Preparation And Characterization Of Highly Active Self-supporting Electro-Fenton Cathode

Electro-Fenton,as an attractive electrochemical advanced oxidation process for wastewater treatment,has recently received considerable attention.The principle of the electro-Fenton process lies in the in-situ generation of H₂O₂ and Fe²⁺at the cathode,which react with each other to generate hydroxyl radical（·OH）with strong oxidizing capacity for degrading organic pollutants in wastewater.The yield of H₂O₂ formation and the ability of Fe²⁺regeneration are critical in determining the oxidation efficiency of the electro-Fenton system,which are closely linked to the characteristics of cathode materials.Carbonaceous materials have been widely utilized as cathodes in H₂O₂production owing to their large specific surface areas and good chemical stability.However,the frequent requirement of extra binders for electrode fabrication leads to a complex fabrication process,low catalyst utilization,weak conductivity and stability.Herein,a self-supporting and binder-free electrode for efficient H₂O₂ formation and Fe²⁺regeneration was fabricated and applied to an electro-Fenton process.The effects of calcination temperature,applied cathode potential,and p H on the production of H₂O₂were discussed.The degradation performance of different organic pollutants and organics in iron-laden acid wastewater was clarified.The main findings are as follows:（1）A self-supporting electro-Fenton cathode was successfully fabricated and used for efficient H₂O₂ formation and organic compounds degradation.N-doped mesoporous carbon firmly supported on Ni foam（N@C-NF）was attained by a simple one-step pyrolysis of an as-prepared mixture containing zeolitic imidazolate framework-8（ZIF-8,a MOF providing hierarchical porosity,uniform N doping,and carbon defects）and a piece of Ni foam under inert atmosphere.The resulting 900/N@C-NF cathode（synthesized at 900°C）was high in surface area and N content,and moderate in degree of graphitization,achieved high performance of H₂O₂ production(2.58 mg L^-1 h^-1H₂O₂/mg catalyst)at–0.7 V（vs.SCE）,and enabled prompt regeneration of Fe²⁺.The electro-Fenton system equipped with the 900/N@C-NF cathode was effective in removing a diverse range of organic pollutants,including rhodamine B（Rh B）,phenol,bisphenol A（BPA）,nitrobenzene（NB）,and Cu-ethylenediaminetetraacetic acid（EDTA）.Through quenching experiments,the role of·OH_ads（adsorbed on the electrode surface）and·OH_free（dissolved in the solution）in the degradation of pollutants was distinguished.Moreover,there was no significant difference regarding the Rh B removal percentage recorded over 10 cycles for the electro-Fenton system equipped with the900/N@C-NF cathode.（2）In addition,to avoid the addition of external iron sources,an Fe-doped self-supporting electro-Fenton cathode was fabricated and the heterogeneous electro-Fenton system was constructed for degrading organic compounds.Analogous to the preparation of the self-supporting electrode 900/N@C-NF,the Fe-doped electrode was obtained by introducing the iron source during the precursor synthesis process.The resulting 900/25Fe-N@C-NF cathode also exhibited high surface area（the iron mass ratio was 0.25%）,and was effective in removing Rh B.However,it was revealed that the heterogeneous electro-Fenton cathode delivered relatively poor stability（decay of20.7%performance for cycle 4 compared to cycle 1）,owing to the dissolution of iron upon repetitive cycling.（3）The feasibility of self-supporting electro-Fenton cathode in wastewater treatment was verified.To further explore the application of 900/N@C-NF electrode in actual acidic iron-laden wastewater,the electro-Fenton system equipped with the900/N@C-NF cathode at–0.7 V（vs.SCE）significantly attenuated the concentration of chemical oxygen demand（COD）of the real acid wastewater,exhibiting superior activity and stability.Note that neither extra Fe³⁺/Fe²⁺nor acids were supplemented during treatment of pickling wastewater or AMD.A qualitative and semi-quantitative analysis of the types of soluble organic pollutants in metal surface cleaning wastewater was conducted by three-dimensional fluorescence spectroscopy.The results showed that the soluble organic pollutants are composed of various types of oils（e.g.,lubricating oil and diesel）.The decrease in the fluorescence signals with the increasing time further proved that the electro-Fenton system equipped with the 900/N@C-NF cathode exhibited outstanding performance for pickling wastewater treatment.