An open superstructure of hydrangea-like carbon with highly accessible Fe-N4 active sites for enhanced oxygen reduction reaction

Single-atom catalysts (SACs) have sparked intense interest in heterogeneous catalysis lately, but insufficient accessibility of active sites restricts their further applications. Herein, a hydrangea-like carbon nanoparticle possessing highly open nanospaces and enhanced external surface area that fully exposes the catalytic sites is reported. By introducing mesoporous silica nanoparticles (MSNs), Fe-doping zeolitic imidazolate framework-8 (ZIF-8) precursors are assembled into a flower-like superstructure interlaced by porous nanosheets, which expand from inside out constructing an open and interconnected architecture. This unique structure can substantially liberate the densely dispersed but deeply buried Fe-N4 sites inside carbon nanoparticles and accelerate mass transfer in the catalyst layer that increases active-site utilization. Furthermore, the physical and chemical properties of the assembled catalysts can be adjusted by the diameter (200, 300, and 500 nm) of MSNs. When the diameter was 500 nm, the optimal FeNC-F3 catalyst exhibited large specific surface area, maximum metal centers utilization, high oxygen reduction reaction efficiency, and excellent stability. The novel cathodic electrocatalyst achieved high power density of 0.32 mW cm−2 in microbial fuel cell and 288 mW cm−2 in Zn-air battery. This work established a realistic strategy for promoting the practical development of fuel cell catalysts.