Symmetric Electronic Structures of Active Sites to Boost Bifunctional Oxygen Electrocatalysis by MN4+4 Sites Directly from Initial Covalent Organic Polymers

Transition metal-nitrogen-carbon (M-N-C) catalysts with CoN4 centers have attracted great attention as a potential alternative to precious metal catalysts for bifunctional oxygen electrocatalysis. However, the asymmetric charge environment of the active site of MN4 obtained by conventional pyrolysis strategy makes the unbalanced adsorption of oxygen molecules, which restricts the activities of both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a series of well-defined quasi-phthalocyanine conjugated 2D covalent organic polymer (COPBTC-M) is developed with MN4+4 active sites through a pyrolysis-free strategy. Compared to CoN4 site, the additional subcentral N-4 atoms in MN4+4 site in COPBTC-Co catalyst balance the charge environment and form a symmetric charge distribution, which changes the antibonding orbital of the active metal and regulate the oxygen species adsorption, thus improving the activity of the bifunctional oxygen electrocatalysis. In Silico screening demonstrates that cobalt has the best ORR and OER activity for COPBTC-M with MN4+4 sites, which can be attributed to the fewer anti-bonding orbital below the Fermi level, which weakens the oxygen species adsorption. Both theoretical and experimental results verify that the COPBTC-Co possesses unique CoN4+4 active sites and the harmonious coordinating environment can lead to superior bifunctional oxygen catalytic activity with a high bifunctional oxygen catalytic activity (Delta E [Ej(10) - E-1/2] = 0.76 V), which is comparable with the benchmark Pt/C-IrO2 pairs. Accordingly, the as-assembled Zn-air battery exhibits a maximum power density of 157.7 mW cm (-2) with stable operation for >100 cycles under an electric density of 10 mA cm (-2). This study provides a characteristic understanding of the intrinsic active species toward MNx centers and could inspire new avenues for designation of advanced bifunctional electrocatalysts that catalyze ORR and OER processes simultaneously.

Automated summary

A two-dimensional covalent organic polymer catalyst with well-defined four-nitrogen coordination structure is developed through a pyrolysis-free strategy. The catalyst exhibits excellent activity for oxygen electrocatalysis by balancing the charge environment and regulating the adsorption of oxygen species. Zn-air batteries assembled with this catalyst show stable high power density.