Self-Protection Mechanism and Mass Transport Governing O₂ Tolerance in an Iron Porphyrin Homogeneous Catalyst for CO₂ Electroreduction

The electrochemical reduction of CO₂, coupled with renewable energy, offers a promising approach to convert CO₂ to valuable products. However, the presence of O₂ in operating environments presents challenges such as catalyst degradation. Transition metal complexes, such as iron tetraphenylporphyrin (TPPFe), are molecular electrocatalysts with tunable structures and redox properties that make them attractive for CO₂ reduction. A critical challenge for practical application is achieving O₂ tolerance─the ability of the catalyst to sustain CO₂ reduction without degradation in the presence of O₂. This study highlights the self-protection mechanism of TPPFe in homogeneous electrocatalysis against O₂ and reactive oxygen species (ROS). Using rotating disk voltammetry, constant potential electrolysis, and spectro-electrochemistry, we demonstrate that lesser reduced TPPFe states selectively reduce O₂, form a protective layer that shields the active catalyst for CO₂ reduction. Furthermore, we reveal that mass transport significantly influences the efficiency of CO₂-to-CO conversion in O₂-containing environments, with stirring rates during electrolysis directly affecting CO faradaic yields. This self-protection mechanism, applicable to other catalysts with multiple redox states and adaptable to molecular catalysts immobilized in thick films, underscores the importance of optimizing mass transport conditions and catalyst design to achieve an O₂-tolerant CO₂ reduction.