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Unpaired 3d Electron on Atomically Dispersed Cobalt Centre in Coordination Polymers to Regulate both ORR Activity and Selectivity.

Reversible oxygen conversion is of vital importance for various green energy technologies. Herein we synthesize a series of bimetallic coordination polymers by varying the Ni/Co ratio and using HITP (HITP = 2, 3, 6, 7, 10, 11-hexaiminotriphenylene) as the ligand, with the aim to interrogate the role of metal centers in modulating the activity of oxygen reduction reaction (ORR). The differences in crystallographic structure, spin character, electric conductivity, and coordination states are elaborately compared for both Co3HITP2 and Ni3HITP2 , revealing the former with unpaired 3d electrons possessing less coplanarity but more radical features. As a result, despite of a reduced crystallinity and conductivity, the best ORR activity comparable to 20% Pt/C is obtained for Co3HITP2 , signifying the 3d orbital configuration of the metal center in promoting ORR. More importantly, both experimental and DFT investigations unveil a transition of ORR pathway from four-electron for Co3HITP2 to two-electron for Ni3HITP2 owing to different intermediate binding energetics. Further in conjunction with the considerable OER activities, rechargeable zinc-air batteries using Co3HITP2 as the air cathode catalyst demonstrate excellent energy efficiency and operation stability.