In situ Confocal Microscopy of Electrochemical Generation and Collision of Emulsion Droplets in Bromide Redox System

For redox flow battery, 1-ethyl-1-methylpyrrolidinium bromide (MEPBr) is a promising bromine-complexing agent that forms insoluble organic phase of MEPBr3. A series of optical images acquired by in-situ confocal microscopy with tens of millisecond interval visualize how MEPBr3 emulsion droplets are electrochemically generated and collide with the electrode surface. Two types of electrodes, i.e. a Pt microdisk of 10 μm diameter and a 2.5 mm long Pt wire of 25 μm diameter, show clear correlation between electrochemical behavior and optical images. The droplets starts growing on the electrode surface at the potential at which oxidative faradaic current starts flowing. As the overpotential increases, the droplets become larger adhered at the electrode surface, from which small droplets start detaching at 0.925 V or higher potential. Some of the droplets leave the surface, move back and collide with the electrode surface, thus producing current spikes, which are detected by chronoamperometry simultaneously. In-situ confocal microscopy in this study confirms that the droplets are heterogeneously generated by electrochemical reaction and that individual droplets collide back onto the electrode surface, thus providing a better understanding of the phenomena that happen at redox flow battery electrode surfaces.