Supercapacitors

Fullerenes are characterized by their high electron affinity, which makes them interesting as electron acceptors. We focus on achieving a fundamental understanding of the interaction between 3D graphene networks and fullerenes, specifically in the context of stability and charge transfer in an electrochemical environment.

Our project demonstrated that the double layer capacity of the 3D graphene network is significantly improved upon the addition of carbon 60 (C₆₀) and C₆₀ monoadducts by providing additional acceptor states in the form of the low-lying lowest unoccupied molecular orbitals (LUMOs) of C₆₀ and its derivate. Guided by experimental results and first-principle calculations, we synthesized and tested a variety of different C₆₀ monoadducts, demonstrating that the stability can be increased by strengthening the van der Waals interactions.

Using atomistic simulations, we also showed that the introduction of C₆₀ molecules significantly alters the electric double layer response through a complex interplay between surface morphology and charge localization character, which ultimately dominates the overall capacitive improvement in the hybrid system.

Our synthesis method and stabilization strategy are expected to benefit the integration of graphene/C₆₀ hybrid materials in solar cell and charge storage applications.

Publications

People

Macro-assemblies of graphene exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture.

We fabricated 3D periodic graphene composite aerogel microlattices for supercapacitor applications through a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention and power densities that equal or exceed those of reported devices made with electrodes 10–100 times thinner.

We are working to further improve the capacitance of 3D-printed graphene aerogels while still retaining their excellent rate capability. This method involves two ion‐intercalation steps (lithium‐ion intercalation and perchlorate‐ion intercalation) followed by hydrolysis of perchlorate ion intercalation compounds. Lithium ion intercalation mainly leads to surface exfoliation, while the hydrolysis of perchlorate ion intercalation compounds functionalize the graphene surface with oxygen moieties. Electrochemically treated graphitic paper electrodes show 1000 times enhancement in areal capacitance.

Polypyrrole (PPy) is one of the most widely used pseudocapacitive materials, which generally boosts specific capacitance of graphene through synergetic effects. We demonstrated a facile strategy for creating PPy–GA composite‐based, highly compressible electrodes with designed 3D shapes and architectures. The 3D printed GA scaffolds facilitate the chemical deposition of PPy. The addition of PPy not only greatly increases the specific capacitance but also enhances the mechanical properties. This concept will open a new paradigm for the fabrication of arbitrarily shaped, deformable 3D GA‐based composite supercapacitors and broaden their range of applications.

Publications

People

Commercial supercapacitors, while excelling in power density (fast charge/discharge), generally suffer from low energy densities (overall storage capacity). Pseudocapacitors are an important class of energy storage devices that could balance the need of high energy density and fast charging/discharging. Among many pseudocapacitive materials, manganese oxide (MnO₂) stands out for its high theoretical specific capacitance, wide availability, and environmental friendliness. Researchers have employed various strategies to improve the capacitive performance of MnO₂, including crystallinity engineering, crystal structure selection, morphology control, defect introduction and elemental doping.

Electrodes with small mass loadings of MnO₂ provided excellent specific capacitance. However, increasing MnO₂ loading often causes severe deterioration of capacitive performance due to inefficient electron transport and ion diffusion. Deposition of MnO₂ nanomaterials onto conductive and high surface area current collectors have proven to be an effective way to improve electron transport, but they are less effective in addressing the sluggish ion diffusion in thick MnO₂ films, especially for thick electrodes.

Recent advances in 3D printing technologies offer new possibilities in addressing this outstanding challenge for pseudocapacitors. Our previous studies show that a 3D printed carbon electrode demonstrates improved rate capability compared its bulk counterpart. The enhancement may have been due to periodic macropores, which facilitate the diffusion of electrolyte ions, and thus, improve the electric double layer capacitance at high current densities. Subsequently, we demonstrated that 3D printed carbon aerogels also serve as good scaffolds for pseudocapacitive materials, enabling even record-high loadings of MnO₂ on electrodes several millimetres thick.

Publications

People