Ce⁴⁺/Ce³⁺ Redox-Promoted Electron Transfer for Efficient Neutral H₂O₂ Electrosynthesis from Two-Electron Oxygen Reduction

Electrochemical production of H2O2 via a 2-electron oxygen reduction reaction (2e– ORR) provides a clean alternative to the traditional industrial process. H2O2 electrosynthesis in noncaustic neutral electrolytes is desirable for broader applications; however, it requires larger overpotentials compared to those in alkaline electrolytes where a high 2e– ORR activity can be achieved even with metal-free carbon electrocatalysts. Although ceria has been widely adopted as a catalytic promoter in thermo- and electrocatalytic reactions, its roles in enhancing the neutral 2e– ORR have seldom been explored and remain unclear. In this work, we prepared ceria nanoparticles supported on carbon nanotube (CeOx/CNT) composite catalysts and investigated ceria's promotional effect on the neutral 2e– ORR. The optimal CeOx/CNT catalyst demonstrated a 1.5-fold increase in ORR activity compared to CNT alone, with high H2O2 selectivity over 87%. Electrochemical impedance spectroscopy indicated that the activity improvement correlated with an enhanced electron transfer (ET) rate. In situ X-ray absorption near-edge structure analysis revealed a counterintuitive decrease in the Ce3+/Ce4+ ratio as the applied potential was lowered. This finding suggested ET from Ce3+ to O2, as supported by further electrochemical measurements. In addition, in situ Raman spectroscopy indicated the participation of the CNT in the electrocatalysis. The combination of electrochemical tests and in situ spectroscopies proposes a cascade reaction pathway, where O2 is initially reduced by Ce3+ and subsequently adsorbed onto the active carbon sites.