An Investigation of the Interaction Between Graphene and Hydrated Ions

Abstract

The ability of graphene and carbon nanotubes to generate an electric potential from flowing fluids has attracted much interest. The effect is thought to occur because certain ionic and molecular species bind to the surface of these materials more strongly than others. Although several physical models have been proposed, none has yet been rigorous enough to be confirmed by experiment. In this work, I describe an electromechanical device, i.e. a supercapacitive electrical energy generator (SCEEG) made with graphene electrodes, that generates electricity from oscillating droplets of electrolyte solutions and ionic liquids. I provide a theoretical model of how the graphene SCEEG (G-SCEEG) works that quantitatively agrees with experimental results. More importantly, the model allows one to characterize electrochemically-useful properties of the interaction between graphene and hydrated ions from the device’s experimentally measured output. The identity of the ions that preferentially adsorb to graphene as well as the effective surface density and effective binding energy of these ions can all be determined using the G-SCEEG. Additionally, both the capacitance per unit area and the surface potential of the graphene-solution interface can also be determined. Although this work focuses on a SCEEG device whose electrodes are made from graphene, the electrodes can, in principle, be made from any conductive material, e.g. metals, such as gold, and semiconductors, such as indium-tin-oxide. All SCEEGs described in this thesis, however, work by exchanging charge between two supercapacitors, i.e. capacitors that spontaneously form at the interface between an electrode and an electrolyte solution (or ionic liquid) as a result of ionic adsorption. Charge exchange occurs not by charging and discharging these interfacial capacitors but instead by increasing and decreasing their capacitance. This is accomplished mechanically as a moving droplet wets and dewets the electrodes. The changing interfacial capacitance acts as a source of current; the flow of current through the device’s internal impedance creates an electric potential; and, when connected to an external load, the SCEEG is then capable of generating electric power. I show that a G-SCEEG can generate a peak power of up to 7 μW from the oscillatory motion of two 20 μL droplets of 6.0 M HCl. I demonstrate that the device can be successfully modeled as a source of alternating current that is in parallel with a time-varying internal impedance. Using this model and the G-SCEEG’s output, I determine that the chloride anions in 6.0 M HCl adsorb to graphene with the greatest effective surface density (5.6 x 10^12 ions/cm^2) and induce in it the largest charge density (900 nC/cm^2) of all the solutions I studied. These chloride anions adsorb to graphene with an effective binding energy of 350 meV, in turn producing an electric potential of 690 mV at the graphene-solution interface. I also find that the density and the sign of the surface charge induced in graphene are dependent on the ionic species present in each solution, the concentration and pH of the solution, and the presence of multi-layers in the graphene electrodes with the solution’s pH providing the greatest effect. Finally, I describe in this dissertation the study of the Raman spectral properties of suspended mono- and multi-layer graphene membranes and the use of graphene and graphite to electrostatically trap DNA from an aqueous solution. I conclude by explaining the unique methods I developed during the course of this work including the synthesis of large-grain graphene using chemical vapor deposition (CVD) and the transfer of large areas of CVD-grown graphene to hydrophobic substrates.