Inhibiting Photo-Oxidation and Enhancing Visible-Light-Driven Photocatalytic Water Oxidation over Covalent Organic Frameworks Through the Coordination of Cobalt with Bipyridine

Photocatalytic water splitting using covalent organic frameworks (COFs) is a promising approach for harnessing solar energy. However, challenges such as slow kinetic dynamics in the photocatalytic oxygen evolution reaction (OER) and COFs' self-oxidation hinder its progress. In this study, an enamine-based COF coordinated is introduced with cobalt dichloride, CoCl₂ (CoCl₂-TpBPy). The coordination of cobalt ions with bipyridines in CoCl₂-TpBPy enhances charge-carrier separation and migration, leading to effective photocatalytic OER. Under visible light irradiation, CoCl₂-TpBPy achieves a notable OER rate of up to 1 mmol·g^-1·h^-1, surpassing the reported organic semiconductor analogs. Additionally, CoCl₂-TpBPy shows minimal nitrogen evolution compared to TpBPy and ethanol-treated TpBPy (E-TpBPy), indicating cobalt plays a pivotal role in improving charge utilization and minimizing photo-oxidation. In situ X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analyses revealed that Co(IV) species are key to the high OER efficiency. This work highlights Co(IV) species in the efficient OER and inhibiting photo-oxidation of CoCl₂-TpBPy.