Oxygen Reduction to Hydrogen Peroxide on Hydrophilic Carbon Fiber Paper: Dependence of the Mechanism and Active Site Stability on Electrolyte pH and Potassium Ion Concentration

High Resolution Image Download MS PowerPoint Slide Electrocatalytic H 2 O 2 synthesis enables decentralized production and reduces reliance on energy-intensive large-scale infrastructure. Practical application, however, requires catalyst materials that are affordable, scalable, and durable. Here, we show that oxygenated carbon fiber paper, hydrophilized through a rapid mild chemistry process developed in-house, serves as an efficient electrocatalyst for the oxygen reduction reaction (ORR) to H 2 O 2 . This catalyst achieves (95 ± 4)% faradaic efficiency and long-term stability for more than 31 h in a divided cell and 100 h in an undivided cell, significantly surpassing traditional particulate carbon catalysts while eliminating the need for supporting electrodes or binders. The analysis of onset potentials versus the reversible hydrogen electrode reveals pH dependence, indicating a nonproton-coupled electron transfer mechanism. When referenced to the standard hydrogen electrode, the onset potentials further suggest that the rate-determining step of the ORR is proton-dependent. Mechanistic studies highlight the coupled roles of oxygenated carbon sites, electrolyte pH, and spectator potassium ions in steering ORR pathways and show that binder-free catalysts are essential for probing the true reaction environment. Higher H 2 O 2 production rates are obtained at elevated pH, attributed to the greater stability of oxygenated active sites, as confirmed experimentally and supported by density functional theory (DFT) calculations. Hydrophilic carbon fiber paper thus emerges as a robust and viable platform for H 2 O 2 electrosynthesis. These results also provide mechanistic insight into how oxygen functional groups, electrolyte pH, and potassium cations govern activity and selectivity in ORR.