With the rapid increase of greenhouse gas emissions, there is an urgent need to produce fuels like dihydrogen (H2) and chemicals like ammonia (NH3) and hydrogen peroxide (H2O2) in a cleaner and more efficient way. Electrochemical conversions of dinitrogen (N2), dioxygen (O2), carbon dioxide (CO2) and water (H2O) can afford fuels and chemicals with high selectivity, efficiency and reaction rates under mild conditions. These processes also help to store and redistribute renewable energy to better match societal needs. However, electrocatalysts with high efficiency are still in need of further development. Carbon materials, which exhibit high electrical conductivity, larger specific surface area and good chemical stability, are promising candidates for making electrocatalysts. Since well-defined hexagonal defect-free carbon materials are electrochemically inert, it is difficult to synthesize highly active catalysts with single-atomic sites and requisite carbon defects which are required for efficient conversion reactions. This thesis begins with the introduction of single-atomic sites on carbon materials. In Chapter 1, single-atomic Mo was anchored on holey nitrogen-doped graphene, which proved to be an efficient catalyst to selectively reduce N2 to ammonia (NH3). This method could be a possible alternative to the traditional Haber-Bosch process that operates at a high temperature and pressure. In Chapter 2, a new method to introduce single atoms into the carbon frame, namely flash Joule heating, was present, with which high-purity pyrrole-type FeN4 active sites were prepared. These iron-rich carbon materials exhibited good performance in both the oxygen reduction reaction and the CO2 reduction reaction. In both chapters, the single-atomic nature and coordination environment were confirmed with a series of microscopic and spectroscopic methods, including X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption near-edge structure (XANES). O-containing functional groups and N-dopants are normally considered as the active sites for the 2-electron oxygen reduction reaction. However, the carbon defects formed during the preparation process were ignored. By decoupling the formation of carbon defects with heteroatom doping, the critical role of carbon defects in the catalytic process was illustrated in Chapter 3 and Chapter 4. The synergistic effect of carbon defects and semiconductor materials in the 2-electron oxygen reduction reaction was analyzed using commercial carbon black and titanium dioxide (TiO2) as the model in Chapter 5.