Understanding the Three-Shell Coordination Structure–Performance Relationship of Single-Atom Sites for Oxygen Reduction Using Molecular Model Catalysts
Long He, Tan Li, Hao Hu, Pengfei Xie, Jincheng Li
Abstract
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–N 4 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–N 4 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–N 4 site and reduce the ORR energy barrier. Furthermore, when used in a Zn–O 2 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.