High-Resolution Imaging of the Electrochemical Interface by Operando Fluorescence Confocal Laser Scanning Microscopy

Coupling electrochemistry with optical techniques gives in-depth insights into the interfacial processes in action. In that context, fluorescence confocal laser scanning microscopy (F-CLSM) enables an electrode surface characterization with spatial resolution in the lateral plane (<i>xy</i>) as well as in the axial direction (<i>z</i>), perpendicular to the electrode surface. However, like most optical techniques, fluorescence microscopy has intrinsic limitations, notably in terms of resolution and sensitivity, which are investigated in this contribution by conducting F-CLSM experiments with two disk electrodes of different sizes: a large microelectrode (LME, Ø = 250 μm) and a much smaller so-called ultramicroelectrode (UME, Ø = 18 μm). We demonstrated that the diffusion layers of both microelectrodes can be imaged with sufficient resolution and sensitivity to be quantitatively compared with the simulated concentration profiles. This work highlights the intrinsic technical challenges associated with this kind of coupled experiments, and it discusses the conditions that should be fulfilled to obtain reliable results at the microscale. These results pave the way toward reaction layer imaging down to micrometric resolution and could help decipher complex electrochemical reactions possibly involving transient species.