Nowadays, the world energy supply comes mainly from carbon-based fuels, which is highly involved with environmental issues. Several decades ago, scientific studies about photosynthesis stated the ability of plants to oxidize water into oxygen powered by sunlight, storing energy as chemical bonds. Taking nature as inspiration, water splitting appears to be the most suitable process to produce clean energy from water, with the water oxidation reaction as the critical step. In this thesis, state-of-the-art periodic DFT methods are used to understand the key factors that control water adsorption and the catalytic performance of RuO2 on the oxygen evolution reaction (OER). For the first time, not only the most stable surface, but also all the surfaces that constitute the Wulff shape were taken into account for both the water interface and their catalytic activity. Wulff theorem was employed to build atomistic models of RuO2 nanoparticles of different sizes. The OER performance of RuO2 has been explored through the water nucleophilic attack (WNA) and oxo-coupling (I2M) mechanisms for both surfaces and nanoparticle models. Finally, two OER mechanisms have been proposed for an Iridium single site catalyst grafted on an Indium Tin Oxide (ITO) support as the work done during a predoctoral stay in ETH-Zürich (Switzerland). Results show that water dissociation onto the RuO2 main surfaces is controlled by the intrinsic Ru site acidity, the basicity of the Obr groups coming from the surface and cooperative effects between adsorbed water molecules. Concerning the OER mechanism, the WNA is the applying mechanism for both the main surfaces and nanoparticles. However, the I2M mechanism on nanoparticles seems to be significantly more favorable, because of the higher flexibility of the nanoparticle surface. Consequently, the I2M mechanism could be competitive on small clusters. Furthermore, results for the Iridium supported catalyst indicate that the highly oxidized Ir(VI) bis-oxo is a key intermediate in the OER mechanism.