Zn/Br flow batteries are one of the promising technologies for stationary energy storage. Bromine complexing agents have been used to form phase-separated liquid polybromide charging products. However, dynamic and microscopic understandings of the nucleation and growth of the polybromides are limiteddue to the beam-sensitivity and complexity of these compounds and their surrounding electrolytes. In this communication, we report an in operando platform composed of dark field light microscopy and a transparent planar electrochemical cell to dynamically reveal the electrochemical nucleation and formation of polybromides. Using our platform, we confirm the liquid nature, reveal the pinning effect (strong interaction with Pt), memory effect (surface residual), self-discharge of the polybromide product, and discover over-oxidation as a side reaction for ZBB. We also conclude that the electrochemically formed droplets are mainly MEPBr_5,rather than previously reported MEPBr₃ with in situ Raman spectroscopy. These results provide mechanistic insightsinto the role of ionic liquid complexing agents and Zn/Br battery design. The in operando visualization platform can also be applied to study other unstable or transient products or intermediates in electrochemical reactions in ambient environment.