Revealing Decay Mechanisms of H₂O₂-Based Electrochemical Advanced Oxidation Processes after Long-Term Operation for Phenol Degradation

Abstract

Hydrogen peroxide (H₂O₂)-based electrochemical advanced oxidation processes (EAOPs) have been widely attempted for various wastewater treatments. So far, stability tests of EAOPs are rarely addressed and the decay mechanism is still unclear. Here, three H₂O₂-based EAOP systems (electro-Fenton, photoelectro-Fenton, and photo+ electro-generated H₂O₂) were built for phenol degradation. More than 97% phenol was removed in all three EAOPs in 1 h at 10 mA·cm^-2. As a key component in EAOPs, the cathodic H₂O₂ productivity is directly related to the performance of the system. We for the first time systematically investigated the decay mechanisms of the active cathode by operating the cathodes under multiple conditions over 200 h. Compared with the fresh cathode (H₂O₂ yield of 312 ± 22 mg·L^-1·h^-1 with a current efficiency of 84 ± 5% at 10 mA·cm^-2), the performance of the cathode for H₂O₂ synthesis alone decayed by only 17.8%, whereas the H₂O₂ yields of cathodes operated in photoelectro-generated H₂O₂, electro-Fenton, and photoelectro-Fenton systems decayed by 60.0, 90.1, and 89.6%, respectively, with the synergistic effect of salt precipitation, ^•OH erosion, organic contamination, and optional Fe contamination. The lower current decay of 16.1-32.3% in the electrochemical tests manifested that the cathodes did not lose activity severely. Therefore, the significant decrease of H₂O₂ yield was because the active sites were altered to catalyze the four-electron oxygen reduction reaction, which was induced by the long-term erosion of ^•OH. Our findings provided new insights into cathode performance decay, offering significant information for the improvement of cathodic longevity in the future.