Optical quantification of multiphase flow near a hydrogen-evolving electrode

Abstract

In this research project, multiphase behaviour in zero gap alkaline water electrolyzers is investigated, focusing on the influence of various parameters including current density, electrode-wall distance and electrode height. This involves a cell containing a vertical electrode pair separated by a membrane submerged in an electrolyte that has a liquid-air interface near the top of the electrodes. The study aims to identify and understand turbulent phenomena through different analysis methods. Additionally, the outcomes derived from the conducted experiments serve as validation data for concurrent modeling research.
The analysis begins by examining vortex formation on the cathodic side, where hydrogen bubbles originate, using Particle Image Velocimetry (PIV). The results indicate the presence of bubble vortices, especially near the top of the channel. At electrode-wall distances of 1.0 cm or more, the bubbles that fail to reach the liquid-air interface tend to circulate extensively within the channel without exhibiting any noticeable discrepancies, such as small vortices. It should be noted that some low-velocity vortices, typically on the order of millimeters per second, can be observed within the bulk of the channel. As a general rule, shorter electrode-wall distances lead to increased vortex formation throughout the entire channel. Additionally, higher current densities induce a greater amount of these vortices or chaotic and irregular motion of the bubbles in the bulk region. In modern configurations, elastic elements, such as materials of thin wires with high porosity and low solid volume fraction, are strategically incorporated into the electrolyzer design to enhance mechanical resilience and accommodate variations in pressure and temperature. These elements serve to ensure the stability and reliable performance of the electrolyzer system under diverse operational conditions. Much of the chaotic behaviour of the bubbles seems to be counteracted when the electrode-wall distance is large (1.5 cm) while an elastic element is placed in between the cathode and opposing wall. When the whole channel is filled by the elastic element (0.6 cm electrode-wall distance), this results in more small-velocity vortices than without the element.
Another examined phenomenon is the presence of a bubble mist. This mist arises when the bubbles that originate at the electrode do not escape the channel at the liquid-air interface, but recirculate towards the opposing wall and downwards from there. Light intensity coming from the bubble clouds is used to gain and calibrate results. Increasing the current density and decreasing the electrode-wall distance lead to deeper bubble mist plumes. Surprisingly, elevated electrolyte height directly influences mist depth, with larger electrolyte heights showing the highest mist depths, even when dividing these by the exposed electrode height. For these measurements, diamond shaped apertures on the electrodes are used. For another series of experiments where higher current densities are examined, slotted apertures are used and the effect of lowering the electrode-wall distance on the bubble mist depth is diminished. Whether this is due to the type of electrodes or the different measurement circumstances (for example lighting) can not be concluded from the results. This emphasizes the need for consistent experimental conditions.
Thirdly, the continuous release and upward movement of gas bubbles originating at the cathode, i.e. the bubble plumes, are investigated. The effect of changing the current density is investigated along different heights of the electrodes. The analysis performed employs three different strategies to assess turbulent behaviour. This includes the PIV method used initially to depict vortices, the light intensity coming from the bubble clouds and visual analysis to find discrepancies like bubble bursts and big bubble trajectories. The PIV method shows capabilities in tracking the plume width and depicting turbulent behaviour. The light intensity analysis however proves challenging due to disparities in lighting. Improved lighting and higher-resolution cameras could yield more reliable results, enabling further investigation into correlations between parameters and turbulence, such as bubble bursts.