About

Gas producing (electro)chemical reactors are central to the energy transition in Europe. A specific example is water electrolyzers, which are expected to generate the large volumes of green hydrogen needed for a net-zero Europe. In these systems, the presence of gas bubbles significantly influences the energy efficiency and overall performance. The scientific scope of the colloquium is discussing how to optimize the gas bubble evolution and the evolving multiphase flow. The matter is complex, as gas evolution and multiphase flow in such devices are strongly coupled to electrochemical, electrical and thermal aspects.

The proposed colloquium aims to address fundamental fluid dynamics questions that are relevant in improving these systems. The focus will first be on local physical phenomena, in particular interfacial phenomena during gas evolution, that determine eventually the bubble departure and thus the performance at global level. These include, for instance, nucleation, nano-bubble phenomena and bubble growth, wetting and contact line dynamics, bubble coalescence and micro-jetting, the effects of surfactants, ions and electric fields as well as of Marangoni and shear flows on bubble departure. Special attention will be given to emerging trends such as surface-texturing and novel electrode design, e.g. porous electrodes.

We will further discuss related up-to-date numerical and experimental techniques and advancements, which include high-speed video techniques, studies at micro-electrodes and microfluidic setups, data-driven processing methods for bubbly flows, monitoring techniques, and advanced interface-resolving and dispersed multiphase methods. Finally, we intend to discuss the impact of multiphase flow on the device scale, including mass and ion transport across the cell, gas holdup, concentration and Ohmic losses and thermal aspects.

The discussion will also include alternating and emerging designs as e.g. membraneless electrolyzers and industry prospects.

We aim to bring together European researchers using both numerical and experimental methods to tackle these questions, in an attempt to (1) unify and consolidate the existing knowledge, (2) identify the most urgent knowledge gaps considering the applied interest.

Scientific Questions to be addressed

The performance of energy conversion devices that employ gas-producing electrodes (e.g. water electrolyzers) is to a high extent influenced by the bubble behavior; the layer of ‘attached’ bubbles on an electrode can impede the active reaction sites and the bubble curtain adjacent to the electrode can hamper the mass transfer. Local changes in species concentration and temperature caused by the electrochemical reaction can lead to interfacial flows and affect the reaction efficiency. The chemical composition of the electrolyte is known to have an impact on bubble coalescence, and thereby, on electrode coverage. The electric field is debated to affect wetting and bubble departure. The existence and role of nano-bubbles is intensively discussed in literature. The increasing use of porous electrodes further highlights the importance of understanding bubble-surface interactions. The industry’s current push towards reactors with higher current densities only further amplifies the impact of the bubble phenomena.

In view of the above, understanding the interplay between the interfacial, electrochemical, and transport phenomena in gas-producing devices shapes the core scientific question of the proposed colloquium. Such an understanding is crucial to determine what interactions govern the bubble behavior in such systems and how they can be predicted or manipulated towards improvements of the energy efficiency. The organizers’ approach is to mainly focus on the flow physics rather than the technological aspects as a deep physical understanding can precede significant applied improvements.

Additionally, attention will be paid to the methodological advances, both numerical and experimental, that allow new physical discoveries on the topic. To this end, the organizers foresee sessions with special focus on numerical or experimental techniques.