Preparation And Properties Of The Graphite/PTFE Gas Diffusion Electrode

With the graphite powder as the main material, polytetrafluoroethylene （PTFE） asadhesive and hydrophobic agent, the graphite/polytetrafluoroethylene gas diffusionelectrode was prepared. The diffusion electrode was characterized by TG-DSC and BETtechniques, and the application was studied subsequently. The effects of various operatingparameters including pH, cathodic current density, electrolyte concentration, electrolytetype and temperature on the generation of Hydrogen peroxide （H₂O2） were investigated.The electrochemical impedance spectroscopy （EIS） technique was adopted to study theoxygen reduction reaction on the graphite/PTFE gas diffusion cathode. The oxygenmolecule adsorption and reduction mechanism of its on the graphite surface were studiedthrough the density functional theory （DFT）. The influence of pH, electrolyteconcentration, initial dye concentration and ferrous ion (Fe²⁺) concentration on thedecolorization of orange Ⅱ were also evaluated. Furthermore, the application ofthe gasdiffusion electrode for microbial fuel cells （MFC） was also investigated.TG-DSC and BET results suggest that the graphite/PTFE gas diffusion electrode hasa microporous structure, and exhibits a heat-resistant stability. The results fromelectrochemical generation H₂O2experiments show that the influences of pH and cathodiccurrent density on the generation of H₂O2are more remarkable than that of electrolyteconcentration, electrolyte type and temperature. When the current density and solution pHis3mA/cm2and3respectively, the concentration of generated H₂O2is438.9mg/L. Theanalyzed results of EIS indicate that the rate-determining step of the oxygen reductionreaction involves the adsorption of oxygen molecule and the intermediate species. Thevalue of double layer capacity increases with the increase of cathodic current density,which indicates that the electroactive area of the gas diffusion cathode also increases. TheEIS spectra with the existence of inductive ring demonstrate that two electrochemicalsteps will take pace in the oxygen reduction reaction, at least.The density functional theory （DFT） was used to study the oxygen reductiongeneration of H₂O2in acidic medium and alkaline medium. The calculated results showthat in the acidic medium, the oxygen molecule of adsorption on the surface of graphite first gets two electrons to form HO₂^-, and then combined with a H+to generate hydrogenperoxide molecule. This process is an exothermic reaction. In alkaline medium, H₂O₂isgenerated via two kinds of path. One path is that the oxygen molecule directly gets twoelectrons and combines with water to form HO₂^-and then binds H+to generate H₂O₂. Theother path is that the oxygen molecule soaks up a electron and O₂-is generated, then O₂-acts with water to obtain HO₂^-, and then the formed HO₂^-combines with H+to generateH₂O₂. However, this kind of process is an endothermic reaction.The results obtained from decoloring experiment show that the decoloring efficiencyof orange Ⅱ is89.9%, when the initial concentration of the dye and pH of the solutionwas kept as100mg/L and2, respectively. The decolorization of orangeⅡ increases withthe increase of electrolyte concentration, however, it reduces with the increase of theinitial concentration of dye. The presence of Fe2+improves the decolorization of orangeⅡmarkedly. In the electrochemical degradation process,92.8%decolorization efficiencycan be achieved as the concentration of Fe2+and reaction time kept as0.1mmol/L and110min. In this regard, it shows that the electro-Fenton technique can be regarded as anexcellent method for the treatment of dye wastewater. When the graphite/PTFE gasdiffusion electrode as the cathode in MFC, the output voltage can reach0.219V, and therate of COD removal in the anode chamber is75%. Therefore, it can be inferred that thecombination technique of cathode oxygen reduction and MFC, has a potential applicationfor wastewater treatment.