Spatiotemporal Mapping of Electrochemical Diffusion Layers

Abstract

Although pure electrochemical techniques can provide substantial knowledge about electrochemical reaction mechanisms, they lack the ability to provide direct molecular structure information about the species involved. This inability to extract molecular information can result in mechanistic ambiguity with respect to the identity of the reaction intermediates. This information can be obtained by coupling in situ spectroscopic methodologies with electrochemical techniques. This is known as spectroelectrochemistry. A particularly problematic area of study with spectroelectrochemical techniques is the analysis of the mass transport of species within the diffusion layer of the electrode. The visualization of the diffusion layer surrounding electrodes allows for the unambiguous determination of electrode processes. However, a high degree of spatial and temporal resolution is needed as the diffusion layer in a typical electrochemical reaction extends to a thickness of hundreds of microns in tens of seconds. While traditional infrared spectroelectrochemical techniques have been valuable for the study of electrochemical processes, they do not provide the spatial and/or the temporal resolution that is needed to examine the diffusion layers produced at electrodes. This thesis focuses on the development of an IR technique that couples synchrotron based IR radiation (SIR) with electrochemistry, allowing for the concentrations of species present within the diffusion layer of an electrode to be mapped during an electrochemical reaction. The reduction of ferricyanide and oxidation of hydroquinone (HQ) are used as test redox systems to study the ability of SIR to map electrochemical diffusion layers. The resulting diffusion coefficients of ferricyanide, ferrocyanide, hydroquinone and benzoquinone are extracted using the IR method developed here and compared to those determined independently by hydrodynamic linear sweep voltammetry (HLSV). The diffusion coefficients of all species as determined by SIR diffusion layer mapping will be shown to be consistent with the diffusion coefficients determined by HLSV. This validates the ability of SIR diffusion layer mapping to monitor electrochemically generated diffusion layers.