The mechanism of ultrafast supercapacitors
While microporous carbon is considered as a prime candidate for the energy storage application of supercapacitors, graphene is usually used for AC line-filtering capacitors. Surprisingly, the rate capability of the latter is a few orders of magnitude higher than that of the former. The interesting point is that such ultrafast performance can be achieved when the graphene sheets are vertically aligned, as the rate capability of ordinary graphene electrodes is similar to that of microporous supercapacitors. Understanding the actual role of the graphene alignment can shed light on the underlying mechanism to design faster supercapacitors for the energy storage purpose too. It is discussed that the classic double layer models cannot explain the critical importance of the graphene orientation. By reviewing the experimental evidence reported in the literature, the possibility of a different mechanism is justified. Furthermore, the impedance model of supercapacitors, which can be a guiding star in this direction, is explained from a practical perspective.