Photocoupled electroreduction of CO2 over photosensitizer decorated covalent organic frameworks

Electrocatalytic CO₂ reduction reaction (CO₂RR) is thought to be an ecologically favorable technique for using CO₂ as a cheap and abundant C1 feedstock for the production of value-added chemicals such as CO. However, the thermodynamically stable nature of the CO₂ molecule featuring high C=O bond energy frequently leads to sluggish kinetics of CO₂→CO and high energy inputs.

Introducing an additional visible-light field would be a potential way to explore the optimized reaction conditions and develop effective catalysts for efficient CO₂RR toward CO, as the highly energetic photo-induced excited state of light-sensitive electrocatalysts can effectively optimize the formation barrier of reactive intermediates and lower the energy of the rate determining steps (RDS) of electrocatalytic CO₂RR. The efficient excited state lifetime of the active site is an important influencing factor in visible photocoupled electrocatalysis.

In a study published in the Journal of the American Chemical Society, a group led by Prof. Cao Rong and Prof. Huang Yuanbiao from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences incorporated Ru(bpy)₃Cl₂ photosensitive donors into a 2,2'-bipyridine functionalized Co-porphyrin-based COF (Co-Bpy-COF) by a post-synthetic method (PSM), providing Co-Bpy-COF-Ru_x (X is the molar ratio of Ru and Co species, X = 1/2 and 2/3) with longer excited state lifetime for CO₂ reduction under photo-electric conditions. Donor–acceptor characteristic and giant built-in electric field in Co-Bpy-COF-Ru_x efficiently accelerate the photo-induced electron transfer from Ru(bpy)₃Cl₂ to the cobalt porphyrin under the external light.

The researchers found that compared with the unmodified counterpart Co-Bpy-COF, 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 CO partial current density of 16.27 mA cm^-2 at -1.1 V vs. RHE under the assistance of light, both of which far surpassed the values observed in the dark. Nonetheless, for Co-Bpy-COF, the values of FE_CO and j_CO tested in the light displayed no significant improvement than those recorded in the dark, which could be attributed to the rapid relaxation of the excited state in the mono-photosensitive unit system.

In addition, the researchers revealed the trend of excited-electron transfer from Ru(bpy)₃Cl₂ to cobalt porphyrin by highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the building units. The fluorescence quenching experiment and X-ray photoelectron spectroscopy (XPS) under visible light further confirmed the real existence of excited-electron transfer from Ru(bpy)₃Cl₂ to cobalt porphyrin.

Furthermore, the researchers verified the formation of the giant built-in electric field in Co-Bpy-COF-Ru_x by electrochemical impedance spectroscopy (EIS) and conductivity measurements. Ultrafast transient absorption (TA) spectra indicated that the powerful donor (Ru(bpy)₃Cl₂) and the giant built-in electric field dramatically extend the excited state lifetime of cobalt-porphyrins up to 3.4 times higher than the unmodified counterpart Co-Bpy-COF. The effect of external light irradiation lowering the energy barrier to the formation of CO was verified by Tafel slopes and density function theory (DFT) calculations.

This study focuses on extending the long, excited state lifetime of photo-coupled electrocatalysts for efficient CO₂RR to CO products and provides a guideline for the development of efficient photocoupled electrocatalysts.