Formation of Nitrogen-Coordinated Metal Sites (M = Fe, Co) via Solution-Phase Coordination on Nickel- And Nitrogen-Co-Doped Carbon Templates with Metal Vacancy-Nx Sites

Platinum group metal-free catalysts based on transition metal and nitrogen co-doped carbon materials are gaining attention for their applications in electrocatalysis. Nitrogen-coordinated metal sites (MNx) have been identified as the main active sites in PGM-free catalysts. Current synthetic approaches rely on the high-temperature treatment to introduce targeted electroactive MNx sites into the carbon matrix. In this study, we demonstrated the formation of MNx sites on Ni- and N-co-doped carbon templates via the solution-phase coordination with target metal ions, including Fe2+, Fe3+, and/ Co2+. The formation of MNx sites was confirmed via a combined approach of various physical characterization techniques, elemental analysis, and electrochemical analysis. The results indicate that the metal-vacancy-Nx sites in the template can coordinate with target ions to form electrochemically active MNx sites. By varying heat-treatment temperature, the amount of FeNx sites formed via coordination with Fe2+ ion can be tuned and correlated with the change of activity toward oxygen reduction reaction. The amount of MNx sites formed via solution-phase coordination with various ions followed the sequence of Co2+ > Fe2+ > Fe3+. These findings may further guide the future development of MNx-C electrocatalysts through this solution-phase coordination approach.