Bifunctional catalysts for heterogeneous electro-Fenton processes: a review

Fenton processes allow to degrade and mineralize toxic organic contaminants, yet classical Fenton processes require continuously adding hydrogen peroxide and ferrous ions, costly solution pH adjustment, and treatment of secondary iron sludge pollution. Heterogeneous electro-Fenton processes deliver oxidizing radicals with only oxygen and electricity consumed. Bifunctional catalysts allow the synthesis and activation of hydrogen peroxide simultaneously, eliminate additional chemical reagents, and yield no metal residues in treated water. Here, we review bifunctional catalysts for heterogeneous electro-Fenton processes. We describe the mechanisms of oxidizing radical generation from oxygen. Then, we compare different types of bifunctional catalysts based on their elemental compositions: (1) metal/carbon composite catalysts, i.e., monometallic iron/carbon composite catalysts, bimetallic/trimetallic carbon composite catalysts, and transition metal single-atom catalysts; (2) metal composite catalysts without carbon; and (3) metal-free carbon catalysts. Then, we present five other approaches beyond electrocatalysts, which have been used to improve the performance of heterogeneous electro-Fenton processes.