Mg(OH)2 is a support material that is appropriate for dispersing the single atoms through a moderate interaction of Mg(OH)2 with the metal species as well as its inertness towards the reaction with metal ions during high temperature pyrolysis process. The single-atom Co precatalyst dispersed on a N-doped carbon support (Co-N-C) was synthesized by calcining a mixture of a Co(II) complex of phenanthroline (Phen), extra Phen ligands, and Mg(OH)2 followed by acidwashing of MgO (Scheme 1, detailed synthetic procedure is provided in Supporting Information). The Mg(OH)2 served as a template and dispersing reagent. The X-ray diffraction (XRD) pattern of Co-N-C showed only broad peaks at 26.2° and 42.9°, corresponding to the (002) and (004) planes of graphitic carbon (Fig. ). No peaks from either crystalline metal oxides or metallic species were present. Raman spectrum of Co-N-C showed only two bands characteristic of graphite carbon (G band at 1600 cm -1 ) and defective carbon (D band at 1350 cm -1 , Fig. ), respectively, while no peaks corresponding to metallic cobalt or cobalt oxide were observed. According to the physical adsorption-desorption measurement, the Co-N-C exhibits a high BET (Brunauer-Emmett-Teller) surface area of 496 m 2 /g and a mesoporous feature (type IV isotherm with a hysteresis loop), with a pore size distribution centered at 3.7 nm and a large pore volume of 0.478 cm 3 /g (Fig. ). Transmission electron microscopy (TEM) images further showed that Co-N-C is multilayered and porous (Fig. ). The corresponding selected area electron diffraction (SAED) pattern (Fig. inset) indicated poor crystallinity, which was in accordance with the XRD results. The highresolution (HR-)TEM image also revealed a mostly amorphous carbon support, with only some irregular lattice fringes (Fig. ). Neither CoOx nor metallic Co nanoparticles were observed. Elemental energy dispersive X-ray spectroscopy (EDXS) mapping images recorded in highangular annular dark field scanning transmission electron microscopy (HAADF-STEM) mode showed the material is composed of C, N, O, and Co (Fig. and 1c, S5), in which Co is homogeneously distributed across the whole material. Spherical aberration corrected HAADF-STEM images (Fig. and Fig. ) showed many well-dispersed bright dots, each having a size of below 0.2 nm. These dots are attributed to single Co atoms dispersed on the carbon support. Note that as the Co atoms are distributed through a carbon support that is amorphous, multilayered, and not flat, only the Co atoms which are at the focal height of the electron probe appear sharp, while Co atoms at other heights are imaged as blurry spots. X-ray photoelectron spectroscopy (XPS) reveals the chemical composition and oxidation states of Co-N-C. In Co 2p region, the Co2p3/2 (780.7 eV) and Co2p1/2 (796.3 eV) subpeaks and two satellite peaks at 785.1 and 801.3 eV suggested that the oxidation state of Co is +2 (Fig. ). The high-resolution N 1s XPS spectrum (Fig. ) was deconvoluted into four different types of N species including pyridinic N (398.6 eV), pyrrolic N (399.7 eV), graphitic N (401.1 eV), and oxidized N (402.9 eV). The peak at 406.2 eV indicates the existence of a little amount of adsorbed nitrate anions caused by nitric acid post-treatment. A large proportion of pyridinic and pyrrolic N provide sufficient sites for anchoring and stabilizing Co single atoms. The subpeaks at 286.1, 287.8, 289.9 eV in C1s region (Fig. ) suggested that the material contains hydroxyl (C-OH), carbonyl (C=O), C=N, and carboxylic functional groups. The O 1s spectrum was deconvoluted into two peaks at 531.5 and 533.2 eV, which were attributed to C=O and C-OH groups, respectively (Fig. ). The oxygen groups were inadvertently introduced during the synthetic process.