Fundamentals of capacitive charge storage in carbon-based supercapacitors

Supercapacitors are electrochemical energy storage devices known for their large power densities and long lifetimes yet limited energy densities. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer (EDL), and the low energy to limited electrode surface area (SA). As such, carbon-based nanomaterials have been extensively explored as high SA electrode materials. Interestingly, anomalous and nonlinear relationships between observed capacitances and SAs have recently emerged. These observations suggest that a gap exists in our fundamental understanding of charge storage mechanisms due to the introduction of low-dimensional materials. In this chapter, we review new physical insights from both quantum mechanical calculations and atomistic simulations of two broad types of carbon-based nanomaterials. First, the study of graphene-derived materials has highlighted the importance of competing contributions from the electrode and EDL capacitance. Furthermore, the study of nanoporous carbons has elucidated the importance of charging dynamics. Taken together, these findings can be generalized to establish design principles for future electrode materials.