Boosting CO₂ Electroreduction over a Covalent Organic Framework in the Presence of Oxygen

Abstract

Herein, we propose an oxygen-containing species coordination strategy to boost CO₂ electroreduction in the presence of O₂. A two-dimensional (2D) conjugated metal-covalent organic framework (MCOF), denoted as NiPc-Salen(Co)₂-COF that is composed of the Ni-phthalocyanine (NiPc) unit with well-defined Ni-N₄-O sites and the salen(Co)₂ moiety with binuclear Co-N₂O₂ sites, is developed and synthesized for enhancing the CO₂RR under aerobic condition. In the presence of O₂, one of the Co sites in the NiPc-Salen(Co)₂-COF that coordinated with the intermediate of *OOH from ORR could decrease the energy barrier of the activation of CO₂ molecules and stabilize the key intermediate *COOH of the CO₂RR over the adjacent Co center. Besides, the oxygen species axially coordinated Ni-N₄-O sites can favor in reducing the energy barrier of the intermediate *COOH formation for the CO₂RR. Thus, NiPc-Salen(Co)₂-COF exhibits high oxygen-tolerant CO₂RR performance and achieves outstanding CO Faradaic efficiency (FE_CO) of 97.2 % at -1.0 V vs. the reversible hydrogen electrode (RHE) and a high CO partial current density of 40.3 mA cm^-2 at -1.1 V in the presence of 0.5 % O₂, which is superior to that in pure CO₂ feed gas (FE_CO=94.8 %, j_CO=19.9 mA cm^-2). Notably, the NiPc-Salen(Co)₂-COF achieves an industrial-level current density of 128.3 mA cm^-2 in the flow-cell reactor with 0.5 % O₂ at -0.8 V, which is higher than that in pure CO₂ atmosphere (j_CO=104.8 mA cm^-2). It is worth noting that an excellent FE_CO of 86.8 % is still achieved in the presence of 5 % O₂ at -1.0 V. This work provides an effective strategy to enable the CO₂RR under O₂ atmosphere by utilizing the *OOH intermediates of ORR to boost CO₂ electroreduction.