Electrochemical quartz crystal microbalance studies of potentiodynamic electrolysis of aqueous chloride solution: surface processes and evolution of H 2 and Cl 2 gas bubbles

Abstract

The industrially important electrolysis of aqueous chloride was investigated under potentiodynamic conditions, using a platinum-coated electrochemical quartz crystal microbalance (EQCM) for the first time. It is shown that chloride ions are adsorbed on the platinum surface at potentials between approx. −0.5 and 0.6 V vs. SCE. The coverage of the chloride is essentially 100% in a solution containing 0.9 M NaCl and 0.1 M HCl. Anodic desorption of the adsorbed chloride and adsorption of approx. 25% of a monolayer of atomic chlorine are observed in the range 0.6-1.1 V. The coverage of atomic chlorine increases with the concentration of molecular chlorine in the solution near the electrode surface, reaching about 50% under the conditions given. During chlorine evolution between 1.3 and 1.6 V at a sweep rate of 50 mV s −1, up to half a monolayer of platinum oxide (i.e. one oxygen atom on every two platinum atoms) is deposited. Reduction of the oxide during the negative scan to potentials below 0.8 V results in dissolution of the platinum substrate by up to half a monolayer in every redox cycle. The dissolution process was confirmed by quantitative atomic absorption analysis. As expected, partial replacement of the bathing electrolyte at the electrode surface by hydrogen and chlorine gas bubbles causes a significant decrease in the apparent areal density, suggesting the suitability of the EQCM for the study of electrolytic gas bubbles.