Behavior of Dielectrics in Capacitors and Theories of Dielectric Polarization

Abstract

In the case of electrochemical capacitors of the double-layer type, the behavior of the dielectric of the capacitor has a special significance since it is the solvent of the electrolyte solution that constitutes locally the dielectric of the double layer and provides the solvation shells of the ions in that medium. As shown in more detail in Chapter 6, the double layer at an electrode/solution interface consists of one real, electronically conducting plate (metal, semiconductor, oxide, or carbon surface) and a second virtual plate that is the inner interfacial limit of a conducting electrolyte solution phase. The double-layer distribution of charges is established across this interphasial region, which is composed of a compact layer having dimensions of about 0.5 to 0.6 nm, corresponding to the diameters of the solvent molecules and ions that occupy it, and a wider region of thermally distributed ions over 1 to 100 nm, depending on ionic concentration (Chapter 6). It is because of this very small thickness of the compact molecular interphasial layer that a relatively large specific capacitance of 20 to 50 μF cm-2 can arise.