Change of self-discharge mechanism as a fast tool for estimating long-term stability of ionic liquid based supercapacitors

A systematic study is presented on self-discharge (SD) characteristics and leakage current levels of supercapacitors based on various carbon materials in ionic liquids: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4). The higher SD rate occurring with partly mesoporous carbon xerogels, opposed to microporous activated carbon, suggests underlying faradaic induced processes rather than SD governed by charge redistribution. The passivation of some surface active sites of carbon xerogels by high-temperature di-hydrogen treatment resulted in reduced SD extent. The cell potential hold dependence of SD was studied for individual electrodes, showing changes of SD mechanism in EMImTFSI for both electrode polarities at cell potential higher than 2.8 V, against ∼3.4 V and >3.6 V for the negatively and positively charged electrodes in BMImBF4. A good correlation was established between the electrochemical stability limits determined by long-lasting floating tests and the limits suggested by changes in the SD mechanism and increase in measured leakage current. Hence, SD and leakage current analysis is a sensitive and much faster tool for determining the maximum cell potential applicable to supercapacitors than life time tests by floating, and should be more extensively applied in the future for such objective.