▎ 摘　　要

Recently, metal-free nitrogen-doped graphene quantum dots (NGQDs) have been experimentally demonstrated to electrochemically convert CO2 into high-order hydrocarbons and oxygenates, after more than 30 years since the identification of copper as an active metal catalyst for such conversions. However, the physicochemical principle of such catalytic activity for NGQDs has remained unclear. Here, by performing first-principles simulations, we have systematically investigated the underlying mechanisms governing the whole process. The introduction of N atoms into edges of graphene quantum dots enhances their bonding with *COOH, effectively promoting the reduction of CO2 to CO. By including the influences of water, we reveal that the selective production of CH4 over CH3OH is attributed to a much lower kinetic barrier for the conversion of adsorbed *CH2OH to *CH2 via water molecule mediated proton shuttling. Further, adsorbed *CH2 provides active sites for the coupling with CO to generate C-2 products, including both C2H4 and C2H5OH. These results offer theoretical insights into the reduction pathways of CO2 on NGQDs, which may facilitate the design of metal-free carbon-based catalysts for efficient CO2 reduction.