Supercondensadores basados en electrolitos redox activos

Resumen

[EN] The main challenge in the supercapacitor technology is to achieve higher energy density levels that allow them to be applied in a growing number of applications. Routes developed for energy enhancing of these systems are based in capacity increasing, voltage opening or the simultaneous enhancing of both parameters. These have mainly arisen from the use of pseudocapacitive materials, high-voltage electrolytes and asymmetric configurations, which have usually been associated with limitations in their practical application due to their low energy, high cost or high contaminant power. Moreover, the need to achieve higher energy densities has motivated the research in carbon materials able to promote high capacitance and power density simultaneously. In this Doctoral Thesis, a new strategy for the increase in energy density of the electrochemical capacitors built with carbon materials has been developed by the employ of the faradaic reactions of electrochemically active organic molecules incorporated in an aqueous electrolyte. This work provides an alternative route for enhancing of energy density of supercapacitors through the increase of capacitance, by simply modifying the composition of the electrolyte. The effect of the incorporation of several redox molecules (indigo carmine, hydroquinone and methylene blue) in the electrolyte of supercapacitors built with different carbon materials (multiwalled carbon nanotubos, a carbon aerogel and two activated carbons) was studied. The positive effect of the addition of a redox molecule was initially demonstrated by the incorporation of indigo carmine into an ideal double layer system built with nanotube-based electrodes in sulphuric medium. The capacitance increase achieved with this system was optimized by the use of hydroquinone, which led to significant capacitance enhancements with all the aforementioned materials. With the employ of an activated carbon of high specific surface area, the presence of hydroquinone trebled the original capacitance of the initial capacitor, reaching an energy density value equal to 30 Wh kg-1, which constitutes the target for the new generation of supercapacitors. The study of the energy storage mechanisms revealed that the battery-type behaviour of the electrode where the faradaic process of the redox additive takes place is the cause of the capacitance enhancement and, therefore, of the energy enhancement. Moreover, the relevance of the charge compensation ability of the capacitor type-electrode in the energy enhancement process by the employ of redox electrolytes was demonstrated. Devices modified with redox electrolytes showed a significant increase in the capacitance of the starting system and, therefore, in the energy density, without an important detriment in the power density, as the resistance does not result significantly modified. On the contrary, the response shown by these systems under long-term cycling conditions was worse than that of the corresponding unmodified supercapacitors, although clearly better than that of any known battery.