Long-term cycling of carbon-based supercapacitors in aqueous media

Abstract

Abstract.-The behaviour of mesophase-derived electrodes on long-duration cycling conditions was studied in 1M sulfuric acid and 6M potassium hydroxide. Variation in the specific capacitance values with the number of cycles shows a good cycling life performance in acidic media but very poor in an alkaline electrolyte. The total loss of capacitance after 7,000 cycles in acidic media is 8 % at 0.6 V and 16 % at 1 V, whereas in the basic electrolyte the reduction in the capacitance values is 72 %, even at a very small operating voltage (0.6 V). This wide range of applications related to their fast energy delivery (e.g. telecommunications systems, maintenance-free traffic lights, uninterruptible power sources, and hybrid electric vehicles among others) [1]. The storage of energy in electrochemical capacitors can arise from either electrostatic charging or from pseudocapacitative chage-discharge [2]. In the first case, electrolyte ions are accumulated at the electrode/electrolyte interface forming the socalled double layer in a similar way as in conventional capacitors. By utilizing high surface area electrodes the amount of charge stored, and consequently the capacitance of the device, is several orders of magnitude higher than that of conventional electrolytic capacitors. In the second case, pseudocapacitance arises from faradaic processes of electroactive species, such as polymeric materials [3], metal oxides [4,5] and certain functionalities at the surface of carbon materials [6,7,8].It is generally believed that pseudocapacitance in carbon materials is largely based on the redox reactions of surface quinoid functionalities, whose reduction requires protons to proceed [9,10]. Nevertheless, some other oxygenated functional groups might be electrochemically active at the working potentials of the acidic solutions, such as some pyrone groups [11,6]. Therefore, and as may be expected, faradic phenomena have a large dependence on the pH of the solution. In this sense, enormous differences in capacitance values can be found depending on the electrolyte used. For carbon materials in neutral solutions, specific capacitance values between 8-12 μF cm -2 have been reported and between 15-20 μF cm -2 in the case of acidic media [12]. Despite that its study is not so frequent, redox reactions in basic media are also possible [13]. In general terms, the presence of the so-called pseudocapacitance has been related in literature to poor cycle life [19] contrary to what occurs in organic media, where only purely charge separation occurs and good cycling stability is typically reported [20]. Investigations in this field are rarely reported, despite that the cyclability of supercapacitors is a property of mayor importance. The present work investigates the effect of the electrolyte (basic or acidic)in the long-term stability of carbon-based supercapacitors. The electro-oxidation of the activated carbon M-AC was carried out in a conventional three-electrode system. Around 100 mg of material was subjected to +0.4 V vs. NHE in a N 2 bu...

Enhanced electro-oxidation resistance of carbon electrodes induced by phosphorus surface groups

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Self-discharge of AC/AC electrochemical capacitors in salt aqueous electrolyte

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Stability studies of polypyrole- derived carbon based symmetric supercapacitor via potentiostatic floating test

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