First-principles investigation of carbon-based nanomaterials for 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). To improve the energy density, the capacitance may be enhanced by using high SA electrode materials such as carbon-based nanomaterials. While promising results have been experimentally reported, capacitances have also been noted to exhibit a highly non-linear relationship with SA. These interesting observations suggest that a gap exists in our fundamental understanding of charge storage mechanisms in the EDL of carbon nanomaterials. Given that EDLs are typically on the order of 1-3 nm thick, theoretical simulations can elucidate these unknown physical insights in order to identify new design principles for future electrode materials.

In this dissertation, we explore two broad types of carbon-based nanomaterials, which are separated into two Parts, using a combined density functional theory and classical molecular dynamics computational approach. In Part I, we study the capacitance using various chemically and/or structurally modified graphene (or graphene-derived) materials which is motivated by previous accounts of the limited capacitance using pristine graphene. Our analysis demonstrates the viability of dramatically improving the capacitance using graphene-derived materials owing to enhancements in the quantum capacitance with marginal effects on the double layer capacitance. In Part II, we investigate the capacitance using nanoporous carbon materials which is motivated by experimental observations that relate capacitance to pore width rather than SA. Our findings confirm that promoting ion confinement through pore width control can enhance capacitance, but also identify pore shape dispersity as another important structural feature that facilitates fast ion dynamics during charging/discharging.

The work in this dissertation presents an overview of new insights into charge storage mechanisms using low-dimensional carbon-based nanomaterials and future directions for materials development. Moreover, we anticipate that the established methodologies and analyses can be broadly applicable to the study of other applications utilizing electrified interfaces, including capacitive deionization and liquid-gated field effect transistors.