Alkaline water electrolysis holds significant promise for the production of green hydrogen, playing a crucial role in decarbonization across various industrial sectors. The operational effectiveness of this technology hinges on its electrical efficiency. This is intrinsically tied to the formation and behaviour of gas bubbles at the electrode-electrolyte interface, influencing its overall performance. In the Zero-Gap configuration, the electrodes and the membrane are directly pressed against each other to reduce the ohmic resistance between the electrodes. However, this possibly introduces large bubble overpotentials, reduces active surface area, or facilitates gas crossover. Hypotheses for avoiding these shortcomings include introducing a small space between the electrode and membrane to allow bubbles to escape. This study aims to visualise and quantify the behaviour of bubbles in the near-electrode space, presenting numerous experimental findings and images from high-speed photography. The analysis encompasses multiple physical phenomena influencing bubble behaviour, offering comprehensive insights into their dynamics.
The investigation begins by examining the effect of electric fields on electrolytic bubbles. First, an electrowetting experiment was conducted on a polished nickel electrode. Surprisingly, it was discovered that this effect had no observable influence on the contact angle of bubbles with the electrode when subjected to voltages up to 1.7 V. Furthermore, an electrophoresis experiment revealed that the application of an electric field was found to have a minimal impact on the trajectory of both oxygen and hydrogen bubbles. The electric field had a maximum strength of 15 kV/m. This finding implies that electric fields do not significantly alter the behaviour of electrolytic bubbles in this context.
Two distinct cell configurations were employed further to assess bubble behaviour and bubble influence on AWE overpotentials. The classical Zero-Gap cell design was evaluated, equipped with a spacer to introduce a small gap between the electrode and membrane. Interestingly, linear sweep voltammetry experiments conducted on the Zero-Gap cell configuration contradicted previous literature, suggesting that imperfect gap control might explain this discrepancy.
In parallel, a novel cell design allowed for imaging an electrode's membrane side, which was analyzed using chronopotentiometry. Surprisingly, bubble resistances were found to be independent of current density. Additionally, the study revealed that electrode geometry is essential in influencing the potential increases due to double-layer charging and bubble evolution. The analysis of chronopotentiometry data provided insights into the periodic flow of large bubble slugs on the membrane side of the electrode, with the characteristic time of the flow features being inversely proportional to the current density. The utilization of this configuration led to the acquisition of unforeseen images from the electrode's membrane side.
Finally, a Particle Image Velocimetry (PIV) analysis of the recorded data yielded insights into the velocity fields surrounding an expanded metal electrode in a Zero-Gap configuration. The investigation revealed that bubble highways were established owing to the specific electrode geometry, exhibiting a vertical velocity approximately 1.4 times greater than the average vertical velocity.
In summary, this study delivers comprehensive visualizations of bubble dynamics near the electrode and quantifies their influence on the performance of alkaline water electrolysis. Moreover, the utilization of High-Speed - High-Resolution images enables unprecedented insights into the visualization of bubble behaviour on the electrode. These findings significantly contribute to our understanding of bubble behaviour in the vicinity of water-splitting electrodes and provide essential information for validating more comprehensive models of this technology, ultimately aiding in advancing green hydrogen production and the decarbonization of industrial sectors.