Investigations on the Aging Effect of Supercapacitors
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Investigations on the Aging Effect of Supercapacitors
F. Parigi, Y. Gao, M. Casares, T. Gachovska, Y. S. Zhou, Y. F. Lu, D. Patterson and J. L. Hudgins. Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588, U.S.A. ABSTRACT In this paper, the degradation processes of commercial supercapacitors aged at 2.7 V and 65 °C for 2000 h were studied. The crystallinity, thermal stability, and specific surface areas of the carbon electrodes of the supercapacitors were measured. Significant changes and degradations in the carbon electrodes were observed for the aged supercapacitors. New functional groups were also found on the surface of the electrodes. The degradation of the lattice structures and the reduction in the specific surface area were as well observed for the aged supercapacitors. It was suggested that the aging of supercapacitors significantly changed the electrode surface which affects considerably electrical properties and functionality of supercapacitors. We have also performed experiments which suggest that the aging effect on the electrode is not uniformly distributed through its length. INTRODUCTION Supercapacitors (SCs), also called ultracapacitors, are short-term energy storage devices. SCs are suitable either for industrial purpose, backup power, quick charge or regenerative power and applications such as pitch control in wind turbine, solar system, and transportation [1-3]. SCs have higher specific energy than regular electrolytic capacitor and higher specific power than regular lithium-ion batteries or fuel cells [9, 13, 14]. SCs are characterized by the capability of delivering high specific energy in a short amount of time and a high cycling life time. The life of a regular SC is > 100 000 cycles [4, 7, 8]. The difference in cycling life is ascribed to the fact that SCs store energy in an electrostatic way, in comparison to batteries which store energy through oxidation/reduction processes. The specific capacitance of a SC can reach 150F/g [6, 7] and the energy density can reach 6.5 Wh/kg [12]. The operating voltage of a SC varies according to the electrolyte as for the aqueous electrolytes the voltage is around 1 V and the non-aqueous the voltage can be to 4 V approximately [5, 8]. It has been shown that during the cycling the SCs performance degrades as a result of the equivalent series resistance (ESR) increases and the capacitance decreases. This degradation is ascribed to the physical changes such as gas evolution, mass increases and degradation of the carbon coat on the current collectors [4]. According to numerous related references, the causes of the electrochemical decline of SCs are addressed to the electrolyte decomposition and to the presence of contaminations in the carbon electrodes such as metals, water and oxygen. The surface functional groups formed on the carbon surface lead to the decrease of the capacitance and the increase in the leakage current [4, 9, 11]. At voltages above 2.5 V, the surface functional groups will react with water and generate ga
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