Photocoupled Electroreduction of CO₂ over Photosensitizer-Decorated Covalent Organic Frameworks

Introducing an external visible-light field would be a promising strategy to improve the activity of the electrocatalytic CO₂ reduction reaction (CO₂RR), but it still remains a challenge due to the short excited-state lifetime of active sites. Herein, Ru(bpy)₃Cl₂ struts as powerful photosensitive donors were immobilized into the backbones of Co-porphyrin-based covalent organic frameworks (named Co-Bpy-COF-Ru, x is the molar ratio of Ru and Co species, x = 1/2 and 2/3) via coordination bonds, for the photo-coupled CO₂RR to produce CO. The optimal Co-Bpy-COF-Ru_1/2 displays a high CO Faradaic efficiency of 96.7% at -0.7 V vs reversible hydrogen electrode (RHE) and a CO partial current density of 16.27 mA cm^-2 at -1.1 V vs RHE under the assistance of light, both of which were far surpassing the values observed in the dark. The significantly enhanced activity is mainly attributed to the incorporation of a Ru(bpy)₃Cl₂ donor with long excited-state lifetime and concomitantly giant built-in electric field in Co-Bpy-COF-Ru_1/2, which efficiently accelerate the photo-induced electron transfer from Ru(bpy)₃Cl₂ to the cobalt-porphyrin under the external light. Thus, the cobalt-porphyrin active sites have a longer excited-state lifetime to lower the rate-determining steps' energy occurring during the actual photo-coupled electrocatalytic CO₂RR process. This is the first work of porphyrin-based COFs for photo-coupled CO₂RR, opening a new frontier for the construction of efficient photo-coupled electrocatalysts.