Molecular Dynamics Simulation Studies of Graphene Oxide Supercapacitors

Electric double layer capacitors (EDLCs), commonly known as supercapacitors, hold tremendous promise as energy storage systems. Graphene has emerged as an effective electrode material, but given the challenge of obtaining pristine graphene sheets, especially in the persistence of oxidative impurities, graphene may be more accurately described as graphene oxide (GO) or reduced graphene

oxide (rGO). GO and rGO have been successfully used as electrodes in supercapacitors containing a variety of electrolytes, including ionic liquids (ILs), but there has

been a lack of clarity on how oxidation impurities influence device capacitance. To address this, and to gain molecular-level insight into electrode–electrolyte interactions, in this thesis we report molecular dynamics (MD) simulations of

parallel plate GO supercapacitors containing a prototypical IL, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF–

4 ). GO supercapacitors with three types of electrodes are simulated. Hydroxylbased GO electrodes in both pure IL and an organic electrolyte (OE) (i.e., a solution of EMI+BF–

4 in acetonitrile) are simulated. The full range of electrode oxidation, from 0% (pristine graphene) to 100% (fully oxidized GO), is considered. Capacitance tends to decrease with increasing electrode oxidation in both electrolytes.

This trend is attributed to the decreasing reorganization ability of ions and a widening gap in the double layer structures as the density of hydroxyl groups on the electrode surface increases. Epoxide-based GO electrodes in pure IL are also simulated, with the full oxidation range considered. The capacitance of epoxide-based GO is larger than that

of hydroxyl-based GO at a given oxidation level. This is attributed to the epoxide groups’ structure and charge distribution, and to the enhanced reorganization ability of ions resulting from a lack of significant specific electrolyte–ion interactions. Finally, GO supercapacitors with EMI+-based poly(ionic liquid)s (PILs) in pure IL are simulated. Capacitance values of such PIL systems are lower than the corresponding systems containing only pure IL. This is attributed to the PIL systems’ lower ion density and diminished reorganization ability. This work provides a systematic and comparative baseline analysis of GO supercapacitors and provides insight into the specific interactions that influence observable quantities such as capacitance. This, in turn, lays the groundwork for

reasoning about tuning supercapacitor properties via control of oxidation group configurations.