| Cathodic oxygen reduction reaction (ORR) is considered to proceed 2-electron and direct 4-electron pathway. The ORR selectivity is closely related to the category and structure of cathode materials. One-step synthesis of H2O2 from oxygen gas or air can be realized via 2-electron ORR, which is mainly applied in electro-Fenton degradation of organic pollutants. Due to the advantages such as continuous in-situ generation of H2O2, high oxidation efficiency and free secondary pollution, the process has aroused wide concern. Suitable cathode material is the key to guarantee high oxidation efficiency in electro-Fenton process.N-containing structures’contribution to catalyse ORR of N-doped carbon materials has been demonstrated by many researchers, so a variety of N-doped carbon materials were synthetized and characterized but with complex synthetic steps which limits its large-scale manufacture. Commercial polyacrylonitrile-based carbon fiber (PAN-CF) is produced from acrylonitrile monomer after spinning, polymerization, high-temperature carbonization, etc., with natural N-doped structures, which may exhibit excellent ORR activity. Therefore, PAN-CF was firstly used as a cathode in electro-Fenton process and a brush cathode (PAN-CFB) in favor of oxygen mass transfer was adopted. H2O2 electrogeneration performance via 2-electron ORR and organics degradation performance in electro-Fenton process using PAN-CFB cathode were studied; EMPAN-CF with obviously improved ORR activity was obtained via electrochemical modification of PAN-CF and the correlation between the changes of N-doped structures and the improvement of ORR activity was studied. Finally, H2O2 electrogeneration performance, energy consumption and organics degradation performance in electro-Fenton process using EMPAN-CFB cathode were evaluated. The main research contents and conclusions are as follows:1. ORR activity and H2O2 electrogeneration performance of PAN-CFB in H2SO4, Na2SO4 and NaOH solution were studied. H2O2 can be efficiently electrogenerated in all three medium with high current efficiency> 90%, indicating 2-electron selectivity of PAN-CF independent of medium (acidic, neutral or alkaline condition). The type of N-containing functional groups present on the surface of PAN-CF was identified by XPS analysis and the ORR activity of PAN-CF was confirmed to be related to its N-doped structures. Influence of cathodic potential, electrolyte concentration, volume and the distance between the anode and the cathode was studied in detail.2. Electro-Fenton decolorization and degradation of cationic dye methylene blue (MB) and degradation of phenol using PAN-CFB cathode with additionally added Fe2+ were studied. Influence of initial pH、Fe2+ and dye concentration on MB decolorization was studied firstly. Different initial pH leads to unobvious influence on MB decolorization in a wide pH range from 3 to 7 when treating 5 mg L-1 MB, with almost 100% color removal. The acidification of the solution at initial pH 5,7 results in similar high color removal percentage as at the optimum pH 3. Moreover, under the conditions of 300 mA current, pH 3.00 and 0.3 mmol L-1 Fe2+, a nearly 100% color removal after 30 min,95.9% CODCr removal and the low electrochemical energy consumption of 11.6 kWh kg-1 COD after 60 min for 100 mg L-1 MB solution were obtained. Phenol (100 mg L-1) can also be degraded efficiently with CODCr removal of 86.2%, average current efficiency of 74.2% and electrochemical energy consumption of 35.6 kg-1 COD.3. Decolorization of MB via electro-Fenton process using Fe sacrificial anode and PAN-CFB cathode (peroxi-coagulation) was studied. H2O2 electrogeneration performance of PAN-CFB cathode is unrelated to the electrolyte type. However, the accumulated concentration of H2O2 was significantly influenced by anode and electrolyte. The accumulated H2O2 concentration decreased with the increasing NaCl concentration due to the reaction between active chlorine from anodic reaction and H2O2 from cathodic ORR when using dimensionally stable anode (DSA). In the peroxi-coagulation process at initial pH 3, the color removal percentage for MB with different concentrations (5～20 mg L-1) all reached> 98% after electrolysis of 30 min. While at initial pH 7～8 (without pH regulation), MB decolorization process successively proceeds three stages in which oxidation, "oxidation + coagulation" and coagulation respectively plays the dominant role; the color removal percentage reached more than 80% after 20 min and stopped to increase because Fenton oxidation was almost completely inhibited by coagulation.4. PAN-CF was modified using recurrent galvanic pulse method, converting its inherent pyridinic-N into 2-pyridone (or 2-hydroxyl pyridine) functional group existing in three-dimensional active layers with remarkable ORR catalytic activity and stability. The C atom of pyridone adjacent to N and O is prone to act as an active site to efficiently catalyze a two-electron ORR process; However, after coordinating pyridone to the Cu2+, together with the electrochemical reaction, the chemical redox between Cu+ and ORR intermediates synergistically tends towards a 4-electron pathyway in alkaline solution. In different medium, the complexation and dissociation can induce the charge transfer and reconstruction among proton, metal ion and pyridone functionalities, leading to the changes of ORR performance.5. EMPAN-CFB cathode was used for H2O2 electrogeneration and electro-Fenton decolorization of azo dye methyl orange (MO). H2O2 electrogeneration performance and energy consumption of PAN-CFB cathode before and after modification was compared. EMPAN-CFB cathode exhibits high stability for H2O2 electrogeneration with ca.500 mV positive shift of the optimum cathodic potential and significantly reduced energy consumption (EC). The minimal EC was only 5.02 kWh kg-1 H2O2 (0.5 mol L-1 Na2SO4,I= 400 mA, electrode distance= 1 cm). Rapid and effective decolorization of MO via electro-Fenton oxidation was realized in 5 min, following the pseudo-first-order kinetic model with high correlation coefficients. |
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