood, with unidirectional honeycomb channels with thin channel wall (0.7 μm) and unique interconnected macro-mesoporous pathway on the channel wall, as well as meso-micropores on the carbon sheets. The ultra-thick 1 mm WBM electrode that is pyrolyzed at 800°C with a specific surface area of 486 m2 g−1 shows the optimal electrochemical performance of 133 F g−1 (27 F cm−3) at 10 mA cm−2, which is higher than other reported wood biochar monoliths. Even the 10 mm thickness electrode retains 86% of the capacitance above. What’s more, the electrode in a symmetric electrochemical capacitor delivers a competitive energy density of 9.4 W h kg−1 and an outstanding stability of 111% after 10,000 cycles. It is demonstrated that cotton rose wood is a promising carbon precursor for the production of ultra-thick electrodes.

Supercapacitors with lithium-ion electrolyte: An experimental study and design of the activated carbon electrodes via modelling and simulations

Electrochemical double layer capacitors (EDLCs) are investigated with activated carbon electrodes and a lithium-ion electrolyte, in anticipation of potential future applications in hybridised battery-supercapacitor devices and lithium ion capacitors. An experimental study of a symmetric electrochemical double layer capacitor (EDLC) with activated carbon (AC) electrodes on aluminium foil current collectors and electrolyte 1 M LiPF6 in EC:EMC 50:50 v/v concludes a stability window to a maximum potential of 3 V, an equivalent in series resistance of 48 Ω for 1 cm2 cell area (including the contact resistance between electrode and current collector) and an average specific electrode capacitance of 50.5 F g−1. Three AC electrode materials are assessed via computer simulations based on a continuum ion and charge transport model with volume-averaged equations, considering the pore size distribution for each electrode material and, depending on pore size, transport of tetrahedral solvated or flat solvated Li+ ions and solvated or desolvated PF6− ions. The computer simulations demonstrate that the best electrode material is an AC coating electrode with a hierarchical pore size distribution measured in the range of 0.5–180 nm and bimodal shape, and specific surface area BET = 808 m2 g−1.

High-performance wearable asymmetric electrochemical capacitors based on composite aramid nonwovens with unique surface architecture

High-performance flexible asymmetric electrochemical capacitors (ANW-AECs) for high energy-density wearable devices have been developed through assembling unique aramid nonwovens (ANWs) with different surface structures, that is, ANWs chemically coated with Ag particles and polypyrrole (ANW/Ag/PPy) act as a positive electrode, while ANWs chemically coated with Ag particles and multi-walled carbon nanotubes (ANW/Ag/CNT) play as a negative electrode. The influences of different compositions of surface modified ANWs on structure and integrated performances of ANW-AECs were intensively studied and compared with those of symmetric electrochemical capacitors (ANW-SECs). The prepared ANW-AECs not only have high energy density, but also process good wearability and cyclability. Their areal, volumetric or gravimetric energy density reaches 2.55 × 10−4 W h cm−2, 3.27 × 10−3 W h cm−3 or 8.61 × 10−3 W h g−1 at a high operating voltage of 1.6 V; their capacitance is still as high as 95.0% or 95.9% when the electrochemical or bending cycle reaches 1000 or 500. These outstanding performances originate from asymmetric design of devices as well as unique chemical and space structures of electrodes, including interactions between different components, high electrical conductivity and specific surface area.

Interfacial aspects induced by saturated aqueous electrolytes in electrochemical capacitor applications

This paper reports on the “water-in-salt” electrolyte concept, applying CsF as an ionic compound in a symmetric electrochemical capacitor. CsF has been selected as a highly soluble inorganic salt that allows the amount of free water in the electrolyte to be significantly reduced. Several activated carbons with various microtextural parameters have been investigated to identify the most suitable electrode material. A wide range of electrolyte concentrations have been investigated in terms of conductivity, pH and viscosity. The electrochemical characteristics have been determined for three electrolyte concentrations and indicate critical changes in the electrolyte physicochemical properties. It seems that ion-solvent interactions, residual water content, electrolyte viscosity and charging/discharging conditions remarkably affect the electrochemical performance of capacitors with elevated concentrations of electrolyte. In addition to the capacitance increase, a positive impact on the self-discharge and charge/discharge efficiency has been observed for higher CsF concentrations.

Electroanalytical characterization of electrochemical capacitor systems

Herein we report on an in-situ electroanalytical technique to investigate electrode-electrolyte behaviour during cycling in a conventional symmetrical electrochemical capacitor system based on an activated carbon (AC) electrodes with 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (ACN) as the electrolyte. The electrochemical cell is based on a Swagelok T-junction, with the perpendicular port being used for a Ag/Ag+ (0.01 M AgNO3 in ACN) reference electrode. Situated between the opposing AC electrodes, between two separators, is a Pt sensing electrode, the potential of which is either monitored or controlled to explore either electrolyte fluxes during cycling, or the formation of soluble species in the electrolyte, respectively. It was deduced that electrolyte ions are strongly associated with the AC electrodes rather than between the electrodes even without an applied potential. Furthermore, aging of the electrochemical system with cycling (cyclic voltammetry) is also explored. Even with identical electrodes it is noted that the individual electrodes behaved differently as a result of anion versus cation association. Finally, at cell voltages approaching 2 V, oxidation of the positive electrode is observed, liberating species into the electrolyte. The implications of these observations have been discussed.

Vertically aligned nanostructured FeOOH@MnO2 core shell electrode with better areal capacitance

Present work focuses on hydrothermal preparation of FeOOH@MnO2 core shell as a vertically aligned 1-D nanostructure and improvement in electrochemical charge storage by utilizing MnO2 nanostructure as an active electrode material and vertically aligned FeOOH nanorods for one dimensional electron transfer paths. A FeOOH@MnO2 core shell thin film electrode gives maximum capacitance of 0.252 Fcm−2 for the applied current density of 1 mAcm−2 and maintains 99.5% capacitance retention for 2000 charge discharge cycles at 5 mAcm−2 that attributes to better capacitive charge storage in the material. Assembled solid state symmetric electrochemical capacitor of FeOOH@MnO2 electrode yields maximum of 0.05 mWhcm−2 energy density with power delivery of 1.5 mWcm−2.

High Voltage Window of the Symmetrical Electrochemical Capacitor Made By Using Dual Redox Additives Enhanced Quasi-Solid like Electrolyte

Recently, researchers have shown an increased interest in redox-active electrolyte because of its high performance and simple fabrication. However, most studies have only focused on aqueous electrolyte system, while suffering from limiting potential window of the electrochemical capacitor(EC) due to water decomposition voltage at 1.23 V. In addition, leakage problem results in safety concern in liquid system. In the light of this, we introduce dual redox additives TEMPO and pBQ into PVDFHFP-BMIMTFSI-EC quasi-solid like polymer electrolyte (QSPE) as dual redox active electrolyte (RQSPE). The TEMPO and pBQ enable rapid charge transfer and highly solubility in ionic liquid, promoting the synergic effect of electric double layer capacitance(EDLC) and redox reaction. The coupling of battery-like reversible redox reaction and supercapacitor-like EDLC leads to provide high energy density and power density at the same time. The electrochemical properties of the fabricated EC involving RQSPE electrolyte and carbon electrodes are compared to those of the EC using QSPE without redox additives. The excellent specific energy of 61.12 Wh kg-1 is exhibited by the EC with RQSPE, which is over three times higher than the one without redox additives. In addition, the specific power density of the EC with RQSPE is up to 3.43 kW kg-1 at 17.10 Wh kg-1. Furthermore, the addition of redox additives can expand the voltage window of the EC to 3 V, while the EC without such additives has less value of 2.5 V. The ionic conductivity of the electrolyte is 8.85 mS cm-1, also indicating that adding redox additives have positive effect to the mobility of electrolyte. Finally, the EC with RQSPE exhibits stable cycling stability over 2000 charge/discharge cycles. Figure 1

Modelling and Optimization of Electrodes Utilization in Symmetric Electrochemical Capacitors for High Energy and Power

Expressions and guidelines for determination of electrode's effective thickness, optimum charging current density and electrode utilization in device of certain electrode's and electrolyte's effective conductivity were developed, and systematically used to study the performance of electrochemical capacitors (ECs). Effective thickness of electrode increases along with increase in effective conductivity of electrolyte and decreases as charging current density is increase. It was seen that every current density applied to device of specific electrode's and electrolyte's effective conductivity has corresponding effective thickness of electrode, and when charged at current density higher than its maximum current density, materials (electrodes) utilization were less than 100%. Also, when device with electrode's thickness higher than the effective thickness was charged at its maximum current density, materials (electrodes) utilization reduced below 100%. Materials utilization decreases along with increase in charging current density and electrode thickness, but increases as effective conductivity of electrode and electrolyte were increase. Therefore, optimum/effective thickness of electrode and optimum current density must be employed in charging device of given electrode's and electrolyte's effective conductivity for maximum materials utilization and performance (with minimum or no potential drop). Optimum current density beyond which energy density decays increases along with increase in electrode's and electrolyte's effective conductivity and decrease in electrode thickness. Use of optimum current density and effective electrode's thickness to maximize energy and power densities is inevitable, because increase in current density results in increase in power density and decrease in energy density. Key words: Electrode thickness; Current density; Effective conductivity; Modelling and simulation; Electrode utilization; and Potential drop.

Nitrogen/sulfur dual-doped sponge-like porous carbon materials derived from pomelo peel synthesized at comparatively low temperatures for superior-performance supercapacitors

Pomelo peel is a popular biomass material that is eco-friendly and widely distributed worldwide. However, a large amount of pomelo peel is directly discarded instead of being utilized. This study synthesizes nitrogen/sulfur dual-doped porous carbon materials using pomelo peel and ammonium persulfate, followed by potassium hydroxide (KOH) activation at 600°C, which is lower than most average activation temperature. The as-obtained products are electrode materials applied in supercapacitors. Results indicate that the as-prepared nitrogen/sulfur dual-doped porous carbon material (NSC-600) has high nitrogen (5.89%) and sulfur (0.46%) contents, and a large surface area (1193m2g−1). As the current density is 0.5 A g−1, the specific capacitance of the NSC-600 material up to 310Fg−1. After 10,000cycles, the capacity retention is approximately 98.8% at a current density of 2 A g−1 in 6M KOH, which indicates outstanding cycling performance. Moreover, the assembled symmetric electrochemical capacitor exhibits a high energy density of 21.4Whkg−1 at a power density of 259.9Wkg−1. Therefore, the nitrogen/sulfur dual-doped porous carbon materials synthesized at comparatively low temperatures exhibit promising applications in superior-performance supercapacitors.

One‐Step Synthesis of Vacancy‐Rich MnO2−x/Reduced Graphene Oxide Composite Film for High Electrochemical Performance

AbstractHigh electrochemical performance is important to achieve practical application of energy storage devices. It has been found recently that it is hard to achieve high capacity and high stability simultaneously using a single electrode material. Thus, constructing composite electrode materials which can combine the advantages of different materials together is an efficient approach to improve the electrochemical properties. In this work, we have built a vacancy‐rich MnO2−x/reduced graphene oxide (rGO) composite film through a facile one‐step route. In the synthesis process, GO nanosheets were reduced into rGO to improve the electronic conductivity, while Mn2+ was oxidized to MnO2 with vacancies. The obtained vacancy‐rich MnO2−x/rGO composite showed a superior electrochemical performance: The specific capacitance reached 675.5 F/g at a current density of 0.5 A/g and the material demonstrated high capacitance retention (96.1 % after 10000 cycles). Besides, the symmetric electrochemical capacitor based on the vacancy‐rich MnO2−x/rGO composite electrode can achieve a high energy density (8.42 mWh/cm3) and power density (3.78 W/cm3) as well as excellent stability.

MXene/Polymer Hybrid Materials for Flexible AC-Filtering Electrochemical Capacitors

Summary Energy storage devices are limited by the trade-off between the transport properties and charge storage ability of materials. Electrolytic capacitors are kinetically fast, operating at kilohertz frequency, but limited by low capacitance. Electrochemical capacitors (ECs) provide high capacitance, yet their sluggish kinetics limit frequency response to a few hertz. Here, we devise strongly interacting, porous MXene/conducting polymer hybrids for large-scale flexible alternating current filtering symmetric ECs with high areal and volumetric capacitances of 0.56 mF cm−2 and 24.2 F cm−3 at 120 Hz, respectively. The high capacitance was maintained up to 1,000 V s−1 and originates from synergy of MXene/polymer hybrids. The operation of tandem ECs that filter a pulsating voltage from 60 to 10,000 Hz is demonstrated with device flexibility and durability over 30,000 cycles. These MXene hybrid-based ECs are expected to bridge the performance gap between high capacitance and the high-frequency response toward the form-factor-free miniature and scalable devices.

Solid State Electrochemical Capacitors

Solid-state electrochemical capacitors were prepared by combining composite electrodes based on a carbon paper/carbon nanotubes assembly coated with a layer of a pseudocapacitive electrode materials such as manganese dioxide or ruthenium dioxide separated by a solid state membrane to support the electrolyte. The composite electrodes were prepared by using a simple slow filtration technique. The electrolytes consist of ion-exchange membranes (such as Nafion or CMX from Neosepta) that are impregnated with an appropriate electrolyte. Acidic and neutral electrolytes were investigated in this work. Symmetrical electrochemical capacitors were prepared with two similar electrodes. Different combinations of electrode and membrane were assembled and characterized electrochemically by cyclic voltammetry and constant current charge/discharge cycling. The effect of cycling rate and temperature (from – 60 to + 60 °C) on the electrochemical performance of the solid-state electrochemical capacitors was evaluated.

Effect of physical activation/surface functional groups on wettability and electrochemical performance of carbon/activated carbon aerogels based electrode materials for electrochemical capacitors

Polymeric carbon/activated carbon aerogels were synthesized through sol-gel polycondensation reaction followed by the carbonization at 800 °C under Argon (Ar) atmosphere and subsequent physical activation under CO2 environment at different temperatures with different degrees of burn-off. Significant increase in BET specific surface area (SSA) from 537 to 1775 m2g−1 and pore volume from 0.24 to 0.94 cm3g−1 was observed after physical activation while the pore size remained constant (around 2 nm). Morphological characterization of the carbon and activated carbons was conducted using X-ray diffraction (XRD) and Raman spectroscopy. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of thermal treatment (surface cleaning) on the chemical composition of carbon samples.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to analyse the capacitive and resistive behaviour of non-activated/activated/and surface cleaned activated carbons employed as electroactive material in a two electrode symmetrical electrochemical capacitor (EC) cell with 6 M KOH solution used as the electrolyte.CV measurements showed improved specific capacitance (SC) of 197 Fg−1 for activated carbon as compared to the SC of 136 Fg−1 when non-activated carbon was used as electroactive material at a scan rate of 5 mVs−1. Reduction in SC from 197 Fg−1 to 163 Fg−1 was witnessed after surface cleaning at elevated temperatures due to the reduction of surface oxygen function groups.The result of EIS measurements showed low internal resistance for all carbon samples indicating that the polymeric carbons possess a highly conductive three dimensional crosslinked structure. Because of their preferred properties such as controlled porosity, exceptionally high specific surface area, high conductivity and desirable capacitive behaviour, these materials have shown potential to be adopted as electrode materials in electrochemical capacitors.