Effect of surfactants on capacitance properties of carbon electrodes
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Effect of surfactants on capacitance properties of carbon electrodes Krzysztof Fic, Grzegorz Lota and Elzbieta Frackowiak Poznan University of Technology, Institute of Chemistry and Technical Electrochemistry, Piotrowo 3, 60965 Poznan, Poland ABSTRACT Effect of surfactants present in alkaline solutions on the capacitance of carbon electrodes has been studied. Different types of surfactants have been selected for this target. Concentration of these electrolyte additives was 0.005 mol L -1. Decreasing the surface tension in the electrode/electrolyte interface allows better penetration of electrolyte into the pores. Detailed analysis of capacitance versus current load, frequency dependence as well as self-discharge, cyclability and behaviour in wider voltage window proved a useful effect of Triton X-100 on capacitor operating in alkaline solution. Influence of surfactant concentration has also been investigated.
INTRODUCTION Energy storage phenomenon in electrochemical capacitors is mainly attributed to fast electrostatic charge accumulation on the electrode/electrolyte interface [1], called usually as electrical double layer. Taking into account the formula C = S d-1, where C is supercapacitor capacitance, is dielectric constant, S is electrochemically available surface for charge accumulation and d is the thickness of electrical double layer (usually equal to about 1 nm), enhancement of electrode/electrolyte interface seems to be the most effective approach to improve the total capacitance. Unfortunately, hydrophobic character of carbon surface and its poor wettability by electrolyte is often responsible for limited pore accessibility by ions. Hence, the improvement of the electrode wettability is a quite reasonable approach for more efficient utility of the total carbon surface. Slight improvement of carbon wettability is observed when oxygen functional groups are introduced onto activated carbon surface; however, observed increase in capacitance is mainly caused by some redox reaction between functional group and electrolyte [2]. Apart from introducing oxygen functional group onto carbon surface, improvement of carbon electrodes wettability may be realized by surfactant addition to electrolyte solution. It is well known that water solutions have relatively high surface tension, mainly due to strong interactions between water molecules. In this paper, we presented the influence of surfactants with various ionic part (cation or anion) on electrode behaviour and we proved the influence of surfactant concentration on capacitance and interesting possibility of operational voltage range increase when aqueous (alkaline) electrolyte is modified by non-ionic surfactant. EXPERIMENTAL Two-electrode capacitors were assembled in a Swagelok® system with pellets of comparable mass. Each electrode was composed of 85 wt. % of activated carbon, fully characterized in Ref. [3], 10 wt. % of polyvinylidene fluoride (PVDF Kynar Flex 2801) and 5 wt. % of acetylene black. Briefly, commercial carbon Norit® XS32 was activated by KOH (1:
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