Insight into the overpotential and thermodynamic mechanism of hydroxyl radical formation on diamond anode

Abstract

Electrochemical advanced oxidation process (EAOP) to produce highly reactive hydroxyl radicals (OH) in aqueous media offers an intriguing water treatment strategy for the removal of organic compounds. How to efficiently oxidize H2O to OH in EAOP is a great challenge due to the OH production mechanism is ambiguous. In this study, the state-of-the-art anode, boron-doped diamond (BDD) anode, was chosen as a prototype to unveil the water oxidation and OH production mechanism by using density functional theory (DFT). The free energy of adsorbed oxygen intermediates (OH*, O* and HOO*) was elucidated in detail, and the max {ΔG1, G2, G3 and G4} of step oxidation reactions was used as a descriptor to understand OH production mechanism. The obtained results indicated that the BDD anode was thermodynamically favorable to be involved in the production of OH rather than O2 generation reaction. Besides, the water oxidation mechanism on various dopants (N, P, S and F elements) doped diamond (i.e., NDD, PDD, SDD and FDD) anodes were also investigated. Notably, FDD (2.69 V), SDD (2.43 V) and PDD (1.89 V) exhibited higher ηOER compared with NDD (1.24 V) and BDD (1.59 V). The F, S and P doping in diamond would enhance the ΔG2 or ΔG3 of step H2O oxidation reactions and induce the increase of the ηOER, which made the generation of O2 more difficult, thus increasing the Faradic efficiency of OH production. This allowed us to resolve several long-standing puzzles about the relationship between water oxidation and OH production mechanism on BDD electrodes, and might give inspiration on EAOP anode design for effective water treatment.