Carbon-based Electrical Double-layer Capacitors Research Articles

Investigation of ionic liquid-doped ion conducting polymer electrolytes for carbon-based electric double layer capacitors (EDLCs)

Ionic liquid-based poly(vinyl alcohol) (PVA)/ammonium acetate (CH3COONH4) ion conducting polymer electrolytes are prepared by solution casting technique. Ionic liquid, 1-butyl-3-methylimidazoliumtrifluoromethanesulfonate (BmImTf) is then added to increase the ionic conductivity of polymer electrolytes by reducing the glass transition temperature (Tg).It is found that ionic conductivity of polymer electrolytes increases with temperature. The polymer electrolytes are found to obey the Vogel–Tamman–Fucher (VTF) rule in the conduction mechanism which is related to free volume model. Subsequently, two symmetrical electrodes containing activated carbon, carbon black and carbon nanotubes (CNT) are prepared through the dip coating method. Electric double layer capacitors (EDLCs) are then assembled using these electrodes and polymer electrolytes. Excellent electrochemical properties such as higher specific capacitance were obtained for EDLC comprising ionic liquid–doped polymer electrolytes. Power density of 18.37kWkg−1 was obtained for the EDLC containing the most conducting ionic liquid–added polymer electrolyte.

ChemInform Abstract: Cobalt‐based Compounds and Composites as Electrode Materials for High‐performance Electrochemical Capacitors

AbstractReview: 391 refs.

Performance comparison for 12 V lead-carbon hybrid ultracapacitors with substrate-integrated and conventional pasted positive plates

Lead-carbon hybrid ultracapacitors comprise positive lead dioxide plates of the lead-acid battery and negative plates of carbon-based electrical double-layer capacitors (EDLCs). Accordingly, a lead-carbon hybrid ultracapacitor has the features of both the battery and that of an EDLC. In this study, the development and performance comparison between the two types of lead-carbon hybrid ultracapacitors, namely those with substrate-integrated and conventional pasted positive plates, is presented as such a study is lacking in the literature. The study suggests that the faradaic efficiencies for both types of lead-carbon hybrid ultracapacitors are comparable. However, their capacitance values as well as energy and power densities differ significantly. For substrate-integrated positive plate hybrid ultracapacitor, capacitance and energy density values are lower, but power density values are higher than pasted positive plate lead-carbon hybrid ultracapacitors due to their shorter response time. Accordingly, internal resistance values are also lower for substrate-integrated lead-carbon hybrid ultracapacitors. Both types of lead-carbon hybrid ultracapacitors exhibit good cycle life of 100,000 pulse charge-discharge cycles with only a nominal loss in their capacitance values.

Electric double-layer capacitors based on highly graphitized nanoporous carbons derived from ZIF-67.

Nanoporous carbons (NPCs) have large specific surface areas, good electrical and thermal conductivity, and both chemical and mechanical stability, which facilitate their use in energy storage device applications. In the present study, highly graphitized NPCs are synthesized by one-step direct carbonization of cobalt-containing zeolitic imidazolate framework-67 (ZIF-67). After chemical etching, the deposited Co content can be completely removed to prepare pure NPCs with high specific surface area, large pore volume, and intrinsic electrical conductivity (high content of sp(2) -bonded carbons). A detailed electrochemical study is performed using cyclic voltammetry and galvanostatic charge-discharge measurements. Our NPC is very promising for efficient electrodes for high-performance supercapacitor applications. A maximum specific capacitance of 238 F g(-1) is observed at a scan rate of 20 mV s(-1) . This value is very high compared to previous works on carbon-based electric double layer capacitors.

Capacitance of carbon-based electrical double-layer capacitors

Experimental electrical double-layer capacitances of porous carbon electrodes fall below ideal values, thus limiting the practical energy densities of carbon-based electrical double-layer capacitors. Here we investigate the origin of this behaviour by measuring the electrical double-layer capacitance in one to five-layer graphene. We find that the capacitances are suppressed near neutrality, and are anomalously enhanced for thicknesses below a few layers. We attribute the first effect to quantum capacitance effects near the point of zero charge, and the second to correlations between electrons in the graphene sheet and ions in the electrolyte. The large capacitance values imply gravimetric energy storage densities in the single-layer graphene limit that are comparable to those of batteries. We anticipate that these results shed light on developing new theoretical models in understanding the electrical double-layer capacitance of carbon electrodes, and on opening up new strategies for improving the energy density of carbon-based capacitors.

Super-capacitive electro-chemical performance of polymer blend gel polymer electrolyte (GPE) in carbon-based electrical double-layer capacitors

This study reports on the fabrication and comparative performance characteristics of a symmetrical electrical double-layer capacitor (EDLC) employed gel polymer electrolyte (GPE) assembled between carbon based electrodes. Three cells, A, B and C were fabricated using different composition of active materials (activated or porous carbon), binder (PVdF-HFP) and conductivity enhancer (super-P). The configuration of cell A: 0.9 porous carbon/0.1 PVdF-HFP, cell B: 0.45 activated carbon/0.45 porous carbon/0.1 PVdF-HFP and cell C: 0.8 activated carbon/0.1 super-P/0.1 PVdF-HFP. The GPE, comprising a poly(vinyl pyrrolidone) (PVP)/poly(vinylidene fluoride co-hexafluoroproplyne) (PVdF-HFP) blend complexed with magnesium triflate, Mg(CF3SO3)2, was prepared by the solution casting technique at 60°C. The physico-chemical properties of the GPEs were characterized by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The ionic conductivity at the ambient temperature of the GPE is 2.16×10−4Scm−1 at 7.5wt.% of Mg(CF3SO3)2 with a ∼2.6V electro-chemical stability window. At the 1000th cycle, the specific capacitance, Cs of cell A is 89Fg−1 while cell B and C are 63 and 49Fg−1. Cell A shows excellent long-term cyclic stability (less than a 5% decrease in specific capacitance after 1000 cycles). The best operating voltage for cell A is 1.6V with the specific capacitance 106Fg−1 after 500 cycles.

Scaling laws for carbon-based electric double layer capacitors

This paper presents scaling laws for the integral capacitance of electric double layer capacitors (EDLCs) supported by rigorous analysis and experimental data for porous carbon-based electrodes. First, dimensionless similarity parameters were identified based on dimensional analysis of the modified Poisson–Boltzmann model for binary and symmetric electrolytes. Then, a correlation for the equilibrium potential at the Stern/diffuse layer interface near planar electrodes and near cylindrical and spherical pores was obtained as a function of previously identified dimensionless numbers for a wide range of parameters. Similarly, an analytical expression was proposed for the dimensionless double layer integral capacitance of planar electrodes. Finally, scaling analysis was applied to experimentally measured integral capacitance of mesoporous carbon electrodes with a wide range of morphology and different electrolytes. To maximize the integral areal capacitance, the electrolyte should have small ion effective diameter and large dielectric constant. The electrode pore diameter should be tailored to match the ion diameter.

Important parameters affecting the cell voltage of aqueous electrical double-layer capacitors

This study discusses and demonstrates how the open-circuit potential and charges stored in the working potential window on positive and negative electrodes affect the cell voltage of carbon-based electrical double-layer capacitors (EDLCs) in aqueous electrolytes. An EDLC consisting of two activated carbon electrodes is employed as the model system for identifying these key parameters although the potential window of water decomposition can be simply determined by voltammetric methods. First, the capacitive performances of an EDLC with the same charge on positive and negative electrodes are evaluated by cyclic voltammetric, charge–discharge, electrochemical impedance spectroscopic (EIS) analyses, and inductance–capacitance–resistance meter (LCR meter). The principles for obtaining the highest acceptable cell voltage of such symmetric ECs with excellent reversibility and capacitor-like behaviour are proposed. Aqueous charge-balanced EDLCs can be operated as high as 2.0 V with high energy efficiency (about 90%) and only 4% capacitance loss after the 600-cycle stability checking. The necessity of charge balance (but not capacitance balance) for positive and negative electrodes is substantiated from the lower acceptable cell voltage of charge-unbalanced EDLCs.

Application of a novel binder for activated carbon-based electrical double layer capacitors with nonaqueous electrolytes

In this work, we have fabricated activated carbon electrodes using the binder LA135 and assembled electrical double layer capacitors with nonaqueous electrolytes of 1 M tetraethyl ammonium tetrafluoroborate (Et4NBF4) in propylene carbonate (PC), 1 M Et4NBF4 in acetonitrile (AN), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid, respectively. The main chemical compositions of the binder are polyacrylonitrile and styrene–butadiene rubber. Scanning electron microscope images show that the conductive agents have been uniformly dispersed on the activated carbons in the electrode. The thermal stabilities of electrodes using different binders are studied by thermogravimetric analysis. The electrochemical properties of cells in different nonaqueous electrolytes are characterized by cyclic voltagramms, electrochemical impedance spectra, galvanostatic charge–discharge, leakage current, and cycle life measurements. The capacitor in Et4NBF4/AN has the lowest internal resistance and superior high-rate capability, and the one in Et4NBF4/PC has the smallest leakage current. The capacitor in EMIMBF4 has the energy density as high as 35.4 Wh kg−1 at a current density of 0.2 A g−1 (based on the total mass of active materials), which is 1.6 times higher than that of capacitor in PC electrolyte. Besides, the electrochemical properties of capacitors with different binders are comparatively studied. The capacitor using LA135 has the highest specific capacitance and moderate internal resistance comparing with the ones using poly(tetrafluoroethylene), sodium carboxymethyl cellulose + styrene–butadiene rubber or poly (vinylidene fluoride).

Scalable High-Power Redox Capacitors with Aligned Nanoforests of Crystalline MnO2Nanorods by High Voltage Electrophoretic Deposition

It is commonly perceived that reduction-oxidation (redox) capacitors have to sacrifice power density to achieve higher energy density than carbon-based electric double layer capacitors. In this work, we report the synergetic advantages of combining the high crystallinity of hydrothermally synthesized α-MnO2 nanorods with alignment for high performance redox capacitors. Such an approach is enabled by high voltage electrophoretic deposition (HVEPD) technology which can obtain vertically aligned nanoforests with great process versatility. The scalable nanomanufacturing process is demonstrated by roll-printing an aligned forest of α-MnO2 nanorods on a large flexible substrate (1 inch by 1 foot). The electrodes show very high power density (340 kW/kg at an energy density of 4.7 Wh/kg) and excellent cyclability (over 92% capacitance retention over 2000 cycles). Pretreatment of the substrate and use of a conductive holding layer have also been shown to significantly reduce the contact resistance between the aligned nanoforests and the substrates. High areal specific capacitances of around 8500 μF/cm(2) have been obtained for each electrode with a two-electrode device configuration. Over 93% capacitance retention was observed when the cycling current densities were increased from 0.25 to 10 mA/cm(2), indicating high rate capabilities of the fabricated electrodes and resulting in the very high attainable power density. The high performance of the electrodes is attributed to the crystallographic structure, 1D morphology, aligned orientation, and low contact resistance.

Effect of crystallinity on the electrochemical properties of carbon black electrodes

Carbon-based electric double-layer capacitors are being evaluated as potential energy-stor- age devices in an expanding number of applications. In this study, samples of carbon black (CB) treated at different temperatures ranging from 650°C to 1100°C were used as elec- trodes to improve the efficiency of a capacitor. The surface properties of the heat-treated CB samples were characterized by X-ray photoelectron spectroscopy and X-ray diffraction. The effect of the heat-treatment temperature on the electrochemical behaviors was investigated by cyclic voltammetry and in galvanostatic charge-discharge experiments. The experimental results showed that the crystallinity of the CBs increased as the heat-treatment temperature increased. In addition, the specific capacitance of the CBs was found to increase with the increase in the heat-treatment temperature. The maximum specific capacitance was 165 F·g- 1 for the CB sample treated at 1000 o C.

Effect of oxygen-containing functional groups on the impedance behavior of activated carbon-based electric double-layer capacitors

The surface chemistry of activated carbon was treated with sulfuric acid and hydrogen to analyze the oxygen-containing functional groups on the impedance behavior of electric double-layer capacitors. Based on the electrochemical impedance spectrum (EIS), an equivalent circuit model was proposed considering the kinetic and charge transfer characteristics, and Marquardt fit procedure was applied to the EIS data. The simulated results indicate that the oxidation treatment made the ionic resistance within the pore of carbon electrode decrease, and the ion diffusion coefficients significantly increase, which leads to improvement of power capability of the carbon electrode.

Basic Characteristic of Electric Double Layer Capacitor Mixing Ketjen Black as Conducting Filler

In this study, ketjen black was used for polarized electrodes instead of conventionally used acetylene black in order to improve the capacitance of activated carbon-based Electric Double Layer Capacitor (EDLC). Usually, ketjen black exhibits higher electronic conductivity compared to acetylene black, so that ketjen black could be considered as conducting filler. Moreover, N2 adsorption-desorption isotherm as well as TEM observation revealed that ketjen black used in this study has different type of pore structures compared to acetylene black. The capacitances of prepared EDLC were evaluated from discharge characteristics. It was found that the ketjen black containing EDLC showed fairly high capacitance compared to acetylene black containing one. Especially, 8 wt% ketjen black containing EDLC showed the highest capacitance among the prepared samples. The specific capacitance of the best one was evaluated as 59.2 F/g. It was also found that the 8 wt% ketjen black containing EDLC exhibited fairly stable capacitance at elevated tempareture from temperature dependence of capacitance for 10 wt% acetylene black containing EDLC and 8 wt% ketjen black containing one.