The discovery of single atom catalysts (SACs) paths new frontier in heterogeneous catalysis. Recently, increasing interests have been focused on the construction of atomically-dispersed transition metal centers in nitrogen doped carbon through supports-assisted pyrolytic synthesis for various applications, such as electrocatalysis, CO oxidation, and organic synthesis. However, the evolution, activation and stabilization of SAC in synthesis and catalysis, which is critical for its applications, still remain unexplored yet. Here, we examined the critical importance of supports and anions for the single-atom transition-metal-sites(Co, Fe, and Ni) construction and modification, combing synchrotron X-ray absorption studies, and the electron microscopic characterizations, and electrochemical measurements.

The supports and anions are essential parts which are involved in the pyrolytic synthesis. However, both of them are often excluded from construction of catalytic sites in SACs. The formation of atomic metal sites is often accompanies with the evolution of metallic metal/metal carbide nanoparticles encased by graphite layers. Here, we demonstrate a versatile anions-regulated selective generation of metal sites in carbon and demonstrate the metal-sites dependence catalytic performances for oxygen reduction reaction (ORR) and hydrogen evolution reaction.

Besides achieving the controlled evolution of atomically-dispersed metal sites, we also demonstrate the roles of supports on the structural reconstruction and activation of SAC and the functional relationship between the electrocatalytic activity and electrochemical stability in SAC. Different from the previous models of integration of fragments on carbon, we initially reveal the supports-assisted structural reconstruction on the evolution of electrocatalytic SAC. More importantly, the enhanced catalytic activity and selectivity are found to result from more unoccupied 3d orbitals at low energy positions for single-atom-dispersed metal sites within non-planar coordination. This is mainly due to the facilitated interactions between metal sites and adsorbates. More importantly, such activity descriptors can further modified through controlling the atomic coordination geometry or structures. Interestingly, decreasing the atomic coordination numbers enhances the ORR activity, however, also damages the electrochemical stability of atomically dispersed metal sites under potentials, demonstrating the coordination-structure sensitivity of its electrochemical stability. Therefore, we present a functional relationship between the catalytic activity and electrochemical stability of SAC for electrocatalysis. Our studies offer molecular insights on the evolution of SAC, pathing new directions to SAC engineering with improved activity and stability for various reactions, such as ORR, water splitting, and CO2 reduction.