Guidelines for the optimization of EDLC carbon electrodes

摘要

Electric double-layer capacitors are an advantageous high-power modern technology, harvesting energy by ions electro-sorption on high surface area carbons. Broadening the application spectrum of such devices requires a substantial increase in their energy. Since the later is defined by E=0.5CU~2, energy improvements can be obtained by increasing capacitance or/and voltage. The voltage is imposed by the stability window of the electrolyte. Organic-based industrial systems, able to operate at a maximum voltage of 2.7 V, are preferred to the aqueous ones working only up to 0.8-1.0 V.rnRecent findings demonstrated that the specific capacitance can be increased by the maximum use of the electrode porosity, matching the sizes of pores and ions [1,2]. Remarkable values of specific capacitance were observed for ultramicroporous carbons (pores lower than 0.7 nm), suggesting that ions might be desolvated. However, here we will show that, upon reducing the useless porosity by designing carbons containing essentially sub-nanometer pores, charge storage saturation can occur in organic electrolyte before reaching 2.7 V, because the porosity is not enough developed [3]. This conclusion is further supported by using organic electrolytes based on symmetrical bulky tetra-alkyl ammonium cations with a different length of the alkyl substituent [4].