Comparison of pressure evolution in supercapacitor devices using different aprotic solvents R. Ko ¨tz a, * , M. Hahn a,1 , P. Ruch a , R. Gallay b a Electrochemistry Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland b Maxwell Technologies SA, CH-1728 Rossens, Switzerland Received 30 November 2007; accepted 12 December 2007 Available online 23 December 2007 Abstract Gas evolution and associated pressure build up in supercapacitor devices was monitored using commercial electrode rolls and 1 mol/l solutions of TEABF 4 in acetonitrile (AN), propylene carbonate (PC), and c-butyrolactone (GBL). During subsequent constant voltage periods (24 h) at 2.5 V, 2.75 V, 3.0 V and 3.25 V, significant gas evolution was observed in GBL already at 2.5 V. Under the same conditions, severe gas evolution in PC started only at 3.0 V, while in AN rate of gas evolution remained low even at 3.25 V. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Electrochemical double layer capacitor; Aprotic solvent; Electrolyte decomposition; Gas evolution 1. Introduction Electrochemical double layer capacitors (EDLC), also called supercapacitors, gained significant attention during recent years and are considered to have the potential to replace batteries in certain applications or complement bat- teries or fuel cells as power boosters. EDLCs are famous for their high power density and the unsurpassed life-time during deep discharge cycling. However, in many applica- tions the relatively small energy density of EDLCs is a drawback. Therefore, many research efforts aim at an increase of the capacitor’s energy density. One way to increase energy density is to increase the voltage of these capacitors beyond the typical nominal voltage of 2.7 V. It is known that the life-time of the EDLCs suffers from higher voltages and an increase of the nominal voltage by 0.1 V results in a doubling of the degradation rate [1,2]. One significant problem, which may contribute to the observed life-time reduction, is the increase of internal pressure, most probably due to gas evolution at the high surface area carbon electrodes. Besides life-time issues pres- sure increase is also an important safety aspect. Exploiting a supercapacitor at high voltages may result in a rupture of the capacitor case or at least in electrolyte leakage. There- fore, manufacturers include measures against overpressure in the capacitor housing such as predetermined breaking points for controlled pressure release. The stability window of electrolytes for supercapacitors was studied by Ue at glassy carbon working electrodes [3]. In this study, he investigated propylene carbonate (PC), acetonitrile (AN) and c-butyrolactone (GBL) and claimed a stability window of more than 5 V. In a recent publication [4], Ue measured the degradation of coin cell type capacitors based on AN, PC and GBL at a voltage of 3 V and found different results. In particular, AN showed a very low performance in terms of capaci- tance loss and resistance increase, quite comparable to that of GBL, but much worse than that of PC. Gas evolution at the carbon electrodes is a surface reac- tion which can be influenced by the choice of carbon, salt 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.12.016 * Corresponding author. Tel.: +41 56 310 2057; fax: +41 56 310 4415. E-mail address: ruediger.koetz@psi.ch (R. Ko ¨ tz). 1 Present address: Honeywell Specialty Chemicals, DE-30926 Seelze, Germany. www.elsevier.com/locate/elecom Available online at www.sciencedirect.com Electrochemistry Communications 10 (2008) 359–362