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Supercapacitors (SCs), which are also known as ultracapacitors, are commonly based on porous activated carbon electrodes and electrostatic charge-storage mechanisms. Carbon electrodes are supposed to be chemically and electrochemically inert, and the electrostatic nature of the charge-storage mechanism is highly reversible. These properties should assure that SCs have an infinite shelf life. However, in practice, SC cells exhibit performance fading when they are used for months. The purpose of this paper is to compare the performance fading of SCs in two types of tests. The first accelerated test is the calendar life test, which evaluates cell degradation when cells are used at elevated voltages and temperatures; these degradations are observed in cells during rest periods. The second test is the power cycling test, which corresponds to degradations during the actual use of SCs as peak power sources in hybrid electric applications. In this paper, we use an impedance model to monitor changes in cell behavior. This analytical impedance model is based not only on physical quantities, such as double-layer capacitance, specific area, and electrolyte conductivity, but on elementary electrode dispersion as well. This model is validated in the frequency domain using the initial state of the tested SC for different temperature and voltage configurations. Then, we present and specify two types of tests. A periodic characterization based on impedance spectroscopy is done to quantify impedance changes during the ageing tests. The obtained results confirm that the impedance real part is increasing and that the capacitance is decreasing during the two ageing tests; however, the way they change is different. Indeed, the shape of the impedance real part is conserved in the calendar life test, whereas it is distorted in the power cycling test. This difference will be explained in terms of the evolution of the impedance model parameters. This paper confirms that the two ageing tests - - affect the SC performances differently.
Date of Publication: Oct. 2009