Understanding the Three-Shell Coordination Structure–Performance Relationship of Single-Atom Sites for Oxygen Reduction Using Molecular Model Catalysts

Atomic Fe/Co–N–C materials represent one promising type of noble-metal-free oxygen reduction reaction (ORR) catalyst for metal–air batteries and fuel cells, but their inherent features of complex and indistinct active sites derived from high-temperature treatment impede further development in terms of high-performance catalyst design. Herein, molecular model catalysts with defined active sites are developed to understand the structure–performance relationship. Specifically, cobalt phthalocyanine and cobalt porphyrin with identical Co–N4 moieties but completely different three-shell coordination structure are respectively anchored on reduced graphene oxide to synthesize molecular catalysts of CoPc@rGO and CoPr@rGO by π–π coupling interactions. Experimental results show that CoPc@rGO with three-shell N-modified Co–N4 sites holds a fairly good ORR activity, which is more than 3 times that for CoPr@rGO (7.36 vs 2.34 e–1 site–1 s–1 at 0.85 V). Density functional theory calculations reveal that three-shell N can modulate the electron structure of the Co–N4 site and reduce the ORR energy barrier. Furthermore, when used in a Zn–O2 battery, CoPc@rGO also exhibits an ultrahigh peak power density of 425 mW cm–2. This study offers molecular model catalysts to reveal the structure–performance relationships of Co–N–C and beyond.