The production of hydrogen through water electrolysis and its application as an energy carrier have become a subject of great relevance in view of the worldwide growing operation of renewable energies. However, the efficiency of this process is limited by the formation of hydrogen and oxygen gas bubbles, among other effects. These bubbles reduce the active electrode area and the effective electric conductivity of the electrolyte. The application of forced convection generated by magnetic fields shows great potential for improving the efficiency by accelerating both the detachment of these bubbles from the electrodes and their transport out of the electrolyzer. The purpose of this work is to gain further insight into the underlying magnetohydrodynamic (MHD) flow and its effect on the bubble growth, detachment and transport in the electrolyte. There are two basic configurations, electrode-normal and electrodeparallel magnetic fields, both of which generate a completely different Lorentz force distribution in the electrolyte, which in turn gives rise to different flow structures around the evolving bubbles. In this work, both magnetic field configurations were investigated in various electrochemical setups using optical flow measurement techniques. A major part of the studies has been carried out in a small electrochemical cell employing a Pt microelectrode (100 mm in diameter), which allows for a controlled generation and undisturbed observation of single hydrogen bubbles. The first study performed in this setup aimed to investigate the baseline case without the action of a magnetic field. The bubble growth dynamics and the surrounding flow field were measured using microscopic high speed imaging and Particle Tracking Velocimetry (PTV). The strong electrolyte convection observed close to the gas-liquid interface of the bubble points to the presence of a Marangoni effect, presumably caused by temperature gradients and concentration gradients of dissolved hydrogen. The latter is associated with the fact that some simple gases such as hydrogen are known to have strong surface-active properties. The observed Marangoni effect not only affects the mass transfer close to the bubble, but also induces shear stresses at the its interface, which impose a stabilizing force on the bubble and thus can influence its detachment. A similar Marangoni flow could also be observed in the following studies, which focused on the effect of the magnetic fields. When an electrode-normal magnetic field is applied, the generated Lorentz forces drive a rotating flow around the evolving bubble, as shown by 2D PTV as well as 3D Astigmatism Particle Tracking Velocimetry (APTV) flow measurements. The generated MHD flow leads to a relative pressure reduction on the lower side of the bubble, which ultimately causes a stabilizing force that tends to retard the bubble detachment. However, in the case of large electrode surfaces, as commonly used in practical applications, the magnitude of the Lorentz force generated on the upper side of the bubble is significantly larger compared to the case of a microelectrode due to a different electric field distribution. As a result, a counter-rotating flow is formed on both sides of the bubble. This could be demonstrated by further three-dimensional flow measurements around a magnetized sphere, which mimicked the Lorentz force distribution around a stationary bubble on a large electrode surface. The pressure distribution resulting from this MHD flow causes a lift force acting in favor of the bubble detachment. However, additional simulations for both the case of a large electrode and a microelectrode demonstrate that the imposed forces are not sufficient to significantly affect the bubble detachment size and thus cannot explain the observations reported in previous studies. Other effects, such as the interaction of the flow with the contact area between the bubble and the electrode or the large-scale flow generated by bubble groups as opposed to single bubbles, may play a relevant role in this context. In the second investigated case of electrodeparallel magnetic fields, the generated Lorentz forces give rise to an intense shear flow along the entire electrode surface. Further flow measurements carried out at the microelectrode by means of Particle Image Velocimetry (PIV) and PTV showed the formation of a strongly asymmetrical flow field around the bubble. This asymmetry can be attributed to the asymmetric electric field distribution that occurs as the MHD flow forces the bubble to slide along the microelectrode in flow direction. Furthermore, the experimental data showed that with increasing magnetic induction a considerable decrease of the bubble detachment size can be achieved. Additional simulations revealed the influence of the MHD-induced drag and lift force on the lateral motion of the bubble and its accelerated detachment. The experimental observations also indicate that the rapid formation and coalescence of tiny bubbles at the foot of the main bubble plays an important role in the effective surface tension force between the bubble and the electrode, thus influencing the detachment behavior of the bubble. Compared to the relatively simple case of bubble growth at a microelectrode, the fluid dynamics at large electrode surfaces become far more complex as many gas bubbles evolve simultaneously at the electrode. To characterize the convection induced by both the bubble movement and the magnetic field, another experiment with two large vertical electrodes, similar to common industrial electrolyzers, was used. The dense layer of rising bubbles that forms near the vertical electrode generates a strong unsteady shear flow, as demonstrated by PIV and PTV flow measurements. An estimation of the stability characteristics of this flow indicates that a shear–induced instability is triggered by the bubble-driven flow and thus contributes to the unsteady flow behavior. Finally, it was shown that the use of simple permanent magnets allows for a strong acceleration of the flow in the entire interelectrode gap, especially in the area near the electrode. This in turn helps to accelerate the detachment of gas bubbles and their transport out of the electrolyzer.    «

The production of hydrogen through water electrolysis and its application as an energy carrier have become a subject of great relevance in view of the worldwide growing operation of renewable energies. However, the efficiency of this process is limited by the formation of hydrogen and oxygen gas bubbles, among other effects. These bubbles reduce the active electrode area and the effective electric conductivity of the electrolyte. The application of forced convection generated by magnetic fields...    »