Capacitor Theory

Fig 2.1 schematically shows how the theories proposed regarding electric double layer capacitors have changed over time. The first model was introduced by Helmholtz38. Helmholtz proposed that if a charged surface is immersed into an electrolyte containing mobile ions, the surface would attract the counter-ions towards the surface and repel the like charge ions at the surface (9,10). His hypothesis further suggests that a monolayer would be formed of counter-ions near the surface, separated by a distance d, which is approximately equal to half of the diameter of the ion. This system is analogous to a conventional capacitor. The only drawback with this theory was the fact it allowed the ion to come unrealistically close to the charged surface(11)(9)

They suggested that the mobile ions in the electrolyte are always affected by two forces and their overall motion is governed by the net the combined forces – force driven by the electric field towards the oppositely charged surface and concentration driven forces towards the bulk of the electrolyte. This results in a layer, which they termed as “diffuse layer”. However, their theory treats the ions as point charges, leading to large concentrations of ions at the electrolyte-electrode interface (9,10) .
The two models: Helmholtz and Gouy-Chapman theories were later on modified and combined in 1924 by Stern (15) into a new model called as Gouy-Chapman-Stern model. The new model suggested that there are two layers: 1) Stern layer – inner region of H thickness 2) Diffuse layer – governed by Gouy and Chapman model. Graham in 1947, made some correction to this model by stating that the inner layer consisting of two sub-layers 1) Inner Helmholtz layer consisting of ions adsorbed by covalent forces and 2) Outer Helmholtz layer or the Stern layer consisting of ions held by non-specific forces (electrostatic

One of the most commonly used as electrode materials at commercial scale is Activated carbons. The aim of the research is to determine the factors that contribute to the specific capacitance i.e. capacitance (C) per mass of the electrode (m) and series resistance (diffusion resistance, ohmic resistance and mass transfer resistance) in such materials. At the same time, a correlation between pore size, pore size distribution and pore length with specific capacitance (24) have been researched. Studies at the Chinese Academy of Science have shown that activated carbon nanotubes achieve higher specific capacitance than normal carbon nanotubes (25). Apart from activated carbon, metal oxides and polymeric materials have also gained considerable interest as research suggests that higher specific capacitances compared to activated carbon can be attainable using these materials (26).
Table 2.2: Different types of capacitors prepared with their capacitance
MATERIALS ELECTROLYTE PROCESS USED CAPACITANCE REF.
Activated Carbon 6 M KOH Microwave Heating 361 F/g (27)

C2H4 + C2H3N (C2H5)4N(BF4) CVD 146 F/g