A hierarchical CoFe-layered double hydroxide modified carbon-felt cathode for heterogeneous electro-Fenton process

Hierarchical CoFe-layered double hydroxide (CoFe-LDH) was grown on carbon felt (CF) as a heterogeneous catalyst by in situ solvothermal growth. The CoFe-LDH/CF serves as a cathode as well as a Fe2+ (catalyst) source in the electro-Fenton (EF) process. A combined structural and electrochemical characterization revealed highly ordered and well crystallized CoFe-LDH anisotropically grown on a CF substrate with highly dense urchin-like structures that were highly stable at circumneutral pH. EF experiments with the CoFe-LDH/CF cathode showed excellent mineralization of Acid Orange II (AO7) over a wide pH range (2-7.1). The mineralization of AO7 with CoFe-LDH/CF was by both homogeneous and surface-catalyzed processes at low acidic pH, whereas only the surface-catalyzed process occurs at circumneutral pH due to the stability of the LDH as well as precipitation of the catalyst. Higher mineralization was achieved with CoFe-LDH/CF compared to homogeneous EF with raw CF using the Fe2+/Co2+ catalyst at all pH values studied and the TOC removal with the CoFe-LDH/CF cathode was at least 1.7 and 3.5 times higher than that with the homogeneous system with Fe2+/Co2+ at pH 5.83 and 7.1, respectively. The enhanced performance observed with CoFe-LDH/CF was ascribed to (i) the surface-catalyzed reaction occurring at the surface of the cathode which can expand the working pH window, avoiding the precipitation of iron sludge as pH increases, (ii) enhanced generation of H2O2 due to the improved electroactive surface area of the cathode and (iii) the co-catalyst effect of the Co2+ in the LDH that can promote regeneration and additional production of Fe2+ and the hydroxyl radical, respectively. The CoFe-LDH/CF cathode exhibited relatively good reusability as the TOC removal after 2 h was still above 60% after 7 cycles of degradation, indicating that the prepared CoFe-LDH/CF is a promising cathode for the removal of organic pollutants by EF technology.