Symmetric Electric Double-layer Capacitors Research Articles

Improving Performance of Electric Double Layer Capacitors with a Mixture of Ionic Liquid and Acetonitrile as the Electrolyte by Using Mass-Balancing Carbon Electrodes

In the present study, we demonstrate the beneficial effect of mass-balancing electrodes in electric double layer capacitors (EDLCs) based on an acetonitrile/ionic liquid electrolyte. Cyclic voltammetry of single carbon electrodes in a mixture of acetonitrile and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) 50% wt was performed to calculate the optimum mass ratio of electrodes (m+/m−) that was found to be 2.0. Asymmetric EDLCs were assembled using this mass ratio and long-term cyclability testing was performed by galvanostatic charge/discharge at different cutoff voltages for 10000 cycles. Symmetric EDLCs were also assembled and characterized for comparison purposes. Asymmetric EDLCs showed an improved performance retaining the 99% of initial specific capacitance (CEDLC), 96% of initial specific power (Paverage) and the 86% of the initial real specific energy (Ereal) after 10000 cycles at 3.2 V. On the contrary, the maximum operating voltage for the symmetric EDLC was only 2.7 V since a dramatic decrease in performance was observed at 3.0 V and 3.2 V. By using the mass-balancing strategy, it was possible to extend the maximum operating voltage from 2.7 V to 3.2 V. This increment in operating voltage resulted in higher values of real specific energy (Ereal) that increased from 25 Wh kg−1 to 33 Wh kg−1.

Gel Electrolyte Derived from Poly(ethylene glycol) Blending Poly(acrylonitrile) Applicable to Roll‐to‐Roll Assembly of Electric Double Layer Capacitors

AbstractThe synthesis of a gelled polymer electrolyte (GPE) using poly(ethylene glycol) blending poly(acrylonitrile) (i.e., PAN‐b‐PEG‐b‐PAN) as a host, dimethyl formamide (DMF) as a plasticizer and LiClO4 as an electrolytic salt for electric double layer capacitors (EDLCs) is reported. The PAN‐b‐PEG‐b‐PAN copolymer in the GPE has a linear configuration for high ionic conductivity and excellent compatibility with carbon electrodes. When assembling the GPE in a carbon‐based symmetric EDLC, the copolymer network facilitates ion motion by reducing the equivalent series resistance and Warburg resistance of the capacitor. This symmetric cell has a capacitance value of 101 F g−1 at 0.125 A g−1 and can deliver an energy level of 11.5 Wh kg−1 at a high power of 10 000 W kg−1 over a voltage window of 2.1 V. This cell shows superior stability, with little decay of specific capacitance after 30 000 galvanostatic charge‐discharge cycles. The distinctive merit of the GPE film is its adjustable mechanical integrity, which makes the roll‐to‐roll assembly of GPE‐based EDLCs readily scalable to industrial levels.

Structural Feature and Double-Layer Capacitive Performance of Porous Carbon Powder Derived from Polyacrylonitrile-Based Carbon Fiber

A microporous carbon powder derived from pulverizing a polyacrylonitrile-based activated carbon fiber showed an excellent performance serving as electrodes for symmetric electric double-layer capacitors using sulfuric acid as the electrolyte. In comparison with conventional activated carbon powders derived from a phenol-formaldehyde resin, this fiber-derived carbon showed a large ultimate capacitance value and could still retain a high capacitance at high current rates. Quantitative characterization on the pore structure of these carbons was conducted. The electrochemical impedance spectra of the capacitors were adequately fitted to an equivalent circuit containing a Warburg element. The fiber-derived carbon was found to have a greater effective diffusivity for electrolyte transport due to its smaller tortuosity factor, which had a value of 4–7 times smaller than those of the conventional carbon powders. The results of oxygen chemisorption on the carbons suggested that the pore walls of the fiber-derived carbon were more populated with graphite-like crystallite edges. This feature would lead to a stronger specific adsorption and thus a higher double layer capacitance.