High-spin atomically dispersed Mn(II)N₄ site: Unraveling the catalytic activity and selectivity of oxygen reduction reaction in microbial fuel cell

Microbial fuel cell (MFC) presents an innovative eco-friendly technology, but its development is greatly hindered by expensive and inefficient cathodic oxygen reduction reaction (ORR) catalysts. Currently, CN-coordinated single-atom Fe-based or Co-based materials report have been widely recognized as a promising ORR catalyst. However, this application is constrained by the Fenton reaction. Consequently, it is particularly necessary to further advance innovative non-precious metal ORR catalysts. Herein, atomically dispersed Mn-N-C catalysts with a precise Mn(II)N₄ structure are developed using a one-step calcination method, which is served as MFC cathodes for the ORR. The optimized Mn-N-C catalyst demonstrates a half-wave potential (E_1/2) of 0.864 V, surpassing that of commercial Pt/C (0.855 V). Specifically, the catalyst exhibits outstanding four-electron ORR selectivity with H₂O₂ yields below 4 %. Theoretical calculations indicate that the generation of H₂O₂ by *OOH protonation at the Mn(II)N₄ site is a non-spontaneous process. The high-spin Mn(II)N₄ site greatly enhances catalytic activity through increased electron delocalization and effective interaction between σ and π orbitals near the Fermi energy level. Accordingly, Mn-N-C present excellent power density and high chemical oxygen demand (COD) removal in MFC. This study provides new insight about the metal valence state at the center of Mn single-atom materials in relation to ORR activity and selectivity.