Electrodes For Electrochemical Capacitors Research Articles

Observation of electrode potential in electrochemical double layer capacitors with variations in temperature, scan rate, and ion size

Owing to different ionic sizes, the ionic transport rates of anions and cations are not identical under the same electric field. However, both ions in the electrodes of electrochemical double layer capacitors (EDLCs) should have the same transport rate in order to maintain the charge neutrality of the entire cell. The faster-moving ion requires a lower electric field than its slower-moving counterion in EDLCs. Anions (BF4−) smaller than quaternary ammonium cations require a lower electric field than cations. The relative ratios of ionic transport resistance between anions (positive electrode, R+) and cations (negative electrode, R−) were obtained with 1.0 M tetraethylammonium BF4 in acetonitrile. The value of R+/R− was 0.037 at 25 °C and increased to 0.41 at 55 °C, as the cationic transport rate improved with temperature. Although the positive electrode exhibited narrower potential ranges than the negative electrode, the electrolysis of electrolytes was more active on the positive electrode than on the negative electrode. The use of large cations and high operation rates was identified as a way to narrow the range of the positive electrode, and thus reduce electrolysis.

Heteroatom doped high porosity carbon nanomaterials as electrodes for energy storage in electrochemical capacitors: A review

Abstract At present it is indispensable to develop and implement new/state-of-the-art carbon nanomaterials as electrodes in electrochemical capacitors, since conventional activated carbon based supercapacitor cells cannot fulfil the growing demand of high energy and power densities of electronic devices of the present era, as a result of the rapid developments in this field. Functionalized carbon nanomaterials symbolize the type of materials with huge potential for their use in energy related applications in general and as an electrode active material for electrochemical capacitors in particular. Nitrogen doping of carbons has shown promising results in the field of energy storage in electrochemical capacitors, gaining attention of researchers to evaluate the performance of new heteroatoms functionalised materials such as sulphur, phosphorus and boron lately. Literature is widely available on nitrogen doped materials research for energy storage applications; however, there has been a limited number of review works on other functional materials beyond nitrogen. This review article thus aims to provide important insights and an up-to-date analysis of the most recent developments, the directions of future research, and the techniques used for the synthesis of these functional materials. A critical review of the electrochemical performance including specific capacitance and energy/power densities is made, when these single doped or co-doped active materials are used as electrodes in electrochemical capacitors.

Niobium pentoxide nanoparticles @ multi-walled carbon nanotubes and activated carbon composite material as electrodes for electrochemical capacitors

We report here on a novel method to produce nanostructured porous carbon composite electrodes decorated with niobium pentoxide nanoparticles. The carbon support is composed of multiwalled carbon nanotubes and activated carbon composite material. Nanotubes work simultaneously as binder and additive for activated carbon supported on the nickel-aluminide current collector. The hydrated niobium pentoxide nanoparticles attached on the carbon surface improved the charge storage process, introducing Faradaic reactions (pseudocapacitance) to the storage mechanism. The pseudocapacitive process involving the niobium pentoxide was stable since the oxide nanoparticles partially covered with a porous carbon layer were not deactivated. This approach improved the electrical conductivity and chemical stability and also avoided reaggregation and deactivation of niobium pentoxide nanoparticles by passivation. The electrochemical performance of the symmetric coin cell using an aqueous lithium sulfate solution was evaluated by cyclic voltammetry, galvanostatic (re)charge/discharge curves, and the electrochemical impedance spectroscopy techniques. In short, our results showed that the composite material has good electrochemical properties, including high specific capacitance (∼220 F g−1), long lifespan (more than 200 thousand cycles), and high energy (0.11–6.5 kW kg−1) and power (3.1–6.1 Wh kg−1) densities for the applied gravimetric currents in the range of 0.5–30 A g−1. We additionally performed in situ Raman analyses (operando studies) using the composite electrode under dynamic potential conditions. We observed reversible shift on the D band position, and the intensity of the Raman signal decreased during cycling due to SO42− adsorption.

N-doped porous carbon film electrodes for electrochemical capacitor, made by electrospray of sol precursors

Heteroatom doping in carbon structure enhances surface wettability, electrical conductivity, and electron-donor/acceptor properties that in turn, improves the electrochemical performance of carbon electrodes. In this article, in situ grown carbon film electrodes are obtained through electrospray of polymeric sol on carbon paper, followed by curing, and carbonization. The nitrogen functionality in these binder-free carbon film electrodes is successfully incorporated from melamine addition in resorcinol-formaldehyde sol. The film electrode shows a capacitance of 223 mF cm−2 (446 F g−1), and volumetric energy density of 15.2 mWh cm−3 at 1 mA cm−2 in a symmetric cell with 2 M KOH as electrolyte. Furthermore, these electrodes exhibit excellent cyclic stability even after 5000 charge-discharge cycles with almost complete capacitance retention at 5 mA cm−2. The outstanding electrochemical performance of carbon film electrodes are due to the combined effect of following steps: (i) incorporation of nitrogen and oxygen in right amount, (ii) retention of the desired pore size distribution, and fraction of micropores through systematic balancing of resorcinol to melamine ratio in precursor sol, (iii) disintegration of sol by electrospray prior to deposition on carbon paper, that in turn produces a porous three-dimensional interconnected network of nanoparticulate film, and (iv) lyophilization of wet gel layer that retains the pore spaces during solvent removal.

Pseudo-capacitance behaviour of reactively sputtered vanadium nitride electrodes deposited at different working pressures: The critical role of surface chemistry

Vanadium nitride (VN) films were deposited at different working pressures over silicon substrates using DC plasma reactive sputtering. These films were tested as electrodes for electrochemical capacitors in a 1 M KOH aqueous electrolyte solution. It was found that the working pressure influences the film surface chemistry and crystalline quality, as probed by X-ray photoelectron spectroscopy and X-ray diffraction, and thus, the pseudo capacitive behaviour of the electrodes. The cycling stability and capacitance values, measured for the different VN electrodes, were discussed according to the change of surface chemistry and morphology of VN deposits. In particular, the effects of oxygen content and different oxide compositions on the electrode surfaces are highlighted. Such results demonstrate the benefit of controlling the working pressure in order to improve the pseudo-capacitive storage properties of VN, and add a piece of information on the role of VN complex surface chemistry to achieve high specific capacitance.

(Invited) Redox Contributions to the Behaviour of Electrochemical Capacitors

The use of electrochemical capacitors is expanding rapidly in the electronics industry due to their excellent performance characteristics, including high specific power and cyclability. However, their specific energy, and hence cost per unit energy, is somewhat limited, thus prompting significant global research to address this shortcoming. Energy storage in these systems is a combination of electrical double layer formation, or charge separation at the electrode-electrolyte interface, and pseudo-capacitance, which is based on facile redox reversible reactions at this interface. One of the key characteristics of these systems is the relative contributions that these processes make to overall charge storage. Much ado has been made about the contributions that redox processes made to pseudo-capacitive electrode performance. Many apparently similar materials such as RuO2, MnO2 and NiOOH contribute charge through redox processes at the electrode surface; however, their performance can be quite different depending on the electrode material itself and the conditions under which it is cycled. The typical response for an electrochemical capacitor electrode is a box-like voltammogram; however, materials such as these have a basis of such behaviour, but may also include distinct redox peaks within the voltammogram. Here we will examine the role that the redox mechanism for these pseudo-capacitive materials has on electrode performance, as well as investigate its impact on electrode properties, in particular crystal size. Outcomes will allow further differentiation of materials used in electrochemical capacitors.

Core-niobium pentoxide carbon-shell nanoparticles decorating multiwalled carbon nanotubes as electrode for electrochemical capacitors

We report here a method to produce nanostructured and porous electrodes by electrospraying niobium pentoxide nanoparticles onto entangled spaghetti-like multiwalled carbon nanotubes. The novel niobium pentoxide and carbon nanotubes composite material exhibits promising electrode properties for the aqueous-based electrochemical capacitors. Monoclinic and orthorhombic niobium pentoxide nanoparticles are partially covered with a thin carbon layer and anchored onto carbon nanotubes. Not only does this electrode material possess the excellent, well-known characteristics associated with carbon nanotubes (large voltage window and chemical stability), but it also has pseudocapacitive properties due to niobium pentoxide nanoparticles. This approach improves the electrical conductivity and chemical stability and also avoids reaggregation and deactivation of niobium pentoxide nanoparticles. These nanostructured carbon nanotubes and niobium pentoxide composite electrodes are characterized by ex-situ techniques, such as scanning and transmission electron microscopy, Raman, X-ray photoelectron spectroscopy, and X-ray diffraction. In-situ studies are performed on cyclic voltammetry, galvanostatic (re)charge/discharge curves, and electrochemical impedance spectroscopy techniques. Our results show that carbon nanotubes and niobium pentoxide composite has outstanding electrochemical properties, including high specific capacitance (~320 F g−1), long lifespan (more than 100,000 charge and discharge cycles), and high energy and power densities.

Synthesis of Diverse Green Carbon Nanomaterials through Fully Utilizing Biomass Carbon Source Assisted by KOH

With multiple properties, green carbon nanomaterials with high specific surface area have become extensively attractive as energy storage devices with environmental-friendly features. The primary synthesis attempts were based on alkalis activation, which, however, faced the dilemma of low utilization rate of carbon sources. Herein, the green carbon with ultrahigh surface area (up to 3560 m2/g) was prepared by the KOH-assisted biomass carbonization. Moreover, the redundant K2O steam and CxHy flow were further utilized; as a result, the carbon materials with a wide range of morphological diversity were collected on the Cu foam. Accordingly, we carried out density functional theory simulations to reveal the mechanism of O-adatom-promoted CH4 dissociation over the Cu surface for carbon formation. The electrodes of electrochemical capacitor fabricated by carbon synthesis possess a 170% higher specific capacitance compared with commercial carbon electrodes. As such, this strategy might be promising in developing hierarchical carbons along with sufficient carbon sources for broadening their potential applications.

Enhancing the performance of electrochemical capacitor electrodes by modifying their carbon nanopores with redox-active materials

Enhancing the performance of electrochemical capacitor electrodes by modifying their carbon nanopores with redox-active materials

Multi-walled carbon nanotubes and activated carbon composite material as electrodes for electrochemical capacitors

We report on novel multi-walled carbon nanotubes (MWCNTs) and activated carbon (AC) composite material as electrode for electrochemical capacitors (ECs). Here MWCNTs work simultaneously as binder and additive for AC electrodes. MWCNTs were synthesised directly onto AC placed on nickel coated aluminium foils binding all of them. During MWCNT synthesis, nickel coated aluminium foils were converted on nickel aluminide alloy, which is extremely stable in neutral aqueous media and also permit welding in the case of large devices manufacturing or association.This novel material brings two significant features to ECs devices: (i) very low equivalent series resistance, and (ii) operational stability in aqueous based electrolytes. That is an excellent opportunity to explore because nonaqueous electrolytes itself and its handling to keep them humidity-free are ones of the most expensive part of ECs manufacturing. Replacing them using a better cost-effective and environmentally friendly electrolyte is desirable. Our approach is simple, fast, very reproducible, and low cost and in the end of the day devices showed large cell voltage of 1.5 V in aqueous electrolyte with specific capacitance of electrode material up to 105 F g−1 (e.g. very similar to several reports on literature using standard binders). Our cyclability test suggested a long service life with no significative lost on capacitance retention after 70 thousand cycles using a voltage range of 1.5 V at 25 A g-1. A detailed electrochemical analysis of device is also presented. On the Ragone plot, our devices fit between Li-ion capacitor and electrochemical capacitors.

Catalytic Carbonization of Acenaphthene for the Preparation of Ordered Mesoporous Carbon CMK-1 toward Application as Electrochemical Double-layer Capacitor Electrode with Ionic Liquid Electrolyte

Ordered mesoporous carbon CMK-1 with partially graphitized domains was prepared from acenaphthene in Ni-loaded MCM-48, which showed an extremely high electric double-layer capacitance of 13.6 µF cm...

Facile preparation of binder–free electrode for electrochemical capacitors based on reduced graphene oxide composite film

Binder-free electrode based on reduced graphene oxide (rGO) is attracting attention for electrochemical capacitors. Here the binder-free electrodes have been fabricated by activating the composite films of GO and carbonized cornstalk-cores. In the composites, the rGO sheets are found to be tightly adhered to the activated bio‑carbon which is intercalated between the rGO sheets to prevent them from stacking. Moreover, the influences of the mass ratios of GO to bio‑carbon on the electrochemical performance have been systematically investigated. The optimal binder-free electrode (rGO/ACCS1/1) prepared from the composite films with 1:1 mass ratio of GO to bio‑carbon exhibits a specific capacitance as high as 278Fg−1 at scan rate of 1mVs−1 and shows excellent stability in 6M KOH electrolyte. Furthermore, the optimal electrode can retain 47.8% of the initial specific capacitance even at current density of 100Ag−1, indicating a fast charge-discharge capability. This work provides a simple method to prepare binder-free composite electrodes by introducing renewable and inexpensive biomass based carbon into activated rGO films.

Monitoring the length change of Ni@C composite electrodes during charging/discharging processes

The iterative volume change of electrode materials during electrochemical charging/discharging processes could generally result in the performance degradation of electrochemical capacitors. This work investigates the mechanical stability of the electrode material during the electrochemical charging/discharging processes, which can be monitored by means of in situ dilatometry technique with a focus on the length change of the electrode. As a simple model, the porous Ni@C composite was prepared with a core-shell structure as the electrode for electrochemical capacitor, which exhibits excellent electrochemical cycling stability in a dilute electrolyte. This stability of Ni@C electrode may benefit from the small length change during the electrochemical cycling. The experimental results exhibit that the expansion and contraction of the electrodes in response to the charge state at the surface of the electrode is weakly asymmetric. This finding of an unbalanced strained state may contribute to a better understanding of the performance degradation of electrochemical capacitors.

Effect of thermally decomposable spacers on graphene microsphere structure and restacking of graphene sheets during electrode fabrication

Graphene has been proposed as a suitable material for electrodes in electrochemical double layer capacitors because of its excellent properties. However, a drawback is the irreversible restacking tendency of graphene during electrode preparation which, consequently, decreases the electrochemically accessible surface area. Therefore, to preserve the electrochemically accessible surface area of graphene, the development of controllable graphene structures with restacking-resistive property is highly desirable. In this work, spherical graphene with high restacking-resistive property during the re-dispersive process are synthesized using ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as a decomposable spacer. During graphene assembly, the IL is not only used as a guide to obtain a spherical morphology, but also as a spacer that prevents restacking of graphene oxide (GO). Through our experiment, it was observed that, during the thermal treatment of spherical GO/IL to reduce GO to reduced graphene oxide (rGO), the IL could not only prevent restacking of GO sheets, but also protect its spherical morphology from the gas explosion generated by the reduction of GO. In particular, spherical rGO/IL has high surface area of 474 m2 g−1 and high restacking-resistive property such that it lost only 5% of its initial surface area after soaking in NMP, which exhibit high specific capacitance.

Expanding the potential window of aqueous electrochemical capacitors with binder-free electrostatically sprayed manganese oxide composite cathode films

In this study, three binder-free manganese oxide-carbon nanotube (MnOx-CNT) composite films were synthesized using electrostatic spray deposition (ESD) and evaluated as electrochemical capacitor (EC) electrodes in aqueous media. The MnOx-CNT 9-1 composition exhibited superior electrochemical performance as compared to both the pristine MnOx and MnOx-CNT 8-2 composite films, exhibiting a high specific capacitance of 281 F g−1 in addition to excellent rate handling and cyclability. In order to investigate the practical viability of the MnOx-CNT 9-1 electrodes, a comprehensive two-electrode analysis was performed using activated carbon (AC) counter electrodes for different mass ratios to obtain optimal energy-power trade-off. The asymmetric capacitor with the AC electrode with a mass 2.6 times that of the MnOx-CNT 9-1 electrode exhibited the best energy-power characteristics reaching a maximal gravimetric energy density of 30.3 Wh kg−1; the excellent electrochemical performance of the asymmetric capacitor is attributed to the wider cell voltage of 2V as well as the high gravimetric capacitance of the ESD- based MnOx-CNT composite cathode.

Hierarchical nanoflake-assembled flower-like NiCo double hydroxide@NiC2O4 microspheres for high-performance supercapacitor

ABSTRACTAssembly of NiCo double hydroxide@NiC2O4 porous microspheres on Ni foam was fabricated by the direct anodisation of nickel foam (NF) and the subsequent electrodeposition of NiCo double hydroxide (NiCo-DH). The obtained Ni foam-supported NiCo-DH@NiC2O4 (NiCo-DH@NiC2O4/NF) was used as the binder-free electrode for electrochemical capacitor (EC). The combination of NiC2O4 and NiCo-DH increases the amount of active sites for redox reactions while the hierarchical porous morphology significantly enhances the diffusion of electrolyte. With Ni foam as 3D conductive substrate and current collector, the electronic conductivity of the electrode material was significantly improved and the diffusion of the electrolyte on the electrode surface was effectively enhanced. NiCo-DH@NiC2O4/NF demonstrates a high areal capacity, a good cycling stability and a good rate capability . In addition, a hybrid supercapacitor (HSC) assembled with NiCo-DH@NiC2O4/NF as the positive electrode and activated carbon (AC) as the negative electrode exhibited good performance with high energy density and power density.

A 3D self-supported coralline-like CuCo2S4@NiCo2S4 core–shell nanostructure composite for high-performance solid-state asymmetrical supercapacitors

Rational construction of three dimensional (3D) composite structure is an important method to flexible supercapacitor electrodes and has been extensively developed. In this work, a 3D self-supported CuCo2S4@NiCo2S4 core–shell nanostructure grown on Nickel (Ni) foam, constructed by a hydrothermal method, was used as a novel supercapacitor electrode material. The unique structure possesses a large, specific surface area, rapid diffusion of electrolyte ions by numerous channels and avoids the use of additives and adhesives. The high electrical conductivity of the CuCo2S4 nanoneedle arrays can speed up electronic transmission. At a current density of 1 A g−1, the electrode material exhibits a high specific capacity of 539.2 C g−1 and cycling stability with 100% capacity retention after 5000 cycles in 3 M KOH. Furthermore, when the obtained CuCo2S4@NiCo2S4 was used as the positive electrode and an activated carbon was used as the negative electrode, a solid-state asymmetric supercapacitor was assembled. More importantly, the obtained solid-state asymmetric supercapacitor demonstrated excellent electrochemical performance. When the power density was 400 W kg−1, it delivered a high density of 23.4 W h kg−1 with a high voltage window of 1.6 V, thus demonstrating that the material has the potential for use as an efficient electrode for electrochemical capacitors. Due to its comprehensive electrochemical performance, the CuCo2S4@NiCo2S4 solid-state asymmetric supercapacitor effectively operated a red LED.

Sulphur-linked graphitic and graphene oxide platelet-based electrodes for electrochemical double layer capacitors

This study presents novel investigations of sulphur-graphitic nanoplatelet (S-GNP) and sulphur-microwave expanded graphene oxide (S-MWGO) composite electrodes for structural electrochemical double layer capacitors (EDLCs) with liquid organic electrolyte 1 M TEABF4 (tetraethylammonium tetrafluoroborate) in propylene carbonate (PC). Elucidating the chemical structure of these electrodes, XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy indicated the presence of CSSC links while mixed EDX (energy dispersive X-ray spectroscopy) elemental maps displayed elemental S outlining the edges of nanoplatelets, concluding the presence of S-links between nanoplatelets. While S-linking improved the mechanical properties and ensured structural integrity of the produced monoliths without the need of any binder, it also decreased the specific surface area of the resulting materials. Furthermore, additional sulphur might have been trapped in other forms, amounting to up to 26 wt% sulphur in the composite graphitic and graphene oxide-based electrodes. Three-point bend testing yielded that an S-GNP-MWCNT monolith with 20 wt% S and 0.24 wt% MWCNT exhibited similar mechanical properties to those of a rigid polyurethane foam. The same S-GNP-MWCNT monolith exhibited an average electrode capacitance of 12.2 F g−1 during discharge at 2.2 mA/cm2. An S-MWGO-MWCNT monolith electrode with 9.6 wt% S, 16.4 wt% carbon black and 0.24 wt% MWCNT exhibited an average electrode capacitance of 64.9 F g−1 during discharge at 2.2 mA/cm2 but higher resistance than the S-GNP electrodes.

Biomass derived chars for energy applications

Biomass-derived chars present energy density values close to those of fossil fuels and therefore they are good candidates in electricity or heat production plants with only minor drawbacks compared to fossil fuels. Even if co-firing seems the most attractive solution for near-term applications, processes based on combustion and gasification (which are competing in dependence of the need of heat or electricity) are receiving renewed attention. Thanks to their high carbon content, and their high specific surface area and developed porous structure, biomass-derived chars can be treated and converted into activated carbons and applied in many different field (as energy storage materials for gaseous fuels, mainly hydrogen and methane, or as electrodes). They can constitute the raw materials for preparing synthetic graphite, which can be used in some types of batteries and fuel cells, and in carbon electrodes for electrochemical capacitors. The performances in terms of capacitance, electrical conductivity, potential, charge and discharge rates, power density, etc. have been reported to be very close to those of commercial devices. The recent progress in the activation protocols brought to higher fuel gas storage capacities, especially in cryogenic conditions and under high pressure, and opened the possibility to apply these materials in new application fields. In catalysis, advances in the use of biomass-derived chars and active carbons have been made thanks to the improvement of the modification techniques. The optimization of the engineering methodologies allows to lower the cost of the activation processes of biomass-derived chars and to tune the char properties to adapt them to the final application. The present paper aims to give a comprehensive survey of already-well-established or future potential energy applications of biomass-derived chars. A critical comparison of their use in different processes is reported and their modification by various catalytic, physical and chemical routes is detailed.

Electrochemical Capacitors Based on Electrodes Made of Lignocellulosic Waste Materials

Activated carbons have been obtained from the following organic precursors (lignocellulosic waste materials): willow, rapeseed straw, hemp shives, flax shives, switch grass, Virginia mallow and giant miscanthus. The activation process has been performed by using potassium hydroxide at 750 °C. Elemental analysis has been performed in order to determine the presence of heteroatoms in the carbon structure. Boehm titration was used to estimate the functional groups of carbon materials. The surface area was characterized using gas adsorption method. All the obtained samples have been used as electrodes for electrochemical capacitors operated in 1 M H2SO4. The electrochemical characterization was carried out using cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. All measurements proved that carbon materials obtained from different lignocellulosic wastes are suitable for energy storage applications. Moreover, the content of oxygen groups has great influence on the charge storage mechanism. Excessive oxygen content causes the disruption of surface conjugation of the carbon material.Graphical