Charge redistribution and ionic mobility in the micropores of supercapacitors

It is increasingly recognized that a significant component of apparent self-discharge in supercapacitors is, in fact, due to charge redistribution. This charge redistribution is due to the limited mobility and hence high resistance of ions moving in the smallest pores of the carbon electrodes. A simple two-branch equivalent circuit has been used to model this charge redistribution and has been applied to an ultra-microporous aqueous electrochemical capacitor. The results indicate that a substantial fraction of the total capacitance is found in the smallest pores and that the resistance to ionic movement is four orders of magnitude higher than in the bulk electrolyte. A survey of the literature indicates that many commercial capacitors with organic electrolyte have similar electrical properties. These properties mean that a significant fraction of the overall pore surface and hence capacitance of the electrode is not accessible during time frames of practical interest and is therefore wasted. A mechanism for the low mobility of ions in micropores is proposed. (C) 2013 Elsevier B.V. All rights reserved.