Activating the Mn Single Atomic Center for an Efficient Actual Active Site of the Oxygen Reduction Reaction by Spin-State Regulation

The ligand engineering for single-atom catalysts (SACs) is considered a cutting-edge strategy to tailor their electrocatalytic activity. However, the fundamental reasons underlying the reaction mechanism and the contemplation for which the actual active site for the catalytic reaction depends on the pyrrolic and pyridinic N ligand structure remain to be fully understood. Herein, we first reveal the relationship between the oxygen reduction reaction (ORR) activity and the N ligand structure for the manganese (Mn) single atomic site by the precisely regulated pyrrolic and pyridinic N4 coordination environment. Experimental and theoretical analyses reveal that the long Mn-N distance in Mn-pyrrolic N4 enables a high spin state of the Mn center, which is beneficial to reduce the adsorption strength of oxygen intermediates by the high filling state in antibond orbitals, thereby activating the Mn single atomic site to achieve a half-wave potential of 0.896 V vs RHE with outstanding stability in acidic media. This work provides a new fundamental insight into understanding the ORR catalytic origin of Mn SACs and the rational design strategy of SACs for various electrocatalytic reactions.