Preparation And Performance Evaluation Of Boron-Doped Diamond Electrodes

Electrode materials are very significant in the electrochemical wastewater treatment process, and the preparation process and electrode structure are the main factors affecting the electrocatalytic characteristics of the electrodes. The thesis focused on the preparation and performance evaluation of boron-doped diamond (BDD) electrode, carrying out research on preparation, parameters optimization and structure characterization of electrodes as well as electrochemical oxidation of environmental contamination. By comparing the electrocatalytical characterization differences between different BDD electrodes and between BDD and SnO₂ electrodes, the relationship between material and performance as well as the corresponding electrocatalytical mechanism were discussed.Direct current plasma chemical vapor deposition (DC-PCVD) system was used to prepare diamond films and the optimal parameters were 1000℃for substrate temperature and 2% for CH₄ concentration. Based on the optimal parameters, BDD electrodes were prepared by doping boron dopant in the source gases. Taken phenol degradation efficiency and electrode stability as target, combined with electrode structure characterization results, the optimal parameters for BDD electrodes preparation were obtained, which included CH₄:H₂:B(OCH₃)₃ ratio of 5:190:10 and substrate temperature of 1000℃.The performance of electrochemical oxidation of phenol was investigated at different initial concentrations and at different current densities. The results showed that phenol degradation and total organic carbon (TOC) removal followed first-order rate kinetics. Phenol with different initial concentrations was completely mineralized to CO₂ and H2O at BDD electrode. Increasing current density lead to side reaction of oxygen evolution, resulting in a decrease in current efficiency. ?OH radicals were proved to be the major active oxidants for indirect electrochemical oxidation of phenol by fluorescence spectrum test. Phenol degradation pathway was analyzed by high performance liquid chromatography (HPLC). It was found that the para site of phenol was attacked by ?OH radicals first, then formed hydroquinone and the ring was broken into maleic acids, fumaric acids and acetic acids. Maleic acids, fumaric acids were reduced into succinic acids at cathode which were further oxidized into malonic acids, acetic acids or formic acids. These small organic acids were finally oxidized into CO₂.The performance and pathways of electrochemical degradation of bisphenol A (BPA) at BDD electrodes were investigated. Similar results to phenol degradation were also found for BPA degradation that the degradation and TOC removal followed first-order rate kinetics. But the kinetic parameters of degradation and TOC removal for BPA were lower than that for phenol, meaning that BPA was more refractory. BPA degradation pathway was analyzed by gas chromatography mass spectrometer (GC-MS). It was found that BPA was first broken into hydroquinone and 4-isopropylphenol, and then the two compounds were further oxidized to several small organic acids which were finally mineralized to CO₂.Taking BDD electrodes with different boron doping concentration as representatives, relationship between structure and activity of BDD electrodes was investigated. Results showed that the carrier concentration and the distortion of lattice were the main factors affecting electrocataltyic characterization of BDD electrodes. Higher carrier concentration and lower distortion of lattice favored high activity. The electrochemical behavior of the optimal BDD electrode was studied, which revealed that the direct electrochemical oxidation of phenol was irreversible, and that the diffusion step was the rate determining step. The interface structure between BDD electrode and electrolyte consisted of space charge layer and electrical double layer. The interface charge transfer resistance was reduced by increasing reaction potential, resulting in the reaction acceleration. When phenol existed in the solution, the interface charge transfer resistance was increased and the oxygen evolution reaction was inhibited.Two high oxygen evolution over-potential electrodes, BDD and SnO₂ electrodes, were compared in their electrocatalytic characterization and electrode structure. It was found that the electrocatalytic activities of BDD electrodes were better than those of SnO₂ electrodes and that the intermediate products accumulation for BDD electrodes were much less. These results were attributed to the high oxygen evolution over-potential, wide potential window and low background current for BDD electrodes. The service life and corrosion resistance capacity of BDD electrodes were much higher than those of SnO₂ electrodes. X-ray photoelectron spectroscopy (XPS) was employed to study the composition and chemical state of different elements on BDD and SnO₂ electrode surfaces. Results showed that the different oxygen chemical state at the two electrodes was the main reason affecting their electrocataltyic characterization. The chemical absorbed"active oxygen"at SnO₂ electrodes had low activity, while the physical absorbed"active oxygen"at BDD electrodes possessed high activityThe dissertation developed BDD electrodes with high activity and stability, which gived references for electrocatalytic electrodes application in electrochemical wastewater treatment process. This should be helpful for the application of this type of electrodes in the field of environmental engineering.