A first-principles study of graphene and its derivatives as supercapacitor electrodes

Open Access
Author:
Chichester-constable, Alexander James
Area of Honors:
Materials Science and Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Ismaila Dabo, Thesis Supervisor
  • Robert Allen Kimel, Honors Advisor
Keywords:
  • density functional theory
  • DFT
  • SCCS
  • self-consistent continuum solvation model
  • implicit solvation
  • supercapacitor
  • graphene
Abstract:
As world energy demands increase, new and more efficient energy generation and storage methods must be developed. Supercapacitors are one class of energy storage devices, comprised of electrodes and an electrolyte, that are capable of delivering higher power density than batteries but typically have lower energy density. Optimizing the materials used to manufacture supercapacitors will enable improved energy and power densities, ultimately complimenting and benefiting technologies dependent on electrical energy storage such as electric cars. This is achievable through the electrochemical interactions between an electrode and an electrolyte resulting in an electronic double layer that can store electrical energy in the potential difference between layers of atoms and molecules. One group of materials of interest includes graphene and novel graphene-derivative structures, which are based upon the honeycomb lattice of carbon atoms in a single layer. Such 2-dimensional materials exhibit unique quantum phenomena and properties because they interact with solvents and store charge differently than traditional metal electrodes. In the design and selection of materials for such purposes, computational techniques are fulfilling an increasingly important role. The development of computational techniques to accurately predict the properties of materials allows for the acceleration of material testing for specific applications. Furthermore, it enables a high level of understanding of the underlying phenomena that govern materials properties. Density functional theory (DFT) is a powerful computational tool that can be used to simulate the electronic and nuclear interactions of individual atoms through the iterative solution of Schrodinger’s equation. These calculations can become computationally expensive when dealing with solvated systems; the implementation of the self-consistent continuum solvation (SCCS) model allows for the simulation of the solvent as a continuous dielectric medium, reducing the required computational resources while retaining the key features of charge storage in the solvent media. By applying these computational techniques to novel graphene and graphene-derivatives, the solvation effects of 2D materials can be better understood. Furthermore, the results can be compared to experimental benchmarks thereby validating the reliability of the SCCS model coupled with DFT so that the model can be applied in future material selection for electrochemical systems. Using an implicit water solvent as a medium for graphene, graphene with a nitrogen point substitution, graphene with a vacancy, and graphene with adsorbed NO2 molecules, the capacitance of small systems can be calculated thereby accounting for the quantum effects that dictate the charge-voltage response of the system. Starting from the structural optimization of the various systems under charge using converged parameters for wavefunction energy cutoffs and Gaussian spreads, subsequent self-consistent calculations of solvation energies were performed to determine the change in potential of the charged system. By employing the simple relationship between capacitance, applied charge, and voltage, the electrical response of the capacitance of the varying systems can be calculated. Subsequent comparison to experimental data shows the power of the SCCS model coupled with DFT and reaffirms in a new light that graphene and graphene-derivatives show significant promise as electrodes for next generation energy storage devices such as supercapacitors.