Synchrotron X-ray Radiography and Tomography of Vanadium Redox Flow Batteries - Cell Design, Electrolyte Flow Geometry, and Gas Bubble Formation.

The wetting behavior and affinity to side reactions of carbon-based electrodes in Vanadium Redox Flow Batteries are highly dependent on the physical and chemical surface structures of the material, as well as on the cell design itself. To investigate these properties, we propose a new cell design to facilitate synchrotron X-ray imaging. We studied three different flow geometries towards the impact on the flow dynamics, and the formation of hydrogen bubbles. Via electrolyte injection experiments we show that the maximum saturation of carbon felt is achieved by a flat flow field after the first injection and by a serpentine flow field after continuous flow. Furthermore, the average saturation of the carbon felt is correlated to the cyclic voltammetry current response, and the hydrogen gas evolution was visualized in 3D via X-ray tomography. We outline the capabilities of our cell design and experiments, which are essential for the evaluation and optimization of cell components of VRFBs.