Clean electrochemical CO2 reduction reaction (eeCO2RR) to produce value-added chemicals and fuels using renewable electricity is a promising strategy for closing the carbon cycle. However, this early-stage technology is challenged by the low activity and selectivity and the competing hydrogen evolution reaction (HER). Thus, it is important to design catalysts with economically viable current density at low overpotential and high selectivity toward certain eCO2RR products. Single-site carbon-based materials are alternative candidates for CO2 conversion. This thesis aims to model carbon materials under electrochemical conditions using Density Functional Theory (DFT) coupled with Computational Hydrogen electrode (CHE) formalism to investigate their selectivity and activity. The main goal is to establish experimental and theoretical correlations between physicochemical and catalytic properties for the eCO2RR over different types of catalysts. In particular, metal-nitrogen-carbon (MNC) catalysts with metal atoms present as atomically dispersed MNx centers were investigated as model systems. The distinct current for CO formation (JCO) observed along the series of catalysts is attributed to the nature of the transition metal in MNx moieties. Among them, the Nickel-Nitrogen-Carbon single-atom catalyst exhibits the highest efficiency for producing CO at different potentials. The variation in the material synthesis produces defects with coordinatively saturated and unsaturated N-doped cavities, and once the metals are placed there, they can present different oxidation states depending on the type of cavity. I address the electrochemical stability of the reconstruction and redispersion of supported nanoparticlesthrough the Oswald formalism. At high CO coverages, Ni nanoparticles reconstruct by forming Ni(CO)4 species that can redisperse into active single atoms. Moving to different systems, I also unravel the production of C1 hydrocarbons with molecular catalysts. I focus on the potential influence of the product selectivity of eCO2RR over Co(II)-tetraphenyl porphine and Co(II)-phthalocyanine deposited on multiwalled carbon nanotubes. I identify the key difference between the binding energies of oxygen or carbon bonding of CH2O, the binding strength of CO, and the protonation of *CHO. Taken together, this thesis presents a computational understanding of activity and selectivity of single-site carbon-based catalysts for CO2 reduction.
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