Self-discharge of carbon materials-based electrochemical capacitors

Zhang, Qing
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University of Delaware
Suffering from poor energy retention, electrochemical capacitors (ECs), with exceptional power capability and long-term cyclability compared to batteries, have rarely been considered as an energy storage device that can store energy enduringly. To develop ECs as a competitive alternative to batteries, it is critical and necessary to realize control over ECs' self-discharge for desired energy retention. This dissertation covers the pioneering progress made in establishing self-discharge mechanisms, realizing tunable self-discharge and making further insights into capacitive behaviors of ECs beyond self-discharge. Self-discharge mechanisms have been explored on ECs built with three typical carbon materials: single-walled carbon nanotubes (SWNT), graphene oxide (GO), and activated carbon fibers (ACF). Our research indicates that ECs' structures determine the relative strength between the two self-discharge driving forces: the potential field Δ E and the concentration gradient ∂c/∂x, and eventually affect the self-discharge mechanisms. For the ACF-LiPF6 EC with a high specific capacitance (i.e., high ∂c/∂x ) and relatively poor charge transportation (i.e., low ΔE ), its self-discharge is driven by ∂c/∂x and obeys the diffusion control model; for the SWNT-TEABF4 EC with good charge transportation and a relatively low specific capacitance, its self-discharge is driven by Δ E and obeys the potential driving model; and for the ACF-TEABF 4 EC with the two comparable driving forces, the dual mechanism model has been proposed. The self-discharge mechanism is extendable to GO based solid-state ECs. Nevertheless, factors such as temperature have no effects on self-discharge mechanisms though can affect the discharge rate of the self-discharge process. Moreover, through tailoring the functional groups on the SWNT surface, tuning of the self-discharge has been realized for the first time on the SWNT-TEABF 4 EC. This breakthrough will offer a facile and feasible solution to the appeal for capacitor components with specialized energy retention in electric circuit designs. Interesting contradiction between considerable energy density and undetectable ionic diffusion has been observed on the GO based solid-state ECs, which challenges the conventional understanding of ECs and dielectric capacitors (DCs). From systematical characterizations and modeling calculations, the Charge Close-Packed model has been proposed, featuring periodical layers of charges as well as nanoscaled ionic diffusion and well explaining the contradiction observed on this GO based EC.