Insights into the hydrogen evolution reaction in vanadium redox flow batteries: A synchrotron radiation based X-ray imaging study

The parasitic hydrogen evolution reaction (HER) in the negative half-cell of vanadium redox flow batteries (VRFBs) causes severe efficiency losses. Thus, a deeper understanding of this process and the accompanying bubble formation is crucial. This benchmarking study locally analyzes the bubble distribution in thick, porous electrodes for the first time using deep learning-based image segmentation of synchrotron X-ray micro-tomograms. Each large three-dimensional data set was processed precisely in less than one minute while minimizing human errors and pointing out areas of increased HER activity in VRFBs. The study systematically varies the electrode potential and material, concluding that more negative electrode potentials of -200 mV vs. reversible hydrogen electrode (RHE) and lower cause more substantial bubble formation, resulting in bubble fractions of around 15%-20% in carbon felt electrodes. Contrarily, the bubble fractions stay only around 2% in an electrode combining carbon felt and carbon paper. The detected areas with high HER activity, such as the border subregion with more than 30% bubble fraction in carbon felt electrodes, the cutting edges, and preferential spots in the electrode bulk, are potential-independent and suggest that larger electrodes with a higher bulk-to-border ratio might reduce HER-related performance losses. The described combination of electrochemical measurements, local X-ray microtomography, AI-based segmentation, and 3D morphometric analysis is a powerful and novel approach for local bubble analysis in three-dimensional porous electrodes, providing an essential toolkit for a broad community working on bubble-generating electrochemical systems. (c) 2023 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.