Flow Geometry of the Electrolyte and Gas Bubble Formation in Redox Flow Batteries - a Synchrotron Imaging Study
Authors: Eifert László , Bevilacqua Nico , Köble Kerstin , Fahy Kieran F. , Xiao Liusheng , Li Min , Duan Kangjun , Bazylak Aimy , Sui Pang-Chieh , Zeis Roswitha 
Published: December 2020

Two major limitations of Vanadium Redox Flow Batteries (VRFBs) are (1) The transport losses of getting the electrolyte into the electrode and to the reaction sites with minimum resistance and (2) Lack of access to all reaction sites due to relatively low saturation levels of the electrolyte.1,2 Although the porous carbon electrodes have high porosity, a large pressure may be required to pump the electrolyte through the electrode during operation. A key factor that influences the saturation and pumping pressure is the design of flow fields.3 Also, the presence of hydrogen bubbles can affect the saturation of the carbon electrode, which is formed as a parasitic side product of the V3+ reduction reaction at the anode in VRFBs.4 This affects the efficiency and stability of the VRFB cells. Besides mixed potentials, the Hydrogen bubbles could also damage the pore structure of the electrode and create an inaccessible surface area where otherwise the reaction would occur. To investigate the formation of gas bubbles inside the small pores of the carbon electrode, as well as the influence of flow geometry, we present a novel vanadium redox flow full-cell (Fig. 1(a)) that facilitates synchrotron X-ray imaging. 5 Three different flow geometries, i.e., serpentine, interdigitated, and flow-through, were tested as shown in Fig. 1(b).
During the experiment, the electrolyte was injected into the cell using a peristaltic pump. Radiography was conducted during the injection process to track the flow of the electrolyte through the electrode and to calculate the average saturation for each flow geometry. Further experiments include the potential-dependent changes of the saturation and the 3D-visualization of the hydrogen bubble formation during constant potential measurements via tomography (Fig. 1(c)).
This setup offers great flexibility in designing experiments for redox flow batteries, allowing the investigation of various electrode materials with different compression ratios and flow geometries under potential control. In the future, the measurements will help us develop theoretical models for a better understanding of the multiphase and interfacial flow phenomena within the porous electrode. These experiments are essential for the evaluation and optimization of electrode materials and manifolds currently being used in VRFBs. References

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Figure 1