Cation trapping in highly porous carbon electrodes for EDLC cells

We report on the electrochemical characterization of activated, highly porous carbons in TEABF(4) (TEA=tetraethylammonium)/propylene carbonate solutions. One of two selected carbons had a large number of micropores, whereas the other contained mesopores. The behavior of charged carbon electrodes upon prolonged storage at elevated temperatures was explored. A combined application of various electroanalytical techniques enabled the determination of the electrodes' capacitance retention and serial resistance as functions of storage time. The loss of capacitance of the charged electrodes during storage of electric double-layer capacitor (EDLC) pouch cells, determined by their postmortem electrochemical characterization, was more pronounced for the carbon electrode of higher porosity. The capacitance loss concerned the negative carbon electrode only and was attributed to the partial trapping of cations in the porous structure. The trapped cations could be completely released at moderate anodic polarizations of the carbon electrode. The mechanism of the trapping phenomenon seems to be complex, comprising a number of potential and time-dependent steps, such as penetration of the cations into the smallest pores of the carbon electrode (less than 1 nm) and adsorption of the cations within the walls of the smallest pores containing energetically different types of carbon sites. (C) 2008 The Electrochemical Society.