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

Abstract

Electrochemical production of H 2 O 2 via a 2electron oxygen reduction reaction (2e − ORR) provides a clean alternative to the traditional industrial process. H 2 O 2 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 thermoand 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 (CeO x /CNT) composite catalysts and investigated ceria's promotional effect on the neutral 2e − ORR. The optimal CeO x /CNT catalyst demonstrated a 1.5-fold increase in ORR activity compared to CNT alone, with high H 2 O 2 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 Ce 3+ /Ce 4+ ratio as the applied potential was lowered. This finding suggested ET from Ce 3+ to O 2 , 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 O 2 is initially reduced by Ce 3+ and subsequently adsorbed onto the active carbon sites.