Double coordination shell modulation of nitrogen-free atomic manganese sites for enhancing oxygen reduction performance.

The rational design and fabrication of the active sites of single-atom catalysts (SACs) remains the main breakthrough for efficient electrocatalytic oxygen reduction reaction (ORR). Although metal-nitrogen-carbon (M-N-C) materials have been reported to exhibit good ORR performance, the M-N bond is prone to oxidation and subsequent destruction in Fenton-like reactions. Here, we report a nitrogen-free Mn-based SAC (Mn-SOG-600) anchored on a nitrogen-free graphene substrate, where manganese is bound to four oxygen atoms and one sulfur atom across two different coordination shells. In 0.1 M KOH, the Mn-SOG-600 demonstrates a half-wave potential of 0.86 V and a high kinetic current density of 10.3 mA cm at 0.6 V. Due to the lack of nitrogen coordination, the Mn-SOG-600 has good anti-Fenton reaction properties. Notably, the Mn-SOG-600-based zinc-air battery displays an open circuit voltage of 1.46 V and outstanding cycle stability, which is superior to Pt/C. Theoretical calculations reveal that the introduction of the second S-coordination layer increases the charge density of the manganese center and decreases the energy barrier. This work identifies a novel active coordination configuration model for the ORR, paving the way for innovative design and synthesis of efficient SACs.