Supercapacitors and interfacial charge accumulation devices

The first attempts to achieve such a device were made in the 1970s. The cell had a non-symmetric Ag/RbAg4l5/C structure with a double-layer capacitance at the carbon electrode in the range of 10-40 nF cm interface area However, due to the redox reaction, the working voltage was too low ( 0.7 V) for the electronic devices of the time. Furthermore, the reversibility of the Ag electrode was poor and it was difficult to use fully the surface area of the ultrafine carbon. More recently, double-layer capacitors using acidic solution (H2SO4) as liquid electrolyte were developed by NEC (Nippon Electric Co. Ltd). A liquid electrolyte allows most of the surface area of the carbon electrode to be used. The electrical characteristics of the devices can be classified in relation to the properties of each material as follows. 

The working voltage is fixed by the voltage gap between anodic and cathodic reactions (typically 0.5-3 V), and is thus also a function of the device structure (symmetric or non-symmetric). 

Parallel connection of each elementary cell allows the resulting voltage to increase, but its resistance is also increased. 

The autodischarge of the device is related to the electrical properties of the electrolyte. Solid electrolytes exhibit only one type of ionic carrier and have a very low electronic conductivity. They thus give a lower leakage current and have a larger voltage safeguard than liquid electrolytes in which other charge carriers are also present. 

The series resistance is directly related to the conductivity of the electrolyte and the electrodes used. 

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