supercapacitors as energy storage devices - Acta Energetica

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74Anna Lisowska-Oleksiak; Andrzej P. Nowak; Monika Wilamowska / Gdańsk University of TechnologySeparatorwith electrolyteConductivecasing wallElectrode material in contactwith electrolyteFig. 2. Schematic diagram of an electrochemical capacitorIn the process of capacitor charging or discharging an internal change of electrode potential V is continuouslyobserved. It follows the equation:C = q/V or = C × V (3)Curves for discharge of an electrochemical capacitor and a battery are presented in fig. 3.Battery discharge curveU [V]Electrochemical capacitor discharge curveFig. 3. Discharge curves of an electrochemical capacitor and a batteryIn contrast to that characteristic, charging/discharging of galvanic cells is carried out at constant potential,except for values near 0 and 100% (fig. 2).This difference results in the fact that energy E accumulated by a capacitor is:E = 1/2 C × U 2 or E = 1/2 q × U (4)Timewhile for a battery the energy level is q × U, i.e. it is twice higher than the energy level in a capacitor ofthe same voltage U = ΔV. Power of a supercapacitor is expressed by the equation:
Supercapacitors as Energy Storage Devices75where R stands for the device’s resistance. 2UP 4 REquation (4) indicates that increase of energy accumulated in a capacitor can be achieved by:1) capacitance increase, which may be realized by:a. increasing electrode active surface,b. decreasing plate separation,c. increasing relative dielectric permittivity of the medium2) voltage increase.According to the equation (5) power can be increased by:1) voltage increase2) resistance decrease.When designing a device we can influence the useful power level by appropriate selection of materials,electrode geometry and electrochemical stability of the electrolyte.Electrochemical capacitors as energy storage and conversion devices can be placed between electrolyticcapacitors 2 and batteries. This is illustrated by the Ragone diagram (fig. 4).(5)Specific power [W/kg]ElectrolytecapacitorElectrochemicaldouble-layercapacitorAcid-head cellsCadmium nickel and nickel-metalhydride cellsUltracapacitorLithium-ion batteriesFuelcellsSpecific energy [Wh/kg]Fig. 4. Ragone diagram for various electrochemical devices [8, 9]Time constants (RC) (dashed lines in the diagram) indicate that the charging/discharging time for reversiblegalvanic cells is considerably longer than corresponding times for electrochemical capacitors.Batteries, just like low-temperature fuel cells, display low power density when compared to electrolyticcapacitors. At the same time batteries have higher energy density than capacitors. Using both batteries andelectrochemical capacitors in the same device can improve its operational parameters. The number of charging/dischargingcycles for electrochemical capacitors is much higher than for batteries. This results from thefact that in the cells new phases are created during the charge transfer process and difficulties resulting fromside effects occur. An electrochemical capacitor uses mainly electrostatic charge, so from a theoretical point ofview there is no limit for the number of charging/discharging cycles.2 In contrast to electrochemical capacitors, electrolytic capacitors (with dimensions of several centimetres) have very small capacitances, measuredin micro- or nanofarads. Electrolytic capacitors owe their name to the method of separator creation between the plates. Plates of electrolyticcapacitors are made of metals like aluminium, tantalum, titanium, niobium, etc. Plates are separated with a thin (10…100 nm) film of an oxide of therespective metal. This film occurs as a result of anodic polarization of both plates. Electrolytic capacitors should not be confused with electrochemicalcapacitors.
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