Characterisation and Application of Lead Dioxide Electrodeposited from Methanesulfonate Electrolytes

Abstract

This work focuses on the electrodeposition and modification of lead dioxide (PbO2) electrode. PbO2 has been actively studied as one of the critical anode materials in electrochemical engineering, and its applications have been extensively explored in catalysis fields including effluent treatment, electroanalysis, and electrosynthesis. A modern electrochemical industry commonly employs anodic electrodeposition to manufacture PbO2 electrodes based on traditional oxidising and toxic electroplating systems. However, a cleaner and more efficient electrodeposition mode using the methanesulfonate electrolyte has sparked increasing research interest in recent times. This electrolyte type benefits from its high Pb2+ solubility, good biodegradability and high chemical stability. In spite of its superior properties, the presence of chelating methanesulfonate anions causes difficulty in investigating the PbO2 preparation and modification processes. Hence, the primary aim of this research thesis is the characterisation and exploration of titanium-based PbO2 electrodes deposited from the methanesulfonate electroplating system. In this project, a series of PbO2 coatings were electrosynthesised on the Ti/SnO2-Sb substrate. An interlayer of SnO2-Sb on Ti substrate was used to promote the electrodeposition of PbO2 coatings. Structural and electrochemical features of the resulting PbO2 coatings were characterised. Efforts have been made to establish correlations between the PbO2 physicochemical properties and some determining deposition parameters. The concentrations of lead methanesulfonate and methanesulfonic acid influence the electrodeposition in a synergistic manner, due to the sophisticated effects of methanesulfonate (MS) anions. Although MS anions normally cause inhibitive effects on the deposition process, the aggregated MS anions near the anode surface promote the Pb2+ mass transfer, which considerably facilitates the PbO2 electrodeposition under the insufficiency of Pb2+ from bulk ii solutions. The marked shift in deposition kinetics when changing the electrolyte composition concurrently changes the phase constitution of PbO2 coatings. In addition, the study of current density and deposition times provides further information concerning the PbO2 electrodeposition. The increased applied current density changes the structural properties and lowers the crystallinity level of PbO2 coatings. During the initial deposition, random grain growth occurs near the substrate, while textured PbO2 forms as the deposition proceeds. It is worth noting that strong textures were generated along several preferential crystalline planes in all the PbO2 coatings obtained in our research. The surface morphologic features of the electrodeposited PbO2 depend on the electrodeposition conditions and are primarily responsible for its surface electrocatalytic activity - an ordered, rough surface structure is favoured in surface redox reactions. The subsequent exploration focuses on the modification of PbO2 electrode to improve its electrochemical performance in wastewater treatment, either by the addition of polyethylene glycol (PEG) in electrolytes or by a pulse reverse current (PRC) electrodeposition technique. The PEG addition suppresses the PbO2 electrodeposition process and refines the PbO2 coating. The PEG addition (≤4g/L) leads to a more compact surface without changing the pyramidal morphology. Enhanced degradation ability against organic pollutant was observed for PbO2 electrodes modified with PEG addition at 4g/L. This originates from its improved surface activity and higher resistance to oxygen generation - the side reaction. Moreover, our study investigates the PRC deposited PbO2 electrodes using different duty cycles, revealing that PRC is an efficient and promising approach to modify PbO2 deposits. PRC deposition distinctly alters the phase constituent and surface microstructure, transforming the PbO2 deposits into pure β-PbO2 with a rough surface structure. This phenomenon originates from the preferred formation of β-PbO2 in the dissolution/re-deposition reactions. The degradation tests reveal that higher catalytic ability is achieved for the PRC deposited PbO2 electrode, implying better iii performance in effluent treatment. Based on the analyses presented in this thesis, a uniform and compact surface with three-dimensional pyramidal features offers excellent degradation ability towards organic pollutants. To conclude, the new knowledge arising from this study may help to elucidate the PbO2 electrodeposition and may shed light on the future of PbO2 electrodes in environmental engineering.