Marangoni effects on hydrogen bubbles on a microelectrode

In the realm of electrochemistry, understanding the intricate dynamics of bubbles on microelectrodes is essential for optimizing processes. Aled Meulenbroek's PhD research delves deep into the nuanced interplay of capillary effects governing bubble behavior. Through rigorous simulations and analysis, Meulenbroek's work sheds light on key phenomena such as surfactant-induced interface stagnation, electrolyte-mediated Marangoni forces, and the nuanced influence of microbubble carpet thickness. These findings contribute significantly to our fundamental understanding of electrochemical processes, paving the way for future advancements in the field.

During electrochemical production of hydrogen, bubbles nucleate on the electrodes. These bubbles grow and coalesce with neighboring bubbles before they detach from the electrodes. An important aspect of bubble dynamics are capillary flows that occur at the gas-liquid interface. Capillary flows are driven by surface tension gradients, and originate from temperature, concentration, and electric potential variations along the gas-liquid interface. Numerical modeling is used to predict capillary flow driven by three types of capillary convection: thermocapillary, solutocapillary (by surfactant or electrolyte concentration) and electrocapillary convection. Our numerical models are compared with microelectrode setup measurements available in literature.

Simulations of capillary convection

First, simulations of combined thermo- and solutocapillary convection are performed in which the solute is a surfactant at the bubble surface. They show that the transport of surfactants leads to a competition between thermo- and solutocapillary convection and explains the stagnant cap at the top of the bubble, as observed in measurements. Also simulations of solutocapillary convection in which the solute is the electrolyte were performed. These simulations are inspired by measurements from literature on bubble detachment radius for several acidic electrolytes. In this literature it was deduced from measurements that the Marangoni force on a hydrogen bubble was directed towards or away from the electrode depending on the electrolyte type. Meulenbroek’s simulations confirm this outcome and quantify the Marangoni flow and resulting force on a hydrogen bubble at the cathode in various acidic electrolytes. Next, a thermocapillary effect was applied, and in combination with the solutocapillary effect, the Marangoni force on the hydrogen bubble significantly decreases. As a result, the direction of the force is more difficult to predict; the prediction for the effect on bubble detachment is somewhat revised. He also performed simulations of oxygen bubbles in acidic electrolytes, and found that for the same acidic electrolyte the Marangoni force on the bubble is in the opposite direction: if the force delays detachment at the cathode it accelerates at the anode, or vice versa. Furthermore, a similar behavior is simulated for bubbles in alkaline electrolysis. The solutocapillary effect provides an explanation why oxygen bubbles are larger than hydrogen bubbles in alkaline water electrolysis.

Studying electrocapillary convection effects

A more obscure capillary effect, the electrocapillary effect, is also studied. Due to ion adsorption at the bubble interface an electric double layer is formed. In a model proposed by others in literature the Lippmann equation is applied to relate surface tension variations to surface charge density and variations in the electric field tangential to the bubble interface. We use this model to simulate electrocapillary convection for various surface charge densities. When we use the surface charge density, deduced in the same literature, in our simulation, the electrocapillary Marangoni flow is much larger than the measured Marangoni flow. We thus encounter a contradiction. It seems that either the surface charge density on the bubble should be lower or the simulation model proposed has shortcomings.

Impact of bubble growth dynamics

Finally, the influence of bubble growth on thermal Marangoni convection is investigated. We developed a method to simulate a growing bubble on a microelectrode. For bubbles growing on a carpet of microbubbles it is crucial to consider the thickness of the bubble carpet. It appears that a thicker microbubble carpet reduces the Marangoni velocity and the Marangoni force. We find that the inclusion of the bubble growth dynamics is only relevant to the Marangoni flow when the growth rate of the bubble is large. This allows us to introduce a new type of stagnant bubble simulation.

Title of PhD thesis: Marangoni effects on hydrogen bubbles on a microelectrode. Supervisors: Prof. Bert Vreman, and Prof. Niels Deen.