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

Abstract

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.