Generally, capacitors are constructed with a dielectric (insulator) placed between two metal plates that serve as conductors; energy is stored when, during the charging process, electrons leave one plate and accumulate on the other, building a positive charge on one plate and a negative charge on the other as the dielectric prevents the negatively charged electrons from returning to the positively charged plate. This charge separation creates a potential between the two plates, which can be harnessed in an external circuit. The total energy accumulated in this fashion is a combination of the number of charges stored and the potential between the plates. The number of charges stored is essentially a function of the size and material properties of the plates, while the potential between the plates depends on the dielectric material used.
Ultracapacitors, by contrast, generally do not have any dielectrics but rather utilize a thin coating on the metal plates to keep the positive and negative charges in place. The porous nature of the carbon coating used in this "electric double layer" gives the plates a larger surface area, which allows for a higher number of charges to be stored on them. The carbon coating used is also much thinner than any dielectric used in a traditional capacitor, which means that the distance between the separated charges is much smaller in ultracapacitors. Combined, this very small charge separation and increased plate surface area give ultracapacitors a much higher energy density than that of traditional capacitors.
Ultracapacitor Charge Separation
To charge them, ultracapacitors are submerged in an electrolyte consisting of positive and negative ions dissolved in a solvent. There are two types of electrolytes used by ultracapacitor manufacturers: One is water-soluble, and the other is non-water-soluble. The non-water-soluble electrolyte increases the withstand voltage per cell compared to that of a water-soluble electrolyte, producing a higher energy density. Tecate Group cells are made with non-water-soluble electrolytes.