Uncovering mechanistic pathways of two-electron water oxidation reaction in KHCO3 and K2CO3 electrolytes

Two-electron water oxidation reaction (2e-WOR) is a promising approach to produce H 2 O 2 under mild conditions without pollution. Despite of the crucial roles of electrolytes in 2e-WOR, the effects of electrolytes on the performance and reaction pathway remain in debate and seldom reported. In this work, the effects of KHCO 3 and K 2 CO 3 electrolytes on the reaction pathways in 2e-WOR reactions were systematically explored using experimental approaches and DFT calculations. Our results highlight that the crucial distinction in 2e-WOR reaction mechanism lies in the formation of CO 3 ·- , which arises from the reactions of ·OH with HCO 3 - in KHCO 3 but ·OH with CO 3 2- in K 2 CO 3 . DFT calculations reveal that H 2 O 2 formation via ·OH+CO 3 2- →CO 3 ·- +OH - in K 2 CO 3 is more thermodynamically favorable than that via ·OH+HCO 3 - →CO 3 ·- +H 2 O in KHCO 3 . This finding aligns well with experimental results showing a significantly higher H 2 O 2 generation rate in K 2 CO 3 compared with KHCO 3 . Moreover, another pathway of OH - -e→·OH followed by 2·OH→H 2 O 2 at the anode was unanimously ruled out, as no H 2 O 2 was detected in KOH electrolyte and the high endothermicity of OH - -e→·OH (Δ E = 2.95 eV) was verified by DFT calculation. ·OH plays a crucial role in 2e-WOR by reacting with CO 3 2- or HCO 3 - to form CO 3 ·- , as well as the subsequent formation of HCO 4 - , which ultimately hydrolyzes to produce H 2 O 2 . This work provides a novel insight into the reaction mechanism of 2e-WOR in different electrolytes, laying a solid foundation for the design of high-efficient electrolytes and the development of high-performing catalysts, as well as the optimization of operational conditions in the 2e-WOR process.