Electrochimica Acta 130 (2014) 344–350 Contents lists available at ScienceDirect Electrochimica Acta j our na l ho me pa g e: www.elsevier.com/locate/electacta Effect of accelerated ageing on the performance of high voltage carbon/carbon electrochemical capacitors in salt aqueous electrolyte Paula Ratajczak, Krzysztof Jurewicz, Piotr Skowron, Qamar Abbas, Franc ¸ ois Béguin ∗ Faculty of Chemical Technology, Poznan University of Technology Piotrowo 3, 60-965 Poznan, Poland a r t i c l e i n f o Article history: Received 31 December 2013 Received in revised form 13 February 2014 Accepted 25 February 2014 Available online 12 March 2014 Keywords: AC/AC electrochemical capacitor Supercapacitor Salt aqueous electrolyte Accelerated ageing Floating High voltage a b s t r a c t An accelerated ageing protocol has been developed and validated to determine the factors influencing the life-time of high voltage carbon/carbon electrochemical capacitors (ECs) in aqueous lithium sulfate elec- trolyte, and to offer solutions reducing ageing of the cells. The study allowed the internal overpressure, increase of equivalent series resistance and corrosion of the positive current collector to be distinguished as the main failures which can appear during operation of these ECs at high voltage. To assure stability dur- ing cycling, the capacitor should not operate above 1.5 V at room temperature. Adding sodium molybdate (Na 2 MoO 4 ) to the electrolyte reduces the corrosion of the positive collector and lowers the equivalent series resistance which remains constant during floating at 1.5 V; additionally, the initial capacitance is enhanced due to a pseudo-faradaic contribution of the molybdate additive. The internal resistance is further lowered when the pellet electrodes are glued to the current collectors with a carbon conductive adhesive. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Due to the depletion of fossil fuels and increasing CO 2 emissions, researches have been recently concentrated on the improvement of energy management by developing high performance storage devices. Among various energy storage elements and power deliv- erers, electrochemical capacitors (ECs) attract a lot of attention [1,2]. Although high power and long cycle life [1,3] are dominant advantages of ECs over accumulators and fuel cells, it is still very important to enhance their energy density and reduce their cost in order to broaden the possibilities of applications. The maximum energy, E (J), of ECs is directly proportional to the capacitance, C (F), and the square of voltage, U (V), according to formula (1): E = 1 2 CU 2 (1) The operating voltage is essentially limited by the stability of the combined electrode and electrolyte system, and it can reach 3 V with organic electrolytes [4] and usually less than 1.0 V with H 2 SO 4 and KOH aqueous electrolytes [5,6]. Therefore, organic solu- tions are preferred for commercial systems, although the aqueous ones demonstrate a better conductivity. However, when using ∗ Corresponding author. E-mail address: francois.beguin@put.poznan.pl (F. Béguin). aqueous electrolytes, the production costs are dramatically reduced, because it is not necessary to eliminate moisture from the various components before closing the capacitors. Consequently, it is interesting to search for better technical solutions which would allow the operating voltage of ECs to be increased while using aque- ous electrolytes. Lately, it has been demonstrated that carbon/carbon electro- chemical capacitors can operate efficiently up to 1.6 V in 0.5 mol . L -1 Na 2 SO 4 [6,7] and even achieve around 2 V when using 1 mol . L -1 Li 2 SO 4 [8,9]. However, the test conditions used in the publica- tions were based on galvanostatic charge/discharge during around 10,000 cycles, and are far below the expected cycle life of these sys- tems, which should be one or two orders of magnitude higher. Since such tests would obviously require extremely long periods of time, it is necessary to develop an alternative methodology which would be sufficient to prove the lifetime stability of the device. Consider- ing some of the envisioned applications of ECs [10,11], floating at an imposed high voltage is usually applied to accelerate the age- ing. Such floating tests are currently applied on activated carbon based ECs in organic electrolytes on commercial capacitors [12–14], but they have not been yet performed in presence of salt aqueous electrolytes. The main failures which can appear during the operation of an EC under critical conditions, such as high temperature and/or high voltage, may be a decrease of capacitance, an increase of equiva- lent series resistance (ESR) [15] and unsealing of the system due to internal overpressure provoked by electrolyte decomposition and http://dx.doi.org/10.1016/j.electacta.2014.02.140 0013-4686/© 2014 Elsevier Ltd. All rights reserved.