Electrodes For Electrochemical Capacitors Research Articles

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.

High utilization efficiencies of alkylbenzokynones hybridized inside the pores of activated carbon for electrochemical capacitor electrodes.

Benzoquinone derivatives (BQDs) are hybridized inside activated carbon (AC) pores via gas-phase adsorption to prepare electrochemical capacitor materials. In this study, 2 mmol of BQDs are hybridized with 1 g of AC. The hybridization of alkylbenzoquinones (ABQs) with AC enhances the volumetric capacitances of the hybrids from 117 to 201 F cm-3 at 0.05 A g-1 and the capacitances are retained up to 73% at 10 A g-1. Meanwhile, the volumetric capacitances are increased to 163 F cm-3 at 0.05 A g-1 by the hybridization of halobenzoquinones (HBQs) and the capacitance retentions at 0.05 A g-1 are ∼62%, which are higher than that of AC (46%). The results of electrochemical measurements suggest that HBQs exist as agglomerates while ABQs are finely dispersed inside the pores. The ABQs have good contact with the conductive carbon pore surface compared to the HBQs. Consequently, most of the ABQ molecules undergo reversible redox reactions (i.e., high utilization efficiencies), and a large contact area facilitates charge transfer at the large contact interface, thereby endowing the hybrids of ABQs with fast charging and discharging characteristics. HBQ molecules can be finely dispersed by liquid-phase adsorption, but the finely dispersed HBQ molecules are mobile inside the pores at room temperature and gradually form agglomerates. The difference in the existing form of BQDs is explained by the dominant interaction affecting the BQD molecules. ABQs have a strong interaction with the carbon pore surface while the intermolecular interaction is dominant for HBQs.

Optimization of porous carbon nanofiber/MnO2 electrode-based supercapacitor performance using γ‑cyclodextrin: Effects of graphene and MnCl2 contents

• Graphene and MnCl 2 content is optimized for supercapacitor performance of PCM(5)G(3). • PCM(5)G(3) has high surface area and active material of MnO 2 embedded in the fiber. • PCM(5)G(3) maximize the synergistic effect of the pseudocapacity and EDLC. • PCM(5)G(3) have great potential in electrochemical capacitor electrode materials. We systematically evaluate the influence of graphene and MnCl 2 contents on the structure and capacity of γ-cyclodextrin/graphene-based carbon nanofibers and MnO 2 (PCMG). The PCMG electrodes are stable long-term and had good capacitances, e.g., up to 236Fg -1 at 1mAcm -2 and 25.5Whkg -1 at 400Wkg -1 . The electrochemical performance is maximized by the synergistic effect between the pseudocapacity of MnO 2 well-embedded in the fiber and the electric double layer capacity induced by the well-developed surface pores.

Ion-Conducting Robust Cross-Linked Organic/Inorganic Polymer Composite as Effective Binder for Electrode of Electrochemical Capacitor

Poly(ionic liquid)s (PILs) are used in many electrochemical energy storage/conversion devices owing to their favorable physical properties. Therefore, PIL binders have been examined as polymeric binders for electrodes in energy storage systems (ESSs) and have shown superior performance. Several innovative technologies have been developed to improve the properties of polymers, with cross-linking being the most effective and easy strategy to achieve this. In this study, we designed a breakthrough complex cross-linking and composite technique that could successfully develop the physical properties of a polymer in a simple one-step process. Additionally, the technique could improve the thermal stability and mechanical properties of the polymer. The proposed polymeric binder showed better adhesion, higher capacitance, and good energy density with improved cyclic stability compared to that shown by conventional polyvinylidene fluoride (PVDF). This study revealed that cross-linked networks in polymeric binders are long-cycle-life features for electrochemical redox capacitors.

Flexible supercapacitor electrodes based on modified binary metal phosphites on three dimensional graphene sponge

In this paper, two flexible graphene-based materials were prepared by electrochemically depositing FeCoP and FeZnP on graphene sponges (GS) as self-standing electrodes for high-performance electrochemical capacitors. The structural and morphological properties of FeCoP/GS and FeZnP/GS electrodes were characterized using different spectroscopic techniques. As a result, the FeCoP/GS and FeZnP/GS electrodes demonstrated a high specific capacitance of 2100 and 1100 F/g at 1.0 A/g, great cycling stability after 10,000 cycles maintaining 80 % and 74 % of its initial capacitance, respectively, which showed better performance than GS electrode. Moreover, two asymmetric supercapacitors were fabricated with FeCoP/GS and FeZnP/GS electrodes and GS; FeCoP/GS//GS and FeZnP/GS//GS circuits displayed a specific capacitance of 1100 and 780 F/g at 1.0 A/g and an outstanding energy density of 185 and 98 Wh/kg at a high power density of 5.6 and 3.8 kW/kg, respectively. Therefore, the as-fabricated electrodes hold great potential in the field of energy storage.

Metal-Free Homocoupling of Pyrene inside the Pores of Mesoporous Carbons via Electrochemical Oxidation: Application for Electrochemical Capacitors.

A pyrene dimer (PYD) is synthesized by electrochemical oxidation via homocoupling of pyrene (PY) inside the pores of MgO-templated mesoporous carbons without any metal catalysts or organic solvents. The resulting MgO-C/PYD hybrids can be used as high-performance aqueous electrochemical capacitor electrodes due to the reversible redox property of PYD and large contact area between the hybridized PYD and conductive carbon surfaces, which enable rapid charge transfer at the large contact interface. In our previous study, PY was considered to polymerize through electrochemical oxidation, and activated carbon with the pore sizes of ∼4 nm was used as a porous carbon substrate. In this study, the MgO-templated carbons have the average pore sizes of 5, 10, and 30 nm, and their large mesopore volumes can accommodate a large amount of PYD for enhancing the capacitance. To develop high-performance electrochemical capacitors, the dependence of the capacitance enhancement and the capacitance retention on the amount of PY and the pore sizes of MgO-templated carbons are studied. It is found that mesopores are necessary for fast charging/discharging, but the capacitance retention and capacitance enhancement decrease with increasing the mesopore sizes and the amount of PY due to the decreased utilization ratio of PY.

Ag, N and O co-doped carbon cloth as high-capacitance electrodes for high-energy capacitors

Binder-free carbon cloth supported activated carbon electrode has become one of the most promising electrodes in electrochemical capacitors due to its advantages of large specific capacity and long cycle stability, but its application is limited by its inherent low electric double layer capacitance and low electrical conductivity. In this study, a nitrogen-doped carbon layer with Ag particles is formed on the carbon cloth via carbonizing polydopamine layer, which is formed via polymerization reaction, and introducing Ag particles into the obtained carbon layer can improve the electrical conductivity. Subsequently, oxygen-containing functional groups were introduced on the surface of the carbon layer by means of electro-oxidation to provide additional pseudo capacitance to the carbon layer. The obtained electrode exhibits a high area specific capacitance of 2740 mF cm−2 at a current density of 1 mA cm−2, and the capacitance retention rate is 69.1 % when the current density is increased by 15-fold. The symmetric capacitor assembled by this electrode maintains a specific capacity of 87.7 % after 20,000 cycling cycles. The electrode's high specific capacity and long-cycle stability make it as a promising candidate for practical application in energy storage.

Facile synthesis of nitrogen-doped interconnected porous carbons derived from reed and chlorella for high-performance supercapacitors

Three-dimensionally (3D) nitrogen (N)-doped interconnected porous carbons (IPC) derived from cost-effective biomasses were facilely prepared by one-step method and utilized as electrochemical double layer capacitor (EDLC) electrodes. The reed and chlorella derived interconnected porous carbons (denoted as IPC-R and IPC-C) through carbonization with chemical activation both exhibited typical capacitive behaviors; importantly, by introducing N-rich chlorella into N-free reed by interactive‑carbonization, the resultant N-doped interconnected porous carbons (IPC-RC) positively demonstrated a high N content, a large specific surface area and pore volume as well as a high graphitization, which are crucial for fast ion diffusion and thus enhancing the electrochemical properties of supercapacitor. In the three-electrode system, the IPC-RC1.2 showed a superb specific capacitance of 340.4 F/g at a current density of 1 A/g, one of the highest capacitances for biomass derived‑carbon electrodes reported so far. Even at 20 A/g, the capacitance could achieve 265.5 F/g, revealing the superb rate capability. In the symmetric two-electrode system, the maximum power density and energy density reached up to 23.6 Wh/kg and 15,000 W/kg, respectively. Besides, an outstanding cycling stability was observed after 10,000 cycles.

Electrophoretic deposition of Ti3C2Tx MXene nanosheet N-carbon cloth as binder-free supercapacitor electrode material

Despite the existing advances of Ti3C2Tx MXene as supercapacitor electrode material, its capacitance can be further enhanced through the composite strategy. In this work, the nitrogen-doped superhydrophilic carbon cloth (ENCC) was firstly prepared by N-doping of carbon cloth (CC), and then Ti3C2Tx MXene nanosheets were electrophoretically deposited to yield the binder-free Ti3C2Tx(EPD)/ENCC as supercapacitor electrode material. As a result, the composite electrode exhibited an area-specific capacitance of 2080.1 mF·cm−2 at 1 mA·cm−2 current density. The analysis implied that the varying Ti valence states provided certain psedocapacitance to the capacitance of the as-prepared electrode, though the main contribution was still dominantly from the electric double layer capacitance (EDLC). Further, the assembled symmetric supercapacitor yielded a wide voltage window of 1.8 V, and a good cyclic stability with 91% capacitance retention after 10,000 charge/discharge cycles at 20 mA·cm−2. Overall, the employed composite strategy enabled the good dispersion of the MXene flakes on the surface of the flexible substrate. Meanwhile, the large number of functional groups (-OH, -F, etc.) on the surface of Ti3C2Tx MXene interacted with the carbon layer through hydrogen bonding effects, which endows it as an ideal electrode for electrochemical capacitors.

Graphene-Wine Waste Derived Carbon Composites for Advanced Supercapacitors

In this work, we investigate the potential of a novel carbon composite as an electrode for high-voltage electrochemical double-layer capacitors. The carbon composite was prepared following a sustainable synthetic approach that first involved the pyrolysis and then the activation of a precursor formed by winery wastes and graphene oxide. The composite prepared in this way shows a very high specific surface area (2467 m2·g−1) and an optimum pore size distribution for their use in supercapacitor electrodes. Graphene-biowaste-derived carbon composites are tested as active electrode materials in two different non-aqueous electrolytes, the ammonium salt-based conventional organic electrolyte and one imidazolium-based ionic liquid (1 M Et4NBF4/ACN and EMINTFSI). It was found that the presence of graphene oxide led to significant morphological and textural changes, which result in high-energy and power densities of ~27 W·h·kg−1 at 13,026 W·kg−1. Moreover, the devices assembled retain above 70% of the initial capacitance after 6000 cycles in the case of the organic electrolyte.

Oaks-derived activated carbon by trace alkali-induced catalytic steam activation for electrochemical capacitor applications

With the aim of more efficiently using oaks resources, this study proposed a green and efficient strategy of trace KOH-induced catalytic activation to generate oaks-derived activated carbons (AC) for electrochemical capacitor applications. By studying the pore formation, trace KOH is found to be associated with mesopore formation and promotes the development of micropores in ACs, regulating the hierarchical structure of pores. Furthermore, trace KOH can enlarge the specific surface areas and change the surface functionalities of ACs. As the result, the obtained sample has a high specific surface area of 958 m2 g−1 and a total volume of 0.448 cm3 g−1. Evaluated as electrochemical capacitor (EC) electrode material, the obtained sample achieves a high specific capacitance of 157.5 F g−1 at a current density of 1 A g−1 and shows an excellent cycling performance with 97 % retention over 10,000 cycles at 2 A g−1. This work not only provides the mechanism understanding of the KOH-induced catalytic activation but also proves that the trace KOH-induced catalytic activation is a viable and green method for developing high-performance activated carbons for the demand of ECs in industrial applications.

Performance evaluation of electrochemical capacitors with activated carbon spheres as electrode material and aqueous electrolyte

The model activated carbon spheres (ACSs) were optimized to be used as negative electrode in asymmetric electrochemical capacitors (ECs). A microporous commercial activated carbon was used as positive electrode.Carbon spheres (CSs) were synthesized by precipitation polymerization at room temperature followed by pyrolysis and activation. By enhancing the carbon:KOH ratio from 1:2 to 1:4, an increase in the specific surface area from 380 m2 g−1 to 2835 m2 g−1 (1:4) was achieved along with an increase in pore volume/size. Consequently, the electrochemical performance in the aqueous electrolyte was improved. After activation, low-temperature-pyrolyzed polymer spheres (400 °C) result in random-like particles with macroporous structure, while intermediate- and high-temperature-pyrolyzed materials (550 °C and 700 °C) lead to partially and totally conserved spheres.The interaction between the electrode material and electrolyte is important and is related to the texture, morphology, and surface chemistry of ACS. In 1 mol L−1 H2SO4, capacitance and rate handling are mostly affected by electrode morphology and wettability with the electrolyte. In 1 mol L−1 Li2SO4 electrolyte solution, texture and surface chemistry of the electrode material are crucial to obtain high-performance EC. Regardless of morphology, poor wetting properties of synthesized materials were found in Li2SO4, compared to the H2SO4 solution.

Chemical co-activated modified small mesoporous carbon derived from nature anthracite toward enhanced supercapacitive behaviors

Chemical co-activation is a valid avenue to achieve the surface chemical modification and pore configuration regulation of carbon electrode materials, thus greatly facilitating their capacitive behaviors. In the study, ultrapure anthracite-derived porous carbons (PCs) with small mesopore development are synthesized via KOH pre-etching of natural coal for pore-creating, closely followed by H3PO4 deep activation for pore-expanding and carbon surface stabilization. Phosphoric acid post-activation under increased temperature not only creates abundant small mesopores for fast ions transport, but improves the surface chemistry on carbon skeleton to boost the capacitance reversibility. When assessed as electro-chemical double-layer capacitor (EDLC) electrodes in KOH electrolyte, the resulting co-activated PC demonstrates low internal resistance of less than 2.50 mΩ, high specific capacity of up to 212F g−1, and superior rate capability of 71.82% at 10 A g−1. More importantly, due to the enhancement of oxidation stability of optimal electrode by the reduction of unstable chemical groups on carbon surface, the constructed organic supercapacitor can be quickly charged or discharged for a long period in 1 m (C2H5)4NBF4 electrolyte, so as to deliver an excellent cycling stability of 97.52% capacitance retention. This study realizes the high value-added and efficiently clean utilization of anthracite as a non-renewable resource, as well as offering profound electrochemical insights for large scale application of small mesoporous carbon towards next-generation high-power smart devices.

Redox activity from the electrolyte and electrode in electrochemical capacitors

In electrochemical capacitors, also known as supercapacitors (SC), electrolytes and electrodes are the most important constituents and control the main characteristics of the device. The high energy density of a SC can be obtained by designing functional electrode materials along with appropriate electrolytes. Redox activity represents a groundbreaking boost to the performance of SCs by increasing the charge storage capability through Faradaic reactions. This can be accomplished by using electrolytes with redox additives or by combining electrode materials showing charge transfer phenomena. Among many pseudocapacitive compounds, transition metal disulfides (TMDs) have been considered for use in SCs owing to their remarkable properties. In this paper, the electrochemical performance of SCs and the current state-of-the-art of redox activities provided by electrolytes and electrodes are discussed.

Synergistic effect of hydrothermal and physical activation approaches to fabricate activated carbon for energy storage applications

Carbon is the main constituent of all waste biomass. The extraction of activated carbonaceous materials from the waste biomass and their utilization in energy-storage applications is a green, cost-effective strategy. Herein, honeycomb-like activated carbon (AC) with a large surface area (2344 m2g-1) and improved conductivity was derived from palm seeds via hydrothermal followed by CO2 activation approaches. The AC sample was decorated over a carbon cloth (CC) current collector to form a flexible working electrode for next-generation electrochemical capacitors. As-prepared palm seed-derived activated carbon electrode delivered a good specific capacitance (Cs) of 294.9 Fg−1 @ 01 Ag−1, with exceptionally high cycling stability, showing 97.36% retention of its initial capacitance, even after 10,000 charge-discharge cycles in an alkaline electrolyte. The porous structure with microsized cavities enables the activated carbon-based electrode to show excellent rate capability (93.1% capacitance retention) even at nine-fold higher current density. As-prepared flexible electrode exhibits excellent electrochemical performance due to their unique three-dimensional network structure combined with porous architecture and good conductivity. This work provides a facile way to enhance activated carbon's capacitive performance and opens up new avenues for developing high-performance capacitive materials using waste biomass to reduce environmental concerns.