Supercapacitor or ultracapacitor basics: What you need to know

A supercapacitor, also known as an ultracapacitor, is a high-capacity capacitor that possesses a lower energy density than batteries, but higher than conventional capacitors. In the race for longer smartphone life, batteries have not made as much progress in the capacity as semiconductors have made in power efficiency. However, semiconductors are fast approaching a point where they can gain less additional power efficiency versus cost. One goal has been to replace batteries with supercapacitors someday, due to the supercapacitor’s ability to charge and discharge in seconds and the ability to retain charging capacity over time.

Unlike rechargeable batteries, which eventually experience “charging fatigue,” supercapacitors can recharge and discharge hundreds or thousands of cycles more than rechargeable batteries. This is because supercapacitors store energy in an electric field, whereas batteries use chemical reactions to store and release energy.

The great downside to supercapacitors is that they have nowhere near the lasting power of batteries since total discharge occurs at most over several minutes, not hours, for present-day supercapacitors. However, new discoveries with new materials show great promise to “render lithium-ion batteries obsolete.”

The charging plates of a capacitor have an insulating material that separates the plates from each other, called a dielectric. Dielectrics are insulators and extremely poor conductors. However, dielectrics support an electrostatic field, the mechanism by which capacitors store energy. Supercapacitors do not have a dielectric, per se; they have an electric double layer separating the plates, and thus were once referred to as electric double layer capacitors (EDLC).

John Bird, in Electrical Circuit Theory and Technology, states that “The separation of charge distance in a double-layer is of the order of a few Angstroms (0.3 – 0.8nm) and is static in origin.”[i] Therefore, the thickness of the charge distance is on the molecular level.

Figure 1: Supercapacitors charge and discharge rapidly. Batteries discharge at a steady, flat rate up until almost exhausted, at which point energy drop-off increases the pace.  (Source: Elcap, Creative Commons CC0 1.0 Universal Public Domain)

All capacitors accumulate charge, like static energy, in the material between two opposing electrodes. Several factors influence the capacitance (farad values) of supercapacitors including size, composite materials, and geometry.

Table 1: Supercapacitors vs. Rechargeable Batteries

[i] Bird, J. O. Electrical Circuit Theory and Technology. Taylor & Frances, 2014. Print.