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

Abstract

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–S₁O₄G-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–S₁O₄G-600 demonstrates a half-wave potential of 0.86 V and a high kinetic current density of 10.3 mA cm⁻² at 0.6 V. Due to the lack of nitrogen coordination, the Mn–S₁O₄G-600 has good anti-Fenton reaction properties. Notably, the Mn–S₁O₄G-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.