High Electric Double Layer Capacitance Research Articles

Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids.

Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfill these two requisites. Here we report that an oligomeric ionic liquid such as IL4TFSI with four imidazolium ion units in its structure provides a wide electrochemical window of ∼5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4TFSI exhibits an exceptionally high EDL capacitance of ∼66 μF/cm2, which is ∼6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4TFSI as a gate dielectric material and SrTiO3 as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ∼64 μF/cm2, as determined by Hall-effect measurements.

Improving pH sensitivity by field-induced charge regulation in flexible biopolymer electrolyte gated oxide transistors

Electrical manipulation of charged ions in electrolyte-gated transistors is crucial for enhancing the electric-double-layer (EDL) gating effect, thereby improving their sensing abilities. Here, indium-zinc-oxide (IZO) based thin-film-transistors (TFTs) are fabricated on flexible plastic substrate. Acid doped chitosan-based biopolymer electrolyte is used as the gate dielectric, exhibiting an extremely high EDL capacitance. By regulating the dynamic EDL charging process with special gate potential profiles, the EDL gating effect of the chitosan-gated TFT is enhanced, and then resulting in higher pH sensitivities. An extremely high sensitivity of ∼57.8mV/pH close to Nernst limit is achieved when the gate bias of the TFT sensor sweeps at a rate of 10mV/s. Additionally, an enhanced sensitivity of 2630% in terms of current variation with pH range from 11 to 3 is realized when the device is operated in the ion depletion mode with a negative gate bias of −0.7V. Robust ionic modulation is demonstrated in such chitosan-gated sensors. Efficiently driving the charged ions in the chitosan-gated IZO-TFT provides a new route for ultrasensitive, low voltage, and low-cost biochemical sensing technologies.

Computational insight into the capacitive performance of graphene edge planes

Recent experiments have shown that electric double-layer capacitors utilizing electrodes consisting of graphene edge plane exhibit higher capacitance than graphene basal plane. However, theoretical understanding of this capacitance enhancement is still limited. Here we applied a self-consistent joint density functional theory calculation on the electrode/electrolyte interface and found that the capacitance of graphene edge plane depends on the edge type: zigzag edge has higher capacitance than armchair edge due to the difference in their electronic structures. We further examined the quantum, dielectric, and electric double-layer (EDL) contributions to the total capacitance of the edge-plane electrodes. Classical molecular dynamics simulation found that the edge planes have higher EDL capacitance than the basal plane due to better adsorption of counter-ions and higher solvent accessible surface area. Our work therefore has elucidated the capacitive energy storage in graphene edge planes that take into account both the electrode's electronic structure and the EDL structure.

Water-Gated n-Type Organic Field-Effect Transistors for Complementary Integrated Circuits Operating in an Aqueous Environment.

The first demonstration of an n-type water-gated organic field-effect transistor (WGOFET) is here reported, along with simple water-gated complementary integrated circuits, in the form of inverting logic gates. For the n-type WGOFET active layer, high-electron-affinity organic semiconductors, including naphthalene diimide co-polymers and a soluble fullerene derivative, have been compared, with the latter enabling a high electric double layer capacitance in the range of 1 μF cm–2 in full accumulation and a mobility–capacitance product of 7 × 10–3 μF/V s. Short-term stability measurements indicate promising cycling robustness, despite operating the device in an environment typically considered harsh, especially for electron-transporting organic molecules. This work paves the way toward advanced circuitry design for signal conditioning and actuation in an aqueous environment and opens new perspectives in the implementation of active bio-organic interfaces for biosensing and neuromodulation.

Electrostatic-Induced Assembly of Graphene-Encapsulated Carbon@Nickel-Aluminum Layered Double Hydroxide Core-Shell Spheres Hybrid Structure for High-Energy and High-Power-Density Asymmetric Supercapacitor.

Achieving high energy density while retaining high power density is difficult in electrical double-layer capacitors and in pseudocapacitors considering the origin of different charge storage mechanisms. Rational structural design became an appealing strategy in circumventing these trade-offs between energy and power densities. A hybrid structure consists of chemically converted graphene-encapsulated carbon@nickel-aluminum layered double hydroxide core-shell spheres as spacers among graphene layers (G-CLS) used as an advanced electrode to achieve high energy density while retaining high power density for high-performance supercapacitors. The merits of the proposed architecture are as follows: (1) CLS act as spacers to avoid the close restacking of graphene; (2) highly conductive carbon sphere and graphene preserve the mechanical integrity and improve the electrical conductivity of LDHs hybrid. Thus, the proposed hybrid structure can simultaneously achieve high electrical double-layer capacitance and pseudocapacitance resulting in the overall highly active electrode. The G-CLS electrode exhibited high specific capacitance (1710.5 F g-1 at 1 A g-1) under three-electrode tests. An ASC fabricated using the G-CLS as positive electrode and reduced graphite oxide as negative electrode demonstrated remarkable electrochemical performance. The ASC device operated at 1.4 V and delivered a high energy density of 35.5 Wh kg-1 at a 670.7 W kg-1 power density at 1 A g-1 with an excellent rate capability as well as a robust long-term cycling stability of up to 10 000 cycles.

Ni-catalyzed carbonization of furfuryl alcohol polymer in ordered mesoporous silica MCM-48 giving ordered mesoporous carbon CMK-1 with high electric double-layer capacitance

Ordered mesoporous carbon CMK-1 was partially graphitized during carbonization treatment of furfuryl alcohol (FA) polymer inside the ordered mesopores of Ni-loaded MCM-48. FA polymer was successfully produced within the mesopores of MCM-48 by thermal treatment at 90 °C. A typical weight decrease of the FA-polymer/MCM-48 composite occurred from 300 to 400 °C by carbonization treatment. Ni(II) species acts as a catalyst for dehydrogenation from 500 to 640 °C, and then CH4 fragments formed in situ can reduce Ni(II) to Ni(0) at 640 °C. The metallic Ni thus formed can then act as a catalyst for the formation of the graphitic structure within the mesopores of MCM-48 at 1000 °C. The CMK-1 obtained with the aid of the Ni catalyst exhibited a higher electric double-layer capacitance with a tetraethylammonium tetrafluoroborate/propylene carbonate electrolyte, compared to normal CMK-1. The results suggest that the edge planes of the graphitic domains in CMK-1 are predominantly exposed on the surface of carbonaceous walls, which leads to an increase in the number of adsorptive sites for the electrolyte during the capacitance measurements.

Activation of Petroleum Pitch for Production of Porous Carbons with High Electric Double Layer Capacitance

Petroleum pitch can be an alternative of coal tar pitch as the precursor of activated carbon because of their lower contents of detrimental impurities and less pollution to the environment when compared with coal tar pitch. High-porosity carbon derived from activation of mesophase pitch with KOH has been successfully used as the electrode materials for electric double layer capacitors (EDLCs).1 Formation of mesophase structure through thermal treatment coordinates the pitch molecules and eventually reinforces the structure of porous carbons obtained from activation. The present study devises a temperature-programmed heating strategy to convert petroleum pitch to mesophase that has a framework advantageous for producing porous carbons suitable for EDLCs. The porous carbon is synthesized by KOH activation and has a Brunauer-Emmett-Teller surface area exceeding 3000 m2/g, When assembled in a symmetric two-electrode cell, the carbon delivers capacitance values of 170 and 102 F g-1 at current densities of 0.5 and 100 A g-1, respectively, in 1 M tetraethylammonium tetrafluoroborate/acetonitrile electrolyte. The energy density of the cell based on the total carbon mass reaches 23 Wh kg- 1 at a power density of 40 kW kg- 1. The results of the present study demonstrate the feasibility of using petroleum pitch as the precursor for porous carbon production and the significance of mesophase pitch structure for regulating the structure of pores suitable for charge storage.

Enhanced electrical and mass transfer characteristics of acid-treated carbon nanotubes for capacitive deionization

Capacitive deionization (CDI) has attracted significant attention for the next generation water treatment due to its low energy consumption and environment friendly properties in comparison to widely established methods. For CDI technology to move forward, however, the development of carbon electrodes having superb electrosorption behavior is essential. In this study, we demonstrate the functionalization of carbon nanotubes (CNTs) via acid treatment shows enhanced electrochemical characteristics and effectively improves the wettability of the acid-treated CNTs (a-CNTs) via the addition of oxygen functional groups, leading to a higher electric double layer capacitance. Furthermore, defect formation in a-CNTs increases the conductivity and decreases the mass transfer resistance during CDI operation. CDI measurements confirmed a 270% increase in performance of a-CNTs in contrast to pristine CNTs (p-CNTs), attributable to the improved characteristics outlined above.

Electrochemically synthesized and vertically aligned carbon nanotube–polypyrrole nanolayers for high energy storage devices

In this work, we successfully demonstrate the fabrication of vertically aligned carbon nanotube (VACNT)-polypyrrole (PPY) nanocomposites as a “hybrid supercapacitor” material directly integrated on silicon-based electrodes. In contrast to previous works, three distinctive achievements have been accomplished: (1) a “hybrid supercapacitor” using VACNT forest with electroplated PPY and dodecylbenzenesulfonate (DBS) as a dopant in acetonitrile, (2) realizing 500% higher capacitance as compared to the capacitance of electrodes made of VACNT or DBS-doped PPY alone, and (3) highly reversible cycling between −1V and +1V with improved knee frequency at 797Hz. As such this hybrid nanocomposite could become a new class of material for future supercapacitors. We also demonstrate the life-cycle stability of the VACNT-PPY nanocomposite supercapacitor, as well as its rapid charge-discharge capabilities and low leakage current. Electrochemical impedance modeling results suggest that the VACNT-PPY supercapacitor has higher electric double layer capacitance in addition to higher pseudocapacitance compared to uncoated VACNTs.

Preparation of Nd2Fe14B/C magnetic powder and its application in proton exchange membrane fuel cells

Nd2Fe14B and Nd2Fe14B/C magnetic powders are prepared by the ball-milling and high-temperature baking methods, respectively. The effect of the magnetic powder in the oxygen transfer process is studied using the three-electrode electrochemical system, rotating disk glassy carbon electrode, and proton exchange membrane fuel cells (PEMFCs). Results show that the magnetic electrode has higher electric double-layer capacitance and lower charge-transfer resistance than the nonmagnetic electrode at different Nd2Fe14B/C load densities. In addition, the oxygen diffusion coefficient and transfer coefficient for the magnetic electrode are both larger than the nonmagnetic electrode. At 0.40 mg cm−2 Nd2Fe14B/C load density in the PEMFC cathode, the magnetic PEMFC discharge current increases by 39.874% compared with the nonmagnetic PEMFC at 0.20 V discharge voltage. The magnetic PEMFC discharge performance at 0.80 mg cm−2 Nd2Fe14B/C load density is lower than the magnetic PEMFC at 0.40 mg cm−2 load density. These factors result in the decline of magnetic PEMFC discharge performance at higher Nd2Fe14B/C load density, including decreased Pt/C actual catalyst area and increased magnetic interactions among different magnetic particles.

Ethanol-based synthesis of hierarchically porous carbon using nanocrystalline beta zeolite template for high-rate electrical double layer capacitor

A porous carbon with a mesopore–micropore hierarchy has been synthesized using ethanol as a carbon source within a beta zeolite template, which possesses mesopores and micropores in a hierarchical manner. Various compounds have been evaluated as the carbon source in the carbon synthesis. The result indicates that ethanol vapor is suitable for the hierarchical carbon synthesis, as compared with propylene or acetylene gas. The advantage of ethanol is attributed to the formation of water from the ethanol decomposition during carbonization process. The water seems to assist in delaying the deposition of carbons at the exterior (or the mesopore walls). The resultant carbon after removal of the template is composed of regular micropores of 1-nm diameter and irregular mesopores of 10–30nm arranged in a hierarchical manner. The hierarchical carbon exhibits a very high electrical double-layer capacitance both at low and high discharge current densities in galvanostatic measurements, as compared to solely microporous or solely mesoporous carbons. The high performance is attributed to facile transport of electrolytes in the hierarchically porous structure.

A BCN material synthesized from melamine resin and the factors influencing its high electric double layer capacitance

A BCN material synthesized from melamine resin and the factors influencing its high electric double layer capacitance

Supercapacitance of ruthenium oxide deposited on titania and titanium substrates

Titanium planar sheet formed by a chemical polishing process and titania nanotube array formed by an electrochemical anodization process are used as electrode substrates, on which electroactive ruthenium oxides are deposited by an electroreduction and electrooxidation process for supercapacitor applications. Morphological characterization and electrochemical properties of the electrode substrates and ruthenium oxide electrodes have been investigated. Crystalline titania nanotube array shows a much higher electric double layer capacitance than titanium planar sheet due to its high surface area of nanotube walls. Additionally, the well-defined ruthenium oxide–titania/titanium nanotube array electrode exhibits a much higher redox supercapacitance and a lower capacitance decay than ruthenium oxide/titanium planar film electrode. Such a superior energy-storage performance of ruthenium oxide–titania/titanium is ascribed to highly accessible nanotube channels for the reversible redox reaction of ruthenium oxide. The modification strategy of ruthenium oxide electrode by introducing highly ordered nanotube array structure instead of planar film structure can significantly improve specific capacitance as well as cyclic charge-discharge stability.

Enhanced Electric Double Layer Capacitance of Graphite Oxide Intercalated by Poly(sodium 4-styrensulfonate) with High Cycle Stability

We propose a new material for high power and high density supercapacitors with excellent cycle stability. Graphite oxide (PSS-GO) intercalated with poly(sodium 4-styrensulfonate) showed high performance of electric double layer capacitance (EDLC) compared to that of the pristine graphite oxide. Specific capacitance of the PSS-GO reached 190 F/g, and the energy density was much improved to 38 Wh/kg with a power density of 61 W/kg. Cycle test showed that the specific capacitance decreased by only 12% after 14860 cycles, providing excellent cyclic stability. The high EDLC performance of PSS-GO composite was attributed to the wide interlayer distance and simple pore structures accommodating fast ion kinetics.

Photochemical performance and electrochemical capacitance of titania nanocomplexes

Titania nanocomplexes, comprising the disordered nanoribbons or nanowires on the top surface and highly ordered nanotube array on the underlaying layer, has been fabricated by longitudinally splitting off nanotubes in a controlled anodization process. Anatase titania nanocomplexes show higher photovoltage and photocurrent responses and photocatalysis activity than titania nanotube array due to the enhanced light harvesting caused by nanoribbons and nanowires. Furthermore, titania nanowire–nanotube demonstrates a higher photoelectrical performance than nanoribbon–nanotube due to its thicker space charge layer caused by long nanotubes and more effective surface area contributed by nanowires. Cyclic charge–discharge measurements show that titania nanotube array exhibits a much higher electric double layer capacitance than titania nanocomplexes because the surface nanoribbons or nanowires inhibit the free diffusion and transportation of electrolyte ions into the underlaying nanotubes. Therefore, titania nanocomplexes can act as a photoactive material for photocatalysis applications and titania nanotube array can act as an electrode substrate for electrochemical supercapacitor applications.

Investigation on Capacitance Mechanisms of Fe3O4 Electrochemical Capacitors

The capacitance mechanisms of magnetite electrochemical capacitor in , , and KOH aqueous solutions have been investigated by electrochemical quartz-crystal microbalance analysis, along with cyclic voltammetry and X-ray photoelectron spectroscopy. The oxide thin-film electrode was prepared by an electroplating method, and exhibits a capacitance of , 25, and in , , and , respectively. Strong specific adsorption of the anion species was evidenced in all solutions. Experimental results indicate that, in , the capacitive current of magnetite electrode originates from the combination of electric double-layer capacitance (EDLC) and the pseudocapacitance that involves successive reduction of the specifically adsorbed sulfite anions, from through, e.g., , and vice versa. In , the current is due entirely to EDLC. Furthermore, due to the specific adsorption behavior, magnetite exhibits high EDLC, , in both and solutions. The lowest capacitance of magnetite was observed in KOH, which is attributed to the formation of an insulating layer on the magnetite surface.

Assessing optimal pore-to-ion size relations in the design of porous poly(vinylidene chloride) carbons for EDL capacitors

In order to maximize the capacitance of electrical double layer (EDL) capacitors per unit electrode volume, higher surface areas are required. This leads to an increasingly greater subdivision of the carbon electrode, namely to pore systems of smaller pore size. When the pore size approaches the ion size, the EDL charging kinetics lows down because of multiple interactions of the ions with the surrounding pore walls. On the other hand, the ion electroadsorption capacity increases just because of this enhanced interaction. Therefore, there is a conflict between improving discharge kinetics and improving capacity. Knowing the effective ion size relative to the pore size can be helpful in optimizing the pore system design in electrodes for EDL capacitors. A thorough technique based on the adsorption of molecular probes in the gas phase and the electroadsorption of different ions was developed to assess pore dimensions. In this report, the technique is applied to the unique case of polyvinylidene chloride (PVDC) based carbon electrodes in an attempt to elucidate its extraordinary high EDL capacitance, in terms of the relation between effective ion size and pore size.

Structural characterization and electric double layer capacitance of template carbons

Novel carbon structure has been prepared from heterocyclic hydrocarbons, which contain a nitrogen atom, using a fluorine expandable mica as a template. The obtained carbon particles as a residue after acid washing of carbon/mica nanocomposite comprised two structural parts: very thin film and surrounding rim part. The latter rim part has constant width of about 20 nm. In addition, on account of the nitrogen atoms in the starting hydrocarbons, a certain amount (10–18 wt.%) of nitrogen remained in the resulting template carbon. These template carbons indicated a high electric double layer capacitance amounting to more than 100–180 F/g in H 2SO 4 electrolyte. Because of low surface area of the samples, specific capacitance per unit surface area could reach 1.2–2.2 F/m 2, which is over 10 times as high as the capacitance of ordinary activated carbons. The capacitance decreased markedly with heat treatment temperature (>1000 °C) of the template carbon. Since the heat treatment accompanied the elimination of nitrogen from the sample, the presence of residual nitrogen may account for such a high capacitance.

Preparation of spherical activated carbon with high electric double-layer capacitance

Preparation of spherical activated carbon with high electric double-layer capacitance