Measuring three-dimensional bubble dynamics for hydrogen production via water electrolysis

The commercialization of mass hydrogen production via water electrolysis is presently limited by low operational current densities. The optimal performance of electrolysis cells is significantly influenced by the substantial formation and residence of bubbles at high current densities. Thus, it is crucial to design electrodes with the ability for rapid bubble discharge to ensure appropriate bubble management. However, the quantitative volumetric measurements required to determine the bubble discharge ability of an electrode are not yet sufficiently accurate. This paper describes a quantitative volumetric method that combines a stereoscopic shadowgraph imaging system with particle tracking velocimetry (PTV) to measure the three-dimensional position, size, and velocity of micrometer-sized bubbles. The proposed method successfully captures hydrogen bubbles larger than 30 mu m bubbles in an alkaline water electrolyzer. Considering the different luminance patterns of small bubbles (r <= 4 pixels) and large bubbles (r > 4 pixels) in the current imaging system, a bubble-size adaptive detection algorithm is established based on the cascade correlation method to obtain the two-dimensional centroid coordinates and radius of the observed bubbles. The bubble size information is also introduced into a two-view PTV algorithm for retrieving the Lagrangian trajectory of each bubble. Both the bubble detection and PTV algorithms are validated using synthetic datasets. Once the bubble trajectories are resolved successfully, the three-dimensional bubble velocity is obtained, and the actual bubble sizes are further corrected using the depth information. Analysis of the trajectory and velocity components indicates the existence of lateral bubble motion, reflecting interactions among bubbles. The rise velocity is positively correlated with the bubble radius in two regions, respectively, and the deviation from the theoretical value reveals the influence of non-buoyancy factors. The proposed technique provides effective diagnostics of the three-dimensional dynamic characteristics of micrometer-sized bubbles and can be used to evaluate and design bubble management systems for various electrochemical energy conversion devices.