Mercury Amalgam Electrodeposition on Metal Microelectrodes

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Saillard, Audric
Anei, Fedorov
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Mercury amalgam microelectrodes, typically fabricated by electrodeposition of mercury onto metal (platinum, gold, silver) inlaid disks, possess certain advantageous properties for scanning electrochemical microscopy (SECM) and electroanalysis. But as applications require more and more precision, fundamental questions concerning the exact shape and constitution of the amalgam can become important for interpreting SECM experimental data. The purpose of this study is to analyze in depth the formation of the amalgam, in order to provide a better understanding of the key physical processes, and so be able to judge of the accuracy of the currently used models and refine them when necessary. The amalgam formation is the result of several processes that occur roughly at two different scales: the global scale, which is microscopic, and the local scale, of the order of few nanometers. On the global scale, the dominant physical process is the mass transport, driven almost entirely by diffusion, which determines the rate of mercury deposition. Other phenomena occur at the smaller local scale. Their understanding is essential to predict precisely the volume and shape of the amalgam at shorter times. Among these local phenomena, nucleation and droplet interactions appear critical. The former sets the formation rate and the size of the isolated mercury droplets that are initially formed at the surface of the electrode. An understanding of the latter is necessary to determine the droplet coalescence process. Among the specific accomplishments of this Master thesis work, a time scale analysis of the global phenomena has been performed leading to the conclusion that quasi-steady state diffusion of mercury ions in the bulk mainly defines the electrodeposition rate. Then, a series of analytical formulations for diffusion-limited electrodeposition current available in the literature has been quickly analyzed, leading to development of analytical/numerical models. These latter have been implemented, and results were critically compared with experimental data, leading to the conclusion that the early electrodeposition was not enough finely modeled. Mercury droplets nucleation and surface interaction have been identified as relevant processes of this period. They have next been investigated in detail, leading to the characterization of the nucleation process, and the derivation of two complimentary approaches on charged droplet stability. Regime maps have been developed, providing first explanations and quantitative information on charged droplet stability dependence on potential applied, electrolyte and droplet size. Finally, through analysis of theoretical predictions, a series of electroanalytical experiments have been proposed for the future validation of the suggested theoretical models.
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