Electrochemical capacitors, usually called also supercapacitors, are of great interest as high power electrochemical energy storage devices due to the fact that they combine high power density with long cycle life and wide operational temperature range, properties that are currently unattainable in Li-ion batteries. However, the energy density achieved by commercial products (about 5 Wh/kg) limits their use to few seconds of charge/discharge, hampering their utilization in energy harvesting applications. Therefore, efforts are focused now on increasing the energy density of super capacitors. Based on the charge storage mechanism, electrochemical capacitors can be divided in:
i) electrochemical double layer capacitors-where the energy storage is based on the electrostatic adsorption of electrolyte ions on the surface area of electrically conductive porous electrodes and
ii) pseudo-capacitors-where the energy is stored through redox reactions at the electrode/electrolyte interface.
Porous carbon materials are the main candidate for supercapacitors in terms of cost, availability, large surface area, versatility with regards to porosity development and surface chemistry, good conductivity and a lack of negative environmental impact. They behave mainly as electrochemical double layer capacitors, with their large surface area providing high capacitance values. Nevertheless, many carbons possess surface functional groups,which gives rise to an additional pseudo-capacitance contribution. Thus, nitrogen and oxygen functionalities are known to give rise to faradaic redox reactions, increasing the capacitance values and thus the energy density of the supercapacitor. An additional advantage of the presence of oxygen and/or nitrogen is that they improve the wettability of the electrodes and in the case of nitrogen, also the electronic conductivity of the material.
In our group we synthesize well defined porous carbon materials with tailored porosity, functionality and conductivity based on low-cost and renewable bio-waste precursors and test their performance as electrodes in super capacitors.
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