Investigation of the Interfaces between Cu(111)-based Electrodes and Water Using Density Functional Theory

This thesis presents a fundamental study of the interface between Cu(111)-based electrodes and water. Density functional theory is used to investigate the interaction between water and Cu(111) both in the absence and presence of an external electric field. To analyze the water-electrode interaction in depth, a monomeric adsorption of water is studied and compared with other similar types of molecules. The orientation of the adsorbed molecules is found to be influenced by the energy gap between the Fermi level of Cu(111) and the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the molecules. A simple relationship is observed where a stronger dipole moment of the molecule and a lower adsorption energy lead to an interface which responds more sensitively to the external electric field. The dipole moment is a more critical parameter.;The behaviour of a water overlayer is then investigated on Cu(111). A differential capacitance of the Cu(111)/water interface remains constant when the molecular movements of water is ignored. The change in the orientations of the water molecules causes the differential capacitance of the interface to be potential dependent. A sharp increase in the differential capacitance is found when the water layer undergoes a structural transition. A new method, namely a "constant field" method is devised to determine the potential of the electrode at a given external electric field.;The impact of coating Cu(111) with a layer of graphene is also studied. A quantum capacitance, an intrinsic capacitive property of graphene, is found to influence the differential capacitance of the graphene-coated Cu(111). However, it was found that the electronic interaction between graphene and Cu(111) alters the differential capacitance of the interface at certain electrode potentials. The change in the differential capacitance cannot be predicted by the standard modelling approaches and thus, ab initio modelling of the electrified interface is necessary. The water layer does not heavily influence the interaction between graphene and Cu(111), which agrees with the conventional understanding that graphene-coating suppresses the oxidation of the Cu surfaces.