Supercapacitors based on functionalized carbon materials

Abstract

Electrical double layer capacitors store energy in the form of electrical charges at the interface between an electrolyte and a high surface area electrode. As their energy storage mechanism relies on physical interactions, EDLCs have high power densities, unlike batteries, which are often limited by the slow charge- and mass- transfer kinetics. Additionally, EDLCs can sustain millions of charging/discharging cycles. Nevertheless, they suffer from low energy densities. For a high energy density EDLC, it is critical to maintain a compact architecture with large ion-accessible surface area while also ensur ing low ion transport and electrical resistance. We incorporate carbon quantum dots into thermally exfoliated graphene oxide sheets in the presence of a room temperature ionic liquid to form conductive carbon networks with improved ion transport networks, enhancing ion transport kinetics and storage. This yields an electrode in which both the carbon quantum dots and the ionic liquid serve as spacers to effectively separate the thermally exfoliated graphene oxide sheets, while the ionic liquid also functions as the electrolyte and carbon quantum dots provide a conductive network. Using this approach, we achieve a gravimetric capacitance of 165 F/g at 30/70 wt% CQD/TEGO composition with 4.5 M EMIM-BF4 electrolyte at 20 mV/s. The electrodes demon strate 70 % capacitance retention at 500 mV/s. When a 3.4 M EMIM-BF4 electrolyte is used instead, capacitance reaches 206 F/g, and the electrodes retain 70% of its ca pacitance at 500 mV/s. This demonstrates that we can simultaneously improve both energy and power density by tailoring the electrolyte composition.