Conventional capacitors store little energy due to the limited charge storage areas
and geometric constrains of the separation distance between the two charged plates.
However, supercapacitors based on the EDL mechanism can store more energy
because of the large interfacial area and the atomic range of charge separation
distances. As schematically illustrated in Figure 2.2a (Zhang and Zhao, 2009), the
concept of the EDL was first described and modeled by von Helmholtz in the 19th
century when he investigated the distribution of opposite charge at the interface of
colloidal particles (Helmholtz, 1853). The Helmholtz double layer model states two
layers of opposite charges formed at the electrode-electrolyte interface separated by an
atomic distance. The model is similar to that of two-plate conventional capacitors. This
simple Helmholtz EDL model was further modified by Gouy (1910) and Chapman
(1913) on the consideration of a continuous distribution of the electrolyte ions (both
cations and anions) in the electrolyte solution because of thermal motion, which is
referred as a diffuse layer (Figure 2.2b). However, the Gouy-Chapman model leads to
an overestimation of the EDL capacitance since the capacitance of two separated
arrays of charges increases inversely with their separation distance, hence a very large
capacitance value would arise in the case of the point charge ions close to the electrode
surface. Later, Stern (1924) combined the Helmholtz model with the Gouy-Chapman