Energy technologies integrated with carbon capture contribute to energy production by lowering the environmental impact. Chemical looping combustion (CLC), as such technology, can produce energy from fossil fuels while capturing CO2. Improving the performance of Oxygen Carrier (OC) material is essential for the improvement of CLC technology. Understanding the effect of support material on OC performance is crucial to develop a framework for the selection of suitable support and an optimal design for OC. To develop such an understanding, the synergetic (atomic-level electronic) effects of support material, oxygen vacancy defects and their interplay on the performance of OC are investigated by Density Functional Theory. The effect of different reacting species on the OC performance is also investigated.In the first three parts of the thesis, NiO is considered as oxygen carrier, with TiO2 and MgO as supports, and H2 as fuel. Firstly, the results indicate that the electronic interaction of the TiO2 support with NiO lowers the energy of intermediate states and the energy of the reaction. The interplay between the electronic effect of TiO2 and the geometric effect of oxygen vacancy results in a more stable chemisorbed state, as the reaction proceeds. Secondly, it was found that by addition of an oxygen vacancy and change of hydrogen adsorption sites, the magnitude of H–Ni interaction increases, which results in the enhanced reactivity of NiO after O vacancy creation. Thirdly, by comparing the synergetic effect of TiO2 and MgO supports, it was found that adding a more electronegative support of TiO2 results in creating a more stable chemisorbed state. This shows that we can correlate the reactivity of OC with electronegativity as an atomic-scale property of the support to establish a framework for support material selection. In the last part of this study, the synergetic effect of reacting species on the performance of FeTiO3 oxygen carrier was investigated by deriving the micro-mechanisms and calculating the energetics of CH4 oxidation reaction and CO2, an unwanted reacting species, reduction reaction. The results explain the observed trend of drop in OC performance and syngas production due to the presence of CO2.