Fundamentals of Electrochemical Double-Layer Supercapacitors

The early concept of an electrochemical supercapacitor (ES) was based on the electric double-layer existing at the interface between a conductor and its contacting electrolyte solution. The electric double-layer theory was first proposed by Hermann von Helmholtz and further developed by Gouy, Chapman, Grahame, and Stem. The electric double-layer theory is the foundation of electrochemistry from which fhe electrochemical processes occurring at an electrostatic interface 

At the same time, a fundamental understanding of supercapacitor design, operation, performance, and component optimization led to improvements of supercapacitor performance, particularly increasing their energy density. To further increase energy density, more advanced supercapacitors called pseudocapacitors, in which the electroactive materials are composited with carbon particles to form composite electrode materials, were developed. The electrochemical reaction of the electroactive material in a pseudocapacitor takes place at the interface between the electrode and electrolyte via adsorption, intercalation, or reduction-oxidation (redox) mechanisms. In this way, the capacitance of the electrode and the energy density can be increased significantly. 

This chapter will provide a comprehensive discussion of the fundamentals of double-layer supercapacitors, including the electric double-layer charging and discharging mechanism, the theoretical principles that govern their operation, and their structural designs. 

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