Fabrication of a double-layer membrane cathode based on modified carbon nanotubes for the sequential electro-Fenton oxidation of p-nitrophenol.

To improve the electrocatalytic efficiency of the cathode and provide a wider pH range in the electro-Fenton process, N-doped multi-walled carbon nanotubes (NCNTs) and ferrous ion complexed with carboxylated carbon nanotubes (CNT-COOFe2+) were used to fabricate the diffusion layer and catalyst layer of a membrane cathode, respectively. The morphology, structure, and composition of CNT-COOFe2+ were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The oxygen reduction performance of NCNT was evaluated using cyclic voltammetry (CV) and the rotating disk electrode technique (RDE). In addition, a potential application of the cathode in sequential electro-Fenton degradation of p-nitrophenol (p-NP) was investigated. The results revealed that iron was successfully doped on the carboxylated carbon nanotubes in ionic complexation form and the content of iron atoms in CNT-COOFe2+ was 2.65%. Furthermore, the defects on the tube walls provided more reactive sites for the electro-Fenton process. A combination of CV and RDE data indicated that NCNT had better electrocatalytic H2O2 generation activity with a more positive onset potential and higher cathodic peak current response than CNT. A p-NP removal rate of 96.04% was achieved within 120 min, and a mineralization efficiency of 80.26% was obtained at 180 min in the sequential electro-Fenton process at a cathodic potential of - 0.7 V vs SCE and neutral pH. The activity of the used cathode was restored simply through electro-reduction at - 1.0 V vs SCE, and a p-NP removal rate of more than 70% was obtained at 60 min after six regeneration cycles.