TECHNOLOgy REViEW > continued


What are “supercapacitors” and what design benefits do they bring to key applications?


thickness of the electrodes can reduce the equivalent series resistance (ESR), allowing even faster charge/discharge rates and improved operation at low-temperature. Similarly, the use of propylene carbonate- based solvent for the electrolyte rather than the more usual acetonitrile electrolyte, provides a number of benefits: its lower viscosity contributes to lower resistance and unlike acetonitrile, which can give off toxic gasses if burnt, it provides safer solution with lower environmental impact.


how do supercapaciTors differ from oTher energy sTorage devices?


Figure 1: Ragone chart showing energy density vs power density for various energy storage devices


This article will examine EDLC technology in a little more detail to better understand its technical characteristics and how these contribute to the features and benefits of commercial products targeted at these markets and applications. In doing so it will focus on some selected products from leading manufacturers including Panasonic, Murata, Nichicon, Cooper Bussmann, and AVX.


whaT are supercapaciTors and how do They work? The Electric double-layer capacitor (EDLC) is a type of electrochemical capacitor that stores energy electrostatically without employing a chemical reaction. Such supercapacitors do not have a conventional solid dielectric; rather their construction uses an electrolyte solution and conductor electrodes. The EDLC description refers to the charge separation that occurs in a Helmholtz double layer at the interface between the electrode and electrolyte. This double layer is very thin, less than a nanometre, and consequently electrodes with a large surface area, such as those made from activated carbon, yield very large capacitance values relative to their volume.


The electrical characteristics of supercapacitors provide performance that lies between the energy storage capabilities of traditional capacitors and rechargeable batteries. They enjoy the fast charging characteristic of regular capacitors but with the increased storage capacity normally only associated with battery technology. Compared to other capacitor technology, supercapacitors have the greatest energy density (capacitance/unit volume), enabling capacitance values up to 10,000 times that of electrolytic capacitors. However their energy density is still only around 10% of a conventional battery and batteries can still provide much higher total capacities.


Where supercapacitors really win over batteries is in power density, which relates to the speed at which energy can be delivered to the load. The low ESR of supercapacitors (measured in milliohms) enables quick charging without the need for any charge protection circuitry and their ability to discharge equally quickly allows them to respond to demand in pulse power and battery assist applications. Additionally, supercapacitors have no charge/discharge cycle life limitation (i.e. no memory effect), with the consequent cost-saving benefits of a long and maintenance free service life. Supercapacitors are also attractive compared to batteries for their low


environmental impact because they contain no lead or other potentially harmful substances and are therefore compliant to EU directives such as RoHS.


Figure 3: EVerCAP electrical discharge characteristics (source Nichicon) Figure 2: Charge storage in a supercapacitor


The basic principles of supercapacitors have been refined over the years as manufacturers compete to produce devices with ever better performance. For example, optimising the pore size of the activated charcoal and the


which applicaTions can benefiT from using supercapaciTors? Supercapacitors are attractive for a wide variety of applications and consequently there are products on the market that span a range of capacitance values as the examples selected for this article illustrate. At one end of the scale are devices that provide reliable, long-life replacements for the coin-cell batteries that backup real-time clocks in microcontroller systems. At the other end are higher capacity


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focus magazine - issue 16


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