Spin-Dominated Electroreduction of Oxygen to Hydrogen Peroxide: A Case Study With Molecular Model Catalysts

Oxygen reduction reaction (ORR) represents a critical process in advanced electrochemical energy technologies. Yet, the fundamental mechanism of ORR selectivity has remained largely elusive. Herein, electron spin state is identified as the underlying factor governing ORR selectivity for hydrogen peroxide (H₂O₂) production using model-definite and site-identical molecular catalysts as testing platforms. Experimentally, a series of cobalt phthalocyanine (CoPc) derivatives are synthesized, and an explicit correlation is found between the Co spin state and ORR selectivity, where H₂O₂ production increases with elevated spin states. Combined theoretical orbital analysis and in situ spectroscopy investigations unveil that the spin state transition and subtle d-orbital rearrangements optimize multiple orbital hybridization with key intermediates and facilitate the selective two-electron ORR. Among the series, tetra-hydroxyl modified CoPc with a high spin state achieves a two-electron ORR performance in neutral media superior to those of low-spin state CoPc and previously reported catalysts, with H₂O₂ selectivity over 95% within the potential range of +0.1 to +0.42 V and a remarkable H₂O₂ yield of 191.22 mg cm^-2 h^-1 at -350 mA cm^-2. These findings advance the fundamental understanding of the electronic structure effect on catalytic behaviors.