Electrodes For Electrochemical Capacitors Research Articles

A Strategy for Synthesis of Carbon Nitride Induced Chemically Doped 2D MXene for High‐Performance Supercapacitor Electrodes

AbstractA step‐by‐step strategy is reported for improving capacitance of supercapacitor electrodes by synthesizing nitrogen‐doped 2D Ti2CTx induced by polymeric carbon nitride (p‐C3N4), which simultaneously acts as a nitrogen source and intercalant. The NH2CN (cyanamide) can form p‐C3N4 on the surface of Ti2CTx nanosheets by a condensation reaction at 500–700 °C. The p‐C3N4 and Ti2CTx complexes are then heat‐treated to obtain nitrogen‐doped Ti2CTx nanosheets. The triazine‐based p‐C3N4 decomposes above 700 °C; thus, the nitrogen species can be surely doped into the internal carbon layer and/or defect site of Ti2CTx nanosheets at 900 °C. The extended interlayer distance and c‐lattice parameters (c‐LPs of 28.66 Å) of Ti2CTx prove that the p‐C3N4 grown between layers delaminate the nanosheets of Ti2CTx during the doping process. Moreover, 15.48% nitrogen doping in Ti2CTx improves the electrochemical performance and energy storage ability. Due to the synergetic effect of delaminated structures and heteroatom compositions, N‐doped Ti2CTx shows excellent characteristics as an electrochemical capacitor electrode, such as perfectly rectangular cyclic voltammetry results (CVs, R2 = 0.9999), high capacitance (327 F g−1 at 1 A g−1, increased by ≈140% over pristine‐Ti2CTx), and stable long cyclic performance (96.2% capacitance retention after 5000 cycles) at high current density (5 A g−1).

Electrospun porous carbon nanofibers with controllable pore sizes by boron trioxide for electrochemical capacitor electrodes

Abstract Carbon nanofibers (CNFs) with controllable pore sizes are fabricated by a simple electrospinning method with the help of boron trioxide (B2O3), and their electrochemical properties as electrodes are investigated in aqueous electrolyte. The optimized result is obtained with a B2O3 concentration of 10 wt% in a spinning solution, offering a large specific surface area of 1065 m2/g with a mesopore volume fraction up to 35%, pore size range of 2–3 and 20–50 nm, and many heteroatoms. The CNF composite with 10 wt% B2O3 concentration (PMB-10) shows a high gravimetric capacitance of 126.31 F/g with a capacitance retention of 84%, and a high energy density of 12–18 W h/kg in the power density range of 400–10,000 W/kg. Thus, the porous structures and heteroatom contents of the PMB composites can be controlled by varying the B2O3 content as the pore expanding agent, as well as heteroatom generator, to optimize the electrochemical performance in an aqueous electrolyte.

Synthesis and characterization of porous carbon derived from activated banana peels with hierarchical porosity for improved electrochemical performance

Banana peels, a common fruit waste was adopted as a material precursor in this study to synthesize highly porous activated carbon from banana peels (ABP) which serves as an electrode material for a symmetric supercapacitor device. The activation was done using KOH pellets at different carbonization temperatures ranging from 750 °C to 950 °C. The ABP sample obtained from the 900 °C carbonization temperature (ABP900) exhibited unique material properties such as hierarchical porous nano-architecture containing micropores, and mesopores with the highest specific surface area (1362 m2 g−1). Electrochemical performance investigation in different neutral aqueous electrolytes showed that the best response was obtained in NaNO3 for the ABP900 electrode. The symmetric device subsequently assembled using 1 M NaNO3 operated in a potential window of 1.8 V, exhibited a specific capacitance of 165 F g−1 with a corresponding energy density of 18.6 W h kg−1 at 0.5 A g−1. A 100% capacitance retention and columbic efficiency were obtained after 10000 continuous charge-discharge cycles at 5 A g−1. Remarkably, after subjecting the symmetric device to a voltage holding test for 60 h, the specific capacitance was observed to increase from 165 F g−1 to 328 F g−1 with a corresponding increased energy density to about 36.9 W h kg−1 at 0.5 A g−1, suggesting a 98% increase in device energy density from its initial value after voltage holding. Thus, the results reported showcase the ABP900 material as a potential nanostructured porous material useful in the design of high-performance electrodes for stable electrochemical capacitors.

Ti3C2Tx on copper and nickel foams with improved electrochemical performance produced via solution processing for supercapacitor

Herein, we demonstrate the fabrication of Ti3C2Tx electrodes using solution processing. Two-dimensional (2D) nanometer Ti3C2Tx flakes are site-aggregated on copper and nickel foam papers with a reconstituted threedimensional (3D) structure consisting of overlapping and open-pore 2D flakes. When Ti3C2Tx was used as electrochemical capacitor electrodes in 1M Na2SO4 solution, the capacitances were comparable to literature values. There is a quantitative linear relationship between the capacitance and foam thicknesses. Given the process scalability and the morphological control that is possible, these results provide a promising road map for convenient and economical supercapacitors.

In situ generation of CoS1.097 nanoparticles on S/N co-doped graphene/carbonized foam for mechanically tough and flexible all solid-state supercapacitors

Flexible, high-strength and free-standing CoS1.097/GF/KOH/PVA composite films are synthesized by a simple squeeze-dip-coating and sulfidation process and utilized as positive electrode for asymmetric electrochemical capacitor with ultrahigh specific capacitance, high energy density and excellent mechanical stability.

Facile synthesis of pyrite (FeS2/C) nanoparticles as an electrode material for non-aqueous hybrid electrochemical capacitors.

Pyrite (FeS2) is a promising electrode material for lithium ion batteries (LIBs) because of its high natural availability, low toxicity, cost-effectiveness, high theoretical capacity (894 mA h g-1) and high theoretical specific energy density (1270 W h kg-1, 4e-/FeS2). Nevertheless, the use of FeS2 in electrochemical capacitors was restricted due to fast capacity fading as a result of polysulfide (S/Sn2-) formation during the initial electrochemical cycling. In order to avoid the formation of polysulfides, we employed the strategy of utilizing an ether based electrolyte (1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/diglyme (DGM)). Herein, we introduce FeS2/C as the Faradaic electrode for a non-aqueous hybrid electrochemical capacitor (NHEC) in combination with activated carbon (AC) as a non-Faradaic electrode, and 1.0 M LiTFSI/DGM as a non-aqueous electrolyte. Specifically, FeS2/C nanoparticles have been prepared via the sulfidation of a room temperature synthesized Fe-based MOF (metal organic framework) precursor. The fabricated FeS2/C∥AC NHEC, operating within the chosen voltage window of 0-3.2 V, delivered energy densities in the range of 63-9 W h kg-1 at power densities of 152-3240 W kg-1. Remarkable cycling stability with stable energy density retention for 2500 cycles at high power densities (729, 1186 and 3240 W kg-1) was observed.

Influence of the Concentration of Electrolyte on the Capacitive Properties of Vanadium Nitride Electrode for Electrochemical Capacitors

Cyclic voltammetry and electrochemical impedance spectroscopy were applied to investigate the effect of the concentration of KOH solution on the capacitive performance of the nanostructured vanadium nitride. The results reveal that the required overpotential for the transformation of V2+ to V3+ in concentrated KOH solutions is much lower than that in diluted solutions, indicating that concentrated solutions are kinetically more favorable than diluted solutions. Additionally, a marked decrease in charge transfer resistance and electrolyte series resistance were observed as the concentration of KOH solution increased. Thus, the specific capacitance and the rate capability of the vanadium nitride electrode are improved with increasing of the concentration of KOH solution. Moreover, the specific capacitance of the vanadium nitride material in 6 M KOH solution is the highest, and the vanadium nitride material exhibits great long term stability in KOH solution.

Cyclic Voltammetry Peaks Due to Deep Level Traps in Si Nanowire Array Electrodes

When metal-assisted chemical etching (MACE) is used to increase the effective surface area of Si electrodes for electrochemical capacitors, it is often found that the cyclic voltammetry characteristics contain anodic and cathodic peaks. We link these peaks to the charging–discharging dynamics of deep level traps within the nanowire system. The trap levels are associated with the use of Ag in the MACE process that can leave minute amounts of Ag residue within the nanowire system to interact with the H2O layer surrounding the nanowires in a room temperature ionic liquid. The influence of the traps can be removed by shifting the Fermi level away from the trap levels via spin-on doping. These results in lower capacitance values but improved charge–discharge cycling behavior. Low-frequency noise measurements proof the presence or absence of these deep level traps.

Direct growth of 2D nickel hydroxide nanosheets intercalated with polyoxovanadate anions as a binder-free supercapacitor electrode.

A mesoporous nanoplate network of two-dimensional (2D) layered nickel hydroxide Ni(OH)2 intercalated with polyoxovanadate anions (Ni(OH)2-POV) was built using a chemical solution deposition method. This approach will provide high flexibility for controlling the chemical composition and the pore structure of the resulting Ni(OH)2-POV nanohybrids. The layer-by-layer ordered growth of the Ni(OH)2-POV is demonstrated by powder X-ray diffraction and cross-sectional high-resolution transmission electron microscopy. The random growth of the intercalated Ni(OH)2-POV nanohybrids leads to the formation of an interconnected network morphology with a highly porous stacking structure whose porosity is controlled by changing the ratio of Ni(OH)2 and POV. The lateral size and thickness of the Ni(OH)2-POV nanoplates are ∼400 nm and from ∼5 nm to 7 nm, respectively. The obtained thin films are highly active electrochemical capacitor electrodes with a maximum specific capacity of 1440 F g-1 at a current density of 1 A g-1, and they withstand up to 2000 cycles with a capacity retention of 85%. The superior electrochemical performance of the Ni(OH)2-POV nanohybrids is attributed to the expanded mesoporous surface area and the intercalation of the POV anions. The experimental findings highlight the outstanding electrochemical functionality of the 2D Ni(OH)2-POV nanoplate network that will provide a facile route for the synthesis of low-dimensional hybrid nanomaterials for a highly active supercapacitor electrode.

Electrochemical polymerization of pyrene and aniline exclusively inside the pores of activated carbon for high-performance asymmetric electrochemical capacitors

An asymmetric polymer capacitor was prepared from pyrene (PY), aniline (ANI), and commercially available activated carbon (AC) through a solvent-free preparation. PY and ANI were adsorbed into the AC host material in the gas phase and electrochemically polymerized exclusively inside the AC pores in an aqueous H2SO4 electrolyte (1 M). No volumetric expansion of the AC particles occurred upon the adsorption of monomers and their subsequent polymerizations; thus, the volumetric capacitance was enhanced by the inclusion of pseudocapacitive polypyrene (PPY) and polyaniline (PANI). The PPY and PANI structures formed inside the AC pores are very thin and have a large contact area with the conductive carbon surfaces. Therefore, the charge transfer distance between the polymers and the carbon surfaces was drastically shortened, significantly reducing the charge transfer resistance; i.e., high power density. The maximum volumetric capacitances for the PPY- and PANI-hybridized AC reached 314 and 299 F cm-3, respectively. Moreover, the strong adhesion derived from their large contact areas and adsorption capability of AC endow these materials with long cycle lifetimes. The PPY- and PANI-hybridized AC have different redox potentials and can be assembled into an asymmetric capacitor. The volumetric capacitance obtained for the asymmetric capacitor further surpassed that of the symmetric capacitor consisting of pristine AC, with high power density and long cycle lifetimes.

Non-polymeric hybridization of a TEMPO derivative with activated carbon for high-energy-density aqueous electrochemical capacitor electrodes

High-dispersion of a hydrophobic TEMPO derivative inside the pores of activated carbon provides high-performance aqueous electrochemical capacitor electrodes without polymerization.

High-performance flexible wire-shaped electrochemical capacitors based on gold wire@reduced graphene oxide

Flexible gold @reduced graphene oxide sheathed wires (WAu@rGO) were prepared by soaking a gold wire (100μm in diameter) in the center of glass capillaries with inner diameters of 1.0, 1.3 and 1.8 mm in a suspension of graphene oxide (GO) in a L-ascorbic acid aqueous solution at 30 °C for 48 h. The WAu@rGO wires were obtained after removal of the capillaries, and their microstructures were characterized by SEM and their electrochemical properties as flexible electrodes in electrochemical capacitors were investigated by electrochemical tests. Results show that the specific capacitance of the WAu@rGO wire with the smallest diameter is 5.47 mF cm−1 or 176.7 F g−1 at a scan rate of 1 mV s−1. A symmetric all-solid-state flexible electrochemical capacitor based on the two smallest wires and a polyvinyl alcohol/H3PO4 gel electrolyte shows a maximum capacitance of 2.06 mF cm−1, or 6.87 mF cm−2 or 411.9 mF cm−3. The solid-state capacitor has energy and power densities of 9.48×10−4 mWh cm−2 and 0.017 mW cm−2, respectively, based on the whole device. Three devices connected in series can light a light-emitting diode with a threshold voltage of 2.5 V. Moreover, the device also shows a long cyclic stability and a good bending ability.

Metal sputtered graphene based hybrid films comprising tin oxide/reduced graphene oxide/Ni as electrodes for high-voltage electrochemical capacitors

The high voltage aqueous electrochemical capacitor (EC) is a promising energy storage device because of eco-friendliness and high electrochemical performance with a wide operational voltage and high energy density. However, it typically experiences a stability problem that includes cell aging and capacitance loss. Here, to overcome the stability issue, a thin metal layer of Ni is created on one side of a SnO2/reduced graphene oxide (rGO) hybrid film to produce a binder-free film of SnO2/rGO/Ni. Due to the formation of the highly conductive metal layer of Ni, the fabricated film can be well interconnected with the current collector and have lower contact resistance and open-circuit potential compared with untreated SnO2/rGO film, which results in a remarkable enhancement of electrochemical performance, including a wide operational voltage (1.8 V), semi-permanent cycle-life (95% retention after 10k cycles), and ultrahigh volumetric energy density with a high power density, all of which are superior values compared to bare SnO2/rGO film based devices. We anticipate that the fabricated SnO2/rGO/Ni film could be utilized as a promising electrode for high voltage ECs, and our simple surface engineering technique will provide an effective electrode design for the fabrication of high performance thin-film ECs.

Fast in situ synthesis of CoFe layered double hydroxide onto multi-layer graphene for electrochemical capacitors

Homogeneous CoFe layered double hydroxide (LDH) was in situ synthesized onto the surfaces of multi-layer graphene (MLG) based on π-π interaction method just for 1-h reaction. The morphology, crystal structure, and electrochemical properties of the composite were systematically studied. The results showed that crystalline CoFe-LDH grown onto MLG was composed of two layers. The bottom layer was parallel to graphene sheet to increase the contact with graphene. The external layer was vertically assembled to allow the easy immersion of electrolyte into the pores between CoFe-LDH sheets. When being tested as electrochemical capacitor electrode, the composite could deliver a specific capacitance of 882.5 F g−1 at a current density of 1 A g−1. When prolonging the reaction time to 3 h, homogeneous CoFe oxide nanoparticles with a size of about 5 nm would be formed on the surface of CoFe-LDH. Its specific capacitance decreased to 462.5 F g−1 at 1 A g−1, but its cycle stability and rate capability were both improved. All CoFe-LDH sheets were transformed into CoFe oxide nanoparticles when the reaction time reaches 10 h, resulting in a further decrease of specific capacitance to 82.5 F g−1. Thus, to obtain a high electrochemical performance, the reaction time should be controlled because the phases and morphologies of the products are dependent on it.

Superelastic Graphene Aerogel/Poly(3,4-Ethylenedioxythiophene)/MnO2 Composite as Compression-Tolerant Electrode for Electrochemical Capacitors

Ultra-compressible electrodes with high electrochemical performance, reversible compressibility and extreme durability are in high demand in compression-tolerant energy storage devices. Herein, an ultra-compressible ternary composite was synthesized by successively electrodepositing poly(3,4-ethylenedioxythiophene) (PEDOT) and MnO2 into the superelastic graphene aerogel (SEGA). In SEGA/PEDOT/MnO2 ternary composite, SEGA provides the compressible backbone and conductive network; MnO2 is mainly responsible for pseudo reactions; the middle PEDOT not only reduces the interface resistance between MnO2 and graphene, but also further reinforces the strength of graphene cellar walls. The synergistic effect of the three components in the ternary composite electrode leads to high electrochemical performances and good compression-tolerant ability. The gravimetric capacitance of the compressible ternary composite electrodes reaches 343 F g−1 and can retain 97% even at 95% compressive strain. And a volumetric capacitance of 147.4 F cm−3 is achieved, which is much higher than that of other graphene-based compressible electrodes. This value of volumetric capacitance can be preserved by 80% after 3500 charge/discharge cycles under various compression strains, indicating an extreme durability.

High-performance energy storage of poly (o-methoxyaniline) film using an ionic liquid as electrolyte

Abstract Different materials have been proposed as active electrode in electrochemical capacitors, including conducting polymers. Among them, several papers describe the use of polyaniline and its derivatives for such purpose. However, the use of poly(o-methoxyaniline), a polyaniline derivative, in ionic liquid electrolytes as pseudocapacitors has never been proposed in the literature. Therefore, the objective of this work is to investigate the electrochemical properties and stability of POMA casting film in ionic liquid electrolyte as electrode for supercapacitors. Cyclic voltammetry and galvanostatic charge/discharge experiments have been used to analyze the behavior of the films. The results show that POMA films have high specific capacitance and good electrochemical stability during 3000 cycles. Its initial value is 260 F g−1 in a mixture of ionic liquid and polyethylene glycol (80:20 v/v). The specific energy shows 70% retention from the initial value after 3000 cycles. Furthermore, the specific power remains remarkably stable during 3000 cycles, as well as the coulombic efficiency, which is about 99%. For these reasons this work opens up the possibilities of using conducting polymers in ionic liquids and illustrates its good behavior and stability for supercapacitor application.

Competition between ionic adsorption and desorption on electrochemical double layer capacitor electrodes in acetonitrile solutions at different currents and temperatures

The operation of electrochemical double layer capacitors at high currents and viscosities and at low temperatures is difficult. Under these conditions, ion transport is limited, and some of the electrode area is unavailable for adsorption, which results in a low capacitance. Increasing the temperature helps to increase the ionic movement, leading to enhanced adsorption and increased capacitance. In contrast, ion desorption (self-discharge) surpasses the capacitance improvement when ions gain a high amount of energy with increasing temperature. For example, temperatures as high as 70 °C cause a very high rate of ionic desorption in acetonitrile solutions in which the individual properties of the two electrolytes—tetraethylammonium tetrafluoroborate (TEA BF4) and ethylmethylimidazolium tetrafluoroborate (EMI BF4)—are not distinguishable. The capacitance improvement and self-discharge are balanced, resulting in a capacitance peak at mid-range temperatures, i.e., 35–45 °C, in the more viscous electrolyte, i.e., TEA BF4. The less viscous electrolyte, i.e., EMI BF4 has a wider capacitance peak from 25 to 45 °C and higher capacitance than that of TEA BF4. Because the maximum power is obtained in the mid-temperature range (35–45 °C), it is necessary to control the viscosity and temperature to obtain the maximum power in a given device.

Polymerized fuchsin and modified carbon nanotube electrodes for electrochemical capacitors

Chemically polymerized fuchsin (CpF) and electrochemically polymerized fuchsin (EpF) were developed to modify multi-walled carbon nanotubes (MWCNT) as composite electrodes for electrochemical capacitors (EC). The cyclic voltammograms (CV) of both modified electrodes exhibited multiple highly reversible redox features indicating pseudocapacitive behavior. The capacitance was ca. 2x greater than that of bare MWCNT electrode. While all electrodes exhibited excellent cycle life, CpF-MWCNT performed even better than its electropolymerized counterpart. Scanning electron microscopy (SEM) images showed the deposition of a 1-nanometer thin layer on MWCNT, with good adhesion. The chemical structure and thermal analyses confirmed that H2O2 is an effective reactant to polymerize fuchsin, and the resulting polymer is semi-crystalline and stable for EC applications.

Flexible three-dimensional carbon cloth/carbon fibers/NiCo2O4 composite electrode materials for high-performance all-solid-state electrochemical capacitors

A flexible three-dimensional carbon cloth/carbon fibers/NiCo2O4 composite electrode is prepared via a two step method: first growth carbon fibers on flexible carbon cloth; second coating NiCo2O4 on the carbon cloth/carbon fibers composite. The carbon cloth guarantees the flexibility of electrodes. The three-dimensional carbon cloth/carbon fibers conductive network enhances the ionic transportation property and offer high conductivity. The ultrathin and porous NiCo2O4 nanosheet layers contribute high faradic capacity. The hierarchically porous electrode structure further facilitates to shorten the electrolyte diffusion path. When used as electrodes for electrochemical capacitor, the composite electrode exhibits superior energy storage performances with high specific capacitance of 1221Fg−1 at 1Ag−1, outstanding cycle stability (81% capacitance retention after 3000 cycles) and excellent flexibility. To further investigate the performances of the carbon cloth/carbon fibers/NiCo2O4 composite electrode, we assemble an all-solid-state symmetric electrochemical capacitors device using identical carbon cloth/carbon fibers/NiCo2O4 composite as electrodes. The assembled device delivers a high energy density of 27.2Whkg−1 at a power density of 1.46kWkg−1 and outstanding cycling stability with 91% capacitance retention after 2000 cycles. The combination of excellent flexibility and outstanding energy storage performance of the device will help to pave the way to wearable electronics and devices.

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived Electrocatalysis.

Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensively in the context of their potential applications in energy storage and conversion. Here, we report on the properties of core-shell (gold-polypyrrole) hybrid nanowires and their suitability as electrodes in electrochemical capacitors and as electrocatalysts. In general, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transfer resistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate. The specific capacitance of the hybrid electrodes is enhanced due to the pseudocapacitance of the polypyrrole shells; moreover, the electrodes operate as an ideal capacitive element and maintain their specific capacitance even at fast charge/discharge rates of 4690 mA/cm3 and 10 V/s. These rates far exceed those of other types of pseudocapacitors, and are even superior to electric double layer-based supercapacitors. The mechanisms behind these fast charge/discharge rates are elucidated by electrochemical impedance spectroscopy, and are ascribed to the reduced internal resistance associated with the fast charge transport ability of the gold nanowire cores, low ionic resistance of the polypyrrole shells, and enhanced electron transport across the nanowire's junctions. Furthermore, the hybrid electrodes show great catalytic activity for ethanol electro-oxidation, comparable to bare gold nanowires, and the surface activity of gold cores is not affected by the polypyrrole coating. The electrodes exhibit improved stability for electrocatalysis during potential cycling. This study demonstrates that the gold-polypyrrole hybrid electrodes can store and deliver charge at fast rates, and that the polypyrrole shells of the nanowires extend the catalytic lifetime of the gold cores.