Constructing of single atom catalysts that can stably exist in various energy conversion and storage devices is still in its infancy. Herein, a geometrically optimized three-dimensional hierarchically architectural single atomic Fe-N-C catalyst with fast mass transport and electron transfer is rationally developed by post-molecule pyrolysis assisted with silicon template and reconstructs by ammonia treating. The ammonia-assisted secondary pyrolysis not only compensates for the volatilization of nitrogen species contained in organic precursors but also optimizes the surface structure of Fe-N-C catalyst, thus increasing the content of pyridinic nitrogen and boosting the density of active sites (Fe-N-x) in Fe-N-C samples. In addition, the pyridinic nitrogen adjusts the electronic distribution in Fe 3d active center and promotes the catalytic performances. Therefore, this hollow spherical atomically dispersed Fe-N-C catalyst delivers outstanding oxygen reduction reaction (ORR) activity in pH-universal electrolyte and surpasses the most reported values. (c) 2020 Published by Elsevier Inc.