Overlooked Impacts of Alcohols in Electro-H2O2 and Fenton Chemistry

Alcohols are promising fuels for direct alcohol fuel cells and are common scavengers to identify reactive oxygen species (ROS) in electro-Fenton (EF) systems. However, the side impacts of alcohols on oxygen reduction reactions and ROS generation are controversial due to the complex interactions between electrodes and alcohol-containing electrolytes. Herein, we employed synchrotron-Fourier-transform infrared spectroscopy and electron paramagnetic resonance technologies to directly observe the changes of chemical species and electrochemical properties on the electrode surface. Our studies suggested that alcohols exhibited different limiting degrees on proton (H⁺) mass transfer toward the catalytic surface, following an order of methanol < ethanol < isopropanol < tert-butyl alcohol (TBA). In addition, the formation of hydrophobic TBA clusters at high concentrations (>400 mM) resulted in a significant reduction in ionic conductivity and an elevation in charge transfer resistance, which impedes H⁺ mass transfer and raises the energy barrier for 2e^- oxygen reduction reaction processes. Moreover, the organic radical ^•CH₂(CH₃)₂CH₂OH produced by the interaction of Fe³⁺ and ^•OH with the alcohol in the EF system serves as a crucial intermediate in facilitating H₂O₂ regeneration, which complicates the quenching effect of alcohols on ^•OH identification. Therefore, it is recommended that methanol should be used as the scavenger instead of TBA and the concentration should be less than 400 mM in EF systems.