Metal-Free Electrocatalysts for Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H2O2), as an environmentally friendly and highly efficient oxidation reagent, is now widely and increasingly used in many areas. The traditional anthraquinone synthesis process, however, is energy and capital intensive with low efficiency. Electrochemically reducing oxygen (O-2) to H2O2 with metal-free catalysts is considered a promising alternative. This work describes the two-electron oxygen reduction process from both thermodynamic and kinetic aspects and summarizes the designing rules for effective H2O2 formation catalysts. This includes tuning the binding energy of the intermediate species, the concentration of dissociation active sites, and adjusting the dielectric constant of solvents. These principles are successfully applied to the design of various metal-free catalysts by proper structure engineering and heteroatom doping, which shows both high activity and efficiency. Furthermore, pH and temperature effects are presented with a view toward reaction optimization. An outlook of future challenges is also discussed.