Identifying high-spin Fe3+(S=5/2) as active center for acidic oxygen reduction

Abstract The active spin state of a single Fe catalytic center in acidic oxygen reduction reaction (ORR) remains uncertain, posing a significant challenge for the precise design of Fe-based ORR catalysts. To address it, we synthesized a conjugate-bridged dual-metal iron phthalocyanine molecule that exhibits an identical Fe site and high catalytic activity comparable to real catalysts. This molecular model addresses the limitations of current model catalysts that fail to meet simultaneously both structural and activity requirements. Compared to the non-conjugate-bridged molecule, the conjugate-bridged dual-metal iron phthalocyanine molecule displayed a 76-fold increase in turnover frequency. By combining spin characterization techniques, in-situ electrochemical infrared spectroscopy, and theoretical calculations, we proposed that the conjugate bridging of two FeN4 sites induces a high-spin Fe3+(S=5/2) state. This state allows for optimal adsorption strength of hydroxyl groups, neither too strong nor too weak, resulting in high catalytic activity that approaches the peak performance. The conclusion of Fe3+(S=5/2) state as the active site was further confirmed by a linear correlation between the content of Fe3+(S=5/2) state and catalytic activity. This study deepens our comprehension of active sites and contributes to the precise design of Fe-based ORR catalysts and beyond.