AC impedance studies on copper deposition and aluminum electrolysis.

摘要

The influences of cathodic potential and surface roughness on copper deposition on a rotating cylindrical electrode from a copper sulfate/sulfuric acid bath were studied experimentally and theoretically using electrochemical impedance analysis. The influence of methylthiourea, a sulfur containing organic compound often added to sulfate bath for the purpose of improving the physical properties of deposited copper, on copper deposition onto a copper electrode was investigated. In aluminum electrolysis, the evaluation of the inert anode, the development of which is important for the aluminum industry, was tried using electrochemical impedance analysis and cyclic voltammetry. Four anode materials were tested: carbon, platinum, cermet, and SnO{dollar}sb2{dollar}.; The rate determining step of copper deposition was determined through Nyquist impedance plots as a function of cathodic potential. At low cathodic potentials ({dollar}-{dollar}60 mV and {dollar}-{dollar}110 mV), the total rate was controlled by the redox reaction of cupric ion, while mass transfer, the diffusion of cupric ion to electrode surface, became the rate determining step at high cathodic potentials ({dollar}-{dollar}200 mV and {dollar}-{dollar}250 mV). The modeling of the impedance behavior agreed with experimental results. The change of effective surface area of electrode due to surface roughness was monitored through impedance diagrams.; The addition of methylthiourea to the sulfate bath reduced the cathodic current and changed the current-voltage relation. A passivation-like phenomenon related to the adsorption of methylthiourea on the cathodically charged electrode was observed. The passivation-like phenomenon, demonstrated in cyclic voltammograms and impedance plots, was established to occur in the potential range of {dollar}-{dollar}150 mV to {dollar}-{dollar}260 mV, above 0.3 M cupric sulfate, and with methylthiourea concentration higher than 0.005 g/l. The impedance diagram was shown to be a sensitive indicator often whether effective conditions for electroplating were present in the bath.; Platinum, cermet, and SnO{dollar}sb2{dollar} electrodes showed lower anodic overpotentials than the carbon anode presently used as an anode in aluminum production. The reaction of combining the aluminate ion with carbon to produce carbon dioxide was the rate determining step in carbon anode with saturated alumina. The mass transfer of aluminate ion controlled the total rate in cases of platinum, cermet, and SnO{dollar}sb2{dollar} anodes but in SnO{dollar}sb2{dollar}, the charge transfer reaction was not negligible at high current density (26.8 mA/cm{dollar}sp2{dollar}). Cermet was found to be a promising practical inert anode material since it was chemically stable due to the copper oxide film and its electrical conductivity was relatively high.