Electrochemical capacitors, also called supercapacitors, are high power devices that exhibit moderate energy density. One interesting strategy to improve this latter parameter is the use of additional redox reactions that can enhance the double layer capacity of carbon electrodes while maintaining high rate capability. Diazonium chemistry is a powerful tool for synthesizing carbonaceous materials with active redox sites over the surface. Grafting of 2-aminoanthraquinone on Kynol active carbon fibers was successfully achieved by reduction of the corresponding diazoquinone. The diazotization reactions were fast and efficient, and the attachment of the quinone groups resulted in a 2.5-fold increase in capacity of the modified carbonaceous material -65 vs. 25 mAh/g for the unmodified carbon cloth. This significant increase in capacity reflects the contribution of the redox reaction of the grafted quinone molecules. Nitrogen gas adsorption measurements showed that the attachment of anthraquinone molecules significantly reduced the specific surface area of the carbon, mainly affecting the micro-porosity of the carbon powder. The electrochemical performance of the modified carbon electrodes was assessed by prolonged cycling experiments, during which capacity fading was observed. The modified carbon electrodes clearly showed very high cycling ability compared to other grafted carbons reported in the literature.
Ionic liquids (ILs) are attractive candidates for high‐voltage electrochemical energy storage systems, owing to their high electrochemical stability. Recently, a unique eutectic mixture of ILs was reported to demonstrate outstanding performance in supercapacitor systems at low temperatures. Yet, many publications using this or similar IL mixtures reported only a limited voltage or cyclability when utilizing them with practical activated carbon electrodes. With supercapacitors consisting of symmetric electrodes, in which voltages higher than 3 V are applied, fast capacity fading and activity termination are observed. In order to exceed the limit of 3 V for supercapacitors that use electrolyte solutions possessing wide electrochemical windows, we thoroughly investigated the (unexpected) failure mechanism, using several analytical methods. This is the most important aspect of the paper. By this, we discovered a pronounced difference in the electrochemical behavior of the negative and the positive electrodes, which has significant implications on the operation of full symmetric cells at high voltages. Finally, we propose a solution that enables stable operation of cells up to 3.4 V. By balancing the mass of the electrodes, we prevent high‐voltage failure and control the voltage split to use the full electrochemical window of each electrode and obtain a higher cell voltage of 3.4 V and an energy density higher than 40 Wh/kg (of the electrode materials). The most important aspect of this work was a rigorous study of the failure mechanism.
Diazonium chemistry was used to enrich Kynol carbon cloth with catechol (dihydroxybenzene) moieties as redox agents. Comprehensive surface analyses (combining BET, SEM, TGA, and TGA-GC/MS) were carried out to evaluate the effect of redox molecules grafted over the carbon cloth surface. Electrochemical deprotection of 3,4-dimethoxyaniline grafted to the electrochemically active catechol, followed by electrochemical assessment in a three-electrode cell, shows a faradaic contribution due to redox reactions from the catechol-grafted moieties. Galvanostatic measurements underline the remarkably stable performance of this redox reaction, which permits over 3,000 cycles.
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