Parameters determining dimensional changes of porous carbons during capacitive charging

The expansion of porous carbons used in electrochemical double layer capacitors during positive or negative charging has been investigated in several studies, however, the underlying mechanisms are not well understood yet. We therefore investigated dimensional changes of three different carbons in various electrolytes including ionic liquids e.g. [EMIM][BF4] with and without solvent and [EMIM][TFSI]. For all carbon/electrolyte combinations the expansion during positive charging was smaller than for negative charging. The expansion increased with decreasing average pore size of the carbon. Addition of solvents, such as acetonitrile or propylene carbonate, significantly increased the expansion for negative charging. These observations are discussed in the view of various existing models for charge induced strain of carbons.