A stable CoSP/MWCNTs air-diffusion cathode for the photoelectro-Fenton degradation of organic pollutants at pre-pilot scale

CoS2/MWCNTs have been previously described as potentially viable catalysts to enhance the classical two-electron H2O2 production from O2 reduction reaction (ORR) for in situ water treatment, but their poor stability still limits their large-scale application. Here, the synthesis and characterization of a novel electrocatalyst made of CoSP nanoparticles supported onto multi-walled carbon nanotubes (MWCNTs) is reported. X-ray diffraction data demonstrated the much higher stability conferred upon partial sulfur substitution by phosphorus. Linear and cyclic voltammograms of CoSP/MWCNTs showed a potential window from 0.9 to 0.1 V for the ORR at pH 3.0, along with greater H2O2 production ability. Large area air-diffusion cathodes were manufactured by depositing the catalyst onto carbon paper, being further used in a pre-pilot filter-press cell containing a boron-doped diamond anode. A stable H2O2 accumulation, with maximum current efficiency of 72.0%, was found upon electrolysis of 2.5 L of 0.050 M Na2SO4 at pH 3.0 and 25 mA cm-2. As a crucial finding, Co leaching was negligible. Solutions with 20 mg L-1 of the herbicide bentazon in the same electrolyte could not be mineralized by electrochemical oxidation, whereas photoelectro-Fenton with an UVA lamp and 0.50 mM Fe2+ led to total removal of the herbicide with 77.0% mineralization.