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

•The reaction pathway of ·OH+HCO3-→CO3·- in KHCO3 is firstly reported.•OH+CO32-→CO3·-+OH- and OH-+HCO3-→CO32-+H2O in K2CO3 is firstly proposed.•H2O2 formation in K2CO3 is more thermodynamically favorable than that in KHCO3.•OH--e→·OH followed by 2·OH→H2O2 at anode is unanimously ruled out.•OH plays a crucial role in 2e-WOR by reacting with CO32- or HCO3- to CO3·-. Two-electron water oxidation reaction (2e-WOR) is a promising approach to produce H2O2 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 KHCO3 and K2CO3 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 CO3·-, which arises from the reactions of ·OH with HCO3- in KHCO3 but ·OH with CO32- in K2CO3. DFT calculations reveal that H2O2 formation via ·OH+CO32-→CO3·-+OH- in K2CO3 is more thermodynamically favorable than that via ·OH+HCO3-→CO3·-+H2O in KHCO3. This finding aligns well with experimental results showing a significantly higher H2O2 generation rate in K2CO3 compared with KHCO3. Moreover, another pathway of OH--e→·OH followed by 2·OH→H2O2 at the anode was unanimously ruled out, as no H2O2 was detected in KOH electrolyte and the high endothermicity of OH--e→·OH (ΔE = 2.95 eV) was verified by DFT calculation. ·OH plays a crucial role in 2e-WOR by reacting with CO32- or HCO3- to form CO3·-, as well as the subsequent formation of HCO4-, which ultimately hydrolyzes to produce H2O2. 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. [Display omitted] H2O2 formation via ·OH+CO32-→CO3·-+OH- in K2CO3 is more thermodynamically favorable with high performance than that via ·OH+HCO3-→CO3·-+H2O in KHCO3.