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

Abstract

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 (MN x ) 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 MN x sites into the carbon matrix. In this study, we demonstrated the formation of MN x sites on Ni- and N-co-doped carbon templates via the solution-phase coordination with target metal ions, including Fe 2+ , Fe 3+ , and/ Co 2+ . The formation of MN x sites was confirmed via a combined approach of various physical characterization techniques, elemental analysis, and electrochemical analysis. The results indicate that the metal-vacancy-N x sites in the template can coordinate with target ions to form electrochemically active MN x sites. By varying heat-treatment temperature, the amount of FeN x sites formed via coordination with Fe 2+ ion can be tuned and correlated with the change of activity toward oxygen reduction reaction. The amount of MN x sites formed via solution-phase coordination with various ions followed the sequence of Co 2+ > Fe 2+ > Fe 3+ . These findings may further guide the future development of MN x -C electrocatalysts through this solution-phase coordination approach.