The energy crisis and environmental pollution are the two major global issues caused by the excessive utilization of fossil fuels. In recent decades, developing renewable energy via electrocatalytic conversion technology has been considered as a feasible approach to replace fossil fuels. However, the scarcity and high price of commercial catalysts (e.g., Pt/C catalyst for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER); RuO2 for oxygen evolution reaction (OER)) seriously hinder the industrialization of the electrocatalytic technology. Therefore, it is highly urgent to develop efficient and cost-effective electrocatalysts to accelerate the further development of renewable energy technologies. Defective carbon-based materials (DCMs) have recently been considered as one of the most promising alternatives to replace precious metal electrocatalysts with the merits of high-performance, abundance and low-cost. However, structural tailoring of carbon defects at atomic scales poses great challenges in regulating defect types and density to maximize the activity. In this thesis, we aim to develop new synthetic strategies to precisely control the structural reconstruction and surface modification of carbons, which involves a series of intensive thermal redox reactions and oxygen atom modification. Specifically, For the first research work, an interfacial self-corrosion strategy was developed to control the removal and reconstruction of carbon atoms via a series of thermal redox reactions of ZnO quantum dots and formed CO2 gas in confined carbon cavity, which results an ultra-dense carbon defects on carbons (HDPC). Such ultra-dense carbon defects (2.46 × 1013 cm-2) were served as efficient active sites for oxygen reduction, resulting in an excellent catalyst in both base and acid media (half-wave potentials of 0.90 or 0.75 V in 0.1 M KOH or HClO4). For the second research work, in consideration of the difficulty of identification of active sites on hierarchical porous carbon, we employed graphene as a model catalyst to control carbon defect density and surface oxygen groups (O-groups) on graphene. Firstly, the as-synthesized catalyst with the highest defect density (DG-30) shows the best four electronic pathway oxygen reduction reaction (4e-ORR) performance. After modifying O-groups (named as O-DG-30), the ORR of the catalyst turns into a 2e- pathway. Moreover, the dynamic evolution processes and catalytic mechanisms were revealed through multiple in-situ technologies and theoretical simulations. This work further demonstrated the significance of defect density towards ORR performance. In summary, we develop a new synthetic strategy to fabricate ultra-dense defect density on carbon, emphasizing the importance of defect density towards ORR. Based on this knowledge, we further control defect density and surface chemical environment on graphene to identify the real active sites of DCMs. This thesis provides new knowledge and perspectives in materials synthesis and electrocatalytic mechanisms via 1) developing a new synthetic methodology for ultra-dense defects construction and 2) identifying the real active site and catalytic mechanism of DCMs.