Manipulating the Spin State of Perovskite Cs ₃ Bi ₂ Br ₉ by Co‐Doped for Efficient Photocatalytic CO ₂ Reduction

Abstract The photocatalytic conversion of CO 2 into the renewable fuels is a promising strategy to address energy and environmental challenges, however, its limited application is mainly attributed to the inefficient charge separation and lack of active sites in conventional catalysts. Here, a spin‐polarization strategy using Co 2 ⁺ doping in lead‐free perovskite Cs 3 Bi 2 Br 9 (CBB) synergized with an external magnetic field (MF), is reported to achieve highly efficient CO 2 reduction. The optimized Co‐doped CBB (0.2CBB) exhibited a 2.6‐fold enhancement in CO production rate (35.04 µmolg −1 h −1 ) compared to the pristine CBB, with further improvement to 86.56 µmolg −1 h −1 under 200 mT MF. Advanced characterizations together with the density functional theory calculations further revealed that the Co doping introduces spin‐polarized electrons, suppresses charge recombination, and elongates the carrier lifetime (6.68 ns vs 5.20 ns in CBB). The Zeeman effect under MF activates the additional spin‐polarized carriers, while the Co sites lower the energy barrier for * COOH intermediate formation (ΔG = −0.59 vs −0.38 eV in CBB), as confirmed by the in situ FT‐IR and Gibbs free energy analysis. This work pioneers the integration of spin manipulation and MF‐assisted catalysis in perovskites, offering a novel pathway for the design of high‐performance photocatalytic systems.