Yutong Wu1 Nian Liu1

1, Georgia Institute of Technology, Atlanta, Georgia, United States

Zn-Br battery is a promising technology for large-scale energy storage for the electric grid. Fundamental understanding of Br2/Br- redox, which is key to Zn-Br battery, has only been provided by electrochemical measurements. My new research group at Georgia Tech (started Jan 2017) has successfully developed a methodology based on optical microscopy to visualize Br electrochemical reactions in real time and in native liquid electrolyte. In this presentation, I will show our results including electrochemically-correlated time-lapse microscopic videos supporting these discoveries: (1) bromine complexing agent such as N-methyl-N-ethyl pyrrolidinium bromide is necessary for the formation of phase-separated bromine; (2) individual bromine microdroplets (1~20 um) tend to adhere to electrode surface at low current density, while leave electrode surface and enter electrolyte solution at high current density; (3) bromine forms with different morphology on metal and carbon electrode; (4) bromine microdroplets can be reversibly consumed by applying reductive current; (5) the morphology of bromine microdroplets that form on cycled electrode surface is different from that on fresh surface, indicating the change of surface chemistry after only one cycle; (6) bromine microdroplets slowly dissolve without electrical current, which explains their self-discharge; (7) bromine microdroplets are stable at temperature much lower their melting point, when they are super-cooled. Our findings will be interesting to a broad audience in the field of flow battery, by revealing mechanistic insight, and provide guidance on the optimization of materials and system. This platform we have developed can be adapted to study other electrochemical systems as well.