Electrodes For Electrochemical Capacitors Research Articles

Effect of Thermal Activation on the Structure and Electrochemical Properties of Carbon Material Obtained from Walnut Shells

A simple activation method has been used to obtain porous carbon material from walnut shells. The effect of the activation duration at 400 °C in an atmosphere with limited air access on the structural, morphological, and electrochemical properties of the porous carbon material obtained from walnut shells has been studied. Moreover, the structure and morphology of the original and activated carbon samples have been characterized by SAXS, low-temperature adsorption porosimetry, SEM, and Raman spectroscopy. Therefore, the results indicate that increasing the duration of activation at a constant temperature results in a reduction in the thickness values of interplanar spacing (d002) in a range of 0.38–0.36 nm and lateral dimensions of the graphite crystallite from 3.79 to 2.52 nm. It has been demonstrated that thermal activation allows for an approximate doubling of the specific SBET surface area of the original carbon material and contributes to the development of its mesoporous structure, with a relative mesopore content of approximately 75–78% and an average pore diameter of about 5 nm. The fractal dimension of the obtained carbon materials was calculated using the Frenkel–Halsey–Hill method; it shows that its values for thermally activated samples (2.52, 2.69) are significantly higher than for the original sample (2.17). Thus, the porous carbon materials obtained were used to fabricate electrodes for electrochemical capacitors. Electrochemical investigations of these cells in a 6 M KOH aqueous electrolyte were conducted by cyclic voltammetry, galvanostatic charge/discharge, and impedance spectroscopy. Consequently, it was established that the carbon material activated at 400 °C for 2 h exhibits a specific capacity of approximately 110–130 F/g at a discharge current density ranging from 4 to 100 mA/g.

Utilizing Activated Carbon Nanopores for Electrochemical Oxidation of Perylene to Redox-Active 3,10-Perylenedione: Application in Electrochemical Capacitor Electrodes.

We demonstrate that nanopores of activated carbon (AC) function as nanoreactors that oxidize perylene (PER) to a redox-active organic compound, 3,10-perylenedione (PERD), without any metal catalysts or organic solvents. PER is first adsorbed on AC in the gas phase, and the PER-adsorbed AC is subjected to electrochemical oxidation in aqueous H2SO4 as the electrolyte. Because gas-phase adsorption is solvent-free, PER is completely adsorbed on AC as long as the amount of PER does not exceed the saturated adsorption capacity of the AC, which enables accurate control of the amount adsorbed. PER is electrochemically oxidized to PERD in the nanopores of AC at above 0.7 V vs Ag/AgCl. The hybridized PERD undergoes a rapid reversible two-electron redox reaction in the nanopores owing to the large contact interface between the conductive carbon pore surfaces and PERD. The resulting AC/PERD hybrids serve as electrodes for electrochemical capacitors, utilizing the rapid redox reaction of PERD. The hybridization method is advantageous for quantitatively optimizing electrochemical capacitor performance by adjusting the amount of adsorbed PER. Moreover, because PERD hybridization in the AC nanopores does not expand the electrode volume, the volumetric capacitance increases with increasing hybridized PERD content. In three-electrode cell measurements, the volumetric capacitance at 0.05 A g-1 reaches 299 F cm-3, and 61% of this capacitance is retained at 10 A g-1 when 5 mmol of PER is used per gram of AC. Meanwhile, pristine AC delivers 117 F cm-3 at 0.05 A g-1 with a capacitance retention of 46% at 10 A g-1. Two-electrode cell measurements reveal that self-discharge is significantly suppressed by the hybridized PERD when AC/PERD hybrids and AC are used as cathodes and anodes, respectively, compared to that of a symmetrical AC cell. Moreover, PERD does not undergo cross-diffusion in the asymmetrical cells during self-discharge tests for 24 h.

Effect of Aggregation Structure on Capacitive Energy Storage in Conducting Polymer Films.

Conducting polymers (CPs), a significant class of electrochemical capacitor electrode materials, exhibit exceptional capacitive energy storage performance in aqueous electrolytes. Current research primarily concentrates on enhancing the electrical conductivity and capacitive performance of CPs via molecular design and structural control. However, the absence of a comprehensive understanding of the impact of molecular chain spatial order on ion/electron transport and capacitive performance impedes the development and optimization of advanced electrode materials. Here, a solvent treatment strategy is employed to modulate the molecular chain spatial order of PEDOT:PSS films. The results of electrochemical performance tests and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) show that PEDOT:PSS films with both face-on and edge-on orientations exhibit exceptional electronic conductivity and ion diffusion efficiency, with capacitive performance 1.33 times higher than that of PEDOT:PSS films with only edge-on orientation. Consequently, molecular chain orientations conducive to charge transport not only enhance inter-chain coupling, but also effectively reduce ion transport resistance, enabling efficient capacitive energy storage. This research provides novel insights for the design and development of higher performance CPs-based electrode materials.

A Highly Robust and Conducting Ultramicroporous 3D Fe(II)-Based Metal-Organic Framework for Efficient Energy Storage.

Exploitation of metal-organic framework (MOF) materials as active electrodes for energy storage or conversion is reasonably challenging owing to their poor robustness against various acidic/basic conditions and conventionally low electric conductivity. Keeping this in perspective, herein, a 3D ultramicroporous triazolate Fe-MOF (abbreviated as Fe-MET) is judiciously employed using cheap and commercially available starting materials. Fe-MET possesses ultra-stability against various chemical environments (pH-1 to pH-14 with varied organic solvents) and is highly electrically conductive (σ=0.19Sm-1) in one fell swoop. By taking advantage of the properties mentioned above, Fe-MET electrodes give prominence to electrochemical capacitor (EC) performance by delivering an astounding gravimetric (304Fg-1) and areal (181mFcm-2) capacitance at 0.5Ag-1 current density with exceptionally high cycling stability. Implementation of Fe-MET as an exclusive (by not using any conductive additives) EC electrode in solid-state energy storage devices outperforms most of the reported MOF-based EC materials and even surpasses certain porous carbon and graphene materials, showcasing superior capabilities and great promise compared to various other alternatives as energy storage materials.

Enhancing electrochemical capacitor performance of N-doped tannin-derived carbons by hydrothermal treatment in ammonia

This study demonstrates that ammonia concentration when doping carbons using hydrothermal treatment has crucial impact on textural properties. The latter translates into an improvement of the electrochemical performance of carbon materials, when used into electrochemical capacitors, especially at high-rate performance. Nitrogen-doped carbons were synthesized by subjecting tannin to hydrothermal carbonization in ammonia solutions of varying concentrations. After carbonization and CO2 activation, the as-produced activated carbons (ACs) were tested as electrodes for electrochemical capacitors in symmetric configuration using 1 M H2SO4 as aqueous electrolyte. Interestingly, ammonia concentration during the hydrothermal synthesis step did not significantly affect the final N content (ca. 3 and 4 at. %) nor the nitrogen and oxygen functionalities on the surface. However, the use of ammonia had crucial impact on the textural properties developed by CO2 activation, and therefore on the electrochemical performance of the ACs. The best-performing N-doped AC showed specific electrode capacitance values, based on carbon material, of 212 F g−1 at 0.5 A g−1 and outstanding capacitance retention of ca. 71 % at 40 A g−1. It also showed high cycling stability, with capacitance retention of ca. 96 % after 30,000 cycles. Furthermore, this AC outperformed similar reported materials, achieving a specific energy of 4.6 W h kg−1 at 12.1 kW kg−1.

Graphene Nanocomposite Materials for Supercapacitor Electrodes

Graphene and related materials (graphene oxide, reduced graphene oxide) as a subclass of carbon materials and their composites have been examined in various functions as materials in supercapacitor electrodes. They have been suggested as active masses for electrodes in electrochemical double-layer capacitors, tested as conducting additives for redox-active materials showing only poor electronic conductivity, and their use as a coating of active materials for corrosion and dissolution protection has been suggested. They have also been examined as a corrosion-protection coating of metallic current collectors; paper-like materials prepared from them have been proposed as mechanical support and as a current collector of supercapacitor electrodes. This entry provides an overview with representative examples. It outlines advantages, challenges, and future directions.

Synthesis and performance of binder-free porous carbon electrodes in electrochemical capacitors.

Porous binder-free carbon electrodes were obtained by impregnating cellulose filter papers with phenolic resin through soft-salt template synthesis and thermal pyrolysis. These self-standing electrodes were used directly in a supercapacitor device. To understand the impacts of filter paper (FP) thickness on carbon filter paper (CFP) morphology, porosity, and surface functionalities, five different materials were examined. The CFP electrode thickness was adjusted linearly with the FP thickness to produce electrodes ranging from 100 to ∼800 μm. As the thickness of the CFP increased, there was an increase in the specific surface area and oxygen-based functionalities. Electrochemical testing in a 1 M KOH aqueous electrolyte demonstrated that electrode thickness played a key role in electrochemical capacitor (EC) performance, i.e., capacitance enhances linearly when the electrode becomes thinner. For low thicknesses (<280 μm), the capacitance and rate capability decreased slightly with increasing thickness; for high thicknesses, the performance drastically degraded despite the high specific surface area and oxygen surface functionalities. This effect was amplified at high regimes, indicating that high electrode thicknesses and fiber diameters limited electrolyte diffusion in the applied synthesis conditions. Thus, the high material porosity was inaccessible to ion adsorption. Consequently, thinner electrodes showed the highest capacitance (197 F g-1 at 0.1 A g-1) and rate capability (81%) values, exceeding those of their traditional binder-electrode counterparts prepared under similar conditions. Finally, for alkali metal hydroxide solutions, 1 M KOH exhibited a cation match with the salt template (KCl), while CsOH achieved additional capacitance retention (88% at 10 A g-1). Complex ion adsorption mechanisms were observed through a quartz crystal microbalance.

Extraordinary Capacitance and Stability of Carbon Electrode for Electrochemical Capacitors

Extraordinary Capacitance and Stability of Carbon Electrode for Electrochemical Capacitors

Step Potential Electrochemical Spectroscopy (SPECS) Technique to Investigate the Ageing of Carbon Electrodes in Organic-Based Electrochemical Capacitors

Current research on electrode degradation in electrochemical capacitors (ECs) mainly focuses on the application of new materials and the establishment of their stability on the basis of various standards and qualitative tests. However, there exists limited research that focuses on the elucidation and determination of the degradation mechanisms and pathways of carbon electrodes. The proposed study uses an innovative approach with the use of the Step Potential Electrochemical Spectroscopy (SPECS) technique, which allows one to quantify the contribution of different charge-storing mechanisms, and successfully track any changes that take place after accelerated ageing, including structural changes of the electrode in the organic medium. Additionally, the role of oxygen content on the degradation of ECs was explored.In this report carbon material, namely binder-free Activated Carbon Cloth (ACC) Kynol® 507-20 was prepared in a furnace under a nitrogen atmosphere at different temperatures, namely 120, 500, and 1000ᵒC. This is so that the effect of the presence of different oxygen functional groups on the surface of carbon could be investigated. The material was selected as a self-standing electrode to eliminate any influence the binder would have on the electrode porosity. Subsequently, these materials were prepared as electrode materials and assembled in a symmetric two-electrode setup with 1M TEABF4/AN (tetraethylammonium tetrafluoroborate in Acetonitrile solvent). In each cell, the potential range of both electrodes was first determined in a 2-electrode Swagelok® cell with RE using galvanostatic charge/discharge (1 A g-1). The further procedure involved 3 CV cycles (1 mV s-1) at a predetermined potential for each electrode in a 3-electrode set-up, followed by SPECS measurement where 5-minute potentiostatic hold with 10 mV potential steps was applied. The obtained current vs. time plots were used to calculate the corresponding capacitance, diffusion and residual parameters. Floating tests at 2.7 V were then carried out for each system. According to the international standard (IEC 62391-1), system failure is reported when the initial capacitance drops below 80% of its initial value; thus floating experiments were halted after that value was reached for each system or when the resistance of the cell doubled in value. The electrochemical investigation of the individual aged electrodes was then repeated in the same manner.Our study allowed for a thorough examination of individual electrodes prior to and after floating experiments, which helped to elucidate the degradation causes and pathways in organic medium.

Hierarchical Porous Carbon Aerogel Derived from Sodium Alginate for High Performance Electrochemical Capacitor Electrode

To improve the performance of electrochemical capacitors, there is a notable focus on carbon materials characterized by a large surface area, reasonable pore size, pore size distribution, appropriate electronic conductivity, and excellent chemical durability. Herein, the hierarchical porous carbon aerogel originating from sodium alginate (SA) with well-defined porosity are proposed. The resultant hierarchical porous carbon aerogel shows a substantial specific surface area of 2050.6 m2 g−1 with macropores, mesopores and micropores confirmed by techniques such as TEM, SEM, BET, etc., resulting from a sequence of aerogel formation-carbonization-activation. By electrochemical measurement, the hierarchical porous carbon aerogel exhibits a specific capacitance of 204 F g−1 at an operating current density of 0.2 A g−1 employing 6 M KOH aqueous solution. The hierarchical carbon aerogel displays outstanding cycling stability with a 96.2% capacity maintenance for 10,000 cycles at an operating current density of 1 A g−1. This study presents a viable method for for preparing hierarchical porous carbon aerogel derived from biopolymer for electrochemical capacitors.

Analysis of voltametric data from electrochemical capacitor electrode materials: Method refinement for improved outcomes

A revised model is proposed to improve analysis outcomes from the voltametric analysis of electrochemical capacitor materials and electrodes at different sweep rates (v). Conventional analysis involves breaking down the voltametric current (i) into contributions from capacitive processes (i∝v) and diffusional processes (i∝v1/2). The proposed model introduces a resistance or residual current term, improving the goodness of fit and the quality of data interpretation. This is demonstrated by application of the revised model to artificial voltametric data, electrical double layer capacitor data, as well as pseudo-capacitive electrode data. Analysis with this model also demonstrates the importance of understanding resistive processes in electrode systems, in particular differentiating resistance in the act of charge storage (double layer and redox processes), and ohmic electrode resistance.

Facile hydrothermal synthesis and fabrication of nickel cobalt oxide as an advanced electrode for electrochemical capacitors

The surface chemistry and decent morphology of electrode materials are highly desirable for high-performance energy storage/conversion devices. Bimetallic oxides especially nickel cobalt oxide (NCO) have gained substantial attention in energy storage/conversion devices owing to their exclusive electronic properties. Herein, flower-like nickel cobalt oxide (NiCo2O4) was prepared by a facile hydrothermal method for different reaction times. The properties of obtained samples were characterized through X-ray diffraction (XRD), EDS, field-emision scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM). The cyclic voltammogram (CV), Galvanostatic changing discharging (GCD), and electrochemical impedance spectroscopy (EIS) were employed for the evaluation of the electrochemical performances of the resultant product. The flower-like NiCo2O4 electrode exhibits high areal capacity (392 mF cm−2 at 0.5 mA cm−2) and the electrode shows an energy density of about 9.1 µWh cm−2 at power density 102 µW cm−2. The excellent electrochemical performance of the fabricated supercapacitor offers good areal capacitance and energy density that exceeds several carbon-based materials.

Binder mediated enhanced electrochemical capacitance in sodium silicate-graphite paint coated filter paper electrodes for all-solid-state symmetric electrochemical capacitors

In this study, we have prepared a conductive paint by mixing graphite powder, fumed SiO2 particles, NaOH and water in a suitable ratio. The paint was coated on the surface of low-cost laboratory filter paper to prepare paper-based electrodes for electrochemical capacitors. Electrochemical parameters such as specific capacitance, specific energy and specific power of the electrodes were determined for 1 M H2SO4, H3PO4 and KOH electrolytes using 3-electrode electrochemical cells. All-solid-state symmetric electrochemical capacitors were constructed by using three different gel-electrolytes: PVA-H2SO4, PVA-H3PO4 and PVA-KOH. The areal capacitance values obtained for all-solid-state symmetric electrochemical capacitors with PVA-H2SO4, PVA-H3PO4 and PVA-KOH gel-electrolytes were 123.8, 94.8 and 74.7 mF cm−2 respectively. In presence of acidic electrolyte, there is significant pseudocapacitance contribution from the electrode due to presence of binder. Thus, the value of areal capacitance and hence the energy density of electrochemical capacitor with PVA-H2SO4 is highest followed by PVA-H3PO4 and lowest with PVA-KOH gel-electrolyte. It is interesting to note that in acidic electrolyte, although there is enhancement of areal capacitance, it is associated with significant reduction of cyclic stability due to the structural change of the binder. Thus, electrochemical capacitor with PVA-KOH gel-electrolyte has shown highest cyclic stability.

High utilization efficiency of tetramethylbenzoquinone hybridized in the pores of activated carbon for high-performance electrochemical capacitor electrodes

High utilization efficiency of tetramethylbenzoquinone hybridized in the pores of activated carbon for high-performance electrochemical capacitor electrodes

Gold coated vertically aligned carbon nanotubes as electrode for electrochemical capacitors

In this work, gold (Au) films were deposited onto vertically aligned carbon nanotubes (CNTs) by Dc magnetron sputtering method for use as super-capacitor electrodes. By so doing, we synthesize a porous Au deposit anchored on the CNTs with a high apparent surface area. The structure and microstructure studies revealed that the Au@CNTs have a porous nanostructure morphology. The porous Au/CNTs electrodes exhibit an a good areal capacitance compared to state of art Au based electrode (25.6 mF cm-2 at 5 mV.s-1) and excellent cycling stability (90% retention after 10.000 cycles) in 0.5 M H2SO4 aqueous solution. These results demonstrate the beneficial use of CNTs to fabricate a porous Au deposit with improved electrochemical energy storage. In addition, these porous structures can also be used as templates for another type of materials for electrochemical energy storage.

Ambiguous Role of Cations in the Long-Term Performance of Electrochemical Capacitors with Aqueous Electrolytes

A comprehensive comparison of electrochemical capacitors (ECs) with various aqueous alkali metal sulfate solutions (Li2SO4, Na2SO4, Rb2SO4, and Cs2SO4) is reported. The EC with a less conductive 1 mol L-1 Li2SO4 solution demonstrates the best long-term performance (214 h floating test) compared to the EC with a highly conductive 1 mol L-1 Cs2SO4 solution (200 h). Both the positive and negative EC electrodes are affected by extensive oxidation and hydrogen electrosorption, respectively, during the aging process, as proven by the SBET fade. Interestingly, carbonate formation is observed as a minor cause of aging. Two strategies for optimizing sulfate-based ECs are proposed. In the first approach, Li2SO4 solutions with the pH adjusted to 3, 7, and 11 are investigated. The sulfate solution alkalization inhibits subsequent redox reactions, and as a result, EC performance is successfully enhanced. The second approach exploits so-called bication electrolytic solutions based on a mixture of Li2SO4 and Na2SO4 at an equal concentration. This concept allows the operational time to be significantly prolonged, up to 648 h (+200% compared to 1 mol L-1 Li2SO4). Therefore, two successful pathways for improving sulfate-based ECs are demonstrated.

Toward better porous carbon-based electrodes by investigation of the viscoelastic properties of carbon suspension

In this article, an electrode slurry based on activated carbon, which is used to form electrodes of electrochemical capacitors, was assigned to a given group of rheological fluids. The interactions between the textural properties of carbon, the components of the slurry and their impact on the flow nature were explained. Moreover, the viscosity window and optimal ratio between components have been specified, which are necessary to form the right coating on the stainless steel foil as a current collector. The equation that correlates electrode thickness, viscosity, and composition of the slurry (mass of solid materials, solvent, and ratio between components) was obtained by mathematical consideration. Electrochemical measurements were performed to verify the impact of coating quality and thickness on device performance. Finally, the effect of the microstructure formed after slurry preparation and the phenomena accompanying the selected mixing technique are discussed. The paper ends with an economic analysis of electrode slurry production.

Ultralong-Cycling and Free-Standing Carboxylated Graphene/PEDOT:PSS Films as Electrode for Flexible Supercapacitors

The rapid development of current wearable electronics motivates the promotion of energy storage devices. Supercapacitor with flexible, lightweight, highly efficient, and long cycling life is one of the potential and glittering candidates. Here, the composite film of carboxylated graphene (CG) and commercially available poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) are prepared as electrodes for flexible electrochemical capacitors by ultrasonication and vacuum filtration. The CG/PEDOT:PSS films were roundly characterized by SEM, XPS, FTIR, XRD and Raman spectrometer. The as-prepared free-standing films exhibited high specific capacitance (192.7 F g-1 at 0.5 A g-1), excellent rate capability (up to 80.4% from 0.5 to 20 A g-1), and good electrochemical stability (95.5% of specific capacitance retention after 50000 cycles). Surprisingly, the post treated free-standing films by Dimethyl Formamide (DMF) shows better capacitive properties than those of the pristine electrode, such as higher specific capacitance (204.3 F g-1 at 0.5 A g-1), and ultralong-cycling stability (104.6% retention at a high current density of 20 A g-1 after 50000 cycles). It is expected that the free-standing CG/PEDOT:PSS films can be used as flexible electrodes for various wearable energy storage devices.

Effect of applied potential polarity on electrochemical properties of electrophoretically deposited activated carbon on an indium tin oxide substrate

Herein, an activated carbon (AC) film was deposited on a transparent indium tin oxide (ITO) substrate via electrophoresis, assembling electrodes for electrochemical capacitors. The colloidal solution for deposition comprised AC, conducting carbon black (CB), and an ethyl cellulose binder. The polarity of the practical applied voltage was investigated to determine the optimum electrochemical performance because the different zeta potentials of the individual elements influence the type and quantity of the deposit. Potentials with different polarities but same magnitude (+100 V, −100 V) were applied and the results were compared with those of dip coating. The composition of an electrode formed at different conditions was evaluated based on the particle size and pore size distribution of AC and CB. Owing to the high negative zeta potential, CB with 1555 m2 g−1 was rapidly deposited in large quantities (0.9 mg cm−2) under an applied potential of +100 V. Dip coating primarily resulted in the precipitation of CB owing to its high dispersion due to the negative zeta potential. Both AC and CB were deposited at an appropriate ratio at the negative applying potential of −100 V, although both carbons have the negative zeta potential. However, the overall amount (0.3 mg cm−2) of deposit was reduced. The highest specific capacitance was achieved at −100 V based on the large specific surface area (1945 m2 g−1) of AC. Furthermore, adhesion to the ITO substrate was stronger at −100 V than at other conditions.

Temperature-dependent tailoring of the pore structure based on MOF-derived carbon electrodes for electrochemical capacitors.

The pore structures of carbon play a critical role in the charge storage process of electrochemical capacitors; however, the involvement of other varying characteristics, such as electrical conductivity and surface functionalities, complicate the research of the pore size effects on various electrochemical phenomena. In this study, by carbonizing MOF-5 at a selected temperature range of 500-700 °C, a series of MOF-derived carbon materials were obtained with pore size distribution concentrated in different size ranges while admitting similar results in the graphitization degree and surface functionalities. The related morphological changes of ZnO were systematically investigated by changing the carbonization temperature and dwelling time, demonstrating a "from thin to thick, from inside to outside" growth routine of ZnO crystals. With the pore size approximated as the sole variable, the as-assembled electrochemical capacitors present a linear relationship between the 1-10 nm pores and the impedance resistance, which for the first time demonstrate how 1-10 nm pores is beneficial to ion diffusion. The results of this study not only provide a useful approach to manipulating the pore structure in carbon electrodes but also pave the way to establish the numerical relationship between the pore structure and various phenomena in electrochemistry or other related areas.