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

摘要

Transition metal-nitrogen-carbon (M-N-C) catalysts with CoN_4 centers haveattracted great attention as a potential alternative to precious metal catalystsfor bifunctional oxygen electrocatalysis. However, the asymmetric chargeenvironment of the active site of MN_4 obtained by conventional pyrolysisstrategy makes the unbalanced adsorption of oxygen molecules, whichrestricts the activities of both oxygen evolution reaction (OER) and oxygenreduction reaction (ORR). Herein, a series of well-definedquasi-phthalocyanine conjugated 2D covalent organic polymer (COP_(BTC)-M) isdeveloped with MN_(4+4) active sites through a pyrolysis-free strategy.Compared to CoN_4 site, the additional subcentral N_4 atoms in MN_(4+4) site inCOP_(BTC)-Co catalyst balance the charge environment and form a symmetriccharge distribution, which changes the antibonding orbital of the active metaland regulate the oxygen species adsorption, thus improving the activity of thebifunctional oxygen electrocatalysis. In Silico screening demonstrates thatcobalt has the best ORR and OER activity for COP_(BTC)-M with MN_(4+4) sites,which can be attributed to the fewer anti-bonding orbital below the Fermilevel, which weakens the oxygen species adsorption. Both theoretical andexperimental results verify that the COP_(BTC)-Co possesses unique CoN_(4+4)active sites and the harmonious coordinating environment can lead tosuperior bifunctional oxygen catalytic activity with a high bifunctional oxygencatalytic activity ( E Ej_(10) – E_(1/2) = 0.76 V), which is comparable with thebenchmark Pt/C-IrO_2 pairs. Accordingly, the as-assembled Zn–air batteryexhibits a maximum power density of 157.7 mW cm ~(?2) with stable operationfor >100 cycles under an electric density of 10 mA cm ~(?2). This study providesa characteristic understanding of the intrinsic active species toward MN_xcenters and could inspire new avenues for designation of advancedbifunctional electrocatalysts that catalyze ORR and OER processessimultaneously.