Bubble up: Tracking down the vertical velocity of oxygen bubbles in parallel plate electrolyzers using CNN

Bubble-induced convection governs the flow pattern inside parallel plate electrolyzers, independent of the superficial electrolyte velocity. At the electrode surface, gas bubbles nucleate, grow and detach, increasing the gas volume fraction and accelerating the electrolyte in the proximity of the electrode. This acceleration due to buoyancy-induced bubble velocity enhances the mixing and mass transport, impacting the local concentration and, hence, the electrochemical reaction. To study the velocity and size of electrogenerated gas bubbles, we present a particle tracking velocimetry method that enables the velocity measurement directly inside the bubble curtain of a membrane-separated, parallel plate electrolyzer. By decoupling the effect of the bubble size on the bubble velocity, we study the impact of different current densities and superficial velocities of the electrolytes on the vertical bubble velocity. Our results reveal the strong dependence of the bubble velocity on the total net volume of produced gas and the thereby linked acceleration of the electrolyte near the electrode. Under no net electrolyte flow conditions, the determined vertical bubble velocities inside the bubble curtain double to triple values of single bubble experiments and predictions by commonly used drag correlations. By applying forced convection, the measured vertical velocity of equally sized bubbles decreases and shifts towards the superficial electrolyte velocity. Additionally, the horizontal bubble velocities increase at higher electrolyte velocities, indicating a broadening of the bubble curtain, as also proposed by numerical studies. The presented findings improve the understanding of gas-liquid flows in electrolyzers and, thus, the efficiency of gas-evolving parallel-plate electrolyzers.