Atomically Dispersed Electrocatalysts for Oxygen Reduction Reaction: Understanding the Synthetic Processes for Tuning Structure, Surface Chemistry, and Formation of Different Active Sites

The interest in atomically dispersed transition metal (TM-Nx-C) electrocatalysts for a plethora of electrochemical reactions has risen dramatically due to their superior selective activities and operational durability. Concerning the oxygen reduction reaction (ORR), diverse simple and complex synthetic routes are used to synthesize TM-Nx-C. However, each known route exploits a pyrolysis step to (i) stabilize the TM-Nx over the carbon support, (ii) create active sites, and (iii) enhance graphitization. Commonly, the synthetic routes also involve a postpyrolytic treatment to enhance atomic level homogenization/distribution of the active moieties and remove undesired active sites such as nanoparticles and oxides. Synthetic processes are discussed and the reaction mechanisms are highlighted. Recently, in situ characterization methods and techniques have unraveled the evolution, growth, and then transformation of active site structure and morphological attributes during the pyrolytic process. In this review, the synergistic effects of surface chemistry and morphology of these electrocatalysts are reported and discussed.