Evaluation of the stability of polyacrylonitrile-based carbon fiber electrode for hydrogen peroxide production and phenol mineralization during electro-peroxone process

This study evaluated the stability of polyacrylonitrile-based carbon fiber cathode for hydrogen peroxide (H2O2) production and phenol mineralization during multiple cycles of electro-peroxone (E-peroxone) process. Results show that the oxidation of bulk carbon fiber by electro-generated H2O2 and bubbled ozone (O-3) is negligible during the E-peroxone process. Nevertheless, the carbon fiber surface was oxidized to some extent as the cathode was repeatedly used in the multi-cycle E-peroxone process. Due to the oxidation by H2O2 and O-3, nitrogen-containing groups on the carbon fiber surface were converted from pyridinic-N to pyridonic-N during the E-peroxone process. These changes resulted in an increase in the activity of the cathode for oxygen reduction reaction (ORR), but a decrease in the selectivity of the cathode for two-electron ORR to H2O2. After the carbon fiber cathode was used for 30 cycles of the E-peroxone treatment of phenol solutions, the cathodic potentials for ORR shifted positively by similar to 450 mV, which is beneficial to reduce the energy consumption of electrochemical H2O2 production. Nevertheless, the apparent current efficiency (ACE) for H2O2 production decreased from similar to 91.5% for the virgin cathode to similar to 48.2% for the used cathode. Despite the decrease in the ACE for H2O2 production, sufficient amounts of H2O2 could still be produced during the E-peroxone process with the used cathode. Therefore, complete phenol mineralization was maintained during all 30 cycles of the E-peroxone treatment of phenol solutions. These results suggest that the polyacrylonitrile-based carbon fiber is a promising cathode material for long-term E-peroxone operations.