(Co, S, P)-decorated multiwalled carbon nanotubes (MWCNTs) have been synthesized following a hydrothermal route as electrocatalysts to manufacture large surface area air-diffusion cathodes with carbon cloth as substrate. The enhanced electrocatalytic H2O2 production as compared with Co-free MWCNTs cathodes was demonstrated in a 2.5-L pre-pilot plant with either a RuO2-based or boron-doped diamond (BDD) anode, accumulating between 2- and 3-fold greater H2O2 contents with the catalyzed cathode. The good stability of this new material was ensured from the low Co leaching, with less than 9% Co released to solutions upon repeated usage. Aqueous solutions of the brominated organic preservative bronopol with 0.050 M Na2SO4 at pH 3.0 were comparatively treated by electro-oxidation (EO-H2O2), electro-Fenton (EF), UVA-assisted photoelectro-Fenton (PEF) and solar PEF (SPEF) at constant current density. SPEF with BDD anode and the catalyzed cathode showed the best performance, with total bronopol removal at 210 min and 94% mineralization after 360 min at 40 mA cm−2, thanks to the action of OH, BDD(OH) and sunlight. Formic acid was identified as main reaction by-product, whereas Br and N atoms were mainly converted to Br-, BrO3- and NO3-. Some unidentified organic by-product containing Br and N was formed as well.