N-Doped Carbon Nanosheet Networks with Favorable Active Sites Triggered by Metal Nanoparticles as Bifunctional Oxygen Electrocatalysts

Developing noble metal-free bifunctional oxygen electrocatalysts is vital for metal–air batteries. Herein, we present a facile approach to fabricating N-doped carbon nanosheet networks with metal nanoparticles (M/N-CNSNs) readily converted from metal–organic frameworks. The resultant Co/N-CNSNs show superior bifunctional oxygen catalytic activity attributed to the efficient active sites and fast mass diffusion enabled by the nanosheet structure. It is worth noting that the first-principles studies prove the Co/N–C sites to be the oxygen reduction reaction active sites, where the most favorable ones are the carbon atoms next to Co-coordinated pyridinic N. Interestingly, the cobalt content plays an important role in Co/N–C sites but was not directly involved in the catalytic process. In a Zn–air battery, a small voltage gap without obvious voltage loss is found for the Co/N-CNSNs. This facile approach enables scalable synthesis, representing an essential step toward the popularization of metal–air batteries.