Detachment characteristics of electrolytic bubbles
Çayan Demirkır is a PhD student in the Department Physics of Fluids. Promotors are prof.dr. D. Lohse and dr. D.J. Krug from the Faculty of Science & Technology.
The global transition from fossil fuels-based energy towards renewable sources-based energy is a pressing challenge of our time. In this context, hydrogen has emerged as a promising energy carrier. Water electrolysis is a technique by which hydrogen gas (H2) can be obtained by splitting the H2O molecules. This method is especially attractive due to its compatibility with renewable power sources, offering a sustainable pathway for hydrogen production. However, the efficiency of water electrolysis is significantly affected by the formation of gas bubbles during the electrochemical water splitting. These bubbles pose a considerable challenge by blocking the active surface area of electrodes, impeding mass transfer and increasing overall ohmic resistance, thereby reducing overall system efficiency. Understanding and optimizing bubble dynamics in water electrolysis is therefore crucial for advancing this technology and supporting the broader transition to sustainable energy systems.
This thesis investigates various aspects of bubble dynamics during water electrolysis, with a focus on bubble detachment characteristics under different conditions. The research is divided into four main chapters, each addressing specific aspects of bubble behavior.
Chapter 1 examines the detachment of isolated hydrogen bubbles due to buoyancy during electrolysis. Using a horizontal transparent disk electrode, two types of bubbles were identified based on contact line formation: pinned bubbles and spreading bubbles. The study revealed that electrolyte concentration influences the predominant bubble type, with pinned bubbles dominating at lower acid concentrations and spreading bubbles at higher concentrations. Experimental evidences demonstrated that the well-known Fritz expression is not applicable for predicting bubble detachment on real (non-ideal) surfaces. We developed a model to predict the detachment radius of isolated bubbles, considering both receding and advancing contact angles.
Chapter 2 focuses on bubble dynamics during the hydrogen evolution reaction on a platinum microelectrode, investigating the influence of electrolyte composition. Experiments in four different acidic electrolytes revealed that the applied potential and electrolyte type affect bubble detachment size and periodicity. The study found that microbubble coalescence efficiency follows the Hofmeister series of anions across different electrolytes. Solutal Marangoni convection, driven by ion concentration gradients, was identified as a significant factor affecting bubble detachment which can either slow down or accelerate the departure from the electrode.
In Chapter 3, we investigated the detachment conditions of wall-attached bubbles through coalescence-induced detachment. Combining experimental observations and numerical simulations, the study revealed new insights into the contact line motion of coalescing spreading bubbles. Adhesion energy was identified as a crucial factor in determining coalescence outcomes for bubbles of comparable sizes. An existing predictive model was enhanced by including adhesion energy, expanding its applicability to different configurations and improving its success rate in predicting coalescence events.
Finally, the dynamics of hydrogen bubble pairs produced during water electrolysis using a dual platinum microelectrode setup are studied in Chapter 4. The effects of varying electrode distance and cathodic potential were systematically investigated. The research revealed different modes of bubble departure following the coalescence event, depending on electrode distance and applied potential. The study demonstrated that bubble coalescence leads to earlier departure and higher reaction rates compared to buoyancy-driven detachment alone. The electrode spacing optimisation was found to significantly enhance mean current in electrolysis systems, resulting in performance improvements of up to 2.4 times compared to using a single electrode under the same conditions.
