Electrochemical Double Layer Capacitor Electrodes Research Articles

The effects of cell assembly compression on the performance of carbon electrochemical double-layer capacitor electrodes

Our previous work concluded that the application of force altered the physical structure of the activated carbon electrodes, which resulted in a decrease in the accessible surface area and a displacement of the electrolyte. In this work, the response that different carbon material electrodes exhibit to an applied force was evaluated. Activated carbon powders possess different porous structures, which would exhibit different behaviors when subjected to an applied force and after the release of that force. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used to characterize the response behaviors of the different carbons. Furthermore, a porosimetry analysis was conducted on the carbon material of the electrode before and after the application of force. It was concluded that the application of force shifted the pore distribution toward overall smaller pores through a compression of the porous structure of the carbon. This resulted in a decrease in the more easily accessible surface area, which was exhibited as a decrease in the capacitance values as calculated from the cyclic voltammetry data. There was no longer sufficient time to access the now smaller powers at the given time scale of the cyclic voltammetry analysis, which negatively impacted the formation of the double layer.

Electric Pulse Discharge Activated Carbon Supercapacitors for Transportation Application

ScienceTomorrow is developing a high-speed, low-cost process for synthesizing high-porosity electrodes for electrochemical double-layer capacitors. Four types of coal (lignite, subbituminous, bituminous, and anthracite) were used as precursor materials for spark discharge activation with multiscale porous structure. The final porosity and pore distribution depended, among other factors, on precursor type. The high gas content in low-grade carbon resulted in mechanical disintegration, whereas high capacitance was attained in higher-grade coal. The properties, including capacitance, mechanical robustness, and internal conductivity, were excellent when the cost is taken into consideration.

Multiple time constant modelling of a printed conducting polymer electrode

Constant phase elements (CPE) are routinely used to describe the frequency response of electrochemical systems. However, this approach is often scientifically unsatisfactory because the physical origin of the phase is unclear. Here we observe CPE-like behaviour in a conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) (PEDOT:PSS) film that was inkjet printed onto paper to form a flexible electrochemical double layer capacitor electrode. We show that the response of the electrochemically active film can also be described using a physical model with multiple parallel finite RC (resistor–capacitor) transmission lines whose lengths and time constants are determined by the distribution of the measured film thickness. The modeled volumetric capacitance and ionic conductivity match those determined experimentally, suggesting that the physical origin of the constant phase response is a distribution of mass transport limited time constants.

Analytical Impedance Model for Electrochemically Driven Conducting Polymer Devices

An analytical model is presented to describe the electrochemical impedance of conducting polymer based devices. The analytical expression of the impedance is obtained from a two dimensional finite transmission line equivalent circuit. The model relates impedance to cell geometry, electrolyte conductivity, polymer ionic and electronic conductivities and capacitance. These parameters were measured for a hexafluorophosphate (PF6-) doped polypyrrole material (the conducting polymer used in this study) and entered to the model to predict its impedance as a function of frequency. The model is unique in representing the two dimensional charging of the polymer, namely ionic mass transport through the thickness of the polymer structure and electronic resistance along its length. Close agreement is observed between impedance spectroscopy results and model prections of the charging of a polypyrrole film electrically connected at one end. The provides a means of modeling the electrochemical charging of conducting polymers and electrochemical double layer capacitor electrodes having significant ionic and electronic conductivities.

Ferrocene Derived Carbon Nanotubes and Their Application as Electrochemical Double Layer Capacitor Electrodes

We report on the synthesis of carbon nanotubes (CNTs) by direct thermal decomposition of ferrocene (Fe(C5H5)2). Our studies indicate that presence of a small amount of sulfur along with the ferrocene in the decomposition process strongly affects the quality of the CNTs produced and is crucial for obtaining thin diameter nanotubes. Raman spectroscopic investigations suggest that the atomic ratio of sulfur to the total iron plus sulfur content of approximately 0.09 yields CNTs with highly crystalline structure having diameters ranging from 0.85 nm to 1.75 nm. Electrochemical double layer capacitor electrodes fabricated from these CNTs show impressive energy storage properties, capable of delivering a maximum power density of approximately 27 KW/kg and energy density of approximately 2.12 Wh/Kg.

KOH activation of ordered mesoporous carbons prepared by a soft-templating method and their enhanced electrochemical properties

Ordered mesoporous carbon (COU-2) was synthesized by a soft-templating method. The COU-2 mesoporous carbon was activated by using KOH to improve its porosity. The mesopore size of COU-2 was 5.5nm and did not change by the KOH activation. But, the BET surface area of COU-2 largely increased from 694 to 1685m2/g and total pore volume was increased from 0.54 to 0.94cm3/g after the KOH activation. The large increase of micropore volume is due to the increase of the surface area. Electrochemical cyclic voltammetry measurements were conducted in aqueous (1M sulfuric acid) and organic (1M tetraethyl ammonium tetrafluoroborate/polypropylene carbonate) electrolyte solutions. The KOH-activated COU-2 carbon shows superior capacitances over the COU-2 carbon and a commercial microporous carbon both in aqueous and organic electrolyte solutions. These results suggest that the carbons having regularly-interconnected uniform mesopores and micropores in thin pore walls are desirable for the electrodes in electrochemical double-layer capacitors.

A dilatometric and small-angle X-ray scattering study of the electrochemical activation of mesophase pitch-derived carbon in non-aqueous electrolyte solution

The electrochemical activation of certain pitch-derived carbons has been proposed as a promising route towards obtaining high-capacitance electrodes for electrochemical double-layer capacitors. In the present work, the mechanism of electrochemical activation of a graphitizable carbon after calcination and KOH-activation was studied by nitrogen adsorption, electrochemical dilatometry and in situ small-angle X-ray scattering (SAXS). During electrochemical activation, a large capacitance gain from 1 to 121F/g (at 0V in a 1mol/L solution of Et4NBF4 in propylene carbonate) was accompanied by a significant irreversible swelling of the electrode by 24% (6%) for activation in the negative (positive) potential range, respectively. In situ SAXS provided clear evidence for the insertion of ions into the latent microporosity of the carbon during electrochemical activation. Thus, the mechanism of electrochemical activation of weakly activated graphitizable carbon is not strictly due to ion intercalation between parallel graphene sheets.

Qualitative Electrochemical Impedance Spectroscopy study of ion transport into sub-nanometer carbon pores in Electrochemical Double Layer Capacitor electrodes

Ion adsorption onto high surface area microporous Carbide Derived Carbons (CDCs) with pore sizes in the sub-nanometer range was studied by means of the Electrochemical Impedance Spectroscopy (EIS) technique in two electrolytes, Tetraethylammonium Tetrafluoroborate (NEt 4BF 4) in Acetonitrile (AN) and in Propylene Carbonate (PC). Polarization at two bias voltages (0.5 V/Ref and −1 V/Ref) for EIS measurements enabled comparing the capacitive behaviors resulting from anions and cations adsorption, respectively, it was confirmed that the effective size of NEt 4 + is bigger than the one of BF 4 −. Higher transport limitation was then observed for cations and was exalted in PC-based electrolyte. Although slow ion transport kinetics, it was found that the low frequency vertical line observed on the Nyquist plots was preserved meaning that carbon electrodes were fully charged. This study confirmed the importance of choosing an electrode carbon pore size adapted to the effective ion size. Finally, the best performances would be got in 1.5 M NEt 4BF 4 AN-based electrolyte with a 0.76 nm pore size negative electrode and a 0.68 nm pore size positive electrode.

Preparation and performance of straw based activated carbon for supercapacitor in non-aqueous electrolytes

The activated carbon with honeycomb-like morphology for electrochemical double layer capacitors (EDLCs) was prepared from wheat straw by carbonization and subsequent activation with KOH. The product presented micro- and mesoporous structure with the peak pore size of 2.1 nm and high specific surface area of 2316 m 2/g. The electrochemical capacitance performances of the activated carbon as EDLC electrodes were characterized by using cyclic voltammetry (CV), alternating current (AC) impedance. The specific capacitance of the activated carbon reached 251.1 F/g at 2 mV/s scan rate in the electrolyte of MeEt 3NBF 4/AN. The thermal gravimetric analysis of AN impregnated carbon was explored as a new approach to investigate the effectivity of pore for the electrolytes. The relationships between specific capacitance and scan rate, surface area and pore diameter were discussed.

Electrochemical double layer capacitor electrodes using aligned carbon nanotubes growndirectly on metals

We report on the fabrication of electrochemical double layer capacitor(EDLC) electrodes with aligned carbon nanotubes (CNTs) grown directly onconductive substrates using an air assisted chemical vapor deposition technique.The fabricated EDLCs showed very small equivalent series resistances (∼few hundredsof mΩ),a direct consequence of integrating CNTs with metal current collectors. The specific capacitance ofthe CNTs used for EDLC electrodes increased with decreasing CNT lengths and ranged from 10.75 F g−1 to21.57 F g−1 with maximum energy and power density ranging from 2.3 to 5.4 Wh kg−1 and 19.6to 35.4 kW kg−1, respectively. These results indicate that the integrated CNT electrodes fabricated using asimple single step process hold significant promise in applications related to electrochemicalenergy storage.

Acetylene black agglomeration in activated carbon based electrochemical double layer capacitor electrodes

The influence of acetylene black (AB) content (0–20 wt.%) on the electrochemical performance of activated carbon based electrochemical double layer capacitor (EDLC) electrodes has been studied systematically. The electrochemical performance was evaluated by galvanostatic charging/discharging, cyclic voltammetry, and alternating current impedance. Experimental results indicated that 5 wt.% AB was the best choice for electrodes presenting high energy density and good rate capability. AB ensures that the composite electrodes present high electronic conductivity; however, scanning electron microscope images show that excessive AB form large agglomerates, which stuff the voids among activated carbon particles and hinder the electrolyte ion transfer during the electrochemical double layer charging/discharging. Thus the composite electrodes with excessive AB lose their hierarchically porous structure and the subsequent ion-sieving effect deteriorates the electrochemical performance.

Comparison Between Electrochemical Properties of Aligned Carbon Nanotube Array and Entangled Carbon Nanotube Electrodes

Carbon nanotubes (CNTs) are promising electrochemical double-layer capacitor electrode materials because they have excellent conductivity and mesopores dominated pore structures. Electrochemical properties of an ultralong aligned carbon nanotube array (ACNTA) electrode and an entangled CNT (ECNT) electrode in ionic liquid electrolyte were studied by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance. The ACNTA electrode obtains higher specific capacitance, lower equivalent series resistance, and better rate capability than the ECNT electrode. The reason is that ACNTA electrode possesses larger pore size and more regular pore structure and conductive paths, which are revealed by adsorption and scanning electron microscopy results, than ECNT electrode.

On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated material—A review

Supercapacitors or electrochemical double-layer capacitors (EDLCs) have capacitance value up to thousands of Farads at the same size as for conventional capacitors. At such capacitance value EDLCs are of interest for electrical energy storage. The specific energy of commercial supercapacitors is limited to 5–6 Wh/kg, whereas for batteries the lower limit is 35–40 Wh/kg. Nonetheless other advantages of supercapacitors make them already useful in conjunction with batteries in power applications. Main results related to supercapacitor performance improvement available in literature are presented. Research efforts have been done to increase the specific capacitance of supercapacitor electrodes based on activated or porous carbon material, already used in commercial products. By using available activated carbon with a specific surface area reaching 3000 m 2/g, specific capacitance values up to 300 F/g have been reported for the investigated experimental supercapacitors. Nonetheless, further optimization of activated carbon properties and its use in supercapacitor electrodes is required for 300 F/g and higher value. By addition of metallic oxides or conductive polymers in the activated carbon used for EDLC electrodes, specific capacitance enhancement takes place. Carbon nanotubes used in experimental supercapacitor electrodes resulted in specific capacitance as high as 180 F/g but higher electrical conductivity and consequently, specific power than in the case of activated carbon was observed. Addition of a small percent of carbon nanotubes in the activated carbon for electrodes results in performance improvement (higher capacitance and conductivity). Nevertheless, high cost of carbon nanotubes prevents their use in commercial products.

Electrochemical properties of ultra-long, aligned, carbon nanotube array electrode in organic electrolyte

The electrochemical properties of an electrochemical double-layer capacitor electrode based on an ultra-long (500 μm), aligned, carbon nanotube array (ACNTA) in Et 4NPF 6/propylene carbonate electrolyte are examined. The specific capacitance of the ACNTA electrode in an organic electrolyte is 24.5 F g −1, which is larger than that obtained in an aqueous electrolyte. The results of ac impedance measurements show that the ACNTA electrode gives a high power density and an excellent rate capability in an organic electrolyte. It is shown that the ACNTA electrode has a lower equivalent series resistance and a better rate capability than activated carbon electrode. This is due to the fact that ACNTA possesses a larger pore size and a more a regular pore structure. Both these features are conformed by scanning electron microscopic and nitrogen gas adsorption studies.

Comment on “ Studies on nanoporous glassy carbon as a new electrochemical capacitor material [Y. Wen, G. Cao, Y. Yang, J. Power Sources 148 (2005) 121–128

Gas-phase activated monolithic glassy carbon was used as electrochemical double layer capacitor electrode by a research team of Siemens AG in the early 1980s [1], J. Miklos, K. Mund, W. Naschwitz, Siemens AG, Offenlegungsschrift DE 30 11 701 A1, German Patent (1980). Wen et al. [2] (Y.H. Wen, G.P. Cao, J. Cheng, Y.S. Yang, New Carbon Mater. 18(3) (2003) 219, and [3] Y.H. Wen, G.P. Cao, Y.S. Yang, J. Power Sources 148 (2005) 121, have repeatedly questioned the performance of this glassy carbon based supercapacitor electrode concept. This asks for some comments.

Pressure evolution in propylene carbonate based electrochemical double layer capacitors

Increasing the available cell voltage for electrochemical double layer capacitors (EDLC) is one route to simultaneously increase energy density and power density of the EDLC. Increased cell voltage may, however, introduce faradaic reactions such as ion insertion and electrolyte decomposition, which potentially limit the lifetime of the device. Using a purpose designed pressure cell, we have, for the first time, measured the pressure increase in capacitor cells based on real EDLC electrode coils in 1 M (C 2H 5) 4NBF 4/propylene carbonate electrolyte during cycling between 0 and 2.5 V and for constant cell voltages up to 3 V. During cycling a reversible pressure decrease was observed upon charging. An irreversible pressure increase was monitored during load tests at constant cell voltages above 2.5 V. The absolute amount of gases evolved could be determined by means of simultaneous compressibility monitoring.

Mesoporous Carbon Materials as Electrodes for Electrochemical Double-Layer Capacitor

ABSTRACTNanostructured carbon materials with a regular array and narrow size distribution of mesopores have been synthesized at Oak Ridge National Laboratory via the self-assembly of block copolymers as soft templates. One promising application for these materials is as electrodes for electrochemical double-layer capacitors. To evaluate the performance, electrodes were prepared by coating the precursor onto fibrous carbon paper, followed by curing and pyrolysis at 850°C. The resulting mesoporous carbon has a surface area of 310 m2/g and an average pore size of 8.5 nm. Results from cyclic voltammetry and impedance spectroscopy experiments using a sulfuric acid electrolyte showed high specific capacitance values of up to 300 F/g. For comparison, a commercially available aerogel carbon coated paper was also examined.

Chemical treatment of carbon nanotubes as electrodes in electrochemical double-layer capacitors

Multi-walled carbon nanotubes with homogeneous diameters (40–60 ran), produced by chemical vapor deposition of hydrocarbon gas, are purified by nitric acids. Infrared and Raman studies indicate that oxygen containing surface groups, which are predominately carboxylic, phenolic and lactonic groups, are introduced into purified carbon nanotubes. Then three kinds of block-form porous tablets of carbon nanotubes are fabricated as electrodes in electrochemical double-layer capacitors. Using mounded mixture comprising carbon nanotubes and binder powders provides these tablets. Comparison of the effect of different processing on the structural performance of the capacitors is specifically investigated. Using chemically treated electrodes, electrochemical double-layer capacitors with a specific capacitance of about 33 F/g are obtained with 38 wt% H2 SO4 as the electrolyte.

Carbon based double layer capacitors with aprotic electrolyte solutions: the possible role of intercalation/insertion processes

The extraordinary stability and cycle life performance of today’s electrochemical double-layer capacitors (EDLCs) are generally ascribed to the fact that charge storage in activated carbon (AC) is based on pure double-layer charging. In contrast, Faradaic charge-transfer reactions like those occurring in batteries are often connected with dimensional changes, which can affect the cycle life of these storage devices. Here we report the charge-induced height change of an AC electrode in an aprotic electrolyte solution, 1 mol/l (C2H5)4NBF4 (TEABF4) in acetonitrile. The results are compared with those obtained for a graphite electrode in the same electrolyte. For both electrodes, we observe an expansion/contraction of several percent for a potential window of ±2 V vs. the immersion potential (ip). For the EDLC electrode, significant expansion starts at about 1 V remote from the ip and hence is well within the normal EDLC operation range. For the graphite electrode, the height changes are unambiguously caused by intercalation/deintercalation of both anions and cations. The close analogies between the graphite and the EDLC electrode suggest that ion intercalation or insertion processes might play a major role for charge storage, self discharge, cyclability, and the voltage limitation of EDLCs.

Simple Synthesis of Uniform Mesoporous Carbons with Diverse Structures from Mesostructured Polymer/Silica Nanocomposites

Uniform-sized mesoporous carbons having high surface areas and various structures were synthesized directly by the carbonization of P123 triblock copolymer/phenol−resin/silica nanocomposites, which were prepared from the sol−gel polymerization of silica in the presence of Pluronic P123 triblock copolymer and phenol and subsequent silica removal. Mesoporous carbon, with uniform 5.6 nm wormhole-like pores, was synthesized using sodium silicate as a silica source. When the amount of phenol relative to block copolymer was reduced in the synthesis, mesocellular carbon foams with bimodal pore structures were obtained. In addition, mesoporous carbons were synthesized using TEOS (tetraethyl orthosilicate) as a silica source. The pore size of the mesoporous carbons was controlled by varying the ratio of phenol and P123 triblock copolymer. At an optimal molar ratio of phenol to block copolymer, mesoporous carbon with a nanofiber morphology was obtained. The mesoporous carbons were successfully used as the electrodes for electrochemical double-layer capacitors.