Electrochemical quartz crystal microbalance and impedance analysis investigations of surface processes at platinum electrodes.
Creators: name: De Cliff, Steve V.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1997
Subjects: subject: Chemistry, Physical.

description: This thesis is concerned with the application of the Electrochemical Quartz Crystal Microbalance (EQCM) and of Impedance Analysis (IA) of EQCM electrodes in investigations of surface processes, particularly adsorption, at electrodeposited platinum electrodes. In the first part of the present work, we have studied effects that can influence the mass (frequency) response that is measured by the EQCM technique. The factors investigated include: the preparation (platinization) and pre-treatment (cycling) of the electrode material itself which changes the electrode area, the surface roughness of the electrode and the identity and concentration of the anions of the background electrolyte. Anions of the background electrolyte are found to influence the observed mass response, but analysis of the data is difficult. The most important discovery in this thesis is that the EQCM reveals large frequency changes in the region of adsorbed hydrogen at Pt electrodes that have been extensively cycled. EQCM and impedance experiments carried out in different electrolytes across a range of pH suggest that the large change is due to an interaction between weakly adsorbed hydrogen and water which leads to an increase in the structure of the solvent close to the interface. This work has shown that the surface state of an electrode and its hydrophilicity or hydrophobicity can have large effects on the frequency (mass) response. These changes cannot be seen from the electrochemical response alone. In addition to the study of the adsorption of anions from the supporting electrolytes, the adsorption of another strongly adsorbing anion (chloride) and of a neutral adsorbate (thiourea) has been investigated. Thiourea adsorbs strongly to Pt and when present at or close to monolayer coverage has the effect of removing mass changes seen in the background electrolyte. In solutions where thiourea is present and the potential of the electrode is slowly increased, the mass is constant until the electrode surface is oxidised. This suggests that re-adsorption of products of thiourea oxidation or reactant occurs when adsorbates are removed at potentials below those where the electrode surface is oxidised. When the bulk thiourea concentration is very small, the re-adsorption is prevented and a mass decrease is seen as the adsorbed species are removed from the electrode surface by oxidation. In contrast to thiourea, adsorbed chloride influences the mass response in the H adsorption region of potential but does not produce complete blocking of mass changes of the background electrolyte. The most interesting responses were in the region of potential where the electrode is oxidised. A series of mass responses that change substantially with potential and chloride concentration were suggested to result.from a shifting balance between chloride desorption, possible adsorption of Cl$\sp-,$ oxide formation and electrode dissolution. (Abstract shortened by UMI.)