Mechanistic insight into the selective singlet oxygen production at cathode via the activation of anodic water...

Selective and efficient production of singlet oxygen (¹O₂) with a Faraday efficiency of 35.1 % through the direct cathodic activation of oxygen bubbles (OBs) is studied in this work, the OBs are generated from the anodic water splitting process in an electrochemical reactor with the cathode-above-anode configuration. Dramatically, ¹O₂ is derived from the utilization of gaseous O₂ molecules in OBs rather than the dissolved oxygen in water. A series of experimental investigations were conducted and analyzed to study the efficiency and mechanism of ¹O₂ production with an undoped hydrophobic carbon plate cathode, including luminol tests for fluorescent visualization of ¹O₂, aqueous organic pollutants degradation tests with sulfamethoxazole (SMX), ¹O₂ probe tests with furfuryl alcohol (FFA), electron paramagnetic resonance (EPR) tests, H₂¹⁸O isotope-labeling experiments, and the determination of cathodic electron transfer number. Density functional theory (DFT) calculations were also employed to study the thermodynamics of the key molecular reaction processes. The unique mechanism for ¹O₂ production from the OBs activation is proposed, because the reduced interfacial proton flux in the solid-gas interface between the hydrophobic cathode and OBs facilitates the cathodic desorption of *•O₂^-, the multiple electrons transfer for oxygen reduction reaction is prevented, while amplifying the radical-radical coupling of •O₂^-_(g)/ΗΟ₂•_(g) to selectively produce ¹O₂ in the OBs. This research provides insights into the pathways of ¹O₂ generation utilizing self-produced OBs in electrochemical systems, and discovers a new avenue to enhance the efficiency and selectivity of cathodic ¹O₂ production for oxidative treatment of aqueous pollutants or chemicals.