Revealing the Critical Role of the Lone Pair Electrons of Bi³⁺ in Electrocatalytic Oxygen Reduction Reaction on Mn‐Based BiMn₂O₅ Mullite Surface

Abstract There is a research gap regarding the role of lone pair electrons of post‐transition metal cations in catalysis field, yet such studies hold significant implications for expanding their applications. Herein, the role of Bi 3+ ion lone pair electrons in electrocatalytic oxygen reduction reaction (ORR) is elucidated using ternary mullite oxides as prototype catalysts. Among the three synthesized mullites ((Sm/Y/Bi)Mn 2 O 5 ), BiMn 2 O 5 exhibits notably superior ORR catalytic activity. Through advanced characterization techniques, including in situ Raman, in situ infrared, and X‐ray absorption spectroscopy, combined with theoretical calculations, it is determined that the superior electrocatalytic performance of BiMn 2 O 5 originates from the lone pair electrons that activate catalytic sites through stereochemical effects. Specifically, the Bi 3+ lone pair electrons in the 6 s orbital near Fermi level interact with O 2 p orbitals, causing a shift of the oxygen ligands, and significant variance in A─O bond lengths. This structural distortion of BiO 8 coordination unit directly results in a large Mn─O bond angle at the active center of BiMn 2 O 5 and strong covalent interactions, facilitating charge transfer and refining the adsorption behavior of oxygen intermediates to achieve the overpotential of 0.25 V vs. RHE. This study provides insights into developing next‐generation catalysts by harnessing the lone pair electrons of post‐transition metal cations.