Supercapacitor devices, also known as electrical double-layer capacitors (EDLCs), store charge by adsorption of electrolyte ions onto the surface of electrode. No redox reactions are required, so the response to changes in potential without diffusion limitations is rapid and leads to high power. However, the charge is confined to the surface, so the energy density of EDLCs is less than that of batteries. In the 1970s, Conway and others recognized that reversible redox reactions occurring at or near the surface of an appropriate electrode material lead to EDLC-like electrochemical features but the redox processes lead to much greater charge storage. This pseudocapacitance represents a second mechanism for capacitive energy storage.

It seems now well-established that the term “hybrid” supercapacitor should be used when pairing two electrode with different charge storage behavior, i.e., one capacitive and one faradaic, and the resulting device is in-between a supercapacitor and a battery. “Asymmetric” supercapacitor covers a wider range of electrode combinations because it can be used for supercapacitors using electrodes of the same nature but with different mass loading, or two electrodes using different materials. We suggest the term “asymmetric” should be used only when capacitive or pseudocapacitive electrodes are involved (such as activated carbon//MnO₂ asymmetric supercapacitor) in order to avoid confusion with true “hybrid” devices.

More useful information on this subject can be found in informative published papers like:

1- Gogotsi et al. Where Do Batteries End and Supercapacitors Begin? Science, 2014.

2- Brousse et al. To Be or Not To Be Pseudocapacitive? journal of the Electrochemical Society, 2015.